Experimental laser parameters of Nd:YAG lasers at 1064 nm with effective pumping energy of 390W.
\r\n\tIn the book the theory and practice of microwave heating are discussed. The intended scope covers the results of recent research related to the generation, transmission and reception of microwave energy, its application in the field of organic and inorganic chemistry, physics of plasma processes, industrial microwave drying and sintering, as well as in medicine for therapeutic effects on internal organs and tissues of the human body and microbiology. Both theoretical and experimental studies are anticipated.
\r\n\r\n\tThe book aims to be of interest not only for specialists in the field of theory and practice of microwave heating but also for readers of non-specialists in the field of microwave technology and those who want to study in general terms the problem of interaction of the electromagnetic field with objects of living and nonliving nature.
",isbn:"978-1-83968-227-8",printIsbn:"978-1-83968-226-1",pdfIsbn:"978-1-83968-228-5",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"8f6a41e4f5ce0e9c48628516d7c92050",bookSignature:"Prof. Gennadiy Churyumov",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/10089.jpg",keywords:"Electromagnetic Wave, Microwave Energy Application, Electromagnetic Energy Generation, Intelligent Microwave Heating, Microwave Organic Chemistry, Microwave Reactor, Microwave Discharge, Microwave Plasma, Microwave Drying System, Tissue Microwave Heating, Measurement Automation, Industrial Microwave Process",numberOfDownloads:224,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"July 3rd 2020",dateEndSecondStepPublish:"July 24th 2020",dateEndThirdStepPublish:"September 22nd 2020",dateEndFourthStepPublish:"December 11th 2020",dateEndFifthStepPublish:"February 9th 2021",remainingDaysToSecondStep:"7 months",secondStepPassed:!0,currentStepOfPublishingProcess:5,editedByType:null,kuFlag:!1,biosketch:"Prof. Gennadiy I. Churyumov is a professor at two universities: Kharkiv National University of Radio Electronics, and Harbin Institute of Technology and a senior IEEE member.",coeditorOneBiosketch:null,coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"216155",title:"Prof.",name:"Gennadiy",middleName:null,surname:"Churyumov",slug:"gennadiy-churyumov",fullName:"Gennadiy Churyumov",profilePictureURL:"https://mts.intechopen.com/storage/users/216155/images/system/216155.jfif",biography:"Gennadiy I. Churyumov (M’96–SM’00) received the Dipl.-Ing. degree in Electronics Engineering and his Ph.D. degree from the Kharkiv Institute of Radio Electronics, Kharkiv, Ukraine, in 1974 and 1981, respectively, as well as the D.Sc. degree from the Institute of Radio Physics and Electronics, National Academy of Sciences of Ukraine, Kharkiv, Ukraine, in 1997. \n\nHe is a professor at two universities: Kharkiv National University of Radio Electronics, and Harbin Institute of Technology. \n\nHe is currently the Head of a Microwave & Optoelectronics Lab at the Department of Electronics Engineering at the Kharkiv National University of Radio Electronics. \n\nHis general research interests lie in the area of 2-D and 3-D computer modeling of electron-wave processes in vacuum tubes (magnetrons and TWTs), simulation techniques of electromagnetic problems and nonlinear phenomena, as well as high-power microwaves, including electromagnetic compatibility and survivability. \n\nHis current activity concentrates on the practical aspects of the application of microwave technologies.",institutionString:"Kharkiv National University of Radio Electronics (NURE)",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"2",totalChapterViews:"0",totalEditedBooks:"0",institution:null}],coeditorOne:null,coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"24",title:"Technology",slug:"technology"}],chapters:[{id:"74623",title:"Influence of the Microwaves on the Sol-Gel Syntheses and on the Properties of the Resulting Oxide Nanostructures",slug:"influence-of-the-microwaves-on-the-sol-gel-syntheses-and-on-the-properties-of-the-resulting-oxide-na",totalDownloads:94,totalCrossrefCites:0,authors:[null]},{id:"75284",title:"Microwave-Assisted Extraction of Bioactive Compounds (Review)",slug:"microwave-assisted-extraction-of-bioactive-compounds-review",totalDownloads:12,totalCrossrefCites:0,authors:[null]},{id:"75087",title:"Experimental Investigation on the Effect of Microwave Heating on Rock Cracking and Their Mechanical Properties",slug:"experimental-investigation-on-the-effect-of-microwave-heating-on-rock-cracking-and-their-mechanical-",totalDownloads:28,totalCrossrefCites:0,authors:[null]},{id:"74338",title:"Microwave Synthesized Functional Dyes",slug:"microwave-synthesized-functional-dyes",totalDownloads:21,totalCrossrefCites:0,authors:[null]},{id:"74744",title:"Doping of Semiconductors at Nanoscale with Microwave Heating (Overview)",slug:"doping-of-semiconductors-at-nanoscale-with-microwave-heating-overview",totalDownloads:45,totalCrossrefCites:0,authors:[null]},{id:"74664",title:"Microwave-Assisted Solid Extraction from Natural Matrices",slug:"microwave-assisted-solid-extraction-from-natural-matrices",totalDownloads:25,totalCrossrefCites:0,authors:[null]}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"252211",firstName:"Sara",lastName:"Debeuc",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/252211/images/7239_n.png",email:"sara.d@intechopen.com",biography:"As an Author Service Manager my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review, to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. Whether that be identifying an exceptional author and proposing an editorship collaboration, or contacting researchers who would like the opportunity to work with IntechOpen, I establish and help manage author and editor acquisition and contact."}},relatedBooks:[{type:"book",id:"6826",title:"The Use of Technology in Sport",subtitle:"Emerging Challenges",isOpenForSubmission:!1,hash:"f17a3f9401ebfd1c9957c1b8f21c245b",slug:"the-use-of-technology-in-sport-emerging-challenges",bookSignature:"Daniel Almeida Marinho and Henrique Pereira Neiva",coverURL:"https://cdn.intechopen.com/books/images_new/6826.jpg",editedByType:"Edited by",editors:[{id:"177359",title:"Dr.",name:"Daniel Almeida",surname:"Marinho",slug:"daniel-almeida-marinho",fullName:"Daniel Almeida Marinho"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"8494",title:"Gyroscopes",subtitle:"Principles and 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Theophile"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3092",title:"Anopheles mosquitoes",subtitle:"New insights into malaria vectors",isOpenForSubmission:!1,hash:"c9e622485316d5e296288bf24d2b0d64",slug:"anopheles-mosquitoes-new-insights-into-malaria-vectors",bookSignature:"Sylvie Manguin",coverURL:"https://cdn.intechopen.com/books/images_new/3092.jpg",editedByType:"Edited by",editors:[{id:"50017",title:"Prof.",name:"Sylvie",surname:"Manguin",slug:"sylvie-manguin",fullName:"Sylvie Manguin"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}},{type:"book",id:"3161",title:"Frontiers in Guided Wave Optics and Optoelectronics",subtitle:null,isOpenForSubmission:!1,hash:"deb44e9c99f82bbce1083abea743146c",slug:"frontiers-in-guided-wave-optics-and-optoelectronics",bookSignature:"Bishnu Pal",coverURL:"https://cdn.intechopen.com/books/images_new/3161.jpg",editedByType:"Edited by",editors:[{id:"4782",title:"Prof.",name:"Bishnu",surname:"Pal",slug:"bishnu-pal",fullName:"Bishnu Pal"}],productType:{id:"1",chapterContentType:"chapter",authoredCaption:"Edited by"}}]},chapter:{item:{type:"chapter",id:"17616",title:"Laser Applications of Transparent Polycrystalline Ceramic",doi:"10.5772/17532",slug:"laser-applications-of-transparent-polycrystalline-ceramic",body:'\n\t\tHigh power lasers are widely used in a variety of applications, including materials processing, remote sensing, free-space communications, laser particle acceleration, gravitational wave interferometers, and even inertial confinement fusion (ICF) [1]. The optical gain media of the system is the key factor for efficient laser oscillation. Since Maiman discovered the first ruby laser in 1960, numerous materials have been developed and improved to achieve high efficiency and high power for all-solid-state lasers. There are three primary groups of solid state host materials: single crystals, glasses and ceramics. Among them Nd:YAG single crystal may be the most widely used laser media. But Nd:YAG single crystal grown by conventional Czochralski method has its own insurmountable disadvantages such as expensive, time-consuming, small size and low concentration [2], which has limited its applications in high power lasers. And for Nd-doped glass material, though it is very easy to get large size and high concentration, but its thermal conductivity and gain are quite low and the laser efficiencies were not satisfying. Polycrystalline ceramics is an aggregate of crystalline grains, each randomly oriented with respect to neighboring grains. Since the 1960s, it has been speculated that a dense polycrystal of an isotropic, pure material would be optically indistinguishable from a single crystal of the same material. The only problem has been finding a fabrication method. Now materials scientists in Japan have come up with a way to mass-produce polycrystalline ceramics materials that maintain high conversion efficiency and good optical characteristics as well as single crystals. Further more, the ceramics laser materials manufacturing method has five distinct advantages over single-crystal growth:
\n\t\t\tEase of manufacture: It takes 4-6 weeks to grow crystals using the Czochralski method, but thes method makes rods in just a few days.
Less expensive: Single crystals have to be grown in an expensive iridium crucible. Ceramics rod growth requires no crucible and is also faster. The cost of a single crystal increases dramatically with its size, unlike ceramics.
Fabrication of large size and high concentration laser medium: Size of rods Single-crystal growth limits crystal size, which in turn limits the potential output power. The maximum crystal size is about 23 cm long and the Nd3+ doping concentration is no more than 2 at.%. But polycrystalline ceramic YAG can be made as large as 1 m×1 m×0.02 m and up to 4 at.% doping with no gradient.
Multi-layer and multi-functionality Ceramics structure: Ceramics fabrication could enable the incorporation of Q-switching and Raman shifting within the source, which is impossible with a single crystal.
Mass-production: Suitability Ceramics materials can be fabricated in a production-line fashion, reducing the time and cost required for single-crystal YAG rod manufacturing.
Since 1960’s, a number of researchers had speculated that a theoretically dense polycrystal of an isotropic, pure material would be optically indistinguishable from a single crystal of the same material. In 1966, Hot-pressed CaF2 doped dysprosium appears to be the first reported polycrystalline material which established laser oscillation [4]. Then several decades passed, no remarkable development had been acquired. The problem with making laser materials from ceramics material is that ceramics are polycrystalline and some of their characteristics, such as grain boundaries, pores, composition gradients and lattice imperfections, increase the scattering of light in the host. This adds to the opacity of the material, making it unsuitable for laser action. The key is to find a manufacturing method in which the crystals that make up the rod are very similar in size and small enough to have little effect on incident light with a wavelength of around 1 µm. Only in the last decade have ceramics laser materials received much attention, after manufacturing breakthrough coming with highly transparent nanocrystalline YAG doped with Ln3+ activators, in particular Nd3+ ions. In 1995, the first Nd:YAG ceramics laser was developed by Akio Ikesue and colleagues at Japan\'s Krosaki Corporation. Ikesue used a hot-press method to make the ceramics laser materials. [5] Later in 1999, another research team led by Ken-ichi Ueda improved Nd:YAG ceramics successfully by combining liquid-phase chemical reaction with vacuum sintering technique to produce the similarly-sized nanoparticles for ceramics formation. The nanoparticles are homogenous, so any pressure need not use. [6-7]. High quality, high transparent Nd:YAG ceramics with much low scattering losses have been fabricated. Optical absorption, fluorescence and emission spectra, physical and laser properties of Nd: YAG ceramics have been measured and compared with those of Nd: YAG single crystals, and almost identical superiority features have been obtained in qualitative analysis [8-10]. It shows that Nd:YAG ceramics are indeed potential superexcellent gain media for high efficient and high power lasers. Using these Nd:YAG ceramics samples, high slope efficiency of 68 % was achieved under end-pumping disk laser [11]. And for high power laser oscillation, output power from 499mW→31W→72W→88W→128W→1460W were reported one by one [12-15]. Now Nd:YAG ceramics slabs for solid-state heat capacity laser were reported hit 67kW high power. In order to suppress parasitic oscillation, Sm:YAG ceramics was fabricated as for an absorber, and its optical properties were investigated. Higher power ceramics laser is still-evolving. The biggest advantage is the scaling to the meter-size plate. As a result, the ceramics laser is the most promising active medium for the laser fusion drivers.
\n\t\t\tMaking ceramics YAG crystals is not restricted to neodymium-doped material or YAG crystals. Er3+, Yb3+, Nd3+, Eu3+, Dy3+ and Cr4+ as well as Sesquioxide host crystals can be made also. Nd:Y2O3 and Yb:Y2O3 ceramics laser materials as having an extra advantage over single crystals. It is very hard to grow a single Y2O3 crystal because its melting temperature is 2430 °C. The sintering temperature for Y2O3 is some 700 °C lower than its melting point, meaning that large Y2O3 ceramics could be manufactured using a vacuum sintering method. One of the advantages of Y2O3 is its thermal conductivity, which is twice that of YAG for ceramics materials. This could make it more appropriate for using in the femto-second lasers for industry. Ceramics Y2O3 laser generated Sub-200 fs Fourier-limited pulses in the SESAM mode locking.
\n\t\t\tAnother possibility is that ceramics laser rods could incorporate multiple-laser functionality. All ceramics passively Q-switched Yb:YAG/Cr4+:YAG microchip laser with shortest pulse width of 380 ps has been achieved.
\n\t\t\tThe still developing Nd:YAG ceramics are very good alternative to Nd:YAG single crystals for high energy pulse laser applications in the near future.
\n\t\tOptical absorption and emission measurements were carried out as follows. The normalized intensity of room temperature absorption spectrum of 1at.% Nd:YAG ceramics and 1.1at.% Nd:YAG single crystal is shown in Figure. 1(a). From this figure, we see that the main absorption peak of 2% ceramics is centered at 808.56 nm which is slightly red shifted compared to that of single crystal ~808.48 nm! Because of a slight change in the crystal field in the high neodymium concentration samples. Figure. 1(b) shows the room temperature fluorescence spectra for 1at.% Nd:YAG ceramics and 1.1at.% Nd:YAG single crystal, respectively. For comparison, the fluorescence spectrum for single crystal and ceramics are normalized and put together. A slight redshift was also observed in emission spectrum because of high neodymium concentration. The emission peak of Nd:YAG ceramics is centered at 1064.2 nm which is 0.1 nm redshifted away from that of Nd:YAG single crystal. Except the slight redshift, the two spectra are almost identical to each other.
\n\t\t\t\ta). Comparison of room-temperature absorption spectrum from 770 nm to 850 nm between Nd:YAG ceramic and single crystal. (b). Comparison of room-temperature fluorescence spectrum from 1045 to 1085nm between Nd:YAG ceramic and single crysPulse trains from Q-switched ceramic
As reported by Konoshima Chemical, Co., Ltd. and Ueda’s research group, [16] the fluorescence lifetime for single crystal and ceramics have been obtained through curve fitting on the fluorescence decay curve. The fluorescence lifetime of Nd:YAG ceramics and single crystal versus neodymium concentration. The fluorescence lifetime for 0.6% Nd:YAG single crystal and 0.9% Nd:YAG single crystal are 256.3 μs and 248.6 μs, respectively( Figure. 2), which agrees well with the earlier reports [17]. Fluorescence lifetimes of 257.6 μs, 237.6 μs, 184.2 μs and 95.6 μs have been measured, respectively, for 0.6%, 1%, 2% and 4% Nd:YAG ceramics. These data also agree well with the results in [17]. The fluorescence
\n\t\t\t\tFluorescence lifetime of Nd:YAG ceramics and single crystal versus neodymium concentration. Solid line is the fitted curve for ceramics fluorescence lifetime.
lifetime decreases dramatically when neodymium concentration exceeds 1%. The fluorescence lifetimes for 0.6% doped single crystal and ceramics are almost identical (only 1.3 μs difference). The fluorescence lifetime difference between 0.9% Nd:YAG single crystal and 1% Nd:YAG ceramics is 11 μs. It can be predicted that for the same concentration of Nd:YAG single crystal and ceramics, for example, 0.9% concentration, the lifetime difference should be less than 11 μs. From the fitted curve for ceramics fluorescence lifetime, the lifetime for 0.9% Nd:YAG ceramics is 244.2 μs, which is only 4.4 μs different from that of 0.9% Nd:YAG single crystal. It indicated that the neodymium ions inside the grain have the same conditions as those of single crystal, and the fluorescence lifetime difference is caused only by the neodymium ions in the vicinity of grain boundaries.
\n\t\t\t\tThe wavefront distortion picture of a single crystal YAG slab and ceramics YAG slab near the facet part measured by a Zygo interferometer is show in Figure. 3. From this figure, one can see that near the facet part, the wavefront was seriously distorted for the single crystal YAG. But for a ceramics Nd : YAG slab, because there is no facet problem, the wavefront distortion picture (right) shows a homogeneous pattern, which is much better than that of a single crystal. A crystalline YAG has poor optical homogeneity because of its facet structure during growing process. The optical homogeneity of ceramics YAG is good as well as glass.
\n\t\t\t\tThe wavefront distortion picture of a single crystal YAG and ceramics YAG slab
By using a quite uniformly side-around arranged compact pumping system, A high efficiency high power quasi-CW laser with a Nd:YAG ceramics rod has been demonstrated. With 450 W quasi-CW stacked laser diode bars pumping at 1064 nm, 236 W optimum output laser at 1064 nm was obtained. The optical-to-optical conversion efficiency was 52.5% and corresponding slope efficiency was 62%.
\n\t\t\t\tA schematic diagram of the laser setup is shown in Figure. 4. The Nd:YAG ceramics rod used in the experiment was 75 mm in length and 5 mm in diameter with neodymium doping level of 1 at.%. Both the end facets of the rod were flat and antireflection coated at 1064 nm in order to reduce the intra-cavity losses, and the lateral surface was frosted. The rear mirror of the laser cavity was high-reflection mirror at 1064 nm and a series of output coupling mirrors were prepared with reflectivity from 30% to 84% at 1064 nm. Thus we could find the optimized output in experiment. The cavity length was about 195 mm. The pump source was operated at 808 nm. Liquid cooling was employed to remove heat from the ceramics rod and diode heat sink. The operation temperature was kept at about 16 ℃.
\n\t\t\t\tSchematic diagram of experimental setup for side-pumped Nd:YAG ceramics rod laser.
In order to optimize the uniformity and radial profile of the pump distribution within the gain medium and decrease the coupling losses, we designed a compact side-around arranged direct radial-pumping head, of which cross-section configuration was illustrated in Figure. 5. The optical pump head consisted of nine LD stacked arrays mounted around the rod from 9 directions with proportional angle. The ceramics rod was mounted inside a flow-tube. The side-face of the ceramics rod and the emitting surface of the laser diodes were close proximity, and no coupling optics was employed between them. The coupling efficiency was by far the most desirable. Each LD stacked array consisted of five quasi-CW types LD bars, which were placed along the length of the laser rod and pumped perpendicularly to the direction of propagation of the laser radiation. Each bar generated 60 W peak powers. The arrays operating at 20% duty cycle were pulsed at a repetition rate of 1 kHz with a pulse width of 200 μs. The design of 9 LD arrays arranged around the ceramics rod symmetrical allowed optimizing the uniformity and radial profile of the pump distribution within the gain medium with good spatial overlap between pump radiation and low-order modes in the resonator, which in turn leads to a high-brightness laser output. Figure. 6 showed the 2D contour plot of pump intensity distribution simulated by computer with ray tracing method.
\n\t\t\t\tCross-section of large diode arrays compact side-pumped Nd:YAG ceramics laser head.
Contour plot of pump intensity distribution simulated by computer with ray tracing method.
By changing the rear mirror with different reflectivity of 30%, 50%, 62.5%, 78%, and 83.4%, we get a relationship laser output power as a function of the average pumping power, which was shown in Figure. 7. The output power increased almost linearly with the pumping power, and the optimum output appeared with the coupling mirror of the reflectivity near 78%. When the pump current rose to 60 A, the total average pump power was about 450 W, and the maximum average power of 236 W multi-mode laser output was obtained by using optimum output coupling mirror. The optical-to-optical conversion efficiency was as high as 52.5% and corresponding slope efficiency was 62%. No obvious evidence of saturation was observed from the output curve, which means higher output power is possible if higher pump power is available. It also indicated that the laser cavity is stable enough.
\n\t\t\t\tOutput power versus pump power for Nd:YAG ceramics laser with different coupling mirrors.
Referring to the former experimental record of a Nd:YAG single crystal with the same concentration and size using in this system with an output coupling mirror of T=70%, we made a comparison between ceramics and crystal, which was shown in Figure. 8. The optical to optical efficiencies were 29% and 27% for the ceramics laser and for the single crystal laser, respectively. The corresponding slope efficiency was 46% for ceramics laser, and 44% for single crystal laser. It showed that these two kinds of laser materials share extraordinary the same laser output properties in quasi-CW operating.
\n\t\t\t\tComparing output power of Nd:YAG ceramics and Nd:YAG single crystal at the same condition.
\n\t\t\t\t\tFigure. 9 showed the two and three-dimensional beam profiles of the Nd:YAG ceramics laser from CCD. Some interference stripes could be seen because the cavity length was fixed and the pass length differences between the transmitted beams were multiple numbers of the laser wavelength. It can be eliminated just by adjusting the cavity length slightly. The divergence angle of laser beam was measured about 12 mrad. For high power rod Nd:YAG lasers, thermal lensing and thermal stress-induced birefringence play very important roles. They would result a distortion of the laser beam and cause a significant decrease in beam quality and optical efficiencies. The detailed study will be explored later.
\n\t\t\t\tTwo and three-dimensional beam profiles o of a 236W Nd:YAG ceramics laser from CCD.
In conclusion, a high efficiency high power quasi-CW Nd:YAG ceramics rod laser operating at 1064 nm was demonstrated by using compact quasi-CW LD stacked arrays side-pumping system. High average output power of 236 W was achieved under 450 W pumping, corresponding to an optical-to-optical efficiency of 52.5% and slope-efficiency of 62%.
\n\t\t\tBased on previous work, we improved the system and thus demonstrated a high energy electro-optical Q-switched Nd:YAG ceramics laser. With 420 W quasi-CW LDA pumping at 808 nm and Q-switched repetition rate at 100 Hz, 50 mJ pulsed laser at 1064 nm was obtained with pulse width of 10 ns, an average output power of 5 W and peak power of 5 MW. Its corresponding slope-efficiency was 29.8%.
\n\t\t\t\tThe experimental setup of LDA side-pumped electro-optical Q-switched Nd:YAG laser was shown schematically in figure. 10. The radiation light emitted from the ceramics rod was first linearly polarized by a polarizer and then introduced a phase difference of a quarter of a wavelength through the quarter-wave plate. A KD*P nonlinear crystal was employed as a Pockels cell Q-switch with longitudinal field. The total length of the cavity was about 260 mm.
\n\t\t\t\tSchematic diagram of experimental setup for side-pumped E-O switched Nd:YAG ceramics rod laser.
We employed two Nd:YAG samples with the same concentration and size in our experiment. One was ceramics, and the other was single crystal. Figure. 11. showed the comparative laser output power of the two samples with different conditions. At first, the two Nd:YAG lasers were easy to operate at quasi-CW mode without the polarizer, quarter-wave plate and KD*P Q-switch. Their average output power and pulse energy increased almost linearly with the increasing of the pumping energy. The corresponding slope efficiency was 46% for ceramics laser, and 44% for single crystal laser. And the optical to optical efficiencies were 29% and 27% for the ceramics laser and for the single crystal laser, respectively. When those modulating devices were inserted into the laser cavity, the actively
\n\t\t\t\tComparing output power of Nd:YAG ceramics and single crystal at the same condition.
Q-switched operation has been observed. Both of the single crystal and ceramics Nd:YAG lasers are affected by the thermal depolarization losses, so caused a little roll over of the E-O Q-switched output power curves and a significant decrease of the optical efficiencies when compared with quasi-CW operations.
\n\t\t\t\tSingle pulse waveform of Electro-Optical Q-switched Nd:YAG lasers under modulating repetition rate of 1 kHz. (a) ceramics; (b) single crystal.
Under the max average pumping power of 420 W and 1 kHz modulating rate, the slope efficiency of ceramics sample was 15.2 % and its pulse width is 12 ns and those of single crystal sample were 17.5 % and 9.6 ns. Figure. 12. showed the single pulse shape from electro-optical Q-switched Nd:YAG crystal and ceramics lasers. The above data showed that these two kinds of laser material shared very similar laser output characteristics. The ceramics has a little better performance in quasi-CW operating while the single crystal was better in pulse operation. We speculated that the polycrystalline structure inside the ceramics body, which changes the path length of photons in the rod and adds the scattering losses of the cavity, extended the waveform distortion of the Q-switched laser pulse, and resulted in lower efficiency and broaden pulse width. As well as Nd:YAG single crystal, Nd:YAG ceramics are affected by the thermal effects when high energy pulse operation. The detail research on the thermal-optical effects of Nd:YAG ceramics laser is to be explored in another paper.
\n\t\t\t\tPaverage power, pulse energy vs. Ppump and repetition rate of Nd:YAG ceramics laser.
Next we changing the pumping condition and modulating rate to 100 Hz operation, and compared the pulse performances of Nd:YAG ceramics laser under different repetition rates. The average output power and pulse energy as functions of the pumping energy with different repetition rates have been measured and plotted in Figure. 13. With 420 W max average pumping power, an average output power of 28.3 W was achieved under the repetition rate of 1 kHz. The pulse energy was 28.3 mJ and its peak power was 2.36 MW with pulse width of 12 ns. Its slope-efficiency was 15.2%. While under the modulating repetition rate of 100 Hz, the average output power of 5 W with pulse width of 10 ns was observed. The pulse energy was 50 mJ and its peak power was 5 MW. And the corresponding slope-efficiency was 29.8%. Electro-optical Q-switched ceramics laser with higher modulating repetition rates generated higher average output power but broader pulse width and lower pulse energy and peak power. No saturation phenomenon was observed and higher output energy could be in expectation. Because the thermal build up of higher repetition rate pulse laser is more serious than that of lower repetition rate pulse laser, so the thermal depolarization losses of 1k Hz pulse laser were higher than those of 100 Hz pulse laser, which resulted lower efficiency than the latter.
\n\t\t\t\tBeam profile of Nd:YAG ceramics laser under different modulating rates from CCD.
\n\t\t\t\t\tFigure. 14. showed the three-dimensional beam profiles of the pulse Nd:YAG ceramics laser with different modulating rates and under the max pumping power of 420 W from CCD. They were approximate Gaussian beam intensity distribution, but a little distortion indicated some thermal stress-induced birefringence was existed.
\n\t\t\t\tIn conclusion, a high energy electro-optical Q-switched Nd:YAG ceramics laser has been demonstrated by employing a quite uniformly compact side-pumping system. The laser parameters between ceramics and single crystal Nd:YAG lasers have been compared and the pulse characteristics of ceramics laser with different repetition rates have been discussed in detail. With 100 Hz modulating rate, output energy of 50 mJ has been attained with pulse width of 10 ns and average output power of 5 W. And its corresponding peak power was 5 MW. While with 1 kHz modulating rate, output energy of 28.3 mJ has been achieved with pulse width of 12 ns and an average output power of 28.3 W. Table.1. Summarized the measured laser parameters with the effective pumping energy of 420 mJ at 1064 nm. It approved in experimental that Nd:YAG ceramics has comparable good performance with Nd:YAG single crystal in mJ-level energy laser output. By optimizing the design of the laser cavity, adopting higher pumping power and choosing proper repetition rate, the Nd:YAG ceramics Electro-optical Q-switched laser will obtain better performance with higher pulse energy and narrower line width as well as better beam quality.
\n\t\t\t\t1at.% Ceramics | \n\t\t\t\t\t\t\tAverage output Power | \n\t\t\t\t\t\t\tPulse energy | \n\t\t\t\t\t\t\tPulse width | \n\t\t\t\t\t\t\tPeak power | \n\t\t\t\t\t\t\tSlope efficiency | \n\t\t\t\t\t\t
Quasi-CW | \n\t\t\t\t\t\t\t236 W | \n\t\t\t\t\t\t\t236 mJ | \n\t\t\t\t\t\t\t160μs | \n\t\t\t\t\t\t\t1.6 kW | \n\t\t\t\t\t\t\t62 % | \n\t\t\t\t\t\t
Pulse (1 kHz) | \n\t\t\t\t\t\t\t28.3 W | \n\t\t\t\t\t\t\t28.3 mJ | \n\t\t\t\t\t\t\t12 ns | \n\t\t\t\t\t\t\t2.36 MW | \n\t\t\t\t\t\t\t15.2 % | \n\t\t\t\t\t\t
Pulse (100 Hz) | \n\t\t\t\t\t\t\t5 W | \n\t\t\t\t\t\t\t50 mJ | \n\t\t\t\t\t\t\t10 ns | \n\t\t\t\t\t\t\t5 MW | \n\t\t\t\t\t\t\t29.8 % | \n\t\t\t\t\t\t
Experimental laser parameters of Nd:YAG lasers at 1064 nm with effective pumping energy of 390W.
Since the emergence of semiconductor laser diodes (LD) that emit at 900 ~ 1100 nm, high power LD array are used as stabilized pumping source. The Yb doped laser material with the pumping wavelength requirement at this wavelength range attracts a lot of attention. [17] Figure. 15. shows the energy level of Yb3+ ion in the crystal Yb:YAG.[18] Yb3+ ion has very simple energy diagram with 2F7/2 as lower level and 2F5/2 as excited state manifolds separated by about 10,000 cm-1. The laser wavelength of ~ 1030 nm with transition of 2F5/2 - 2F7/2 has a terminal level of 612 cm-1 above the ground states. While the thermal energy at room temperature is 200 cm-1, the terminal state is thermally populated making the Yb:YAG a quasi-three level system. At room temperature, the thermal population of the lower laser level is about 5.5%.
\n\t\t\t\tEnergy level of Yb [2]
\n\t\t\t\t\tTable 2 summarized physical, chemical and laser properties of Yb:YAG single crystal. [19-21] Comparing to the Nd:YAG laser material, Yb doped laser material have the merits of (1) wide pumping range, (2) high quantum efficiency of over 90%, (3) longer upper-state lifetime of ~1ms, (4) no excited state absorption, (5) no up-conversion, and (6) minimal concentration quenching. With the fast development of the Nd:YAG transparent ceramics, the Yb doped laser ceramics also shows its potential as one of the ideal candidates for high-power laser application.
\n\t\t\t\tCrystal Structure | \n\t\t\t\t\t\t\tCubic | \n\t\t\t\t\t\t
Lattice Parameters (nm) | \n\t\t\t\t\t\t\t1.201 | \n\t\t\t\t\t\t
Melting Point (K) | \n\t\t\t\t\t\t\t2243 | \n\t\t\t\t\t\t
Moh Hardness | \n\t\t\t\t\t\t\t8.5 | \n\t\t\t\t\t\t
Density (g/cm3) | \n\t\t\t\t\t\t\t4.56±0.04 | \n\t\t\t\t\t\t
Specific Heat (0-20) (J/g.cm3) | \n\t\t\t\t\t\t\t0.59 | \n\t\t\t\t\t\t
Modulus of Elasticity (GPa) | \n\t\t\t\t\t\t\t310 | \n\t\t\t\t\t\t
Young\'s Modulus (Kg/mm2) | \n\t\t\t\t\t\t\t3.17*104 | \n\t\t\t\t\t\t
Poisson Ratio (est.) | \n\t\t\t\t\t\t\t0.3 | \n\t\t\t\t\t\t
Tensile Strength (GPa) | \n\t\t\t\t\t\t\t0.13 ~ 0.26 | \n\t\t\t\t\t\t
Thermal Expansion Coefficient(/K) (0~250℃) | \n\t\t\t\t\t\t|
[100]Direction | \n\t\t\t\t\t\t\t8.2*10-6\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
[110]Direction | \n\t\t\t\t\t\t\t7.7*10-6\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
[111]Direction | \n\t\t\t\t\t\t\t7.8*10-6\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Thermal Conductivity (W/m/K) | \n\t\t\t\t\t\t\t14 @ 20℃, 10.5 @ 100℃ | \n\t\t\t\t\t\t
Thermal Optical Coefficient (dn/dT, /K ) | \n\t\t\t\t\t\t\t7.3*10-6\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Thermal Shock Resistance(W/m) | \n\t\t\t\t\t\t\t790 | \n\t\t\t\t\t\t
Laser Transition | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t2F5/2 →2F7/2\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Laser Wavelength | \n\t\t\t\t\t\t\t1030nm,1048nm | \n\t\t\t\t\t\t
Photon Energy (J) | \n\t\t\t\t\t\t\t1.93*10-19 @ 1030nm | \n\t\t\t\t\t\t
Emission Linewidth (nm) | \n\t\t\t\t\t\t\t9 | \n\t\t\t\t\t\t
Emission Cross Section (cm2) | \n\t\t\t\t\t\t\t2.0*10-20\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Fluorescence Lifetime (ms) | \n\t\t\t\t\t\t\t1.2 | \n\t\t\t\t\t\t
Diode Pump Band (nm) | \n\t\t\t\t\t\t\t940 or 970 | \n\t\t\t\t\t\t
Pump Absorption Band Width (nm) | \n\t\t\t\t\t\t\t8 | \n\t\t\t\t\t\t
Index of Refraction | \n\t\t\t\t\t\t\t1.82 | \n\t\t\t\t\t\t
Thermal Optical Coefficient (/K) | \n\t\t\t\t\t\t\t9*10-6\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Loss Coefficient (cm-1) | \n\t\t\t\t\t\t\t0.003 | \n\t\t\t\t\t\t
Physical and chemical property of Yb:YAG
Comparing to rod shape medium in which heat is along the radius of the rod, so there is strong thermal gradient induced lensing and birefringence, in the thin disk shaped gain medium heat is extracted through the large faces with thermal gradients which is established across the smallest dimension and aligned with the beam propagation direction. [22] But thermo-mechanical distortion is still the bottleneck of high-power thin disk laser. Researchers brought up the idea of using composite media. During the pumping process, the undoped part of the medium helps to defuse the heat generated by the doped part, because the thermal conductivity of undoped part is usually higher than that of doped part. In the case of Yb:YAG/YAG medium, the undoped YAG acts as a passive heat sink and rebuilds the temperature field, especially along the thickness direction, and it seems that there is an imaginary cooling effect on the front face of the gain medium. [23] So by using a composite gain medium, which consists of both Yb:YAG and undoped YAG, the bending of the medium can be eliminated to some degree. Additionally, the composite medium eliminates the radiation trapping to a larger degree because the undoped YAG mitigates the effects of total internal reflection at the undoped-YAG-air interface. [24]
\n\t\t\t\t\n\t\t\t\t\tFigure. 16. shows the pictures of the composite Yb:YAG/YAG thin disk ceramics made by BAIKOWSKI, Japan. The thin disk is ~10 mm in diameter with very thin absorbing part of the disk (~0.6 mm) bonded together with a thicker undoped piece of YAG ceramics (~2.5 mm). The doping concentration is 9.8 at.% in the doped part. The composite ceramics disk is AR coated for the wavelength of ~930-970 nm and laser radiation 1030 nm at the front side and HR coated for both wavelengths at the back side. Figure.17. shows the double-pass absorptivity of the disk ceramics. There are mainly three absorption peaks in the range of 900 nm ~ 1100 nm: 937nm,968nm and 1027nm, with absorption efficiency of ~ 75%, 58% and 38.7%, absorption bandwidth of ~ 37nm, 10nm and 14nm respectively.
\n\t\t\t\tPictures of Yb:YAG/YAG composite transparent ceramics disk
Double-pass absorptivity of the composite ceramics
In order to lengthen the effective absorbing length in the thin-disk medium and make a good overlap between pump and resonator mode, a face-pumped CAMIL structure is chosen. With this structure, diode pump radiation is injected into the back face of the disk and then reflected by the face several times. The schematic diagram of the experimental setup is shown in 18. The laser medium is a composite Yb:YAG/YAG thin disk ceramics as described above. It is fixed with a layer of indium onto a heat sink, which is cooled with water from the back side. A collimated LD array with central wavelength at 970 nm working at 15 oC is used as pump source. By a focal length of ~ 9.4 cm lens, the pumping light is focused on the back side of the ceramics and the unabsorbed pumped radiation is reflected for another turn of absorption, i.e., the effective absorbing length is twice the length of the doped ceramics. A dichroic beam splitter (45o) which is coated with AR film at 970nm and HR film at 1030nm is inserted between the focusing lens and the composite ceramics for redirecting the laser to the output couplers.
\n\t\t\t\tIn the CW mode, output couplers with the same radius of curvature of 100 mm, and transmissions of 1%, 2%, 5% and 10% are used respectively. The whole cavity length is ~80 mm. In the Q-switched mode, output coupler with transmission of 10% is used. The output laser power is measured by a power meter (OPHIR, NOVA II) and the spectrum is recorded by a spectroscopy (YOKOGAWA, AQ6370), while the pulse width is recorded by an oscillograph (Lecroy, WR62XR).
\n\t\t\t\tIn the CW mode, the laser output power increases as the pump power increases with different output couplers, as shown in figure. 19. Up to 1.05W CW power is achieved with optical to optical efficiency of 5.25% with 2% output coupler. Central laser wavelength is at 1031 nm, as shown in Fig.20. We also get Q-switched output of the laser using an acousto-optic (A-O) Q-switch. We insert the A-O Q-switch device (Gooch & Housego, M080-2G) into the cavity with 10% transmission output coupler. Stable operation is achieved with the repetition rate of 1 kHz, 5 kHz, 10 kHz, 20 kHz and 30 kHz, along with the average output power of 0.44 W, 0.446 W, 0.452 W, 0.461 W and 0.47 W respectively. Figure. 21. shows the width of the pulse enlarges with the increasing repetition rate. Figure 22 shows the pulse waveform at 1 kHz: a minimal pulse width of 166 ns and corresponding peak power of 2.6 kW. Figure. 22. inset also shows the pulse serial, which appears to be a bit unstable but acceptable.
\n\t\t\t\tSchematic diagram of the experimental setup
CW laser output power vs. Pump power with different output transmission
Laser output spectrum
Average output power and Pulse width vs. Repetition rate
Both for CW and AO Q-switched mode, the optical to optical efficiency is low according to the data figure 19 and figure 21. But when we considered the actual absorbed pump power, the case would be different. There is only 53.5% of the pump power can be absorbed at the pump wavelength 970 nm, as shown in Fig.17. Moreover, further measurement reveals that the pumping wavelength drifted dramatically along with the increasing pump power. Figure 23 shows the measured relationship of pump wavelength and pump power while maintaining the temperature of the cooling water at 15oC. The pumping central wavelength drifts from 970 nm to 979 nm with the decreased pumping absorptivity from 53.5% to 38%, respectively. Figure. 19. also suggests that in higher pump power region, the laser power tended to be “saturated”, which is possibly caused by the decreasing absorption efficiency of the medium. Form this experiment, we found that it’s difficult to control the pump wavelength only by cooling in this pump source. Figure 17 indicates that the lengthened absorbing length inside the laser medium brought about 28% background absorption of the pump power, which might be caused by the quality of the media. It would raise the laser threshold. Figure 17 also shows that there is another absorption peak at around 1031 nm where is exactly the output laser wavelength located, indicating the reabsorption effect at 1031 nm. Thus, the increasing pumping power would lead to a stronger reabsorption results in a quick saturation at this wavelength. Moreover, the unabsorbed pump energy would contribute to the difficulty of the population inversion, thermal lensing, which would further reduce the efficiency and the laser output power.
\n\t\t\t\tPulse profile of minimum pulse width at 1 kHz. Inset shows the pulse serial.
Pump wavelength drifted with pump power and their corresponding absorptivity
We also study the CW and AO Q-switched laser performance of this Yb:YAG/YAG composite ceramics disk under the pumping wavelength of ~933 nm in order to further explore the high-power potential of this material by increasing the media absorption of pumping power.
\n\t\t\t\t\n\t\t\t\t\tFigure 24 shows the schematic diagram of the experimental setup using 933 nm pump source. The experimental setup is similar to that of ~970 nm pump source. The pump source is a fiber coupled LD array with central wavelength at 933 nm working at 20℃.
\n\t\t\t\tSchematic diagram of the experimental setup
In the CW mode, output couplers with the same radius of curvature of 100 mm, and transmissions of 1%, 2%, 5% and 10% are used respectively. The whole cavity length is ~80 mm. In the Q-switched mode, output coupler with transmission of 10% is used. The output laser power is measured by a power meter (OPHIR, NOVA II) and the spectrum is recorded by a spectroscopy (YOKOGAWA, AQ6370), while the pulse width is recorded by an oscillograph (Lecroy, WR62XR).
\n\t\t\t\tIn the CW mode, the laser output power increases as the pump power increase with different output couplers, as shown in Figure. 25. When the transmission of the output coupler is 2%, up to 2.575 W CW power is achieved with optical-optical efficiency of 17.6% and slope efficiency of 31.2%. Central laser wavelength is at 1030.2 nm, as shown in Fig.26. Because of the limited output power of the pump source, the maximum output laser power is not high enough. But from figure 25, the output laser shows no saturated intention, which means higher laser output can be achieved in the future.
\n\t\t\t\tWe also get Q-switched output of the laser using an acousto-optic (A-O) Q-switch. We insert the A-O Q-switch device (Gooch & Housego, M080-2G) into the cavity with 10% transmission output coupler. Stable operation is achieved with the repetition rate of 1.1 kHz, 5 kHz, 10 kHz, 20 kHz, 30 kHz, and 40 kHz along with the average output power of 1.29 W, 2.119 W, 2.221 W, 2.237 W, 2.246 W and 2.249W respectively. Figure 27 shows the width of the pulse enlarges and the maximum peak power of the pulse decreases with the increasing repetition rate. Fig.. shows the pulse waveform at 1.1 kHz: a minimal pulse width of 29 ns and corresponding peak power of 40.4 kW, single pulse energy of 1.17mJ.
\n\t\t\t\tCW laser output power vs. Pump power with different output transmission
Laser spectrum
Maximum peak power and Pulse width vs. Repetition rate
Pulse profile of minimum pulse width at 1.1 kHz.
We demonstrated a CW and Q-switched laser with composite Yb:YAG/YAG ceramics pumped by 970 nm and 933 nm LD. For the 970 nm pumping experiment, a maximum laser power of 1.05W with central wavelength at 1031 nm is obtained. A minimal pulse-width of 166ns and the maximal peak power of 2.6KW at 1 kHz are achieved, corresponding to an average output power of 0.44W. The repetition ranged from 1 kHz to 30 kHz. For the 933 nm pumping experiment, a maximum laser power of 2.575 W with central wavelength at 1030.2 nm is obtained. A minimal pulse-width of 29ns and the maximal peak power of 40.4 KW at 1.1 kHz are achieved. The repetition ranged from 1.1 kHz to 40 kHz. Table 3 summarizes the detailed results.
\n\t\t\t\tPump source | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t970 nm\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t\t\n\t\t\t\t\t\t\t\t933 nm\n\t\t\t\t\t\t\t | \n\t\t\t\t\t\t
Ceramics absorptivity of pump power | \n\t\t\t\t\t\t\t53.5%@970 nm | \n\t\t\t\t\t\t\t73%@933 nm | \n\t\t\t\t\t\t
Diameter of footprint of focusing pump laser | \n\t\t\t\t\t\t\t500 μm | \n\t\t\t\t\t\t\t600 μm | \n\t\t\t\t\t\t
Central wavelength of laser output | \n\t\t\t\t\t\t\t1031 nm | \n\t\t\t\t\t\t\t1030.2 nm | \n\t\t\t\t\t\t
FWHM of output laser | \n\t\t\t\t\t\t\t2 nm | \n\t\t\t\t\t\t\t5 nm | \n\t\t\t\t\t\t
CW maximum output power | \n\t\t\t\t\t\t\t1.05 W | \n\t\t\t\t\t\t\t2.575 W | \n\t\t\t\t\t\t
CW optical-optical efficiency | \n\t\t\t\t\t\t\t5.25% | \n\t\t\t\t\t\t\t17.6% | \n\t\t\t\t\t\t
CW slope efficiency | \n\t\t\t\t\t\t\t7% | \n\t\t\t\t\t\t\t31.2% | \n\t\t\t\t\t\t
Q-switched repetition rate | \n\t\t\t\t\t\t\t1 kHz-30 kHz | \n\t\t\t\t\t\t\t1.1 kHz-40 kHz | \n\t\t\t\t\t\t
Maximum average power under Q-switched mode | \n\t\t\t\t\t\t\t0.47 W | \n\t\t\t\t\t\t\t2.249 W | \n\t\t\t\t\t\t
Shortest pulse-width | \n\t\t\t\t\t\t\t166 ns | \n\t\t\t\t\t\t\t29 ns | \n\t\t\t\t\t\t
Maximum peak power | \n\t\t\t\t\t\t\t2.6 kW | \n\t\t\t\t\t\t\t40.4 kW | \n\t\t\t\t\t\t
Experimental results of 970nm and 933nm pumping experiment
The Nd:YAG ceramics has proven its advanced merits [25,26] and can be manufactured commercially. But scientists also exploree forward to develop new kind of ceramics in order to overcome the disadvantage exists in Nd:YAG single crystal. Recently, a new kind of laser material based on Nd:YAG-Nd:YSAG has been prepared, in which Sc3+ replaces Al3+ in YAG. Because Sc3+ has a larger size than Al3+, the entrance of Sc3+ leads to lattice expansion. In this way, more Nd3+ can be accommodated in the lattice, in other words, higher doping level is expected comparing to Nd:YAG. By far, highly and homogeneous doped Nd:YSAG has been fabricated successfully. Due to the associated increase of the absorption coefficient, it’s possible for us to use thinner laser media, which promises a higher cooling efficiency because of the higher surface area per unit volume. Thus we can reduce optical distortion and thermal stress, which are important for improving the laser beam quality. Moreover, the enhanced emission intensity, prolonged fluorescence lifetime [27, 28] and lower threshold comparing to the quasi-four-level Yb3+ doped material together highlight this novel material. It’s a suitable media for short-pulse microchip laser. 10ps ultra short-pulse laser has been generated from a passive mode-locked Nd:YSAG ceramics laser [29]. Therefore, transparent ceramics Nd:YSAG will find its way to the application of thin-disk laser and high-power miniature laser.
\n\t\t\tBut Nd:YSAG ceramics is an interesting material besides its merits discussed above. In our experiment, we demonstrated a dual-wavelength competitive laser output in Nd:Y3Sc1.5Al3.5O12 ceramics disk. In former published papers, many scientists have reported dual-wavelength in many laser materials, such as J. Lu et al. [30]. Yoichi Sato et al. also reported the appearance of dual-wavelength in Nd:YSAG [29]. We further compare the laser spectra in Nd:YAG and Nd:YSAG and then figures out possible reasons for this interesting phenomenon from the view of the material structure. The competitiveve behavior of these two wavelengths prognosticates a possible simpler way to generate Terahertz radiation.
\n\t\t\tSplitting laser level of Nd3+ in YSAG and Fluorescence spectra of Nd:YSAG ceramics
\n\t\t\tSince introducing Sc3+ into the Nd:YAG, which means that the surrounding of the optical center Nd3+ is modified, the energy level structure would be altered. By using a model Hamiltonian that assumes D2 site symmetry for the Nd3+ ions in the garnet lattice, John B. Gruber et al. figured out the energy level of Nd:YSAG [31]. The splitting laser levels of Nd3+ in YSAG-4F3/2 and 4I11/2 -are shown in Figure. 29.
\n\t\t\tThe splitting laser level of Nd3+ in YSAG, 4F3/2 and 4I11/2\n\t\t\t\t\t
Fluorescence spectra is influenced by x in Nd:Y3ScxAl5-xO12\n\t\t\t\t\t
The Nd:YSAG ceramics disks used in our experiment are made by Shanghai Institute of Ceramics, Chinese Academy of Science. By changing the x in Nd:Y3ScxAl5-xO12, we obtain the fluorescence spectra, as shown in figure 30. The two strongest peaks locate from 1060.5nm to 1062.5nm depending on the concentration of Sc, corresponding to the transition of R1-Y1 and R2-Y3 in in figure 29.Along with the increasing amount of Sc3+, the bandwidth of fluorescence broadens. Moreover, the branching ratio and position of emission peaks also change. We attributed this inhomogeneous line broadening to the expansion of lattice and complex surroundings of Nd3+ ions in the disordered YSAG lattice.
\n\t\t\tThe schematic diagram of experiment setup is shown in Figure. 31. The size of Nd:Y3Sc1.5Al3.5O12 ceramics disk is Ø12mm*1mm with Nd3+ 4 at. % doped and both the surfaces of the sample are coated with antireflection-film at 1064nm. The laser experiments are carried out at room temperature without active cooling system. A fiber-coupled LD working at the central wavelength of 808nm is used as pump source. The fiber core diameter is 200 μm with the numerical aperture of 0.22. By 2 lenses coupling system, the pump beam is focused on the ceramics’ surface to produce a pump light footprint of about 60μm in diameter. We apply a plan-plan cavity with an overall length of ~7 mm. The front mirror is antireflection coated at 808nm and highly reflecting at 1064nm. The rear mirror is the plane-parallel one as output coupler. A dichroic beam splitter (450) is used to reflect the laser and filter out the pump light. The laser output characteristics are analyzed for their spectral content and power with an optical spectrum analyzer (YOKOGAWA AQ6370) and a power meter (Spectra-Physics 407A), respectively.
\n\t\t\tSchematic diagram of the experimental setup
In the experiment, the ceramics absorbs about 64% of the pump power set on its surface. The output couplers with different transmission 3%, 3.9%, 6% and 10% at 1064nm are used for laser output experiments respectively. The results are shown in Figure. 32. The laser threshold increases from 0.345W to 1.03W with the increasing of transmission of the output coupler. The maximum output power of 0.356W is achieved at absorbed pumping power of 1.96W with the output coupler of Toc=10%. Correspondingly, the optical-optical efficiency is 18.2% and the slope efficiency is 23.2%. The emission spectra of the laser output is shown in Figure. 33., in which the absorbed pump power is 1.52 W with the output coupler transmission of 10 %. We observes two wavelengths oscillate simultaneously.
\n\t\t\tOutput power of the laser emission vs. absorbed pump power for different output couplers
Emission spectra of the laser: two wavelengths oscillates simultaneously
In order to make a comparison and get more conclusive results, we apply the same experimental environment to another 2 at. % doped Nd:YAG ceramics disk. The output transmission is Toc=3%. During the experiment, two-wavelengths’ oscillation is also observed in Nd:YAG ceramics disk. But the behavior of the two wavelengths in Nd:YAG is totally different with that in Nd:YSAG when the pump power is increasing. In Figure. 34(a), at the absorbed pump power of 0.347W which is just above the threshold, only 1064nm can oscillate in Nd:YAG. When the pumping intensity enhanced, another wavelength at 1061nm appears. Further increasing the pumping power, the intensity of 1061nm and 1064nm increase synchronously. In Figure. 34 (b), the first laser wavelength operated at 1059.9nm in Nd:YSAG. Increasing the pump power, signal laser at the wavelength of 1063.8nm and 1059.9nm radiate form Nd:YSAG simultaneously. Boosting the pumping power, a competitive laser output is shown, in which the intensity of laser at 1059.9nm decreases and that of laser at 1063.8nm increases.
\n\t\t\tDifferent behavior of (a) Nd:YAG and (b) Nd:YSAG along with increasing pump power.
Experimental and deducted data of the two wavelengths
By assuming that the total laser output only contains the power of 1059.9nm and 1063.8nm, which ignores noise, we can safely calculate the ratio of each wavelength contributed to the laser output power from the recorded 6 groups of spectrum data for different absorbed pump power. The results are shown in Figure. 35.(a), the ring (o) for 1059.9nm and the plus (+) for 1063.8nm. The green solid line is polynomial fitting curve for 1059.9nm,and the red line is for 1063.8nm. Correspondingly, the output power of each wavelength can be deducted from the recorded experimental data for Toc=3%, as shown in Figure. 35. (b). We also work out the fitted functions of the power of 1059.9nm and 1063.8nm. Laser 1059.9nm experienced a whole climbing-hill process, in which its power ascended when absorbed pump power is under 1.7W and then descended. On the other hand, laser 1063.8nm exhibited an always-climbing process. The blue line, corresponding to the sum up fitted function, is consistent with the experimental data obtained by power meter marked by start (*) in (b).
\n\t\t\tWhile considering the different laser behavior of the two ceramics samples, we attribute these to the different structure of the ceramics. By inviting Sc3+ to the Nd:YAG to make Nd:YSAG, we actually change the structure of the crystal lattice within the ceramics. When the Sc3+ ions (with ion radii larger than Al3+ ions but smaller than Y3+ and Nd3+ ions) enter the lattice, part of the Al3+ will be replaced. But the replacement is random. The difference of ion radii, chemical and physical properties between Sc3+ and Al3+ ions would lead to an almost unpredictable replacing situation. It is highly possible that around the optical center-Nd3+, one site is covered with Al3+ and the other site covered with both Al3+ and Sc3+, as illustrated in Figure.36. Besides, at different part within the ceramics, Nd3+ are affected by different but similar crystal field, originated from the disorder nature of this new material. Thus the introduction of Sc3+ creates different local environments for the Nd3+ ions which results in multiple sites having different symmetries. The effect of this substitutional disorder is also illustrated: the more Sc3+ enters YAG, the more asymmetric the lattice is, and the more evident the inhomogeneous broadening is presented. Moreover, the transition possibility between different stark levels is also changed. We assume it as multi-sites. The grain boundary within the ceramics material would produce even more complex multi-sites of optical centers, such as Nd3+ ions right at the grain boundary or within a single-crystal grain, for instance.
\n\t\t\t\tPositions of Y3+, Al3+ in YAG lattice: when Sc3+ enters the lattice, the replacement of Al3+ in octahedral site is random
In order to give a reasonable explanation of this competitive phenomenon, mutual interactions between ions (instead of isolated ion) are considered and an energy transfer model is applied, as illustrated in Figure.37. When the concentration of active ions is increased, such as in high doped materials, long before the appearance of new lines due to pairs or modifications in radiative transition probabilities, a migration of energy between centers is found. In fact, the energy transfer probability is proportional to the activator concentration [32]:
\n\t\t\t\tWt=UNA,
\n\t\t\t\twhere U is a constant that depends on the type of interaction; NA is the activator concentration.
\n\t\t\t\tLet us consider the simple case of two ions with excited states of different energies, see Figure.37. (a). Then for small energy mismatch (about 100cm-1), energy transfer assisted by one or two phonons can take place [33]. As far as Nd:Y3Sc1.5Al3.5O12 ceramics is concerned, the fluorescent intensity around 1059nm is stronger than that of around 1062.5nm. Without any control of the output laser, laser at 1059.9nm will certainly oscillate first. Meanwhile, when energy transfers from one particular site to another which has slightly different surroundings, the lattice will absorb energy as non-radiative transitions. Thus the “losing of frequency” (lowered energy) leads to the switching to longer wavelength. From the fluorescence spectra, the second highest peak is at around 1062.5nm. Since the energy mismatch between R1 (upper laser state for 1059.9nm) and R2 (upper laser state for 1063.8nm) is small (~ 82cm-1), along with the high doping concentration (4 at. % doped), it is possible that energy transfers from centers (lattice A) which radiate mainly at 1059. 9nm, to the other centers (lattice B) mainly radiating at 1063.8nm. In fact, we can assume the 1059.9nm center as sensitizer and the 1063.8nm center as activator see Figure.37. (a). With the increasing of pumping intensity, more and more transfer would take place. As a result, the 1063.8nm reaches its threshold later and forms the second laser. For there is no outside assistance to influence the transfer, it is natural that higher energy from part of active ions is transferred to other different part of ions and emitted photon with lower energy there. The thermal load of the ceramics will enhance such process. That is the reason for the competitiveve output between 1059.9nm and 1063.8nm.
\n\t\t\t\tDifferent energy transfer styles between Nd:YAG and Nd:YSAG
If now we consider another situation: two ions with their nearly equal energy of the excited state, which is the case of Nd:YAG. Bbecause the Nd3+ ions occupy identical sites in the ordered lattice and the doping concentration is much lower (2 at. % doped), the excitation will jump from one ion (lattice A) to the nearby ion (lattice B) and resulted in almost no energy loss, see Figure. 37. (b). Therefore, with the development of pumping intensity, both the two laser output power increased correspondingly. Again, this proves that the dual-wavelength output behavior is the result of Nd:YSAG’s own special and complex structure.
\n\t\t\t\tTerahertz wave attracts many scientists because of its ability to penetrate common materials without harming human tissue like typical X-rays. Several methods are developed to generate it. A very effective way to achieve that is by Difference Frequency Mixing (DFM) of near-IR lasers, usually using 2 seed sources. One of these outstanding jobs is done by Daniel Greeden et al. [9]. Since they used two seed diodes whose wavelengths are 1064.2nm and 1059nm respectively, there is highly possible that this kind of Nd:YSAG ceramics disk can be used to replace the two seed sources in the future. From Figure. 33., when absorbed pump power is 1.96W, the output of 1059.9nm and 1063.8nm are the same. From this point, utilizing this kind of ceramics laser as seed source to output two near-IR lasers amplified by double-clad fiber laser and then applying DFM methods to generate Terahertz radiation is our future blueprint of a compact Terahertz source.
\n\t\t\tIn our experiment on Nd:YSAG thin disk ceramics, we get CW laser output and demonstrated the dual-wavelength competitive output phenomenon. By comparing the different laser performance between Nd:YAG and Nd:YSAG and applying an energy transfer model, we discuss and give reasonable explanation for the dual-wavelength competetive output in Nd:YSAG as the disordered replacing of Al3+ ions by Sc3+ ions. This disordered replacing leads to a different energy transfer system in Nd:YSAG. Through the analysis of the behavior of the two wavelengths, we proposed a possible solution to make compact Terahertz source by using one laser source in the future.
\n\t\tSince the industrial revolution, a considerable increase in air pollution has been noted. According to a World Health Organization air quality report [1], inhalation of trace elements bound to airborne particulates is worsening air pollution in cities of the world, thereby causing more than 2 million premature deaths annually. In urban centers, particulate matters are major pollutants in the atmosphere, as they present health risk to dwellers. Urban particulates are known for their heterogeneous mix with diverse natural and anthropogenic origins. The composition can vary depending on geographical location, resuspended soil, atmospheric deposition and sources, which include traffic related particles such as metallic components, eroded road pavement, building construction and demolition, and power generation [2, 3]. The mean daily concentration of PM of ≤10 μm in diameter (PM10) ranges from <10 μg/m3 to 200/m3 [4]. In 2002 the USEPA reported a range of maximal city concentrations of 25–534 μg/m3 [5]. These toxic contaminants originated mainly from the anthropogenic emission sources, through ubiquitous applications of elements in urban centers including automobile, industries and domestic fuels combustion [2].
\nQuite a lot of researchers have investigated elemental compositions of suspended particulate matters in cities worldwide [4, 5, 6, 7, 8, 9]. In most of these studies, elevated levels of trace elements have been observed in atmospheric suspended dust in most cities. For example, Okunola et al. [8] reported the presence of Cd, Cr, Ni, Pb, Cu, and Zn in atmospheric settling dust in Kano metropolis of Nigeria. Meanwhile, Mafuyai et al. [9] reported that the concentrations of some trace elements were found to be far above the standard limits prescribed by WHO for respirable dust in Jos, Nigeria. Therefore, urban dwellers are exposed to considerable amounts of these elements through inhalation of airborne particulates.
\nOnce inhaled, these particles are deposited in the lung and thereby cause serious health effects. Ruby et al [10] reported that more than 80% of the binding mass of particles smaller than 2.5 μm reaches the pulmonary alveoli, where a small fraction is deposited and can stay for months to years. Zwozdziak et al [11] has also observed that elements deposition in human respiratory tract decreases with increase depth. Recognizing that dissolution of inhaled particulate-bound metal in the body has been observed to depend on the ability of such metal to be solubilized in body fluids [8], therefore it is only such soluble fraction of the elements which can be taken across the cell membrane through lung pathway that have direct effects on health. Hence, it is important to assess the bioaccessibility of trace elements bound to inhale particles over total metal concentration in particle’s matrix.
\nIn this chapter, we aimed to discuss the fates, mechanism of toxicity, and recent trends in assessment of bioaccessibility of trace elements. Attempt was made to understand influence of serum levels on trace elements in some respiratory disorders such as chronic obstructive pulmonary disease (COPD), bronchial asthma. This presentation will not consider routes of exposure other than inhalation of particulate matters.
\nTrace elements are elements present in natural materials at concentration of <1000 mgkg−1 [11]. Some of them are essential micronutrients that exist in very low concentrations in the body, forming 0.01% of the total body weight [12] while others are classified as non-essential. Generally, the major trace elements in atmospheric dust are: iron, manganese, zinc, vanadium, chromium, nickel, copper, cobalt, lead, cadmium, mercury.
\nSome trace elements are essential for human body; for cell metabolism regulation, including activation or inhibition of enzymatic reactions, and regulation of gene and membrane functions.
\nMany enzymes have trace elements within their structures and these trace elements act as a cofactor to them [13]. These enzymes play important roles in protection of the body by their activatory or inhibitory and antioxidant activities, with defense system molecules in diseases. For example, Iron is an important constituent of succinate dehydrogenase as well as part of heme of the haemoglobin, myoglobin and the cytochromes [14]. Zinc is involved in carbonic acid (Carbonic anhydrase) and in alcohol (alcohol dehydrogenase) formation, and in proteolysis (Carboxypeptidase, leucine, aminopeptidase etc) [15]. Copper is present in many enzymes involved in oxidation (tyrosinase, ceuloplasmin, amino oxidase, cytochrome oxidase) [16]. Changes in the levels of these trace elements decrease the efficiency of the antioxidants systems and lead to hyper-reactivity and inflammation in the respiratory tract [17].
\nAlthough, trace elements play important roles in various physiological processes and are crucial for functioning of the immune system. However, excessive accumulation or deficiency of some of these elements in human body may be associated with metabolic disturbance, tissue damage and infectious diseases.
\nHuman activities have been found to contribute more to environmental pollution due to the everyday manufacturing of goods to meet the demands of the large population [19]. Particulate matters in the environment emanate from two main sources: (i) Environmental sources: this include processes like forest fires, marine water sprays, and volcanic emissions, and (ii) Human-derived sources include a variety of largely industrial sources, like cement and metals manufacturing, incinerators, power plants, refineries, smelters, and vehicular exhaust and dust. Include volcanic products, minerals which occur naturally in the environment Anthropogenic activities such as Oil, natural gas production, petroleum utilization, combustion products (ie, lead in gasoline), manufacturing/industrial wastes and byproducts; commercial products (ie, lead paint in houses), or spills thereof (ie, commercial chemicals), municipal waste incinerators, landfills, sewage sludge disposal etc. Figure 1 illustrates the cycle of trace elements in atmosphere of urban centers.
\nCycling of trace elements in the urban atmosphere. Source: http://doi.org/10.1016/j.scitotenv.2019.13447.
Meanwhile, trace elements in the atmosphere originate mainly from anthropogenic emission sources, through ubiquitous applications of elements in urban centers including automobile, industries and domestic fuels combustion [20]. Trace elements emitted in wind-blown dusts are mostly from industrial areas. Some important anthropogenic sources which significantly contribute to the atmospheric pollution in urban centers include automobile exhaust which releases lead; smelting which releases arsenic, copper and zinc; insecticides which release arsenic and burning of fossil fuels which release nickel, vanadium, mercury, selenium and tin. Other metals reported on the particles are iron (Fe), Zinc (Zn), and Nickel (Ni), and recently with the use of the catalytic converters an increase in the presence of Platinum (Pt), Paladium (Pd) and Rhodium (Rh) in the particles inhaled has been observed.
\nFor a better understanding of the significances of trace element in human health, it is important to have some knowledge of their routes of exposure. Human are exposed to trace elements in the environment through different routes including ingestion, inhalation of dusts, gases, aerosols and dermal absorption (through skin). The main routes of exposure to trace elements bound to particulate matter (PM) in urban centers include occupational exposure through activities listed below for some specific elements such as:
\nCd is an environmentally widespread toxic element. It is classified as a group I carcinogen by IARC (International Agency for Research on Cancer) and has been associated with lung cancer [21]. The modes of human exposure are contamination food, drinking water, occupational or by inhalation in polluted air. Occupational exposure to cadmium primarily takes place in industrial factories such as zinc smelters, battery manufacturing and metal-recovering factories, cadmium-refining companies, production units for paint and pigment. The threshold safety cadmium exposure level has been set at 2.5 μg/kg body weight per week [21]. Cadmium (Cd) exposure is known to induce pulmonary damage such as emphysema and lung cancer [22].
\nWorldwide, lead in atmosphere originates from human activities following its uses as; gasoline additive, paints, cosmetics, ceramic glaze, etc. [23]. Lead enters the human body by ingestion or inhalation. According to the WHO-OSHA, the established safety standard for blood lead in workers is 40 μg/dL. However, it has been suggested that the criterion for elevated blood levels in children is too high in adults therefore recommended a new set of guidelines levels >15 μg/dL [24].
\nAtmospheric Manganese originated from gasoline additive, methylcyclopentadienyl manganese tricarbonyl (MMT) is a putative modulator of dopamine biology (the primary target of Mn neurotoxicity) [25].
\nChromium is widely used in the industry for the production of stainless steel, chromium plating, and spray-painting. According to World Health Organization (WHO) [26], the long term exposure of Cr (VI) levels of over 0.1 ppm causes respiratory problems, liver and kidney damage, and carcinogenicity. According to epidemiological studies, the hexavalent form [Cr (VI)] of this metal, appears to be drastically toxic and carcinogenic, thus it has been classified as carcinogenic to humans by the IARC [27].
\nAluminum and its compounds [28] are released into the atmosphere during activities such as aluminum mining, processing, production and recovery. The skin, nose, lung and gastrointestinal tract is a route for the uptake of aluminum in the body [29]. Therefore, people close to industrial areas may be exposed to aluminum through inhalation of airborne particulates.
\nElemental arsenic is a metalloid that exists in valency states; trivalent ASIII, pentavalent Asv in the environment. The main sources of exposure to arsenic include; occupational, environmental and medicinal sources. The safety level of arsenic has been lowered from 50 ppb to 10 ppb by United State Environmental Protection Agency [30]. The presence of arsenic in airborne particulate matter is considered a risk for certain diseases. All the potential pathways of its exposure seem to have adverse effect on human health [31]. Arsenic exposure has been repeatedly associated with lung carcinogenesis [32].
\nVanadium is a major transition element that is released primarily by the burning of fossil fuels, including petroleum, oil, coal, tar, bitumen, and asphaltite. Among Vanadium compounds, Vanadium pentoxide is highly toxic [33]. The IARC classified it as a possible carcinogen to humans (Group 2B) in 2003 [34].
\nOccupational studies of workers exposed to zinc by inhalation (usually in the presence of other trace elements such as copper, lead, arsenic, and chromium) have not implicated zinc as a risk factor for cancer [35].
\nThe fate and behavior of trace elements in respiratory tract are fundamental to understanding of their health effects and in recent time has become a key aspect of potential health risk assessment.
\nParticulate matters are inhaled during breathing. Upon inhalation, deposition of the particles in the lung may occur through five different mechanisms: sedimentation (gravity), inertial impaction, interception (particle-surface contact), electrostatic deposition, and diffusion. These mechanisms generally occur in different regions of the respiratory tract [36, 37]. Human respiratory tract can be divided into the upper respiratory region (nasal airway, pharynx and larynx), the lower respiratory region (trachea and bronchi) and the alveolar region. Figure 2a shows the particle size distribution in human respiratory tract. Meanwhile Figure 2(b) llustrates the health risk of trace elements and bioaccessibility questions. The extent of particle deposition in the lung is determined by the physicochemical properties of the particles, such as size, shape, density, and surface chemistry [38] (see Figure 2a). Breathing conditions, like ventilation rate, mouth or nose breathing, and airway geometry are other factors that affect particle deposition [39]. The transportation of particles into the lung can be explained by their aerodynamic diameter [40]. Meanwhile, materials with an aerodynamic diameter below 5 μm are predominantly deposited in the alveolar regions of the airways [41].
\n(a) Dust particle sizes distribution in human respiratory tract (b) human health risk and bioaccessibility questions.
When trace elements are absorbed through respiratory tract, it is transported in blood bound to metallothionen [42]. Figure 3 shows an example of such complex, where they form complex with glutathione. This is then followed by alteration of homeostasis [43], thus directly increasing the oxidative stress and lipid peroxidation.
\nGlutathione-trace element complex.
A primary mechanism for most trace elements toxicity is their effects on cells which has been ascribed to the oxidative stress promoting actions, as observed in in vivo [44] and most importantly, the inactivation of enzyme systems by binding to sulfhydryl groups [45] of proteins. The mechanisms of their actions include genetic change reactions; reactive oxygen free radicals and adduct formations, oxidative stress, and inflammation [46].
\nReactive oxygen species (ROS) such as superoxide, hydroxyl radical, nitric oxide radical are byproducts of metabolic processes. External substances such as smoke, cigerate, pesticides and inhalation of trace elements -bound particulate matters can also cause the formation of free radicals in the body. Trace elements in particulate matters have been reported to cause oxidative stress. For example, pentavalent form of vanadium is reported to cause ROS generation, thus induce oxidative stress, DNA damage, and activation of hypoxia signaling [47]. Oxidation stress is a phenomenon caused by an imbalance between production and accumulation of oxygen reactive species in cell and tissues and the ability of a biological system to detoxify these reactive products [48]. Cadmium causes liver damage mainly by induction ROS inducing lipoperoxidation via Fenton reaction [49]. The increment of ROS induces DNA damage, proteins oxidation and lipid peroxidation. Copper ions are well suited to facilitate formation of ROS that can damage biomolecules, including DNA and chromatin.
\nThe genetic changes reaction of trace elements involves: formation of DNA-protein cross-links, single and double strand DNA breaks [49, 50]. The reaction of elemental ions with nucleic acid lead to a variety of dramatic effects on the nucleic acid structure e.g. crosslinking of polymer strands, degradation to oligomer and monomers, stabilization or destabilization, and the mispairing of bases. For example, Copper can directly bind with high affinity to DNA molecule; this binding can modify the conformational structure of DNA promoting carcinogenesis [51]. Cadmium also produces genotoxicity by the production of DNA single strand breaks and damage and competes for binding at sites (specifically with a zinc finger motifs that are important in gene regulation, enzyme activity, or maintenance of genomic stability [52]).
\nIn toxicological study, the potential health risks of individual elements bound to inhale particulate matter depend on particle size, inhalability, bioavailability/bioaccessibility, exposure dose and deposition/retention in respiratory tract [53, 54]. Recently, it was emphasized that bio-toxicities of trace metals depend not only on the concentration as expressed by total amount, but also on their geochemical fractions and bioavailability [55]. Bioavailability is the fraction of total elements that can enter the human systemic circulation and exert toxicity on the organs [56]. Meanwhile, bioaccessibility refers to the fraction of contaminant that may become available for absorption e.g., solubilized in the respiratory tract fluid or volatilized into inhaled air and released from the matrix in a topically absorbable form. Bioaccessibility (%) can be defined as the ratio of soluble fraction of trace elements in simulated lung fluids (SLF) to the total concentrations.
\nThe dissolution of particulate-bound metal in the body has been observed to depend on the ability of such element to be bioaccessible (solubilized) in body fluids after inhalation [57]. Different particulate-bound elemental species behaves differently in human body after inhalation and deposition, depending on their bioaccessibility in lung fluids. In general, high bioaccessible elements are easily taken up by the lung fluids and get introduced to human circulatory system. Recognizing that only soluble fraction of the metals which can be taken across the cell membrane through lung pathway has more direct effects on health. Thus, bioaccessibility of trace elements bound to inhale particles over total metal concentration in particle’s matrix is being considered important for assessment of the overall health risk associated with inhalation of particulate matters.
\nEmerging studies [58, 59, 60] have shown risk assessment using bioaccessibility presents better understanding of the fate of trace elements upon inhalation by children and adults. However, one of the challenges for environmental toxicologist has been development of fluid with properties similar to human tracheobronchial fluids, so as to enable systematic investigation into bioaccessibility and lung deposition of particles in respiratory tracts [61]. Several fluids have been explored to mimic human respiratory tract fluids in investigation of trace elements bioaccessibility. These range from the traditional Gamble’s solution to simulated artificial lung fluids (SALF), which is simply a modification of Gamble’s solution.
\nIn one of such previous study, [62] reported that pulmonary bioaccessible fraction of Pb and Cd were relatively high (69 and 74% respectively) when lung stimulating solution (artificial lysosome fluid, ALF) was used to extract fine particles. Similarly, [63, 64, 65, 66, 67] reported higher bioaccessibility for Cd (88 ± 6.4% for PM10 and 91 ± 6.6% for PM2.5) when ALF was used as extraction fluid compared to Gamble’s solution. Tang et al [64] reported that As, Pb, V and Mn showed higher inhalation bioaccessibility extracted by the artificial lysosomal fluid (ALF); while V, As, Sr. and Cd showed higher inhalation bioaccessibility using the simulated lung fluid (SLF), suggesting differences in elemental inhalation bioaccessibility between ALF and SLF extraction. Table 1 presents the bioaccessibility values of trace elements in the three lung fluids in different reference materials, as reported by [62]. In general, one of the important factors affecting bioaccessibility of trace elements is the influence of fluid’s composition and pH.
\nLung Fluids | \nBCR-723 | \n||||||
---|---|---|---|---|---|---|---|
Trace Element | \n|||||||
\n | Cd | \nCr | \nCu | \nMn | \nNi | \nPb | \nZn | \n
PBS | \n<LD | \n0.8 ± 0.5 | \n4.1 ± 1.5 | \n0.9 ± 0.0 | \n<LD | \n<LD | \n6.8 ± 0.8 | \n
Gamble’s | \n<LD | \n0.5 ± 0.3 | \n49.9 ± 5.6 | \n1.7 ± 0.0 | \n0.8 ± 0.0 | \n7.8 ± 0.6 | \n44.6 ± 0.8 | \n
ALF | \n81.4 ± 7.6 | \n8.7 ± 0.0 | \n65.2 ± 3.7 | \n5.5 ± 0.1 | \n24.1 ± 3.7 | \n62.0 ± 3.2 | \n76.8 ± 2.2 | \n
\n | NIST2710 | \n||||||
PBS | \n44.2 ± 21.2 | \n7.8 ± 0.0 | \n8.3 ± 0.2 | \n28.7 ± 0.4 | \n<LD | \n0.04 ± 0.00 | \n6.2 ± 0.1 | \n
Gamble’s | \n86.0 ± 2.8 | \n<LD | \n47.6 ± 1.4 | \n40.1 ± 0.7 | \n<LD | \n7.8 ± 0.4 | \n23.7 ± 0.1 | \n
ALF | \n85.3 ± 8.4 | \n<LD | \n59.7 ± 1.4 | \n44.3 ± 0.2 | \n<LD | \n55.0 ± 0.5 | \n35.3 ± 0.1 | \n
\n | NIST 1648 | \n||||||
PBS | \n24.1 ± 6.2 | \n1.3 ± 0.4 | \n7.3 ± 1.8 | \n16.4 ± 1.4 | \n<LD | \n<LD | \n4.3 ± 0.2 | \n
Gamble’s | \n45.2 ± 4.0 | \n2.7 ± 1.0 | \n49.9 ± 2.7 | \n29.6 ± 0.2 | \n3.3 ± 1.2 | \n9.1 ± 0.9 | \n43.2 ± 0.2 | \n
ALF | \n65.6 ± 5.5 | \n8.7 ± 0.9 | \n55.0 ± 1.1 | \n46.8 ± 2.6 | \n12.2 ± 4.1 | \n75.9 ± 2.2 | \n66.2 ± 2.3 | \n
Bioaccessibility (%; mean ± SD; n = 3) values of trace elements in the three lung fluids (adopted from [62]).
LD, Limit of detection.
Inhalation exposure to trace elements can have significant health impacts on urban dwellers and nearby workers. Unlike other organs, lungs are directly and continuously exposed to high oxygen concentrations, exogenous oxidants, and pollutants: thus, they have the greatest susceptibility to oxidative stress and pollutant toxicity. The existence of concentration gradient within the lung and inter-individual concentration differences reveals the existence of two groups of elements: (i) homogeneously distributed over the lung e.g. elements Br, Cs, Cu, K, Na, Rb, Se and Zn, and (ii) heterogeneously distributed e.g. elements such as Cd, Co, Cr, Pb, Sb, Sc and V [68].
\nThe enrichment of trace elements in the lung tissue is known to result a number of lung diseases. These diseases have been associated with disturbance of trace elements balance [69]. Here, we discussed recent observations on variation of serum levels in diseases such as chronic obstructive pulmonary disease (with or without hypertension), emphysema, bronchiectasis and bronchial asthma, non-tuberculose mycobacterial (NTM) lung disease, idiopathic pulmonary fibrosis (IPF).
\nMany trace elements have activator or inhibitory roles in the antioxidants defensive mechanism in diseases. Recent study [70] showed that serum levels of Co, Cu and Fe were higher in COPD patients with pulmonary hypertension compared to COPD patients without pulmonary hypertension. Similarly, [70] reported that the serum copper (Cu) in COPD patients were higher than the control group.
\nBronchial asthma is a chronic inflammatory disease of the respiratory tract with an unknown etiology where inflammation is often associated with an increase generation of ROS [71]. Several trace elements are known to be capable of causing bronchial asthma, such as nickel (Ni), Chromium (Cr), Cobalt (Co) etc. Table 2 presents the variations in concentrations of some trace elements (Zn, Cu and Se) in serum of asthmatic, as observed in a study [66]. The results showed higher Cu concentration, and Cu/Zn and lower Cu/Se ratios.
\n\nIdiopathic pulmonary fibrosis is an interstitial lung disease with poor prognosis and an undefined etiopathogenesis [72] leading rapidly to death. It is the most common lung disease with estimated incidence of 2.8–9.3% per 100,000 per year in Europe and America [73]. Particulate matters bound trace elements deposited in the lung may give rise to more or less marked pulmonary fibrosis, depending on intrinsic properties and amount of the particulate matters. Oxidative stress by trace elements contributes to alveolar injury and fibrosis development in patients. A study [74] reported that IPF patients had significantly increased sputum levels of Cd, Cr, Cu and Pb respect to control. Table 3 presents the variations in concentrations of some trace elements in serum in patents with NTM, TB and healthy as control, as reported by [74].
\nElement (μg/L) | \nPatients with NTM (n = 95) | \nPatient with TB (n = 97) | \nHealthy control (n = 99) | \n
---|---|---|---|
Co (μg/L) | \n0.24(0.20–0.35) | \n0.54(0.22–0.83) | \n0.23(0.19–0.27) | \n
Cu (μg/L) | \n109(97–134) | \n129(111–153) | \n91(82–102) | \n
Cr (μg/L) | \n0.23(0.19–0.27) | \n0.23(0.18–0.27) | \n0.23(0.19–0.28) | \n
Mn (μg/L) | \n0.90(0.81–1.07) | \n0.93(0.71–1.31) | \n0.92(0.80–1.23) | \n
Se (μg/L) | \n105(95–116) | \n108(99–119) | \n115(105–123) | \n
Zn (μg/L) | \n94(84–107) | \n84(75–93) | \n102(92–116) | \n
Serum levels of trace elements in patents with NTM, TB and healthy [75].
Non-tuberculose mycobacterial lung diseases are emerging cause of pulmonary infection and are becoming more common in the clinical setting. A recent study [75] showed that serum concentration of copper and molybdenium (Table 4) were higher in patients with NTM lung disease (109 vs. 91 μg/dL, p < 0.001 and 1.70 vs. 0.96 μg/L, p < 0.001). In contrast, the media serum concentrations of Selenium and Zinc were significantly lower in patients with non-tuberculose mycobacterial lung diseases than in healthy control (105 vs. 115 μg/L, p < 0.001 and 94 vs. 102 μg/dL, p < 0.001).
\nElement (μg/mL) | \nPatient | \nControl | \n
---|---|---|
Cd (μg/mL) | \n110 | \n54 | \n
Cu (μg/mL) | \n330 | \n635 | \n
Pb (μg/mL) | \n1217 | \n1444 | \n
Mn (μg/mL) | \n399 | \n522 | \n
Se (μg/mL) | \n1496 | \n1443 | \n
Zn (μg/mL) | \n2515 | \n2699 | \n
Serum levels of trace elements in patents with Haemodialysis compare with control [76].
Oxidants-antioxidants balance is essential for the normal lung function. Both, an increased oxidant and/or decrease antioxidant may reverse the physiologic oxidants-antioxidants balance, leading to lung injury. Available data (Table 4) suggested that the levels of Cd, Cr, Pb, and V were higher and the levels of Se, Zn and Mn were lower in hemodialysis patients compare with controls [76].
\nParkinson disease, also known as manganism is an extrapyramidal neurological disease characterized by rigidity action tremor, bradykinesia, memory and cognitive dysfunction that occurs in workers exposed to airborne Mn. The element (Mn) in blood crosses the blood brain barrier and accumulates inside the neuron disrupting the synaptic transmission and inducing glial activation [77].
\nTrace elements bound to particulate matter could be trapped and deposited along the nasal cavity through inhalation of air-borne particulate matter. In this chapter, we attempted to understand influence of serum levels and bioaccessibility of trace elements in some respiratory fluids. Our investigation provides evidence that enrichment of trace elements in the lung tissue is known to result a number of lung diseases, such as chronic obstructive pulmonary disease (with or without hypertension), bronchial asthma, non-tuberculose mycobacterial (NTM) lung disease, and idiopathic pulmonary fibrosis (IPF). The findings suggest that serum Cu were higher in asthmatic patients and COPD patients than the healthy. Meanwhile, the levels of Se, Zn and Mn were lower in hemodialysis patients and non-tuberculose mycobacterial lung diseases than in healthy control.
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