Polarizations states of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers.
1. Introduction
Ytterbium doped laser materials have been intensely investigated for developing high power laser-diode pumped solid-state lasers around 1 m (Krupke 2000). Yb:YAG as crystals and polycrystalline ceramics are one of the dominant laser gain media used for solid-state lasers (Lacovara et al., 1991; Brauch et al., 1995; Bruesselbach et al., 1997; Taira et al., 1997;
Dong et al., 2006
) owing to the excellent optical, thermal, chemical and mechanical properties (Bogomolova et al., 1976). Owing to the small radius difference between yttrium ions and ytterbium ions (Dobrzycki et al., 2004), Yb:YAG single-crystal doped with different Yb concentrations can be grown by different crystal growth methods and efficient laser performance has been achieved (Brauch et al., 1995; Patel et al., 2001;
Dong et al., 2006
). Transparent laser ceramics (Lu et al., 2000; Lu et al., 2001; Lu et al., 2002; Takaichi et al., 2003;
Dong et al., 2006
) fabricated by the vacuum sintering technique and nanocrystalline technology (Yanagitani et al., 1998) have been proven to be potential replacements for counterpart single crystals because they have several remarkable advantages compared with single-crystal laser materials, such as high concentration and easy fabrication of large-size ceramics samples, multilayer and multifunctional ceramics laser materials (Yagi et al., 2006;
Dong et al., 2007
). Efficient and high power laser operation in Nd3+- and Yb3+-ions doped YAG ceramics has been demonstrated (Lu et al., 2002;
Dong et al., 2006
). Yb:YAG has been a promising candidate for high-power laser-diode pumped solid-state lasers with rod (Honea et al., 2000), slab (Rutherford et al., 2000), and thin disk (Giesen et al., 1994; Stewen et al., 2000) configurations. The quasi-three-level laser system of Yb:YAG requires high pumping intensity to overcome transparency threshold and achieve efficient laser operation at room temperature (Dong & Ueda 2005). The thin disk laser has been demonstrated to be a good way to generate high power with good beam quality owing to the efficiently cooling of gain medium and good overlap of the pump beam and laser beam (Giesen et al., 1994). However, in the thin disk case, the pump beam must be folded many times into thin laser gain medium disk with mirrors in order to absorb sufficient pump power, which makes the laser system extremely complicated. Some applications require that the lasers should be compact and economic; therefore, the cooling system is eliminated in compact and easily maintainable laser system. Therefore, laser-diode end-pumped microchip lasers are a better choice to achieve highly efficient laser operation under high pump power intensity. The thinner the gain medium, the better the cooling effect, therefore, heavy doped Yb:YAG gain media are the better choice for such lasers. The development of Yb:YAG ceramics doped with 1 at.% Yb3+ ions have been reported (Takaichi et al., 2003), but the efficiency of such Yb:YAG ceramic laser is low owing to the deficient activator concentration. In principle, there is no concentration quenching effect in Yb:YAG, however, the unwanted impurities (such as Er3+, Tm3+, Ho3+, and so on) from raw materials will be deleterious to the laser performance owing to the high activator doping. Concentration dependent optical properties and laser performance of Yb:YAG crystals have been reported(Yin et al., 1998; Qiu et al., 2002; Yang et al., 2002;
Dong et al., 2007
). The concentration quenching of Yb:YAG crystals has been investigated and it was found that fluorescence lifetime decreases when the Yb concentration is greater than 15 at.% and lifetime decreases up to 15% when the Yb concentration reaches to 25 at.% (Sumida & Fan 1994; Yang et al., 2002). The fluorescence lifetime of Yb3+ doped materials is usually affected by the radiative trapping and concentration quenching effects (M. Ito et al., 2004). Radiative trapping and concentration quenching effects become stronger with Yb concentration and there is a concentration region (from 15 to 25 at.% for Yb:YAG crystal), two trends compete each other and consequently compensate each other, leading to a constant value of measured fluorescence lifetime. Therefore special technologies have been taken to eliminate the radiative trapping effect when the fluorescence lifetime is measured for Yb doped materials. Optical-thin samples or powder sandwiched between two undoped YAG crystals were used to measure the radiative lifetime of Yb:YAG crystals (Sumida & Fan 1994; Patel et al., 2001). The radiative lifetime of Yb:YAG crystal was found to decrease with Yb concentration. Optical spectra of Yb:YAG ceramics doped with different Yb3+-lasant concentration (
Compact, high beam quality laser-diode pumped passively Q-switched solid-state lasers with high peak power are potentially used in optical communications, pollution monitoring, nonlinear optics, material processing and medical surgery, and so on(Zayhowski 2000). Passively Q-switched solid-state lasers are usually achieved by using neodymium or ytterbium doped crystals as gain media and Cr,Ca:YAG as saturable absorber(Zayhowski & Dill III 1994; Lagatsky et al., 2000; Takaichi et al., 2002; Dong et al., 2006 ) or semiconductor saturable absorber mirror (SESAM)(Spuhler et al., 2001) as saturable absorber. Compared with SESAM, Cr4+ doped bulk crystals as saturable absorber have several advantages, such as high damage threshold, low cost, and simplicity. The output pulse energy from passively Q-switched solid-state lasers is inversely proportional to the emission cross section of gain medium and reflectivity of the output coupler according to the passively Q-switched theory(Degnan 1995). Besides the broad absorption spectrum (Bruesselbach et al., 1997), longer fluorescence lifetime(Sumida & Fan 1994), high quantum efficiency (over 91% with pump wavelength of 941 nm and laser wavelength of 1030 nm) (Fan 1993) of Yb:YAG gain medium and easy growth of high quality and moderate concentration crystal without concentration quenching (Patel et al., 2001), smaller emission cross section of Yb:YAG (about one tenth of that for Nd:YAG) (Dong et al., 2003) is more suitable to obtain high pulse energy output than Nd:YAG in passively Q-switched solid-state lasers. Another interest in Yb:YAG lasers is that the frequency doubled wavelength of 515 nm matches the highest power line of Ar-ion lasers, thereby leading to the possibility of an all solid-state replacement (Fan & Ochoa 1995). Linearly polarized laser output was observed in these compact passively Q-switched lasers (Li et al., 1993; Yankov 1994; Kir'yanov et al., 1999; Yoshino & Kobyashi 1999; Dong et al., 2000; Bouwmans et al., 2001). The causes of the linearly polarized output in these passively Q-switched lasers were attributed to the influence of the pump polarization(Bouwmans et al., 2001), relative orientations of the switch and an intracavity polarizer(Kir'yanov et al., 1999), temperature change induced weak phase anisotropy(Yoshino & Kobyashi 1999), and the anisotropic nonlinear saturation absorption of Cr,Ca:YAG crystal under high laser intensity(Eilers et al., 1992). The anisotropic nonlinear absorption of Cr,Ca:YAG crystal induced linearly polarization in passively Q-switched lasers with Cr,Ca:YAG as saturable absorber held until appearing of transparent rare-earths doped YAG laser ceramics(Lu et al., 2002; Dong et al., 2006 ). Efficient laser operation in Nd3+:YAG and Yb3+:YAG ceramic lasers has been demonstrated(Lu et al., 2002; Dong et al., 2006 ; Nakamura et al., 2008). Chromium doped YAG ceramic has also been demonstrated to be a saturable absorber for passively Q-switched Nd:YAG and Yb:YAG ceramic lasers(Takaichi et al., 2002; Dong et al., 2006 ). Recently, laser-diode pumped passively Q-switched Yb:YAG/Cr:YAG all-ceramic microchip laser has been demonstrated( Dong et al., 2006 ), and pulse energy of 31 J and pulse width of 380 ps have been achieved with 89% initial transmission of the Cr,Ca:YAG ceramic as saturable absorber and 20% transmission of the output coupler. However, there is coating damage occurrence because of the high energy fluence with low transmission of the output coupler. There are two ways to solve the coating damage problem: one is to improve the coating quality on the gain medium which is costly; the other is to increase the transmission of the output coupler to decrease the intracavity pulse energy fluence. Therefore, 50% transmission of the output coupler was used to balance the output pulse energy and intracavity pulse energy, for this case, the initial transmission of Cr,Ca:YAG can be further decreased to obtain high energy output according to the passively Q-switched solid-state laser theory(Degnan 1995). The laser performance of passively Q-switched Yb:YAG/Cr,Ca:YAG all-ceramic microchip laser was further improved by using 20% initial transmission of the Cr,Ca:YAG ceramic as saturable absorber and 50% transmission of the output coupler, and no coating damage were observed with high pump power( Dong et al., 2007 ). Highly efficient, sub-nanosecond pulse width and high peak power laser operation has been observed in Yb:YAG/Cr4+:YAG composite ceramics( Dong et al., 2007 ; Dong et al., 2007 ). Although linearly polarized states was reported in passively Q-switched Nd:YAG/Cr,Ca:YAG ceramic lasers(Feng et al., 2004), the extinction ratio was very small. The crystalline-orientation self-selected linearly polarized, continuous-wave operated microchip lasers were demonstrated by adopting [111]-cut Yb:YAG crystal(Dong et al., 2008) and [100]-cut Nd:YAG crystal(McKay et al., 2007) as gain medium.
Here, we report on the systematical comparison of the performance of miniature Yb:YAG (
2. Experiments
To compare the laser performance of Yb:YAG ceramics and single-crystals, double-pass pumped miniature lasers were used in the experiments. To absorb sufficient pump power, high doping concentration was needed for thin gain medium. Therefore, high doping concentration Yb:YAG single-crystals and ceramics were used in the laser experiments. Three Yb:YAG ceramics samples (
Figure 2 shows a schematic diagram of the experimental setup for laser-diode pumped Yb:YAG miniature laser. One surface of the sample was coated for antireflection both at 940 nm and 1.03 m. The other surface was coated for total reflection at both 940 nm and 1.03 m, acting as one cavity mirror and reflecting the pump power for increasing the absorption of the pump power. Plane-parallel fused silica output couplers with transmission (
Figure 3 shows a schematic diagram of experimental setup for passively Q-switched Yb:YAG microchip laser with Cr,Ca:YAG as saturable absorber. Two Yb:YAG samples are used as gain media, one is Yb:YAG ceramic doped with 9.8 at.% Yb, the other is [111]-cut Yb:YAG crystal doped with 10 at.% Yb. The thickness of Yb:YAG samples is 1 mm, and the Yb:YAG samples are polished to plane-parallel. One surface of the gain medium was coated for anti-reflection at 940 nm and total reflection at 1.03 m acting as one cavity mirror. The other surface was coated for high transmission at 1.03 m. Two 1-mm-thick, uncoated Cr,Ca:YAG ceramic and [111]-cut Cr,Ca:YAG crystal with 80% initial transmission, acting as Q-switch, was sandwiched between Yb:YAG sample and a 1.5-mm-thick, plane-parallel fused silica output coupler with 50% transmission. Total cavity length was 2 mm. The initial charge concentration of CaCO3 and Cr2O3 in growth of Cr,Ca:YAG crystal and fabrication of Cr,Ca:YAG ceramic are 0.2 at.% and 0.1 at.%, respectively. The absorption center of Cr,Ca:YAG sample centered at 1 m is also strongly affected by the annealing process and the exact concentration of this absorption center is difficult to determine, the concentration center of this absorption is roughly about 4% of the initial Cr doping concentration (Okhrimchuk & Shestakov 1994). Therefore, the initial transmission of the Cr,Ca:YAG saturable absorber is usually used in comparing the laser performance of passively Q-switched lasers. The initial transmission of Cr,Ca:YAG is governed by the doping concentration and the thickness of the sample, to fully compare laser performance with our previously passively Q-switched Yb:YAG/Cr,Ca:YAG all-ceramic microchip laser and the effect of polarization states on the passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers, 1-mm-thick Cr,Ca:YAG crystal with 80% initial transmission was used in the experiment. It should be noted that the polarization behavior keeps the same if a different modulation depth of Cr,Ca:YAG saturable absorber is used. A high-power fiber-coupled 940 nm laser diode with a core diameter of 100
3. Results and discussion
3.1. Continuous-wave Yb:YAG miniature lasers
Figure 4 shows the output power of miniature Yb:YAG ceramics and single-crystal lasers as a function of the absorbed pump power for different Yb concentrations and
The absorbed pump power for reaching laser thresholds of Yb:YAG single-crystal (
However, for Yb:YAG ceramics, owing to the random distribution of Yb:YAG crystalline particles, the absorbed pump power threshold can be achieved more easily. Output power increases linearly with absorbed pump power for Yb:YAG single-crystals doped with 10 and 15 at.% Yb. The slope efficiencies of miniature lasers based on Yb:YAG single-crystals doped with 10 and 15 at.% Yb were measured to be 69, 62% for
For Yb:YAG single-crystal doped with 20 at.% Yb, the output power increases with the absorbed pump power, and tends to increase slowly when the absorbed pump power is higher than a certain value (e.g. 3 W for
Figure 5 shows the optical-to-optical efficiencies of Yb:YAG lasers as a function of absorbed pump power. Under present laser experimental conditions, there is no saturation effect of Yb:YAG lasers with different output couplings for Yb concentration equal to or less than 15 at.% although the optical efficiency increases slowly with the absorbed pump power. However, for 20 at.% Yb:YAG, there is saturation effect for ceramic lasers with
Figure 6 shows the maximum optical-to-optical efficiency under available pump power of Yb:YAG ceramics and single-crystals lasers as a function of Yb concentrations for different output couplings. For Yb:YAG single crystals, the maximum optical-to-optical efficiency decreases with Yb concentrations, there are 45% and 56% dropping for
Figure 7 shows the comparison of the laser emitting spectra of 9.8 at.% Yb:YAG ceramic and 10 at.% Yb:YAG single-crystal miniature lasers under different absorbed pump power for
The output beam transverse intensity profiles were also monitored in all the pump power range.
One example of the beam intensity profile at output power of 2.5 W for 12 at.% Yb:YAG ceramics with
3.2. Passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers
Four combinations of Yb:YAG and Cr,Ca:YAG were used in the laser experiments to investigate the polarization states of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers: C1, Yb:YAG crystal + Cr,Ca:YAG crystal; C2, Yb:YAG ceramic + Cr,Ca:YAG ceramic; C3, Yb:YAG crystal + Cr,Ca:YAG ceramic; C4, Yb:YAG ceramic + Cr,Ca:YAG crystal. The polarization states of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers with different combinations were investigated by measuring the output power after polarizer. Table 1 summaries the polarization states observed in passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers with different combinations of Yb:YAG, Cr,Ca:YAG crystals and ceramics. By rotating the combination of Yb:YAG/Cr,Ca:YAG, the polarization states of these lasers do not change, only the polarization directions are changed by arranging Yb:YAG or Cr,Ca:YAG. Rotating any one of sample does not affect the polarization states and no stronger influence on the polarization was observed.
No. | Combinations | Polarization |
C1 | Yb:YAG crystal + Cr,Ca:YAG crtstal | Linear |
C2 | Yb:YAG ceramic + Cr,Ca:YAG ceramic | Random |
C3 | Yb:YAG crystal + Cr,Ca:YAG ceramic | Linear |
C4 | Yb:YAG ceramic + Cr,Ca:YAG crystal | Linear |
Figure 10 shows the typical polarization states of four combinations. Except for the random oscillation of Yb:YAG/Cr,Ca:YAG all-ceramics combination, other three combinations exhibit linearly polarization output. The extinction ratio of the linearly polarization is greater than 300:1. Some differences between the extinction ratios for different linearly polarization were observed. The extinction ratios of three different linearly polarized combinations are in the order of C1 > C4 > C3. The extinction ratios of three different linearly polarized combinations decrease a little with increase of the pump power, we did not observe significant decrease of the extinction ratio at the maximum pump power used here, this shows that the thermal effect under current available pump power is not strong enough to induce sufficient birefringence and depolarization for Yb:YAG crystals and ceramics. However, we did observe the thermal effect under high pump power level for cw Yb:YAG microchip lasers(Dong et al., 2008), therefore, the thermal effect induced birefringence and depolarization should be considered in high power pumped passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers. The different polarization states between all-ceramics combination and three others are due to the random distribution of nanocrystalline particles in ceramics. To fully understand the nature of polarization states in passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers, we measured the polarization states of Yb:YAG crystals and ceramics by removing Cr,Ca:YAG saturable absorber and found that Yb:YAG crystals oscillate at linearly polarization states selected by the crystalline-orientations in the (111) plane(Dong et al., 2008) and Yb:YAG ceramic oscillates at unpolarization states. Although there is saturation absorption in Cr,Ca:YAG ceramic, the same as that for Cr,Ca:YAG crystal, owing to the random distribution of Cr,Ca:YAG particles in ceramic, the saturation absorption does not exhibit crystalline-orientation dependent anisotropic properties when the sample is rotated, which is different from the anisotropic saturation absorption of Cr,Ca:YAG crystal when the laser propagate along [111] direction(Eilers et al., 1992). Therefore, the polarization states in passively Q-switched microchip lasers with Cr,Ca:YAG as saturable absorber are not only determined by the anisotropic saturation absorption of Cr,Ca:YAG saturable absorber, but also determined by the linearly polarized states of Yb:YAG crystals.
The continuous-wave operation of Yb:YAG crystal and ceramic has been investigated previously by using different transmissions of output coupler( Dong et al., 2006 ; Dong et al., 2007) and found that the laser performance 1-mm-thick Yb:YAG crystal doped with 10 at.% Yb is better than that of 1-mm-thick Yb:YAG ceramic doped with 9.8 at.% Yb. The absorbed pump power thresholds are 0.46 W and 0.54 W for 1-mm-thick Yb:YAG crystal and ceramic, respectively, the slope efficiencies were 49% and 44%, respectively by using 50% transmission of output coupler. The differences of cw laser performance between Yb:YAG crystal and ceramic suggest that the optical quality of ceramic used in the experiments is not as good as that of Yb:YAG crystal, and the slight different doping concentration may be another cause of the difference.
Here we show the effect of different polarization states of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers on the laser performance. Average output power as a function of absorbed pump power for these four combinations of Yb:YAG and Cr,Ca:YAG microchip lasers was shown in Figure 11. The absorbed pump power thresholds were about 0.53, 0.66, 0.75, and 0.6 W for combinations C1, C2, C3 and C4. The higher pump power threshold of these passively Q-switched lasers was due to the low initial transmission of Cr,Ca:YAG and high transmission of the output coupler used in the experiments. Average output power increases linearly with absorbed pump power for the four combinations, the slope efficiencies with respect to the absorbed pump power were estimated to be about 39, 36, 36 and 29% for the four combinations of C1, C2, C3 and C4, respectively. The best laser performance (low threshold and high slope efficiency) of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers was obtained with C1 combination because of the enhancement of linearly polarized laser operation due to the combination of linearly oscillation of Cr:YAG crystal under high intracavity laser intensity(Eilers et al., 1992) and the crystalline-orientation selected linearly polarized states of Yb:YAG crystal(Dong et al., 2008). Maximum average output power of 310 mw was obtained with Yb:YAG/Cr,Ca:YAG all-crystal combination when the absorbed pump power was 1.34 W, corresponding to optical-to-optical efficiency of 23%. The optical-to-optical efficiency is 15% with respect to the incident pump power for C1. The optical-to-optical efficiencies with respect to the incident pump power were measured to be 12, 11 and 11% for C2, C3 and C4 respectively. There is no coating damage occurrence with further increase of the pump power owing to decrease of the intracavity energy fluence by using high transmission output coupler.
Although linearly polarized laser operation was observed in Yb:YAG/Cr,Ca:YAG combinations with at least one crystal, the effect of linearly polarized states on the laser performance was different. The slope efficiency of C4 is lower than that of C3, however the laser threshold of C4 is lower than that of C3 and the average output power is higher than that of C3 for all the available pump power range, as shown in Figure 11. The contribution of polarization states from Cr,Ca:YAG crystal and Cr,Ca:YAG ceramic is different, when Cr,Ca:YAG crystal is used as saturable absorber, even with Yb:YAG ceramic as gain medium, the laser threshold is low. For all-ceramics combination, C2, although the laser threshold is higher than those of C1 and C4, the slope efficiency is better those of C4 and C3. These results show that the polarized states have great effect on the laser performance. Even with random polarized states of all-ceramics combination, passively Q-switched Yb:YAG/Cr,Ca:YAG laser has nearly the same laser performance as that of all-crystals combination. The discrepancies between all-crystals (C1) and all-ceramics (C2) combinations were caused by the optical quality of Yb:YAG crystal and Yb:YAG ceramic, the laser performance of Yb:YAG crystal is better than its ceramic counterpart. The discrepancies between C3 and C4 were attributed to the linearly polarization, with Cr,Ca:YAG crystal as saturable absorber, the extinction ratio of the polarization is stronger than that of with Cr,Ca:YAG ceramic as saturable absorber, the laser prefers to oscillate more efficiently with orientation selected anisotropic saturbable absorption of Cr,Ca:YAG crystal along <111> direction under high intracavity intensity(Eilers et al., 1992).
The output beam profile is close to fundamental transverse electro-magnetic mode. Near diffraction-limited output beam quality with
Owing to the broad emission spectrum of the Yb:YAG materials around 1.03 μm (about 10 nm in FWHM), many longitudinal modes can be excited even for a 1-mm-thick Yb:YAG crystal. Microchip cw Yb:YAG lasers operate in a multi-longitudinal-mode over the whole pump power region(
Dong et al., 2006
). However, single-longitudinal-mode oscillation around 1029.7 nm was observed in passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers when the average output power was kept below 50 mW for different Yb:YAG/Cr,Ca:YAG combinations, the same as that for all-ceramics combinations(
Dong et al., 2007
). Above this value, the laser exhibited two-mode oscillation and three-mode oscillation. A typical example of single-longitudinal-mode and multi-longitudinal-mode oscillations of passively Q-switched Yb:YAG/Cr,Ca:YAG all-ceramic microchip laser under different average output power levels is shown in Figure 12(a). The separation between first and second modes was measured to be 1.16 nm, which is eight times wider than the free spectral range between the longitudinal modes (0.146 nm) in the laser cavity filled with gain medium predicted by(Koechner 1999)
The polarization states of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers have great effect on the characteristics of the output pulses. Figure 13 shows the pulse characteristics (pulse repetition rate, pulse width, pulse energy and pulse peak power) of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers as a function of absorbed pump power. For all four combinations of Yb:YAG and Cr,Ca:YAG, the repetition rate of passively Q-switched laser increases linearly with the absorbed pump power. Pulse width (FWHM) of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers decreases with absorbed pump power at low pump power levels and tends to keep constant at high pump power levels. The shortest pulse width of 277 ps was achieved with C4 combination. Pulse widths for the four combinations are in the order of C4 < C1 < C2 < C3. Pulse energy increases with absorbed pump power and tends to keep constant at high pump power levels. The highest pulse energy was achieved with C1 combination. The pulse energy for the four combinations are in the order of C1 > C4 > C2 > C3. Peak power of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers exhibits the same tendency as those of pulse energy: C1 > C4 > C2 > C3 for the four combinations. Therefore, the overall best laser performance (highest peak power) in passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers achieved by using C1 combination. Linearly polarization operation of passively Q-switched all-crystals lasers is more favorable for laser performance. The combination of Yb:YAG/Cr,Ca:YAG microchip lasers with Cr,Ca:YAG crystal as saturable absorber, C4, has better laser pulse characteristics than those of all-ceramics combination, C2 and combination C3. Although linearly polarized state was achieved in combination C3 with Yb:YAG crystal, the linearly polarized states was attributed to the linearly polarization of Yb:YAG crystal, not from the Cr,Ca:YAG ceramic. The contribution of the linearly polarized state from Yb:YAG crystal in C3 combination is less than that from the nonlinear anisotropic absorption of Cr,Ca:YAG crystal, therefore, therefore, the laser performance of combination C3 is less than those of combination C1 and C4. The effect of depolarization effect on the polarization states observed in Yb:YAG crystal (Dong et al., 2008) may be another cause to less efficient laser operation in C3 combination at high pump power levels.
4. Conclusions
In conclusion, systematic comparison of laser performance was done for Yb:YAG ceramics and single-crystals doped with different concentrations. Although the pump power thresholds of Yb:YAG crystals were higher than their ceramics counterparts due to the pump configuration, the efficient laser operation was obtained by using both Yb;YAG ceramics and single-crystals. The laser performance of 1-mm-thick Yb:YAG ceramics and crystals becomes worse with Yb concentration under present miniature laser configuration. However, the laser performance of Yb:YAG crystals is more sensitive to the Yb concentrations, while the laser performance of Yb:YAG ceramics is less sensitive to the Yb concentrations. The laser performance of low doping Yb:YAG ceramics is worse than those obtaining from Yb:YAG singly crystals. The laser performance of 20 at.% Yb:YAG ceramics is better than its counterpart single crystal. Both Yb:YAG ceramics and crystals miniature lasers oscillate at multi-longitudinal modes, the number of longitudinal-mode increases with absorbed pump power. Strong mode competition and mode hopping were observed in these Yb:YAG lasers. The strong reabsorption and gain curve change under high intracavity laser intensity play important roles on the red-shift of the output laser wavelength. High beam quality lasers with
Random polarized oscillation was observed in passively Q-switched Yb:YAG/Cr,Ca:YAG all-ceramic microchip laser while linearly polarized oscillations were observed with at lease one crystal in the Yb:YAG/Cr,Ca:YAG combinations. The polarization states in passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers show that the linearly polarization states in passively Q-switched laser are not only resulted from the anisotropic saturation absorption of Cr,Ca:YAG crystal, but also from linearly polarization states of Yb:YAG crystal. High peak power pulses with sub-nanosecond pulse-width and nearly diffraction-limited beam quality were obtained in these lasers. The best laser performance was achieved by using Yb:YAG crystal as gain medium and Cr,Ca:YAG crystal as saturable absorber because of the enhancement of linearly polarized state due to the crystalline-orientation selected polarized states of Yb:YAG crystal and linearly polarized oscillation of Cr,Ca:YAG crystal under high intracavity laser intensity. Other combinations of Yb:YAG and Cr,Ca:YAG have less efficient linearly polarized laser oscillation and also affect the laser performance of passively Q-switched Yb:YAG/Cr,Ca:YAG microchip lasers.
References
- 1.
Barabanenkov Y. N. Ivanov S. N. Taranov A. V. Khazanov E. N. Yagi H. Yanagitani T. Takaichi K. Lu J. Bisson J. F. Shirakawa A. Ueda K. Kaminskii A. A. 2004 Nonequilibrium acoustic phonons in Y3Al5O12-based nanocrystalline ceramics.79 7 (342- 345). - 2.
Bisson J. Yagi H. Yanagitani T. Kaminskii A. Barabanenkov Y. N. Ueda K. 2007 Influence of the grain boundaries on the heat transfer in laser ceramics.14 1 (1- 13). - 3.
Bogomolova G. A. Vylegzhanin D. N. Kaminskii A. A. 1976 Spectral and lasing investigations of garnets with Yb3+ ions.42 3 (440- 446). - 4.
Bouwmans G. Segard B. Glorieux P. 2001 Polarisation dynamics of monomode Nd3+:YAG lasers with Cr4+ saturable absorber: influence of the pump polarisation.196 1-6 , (257- 268). - 5.
Brauch U. Giesen A. Karszewski M. Stewen C. Voss A. 1995 Multiwatt diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm.20 7 (713- 715). - 6.
Bruesselbach H. W. Sumida D. S. Reeder R. A. Byren R. W. 1997 Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers3 1 (105- 116). - 7.
Degnan J. J. 1995 Optimization of passively Q-switched lasers.31 11 (1890- 1901). - 8.
Dobrzycki L. Bulska E. Pawlak D. A. Frukacz Z. Wozniak K. 2004 Structure of YAG crystals doped/substituted with erbium and ytterbium.43 24 (7656- 7664). - 9.
Dong J. Bass M. Mao Y. Deng P. Gan F. 2003 Dependence of the Yb3+ emission cross section and lifetime on the temperature and concentration in ytterbium aluminum garnet.20 9 (1975- 1979). - 10.
Dong J. Deng P. Lu Y. Zhang Y. Liu Y. Xu J. Chen W. 2000 LD pumped Cr4+,Nd3+:YAG with self-Q-switched laser output of 1.4 W.25 15 (1101- 1103). - 11.
Dong J. Shirakawa A. Takaichi K. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2006 All ceramic passively Q-switched Yb:YAG/Cr4+:YAG microchip laser.42 20 (1154- 1156). - 12.
Dong J. Shirakawa A. Ueda K. 2008 A crystalline-orientation self-selected linearly polarized Yb:Y3Al5O12 microchip laser.93 10 (101105). - 13.
Dong J. Shirakawa A. Ueda K. Kaminskii A. A. 2007 Effect of ytterbium concentration on CW Yb:YAG microchip laser performance at ambient temperature Part I: Experiments.89 2-3 , (359- 365). - 14.
Dong J. Shirakawa A. Ueda K. Xu J. Deng P. 2006 Efficient laser oscillation of Yb:Y3Al5O12 single crystal grown by temperature gradient technique.88 16 (161115). - 15.
Dong J. Shirakawa A. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2006 Efficient Yb3+:Y3Al5O12 ceramic microchip lasers.89 9 (091114). - 16.
Dong J. Shirakawa A. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2007 Laser-diode pumped heavy doped Yb:YAG ceramic lasers.32 13 (1890-1892). - 17.
Dong J. Shirakawa A. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2007 Near-diffraction-limited passively Q-switched Yb:Y3Al5O12 ceramic lasers with peak power > 150 kW.90 13 (131105). - 18.
Dong J. Shirakawa A. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2007 Ytterbium and chromium doped composite Y3Al5O12 ceramics self-Q-switched laser.90 19 (191106). - 19.
Dong J. Ueda K. 2005 Temperature-tuning Yb:YAG microchip lasers.2 9 (429- 436). - 20.
Dong J. Ueda K. Shirakawa A. Yagi H. Yanagitani T. Kaminskii A. A. 2007 Composite Yb:YAG/Cr4+:YAG ceramics picosecond microchip lasers.15 22 (14516- 14523). - 21.
Eilers H. Hoffman K. R. Dennis W. M. Jacobsen S. M. Yen W. M. 1992 Saturation of 1.064 m absorption in Cr,Ca:Y3Al5O12 crystals.61 25 (2958-2960). - 22.
Fan T. Y. 1993 Heat generation in Nd:YAG and Yb:YAG.29 6 (1457-1459). - 23.
Fan T. Y. Ochoa J. 1995 Tunable single-frequency Yb:YAG laser with 1-W output power using twisted-mode technique.7 10 (1137-1138). - 24.
Feng Y. Lu J. Takaichi K. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2004 Passively Q-switched ceramic Nd3+:YAG/Cr4+:YAG lasers.43 14 (2944- 2947). - 25.
Giesen A. Hugel H. Voss A. Wittig K. Brauch U. Opower H. 1994 Scalable concept for diode-pumped high-power solid-state lasers.58 5 (365- 372). - 26.
Honea E. C. Beach R. J. Mitchell S. C. Sidmore J. A. Emanuel M. A. Sutton S. B. Payne S. A. Avizonis P. V. Monroe R. S. Harris D. 2000 High-power dual-rod Yb:YAG laser.25 11 (805- 807). - 27.
Kir’yanov A. V. Aboites V. Ii’ichev N. N. 1999 A polarisation-bistable noedymium laser with a Cr4+:YAG passive switch under the weak resonant signal control.169 1-6 , (309- 316). - 28.
Koechner W. 1999 . Berlin, Springer-Verlag. - 29.
Kong J. Tang D. Y. Lu J. Ueda K. 2004 Random-wavelength solid-state laser.29 1 (65- 67). - 30.
Krupke W. F. 2000 Ytterbium solid-state lasers- The first decade.6 6 (1287- 1296). - 31.
Lacovara P. Choi H. K. Wang C. A. Aggarwal R. L. Fan T. Y. 1991 Room-temperature diode-pumped Yb:YAG laser.16 14 (1089- 1091). - 32.
Lagatsky A. A. Abdolvand A. Kuleshov N. V. 2000 Passive Q-switching and self-frequency Raman conversion in a diode-pumped Yb:KGd(WO4)2 laser.25 9 (616- 618). - 33.
Li S. Zhou S. Wang P. Chen Y. C. Lee K. K. 1993 Sefl-Q-switched diode-end-pumped Cr,Nd:YAG laser with polarized output.18 3 (203- 204). - 34.
Lu J. Prabhu M. Song J. Li C. Xu J. Ueda K. Kaminskii A. A. Yagi H. Yanagitani T. 2000 Optical properties and highly efficient laser oscillation of Nd:YAG ceramics.71 4 (469- 473). - 35.
Lu J. Prabhu M. Song J. Li C. Xu J. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2001 Highly efficient Nd:Y3Al5O12 ceramic laser.40 6A (L552- L554). - 36.
Lu J. Ueda K. Yagi H. Yanagitani T. Akiyama Y. Kaminskii A. A. 2002 Neodymium-doped yttrium aluminum garnet (Y3Al5O12) nanocrystalline ceramics- a new generation of solid-state laser and optical materials.341 1-2 , (220- 225). - 37.
Ito M. Goutaudier C. Guyot Y. Lebbou K. Fukuda T. Boulon G. 2004 Crystal growth, Yb3+ spectroscopy, concentration quenching analysis and potentiality of laser emission in Ca1-xYbxF2+x.16 8 (1501- 1521). - 38.
McKay A. Dawes J. M. Park J. 2007 Polarisation-mode coupling in (100)-cut Nd:YAG.15 25 (16342- 16347). - 39.
Nakamura S. Yoshioka H. Matsubara Y. Ogawa T. Wada S. 2008 Efficient tunable Yb:YAG ceramic laser.281 17 (4411- 4414). - 40.
Okhrimchuk A. G. Shestakov A. V. 1994 Performance of YAG: Cr4+ laser crystal.3 1 (1- 13). - 41.
Patel F. D. Honea E. C. Speth J. Payne S. A. Hutcheson R. Equall R. 2001 Laser demonstration of Yb3Al5O12 (YAG) and materials properties of highly doped Yb:YAG.37 1 (135- 144). - 42.
Qiu H. Yang P. Dong J. Deng P. Xu J. Chen W. 2002 The influence of Yb concentration on laser crystal Yb:YAG.55 1-2 , (1- 7). - 43.
Rutherford T. S. Tulloch W. M. Gustafson E. K. Byer R. L. 2000 Edge-pumped quasi-three-level slab lasers: design and power scaling.36 2 (205- 219). - 44.
Spuhler G. J. Paschotta R. Kullberg M. P. Graf M. Moser M. Mix E. Huber G. Harder C. Keller U. 2001 A passively Q-switched Yb:YAG microchip laser.72 3 (285- 287). - 45.
Stewen C. Contag K. Larionov M. Giessen A. Hugel H. 2000 A 1-kW CW Thin Disc laser.6 4 (650-657). - 46.
Sumida D. S. Fan T. Y. 1994 Effect of radiation trapping on fluorescence lifetime and emission cross section measurements in solid-state laser media19 17 (1343- 1345). - 47.
Taira T. Saikawa J. Kobayashi T. Byer R. L. 1997 Diode-pumped tunable Yb:YAG miniature lasers at room temperature: modeling and experiment.3 1 (100- 104). - 48.
Takaichi K. Lu J. Murai T. Uematsu T. Shirakawa A. Ueda K. Yagi H. Yanagitani T. Kaminskii A. A. 2002 Chromium-doped Y3Al5O12 ceramics- a novel saturable absorber for passively self-Q-switched 1-m solid-state lasers.41 2A (L96- L98). - 49.
Takaichi T. Yagi H. Lu J. Shirakawa A. Ueda K. Yanagitani T. Kaminskii A. A. 2003 Yb3+-doped Y3Al5O12 ceramics- A new solid-state laser material.200 1 (R5- R7). - 50.
Xu X. Zhao Z. Song P. Jiang B. Zhou G. Xu J. Deng P. Bourdet G. Chanteloup J. C. Zou J. Fulop A. 2005 Upconversion luminescence in Yb3+- doped yttrium aluminum garnets.357 3-4 , (365- 369). - 51.
Xu X. Zhao Z. Zhao G. Song P. Xu J. Deng P. 2003 Comparison of Yb:YAG crystals grown by CZ and TGT method.257 3-4 , (297- 300). - 52.
Yagi H. Yanagitani T. Yoshida K. Nakatsuka M. Ueda K. 2006 Highly efficient flashlamp-pumped Cr3+ and Nd3+ codoped Y3Al5O12 ceramic laser.45 1A (133- 135). - 53.
Yanagitani T. Yagi H. Hiro Y. 1998 Production of yttrium aluminium garnet fine powders for transparent YAG ceramic. Japan Patent10 101411 . - 54.
Yang P. Deng P. Yin Z. 2002 Concentration quenching in Yb:YAG.97 1 (51- 54). - 55.
Yankov P. 1994 Cr4+:YAG Q-switching of Nd:host laser oscillators.27 6 (1118- 1120). - 56.
Yin H. Deng P. Gan F. 1998 Defects in YAG:Yb crystal.83 7 (3825- 3828). - 57.
Yoshino T. Kobyashi Y. 1999 Temperature characteristics and stabilization of orthogonal polarization two-frequency Nd3+:YAG microchip lasers.38 15 (3266- 3270). - 58.
Zayhowski J. J. 2000 Passively Q-switched Nd:YAG microchip lasers and applications.303-304 ,393-400 ). - 59.
Zayhowski J. J. Dill C. III. 1994 Diode-pumped passively Q-switched picosecond microchip lasers.19 18 (1427-1429).