4 All-PolyCrystalline Ceramics Nd : YAG / Cr 4 + : YAG Monolithic Micro-Lasers with Multiple-Beam Output

Laser-induced ignition of air-fuel mixtures in internal combustion engines is a subject that has been investigated extensively during last years. In the beginning, experiments were performed with sized and robust, commercial available lasers that delivered pulses with energy in the range of tens to a few hundreds of mJ and several ns pulse duration (Ma et al., 1998; Phuoc & White, 1999; Weinrotter et al. 2005a; Weinrotter at al. 2005b). These investigations revealed that laser-induced ignition offers significant advantages over a conventional spark-ignition system, such as higher probability to ignite leaner mixtures, reduction of erosion effects, increase of engine efficiency, or shorter combustion time. Thus, developing of an engine ignited by laser could address, even partially, the increase concern of humanity for protecting global environment and preserving fossil resources. Subsequent research (Kofler et al., 2007) concluded that a suitable laser configuration for engine ignition is a Nd:YAG laser, passively Q-switched by Cr4+:YAG saturable absorber (SA). Q-switched laser pulses with energy up to 6 mJ and 1.5-ns duration were obtained from an end-pumped, 210-mm long Nd:YAG-Cr4+:YAG laser. Furthermore, side-pumping technique was employed to realize a Nd:YAG laser passively Q-switched by Cr4+:YAG SA with 25 mJ energy per pulse and pulse duration around 3 ns (Kroupa et al. 2009); the laser resonator was around 170 mm. However, the length of these lasers make difficult to accomplish compactness of an electrical spark plug used in the automotive industry. In recent works our group has realized Nd:YAG-Cr4+:YAG micro-lasers and demonstrated laser ignition of an automobile engine with improved performances in comparison with ignition induced by a conventional spark plug (Tsunekane at al. 2008; Tsunekane et al. 2010). The strategy was to shorten the pulse duration by decreasing the resonator length, and to maximize the laser pulse energy by optimizing the pump conditions, the Nd:YAG doping level and length, as well as Cr4+:YAG initial transmission (T0) and the output mirror transmission (T) (Sakai et al., 2008). A passively Q-switched Nd:YAG-Cr4+:YAG micro-laser with 2.7-mJ energy per pulse and 600-ps pulse duration was realized. This laser, which


Introduction
Laser-induced ignition of air-fuel mixtures in internal combustion engines is a subject that has been investigated extensively during last years. In the beginning, experiments were performed with sized and robust, commercial available lasers that delivered pulses with energy in the range of tens to a few hundreds of mJ and several ns pulse duration (Ma et al., 1998;Phuoc & White, 1999;Weinrotter et al. 2005a;Weinrotter at al. 2005b). These investigations revealed that laser-induced ignition offers significant advantages over a conventional spark-ignition system, such as higher probability to ignite leaner mixtures, reduction of erosion effects, increase of engine efficiency, or shorter combustion time. Thus, developing of an engine ignited by laser could address, even partially, the increase concern of humanity for protecting global environment and preserving fossil resources. Subsequent research (Kofler et al., 2007) concluded that a suitable laser configuration for engine ignition is a Nd:YAG laser, passively Q-switched by Cr 4+: YAG saturable absorber (SA). Q-switched laser pulses with energy up to 6 mJ and 1.5-ns duration were obtained from an end-pumped, 210-mm long Nd:YAG-Cr 4+ :YAG laser. Furthermore, side-pumping technique was employed to realize a Nd:YAG laser passively Q-switched by Cr 4+ :YAG SA with 25 mJ energy per pulse and pulse duration around 3 ns (Kroupa et al. 2009); the laser resonator was around 170 mm. However, the length of these lasers make difficult to accomplish compactness of an electrical spark plug used in the automotive industry. In recent works our group has realized Nd:YAG-Cr 4+ :YAG micro-lasers and demonstrated laser ignition of an automobile engine with improved performances in comparison with ignition induced by a conventional spark plug (Tsunekane at al. 2008;Tsunekane et al. 2010). The strategy was to shorten the pulse duration by decreasing the resonator length, and to maximize the laser pulse energy by optimizing the pump conditions, the Nd:YAG doping level and length, as well as Cr 4+ :YAG initial transmission (T 0 ) and the output mirror transmission (T) (Sakai et al., 2008). A passively Q-switched Nd:YAG-Cr 4+ :YAG micro-laser with 2.7-mJ energy per pulse and 600-ps pulse duration was realized. This laser, which 61 crystal delivered pulses with 11-μJ energy and 337-ps duration, at 6-kHz repetition rate (Zayhowsky and Dill III, 1994). The laser included a composite structure that was made of a thin piece of Nd 3+ :YAG single-crystal gain medium bonded to a short Cr 4+ :YAG SA single crystal. Later, laser pulses with increased energy of 100 μJ and duration of 36 ns at 15-kHz repetition rate were obtained from a passively Q-switched Nd:YAG-Cr 4+ :YAG laser, with medium average output power (Agnesi et al., 1997). Furthermore, laser pulses with high energy E p = 3.4 mJ, but long duration of 99 ns, were achieved by employing side-pumping geometry of a Nd:YAG-Cr 4+ :YAG laser (Song et al., 2000). A sketch of the cw-pumped Nd:YAG-Cr 4+ :YAG laser developed in our laboratory is shown in Fig. 1 (Pavel et al., 2001a). The gain medium was a composite Nd:YAG rod that was fabricated by diffusion bonding of a Nd:YAG single crystal (thickness of 5.0 mm, 1.1-at.% Nd doping) to an undoped, 1.0-mm thick YAG. The concept of combining doped and undoped components was used, in the beginning, to modify the configuration of the thermal field induced by pumping in solid-state laser rods, and successfully employed to improve the output performances of Nd:YAG (Hanson, 1995), Nd:YVO 4 (Tsunekane et al., 1997), or Yb:YAG (Bibeau et al., 1998) lasers. This method has also found applications in the passive Q-switching technique. Thus, microchip structures that consisted of undoped YAG caps, Nd:YAG and Cr 4+ :YAG SA bonded together to form a monolithic resonator were demonstrated to produce linearly polarized, single-longitudinal mode output pulses in quasi-cw or pulsed pumping regimes (Zayhowski et al., 2000;Aniolek et al., 2000). Fig. 1. A passively Q-switched Nd:YAG-Cr 4+ :YAG laser, pumped by cw diode laser is shown. Composite YAG/Nd:YAG was used for thermal management. The gain Nd:YAG medium as well as Cr 4+ :YAG SA were single crystals, discrete elements.
A fiber-bundled diode (OPC-B030-mmm-FC, OptoPower Co.; 1.55-mm diameter, 0.11 NA) was used for the pump at 807 nm ( p ). The YAG/Nd:YAG surfaces (S1 and S2) were antireflection (AR) coated at both the laser wavelength of 1.064 m ( em ) and  p . A planeplane resonator with the pump-mirror M1 coated for high reflectivity (HR) at  em and high transmission (HT) at  p , and that was placed very close to Nd:YAG, was used. A collimating lens (L1) and a focusing lens (L2) were used to image the fiber bundle end into Nd:YAG, to a diameter of 800 m. The focusing point was 2.0 mm below surface S1 of Nd:YAG. Cr 4+ :YAG SA (CASIX Inc., China) with initial transmission T 0 of 0.89, 0.85, and 0.80 and AR coated at  em on both surfaces (F1 and F2) were used for Q-switching. Each Cr 4+ :YAG SA crystal was placed close to the out-coupling mirror (OCM) M2. Figure 2 presents characteristics of Q-switched laser emission obtained from a 40-mm long resonator and an OCM with T= 0.10. A maximum average power of 3.8 W resulted for the Cr 4+ :YAG with T 0 = 0.89 (Fig. 2a) with a beam factor M 2 of 1.4. The laser ran at a frequency as high as 24.8 kHz. The pulse energy and the pulse duration (t p , FWHM definition) were E p = 152 J and t p = 25.4 ns (Fig. 2b), respectively; the pulse peak power was ~6.0 kW. When a SA with a lower transmission T 0 was used, pulses with higher peak power were generated with a reduced average output power. For a Cr 4+ :YAG SA with T 0 = 0.85, pulses with t p = 18.0 ns at a repetition rate of 16.1 kHz resulted for the maximum absorbed power of 21.0 W. Energy E p and pulse peak power was 213 J and 11.8 kW, respectively. For a Cr 4+ :YAG with T 0 = 0.80 an average power of 2.6 W at the absorbed pump power of 18.6 W resulted. The laser ran at 9.1 kHz repetition rate with pulses of E p = 272 J energy and 16.2 kW peak power. The Q-switched laser performances were evaluated with a rate equation model (Pavel et al., 2001a;Degnan, 1995;Zhang et al., 1997). The laser pulse energy is given by general relation: where h is the photon energy at 1.06 m,  g represents Nd:YAG stimulated emission,  g is the inversion reduction factor, A g is the effective area of the laser beam in Nd:YAG, and the OCM reflectivity is R= (1-T). The initial population inversion density, n gi is: where L represents the resonator round-trip residual loss and  g is the Nd:YAG length. The final population inversion density, n gf and n gi are related by equation: (1 ) ln (1 ) ln 1 11 l n 1 0 (l n l n ) (l n l n )  Figure 3 compares experimental (symbols) and calculated (continuous lines) pulse energy for two pump rates. In simulation, spectroscopic parameters of Nd:YAG and Cr 4+ :YAG were σ g = 2.310 -19 cm 2 , σ SA = 4.310 -18 cm 2 , and σ ESA = 8.210 -19 cm 2 (Shimony, et al., 1995). The laser beam sizes in the gain crystal and in Cr 4+ :YAG were evaluated by the PARAXIA software package (Sciopt Enterprises, San Jose, California), in which the active medium was described as a thin lens. For the absorbed power of 9.2 W (near to the threshold) the calculated active element focal length was 33.8 cm and the ratio A g /A SA amounted to 1.1, while for the absorbed pump power of 17.5 W (close to the maximum pump power) the focal length decreases to 13.2 cm and A g /A SA was 1.3. Good agreement between the experimental results and the calculated values was obtained. In order to realize a passively Q-switched Nd:YAG-Cr 4+ :YAG laser with generation into green visible spectrum at 532 nm (  ), the set-up of Fig. 1 was modified such to include a nonlinear crystal. We used a V-type resonator, as shown in Fig. 4 (Pavel et al., 2001b). The YAG/Nd:YAG crystal, the Cr 4+ :YAG SA and a glass plate (BP) positioned at Brewster angle for polarization were placed in the resonator arm (of 80-mm length) made between mirrors M1 and M2. The nonlinear crystal was a 10-mm long LBO (type I, = 90 0 , = 11.4 0 ; operation at 25 o C) that was placed between mirrors M2 and M3 (the arm length was 90 mm). The LBO surfaces were AR coated at both  em and   wavelengths. The concave mirror M3 has a radius of 50 mm. This arrangement makes use of the high peak power available inside the cavity, and enables high conversion efficiency of the fundamental wavelength  em . Figure 5 presents characteristics of the green laser pulses. For a Cr 4+ :YAG with T 0 = 0.90, the maximum average power at 532 nm was 0.95 W (Fig. 5a) at the absorbed pump power of 13.1 W; the laser beam quality was characterized by an M 2 factor of 1.8. The green pulse energy was 226 J (Fig. 5b), and the laser runs with a 4.2-kHz rate of repetition and pulse duration of 86 ns. A slightly higher average power of 1.0 W was obtained with the Cr 4+ :YAG of T 0 = 0.85. However, the pulse energy reduced at 131 J and the pulse duration increased at 96 ns. Green pulse peak power reached 2.6 kW for the Cr 4+ :YAG with T 0 = 0.90 (Fig. 5b). The characteristics of Q-switched laser pulses at 532 nm were described with a model of rate equation for photon density inside the resonator (, for the inversion of population in  Nd:YAG (n g ) and for the population density in Cr 4+ :YAG (n SA ), and in which out-coupling of the cavity field by frequency conversion was considered (Pavel et al., 2001b). The initial population inversion density, n gi is given by relation: The final inversion density n gf and n gi are related by the transcendental equation: where d= k/(c g  g ), and the coefficient k is given by the second-harmonic generation theory (Eimerl, 1987;Honea et al., 1998). The green pulse energy is with A  the green beam effective area, h  the photon energy at   , and  c the optical length of the resonator. Finally, the analytical expression deduced for the pulse energy E  was: www.intechopen.com (1 )ln (l n)1 l n 1 21 (l n) (6) Figure 6 shows modeling of the green pulse energy, for the Cr 4+ :YAG with T 0 = 0.90, versus the absorbed pump power, at various values of the losses L. Agreement with experimental results is good, especially if uncertainties in evaluation of L, or of laser beam variation inside the optical resonator are considered. This was the first passively Q-switched Nd:YAG-Cr 4+ :YAG laser intra-cavity frequency doubled with LBO nonlinear crystal. The laser performances (green pulse of 226 J energy and 2.6 kW peak power, with ~1 W average power) were much higher than previously developed systems. For example, green laser pulses with 2.5-J energy (190-mW average power) were obtained from a Nd:LSB gain medium passively Q-switched by Cr 4+ :YAG and intra-cavity frequency doubled by KTiOPO 4 (KTP) in a linear resonator (Ostroumov et al., 1997). Furthermore, a Nd:YAG laser that was passively Q-switched by GaAs semiconductor and intra-cavity frequency doubled by KTP, in a V-type laser resonator, yielded green laser pulses with 20.5-J energy and ~250-mW average power (Kajava and Gaeta, 1997). Later, a Nd:GdVO 4 -Cr 4+ :YAG-KTP laser with 21-J energy per pulse (average power of ~400 mW) was realized (Liu et al., 2004). More recently, passively Q-switched Nd:YAG  or Nd:LuVO 4 lasers (Cheng et al., 2011) intra-cavity frequency doubled with KTP were reported. Novelty of these last two devices is the use of two SA crystals, Cr 4+ :YAG and GaAs, for the purpose of obtaining shorter and more symmetrical pulses, in comparison with those delivered by a single SA crystal.

4+
:YAG micro-lasers based on single-crystal components The cw pumped, passively Q-switched lasers have large pulse-to-pulse energy fluctuations and large timing jitters (Huang et al., 1999;Tang et al., 2003) due to thermal and mechanical instabilities. The purpose of our next research was to realize a Nd:YAG that is passively Q-switched by Cr 4+ :YAG SA and that can be used for ignition of an automobile engine. The operation frequency of igniters in internal combustion engines is less than 60 Hz, corresponding to an engine speed of 7200 rpm; the duty cycle is less than 5% for automobiles. In such a low frequency range, passively Q-switched lasers that are quasi-cw pumped with a low duty cycle are expected to operate stably due to initialization of the thermal and mechanical conditions during pulses. Figure 7 is a drawing of a passively Q-switched laser module developed in our laboratory for preliminary experiments (Tsunekane et al., 2008). The active medium was a 1.1-at.% Nd:YAG single crystal (Metal Mining Co., Ltd., Japan) with a length of 4 mm. AR (R<0.2%) and HR (R>99.8%) coatings at  p and  em , respectively, were deposited on the pumped surface S1 of Nd:YAG. HR (R>90%) and AR (R<0.2%) coatings at p and  em , respectively, were deposited on the intra-cavity surface S2 of Nd:YAG. AR coatings at  em were deposited on both surfaces of a Cr 4+ :YAG SA (4-mm thick single crystal; Scientific Materials Corp., USA). The output coupler was flat with transmission T= 0.50 at  em . Cavity length was 10 mm. The Nd:YAG was end pumped by a fiber coupled, conductive cooled, 120-W peak power laser diode (JOLD-120-QPXF-2P, Jenoptik, Germany) with emission at  p = 807 nm; fiber core diameter was 600 m and numerical aperture NA was 0.22. The fiber end was imaged into Nd:YAG to a spot size of 1.1-mm diameter. Pump energy was controlled by changing the pump pulse duration, whereas the peak pump power was maintained constant at 120 W. The maximum pump duration was 500 μs and the repetition rate was 10 Hz.  Figure 8a shows energy of the laser pulse delivered by the Nd:YAG-Cr 4+ :YAG laser versus initial transmission T 0 of a Cr 4+ :YAG SA. E p increases from 0.45 mJ for a Cr 4+ :YAG with T 0 = 0.80 to 4.3 mJ for a Cr 4+ :YAG with T 0 = 0.15. Corresponding pump pulse energy (E pump ) was 53 and 5.2 mJ, respectively. The pulse duration was measured with a 10 GHz, InGaAs detector (ET-3500, Electro-Optics Technology, Inc.) and with a 12 GHz oscilloscope (DSO81204B, Agilent Technology). The shortest pulse width of 300 ps was obtained with a Cr 4+ :YAG of T 0 = 0.15 (Fig. 8a). Pulse peak power was 0.16 MW for a Cr 4+ :YAG with T 0 = 0.80 and a record of 14.5 MW for a Cr 4+ :YAG with T 0 = 0.15 (as shown in Fig. 8b). From the experimental observations, stable breakdown in air was observed for laser pulse energy E p larger than 1.5 mJ and pulse duration t p below 1 ns using an aspheric focus lens of 10-mm focal length. Based on these results, a Cr 4+ :YAG SA single crystal with initial transmission T 0 = 0.30 was selected for the laser igniter. The first prototype micro-laser module that was built in our laboratory and that has the same dimensions as a spark plug is shown in Fig. 9. The device includes not only the pumping optics from fiber to the Nd:YAG gain material, but also a beam expanding and focusing optics for ignition. The laser igniter has the same optical design and similar performances as the experimental module shown in Fig. 7, and it is physically possible to ignite a real engine by installing it instead of an electrical spark plug to a plug hole (Tsunekane et al., 2010). For real operation on an engine, however, the mechanical design inside the module has be improved in order to sustain the high temperatures (up to 150°C) and vibrations of a real engine. Fig. 9. Nd:YAG-Cr 4+ :YAG laser with one-beam output. Nd:YAG gain medium and Cr 4+ :YAG SA were single crystals; all optical components (including the output mirror) were discrete elements. Air breakdown is shown and size comparison is made with a spark plug.

Ceramics versus single-crystals Nd:YAG-Cr 4+ :YAG micro-lasers
The Nd:YAG as well as the Cr 4+ :YAG SA media used in the previous reports were single crystals. The advancement in ceramic techniques has reached a maturity stage, especially in obtaining poly-crystalline cubic laser media of very good optical quality. It is recognized that laser ceramics has become a serious challenge to crystalline optics, especially due to an easier manufacturability and a lower price. The use of poly-crystalline ceramics could decrease the price of the Nd:YAG-Cr 4+ :YAG laser, which is a critical condition for realizing and using of a laser spark plug for engine ignition. We have therefore conducted an investigation of laser output characteristics obtained from a passively Q-switched Nd:YAG-Cr 4+ :YAG laser that employs single crystals and poli-crystalline ceramics as Nd:YAG active media as well as Cr 4+ :YAG SA elements. A sketch of the experimental set-up is shown in Fig. 10. The laser media were Nd:YAG single crystals with doping level of 1.0-at.% Nd (sample A; Japan) and 2.0-at.% Nd (sample B; Germany), and poly-crystalline Nd:YAG ceramics with 1.1-at.% Nd (sample A * ; Baikowski Japan Co., Ltd.) and 2.0-at.% Nd (sample B * ; Baikowski Japan Co., Ltd.) doping level. The thickness of sample B * was 3 mm, whereas the other Nd:YAG media had 4 mm in thickness. Side S1 of each Nd:YAG was coated as HR (R> 99.9%) at  em , and as HT (T> 97%) at  p . The other side (S2) was AR coated (T> 99.9%) at  em , and as HR (R> 95%) at  p . The Cr 4+ :YAG SA had initial transmission, T 0 between 0.80 and 0.20, and were single crystals provided by two different venders (SA1 and SA2, China), as well as poly-crystalline ceramics (SA3; Baikowski Japan Co., Ltd.). Both sides of a Cr 4+ :YAG SA were AR coated at  em . Q-switched emission was reported previously in all-ceramics Nd:YAG-Cr 4+ :YAG (Feng et al., 2004) or Yb:YAG-Cr 4+ :YAG (Dong et al., 2006;Dong et al, 2007) compact lasers, the pumping being made with diode lasers of low power (few watts) in cw mode. In our experiments, the optical pumping was made with a fiber-coupled diode laser (JOLD-120-QPXF-2P, Jenoptik, Germany) in quasi-cw regime. The pump repetition rate was 10 Hz and the pump pulse duration was fixed at 250 s; pump energy was controlled by changing the diode current. An optical system made of two L1 and L2 lenses was used to image the fiber end (600-m in diameter, NA= 0.22) into Nd:YAG to a spot size of 1.1 mm in diameter. A linear resonator made between side S1 of Nd:YAG and a plane OCM was employed. Figure 11 presents output performances measured in free-generation regime, using a 35-mm long resonator equipped with an OCM of transmission T= 0.20. The slope efficiency,  s was in the range of 0.65 to 0.61, the highest value being recorded with the 1.0-at.% Nd:YAG single crystal (sample A), as shown in Fig. 11a. The overall optical-to-optical efficiency,  0 for the maximum pump level of 36.1 mJ per pulse is given in Fig. 11b. Laser pulses with 22.7 mJ energy (optical to optical efficiency  0 of ~0.63) were measured from the 1.0-at.% Nd:YAG (sample A). Efficiencies  s and  0 recorded with the highly-doped Nd:YAG were a www.intechopen.com  Figure 12 presents characteristics of Q-switched laser pulses obtained with a Cr 4+ :YAG SA single crystal (SA1) of initial transmission T 0 = 0.40, and various OCM transmission T. Laser pulses with energy E p~1 .7 mJ (Fig. 12a) and duration t p~1 .5 ns were yielded by the 1.0-at.% Nd:YAG single crystal (OCM with T= 0.70). The highly-doped 2.0-at.% Nd:YAG single crystal yielded laser pulses with energy E p = 1.22 mJ and duration t p = 1.45 ns. The corresponding pulse peak power was 1.1 MW for the 1.0-at.% Nd:YAG and 0.84 MW for the 2.0-at.% Nd:YAG single crystal (Fig. 12c). The Cr 4+ :YAG single crystal was then replaced with a Cr 4+ :YAG (SA2) ceramics of the same initial transmission T 0 = 0.40. The Q-switched laser pulse energy was E p = 1.0 mJ for the 1.1-at.% Nd:YAG ceramics (sample A*), and E p ~1.1 mJ for the 2.0-at.% Nd:YAG ceramics (sample B*) (Fig. 12b). Corresponding pulse peak power was 0.53 and 0.73 MW, respectively. Generally, pulse energy E p was lower than that measured with the Cr 4+ :YAG single crystal (SA1), whereas the pulse duration was longer.  The pump pulse energy was E pump = 14.7 mJ for 1.0-at.% Nd:YAG (sample A) (Fig. 13a), lower than 15.4 mJ energy of the pump pulse required for Q-switched emission of the 2.0at.% Nd:YAG sample B (OCM with T= 0.70). On the other hand, E pump necessary for the 1.1at.% Nd:YAG ceramics (sample A*) was only 13 mJ ( Fig. 13b), while E pump required for laser operation of the 2.0-at.% Nd:YAG ceramics was the highest of 22.3 mJ. Generally, higher pump pulse energy was necessary for laser operation of a Nd:YAG gain medium that was Q-switched by Cr 4+ :YAG ceramics (Fig. 13b), compared with emission of the same laser medium that was Q-switched with a Cr 4+ :YAG single crystal (Fig. 13a).  Figure 14 presents performances of the Q-switched laser pulses obtained with the 1.0-at.% Nd:YAG single crystal (Fig. 14a), the 1.1-at.% Nd:YAG ceramics (Fig. 14b), and using the available Cr 4+ :YAG SA. The OCM has transmission T= 0.50 and the resonator length was fixed at 35 mm. Differences between pulses obtained with the SA1 and SA2 Cr 4+ :YAG single crystals were observed at the same initial transmission T 0 . Most probably, the final transmissions of the Cr 4+ :YAG single crystals were a little different, depending of the growth process used by the companies that delivered the SA. Therefore, at this stage of the www.intechopen.com experiments (Pavel et al., 2010a) and with the available Nd:YAG gain media and Cr 4+ :YAG SA components, the best laser performances were obtained with single-crystals elements. The laser pulse peak power (shown in Fig. 14c) was also better when we used Nd:YAG single crystal with Cr 4+ :YAG single crystal (compared with its counterparts ceramics). In the final experiment the resonator length was reduced to 11 mm. Figure 15 summarizes results obtained with a combination of all-poly-crystalline ceramics, 1.1-at.% Nd:YAG gain medium and Cr 4+ :YAG SA with T 0 = 0.40. Laser pulse energy was around 1.4 mJ when OCM's transmission was higher than T= 0.50, whereas the pulse duration was t p~ 550 ps. Therefore, corresponding pulse peak power overcomes 2.5 MW (Fig. 15a). Air breakdown was realized with a focusing lens of 11-mm focal length. The pump pulse energy varied between 9.6 mJ when OCM transmission was T= 0.20 (low pulse energy E p = 0.95 mJ, and t p = 650 ps) and E pump = 14.7 mJ when OCM transmission was increased at T= 0.70 (Fig. 15b). It is known that the intensity required for optical breakdown depends on pulse duration. There are not many reports on this subject: According to (Paschotta, 2008), an optical intensity of ~2×10 13 W/cm 2 is required for air breakdown with laser pulses of 1-ps duration. Therefore, experiments were performed in order to evaluate the optical intensity of nsduration laser pulses that realizes air breakdown. A Nd:YAG-Cr 4+ :YAG laser, as shown in Fig. 10, was used. The laser pulse duration was varied by changing the OCM transmission T, and with the help of two Cr 4+ :YAG that had initial transmission T 0 of 0.39 and 0.29. The resonator length was fixed at 15 mm: The laser beam M 2 factor was ~1.50, as determined by knife-edge method. The air breakdown was observed after a convergent lens of 7.5 mm focal length. A half waveplate and a polarizer were placed after the laser, in order to vary the intensity of the pulse incident on the focusing lens. Figure 16 presents optical intensity that induced air breakdown (in laboratory conditions). In experiments, the pulse duration t p could be varied between 0.4 and 1.1 ns; corresponding laser pulse optical intensities that induced air breakdown were ~0.6510 13 W/cm 2 and ~0.4010 13 W/cm 2 , respectively. We therefore concluded that optical intensity of a laser pulse with 1-ns duration that induced air breakdown is ~0.510 13 W/cm 2 . Fig. 16. Optical intensity of a laser pulse with ns duration that induced air breakdown. Figure 17 is a sketch of the set-up used to demonstrate, in preliminary experiments, a passively Q-switched Nd:YAG-Cr 4+ :YAG laser with two-beam output. Generally, one could choose to use one line for pumping, and to divide the high-energy output laser beam into two (ore more) fascicles, which has to be directed at necessary angle and then focused. This solution increases probability of damaging the laser media (due to the high intensity of the laser beam, or due to thermal effects), and could complicate the guiding line. Therefore, our choice was to employ similar, independent, multi-pumping lines, and then to change the optical path of a laser beam before focusing it. The pump was made at 807 nm ( p ) with two fiber-coupled (600-m diameter and numerical aperture NA=0.22) diode lasers (JOLD-120-QPXF-2P, Jenoptik, Germany). Pump repetition rate and pump pulse duration were 5 Hz and 250 s, respectively. The fiber end was imaged into Nd:YAG to a spot size of 1.1-mm diameter. The two-pump beams were inserted into Nd:YAG with a metal-coated prism. Furthermore, distance between the pumping positions on Nd:YAG input surface was changed by forward and backward translation of this prism. The laser medium was a 1.1-at.% Nd:YAG ceramics (Baikowski Japan Co., Ltd.) with thickness of 5 mm and a 10-mm diameter. Surface used for pumping was coated HR at the lasing wavelength of 1.06 m ( em ) and HT at  p . The second surface was coated AR at both  em and  p . Around 90% of the pump radiation was absorbed in Nd:YAG. Cr 4+ :YAG ceramics with initial transmission T 0 of 0.70, 0.50, and 0.30 were employed for Q-switching, both surfaces of a SA ceramics being coated AR at  em . The resonator length was 12 mm, and a plane output mirror (OCM) was used for out-coupling. The laser beams path was bent with a prism placed after the resonator, whereas air breakdown was observed behind a lens L3 that has an 11-mm long focal length.  Table 1. Characteristics of Q-switched laser pulses obtained from the Nd:YAG-Cr 4+ :YAG allpoly-crystalline ceramics laser that was build of discrete components.

4+ :YAG monolithic micro-lasers with two and three-beam output for engine ignition
Characteristics of the Q-switched laser pulses obtained from the Nd:YAG-Cr 4+ :YAG laser are summarized in Table 1, at various OCM transmission T. Laser pulses of few-ns duration and energy E p of 0.8 mJ were measured for the Cr 4+ :YAG with T 0 = 0.70. Energy E p overcame 2 mJ and t p shortened below 1 ns when combination of Cr 4+ :YAG with T 0 = 0.30 and OCM with T of 0.50 or 0.70 was used. Air breakdown was successfully for the Cr 4+ :YAG with T 0 = 0.30 and all the OCM employed in the experiments. The next step of our investigations constituted realization of a compact Nd:YAG/Cr 4+ :YAG laser with two-beam output and dimensions close to an electrical spark plug. Figure 18a shows the experimental set-up. The laser medium was a composite Nd:YAG/Cr 4+ :YAG ceramics: Research experience of Baikowski Japan Co., as well as available optics on market (for example one could visit: www.thorlabs.com), allowed realization of this medium as a parallelepiped with 1015 mm 2 surface area, as presented in Fig. 18b. The Nd:YAG doping level was 1.1-at.% Nd, and its length was increased at 8 mm: In this way, the gain medium absorption efficiency at  p was better than 0.95, which avoided bleaching effects of Cr 4+ :YAG by the pump beam (Jaspan et al., 2004). The 3-mm thick Cr 4+ :YAG SA ceramics had initial transmission T 0 = 0.30. Surface S1 of Nd:YAG was coated HR at  em and HT at  p , and the OCM with transmission T= 0.50 was coated on surface S2 of Cr 4+ :YAG. The compact pumping line imaged the fiber end to a spot size of 1.0-mm into Nd:YAG. Each laser beam was expanded and then collimated in the "expander" section. Next, the beam was bended with a prism (patent pending), and finally focussed. The composite, all-ceramics, passively Q-switched Nd:YAG/Cr 4+ :YAG monolithic laser with two-beam output is shown in Fig. 19a. Figure 19b presents the air breakdown realized with this laser, whereas an electrical spark plug used in industrial gas engine is given for comparison. Each beam delivered Q-switched laser pulses with ~2.5 mJ energy and ~800 ps duration, which corresponds to a peak power of 3.1 MW. Minimal pump pulse energy was ~27 mJ. The laser pulse jitter, which was estimated from 500 consecutive pulses, improved from 3 s to 1 s when the pump pulse energy was increased from 27 mJ to 34 mJ respectively, while the pulse standard deviation decreases from 0.53 s to 0.18 s. The prism angle was chose such as the distance between the ignition points ( c ) was 13 mm, whereas the depth of the ignition (b c ) was 9 mm (Pavel et al., 2011a).   Once the Nd:YAG/Cr 4+ :YAG monolithic laser with two-beam output was build, the final goal of our work was realization of a laser device with three-beam output and a size that fits an www.intechopen.com  Fig.  20a. Three composite, all-ceramics Nd:YAG/Cr 4+ :YAG media (Baikowski Japan Co., Ltd.), each with a 9-mm diameter were prepared for the experiments. While the Cr 4+ :YAG SA has initial transmission T 0 = 0.30 and a thickness of 2.5 mm, the influence of Nd-doping level on Qswitched laser characteristics was investigated by using Nd:YAG with 1.1-at.% Nd (7.5-mm thick), as well as highly-doped 1.5-at.% Nd (thickness of 5 mm) and 2.0-at.% Nd (thickness of 3.5 mm). Again, surface S1 of Nd:YAG was coated HR at  em and HT at  p . The OCM with T= 0.50 at  em was coated on surface S2 of Cr 4+ :YAG SA. A photo of two composite Nd:YAG/Cr 4+ :YAG ceramics is shown in Fig. 20b. The optical pumping was realized through three independent, similar, and compact pumping lines (marked by 1 to 3 in Fig. 20a), each line containing a pair of an aspheric collimating lens and an aspheric focusing lens with short focal length and high NA. We mention that in order to fulfill dimensions of an automobile spark plug, diameter of all lenses was reduced and a new design of the fiber end was made. The characteristics of the Q-switched laser pulses measured from the Nd:YAG/Cr 4+ :YAG ceramics are given in Table 2. The energy of the laser pulse yielded by the 1.1-at.% Nd:YAG ceramics was 2.37 mJ, with a pulse peak power of 2.8 MW. The laser beam M 2 factor, which was measured by the knife-edge method, was 3.7. In order to explain the influence of pump-beam spot size on Q-switched laser performances, we used a rate equation model (Zhang et al., 2000;Li et al., 2007) in which the pump beam was assumed to have a top-hat distribution of radius w p and the laser beam was taken as Gaussian with a spot size of radius w g . Both w p and w g were considered constant along the Nd:YAG/Cr 4+ :YAG medium. The laser pulse energy is given by Eq.

Nd
(1), while the initial population inversion density, n gi was written as: with a= w p /w g . The final population inversion density, n gf and n gi are related by relation:   Figure 21 presents n gf /n gi versus ratio a= w p /w g . In simulation losses were L= 0.06 (0.01 for Nd:YAG and 0.05 for Cr 4+ :YAG final transmission), while spectroscopic parameter of Nd:YAG was σ g = 2.6310 -19 cm 2 (Taira, 2007). If w p <w g the overlap between pump and laser beam is good, but n gf /n gi increases when w p /w g decreases. Although the initial inversion of population n gi is high, a small fraction of it used for lasing and therefore Q-switched laser pulse energy is low. If w p /w g has a large value, the central part of the inversion of population interacts with laser mode, whereas some outside part could be depleted by spontaneous emission. Increasing w p /w g decreases n gf /n gi : The final inversion of population is low and a pulse laser with high energy is obtained. The expected values of the Q-switched laser pulse at various sizes w g of the laser mode were also shown in Fig. 21. Fig. 21. Ratio n gf /n gi versus w p /w g and Q-switched laser pulse energy for various laser beam radii w g . The pulse energy obtained from the 1.1-at.% Nd:YAG is given by the sign (■).
A plane-plane resonator operates due to thermal effects induced by optical pumping in the laser medium. The focal length f of Nd:YAG thermal lens can be evaluated by relation: f= (K c w p 2 )/[P h (dn/dT)] (Innocenzi et al., 1990). Nd:YAG has thermal conductivity K c = 10.1 Wm -1 K -1 (Sato & Taira, 2006), thermal coefficient of the refraction index is dn/dT= 0.7310 -5 K -1 , while ~0.24 of the absorbed pump power is transformed into heat (P h ) under efficient laser emission at 1.06 m. The average thermal lens of the 1.1-at.% Nd:YAG was evaluated as ~6.5 m, whereas value of w g was determinate by an ABCD description of the resonator as ~420 m. Sign of Fig. 21 is the experimental value of the laser pulse energy obtained from the 1.1-at.% Nd:YAG. Agreement with theoretical modeling is good, especially if uncertainties in evaluation of thermal focal lens or of other system parameters (such as losses L) are considered. The model can be improved by taking into account variation of pump beam radius w p and of laser beam spot size w g along the resonator length. Energies of 2.03 mJ and 1.37 mJ were measured from the 1.5-at.% and 2.0-at.% Nd ceramics, respectively. The pump pulse energy increased from 27 mJ for the 1.1-at.% Nd:YAG to 33 mJ for the 1.5-at.% Nd:YAG, and to 32 mJ for the 2.0-at.% Nd:YAG. The decrease of the 4 F 3/2 upper-level lifetime with Nd-doping level could be a reason for lower laser performances recorded with the highly-doped Nd:YAG compared with the 1.1-at.% Nd:YAG. The OCM transmission has also to be optimized for the highly-doped Nd:YAG ceramics. Very important for performances of the monolithic laser is the uniformity of the ceramic Nd:YAG/Cr 4+ :YAG material. Table 3 presents average values of the laser pulse energy determined along Ox and Oz axes, estimated by scanning the medium with a 0.5-mm step. The laser pulse energies were very close to those measured at the media center. Moreover, standard deviation was small, below 3% for the 1.1-at.% Nd:YAG, and less than 4% for the highly-doped Nd:YAG. The results indicate a very good homogeneity as well as quality of the composite all-ceramics Nd:YAG/Cr 4+ :YAG media, in spite of the high, 9-mm diameter.
As an example, laser pulse energy measured from the highly-doped, 2.0-at.% Nd:YAG/Cr 4+ :YAG medium is given in Fig. 22. The three-beam output micro-laser (Fig. 23a) was realized with the composite, all-ceramics 1.1-at.% Nd:YAG/Cr 4+ :YAG medium. The guiding line (Pavel et al., 2011b) was designed to assure a distance between a focusing point and the laser axis,  c of 4.5 mm and a depth of the focusing point inside the combustion chamber, b c of 9 mm. Air breakdown is illustrated in Fig. 23b, and an automobile electrical spark plug is shown for comparison. Various other characteristics of a Q-switched laser pulse, such us the delay time (i.e. the time between the moment when the pump pulse begins and the moment when the laser pulse develops), or the pulse jitter and standard deviation were determined. Figure 24 presents these parameters function of the pump pulse energy. As expected, delay time decreases with the increase of the pump pulse power. Therefore, the use of independent pumping lines allows control of the air breakdown timing by changing the pump energy of each line. Furthermore, real simultaneous ignition in all three points can be obtained by a small (less than 5%) tuning of the pump energy of each individual line. Time jitter is low (2.1 s at the pump energy of 26.7 mJ and 1.0 s at 32-mJ available energy of the pump pulse) and thus it would not have a negative impact on an automobile engine that is ignited by the laser. A second laser pulse was not observed. Nevertheless, increasing the pump pulse duration would enable obtaining of multiple laser pulses (Weinrotter et al., 2005;Tsunekane et al., 2010), which are useful for engine ignition especially if lean fuel-air mixtures are used. Previous experiments were performed at 5-Hz pump repetition, and temperature of the composite Nd:YAG/Cr 4+ :YAG ceramics was not controlled. However, higher repetition rates are necessary for operation of a car engine, usually up to 60 Hz. Therefore, variation of laser pulse characteristics was investigated versus the pump repetition rate. Experiments concluded that an increase from 5 to 100 Hz of the pump repetition rate improved the laser pulse energy from 2.37 mJ to 2.51 mJ, i.e. by only a 6% fraction, as shown in Fig. 25. This change required a small (5 to 6%) increase of the minimal pump energy, from ~27 mJ at 5 Hz pump repetition rate to 28.4 mJ at the pump repetition rate of 100 Hz. Variation with temperature of the laser performances was behind this work purpose. Nevertheless, in prior papers Dascalu and Pavel, 2009;Pavel et al., 2010b) we have measured only a slight increase of the laser pulse energy when temperature of a Nd:YAG laser passively Q-switched by Cr 4+ :YAG SA (build of discrete, single-crystals components) was increased up to 150 o C. Future experiments would also consider testing of the laser to shock and vibration conditions that are similar to those experienced in a car engine.

Conclusion
A passively Q-switched Nd:YAG/Cr 4+ :YAG giant-pulse emitting micro-laser with up to three-beam output has been realized. The device incorporates a composite, all-ceramics Nd:YAG/Cr 4+ :YAG monolithic structure that was pumped by similar, independent lines. The laser size is comparable to that of an electrical spark plug, being the first demonstration of this kind of device to the best of our knowledge. Laser pulses with energy of ~2.4 mJ and 2.8-MW peak power at 5-Hz repetition rate were obtained from a 10-mm thick Nd:YAG/Cr 4+ :YAG ceramics, just such as "giant micro-photonics". Increasing pump repetition rate up to 100 Hz improved the laser pulse energy by 6% and required only a 6% increase of the pump pulse energy compared with operation at 5 Hz. Pulse timing of the laser beams can by controlled by changing the pump energy of each individual line. On the other hand, simultaneous multi-point ignition is possible by less than 5% tuning of the pump energy of each individual pump line. These lasers will enable studies on the performances of internal combustion engines with multi-point ignition.