SPIE Proceedings [SPIE Lasers and Applications in Science and Engineering - San Jose, CA (Saturday 22 January 2005)] Solid State Lasers XIV: Technology and Devices - Short pulse and

Short pulse and high repetition rate Q-switched Yb:YAG microchip laser

Eric P. Ostbyi, Joseph Fukumotoii, Robert D. Stultz, Steve Matthews, and Dave Filgas

Raytheon Space and Airborne Systems

El Segundo, California 90245

ABSTRACT

In this paper, we report a diode-pumped passively Q-switched Yb:YAG laser that is an excellent candidate for a ladar master oscillator. This microchip laser has 1 ns pulse duration, 68 µJ pulse energy, 700 mW average power, 10 kHz repetition rate, and 29% optical slope efficiency. Additionally, the microchip oscillates in the fundamental TEM00 mode. The peak power was measured as high as 66 kW. We compare the pulse shape and duration, and the beam quality to simulation. We study the effects of Q-switch absorption, output coupler reflectivity, cavity length, and pump power on the laser’s pulse duration, pulse energy, average power, and repetition rate. Keywords: microchip, laser, Q-switch, Yb:YAG, Cr:YAG, nanosecond, solid-state laser, diode-pumped

1. INTRODUCTION

Diode-pumped solid-state microchip lasers are typically used in ladar, metrology, material treatment, and nonlinear frequency conversion. Many ladar applications require an oscillator with short pulse duration, high repetition rate, high efficiency, and diffraction-limited beam quality. Ytterbium (Yb) doped Yttrium Aluminum Garnet (Y3Al5O12, or YAG) has several important advantages over Neodymium (Nd) doped YAG as the gain medium, including its low thermal defect and broad pump absorption. Yb:YAG has a simple electronic structure, which consists of the ground 2F7/2 and the excited 2F5/2 manifolds1,2. As a result, there is no excited state absorption, upconversion, or concentration quenching3. These undesirable laser effects reduce efficiency and generate heat. YAG can be doped with higher concentrations of Yb than Nd because the Yb3+ ions substitute easily for the Yttrium(Y) ions4. Efficient and stable InGaAs laser diodes are commonly used to CW pump Yb:YAG, and the large absorption bandwidth of Yb:YAG (18 nm)3 eliminates the need for temperature control of the diodes. Yb:YAG stores more energy3 than Nd:YAG due to its significantly longer upper-level lifetime (0.95 ms versus 0.23 ms)5,6. The 6 nm emission bandwidth of Yb:YAG is useful for tunable sources7, but also allows oscillation of multiple longitudinal modes more easily than those with Nd:YAG, which has a 0.4 nm emission bandwidth7. The thermal defect is only 9% for Yb:YAG, compared to 26% for Nd:YAG6, resulting in a lower thermal load, and giving Yb:YAG an advantage for high pulse energy and repetition rate microchip lasers9. For its efficiency and thermal properties, Yb:YAG was chosen as the active gain medium for the microchip laser presented in this paper.

We used a Cr:YAG crystal to Q-switch the microchip laser. Saturable absorbers, or passive Q-switches like Cr4+:YAG are simpler and cheaper than active Q-switches like LiNbO3. Cr:YAG has good photochemical and thermal stability, large absorption cross section, and high laser damage threshold10. Active Q-switches have less timing jitter because they are electronically driven, but shorter cavity lengths are possible with passive Q-switches. A combined active and passively Q-switched Nd:YVO4 microchip laser was presented with less than 85 picoseconds of timing jitter with 300 V switching voltage11. Research groups have used dual doped Cr,Yb:YAG crystals to perform Q-switching

i For correspondence: Eric Ostby - E-mail: [email protected]; Telephone: (310) 616-8663; Fax: (310) 647-3250 ii Currently at Northrop Grumman Space Technology, One Space Park, Redondo Beach, California 90278

Solid State Lasers XIV: Technology and Devices, edited by Hanna J. Hoffman,Ramesh K. Shori, Proceedings of SPIE Vol. 5707 (SPIE, Bellingham, WA, 2005)

0277-786X/05/$15 · doi: 10.1117/12.609873

72

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 11/19/2013 Terms of Use: http://spiedl.org/terms

with improved polarization stability, but these crystals have half the upper-level lifetimes of Yb:YAG and lower energy storage12. Several Cr:YAG passive Q-switch small-signal transmittance values were used in this study.

Impressive progress in Yb:YAG lasers continues. Different research groups have built lasers with 340 fs pulse duration13, 1 J pulse energy4, or 2.65 kW average power14. Advances in microchip Yb:YAG lasers are also steady, albeit at lower average powers. To the best of the author’s knowledge, the benchmark Yb:YAG microchip laser performance achievements to date for different laser systems are: 530 ps pulse duration15, 8.1 W average power16, 72 µJ pulse energy17, 50 kHz repetition rate18, or 58% optical slope efficiency19. LADAR systems have certain requirements for the master oscillator. Many applications require nanosecond pulse widths, microjoule pulse energy, modest average power, kilohertz repetition rates, and excellent beam quality. Several Yb:YAG microchip lasers have been presented that individually meet some of these requirements. T. Y. Fan et al. reported a diode-pumped actively Q-switched Yb:YAG microchip laser with 11 ns pulse duration, 72 µJ pulse energy, 71 mW average power, and less than 1 kHz repetition rate17. Z. Qiu-Lin et al. presented a passively Q-switched Yb:YAG microchip laser with 52 ns pulse duration, 3.4 µJ pulse energy, 1.9 W average power, and 7.8 kHz repetition rate12. Finally, Y. Zhou et al. presented a monolithic Yb:YAG integrated passive Q-switch laser with 500 ps pulse duration, 20 µJ pulse energy, 20 mW average power, and 1 kHz repetition rate20.

In this paper, we present a diode-pumped passively Q-switched Yb:YAG laser that has 1 ns pulse duration, 68 µJ pulse energy, 700 mW average power, 10 kHz repetition rate, and 29% optical slope efficiency. The microchip oscillates in the fundamental TEM00 mode, and the peak power was measured as high as 66 kW. We compare the pulse shape and duration, and the beam quality to simulation. We study the effects of Q-switch transmittance, output coupler reflectivity, cavity length, and pump power on the laser’s pulse duration, pulse energy, average power, and repetition rate.

2. MICROCHIP LASER A schematic of the microchip laser is shown in Figure 1. The 6 mm microchip laser cavity contains thin disks of Yb:YAG and Cr:YAG. The Yb:YAG crystal is face-pumped by a 15 W CW InGaAs laser diode (LIMO model HLU15F100-940). The water cooled diode laser output at 940 nm is delivered to the microchip by a multi-mode fiber. The pump beam is then imaged to a 155 µm diameter spot inside the Yb:YAG by two lenses. Modeling shows that the laser will have only one transverse mode for this pump spot size. Gain is provided by a 7-at. % Yb:YAG disk (supplied by Saint-Gobain Crystals and Detectors). It is 10 mm in diameter and 2 mm thick; both faces are polished and uncoated. The pump only passes through the laser crystal once. In the future, coatings will be applied to the crystal to improve the laser performance. The measured pump transmission through the crystal is 19%, neglecting the Fresnel reflection losses. Therefore, the pump absorption coefficient is 7.4 cm-1. This crystal was grown by the Czochralski method. The Q-switches used in this laser are 0.6 mm thick and 10 mm in diameter with 1030 nm anti-reflection coatings on both faces. The initial (small-signal) transmission of the [100] cut Q-switches is either 95% (supplied by Red Optonics) or 90% (supplied by Saint-Gobain). It is important to note that the pump partially bleaches the Q-switch in these experiments.

Proc. of SPIE Vol. 5707 73


Figure 1. Microchip laser schematic

The rear cavity mirror is a flat, coated, and undoped YAG disk, which is 1mm thick and 10 mm in diameter.

The first side is anti-reflection coated at the pump wavelength (940 nm). The second side is coated for high reflection at the microchip laser wavelength (1030 nm), and transmission at the pump wavelength. The flat front cavity mirror, also referred to as the output coupler, is a 6 mm thick and 5 mm diameter rod of UV grade fused silica. The output coupler reflectivities used were 10, 30, or 50% at 1030 nm. The microchip laser is conductively cooled without heat fins or a fan. Clamping of the disks to a surrounding heat sink ensures adequate thermal contact. Initially, we tried operation with the disks free standing (only cooled by the surrounding air); but as expected, the laser is significantly more stable when mounted in the heat sink.

3. MODELING In order to choose the best laser design parameters, we constructed two different models of the laser. The first is a diffraction-based program using GLADiii that models the transverse profile of the output beam. In the model, the laser is gain guided, and oscillation builds up from noise. We assumed 20 diopters of thermal lensing, which is close to the 23 diopters that we later estimated from experiment. For simplicity, we assumed top hat profiles for the pump and gain spots. Using this model, we determined that the optimum pump spot diameter in the Yb:YAG is about 150 µm. Spot diameters significantly smaller than this result in poor laser efficiency, while larger spot diameters result in multi-transverse mode operation. We measured the actual pump spot diameter to be 155 µm and confirmed that the laser operates in a single transverse mode. The second model simulates the laser dynamics, including the saturable absorber, using standard rate equations. This Matlab-based program predicts the output pulse energy, internal cavity intensity, pulse duration, and gain. The model simulates the Yb:YAG microchip laser as a quasi 4-level system. The three rate equations are given below. Equation (1) represents the laser photon population ϕ (m-3), (2) the Yb:YAG gain g (m-1), and (3) the absorber ground state population na (m-3) of the Cr:YAG Q-switch.

( ) ( ) ( )[ ]{ }[ ]faaTaaaatgrt nnnLggLdtd ϕϕσστϕ

+−+−−= 212 (1)

f

o

rt

Rpeff

gggLdtdg

τϕ

τϕσ

−−

−=

2 (2)

iii “General Laser Analysis and Design” software from Applied Optics Research.

74 Proc. of SPIE Vol. 5707


21

2

a

aaTaa

rt

Ra nnnLdt

dnτ

ϕστ

−+−= (3)

τrt is the round trip time and τf = 0.95 ms is the fluorescence lifetime of Yb:YAG. Lg, La, and LR are the gain, absorber, and cavity lengths respectively. σa1 and σa2 are the absorber cross-section in the ground and excited states, naT is the total absorber Cr4+ population density, φf the fluorescence photon density, and φp the pump rate. The effective stimulated emission cross section σeff = 2.2 ×10-20 cm2, gt is the threshold gain, and go is the (negative) unpumped gain. The excited state and ground state cross-sections for Cr:YAG are assumed to be 5×10-19 cm2 and 5×10-18 cm2 respectively21.

The laser pulse model aided in the selection of the Q-switch small-signal transmittance, the output coupler reflectivity, and the cavity length. These parameters were varied to achieve the desired laser performance of 1-3 ns pulse durations with reasonable damage threshold margins. The results of the laser rate equation model indicated important trends in the microchip laser performance. Reducing the output coupler reflectivity increased the pulse energy and required pump power for a given repetition rate. Decreasing the Q-switch unsaturated transmission increased the pulse energy, pump power, and pulse buildup time. It also reduced the pulse duration, and increased the internal cavity fluence. Consequently, the output coupler reflectivity must be reduced to maintain adequate damage margin. Using the numerical results for the internal fluence, the output coupler reflectivity and Q-switch transmission were carefully matched in each experiment to avoid damage previously observed in short pulse Yb:YAG microchip lasers20. The pulse shape and duration simulated using the rate equation model compares well, as shown in Figure 2, to the experimentally measured pulse. The simulated pulse duration was 1.9 ns and the actual pulse duration was 2.0 ns. For this experiment, the output coupler reflectivity, R, and Q-switch small-signal transmission,T0, were 50% and 95% respectively.

Figure 2. Output pulse comparison: a) simulation, and b) measurement

R=50% T0=95%

4. EXPERIMENTAL RESULTS We tested the microchip laser for different values of R {10, 30, 50%} and T0 {85.5, 90, 95%}. One 95% and one 90% Q-switch were placed in series for the 85.5% Q-switch experiments. For each pair of R and T0, we measured the pulse duration, pulse energy, average output power, and repetition rate. Additionally, the beam quality was checked to verify the existence of a single transverse mode. The laser output pulses were detected by a fiber coupled 18.5 ps



NIR photodetector (New Focus model 1444), and measured with a 3 GHz digital oscilloscope (Tektronix model TDS 6946). The pulse duration is a FWHM (Full-Width at Half Maximum) measurement. The average power of the microchip laser was measured using a thermopile sensor and power meter (Newport models 818T-10 and 1825-C). A slow IR detector (Molectron J4-09) was used to measure the pulse repetition rate. The pulse energy was calculated as the average power divided by the repetition rate. The laser optical slope efficiency and pump threshold were calculated from the output power relative to the incident pump power. Some papers plot the output power versus absorbed power. As a result, their reported slope efficiencies will be higher and thresholds will be lower than if they used the incident pump power. We use incident, not absorbed, pump power throughout this publication. Table 1 presents the results of the parametric analysis of the Yb:YAG microchip laser. Numerous measurements were taken over time, and the results given are specific examples representative of the microchip laser performance at that setting. The cavity length was 6 mm for every experiment in Table 1. The representative pulse durations varied from 1.7 to 5.3 ns. The pulse energy varied from 16 to 68 µJ. For each pair of output coupler and saturable absorber, the laser generated pulses with repetition rates from 1 to 10 kHz depending on the pump power.

Table 1. Results of parametric analysis of microchip laser (fixed cavity length)

R, Output Coupler

Reflectivity %

T0, Q-switch Initial

Transmission %

Pump Threshold

W

Optical Slope

Efficiency %

Pulse Duration (FWHM)

ns

Average Power at ~10 kHz

mW

Pulse Energy

µJ

50 95.0 1.1 26 3.2 195 20 30 95.0 1.8 20 4.1 193 19 10 95.0 2.1 18 5.3 177 16 30 90.0 1.5 32 2.1 501 50 10 90.0 2.5 24 4.1 288 29 30 85.5 1.6 29 1.7 705 68 10 85.5 2.2 27 1.8 496 48

The results indicate several important trends. In summary, increasing R, the output coupler reflectivity,

shorted the pulse duration, and increased the pulse energy, average output power, threshold, and optical-to-optical slope efficiency. Decreasing T0, the Q-switch small-signal transmission, shortened the pulse duration and increased the pulse energy. The MATLAB model predicted that decreasing the Q-switch transmission would cause the pulse duration to decrease and the pulse energy to increase, however, the experimental pulse duration increased faster with decreasing output coupler reflectivity than that predicted by the model. The pulse energy increases with decreasing pulse duration. Reducing the Q-switch transmission or increasing the output coupler reflectivity resulted in higher pulse energy. Therefore, to minimize the pulse duration and maximize the pulse energy, one should increase the Q-switch absorption and the output coupler reflectivity. But, each of these changes increases the internal cavity fluence. We carefully selected R and T0 in order to avoid damage. The highest fluence inside the laser cavity was 1.1 J/cm2.

The 90% T0 Q-switch had the best efficiency: 32% optical slope efficiency, and 14% optical efficiency. For fixed output coupling, the pump power threshold did not change significantly for the different Q-switches as expected and previously shown22. But, it is not known why the laser has a lower threshold with the 85.5% Q-switch than with the 90% Q-switch. Increasing the output coupling (reducing R) appreciably increased the laser threshold. For a fixed pump power, decreasing the Q-switch transmission decreased the average output power and repetition rate. The pulse duration is proportional to the cavity length; that is, shorter cavities produced shorter pulses due to shorter photon round trip times15. Changing the cavity length did not change the pulse repetition rate, however. The pulse energy increased slightly with average power as previously documented17,22.

The repetition rate and average power both increased linearly with pump power. Figure 3 plots the average

power as a function of the pump power for the 30% output coupler and three different Q-switch transmissions.



Figure 3. Average output power versus pump power for different Q-switches, R=30%

The linear fit equation is displayed: the slope is the optical slope efficiency. As reported, the best performance to date is for 30% reflectivity (R) and 85.5% initial transmission,T0. Here,

the smallest pulse duration measured was 1.0 ns. For this measurement, the repetition rate was 10.3 kHz, the average power was 705 mW, the pulse energy was 68.4 µJ, and the peak power was 66 kW. Figure 3 shows the oscilloscope recorded trace of the Q-switched pulse with 1.0 ns duration. The cause of the falling-edge ringing is unknown, but it has been observed in a Nd:YAG microchip laser23 and exists for pulse durations less than approximately 1.2 ns. It may be an artifact of the measurement equipment.

Figure 4: Oscilloscope trace of the 1 ns pulse with 68 µJ pulse energy

and 10.3 kHz repetition rate. R=30%, T0=88.5%



This microchip laser oscillates in a single transverse mode and several longitudinal modes. We imaged the beam waist, with a 6.6x magnification, directly onto a COHU 4800 camera. A filter was attached to the camera to block visible light. The transverse beam profile is Gaussian, and the laser beam waist radius is 59 µm (w0). The measured far-field diffraction half-angle, θ, is 0.0125 radians. Therefore, the laser beam’s M2 value is 1.1 as calculated in the equation M2 = πw0θ/2λ. We also measured the spectrum of the microchip laser with a Traix 320 (Jobin Yvon-Spex) that has 0.06 nm spectral resolution. Figure 5 shows that this laser’s center wavelength is 1029.5 nm and the bandwidth is 0.5 nm. This bandwidth indicates the existence of several longitudinal modes, which are due to the large 6 nm emission bandwidth of Yb:YAG. Shortening the cavity will reduce the number of modes.

Figure 5. Yb:YAG microchip laser spectrum with 1029.5 nm center wavelength

The microchip laser usually oscillates in two orthogonal polarizations due to the Cr:YAG anisotropic saturation absorption. But, with careful alignment, the laser output beam could be made to operate in a single linear polarization with greater than 200:1 extinction, for some time. Scanning the tilt of the output coupler alignment produced many cycles of the laser’s polarization from completely vertical to completely horizontal. This behavior is believed to be due to polarization state changes in the Cr:YAG Q-switch, and from a light pulse-induced anisotropy of the Cr:YAG absorption coefficient and refractive index27. The saturation absorption coefficient of Cr:YAG is anisotropic due to phototropic center symmetry24. Cr:YAG cut in the [100], like the ones used in these experiments, oscillate with the electric field vector parallel to either the [001], [010] crystallographic axis or both25. N. Il’ichev et al. reported that linearly polarized 25 ns pulses became elliptically polarized after passing through a saturated Cr:YAG crystal24. It has been reported that dual-doped Cr,Yb:YAG crystals stabilize the polarization20. But, Cr,Yb:YAG has reduced upper-level lifetime and pump efficiency compared to Yb:YAG18. We are currently investigating others ways to force the microchip laser to continuously oscillate in a single polarization.

5. CONCLUSION A conductively-cooled Yb:YAG microchip laser was presented with 1 ns pulse duration, 68 µJ pulse energy, 700 mW average output power, 10 kHz repetition rate, and 29% optical slope efficiency. It operated in a single TEM00 mode and the M2 value was 1.1. The peak power was measured as high as 66 kW, which is one of the highest peak powers to date for a Yb:YAG microchip laser. This short pulse duration, high repetition rate, and single transverse mode laser could be used in a master oscillator power amplifier (MOPA) or a ladar system. Also, the extremely high peak power could be useful for frequency conversion or material treatment.



The effects of the Q-switch absorption, output coupler reflectivity, and cavity length on the laser performance

were studied, and numerical results were presented in Table 1. The laser has smaller pulse duration, larger pulse energy and higher average power for both higher output coupler reflectivity and higher Q-switch initial absorption. The pulse energy is unaffected by the cavity length, while the pulse duration shortens linearly with the cavity length. We compared the experimental results to numerical simulations based on diffraction and laser rate equation models.

In the future, several key improvements will be made. Dichroic coatings will be applied to both sides of the

Yb:YAG crystal to eliminate the uncoated YAG surface reflection losses, increase pump absorption in the Yb:YAG, and reduce bleaching of the Cr:YAG by the pump. By placing the rear mirror (1.03 µm HR) coating on the first surface, the cavity length will be reduced. The second surface coating will reflect the pump light in order to increase its absorption, and also eliminate bleaching of the Q-switch. These coatings will increase the efficiency and pulse energy of the laser, and reduce the pulse duration. In addition, we will improve the heat dissipation and modify the microchip cavity so that the laser oscillates in a single polarization. Finally, the individual crystal disks could be diffusion bonded together into a monolithic architecture.

This investigation was performed by the authors in El Segundo, California as a Raytheon Space and Airborne Systems internal research and development project.

6. REFERENCES 1. D. Sumida and T. Y. Fan, “Room-temperature 50-mJ/pulse side-diode-pumped Yb:YAG laser”, Opt. Lett. 20,

2384-2386, 1995. 2. T. Dascalu, N. Pavel, and T. Taira, “90 W continuous-wave edge-pumped microchip composite Yb:Y3Al5O12

laser”, Appl. Phys. Lett. 83, 4086-4088, 2003. 3. P. Lacovara, H. K. Choi, C. A. Wang, R. L. Aggarwal, and T. Y. Fan, “Room-temperature diode-pumped Yb:YAG

laser”, Opt. Lett. 16, 1089-1091, 1991. 4. H. W. Bruesselbach, D. S. Sumida, R. A. Reeder, and R. W. Byren, “Low-heat high-power scaling using InGaAs-

diode-pumped Yb:YAG lasers”, IEEE J. Quantum Electron. 3, 105-116, 1997. 5. D. S. Sumida and T. Y. Fan, “Effect of radiation trapping on fluorescence lifetime and emission cross section

measurements in solid-state laser media”, Opt. Lett. 13, 1343-1346, 1994. 6. O. A. Buryy, S. B. Ubiszkii, S. S. Melnyk, and A. O. Matkovskii, “The Q-switched Nd:YAG and Yb:YAG

microchip lasers optimization and comparative analysis”, Appl. Phys. B 78, 291-297, 2004. 7. V. V. Ter-Mikirtychev, and V. A. Fromzel, “Directly single-diode-pumped continuous-wave Yb3+:YAG laser

tunable in the 1047-1051-nm wavelength range”, Appl. Opt. 39, 4964-4969, 2000. 8. T. Taira, J. Saikawa, T. Kobayashi, and R. L. Byer, “Diode-pumped tunable Yb:YAG miniature lasers at room

temperature: modeling and experiment”, IEEE J. Quantum Electron. 3, 100-103, 1997. 9. T. Y. Fan, “Heat generation in Nd:YAG and Yb:YAG”, IEEE J. Quantum Electron. 29, 1457-1459, 1993. 10. F. D. Patel, E. C. Honea, J. Speth, and S. A. Payne, “Properties of Yb3Al5O12”, IEEE Proc., 856-857, 1999. 11. M. Arvidsson, B. Hansson, M. Holgren, and C. Lindstrom, “A combined actively and passively Q-switched

microchip laser”, SPIE proc. 3265, 106-113, 1998. 12. Z. Qiu-Lin, F. Bao-Hua, Z. Dong-Xiang, F. Pan-Ming, Z. Zhi-Guo, Z. Zhi-Wei, D. Pei-Zhen, X. Jun, X. Xiao-

Dong, W. Yong-Gang, and M. Xiao-Yu, “Diode-pumped passively Q-switched Yb:YAG microchip laser with a GaAs as saturable absorber”, Chin. Phys. Lett. 20, 1741-1743, 2003.

13. C. Honninger, R. PAschotta, M. Graf, F. Morier-Genoud, G. Zhang, M. Moser, S. Biswal, J. Nees, A. Braun, G. A. Mourou, I. Johannsen, A. Giesen, W. Seeber, and U. Keller, “Ultrafast ytterbium-doped bulk lasers and laser amplifiers”, Appl. Phys. B 69, 3-17, 1999.

14. H. Bruesselbach: in OSA Annu. Meet., Long Beach, CA, paper TuR1, 14–18, October 2001. 15. G. J. Spuhler, R. Paschotta, M. P. Kullberg, M. Graf, M. Moser, E. Mix, G. Huber, C. Harder, and U. Keller, “A

passively Q-switched Yb:YAG microchip laser”, Appl. Phys. B 72, 285-287, 2001. 16. J. Aus der Au, S. F. Schaer, R. Paschotta, C. Hönninger, and U. Keller, “High-power diode-pumped passively

mode-locked Yb:YAG lasers”, Opt. Lett. 24, 1281-1283, 1999.



17. T. Y. Fan, S. Klunk, and G. Henein, “Diode-pumped Q-switched Yb:YAG laser”, Opt. Lett. 18, 423-425, 1993. 18. D. Jun, D. Pei-Zhen, L. Yu-Pu, Z. Ying-Hua, H. Guo-Song, and G. Fu Xi, “Performance of the self-Q-switched

Cr,Yb:YAG laser”, Chin. Phys. Lett. 19, 342-344, 2002. 19. C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hügel, “A 1-kW CW thin disc laser,” IEEE J. Sel. Top.

Quantum. Electron. 6, 650-657, 2000. 20. Y. Zhou, Q. Thai, Y. C. Chen, S. Zhou, “Monolithic Q-switched Cr,Yb:YAG laser”, Opt. Comm. 219, 365-367,

2003. 21. N. Borodin, V. Zhitnyuk, A. Okhrimchuck, and A. Shestakov, “Oscillation of a Y3Al5O12:Cr4+ laser in wavelength

region of 1.34-1.6 µm”, Izvestiya Akademii Nauk SSSR, Seriya Fizicheskaya 54, 1500-1506, 1990. 22. J. Dong, P. Deng, Y. Liu, Y. Zhang, J. Xu, W. Chen, and X. Xie, “Passively Q-switched Yb:YAG laser with

Cr4+:YAG as the saturable absorber”, Applied Optics 40, 4303-4307, 2001. 23. L. Lv, L. Wang, P. Fu, X. Chen, Z. Zhang, V. Gaebler, D. Li, B. Liu, H. J. Eichler, S. Zhang, A. Liu, and Z. Zhu,

“Diode-pumped self-Q-switched single-frequency 946-nm Nd3+,Cr4+:YAG microchip laser”, Opt. Lett. 26, 72-74, 2001.

24. N. N. Il’ichev, A. V. Kir’yanov, P. P. Pashinin, S. M. Shpuga, S Gulyamova, “Changes in the profile and state of polarization of a short light pulse (λ~1.06 µm) during propagation in a YAG:Cr4+ crystal”, Quant. Electron. 24, 771-776, 1994.

25. H. Eilers, K. R. Hoffman, W. M. Dennis, S. M. Jacobsen, and W. M. Yen, “Saturation of a 1.064 µm absorption in Cr,Ca:Y3Al5O12 crystals”, Appl. Phys. Lett. 61, 2958-2960, 1992.



Documents

SPIE Proceedings [SPIE Lasers and Applications in Science and Engineering - San Jose, CA (Saturday 22 January 2005)] Solid State Lasers XIV: Technology and Devices - Short pulse and