- Email: [email protected]
LD side-pumped passively Q-switched Yb:YAG slab laser
ARTICLE IN PRESS
Optics and Lasers in Engineering 42 (2004) 413–419
LD side-pumped passively Q-switched Yb:YAG slab laser Ping Yan*, Haisheng Wu, Mali Gong, Qiang Liu, Chen Li, R.Z. Cui, W.P. Jia Department of Precision Instruments, Tsinghua University, Beijing 100084, China Received 1 November 2003; received in revised form 1 November 2003; accepted 1 December 2003
Abstract A quasi-continuous wave laser diode side-pumped passively Q-switched Yb:YAG slab laser with Cr4+:YAG saturable absorber has been demonstrated in order to understand the pulse properties of Yb:YAG crystal. To our knowledge the maximum 69% extraction efficiency is achieved by the system. 44 mJ pulse energy and 1.64 KW peak power with near diffractionlimited beam quality are presented at 25 Hz repetition rate. The build-up time of the Q-giant in the passively Q-switched laser is shown. r 2004 Elsevier Ltd. All rights reserved. Keywords: Q-switched laser; Yb:YAG crystal; Saturable absorber; Diode-pumped solid state laser
1. Introduction Quasi-three level laser ions have attracted interest since the achievement to high intensity semiconductor lasers capable to pump such a lasing medium. Among them, the trivalent ytterbium ion seems to be the most attractive candidate as its electronic structure is limited to only two levels avoiding the excited states absorption, up conversion and concentration quenching. Recent literature verify enormous potential for CW high power and high efficiency of Yb:YAG lasers [1–4] and Yb:YAG microchip lasers [5,6] because of the good laser properties of Yb ion and mechanical properties of the hosting YAG.
*Corresponding author. Fax: +86-10-62781449. E-mail address: [email protected] (P. Yan). 0143-8166/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2003.12.002
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Compared to Nd:YAG crystal, Yb:YAG crystal has low quantum defect, broad absorption bandwidth, broad emission bandwidth, high-doped level, no excited state absorption, no upconversion, long lifetime and low emission cross-section. Because of these properties Yb:YAG crystal has been widely regarded as good laser material for high power CW laser. However these properties bring out some unique performance of Q-switched Yb:YAG lasers. For actively Q-switched laser, the first diode-pumped Q-switched Yb:YAG was reported in 1993 [7]. In 1999 183 W, M2=2.4 Yb:YAG acousto-optic Q-switched laser was reported [8], this result corresponds to a brightness 6.9 times greater than the result reported previously [9]. In 2001 measurements have been performed at energies to 60 mJ with pulse lengths of 60 ns by KDP Pockels cell Q-switch [10]. For passively Q-switched laser, using a SESAM as semiconductor saturable absorber, 1.1 mJ energy and 530 ps duration Qswitched Yb:YAG microchip laser was reported [11]. Only one reference paper was found to use Cr4+-doped garnets as saturable absorber in Yb:YAG laser [12]. It presented that pulse energy of 0.5 mJ and extraction efficiency of 30% at high repetition rate were achieved in CW diode-end-pumped passively Q-switched Yb:YAG laser. In this paper we report what to our knowledge is the first quasi continuous wave laser diode (QCW LD) side-pumped Cr4+:YAG passively Q-switched Yb:YAG slab laser and its properties.
2. Experimental setup The QCW LD side-pumping geometry and Q-switched laser setup are shown in Fig. 1. The Yb:YAG slab crystal dimensions are 2 mm 3 mm 8 mm with 5% Ybdoped. A quasi continuous wave (QCW) InGaAs diode arrays, consisting of eight
water in
water out 4+
Cr : YAG rear mirror
heat sink
lens duct
output mirror
Yb:YAG
lensless laser diode
Fig. 1. Schematic of the passively Q-switched diode-pumped Yb:YAG laser.
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lensless diode bars, is used as pump source. A lensduct transforms the pump beam into a 3 mm-wide 1.2 mm-high rectangle shape focus that enters the Yb:YAG laser crystal through the 3 mm 8 mm face. This face is antireflection coated at the pump wavelength of 941 nm. The pump beam double passed the laser crystal in the 2 mm dimension after reflecting off the high-reflection-coated back surface. Eighty eight percent of pumping power can be absorbed by Yb:YAG crystal. In order to obtain a good thermal contact, another 3 mm 8 mm face of Yb:YAG crystal is mounted to a copper heat sink by indium foil. The copper heat sink is cooled by water. The laser resonator consists of an output coupler with flat mirror of 97% reflectivity at 1030–1060 nm in the end face of saturable absorber. A rear flat mirror is high-reflectivity coating at 1030–1060 nm. The length of the cavity is around 35 mm. The laser axis is perpendicular to the 2 mm 3 mm Yb:YAG slab faces. Both of these faces are antireflection coated at the laser wavelength. Initial transmission of the Cr4+:YAG saturable absorber is 93.3%. The Q-switched pulse is detected by a New Focus 12G photo-detector and an Agilent 1.5 G Oscilloscope. The output power is measured by the Molectron EPM2000 power meter and the output beam quality is measured by the Spiricon M2200-ACC beam analyzer.
3. Results and discussions In the laser system transform efficiency of lensduct is measured to be 80% when the fast axis beam is suppressed by 2.4 times and the slow axis beam is suppressed by 8.75. Laser diode (LD) temperature is tuned to match the maximum absorption peak of the Yb:YAG crystal. The temperature effects on the three-level laser performance are clearly observed. So Yb:YAG crystal heat sink temperature is set at 5 C71 C which is the lowest temperature in our cooling system. By our experiment, 140 mJ absorbed pump threshold energy (at 1 ms pump duration) is required for getting Q-switched pulses. Correspondingly the threshold peak power intensity is around 3.9 KW/cm2. It is usually easy to operate a diodepumped passively Q-switched Yb:YAG laser with Cr4+:YAG. From analysis of the coupled equations, the criterion for a giant pulse to occur (second threshold) is given by [13] sgs A lnð1=T02 Þ g ; > þ lnð1=RÞ þ L s As 1 b
lnð1=T02 Þ
where T0 is the initial transmission of the saturable absorber. R is the reflectivity of the output mirror. L is the nonsaturable intracavity round-trip dissipative optical loss. sgs is ground state absorption cross-section of the saturable absorber, s is the stimulated emission cross-section of the gain medium. A/As is the ratio of the effective area in the gain medium and in the saturable absorber. g is the inversion reduction factor of quasi-three-level system. b is the ratio of the excited-state absorption cross section to that of the ground-state absorption in the saturable absorber. The small emission cross-section s (3.1 1020 cm2 at 5.5 at%) [14] of
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P. Yan et al. / Optics and Lasers in Engineering 42 (2004) 413–419
Yb:YAG crystal in comparison with that of Nd:YAG is favorable for obtaining passively Q-switched operation in Yb:YAG laser. We obtain the Q-switched pulse output in the passively Q-switched Yb:YAG laser system. The output pulse energy of 44 mJ and FWHM duration of 26.7 ns are presented at 25 Hz repetition rate when the Yb:YAG crystal absorbed energy is around 141 mJ, the LD pump duration is 1ms and the rising time of pumping pulse is 8 ms. The output peak power is to be 1.64 KW. The output beam M2 factor is measured as 1.03 and 1.16, respectively in vertical and horizontal directions with the beam waist of 633 and 387 mm, respectively. The ellipse output shape is caused by the pumping non-uniformity of the one-side pumping. Pulse to pulse energy fluctuation is 12.5% and pulse width fluctuation is 2.3%. Usually passively Q-switched laser has not very good energy stability. But at low pulse repetition rate, passively Q-switched lasers have better pulse to pulse amplitude and pulse-width stability than at high pulse repetition rate. Except the instability of pulse energy caused by Q-switch mechanism, we estimate that the energy fluctuation in our system is mainly due to the instability of initial population inversion densities of every Q-pulse in the cavity. In QCW-running operation it brings the relaxation oscillations before steady-state is reached in every pumping cycle. Passively Q-pulse is generally established shortly after the relaxation oscillations in QCW pumping mode. Fig. 2 shows the passively Q-switched and QCW-running output energy versus absorbed energy curve. When absorbed energy is 141 mJ, 64 mJ QCW-running pulse
Fig. 2. Output energy versus absorbed input energy in passively Q-switched and QCW running Yb:YAG lasers. The pulse trace is the typical Q-switched pulse output when the absorbed input energy is 141 mJ (50 ns/div).
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energy and 44 mJ Q-switched single pulse energy are obtained. The maximum 69% extraction efficiency is achieved with single pulse mode in our laser system. The fraction of the Q-switched energy relative to the QCW-running output energy under the same experimental conditions is defined by extraction efficiency. Compared to the maximum 74% extraction efficiency in the QCW LD pumped Nd:YAG/ Cr4+:YAG laser system [15], it shows that extraction efficiency in Yb:YAG/ Cr4+:YAG laser can reach the same high level by optimizing the laser parameters. In the LD pumped passively Q-switched laser system, if the absorbed pump energy of above 148 mJ at 1 ms pump duration, multi-pulse oscillation is observed in one modulation interval. In order to get single pulse oscillation and high extraction efficiency, the QCW LD pump width is correspondingly decreased with the increase of the pump peak power. In the system when decreasing the pump duration and increasing the pump peak power, the Q-switched single pulse energy keep unchanged and the extraction efficiency does not change a lot either. High extraction efficiency with Cr4+:YAG passively Q-switched Yb:YAG laser indicates that the Yb:YAG crystal is a potential good laser material as Q-switched laser and its amplifier. Fig. 3 shows the Q-pulse build-up time versus absorbed pump peak power while keeping single pulse output. Here the build-up time defines as the time-delay from the beginning of LD pump to get the 90% peak Q-switch pulse. When the Yb:YAG crystal parameter and saturable absorber parameter are fixed, build-up time of the Q-giant pulse mainly depends on the pump level. With the increase of pump rate the population inversion density is increased, so the build-up time is decreased. While increasing the absorbed pump peak power from 141 to 230 W (corresponding intensity from 3.9 to 6.4 KW/cm2) in Fig. 3, the build-up time decreases from 985 to
Q-pulse bulid-up time(us)
1000
900
800
700
600
500 140
160
180
200
220
absorbed pump peak power(W) Fig. 3. Q-pulse build-up time versus absorbed pump peak power.
240
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P. Yan et al. / Optics and Lasers in Engineering 42 (2004) 413–419
495 ms. The build-up time of Yb:YAG laser is longer than that of Nd:YAG laser because of the longer fluorescence lifetime (0.95 ms), lower pump transition peak cross-section (0.7 1020 cm2) and lower emission cross-section (3.1 1020 cm2) of Yb:YAG crystal. The experimental results show that even though the Yb:YAG crystal has longer spontaneous time than Nd:YAG crystal, higher repetition rate (up to 2 KHz) can still be achieved by increasing the pumping intensity in our Q-switched Yb:YAG laser system. For some Yb:YAG MOPA (master oscillation power amplifier) systems, this Q-pulse build-up time presents relevant time magnitude as minimum build-up time of storage energy which is important for the laser engineer. It is reasonable to find that there is a low optical efficiency in the Q-switched laser system on single pulse mode at low repetition rate. Because every shot needs the bleaching investment on the low-level population of Yb:YAG crystal and the single mode operation requires short pump duration. With the multi-pulses Q-switched Yb:YAG operation in one modulation cycle, the better optical to optical efficiency can be obtained by higher pump duty ratio. The newly established QCW LD pumped passively Q-switched Yb:YAG laser system has advantages of well-controlled repetition rate, compact and miniature. Based on our experimental results it can be shown that the LD pumped passively Qswitched Yb:YAG laser has good laser properties. It is anticipated that the Yb/Cr4+ laser system will possess significant advantages related to Q-switched lasers and their applications for example pulse seeding amplifier, micro-machining, remote sensing, target ranging and microsurgery. By the way in order to scale the pulse energy the electro-optic Q-switched LD side-pumped Yb:YAG slab laser is in procession.
4. Summary In conclusion, we have developed a QCW side-diode-pumped passively Qswitched Yb:YAG slab laser with Cr4+:YAG. To our knowledge the maximum 69% extraction efficiency at 25 Hz repetition rate is achieved for single pulse operation. 44 mJ Q-pulse energy with good beam quality is presented. When the absorbed pump peak power intensity is 6.4 KW/cm2, the build-up time of Q-switch is 495 ms. Compared to the Nd:YAG/Cr4+:YAG system, Yb:YAG/Cr4+:YAG laser can reach the high extraction efficiency, is easy to get Q-switched pulse and have a longer build-up time. Yb:YAG crystal is a good laser material not only for CW operation but also for Q-switch operation.
References [1] Sumida DS, Betin AA, Brusselbach H, Byren R, Matthews S, Reeder R, Mangir MS. Diode-pumped Yb:YAG catches up with Nd: YAG. Laser Focus World 1999;35(6):63–70. [2] Krupe WF. Ytterbium solid-state lasers-the first decade. IEEE J Sel Top Quantum Electron 2000;6(6):1287–96. [3] Stewen C, Contag K, Larionov M, Giesen A, Hugel H. A 1-kW CW thin disc laser. IEEE J Sel Top Quantum Electron 2000;6(4):650–7.
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[4] Beach RJ, Honea EC. High-average-power diode-pumped Yb:YAG lasers. SPIE 2000;3889:246–60. [5] Griebner U, Grunwald R, Schonnagel H. Thermally bonded Yb:YAG planar waveguide laser. Opt Commun 1999;164:185–90. [6] Brauch U, Giesen A, Karszwski M, Chr.Stewen, Voss A. Diode-pumped Yb:YAG thin disk laser continuously tunable between 1018 and 1053 nm. Opt Lett 1995;20(7):713–5. [7] Fan TY, KlunK S, Henein G. Diode-pumped Q-switched Yb:YAG laser. Opt Lett 1993;18(6):423–5. [8] Honea EC, Beach RJ, Mitchell SC. 183-W, M2=2.4 Yb:YAG Q-switched laser. Opt Lett 1999;24(3):154–6. [9] Bibeau C, Beach RJ, Mitchell SC, Emanuel MA, Skidmore JA, Ebbers CA, Sutton SB, Jancaitus KS. High-average power 1 mm performance and frequency conversion of a diode-end-pumped Yb:YAG laser. IEEE J Quantum Electron 1998;34:2010–9. [10] Mandl A, Klimek DE. Compact diode-pumped Yb:YAG slab laser. Proc SPIE 2001;4267:1–8. [11] Spuhler GJ, Paschotta R, Kullberg MP, Graf M, Moser M, Mix E, Huber G, Harder C, Keller U. A passively Q-switched Yb:YAG microchip laser. Appl Phys B 2001;72:285–7. [12] Kalisky Y, Labbe, C, Waichman, K, Kravchik L, Rachum U, Deng, P, Xu J, Dong J, Chen W. Passively Q-switched diode-pumped Yb:YAG laser using Cr4+-doped garnets. Opt Mater 2002;19(4):403–13. [13] Chen YF, Lan YP, Chang HL. Analytical model for design criteria of passively Q-switched lasers. IEEE J Quantum Electron 2001;37(3):462–8. [14] Gupta MC, Miniature solid-state lasers. In: Zayhowski JJ, Harrison J, editors. Handbook of photonics. New York: Boca Raton; 1997. [15] Anfu, F, Chenglong, Y, Xu Tianhua, Wang Weimin, Chen Jinyan, Zhou Xin. Study of performance of a Cr4+:YAG passively Q-switched QCW slab Nd:YAG laser. Chinese Laser 1998;25(9):781–4.
Optics and Lasers in Engineering 42 (2004) 413–419
LD side-pumped passively Q-switched Yb:YAG slab laser Ping Yan*, Haisheng Wu, Mali Gong, Qiang Liu, Chen Li, R.Z. Cui, W.P. Jia Department of Precision Instruments, Tsinghua University, Beijing 100084, China Received 1 November 2003; received in revised form 1 November 2003; accepted 1 December 2003
Abstract A quasi-continuous wave laser diode side-pumped passively Q-switched Yb:YAG slab laser with Cr4+:YAG saturable absorber has been demonstrated in order to understand the pulse properties of Yb:YAG crystal. To our knowledge the maximum 69% extraction efficiency is achieved by the system. 44 mJ pulse energy and 1.64 KW peak power with near diffractionlimited beam quality are presented at 25 Hz repetition rate. The build-up time of the Q-giant in the passively Q-switched laser is shown. r 2004 Elsevier Ltd. All rights reserved. Keywords: Q-switched laser; Yb:YAG crystal; Saturable absorber; Diode-pumped solid state laser
1. Introduction Quasi-three level laser ions have attracted interest since the achievement to high intensity semiconductor lasers capable to pump such a lasing medium. Among them, the trivalent ytterbium ion seems to be the most attractive candidate as its electronic structure is limited to only two levels avoiding the excited states absorption, up conversion and concentration quenching. Recent literature verify enormous potential for CW high power and high efficiency of Yb:YAG lasers [1–4] and Yb:YAG microchip lasers [5,6] because of the good laser properties of Yb ion and mechanical properties of the hosting YAG.
*Corresponding author. Fax: +86-10-62781449. E-mail address: [email protected] (P. Yan). 0143-8166/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2003.12.002
ARTICLE IN PRESS 414
P. Yan et al. / Optics and Lasers in Engineering 42 (2004) 413–419
Compared to Nd:YAG crystal, Yb:YAG crystal has low quantum defect, broad absorption bandwidth, broad emission bandwidth, high-doped level, no excited state absorption, no upconversion, long lifetime and low emission cross-section. Because of these properties Yb:YAG crystal has been widely regarded as good laser material for high power CW laser. However these properties bring out some unique performance of Q-switched Yb:YAG lasers. For actively Q-switched laser, the first diode-pumped Q-switched Yb:YAG was reported in 1993 [7]. In 1999 183 W, M2=2.4 Yb:YAG acousto-optic Q-switched laser was reported [8], this result corresponds to a brightness 6.9 times greater than the result reported previously [9]. In 2001 measurements have been performed at energies to 60 mJ with pulse lengths of 60 ns by KDP Pockels cell Q-switch [10]. For passively Q-switched laser, using a SESAM as semiconductor saturable absorber, 1.1 mJ energy and 530 ps duration Qswitched Yb:YAG microchip laser was reported [11]. Only one reference paper was found to use Cr4+-doped garnets as saturable absorber in Yb:YAG laser [12]. It presented that pulse energy of 0.5 mJ and extraction efficiency of 30% at high repetition rate were achieved in CW diode-end-pumped passively Q-switched Yb:YAG laser. In this paper we report what to our knowledge is the first quasi continuous wave laser diode (QCW LD) side-pumped Cr4+:YAG passively Q-switched Yb:YAG slab laser and its properties.
2. Experimental setup The QCW LD side-pumping geometry and Q-switched laser setup are shown in Fig. 1. The Yb:YAG slab crystal dimensions are 2 mm 3 mm 8 mm with 5% Ybdoped. A quasi continuous wave (QCW) InGaAs diode arrays, consisting of eight
water in
water out 4+
Cr : YAG rear mirror
heat sink
lens duct
output mirror
Yb:YAG
lensless laser diode
Fig. 1. Schematic of the passively Q-switched diode-pumped Yb:YAG laser.
ARTICLE IN PRESS P. Yan et al. / Optics and Lasers in Engineering 42 (2004) 413–419
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lensless diode bars, is used as pump source. A lensduct transforms the pump beam into a 3 mm-wide 1.2 mm-high rectangle shape focus that enters the Yb:YAG laser crystal through the 3 mm 8 mm face. This face is antireflection coated at the pump wavelength of 941 nm. The pump beam double passed the laser crystal in the 2 mm dimension after reflecting off the high-reflection-coated back surface. Eighty eight percent of pumping power can be absorbed by Yb:YAG crystal. In order to obtain a good thermal contact, another 3 mm 8 mm face of Yb:YAG crystal is mounted to a copper heat sink by indium foil. The copper heat sink is cooled by water. The laser resonator consists of an output coupler with flat mirror of 97% reflectivity at 1030–1060 nm in the end face of saturable absorber. A rear flat mirror is high-reflectivity coating at 1030–1060 nm. The length of the cavity is around 35 mm. The laser axis is perpendicular to the 2 mm 3 mm Yb:YAG slab faces. Both of these faces are antireflection coated at the laser wavelength. Initial transmission of the Cr4+:YAG saturable absorber is 93.3%. The Q-switched pulse is detected by a New Focus 12G photo-detector and an Agilent 1.5 G Oscilloscope. The output power is measured by the Molectron EPM2000 power meter and the output beam quality is measured by the Spiricon M2200-ACC beam analyzer.
3. Results and discussions In the laser system transform efficiency of lensduct is measured to be 80% when the fast axis beam is suppressed by 2.4 times and the slow axis beam is suppressed by 8.75. Laser diode (LD) temperature is tuned to match the maximum absorption peak of the Yb:YAG crystal. The temperature effects on the three-level laser performance are clearly observed. So Yb:YAG crystal heat sink temperature is set at 5 C71 C which is the lowest temperature in our cooling system. By our experiment, 140 mJ absorbed pump threshold energy (at 1 ms pump duration) is required for getting Q-switched pulses. Correspondingly the threshold peak power intensity is around 3.9 KW/cm2. It is usually easy to operate a diodepumped passively Q-switched Yb:YAG laser with Cr4+:YAG. From analysis of the coupled equations, the criterion for a giant pulse to occur (second threshold) is given by [13] sgs A lnð1=T02 Þ g ; > þ lnð1=RÞ þ L s As 1 b
lnð1=T02 Þ
where T0 is the initial transmission of the saturable absorber. R is the reflectivity of the output mirror. L is the nonsaturable intracavity round-trip dissipative optical loss. sgs is ground state absorption cross-section of the saturable absorber, s is the stimulated emission cross-section of the gain medium. A/As is the ratio of the effective area in the gain medium and in the saturable absorber. g is the inversion reduction factor of quasi-three-level system. b is the ratio of the excited-state absorption cross section to that of the ground-state absorption in the saturable absorber. The small emission cross-section s (3.1 1020 cm2 at 5.5 at%) [14] of
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Yb:YAG crystal in comparison with that of Nd:YAG is favorable for obtaining passively Q-switched operation in Yb:YAG laser. We obtain the Q-switched pulse output in the passively Q-switched Yb:YAG laser system. The output pulse energy of 44 mJ and FWHM duration of 26.7 ns are presented at 25 Hz repetition rate when the Yb:YAG crystal absorbed energy is around 141 mJ, the LD pump duration is 1ms and the rising time of pumping pulse is 8 ms. The output peak power is to be 1.64 KW. The output beam M2 factor is measured as 1.03 and 1.16, respectively in vertical and horizontal directions with the beam waist of 633 and 387 mm, respectively. The ellipse output shape is caused by the pumping non-uniformity of the one-side pumping. Pulse to pulse energy fluctuation is 12.5% and pulse width fluctuation is 2.3%. Usually passively Q-switched laser has not very good energy stability. But at low pulse repetition rate, passively Q-switched lasers have better pulse to pulse amplitude and pulse-width stability than at high pulse repetition rate. Except the instability of pulse energy caused by Q-switch mechanism, we estimate that the energy fluctuation in our system is mainly due to the instability of initial population inversion densities of every Q-pulse in the cavity. In QCW-running operation it brings the relaxation oscillations before steady-state is reached in every pumping cycle. Passively Q-pulse is generally established shortly after the relaxation oscillations in QCW pumping mode. Fig. 2 shows the passively Q-switched and QCW-running output energy versus absorbed energy curve. When absorbed energy is 141 mJ, 64 mJ QCW-running pulse
Fig. 2. Output energy versus absorbed input energy in passively Q-switched and QCW running Yb:YAG lasers. The pulse trace is the typical Q-switched pulse output when the absorbed input energy is 141 mJ (50 ns/div).
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energy and 44 mJ Q-switched single pulse energy are obtained. The maximum 69% extraction efficiency is achieved with single pulse mode in our laser system. The fraction of the Q-switched energy relative to the QCW-running output energy under the same experimental conditions is defined by extraction efficiency. Compared to the maximum 74% extraction efficiency in the QCW LD pumped Nd:YAG/ Cr4+:YAG laser system [15], it shows that extraction efficiency in Yb:YAG/ Cr4+:YAG laser can reach the same high level by optimizing the laser parameters. In the LD pumped passively Q-switched laser system, if the absorbed pump energy of above 148 mJ at 1 ms pump duration, multi-pulse oscillation is observed in one modulation interval. In order to get single pulse oscillation and high extraction efficiency, the QCW LD pump width is correspondingly decreased with the increase of the pump peak power. In the system when decreasing the pump duration and increasing the pump peak power, the Q-switched single pulse energy keep unchanged and the extraction efficiency does not change a lot either. High extraction efficiency with Cr4+:YAG passively Q-switched Yb:YAG laser indicates that the Yb:YAG crystal is a potential good laser material as Q-switched laser and its amplifier. Fig. 3 shows the Q-pulse build-up time versus absorbed pump peak power while keeping single pulse output. Here the build-up time defines as the time-delay from the beginning of LD pump to get the 90% peak Q-switch pulse. When the Yb:YAG crystal parameter and saturable absorber parameter are fixed, build-up time of the Q-giant pulse mainly depends on the pump level. With the increase of pump rate the population inversion density is increased, so the build-up time is decreased. While increasing the absorbed pump peak power from 141 to 230 W (corresponding intensity from 3.9 to 6.4 KW/cm2) in Fig. 3, the build-up time decreases from 985 to
Q-pulse bulid-up time(us)
1000
900
800
700
600
500 140
160
180
200
220
absorbed pump peak power(W) Fig. 3. Q-pulse build-up time versus absorbed pump peak power.
240
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495 ms. The build-up time of Yb:YAG laser is longer than that of Nd:YAG laser because of the longer fluorescence lifetime (0.95 ms), lower pump transition peak cross-section (0.7 1020 cm2) and lower emission cross-section (3.1 1020 cm2) of Yb:YAG crystal. The experimental results show that even though the Yb:YAG crystal has longer spontaneous time than Nd:YAG crystal, higher repetition rate (up to 2 KHz) can still be achieved by increasing the pumping intensity in our Q-switched Yb:YAG laser system. For some Yb:YAG MOPA (master oscillation power amplifier) systems, this Q-pulse build-up time presents relevant time magnitude as minimum build-up time of storage energy which is important for the laser engineer. It is reasonable to find that there is a low optical efficiency in the Q-switched laser system on single pulse mode at low repetition rate. Because every shot needs the bleaching investment on the low-level population of Yb:YAG crystal and the single mode operation requires short pump duration. With the multi-pulses Q-switched Yb:YAG operation in one modulation cycle, the better optical to optical efficiency can be obtained by higher pump duty ratio. The newly established QCW LD pumped passively Q-switched Yb:YAG laser system has advantages of well-controlled repetition rate, compact and miniature. Based on our experimental results it can be shown that the LD pumped passively Qswitched Yb:YAG laser has good laser properties. It is anticipated that the Yb/Cr4+ laser system will possess significant advantages related to Q-switched lasers and their applications for example pulse seeding amplifier, micro-machining, remote sensing, target ranging and microsurgery. By the way in order to scale the pulse energy the electro-optic Q-switched LD side-pumped Yb:YAG slab laser is in procession.
4. Summary In conclusion, we have developed a QCW side-diode-pumped passively Qswitched Yb:YAG slab laser with Cr4+:YAG. To our knowledge the maximum 69% extraction efficiency at 25 Hz repetition rate is achieved for single pulse operation. 44 mJ Q-pulse energy with good beam quality is presented. When the absorbed pump peak power intensity is 6.4 KW/cm2, the build-up time of Q-switch is 495 ms. Compared to the Nd:YAG/Cr4+:YAG system, Yb:YAG/Cr4+:YAG laser can reach the high extraction efficiency, is easy to get Q-switched pulse and have a longer build-up time. Yb:YAG crystal is a good laser material not only for CW operation but also for Q-switch operation.
References [1] Sumida DS, Betin AA, Brusselbach H, Byren R, Matthews S, Reeder R, Mangir MS. Diode-pumped Yb:YAG catches up with Nd: YAG. Laser Focus World 1999;35(6):63–70. [2] Krupe WF. Ytterbium solid-state lasers-the first decade. IEEE J Sel Top Quantum Electron 2000;6(6):1287–96. [3] Stewen C, Contag K, Larionov M, Giesen A, Hugel H. A 1-kW CW thin disc laser. IEEE J Sel Top Quantum Electron 2000;6(4):650–7.
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