ISSN 1054�660X, Laser Physics, 2012, Vol. 22, No. 7, pp. 1169–1172.© Pleiades Publishing, Ltd., 2012.Original Text © Astro, Ltd., 2012.

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1 1. INTRODUCTION

LD pump picosecond laser has been widely used invarious applications, such as laser ablation and micro�fabrication, nonlinear frequency�conversion pro�cesses, satellite ranging [1–4]. However, due to thevery short pulse duration, high pulse energy may easilyinduce optical damage and nonlinear effect, so it is achallenge in developing high�energy pulse in picosec�ond range. In recent years, high average power highrepetition rate picosecond lasers with relative lowpulse energy (μJ) attract many attentions, and theyhave shown excellent performance in micromachin�ing. These kind of laser includes Nd:YVO4 bouncepicoseconds amplifier with 16 W output power of20 MHz repetition rate [5]. Diode pumped Yb:YAGregenerative amplifier with 10 W output power at100 kHz with 6.2 ps pulse width [6], etc. In high pulseenergy amplification, Kube ek and Jelínek havereported a novel grazing�incident Nd:GdVO4 slablaser and amplifier system. Single pulse with 30 Hzand 55 ps duration was cavity dumped from the mode�locked resonator and amplified by the double passamplifier to the energy of 0.4 mJ [7]. This is an attrac�tive structure in producing energy of hundreds ofmicro�joule. In addition, chirped pulse amplification(CPA) is often used, Curtis and Furch reported a com�pact all�diode�pumped Yb:YAG laser which produced100 mJ pulses of 5 ps duration at 100 Hz repetition rate[8]. They also reported laser system produced 8.5 pspulses with up to 1 J energy at 10 Hz repetition rate [9].In these laser systems, Yb doping gain medium is oftenused because it is suitable for CPA, and cryogenic�cooling for improving thermal performance isadopted, too [10]. Although these techniques show

1 The article is published in the original.

c

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remarkable advantages in scaling ps laser pulse energy,the systems are bulky and difficult to align whichincreased the complexity and cost. For some applica�tions, there is no need to employ such high energy likethe order of Joule. In this letter, we have developed acompact and inexpensive five�pass laser amplifier.With 1 mJ 10 Hz pulses as seed laser, without using anypulse stretching means, the amplifier generated 20 mJpulses at 1053 nm, with pulse duration of 12.9 ps.

2. EXPERIMENTAL SETUP

The system used in this work is shown schemati�cally in Fig. 1. The seed laser system contains a mode�locking laser which generates 10.3 ps pulses, at a aver�age power of 30 mW, 60 MHz repetition rate and a sidepumped regenerative amplifier. The 500�pJ modelocking pulse is amplified to 1–2 mJ after about50 round trips in the regenerative amplifier cavity. Dueto the small beam size at a diameter of 1 mm, the opti�cal elements are at the risk of damage, so the output ofregenerative amplifier is limited to 1 mJ, and then amultipass amplifier is used to generate a higher energyoutput with lager beam size. All the gain mediumsused in the systems are Nd:YLF. Naturally birefringentNd:YLF is advantageous for generating linearly polar�ized lasers, and it has a long fluorescence lifetime(480–520 μs), which is adapted to generate highenergy pulse [11].

The traditional four passes amplifier is an effectivedevice for energy scaling [12, 13]. A faraday rotator isplaced between the amplifier head and the end mirrorto produce 90° rotation for the injected pulses everyround trip, as a result, in the active medium, the polar�ization direction of injected pulses have changedtwice, and the polarization direction of seed laserbeam makes no difference in a isotropic medium.

SOLID STATEAND LIQUID LASERS

Picosecond Nd:YLF Five�Passes Laser Amplifier with 20 mJ Pulse Energy1

Q. K. Aia, *, M. Chena, **, Y. Xua, L. Changa, L. Y. Chena, G. Lia, J. H. Yangb, and Y. F. Mab

a Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, Chinab Beijing GK Laser Technology Co., Ltd, Beijing 100085, China

*e�mail: ai.qingkang@gmail.com**e�mail: chenmeng@bjut.edu.cn

December 27, 2011; in final form, December 30, 2011; published online June 12, 2012

Abstract—We have demonstrated an all�diode�pumped Nd:YLF laser amplifier that produces 20.8 mJ pulsesof 12.9 ps duration at 10 Hz repetition rate. The laser system consists of a mode�locking oscillator, a regen�erative amplifier and a five�pass amplifier. The small�angle off�axis five�pass configuration combining withthe gradual expansion of the laser beam cross section makes the amplifier compact and effective.

DOI: 10.1134/S1054660X12070018

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However, when it comes to an anisotropy medium, thepolarization direction and axis direction of mediummust be taken into account. Nd:YLF is a uniaxialanisotropy crystal, the radiation of 1047 nm (π polar�ization) parallels to the c�axis of the crystal, and the1053 nm (σ�polarization) is perpendicular to the c�axis [14, 15]. Therefore, for the amplification of1053 nm laser, the polarization orientation of seedlaser should be perpendicular to the c�axis of Nd:YLF,so the polarization�dependent four passes amplifiermentioned above is not suitable for the Nd:YLF. Tomake full use of the energy stored in the gain medium,we employed an off�axis multi�pass amplifier. Theseed laser beam goes through the rod medium repeat�edly for five passes, and the polarization direction ofseed laser beam transmitting in the amplifier mediumkeeps perpendicular to c�axis all the time. Usuallywide�angle off�axis amplification is used for short

length and larger aperture gain medium [5, 16]. This isthe first time using a non�coaxial way in such a longlength and small aperture medium for five passes. Asshown In Fig. 1, the amplifier is comprised of a sidepumped a�cut Nd:YLF rod(∅6 × 66 mm, 0.9 at %).The maximal angle of the incident seed laser with theamplifier rod axis is only about 5°, so we must thinkover the layout to arrange the reflecting mirrors HR4–HR12.

Considering that the peak power of amplifiedpulses get higher after each pass, a concave lens (focallength f = –800 mm) were inserted into the opticalpath between regenerative amplifier and the five passesamplifier to make the beam cross section bigger onevery passage through the rod. This large beam sizewill balance the increased pulse energy intensity, andthus can keep the laser intensity away from the damagethreshold of the optical components. The beam size ofoutput pulses was measured for each pass, which is 2.0,2.7, 3.6, 4.6, and 5.8 mm in diameter, respectively.

A 1053 nm half�wave plate was also employed tooptimize the rotation of the polarization of the regen�erative amplifier output for a better matching with thea�axis of Nd:YLF rod to maximize gain [7, 17].

3. EXPERIMENTAL STUDIES AND RESULTS

With 1 mJ output pulses from the regenerativeamplifier as seed beam, after five passes through theactive region, the multipass amplifier produces20.8 mJ energy, and the corresponding maximal pumpcurrent and voltage are 80 A and 46 V, respectively. Thepump pulse duration is 500 μs, and the correspondinge–o efficiency is about 1.1%.

Figure 2 shows the output energy of each pass atdifferent pump current. The solid lines are based onthe theory of ultrashort pulse amplification [18, 19]:

(1)JoutN( )

Jsat 1 G0N( )

JinN( )

/Jsat[ ]exp 1–{ }+( ),ln=

Mode�locking oscillator

M1

M2

HR1

HR2

HR4HR8

HR12

HR11 HR7 HR5 HR9

HR10

HR3

HR6

TFPTFP

TFPPC

1/4WP

1/2WP

Len

Nd:YLF

Nd:YLF

Isolator

Fig. 1. Schematic of the five passes amplifier with the seed laser. PC, Pockels cell; WP, waveplate; TFP, thin film polarizer; M1,M2, plano�concave cavity mirror; HR, high reflective mirror.

21

17

13

9

5

1

36 44 52 60 68 76Pump current, A

Output energy, mJ

One passTwo passesThree passesFour passesFive passes

Fig. 2. Output energy versus diode current, the dots repre�sent the experiment result, the solid lines obtained fromthe amplification theory.

LASER PHYSICS Vol. 22 No. 7 2012

PICOSECOND Nd:YLF FIVE�PASSES LASER AMPLIFIER 1171

where Jsat is the saturation fluence specific to the laseractive center in the amplifying medium, N is the pass

numbers, and is the small signal given by:

(2)

is the stored energy per unit cross�section in the

gain medium. is the initial fluence which propor�tional to the pump fluence. The discrete mark in Fig. 2is the experiment result from one pass to five passes,the output energy are 2.4, 4.7, 8.2, 13.4, and 20.8 mJ,corresponding to the gains of 2.39, 1.96, 1.74, 1.63,and 1.55. We can see that single pass amplification isnot satisfying even when the pump current is high.Most of the inversed population cannot be used by thesignal passed laser, although the gain is high. Whileafter five passes, the output is increasing rapidly, so themultipass amplification is an effective way to extract

G0N( )

G0N( )

JstoN( )

/Jsat[ ].exp=

JstoN( )

Jsto0( )

energy from gain medium. However, the number ofpasses is also needed to consider. From Fig. 3a we cansee that at a given pump and seed laser energy, thereexist a optimum passes number, 12 or 13 times, this isthe gain�saturation regime for amplification. If thenumber is more than 13, the loss is larger than thegain, and the output will fall down quickly, if less than13, the potential of the amplifier is not fully exploit.According to Fig. 3a, the five�pass amplifier works faraway from the gain�saturation regime, so increasingthe pass numbers the output of the amplifier can stillincrease. However, considering the surface of crystal isprone to damaged due to the high peak power, and thelaser beam size inside the multipass could not enlargedanymore, the pass numbers has to be limited to five. Inaddition, the loss factor l in each pass is also impor�tant. In Fig. 3a, the output changed a lot with four dif�ferent loss factors, so in order to reach a high outputlevel, the loss must be reduced as much as possible. InFig. 3b, the numerical modeling shows that the outputenergy will also increase as the seed laser energy rises,and the pass numbers that reach the gain saturationwill also reduce.

Figure 4 shows the intensity autocorrelation tracesusing an autocorrelator (APE, PulseCheck). The pulseduration of mode locking master oscillator laser is10.3 ps, while the pulse duration of multipass amplifieroutput is 12.9 ps. This difference is due to the gain nar�row effect. Because the gain in the centre wavelengthof spectrum is higher than that in the edge, a competi�tion of population inversion between the longitudinalmodes of the center wavelengths and that at edgeappears [20]. Compared with other laser medium likeNd:YAG, Nd:YLF has a broader fluorescent spectrumwidth (1.35 nm), and the corresponding linewidth is

120

100

80

60

40

20

0 5 10 15 20 25 30

Ou

tpu

t en

ergy

, m

J

l = 0

l = 0.04

l = 0.08

l = 0.12

80

60

40

20

0 5 10 15 20 25 30Pass number

Ou

tpu

t en

ergy

, m

J

Ein = 4 mJ

Ein = 3 mJ

Ein = 2 mJ

Ein = 1 mJ

(a)

(b)

Fig. 3. (a) The output as a function of the pass numbers fordifferent single�pass losses, with the same input laserenergy and the same pump energy. l is the loss factor.(b) The output as a function of the pass numbers, with dif�ferent initial input seed energy, and for the same loss factor(l = 0.05) and the same pump energy.

650

500

350

200

50

40 50 60 70 80 90 100Delay, ps

Autocorrelation signal, arb. units

Five�passes amplifier pulse

Mode�locking pulse

Fig. 4. The second harmonic autocorrelation traces ofmode�locking pulses, the FWHM duration is 10.3 ps(Gauss fit), and five�passes amplification pulses, the pulseduration is 12.9 ps FWHM (Gauss fit). The inset imageshows the 20 mJ laser beam outline exiting the amplifier.

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368 GHz, so the amplified pulse doesn’t broadenmuch. The beam profile is still keep a good mode qual�ity after five passes, although the edge of the beamcross section is cut by the medium aperture.

4. CONCLUSIONS

By using small�angle off�axis optical path designand the spatial expansion of the laser beam size in eachpass, the simple and low cost five passes amplifierworks at an effective way to extract energy from a singlerod. With 1 mJ seed pulses from the regenerativeamplifier, the five�passes amplifier produces 20.8 mJpulse energy of 12.9 ps duration at 10 Hz repetitionrate. Based on the guidelines of the numerical model�ing, further energy scaling can be realized by a largeraperture rod and more pass numbers.

ACKNOWLEDGMENTS

The authors thank Bai Zhenao and Yang Chao fortheir assistance with measurements. This work wassupported by The National High TechnologyResearch and Development Program of China(2011AA030205).

REFERENCES

1. J. Jandeleit, A. Horn, R. Weichenhain, et al., Appl.Surf. Sci. 127, 885 (1998).

2. X. Wushouer, P. Yan, H. Yu, et al., Laser Phys. Lett. 7,644 (2010).

3. M. Jelínek, V. Kube ek, M. ech, and P. Hiršl, LaserPhys. Lett. 8, 205 (2011).

4. M. Jelínek, Jr. and V. Kube ek, Laser Phys. Lett. 8, 657(2011).

5. Naoki Shiba, Yasuhito Morimoto, Kenji Furuki, et al.,Opt. Express. 16, 16382 (2008).

6. K. Sueda, S. Kawato, and T. Kobayashi, Laser Phys.Lett. 5, 271 (2008).

7. V. Kube ek, M. Jelínek, M. ech, et al., Laser Phys.Lett. 7, 130 (2010).

8. A. H. Curtis, B. A. Reagan, K. A. Wernsing, et al., Opt.Lett. 36, 2164 (2011).

9. F. J. Furch, B. A. Reagan, B. M. Luther, et al., Opt.Lett. 34, 3352 (2009).

10. J. Kawanaka, Y. Takeuchi, A. Yoshida, et al., LaserPhys. 20, 1079 (2010).

11. Y. Sun, H. Zhang, Q. Liu, et al., Laser Phys. Lett. 7, 722(2010).

12. J. Fu, Q. S. Pang, L. Chang, et al., Laser Phys. 21, 1042(2011).

13. M. J. P. Dymott and K. J. Weingarten, Appl. Opt. 40,3042 (2001).

14. N. U. Wetter, E. C. Sousa, I. M. Ranieri, and S. L. Bal�dochi, Opt. Lett. 34, 292 (2009).

15. Y. F. Lü, X. D. Yin, J. Xia, et al., Laser Phys. Lett. 6, 860(2009).

16. Shigeki Tokita, Masaki Hashida, Shinichiro Masuno,et al., Opt. Express. 16, 14875 (2008).

17. Ch. Bollig, C. Jacobs, M. J. Danie Esser, et al., Opt.Express. 18, 13993 (2010).

18. N. N. Il’ichev, A. V. Larikov, and A. A. Malyutin, LaserPhys. 1, 205 (1991).

19. F. P. Strolikendl, D. J. Files, and L. R. Dalton, J. Opt.Soc. Am., Ser. B 11, 742 (1994).

20. Q. S. Pang, Y. Liu, L. Z. Xu, et al., Laser Phys. 21, 1035(2011).

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