a 108 w, 500 khz q-switching nd:yvo_4 laser with the mopa configuration

A 108 W, 500 kHz Q-switching Nd:YVO4

laser with the MOPA configuration

Xingpeng Yan*, Qiang Liu, Xing Fu, Yunxiang Wang, Lei Huang,Dongsheng Wang, and Mali Gong

Center for Photonics and Electronics, State Key Laboratory of Tribology, Department ofPrecision Instruments, Tsinghua University, Beijing 100084, China

[email protected]

Abstract: A high power, dual-end-pumped Nd:YVO 4 laser with a MOPAconfiguration was stably Q-switched at repetition rate up to 500 kHz.The thermally bonded Nd:YVO4 crystal was used in our experiment. Inacousto-optically Q-switching operation at repetition rate of 500 kHz,35 W average power was produced by the oscillator, with optical-opticalefficiency of 41.7%. 108 W average power was obtained by a masteroscillator power amplifier (MOPA) configuration including two amplifierstages, corresponding to the total optical-optical efficiency of 42.2%. Thepulse duration was 48 ns, with a stability of pulse peak value < 2.5%.The beam quality was better than two-times diffraction-limit (M 2

x = 1.99,M2

y = 1.76).

© 2008 Optical Society of America

OCIS codes: (140.3540) Lasers, Q-switched; (140.3480) Lasers, diode-pumped; (140.3530)Lasers, neodymium.

References and links1. N. N. Arev, B. F. Gorbunov, G. V. Pugachev, and Y. A. Bazlov, ”Application of a Laser Ranging System to the

Metrologic Certification of Satellite Radar Measurement Systems,” Meas. Tech. USSR 36, 524–525,(1993).2. N. D. Lai, M. Brunel, F.Bretenaker, and A. L. Floch, ”Stabilization of the repetition rate of passively Q-switched

diode-pumped solid-state lasers,” Appl. Phys. Lett. 79, 1073–1075 (2001).3. W. Koechner, Solid-State Laser Engineering, 5th ed. (Springer-Verlag Publications, Berlin,1999).4. R. A. Fields, M. Birnbaum, and C. L. Fincher, ”Highly efficient Nd:YVO4 diode-laser end-pumped laser,” Appl.

Phys. Lett. 51, 1885–1886 (1987).5. A. Brignon, G. Feugnet, J. P. Huignard, and J. P. Pocholle, ”Compact Nd:YAG and Nd:YVO4 amplifiers end-

pumped by a high-brightness stacked array,” IEEE. J. Quantum Electron. 34, 577–585 (1998).6. J. H. Garcıa-Lopez, V. Aboites, A. V. Kir’anov, M. J. Damzen, and A. Minassian, ”High repetition rate Q-

switching of high power Nd:YVO4 slab laser,” Opt. Commun. 218, 155–160 (2003).7. A. Minassian, B. A. Thompson, G. R. Smith, and M. J. Damzen, ”104W Diode-Pumped TEM00 Nd:GdVO4

Master Oscillator Power Amplifier,” in Advanced Solid-State Photonics, pp. MF46 (Optical Society of America,2005). http://www.opticsinfobase.org/abstract.cfm?URI=URI=ASSP-2005-MF46.

8. A. Minassian, G. Smith, T. hompson, and M. Damzen, ”Ultrahigh repetition rate Q-switched 101W TEM00Nd:GdVO4 laser system,” in Conference on Lasers and Electro-Optics Europe-Technical Digest, pp. 1567802(Munich, Germany, 2005).

9. T. Omatsu, M. Okida, A. Minassian, and M. Damzen, ”High repetition rate Q-switching performance in trans-versely diode-pumped Nd doped mixed gadolinium yttrium vanadate bounce laser,” Opt. Express 14, 2727–2734(2006). http://www.opticsexpress.org/abstract.cfm?URI=oe-14-7-2727.

10. F. He, L. Huang, M. Gong, Q. Liu, and X. Yan, ”Stable acousto-optics Q-switched Nd:YVO4 laser at 500 kHz,”Laser Phys. Lett. 4, 511–514 (2007).

11. Y. Wang, L. Huang, M. Gong, H. Zhang, M. Lei, and F. He, ”1 MHz repetition rate single-frequency gain switchedNd:YAG microchip laser,” Laser Phys. Lett. 4, 580–583 (2007).

12. X. Yan, Q. Liu, L. Huang, Y.Wang, X. Huang, D.Wang, and M. Gong, ”High efficient one-end-pumped TEM00laser with optimal pump mode,” Laser Phys. Lett. 5, 185–188 (2008).

(C) 2008 OSA 3 March 2008 / Vol. 16, No. 5 / OPTICS EXPRESS 3356#91328 - $15.00 USD Received 3 Jan 2008; revised 23 Feb 2008; accepted 24 Feb 2008; published 27 Feb 2008

13. J. C. Bermudez, V. J. Pinto-Robledo, A. V. Kir’yanov, and M. J. Damzen, ”The thermo-lensing effect in a grazingincidence, diode-side-pumped Nd:YVO4 laser,” Opt. Commun. 210, 75–82 (2002).

14. N. G. Lv, ed., Laser Optics, 3rd ed. (Higher Education Press, Beijing, 2003).15. I. Musgrave, W. Clarkson, and D. Hanna, ”Detailed study of thermal lensing in Nd:YVO4 under intense diode

end-pumping,” in Conference on Lasers and Electro-Optics Europe-Technical Digest, pp. 171–172(Baltimore,MD, 2001).

1. Introduction

High repetition rate diode-pumped solid state lasers (DPSSL) are attractive in a variety of appli-cations, such as material processing, remote sensing, laser radar, medicine and so on [1, 2]. Thehigh repetition rate (> 100 kHz) operating can be achieved by Q-switching, especially activelyQ-switching for its stable pulse energy and low temporal jitter at high repetition rates. However,the pulse duration becomes excessively lengthened for materials with long upper-state lifetime,like Nd:YAG and Nd:YLF. Nd:YVO4 is an important laser crystal owing to its large stimulatedemission cross section [3], wide pumping wavelength bandwidth [4] and short upper-state life-time [5]. Therefore, Nd:YVO4 is widely used for high repetition rate Q-switched operating inDPSSL.

In 2003, Garcıa-Lopez et al. reported a 500 kHz acousto-optics (AO) Q-switching slabNd:YVO4 laser with a grazing incidence cavity geometry. The average power is 16.4 W andthe pulse duration is 30 ns [6]. In 2005, Minassian et al. demonstrated a 400 kHz AO Q-switching slab Nd:GdVO4 laser with averge power of 101 W based on a diode-side pumpedbounce MOPA geometry, and the pulse duration is 20 ns [7, 8]. In 2006, Omatsu et al. con-figured a Nd doped mixed gadolinium yttrium vanadate bounce laser at repetition rate of 650kHz with a pulse-width of less than 40 ns, and the average power is 17 W [9]. The stability ofpulse in Ref. [6]–[9] are not reported. In 2007, F. He et al. reported a 1 MHz AO Q-switchingslab Nd:YVO4 laser with a grazing incidence geometry, and the stability of pulse peak value< 15% (RMS) [10]. Y. Wang et al. demonstrated a 1 MHz repetition rate low power gain-switched Nd:YAG microchip laser with 32 ns pulse duration, and the interpulse timing jitterand peak-peak instability were measured to be around 5% and 6% [11].

The above-mentionedhigh repetition rate AO Q-switching lasers were all operated with graz-ing incidence slab configuration. In this work, a dual-end-pumped configuration with fiber-coupled diode was configured, since with this pumped geometry, high degree of spatial overlapbetween pump volume and laser mode can be offered, and high pump power intensity canbe obtained for high repetition operation. The thermally bonded Nd:YVO 4 crystals were usedto reduce the thermal effect. An AO Q-switching oscillator at repetition rate up to 500 kHzwas configured, and the MOPA configuration of two amplifier stages was used. 108 W aver-age power, 500 kHz stable pulse output was obtained, to our knowledge which is the highestaverage power stable pulse output with repetition rate up to 500 kHz of AO Q-switching.

2. Experimental setup

The experimental arrangement of the Q-switching Nd:YVO 4 laser with the MOPA configura-tion is shown in Fig. 1. The MOPA configuration was built up with the oscillator stage and twoamplifier stages.

The thermally bonded Nd:YVO4 crystals were used to reduce thermal effect of high intensitypumping. The bonded Nd:YVO4 crystal consisted of two un-doped YVO4 end caps and a a-cut, 16 mm long, < 0.5 at. % doped Nd:YVO4 laser crystal. This scheme of using end capand low doping concentration can reduce the thermal loading density. This in turn, reduces thelikelihood of thermal fracture in the crystals for high intensity pumping and leads to a reductionin the loss of pump power due to energy-transfer upconversion (ETU).The laser crystal was


Fiber

LD

HR

OC

Nd:YVO4L1

L2

Dichroicmirror

c-axis

AOQ

Couplinglenses

c-axis

LD

LD LD

c-axis

LD

LD

L12

L23

Oscillator Amplifier 1 Amplifier 2

Fig. 1. Schematic of the Q-switching Nd:YVO4 laser with the MOPA configuration.

wrapped with indium foil and placed in copper heat sinks. Both surfaces of the boding crystalwere coated with AR films at 808 nm and 1064.3 nm with the reflectivity of 1064 nm smallerthan 0.1%.

The dual-end-pumped configuration was used. The diode laser modules of Jenoptik JOLD-45-CPXF-1L by fiber-coupled were applied as the pumping source in the experiment. The cou-pling fiber was with a core diameter of 400μm and a numerical aperture (NA) of 0.22. Thetemperature of the laser diode was controlled by temperature control unit to obtain the best ab-sorption. The pump laser at 808 nm was focused into the crystal by a coupling-lenses system.The mode matching coefficient can be defined by [12]

ηm =

(∫∫∫

Vζp(x,y,z) · ςl(x,y,z)dxdydz

)2

∫∫∫

Vζp(x,y,z) · ς2

l (x,y,z)dxdydz(1)

where ζp and ςl are the normalized intensity distribution of pump mode and cavity moderespectively. Optimal pumping mode with highest mode matching coefficient can be obtainedby adjusting the imaging ratio of the coupling-lenses and the location of the pumping waistspot in the crystal. The radial distribution of the pump mode was measured as a 4-order super-Gaussian function, with the focused spot radius of ∼ 400μm. The pump spot was focused intothe doped Nd:YVO4 about 2.3 mm, corresponding to a mode matching coefficient of ∼ 90%provided from Eq. (1). The dichroic mirrors were antireflection coated at 808 nm and highreflection coated at 1064nm for lights at the incidence angle of 45 degree. An acousto-opticQ-switch of quartz crystal was used for intracavity Q-switching operation.

A planar-planar cavity was used in the oscillator, and the transmissivity of the output-coupler(OC) mirror is 38%. The cavity length of L1 and L2 is very important. To obtain stable pulsewith narrow pulse width at high repetition rate, the high pumping power density is essential,which, however, induces heavy thermal lensing effect for a relatively high temperature de-pendence of refractive index (dn/dT ≈ 8.5×10−6K−1)[13]. Therefore, the thermal-stabilizedoscillator was optimally designed with appropriate cavity length. With the theory of resonantcavity [14], the cavity length of L1 = 115 mm and L2 = 85 mm were optimized. The laser mode


0 20 40 60 80 100 1200

0.2

0.4

0.6

0.8

1

1.2

Pump Power PPump

(W)

Mo

de

rad

ius

ω o

f L

aser

, (m

m)

Mode radius of ω

1 on crystal

Mode radius of ω2 on OC

Fig. 2. The laser mode size varies with pump power in oscillator cavity.

size varying with pump power is shown in Fig. 2, from which we can see that the oscillator cav-ity worked in the thermal-stabilized state with the full pump power of 84 W.

Two single-pass amplifier stages were used for power amplification. The pumping/crystalmodules in amplifier stages were all the same as that of the oscillator. In order to obtain asimple and compact configuration, the isolator and coupler were not used. The thermal lens ofthe 1-st amplifier maps the spot sizes of signal beams from the crystal of oscillator to the crystalof the 2-nd amplifier. The mode matching between pump volume and laser mode in amplifierscan be realized with optimal length of L12 = 24 cm and L23 = 28 cm.

3. Experimental results and discussion

0 50 100 150 200 2500

50

100

150

Po

wer

Ou

tpu

t PL

aser

(W

)

Pump Power PPump

(W)0 50 100 150 200 250

0

20

40

60

Op

tica

l−o

pti

cal E

ffic

ien

cy η

O (

%)

Amplifier 2Amplifier 1Oscillator

Fig. 3. The laser power and the optical-optical efficiency vary with pump power in theMOPA laser.

The Nd:YVO4 laser was AO Q-switched at repetition rate up to 500 kHz. Figure 3 showsthe output power and extraction efficiency varying with pump power. The output power of the


oscillator was as high as 35 W, corresponding to the optical-optical efficiency of 41.7%. Theextraction efficiency of the first single-pass amplifier (Amplifier 1) was 34.1% compared withthe extraction efficiency as high as 51.2% of the second single-pass amplifier (Amplifier 2),since higher extraction efficiency can be obtained with higher signal laser intensity. The totaloutput power was 108 W, corresponding to the total optical-optical efficiency of 42.2% with256 W total pumping power. The stability of the average power was less than ±0.5%.

M2y=1.76

M2x=1.99

Fig. 4. Beam quality measurement of the 108 W, 500 kHz Q-switching Nd:YVO4 laserwith the MOPA configuration.

Fig. 5. Spatial form of 108W output on far-field.

The beam quality factors were measured with a Spiricon M2–200 laser beam analyzer. TheM2 factors of the oscillator were measured as M2

x = 1.55, M2y = 1.60. Associated with the

thermally induced high-order phase aberration terms after being amplified by the two amplifierstages [15], the M2 factors degenerated as M2

x = 1.99, M2y = 1.76 (see Fig. 4), i.e., the beam

quality was better than 2-times diffraction-limit. Figure 5 shows the spatial distribution of laserintensity on far-field.

At the repetition rate of 500 kHz, the pulse duration of the oscillator was 51 ns (FWHM).


Fig. 6. Oscilloscope traces of the stable AO Q-switching pulse series at 500 kHz (1μs/div)

After the amplification of two amplifier stages, the pulse duration of the output laser was 48 ns(FWHM), with a stability of pulse peak value < 2.5% (RMS), compared with the pulse stabilityof < 6.5% and < 3.8% in the oscillator and the first amplifier respectively. The energy of eachpulse was 216μJ with pulse peak power of 4.5 kW. Figure 6 shows the oscilloscope traces ofthe stable pulse series.

When the repetition rate was enhanced up to 550 kHz, the stability of pulse peak valuebecame greater than 15% (RMS). Further increasing the repetition rate up to 600 kHz, thephenomenon of pulses missing occurred, which is due to insufficient gain for repetition rate upto 550 kHz.

In CW operation, 117 W stable power output was obtained corresponding to the total optical-optical efficiency of 45.7%.

4. Conclusion

We have presented a 108 W, 500 kHz AO Q-switching Nd:YVO4 laser with the MOPA con-figuration, with the pulse duration of less than 50 ns, pulse peak power of 4.5 kW, and thestability of pulse peak value of less than 2.5%. By applying the bonded Nd:YVO 4 crystals andoptimal pumping mode, the global optical-optical efficiency was 42.2%, and the beam qual-ity was better than 2-times diffraction-limit. A compact and simple MOPA laser was obtainedwithout isolators and couplers. Higher repetition rate and higher optical-optical efficiency canbe obtained applying with higher pumping power density or with appropriately larger dopingconcentration crystal.

Acknowledgment

The research was supported in part by the National Natural Science Foundation of China (GrantNo. 60778014), and the Program for New Century Excellent Talents in University.


a 108 w, 500 khz q-switching nd:yvo_4 laser with the mopa configuration

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