ISSN 1054�660X, Laser Physics, 2012, Vol. 22, No. 5, pp. 914–917.© Pleiades Publishing, Ltd., 2012.Original Text © Astro, Ltd., 2012.

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1 1. INTRODUCTION

Diode pumped active Q�switched lasers with goodbeam quality, high output power have been widely usedin material processing and nonlinear frequency con�version [1–5]. Nd:YVO4 crystal is a good candidate forthese Q�switched applications at high repetition rate(>50 kHz), but the maximum pulse energy is limitedby its short upper�state lifetime (90–100 µs) and thepower tends to drop at moderate repetition rates(<30 kHz) [6]. By comparison, Nd:YAG crystal haslonger upper�state lifetime (230 µs), its superior ther�mal�mechanical and optical properties make it easy togenerate high power, high pulse energy Q�switchedlaser at moderate repetition rate [7].

It is easy for side pumped Nd:YAG to achieve highpower 1064 nm laser, but the o�o efficiency in TEM00

mode operation is relatively low, typically between10% and 15% [8–12]. In comparison, end�pumpingcan reach higher o�o efficiency in TEM00 mode oper�ation. But thermal induced stress is the main limitingeffect for power scaling of end�pumped laser in con�ventional 808nm pumping, it needs dual end�pumpconfiguration to reduce mechanical stress at the crys�tal surface. Pumping of Nd:YAG at 885 nm allows highpump power to be optimally absorbed in long crystalthanks to a low absorption at this wavelength [13].Therefore spreading the thermal load on the wholecrystal length and limiting the critical stress on theinput facet. Furthermore, the low quantum defectleads to a reduced thermal load and a higher opticalefficiency [14], which all contributing to achieve ahigh output power. At present, 885 nm end pumped1064 nm lasers have been reported for continuouswave (CW) [13, 15–20] and quasi continuous wave

1 The article is published in the original.

(QCW) operation [21], but no polarized, active Q�switched Nd:YAG laser has been reported under885 nm diode end pumping.

In this letter, a polarized high�power high�pulseelectro�optical Q�switched Nd:YAG laser under885 nm LD end pumping was reported for the firsttime (to our knowledge). At the absorbed pump powerof 59.5 W, a 10.2 W 1064 nm laser at 2 kHz wasachieved with pulse duration of 14.5 ns, the calculatedpeak power was 352 kW. The maximum 22.7 W outputpower at 10 kHz was achieved with pulse duration of28.9 ns, corresponding to the maximum o�o efficiency

was up to 38.1%, and the M2 factor was = 1.322

and = 1.235, respectively.

2. EXPERIMENTAL SETUP

A schematic of the experimental setup is shown inFig. 1. The Nd:YAG laser rod had a diameter of 4 mmand a length of 38 mm. The central 28 mm of the rodwas Nd doped (0.6 at %) while the two outer 5 mmsegments consisted of diffusion�bonded, undopedYAG endcaps. Endcaps were added to ensure watercooling of the entire heated section of the rod andreduce the thermal lens effect [22]. Both sides of theNd:YAG crystal were antireflection (AR) coated at1064 and 885 nm to reduce the optical loss. The tem�perature of cooling water was set at 25°C during theoperation. The beam from a high power 885 nm diodelaser is coupled into a 400 µm diameter, 0.18 N.A.fiber to pump the crystal. The pump light is focused ina 1200 µm diameter pump spot in the center of thecrystal by a coupling lens, providing an almost colli�mated pump volume along the whole crystal length. Inorder to avoid serious damage of the LD caused by

Mx2

My2

SOLID STATEAND LIQUID LASERS

885 nm LD End�Pumped 22.7 W High Beam Quality Electro�Optical Q�Switched Nd:YAG Laser1

L. Changa, *, C. Yanga, Q. S. Panga, Q. K. Aia, L. Y. Chena, M. 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: constantvip@yahoo.com.cn**e�mail: chenmeng@bjut.edu.cn

Received December 15, 2011; in final form, December 21, 2011; published online April 3, 2012

Abstract—We report an 885 nm laser diode (LD) end�pumped high beam quality ( = 1.322, = 1.235)electro�optical Q�switched Nd:YAG laser with TEM00�mode output for the first time. At the absorbed pumppower of 59.5 W, a 22.7 W 1064 nm laser was achieved at 10 kHz repetition rate with optical�to�optical effi�ciency of 38.1%. The maximum pulse energy and shortest pulse width were 5.1 mJ and 14.5 ns at 2 kHz rep�etition rate, and the calculated peak power was 352 kW.

DOI: 10.1134/S1054660X12050040

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LASER PHYSICS Vol. 22 No. 5 2012

885 nm LD END�PUMPED 22.7 W HIGH BEAM QUALITY 915

unabsorbed pump light during the process of increas�ing the pump current [13], we didn’t use a lens�mirrorstructure [6] to backreflection the unabsorbed pumplight to Nd:YAG crystal.

The cavity was a V�folded cavity with three mirrors,a pair of thin�film polarizers (TFP1, TFP2), two quar�ter wave plates (QWP1, QWP2) and a BBO pockels cell.M1 was an R = –800 mm convex 1064 nm HR endmirror, M2 was an R = –1000 mm convex dichroicmirror, exhibiting high reflectivity (R > 99.8%) at1064 nm and high transmission (T > 99.0%) at885 nm, the out�coupling factor of output coupler(OC) was 30% and the total cavity length was 310 mmdefined by M1 and OC. The pair of TFPs was insertedin the arm defined by M2 and the OC, Each TFP pro�viding high reflectivity for the s�polarization andtransmitting 97% of the p�polarized light. They wereoperating in the reflective mode to produce s polariza�tion laser light and enhance the extinction ratio of theintra�cavity laser. In order to reduce the depolariza�tion loss caused by thermally induced birefringence,

the QWP1 was placed between laser rod and HR endmirror M1 with its fast axis aligned parallel to the planeof s�polarization laser [23]. Both faces of the QWP1

were AR coated at both the pump and the lasing wave�lengths. The QWP2 axis was oriented at 45° to s�polar�izations, rotating the incident s�polarization 90° indouble pass. The BBO pockels cell was chosen for itslow insertion losses, high damage threshold, and lowpiezoelectric ringing, allowing high�repetition�rateoperation. When no voltage was applied to the BBOpockels cell, the cavity was in low Q stage without laseroscillation in the cavity.

3. EXPERIMENTAL RESULTS

In order to evaluate the compensation effect ofdepolarization compensation quarter wave plate,firstly we drove the cavity in CW operation with QWP2

and BBO pockels cell removed. The comparisonbetween output power with and without the QWP1 ver�sus absorbed pump power is shown in Fig. 2. When theincident pump power was 67.5 W, 59.5 W of pumppower was absorbed by the laser gain medium, and themaximum 1064 nm output power was 27.4 W and21.6 W respectively, and the maximum o�o efficiencywas 46%.

After we inserted the BBO pockels cell betweenTFP2 and OC, the maximum output power decreasedfrom 27.4 W down to 25.3 W, this was mainly caused byBBO cell insertion loss and thermally induced depo�larization in the BBO crystal as it absorbed 1064 nmlaser oscillated in the cavity [6].

In order to operate the resonator in On Q�switch�ing state, we inserted QWP2 between the TFP2 andBBO pockels cell, and its axis was oriented at 45° tos�polarization. If no voltage is applied to the BBO cell,the laser resonator is blocked. The Pockels cell is a4 mm aperture with a quarter wave voltage of 4.8 kV,and the high voltage switch is capable of switching to4.8 kV in <4 ns and at a repetition rate up to 10 kHz.When a 300 ns duration high voltage was applied on

885 nm,� 400 µm0.18 N. A.

Couplinglens M2

M1

Nd:YAG QWP1

TFP1

TFP2

QWP2

BBO

OC

Fig. 1. The schematic diagram of the laser system.

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25

20

15

10

5

0 10 20 30 40 50 60Absorbed pump power, W

Output power, W

Compensation

Without compensation

Fig. 2. Output power with and without the depolarizationcompensation QWP1 versus absorbed pump power.

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LASER PHYSICS Vol. 22 No. 5 2012

CHANG et al.

the BBO cell, the resonator trended to high Q stateand permited the emission of laser light. The system’soutput power and corresponding pulse width(FWHM) are plotted versus repetition rate in Fig. 3when the absorbed pump power is 59.5 W.

In Q�switched operation, the cavity delivered10.2 W average power at 2 kHz and 22.7 W averagepower at 10 kHz, respectively, and the beam propaga�

tion factor M2 was = 1.322 and = 1.235 alongthe horizontal and vertical directions. Figure 4 showsthe measured M2 factor and the beam profile. Themaximum pulse energy was 5.1 mJ and the shortestpulse width (FWHM) was 14.5 ns at 2 kHz repetitionrate. The calculated laser peak power was 352 kW. Wedidn’t operate the laser at lower repetition rate in orderto avoid serious damage on optics surface.

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My2

4. CONCLUSIONS

In conclusion, we have demonstrated an 885 nm

LD end�pumped high beam quality ( = 1.322,

= 1.235) electro�optical Q�switched Nd:YAGlaser with TEM00�mode output for the first time. Atthe absorbed pump power of 59.5 W, a 22.7 W 1064 nmlaser was achieved at 10 kHz repetition rate with o�oefficiency of 38.1%. The maximum pulse energy andshortest pulse width were 5.1 mJ and 14.5 ns at 2 kHz,the calculated peak power was 352 kW. Further powerscaling can be realized by reducing doping concentra�tion of Nd:YAG crystal and increasing the crystallength to reduce the thermal lens effect and thermalinduced birefringence at higher pump power.

ACKNOWLEDGMENTS

The authors thank Guo Weirong, Bai Zhen’ao, andXu Yang for their valuable discussion. This work wassupported by National High�tech R and D Program ofChina (2011AA030205 and National Nature ScienceFoundation of China (61144007).

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0 2 4 6 8 10 12Repetition rate, kHz

Ou

tpu

t p

ow

er,

W

Pu

lse

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Mx2 = 1.322

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885 nm LD END�PUMPED 22.7 W HIGH BEAM QUALITY 917

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