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Optics and Lasers in Engineering 44 (2006) 589–596

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doi:10.1016/j

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One hundred and twenty one W green lasergeneration from a diode-side-pumped Nd:YAGlaser by use of a dual-V-shaped configuration

Aicong Genga,b,�, Yong Boa, Yong Bic, Zhipei Suna,b,Xiaodong Yanga,b, Qinjun Penga,b, Huiqing Lia,b, Ruining Lia,

Dafu Cuia, Zuyan Xua

aBeijing National Laboratory for Condensed Matter Physics, Institute of Physics,

Chinese Academy of Sciences, Beijing 100080, ChinabGraduate School of the Chinese Academy of Sciences, Beijing 100080, China

cAcademy of Opto-electronics, Chinese Academy of Sciences, Beijing 100080, China

Received 1 November 2004; accepted 1 June 2005

Available online 12 September 2005

Abstract

To ensure high-average-power and long-lifetime second harmonic generation, a dual-V-

shaped configuration was designed. In this thermally near-unstable resonator, large

fundamental mode size in Nd:YAG rod and small one in LBO crystal could be generated,

which improved both the beam quality and the power density. A green output power of 121W

with pulse width of 68 ns and repetition rate of 10 kHz was obtained. To the best of our

knowledge, the dual-V-shaped configuration is the first time used for green laser with average

output power of more than 100W.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Nd:YAG laser; Green laser; Diode-pumped solid state laser; Second harmonic generation;

Birefringence compensation

see front matter r 2005 Elsevier Ltd. All rights reserved.

.optlaseng.2005.07.003

nding author. Institute of Physics Group CL06, P.O. Box 603, Beijing 100080, China.

82648093; fax: +86 10 82649542.

dress: [email protected] (A. Geng).

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A. Geng et al. / Optics and Lasers in Engineering 44 (2006) 589–596590

1. Introduction

Scaling nonlinear frequency conversion to high powers is an area that has receivedgrowing interest over recent years, owing to the numerous applications such asmaterial processing, sensing, entertainment, and pumping sources for tunable lasersor optical parameter oscillators [1]. The generation of high-power green laser isparticularly important for these applications [2–6]. Second harmonic generation(SHG) of Nd3+-doped solid-state lasers with the advantages of high efficiency,compactness, and robustness becomes the most effective way of generating high-average-power green coherent radiation [1–6]. KTiOPO4 (KTP) crystal and LiB3O5

(LBO) crystal are often chosen as the SHG nonlinear crystal. The former has higheffective nonlinear coefficient, large acceptance angle and large temperaturebandwidth [7,8]. However, the graytracking problem, which leads to a photochromiceffect at a threshold intensity of 80MW/cm2, limits the lifetime of KTP crystal for ahigh-average-power SHG [3,9] LBO is one of the most promising candidatesespecially for a high-average-power SHG owing to the high damage threshold(greater than 2.5GW/cm2). However, the effective nonlinear coefficient of LBO,which is deff ¼ 0:85 pm=V, approximately 1/4 of that of KTP [10], limits its SHGconversion efficiency. Therefore, in order to obtain high-average-power and longlifetime SHG, the key technology is to improve the frequency conversion efficiency(FCE) of LBO crystal.

In this paper, in order to improve the FCE and take full advantage of high damagethreshold of LBO crystal at the same time, a dual-V-shaped symmetricalconfiguration is designed, which runs with large fundamental mode size in laseractive medium and small one in LBO crystal in the presence of strong thermallensing. By this way, both the beam quality and the power density are improved atthe fundamental wavelength, so high PCF can be ensured. Moreover, the laseroperates as a thermally near-unstable resonator and compensates thermalbirefringence, which also ensures high beam quality [11,12]. The average greenoutput power of 121W is obtained with pulse width of 68 ns and repetition rate of10 kHz. To the best of our knowledge, the dual-V-shaped configuration is the firsttime used for green laser with average output power of more than 100W.

2. Experimental setup

Fig. 1 shows the schematic of the diode-side-pumped dual-V-shaped two-rodNd:YAG laser. The symmetrical laser consists of four mirrors (M1–M4), twoidentical pump modules (Model RD40-1C2, CEO Inc.), two identical acousto-opticmodulators, a polarization rotator (901), and a LBO crystal. The four concavemirrors M1–M4 have same curvature radius, which is 150mm. M1 and M2 havehigh reflectance at 1064 nm. M3 and M4 are dichroic mirrors with high reflectance at1064 nm and high transmittance at 532 nm. The two acousto-optic modulators areplaced orthogonally to hold off the very high pump gain for Q-switching operation.

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Quartzrotator

AO-Qswitch

AO-Qswitch

Pumpmodule

Pumpmodule

Nd:YAGrod

M1M2 M3

M4LBO

Fig. 1. Schematic of the diode-side-pumped dual-V-shaped two-rod Nd:YAG laser. The dash lines

represent the profile of calculated fundamental mode size along the resonator symmetric axis when the

pump power is fixed at 1050W.

A. Geng et al. / Optics and Lasers in Engineering 44 (2006) 589–596 591

The 4mm� 4mm� 20mm dual-wavelength antireflection-coated LBO crystal istype-II critically phase-matched.

Two identical pump modules have the same geometry, and each module consistsof a Nd:YAG rod and thirty long lifetime diode bars. A Nd:YAG rod has a diameterof 4mm, a length of 120mm, and a doping concentration of 0.6%. The two end facesof each Nd:YAG rod are coated with antireflective film at 1064 nm. The 30continuous wave (cw) diode bars with summed pump power of 600W at thewavelength of 808 nm, are arranged in a five-fold symmetry around the Nd:YAG rodand efficiently pump the rod. The pump module is powered by a reliable solid-statedriver, and cooled by re-circulating filtered water whose temperature is set to 22.5 1Cwith a precision of 70.1 1C.

3. Theory

To achieve high FCE, it is reasonable that the laser resonator is designed tooperate with large fundamental mode size in two Nd:YAG rods while smallfundamental mode size in LBO crystal. On the one hand, laser medium rod withlarge fundamental mode size itself will act as a spatial filter [13], which limits thenumber of transverse modes beyond the fundamental without incurring largediffractive losses, thus providing high-beam quality light at fundamental wavelengthand then increasing the FCE. On the other hand, small fundamental mode size inLBO crystal will increase the fundamental power density, which is in proportion tothe FCE.

By using ABCD ray propagation matrix, the round-rip propagation matrices Mrod

and MLBO from the rod and the LBO are given by

Mrod ¼Arod Brod

Crod Drod

" #

¼ ½M�½Mrr�½M�½MrL�½ML4�½M4L�½MLr�½M�½Mrr�½M�½Mr1�½M1r�, ð1Þ

MLBO ¼ALBO BLBO

CLBO DLBO

" #

¼ ½ML4�½M4L�½MLr�½M�½Mrr�½M�½Mr1�½M1r�½M�½Mrr�½M�½MrL�, ð2Þ

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where the matrix [M] is the propagation matrix of the laser rod. The matrix [Mrr]represents a ray propagation matrix between two Nd:YAG rods. Similarly, [MrL]and [ML4] are those from the rod to the LBO crystal and from the LBO crystal to themirror M4, respectively, whereas [MLr] and [M4L] describe those from the LBOcrystal to the rod and from the mirror M4 to the LBO crystal, respectively. For thissymmetrical resonator, ½Mr1� ¼ ½MrL�½ML4� and ½M1r� ¼ ½M4L�½MLr�. From Eqs. (1)and (2), the beam radius of fundamental mode on the Nd:YAG rod and the LBOcrystal is given as

orod ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffil � Brod

p

r� 1�

Arod þDrod

2

� �2" #�1=4

, (3)

oLBO ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffil � BLBO

p

r� 1�

ALBO þDLBO

2

� �2" #�1=4

, (4)

where l ¼ 1064 nm. Therefore, the beam radius of fundamental mode as a functionof the pump power is calculated, which corresponds to different thermal focallengths of the Nd:YAG rod. In this symmetrical resonator, same fundamentalmode size is generated in two Nd:YAG rods, and a beam waist occurs in LBOcrystal. Fig. 2 shows the comparison between the beam radius in Nd:YAG rod andthat in LBO crystal. It can be found from Fig. 2 that the beam radius in twoNd:YAG rods is much larger than that in LBO crystal all along from the thresholdto the maximum of the pump power. With the increase of the pump power, the beamradius in two Nd:YAG rods falls sharply initially, then retains a small value till thepump power exceeds 900W, and increases to a very large value. On the contrary,

600 700 800 900 1000 11000.0

0.4

0.8

1.2

1.6

2.0

2.4

In Nd:YAG rod

In LBO crystal

Bea

m r

adiu

s / m

m

Pump power / W

Fig. 2. Comparison between the calculated beam radius of fundamental mode in Nd:YAG rod (solid line)

and that in LBO crystal (dash lines) as functions of the pump power.

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although the beam radius in LBO crystal also falls sharply initially and then retains asmall value, it decreases to an even small value when the pump power exceeds 900W.That is to say, as long as the pump power exceeds 900W, the resonator will meet ourdesired request and then achieve high FCE.

Conventional lasers were designed to operate at the middle of thermally stablezones, where the fundamental mode size was small and insensitive to thermalperturbations. Therefore, those lasers gave rise to inconvenience or even difficulty inpractice when large mode size was required. However, when the pump power exceeds900W, the laser will operate at the border of stability zone instead, namely thermallynear-unstable resonator, which can easily improve the beam quality on a large scale.Details were reported elsewhere [11].

Solid-state laser materials are usually heated throughout their volume, byabsorbed pump power that is not converted to laser light, and are generally surfacecooled by use of a cooling liquid. This situation causes a temperature gradientbetween a maximum value at the center and a minimum value near the coolingliquid. In the case of laser rod, the temperature gradient causes thermal bifocusingand thermal birefringence, which results in depolarization of the beam, polarizationdistortion and bipolar focusing. Therefore, the beam quality will be deterioratedunless the thermal bifocusing and thermal birefringence are compensated. A 901quartz rotator between two identical rods can compensate thermal birefringence [13].In this way, the output polarization after the rotator is rotated by 901, and the radialand tangential components of the polarization will accumulate the same averagephase delay after passing through two rods. Therefore, every spot of the beam in thecross section must pass through the same position in the two birefringent rods, whichcan be satisfied by a symmetrical resonator [12].

4. Experimental results

On the basis of simulated results, the total cavity length of the symmetricalresonator (between M1 and M4) is designed to be 1325mm. The distance from M1to M2 (and from M3 to M4) is 155mm, and that from M2 (and M3) to the end faceof the Nd:YAG rod is 360mm. Fig. 1 also illustrates the profile of calculatedfundamental mode size (dash lines) along the resonator symmetric axis when thepump power is fixed at 1050W. The beam radius of the fundamental mode in theNd:YAG rod and LBO crystal is estimated to be 1.70 and 0.08mm, respectively. It iscan be seen that this resonator can satisfy our needs. Moreover, to ensure thecompensation of thermal birefringence, two identical pump modules with sameconfiguration are pumped by two identical drivers and cooled by re-circulatingfiltered water with the same temperature and same velocity of flow.

In our experiment, the 1064 nm operation is obtained by removing LBO crystalfrom the resonator and replacing the total reflector M2 by an optimal output mirror.The high gain of the laser rod allows us to use an output mirror with 40%transmittance at 1064 nm. As shown in Fig. 3, at 1050W pump power, 279W output

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600 700 800 900 1000 11000

50

100

150

200

250

300

Out

put

pow

er /

W

Pump power / W

cw

At 1064nm

quasi-cw (10 kHz)

At 1064nm

At 532nm

Fig. 3. 1064 and 532 nm output powers as functions of the pump power.

Time

Am

plit

ude

Pulse width: 68ns

(200 ns/div)

Fig. 4. Oscilloscope traces for 532 nm laser at 121W output power. Horizontal scale is 200 ns/div.


power is obtained in cw operation, whereas 175W average output power is achievedin quasi-cw operation with repetition rate of 10 kHz.

Then the output coupler is replaced by the total reflector M2 and LBO crystal isplaced in the resonator. The LBO crystal is mounted in an oven of 110.0 1Cmaintained by a temperature controller with a precision of 70.1 1C. The 532 nmgreen laser is extracted from the mirrors M3 and M4. It can be found from Fig. 3that a maximum green laser output power of 121W is generated at 1050W pumppower, corresponding to an optical-to-optical conversion efficiency of 11.5%. Thepulse width of the green laser, as shown in Fig. 4, is 68 ns at this output power. The

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fundamental incident intensity in the LBO crystal is given by

p ¼Poutð2� TÞ=T

RtppM2o2, (5)

where Pout and T represent the fundamental output power and the transmittance ofthe output coupler, respectively; R and tp represent the repetition rate and the pulsewidth, respectively; M2 and o represent the beam quality factor and the beam radiusof the fundamental mode, respectively. When Pout ¼ 175W, T ¼ 40%, o is estimatedto be 0.08mm and M2 is 20. According to these parameters, the fundamental incidentintensity on LBO crystal can be figured out to be 256MW/cm2, which isapproximately 1/8 of the damage threshold of LBO crystal. Therefore, this intensityis appropriate for the SGH of the LBO and does no harm to the LBO crystal.

We only replace the optimal output at fundamental wavelength by the loss owingto the SHG, so two laser operations above mentioned are almost the same, and theFCE is equal to the ratio of the green output power to the output power at 1064 nmwith the optimal transmittance, which is 69.1% when the green output power is121W. In addition, at 121W green output power, the laser can remain stable with anoutput power fluctuation of less than 2% for several hours.

5. Conclusions

In conclusion, by the combination of the dual-V-shaped configuration and thethermal birefringence compensation resonator design, efficient and stable SHG isachieved. The laser runs as a thermally near-unstable resonator and gains large modesize in the laser medium and a beam waist in the nonlinear crystal. By using thisnovel resonator, green power of 121W is obtained with pulse width of 68 ns andrepetition rate of 10 kHz. To our best knowledge, the dual-V-shaped configurationhas never been used to generate green laser with output power of more than 100Wbefore. Further enhancement of the output power, beam quality and efficiency of thelaser system is under study.

Acknowledgments

This work was supported by the Pilot Project of Knowledge Innovation Programof the Chinese Academy of Sciences, 863 Key Program Foundation of China underGrant No 2002AA311120 and No 2002AA311040, and the council for Science andTechnology of Beijing under Grant No H020420060060110.

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