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Optics & Laser Technology 36 (2004) 69–73www.elsevier.com/locate/optlastec

Experimental study of a high power and high e'ciency CWdiode-side-pumped Nd:YAG laser

Hailin Wang∗, Weiling Huang, Zuoyou Zhou, Hongbin CaoLaser Institute, Huazhong University of Science and Technology, Wuhan 430074, China

Received 1 April 2003; accepted 7 July 2003

Abstract

This paper reports on the characterization of a diode-side-pumped CW Nd:YAG laser. A side-pumped con9guration with 9 laser diodesis used for the laser. Pump light is directly coupled into the Nd:YAG rod without focusing lenses and the pump light distribution in theNd:YAG rod was calculated. A maximum output power of 150 W in multimode operation is obtained for a pumping power of 400 W.The optical–optical e'ciency is 37%. Output power of the laser under di=erent output couplers, resonator lengths and temperatures ofthe cooling water have been studied.? 2003 Elsevier Ltd. All rights reserved.

Keywords: Nd:YAG laser; Diode-side-pumped; High e'ciency

1. Introduction

Diode-pumped solid-state lasers o=er many advantagesover lamp-pumped solid-state lasers. Among its advantagesare high e'ciency, reliability, compactness and long op-eration lifetime. For many years low power end-pumpedsolid-state lasers have demonstrated their superiority tolamp-pumped lasers [1–4]. But power-scaling possibilitiesof end-pumped con9guration are limited because of thethermal induced stress fracture of laser materials. Thereforeside-pumped con9guration has to be used for high outputpower.Nd:YAG is currently one of the most popular laser crystal

for diode-side-pumped solid-state lasers. And side-pumpedNd:YAG lasers operating at high CW power level are at-tractive sources for various applications in materials pro-cessing. It is therefore natural that the study of improvinge'ciency and beam quality, has been the focus of such re-search. Several techniques in delivering pump light fromlaser diodes (LDs) to the Nd:YAG rod have been proposedin order to improve the e'ciency and the beam quality of adiode-side-pumped laser [5–10]. A cylindrical lens is widely

∗ Corresponding author. Tel.: +86-27-8754-1741; fax: +86-27-8754-2997.

E-mail address: wanghl@mail.hust.edu.cn (H. Wang).

used for coupling the pump light from LDs into the Nd:YAGrods with a loss of nearly 10%. This loss leads to low e'-ciency of the diode-side-pumped lasers.In this paper we report a detail investigation of high e'-

ciency and high output power of a CW Nd:YAG laser sys-tem, especially its directly coupling con9guration of pumplight to Nd:YAG rod, which carried out the maximum out-put power of 150 W for a pump power of 400 W with 37%optical–optical e'ciency.

2. Experiment setup

In order to pump the full cross section of the laser rodand to achieve a high spatial overlap between the pumpradiation and the resonator mode, a side-pumped con9gu-ration as shown in Fig. 1 is designed. The pump moduleis a three-fold con9guration consists of 9 CW LDs with anominal output power of 50 W each at 808 nm. But due tothe limit of operating current of power supply, the maxi-mum output power of each LD is 44 W and total pumpingpower is approximately 400 W. Three pump units are spa-tially displaced symmetrically along the crystal, and eachpump unit consists of 3 LDs. The Nd:YAG rod (diameter4 mm, length 76 mm, Nd-doping 0.8 at%, end face Iat/Iat)is mounted inside a Iow tube, and the e=ectively pumped

0030-3992/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0030-3992(03)00135-X

70 H. Wang et al. / Optics & Laser Technology 36 (2004) 69–73

Fig. 1. Pumping con9guration of the side-pumped Nd:YAG laser system:(a) traverse view and (b) side view.

length is approximately 40 mm. Hence a linear pump powerdensity in the Nd:YAG rod of 100 W=cm is available. TheLDs and Nd:YAG rod are cooled by water. The Iow tubehas silver coating on the outside surface with 3 narrow win-dows of 1 mm width, 50 mm long. The emitting windows ofLDs are positioned very close to the 3 narrow windows andpump radiation of LDs is coupled into the laser rod directly.The Iow tube serves as a reIector, reIecting the LDs radi-ation that does not absorb by the Nd:YAG rod back into therod again, to improve the e'ciency of the Nd:YAG laser.A symmetrical resonator consists of two Iat mirror is usedin the experiments.The pump light distribution inside the Nd:YAG rod

has been calculated by using ray trace program, withoutconsidering the internal reIection of silver coating on theIow tube. For the calculated, a Gaussian intensity pro-9le of the LD radiation is assumed perpendicular to thep–n junction, and a constant intensity pro9le is assumedparallel to the p–n junction. The computer calculated lightpro9le as shown in Fig. 2 for a 4 mm Nd:YAG rodillustrates a nonuniform distribution with a peak at thecenter and sub-peak at the 3 entrances of LDs radiation.The nonuniform distribution is caused by the three-foldpumping con9guration.The temperature of cooling water is controlled to

regulate the temperature of the LDs within an accuracy of±0:2◦C. LDs with similar wavelength were selected with2 nm peak-to-peak spectral width.

Fig. 2. Calculated pump light distribution in the Nd:YAG rod.

3. Experiment results

The e'ciency of side-pumped Nd:YAG laser is strongestdepends on the absorption coe'cient of pump light byNd:YAG rod. The absorption coe'cient in Nd:YAG rod isdetermined by the wavelength of pump light, and 808 nmis a peak absorption line in Nd:YAG rod.Due to the narrow absorption bandwidth of the Nd:YAG

rod, the spectrum shift of pump light from LDs would leadto instability of the diode-pumped laser system. The theorythat causes the spectrum shift is complex. The variation ofthe spectrum shift is a function of the temperature at p–njunction of the LDs. If the Iow rate of cooling water is 9xed,there are two key factors that a=ect the temperature at p–njunction of the LD: The electrical input power (or the outputpower of LD) and the temperature of cooling water.We measured the wavelength of pump light by a spec-

trum analyzer (SM-240). The variation of wavelengthof LDs radiation with increase of output power of LDunder di=erent temperature of the cooling water is de-picted in Fig. 3. The Iow rate of cooling water foreach LD is 0:6 l=min. As can be seen in the 9gure thewavelength is sensitive to the output power and cool-ing water temperature. The wavelength increases linearwith the increase in output power under the conditionof the same cooling water temperature, and an increasefactor of 0:14 nm=W was calculated. At the same out-put power, the wavelength increases while the coolingwater temperature rise, and the variation is approxi-mately 0:4 nm=◦C. The 808 nm wavelength only can bereached at high output power of 44 W and high coolingwater temperature of 28◦C. At low output power and lowcooling water temperature, the wavelength is far away from808 nm, hence the absorption coe'cient is low.Since the wavelength of LD radiation is sensitive to the

cooling water temperature, the temperature is carefully con-trolled within an accuracy of ±0:2◦C to keep wavelength

H. Wang et al. / Optics & Laser Technology 36 (2004) 69–73 71

Fig. 3. Wavelength of LD radiation vs. output power of LD under di=erentcooling water temperature.

constant, or the output power of Nd:YAG laser could varyunder the same pumping power.The temperature di=erence between center and surface

of the Nd:YAG rod cause thermal lens e=ect. The beamquality of the Nd:YAG laser deteriorates with the increase inthe pumping power due to the thermal lens e=ect [11]. Thethermal focal length depends on both temperature gradientsand stresses in the Nd:YAG rod. The thermal lens e=ect ofthe Nd:YAG rod was measured by method of stable plane–plane resonator, which becomes unstable when the criticalpumping power is achieved [12]. During the measurementthe Nd:YAG laser operates. In agreement with theoreticalestimations, the focal length of the thermally induced lens isproportional the inverse of pumping power, or the refractingpower is linear increases with the increase in pump power, asshown in Fig. 4. At the maximum pumping power of 400 Wa focal length of 130 mm is measured. And the calculatedvalue of refractive power is 1:9 m−1 per 100 W of pumpingpower for the Nd:YAG rod we used.The output characteristics of the side-pumped Nd:YAG

rod laser at a wavelength of 1064 nm are investigated ina symmetrically Iat–Iat resonator. The cavity mirrors areseparated by 256 mm and the temperature of cooling wateris set as 25◦C. Fig. 5 shows the characteristics of the laseroutput power under di=erent transmission of output couplerswith the same Iat–Iat symmetrically resonator. A maxi-mum CW output power of 150 W in multimode operation isachieved with an output coupler of 20% transmission. Fromthese data a maximum optical–optical e'ciency of 37% andthe electrical–optical e'ciency of 18% were achieved, anda corresponding pumping power at laser threshold is 64 W.The beam parameter product (BPP) is determined to ap-proximately 8 mm mrad at the maximum output power of150 W, and the multimode beam pro9le detected with laserbeam diagnostic system (Prometec Model laserscope UFF100) is shown in Fig. 6.The output power of laser as a function of pumping power

for di=erent resonator length is shown in Fig. 7 with thesame output coupler of 20% transmission. Fig. 7 shows

Fig. 4. Thermal lens at di=erent pumping power: (a) thermal focal lengthand (b) refracting power.

Fig. 5. Output power of Nd:YAG laser as a function of the pumpingpower of LDS under di=erent transmission of the output couplers.

that a symmetrically resonator length of 256 mm gave themaximum laser output power, and the laser threshold isnearly the same for di=erent resonator length.Since the wavelength of LD radiation varies when the

cooling water temperature increase or decrease as shown inFig. 3, the Iuctuation of cooling water temperature couldcause the output power of Nd:YAG laser unstable due to

72 H. Wang et al. / Optics & Laser Technology 36 (2004) 69–73

Fig. 6. Spatial pro9le of a multimode output beam detected with a laser beam diagnostic system.

Fig. 7. Output power of the laser with di=erent resonator lengths.

Fig. 8. Output power vs. cooling water temperature under di=erent pump-ing power.

the change of absorption coe'cient. The output power as afunction of cooling water temperature under di=erent pump-ing power is shown in Fig. 8. From this 9gure, we foundthe output power with low pumping power is more sensitiveto the temperature of cooling water than that with the high

pumping power. At the low pumping power of 150 W, theoutput power is nearly doubled from 23 to 44 W while thecooling water temperature increases from 14◦C to 25◦C. Atthe high pumping power of 400 W, the output power risesonly by 9% from 136 to 150 W while the temperature in-creases from 14◦C to 21◦C, and the output power becomestable when the temperature is higher than 21◦C.

4. Conclusion

In summary, a diode-side-pumped 150 W CW Nd:YAGlaser with an optical–optical e'ciency of 37% and anoverall electrical e'ciency of more than 18% has beendemonstrated. The directly close-pumping con9guration ishelpful to increase e'ciency of the LD-pumped laser sys-tem. The cooling water temperature should be controlledcarefully within an accuracy of ±0:2◦C, or the stabilityof the output power of Nd:YAG laser is poor due to thevariation of absorption coe'cient caused by the spectrumshift of LD radiation. In order to reach the high e'ciency,the wavelength of LDs radiation should be tuned to near808 nm by tuning cooling water temperature, matching theabsorption line in Nd:YAG rod.

Acknowledgements

This work was 9nancially supported by the NationalHigh-tech 863 plan of People’s Republic of China un-der the contract No. 2001AA421012. The authors wouldlike to thank Mr. Zhao Xia for providing the Laser BeamDiagnostic System.

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