Stable High Power and High Beam Quality Diode-Side-Pumped

Stable High Power and High Beam Quality Diode-Side-Pumped Continuous-Wave Intracavity Frequency-Doubled Nd:YAG Laser *

CHENG Xian-Kun(程贤坤)1,3**,ZHOU Yong(周勇)1,3,CUI Qian-Jin(崔前进)1,3,YANG Feng(杨峰),

BO Yong(薄勇)2,PENG Qin-Jun(彭钦军)2,CUI Da-Fu(崔大复)1,2,XU Zu-Yan(许祖彦)1,2

1

Laboratory of Optical Physics,Institute of Physics,Chinese Academy of Sciences,Beijing 100190

2

Research Center for Laser Physics and Technique,Technical Institute of Physics and Chemistry,Chinese Academy of

Sciences,Beijing 100190

3

Graduate School of the Chinese Academy of Sciences,Beijing 100190

(Received 27October 2008)

We report a stable high power and high beam quality diode-side-pumped cw green laser from intracavity frequency-doubled Nd:YAG laser with KTP.By using a L-shaped concave–convex resonator,designed with two Nd:YAG rods birefringence compensation,a large fundamental mode size in the laser crystal and a tight focus in the nonlinear crystal could be obtained simultaneously.The green laser delivers a maximum 532nm output power of 23.2W.Under 532nm output power of 20.9W,the beam quality factor is measured to be 4.1,and the fluctuation of the output power is less than 1.4%in an hour.

PACS:42.55.Rz,42.55.Xi,42.65.Ky

Stable high power all-solid-state green lasers have been widely studied for various applications such as laser display,[1]pumping tunable lasers and optical parametric oscillators,[2]material processing,[3]under-water communication and so on.At present,a main approach to produce high power cw green output is to take diode-end-pumped intracavity frequency-doubled all-solid-state lasers,and in this kind of systems,high beam quality can be easily obtained.Diode-side-pumped intracavity frequency-doubled all-solid-state is another way to generate high-power cw output.With this method,Mukhopadhyay et al .[4]have made use of a special laser head and have already achieved 30.5W green output power,however their beam qual-ity factor M 2is almost 20.By compensating ther-mal lensing of the nonlinear crystal and that of the Nd:YAG rods with an advanced resonator design,Ko-jima et al .[5]have reported 27W green output power at 532nm with M 2=8,however,the output power is only 24W with M 2=11when the laser operated in stable state.[5]Up to date,we have not found any re-port of a 20W level cw green laser with M 2<5for a diode-side-pumped cw intracavity frequency-doubled all-solid-state

laser.

Harmonic separaror

Total reflector

1

2

Fig.1.Schematic diagram of the green laser system.

In this study,a resonator with two Nd:YAG rods in L-shaped concave–convex cavity was designed.As a result,the maximum of 23.2W output power at 532nm was obtained.When the 532nm average out-put power was 20.9W,the beam quality factor M 2was measured to be 4.1and the fluctuation of the out-put power was less than 1.4%in an hour.

The schematic diagram of the resonator is shown in Fig.1.For the experiment,in order to obtain high beam quality,we use two Nd:YAG rods of rel-atively small diameter and a method of two-rod bire-fringence compensation.Each head consists of a Nd:YAG rod (diameter,3mm;length,76mm;Nd doping,0.7at%)and three diode arrays.Each array contained four maximum 20W continuous-wave diode bars.A quartz 90∘polarization rotator was placed be-tween two Nd:YAG rods for thermally-induced bire-fringence compensation.[6]The rods were surrounded by glass flow tubes,and the temperature of the wa-ter used to cool the rods was maintained at 20∘C (±0.1∘C).Concave end mirror M 1(R =150mm)with dual wavelength high-reflection coatings at 1064nm (R >99.7%)and at 532nm (R >99.5%)was used to create a tight focus at the location of the KTP crys-tal.The other convex end mirror M 2(R =−500mm)was coated at the fundamental wavelength at 1064nm (R >99.7%).On the one hand,the convex mirror was used to obtain a large fundamental mode size in the rods,so the total fundamental frequency power in the cavity increased.On the other hand,the convex mirror made the diffraction loss of the higher-order mode greater than that of fundamental mode.The calculated result from ABCD matrix indicated that with a convex end mirror M 2(R =−500mm),the

*Supported

by the National Basic Research Programme of China under Grant Nos 2004CB619006and 2004CB619001,and the

National Natural Science Foundation of China under Grant Nos 60578030and 50590404.

**Email:kunzhu@

c ○

2009Chinese Physical Society and IOP Publishing Ltd