Study on High power Laser Diode Pumped Nd:YAG Laser and Its Frequency Conversion Jianquan yao**a,b Yizhong Yu, Fan Zhang, Jin Chen, Baigang Zhang, Degang Xu , Peng Wang a) Optoelectronics Information Science and Technology Laboratory, Institute of Lasers and Optoelectronics, College of Precision Instrument and Optoelectronics Engineering, Tianjin University b) State Key Laboratory of Laser Technology ( Huazhong University of Science and Technology) ABSTRACT A high power intracavity frequency doubled Nd:YAG laser with KTP crystal and A-O Q-switcher pumped by 1600 Watt-808 nm laser diodes and its thermal effect are discussed. Also we proved that the title angle of KTP crystal can be to compensate for the phase mismatching and to solve the problem of the drop of green laser output

power along with the increasing temperature of KTP crystal. Then based on optical parametric oscillator (KTP-OPO) pumped by 532 nm laser and their frequency doubling (with KTP and BBO) a Watt-level red and blue laser system which would be provided as RGB laser projection display are described. 1 . High power green laser High power laser diode pumped green laser source (including CW and high repetition rate operation) has many applications. As a new light source it will be used for the replacing ofthe traditional lamp-pumped solid state lasers and the Copper-Vapor laser, because it is an important pump source of laser isolation of the Uranium isotopes for saving electrical energy, diode- pumped high power green laser was considered as the most excellent solution. On the other hand, it will be used in some fields such as ocean exploration, laser probe, and under-sea communication etc. Laser

diode pumped intracavity frequency doubled Nd: YAG laser is one of the best way to obtain a high power green laser source (532 nm). Recent years, some literature on high power green laser even over 100 Watts191.

When an intracavity frequency doubled Nd: YAG laser is operated at high average power level, the thermal effect in the KTP crystal must be considered. Depended on the laser oscillation modes the temperature distribution in KTP crystal will appear nonuniform and will shift from the phase matching condition at room temperature.

Thus KTP crystal will appear different mismatching at different position at its cross section. However, the literature mentioned above has not analyzed those thermal effects in detail. In this letter we report the theoretical and experimental study on thermal effect of KiT crystal during high power operating.

Because the intracavity frequency doubled Nd: YAG laser has flat-flat resonator construct, beam dimensions in the cavity are very small. Owing to high power density, the temperature inside of crystal is significantly increased and appears temperature distribution among cross section of KTP crystal. Optimization of such a laser source in terms of efficiency and low noise over a broad temperature range requires an accurate knowledge not only of the absolute values of the refractive indices but also of their temperature dependence. It is specially critical for intracavity SHG lasers, in which variations in KTP retardation can lead to severe green output fluctuations and phase mismatching. *Suppoed by National Nature Science Foundation of China No.69988003, partially supported by the Foundation of Key Lab of Laser Technology ([2001J0103) ** author Email jcvaoçtjucd1Lcn Tel: 022-27402416; Fax: 022-274024 16 Institute of Laser and Corresponding

Optoelectronics. College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin, China (300072)

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High-Power Lasers and Applications II, Dianyuan Fan, Keith A. Truesdell, Koji Yasui, Editors, Proceedings of SPIE Vol. 4914 (2002) © 2002 SPIE · 0277-786X/02/$15.00

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At room temperature (23°) the optimum phase matching (Type II) angle for SHG of Nd: YAG laser is 0 = 90°, = 23 .27° [10-12] Dependence of the refractive-index temperature coefficients on wavelength can be written by K. Kato

= (0.1 323A3 — 0.438522 + 1 .2307k' + 0.7709) x 105(°C'),

=(0.50142 _2.00302 +3.30162' +0.7498)x105(°C'),

(1)

= (0.38962 — 1 .333222 + 2.27622' + 2. 1 1 51) x 105(°C1), and by Wiechmann et al. [14]:

= (1.4272 473522 +8.7111 +0.952)x106(°C'), = (4.26W3 _14.76122 +21.232k' —2.113)x106(°C'),

(2)

=(12.415 —44.414A2 +59.129k' +12.l01)x106(°C'). At high temperature, considering formula (1) and (2) we obtain the conected formula as follows:

ft =

n2=n+L\n n3

(3)

= n + An

Then we derive the dependence of the optimum phase matching angles on temperature (Fig. 1). The

direction of main maximum value for effective nonlinear coefficient is located on the X-Y plane, increases linearly with temperature and has a change of 1.20 when temperature covers 100°C. The values in Fig. 1 are related values with 27°C.

Diode Array Heat Exchanger

By W.Wiechmann

Flow Tube

.

Nd:YAG Rod Diode Array

-9-

Radial Pump Geometry

0

40

80

120

160

jgJ Optimum phase matching angle versus temperature change with respect to 27°C. Pumping diode array geometry.

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259

In our experiment we used 80 diode bars and the output power of each bar was 20W with the pentagon pump

model, showed in Fig.2, made in CEO company. The water faucet of the pump model can be connected to a water-cool temperature-control system. This design provides some high performance such as pump uniformity and high efficiency. As we all known, the laser wavelength emitted from the laser diode will increase mm when the temperature

has an increase of 3°C. Because the injected power of the laser diode arrays is about several kilowatts, we need to design a water-cool temperature-control system to control the temperature of laser diode, whose precision

iswithin

. I

HRF1at

KTP Frequency Doubled

40

LD pump array

[-1

IE

HRlO64nm HT532nm

____________ _____________________ 12.2

11.4

13.0

13.8

-'L)' 532nm Outnut Drive current (A) :ig.: Experimental setup of L-shape flat-flat intracavity frequency doubled Nd: YAG Laser.

!i1!: Green laser (532 nm) output power versus drives current with L-shape cavity configuration. We have designed and performed some experiments with two cavities that is straight cavity and L-shaped cavity. Figure 3 shows the scheme map of the L-shaped cavity. Because the lower thermal effect and high output power, we selected the L-shaped cavity. The output power curve of this cavity is showed in Figure 4. We obtained the highest output power greater than 40W when the green laser worked stable. In the experiment we found that when pumping current of laser diode arrays increases, the phase matching angle would change because of the thermal effect of KTP crystal. Figure 5 shows the angle change in air related maximum output power versus output power of green laser. Also we tilt the angle of frequency doubled KTP crystal and obtain A4=O.7° when green laser output power is 30W (Fig.5) with the KTP crystal's temperature about 80°C (Fig.6). 2.0 KTP 1064nm frequency doubling 4 angle vs.

1.0

Output (L-shaped flat-flat cavity

1.5

0.6 1.0

-e

0.2

0.5

-0.2

0.0

5

15

25

Green laser output (W)

260

35

0

50

100

AT (°C)

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150

Tilt angle of KTP crystal, , in air versus output power of 532 nm laser. Fig.6 Tilt angle of KTP crystal in air versus temperature change with respect to 27°C. We have calculated the optimum phase matching angles of a KTP crystal at different temperatures. It is very

important for a high power intracavity frequency doubled laser, due to the fact that at high power density, the temperature inside of crystal is significantly increased and appears temperature distribution among cross section of KTP crystal. It is especially critical for intracavity SHG lasers that variations in the KTP retardation can lead to severe green output fluctuations and phase mismatching. In experiment we used cooling system and the title angle

of KTP crystal can be used to compensate for the phase mismatching and to solve the problem of the drop of

green laser output power along with the increasing temperature of KTP crystal when we increased the inject cunent. This compensation method for mismatching is an average effect at the whole crystal. At last we obtained

the highest stable output power greater than 40W with L-shape flat-flat intracavity frequency doubled Nd: YAG laser.

2. High power red and blue laser By high power 808 nm laser diode pumped intracavity frequency doubled Nd:YAG laser with KTP crystal, we modify a KTP —optical parametric oscillator (OPO) pumped by 532nmlaser. Red laser source will come from second harmonic of signal wave (near 920 nm) of OPO device, blue laser source will come from second harmonic of idle wave ( near 1260 nm) of OPO. We calculated the tuning region of signal wave from 860-980 nm, idle wave

from 1395-1164 nm, corresponded angle of KTP crystal (Type II) from 63.9°-76°. By tuning the angles of KTP and BBO crystals for second harmonic generation of signal and idea waves of OPO, we obtain tuning regions of red wave from 698-582 nm ( 0 57.2°-70.7°, at 00), of blue wave from 430-490 nm ( 0 27.1°-24.2°). Fig 7 shows the experimental scheme. The calculated results are showing at Table 1.

532nm

Fig. 7 Experimental scheme for obtain red, green and blue laser By well control whole system we can get few Watt-level red, green and blue laser source (RGB).

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261

Table 1 .

Tuning region of red and blue waves and corresponded angles of KTP and BBO crystals

based on KTP-OPO device

Crystal angle (°) of

KTP-OPO

Wavelength of signal wave

(II)

(nrn)

Wavelength of idle wave

(nm)

Crystal angle (°) of KTP-

Wavelength of red wave

(urn)

SHG (Ift

0

Crystal angle (°) of BBO-

Wavelength of blue wave (urn)

SHG(I)

0

0

63.9

860

1395

57.2

0

698

27.1

430

69.7

922

1257

63.4

0

628

25.5

461

76.0

980

1164

70.7

0

582

24.2

490

Tuning region

of

Tuning region of wavelength

Tuning region of wavelength

Tuning region

of

Tuning region of wavelength

Tuning region of angle

Tuning region of wavelength

angle

.

.

angle

.

.

.

120 nrn

231 urn

•

136 nrn

2.9°

60 nm

. 12.1°

13.5°

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