научная статья по теме ALL-SOLID-STATE DUAL END PUMPED ND:YAG/LBO YELLOW GREEN LASER WITH 10.8 W OUTPUT POWER AT 561 NM Физика

Текст научной статьи на тему «ALL-SOLID-STATE DUAL END PUMPED ND:YAG/LBO YELLOW GREEN LASER WITH 10.8 W OUTPUT POWER AT 561 NM»

ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 4, с. 653-656

ЛАЗЕРЫ ^^^^^^^^^^^^

И ИХ ПРИМЕНЕНИЕ

УДК 631.373:535

All-Solid-State Dual End Pumped Nd:YAG/LBO Yellow Green Laser with 10.8 W Output Power at 561 nm © 2015 г. P. F. Zhu*, C. M. Zhang*, P. Song*, L. Bai*, and Y. Yao**

* College of Fundamental Studies, Shanghai University of Engineering Science,

Shanghai 201620, China ** Changchun New Industries Optoelectronics Tech. Co., Ltd. 130012, China E-mail: zhangchaomin@sues.edu.cn Received September 10, 2014

It is reported that the efficient and compact yellow green laser at 561 nm generated by intracavity frequency doubling of a continuous wave (CW) laser operation of a diode pumped Nd:YAG laser on the 4F3/2 ^ 4F13/2 transition operated at 1123 nm. An LBO crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation of the laser. With dual end pump configurations at total incident pump power of 40 W, as high as 10.8 W of CW output power at 561 nm is achieved with 20 mm long LBO. The optical-to-optical conversion efficiency is up to 27%, and the power stability in 8 h is better than 2.56%.

DOI: 10.7868/S0030403415040248

INTRODUCTION

Diode pumped solid state lasers in the visible spectral range have applications in the fields of measurement technique, printing and display technology, etc. [1—8]. Especially in the fields of under water communications, high density optical storage, medical diagnostics, and color display technologies, there has been great interest in multiwatt level blue laser emission. A laser diode (LD) pumped solid state quasi-three level Nd3+ laser has been proved to be an efficient way to achieve this goal.

After N. Moore et al. had demonstrated a diode-pumped continuous wave (CW)/Nd:YAG laser at 1123nm with 1.7W output power [5], Y.F. Chen et al. and X.P. Guo et al. had reported the average output power of passively and actively acoustic-optical Q-switched diode-pumped 1123 nm Nd:YAG lasers is 150 mW and 3W, respectively [6-8]. The CW output of 1123 nm laser has been enhanced to 10.8 W by S.S. Zhang et al. using a simple plano-concave resonator and ceramic Nd:YAG [9]. The 556 nm laser was firstly demonstrated by Q. Zheng et al. that 102 mW output powers was obtained and it was enhanced to 3.2 W in 2009 using a type-I LBO as the frequency doubling crystal [ 10], but no report about 561 nm laser watt-level output is demonstrated.

In this letter, a high power, compact, and efficient fiber coupled LD pumped Nd:YAG, intracavity frequency doubling LBO CW 561 nm yellow-green laser is demonstrated. With dual end pump configurations at total incident pump power of 40 W, Nd:YAG with 1.0 at. % Nd3+ doped concentration, a long type I crit-

ical phase matching LBO crystal, and a compact three mirror folded cavity, up to 10.8 W of yellow-green laser emission at 561 nm is achieved. The optical to optical conversion efficiency is greater than 27%, and the power stability in 8 h is better than 2.56%.

THEORY ANALYSIS

Comparing the performance of main laser transition lines in Nd:YAG, in order to obtain the laser oscillation at 1123 nm, which has a lower gain cross-section than the other main laser lines, not only 1064 nm oscillation, but also the 1319 and the 946 nm must be

Transmittance, % 1.0

600 800 1000 1200 1400

Wavelength, nm

Fig. 1. Transmissivity of the left side of Nd:YAG crystal.

Transmittance, % 100

50

600 800 1000 1200 1400

Wavelength, nm

Fig. 2. Transmissivity of the left side of the plane mirror M1.

suppressed at the same time. In our experiment the right side of the laser crystal is antireflection coated at 808, 946, 1064, 1123 and 1319 nm, and the left side of the plane mirror Mx is AR coated at 808, 946, 1064, and 1319 nm and HR coated at 1123 nm to reduce loss of the resonating 1123 nm oscillation and suppress the strong three chief laser lines of946, 1064 and 1319 nm, as well as the SHG laser at 561 nm, and highly reflection at 1123 nm. Figure 1 and Fig. 2 are the coating curves of the left side of Nd:YAG crystal and the left side of the plane mirror Mx. However, the second harmonic discrimination of 1123 and 1112 nm is difficult to be accomplished by traditional film designing. A thick etalon could be inserted into the cavity to suppress the oscillation of one fundamental wavelength. Although the etalon plays as a line selector, large inserting loss is disadvantage for the increase of output power.

EXPERIMENTAL SETUP

A schematic of the intracavity yellow-green is shown in Fig. 3. The pump sources are two laser diode arrays which used at both end of the laser crystal. And

one of the pump sources is a 20 W 808 nm fiber coupled LDA with a core diameter of 400 ^m and a numerical aperture of 0.22 for CW pumping. Its emission central wavelength at room temperature could be tuned by changing the temperature of the heat sink to match the best absorption of the laser crystal. The coupling optics consists of two identical plano-convex lenses with focal lengths of 17 mm used to reimage the pump beam into the laser crystal at a ratio of 1:1. The coupling efficiency is 95%. Because the pump intensity is high enough in the pump spot regions, the first lens must be well adjusted to collimate the pump beam, since it will strongly affect the focal spot. However, the distance between the two lenses can be freely adjusted by experiment. For the aberration, the average pump spot radius is about 220 ^m. The relative performance of 1123 nm line is half of 1064 nm line and its emission cross section is much smaller than the main spectral lines of Nd:YAG. In order to obtain the oscillation of 1123 nm, coating loss should be added to the 946, 1064, and 1319 nm line.

The laser crystal is a 3 x 3 x 10 mm, 1.0 at. % Nd3+ doped Nd:YAG. It is wrapped with indium foil and mounted in the copper heat sink. The left side of the laser crystal is coated with antireflection films at the pump wavelength and 1064 nm, and with high reflection films at 1123 nm acting as one mirror of the cavity. A long laser crystal with low doped concentration is used to reduce thermal lensing and the reabsorption of quasi three level emission while guaranteeing that enough pump energy will be absorbed. When the pump wavelength is tuned to match the absorption peak of the Nd:YAG, about 60% of pump power is absorbed. The temperature of the laser crystal is kept at a constant of 23 °C by a thermoelectric cooler (TEC), which helps to yield a small thermal population of the terminal laser level and stable output power. The lower temperature is essential to yield efficient operation at Nd:YAG spectral line of 1123 nm. The right side of the laser crystal is antireflection coated at 808, 946, 1064, 1123 and 1319 nm to reduce loss of the resonating 1123 nm oscillation and suppress the strong lines of 1064, 946 and 1319 nm. The left side of the plane mir-

Fig. 3. The schematic for the intracavity frequency doubled 561 nm Nd:YAG/LBO yellow-green laser.

ОПТИКА И СПЕКТРОСКОПИЯ том 118 № 4 2015

ALL-SOLID-STATE DUAL END PUMPED Nd:YAG/LBO 655

Comparison of the Frequency-Doubling Parameters of KTP and LBO

Crystal Phase-matching type deff, pm/V Accept Angle, mrad cm Walk-Off-Angle

KTP 1123(o) + 1123(e) = 561(o) 3.69 2.41 25.53 mrad

LBO 1123(o) + 1123(o) = 561(e) 0.836 15.15 4.62 mrad

ror Mi is AR coated at 808, 946, 1064, and 1319 nm and HR coated at 1123 nm. The other side of M1 is AR coated at 808 nm. The coating condition of M2 is the same with M1. The concave facet of the M3 is HR coated at 1123 and 561 nm. The plano-concave mirror M2 is the output mirror and the concave face is HR coated at 1123 nm and AR coated at 561 nm. The plane facet of M2 is AR coated at 561 nm. When the coating requirements on the both sides of the Nd:YAG and the mirrors are satisfied, the 1123 nm spectral line could oscillate independently. The LDA, the whole cavity, and the crystal are cooled by TEC for an active temperature control with stability of ±0.1°C. LBO is a 2 x 2 x 20 mm nonlinear crystal (6 = 90°, O = 21.7°).

The nonlinear crystal such as KTP and LBO is always used in the second harmonic generation field. As shown in a comparison of the characteristics of the two crystals in Table. Comparing the characteristic of the two crystal, although the effective nonlinear optical coefficient of KTP is 3.69 pm/V, which is much larger than that of LBO at 0.83 pm/V, the walk-off angle of LBO is as small as 4.62 mrad than 25.53 mrad of KTP. An LBO crystal with longer length could be used to obtain higher SHG efficiency. Due to the characteristic of KTP, high intracavity power will incur gray trace, which makes the output of harmonic wave unstable and become lower for a long time. So LBO is se-

Fig. 4. The output power at 561 nm versus the total incident pump power.

lected as the frequency doubling material in our experiment for its small walk off angle and large spectral and angular acceptance bandwidth. Both facets of the LBO crystal are coated for antireflection at 561 and 1123 nm to reduce the reflection losses in the cavity. It is mounted in a copper block, which is also fixed on a TEC for an active temperature control.

RESULTS

When tuning the temperature control of the pump source and aligning each component to good state, the color of the fluorescence in Nd:YAG is blue due to the excited state absorption (ESA) of the upper laser level. Although the Nd:YAG is isotropy, the frequency doubling crystal LBO and LD with high polarization ratio make the fundamental wave with high polarization characteristic, so it is not necessary to insert a Brewster plate for the frequency doubling. For the second harmonic generation (SHG) experiment, a 20 mm LBO is inserted into the cavity close to the end mirror M3. The yellow-green laser output power versus the incident pump power is shown in Fig. 4. The threshold of the blue laser is about 2 W, with the total incident pump power of 40 W, corresponding to an output

Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.

Показать целиком