научная статья по теме ALL-SOILD-STATE 360 NM ULTRAVIOLET LASER GENERATED BY INTRACAVITY FREQUENCY-DOUBLING OF DIODE-PUMPED PR3+ : YLIF4 LASER Физика

Текст научной статьи на тему «ALL-SOILD-STATE 360 NM ULTRAVIOLET LASER GENERATED BY INTRACAVITY FREQUENCY-DOUBLING OF DIODE-PUMPED PR3+ : YLIF4 LASER»

ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 6, с. 1032-1035

ЛАЗЕРЫ И ИХ ПРИМЕНЕНИЕ

УДК 621.373:535

ALL-SOILD-STATE 360 nm ULTRAVIOLET LASER GENERATED BY INTRACAVITY FREQUENCY-DOUBLING OF DIODE-PUMPED Pr3+ : YLiF4 LASER

© 2015 ^ C. M. Zhang*, W. X. Yu**, C. G. Zhang**, Y. Yao***, P. F. Zhu*, P. Song*, and L. Bai*

*College of Fundamental Studies, Shanghai University of Engineering Science, Shanghai, 201620, China **Tianjin Jinhang Technical Physics Institute, 300308, China ***Changchun New Industries Optoelectronics Tech. Co.,Ltd. 130012, China E-mail: zhupengfei@sues.edu.cn Received February 6, 2014

We demonstrate an efficient and compact ultraviolet laser at 360 nm generated by intracavity frequency doubling of a continuous wave (CW) laser diode-pumped Pr3+ : YLiF4 laser at 721 nm. A lithium triborate crystal, cut for critical type I phase matching at room temperature, is used for second-harmonic generation of the fundamental laser. By using an InGaN laser diode emitting at 444.3 nm with a maximum incident power of 5 W, as high as 460 mW of CW output power at 360 nm is achieved. The optical-to-optical conversion efficiency is as high as 9.2%, and the output power stability in 4 h is better than 3.86%. To the best of our knowledge, this is high efficient UV laser generated by frequency doubling of an InGaN diode-pumped Pr3+: YLiF4 laser.

DOI: 10.7868/S0030403415060252

INTRODUCTION

Pr3+ : YLiF4 have been found to be the most promising materials for diode-pumped lasers due to its transitions of trivalent Praseodymium ions (Pr3+) in the visible spectral range (red at 721, 698 and 640 nm, orange at 607 and 60 4 nm, green at 523 nm and blue at 485 nm) [1—3]. Except the wavelength in the blue spectral region, the mentioned wavelengths are four level laser transitions. The 721 nm spectral line is an important Pr3+-doped laser line and the second harmonic generation (SHG) of this line is approximate 360 nm which is close to the wavelength of the third harmonic of Nd : YAG lasers. It has potential application in the areas of material processing, semiconductor performance testing, display technology, holographic and lithographic techniques. While the Pr3+ lasers were realized pumped by pulsed dye laser systems [4] or flash lamps [5], different approaches for direct blue laser pumping of Pr3+ ions were performed for example by Ar-ion lasers [6] or frequency doubled Nd : YAG ground state lasers [7] in the past years. Recently, the lack of suitable pump sources for Pr3+ doped materials at 444 nm or 479 nm has been overcome due to progress in InGaN based pump diodes operating at 444 nm and optically pumped semiconductor lasers (OPS) operating at 479 nm. The output powers available at 444 nm and 479 nm are up to 2W and 5W, respectively. These developments enable

Pr3+-doped crystals performed as gain media for compact and efficient solid-state lasers.

In comparison to other solid state lasers, Pr3+ lasers need only a second harmonic generation processing for generation of several wavelengths in the UV. Continuous wave at 320 nm UV radiation has been generated by intracavity frequency doubling of the OPS pumped Pr3+ : YLiF4 laser [8]. In 2007, Coherent corporation achieved 360 nm laser with the output power of1.3W pumped by 479 nm OPS laser [9]. In 2008, Os-troumov et al. reported on 1W 261 nm Pr3+ : YLiF4 CW laser pumped by two OPS lasers at 479 nm [10]. Compared with the OPS laser working as pump source operating at 479 nm, InGaN diodes available at 444 nm are much more compactness and commercial availability. In 2011, Teoman Gun et al. demonstrated the output power at 261.3 nm of 481 mW pumped by using two InGaN laser diodes [11]. In 2013, Liu et al. demonstrated 698 nm deep red laser output using an InGaN laser diode emitting at 444 nm with a maximum output power of 760 mW [12].

In this paper, we demonstrate an efficient and compact ultraviolet laser at 360nm generated by intracavity frequency doubling of a continuous wave (CW) laser diode-pumped Pr3+ : YLiF4 laser at 721 nm. A lithium triborate (LBO) crystal, cut for critical type I phase matching at room temperature, is used for second-harmonic generation (SHG) of the fundamental laser. By using an InGaN laser diode emitting at 444.

ALL-SOILD-STATE 360 nm ULTRAVIOLET LASER GENERATED

1033

AR @ 444 nm

360 nm

M2 HR @ 360 nm/720 PR @ 479 nm/604 nm 607 nm/640 nm

Fig. 1. Schematic for the intracavity frequency-doubled 360 nm Pr3+ : YLiF4/LBO UV laser.

Transmittance, % 100

50

300 400 500 600 700 800 900 1000

Wavelength, nm

Fig. 2. Transmissivity of output coupler Ml of 360 nm laser.

Intensity, counts

11000

9500

360.175

Aj=

207 271 335 399 463

Wavelength, nm

Fig. 3. Spectrum of the 360 nm UV laser.

3 nm with a maximum incident power of 5 W, as high as 460 mW of CW output power at 360 nm is achieved. The optical-to-optical conversion efficiency is as high as 9.2%, and the output power stability in 4 h is better than 3.86%. To the best of our knowledge, this is high efficient UV laser generated by frequency doubling of an InGaN diode-pumped Pr3+ : YLiF4 laser.

EXPERIMENTAL SETUP

The schematic of the intracavity frequency-doubling 360 nm Pr3+ : YLiF4/LBO UV laser is shown in

Fig. 1. The pump source is a 5W 444 nm InGaN diode which is coupled into the fiber with a core diameter of 400 ^m and a numerical aperture of 0.22 for CW pumping. Its emission central wavelength is 444 nm at room temperature and can be tuned by changing the temperature of the heat sink to match the best absorption of the laser crystal. The spectral width (FWHM) of the pump source is about 3 nm. The coupling optics consists of two identical plano-convex lenses with focal lengths of 10 mm used to reimage the pump beam into the laser crystal at a radio ofl : 1. The coupling efficiency is 96%. Because the pump intensity is high enough in the pump spot region, 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 laser crystal is a 0.3% Pr3+ doped, 3 x 3 x 5 mm Pr3+ : YLiF4 crystal. The left side of the Pr3+ : YLiF4 is coated at 444 nm AR and at 721 nm HR. The other facet of the Pr3+ : YLiF4 is AR coated at 444, 607, 640 and 721 nm, which is wrapped with indium foil and mounted at a thermal electronic cooled (TEC) cooper block. The temperature is maintained at 23°C.

The cavity configuration is a three-mirror folded cavity, which has two separate beam waists. One waist could satisfy the mode-matching condition, and the other could enhance the frequency-doubling efficiency. The radiuses of the concave faces are 200 and 600 mm for M1 and M2,respectively. L1 and L2 are the lengths of the arms in the cavity. L1 and L2 are about 90 and 67 mm, respectively, under the numerical simulation by MATLAB software. The incident angle of the beam on the folded mirror is set to be as small as possible to reduce the astigmatism without additional optical astigmatism-compensating elements. The relative performance of 721 nm laser line is half of 640 nm line and its emission cross section is smaller than the main spectral lines of Pr3+ : YLiF4. In order to obtain the oscillation of 721nm, coating loss should be added to the 479, 604, 607, and 640 nm line. The concave facet of M2 is HR coated at 360 and 721 nm, and PR at 479, 604, 607, and 640 nm line. The concave facet of M1 is HR coated at 721 nm and AR coated at 360 nm. The other facet is AR at 360 nm. Figure 2 shows the coating curves of the concave surface of the end mirror M1.

The nonlinear crystal such as LBO and BBO is always used in the second harmonic generation field. Comparing the characteristic of the two crystal, although the effective nonlinear optical coefficient of BBO is 1.97 pm/V which is much larger than that of LBO at 0.68 pm/V, the large walk off angle of BBO is 73.5 mrad which yields a beam spot with low beam quality, and this makes BBO not suitable for this application. LBO is selected as the frequency-doubling material in our experiment for its relatively smaller

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0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0

-1.0

-0.5 0 0.5 Major-Position, mm

1.0 -0.5 0 0.5 Minor-Position, mm

Fig. 4. Beam profile distribution of 360 nm UV laser.

walk-off angle of 18.54 mrad. Although the nonlinear coefficient of LBO is 0.678 pm/V, the length of the LBO could be extended to compensate for the relatively smaller nonlinear coefficient. LBO is a frequency doubler with dimensions 2 x 2 x 10 mm, cutting for critical type I phase matching (9 = 90°, ^ = 40.5°) and antireflection coated at 721 and 360 nm on both sides to reduce the intracavity loss of fundamental laser and the UV frequency doubling of the 721 nm laser. The whole cavity and the crystal are cooled by a thermo-electric controller for active temperature control with a stability of ±0.1°C.

RESULTS

When tuning the temperature control of the pump source and aligning each component to a good state, the color of the fluorescence in the Pr3+ : YLiF4 is orange-yellow owing to the excited-state absorption (ESA) of the upper laser level. The frequency-doubling LBO crystal and the LD with high polarization ratio create a fundamental wave with high polarization, and it is not necessary to insert a polarizer such as a Brewster plate into the cavity. When the LBO is inserted into the cavity close to the end mirror M2 and the aligning angle of the LBO is tuned, the maximal output power of 460 mW at 360 nm is achieved with a maximum incident power of 5W. Using the Ocean spectrum analyzer to scan the SHG laser and dealing with the data by software, the spectrum of the SHG laser is shown in Fig. 3. Figure 4 is the beam profile testing result which shows that the 360 nm laser is operating at near TEM00 mode and the far-field intensity distribution is near Gaussian distri

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