Compact second-harmonic generation laser module ... - OSA Publishing

Compact second-harmonic generation laser module with 1 W optical output power at 490 nm Christian Fiebig,* Alexander Sahm, Mirko Uebernickel, Gunnar Blume, Bernd Eppich, Katrin Paschke, and Götz Erbert Ferdinand-Braun-Institut für Höchstfrequenztechnik,Gustav-Kirchhoff-Str.4 12489 Berlin, Germany *[email protected]

Abstract: We demonstrate continues-wave 1 W at 490 nm on a 2.5 cm3 micro-optical bench using single-path second-harmonic generation with a periodically poled MgO:LiNbO3 bulk crystal. The pump laser is a distributed Bragg reflector tapered diode laser having a single-frequency spectrum and a pump power of 9.5 W. Based on that 1 W blue light could be achieved resulting in an optical conversion efficiency of 11%. Furthermore, the module has an output power stability of better than 2% and the blue laser beam shows an nearly diffraction limited beam quality of M2σ = 1.2 in vertical and M2 σ = 2 in lateral direction. ©2009 Optical Society of America OCIS codes: (140.2020) Diode Lasers; (140.3515) Lasers frequency doubled; (190.2620) Harmonic generation and mixing; (240.3990) Micro-optical devices

References and links 1.

P. Adamiec, B. Sumpf, I. Rüdiger, J. Fricke, K.-H. Hasler, P. Ressel, H. Wenzel, M. Zorn, G. Erbert, and G. Tränkle, “Tapered lasers emitting at 650 nm with 1 W output power with nearly diffraction-limited beam quality,” Opt. Lett. 34(16), 2456–2458 (2009). 2. A. Michiue, T. Miyoshi, T. Kozaki, T. Yanamoto, S. Nagahama, and T. Mukai, “High-Power Pure Blue InGaN Laser Diodes,” IEICE Trans. Electron. E92(194), 194–197 (2009). 3. Y. Enya, Y. Yoshizumi, T. Kyono, K. Akita, M. Ueno, M. Adachi, T. Sumitomo, S. Tokuyama, T. Ikegami, K. Katayama, and T. Nakamura, “531 nm Green Lasing of InGaN Based Laser Diodes on Semi-Polar {2021} FreeStanding GaN Substrates,” Applied Physics Express 2, 082101 (2009). 4. H. K. Nguyen, M. H. Hu,Y. Li, K. Song, N. J. Visovsky, S. Coleman, C. Zah, “ 304 mW green light emission by frequency doubling of a high-power 1060-nm DBR semiconductor laser diode”, 6890, 68900I–68900I–6 (2008). 5. M. Iwai, T. Yoshino, S. Yamaguchi, M. Imaeda, N. Pavel, I. Shoji, and T. Taira, “High-power blue generation from a periodically poled MgO:LiNbO3 ridge-type waveguide by frequency doubling of a diode end-pumped Nd:Y3Al5O12 laser,” Appl. Phys. Lett. 83(18), 3659–3661 (2003). 6. K. Sakai, Y. Koyata, N. Shimada, K. Shibata, Y. Hanamaki, S. Itakura, T. Yagi, and Y. Hirano, “Masteroscillator power-amplifier scheme for efficient green-light generation in a planar MgO:PPLN waveguide,” Opt. Lett. 33(5), 431–433 (2008). 7. C. Fiebig, G. Blume, M. Uebernickel, D. Feise, C. Kaspari, K. Paschke, J. Fricke, H. Wenzel, and G. Erbert, “High-power DBR tapered laser at 980nm for single path second harmonic generation,” IEEE J. Sel. Top. Quantum Electron. 15, 978–983 (2009). 8. O. B. Jensen, P. E. Andersen, B. Sumpf, K. H. Hasler, G. Erbert, and P. M. Petersen, “1.5 W green light generation by single-pass second harmonic generation of a single-frequency tapered diode laser,” Opt. Express 17(8), 6532–6539 (2009). 9. H. Wenzel, F. Bugge, M. Dallmer, F. Dittmar, J. Fricke, K. H. Hasler, and G. Erbert, “Fundamental-lateral mode stabilized high-power ridgewaveguide lasers with a low beam divergence,” Photon. Technol. Lett. 20(3), 214– 216 (2008). 10. G. D. Boyd, and D. A. Kleinman, “Parametric interaction of focused gaussian light beams,” J. Appl. Phys. 39(8), 3597–3639 (1968). 11. M. Uebernickel, G. Blume, C. Fiebig, C. Kaspari, D. Feise, K. Paschke, A. Ginolas, R. Güther, B. Eppich, and G. Erbert, “Beam quality dependent SHG using edge-emitting lasers and a 50mm bulk PPLN crystal”, Proc. of the SPIE, 7197, 71970F–71970F–8 (2009). 12. K. Paschke, G. Blume, C. Fiebig, A. Sahm, D. Feise, M. Uebernickel, G. Erbert, and G. Tränkle, “Compact Watt-class visible light sources using direct frequency-doubled edge-emitting diode lasers,” Proc. SPIE 7193, 71931C (2009).

#117216 - $15.00 USD Received 14 Sep 2009; revised 17 Nov 2009; accepted 17 Nov 2009; published 30 Nov 2009

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7 December 2009 / Vol. 17, No. 25 / OPTICS EXPRESS 22785

1. Introduction High power visible lasers with nearly diffraction limited beam quality are required for many applications, such as spectroscopy, optical data storage or display technology. Visible gas and solid state laser systems are often large-scale and limited to their specific wavelengths. Due to their small sizes, reduced costs and variable wavelength range semiconductor lasers are well suited for such applications. In the red wavelength region [1] GaAs based diode lasers reach reliable power level of 1 W. Aiming to Watt class blue diode lasers nitride-based laser chips were presented [2] but these emit multiple modes spectrally and spatially. Green diode laser are also shown [3] but the output power is limited to a few mW. Therefore, second-harmonic generation (SHG) of diode laser radiation could be an alternative to develop high-power and efficient visible laser systems. Using edge-emitting distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) diode laser [4] in combination with nonlinear waveguide crystals promise high conversion efficiency but show saturation effects at output powers above a few hundreds of milliwatts [5]. Alternatively Sakai et al. [6] showed a setup using a planar waveguide with a master-oscillator power-amplifier (MOPA) as pump source delivering 346 mW blue light with an optical conversion efficiency of 38%. However, due to the complexity of coupling only 20% of infra-red light could be coupled into the crystal. By using a DBR tapered diode lasers as pump source and a periodically poled MgO:LiNbO3 (PPLN) bulk crystal blue [7] and green [8] light with optical output powers of more than 1 W could obtained in simple single-pass bench-top experiments. In this work we show a miniaturization of a single-pass SHG setup on a compact microoptical bench (MIOB) of only 2.5 cm3 with an optical output power of 1 W at 490 nm. The design of the MIOB as well as the properties of the used DBR tapered laser and the PPLN will be given. Furthermore, we will discuss the conversion efficiency and the beam quality of the generated SH beam. Finally, we present the temporal stability of the SH optical output power and wavelength. 2. Micro optical bench Figure 1(a) shows the MIOB (1) having a footprint of (50 x 10 x 5)mm3. It contains a laser device (2), the coupling optics (3), a λ/2 plate (4), the PPLN (5) on a heater (6) and mounting rails (7) for assembling the individually components. For our experiments this subassembly was mounted onto a conduction cooled package (CCP) [Fig. 1(b)] with a footprint of (50 x 25 x 15)mm3.

Fig. 1. (Color online) Micro-optical bench for single-pass second-harmonic generation. (a) Schematic design of the AlN based MIOB having a footprint of (50 x 10 x 5)mm3. (b). Picture of the MIOB mounted onto a CCP measurement holder (50 x 25 x 15)mm3.

2.1. DBR tapered laser as pump source The pump source on the MIOB is a DBR tapered diode laser (2). Such lasers meet all the requirements for efficient frequency conversion: high optical output power, small line width,


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and good beam quality. The vertical layer structure of the laser is based on an AlGaAs superlarge optical cavity as described in [9]. Therefore, the device has a vertical far field angle of only 15° at full width half maximum (FWHM) hereby simplifying the coupling schemes. The laser is 6 mm long and divided into two individual electrically controllable sections: a 4mm long gain-guided tapered (TA) section having a flared angle of 6°, and a 2 mm long and 4 µm wide index-guided ridge-waveguide (RW) section. The RW section contains a 1 mm passive DBR subsection at the rear side. The reflectivity of the grating is about 30%. The output facet is 425µm wide and has a reflectivity of 1%. More details can be found in [7]. The laser chip (2) has been mounted p-side up on a CuW heat spreader using a Flip Chip Bonder for exact alignment and then onto the AlN MIOB (1). In Fig. 2(a) the output power vs. taper current ITA for a constant RW current of IRW = 450 mA at 20°C is given. The laser achieves an optical output power of 9.5 W at about ITA = 14 A. The maximum efficiency is η = 38% and is accomplished at 6 W optical output power. Figure 2(b) shows the spectral intensity distribution at 9.5 W (circles). The data points are fitted with a Lorentz function (red solid curve), resulting in a spectral line width of ∆λ < 10 pm (FWHM). The measurement is limited by the resolution of the spectrum analyzer (λ/100000).

Fig. 2. (a) NIR optical output power (black solid line) and conversion efficiency (red dashed line) vs. injection current ITA. The current IRW is 450mA. (b) Measured spectral intensity distribution (black circles) with Lorenz fit (red solid line) at 9.5W.

2.2. SHG crystal mounting For the single-pass SHG we used a periodic poled MgO:LiNbO3 bulk crystal that is 30 mm long (longitudinal), 2 mm wide (lateral) and 0.5 mm high (vertical). To avoid backreflections the facets of the crystal are anti-reflection-coated (ARC) below 1% for the fundamental and the second-harmonic wavelength and have a wedged angle of 2° in lateral direction. The poling period of the crystal is 5.25 µm at 50°C. The quasi phase-matching temperature for our DBR tapered laser wavelength of 981nm is achieved at about 90°C. The crystal (5) is mounted onto a thermal expansion coefficient matched heat spreader (6) containing a resistance heater. This crystal package is fitted between the mounting rails (7) and is finally fixed with UV adhesive. 2.3 Beam forming for SHG The optimum focus conditions for the SHG of coherent Gaussian shaped beams is well described by Boyd-Kleinmann [10]. Several groups discussed that the focus condition for non-Gaussian beam e.g. those of diode lasers differ to that model [7, 8, 11]. Therefore, the focus diameters had to be adapted according to the measured beam propagation parameters. To measure the beam quality of the second-harmonic (SH) or fundamental light the diverging beam was focused by a spherical lens to create an accessible caustic. The beam diameter near the focal region were measured with an high resolution CCD camera from PointGrey Research Inc (Grasshopper). We estimated the M2σ value from a hyperbolic fit to #117216 - $15.00 USD Received 14 Sep 2009; revised 17 Nov 2009; accepted 17 Nov 2009; published 30 Nov 2009

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the beam diameters based on the second order moments according to ISO 11146. Up to an output power of 9.5 W the vertical beam propagation factor is stable at about M2σ = 1.6. In lateral direction this value increases from M2 σ = 3 at 1 W to M2 σ = 14 at 9.5 W [12]. Based on the above mentioned considerations for non-Gaussian beams and the measured beam propagation parameters an optical system has been designed consisting of 4 cylindrical micro-optics from INGENERIC GmbH (3) to focus the beam into the bulk crystal. To avoid backreflections these micro-optics had an ARC below 1%. Due to the different polarization of pump source and crystal an half-wave-plate (4) is positioned behind the first two lenses. The optical components were adjusted with a 6 axis controller of Physics Instrument having a resolution of 100 nm. Because of the stable M2σ in vertical direction the corresponding two lenses are mounted onto the MIOB with UV adhesive for all measured working points. In lateral direction the beam propagation ration M2σ as well as the astigmatism of the taper diode laser increase with higher output power. Therefore, the focusing conditions have to be adjusted for every working point. 3. SHG experiments 3.1 Optical output power All measurements were performed at a heat sink temperature of T = 20°C and a RW current of IRW = 450 mA. For the measurement of the SH power the crystal temperature was optimized for every operation point and a short pass filter was inserted to eliminate more than 99.9% of the residual fundamental power. Figure 3(a) shows the measured SH power as function of the fundamental power (circles). At an optical pump power of 9.5 W a maximum optical SH power of 1.05 W was generated. During the experiment the optical conversion efficiency increased from 4% at 1 W pump power to a maximum of 11% at 9.5 W as can be seen in Fig. 3(b). We found that the SH output power was limited by the degradation of the pump source beam quality with increasing output power. By improving the lateral design of the DBR tapered diode laser pump source a further increase of the output power of the visible light should be possible. A macroscopic measurement using the very same PPLN crystal and a comparable DBR tapered diode laser (same setup as in [11]) with the following operation conditions: ITA = 15 A, IRW = 450 mA, P = 9.3 W, M2σ = 14.5, achieved only a slightly higher SH power of 1.15 W (ηopt = 12.3%), see red star in Fig. 3(a). This demonstrates that a small scale integration is possible with only minor reduction in optical conversion efficiency.

Fig. 3. (a) (Color online) SHG output power vs. fundamental input power at 20°C on the MIOB laser system (circles). A value of a macroscopic experiment (red star) is shown for comparison. (b) Optical conversion efficiency (circle) vs. fundamental input power.


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3.2. Beam quality The spatial intensity distribution of the caustics of the fundamental and SH light were measured behind a long pass and short pass filter, respectively, that eliminates more than 99.9% of the undesired light. Figure 4(a) shows the measured beam diameter and the hyperbolic fits of the fundamental beam, whereas Fig. 4(b) shows the same for the SH beam. One can see that the beam quality in vertical (red & triangle) and especially in lateral (black & circle) direction is significantly improved, which we assume to result from increased conversion efficiency of the fundamental mode contained in the source beam compared to higher order modes.

Fig. 4. (Color online) Caustic of measured beam diameter of vertical (red & triangle) and lateral beam (black & circle). The dashed lines are hyperbolic fits according to the second moments. (a) Caustic of the fundamental beam at 1W SH light. (b) Caustic of the SH beam at 1W.

3.3 Stability test of SH To test the reliability of the mounting scheme and the thermal management we performed stability tests of the optical output power as well as of the wavelength at 1 W. The output power and the wavelength were recorded every 10s. The heat sink temperature was stabilized with a Newport 3040 temperature controller, the resistance heater was controlled by a power supply from Agilent (E3631A), and the diode laser was driven by a Newport 525 and a LDX 3650 laser diode controller for the RW and the TA sections, respectively. Figure 5 shows the output power and the center wavelength vs. the measurement time. The MIOB exhibits an output power variation for the SH of ±2% at 1 W. During the test time of more than 1.5 h, the measurement shows only a slight wavelength shift of about ±10 pm. The inset in Fig. 5 shows the spectral intensity distribution at 1 W. The FWHM is about 25pm (40 pm at 980 nm), which is slightly larger then fundamental spectra without optical components [Fig. 2(b)]. This may be due to residual backreflections from the optics and the crystal. Therefore, an improved coating of our optics would reduce the backreflections and hence improve the stability of the laser device.


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Fig. 5. (Color online) Stability tests of the MIOB laser system at 20°C for 1W optical output power. The inset show the spectral intensity distribution.

4. Conclusions We developed a compact laser source using SHG on a 2.5 cm3 MIOB achieving an output power of 1 W at 490 nm. To the best of our knowledge, this represents the highest “bluegreen” output power obtained by single-pass SHG of a diode laser source on a compact micro-optical bench. The MIOB achieves an optical conversion efficiency of 11% at 1 W. First reliability tests show an output power and wavelength stability of± 2% and± 10 pm, respectively. Due to the filtering properties of the PPLN crystal the SH beam shows an nearly diffraction limited beam (M2σ,vertical = 1.2, M2σ,lateral = 2.0). The presented design of our MIOB can be easily adapted to the whole GaAs wavelength range opening an access to the visible laser light between 315 nm – 600 nm. Acknowledgement This work was supported by the Bundesministerium für Bildung und Forschung in the framework of the InnoProfile initiative (#03IP613FKZ). The authors are grateful to S. Schneider and T. Roos for technical assistance.


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