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Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser
Superlattices and Microstructures xxx (2017) 1e6
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Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser Jiang Lidan a, Zhu Renjiang a, Jiang Maohua a, Zhang Dingke a, Cui Yuting a, Zhang Peng a, *, Song Yanrong b, ** a b
College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
We demonstrate a good beam quality 483 nm blue coherent radiation from a frequency doubled InGaAs multiple quantum wells semiconductor disk laser. The gain chip is consisted of 6 repeats of strain uncompensated InGaAs/GaAs quantum wells and 25 pairs of GaAs/AlAs distributed Bragg reflector. A 4 4 7 mm3 type I phase-matched BBO nonlinear crystal is used in a V-shaped laser cavity for the second harmonic generation, and 210 mW blue output power is obtained when the absorbed pump power is 3.5 W. The M2 factors of the laser beam in x and y directions are about 1.04 and 1.01, respectively. The output power of the blue laser is limited by the relatively small number of the multiple quantum wells, and higher power can be expected by increasing the number of the multiple quantum wells and improving the heat management of the laser. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Frequency doubling Multiple quantum wells Semiconductor disk laser Nonlinear crystal
1. Introduction Practical and cost-effective blue coherent radiations are of highly interest in many important applications such as laser display, high density optical storage, underwater communication system, and so on. Compare with others display methods, the advantages of laser display are its large color gamut, high brightness, long lifetime and low power dissipation. As one of the three primary colors which cannot be absent, blue laser plays a key role in the development of laser display [1]. For high density optical storage, the current blue-ray disc can achieve 22 GB storage capacity on a 12 cm disc, six times than that of the existing technology. As the third generation storage technology, blue-ray storage will become the major technology of the forthcoming digital video communications [2]. In underwater communication system, the blue laser communication is the way having the least attenuation and the fastest bit rate [3]. Blue laser diodes are commercially available now, however, the epitaxial growth of the gain wafer is sophisticated and its distortional light beam cannot be used directly for many applications [4]. The dependences of high-volt electrical power of the technological maturity argon ion blue lasers make it inconvenient in some applications, and the gas-state gain medium of the laser limits its output energy density [5]. Another kind of widely used blue laser, solid-state blue laser, has complicated cavity structure with very limited emitting wavelength [6].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Peng), [email protected] (S. Yanrong). https://doi.org/10.1016/j.spmi.2017.12.016 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016
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A new type of semiconductor lasers, optically pumped semiconductor disk laser (OP-SDL), has straightforward and simple cavity and can produce multi-watts output power. Compared with the solid-state laser and the traditional semiconductor laser, the unique feature of a SDL is its successful combination of the excellent beam quality, power scalability and the emitting wavelength adjustability [7e9]. By intra-cavity frequency doubling, the laser wavelength of a SDL can be extended to the visible and ultraviolet waveband [10]. The earliest blue SDL was reported by Raymond et al., in 1999. The laser cavity was composed of two reflective mirrors, a KNbO3 nonlinear crystal was inserted for frequency doubling, and 490 nm blue light of 5 mW output power was produced [11]. By optimizing the number of multiple quantum wells and employing carrier blocking layers, J.Y. Kim et al. demonstrated 1.9 W continuous-wave output power at 460 nm blue wavelength in 2007, the power conversion efficiencies (output power/ input power) of 22.5% and 9.5% were realized for 920 nm and 460 nm, respectively [12]. In 2011, A. Hein et al. presented 460 nm blue output powers between 1.35 W and 1.61 W by the use of a BiBO as the nonlinear crystal, and the optical-tooptical conversion efficiency was 41.5% [13]. So far, frequency doubled SDLs have already been extensively reported in the green and yellow-orange spectral range [14e16], however, there are few reports on the blue spectral range of 460e490 nm. In this paper, InGaAs/GaAs multiple quantum wells (MQWs) are grown in the semiconductor gain wafer to obtain fundamental emission at 966 nm, and GaAs/AlAs distributed Bragg reflector (DBR) is employed to produce about 100 nm bandwidth high reflectivity centering at 966 nm. A type I phase-matched BBO nonlinear crystal is inserted, and a straight linear cavity and a V-shape folded cavity are used respectively for experimental comparison of the second harmonic generation. 210 mW output power 483 nm blue SDL is achieved when the absorbed pumping power reached 3.5 W. The M2 factors of 1.04 and 1.01 for x and y directions show the good beam quality of the blue laser. 2. Experiment setup The epitaxial structure of the gain chip is shown in Fig. 1(a), and the sequence of epitaxial growth is as following: 100 nm buffer layer on GaAs substrate is added to improve the quality of epitaxial growth firstly, then 200 nm Al0.85Ga0.15As is grown as the etch stop layer, 20 nm GaAs cap layer is used to protect the wafer from oxidation when the etch stop layer is removed. Next, a 207 nm thick high band gap Al0.6Ga0.4As window layer is introduced as the high barrier layer to prevent the photoinduced carriers from nonradiative recombination at surface of the wafer. The active region is consisted of 6 repeats strain uncompensated InGaAs/GaAs MQWs and 25 pairs GaAs/AlAs DBR. The every single QW includes 8 nm In0.185Ga0.815As well
Fig. 1. Epitaxial structure of gain chip (a), and the schematics of active region (b).
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Fig. 2. Straight linear cavity of the blue SDL. BS: beam splitter.
and 130 nm GaAs barrier. The epitaxial growth is ended by a 20 nm GaAs layer to protect the chip from oxidation. The DBR at the bottom of the chip and the air-semiconductor interface at the front of the chip will form a microcavity, so to force the building of the standing wave of the laser field. Therefore, The every single well must be located at the anti-nodes of the optical standing wave inside the active region to form the so-called resonant periodic gain structure to obtain the mode gain of the laser as higher as possible (see Fig. 1(b)) [7e9]. To insure that the microcavity is resonant for laser wavelength, two adjust layers are inserted to adjust the total length of the microcavity. The incident pump light is absorbed in the barrier layers, and the generated carriers are captured by the wells, in where the gain of the laser wavelength is provided. To circumvent the thermal problem, the gain chip is cut into 3 3 mm2, a silicon carbide heatsink is directly bonded to the gain chip using the capillary action of water [17], and then the GaAs substrate is removed by the use of wet chemical etching, which includes rapid etching and selective etching. The rapid etching is non-selective and accomplished using NH3$H2O and H2O2, and selective etching is performed using C6H8O7 and H2O2. The etch stop layer is removed using 2% HF and the entire thickness of the remained gain chip is less than 4 mm. The straight linear cavity of the frequency doubled SDL is illustrated in Fig. 2. An 808 nm semiconductor laser is delivered via a multi-mode fiber and focus on the gain chip at incident angle of approximately 30 . This results in a pump spot of about 200 mm diameter at the gain chip. In order to get better frequency conversion efficiency, the BBO crystal is placed to the gain chip as near as possible. The output coupler (OC) is a spherical mirror with the radius of curvature of 100 mm, and is high-reflection (HR) coated at the fundamental wavelength of 966 nm and anti-reflection (AR) coated at the second harmonic wavelength of 483 nm. As shown in Fig. 3, the V-shaped cavity SDL is formed by the DBR at the bottom of the gain chip, the folding mirror (R ¼ 100 mm) and the flat end mirror. The lengths of the first and the second arm of the laser cavity are of 96 and 90 mm, respectively. For efficient second harmonic generation, the BBO crystal is inserted near to the flat end mirror, where the beam waist is situated. A 7 mm length nonlinear crystal BBO is cut for type I phase matching at 966 nm, and is anti-reflection coated for both the fundamental and the blue radiation. In the laser cavity, the end mirror is high-reflection coated for both the fundamental and blue wavelength, and the folding mirror is high-reflection coated for the fundamental laser and antireflection coated for the blue laser. 3. Results and discussions Fig. 4 shows the emission spectrum of the fundamental wave at 966 nm and the blue light at 483 nm when the absorbed pump power is 3.5 W. The full width at half maximum (FWHM) of the infrared and blue laser are 5 nm and 2.5 nm, respectively.
Fig. 3. V-shaped cavity of the blue SDL.
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Fig. 4. Spectrum of the fundamental and blue SDL.
Fig. 5 presents the beam quality M2 factor of the blue output in x (a) and y (b) directions. The spot sizes u of blue laser beam at different distances z are measured using a CCD, and then the experimental curve is fitted by the hyperbolic method, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2 ¼ Az2 þ Bz þ C, and the beam quality M2 factor is calculated by M2 ¼ p AC B2 =4=l. Output powers of the blue SDL with straight linear cavity and V-shaped cavity are plotted in Fig. 6(a) and (b). It can be seen from Fig. 6 that the output power of the blue laser in the straight linear cavity decrease when the absorbed pump power more than 2.4 W, while the output power of the blue laser in the V-shape cavity increase until the absorbed pump power beyond
Fig. 5. Beam quality M2 factors of the blue output in x (a) and y (b) directions.
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Fig. 6. Output powers of the blue SDL with straight linear (a) and V-shaped cavity (b).
3.1 W under condition of 15.5 C heatsink temperature. There are some reasons for the above performance. Firstly, for the straight linear cavity, the nonlinear crystal cannot be put near to the gain chip too much because of the existence of the pump system, so the laser spot on the crystal cannot be small enough and the conversion efficiency of frequency doubling will be discounted. Secondly, the blue laser will back to the gain chip and be absorbed by the gain chip partly (the absorption coefficient of GaAs to blue light is about 1.3/mm), and this not only accelerate the thermal effect in the active region, but also further decrease the conversion efficiency of frequency doubling. On comparison, the BBO can be put to the end mirror as near as possible in the V-shape cavity, and the generated blue laser is coupled out through the folded mirror and will back to the gain chip no longer. The maximum output power of 54.8 mW of the blue laser for the straight linear cavity and 210 mW for the V-shape cavity are produced when the temperature of the heatsink is 13.5 C. We believe that the relatively small number of the quantum wells, which means small mode gain of the laser in the active region, is the essential reason why the output power of the blue laser is relatively small. By the use of strain compensate layer, more quantum wells can be added in the active region and higher mode gain of the laser, thus higher output power can be expected. Another limitation of the power of the blue laser maybe comes from the choice of the nonlinear crystal BBO. The FWHM of the fundamental wave is about 5 nm (see Fig. 4), for the BBO, which has a relatively small acceptance bandwidth, it seems not an ideal candidate for this job. In addition, BBO has bigger walk-off angle (68.9 mrad at 966 nm and 70.2 mrad at 483 nm) compared to the other nonlinear crystals, and 7 mm length may introduce excessive walk-off angle in the intra-cavity frequency doubling and decrease the conversion efficiency, so to the output power of the blue laser. By improving the thermal management of the laser, narrowing the spectrum bandwidth of the fundamental wave and optimizing the quantum design of the gain wafer, the cavity parameters and the length of the nonlinear crystal, further scalability of the output power of the blue laser could be realized. 4. Conclusion We have demonstrated an intra-cavity frequency doubled 483 nm blue coherent radiation from a strain uncompensated InGaAs multiple quantum wells SDL using a type I phase-matched BBO as the nonlinear crystal, the maximum output power Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016
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of the blue laser is 210 mW and the beam quality M2 factors of the blue output are about 1.04 and 1.01 in x and y directions, respectively. To reduce the thermal problem, a silicon carbide heatsink is capillary bonded to the gain chip and the GaAs substrate is removed by wet chemical etching. Our experiment proves that V-shaped cavity is more suitable for intra-cavity frequency doubling of the blue SDLs. Future work includes narrowing the spectrum of the fundamental wave, optimizing the quantum design of the gain wafer and the length of the nonlinear crystal for higher conversion efficiency. Acknowledgments This work is supported by the Chongqing Research Program of Basic Research and Frontier Technology (cstc2015jcyjBX0098), the National Natural Science Foundation of China (61575011), and the Foundation for the Creative Research Groups of Higher Education of Chongqing (CXTDX201601016). References [1] K. Paschke, G. Blume, N. Werner, A. Müller, B. Sumpf, J. Pohl, D. Feise, P. Ressel, A. Sahm, R. Bege, J. Hofmann, D. Jedrzejczyk, G. Tr€ ankle, Compact RGBY light sources with high luminance for laser display applications, Opt. Rev. (2017), https://doi.org/10.1007/s10043-017-0378-z. [2] X.M. Gao, W.D. Xu, F. Zhou, F.X. Gan, Modularized static tester for blue ray optical recording properties, Chin. J. Lasers 32 (8) (2005) 1127e1131. [3] T. Wiener, S. Karp, The role of blue/green laser systems in strategic submarine communications, IEEE T Commun. 28 (9) (1980) 1602e1607. [4] S. Nakamura, S. Pearton, G. Fasol, Blue Laser Diode Complete Story, Springer Science & Business Media, 2013. [5] A. Frank, A. Bla zevi c, P.L. Grande, K. Harres, T. Heßling, D.H.H. Hoffmann, R. Knobloch-Maas, P.G. Kuznetsov, F. Nürnberg, A. Pelka, G. Schaumann, G. €kel, M. Schollmeier, D. Schumacher, J. Schütrumpf, V.V. Vatulin, O.A. Vinokurov, M. Roth, Energy loss of argon in a laser-generated Schiwietz, A. Scho carbon plasma, Phys. Rev. E 81 (2) (2010), 026401. [6] S. Rong, X. Zhu, W. Chen, All-solid-state narrow-line width 455-nm blue laser based on Ti: sapphire crystal, Chin. Opt. Lett. 7 (1) (2009) 43e45. [7] M. Kuznetsov, F. Hakimi, R. Sprague, A. Mooradian, Design and characteristics of high-power (> 0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beams, IEEE J. Sel. Top. Quant. 5 (3) (1999) 561e573. [8] A.C. Tropper, H.D. Foreman, A. Garnache, K.G. Wilcox, S.H. Hoogland, Vertical-external-cavity semiconductor lasers, J. Phys. D. Appl. Phys. 37 (9) (2004) R75. €ki, A. H€ €nen, Optically pumped VECSELs: review of technology and progress, J. Phys. D. Appl. Phys. 50 (38) (2017), 383001. [9] M. Guina, A. Rantama arko [10] S. Calvez, J.E. Hastie, M. Guina, O.G. Okhotnikov, M.D. Dawson, Semiconductor disk lasers for the generation of visible and ultraviolet radiation, Laser Photonics Rev. 3 (5) (2009) 407e434. [11] T.D. Raymond, W.J. Alford, M.H. Crawford, A.A. Allerman, Intra-cavity frequency doubling of a diode-pumped external-cavity surface-emitting semiconductor laser, Opt. Lett. 24 (16) (1999) 1127e1129. [12] J.Y. Kim, S. Cho, S.J. Lim, J. Yoo, G.B. Kim, K.S. Kim, J. Lee, S.M. Lee, T. Kim, Y. Park, Efficient blue lasers based on gain structure optimizing of verticalexternal-cavity surface-emitting laser with second harmonic generation, J. Appl. Phys. 101 (3) (2007), 033103. [13] A. Hein, F. Demaria, A. Kern, S. Menzel, F. Rinaldi, R. Rosch, P. Unger, Efficient 460-nm second- harmonic generation with optically pumped semiconductor disk lasers, IEEE Photonic Tech. L 23 (3) (2011) 179e181. [14] J. Chilla, Q.Z. Shu, H. Zhou, E. Weiss, M. Reed, L. Spinelli, Recent advances in optically pumped semiconductor lasers, Proc. SPIE 6451 (2007), 645109. [15] J.Y. Kim, S. Cho, S.M. Lee, G.B. Kim, J. Lee, J. Yoo, K.S. Kim, T. Kim, Y. Park, Highly efficient green VECSEL with intra-cavity diamond heat spreader, Electron Lett. 43 (2) (2007) 105e107. [16] E. Kantola, T. Leinonen, S. Ranta, M. Tavast, M. Guina, High-efficiency 20 W yellow VECSEL, Opt. Express 22 (6) (2014) 6372e6380. [17] Z.L. Liau, Semiconductor wafer bonding via liquid capillarity, Appl. Phys. Lett. 77 (5) (2000) 651e653.
Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser Jiang Lidan a, Zhu Renjiang a, Jiang Maohua a, Zhang Dingke a, Cui Yuting a, Zhang Peng a, *, Song Yanrong b, ** a b
College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China College of Applied Sciences, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
We demonstrate a good beam quality 483 nm blue coherent radiation from a frequency doubled InGaAs multiple quantum wells semiconductor disk laser. The gain chip is consisted of 6 repeats of strain uncompensated InGaAs/GaAs quantum wells and 25 pairs of GaAs/AlAs distributed Bragg reflector. A 4 4 7 mm3 type I phase-matched BBO nonlinear crystal is used in a V-shaped laser cavity for the second harmonic generation, and 210 mW blue output power is obtained when the absorbed pump power is 3.5 W. The M2 factors of the laser beam in x and y directions are about 1.04 and 1.01, respectively. The output power of the blue laser is limited by the relatively small number of the multiple quantum wells, and higher power can be expected by increasing the number of the multiple quantum wells and improving the heat management of the laser. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Frequency doubling Multiple quantum wells Semiconductor disk laser Nonlinear crystal
1. Introduction Practical and cost-effective blue coherent radiations are of highly interest in many important applications such as laser display, high density optical storage, underwater communication system, and so on. Compare with others display methods, the advantages of laser display are its large color gamut, high brightness, long lifetime and low power dissipation. As one of the three primary colors which cannot be absent, blue laser plays a key role in the development of laser display [1]. For high density optical storage, the current blue-ray disc can achieve 22 GB storage capacity on a 12 cm disc, six times than that of the existing technology. As the third generation storage technology, blue-ray storage will become the major technology of the forthcoming digital video communications [2]. In underwater communication system, the blue laser communication is the way having the least attenuation and the fastest bit rate [3]. Blue laser diodes are commercially available now, however, the epitaxial growth of the gain wafer is sophisticated and its distortional light beam cannot be used directly for many applications [4]. The dependences of high-volt electrical power of the technological maturity argon ion blue lasers make it inconvenient in some applications, and the gas-state gain medium of the laser limits its output energy density [5]. Another kind of widely used blue laser, solid-state blue laser, has complicated cavity structure with very limited emitting wavelength [6].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Peng), [email protected] (S. Yanrong). https://doi.org/10.1016/j.spmi.2017.12.016 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016
2
J. Lidan et al. / Superlattices and Microstructures xxx (2017) 1e6
A new type of semiconductor lasers, optically pumped semiconductor disk laser (OP-SDL), has straightforward and simple cavity and can produce multi-watts output power. Compared with the solid-state laser and the traditional semiconductor laser, the unique feature of a SDL is its successful combination of the excellent beam quality, power scalability and the emitting wavelength adjustability [7e9]. By intra-cavity frequency doubling, the laser wavelength of a SDL can be extended to the visible and ultraviolet waveband [10]. The earliest blue SDL was reported by Raymond et al., in 1999. The laser cavity was composed of two reflective mirrors, a KNbO3 nonlinear crystal was inserted for frequency doubling, and 490 nm blue light of 5 mW output power was produced [11]. By optimizing the number of multiple quantum wells and employing carrier blocking layers, J.Y. Kim et al. demonstrated 1.9 W continuous-wave output power at 460 nm blue wavelength in 2007, the power conversion efficiencies (output power/ input power) of 22.5% and 9.5% were realized for 920 nm and 460 nm, respectively [12]. In 2011, A. Hein et al. presented 460 nm blue output powers between 1.35 W and 1.61 W by the use of a BiBO as the nonlinear crystal, and the optical-tooptical conversion efficiency was 41.5% [13]. So far, frequency doubled SDLs have already been extensively reported in the green and yellow-orange spectral range [14e16], however, there are few reports on the blue spectral range of 460e490 nm. In this paper, InGaAs/GaAs multiple quantum wells (MQWs) are grown in the semiconductor gain wafer to obtain fundamental emission at 966 nm, and GaAs/AlAs distributed Bragg reflector (DBR) is employed to produce about 100 nm bandwidth high reflectivity centering at 966 nm. A type I phase-matched BBO nonlinear crystal is inserted, and a straight linear cavity and a V-shape folded cavity are used respectively for experimental comparison of the second harmonic generation. 210 mW output power 483 nm blue SDL is achieved when the absorbed pumping power reached 3.5 W. The M2 factors of 1.04 and 1.01 for x and y directions show the good beam quality of the blue laser. 2. Experiment setup The epitaxial structure of the gain chip is shown in Fig. 1(a), and the sequence of epitaxial growth is as following: 100 nm buffer layer on GaAs substrate is added to improve the quality of epitaxial growth firstly, then 200 nm Al0.85Ga0.15As is grown as the etch stop layer, 20 nm GaAs cap layer is used to protect the wafer from oxidation when the etch stop layer is removed. Next, a 207 nm thick high band gap Al0.6Ga0.4As window layer is introduced as the high barrier layer to prevent the photoinduced carriers from nonradiative recombination at surface of the wafer. The active region is consisted of 6 repeats strain uncompensated InGaAs/GaAs MQWs and 25 pairs GaAs/AlAs DBR. The every single QW includes 8 nm In0.185Ga0.815As well
Fig. 1. Epitaxial structure of gain chip (a), and the schematics of active region (b).
Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016
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Fig. 2. Straight linear cavity of the blue SDL. BS: beam splitter.
and 130 nm GaAs barrier. The epitaxial growth is ended by a 20 nm GaAs layer to protect the chip from oxidation. The DBR at the bottom of the chip and the air-semiconductor interface at the front of the chip will form a microcavity, so to force the building of the standing wave of the laser field. Therefore, The every single well must be located at the anti-nodes of the optical standing wave inside the active region to form the so-called resonant periodic gain structure to obtain the mode gain of the laser as higher as possible (see Fig. 1(b)) [7e9]. To insure that the microcavity is resonant for laser wavelength, two adjust layers are inserted to adjust the total length of the microcavity. The incident pump light is absorbed in the barrier layers, and the generated carriers are captured by the wells, in where the gain of the laser wavelength is provided. To circumvent the thermal problem, the gain chip is cut into 3 3 mm2, a silicon carbide heatsink is directly bonded to the gain chip using the capillary action of water [17], and then the GaAs substrate is removed by the use of wet chemical etching, which includes rapid etching and selective etching. The rapid etching is non-selective and accomplished using NH3$H2O and H2O2, and selective etching is performed using C6H8O7 and H2O2. The etch stop layer is removed using 2% HF and the entire thickness of the remained gain chip is less than 4 mm. The straight linear cavity of the frequency doubled SDL is illustrated in Fig. 2. An 808 nm semiconductor laser is delivered via a multi-mode fiber and focus on the gain chip at incident angle of approximately 30 . This results in a pump spot of about 200 mm diameter at the gain chip. In order to get better frequency conversion efficiency, the BBO crystal is placed to the gain chip as near as possible. The output coupler (OC) is a spherical mirror with the radius of curvature of 100 mm, and is high-reflection (HR) coated at the fundamental wavelength of 966 nm and anti-reflection (AR) coated at the second harmonic wavelength of 483 nm. As shown in Fig. 3, the V-shaped cavity SDL is formed by the DBR at the bottom of the gain chip, the folding mirror (R ¼ 100 mm) and the flat end mirror. The lengths of the first and the second arm of the laser cavity are of 96 and 90 mm, respectively. For efficient second harmonic generation, the BBO crystal is inserted near to the flat end mirror, where the beam waist is situated. A 7 mm length nonlinear crystal BBO is cut for type I phase matching at 966 nm, and is anti-reflection coated for both the fundamental and the blue radiation. In the laser cavity, the end mirror is high-reflection coated for both the fundamental and blue wavelength, and the folding mirror is high-reflection coated for the fundamental laser and antireflection coated for the blue laser. 3. Results and discussions Fig. 4 shows the emission spectrum of the fundamental wave at 966 nm and the blue light at 483 nm when the absorbed pump power is 3.5 W. The full width at half maximum (FWHM) of the infrared and blue laser are 5 nm and 2.5 nm, respectively.
Fig. 3. V-shaped cavity of the blue SDL.
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Fig. 4. Spectrum of the fundamental and blue SDL.
Fig. 5 presents the beam quality M2 factor of the blue output in x (a) and y (b) directions. The spot sizes u of blue laser beam at different distances z are measured using a CCD, and then the experimental curve is fitted by the hyperbolic method, qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u2 ¼ Az2 þ Bz þ C, and the beam quality M2 factor is calculated by M2 ¼ p AC B2 =4=l. Output powers of the blue SDL with straight linear cavity and V-shaped cavity are plotted in Fig. 6(a) and (b). It can be seen from Fig. 6 that the output power of the blue laser in the straight linear cavity decrease when the absorbed pump power more than 2.4 W, while the output power of the blue laser in the V-shape cavity increase until the absorbed pump power beyond
Fig. 5. Beam quality M2 factors of the blue output in x (a) and y (b) directions.
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Fig. 6. Output powers of the blue SDL with straight linear (a) and V-shaped cavity (b).
3.1 W under condition of 15.5 C heatsink temperature. There are some reasons for the above performance. Firstly, for the straight linear cavity, the nonlinear crystal cannot be put near to the gain chip too much because of the existence of the pump system, so the laser spot on the crystal cannot be small enough and the conversion efficiency of frequency doubling will be discounted. Secondly, the blue laser will back to the gain chip and be absorbed by the gain chip partly (the absorption coefficient of GaAs to blue light is about 1.3/mm), and this not only accelerate the thermal effect in the active region, but also further decrease the conversion efficiency of frequency doubling. On comparison, the BBO can be put to the end mirror as near as possible in the V-shape cavity, and the generated blue laser is coupled out through the folded mirror and will back to the gain chip no longer. The maximum output power of 54.8 mW of the blue laser for the straight linear cavity and 210 mW for the V-shape cavity are produced when the temperature of the heatsink is 13.5 C. We believe that the relatively small number of the quantum wells, which means small mode gain of the laser in the active region, is the essential reason why the output power of the blue laser is relatively small. By the use of strain compensate layer, more quantum wells can be added in the active region and higher mode gain of the laser, thus higher output power can be expected. Another limitation of the power of the blue laser maybe comes from the choice of the nonlinear crystal BBO. The FWHM of the fundamental wave is about 5 nm (see Fig. 4), for the BBO, which has a relatively small acceptance bandwidth, it seems not an ideal candidate for this job. In addition, BBO has bigger walk-off angle (68.9 mrad at 966 nm and 70.2 mrad at 483 nm) compared to the other nonlinear crystals, and 7 mm length may introduce excessive walk-off angle in the intra-cavity frequency doubling and decrease the conversion efficiency, so to the output power of the blue laser. By improving the thermal management of the laser, narrowing the spectrum bandwidth of the fundamental wave and optimizing the quantum design of the gain wafer, the cavity parameters and the length of the nonlinear crystal, further scalability of the output power of the blue laser could be realized. 4. Conclusion We have demonstrated an intra-cavity frequency doubled 483 nm blue coherent radiation from a strain uncompensated InGaAs multiple quantum wells SDL using a type I phase-matched BBO as the nonlinear crystal, the maximum output power Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016
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of the blue laser is 210 mW and the beam quality M2 factors of the blue output are about 1.04 and 1.01 in x and y directions, respectively. To reduce the thermal problem, a silicon carbide heatsink is capillary bonded to the gain chip and the GaAs substrate is removed by wet chemical etching. Our experiment proves that V-shaped cavity is more suitable for intra-cavity frequency doubling of the blue SDLs. Future work includes narrowing the spectrum of the fundamental wave, optimizing the quantum design of the gain wafer and the length of the nonlinear crystal for higher conversion efficiency. Acknowledgments This work is supported by the Chongqing Research Program of Basic Research and Frontier Technology (cstc2015jcyjBX0098), the National Natural Science Foundation of China (61575011), and the Foundation for the Creative Research Groups of Higher Education of Chongqing (CXTDX201601016). References [1] K. Paschke, G. Blume, N. Werner, A. Müller, B. Sumpf, J. Pohl, D. Feise, P. Ressel, A. Sahm, R. Bege, J. Hofmann, D. Jedrzejczyk, G. Tr€ ankle, Compact RGBY light sources with high luminance for laser display applications, Opt. Rev. (2017), https://doi.org/10.1007/s10043-017-0378-z. [2] X.M. Gao, W.D. Xu, F. Zhou, F.X. Gan, Modularized static tester for blue ray optical recording properties, Chin. J. Lasers 32 (8) (2005) 1127e1131. [3] T. Wiener, S. Karp, The role of blue/green laser systems in strategic submarine communications, IEEE T Commun. 28 (9) (1980) 1602e1607. [4] S. Nakamura, S. Pearton, G. Fasol, Blue Laser Diode Complete Story, Springer Science & Business Media, 2013. [5] A. Frank, A. Bla zevi c, P.L. Grande, K. Harres, T. Heßling, D.H.H. Hoffmann, R. Knobloch-Maas, P.G. Kuznetsov, F. Nürnberg, A. Pelka, G. Schaumann, G. €kel, M. Schollmeier, D. Schumacher, J. Schütrumpf, V.V. Vatulin, O.A. Vinokurov, M. Roth, Energy loss of argon in a laser-generated Schiwietz, A. Scho carbon plasma, Phys. Rev. E 81 (2) (2010), 026401. [6] S. Rong, X. Zhu, W. Chen, All-solid-state narrow-line width 455-nm blue laser based on Ti: sapphire crystal, Chin. Opt. Lett. 7 (1) (2009) 43e45. [7] M. Kuznetsov, F. Hakimi, R. Sprague, A. Mooradian, Design and characteristics of high-power (> 0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beams, IEEE J. Sel. Top. Quant. 5 (3) (1999) 561e573. [8] A.C. Tropper, H.D. Foreman, A. Garnache, K.G. Wilcox, S.H. Hoogland, Vertical-external-cavity semiconductor lasers, J. Phys. D. Appl. Phys. 37 (9) (2004) R75. €ki, A. H€ €nen, Optically pumped VECSELs: review of technology and progress, J. Phys. D. Appl. Phys. 50 (38) (2017), 383001. [9] M. Guina, A. Rantama arko [10] S. Calvez, J.E. Hastie, M. Guina, O.G. Okhotnikov, M.D. Dawson, Semiconductor disk lasers for the generation of visible and ultraviolet radiation, Laser Photonics Rev. 3 (5) (2009) 407e434. [11] T.D. Raymond, W.J. Alford, M.H. Crawford, A.A. Allerman, Intra-cavity frequency doubling of a diode-pumped external-cavity surface-emitting semiconductor laser, Opt. Lett. 24 (16) (1999) 1127e1129. [12] J.Y. Kim, S. Cho, S.J. Lim, J. Yoo, G.B. Kim, K.S. Kim, J. Lee, S.M. Lee, T. Kim, Y. Park, Efficient blue lasers based on gain structure optimizing of verticalexternal-cavity surface-emitting laser with second harmonic generation, J. Appl. Phys. 101 (3) (2007), 033103. [13] A. Hein, F. Demaria, A. Kern, S. Menzel, F. Rinaldi, R. Rosch, P. Unger, Efficient 460-nm second- harmonic generation with optically pumped semiconductor disk lasers, IEEE Photonic Tech. L 23 (3) (2011) 179e181. [14] J. Chilla, Q.Z. Shu, H. Zhou, E. Weiss, M. Reed, L. Spinelli, Recent advances in optically pumped semiconductor lasers, Proc. SPIE 6451 (2007), 645109. [15] J.Y. Kim, S. Cho, S.M. Lee, G.B. Kim, J. Lee, J. Yoo, K.S. Kim, T. Kim, Y. Park, Highly efficient green VECSEL with intra-cavity diamond heat spreader, Electron Lett. 43 (2) (2007) 105e107. [16] E. Kantola, T. Leinonen, S. Ranta, M. Tavast, M. Guina, High-efficiency 20 W yellow VECSEL, Opt. Express 22 (6) (2014) 6372e6380. [17] Z.L. Liau, Semiconductor wafer bonding via liquid capillarity, Appl. Phys. Lett. 77 (5) (2000) 651e653.
Please cite this article in press as: J. Lidan et al., Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser, Superlattices and Microstructures (2017), https://doi.org/10.1016/j.spmi.2017.12.016