2.6 W optically-pumped semiconductor disk laser ... - OSA Publishing

2.6 W optically-pumped semiconductor disk laser operating at 1.57-µm using wafer fusion Jussi Rautiainen1*, Jari Lyytikäinen1, Alexei Sirbu2, Alexandru Mereuta2, Andrei Caliman2, Eli Kapon2, and Oleg G. Okhotnikov1 1 Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 3, 33720 Tampere, Finland 2 Ècole Polytechnique Fèdèrale de Lausanne, CH-1015 Lausanne, Switzerland * Corresponding author: [email protected]

Abstract: We report a wafer fused high-power optically-pumped semiconductor disk laser incorporating InP-based active medium fused to a GaAs/AlGaAs distributed Bragg reflector. A record value of over 2.6 W of output power in a spectral range around 1.57 µm was demonstrated, revealing the essential advantage of the wafer fusing technique over monolithically-grown all-InP-based structures. The presented approach allows for integration of lattice-mismatched compounds, quantum-well and quantum-dot based media. This would provide convenient means for extending the wavelength range of semiconductor disk lasers. ©2008 Optical Society of America OCIS codes: (140.3070) Infrared and far-infrared lasers; (140.5960) Semiconductor lasers; (140.7270) Vertical emitting lasers.

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Received 1 Oct 2008; revised 4 Dec 2008; accepted 7 Dec 2008; published 17 Dec 2008

22 December 2008 / Vol. 16, No. 26 / OPTICS EXPRESS 21881

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1. Introduction Semiconductor disk lasers (SDLs) are capable of producing high power levels with excellent beam quality [1-3]. They combine many advantages of solid-state lasers with the versatility of semiconductor gain material and the external cavity allows the use of intracavity optical elements. In addition, high power visible radiation needed for optical pumping, life sciences and low-cost full-colour laser projection displays could be obtained by frequency-doubled, high-power SDLs operating in the near-infrared wavelength range [4,5]. However, increased lattice mismatch between GaInAs and GaAs prevents the use of this materials system at the wavelengths above 1180 nm that are needed particularly for generation of amber emission through frequency-doubling. InP-based lasers around 1.55 μm are needed in telecommunication applications but so far efficient high power disk laser have not been reported. In addition to the limitation on the active region structure, also the distributed Bragg reflector (DBR) critically affects SDL performance. Lasers based on GaInAsP/InP and AlGaInAs/AlInAs suffer from the low quality of the DBR, which stems from low refractive index contrast of compounds lattice-matched to InP. High power operation is limited by the poor thermal conductivity. Consequently, disk lasers operating at λ>1180 nm are difficult to grow monolithically in a single epitaxial process. There exist different approaches to meet this challenge; Tourrenc et al. have proposed a hybrid metal-metamorphic technique for the fabrication of DBR [6]. They reported a 1.55 μm SDL and AlAs/GaAs Bragg mirror made with this technique. The laser produced 77 mW single mode output power at room temperature. Lindberg et al. have published a monolithically grown GaInAsP SDL operating at 1.55 μm with 170 mW single mode output power at room temperature [7]. Recently, wafer fusion has been used to combine disparate materials for application in various optoelectronic devices [8]. This technique allows integrating semiconductor materials with different lattice constant. Due to the difference in lattice constant, monolithical growth of such devices becomes essentially difficult and results in highly defective layers [9]. There are reports of long-wavelength InP/GaAs vertical cavity surface emitting lasers (VCSEL), made with wafer fusion [10-12], but the extension of this technique to SDLs has not been demonstrated to date. In this letter we demonstrate a wafer bonded SDL. A high quality AlGaInAs/InP active region was wafer fused with a AlGaAs/GaAs distributed Bragg mirror, which was grown on GaAs substrate. The laser produced a record 2.6 W of output power at 1.57 μm wavelength. 2. Experimental The active medium of the SDL was grown by low pressure metalorganic vapor phase epitaxy (LP MOVPE) on a (100)-InP substrate. The periodic gain structure comprises of 5×2 compressively strained (1 %) AlGaInAs quantum wells as shown in Fig. 1. The quantum wells are located at the antinodes and the fusion interface at the node of the standing wave pattern created between the DBR and semiconductor-air surface. The wafer fusion technique does not affect the accuracy of the thickness control of the active region as compared to the monolithic semiconductor disk laser. Room temperature photoluminescence of the active region peaked near 1520 nm. The DBR grown by solid-source MBE on top of a (100) GaAs substrate, consists of 35 pairs of quarter wave thick Al0.9Ga0.1As and GaAs layers.

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InPcap Al0.06Ga0.26In0.68As QW Al0.16Ga0.38In0.46As barrier Al0.23Ga0.24In0.53As spacer

Active region

Wafer fusion interface GaAs AlGaAs 35 pairs AlGaAs/GaAs DBR

GaAs Substrate

Fig. 1. Gain structure of the semiconductor disk laser.

The wafers were processed using a 2-inch wafer fusion technique, as described in more detail in [11-14]. After the fusion step, the InP-substrate and GaInAsP etch-stop layer were selectively etched from the top of active region by wet etching and cut into 2×2 mm2 chips. Since the gain mirror operates under intense pumping conditions, thermal management is a crucial issue for the disk laser. In our laser the generated heat was conducted to a heat sink using a transparent intracavity heat spreader. A 3×3×0.3 mm3 intracavity natural type IIa diamond heat spreader was bonded on the top surface of the sample with de-ionized water. InP cap layer and surface of the diamond are pulled together by intermolecular forces of water, as described in [15]. The sample is further mounted between two copper plates with indium foil to ensure reliable contact between the surfaces. The topmost metal plate had a circular aperture for signal and pump beams. The bottom copper block was cooled by a Peltier element, with a water cooled copper block as the heat sink. The temperature of the sample could be tuned from 10 °C to 75 °C. The cavity of the disk laser was of V-type and composed of a 98 %-reflective plane output coupler, curved mirror and the gain mirror, as shown in Fig. 2. The gain mirror was pumped with a 980 nm fiber coupled diode laser capable of producing 25 W of continuous-wave power. The pump is focused to the gain mirror to a spot of 180 μm in diameter. The cavity was simulated numerically to ensure that the mode size at the gain mirror matches the pump spot. This mode size allows for high output power while preserving the fundamental mode operation. The output characteristics for different operation temperatures are shown in Fig. 3. The power exceeded 2.62 W and 2.28 W for a heat-sink temperature of 10 °C and 20 °C, respectively. It should be noted that this achievement demonstrates the superiority of the wafer fusing technique over the technology using monolithically grown 1.5 µm InP lattice#102273 - $15.00 USD

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matched disk lasers, which have produced to date up to 170 mW at 20 °C [7]. Lasing threshold and slope efficiency of the output characteristics for continuous-wave operation of the wafer-fused SDL for various heat-sink temperatures from 10 °C to 75 °C are presented in Fig. 4. The optical spectra are shown in Fig. 5. The multiple-line laser spectrum with the spectral period of 1.87 nm originates from Fabry–Pérot etalon effect induced by the uncoated 0.3 mm-thick intracavity diamond heat spreader.

Diamond heat Peltier element spreader Gain Pump mirror Water cooled copper heat sink

D1=90 mm

RoC=150mm OC=2 %

D2=240 mm

Output Fig. 2. Cavity of the semiconductor disk laser. The temperature of the gain mirror is kept at the desired value with a Peltier element attached to a water cooled heat sink.

3000 T=10 C T=15 C T=20 C T=25 C T=30 C T=35 C T=40 C T=45 C T=50 C T=55 C T=60 C T=65 C T=70 C T=75 C

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2500 2000 1500 1000 500 0 0

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Input power at 980 nm (W) Fig. 3. Output characteristics for different operating temperatures.

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5,0

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Exponential fit, T0=24.1 C 12

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3,5 8 3,0 6 2,5 4

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Threshold pump power (W)

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Temperature (°C) Fig. 4. Threshold pump power and slope efficiency as a function of gain mirror temperature. The T0 parameter was deduced from the exponential fit of the threshold pump power.

An intrinsic temperature rise with increasing pump power at constant temperature of the heat-sink results in a red shift of the optical spectrum with rate of 0.75 nm/W. This value demonstrates that wafer-fused SDL with proper thermal management could operate over a wide temperature range.

0

Intensity (10dB/div)

T=10 C 0 T=40 C 0 T=70 C

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Fig. 5. Optical spectra observed at different heat-sink temperatures. Pump power at 980 nm is kept at the value of 10.9 W for all measurements. The center wavelength red shift is ~0.42 nm/°C. Inset: Output beam profile measured with pyrocamera.

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The spatial light distribution of the emission was studied with a pyrocamera. The transverse beam profile shown as an inset in Fig. 5 suggests that disk laser oscillates on the fundamental cavity mode. 3. Conclusion In conclusion, this work demonstrates an optically-pumped semiconductor disk laser operating around 1570 nm, using InP-based gain material and GaAs-based distributed Bragg reflector integrated using wafer fusion. 2.62 W of output power demonstrated in this study represents the highest power reported to date from a semiconductor disk laser at this wavelength and indicates a substantial improvement by a factor of more than 10 over all-InP based structures. The wafer fusing technique could have great potential for considerable wavelength scaling of semiconductor disk lasers by integration of dissimilar semiconductor materials. Acknowledgments The authors would like to thank V. Iakovlev and G. Suruceanu from BeamExpress S.A. for valuable technical assistance. This research was supported in part by Ulla Tuominen Foundation, Emil Aaltonen Foundation, The Finnish National Graduate School of Nanoscience, and SATW Switzerland.

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