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We report a wafer-fused high power optically-pumped semiconductor disk laser operating at 1.3 µm. An InP-based active medium was fused with a GaAs/AlGaAs distributed Bragg reflector, resulting in an integrated monolithic gain mirror. Over 2.7 W of output power, obtained at temperature of 15 °C, represents the best achievement reported to date for this type of lasers. The results reveal an essential advantage of the wafer fusing technique over both monolithically grown AlGaInAs/GaInAsP- and GaInNAs-based structures.
©2009 Optical Society of America
1. Introduction
Optically-pumped semiconductor disk lasers (SDLs) represent a proven approach for generation of multi-watt output powers with excellent beam quality [1–4]. They combine many advantages of solid-state lasers with the added benefit of wavelength tailoring provided by the semiconductor gain material. Mode locking and frequency-doubling have been demonstrated with SDLs exploiting the versatility of their cavity.
Very few options for the development of 1.3 µm SDL using monolithic wafer growth are currently available. One uses a lattice-matched monolithic structure based on InP technology, another GaInNAs gain material and a GaAs/AlGaAs distributed Bragg reflector (DBR). The critical issue associated with InP monolithic structures is the low DBR quality, which limits their capability for power scaling. The limitation of the dilute nitride approach in shifting the operation of GaInNAs composition towards longer wavelengths is the requirement for a higher concentration of nitrogen in the quantum-wells (QWs) layers, which increases the amount of point defects resulting in a higher rate of non-radiative recombination. The best 1.3 µm disk laser using dilute nitride compounds reported to date produces output powers up to 0.6 W [5].
During the past few years a wafer fusion technique has been used extensively in the producing of vertical-cavity surface-emitting lasers (VCSELs) operating at the telecom wavelengths of 1.3 – 1.55 µm [6,7]. This technique allows the integration of non-lattice-matched semiconductor materials, e.g. GaAs and InP, which cannot be grown monolithically. Recently, this technique has been applied for the first time to a high-power InP based 1.57 µm disk laser exhibiting good performance [8].
In this letter we demonstrate the first wafer fused SDLs operating at the wavelength of 1.3 µm. An AlGaInAs/InP gain wafer has been fused with an AlGaAs/GaAs distributed Bragg mirror grown on a GaAs substrate. The laser produces an output power of 2.7 W, representing the highest power achieved from an SDL at this wavelength to date.
2. Experimental and results
Two different active media grown on InP substrates were studied. Structure A designed for an 808 nm pumping was grown by solid-source molecular beam epitaxy (MBE). The periodic gain structure consisted of 5 × 2 compressively strained (1%) AlGaInAs quantum wells, as shown in Fig. 1 . The QWs were sandwiched by tensile strained AlGaInAs (−0.5%) carrier confining layers and positioned evenly at the antinodes of the standing wave of an optical electric field by InP spacers. The photoluminescence (PL) from the active region peaked at λ = 1270 nm at room temperature, as measured before the wafer fusion. An AlInAs window layer grown after the gain section prevents the carrier surface recombination. A thin InP cap layer terminated the structure. The subcavity resonant wavelength was 1292 nm, resulting in a spectral detuning from the PL peak of 22 nm at room temperature. Structure B, designed for a 980 nm pumping, was grown by low pressure metallorganic vapor phase epitaxy (LP MOVPE). The gain section is similar to that of structure A except that the QW pairs were separated by lattice matched AlGaInAs spacers and an InP window layer was used instead of the AlInAs. The PL spectrum at room temperature was centered at 1263 nm. A subcavity resonance located at 1315 nm results in a detuning of 52 nm from the PL peak. The DBRs for both structures were grown by MBE on GaAs substrates and comprise 35 pairs of quarter-wave thick Al0.9Ga0.1As and GaAs layers.
Fig. 1 Layer structure A for 808 nm pumping, t = thickness. In the structure B designed for 980 nm pumping, the InP spacers are replaced by AlGaInAs and the AlInAs window layer by InP.
The wafers were processed using a 2-inch wafer fusion technique, as described in [7,10]. After the fusion step, the InP-substrate and a GaInAsP etch-stop layer were selectively etched from the top of the active region by wet etching and then the monolithically integrated structure was cut into 2.5 × 2.5 mm2 chips. Excessive heating was prevented by using intracavity 3 × 3 × 0.3 mm3 natural type IIa diamond heat spreaders placed on top of the samples by the water bonding technique [11]. The samples were further mounted between two copper plates with indium foil. The top metal plate had a circular aperture for signal and pump beams. The temperature of the samples was kept at 15 °C throughout the measurements using a water cooled copper block.
The SDL cavity is configured by the gain mirror, a highly reflecting curved mirror and a planar output coupler, as shown in Fig. 2 . The gain mirrors A and B were pumped by 808 nm and 980 nm fiber-coupled diode lasers, respectively, with a pump spot diameter at the gain mirror of 180 μm. The laser cavity was engineered to match closely the mode field diameter with the dimensions of the pump spot at the gain mirror. The output characteristics for the SDLs with the structures A and B are shown in Figs. 3 and 4 , respectively. In these measurements, the power of the 980 nm pump source was limited to 25 W.
Fig. 2 Cavity of the semiconductor disk laser. RoC = Radius of Curvature, OC = Output coupler, D1,2 = Distance.
The maximum output power of structure A reaches 2.1 W, 1.1 W and 1.6 W before the onset of roll-over with 1%, 2% and 2.5% output coupling, respectively. The corresponding slope efficiencies are in the range of 10.6%–11.8% and the threshold pump power varies between 2.9 W and 5.0 W. Reasons for the worst performance of the 2% output coupler for this laser remained unclear. For structure B the output power attains the values of 1.9 W, 2.4 W and 2.7 W for output coupling of 1%, 2% and 2.5%, respectively. The roll-over in the output characteristic was not observed with this gain medium for pump powers up to 25 W which also limited the achievable output power. Slope efficiency for structure B ranges from 8.2% to 12.2% and threshold pump power varies between 2.0 W and 3.1 W. Improved laser performance with 980 nm pumping as compared to 808 nm indicates a detrimental effect of quantum defect (i.e., the relative wavelength difference between pump and laser radiation) on the disk laser performance. The small value of detuning in structure A could also contribute to roll-over in the output characteristics because of the reduced effective gain at operating temperature [12].
Optical spectra are presented in Figs. 5 and 6 for the disk lasers with gain structures A and B, respectively. The spectra show a multiple-line character with an average spectral period of 1 nm, which originated from the Fabry–Pérot etalon effect induced by the surfaces of the uncoated 0.3 mm-thick intracavity diamond heat spreader.
Fig. 5 Optical emission spectrum of SDL using structure A with 1-% output coupler. Inset: Output beam profile measured with a pyrocamera at an output power of 2025 mW.
Fig. 6 Optical emission spectrum of SDL with structure B for 1-% outcoupling. Inset: Output beam profile measured with a pyrocamera at an output power of 1665 mW.
The transverse beam profiles measured with a pyrocamera are shown as an inset in Figs. 5 and 6. The elliptical shape is due to reflections from the uncoated diamond surfaces caused by the off axis pumping arrangement.
3. Conclusion
In conclusion, this work demonstrates an optically-pumped semiconductor disk lasers operating at 1.3 µm using AlGaInAs/InP gain material and AlGaAs/GaAs distributed Bragg reflectors integrated using a wafer fusion process. The 2.7 W of output power achieved in this study represents the highest power reported to date from a semiconductor disk laser at this wavelength. The results demonstrate the high potential of the wafer fusing technique for both wavelength selection and power scaling of semiconductor disk lasers.
Acknowledgments
The authors acknowledge the technical help of Vladimir Iakovlev and Grigore Suruceanu from BeamExpress S.A, Lausanne, CH-1015, Switzerland, and Mika Saarinen from the Optoelectronics Research Centre, Tampere University of Technology, Korkeakoulunkatu 3, 33720 Tampere, Finland.
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