High performance 2.2mm optically-pumped vertical external-cavity surface-emitting laser
J-M Hopkins, R D Preston, A J Maclean, S Calvez, H Sun, J Ng1, M Steer2, M Hopkinson1, and D Burns.
Institute of Photonics, University of Strathclyde, Wolfson Centre, 106 Rottenrow, Glasgow, G4 0NW (UK)
1University of Sheffield, Department of Electronic & Electrical Engineering
Mappin Street, Sheffield, S1 3JD (UK)
2EPSRC National Centre for III-V Technologies, Mappin Street, Sheffield, S1 3JD (UK)
Abstract. We report the operation of an optically-pumped vertical-external-cavity surface-emitting laser (OP-VECSEL) oscillating at wavelengths up to 2.2μm and at output powers greater than 200mW. This versatile platform provides a broad gain bandwidth which may be tuned and/or controlled by the addition of elements into the external cavity. Moreover, the nature of the semiconductor structure permits precise engineering of the operating wavelength when compared with traditional solid state crystalline lasers, and the thin (few microns) pump absorption region, coupled with the external cavity control, permits the mode conversion of a low brightness pump into a high quality single spatial mode TEM00 output.
1. Introduction
Laser sources covering the 2-2.5mm atmospheric transmission window have a large number of potential applications in communications, sensing, spectroscopy and security. Typically, the bandwidth, range and sensitivity requirements for many of these applications demand a laser having diffraction limited, single mode performance. Until recently, however, compact, wavelength versatile, light sources in this range with these qualities have been few; where available they are generally restricted to only a few milliwatts [1]. The recent emergence of the optically-pumped semiconductor vertical-external-cavity surface-emitting lasers (OPS-VECSELS) [2] has provided the ability to obtain high-power performance from semiconductor lasers with high spatial mode quality output at a large range of wavelengths. Figure 1 shows schematically the typical configuration of a VECSEL cavity.
Figure 1 Schematic of a generic optically-pumped vertical external-cavity surface-emitting laser.
Critically, the use of intracavity heat spreaders has permitted the demonstration of high (Watt level) performance in material systems and wavelengths away from the commonly used InGaAs at 980nm [3]. In this paper, we report on the demonstration of a VECSEL device based on a quaternary antimonide (InGaAsSb/AlGaAsSb) active region which is capillary bonded [4] to a diamond heatspreading window for optimal thermal management and therefore efficient performance [5].
2. Semiconductor design and growth
The semiconductor VECSEL structure shown in figure 2 incorporates a carefully designed multi quantum well InGaAsSb/AlGaAsSb gain region containing 14, 7.5nm-thick In0.29Ga0.71As0.19Sb0.81 quantum wells embedded within a thin (<1.2mm) Al0.35Ga0.65As0.03Sb0.97 ‘barrier’ layer.
Figure 2 Schematic of the epitaxial semiconductor structure
The wells are arranged in groups within the active region to overlap with the antinodes of the standing wave intracavity field in order to optimize the pump absorption and signal extraction. This active region is grown on a high reflectivity multilayer 21.5 pair AlAsSb/GaAs Bragg mirror grown on a GaSb substrate. When pumped by a high power diode laser, this entire device then forms an ‘active’ mirror in an external laser resonator, which permits the cavity mode size to be optimized. The laser structure was grown by solid source MBE at around 500°C with the growth rates for the GaSb and AlAsSb DBR layers were are 0.62 and 0.76 monolayers per second respectively. Room-temperature reflectivity and photoluminescence (PL) plots for the sample are shown in figure 3.
Figure 3 Reflectivity spectrum (blue) and room temperature PL emission (red) from the VECSEL structure
3. Experimental set up
The VECSEL sample was first capillary bonded to a 4mm diameter, 250mm thick, plane-parallel, natural diamond heatspreader window. The bonded sample was then compression mounted between 125mm Indium foil sheets in a 1” diameter brass water-cooled heatsink. Pump light was provided by a 100mm core fibre-coupled diode laser (Lissotschenko Mikrooptik GmbH - LIMO) capable of delivering up to 14W at 980nm. The pump light was collimated and re-focussed onto the VECSEL using two short focal length (f=14.5mm, f=11mm) coupling lenses in a custom lens-tube assembly. The laser cavity configuration was typically the 3-mirror type shown in figure 4.
Figure 4 Typical 3-mirror cavity configuration of the 2.2mm VECSEL
The output coupler was nominally a 98% reflectivity flat substrate with wedged rear surface however separate measurements have shown the reflectivity to be closer to 96.5% at 2200nm.
4. Results
With the water bath temperature at 10ºC the laser operated up to 80mW output power for an input pump power of 5.5W. The power transfer and typical free running power spectrum is shown in figure 5.
Figure 5 (a). Power transfer at 10ºC (b). Typical free-running power spectrum for 5.5W incident pump
The laser operated with a circularly symmetric TEM00 mode throughout. With careful cavity alignment, the laser could be operated in single longitudinal mode up to 50mW. The transition to single longitudinal mode operation was accompanied by a characteristic small jump in output power and an obvious improvement in power stability [6], confirming the single spatial mode operation. Figure 6 shows the power transfer when the water bath was reduced in temperature to -10ºC. The laser operated at output powers up to 210mW at an input pump power of 10W.
Figure 6 Power transfer at -10ºC
When the output coupler mirror was replaced with an HR mirror and a 2mm quartz birefringent filter was inserted in the long arm of the cavity, the laser could be tuned over 100nm from 2083nm to 2192nm with output powers up to 5mW. Figure 7 shows the tuning spectrum obtained and the underlying FP etalon substructure caused by the diamond window.
Figure 7 Tuning spectra, blue diamonds – positions of maximum output power with BRF angle, coloured spectra – individual FP filtered spectral features at each BRF position.
The etalon effect caused by the diamond could easily be removed by using a wedged diamond window with an AR coating. The laser would then tune smoothly across this broad gain spectrum. The addition of a separate angle-tuned etalon and feedback electronics would enable stable, tunable single-frequency performance to be achieved across this wavelength range [7]. This would allow the remote sensing of some important atmospheric gasses such as N2O, CH4 and NH3.
5. Conclusions
We have shown high power operation of a 2.1 to 2.2mm VECSEL with high quality single spatial output mode. We believe the performance of such antimonide VECSELs is appropriate to meet the challenging demands of many applications in the 2-2.5mm spectral region. Future work will concentrate on enhanced thermal performance, greater efficiency, higher Watt-level output power and longer wavelength operation as well as the demonstration of continuous, stable, mode-hop free tuning over large wavelength ranges in the 2-2.5mm region. The demonstration of such versatile lasers will rapidly open up new avenues of deployment in this source weak region.
References
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