The absorption lines of many environmentally important gases are found within the λ = 2–3μm wavelength range. Combined with the presence of an appropriate atmospheric window, these spectral lines represent useful diagnostics for gas spectroscopy,1 remote sensing, research in the life sciences, and free-space optical communications. Certain applications, such as laser-based tissue welding, require multiwatt optical sources with high beam quality, while others would benefit from light emitters generating ultrashort optical pulses (e.g., laser radar and free-space communications).
Figure 1. Schematic presentation of a semiconductor disk laser with a simple linear cavity. The smaller arrows indicate the direction of heat flow.
Gallium antimonide (GaSb)-based semiconductor disk lasers2 (sometimes called vertical-external-cavity surface-emitting lasers or VECSELs) could offer a new, compact, and cost-effective alternative for use with the 2–3μm spectral window, which is currently covered by solid-state and fiber lasers as well as optical parametric sources. The VECSEL design combines power scalability and excellent beam quality with wavelength flexibility that is offered by the semiconductor gain material. Additional optical elements can be used inside the laser resonator for spectral control, frequency conversion, and mode locking to produce ultrashort optical pulses.
The gain mirror, deposited onto a GaSb substrate, consists of thin semiconductor films that make up a high-reflectivity Bragg mirror and a gain region. It is typically pumped at an angle using low-cost, poor-beam-quality laser-diode bars. We deployed a transparent diamond heat spreader to cool the gain region efficiently under intense optical pumping (see Figure 1).
Our aim was to demonstrate generation of multiwatt output power, an extended tuning range, and short pulses near 2μm. All gain structures were grown monolithically by solid-state molecular-beam epitaxy. The high-power sample3 included a gain region containing 5×3 In0.2Ga0.8Sb quantum wells embedded in GaSb. Using 980nm pump radiation, we obtained over 4W of output power near room temperature at λ=2μm (see Figure 2). Lasing was observed at mount temperatures of up to 50°C. We pursued wide spectral tuning by creating a gain structure with asymmetric quantum wells. The gain region included three groups of quantum wells of the same composition but different thicknesses, i.e., 3×6.5, 3×9.5, and 2×16nm. Using a V-shaped laser cavity and an intracavity birefringent filter, we obtained a tuning range spanning 156nm from 1924 to 2080nm (see Figure 3).
Figure 2. Output power of the high-power disk laser at different mount temperatures as a function of incident pump power at 980nm. Inset: Beam profile at 4W output power.
Ultrashort optical pulses are often obtained through a passive mode-locking technique and by using a saturable absorber in the laser cavity. We demonstrated active mode locking near 2μm by pulse pumping the gain.4 The pump source included a low-power continuous-wave 1.57μm diode laser, a telecom modulator, and a 1W erbium-doped fiber amplifier to boost the pulses. Our experiments resulted in stable mode locking with pulse widths of approximately 170ps.
GaSb disk lasers can produce multiwatt output power near room temperature. They are characterized by a wide tuning range and can be actively mode locked to produce short optical pulses. These light sources can be fabricated compactly and cost effectively. In the future, they will likely compete with the current solid-state sources in the 2–3μm range. Our future research efforts will include further power scaling and passive mode-locking experiments.
Figure 3. Output power of our widely tunable GaSb disk laser as a function of wavelength.
The work presented here was conducted within the scope of a collaboration between the Optoelectronics Research Centre, Tampere University of Technology (Finland), the Technical Physics group at the University of Würzburg (Germany), and nanoPlus GmbH (Germany). The author would like to thank everyone who contributed to this work.
Optoelectronics Research Centre
Tampere University of Technology
2. L. Cerutti, A. Garnache, E. Genty, A. Ouvrard, C. Alibert, Low threshold, room temperature laser diode pumped Sb-based VECSEL emitting around 2.1μm, IEE Electron. Lett. 39, no. 3, pp. 290-291, 2003. doi:10.1049/el:20030192
4. A. Härkönen, J. Rautiainen, L. Orsila, M. Guina, K. Rößner, M. Hümmer, T. Lehnhardt, M. Müller, A. Forchel, M. Fischer, J. Koeth, O. G. Okhotnikov, 2-μm mode-locked semiconductor disk laser synchronously pumped using an amplified diode laser, IEEE Photon. Technol. Lett. 20, no. 15, pp. 1332-1334, 2008. doi:10.1109/LPT.2008.926921