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Lasers & Sources

Two-micron semiconductor disk lasers achieve higher powers

High-quality semiconductor disk lasers at wavelengths above 2μm promise new applications as they produce powers as high as 5W.
17 June 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.1169

There is an increasing demand for lasers with emission wavelengths at or above 2μm for long-range sensing, such as light detection and ranging (LIDAR) for surveying land, or free-space broadband communication by means of a modulated beam of light transmitted through the atmosphere. Other potential uses of long-wavelength lasers are security-related, or medical applications such as laser surgery. Most of these require both a high output power and a very-high-quality laser beam. Classical semiconductor diode lasers can reach high output powers at this wavelength,1 but fail to deliver the quality of beam necessary for e.g. a small focus spot or long transmission range.

An optically pumped semiconductor disk laser (OPSDL), also referred to as a vertical-external-cavity surface-emitting laser (VECSEL),2 can produce a high-quality, powerful beam in the 1μm wavelength range.3,4,5 Figure 1 illustrates the basic setup, which consists of a semiconductor gain medium placed in an external cavity configuration and pumped using standard laser diodes. The resonator is formed by an external output coupling mirror and a highly-reflective dielectric mirror, which is embedded into the OPSDL chip. However, until recently the realization of high-power OPSDLs for the 2μm range was considered impractical. In particular, relatively poorly understood group-III-antimonide semiconductor materials are required. In addition, there is a high quantum deficit between the 1μm pump and the desired >2μm emission wavelength. Therefore, much of the pump power is converted into heat, which has to be extracted, but this is made less efficient at a 2μm wavelength by the need for a thicker dielectric mirror. Despite these difficulties, we have made semiconductor disk lasers with up to 5W output power in continuous wave (CW) operation at 2.0μm and 2.25μm emission wavelengths.6,7

Figure 1. Schematic setup of an optically pumped semiconductor disk laser (OPSDL). For efficient heat removal, an intracavity heat-spreader is bonded to the OPSDL chip surface.

Figure 2. Compact >2μm OPSDL module, pumped with a fiber-coupled 980nm diode laser.

Precisely controlled growth is necessary to produce high-performance OPSDL chips. Molecular beam expitaxy (MBE) has been widely used for high-quality layered structures using gallium arsenide (GaAs) and related materials. We have developed a mature MBE growth process for structures based on group-III-antimonide materials. In addition, we used comprehensive post-growth analysis together with optical simulations of the OPSDL structures to obtain rapid feedback on growth conditions and a deeper understanding of the functionality of the OPSDL design. The third key to high power operation was improved heat management for our OPSDL chips. As reported previously,8 heat transfer can be significantly improved by using transparent intracavity diamond or silicon carbide (SiC) heat-spreaders. We used an SiC heat-spreader, which dramatically reduced the thermal resistance of our 2μm OPSDL chips (see Figure 2).

Figure 3. CW output power vs. absorbed pump power of a 2.25μm OPSDL, pumped at 980nm for different heat sink temperatures.

Figure 3 shows the power transfer characteristic of a semiconductor disk laser bonded to a SiC heat spreader and emitting at 2.25μm. A fiber-coupled 980nm diode laser module was used as a pump source. A maximum output of 3.4W was measured at a heat sink temperature of -10°C and an absorbed pump power of 21W. No pronounced thermally induced roll-over was observed under these conditions and the maximum output power was limited only by the available pump power. From the linear part of the power transfer curve recorded at −10°C, a slope efficiency of 23.8% was calculated resulting in a differential quantum efficiency of 54.5%. For higher heat-sink temperatures, the slope efficiency decreased slightly to, for example, 17.6% at 20°C and thermally-induced rollover occurred within the available range of pump powers. Nevertheless, we still obtained a maximum output power of >1.6W at 20°C.

OPSDLs emitting at a slightly shorter wavelength of 2.02μm have also been fabricated within the European Versatile Two Micron Light Source (VERTIGO) research project. Using a natural diamond heatspreader, a maximum output power of 5W CW was achieved at -10°C heat sink temperature and an absorbed pump power of 24W, and even 3.8W was recorded at +15°C (see Figure 4).

Figure 4. CW output power vs. absorbed pump power of a 2.0μm OPSDL, pumped at 980nm for different heat sink temperatures.

Figure 5 depicts the far-field profile of a 2.25μm semiconductor disk laser, captured by a camera sensitive in the appropriate part of the IR, showing the Gaussian-shaped, circular beam profile of the OPSDL. Measuring the beam quality parameter M2 in this configuration yielded values of 1.05, very close to the value of M2 = 1 for an ideal diffraction-limited Gaussian beam. The beam quality of an OPSDL changes slightly with resonator and pump optic alignments. Even with the resonator aligned for maximum output power rather than optimized beam quality, the beam quality parameter remains in the range of M2≤5.

Figure 5. Far-field profile of a 2.25μm OPSDL, showing the circular symmetric, Gaussian-shaped beam profile. The measured value for the beam quality parameter M2 was 1.05.

These results set a new standard for high-power, high-efficiency semiconductor disk laser in the long wavelength range above 2μm and enable this type of semiconductor laser to reach new applications. Compared to classical solid-state lasers for this wavelength range (holmium or erbium-doped solid-state or fiber lasers with fixed emission wavelength), the semiconductor disk laser has the advantage that any emission wavelength can be reached by changing the semiconductor composition. Further research includes the optimization of the semiconductor structure and the heat-sinking technique to further increase the output power, as well as the development of prototype lasers for free-space optical communication and long-range sensing.

This research was funded by the six framework program of the European Union within the project VERTIGO, project Number 034692. For further information please visit the project homepage (www.2micron-laser.eu) or contact the project coordinator at marcel. rattunde@iaf.fraunhofer.de

Marcel Rattunde, Benno Rösener, Nicola Schulz, Christian Manz, Joachim Wagner
Fraunhofer Institute for Applied Solid State Physics
Freiburg, Germany

Marcel Rattunde is a research team leader for infrared semiconductor laser at the Fraunhofer Institute for Applied Solid State Physics. His current work includes 2.Xμm diode lasers, laser modules and semiconductor disk lasers. He is the coordinator of the European research project VERTIGO on mid-IR semiconductor disk lasers.

Benno Rösener is completing his PhD at the Fraunhofer Institute for Applied Solid State Physics, working on GaSb-based optically pumped semiconductor disk lasers for the 2.Xμm wavelength range.

Nicola Schulz is a senior reseacher at the Fraunhofer Institute for Applied Solid State Physics. Currently, he is working on the development of (AlGaIn)(AsSb)-based mid-infrared semiconductor disk lasers and quantum cascade laser modules.

Christian Manz is an engineer in physics at the Fraunhofer Institute for Applied Solid State Physics where he is working on the molecular beam epitaxy of III-AsSb semiconductor heterostructures. His work includes the growth of (AlGaIn)(AsSb) VECSELs and quantum cascade lasers as well as the optimization of the respective growth conditions.

Joachim Wagner is deputy director of the Fraunhofer Institute for Applied Solid State Physics and head of the Optoelectronic Modules department. He is also professor at the Physics Department of the University of Freiburg. His current research interest is in III/V-semiconductor heterostructures and their application in optoelectronic devices both for the infrared and the visible/UV spectral range.

John-Mark Hopkins, David Burns
Institute of Photonics
Glasgow, UK

John-Mark Hopkins joined the Institute of Photonics in 2001 where he is now a senior researcher. His research interests currently include doped-dielectric and optically-pumped semiconductor disk lasers in the mid-infrared.

David Burns is an associate director and team leader for solid-state laser development at the Institute of Photonics. His research interests include optically-pumped semiconductor lasers, high-power modelocked solid-state lasers, stabilisation techniques and adaptive optics.