VCSELs head farther into the infrared
Sometimes it's very difficult to take a successful device and replicate it in another materials system. The GaAs VCSEL is a case in point: although it has been a great commercial success, transferring the design to longer wavelengths has been fraught with difficulty. The main stumbling block is the design and fabrication of lattice-matched mirrors with sufficiently high reflectivity.
However, European institutions have been figuring out ways of getting round this problem, and at May s Indium Phosphide and Related Materials Conference in Versailles, France, several speakers outlined novel designs in the 1.5–2.3 µm range.
Alexander Mereuta from Ecole Polytechnique Fédérale de Lausanne described developments in 2 µm VCSELs made by wafer fusing. His group has previously used this approach to construct 1.3 and 1.55 µm equivalents, which are under commercialization at spin-off BeamExpress.
Mereuta s InP VCSEL is designed for sensing water and CO2, which have absorption lines at 2003 and 2004 nm. According to him, distributed-feedback edge-emitting lasers could also be used for this application. However, these power-hungry devices suffer from mode-hopping when their wavelength is tuned, as well as a wider beam profile and a narrower tuning range.
An Aixtron MOCVD tool is used to grow the active region on an InP substrate and each of the two undoped GaAs/AlGaAs distributed Bragg reflectors (DBRs) on separate GaAs substrates. The structures are then fused together. This allows the independent optimization of the active region and the mirrors, and it circumvents problems associated with a monolithic design.
The active region contains InGaAs/InAlGaAs quantum wells that generate the lasing, top and bottom intracavity contacting layers, a reverse-biased junction for lateral current confinement, and a PN and tunnel junction for carrier injection (figure 1).
Chemical etching forms a 9 µm diameter tunnel junction that defines the device s electrical and optical aperture. An n-doped InP spacer layer on top of this structure provides a platform for a top intracavity contact and creates a PN blocking junction that surrounds the tunnel junction and promotes current flow through the aperture.
The researchers selected a quantum-well composition with a room-temperature peak photoluminescence of 1950 nm. This is short of the target wavelength, but it s a good choice. That s because device operation increases the cavity s temperature by 50 °C and brings the emission peak to the target operating wavelength of the laser.
This produced a 0.5 W continuous-wave laser with a side-mode suppression ratio in excess of 30 dB and a maximum operating temperature of 46 °C. Tuning rates of 0.31 nm/mA and 0.14 nm/°C enabled tuning from 2002 to 2007 nm and detection of water and CO2 in air.
Gas sensing is also a target for Alexander Bachmann s team at the Walter Schottky Institute (WSI), Technical University of München, Germany. This team is developing GaSb VCSELs that can span 2.3–3.0 µm. This spectral range offers stronger gas-absorption lines, but it is out of reach for InP-based devices.
WSI produced its VCSEL by bonding a Si/SiO2 dielectric DBR and an MBE-grown epitaxial structure that comprises a GaSb/AlAsSb DBR and an active region (figure 2). Again, a tunnel junction is included in the design to define the aperture size.
The team evaluated the device s performance after mounting the VCSEL onto a temperature-controlled copper stage. Its 5.5 µm diameter aperture laser operated up to a heat sink temperature of 55 °C and had a threshold voltage of 0.95 V. The relatively high voltage indicates that some series resistance is present, which could be caused by an unoptimized active region or a poor-quality epitaxial DBR.
Altering the pump current tunes the VCSEL from 2248 to 2256 nm at a rate of 0.87 nm/mA. The output is just 75 µW and efforts are under way to identify the cause of low power. Prime suspects are a mismatch of the gain spectrum to the cavity resonance and an active region of insufficient quality.
Bachmann revealed that the German electronics giant Siemens has used one of the lasers produced by WSI for gas sensing, and has detected carbon monoxide with a concentration of 57 ppm.
Future targets include improvements in current confinement and an extension of the emission wavelength to 2.5–3.0 µm.
In another presentation from the same technical session, Jean-Philippe Tourrenc from the Photonics and Nanostructures Laboratory in Marcoussis, France, detailed the efforts of a partnership with the Alcatel-Thales III-V lab to develop vertical external cavity surface emitting lasers (VECSELs). Unlike VCSELs, this type of laser has a large gap between one of its mirrors and the active region, and it can produce much higher single-mode output powers, thanks to a larger spot size for the laser beam.
This French partnership has been developing a high-power 1.55 µm source for telecommunications, such as optical sampling and coherent communications. The output power of electrically pumped VECSELs is limited by the thermal conductivity of the materials, but the researchers have started to address this with a hybrid metal-dielectric mirror and a CVD diamond structure.
Their VECSEL featured an InGaAlAs quantum-well active region and a 15-pair GaAs/AlAs mirror. A dielectric stack formed the external mirror.
Tourrenc revealed that pumping the VECSEL optically with an 800 nm multiwatt laser produced an output of more than 120 mW. In comparison, an equivalent device built on SiC produced a thermally limited peak power of just 30 mW.
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