The design, technology and characteristics as well as sensing applications of micromachined long-wavelength
(~1.55μm) tunable vertical-cavity surface-emitting lasers are reported. The laser combines an active optical
component (so-called half-VCSEL) and an agile mechanical component (MEMS) in a hybrid assembly. Electrothermal
actuation expands the enclosed air-gap and continuously shifts the cavity resonance towards longer
wavelengths. A curved mirror membrane is deployed to solely excite the desired fundamental mode with high
output power and high sidemode suppression. The comparatively high stiffness of the MEMS lifts its mechanical
resonance frequency to values around 150 kHz as measured by laser Doppler vibrometry under electrostatic
actuation and - at the same time - reduces its susceptibility to Brownian motion. Laser linewidths as narrow
as 32MHz are demonstrated by using the self-heterodyning technique and the wavelength dependent linewidth
variation is presented for the first time. After successful absorption spectroscopy experiments under steady
laboratory conditions the tunable VCSEL is used for trace gas detection in a combustion process. Preliminary
experimental results are shown and practically encountered problems are discussed.
In this paper, a novel two-chip-concept for an electrically pumped and micro-mechanically tunable vertical-cavity surface-emitting laser (VCSEL) operating in the 1.55 μm wavelength range is presented. One chip contains the active region with 5 quantum wells based on the material system AlGaInAs/InP and a buried tunnel junction (BTJ) to provide current confinement and waveguiding. A dielectric mirror forms the back reflector. The second chip consists of a curved mirror membrane that can be displaced by electro-thermal heating. The main advantage of this approach is that both parts can be optimized separately. Packaged laser devices show continuous-wave operation at room temperature with an output power of up to 200 μW and very good side mode suppression in the order of 45 dB. Single-mode operation was observed across a tuning range of more than 30 nm.
InP-based VCSELs (Vertical Cavity Surface Emitting Lasers) are interesting light sources for applications in spectroscopy and fiberoptical communication. Reviewed are devices with a buried tunnel junction (BTJ) and a dielectric backside reflector directly integrated on a electroplated gold-heatsink in the InGaAlAs/InP material system covering the wavelength range from 1.3 to 2.0 μm. The BTJ accomplishes both current confinement to the active region and wave-guiding by the refractive index distribution to achieve low threshold currents. Furthermore it allows for substitution of p-doped device parts by more suitable n-doped material. This approach already proved excellent device performance such as 7 mW output power (multi-mode) and good high temperature characteristics such as 0.5 mW at 80°C for 1.55 μm. Modulation at 10 Gbit/s was also demonstrated. Since the BTJ VCSEL concept covers a wide wavelength range, there is a high-potential field of applications in Tunable Diode Laser Absorption Spectroscopy (TDLAS). Demonstrated are representative measurements of NH3 and HCl. A specialty of TDLAS with VCSELs is the ability for rapid concentration determination with a time resolution up to the megahertz regime. Recent results and further developments of the device structure are also discussed.