Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications,
gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at
wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions
of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for
high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing).
The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with
side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of
6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of
both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation
bandwidth of 5.5GHz with an output power of 0.8mW.
We present 1.55 μm short-cavity buried-tunnel-junction VCSELs (Vertical-Cavity Surface-Emitting Lasers) with single
mode output powers of 6.7 mW at 20°C and 3 mW at 80°C, respectively. Although the device had been predominantly
optimized for high-power applications and a proper heat management, we are also observing a 3dB-cut-off frequency of
more than 11 GHz and side mode suppression ratios (SMSRs) beyond 54 dB over the whole temperature range. The
tuning range of the devices can be increased from 7 nm based on gain tuning to several tens of nanometers when
replacing the top DBR by a micro-electro-mechanical system (MEMS) distributed Bragg reflector (DBR) composed of
semiconductor or dielectric material being thermally actuated for changing the cavity length. These devices are perfectly
suitable for telecommunication and gas sensing applications and represent outstanding devices for the so called tunable
diode laser absorption spectroscopy (TDLAS) technique.
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.
We present the characterization of silicon oxide (SiOx) and silicon nitride (SiNx) films deposited by inductively
coupled plasma chemical vapour deposition (ICP-CVD) at low temperature (< 100°C). A tunable optical Fabry-
Perot (FP) -filter operating at a wavelength around 1.5μm is realized. It is hybridly assembled with two dielectric
distributed Bragg reflectors (DBR). One of the DBR- mirrors is intentionally curved using the intrinsic stress
inside the films. Our aim is the development of a tunable surface micromachined VCSEL with a curved dielectric
mirror. Therefore ICP-CVD with a low deposition temperature is used for SiOx and SiNx films. As a first step
the realization of a tunable bulk- mircomachined optical FP- filter is presented. The refractive index, deposition
rate, stress and etching rate in buffered hydrofluoric acid (BHF) of thin dielectric films (<500 nm) in dependence
on deposition temperature and on gas flow ratio are investigated. The knowledge of the deposition characteristics of the dielectric films is used to realize DBRs with a given curvature that are applied to electrothermally actuated, optical tunable FP- filters. The presented filter has a free spectral range of 29 nm, an insertion loss of 10 dB and a full width half maximum of 0.16 nm.
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