Tuning dynamics of a > 70 nm continuously tunable MEMS-VCSEL with a hybrid curved mirror

Long-wavelength tunable vertical-cavity surface-emitting lasers (VCSEL) are an attractive tool for many applications, as optical fiber communications and spectroscopy. Large and continuous wavelength tuning and tuning speed are two important attributes of MEMS-VCSEL. They are essential for e.g. wavelength modulation spectroscopy techniques in gas sensing. The MEMS-VCSEL combines an InP-based cavity incorporating 7 InGaAlAs quantum wells, a buried tunnel junction (BTJ = 12 μm) [1] and a concave MEMS-DBR which can be electro-thermally actuated for wavelength tuning. An anti-reflection coating is put on top to eliminate the phase coupling limitation and to enable an extension of the tuning range at expense of slightly reduced optical output power [2]. To achieve a further extension of the tuning range, a new MEMS-DBR has been developed. The MEMS-DBR is a hybrid mirror composed of semiconductor (3.5 pairs) AlGaAs/GaAs and dielectric (8 pairs) SiO/SiN layers. The hybrid mirror combines the advantages of both materials. Due to a high refractive index contrast between SiO/SiN, the mirror has a high theoretical reflectivity of 99.8% and a large stop bandwidth of 224 nm defined by a reflectivity larger than 98.0 %. To enable electro-thermal actuation of the MEMS-DBR, the semiconductor layers are doped with silicon which enables a heating current to flow through the suspension beams of the membrane. The length of the air-gap, which is included in the cavity, increases due to the thermal expansion thus the wavelength can be tuned. The concave bending of the membrane is caused by an implemented compressive mechanical stress gradient inside the layers (In-doping). The membrane parameters are as follow: Ø=190 μm, beam width=58 μm, -length=150 μm, radius of curvature R=4.1 mm, air-gap=6.8 m. More information on the device structure and technology as well as semiconductor and dielectric MEMS-DBRs can be found in [1-3].