Abstract

A simple MEMS technology for wafer-scale integration of tunable VCSELs is presented in Fig. 1 a) [1]. The tunable VCSEL is composed of a ”half-VCSEL”, which is a VCSEL without top distributed Bragg reflector (DBR), and an external mirror, which is a micromachined membrane (”MEMS”). The GaAs-based half-VCSEL comprises a bottom DBR, an active region with 5 quantum wells (QWs), and an oxide aperture for current confinement. The etched mesa is capped with an antireflection coating (AR-c) and embedded in a low-k dielectric (BCB). Reflown photo-resist droplets are used as sacrificial layer and as preform for making curved micro-mirrors, as shown in Fig. 1 b). A dielectric DBR (7.5 pairs TiO2/SiO2) and an actuation layer (50 nm Ni) are deposited onto the half-VCSEL, and then the MEMS structure is etched. Finally, the mirror membrane is released by dissolving the sacrificial layer in acetone and removing the liquid in a critical point dryer. The VCSEL is tuned by injecting a heating current into the actuation layer on the flexible MEMS, which expands and shifts the cavity resonance towards longer wavelengths. In the following we present an optimized half-symmetric cavity design for singlemode emission. Compared to [1] the mesa diameter is enlarged (from 120μm to 200μm) to increase (i. e. flatten) the radius of curvature (RoC) from 420μm to 1.2 mm, while keeping the air-gap at around 3.7 μm. The threshold gain of fundamental mode and higher order mode during tuning are simulated (Fig. 1 c)) using a 3D model based on coupled mode theory [2]. The resulting gain difference for different oxide aperture diameters Doxis plotted in Fig. 1 d). The cavity supports the single fundamental mode for Dox10 μm, while the more expanded higher order transverse modes suffer from clipping at the oxide aperture (for Dox5 μm the fundamental mode is affected, too). A microscope image of a fully processed tunable VCSEL is shown in Fig. 1 e). Each chip contains an array of 8 ×8 tunable VCSELs with a small footprint of 290 μm × 400 μm. The spectrum of a tunable VCSEL with 10μm oxide aperture is shown in Fig. 1 f). The VCSEL emits in fundamental mode with a sidemode suppression ratio SMSR 25 dB over the tuning range of 12 nm. In comparison, conventional non-tunable 850-nm VCSELs with flat top DBR are singlemode only for Dox3 μm and usually operated at higher current densities.

© 2011 Optical Society of America

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