Singlemode tunable VCSELs with integrated MEMS technology

Abstract

A simple MEMS technology for wafer-scale integration of tunable VCSELs is presented in Fig. 1 a) [1]. The tunableVCSEL is composed of a “half-VCSEL”, which is a VCSEL without top distributed Bragg reflector (DBR), and an externalmirror, 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 followingwe 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 1.2mm, while keeping the air-gap at around 3.7µm. 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 Dox is plotted in Fig. 1 d). The cavity supports the single fundamental mode for Dox ≤10µm, while themore expanded higher order transverse modes suffer from clipping at the oxide aperture (for Dox ≤5µm 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 290µm×400µm. 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 Dox ≤3 µm and usually operated at higher current densities.