Widely tunable micromachined filters using stress control of multilayer mirrors

Abstract

We proposed and demonstrated a micromachined filter with a strain control layer, which gives us novel functions including temperature insensitive operation, thermal wavelength tuning, wavelength trimming and 2-D multi-wavelength integration. In this paper, we present the design and the fabrication of micromachined thermally tunable filters with a low tuning voltage. In our micromachined micromachined filter, an air gap is formed between GaAlAs/GaAs DBRs with an upper DBR mirror freely suspended above the substrate by a cantilever structure. A novelty in our devices is to add a GaAs or GaAlAs thermal strain control layer on the upper DBR. We can freely control the temperature dependence of the proposed MEMS cavity. Either temperature insensitive operation or wide wavelength tuning induced by temperature change can be realized. Also, we fabricated a micromachined thermally tunable filter with a heating element. There are two electrodes integrated on the top p-type doped strain control layer of this filter for heating the cantilever. When a voltage is applied between the two electrodes resulting in heating, the micromachined cantilever moves due to thermal strain. The proposed structure enables thermal wavelength tuning either for red shift or blue shift. The amount of wavelength tuning is controlled by the length and the thermal capacity of the cantilever. We can expect much lower tuning voltage than conventional electrostatic force tuning scheme. We measured the tuning characteristics of fabricated filters with changing an applied voltage between two electrodes. We could obtain blue-shift wavelength tuning of over 50 nm with an applied voltage of 6 V.