SiOx-SiCz MEMS-DBR-Based Tunable Optical Devices

Abstract

Wavelength tunable devices are required in many fields like spectroscopy of gases, biomedical absorption experiments, wavelength division multiplexing in optical data networks, among others. Usually the devices are limited by the technology implemented to change the transmitted frequencies and are specific to their usage scenario. Thus extending the regime in which a single device can function, will reduce the need for many different devices with a narrow application window. For example, having a tunable laser for telecommunication networks around 1550 nm, that can support both L- and C-bands (1530 to 1625 nm). It opens much more flexibility, either by applying new coding mechanisms that require wavelength switching. Or simply by reducing storage requirements, because only one type of device serves as hot-backup for all channels. Investigation on tunable lasers on the basis of a VCSEL (vertical cavity surface emitting laser) in combination with a SiO-SiC MEMS-DBR, which offer up to 107 nm of tuning around 1550 nm, went on for more than two decades. It is time to overcome the material-specific limitation of those DBR materials. With a refractive index difference of 0.45 only 120 nm of high reflectivity around 1550 nm are supported by SiN and SiO. In this work, silicon carbide (SiC) is introduced as a replacement for SiN to grow DBR stacks with a refractive index contrast of 1, when paired with SiO. This increases the reflectivity stopband by more than a factor of 2, while the number of layer pairs is reduced for similar maximum reflectivities. So, in the end, wider tuning and smaller devices are feasible. The first step towards a MEMS-DBR tunable Fabry-Perot VCSEL is investigating the behavior of the new material by processing passive filters based on the same principle. MEMS stands for micro-electro-mechanical system. Here a Farby-Perot resonator consisting of two DBRs - one fixed, one movable - and an adjustable air-gap in between, enables continuous shifting of the resonant wavelength through displacement of the MEMS-DBR. After investigations on single layers deposited by low-temperature PECVD to enable compatibility with the active substrates, layer stacks are grown and structured into MEMS-DBRs. They can be actuated electro-thermally and electro-statically to tune the resonator cavity length and ultimately the transmission wavelength. First tests provided proof that the idea is working, but the tuning range was limited by the large cavity length. To increase the free spectral range, the cavity length had to be reduced. By creating a Comsol Multiphysics model for SiO-SiC MEMS-DBR, harnessing packages for structural mechanics and electro-thermal physics, the number of experimental testing could be lowered. Changing several variables led to the need of reducing the lateral MEMS-size by at least a factor of 2. After a redesign of the photolithography masks, new small-sized MEMS were processed successfully, reducing the air-gap to the desired lengths of 1 to 4 micrometer. Those new devices could be tuned over 250 nm, limited only by the measurement equipment. Moreover, increasing the number of layer pairs of the DBR decreased transmission linewidth below 30 pm (or 4 GHz) over a tuning range beyond 250 nm around 1550 nm. MEMS-DBR surface-micro-machining technology was furthermore transferred to both photodiode and half-VCSEL substrates successfully. Both SiO-SiN and SiO-SiC MEMS-DBR tunable photodiodes were able to detect and separate two neighboring lasers in a dense wavelength division multiplexing grid with 100 GHz (or 0.8 nm) spacing. SiO-SiC MEMS-DBR VCSELs showed a tuning range of only 57 nm around 1530 nm due to processing related issues. A much higher potential for wider tuning is available, but could not be achieved within the time frame of this work. Nevertheless, SiO-SiC MEMS-DBR VCSEL were found to be much less prone to temperature changes, considering emission wavelength shift, than SiO-SiN MEMS-DBR VCSEL published previously. This decreases environmentally induced temperature-dependent wavelength changes immensely.