A prototype hybrid micro-electro- mechanically tunable optical filter (MEM-TF) based partly on an electrostatic-actuation principle has been built and tested as an essential component needed for the further development of a prototype micro-electro- mechanically tunable vertical- cavity surface emitting laser (MT-VCSEL). In turn, MT-VCSELs are needed as essential controllable-wavelength sources in diverse advanced optoelectronic devices and systems including, for example, wavelength-division multiplexers in fiber-optic communication systems; lightweight, compact, portable spectroscopic instruments for detecting chemical and biological warfare agents; holographic memory devices; fiber-optic sensors; optoelectronic signal-processing systems; and remote-sensing systems. The development of the prototype MEM-TF also has additional significance in that it demonstrates the merit of the hybrid approach (in comparison with the monolithic-integration approach) to design and fabrication of some advanced optoelectronic devices.

The Prototype Hybrid MEM-TF is shown here in partly schematic cross section and not to scale. Omitted from this view are (1) flexure arms that support the upper subassembly, and (2) polySi dimples that reduce stiction and prevent electrostatic pull-in.

As used here, “hybrid” refers to the design and fabrication of a MEM-actuated optoelectronic device as an assembly, in contradistinction to a monolithic unit: In the hybrid approach the optical component(s) and the micro-electro-mechanical actuation component(s) of a device are first fabricated as separate units, then bonded together. The hybrid approach makes it possible to overcome limitations on design, fabrication, and function inherent in the monolithic-integration approach. In the hybrid approach, there is greater design flexibility in that designs of different components can be optimized separately and the components can be made from different materials that are not amenable to fabrication of the device a monolithic unit. Moreover, in the hybrid approach, unlike in the monolithic-integration approach, defective components identified in wafer-level pre-testing can be discarded prior to completing fabrication of devices.

One notable limitation on design and functionality of a monolithic electrostatically actuated optoelectronic device is that the applied electrostatic-actuation potential must be kept below a value that produces a stroke of about d/3, where d is the distance across an electrostatically variable gap when the electrostatic-actuation potential is zero. Application of a potential large enough to produce a greater stroke causes electrostatic pull-in, which results in catastrophic failure of the device. In the hybrid approach, the design electrostatic pull in potential and the design gap distance can be chosen independently of each other. The hybrid approach also enables the incorporation of polycrystalline silicon (polySi) dimples that can serve to prevent electrostatic pull-in and to reduce stiction (which also results in failure).

The hybrid MEM-TF (see figure) includes a distributed Bragg reflector that is spring-supported, on flexure arms, at a suitable distance from a gold reflector. A piston electrostatic actuator comprising electrodes made of polySi is used to vary the distance between the filter and the reflector and thereby vary the resonance wavelength of the filter. The distributed Bragg reflector, comprising alternating layers of Al0.4Ga0.6As and GaAs, has lateral dimensions of 250 by 250 μm and a thickness of 4.92 μm. The distributed Bragg reflector was fabricated separately, then flip-bonded to the piston electrostatic actuator using SU-8 photoresist as an adhesive. In a test of tunability, the resonance wavelength of the hybrid MEM-TF was found to vary over the range from 936.5 to 989.5 nm when the electrostatic-actuation potential was varied over the range from 0 to 10 V.

This work was done by Edward M. Ochoa of the Air Force Institute of Technology for the Air Force Research Laboratory.


This Brief includes a Technical Support Package (TSP).
Hybrid Micro-Electro-Mechanically Tunable Optical Filter

(reference AFRL-0066) is currently available for download from the TSP library.

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