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Chapter 5:
Optical Waveguides and Silicon-Based Micromachined Architectures
In the past decade, rapid advances in silicon-based microelectromechanical (MEMS) technology have enabled the realization of a variety of novel functions in the micromechanical world similar to the functions performed by microelectronics. One motivation for developing MEMS is their small scale and the ease of integrating them with electronic circuits and sensors, resulting in miniaturized and smart microsystems with moving parts. The dimensional scale of MEMS devices is immediately compatible with the size of integrated optics (IO) and micro-optical devices such as optical fibers, laser diodes, channel waveguides, and diffractive or refractive microlenses. This scale (1 to 500 mm) is compatible with the size of optical beams, and is appropriate for controlling or manipulating optical radiation. MEMS technology exploits photolithographic techniques to fabricate microstructures with extremely high dimensional tolerances, precise positioning on the chip, and well-controlled microactuation. This potential is suitable for fabricating precision-defined optical components and offers relatively easy alignment procedures for optical parts. For all these reasons, MEMS technology has benefited from the integration of microstructures with free-space micro-optics and guided wave optics. This category of photonic devices constitutes a new extension of MEMS, called microoptoelectromechanical systems (MOEMS).Many novelMOEMS architectures, including micromirrors, microlenses, modulators and switches, microchoppers, microplatforms for integrated optics, fiber optics, scanners, optically driven actuators, integrated optical sensors, and other devices with optical functions have been developed. In this chapter we focus our attention on guided-waveMOEMS, combining micromachined structures and optical waveguides on silicon substrates. Since guided-wave MOEMS are fabricated by batch process, these devices are potentially low cost, and their monolithic structure improves their reliability. This approach of integrating optics with micromechanics has applications for integrated sensors, microactuators, and optical communication. A wide variety of fabrication technologies have been developed for optical waveguides on silicon. These include chemical vapor deposition (CVD), flame hydrolysis deposition (FHD), and spin-coating deposition of polymers. Waveguide structures are based on depositing silica layers for the cladding and doped silica, silicon nitride or silicon oxinitride, and polymers for the core layer, for example. The core layer is commonly structured by reactive ion etching (RIE). The waveguide-related structures include directional couplers, Y junctions, multiplexers, and possibly insertion of standard fiber optics via V-groove alignment procedures. Optical waveguides may be integrated with micromechanical deformable structures such as diaphragms, beams, bridges, and membranes. The micro-optomechanic element can serve as a sensor to detect a physical parameter of the mechanical deformation, as well as a microactuator, and also as an optical modulator or switch.
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