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Optical Design & Engineering

Optimal Switching

The choice of actuation technology ultimately affects the optical performance of a tunable optical device.

From oemagazine October 2004
30 October 2004, SPIE Newsroom. DOI: 10.1117/2.5200410.0008

Optical switches remain a key element of communications networks. To date, the majority of next-generation optical switches have relied on micromirror approaches that use microelectromechanical systems (MEMS) technology. These technologies often require engineering tradeoffs that leverage silicon processing at the expense of optical system performance, however.

Micromirror devices have found commercial success in scanning technologies but face more challenges in communications switching. This is the result of using a component (tip/tilt micromirror) optimized around a process technology and adapting it to serve as both a precisely tunable analog device and as the primary optical element in the switch. Intrinsic limitations arise from the low-force, air-gap electrostatic technology used to move the micromirrors. Optical performance requirements also impose tight constraints on the dimensional accuracy of the micromirrors (flatness requirements, pre-stresses, thermal response, etc.) and coatings, which are not always compatible with MEMS processing technologies. Ultimately, these tradeoffs can result in less-than-ideal optical insertion loss, polarization, and wavelength-dependent sensitivities.

Piezoelectric materials are capable of very-high-force actuation, but are limited in stroke, as these load-displacement design curves show. More traditional approaches to optical switching use electrostatic actuation (creating forces using voltage across an air gap), which are unable to move bulk optics because of low force capability.

The solution is to restate the engineering problem such that it is possible to use high-quality bulk optics and decouple the optical performance from the process by which the switching occurs. One approach is beam steering, which requires an actuator with substantially higher force than electrostatics to implement this in a small package. Piezoelectric ceramics, solid-state actuation materials that change size according to voltage applied, are capable of such force.

Nothing comes for free, however, as traditional force-displacement design curves for piezo materials and electrostatic devices show (see figure). Piezoelectric actuators are capable of one million times more force per unit area than electrostatic devices. The displacement of piezoelectric materials is far less, however, only on the order of micrometers. The piezoelectric alone is not sufficient to move optical elements the amount required for reasonably sized switches (64 to 256 channels)—it is necessary to trade some force to obtain more motion.

Our beam-steering implementation uses a simple MEMS-based, impedance-matched mechanical lever to amplify the motion of the piezoelectric material. The new device still offers enough force to move bulk optical components, but now amplifies the several micrometers of piezoelectric motion into several degrees of optics-pointing capability. The design also eliminates micromirrors—every optical path now incorporates only two fibers, and two lenses. The approach removes multiple reflection and light-scattering elements, and results in switching with very low optical insertion loss, polarization-dispersion losses, and channel crosstalk. All of these characteristics are imperative for communication and instrumentation applications.

The design yields a further systems-level advantage. The overall mechanical system remains as much as a thousand times stiffer than electrostatic-based systems, producing frequency response profiles more compatible with closed loop control, and the rejection of external vibrations by passive techniques. Ultimately, the choice of an underlying technology can have a tremendous impact on the overall systems design and resulting device performance. In this case, piezoelectric actuation enabled a new class of device (beam steering), which circumvented the limitations of existing switch architectures and resulted in superior optical switching characteristics. oe

Nesbitt Hagood
Nesbitt Hagood is chief technology officer and Aaron Bent is vice president of marketing and business development at Continuum Photonics Inc., Billerica, MA.