Characterization of diffractive optical lenses for use in MEMS integrated optical monitoring.

摘要

Micro Electro Mechanical Machines (MEMS) are finding use in an increasing number of applications. Many of these applications, especially safety critical ones, require continuous knowledge of the operational status of the MEMS device over its lifetime. In-situ monitoring of the operational status of these devices is better suited to optical methods which provide a non-invasive way of continuously determining device position as a function of time. This data is central to determining the MEMS state and operational condition. Whether the optical source is internal to the MEMS package or external to the package and introduced via optical fiber, lenses will be required within the package to focus the MEMS optical probe beam to and from the test device. The need for hybrid co-integration with the MEMS die within its package results in physical dimension and focal length constraints on the lenses used and points toward the applicability of diffractive optical lenses to realize the goal of MEMS in-situ integrated optical monitoring. The spot diameter must be of a width comparable to any lateral displacement that must be measured so that acceptable resolution of the motion can be observed. Diffractive lenses are capable of meeting all of these requirements. Previous work designed and fabricated Fresnel binary phase diffractive lenses in borofloat glass substrates as a means to achieve the required lens function for either a visible (632 nm wavelength) or near IR (1310 nm) probe beam. This work undertakes the full optical characterization of these previously fabricated lenses, compares their performance (spot size, focal length, efficiency) to that expected from theory, and determines if they meet the original lens design criteria, These results enable an initial assessment of these lenses for use in MEMS in-situ lifetime optical monitoring.