Displacement sensors are used extensively in a host of applications, from simple, straightforward extensiometers to measurement of acceleration, vibration or rotation (e.g. in gyroscopes).
Optical displacement sensors are known. They rely generally on the precise measurement of the micro-displacement of a sub-millimeter flexible component. The displacement is measured with optical techniques, which can be defined as either intensity- or interferometry-based [T. Wang, S. Zheng, and Z. Yang, Sensors and Actuators A 69, 134 (1998)]. Intensity modulation methods are commonly used, but provide inferior sensitivity relative to interferometry ones. Intensity modulation may be obtained by a relative movement between two optical fiber segments separated by a gap. The overlap area between the two segments determines the amount of energy transferred between the segments [H. Golnabi, Rev. Sci. Instrum, 70 2875 (1999)]. Such devices can provide a sensitivity of 1/90 [μm−1] [M. Abdelrafik, L. Pierre, M. Jeanine, R. Christine, and F. Pierre, Appl. Optics 34, 8014 (1995)]. An increased sensitivity may be obtained by placing a ball lens between the two fiber segments or by creating a ball lens on the fiber edge. Integration of such optical sensors in micro-opto-electro-mechanical systems (MOEMS) can be achieved by replacing the optical fibers with dielectric waveguides.
Photonic crystal based displacement sensors are very uncommon. Suh et al in U.S. Patent Application No. 20040080726 disclose a PC displacement sensor that comprises a mechanically tunable photonic crystal structure consisting of coupled photonic crystal slabs. Simulations show that the transmission and reflection coefficients for light normally incident upon such structures can be highly sensitive to nano-scale variations in the spacing between the slabs. Moreover, by specifically configuring the photonic crystal structures, the high sensitivity can be preserved in spite of significant fabrication-related disorders.
Existing MOEMS sensors are limited in the sensitivity that they can provide, and may not be used in applications requiring detection of extremely small absolute displacements. Suh's sensor is not planar and cannot be fabricated on a single chip, a serious deficiency. It would therefore be advantageous to have very sensitive photonic crystal displacement sensors, fully integrable on a single chip.
Photonic crystal waveguides (PCWG) are known, but only in stationary (fixed) configurations. Methods for light coupling into and out of PCWG structures are also well known in the art. The problem of adiabatic coupling from an external light source is also well characterized [se e.g. Y. Xu, R. K. Lee, and A. Yariv, Optics Lett. 25, 755 (2000)].