‘Best-case’ noise performance of feedback interferometers, potential for micromachine applications

We present a theoretical analysis of the noise performance of phase measuring feedback interferometers. We first analyse the operation of this kind of instrument and note that under certain circumstances bistability can occur. The bistable region should be avoided if possible when using feedback interferometry for high-accuracy measurement with low-input powers, as it can give rise to very high-noise levels. We then go on to investigate the effects of shot noise (on the interferometer output signal) and thermal noise (in the feedback loop), and relate these to the phase measurement accuracy of the interferometer. A ‘best-case’ calculation indicates that phase noise of about 0.005λ is possible at input powers in the nanowatt region. In practice, we expect that noise levels will be higher than predicted (particularly at high-input powers) due to the effects of vibration and air turbulence.     

A Linearized Method to Measure Dynamic Friction of Microdevices

We propose and evaluate a linearized method to measure dynamic friction between micromachined surfaces. This linearized method reduces the number of data points needed to obtain dynamic friction data, minimizing the effect of wear on sliding surfaces during the measurement. We find that the coefficient of dynamic friction is lower than the coefficient of static friction, while the adhesive pressure is indistinguishable for the two measurements. Furthermore, after an initial detailed measurement is made on a device type, the number of trial runs required to take the data on subsequent devices can be reduced from 200 to approximately 20.