<title>Control Loop Sensor For An Optical Synthetic Aperture</title>

Abstract

Control Loop Sensor for an Optical Synthetic ApertureDavid H. Jordan Dan R. Morrow Rex W. GoransonThe BDM Corporation, 1801 Randolph Rd. f SE, Albuquerque, New Mexico 87106AbstractThe Research and Development Laboratory at BDM Albuquerque, in cooperation with and for the Air Force Weapons Laboratory has designed and developed an optical phase measurement instrument capable of providing a phase error signal with a resolution of 3 nm at an update rate of 1 kHz. This phase error measurement of better than l/160th of a wave detects the optical path difference (OPD) between two laser beams operating in the 0.5 micron region. The error measurement is used to translate mirrors which adjust the OPD to maintain phase coherence between the beams. By using several mirrors, each corrected to a reference, a large aperture can be synthesized. The 1 kHz update rate provides a high speed control signal for closing the mirror control loop.The design is part of a proof-of-concept program which will identify the feasibility of using feedback control techniques to simulate a large optical aperture with multiple small apertures.IntroductionIn order to simulate a large aperture with multiple small apertures, a feedback control loop is needed. This adjusts the position of individual apertures to maintain phase coherence among the multiple optical paths. Maximizing coherence requires the measurement resolution of the phase difference between the optical paths (optical path difference, OPD) be much less than one wavelength. This OPD adjustment is made by translating turning mirrors to correct the path length. The control signal to these OPD adjustment mirrors must have a bandwidth higher than the mirrors operational bandwidth to prevent aliasing errors.Analyses by R. Buttsl indicated a 1/20 of a wave measurement resolution was necessary to provide the necessary error correction signal. Further analysis by S. Cusumano of the AFWL indicated 1/80 wave measurement precision would ensure the required resolution. Additional analyses by S. Cusumano showed that a 100 Hz error update rate would prevent lab disturbances from affecting the measurement. This 100 Hz update rate translated into a 1 KHz measurement calculation update rate to avoid aliasing errors. The 1/80 wave resolution and the 1 KHz calculation rate became the design constraints for the phase error sensor.The phase error measurement is based on the wave interference principle demon­ strated by Young's slit experiment. This experiment shows that light from two sources passing through two narrow, adjacent ports will cause a well defined fringe pattern to be produced in the far field if the two sources are not independent. This inter­ ference pattern contains information on the phase difference between the two light sources. Work by R. Butts^ indicated an appropriate algorithm could be developed which could extract the phase difference information from the interference pattern.The adaptation of this phase measurement algorithm to a synthetic aperture experi­ ment required the development of a device capable of collecting the interference fringe patterns and operating on the collected data with the algorithms to produce an OPD adjustment signal. The requirement for this sensor/data processor was the genesis for the BDM designed phase measurement data processor (PMDP) .The basic concept for the PMDP was specified by R. Carreras of AFWL. specification called for an optical sensor designed around a Fairchild CCD line scan camera and a processor to be designed around a Motorola 68000 microprocessor. The choice of these components minimized the design risk by providing off-the-shelf operability with advanced capabilities. The following paragraphs detail the design of the PMDP and its present capabilities.109