Optical beam position active sensing and control using acoustooptic satellite beams

This paper presents novel techniques and experimental results for precision active sensing and control of the position of a laser beam using dynamic acoustooptic diffraction patterns. Sensing of the acoustooptic (AO) diffraction pattern higher order beams is used to determine the position of the main optical beam (i.e., zero order beam) while minimally perturbing the main beam. In the experimental system a reduced Q acoustooptic modulator was used in the axial mode to generate symmetrically deviated +1 and -1 diffraction order satellite beams with a small fraction of the zero order power. Sensing the crossing time of scanned satellite beams past differential split bicell detectors to derive precise zero-crossing signals was used to precisely locate them and infer the main beam position. The symmetric configuration is particularly effective for accurately determining and calibrating the main beam position. A Bragg regime acoustooptic deflector was used to steer the position of the main beam over a range of angles, controlling it to a desired position, as well as stabilizing it by means of control of Bragg deflector RF frequency derived from the sensing signals. Experimentally, a 1 milli-radian angular divergence laser beam at 632.8 nm optical wavelength was controlled and settled to within (±1/60) beam diameter in 1 sense/control feedback cycle of 80 msec. The cycle time was limited by the present control electronics; however, it could be much faster: fundamentally response time is limited by the acoustooptic beam transit time on the order of a microsecond. The general method of this paper could be applied at any optical wavelength where suitable acoustooptic modulators and photosensors can be obtained. The method can also be readily extended to operation in two axes, providing two-dimensional position sensing and control.