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The design of a detector array intended for use as a Shack-Hartman wavefront sensor is presented. The chip will output voltages that represent local tilts and currents that represent inferred piston information by processing the optical signals from a lenslet array. Each pixel in the array contains four phototransistors, arranged in a square, along with transistors needed to perform algorithms necessary to output signals. Each pixel has four built-in amplifiers to output a voltage for each of the four phototransistors. By combining these voltages off-chip, tilt information is obtained, and allows for positive and negative tilts to be output from a single-supply chip. All pixels contain a self-biasing circuit, allowing tilt information to be consistent across all pixels for apertures that are not uniformly illuminated. Algorithms for inferring piston information are generated by using CMOS transistors in current mode. The piston information is output as a current with a magnitude that is proportional to the actual piston value. The pixels are selected by means of bit-parallel row and column select. The outputs consist of four voltages, four currents to monitor the phototransistor behavior, and one current representing piston. There are inputs that allow the user to set a global piston offset, and overall bias for overriding the amplification of the tilt voltages. Simulations of this design suggest refresh rates in excess of 2kHz are easily attainable. The design of this wavefront sensor is optimized for low light levels and high refresh rates. Prototype detectors will be fabricated using a 0.5micron CMOS process. This detector could greatly simplify the process of wavefront reconstruction, and could even allow for direct hardware control of deformable mirrors in adaptive optics systems with an order of magnitude more channels than are currently used.
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