A custom CMOS sensor for pyramidal adaptive optics system

This paper reports a custom CMOS light sensor which can be used in a pyramid wavefront sensor based adaptive optics system for eye imaging. A pyramid sensor produces four pupil images and therefore, this CMOS light sensor consists of 4 arrays of 32 times 32 active reset pixels with synchronous analogue outputs. In an eye imaging adaptive optics system the light available for wavefront sensing is of the order of nW. Accordingly the photodiode capacitance size within the pixel is carefully chosen to detect this low light. The pixels were layed out with the fill factor of 56% and the chip consumes 56 mW of power during the read out. The custom CMOS sensor with an area of 12.6 sq. mm was fabricated using an Austria Micro Systems 0.35 mum C35B4 CMOS process. The sensor is mounted on a PCB and packaged in a box of dimensions 6 cm x 5.5 cm x 5 cm, which makes it feasible to install in an optical system. A dedicated high speed camera reading system has been constructed that can read upto 1000 frames per second. The realised complete camera system clearly detects the four pupil images created by a pyramid wavefront sensor.