Integrated optic segment piston sensor for the GMT

Integrated Optic Piston Sensor (IOPS) for the Giant Magellan Telescope (GMT) [1] uses single mode laser written waveguides to measure segment piston of the GMT primary mirrors. Light in the H-band (from 1.5 to 1.6 μm) incident on each segment originating from an off-axis guide star is coupled into separate laser written single mode waveguides in a fused silica substrate. Light from neighbouring segments is interfered in several coupling regions where waveguides are in spatial proximity allowing coupling, in order to produce an interference signal at the output. The output signal intensity is directly related to the phase difference at the waveguide input, originating from the segment piston. IOPS is located in the on-instrument wavefront sensor of the GMT, which includes a deformable mirror for low order aberration correction. IOPS is designed to work with the Laser Tomography Adaptive Optics (LTAO) [2] system. Small residual tip-tilt modes caused by atmospheric dispersion at the IOPS input will reduce performance significantly as these modes are seen by IOPS as segment piston. This aliasing effect also exists for higher order modes, but with a reduced magnitude. The input of IOPS is dithered by a small amount with a steering mirror in order to measure and minimise residual tip-tilt. Segment piston sensitivity of less than 35 nm RMS is achieved with Strehl at the IOPS input greater than 15% with a detector integration time of 7.5 seconds and dithered input.