Coupled waveguide integrated optic segment piston sensor for the GMT

Integrated optic segment piston sensor (IOPS) for the Giant Magellan Telescope (GMT) uses single mode laser written waveguides to measure segment piston (axial alignment) 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. The waveguide inputs are positioned at the focus of a lenslet array with the same hexagonal geometry as the GMT pupil. 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 magnitude is directly related to the phase difference at the waveguide input, originating from the segment piston. Three coupling regions achieve this by splitting the light into one reference, and two signal lines. The reference provides a measurement of the amount of light coupled into the input waveguide, while each signal line is passed on to other couplers to be interfered with light from two neighbouring segments.