Wavefront Sensors

The Shack-Hartman Sensor is the most common wavefront sensor used today due to its simplicity and manufacturability. Using an array of miniature lenses called “lenslets,” the sensor splits light into a number of small beams which is then focused onto a CCD camera. As the incident wavefront is aberrated by the lenslet, the focused spot on the CCD camera moves. Through simple geometry using the displacement of the focused spot and the focal length of the lenslet, the local tilt of the wavefront is calculated by the control system—typically, a computer and the control algorithm software. The control system then calculates the shape required to compensate the wavefront and sends the information to the wavefront corrector.

Compensating a wavefront brings us to the truly adaptive element in adaptive optics, the wavefront corrector. The most prevalent technology used for this function is a deformable mirror – a thin, flexible, reflective surface whose shape is controlled through a variety of competing technologies. The selection criteria for a deformable mirror is application based. Fundamental specifications for deformable mirror systems are spatial resolution, spatial frequency, speed, stroke, and surface finish.

Spatial resolution is a measure of the corrective capabilities of a deformable mirror, i.e. the degree of wavefront complexity for which the deformable mirror is capable of correcting. Spatial resolution is determined by actuator count as well as inter-actuator coupling (the influence of a deformed actuator upon its neighbors). Current technology ranges from 19 actuators (entry-level membrane deformable mirror) to over 4000 actuators (MEMS deformable mirrors), with inter-actuator couplings ranging from 0-100%.

Stroke describes the maximum actuator deflection for a given deformable mirror and presents a significant tradeoff with resolution. Low-resolution bimorph and ferromagnetic deformable mirrors can provide stroke as high as 50 µm, but are only suitable for simple, low order aberrations. Meanwhile, most microscopic, vision science, and laser shaping applications require 1 to 4 µm stroke and higher-order correction, which is achievable with high-resolution MEMS deformable mirrors.