Specialized avalanche photodiode arrays enable adaptive optics uses
The Advanced Imaging Technology Group at Lincoln Laboratory has fabricated arrays of new high-fill-factor Geiger-mode avalanche photodiodes (GM-APDs) for application as optical wavefront sensors. High-quality wavefront sensors are key enablers for laser communications and high-resolution astronomy. GM-APDs differ from conventional photodetectors because they produce a digital pulse in response to a single incoming photon. This capability eliminates analog circuit readout noise and enables sensitive photon counting.
Lincoln Laboratory previously applied GM-APDs to laser radar and imaging applications; however, those devices had relatively low fill factor and were therefore unsuitable for applications as a wavefront sensor. A wavefront sensor converts the incoming light wave to an array of light spots whose locations indicate the wavefront's shape. A high-fill-factor detector array is needed to measure the locations of these light spots.
This new array consists of 2 × 2 subarrays of detectors, known as quad cells, each with high fill factor. Any photoelectron originating in the interior region of a quad cell is collected and detected by the closest APD. Each APD is connected to a digital pixel circuit that counts the detection events. The fact that no photoelectrons are "lost" between diodes enables use of these specialized arrays in adaptive optics systems. The four count values can be used to compute the displacement of the light spot from the center of the quad cell.
|This is a false-color contour plot of the total number of detection events from all four detectors in a quad cell as a function of the location of a small light spot. (Red denotes high counts; blue, low counts.) Photoelectrons incident on the boundary areas between adjacent APD regions are collected and detected smoothly, demonstrating that this design is well suited for wavefront sensor use.
The high-fill-factor arrays will eventually be used in laser-guide-star adaptive optics in which a laser serves as an artificial star, providing a reference point for astronomical imaging. This star can be positioned anywhere the telescope can point, opening up a greater area of the sky to adaptive optics.
Posted January 2009top of page