Intensity Photon-Counting Imager

Another application of the Geiger-mode avalanche photodiode (GMAPD) is passive photon-counting imaging. One particularly stressing example is ground-based astronomy and surveillance of space, in which the images show constantly changing aberrations introduced by atmospheric turbulence. Adaptive optics systems sense the distortion of the wavefront using a bright star or an artificial beacon created by projecting a laser up through the atmosphere and correct for this distortion using deformable mirrors. Any readout noise in the detector array used for wavefront sensing directly limits performance.  

For wavefront sensing applications, Lincoln Laboratory has developed specialized arrays of Geiger-mode detectors hybridized to complementary metal-oxide semiconductor (CMOS) photon-counting circuits. Each 2 × 2 subarray of the focal plane is a high-fill-factor Shack-Hartmann sensor. The  wavefront-sensing GMAPD devices were specifically designed to accept photoelectrons from a wide area of silicon.

Figure 1. Cross section of first-generation APD device design, useful for LADAR applications.Figure 1. Cross-section view of first-generation APD device design, useful for LADAR applications.

Figure 1, in contrast, shows a cross-sectional view of two early types of GMAPD devices suitable for laser detection and ranging (LADAR) applications.  Photoelectrons are able to enter the high field multiplication area between the blue heavily doped p+ layer and the green n+ diode only if they originate inside the dotted lines; photoelectrons originating outside this area are shunted directly to the n+ diode (illustrated by the black arrow), do not cause runaway multiplication, and so are not detected.

Figure 2. Cross-section view of high-fill-factor APD array, suitable for wavefront sensor applications.Figure 2. Cross-section view of high-fill-factor APD array, suitable for wavefront sensor applications.

Figure 2 shows a cross section view of the new APD structure, designed specifically to operate in a Shack-Hartman wavefront sensor. This figure shows how the blue heavily doped p+ area was modified so that photoelectrons are now collected from a much larger silicon region and accelerated through the multiplication area. 

In a wavefront sensor, a closely spaced 2 × 2 array of detectors is used to locate the center of a small spot of light. This location is determined by weighing the number of photoelectron detection events that have taken place in each member of the 2 × 2 array. To locate the center of a small spot of light successfully, it is critical that photoelectrons be collected from a wide silicon area and, particularly, that any photoelectron originating in the region between two adjacent APD devices be collected and detected by the closest APD. The photoelectron represented by the black arrow in Figure 2 is shown reflected from the modified p+ implant layer, allowing it to make its way into the multiplier region of the adjacent APD device and be successfully detected.

Each GMAPD detects the presence of a photon and registers the count in its 10-bit counter. The information is digitized by the pixel and can be read out rapidly without a noise penalty, enabling the wavefront to be sensed with low latency (<20 µs). 

Figure 3. Results of an experiment in which a small spot of light is directed at the central boundary of a 2 × 2 wavefront sensor cell.Figure 3. Results of an experiment in which a small spot of light was scanned across a 2 × 2 wavefront sensor cell.

Figure 3 shows the result of an experiment in which a small (~ 5 µm) spot of light was scanned across a 2 × 2 detector array. The boundaries of each APD area are below the black lines drawn on the diagram (each detector area is a 50 µm square). Each colored pixel in the figure represents the total signal from all four detectors summed together. For each location of the scanned light spot, many detection experiments were run. Red areas represent high numbers of collected photoelectrons for that spot location, while blue areas represent low photoelectron counts.

It is clear that photoelectrons incident near and on the boundary areas between adjacent APD regions are collected and detected with no abrupt changes in the total collected signal here. A quad array with a smooth overall collection function, as demonstrated in Figure 3, is well suited for wavefront sensor use.

 

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