Over the past 20 years, we have developed arrays of custom-fabricated silicon and InP Geiger-mode avalanche photodiode arrays, CMOS readout circuits to digitally count or time stamp single-photon detection events, and techniques to integrate these two components to make back-illuminated solid-state image sensors for lidar, optical communications, and passive imaging. Starting with 4 × 4 arrays, we have recently demonstrated 256 × 256 arrays, and are working to scale to megapixel-class imagers. In this paper, we review this progress and discuss key technical challenges to scaling to large format.
An asynchronous readout integrated circuit (ROIC) has been developed for hybridization to a 32x32 array of single-photon
sensitive avalanche photodiodes (APDs). The asynchronous ROIC is capable of simultaneous detection and
readout of photon times of arrival, with no array blind time. Each pixel in the array is independently operated by a finite
state machine that actively quenches an APD upon a photon detection event, and re-biases the device into Geiger mode
after a programmable hold-off time. While an individual APD is in hold-off mode, other elements in the array are biased
and available to detect photons. This approach enables high pixel refresh frequency (PRF), making the device suitable
for applications including optical communications and frequency-agile ladar. A built-in electronic shutter that de-biases
the whole array allows the detector to operate in a gated mode or allows for detection to be temporarily disabled. On-chip
data reduction reduces the high bandwidth requirements of simultaneous detection and readout. Additional features
include programmable single-pixel disable, region of interest processing, and programmable output data rates. State-based
on-chip clock gating reduces overall power draw. ROIC operation has been demonstrated with hybridized InP
APDs sensitive to 1.06-μm and 1.55-μm wavelength, and fully packaged focal plane arrays (FPAs) have been assembled
At MIT Lincoln Laboratory, avalanche photodiodes (APDs) have been developed for both 2-μm and 3.4-μm detection using
the antimonide material system. These bulk, lattice-matched detectors operate in Geiger mode at temperatures up to 160 K.
The 2-μm APDs use a separate-absorber-multiplier design with an InGaAsSb absorber and electron-initiated avalanching
in the multiplier. These APDs have exhibited normalized avalanche probability (product of avalanche probability and
photo-carrier-injection probability) of 0.4 and dark count rates of ~150 kHz at 77 K for a 30-μm-diameter device. A 1000-
element imaging array of the 2-μm detectors has been demonstrated, which operate in a 5 kg dewar with an integrated
Stirling-cycle cooler. The APD array is interfaced with a CMOS readout circuit, which provides photon time-of-arrival
information for each pixel, allowing the focal plane array to be used in a photon-counting laser radar system. The 3.4-μm
APDs use an InAsSb absorber and hole-initiated avalanching and have shown dark count rates of ~500 kHz at 77 K but
normalized avalanche probability of < 1%. Research is ongoing to determine the cause of the low avalanche probability
and improve the device performance.
We have developed and demonstrated a high-duty-cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, room-temperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.
Geiger-mode avalanche photodiodes (APDs) can convert the arrival of a single photon into a digital logic pulse. Arrays of APDs can be directly interfaced to arrays of per-pixel digital electronics fabricated in silicon CMOS, providing the capability to time the arrival of photons in each pixel. These arrays are of interest for "flash" LADAR systems, where multiple target pixels are simultaneously illuminated by the laser during a single laser pulse, and the imaging array is used to measure range to each of the illuminated pixels. Since many laser radar systems use Nd:YAG lasers operating at 1.06 um, we have extended our earlier work with silicon-based APDs by developing arrays of InGaAsP/InP APDs, which are efficient detectors for near-IR radiation. 32x32 pixel arrays, with 100-um pixel pitches, are currently being successfully used in demonstration systems.
Two low-temperature-grown GaAs photomixers were used to construct a transmit-and-receive module that is frequency agile over the band 25 GHz to 2 THz, or 6.3 octaves. A photomixer transmitter emits the THz difference frequency of two detuned diode lasers. A photomixer receiver then linearly detects the THz wave by homodyne down conversion. The concept was demonstrated using microwave and submillimeter-wave photomixers. Compared to time-domain photoconductive sampling, the photomixer transceiver offers improved frequency resolution, spectral brightness, system size, and cost.
Two low-temperature-grown GaAs photomixers were used to construct a transmit-and-receive module that is frequency agile over the band 25 GHz to 2 THz, or 6.3 octaves. The photomixer transmitter emits the THz difference frequency of two detuned diode lasers. The photomixer receiver then linearly detects the THz wave by homodyne down conversion. The concept was demonstrated using microwave and quasioptical photomixers. Compared to time-domain photoconductive sampling, the photomixer transceiver offers improved frequency resolution, spectral brightness, system size, and cost.
We have combined silicon micromachining technology with planar circuits to fabricated room-temperature niobium microbolometers for millimeter-wave detection. In this type of detector, a thin niobium film, with a dimension much smaller than the wavelength, is fabricated on a 1-micrometers thick Si<SUB>3</SUB>N<SUB>4</SUB> membrane of square and cross geometries. The Nb film acts both as a radiation absorber and temperature sensor. Incident radiation is coupled into the microbolometer by a 0.37 (lambda) dipole antenna with a center frequency of 95 GHz and a 3-db bandwidth of 15%, which is impedance matched with the Nb film. The dipole antennas is placed inside a micromachined pyramidal cavity formed by anisotropically etched Si wafers. To increase the Gaussian beam coupling efficiency, a machined square or circular horn is placed in front of the micromachined section. Circular horns interface more easily with die-based manufacturing processes; therefore, we have developed simulation tools that allow us to model circular machined horns. We have fabricated both single element receivers and 3 X 3 focal-plane arrays using uncooled Nb microbolometers. An electrical NEP level of 8.3 X 10<SUP>-11</SUP> W/(root)Hz has been achieved for a single- element receiver. This NEP level is better than that of the commercial room-temperature pyroelectric millimeter-wave detectors. The frequency response of the microbolometer has a ln(1/f) dependence with frequency, and the roll-off frequency is approximately 35 kHz.