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
Avalanche Photodiode (APD) photon counting arrays are finding an increasing role in defense applications in laser radar
and optical communications. As these system concepts mature, the need for reliable screening, test, assembly and
packaging of these novel devices has become increasingly critical. MIT Lincoln Laboratory has put significant effort
into the screening, reliability testing, and packaging of these components. To provide rapid test and measurement of the
APD devices under development, several custom parallel measurement and Geiger-mode (Gm) aging systems have been
Another challenge is the accurate attachment of the microlens arrays with the APD arrays to maximize the photon
detection efficiency. We have developed an active alignment process with single μm precision in all six degrees of freespace
alignment. This is suitable for the alignment of arrays with active areas as small as 5 μm. Finally, we will discuss a
focal plane array (FPA) packaging qualification effort, to verify that single photon counting FPAs can survive in future
Arrays as large as 256 x 64 of single-photon counting avalanche photodiodes have been developed for defense
applications in free-space communication and laser radar. Focal plane arrays (FPAs) sensitive to both 1.06 and 1.55 μm
wavelength have been fabricated for these applications. At 240 K and 4 V overbias, the dark count rate (DCR) of 15 μm
diameter devices is typically 250 Hz for 1.06 μm sensitive APDs and 1 kHz for 1.55 μm APDs. Photon detection
efficiencies (PDE) at 4 V overbias are about 45% for both types of APDs. Accounting for microlens losses, the full FPA
has a PDE of 30%. The reset time needed for a pixel to avoid afterpulsing at 240 K is about 3-4 μsec. These devices
have been used by system groups at Lincoln Laboratory and other defense contractors for building operational systems.
For these fielded systems the device reliability is a strong concern. Individual APDs as well as full arrays have been run
for over 1000 hrs of accelerated testing to verify their stability. The reliability of these GM-APDs is shown to be under
10 FITs at operating temperatures of 250 K, which also corresponds to an MTTF of 17,100 yrs.
Arrays of photon-counting Geiger-mode avalanche photodiodes (APDs) sensitive to 1.06 and 1.55 μm wavelengths and as large as 256 x 64 elements on 50 μm pitch have been fabricated for defense applications. As array size, and element density increase, optical crosstalk becomes an increasingly limiting source of spurious counts. We characterize the crosstalk by measurement of emitted light, and by extracting the spatial and temporal focal plane array (FPA) response
to the light from FPA dark count statistics. We discuss the physical and geometrical causes of FPA crosstalk, suggest metrics useful to system designers, then present measured crosstalk metrics for large FPAs as a function of their operating parameters. We then present FPA designs that suppress crosstalk effects and show more than 40 times reduction in crosstalk.
Arrays of InP-based avalanche photodiodes operating at 1.06-μm wavelength in the Geiger mode have been
fabricated in the 128x32 format. The arrays have been hermetically packaged with precision-aligned lenslet arrays,
bump-bonded read-out integrated circuits, and thermoelectric coolers. With the array cooled to -20C and voltage biased
so that optical cross-talk is small, the median photon detection efficiency is 23-25% and the median dark count rate is 2
kHz. With slightly higher voltage overbias, optical cross-talk increases but the photon detection efficiency increases to
almost 30%. These values of photon detection efficiency include the optical coupling losses of the microlens array and
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.
Measurements are reported that demonstrate the first use of a photomixer-transceiver system for terahertz rotational spectroscopy of airborne molecules. The photomixer transmitter and receiver were coupled to free space with twin-slot antennas that were optimized for operation in the 1.1 - 1.7 THz range. Both atmospheric-pressure and low- pressure conditions were investigated.
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.
A three-dimensional metallodielectric photonic crystal (MDPC) that utilizes planar metal scattering elements in a dielectric medium has been studied in the microwave regime, both experimentally and theoretically. The metal elements are circular copper patches defined on thin dielectric sheets, which are alternately stacked with thicker polyethylene sheets to form a (111)-oriented face-centered-cubic lattice. A photonic stop band has been measured from this 'flat-atom' MDPC at 8.2 GHz with a rejection level of 18 dB per lattice period and a width of 50% of the center frequency. The photonic stop band persists over a broad range of angles. Finite-Difference Time-Domain calculations show excellent agreement with measured stop band characteristics, including a similar angular dependence and insensitivity to interplane registration. Variation of the stop-band characteristics with thickness of the dielectric layers has also been explored experimentally. Flat-atom MDPC results are compared with measurements made on 'spherical-atom' MDPC structures.
Low-temperature-grown (LTG) GaAs offers the combination of sub-picosecond photocarrier lifetime and high breakdown electric field (greater than 5 X 105 V/cm), and is grown in epitaxial films having excellent quality for microelectronic fabrication. A THz photoconductive mixer (photomixer) is formed on these films by patterning low- capacitance planar electrodes coupled to a coplanar antenna. The photomixer is conveniently pumped by two frequency-offset diode-laser beams focused on the exposed GaAs area between the electrodes. This paper summarizes the operational principles of the photomixer in contrast to a competing technique based on coherent three-wave photonic mixing. It then reviews different configurations of the photomixer as a laboratory tunable source for chemistry and metrology, and addresses some of the challenges in applying the photomixer as a local oscillator in portable spectroscopic and radiometric receivers.
We discuss the design of high-T, superconducting bolometers for use in a far infrared imaging array from wave- lengths 30 - 100 pm. Useful opportunities exist for imaging and spectroscopy with bolometer arrays made on micro-machined silicon membranes.
A sensitive high-Tc superconducting bolometer has been fabricated with a YBCO thin film thermometer on a 20 microns thick sapphire substrate. Electrical measurements showed no noticeable film degradation after bolometer fabrication. Optical measurements gave a noise equivalent power of 5 x 10 to the -11th W/sq rt Hz at 10 Hz and a responsivity of 22 V/W. This performance is comparable to that of the very best pyroelectric detectors. Significant improvement appears possible.