Figure 1: Hermetically packaged APD array with microlens, InP array, CMOS ROIC, and thermoelectric cooler. The APDs are backilluminated mesa structures with diameters ranging from 15-30 µm, depending on the application.

Abstract:
Arrays of photon-counting avalanche photodiodes enable laser-communications receivers with unprecedented sensitivity at 1.06-µm wavelength. Arrays with 64 elements were fabricated in the InGaAsP/InP materials system and were bump-bonded to a custom CMOS integrated circuit. The integrated circuit uses a novel nonblocking architecture to continuously report both time-of-arrival for incoming photons as well as their spatial location on the array. Near room temperature, the best detectors have: 45% photon detection efficiency, 65-kHz dark count rate, and a 1.6-µs reset time to avoid after-pulsing.

Introduction
Laser communication from deep space to Earth is being explored by NASA for back-hauling data from planetary probes [1]. Enabling technologies include fiber lasers compatible with large-alphabet pulse-position modulation, iterative codes for forward-error correction, and photon-counting avalanche photodiodes (APDs). Arrays of APDs were initially developed for imaging laser radars [2], but are now also being developed for laser communications.
In this talk, we describe ongoing APD work at MIT-LL in support of the Mars Laser Communications Demonstrator (MLCD)—a 1.06-µm-wavelength link from Mars to Earth. The link budget for MLCD drives our current research in improving photon-detection efficiency (PDE), dark-count rate (DCR), and Reset Time (Tr). It also drives the architecture of the read-out integrated circuit (ROIC).

Figure 2: ROIC development process: (a) Concept of non-blocking read-out when pixel 7 fires. (b) Floor plan. (c) 100x100-µm pixel cell (mask layout). (d) 0.35-µm CMOS chip (e) Logic analyzer trace demonstrating functionality.

InGaAsP/InP APDs
For systems applications, the size, weight, and power of the APD detector is attractive. Figure 1 shows a hermetic package including a CMOS ROIC, an InP APD array [3], and a GaP microlens array for improving the fill-factor. Figure 1 also shows the device profile of our mesa-etched APDs, which are grown by metalorganic chemical vapor deposition. Near infrared light enters through the InP substrate, traverses the InP multiplier, and is absorbed in an InGaAsP layer whose alloy composition is chosen to absorb 1.06-µm light.
Near room temperature, the best 1.06-µm detectors have: 45% PDE, 65-kHz DCR, and 1.6-µs Tr, the time the APD must be held below breakdown to avoid after-pulsing. The breakdown voltage is typically 50V and the amount of over-bias voltage when the PDE is measured is 5V.

CMOS Read-Out Integrated Circuit (ROIC)
The ROIC has some LADAR heritage in the design of its pixel cell [4]. The pixel cell does the following: applies the 5V over-bias that arms the APD, time-stamps the arrival of a photon with a fast pseudo-random counter, rests the APD below breakdown for either 1.6 or 3.2 µs, and re-arms the APD afterwards. In addition, it can switch its counter into an output path to the frame-store without disturbing the other pixels.
The unique features of this design are the autonomous operation of the pixel cell and the non-blocking read-out method. Figure 2 describes this operation and shows how the concept progressed to a tested 0.35-µm CMOS chip that operates with a clock frequency of 311 MHz. It is this clock that sets the timing resolution of the ROIC. As of this writing, 8x8 InP APD arrays on a 100-µm pitch have been In bump-bonded to the ROIC and initial testing is in progress.

Acknowledgements
This work was sponsored by NASA under United States Air Force Contract No. F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Air Force.

References
[1] D. M. Boroson et al, “MLCD: Overview of NASA’s Mars Laser Communications Demonstration System,” SPIE 5338, 16-28 (2004).
[2] M. A. Albota et al, “Three-dimensional imaging laser radars with Geiger-mode avalanche photodiode arrays,” MIT Lincoln Lab Journal 13, 351-370 (2002).
[3] K. A. McIntosh, et al, “InGaAsP/InP avalanche photodiodes for photon counting at 1.06 µm,” Appl. Phys. Lett. 81, 2505-2507 (2002).
[4] B. F. Aull, et al, “Geiger-Mode avalanche photodiodes for three-dimensional imaging,” MIT Lincoln Lab Journal, 13, 335-350 (2002).