This paper will describe recent developments in the state-of-the art for InP/InGaAs Geiger Mode focal plane arrays developed at MIT Lincoln Laboratory. Fabrication details of highly-dense arrays on a 25-micron pitch (256 x 256) will be presented, along with techniques developed to suppress crosstalk in neighboring pixels. These dense arrays are hybrized to highly efficient read-out circuits capable of simultaneous photon-counting imaging and photon time extraction for multiple user-defined regions of interest. Matching 256 x 256 microlens arrays are attached to the hybrized APD array/ROIC. Performance data and applications of the focal plane arrays will be discussed
A model for the turn-on characteristics of separate-absorber-multiplier InP-based Geiger-mode Avalanche Photodiodes (APDs) has been developed. Verilog-A was used to implement the model in a manner that can be incorporated into circuit simulations. Rather than using SPICE elements to mimic the voltage and current characteristics of the APD, Verilog-A can represent the first order nonlinear differential equations that govern the avalanche current of the APD. This continuous time representation is fundamentally different than the piecewise linear characteristics of other models. The model is based on a driving term for the differential current, which is given by the voltage overbias minus the voltage drop across the device’s space-charge resistance RSC. This drop is primarily due to electrons transiting the separate absorber. RSC starts off high and decreases with time as the initial breakdown filament spreads laterally to fill the APD. With constant bias voltage, the initial current grows exponentially until space charge effects reduce the driving function. With increasing current the driving term eventually goes to zero and the APD current saturates. On the other hand, if the APD is biased with a capacitor, the driving term becomes negative as the capacitor discharges, reducing the current and driving the voltage below breakdown. The model parameters depend on device design and are obtained from fitting the model to Monte-Carlo turn-on simulations that include lateral spreading of the carriers of the relevant structure. The Monte-Carlo simulations also provide information on the probability of avalanche, and jitter due to where the photon is absorbed in the APD.
Dense array slab-coupled optical waveguide lasers (DASCOWLs) consist of several hundred single-mode SCOWL
lasers on a monolithic bar. Near diffraction-limited output of the SCOWLs is preserved with spacing down to 40μm.
Greater than 200W CW operation of a 4% FF, 100-element, 100μm-pitch, centimeter wide DASCOWL bar has
been demonstrated, corresponding to <2W/emitter in array format. We have also demonstrated near 500W
continuous wave (CW) operation from a 10% fill factor (FF) 1-cm wide, 1cm long DASCOWL bar which contains
250 emitters, with a 40μm pitch. The goal of 2W/emitter, 500W/bar represents a 5X increase above the conventional
10-emitter, 10% FF broad area laser diode bar that operates at 10W/100μm-emitter. Some of the reported
DASCOWL performance benefits from SRL’s low thermal resistance EPIC heat sinks.
Slab-coupled optical waveguide lasers (SCOWLs) and amplifiers (SCOWAs) are inherently low-confinement structures
with large nearly-circular modes that are easily coupled to optical fibers or collimated for free-space applications.
Recently SCOWL powers have increased to 3 W by increasing the cavity length to 1 cm and improving the heat
removal. SCOWAs are coherently combined using active phase control to achieve a very high-brightness source. Our
coherent beam combining system consists of single-pass amplifiers with angled-facet SCOWAs that suppress feedback.
Single-pass, 5-mm long, SCOWAs have now been demonstrated with 1.5 W CW output with only 50 mW seed power.
Arrays of 47 SCOWAs have demonstrated a raw power of 57 W with 50 mW of seed power per element. A coherent
beam combining demonstration is currently being 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.
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
A long-standing challenge for semiconductor lasers is scaling the optical power and brightness of many diode lasers by
coherent beam combination. Because single-mode semiconductor lasers have limited power available from a single
element, there is a strong motivation to coherently combine the outputs of many elements for applications including
industrial lasers for materials processing, free space optical communications, and defense. Despite the fact that such a
coherently-combined source is potentially the most efficient laser, coherent combination of semiconductor lasers is
generally considered to be difficult, since precise phase control is required between elements.
We describe our approach to coherent combination of semiconductor lasers. The Slab-Coupled Optical Waveguide
Laser (SCOWL), invented at Lincoln Laboratory, is used as the single-mode diode laser element for coherent
combination. With a 10-element SCOWL array, coherently combined output power as high as 7 W in continuous wave
using an external cavity has been demonstrated, which is the highest output level achieved using a coherent array of
semiconductor lasers. We are currently working on a related approach to scale the coherent power up to 100 W.
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.
Linear arrays of slab coupled optical waveguide lasers (SCOWL) are ideal sources for beam combining of array
elements using techniques such as wavelength beam combining (WBC) and possibly coherent beam combining (CBC).
SCOWL array elements have very high brightness, low divergence nearly diffraction limited output beams. Arrays of up
to 1.2 cm in width containing as many as 240 elements have been demonstrated. In this presentation, the packaging
techniques developed to ensure proper performance of SCOWL arrays will be described, with particular emphasis on the
application to beam combining. A commercial high performance micro impingement cooler (MIC) was used to provide
thermal management for these arrays. Based on performance data for this cooler, a numerical thermal model was
constructed and used to investigate the thermal performance for several packaging schemes. In order to promote
uniform optical performance of SCOWL array elements, assembly procedures, which included fluxless soldering using
In and AuSn solder alloys, along with the use of thermal expansion matching materials were investigated. These
techniques resulted in minimal contraction (≈2 &mgr;m) and smile (≈1 &mgr;m) of the laser bar during the packaging procedure.
Precise control of these parameters is required in order to minimize any detrimental impact on the resultant WBC beam
quality. CBC of SCOWL arrays requires phase control of the array elements. Array packaging providing for individual
electrical addressability of the array elements has been developed and demonstrated, allowing for phase control by
We have been developing a high power, high brightness semiconductor diode laser concept, the Slab-Coupled
Optical Waveguide Laser (SCOWL). This laser concept is based upon slab coupling, in which a large, multimode
waveguide is converted to a large, single mode waveguide by means of slab coupling of the higher order waveguide
modes. SCOWL devices feature large, nearly circular mode sizes (≈4 x 4 &mgr;m and larger) and low modal loss, leading
to low gain per unit length, allowing for the construction of long (≈1 cm cavity length) devices. These characteristics
allow for high single mode output power. For 980-nm AlGaAs/InGaAs/GaAs-based SCOWL devices, we have
demonstrated > 1 W CW output power in a single spatial mode, with brightness levels of > 100 MW/cm2-str. We have
constructed high power arrays of SCOWL devices with bar widths of 1 cm and cavity lengths of 3 mm, and have
demonstrated > 90 W under CW operation. By using the technique of wavelength beam combining (WBC), which is
analogous to wavelength division multiplexing in optical communications, we have been able to combine the outputs
from the elements of a SCOWL array to obtain 50 W peak power (30 W CW) with nearly diffraction-limited beam
quality. These SCOWL arrays combined by WBC have demonstrated record single bar brightness levels, 3.6 GW/cm2-
str. The WBC SCOWL approach is inherently scalable, and offers a route to obtaining kW-class, nearly diffraction
limited output from an all-diode laser source. We have also recently extended single SCOWL devices to the multi-Watt
regime, demonstrating 2.8 W CW output power from a 980-nm SCOWL with a novel design.
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.
A high-brightness semiconductor diode laser design, which utilizes a slab-coupled optical waveguide region to achieve several potentially important advances in performance, is described and experimentally demonstrated using simple rib waveguide quantum well structures. These lasers operate in a large, low-aspect-ratio, lowest-order spatial mode, which can be butt coupled to a single-mode fiber with very high coupling efficiency. The acronym used for this new type of structure is SCOWL, taken from "slab-coupled optical waveguide laser". Initial results on 1.3μmm InGaAsP/InP and 980-nm AlGaAs/InGaAs SCOWLs are presented.
Recent progress in tapered high-brightness lasers emitting in the near infrared region from 1.3 to 2.0 micrometers is reviewed. Improved power and beam quality are obtained for tapered lasers operating near 1.55 micrometers using Gaussian distributed lateral carrier injection profiles. Results for high-brightness 9-element arrays of tapered lasers emitting near 2.0 micrometers are included. Also included is a discussion of the use of mass-transported microlenses for collimating the output of the astigmatic tapered devices and coupling them into optical fibers.
Semiconductor lasers with tapered gain regions are well suited for applications requiring high output powers and good spatial mode quality. In this paper, the development of 1.5-micrometer InGaAsP/InP quantum well (QW) material suitable for this type of device will be discussed and initial results on high-power tapered lasers fabricated in this material presented. Several different 1.5-micrometer QW laser structures grown by metalorganic chemical vapor deposition are being evaluated. Structures containing three compressively strained QWs have shown transparency current densities JT as low as 170 A/cm2 and net gains of approximately equal 40 cm-1 at less than 800 A/cm2. With 5QWs, these parameters were JT approximately equals 275 A/cm2 and net gain approximately 40 cm-1 at 600 A/cm2, respectively. Self-focusing at high current densities and high intensity input into the taper section has been identified as a fundamental problem in these devices that has to be dealt with. Tapered devices with a 0.6-mm-long single-mode gain section coupled to a 1.4-mm-long tapered region fabricated in 5QW material have shown CW output powers of greater than 1.0 W at 3.8 A. Approximately 80% of the 1 W is in the near- diffraction-limited central lobe of the far field-pattern.
Tapered structures fabricated in InGaAsP/InP 1.3-micrometers quantum-well material have been evaluated as lasers and as high-gain high-saturation-power amplifiers. The devices, which had a 1-mm-long ridge-waveguide gain section followed by a 2-mm-long tapered gain region, demonstrated > 1 W output power as lasers, with > 85% of the power in a central diffraction-limited lobe. The amplifiers had an unsaturated gain of 26 dB at 2.0 A and about 30 dB at 2.8 A. Saturated output power at 2.8 A was > 750 mW. At 2.0-A drive current and approximately equals 10-mW input power, the relative intensity noise of the amplified signal was <EQ -160 dB/Hz at frequencies >= 2 GHz.
The OMVPE growth and performance of graded-index separate-confinement heterostructure strained quantum-well InGaAs-AlGaAs diode lasers are reviewed. Broad-stripe lasers have exhibited Jth as low as 60 A cm-2 for a cavity length L equals 1500 micrometers and differential quantum efficiency (eta) d as high as 90% for L equals 300 micrometers . Similar heterostructures have been used to fabricate traveling wave amplifiers with a laterally tapered gain region that emit over 1 W cw in a nearly diffraction-limited spatial lobe at 0.98 micrometers , linear arrays of 200-micrometers -long uncoated ridge-waveguide lasers with average threshold currents of 4 mA and (eta) d approximately 90%, and high-power broad-stripe lasers with power conversion efficiency exceeding 50% at 75 degree(s)C.
Two-dimensional surface-emitting AlGaAs diode laser array modules, each containing two 1 sq cm hybrid arrays, have been fabricated and tested. For quasi-CW operation, peak output powers greater than 300 W/sq cm appear to be easily achievable at repetition rates up to 500 Hz. The measurements also indicate that CW output powers of 100-150 W/sq cm can be achieved from these arrays.