Andrew S. Huntington

Recent advances in avalanche photodiodes

Until the early 2000s, the avalanche photodiode (APD) was widely deployed in high-performance optical receivers that operated up to 10 Gb/s. In subsequent years, the use of APDs for high-capacity systems declined as a result of their limited gain bandwidth, the transition to coherent detection, and the development of high-efficiency modulation techniques. Recently, the rapid growth of optical-fiber communications systems that utilize baud rates up to 25 Gb/s represented by a 100-Gb/s Ethernet has led to a resurgence of research on APDs and the development of low-noise APDs with enhanced gain bandwidth. This paper presents a brief review of APD fundamentals and describes some of the recent advances.

Low-noise impact-ionization-engineered avalanche photodiodes grown on InP substrates

We report low noise multiplication region structures designed for avalanche photodiodes grown on InP substrates. By either implementing a single heterostructure or using a pseudograded structure in the multiplication region, better control of spatial distribution of impact-ionization for both injected and feedback carriers can be achieved; localization of the carrier impact ionization process has resulted in very low excess noise.

Terahertz electro-optic wavelength conversion in GaAs quantum wells: Improved efficiency and room-temperature operation

A 4-μm-thick sample containing 50 GaAs/AlGaAs asymmetric coupled quantum wells was driven with a strong terahertz (THz) electric field of frequency ωTHz and probed with a near-infrared (NIR) laser of frequency ωNIR. The THz beam modulated the probe to generate sidebands at ωNIR+nωTHz, where n is an integer. Up to 0.2% of the NIR laser power was converted into the n=+1 sideband at 20 K, and sidebands were observed up to room temperature. The strong THz fields also induced changes in the NIR absorption of the sample.

Voltage-controlled wavelength conversion by terahertz electro-optic modulation in double quantum wells

An undoped double quantum well (DQW) was driven with a terahertz (THz) electric field of frequency ωTHz polarized in the growth direction, while simultaneously illuminated with a near-infrared (NIR) laser at frequency ωNIR. The intensity of NIR upconverted sidebands ωsideband=ωNIR+ωTHz was maximized when a dc voltage applied in the growth direction tuned the excitonic states into resonance with both the THz and NIR fields. There was no detectable upconversion far from resonance. The results demonstrate the possibility of using gated DQW devices for all-optical wavelength shifting between optical communication channels separated by up to a few THz.

Filtering characteristics of hybrid integrated polymer and compound semiconductor waveguides

This paper reports a study on a compact filter fabricated using hybrid integration of compound semiconductors and polymers. A GaAs epilayer is glued onto a polymer channel waveguide forming a highly asymmetrical directional coupler. This approach results in a narrow band filter due to very different dispersion characteristics of the compound semiconductor and the polymer materials. Furthermore, fiber coupling loss has been significantly reduced, since the input and output coupling is done through the polymer waveguide. Filtering characteristics can be engineered by changing the thickness and the length of the semiconductor epilayer. This can be done precisely using etch stop layers and noncritical lithography. The spectral response of such a filter can also be tuned electronically either using the electro-optic properties of the compound semiconductor or the thermo-optic properties of the polymer.

Probabilistic analysis of linear mode vs. Geiger mode APD FPAs for advanced LADAR enabled interceptors

To meet evolving ballistic missile threats, advanced seekers will include a multi-modal imaging capability in which a passive single- or multi-band infrared focal plane array (FPA) shares a common aperture with an active laser radar (LADAR) receiver - likely, a photon-counting LADAR receiver that can resolve photon times of arrival with sub-nanosecond resolution. The overall success of such a system will depend upon its photon detection efficiency and sensitivity to upset by spurious detection events. In the past, to perform photon counting functions, it has generally been necessary to operate near infrared (NIR) avalanche photodiode (APD) FPAs in Geiger Mode. Linear Mode APDs could not provide enough proportional gain with sufficiently low noise to make the photocurrent from a single photon detectible using existing amplifier technology. However, recent improvements in APDs, sub-micron CMOS technology, and concomitant amplifier designs, have made Linear Mode single-photon-counting APDs (SPADs) possible. We analyze the potential benefits of a LADAR receiver based on Linear Mode SPADs, which include: 1) the ability to obtain range information from more than one object in a pixels instantaneous-field-of-view (IFOV), 2) a lower false alarm rate, 3) the ability to detect targets behind debris, 4) an advantage in the endgame, when stronger reflected signals allow dark current rejection via thresholding, and 5) the ability to record signal intensity, which can be used to increase kill efficiency. As expected, multiple laser shots of the same scene improves the target detection probability.

Multi-Gain-Stage InGaAs Avalanche Photodiode With Enhanced Gain and Reduced Excess Noise

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise and stable linear-mode operation at much higher avalanche gain than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1000.

Time resolved gain and excess noise properties of InGaAs/InAlAs avalanche photodiodes with cascaded discrete gain layer multiplication regions

To predict pulse detection performance when implemented in high speed photoreceivers, temporally resolved measurements of a 10-stage InAlAs/InGaAs single carrier multiplication (SCM) avalanche photodiode (APD)s avalanche response to short multi-photon laser pulses were explained using instantaneous (time resolved) pulse height statistics of the devices impulse response. Numeric models of the junction carrier populations as a function of the time following injection of a primary photo-electron were used to create the probability density functions (pdfs) of the instances of the avalanche buildup process. The numeric pdfs were used to generate low frequency gain and excess noise models, which were in good agreement with analytic models of multiple discrete low-gain-stage APDs and with measured excess noise data. The numeric models were then used to generate the instantaneous and cumulative instantaneous low order statistics of the instances of the impulse response. It is shown that during the early times o...

Densely packed pie shaped vertical-cavity surface-emitting laser array incorporating a tapered one-dimensional wet oxidation

Wavelength-division-multiplexing (WDM) photonic integrated emitter (PIE) vertical-cavity surface-emitting laser (VCSEL) arrays are fabricated using a post growth wet oxidation technique. High-density integration of WDM VCSEL arrays is possible by combining the technique of one-dimensional oxidation and large-scale tapered oxidation. Eight channels are integrated into a circle of 60 /spl mu/m in diameter. Seven channels are found to operate as lasers. The lasing wavelengths range from 823 to 836 nm corresponding with the distance between the VCSEL mesa and the tuning trench. The successful demonstration of incorporating wet oxidation into the wavelength control of the PIE VCSEL array opens a new way of fabricating mask-defined densely packed WDM VCSEL arrays.

High-speed photon counting with linear-mode APD receivers

HgCdTe and InGaAs linear-mode avalanche photodiodes (APDs) were fabricated and tested for properties suitable for high-speed photon counting when integrated with commercially available 2-GHz resistive transimpedance amplifiers (RTIAs). The 2.71-μm, 100-μm-diameter HgCdTe APDs were fabricated in using an n+/p vertical carrier transport architecture designed to reduce carrier drift time and facilitate high-speed operation. At 215 K, a gain of 100 was measured with an excess noise of 2.5. The InGaAs/InAlAs APDs were fabricated using two absorber alloy compositions, one optimized for 950-1300 nm operation and the other for 950-1550 nm operation. Both were fabricated using multiple, cascaded gain regions that allowed for high gain and low avalanche-induced shot noise. Gain exceeding 6000 was observed, and the excess noise factor was measured to be below 20 at a gain of M = 1200 (effective k ~ 0.03). The InGaAs/InAlAs APDs were integrated into receivers consisting of a multi-gain-stage APD coupled to a commercial 2-GHz RTIA and were operated as thresholded photon counters. At a linear gain of M = 1800, a single photon detection efficiency greater than 85% was measured at a maximum count rate of 70 MHz; at a linear gain of M = 1200, single photon detection efficiencies greater than 20% were measured at maximum count rates of 80 MHz. At the temperature tested, 185 K, the receivers dark count rate (DCR) is dominated by electronic amplifier noise from the TIA for low threshold settings, and by dark counts from the APD at high threshold settings.