William H. Farr

Strong Loophole-Free Test of Local Realism

We performed an loophole-free test of Bells inequalities. The probability that local realism is compatible with our results is less than 5.9×10^-9.

InGaAs single photon avalanche detector with ultralow excess noise

An InGaAs single photon avalanche detector capable of sub-Geiger mode (Photomultiplier-tube-like) operation is reported. The device achieves a stable gain at around 106. The gain fluctuation is greatly suppressed through a self-quenching effect, thus an equivalent excess noise factor as low as 1.001 is achieved. In the photon counting experiment, the device is operated in the nongated mode under a dc bias. Because of its unique characteristics of self-quenching and self-recovery, no external quenching circuit is needed. The device shows a single photon response of around 30ns and a self-recovery time of about 300ns.

Fabrication and Characterization of Superconducting NbN Nanowire Single Photon Detectors

We report on the fabrication and characterization of high-speed, single photon detectors using superconducting NbN nanowires at a wavelength of 1064 nm. A 15 by 15 microns detector with a detector efficiency of 40% has been measured. Due to kinetic inductance, the recovery time of such large area detectors is longer than that of smaller or single wire detectors. The recovery time of our detectors (50 ns) has been characterized by measuring the inter-arrival time statistics of our detector.

Communication Limits Due to Photon Detector Jitter

Detector jitter, the random delay from the time a photon is incident on a single-photon-counting detector (SPD) to the time an electrical pulse is produced in response to that photon, is characterized for a number of SPDs. The jitter is modeled as a weighted sum of Gaussians. The performance in detector jitter is measured by determining the capacity of a communications channel utilizing a given detector. It is observed that the loss, measured as the ratio of the signal power required to achieve a specified capacity in the presence of jitter to that in the absence of jitter, goes as the square of the normalized jitter standard deviation (the standard deviation of the jitter in slotwidths). The loss is small when the normalized jitter is less than one, and becomes significant beyond that point. This loss must be taken into account when evaluating detectors for very high throughput channels.

Deep space optical communications link availability and data volume

Optical links from a spacecraft at planetary distance to a ground-based receiver presume a cloud free line of site (CFLOS). Future ground-based optical receiving networks, should they be implemented, will rely on site diversity of cloud cover to increase link availability. Recent analysis shows that at least 90% and as high as 96% CFLOS availability can be realized from a cluster comprised of 3-4 nodes. During CFLOS availability variations of atmospheric parameters such as attenuation, sky radiance and “seeing” will determine the link performance. However, it is the statistical distributions of these parameters at any given node that will ultimately determine the data volumes that can be realized. This involves a complex interaction of site-specific atmospheric parameters. In the present work a simplified approach toward addressing this problem is presented. The worst-case link conditions for a spacecraft orbiting Mars, namely, maximum range (2.38 AU) and minimum sun-Earth-probe (SEP) angle of 3-10° is considered. A lower bound of ~100 Gbits/day under the most stressing link conditions is estimated possible.

Midinfrared Interband Cascade Laser for Free Space Optical Communication

A free space optical (FSO) link utilizing midinfrared (mid-IR) interband cascade lasers has been demonstrated in the 3- to 5-¿m atmospheric transmission window with data rates up to 70 Mb/s and bit-error rate (BER) less than 10-8. The performance of the mid-IR FSO link has been compared with the performance of a near-IR link under various fog conditions using an indoor communication testbed. These experiments demonstrated the lower attenuation and scintillation advantages of a mid-IR FSO link.

A sub-hertz vibration isolation platform for a deep space optical communication transceiver

Mechanical resonators have been extensively used to provide vibration isolation for ground based, airborne, and spaceborne payloads. At low frequency, the effectiveness of these isolation systems is determined mainly by designing a mechanical oscillator with the lowest resonant frequency achievable. The Low Frequency Vibration Isolation Platform (LFVIP) reduces the resonant frequency of the mechanical oscillators into the sub-hertz region to maximize the passive isolation. This mechanical system, which has been expressly designed to isolate spacecraft vibrations from a compact deep space optical communication terminal, is based on the Stewart platform topology. Furthermore, the LFVIP provides tip/tilt functionality for acquisition and tracking of an optical beacon signal. An active control system is used for the DC positioning of the platform and the damping of the resonance of the mechanical oscillator. A summary of the LFVIP system, including analysis design, and preliminary results is presented.

GigaHertz Bandwidth Photon Counting

Early applications driving the development of single photon sensitive detectors, such as fluorescence and photoluminescence spectroscopy, simply required low noise performance with kiloHertz and lower count rate requirements and minimal or no timing resolution. Newer applications, such as high data rate photon starved free space optical communications require photon counting at flux rates into megaphoton or gigaphoton per second regimes coupled with sub-nanosecond timing accuracy. With deep space optical communications as our application driver, we have developed and implemented systems to both characterize gigaHertz bandwidth single photon detectors as well as process photon count signals at rates beyond 100 megaphotons per second to implement communications links at data rates exceeding 100 megabits per second with efficiencies greater than two bits per detected photon. With these systems, we have implemented high bandwidth real-time systems using intensified photodiodes, visible light photon counter detectors, superconducting nanowire detectors, Geiger-mode semiconductor avalanche photodiodes, and negative avalanche feedback photon counters.

Technology development for high efficiency optical communications

Deep space optical communications is a significantly more challenging operational domain than near Earth space optical communications, primarily due to effects resulting from the vastly increased range between transmitter and receiver. The NASA Game Changing Development Program Deep Space Optical Communications Project is developing four key technologies for the implementation of a high efficiency telecommunications system that will enable greater than 10X the data rate of a state-of-the-art deep space RF system (Ka-band) for similar transceiver mass and power burden on the spacecraft. These technologies are a low mass spacecraft disturbance isolation assembly, a flight qualified photon counting detector array, a high efficiency flight laser amplifier and a high efficiency photon counting detector array for the ground-based receiver.

Negative Avalanche Feedback Detectors for Photon-Counting Optical Communications

Negative Avalanche Feedback photon counting detectors with near-infrared spectral sensitivity offer an alternative to conventional Geiger mode avalanche photodiode or phototube detectors for free space communications links at 1 and 1.55 microns. These devices demonstrate linear mode photon counting without requiring any external reset circuitry and may even be operated at room temperature. We have now characterized the detection efficiency, dark count rate, after-pulsing, and single photon jitter for three variants of this new detector class, as well as operated these uniquely simple to use devices in actual photon starved free space optical communications links.