This design constitutes a self-resetting (gain quenching), room-temperature operational semicondu... more This design constitutes a self-resetting (gain quenching), room-temperature operational semiconductor single-photon-sensitive detector that is sensitive to telecommunications optical wavelengths and is scalable to large areas (millimeter diameter) with high bandwidth and efficiencies. The device can detect single photons at a 1,550-nm wavelength at a gain of 1 x 10(exp 6). Unlike conventional single photon avalanche detectors (SPADs), where gain is an extremely sensitive function to the bias voltage, the multiplication gain of this device is stable at 1 x 10(exp 6) over a wide range of bias from 30.2 to 30.9 V. Here, the multiplication gain is defined as the total number of charge carriers contained in one output pulse that is triggered by the absorption of a single photon. The statistics of magnitude of output signals also shows that the device has a very narrow pulse height distribution, which demonstrates a greatly suppressed gain fluctuation. From the histograms of both pulse he...
We propose a solution for pointing and tracking an optical terminal using one or more beacons and... more We propose a solution for pointing and tracking an optical terminal using one or more beacons and a slowly varying background image. The primary application is a deep space optical communication terminal, where multiple source tracking provides robustness against beacon outage. Our solution uses optical orthogonal codes modulated on each beacon to separate the signal from each source for centroiding. This technique allows calculation of the transmit pointing vector from each beacon location as well as from the background image. The latter can be used to track during beacon outages. We present a simple algorithm for performing this separation, and apply it to experimental data from a photon-counting detector illuminated by two beacons and one constant source. Our results show that the photon flux from each source can be accurately estimated even in the low signal, high background regime.
A hardware prototype of a flight receiver for deep space optical communications has been develope... more A hardware prototype of a flight receiver for deep space optical communications has been developed where a single detector array is used for acquisition, tracking, and high-speed data recovery. A counting algorithm accumulates pulses on every pixel in a photon-counting array and extracts signal information encoded with a nested modulation scheme.
the sample ungripped. This design uses about 8-in. (20-cm) of wire per tests. Multiple tests were... more the sample ungripped. This design uses about 8-in. (20-cm) of wire per tests. Multiple tests were conducted at both room and elevated temperature with no failures in the grip region. The novelty of this wire test fixture lies in its simplicity. Its compact features allow the user to install the fixture in a test frame with little or no modifications. The self-alignment feature designed into setup places the wire specimen in perfect alignment with the test frame. The loading spools, when gripped, are in direct contact with the test frame water-cooled wedge grips. This helps to draw temperature away from the fixture for ease of high-temperature testing.
We present a 1024-element imaging array of superconducting nanowire single photon detectors (SNSP... more We present a 1024-element imaging array of superconducting nanowire single photon detectors (SNSPDs) using a 32 × 32 row-column multiplexing architecture. Large arrays are desirable for applications such as imaging, spectroscopy, or particle detection.
Interplanetary Network Progress Report, Feb 1, 2011
We propose a novel deep space optical communication scheme, in which an integrated optical phased... more We propose a novel deep space optical communication scheme, in which an integrated optical phased array (OPA) is used for both phase modulation and fine beam steering. In particular, an optical phase-locked loop (OPLL) based phased array with full electronic control over the phase is introduced and analyzed. The performance of such an array as a beam steering mechanism is evaluated and compared to realistic steering requirements for deep space applications. It is shown that an array with a high fill factor (> 0.7) with about 300 elements per dimension is needed to meet these requirements. The effect of residual phase noise due to limited loop bandwidth is analyzed. Finally the theory is validated by experimental results demonstrating successful beam steering using a two element phased array.
For an optical communications signal, when the noise power dominates, the capacity scales as the ... more For an optical communications signal, when the noise power dominates, the capacity scales as the square of signal power over noise power. In this regime, data rate scales inversely as the 4th power of range, and the high data rate advantage of optical communications that results from a narrower transmit beam cannot be achieved. Before a received signal can be decoded, it first must be acquired from the background noise. In noise-limited optical communications, a background power dependent signal power threshold exists below which acquisition cannot occur, and data cannot be recovered. Decreasing the pulse width, corresponding to increasing the modulation bandwidth B, both increases capacity and aids acquisition. Higher modulation bandwidths than about 10 GHz require higher power in the control electronics due to the energy required to charge and discharge circuit capacitances. Higher complexity and higher power are negative attributes for space optical communications. One solution is to use a mode-locked laser, which enables very high effective peak power pulses with minimum complexity. A mode-locked laser utilizes a non-linear inter-cavity element to generate a fixed-rate train of narrow pulses. Communications requires data encoding on the fixed pulse train carrier. One method is to use a mode-locked laser with right or left circularly polarized output. Initial acquisition is aided by the narrow temporal spectrum and high peak power. Once acquired, all of the background noise outside the single data slot of the symbol can be rejected. In the noise-limited regime, capacity now is proportional to signal power over bandwidth-noise power product. The use of the mode-locked laser, versus a conventional pulse-carved continuous wave laser, allows optical communications links to be established in background noise regimes where the signal could not be acquired using conventional modulation schemes. This laser source and modulation scheme has benefits to CubeSat optical communications, and lander/rover Direct-to-Earth communications. In this paper we investigate the design and performance evaluation of a coded mode-locked laser with right and left circular polarization switching. At the optical receiver a polarizing beam splitter and two photo-detectors are used to detect the data. One method for data detection is to compare the number of received photons from two photo-detectors. This comparison produces 0, or 1, or an erasure output. The erasure symbol is produced when the numbers of photons from the two detectors are equal. This method of detection produces an equivalent binary-input ternary-output discrete memoryless channel. Transition probabilities of this channel are computed based on received signal and background noise fluxes. We evaluate the performance of a very low-rate turbo code (rate 1/31) to be used over this channel. We compare this code's performance with the capacity of the underlying optical channel, and with uncoded system performance. We demonstrate that at least 10 dB coding gain can be achieved with respect to the uncoded case. Moreover, we show that the performance of this well designed code is within 1 dB of channel capacity for an information block size of 16384 bits.
Lightweight, compact, low-power apparatus processes synthetic-aperture-radar (SAR) returns in rea... more Lightweight, compact, low-power apparatus processes synthetic-aperture-radar (SAR) returns in real time, providing imagery aboard moving aircraft or spacecraft platform. Processor includes optical and electronic subsystems that, together, resolve range and azimuth coordinates of radar targets by combination of spatial and temporal integrations.
This design constitutes a self-resetting (gain quenching), room-temperature operational semicondu... more This design constitutes a self-resetting (gain quenching), room-temperature operational semiconductor single-photon-sensitive detector that is sensitive to telecommunications optical wavelengths and is scalable to large areas (millimeter diameter) with high bandwidth and efficiencies. The device can detect single photons at a 1,550-nm wavelength at a gain of 1 x 10(exp 6). Unlike conventional single photon avalanche detectors (SPADs), where gain is an extremely sensitive function to the bias voltage, the multiplication gain of this device is stable at 1 x 10(exp 6) over a wide range of bias from 30.2 to 30.9 V. Here, the multiplication gain is defined as the total number of charge carriers contained in one output pulse that is triggered by the absorption of a single photon. The statistics of magnitude of output signals also shows that the device has a very narrow pulse height distribution, which demonstrates a greatly suppressed gain fluctuation. From the histograms of both pulse he...
We propose a solution for pointing and tracking an optical terminal using one or more beacons and... more We propose a solution for pointing and tracking an optical terminal using one or more beacons and a slowly varying background image. The primary application is a deep space optical communication terminal, where multiple source tracking provides robustness against beacon outage. Our solution uses optical orthogonal codes modulated on each beacon to separate the signal from each source for centroiding. This technique allows calculation of the transmit pointing vector from each beacon location as well as from the background image. The latter can be used to track during beacon outages. We present a simple algorithm for performing this separation, and apply it to experimental data from a photon-counting detector illuminated by two beacons and one constant source. Our results show that the photon flux from each source can be accurately estimated even in the low signal, high background regime.
A hardware prototype of a flight receiver for deep space optical communications has been develope... more A hardware prototype of a flight receiver for deep space optical communications has been developed where a single detector array is used for acquisition, tracking, and high-speed data recovery. A counting algorithm accumulates pulses on every pixel in a photon-counting array and extracts signal information encoded with a nested modulation scheme.
the sample ungripped. This design uses about 8-in. (20-cm) of wire per tests. Multiple tests were... more the sample ungripped. This design uses about 8-in. (20-cm) of wire per tests. Multiple tests were conducted at both room and elevated temperature with no failures in the grip region. The novelty of this wire test fixture lies in its simplicity. Its compact features allow the user to install the fixture in a test frame with little or no modifications. The self-alignment feature designed into setup places the wire specimen in perfect alignment with the test frame. The loading spools, when gripped, are in direct contact with the test frame water-cooled wedge grips. This helps to draw temperature away from the fixture for ease of high-temperature testing.
We present a 1024-element imaging array of superconducting nanowire single photon detectors (SNSP... more We present a 1024-element imaging array of superconducting nanowire single photon detectors (SNSPDs) using a 32 × 32 row-column multiplexing architecture. Large arrays are desirable for applications such as imaging, spectroscopy, or particle detection.
Interplanetary Network Progress Report, Feb 1, 2011
We propose a novel deep space optical communication scheme, in which an integrated optical phased... more We propose a novel deep space optical communication scheme, in which an integrated optical phased array (OPA) is used for both phase modulation and fine beam steering. In particular, an optical phase-locked loop (OPLL) based phased array with full electronic control over the phase is introduced and analyzed. The performance of such an array as a beam steering mechanism is evaluated and compared to realistic steering requirements for deep space applications. It is shown that an array with a high fill factor (> 0.7) with about 300 elements per dimension is needed to meet these requirements. The effect of residual phase noise due to limited loop bandwidth is analyzed. Finally the theory is validated by experimental results demonstrating successful beam steering using a two element phased array.
For an optical communications signal, when the noise power dominates, the capacity scales as the ... more For an optical communications signal, when the noise power dominates, the capacity scales as the square of signal power over noise power. In this regime, data rate scales inversely as the 4th power of range, and the high data rate advantage of optical communications that results from a narrower transmit beam cannot be achieved. Before a received signal can be decoded, it first must be acquired from the background noise. In noise-limited optical communications, a background power dependent signal power threshold exists below which acquisition cannot occur, and data cannot be recovered. Decreasing the pulse width, corresponding to increasing the modulation bandwidth B, both increases capacity and aids acquisition. Higher modulation bandwidths than about 10 GHz require higher power in the control electronics due to the energy required to charge and discharge circuit capacitances. Higher complexity and higher power are negative attributes for space optical communications. One solution is to use a mode-locked laser, which enables very high effective peak power pulses with minimum complexity. A mode-locked laser utilizes a non-linear inter-cavity element to generate a fixed-rate train of narrow pulses. Communications requires data encoding on the fixed pulse train carrier. One method is to use a mode-locked laser with right or left circularly polarized output. Initial acquisition is aided by the narrow temporal spectrum and high peak power. Once acquired, all of the background noise outside the single data slot of the symbol can be rejected. In the noise-limited regime, capacity now is proportional to signal power over bandwidth-noise power product. The use of the mode-locked laser, versus a conventional pulse-carved continuous wave laser, allows optical communications links to be established in background noise regimes where the signal could not be acquired using conventional modulation schemes. This laser source and modulation scheme has benefits to CubeSat optical communications, and lander/rover Direct-to-Earth communications. In this paper we investigate the design and performance evaluation of a coded mode-locked laser with right and left circular polarization switching. At the optical receiver a polarizing beam splitter and two photo-detectors are used to detect the data. One method for data detection is to compare the number of received photons from two photo-detectors. This comparison produces 0, or 1, or an erasure output. The erasure symbol is produced when the numbers of photons from the two detectors are equal. This method of detection produces an equivalent binary-input ternary-output discrete memoryless channel. Transition probabilities of this channel are computed based on received signal and background noise fluxes. We evaluate the performance of a very low-rate turbo code (rate 1/31) to be used over this channel. We compare this code's performance with the capacity of the underlying optical channel, and with uncoded system performance. We demonstrate that at least 10 dB coding gain can be achieved with respect to the uncoded case. Moreover, we show that the performance of this well designed code is within 1 dB of channel capacity for an information block size of 16384 bits.
Lightweight, compact, low-power apparatus processes synthetic-aperture-radar (SAR) returns in rea... more Lightweight, compact, low-power apparatus processes synthetic-aperture-radar (SAR) returns in real time, providing imagery aboard moving aircraft or spacecraft platform. Processor includes optical and electronic subsystems that, together, resolve range and azimuth coordinates of radar targets by combination of spatial and temporal integrations.
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Papers by William Farr