Alan L. Kachelmyer

Optical homodyne PSK demonstration of 1.5 photons per bit at 156 Mbps with rate-½ turbo coding

We report the first demonstration of a near quantum-limited optical homodyne PSK receiver combined with powerful forward-error-correction coding, achieving 1.5 photons/bit sensitivity, within 4.5 dB of the Shannon limit. Phase-locking was achieved at 1.55 microm using an analog dither-based optical phase-locked loop with an external phase modulator. Analysis for this configuration with arbitrary loop damping is given showing a performance advantage for the overdamped case.

Efficiency penalty of photon-counting with timing jitter

Photon-counting is known to be the practically most efficient means for detection of free-space optical communications. Data rates will always be limited, however, by the speed at which such devices can operate. We calculate here the performance one can expect as one demands speeds so fast that device-limiting timing jitter substantially corrupts the measurements.

M-ary orthogonal signaling in the presence of Doppler

The performance degradation of an M-ary orthogonal keying (MOK) system due to relative motion between its transmitter and receiver can be minimized by selecting good signaling sets. A formulation for evaluating good signaling sets, or code sets, is developed for Walsh function data modulations. A union bound performance measure which closely approximates the exact probability of a demodulation error, is devised to allow for convenient evaluation of Walsh function codesets. The best Walsh function codesets for codevectors up to length 10 and for M=2, 4, 8 and 16, found by exhaustive search, are presented, along with their performance. In addition, codesets based on error-correcting codes are presented, along with a performance bound expressed in terms of the codes minimum distance. >

Spectrogram processing of laser vibration data

Laser vibration sensing has traditionally relied on the use of limiters and frequency modulation (FM) discriminators to process frequency modulated laser radar returns. The performance of the traditional FM discriminator approach can be limited by laser radar target characteristics and motion (speckle noise) and laser temporal coherence. In this paper we examine a novel laser vibration signal processor, a spectrogram processor, and compare its performance with the traditional limiter/FM discriminator signal processor used to process laser radar vibration measurements. The two processes are also compared using some laser radar measurement data.

Demonstration of a 1550-nm photon-counting receiver with ≪ 0.5 detected photon-per-bit sensitivity at 187.5 Mb/s

We implemented a photon-counting optical receiver using a periodically-poled lithium niobate waveguide and an emulated array of silicon Geiger-mode avalanche photodiodes. We achieved a sensitivity of < 0.5 detected photons/bit at 187.5 Mb/s.

Centroid tracking of range-Doppler images

This paper deals with the theory of centroid tracking of range-Doppler images created with a heterodyne-detection laser radar. A brief description of the system model is presented and the noise statistics of the image pixels are characterized. A center-of-mass centroid algorithm is used to determine the objects image location within the range-Doppler window. The effects of thresholding and frame averaging upon the performance of the centroid estimator are investigated. An analytical approach for setting the threshold is developed which is based on the criterion of making the estimator unbiased. A constant probability of detection algorithm in conjunction with an acceptable bias condition is described. Performance curves and simulation results are presented which support the theory. Center-of-mass centroid tracking performance, using simulated image data and data collected at the Firepond site in Westford, MA, is shown to be in agreement with the theoretical performance.

Thermal lensing and frequency chirp in a heated CdTe modulator crystal and its effects on laser radar performance

The effects of optical and microwave heatings and thermally-induced birefringence in a CdTe modulator crystal on the performance of a linear FM CO2 laser radar are examined. This is conducted in terms of reductions in beam Strehl ratio and dynamic ranges of the Doppler shift and range for given optical and microwave powers. An analysis of the thermal lenses generated by these heatings is presented.

Laser radar angular tracking

This paper deals with the theory of monopulse angle tracking with a laser radar employing heterodyne detection. The detector is assumed to be of the quadrant type. The targets angular location is inferred from the location of its diffraction pattern in the detector plane. A generalized gain characteristic for the detector can be used to relate the strength of the matched filter outputs, which correspond to the spatial regions (halves) of the detector surface, to the corresponding angular location of the target. The maximum likelihood angle estimator is derived for a speckle target. The performance of an approximation to the maximum likelihood angle estimator is evaluated for both the speckle and glint targets. The angle estimator performance is also evaluated in the presence of atmospheric turbulence and beam jitter.

Effect Of Atmospheric Distortion Of Carbon Dioxide Laser Radar Waveforms

Carbon dioxide (CO2) lasers can be used in coherent optical radars. Some CO2 radars will operate through the Earths atmosphere. Atmospheric CO2 and other gases will absorb the laser radiation. Because this is resonant absorption, dispersion will also occur. The combined effects of absorption and dispersion can significantly degrade the resolution of the radar. These effects are calculated in this paper. The particular example chosen for calculation is for a radar that is used to determine the precise range to a satellite in Earth orbit. This example was chosen to show the effects of CO2 absorption both at low and high altitudes in the atmosphere.

An Airborne Passive/Active Electro‐Optic Sensor System for Theater Ballistic Missile Defense

An airborne passive/active surveillance aircraft-based, electro-optic sensor system, named Gatekeeper, is being developed for the US Navy Theater Ballistic Missile Defense Program. The sensor is designed to detect theater ballistic missiles (TBMs), either in boost or post-boost phase, and make precision three-dimensional measurements of the TBMs post-boost, ballistic trajectory with sufficient state-vector accuracy for handover to naval, air and land based missile defense systems. The sensor includes a dual-band IRST for acquisition, a precision angle tracker (MWIR focal-plane array and high-bandwidth mirror), and a short-pulse, direct detection laser radar. The sensor subsystems are coupled and controlled by a sophisticated multiprocessor computer control system. The system has a highly compressed engagement timeline resulting in a substantial target handling capacity. This paper describes the sensor, its specifications, and performance in terms of the accuracy of the state-vector, and its target handling capability in a realistic engagement scenario.