Philip Gatt

Geiger-mode avalanche photodiode ladar receiver performance characteristics and detection statistics

The performance of single and multielement Geiger-mode avalanche photodiode (GM-APD) devices are investigated as a function of the detectors reset or dead time. The theoretical results, developed herein, capture the effects of both quantum fluctuations and speckle noise and are shown to agree with Monte Carlo simulation measurements. First, a theory for the mean response or count rate to an arbitrary input flux is developed. The probability that the GM-APD is armed is shown to be the ratio of this mean response to the input flux. This arm probability, PA, is then utilized to derive the signal photon detection efficiency (SPDE), which is the fraction of signal photons that are detected. The SPDE is a function of the input flux, the arm probability, and the dead time. When the dead time is zero, GM-APDs behave linearly, PA is unity, and the SPDE theory is simplified to the detectors effective quantum efficiency. When the dead time is long compared to the acquisition gate time, the theory converges to previously published “infinite” dead-time theories. The SPDE theory is then applied to develop other key ladar performance metrics, e.g., signal-to-noise ratio and detection statistics. The GM-APD detection statistics are shown to converge to that of a linear photon counting device when the combined signal and noise flux is much less than the reset rate. For higher flux levels, the SPDE degrades, due to a decreased arm probability, and the detection probability degrades relative to that of a linear device.

Analysis of Geiger-mode APD laser radars

In this paper, the performance of Geiger-mode avalanche photodiode (GAPD) receivers for range detection laser radar sensors is reported. The distribution of the non-linear avalanche detections is developed as a function of laser radar pulse width and energy for a given target and clutter range resolved cross-section with additive background noise. This distribution is then employed to design an efficient signal simulator, which was utilized to model performance and verify theory. Finally, an expression for the pulse energy that optimizes the probability of detection for partially obscured targets is given.

COHERENT OPTICAL ARRAY RECEIVERS FOR THE MITIGATION OF ATMOSPHERIC TURBULENCE AND SPECKLE EFFECTS

A description is given of the design, operation, and test over a 2-km path (roundtrip) of a continuous wave, coherent laser array receiver that uses two independent aperture-receivers whose intermediate frequencies are electro-optically co-phased in real time and then added as a proposed way to overcome effective aperture limitations imposed by atmospheric turbulence and to mitigate signal fading associated with atmospheric turbulence and speckle effects. The experiment resulted in a mean carrier-to-noise ratio increase of 1.8, which is within 1% of the theoretical predictions, when the two signals were phase locked, versus no increase without phase locking. Further, the carrier fading strength, or normalized carrier-to-noise ratio variance, was reduced by a factor of 0.53, which is within 2% of the theoretical prediction. The bandwidth of the electro-optic phase-locked loop was measured to be of the order of 600 Hz, which is adequate to compensate for atmospheric refractive turbulence fluctuations.

Laser radar detection statistics: A comparison of coherent and direct detection receivers

Detection statistics for a coherent laser radar are substantially different from those of a direct detection laser radar. Direct detection ladar detection statistics vary depending upon the detection mode. Speckle noise also impacts the detection statistics. For a single-pixel single- frequency single-polarization coherent detection transceiver, speckle noise can only be suppressed through temporal averaging. Some degree of speckle averaging can also be achieved in coherent detection systems by using a multiple frequencies or dual polarizations. In addition to these, a direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops example performance comparisons. We show that a photon-counting direct detection receiver can exploit spatial diversity to suppress the effects of speckle. This paper develops theory useful for describing the performance of these three receiver architectures against diffuse and glint targets and provides example performance comparisons. We show that a photon-counting direct detection receiver can, in principle, provide superior performance, however practical limitations of current detection technology particularly in the near IR spectral region reduces the performance margin and for many applications a coherent detection receiver provides superior performance.

High-resolution 3D coherent laser radar imaging

The Super-resolution Sensor System (S3) program is an ambitious effort to exploit the maximum information a laser-based sensor can obtain. At Lockheed Martin Coherent Technologies (LMCT), we are developing methods of incorporating multi-function operation (3D imaging, vibrometry, polarimetry, aperture synthesis, etc.) into a single device. The waveforms will be matched to the requirements of both hardware (e.g., optical amplifiers, modulators) and the targets being imaged. The first successful demonstrations of this program have produced high-resolution, three-dimensional images at intermediate stand-off ranges. In addition, heavy camouflage penetration has been successfully demonstrated. The resolution of a ladar sensor scales with the bandwidth as dR = c/(2B), with a corresponding scaling of the range precision. Therefore, the ability to achieve large bandwidths is crucial to developing a high-resolution sensor. While there are many methods of achieving the benefit of large bandwidths while using lower bandwidth electronics (e.g., an FMCW implementation), the S3 system produces and detects the full waveform bandwidth, enabling a large set of adaptive waveforms for applications requiring large range search intervals (RSI) and short duration waveforms. This paper highlights the three-dimensional imaging and camo penetration.

Coherent laser radar using eyesafe YAG laser transmitters

We report the development of an eyesafe YAG laser for coherent laser radar wind sensing applications. The upper-state pumped 1.6 mm Er:YAG laser produces high pulse energies with diffraction-limited beam quality.

Assessment of Lockheed Martin's Aircraft Wake Vortex Circulation Estimation Algorithms Using Simulated Lidar Data

A new algorithm for estimating circulation strength of aircraft wake vortices is under development by Lockheed Martin and is being evaluated by NorthWest Research Associates, Inc. (NWRA) under the National Aeronautics and Space Administration Research Announcement (NASA-NRA) NextGen Airportal Program. The algorithm is being developed with the aid of Lockheed Martin’s Coherent Wind Lidar Simulator, developed under the same NASA-NRA program. A brief description of the simulator tool is presented. An overview of both the legacy estimation algorithm and the new algorithm under development is presented which highlights the differences between the two algorithms. The performance of both algorithms is assessed using simulated lidar data generated from analytic wake models and Large Eddy Simulation (LES) vortex wind fields with ambient turbulence eddy dissipation rates (EDR) of 4 x 10 -5 , 5 x 10 -4 and 1.45 x 10 -2 m 2 s -3 . Algorithm performance is quantified in terms of standard deviation and bias of estimates as a function of SNR.

Widely tunable 2-μm cw local/master oscillator lasers for wind and DIAL measurements

Advances in coherent lidar using eyesafe solid state lasers in recent years have driven the development of increasingly compact, high performance single frequency CW lasers for use as master oscillator and local oscillator sources. In addition to highly stable single-frequency operation for coherent detection, many applications require agile frequency tuning capability. Examples include space-based coherent lidar where the local oscillator must be tunable in order to compensate for the fast platform motion. For a 45 degree conical scan about nadir the 7.5 km/s platform velocity introduces a ± 5.3 GHz Doppler shift. We have recently developed a Tm;Ho:YLF master oscillator producing over 50 mW of single frequency power that can quickly tune over 25 GHz in frequency using a PZT. Over 50 GHz of continuous mode-hop-free single frequency tuning has been demonstrated by temperature tuning. In this paper we review the status of master/local oscillator work at CTI. We also describe the application of this 2.05 μm laser to column content measurements of atmospheric CO 2 and water vapor using a direct detection column content DIAL technique.

Micro-doppler lidar signals and noise mechanisms : Theory and experiment

Lidar remote sensing of micro-Doppler signals is important for a large number of civilian and military applications. The single most important performance metric of these sensors is their velocity measurement precision. The velocity precision of a micro-Doppler lidar is limited by any one of various noise sources, which include shot-noise, local-oscillator frequency noise, speckle decorrelation noise, refractive turbulence advection noise and pointing jitter. In this paper, we present a theory, which describes these noise sources and their wavelength dependence. For example, it will be shown that the turbulence advection noise is wavelength independent while speckle decorrelation noise is proportional to the illumination wavelength and that the noise sources are, to a first-order, independent of the interrogation waveform classification (i.e., pulsed or CW). The results from recent field measurements using a doublet-pulse lidar will be compared with theory.

Agile multiple-pulse coherent lidar for range and micro-Doppler measurement

A novel high time-bandwidth product waveform lidar has been developed. The lidar operates at the eyesafe 2 micrometers wavelength and produces a sequence of two or more cavity- dumped pulselets with a controllable intra-pulse spacing. The number of and spacing for the individual pulselets is adjusted to match the target and atmospheric characteristics. This waveform agility enables the sensor to operate at very long stand-off ranges. Performance predictions and results from recent field demonstrations are described.