Phase locking has been achieved between two adjacent waveguide CO2 lasers separated by a thin plate of ZnSe. Radiation leakage from one waveguide to the other through the ZnSe causes the phase locking, and stationary interference fringes demonstrate the relative phase stability. Both waveguide lasers are excited by dc discharges and operate on one of several P or R transitions in the 10.4-μm band. Phase locking occurs whenever both waveguides are on the same transition. An n by m array of such phase-locked lasers has the potential of nearly an (n×m)2 increase in the peak of the lasers’ far-field irradiance distribution.
In direct detection ladar systems, the received irradiance statistics and, therefore, the detected photon counting statistics are determined by two parameters: the average collected irradiance value and the M parameter. The M parameter is the number of independent speckle cells, per polarization and per independent laser mode, subtended by the receiving aperture and focused onto a detector. In the 1960's Goodman2 analytically determined the M parameter for simple ladar geometries such as circular, square, and Gaussian shaped targets and circular and square shaped receiving apertures. This paper examines the numerical evaluation of this M parameter for arbitrarily shaped target source regions, per pixel or per pixel-range-bin, and arbitrarily shaped receiving apertures using the two dimensional discrete Fourier transform. This evaluation method is capable of treating the cases of high reflectivity target source regions, such as a cylinder' s or cone' s glint-line, and Gaussian spatial mode illumination which effectively reduce the source area. The analyses apply when round-trip atmospheric scintillation effects are negligible.
A simple analytical model of a laser radar's subtended irradiance probability-density-function has been developed for both direct detection and coherent detection laser radar. The vacuum speckle irradiance statistics are developed following Goodman's "M parameter" treatment for direct detection ladar and also by setting the M-parameter equal to one (negative-exponential power statistics) for coherent laser radar. A "turbulence M parameter" is then computed using the round-trip aperture averaging analyses of Gudimetla and Holmes based on the Rytov-variance parameter computation over an atmospheric path of interest. The "vacuum M parameter" and the "turbulence M parameter" are then combined to form an "effective M parameter." This effective M parameter is used in an analytically simple gamma distribution probability-density-function for the laser radar's subtended irradiance. We will show excellent agreement with the more analytically complicated two-parameter K-distribution from the literature. We will also indicate how one may include the turbulence scintillation in addition to the fundamental vacuum speckle, with increasing levels of turbulence to determine ladar performance.
We examine the signal processing of both linear and sinusoidal frequency modulation (FM) coherent ladar returns from resolved and unresolved targets, which are spread in Doppler. The Doppler spread may be due to target spin, tumbling, or vibration as well as to the applied linear or sinusoidal-FM on the transmitted E-field. Monte Carlo realizations of the target surface random phasor reflector elements interact with the incident E-field producing laser speckle, and the speckled returns are analyzed in this study. The speckle signals are processed (1) using several spectrum (periodogram based) estimators, (2) the conventional "spectrogram" approach, and (3) ten joint time-frequency transforms (JTFT). We show that the Born-Jordan JTFT is superior to the other spectral estimators tested here in suppressing local oscillator laser noise and accurately estimating the target's spectrum for signal processing under speckle target return conditions pertaining to coherent laser radar. A new algorithm which sums particular pixels of the JTFT image is introduced and is shown to be much more robust in low CNR conditions than the JTFT maxima or JTFT centroid processing when utilizing the applied linear or sinusoidal-FM modulation waveform.
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