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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

A Λ-type soft-aperture LADAR SNR improvement with quantum-enhanced receiver

Song Yang, Ningjuan Ruan, Xuling Lin, Zhiqiang Wu

Published Aug 5, 2015• 0 citations
Publisher

Abstract

A quantum-enhanced receiver that uses squeezed vacuum injection (SVI) and phase sensitive amplification (PSA) is in principle capable of obtaining effective signal to noise ratio (SNR) improvement in a soft-aperture homodyne-detection LAser Detection And Ranging (LADAR) system over the classical homodyne LADAR to image a far-away target. Here we investigate the performance of quantum-enhanced receiver in Λ-type soft aperture LADAR for target imaging. We also use fast Fourier transform (FFT) Algorithm to simulate LADAR intensity image, and give a comparison of the SNR improvement of soft aperture case and hard aperture case.

DOI
10.1117/12.2192987

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A quantum-enhanced receiver that uses squeezed vacuum injection (SVI) and phase sensitive amplification (PSA) is in principle capable of obtaining effective signal to noise ratio (SNR) improvement in a soft-aperture homodyne-detection LAser Detection And Ranging (LADAR) system over the classical homodyne LADAR to image a far-away target. Here we investigate the performance of quantum-enhanced receiver in Λ-type soft aperture LADAR for target imaging. We also use fast Fourier transform (FFT) Algorithm to simulate LADAR intensity image, and give a comparison of the SNR improvement of soft aperture case and hard aperture case.
The use of quantum resources—squeezed-vacuum injection (SVI) and noise-free phase-sensitive amplification (PSA)—at the receiver of a soft-aperture homodyne-detection LAser Detection And Ranging (LADAR) system is shown to afford significant improvement in the receiver's spatial resolution. This improvement originates from the potential for SVI to ameliorate the loss of high-spatial-frequency information about a target or target complex that is due to soft-aperture attenuation in the LADAR's entrance pupil, and the value of PSA in realizing that potential despite inefficiency in the LADAR's homodyne detection system. We show this improvement quantitatively by calculating lower error rates—in comparison with those of a standard homodyne detection system—for a one-target versus two-target hypothesis test. We also exhibit the effective signal-to-noise ratio (SNR) improvement provided by SVI and PSA in simulated imagery.
Theory has shown [1] that the quantum enhancements afforded by squeezed-vacuum injection (SVI) and phasesensitive amplification (PSA) can improve the spatial resolution of a soft-aperture, homodyne-detection laserradar (ladar) system. Here we show they can improve the range resolution of such a ladar system. In particular, because an experimental PSA-enhanced system is being built whose slow photodetectors imply multi-pulse integration, we develop range-measurement theory that encompasses its processing architecture. We allow the target to have an arbitrary mixture of specular and speckle components, and present computer simulation results demonstrating the range-resolution improvement that accrues from quantum enhancement with PSA.
The phase-sensitive amplification (PSA) with an injected squeezed vacuum field is theoretically investigated in quantum-enhanced laser detection and ranging (LADAR) receiver. The theoretical model of the amplified process is derived to investigate the quantum fluctuations in detail. A new method of mitigating the unflat gain of nonideal PSA is proposed by adjusting the squeezed angle of the squeezed vacuum field. The simulation results indicate that signal-noise ratio (SNR) of system can be efficiently improved and close to the ideal case by this method. This research will provide an important potential in the applications of quantum-enhanced LADAR receiver.
We demonstrate a balanced-homodyne LADAR receiver employing a phase-sensitive amplifier (PSA) to raise the effective photon detection efficiency (PDE) to nearly 100%. Since typical LADAR receivers suffer from losses in the receive optical train that routinely limit overall PDE to less than 50% thus degrading SNR, PSA can provide significant improvement through amplification with noise figure near 0 dB. Receiver inefficiencies arise from sub-unity quantum efficiency, array fill factors, signal-local oscillator mixing efficiency (in coherent receivers), etc. The quantum-enhanced LADAR receiver described herein is employed in target discrimination scenarios as well as in imaging applications. We present results showing the improvement in detection performance achieved with a PSA, and discuss the performance advantage when compared to the use of a phase-insensitive amplifier, which cannot amplify noiselessly.
Abstract : The resolution of a homodyne LADAR can be enhanced significantly by applying quantum optical techniques in the receiver. With a quantum image enhancer (QIE) inserted between the receive optics and the detector of a LADAR, an angular cell resolution improvement of 10x can be realized. The QIE is comprised of two key elements-a squeezed vacuum injector (SVI) that restores the high spatial frequencies lost by attenuation in soft-aperture entrance optics in the LADAR and a phase sensitive amplifier (PSA) that overcomes the inefficiency of the homodyne detector. When the QIE is combined with a phase-conjugate optical coherence tomographic (PC-OCT) technique for a two-fold range resolution enhancement, a 200-fold voxel (angle-angle-range) improvement is possible.
Phase-sensitive amplification (PSA) can enhance the signal-to-noise ratio (SNR) of an optical measurement suffering from detection inefficiency. Previously, we showed that this increased SNR improves LADAR-imaging spatial resolution when infinite spatial-bandwidth PSA is employed. Here, we evaluate the resolution enhancement for realistic, finite spatial-bandwidth amplification. PSA spatial bandwidth is characterized by numerically calculating the input and output spatial modes and their associated phase-sensitive gains under focused-beam pumping. We then compare the spatial resolution of a baseline homodyne-detection LADAR system with homodyne LADAR systems that have been augmented by pre-detection PSA with infinite or finite spatial bandwidth. The spatial resolution of each system is quantified by its ability to distinguish between the presence of 1 point target versus 2 closely-spaced point targets when minimum error-probability decisions are made from quantum limited measurements. At low (5-10 dB) SNR, we find that a PSA system with a 2.5kWatts pump focused to 25μm × 400μm achieves the same spatial resolution as a baseline system having 5.5 dB higher SNR. This SNR gain is very close to the 6 dB SNR improvement possible with ideal (infinite bandwidth, infinite gain) PSA at our simulated system detection efficiency (0.25). At higher SNRs, we have identified a novel regime in which finite spatial-bandwidth PSA outperforms its infinite spatial-bandwidth counterpart. We show that this performance crossover is due to the focused pump system's input-to-output spatial-mode transformation converting the LADAR measurement statistics from homodyne to heterodyne performance.