Low Coherence Fiber Optics for Random Noise Radar

Coherent random noise radar has been developed at the Univei:vity qfNebraska jbr the detection arid identification of shallow subsurface objects such ay land mines. Other application range.sfrom reniote sensing, s.ynthetic aperlzire radar and high-rtmlution interferometer, to covert turget tmcking. i n this paper, we describe the experimental investigation of a novel applicntion of' superfluorescent fiber or erbium doped jiber. amplifier (EDFA) light sources(SFS) for random noise radur. The ultra-+vide band noise coupled with the use of optical fiber delay lines ninkes SFS ideally suited jiw rundoin noise radar applications. Optical jibelmakes long and rnultQde-step delay lines of U f;?w kilometers feasible. lts advantage includes being extremely low-loss, cotnpuct mid lightweighted, and available at U niiich lower cost, rvhile avoiding the dispersion arid non-linearity associated with KF delay lines. We report the analysis oncl experimental results to c haructel-ize the resolution c ha roc teristics ofthe system. INTRODUCTION Random noise radar employs white noise as the signal source. The transmit signal is time delayed and correlated vith the received signal to obtain a correlation peak. The length of delay then determines the target range. A number of noise radar systems have been developed for moving target or landmine detection. Early in 1959, Horton [I] proposed a distance-measuring radar by transmitting modulated noise such that the distance was obtained from the correlation function. It has been suggested that range ambiguities in radar systems 'may be reduced by transmitting wide-band noise [2]. In [3], Dillard discussed the use of optical fiber as signal delay, matched filtering in radar signal processing. Oliver performed theoretical comparisons of radar principles at microwave and optical frequencies[4]. In the system prposed by Forrest and Meeson [ 5 ] , the noise was down converted to an intermediate frequency in order to minimize the delay-line losses. The coherent noise radar was developed by Narayanan [6] to measure both the in-phase and quadrature-phase components. Coherence injection was accomplished using a local oscillator and an intermediate mixing frequency. Theron et. al. has developed an ultrawideband radar operating in the foliage penetration band Traditionally, the time delay is realized with microwave delay lines that are expensive and bulky. Fiber optic delay lines have also been used but they require optical modulators srnd detectors to interface with the microwave signals. The electro-optic conversions have very high loss and linearity limitations. We propose to generate the broadband microwave noise using low-coherence fiber optic light sources. SFS or EDFA light source is based on amplified spontaneous emission (ASE) that has ultra-wide bandwidth of approximately 1,200 GHz. The intense output light is readily available in optical fiber that eliminates the need for electreoptic conversions. The resolution of the radar system that employs SFS is limited only by the bandwidth of the photo-detectors. Optical fiber makes long and multiple-step delay lines of a few kilometers feasible. In our experiments, the low coherence SFS is employed in a novel electro-optical Michelson interferometer arrangement to demonstrate the feasibility of the radar application. In this paper, we analyze the experimental system and report the results from computer simulations and measurements. We also propose a method to optically inject coherence into the radar system that employs EDFA light sources. [71,[81. SYSTEM DESCRIPTION The erbium doped fiber amplifier source is a proven source of broadband light in current applications such as fiber optic gyroscope. In our experiment, the source consisted of a loom-long erbium-doped fiber that was pumped in the backward configuration by a 98Onm laser 0-7803-6512-6/$10.00 (C) 2000 IEEE 907 Published in MILCOM 2000. 21st Century Military Communications Conference Proceedings Vol. 2, pp. 907-911; doi: 10.1109/MILCOM.2000.904062 Copyright 2000 IEEE. Used by permission.