Abstract

Coherent Technologies, Inc., under funding from a Wright Laboratory SBIR contract, has developed a coherent laser radar based on waveform amplification in single mode erbium-doped fiber amplifiers. The transceiver emits a waveform with a large time-bandwidth product that allows imaging with simultaneous high resolution in both range and velocity for enhanced hard target recognition. The ladar is designed for range-resolved measurement rather than range- precise measurement in order to overcome military imaging countermeasures such as camouflage netting. A number of mechanisms are capable of generating wideband signals, such as amplitude mode-locking and intracavity phase modulation (frequency chirping). For the purposes of coherent ladar, most of these approaches require the injection-locking of the ladar transmitter to the master/local oscillator frequency and are therefore sensitive to transmitter perturbations. A more rugged approach adopted by CTI is to use frequency-shifted feedback (FSF) in a multi-pass amplifier to generate a wideband frequency comb on one side of the input master oscillator frequency. The amplifier can be operated in cw mode (below laser threshold) or in quasi Q- switched mode (above laser threshold) without resonator frequency-pulling effects since the axial mode structure of the laser/amplifier resonator is eliminated by FSF. This allows simple and controlled frequency generation which is insensitive to transmitter perturbations.

© 1995 Optical Society of America