LASER-RADAR COMPONENT TECHNOLOGY

The implementation of laser radar systems has required significant adv ances in the technology of both transmitters and receivers. For a grou nd-based carbon dioxide (CO2) range-Doppler imaging radar it was neces sary to develop (for the transmitters) ultrastable master oscillators and local oscillators, high-power amplifiers, frequency modulators, an d isolators and (for the receivers) efficient, wide-bandwidth detector s. Extremely high spectral purity and short-term fractional frequency stabilities of Delta f/f(o) < 15 x 10(-13) have been routinely achieve d with sealed-off ultrastable CW CO2 laser oscillators. Wideband ampli fiers with pulse energies up to 100 J have been built for long-range m easurements of space objects. InSb Faraday rotation isolators with >20 dB isolation and <1 dB insertion loss, and CdTe electrooptic frequenc y modulators producing 1-GHz linear frequency chirps in 1 mu s, are tw o additional key components that were implemented for the CO2 imaging radar transmitter chain. Lower-power CO2 laser transmitters were also developed for short-range portable or airborne radars. The utilization of solid state lasers as radar transmitters has increased in recent y ears, in part through the application of diode lasers as efficient pum p sources, permitting the construction of compact radars, particularly for airborne or space-based systems. Frequency-doubled Nd:YAG lasers have been built and used in direct-detection short-pulse ranging radar s for measurements on space objects. Ultracompact microchip diode-pump ed laser transmitters are currently under development for lightweight inexpensive short-range imaging. Such transmitters have produced e-swi tched pulses as short as 115 ps, enabling range measurements with cent imeter resolution. Eye-safety considerations have recently prompted wo rk on solid state lasers at wavelengths longer than 1.4 mu m. Receiver development has included HgCdTe detectors with 50% heterodyne efficie ncy and several-gigahertz bandwidth for CO2 radars. For the shorter-wa velength solid state laser radars there have been recent significant a dvances in both photoemissive detectors and semiconductor avalanche ph otodiodes for sensitive direct detection at wavelengths up to 1.6 mu m .