Highly efficient photocathode for surveillance and ranging systems.
Laser Focus World- 1996 .- 32, No. 5 .- P. 17, 18.
Highly efficient photocathode for surveillance and ranging systems
The company Inteva (USA) has developed a highly efficient photocathode with electron transfer, which has become the main component of the new surveillance and ranging system with laser illumination LIVAR(Laser Illuminated viewing and ranging system), recently demonstrated by the company. With a quantum efficiency of about 15% in the 1.0-1.6 µm wavelength range of the new photocathode, the InGaAs photodetector will have a sensitivity several orders of magnitude higher than that of a conventional Si photocathode. The 1.5 µm active laser imaging system LIVAR, which uses a new photocathode, has a number of advantages over previous similar systems. The power emitted by these systems (and therefore the range) was limited by the conditions of radiation safety for the human eye. In addition, at the shorter wavelengths at which they operated, the radiation was more strongly absorbed by fog and smoke in the atmosphere.
Conventional visible light television cameras, most effective in daylight and indoor lighting, produce noisy, hard-to-see images in low-light conditions. And although image intensifiers make it possible to use these cameras in low light conditions (down to that created by moonlight), the images are distorted by noise. In the infrared range, detectors respond to temperature differences or thermal radiation from objects, but the resulting images only allow one to distinguish large objects and are not suitable for positive identification of people and small objects.
In the LIVAR systemThe problem of radiation safety for human eyes is minimized by using laser radiation with a wavelength of 1.5 μm, and the effect of radiation scattering by fog and smoke is reduced by strobing the detector signals and synchronizing it with laser pulses. In addition to reducing radiation attenuation, strobing the receiver allows the user to select scenes for observation at a certain distance.
The LIVAR system uses a Nd: YAG laser Bif Sky (from Bozeman) as a radiation source., USA), pumping optical parametric pulse generator with energy of 25 mJ and wavelength of 1.57 μm. Pulse repetition rate is 30 Hz, pulse duration is 10 ns. Laser pulses irradiate the scene, and the reflected radiation is collected by the receiver lens with a diameter of 250 mm and a focal length of 1800 mm and directed to the detector. The matrix of devices with charge coupling produces a video signal, which is converted to the RS 170 format with accumulation of frames and addition of characters from the character generator before outputting the frame to a television monitor or VCR. In the development of the electronic part of the LIVAR system, the company Kaiser Electronic (USA) cooperates with the company Intevac. In clear weather, the range of the system reaches 3250 m with an image refresh rate of 30 Hz. In light fog the range can be reduced to 1000 m. In another system similar to LIVAR, the company uses an erbium-glass laser with a pulse energy of 6 mJ instead of an Nd:YAG laser. The range of this system in clear weather is 800 m. The repetition rate of the laser pulses is only 6 Hz. The advantages of this system are its smaller size, battery power and lower cost.
The images generated by both systems are similar to those seen by the naked eye. They reproduce human faces accurately enough for their positive identification and make it possible to distinguish license plates of cars and identification marks of ships or aircraft. Because the LIVAR system generates high-definition video images of targets regardless of external illumination or the thermal properties of the objects, the company expects it to be of interest to law enforcement agencies and military surveillance and reconnaissance services. Potential civil applications include surveillance and search, rescue, fire protection, landing and airfield movement control for commercial aircraft.