Laser vision system at night and poor weather conditions

Night vision and ranging systems are broadly used in border security management. The key element of these systems is an active source of near infra-Red (NIR) light for illuminating objects during dark time or restricted transparency of atmosphere. This source should meet certain requirements:

  1. The wavelength should belong to so called “eye safe” region of light and fits the windows of atmosphere transparency. From this point of view the 1.54-1.56-micron radiation is the best choice.
  2. To manage large area or big distance the source should have enough power, energy being several mJ per pulse.
  3. For ranging application the source should be pulsed with very short pulse duration for high distance resolution, typically 1-20ns duration.
  4. For night vision application the source also preferably should be pulsed with high pulse energy and high repetition rate (25-50Hz for comfort monitoring).

Available in the market NIR cameras have high resolution sensor matrix and very small integration time at 30-60Hz cadre change.

Main problem is that currently used sources of NIR radiation do not meet all these requirements simultaneously.

For instance, IR light emitting diodes have too low instant power for illuminating large area or long distance. Er doped glass 1,55 micron lasers have too low working frequency (less than 10Hz), Nd:YAG pulsed lasers are dangerous for eye, etc.

We propose eye safe night vision (and ranging) system based on diode pumped solid state (DPSS) pulsed laser source, which should have the following features:

–  wavelength of radiation – 1,54 micron

– pulse energy                     – up 5-8mJ

– pulse duration                  – 10-20 ns (passive Q-switching)

– repetition rate                  – 25-50Hz

– no water cooling

– small size and weight (less than 2 kg)

The instant radiation power of this source during the pulse duration exceeds 0,5MW.  Output laser optics permits to spread this power over a wide angle to monitor large area around, or to collimate the beam along narrow corridor to monitor area on many km distances. In case of ranging application, the laser should be incorporated within range-finder device.

Short pulse illumination permits to use the stroboscopic method of vision, which allows to remove the noise of the receiver video camera, due to the scattering of laser light on droplets of fog, rain or particles of dust and smoke.

Additional advantage of NIR light compared to visible one for objects illumination is its smaller scattering on fog, rain, smoke particles which leads to more reliable monitoring systems.