Introduction
to Laser Radar
Introduction
to Laser Radar
Much Later (approximately 15,000,000,000 years)
Laser radars, also referred to as ladar or lidar, use pulsed laser light instead of continuous microwaves to sense a target. Lasers are extremely pure (coherent) lightwaves, similar to only one (pure) color of light. Note that white light consists of multiple wavelengths (colors) with random (non-coherent) phases. Theodore Maiman of Hughes Aircraft Company (California) built the first working laser using a ruby rod pumped by a flash lamp in May of 1960. Traffic ladars use solid-state diodes to generate laser light.
LASER - Light Amplification by Stimulated Emissions of Radiation
RADAR - RAdio Detection And Ranging
LADAR - LAser Detection And Ranging
LIDAR - LIght Detection And Ranging
Laser radars transmit pulsed laser light to measure target range. The time it takes for a laser light pulse to travel (at the speed of light) from the ladar to the target and back is used to compute the distance from the ladar to target and back (distance pulse travels = speed of light x time). Target range from ladar is half of this distance (Range = 0.5 x speed of light x time). The change in target range over time (1/3 second typical) equals target velocity. Laser radar must transmit a minimum of 2 pulses to get at least 2 range measurements at 2 different times to compute speed. In reality laser radars transmit tens to hundreds of pulses per second.
Apertures for ladars are optical focusing devices (lenses, prisms, and/or mirrors) used to collimate laser energy into a narrow beam. Some models use the same aperture for transmit and receive; some use separate apertures (one for transmit and one for receive). The LTI 20-20 laser radar has two separate apertures; one on top of the other (top aperture transmits). The LR 90-235/p (Europe) has two separate apertures: side-by-side.
Ladars use a semiconductor diode (typically 3 diodes) to generate laser light. Most traffic ladars emit laser light around 904 nm wavelength. Other wavelengths are possible; for example aluminum gallium arsenide (AlGaAs) diodes emit light at a wavelength of 850 nm (some fiber optics use this wavelength). Gallium arsenide (GaAs), classified as an injection laser, emits light between 880 nm to 900 nm between the temperatures of -20 and 140 degrees Fahrenheit. Other wavelengths are possible using other materials or alloys.
Government Regulations
The FCC regulates radiated emissions from high speed circuits such as processing circuits inside a ladar, but not light frequencies. The Federal Drug Administration (FDA) Center for Devices and Radiological Health (CDRH) regulates laser products sold in the United States. Traffic ladars are class 1 devices (by American National Standards Institute definition) and considered eye-safe based on current medical knowledge -- even so it is probability a good idea NOT to stare at a ladar aperture while transmitting, especially at close range.
