Underwater Optical Ranging: A Hybrid LIDAR-RADAR Approach

Proximity detection underwater is performed primarily by sending and receiving some form of magnetic or acoustic radiation. This is done by propagating a known output signal, and measuring the response of or disturbance from some target. However, these systems have limitations in how much range resolution and accuracy they can provide under certain conditions. Recognizing this short-coming, optical techniques for underwater ranging are being investigated. Optical systems can provide update rates on the order of mega-Hertz because it depends on the speed of light instead of the speed of sound. Perhaps most importantly, they can provide much greater resolution and accuracy in their measurements than existing systems, theoretically on the order of centimeters, or even sub-centimeter, depending on the configuration. There is, in fact, a US Navy requirement for a high-precision, large dynamic-range, proximity detector that is as insensitive as possible to changing water conditions, with a range of less than two meters. However, when traveling through a medium such as water, an optical signal will attenuate exponentially because of absorption and scattering. The absorption coefficient, or , characterizes the photons that are extinguished and cannot be recovered by the detector. Physically, when dealing with wavelengths in the blue to IR spectrum about 400 nanometers to 100 micrometers, this loss is caused by the photons interacting with the water molecules and some particulates in the water. Wavelengths in the near infrared NIR spectrum interact more strongly, and absorb more quickly 1. Running out of signal due to absorption is called reaching your photon limit. An analogy would be like driving a car on a clear night- if the vehicles high beams are turned on, the driver can see farther. In other words, sending more photons into the environment i.e. using a greater optical power, will allow the system better range when this type of loss is dominant.