Underwater imaging method and apparatus utilizing multibeam interference

A new underwater imaging method and apparatus has been created and designed. The method is utilizing multibeam interference and is implemented in the following way. First, using mode-locked laser to shoot short light pulse into a mirrored negative dispersion device. Due to dispersion, the pulse width is broadened, that is, the multi-beams formed by pulse spectral components will create destructive interference in the device to reduce their combined intensity. Then, these multi- beams go into the water. Since their combined intensity has been reduced, the water absorption and scattering are reduced too because the water absorption and scattering are all directly proportional to the combined intensity of the multi-beams. Because the water dispersion is positive, the beams with lower frequencies will travel faster in the water which is opposite to what happened in the negative dispersion device. Thus, the width of the broadened light pulse is compressed gradually in the water. If the dispersive characteristic of the mirrored negative dispersion device is designed to match that of the water reversely well, the broadened light pulse can be compressed ideally in the water at a special position. In other words, the multiple beams will create constructive interference to produce a combined intensity maximum in the water, which will form an internal light layer to illuminate the object. The theoretical calculations have proved feasibility of the method and show that the designed apparatus can increase imaging distance in clean ocean water to much more than 100m with possibility of even to 1000m.