Navy

Target imaging and ranging in turbid medium

3-D images in cloudy environments from laser-based sensors

Photonics

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Dr. Linda Mullen demonstrates a laser

Dr. Linda Mullen, at Naval Air Station Patuxent River, patented an encoding method for laser imaging, which offers possibilities for both fleet and commercial use. (U.S. Navy Photo)

Remotely operated vehicles (ROVs)–including underwater, terrestrial, or aerial–typically use cameras for high-resolution imaging. Cameras are effective in clear environments, but perform poorly in murky water, dust, and fog. This is due to the scattering of the light by particulate and organic matter in the operating environment.

Laser-based sensors have been developed to enhance optical imaging, but conventional approaches require that the laser and receiver be located on the same platform. This requirement is not compatible with some weight restrictions of ROVs.

Navy researchers have developed a low-cost optical imager utilizing a bistatic geometry where the laser and receiver are decoupled. An imaging laser beam is steered with a MEMS scanner to sequentially illuminate an object. A distant receiver collects the reflected laser light and reconstructs the image. Communications information, including a synchronization sequence, is encoded onto the modulation which is used by the receiver to build the image.

As seen in the figure below, the apparatus includes two light sources (100, 200), two receivers (300, 400), demodulators (330, 430), and an image processor (500). The first light source generates a beam (105) for scanning the object and a second light source for generating a second beam of intensity-modulated light (205) as a reference signal. The beams have differing wavelengths, and the two receivers each have wavelength matching filters. Reflected beams are demodulated into in-phase and quadrature components, and these components are digitized to obtain magnitude and phase information from each receiver.

The image processor creates the 3-D image using the magnitude of the object-reflected signal and the range computed from the phase difference between the object-reflected signal and the reference signal. This technique provides a way to generate 3-D imagery when the transmitter and receiver are located on separate platforms.

This US patent 9,823,355 is related to US patent 8,373,862, Extended Range Optical Imaging System for Use in Turbid Media, and US patent 8,044,999, Image Enhancer for Detecting and Identifying Objects in Turbid Media.

The ‘999 patent covers the device with the laser, modulator, RF source, detector, demodulator, splitter, filters, and mixers.

The ‘862 patent covers the method using the device including the encoding of information in the laser, scanning the laser over the target, and constructing the image. ‘862 also extends the device claims.

The ‘355 patent incorporates two light sources, two receivers, a demodulator, and an image processor such that a three dimensional image is produced. This patent further enables accurate ranging.

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