19 May 2015 FMCW optical ranging technique in turbid waters
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The performance of a frequency-modulated continuous-wave (FMCW) hybrid lidar-radar system will be presented in the context of an underwater optical ranging application. In adapting this technique from the radar community, a laser is intensity-modulated with a linear frequency ramp. A custom wideband laser source modulated by a new wideband digital synthesizer board is used to transmit an 800 MHz wide chirp into the underwater channel. The transmitted signal is mixed with a reference copy to obtain a “beat” signal representing the distance to the desired object. The expected form of the return signal is derived for turbid waters, a highly scattering environment, indicating that FMCW can detect both the desired object and the volumetric center of the backscatter “clutter” signal. This result is verified using both laboratory experiments and a realistic simulation model of the underwater optical channel. Ranging performance is explored as a function of both object position and water turbidity. Experimental and simulated results are in good agreement and performance out to ten attenuation lengths is reported, equivalent to 100 meters in open ocean or 5 meters in a turbid harbor condition.
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David W. Illig, David W. Illig, Alan Laux, Alan Laux, Robert W. Lee, Robert W. Lee, William D. Jemison, William D. Jemison, Linda J. Mullen, Linda J. Mullen, "FMCW optical ranging technique in turbid waters", Proc. SPIE 9459, Ocean Sensing and Monitoring VII, 94590B (19 May 2015); doi: 10.1117/12.2177051; https://doi.org/10.1117/12.2177051

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