Lidar has been widely used in underwater detection, survey, and seabed mapping. However, the performance of underwater lidar is deteriorated by scattering. Scattering not only contributes to attenuation but also worsens ranging accuracy or imaging contrast. Therefore, it was important to suppress scattering in underwater detection. We present investigations of applying a spiral phase plate (SPP) as a spatial filter to suppress scattering in an underwater lidar system. An SPP was inserted in the echo path to separate target-reflected signals from scattering clutters, because, in the echo, the target-reflected light maintained spatial coherence and was converted into an optical vortex ring after passing through the SPP, while the scattering clutters lost their spatial coherence and cannot be converted to the optical vortex but remained a centrally stronger distribution according to the Mie scattering law. A mask was produced by coating a glass sheet with opaque paint, leaving only a transparent ring for light on the optical vortex ring to pass through. Experimentally, the response of the light in the center of the vortex to the RF modulation frequency decreased with the increase of attenuation length, so it was mainly scattered light, while the light on the vortex was measured to maintain RF frequency modulation at high attenuation length, so it was dominated by signal light. The ranging results showed that the ranging error was significantly reduced in a turbid medium by blocking scattering clutters inside and outside the vortex. Moreover, a high-order SPP was more effective in reducing ranging errors. We reduced the ranging error from 30 cm to 6.6 cm with a 24-order SPP when the attenuation length was 15.
Broadband light absorbers are attractive for their applications in photodetection and thermal detectors. Metal-black coatings have been experimentally proven to have broadband light absorption characteristic. A large area and broadband gold-black coating was fabricated by a low-cost but effective sputtering process. The gold-black films exhibited reduced reflection of 4.81%, 2.48% and 1.93% for sputtering pressure of 50, 65 and 80 Pa in 300-800 nm spectral range, and their size reached 4- inches. A three-dimensional nanocone-like array model was proposed for the gold-black films. Then, the nanocone array of this model was embedded with many gold nanoparticles due to the rough surface of the gold-black films. The results indicated this proposed model of nanocone-like array embedded with nanoparticles can be a good tool to design broadband gold-black absorbers.
A radio-frequency (RF) intensity-modulated light source at 532 nm was built for underwater ranging. The intensity of a narrow-linewidth laser at 1064 nm from a NPRO (Non Planar Ring Oscillator) was modulated via a Mach-Zehnder electrooptical modulator. The modulation frequency was tuned from 10 MHz to 2.1 GHz. The intensity-modulated light was amplified via a 2-stage laser diode-pumped Yb3+ doped large-mode-area fiber amplifier. A 15 mm long magnesium oxide doped periodically-poled lithium niobate (MgO: PPLN) nonlinear crystal was used to convert the 1064 nm light into 532 nm light via frequency doubling. The maximum output power at 532 nm was 2.56 W, the highest efficiency from the fundamental to second harmonic generation (SHG) was 22.6%. The watt level 532 nm light source was applied in underwater ranging experiments. Different modulation frequencies were applied to measure the distance of an object in the water. The turbidity of the water was changed by adding Mg(OH)2 powder, ranging accuracy of 6 cm was obtained at 2.5 m distance when the attenuation coefficient of the water was 1.72 m-1. In turbid water, higher modulation frequency was preferable for obtaining higher ranging accuracy.
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