Application of a frequency chirped RF intensity modulated 532 nm light source in underwater ranging

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 was modulated via a Mach-Zehnder electro-optical modulator. The modulation frequency could be 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. Frequency domain reflectometry (FDR) was applied to retrieve the distances of the objects to the transmitter in a water tank. When the modulation frequency was tuned from 10 MHz to 200 MHz, we obtained ranging accuracy better than 5 cm and ranging resolution smaller than 0.5 cm at 8.6 attenuation length. Two objects spacing by 0.8 m could be resolved simultaneously. The attenuation length at which the object could be detected was limited by the sensitivity of the silicon photodetector. Moreover, we did not use the full bandwidth of the modulation since the lock-in amplifier adopted for data processing had a bandwidth of 200 MHz. Both ranging distance and resolution can be improved if a photomultiplier tube and a high-speed data sampling board were used. Nevertheless, the experimental results showed that the watt level, broadband intensity modulated light source has great potential in underwater detection in terms of ranging accuracy and resolution.