532 nm Chaotic Fiber Laser Transmitter for Underwater Lidar.

摘要

This project develops a new optical source for underwater ranging and imaging work. This source is based on an ultra-long cavity infrared fiber laser that generates a chaotic, wide bandwidth, high frequency signal, as is desirable for underwater applications. Amplification and frequency doubling of the infrared seed signal are performed to generate a green signal that can be used for underwater lidar. This transmitter is novel in that it offers high range resolution, high resilience to backscatter, completely unambiguous range, minimum dwell time, and no external modulator, while operating at high powers for long-range sensing. The seed signal is generated by a fiber laser whose cavity includes a long passive fiber. This fiber forces the lasing of multiple, closely spaced longitudinal modes, which form a wideband intensity modulation on the infrared laser output, with a quasi-continuous power spectral density from DC to >1 GHz. This wideband signal has a sub-nanosecond autocorrelation peak and so offers high resolution for correlation-based ranging systems. Its high frequencies are also ideal for underwater work since they allow suppression of the backscatter. The laser exhibits several physical advantages: its modulation is generated internally, removing the need for external devices; it is light and compact; there are minimal heat dissipation concerns; and it is relatively insensitive to vibration or temperature. Finally, the system scales easily to high powers. A ytterbium-doped fiber laser generates about 50 mW of this continuous-wave wideband signal at 1064 nm. Since water attenuates this wavelength quickly, a nonlinear crystal is used for second-harmonic generation, producing 532 nm green light. This crystal's efficiency increases quadratically with input power, so to achieve efficient frequency doubling the infrared signal is amplified to 5 W.