The basic requirement of underwater communication laser is that the laser output wavelength is 532nm, because the low loss window in seawater is 0.46nm~0.54nm, and the pulse peak energy should be greater than 200mJ, the pulse width is generally 10ns~20ns, the repetition frequency is about 50Hz, and the divergence angle is the smaller the better, generally less than 0.1mrad, the laser spot is as uniform as possible, and the 532nm Nd:YAG laser with electro-optic Q-switching can be applied to these aspects.
Theotry
The main components of device are:
Gain medium - Nd YAG
It is used to generate a laser of 1064 nm to provide fundamental light for generating a laser of 532 nm. The gain medium Nd:YAG is a cubic crystal, optically isotropic, and the Nd3+ ion doping concentration is 1.0%.
In order to get high peak power, it is necessary to concentrate the energy into one pulse by Q-switching technology. LiNbO3 has a large electro-optic coefficient, large acousto-optic coefficient and good nonlinear performance. It is widely used in photoregulators and waveguide material substrates as well as Q-switches for Nd lasers. Insoluble in water, not easy to deliquesce, does not require sealed storage, low transmission loss, stable chemical and mechanical properties.
EO-Q switch：LiNbO3
LBO Frequency doubling
The frequency doubling crystal is a LBO crystal. The LBO crystal is a negative biaxial crystal. The destruction threshold is 4 times that of KTP. The effective nonlinear coefficient is one-third of the KTP crystal, and has a wide allowable angle and a small deviation angle. With Class I phase matching, the two pass surfaces of LBO are plated with a two-color antireflection coating for 1064 nm and 532 nm.
Mirror——M1 – 1064nm total reflection film is plated on the side near electro-optic crystal. M2 – 1064nm output mirror. M3 – 45 ° reflector.
Using a frequency-doubled Q-switched mode-locked Nd:YAG laser, 35 532 nm pulses are generated per second. In general waters, the experimental results in three-dimensional directions are higher than 6 135 mm; in deep water pools, the distance resolution is experimentally obtained，it is 91144m, the lateral resolution is about 6135mm, and the distance resolution is limited by the 9ns pulse width of the Q-switched laser.
Feature
- High photon energy
- Short wavelength
- Sensitive to the human eye
- Regular light spot
- Long distance in water
- Small divergence angle
Application
- Underwater Imaging Technology
The distance gating technique utilizes a pulsed laser and a strobe camera to separate the scattered light at different distances and the reflected light of the target scene by pulse emission and imaging time, so that the radiation pulse reflected from the target scene arrives at camera and imaging within the camera strobe opening time. Most of the underwater distance gating imaging systems and other three-dimensional underwater imaging systems use flash-pumped Nd:YAG lasers. The device is mature in technology, low in cost, and has an output wavelength of 1106Lm. After doubling frequency, 532nm green light can be obtained. At present, the average power can reach several tens of watts, and the working distance can reach several kilometers.
- Underwater Communication
Underwater communication technology based on visible light has the advantages of high communication rate, low cost, compact instrument, easy integration and good confidentiality. Due to the high modulation rate of laser modulation and high single-pulse energy, underwater visible light communication can achieve higher communication rates and wider communication bandwidth.
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