A modulated light detecting and ranging (LIDAR) system has been developed to improve the detection of underwater targets. To study this detection scheme, this paper proposes a Monte Carlo simulation method for modulated pulse LIDAR systems. First, a Monte Carlo model is developed to simulate modulated pulse propagation in sea water. Second, a receiving system model is emulated with advanced signal processing tools like Fourier transform, cross-correlation and filtering. The simulation program based on the model is realized. This program is capable of predicting experimental results and helping in the design of more advanced transmitter and receiver configurations as various parameters of the environment, modulation and geometry are taken into account in our design. The calculations of backscattering frequency responses with different attenuation coefficients exhibit the low-pass filter-like response of backscattering. The modulated LIDAR results for a target depth of 5 m and attenuation coefficient of c = 0.67 and 0.85 m −1 are also computed. It is validated that the modulation approach can suppress volume backscattering clutter and can enhance the target contrast. Furthermore, comparisons with reported experimental results indicate the feasibility of the simulation model.

Compared to conventional floodlighted imaging systems, a pulsed laser range-gated imaging system can get a real-time high performance underwater image when the distance of target is known. Otherwise, optical properties of the water, mainly the attenuation coefficient of the water, should be obtained to automatically set the parameters of the imaging system. Typically, special instrument is required to measure the attenuation coefficient of the water. In this work, a water attenuation coefficient estimation method is proposed merely with the pulsed laser range-gated imaging system. The imaging model of the backscattered light of the pulsed laser range-gated imaging system is built through the light propagation theory. As a result, the water attenuation coefficient is calculated by nonlinear estimation method. Experiments under different water conditions are designed and carried out to verify the proposed method. Results show that the relative errors between the estimation and the reference are about 1% for the water in the water tank and seawater pool, and about 4% for the towing boat tank, respectively.

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Lens for rectangular illumination has been designed in (u,v) coordinate system already. Because of light source in (u,v) coordinate system can be divided into two identical parts, we have found that it is possible to design lenses for illumination of specific shapes. Specific shapes include triangles, as well as quadrangles which can be divided into two equal-area parts by one of the diagonals. We partition two equal-area parts into grids separately with equivalent luminous flux as well as light source, after that we construct freeform lenses by ray mapping method. Simulation results show that lenses for illumination of specific shapes are obtained with a Lambertian point source and favorable efficiency is over 0.86.

We have designed a new TIR(Total Internal Reflection) structure for generating an LED lens which can produce a rectangular beam with small divergence angle in two perpendicular directions for an optical guidance system. The lens can control the divergence angle in the horizontal direction to be a small value of about 8° with a 1 mm × 1 mm LED source, also in the vertical direction it can be about 7°, with optical collection efficiency higher than 0.83. After the lens is manufactured, the work demonstrates that the lens is suitable for an optical guidance system.