Special issue-Ocean optics technology
2020, 49(2): 0203001. doi: 10.3788/IRLA202049.0203001
Ocean is an important part of the earth's ecological environment. Exploration and exploitation of marine resources may easily cause serious damage to ocean, such as large-scale oil spill, pollution and red tide caused by oil and gas exploitation. Hyperspectral imaging technology can obtain both image information and spectral information at the same time, and has important applications in marine in-situ detection. In this paper, some recent works about hyperspectral imagers are reviewed, including a small-scale hyperspectral imager combined with fluorescence technology for the classification of oil spills and the estimation of oil film thickness, a multi-mode hyperspectral marine in-situ detection system (in three modes:common reflection or transmission imaging, telescopic imaging and microscopic imaging) for hyperspectral detection of different algae and spores of some fish infectious disease carriers. Hyperspectral technology combined with lidar technology has great potential in monitoring oil spill, red tide and other marine pollutants. An inelastic hyperspectral Scheimpflug lidar system and a ligh-sheet Scheimpflug lidar system are also reviewed. The former is for the type identification of oil spills through the fluorescence spectrum of oil spills, and the latter is for the detection of the 3D shapes of some manikin, shells and corals with the refraction correction at the air-water interface.
2020, 49(2): 0203002. doi: 10.3788/IRLA202049.0203002
Compared with traditional underwater cameras, the detection distance of underwater range-gated imaging can be increased by 2-3 times. Furthermore, based on this technology, fast high-resolution 3D imaging can be achieved. It has great potentials in underwater target detection and recognition, automatic navigation, marine scientific research and natural resources exploration. Although range-gated imaging can suppress the backscattering noise by space slicing, and achieve higher quality images, the backscattering noise of the sampling water volume still exists in gated images, resulting in low image signal-to-noise ratio and contrast, especially for distant targets or low reflectance targets. This paper systematically introduced author's research of deblurring methods for 2D and 3D range-gated imaging. In 2D imaging, two methods were proposed:the first was an algorithm of self-adaptive double-plateau histogram equalization to improve image contrast and better meet human vision; the second was to use a water-noise-reference denoising algorithm to improve the signal-to-noise ratio of target gated images. In 3D imaging, there were also two methods proposed:one was to use the water-noise-reference denoising algorithm for 3D reconstruction, and the other was to use the threshold-dependent joint bilateral filter algorithm for enhancing 3D images. The four proposed methods can be used independently or in combination for denoising enhancement of underwater range gated imaging.
2020, 49(2): 0203003. doi: 10.3788/IRLA202049.0203003
Ocean optical systems have played an increasingly important role in ocean exploration, development and monitoring. Underwater wireless optical communication and underwater lidar are two types of ocean optical systems that are rapidly developing and have good application prospects. Underwater wireless optical communication is an ideal communication option for short and medium range applications due to high speed and low latency. Underwater lidar is also a highly precise and efficient observation method in applications like deriving geographic information and target detection. However, the complex optical characteristics of seawater channels have brought challenges to the further improvement of the performance of ocean optical systems. In seawater channels, not only strong absorption and scattering, but also dynamic interference factors such as turbulence and bubbles exist in the channel. In order to deal with these challenges, on one hand, signal to noise ratio can be increased with temporal or spatial methods. On the other hand, the conversion between temporal and spatial domains is beneficial to achieving better system performance. This article reviews solutions above and the development trend of ocean optical systems is pointed out.
2020, 49(2): 0203004. doi: 10.3788/IRLA202049.0203004
Blue-green laser has broad application prospects in non-acoustic detection of underwater targets. However, the existing detection models have not considered the problem of matching the detection probability with the self-guided system. A target detection model of underwater laser scanning detecting system was established based on undershoot distance to match the guiding precision. The simulation results shows that an increasing undershoot distance needs an increasing emit angle to get high detection probability at a certain emit frequency, which however leads to a decreasing acting time for the following system. In addition, an increasing undershoot distance needs an increasing emit frequency and a decreasing step angle to avoid missing the target. Finally, the simulation provides an optimal range for undershoot distance and system parameter. The model and the simulation results provide theoretic basis for the matching up design of guiding system and detecting system.
2020, 49(2): 0203005. doi: 10.3788/IRLA202049.0203005
Underwater spectral imaging technology plays an important role in underwater object recognition and ocean ecological monitoring. An underwater spectral imaging system using liquid crystal tunable filter (LCTF) was designed based on the actual engineering environment. The system obtained spectral information by using LCTF as a filter structure. Under the illumination of a wide-spectrum LED, a pool experiment was performed to obtain 31 channel spectral images of the target between 400 and 700 nm. The spectral information of objects with similar colors under water was discussed and analyzed. The results show that the system is helpful for underwater target recognition and classification. In-situ observation of corals in the sea trial successfully obtained underwater spectral images of coral reefs. The system is expected to be applied to ocean remote sensing, ocean ecological environment monitoring and other fields.
2020, 49(2): 0203006. doi: 10.3788/IRLA202049.0203006
A combination of formula derivation and simulation optimization was used to design a LiDAR optical-mechanical system for measuring water depth. The light source of this system used 532 nm and 1 064 nm dual-frequency lasers. Three receiving channels of 532, 647 and 1 064 nm were designed. The planned flight height was 140-500 m, the variable scanning angle is 9°-15°, the divergence angle was less than 0.5 mrad, and the surface point density range was about 0.687-4.170 points/m2. In this paper, Raman band was designed as a water depth measurement band to improve the measurement effect in shallow water. The variable scanning angle was used to realize the functions of variable resolution under a fixable width and the consideration of high precision and large field of view to adapt to different application scenarios.
2020, 49(2): 0203007. doi: 10.3788/IRLA202049.0203007
Oceanic lidar is a vital remote sensing device for detecting the upper ocean and constructing the 3-D ocean observation network. In this paper, the experimental data of lidar were compared with Monte Carlo(MC) simulation, analytical model and in situ data to find their matched-degree. Compared with experimental lidar signals, the MC simulation and analytical model have high agreement (R2>0.97). The normal lidar equation approximately adopting the in situ diffuse attenuation coefficient (Kd) also has good consistency (R2>0.92). The retrieved lidar attenuation coefficients show similar results. The MC simulation and the analytical model have better consistency. The results show that the experimental results of oceanic lidar match well with the simulation results of MC and analytical model.
2020, 49(2): 0203008. doi: 10.3788/IRLA202049.0203008
Underwater visible light communication (UVLC) is an attractive solution to achieve high-speed and large-data transmission but challenging due to the impairments induced by absorption, scattering and turbulence. To combat effects of multipath and fading for the UVLC system over turbulence channels, optical orthogonal frequency division multiplexing (O-OFDM) schemes with the transceiver spatial diversity were proposed, which employed equal gain combining (EGC) at the receiver side. Underwater path loss was calculated by a generalized Lambertian formula, and the fading induced by weak turbulence was modelled as a lognormal-distribution random variable. Based on the channel model and Monte Carlo (MC) simulation, the bit error ratio (BER) performance for quadrature-amplitude modulation (QAM) asymmetrically clipped optical OFDM (ACO-OFDM) and DC-biased optical OFDM (DCO-OFDM) systems in the channel with and without turbulence was evaluated. Furthermore, the diversity gain was estimated for different diversity orders and scintillation indexes. The results demonstrate that the diversity scheme with EGC is an effective measure to reduce the effect of turbulence and could be useful for designing, predicting, and evaluating the performance of O-OFDM UVLC system in a weak oceanic turbulence condition.
2020, 49(2): 0203009. doi: 10.3788/IRLA202049.0203009
In this paper, a spaceborne oceanic lidar simulation system used semianalytic Monte Carlo method was developed. The system can simulate the lidar returns of the atmosphere and the ocean with different optical properties through entering the parameters of the lidar system and the environmental parameters. At the same time, a user-friendly software interface for users was designed to operate input parameters and observe the output results intuitively. A variety of simulations was done, such as different types of water and different scattering phase functions. The simulation results were highly consistent with the theoretical lidar equations. The system was important to the research on the detection mechanism of spaceborne oceanic lidars.
2020, 49(2): 0203010. doi: 10.3788/IRLA202049.0203010
UV-Vis (Ultraviolet and visible) spectrometry was used to measure the absorbance of seawater with different concentrations relative to pure water. In order to meet the requirements for high-precision detection of seawater absorbance, the inherent background signal of the instrument and the difference between the sample containers were used to detect the absorbance of seawater where the error was included in the research category, and the experimental method was further optimized. The subtraction method was used to filter out the interference of the background signal. At the same time, the repeatability of the cuvette empty cup with respect to air and the cuvette with respect to air were verified. The difference in absorbance, and the error caused by the difference between cuvettes was filtered by the method of subtraction, and a certain characteristic wavelength was selected to observe its absorbance data relative to pure water, which verified the use of optimized experimental detection. The method finally achieves the purpose of measuring the absorbance of seawater with high accuracy (<0.000 5 AU). It was of considerable importance for detection of various substances in highly transparent water.
2020, 49(2): 0203011. doi: 10.3788/IRLA202049.0203011
A system for measuring the volume scattering function of water with particles was built, helping the laser and the detector to avoid the shading of each other that causes the reduce of detection angle. A set of double-periscopic optical system was used to separate the detection plane of scattering plane from laser exiting plane, thus the shelter of the laser source was minimized. What's more, as the transmission light was exported from the system, the scattering light of water container was avoided so that the precision of backscatter measurement was promoted. The prism for exporting underwater light was designed according to the manufacturing techniques and by which the scattering measurement equipment was designed to measure the scattering light ranges from 3° to 178°. Next, according to the structure of the system and the principle of optical transmission in water, data rectifying methods was designed to rectify the deviation of measurement optical distances and light absorption. The rectified results were finally compared with simulated results of Mie scattering, the main source of deviation was analyzed and the improvement scheme of the system was proposed.
2020, 49(2): 0203012. doi: 10.3788/IRLA202049.0203012
For the problems of image quality degradation in underwater scene target detection, an algorithm which combined the improved dark channel with MSR was proposed, which could adaptively compute water attenuation coefficient and effectively realize the recovery of underwater target. Through the built-in underwater imaging measurement device, the detection image of the underwater simulated environment with the aid of imaging system was obtained, the underwater detection image was processed step by step according to the algorithm flow chart, and an image for the effective recovery of underwater target radiation information was obtained. In order to objectively evaluate the algorithm effect, contrast, average gradient and information entropy were adopted as quantification to evaluate indexes factors. A quantitative comparison study between this algorithm and the conventional three algorithms was performed. The result show that the improved algorithm to deal with the results is better than the selected compared algorithms under all the quantitative evaluation indexes factors. The research results provide a basic theoretical exploration method for the underwater target detection, as well as have certain guiding significance for the implementation of underwater target detection.
The impact of Gamma Gamma strong oceanic turbulence and pointing error on the average bit error rate(BER) and outage probability of a heterodyne differential phase-shift keying(DPSK) underwater wireless optical communication(UWOC) system with an aperture receiver was investigated. The optical intensity fluctuation due to the combined effects of oceanic turbulence and pointing error was derived. The close-form expressions for the average BER and outage probability were derived. Then the average BER performance and the outage probability performance versus signal to noise ratio(SNR) of the considered UWOC system were investigated with different point errors, source beam widths, receiver aperture sizes and oceanic turbulence parameters. The results indicate that the larger the aiming error is, the worse the system performance is, under the same beam width and channel environment. Choosing a larger radio of source beam width to aperture radius or a bigger aperture receiver can help to improve the system performance. In addition, the system shows a better performance over the strong oceanic turbulence with a smaller ratio of temperature to salinity contributions to the refractive index spectrum ω and the rate of dissipation of mean-squared temperature χT or a larger rate of dissipation of kinetic energy per unit mass of fluid ε and the kinetic viscosity u. This work will provide reference for the construction and performance estimation of UWOC system on strong oceanic turbulence when taking pointing error into consideration.