Aiming at the phenomenon of signal fading and system performance degradation due to the light intensity fluctuation and beam off-axis drift in the line-of-sight (LOS) transmission of laser-based cross-medium communication systems, which are affected by absorption, scattering, and turbulence, a joint coding scheme based on polar codes and pulse position modulation (PPM) is proposed to study the encoding and decoding as well as the method to improve system performance for laser-based signal transmission from atmosphere to underwater. The laser wireless transmission characteristics and the principle of channel merging and splitting of polar codes are analyzed. Furthermore, in terms of bit error ratio (BER) comparisons are made by simulations between the PPM systems with and without polar codes over weak turbulent channels, and meanwhile the effects of code length and rate on BER are discussed. The simulation results show that, compared to the traditional PPM systems without polar codes at the same BER, a system using the joint coding of the Polar Codes and PPM can obtain a code gain from 1 to 2, namely its BER performance and anti-interference ability could be improved. And thus this research can provide a new technical means for practical applications.
Proc. SPIE. 11333, AOPC 2019: Advanced Laser Materials and Laser Technology
KEYWORDS: Signal to noise ratio, Modulation, Error analysis, Field programmable gate arrays, Data transmission, Telecommunications, Turbulence, Wireless communications, Laser communications, Laser systems engineering
Aiming at the characteristics of signal fading caused by the intensity fluctuation and the off-axis drift of the underwater wireless optical communication, a method of laser-based trans-media transmission from atmosphere to underwater employing low density parity check (LDPC) and pulse position modulation (PPM) joint coding is proposed. The encoded frame structure is studied. Furthermore, by simulation, the performance of LDPC and PPM joint coding in Gaussian channel and weak turbulence channel is analyzed and compared with that of the uncoded PPM system. A practical communication system based on Field Programmable Gate Array (FPGA) is established and relevant lab tests are carried out. The simulation results show that the joint coding of LDPC and PPM can significantly improve the system error performance under these two channels, and the performance of Gaussian channel is better than that of weak turbulence channel. It is verified by the tank experiment that the system combining LDPC and PPM can obtain 1.5 dB code gain compared to the uncoded PPM system at the same bit error rate, which fully demonstrates the anti-interference ability of the system combining LDPC with PPM in data transmission.
This work employs the complementary metal–oxide–semiconductor (CMOS) camera to acquire images in a scanning manner for laser line scan (LLS) underwater imaging to alleviate backscatter impact of seawater. Two operating features of the CMOS camera, namely the region of interest (ROI) and rolling shutter, can be utilized to perform image scan without the difficulty of translating the receiver above the target as the traditional LLS imaging systems have. By the dynamically reconfigurable ROI of an industrial CMOS camera, we evenly divided the image into five subareas along the pixel rows and then scanned them by changing the ROI region automatically under the synchronous illumination by the fun beams of the lasers. Another scanning method was explored by the rolling shutter operation of the CMOS camera. The fun beam lasers were turned on/off to illuminate the narrow zones on the target in a good correspondence to the exposure lines during the rolling procedure of the camera’s electronic shutter. The frame synchronization between the image scan and the laser beam sweep may be achieved by either the strobe lighting output pulse or the external triggering pulse of the industrial camera. Comparison between the scanning and nonscanning images shows that contrast of the underwater image can be improved by our LLS imaging techniques, with higher stability and feasibility than the mechanically controlled scanning method.
In the conventional coherent detection method, almost all the techniques of splitting and combining homologous
light source have been adopted, as a result of which the coaxial adjustment of signal and reference light
becomes quite complex and the energy utilization is rather low. This paper presents a new coherent detection
method with a shared line based on acousto-optic deflection. By the way of analyzing the optical structure and
the principle of frequency measurement, it proposes basic measures and solutions to improving the system's
accuracy and resolution of instantaneous frequency measurement. Best use has been made of the characteristics
of the diffracted spots so as to optimize the optical path structure and to make the effective superposition of 0
level spot and 1 level diffracted spot, thus realizing the tracking and measurement of electromagnetic spectrum
signal. Experiments show that application of the acousto-optic signal processing method in coherent detection
can overcome the contradiction between the frequency resolution and the instantaneous bandwidth of traditional
communication receiver and meet the demands of the communication system to high intercepted probability,
large instantaneous bandwidth, as well as rapid measurement of pulse. In comparison with the conventional
coherent detection method, this method can make the power of light source reduce by 50%, and receiver's
sensitivity much better than -65dBm, and then the intercept probability reached up to 100%. Moreover, the
frequency resolution is better than 100KHz.All this can achieve the seamless measurement of carrier frequency.
A technique is developed to detect underwater acoustic signals by utilizing the resultant surface perturbations. It
uses light wave in air and uses acoustic wave under water. Because two best channels are combined together, it is
an ideal method for remotely detecting underwater acoustic signals. The underwater acoustic signal can cause the
perturbations at water-air interface. These perturbations result in an amplitude modulation effect on a laser beam
reflected by the water surface. The modulated underwater laser signal can be detected by using a direct light
intensity detecting method. Based on the theory of modulation of the intensity of laser, a detection system was
developed in the laboratory. By analyzing the massive experimental data, the technique of laser detection of
underwater acoustic signals is validated.
For the measuring demand of surface coating quality, and to keep the surface to be measured from being damaged, laser
is adopted as a optical probe. When contactless measurement of the object surface coating uniformity is carried out, a
laser beam illuminates the object surface to be tested. By analysis of the reflected light signal from the random surface,
relation between the intensity of reflected light and surface coating uniformity is obtained. To decrease the error caused
by the dithering of source, laser source is fitted at a certain height above the surface to be measured. By moving the
optical table, light spot scans over the object surface. Real-time statistic of the data collected is made by computer, and
then deviation report of surface coating uniformity is acquired. Experiment results prove that the measurement system is
simple, stable and reliable, with high speed of response and good interference killing feature. Measurement accuracy of
the system achieves 5μm, and error of measurement value is less than ±1% within an operation scope from 1 μm to 50 μm. The method has the advantage of simple structure and intuitionistic principle, and it can detect online conveniently.