This paper illustrates the principle of several common distributed fiber sensing techniques, especially Brillouin optical time domain reflectometry (BOTDR) and optical time domain reflectometry (OTDR). By measuring the frequency shift of spontaneous Brillouin scattering light in fiber, BOTDR could simultaneously monitor both strain and temperature with a high spatial resolution. But the spontaneous Brillouin scattering signal is so weak that it has a high demand of the laser generator and the signal-to-noise ratio of the whole system. Therefore, the BOTDR system is usually too complex, expensive and difficult to be widely used. Unlike BOTDR, OTDR utilizes Rayleigh scattering to measure the loss of fiber. Rayleigh scattering signal is much stronger than spontaneous Brillouin scattering signal, thus OTDR system has the advantages of high sensitivity, long distance and relatively low price. These advantages make OTDR very suitable for wide application in the field of slope monitoring, especially in remote areas where the geographical environment is complex and are difficult for staff to stay.
This paper designed and implemented a slope monitoring and warning system based on the technology of optical time domain reflectometry(OTDR). The test result shows that the system has high sensitivity, strong real-time and provides user friendly interface.
In order to achieve the goal of constant output light power, the design of high accuracy digital control semiconductor laser constant current source is very important. Semiconductor laser is can be divided into load floating and load grounding two types based on the package. In this paper, a small power grounding type digital control semiconductor laser constant current source is realized by using ARM processor. By using this method, the semiconductor laser can be provided within the range of 0mA-500mA constant current. At same time, some functions such as slow start, temperature control, overcurrent protection are realized. In actual experiment test result show, the design method is stable, reliable and high accuracy.
Basing on the measurement of pulse time-of-flight, 3D imaging LADAR have ability to obtain the profile of target surface. As the convolution results of impulse response function of footprint and probe pulse, the reflected pulse will distort if the footprint contain a variety of distances. After discriminate process, this distort brings in a time error for distance measurement, which becomes the anamorphose on system imaging further more. According to the discussion of time-dependent scattering cross section, this kind of anamorphose is mainly decided by the slope of target surface when the system parameters and target distance are determined. A compensation method for height error based on detected slope has been put forward. First of all, the slope distribution of detected surface could be calculated from the point cloud data by two-way difference method. Then, the approximate compensation height error will be obtained, according to the slope-error relationship by assuming that each footprint be a tilting plane. After adding to the detected data, the first approximate target surface data has been acquired. The compensation result will approaching the real value by repeating the three steps above. As an example, simulation analysis of Gaussian pulse imaging detection has been given. The result shows that this compensation method is effective and efficient.