Yuangang Lu

Improvement of Spatial Resolution for BOTDR by Iterative Subdivision Method

An iterative subdivision method is proposed to improve the spatial resolution of Brillouin optical time-domain reflectometry. By considering the affection of the response time of detection system to the detected Brillouin signal together with the probe pulse, an energy density distribution (EDD) is obtained and the fiber which generates the detected Brillouin signal is divided into short segments with equal length. Then the sub Brillouin signal generated from each segment is extracted out of the detected Brillouin signal based on the EDD. By iteratively using the subdivision method along the fiber, a group of Brillouin signals corresponding to short fiber segments can be obtained. So the Brillouin frequency shift of shorter fiber can be obtained and a higher spatial resolution is achieved. To improve the accuracy, a cross correlation algorithm is also used to enhance the signal-to-noise ratio of the Brillouin signal before the subdivision procedure. A spatial resolution of 1.5 m in a 50 km sensing fiber is obtained by using 100 ns probe pulse, and a spatial resolution of 0.1 m in a 1 km sensing fiber is also obtained by using 10 ns probe pulse.

Determining the Change of Brillouin Frequency Shift by Using the Similarity Matching Method

We have proposed a similarity matching method (SMM) to obtain the change of Brillouin frequency shift (BFS), in which the change of BFS can be determined from the frequency difference between detecting spectrum and selected reference spectrum by comparing their similarity. We have also compared three similarity measures in the simulation, which has shown that the correlation coefficient is more accurate to determine the change of BFS. Compared with the other methods of determining the change of BFS, the SMM is more suitable for complex Brillouin spectrum profiles. More precise result and much faster processing speed have been verified in our simulation and experiments. The experimental results have shown that the measurement uncertainty of the BFS has been improved to 0.72 MHz by using the SMM, which is almost one-third of that by using the curve fitting method, and the speed of deriving the BFS change by the SMM is 120 times faster than that by the curve fitting method.

Investigation on the influence of intensity scintillation and beam wander in space optical uplink DWDM communication system

Abstract Dense wavelength division multiplexing (DWDM) has been widely applied in ground optical communication. However, the technology of DWDM is still not mature enough in the space optical communication system. In order to further advance the use of DWDM into space optical communication, the probability density function (PDF) and the bit-error rate (BER) performance of DWDM is investigated in uplink communications under the influence of atmospheric turbulence, consisting of intensity scintillation and beam wander caused by atmospheric turbulence. Numerical results show that the atmospheric turbulence has a great impact on BER and PDF, and wavelength, divergence angel, and other relevant parameters should be carefully considered in this DWDM system. This work can be conducive for improving DWDM design of space optical uplink communication systems.

Quality Map Generation in Two-Dimensional Phase Unwrapping Process by Using Edge Detection Techniques

Two-dimensional phase unwrapping is the task of recovering the true phase values, given the wrapped phase values in an image. Phase unwrapping arises in several branches of applied optics, physics, medicine and engineering, such as homomorphic signal processing, solid-state physics, optical interferometry, adaptive or compensated optics, magnetic resonance imaging, synthetic aperture radar interferometry, and optical and electron holography (Volkov & Zhu, 2003). In these applications, the measured information is denoted by a two-dimensional phase distribution called the wrapped phase image. In wrapped phase images, the phase is the interval (-π, π] or (0, 2π] due to the use of the mathematical arctangent function (Bone, 1991). Since this wrapped phase suffers from 2-π phase jumps, it is unusable until the phase discontinuities are removed. In order to recovering the true continuous phase values to denote real physical quantity, a phase unwrapping process is needed to recovering the true phase values. The procedure of phase unwrapping is performed by either adding or subtracting integer multiples of 2π to all successive pixels when a phase discontinuity encounters, which are based on some kind of threshold mechanism (Ghiglia et al., 1987). However, many factors, such as surface discontinuities, noise, under-sampling, or shadow, would produce unreliable phase data, which make the recovery of the wrapped phase challenging. To solve the problem, many phase unwrapping algorithms have been developed during the last three decades. These phase unwrapping algorithms can be found in a very good reference book (Ghigli & Pritt, 1998) and review papers such as (Baldi et al., 2002; Jenkinson, 2003; Su & Chen, 2004; Zappa & Busca, 2008). In many phase unwrapping algorithms, a quality map, which evaluated the quality or the reliability of the phase data, is used for completing the phase unwrapping process. In wrapped phase images, the quality of a pixel is low if it is located in areas where the surface discontinuities, noise or undersampling exists. On the contrary, the quality of a pixel is high if it is located in areas where the variation of phase value is low. From the mathematical point of view, quality map is a matrix of the same size of phase image that assign a quality value to each pixel. The quality values are usually normalized in the range [0, 1], where a large value means high reliability of the corresponding pixel. In most phase unwrapping algorithms, a quality map is necessary to guide the phase unwrapping process for achieving desire results. Furthermore,

Investigation on the BER performance of the MSK space downlink laser communication system with a power EDFA

For a space downlink laser communication system with an EDFA as a power amplifier, the performance of its BER deteriorates because the EDFA’s characteristics are badly impacted by space radiation. As is investigated in this paper, small divergence-angle, lower than 30μrad, assures that the BER is lower than10-20 although the increase of radiation dose from 0Gy to 250Gy leads to 20 orders of magnitude increase of the BER. Such perfection results from our selection of optimal parameters. In the case of zenith angle, the BER increases smoothly when the zenith angle is lower than 10 degrees. After the point of 10 degrees, however, the BER starts its linearly fast increase. Increasing the radiation dose makes the BER increase and such evolution trend more smooth. Moreover, the increase of receiving diameter leads to linear reduce of BER. It is interesting to note that the evolution becomes nonlinear in region of low receiving diameter when we change the divergence-angle to a higher value 60μrad. Besides, suffering radiation makes the non-linearity mentioned above more apparent. Another try to change the zenith angle to higher value 45° does not show obvious nonlinear effect but it worsens the performance of BER quite a lot. Commonly, the impact of radiation will reach its saturation when the dose of radiation continues to increase. The work will benefit the design of practical space laser communication system with EDFAs.

The use of BOTDR to evaluate the thermal effects on fiber residual strain of optical fiber cables

The residual axial strain induced in the cabled fiber is a vital factor to evaluate the reliability of optical fiber cables. However, little is known about the thermal effect on the fiber residual strain, especially on the particular short section of the fiber. In order to precisely determine the thermal effects on fiber residual strain of optical cables, a measurement method based on Brillouin Optical Time-Domain Reflectometry (BOTDR) system is proposed. Thermal characteristics of fiber residual strain of optical fiber cables are investigated theoretically and experimentally. Measurements of the residual strain in particular short section of fiber are made with the distributed fiber sensing system based on BOTDR. Experimental results for a double-coated fiber in loose structure cable are in good agreement with those predicted from the theory. It has been found that the fiber residual strain increases linearly with the decrease of the temperature in the range from 50°C to -50°C.