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.
A novel distributed fiber birefringence, strain and temperature measurement based on homodyne Brillouin optical time domain reflectometry (BOTDR) is proposed. Birefringence measurement is based on the beat period detection of Brillouin beat of the same acoustic mode. Strain and temperature are obtained by measuring the power changes of Brillouin beat spectrum (BBS), which corresponds to the optical interaction of different acoustic modes in a complex profile fiber. The birefringence of a 4.3 km large-effective-area fiber (LEAF) was measured with 0.6-m spatial resolution, and strain and temperature at the end of a 4.5 km LEAF were measured at 1.5-m resolution in 140s.
Brillouin Optical Time-Domain Reflectometry (BOTDR) is attracting significant attention due to its immense potential
applications in temperature or strain monitoring. Conventional BOTDR system based on Spontaneous Brillouin
Scattering Spectrum(SBSS) narrow-band detection is not only time-consuming but also controlling complicated. To
overcome the drawbacks of the conventional detection method, a new SBSS wideband detection method is proposed in
this paper. In the new detection method, the whole SBSS is captured by use of a wideband receiver in a single
measurement, and Discrete Fourier Transform (DFT) signal processing algorithm is used to obtain the SBSS. The
configuration of a heterodyne detection BOTDR system based on SBSS wideband detection is presented, and the data
processing method, the spatial resolution of the system and the measurement time is discussed. The BOTDR system
based on SBSS wideband detection has been demonstrated, and a temperature resolution of 3°C and a spatial resolution
of 2 m have been achieved. The measurement time is only about one-tenth that of conventional narrow-band detection
method.
Phase unwrapping is the task of recovering the true phase values, given the wrapped phase values in an image. Flynn's
minimum discontinuity phase unwrapping algorithm can deal with many different kinds of phase unwrapping problems
successfully. However, since it takes the whole wrapped phase image as the improving loop search areas, Flynn's
algorithm has low efficiency in searching for the phase's discontinuity. To overcome this drawback, a new minimum
discontinuity phase unwrapping algorithm based on tabu search is proposed in this paper. In the proposed algorithm, the
nodes of the wrapped image are divided into different N levels according to the corresponding quality of the wrapped
image and the nodes with high levels are added into tabu list. In the iteration process of improving loops search, the tabu
list is updated according to the aspiration criterion to allow the search areas extend to nodes with high levels. By this
means, it searches the highest probability nodes belonging to paths of improving loops first and the lowest probability
nodes last. As a result, it significantly improves the efficiency of the algorithm. The proposed algorithm is described in
detail and verified by use of an experimental wrapped phase image with noise and undersampling. Unwrapped results
show that the proposed algorithm works well and is more effectively than the Flynn's minimum discontinuity algorithm.
Two-dimensional phase unwrapping method is important in a great number of applications. Flynn's minimum
discontinuity phase unwrapping algorithm can be used to solve many different kinds of phase unwrapping problems
successfully, but its main drawback is that, since it takes the whole wrapped phase image as the scanning areas, it
requires a large amount of computations and has low efficiency in searching for the phase's discontinuity within the
improving loop search area. To overcome this drawback, a novel minimum discontinuity phase unwrapping algorithm
using pretreatment and edge detection is proposed. In the new algorithm, a pretreatment operation is carried out on the
wrapped phase image to identify the isolated noisy pixels and exclude them from the search areas. Then, the phase's
discontinuity areas within wrapped phase image are detected based on edge detection techniques and taken as the
improving loop search areas. By these means, the efficiency of the new minimum discontinuity phase unwrapping
algorithm is significantly improved because the required scanning range is cut down to a large extent. This new
algorithm is described in detail and tested by using a noisy IFSAR experimental wrapped phase image. Unwrapped
results make it clear that the new algorithm works very well and is more effectively than the Flynn's minimum
discontinuity algorithm.
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.
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