Source: http://www.google.com/patents/US7995197?dq=5,381,459
Timestamp: 2015-03-03 00:59:02
Document Index: 790510282

Matched Legal Cases: ['Application No. 05733029', 'Application No. 05826466', 'Application No. 2007', 'Application No. 200580009905', 'Application No. 200580009905', 'Application No. 200580009905']

Patent US7995197 - Distributed backscattering - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsThe present invention relates to a method for detecting or inferring a physical disturbances on a communications link, in particular by using distributed backscattering. The method includes the steps of: transmitting test signals onto a link; receiving test signals returned from a remote portion of the...http://www.google.com/patents/US7995197?utm_source=gb-gplus-sharePatent US7995197 - Distributed backscatteringAdvanced Patent SearchPublication numberUS7995197 B2Publication typeGrantApplication numberUS 11/663,954PCT numberPCT/GB2005/003680Publication dateAug 9, 2011Filing dateSep 26, 2005Priority dateSep 30, 2004Also published asDE602005023500D1, EP1794905A1, EP1794905B1, US20080278711, WO2006035205A1Publication number11663954, 663954, PCT/2005/3680, PCT/GB/2005/003680, PCT/GB/2005/03680, PCT/GB/5/003680, PCT/GB/5/03680, PCT/GB2005/003680, PCT/GB2005/03680, PCT/GB2005003680, PCT/GB200503680, PCT/GB5/003680, PCT/GB5/03680, PCT/GB5003680, PCT/GB503680, US 7995197 B2, US 7995197B2, US-B2-7995197, US7995197 B2, US7995197B2InventorsEdmund S R Sikora, Peter HealeyOriginal AssigneeBritish Telecommunications Public Limited CompanyExport CitationBiBTeX, EndNote, RefManPatent Citations (103), Non-Patent Citations (73), Classifications (8), Legal Events (1) External Links: USPTO, USPTO Assignment, EspacenetDistributed backscattering
FIG. 1 shows the basic system architecture of a surveillance system. In general terms, it comprises at a near end of an optical fibre (here a silica glass fibre), an optical source, an unbalanced Mach-Zehender interferometer (with a fibre delay and polarisation scrambler �POL� in one arm), an analogue optical receiver, a filter and a signal processing system. A disturbance is shown at position �d�.
Light from the optical source is split into two paths in the Mach-Zehender interferometer; one path is connected directly and one goes via an optical delay of several kilometres of standard fibre and polarisation scrambler. Thus the fibre under test conveys two copies of the source signal, one delayed by an amount �D� relative to the other. The source signals are backscattered in a continuous fashion as they propagate from the source along the fibre, the backscattered signals being returned towards the source after they have traveled through a portion of the fibre. For signals that had propagated beyond the disturbance point, the phase, polarisation and amplitude of the signals will be perturbed by the disturbance in both the forward and reverse directions of propagation. On returning to the interferometer the differential delay �D� is effectively un-done for one pair of propagating signals. Optical interference takes place at the 2�2 port coupler nearest the receiver creating an intensity modulated output signal that is sensitive to micro disturbances along the fibre under test (different types of disturbance will give rise to different characteristic signatures that can be identified by their respective spectral content).
In the embodiment of FIG. 1, the fibre end 16 c is a free end: the free end could be achieved by simply breaking the fibre, such that it is unprepared (although the free end may reflect some light, typically around 4%, this would not be sufficient without backscattering to allow detection of a disturbance). A fibre with a free end is particularly convenient since the fibre can be permanently or temporarily introduced in/around a region or object to be sensed. For example, the fibre could be introduced into an underground duct, at one end of the duct, without the free end necessarily being recovered at the other end of the duct. Clearly, if a free end only provides 4% reflection or less than 5% or 6%, the distributed backscattering will be responsible for the vast majority of the returned signal�typically at least 90%.
The fibre may be ordinary commercially available fibre, such as single mode silica glass fibre. In such fibre, Rayleigh scattering is normally caused by inhomogeneities in the dielectric medium of the glass whose correlation length is small compared with the wavelength of the light. The inhomogeneities normally arise from the thermal and compositional fluctuations in the glass structure which are �frozen-in� when the glass first solidified. Thus, the backscattering may be due to density or compositional perturbations or variations in the glass which arise during a cooling phase of the manufacturing process. In this way, the imperfections or inhomogeneities (which are often considered undesirable) will normally be introduced unintentionally when the fibre in manufactured. However, further imperfections may be introduced into the fibre, preferably in a backscattering portion thereof, so as to yet further increase backscattering.
We have performed an initial theoretical analysis of the operation of this instrument that explains the nature of the experimental results observed. The analysis shows that the predominant cause of the observed signal is due to phase modulation of the test signal (i.e., the output signal copies). We have also confirmed operation with different types of optical source, ranging from a highly un-coherent source of un-polarised amplified spontaneous emission (ASE) generated by a Erbium doped fibre amplifier (EDFA) (coherence length �0.1 mm) to a typical systems distributed feedback (DFB) laser (coherence length �20 m). We have also used a multi-longitudinal mode Fabry-Perot laser.
A disturbance is likely to change the spectrum of background �noise� as well as the level of noise, such that different disturbances will have different characteristic spectrum types. The distance between the base station and the point where a disturbance is occurring may also affect the spectrum. In general terms, the signal processing system 29 may therefore be configured to compare the noise spectrum at intervals with stored spectrum signatures for known disturbances, and to generate a disturbance alert signal if a monitored spectrum is found to match one of the known signatures. Alternatively, or in addition, the signal processing system 29 may be configured to run a learning algorithm in order to learn the characteristic spectrum types for different disturbances experienced along the particular optical fibre path which is being monitored.
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