Optics: measuring and testing – For optical fiber or waveguide inspection
Patent
1989-01-19
1990-10-09
McGraw, Vincent P.
Optics: measuring and testing
For optical fiber or waveguide inspection
G01N 2188
Patent
active
049616447
DESCRIPTION:
BRIEF SUMMARY
The invention is now described by way of example with reference to the accompanying drawings in which:
FIG. 1 is a schematic illustration of a first embodiment of the invention in which a surveillance signal is launched into a fibre and monitored from the launch end, and
FIG. 2 is a schematic illustration of a second embodiment in which the surveillance signal is monitored at the end remote from the launch of the surveillance signal.
In local or short haul communication networks there is a demand for confidential transmission and it is this type of network that is the principle concern of the invention although it is envisaged that the system may be developed for longer haul applications. In a preferred embodiment a surveillance signal is generated and transmitted along the same path as the data signals. Any available transmission window may be used for the surveillance signal, for example the data transmission may use 1300 nm and the surveillance signal 1310 nm or data tansmissions may be spread around 1300 nm and the surveillance signal transmitted at 850 nm. The surveillance signal is constantly transmitted even when there is no data transmission.
The surveillance signal consists of a cyclically varying signal, for example a laser beam having a square or sinusoidally modulated amplitude. In fact the modulation may take any form in which phase difference or delay can be detected, and for a given maximum amplitude the shorter the modulation period the more sensitive it becomes to phase difference. This amplitude modulated signal is launched into the transmission fibre and propagates along the fibre until it is reflected back towards the launching end by the fibre termination at the far end of the fibre. The proportion of the surveillance signal that is reflected may be increased by providing a metallised coating on the fibre termination or by provision of a grating or other wavelength sensitive device that will selectively reflect the surveillance signal. At the launching end the returning surveillance signal is compared with the launched signal for phase difference and this phase difference provides a reference value. If the transmission path is elongated then the phase difference between the launched and returning signals changes and this change is significant even for minute elongation: picking up a single fibre or disturbing a fibre by blowing on it is sufficient to change the phase difference between the launched and returning surveillance signals at surveillance signal frequencies of the order of 200 MHz. Thus by monitoring for phase difference changes from the reference value an indication of change in length of the fibre transmission line can be detected and this may indicate disturbance of the line. In general it is envisaged that frequencies greater than 50 MHz will be used, preferably frequencies in excess of 100 MHz and less than 300 MHz.
Various factors other than disturbance may also cause changes in length of the fibre, for example thermal expansion and contraction and vibration caused by the environment such as the passage of traffic. In order to eliminate consideration of changes due to these factors the phase difference values are monitored by a microprocessor which revises the reference value taking into account changes in temperature, traffic flow conditions and other variations related to external conditions or following predictable patterns. For example, if ambient temperature increases this can be monitored directly or, as will be necessary if the change in temperature occurs at a part of the transmission line remote from the microprocessor or any related sensor, the microprocessor will take into account comparatively slow and steady changes in the phase and recognise this as a temperature variation pattern and revise the reference value. Likewise vibration patterns caused by traffic will be recognisable both from time of day expectation levels and from the magnitude of the shifts. In contrast to this in order to tap into a fibre transmission line it is necessary to expose the fibre
REFERENCES:
patent: 4207561 (1980-06-01), Steensma
patent: 4514054 (1985-04-01), Stowe
patent: 4634852 (1987-01-01), Shaw
patent: 4636029 (1987-01-01), Johansson et al.
IEEE Instrumentation and Measurement Conference, Mar. 25-27, 1986, Boulder, Colo., IEEE (New York, U.S.), P. Wendland: "Fiber optic measurements; A review of recent methodology", pp. 219-222a.
Patent Abstracts of Japan, vol. 4, No. 175, (E-36)(657) Dec. 3, 1980, & JP. A. 55120240 (Hitachi Seisakusho K.K.), Sept. 16, 1980.
British Telecommunications public limited company
McGraw Vincent P.
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