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Fiber optic cable monitoring method and apparatus including moisture detection and bending loss detection
| Details |
Inventors: Vokey, David E.; Sontag, Kenneth N.; Kraft, Heinrich;
Assignee: Norscan Instruments Ltd. (Winnipeg, CA)
Primary Examiner: Nelms; David C.
Assistant Examiner: Messinger; Michael
Attorney, Agent or Firm: Thrift; Murray E., Ade; Stanley G., Battison; Adrian D.
The invention provides a monitor for monitoring the condition of fibre optic communication cables. The system employs one or more of the optical fibres of a cable to monitor the cable structure for damage or kinks. By using two separate laser sources and simultaneously monitoring the optical losses at two distinct and separate wavelengths, the loss signature of the monitored fibre is determined, analyzed and related to the mechanical condition of the cable structure. To monitor splice points for moisture, a special optical splice sensor unit detects any penetration of water into the splice and transmits a coded alarm signal over the monitored fibre to the optical receiver. Every splice location is assigned a unique sensor code. The optical splice sensor is driven by moisture detection cell, which forms a single cell water activated battery. Water entering the monitored splice closure wets the tape, which activates the cell. The cell generates sufficient voltage and current to power timing, code generation and modulator circuits in the sensor unit for an extended period of time. The activated circuit drives a modulator which modulates the light travelling down the fibre. The modulated light is monitored at the equipment office and the address of the alarming sensor decoded. |
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DETAILED DESCRIPTION Referring to the accompanying drawings, and especially FIG.
1, there is illustrated a fibre optic cable monitoring system 10 associated with a fibre optic cable 12.
The illustrated cable is a single mode (sm) fibre cable with a length up to 100 km.
The cable is shown as including a splice 14 part way along its length.
Conventionally, the complete cable would include a number of splices.
The cable includes a number of optical fibres 16.
The system 10 includes an optical transmitter unit 18 at one end of the cable and an optical receiver 20 at the other.
As illustrated most particularly in FIG.
2, the optical transmitter is a dual wavelength laser transmitter including a stabilized laser source 22 emitting light with a wave length of 1300 nm and second stabilized laser source 24 emitting light with a wavelength of 1550 nm.
The laser source 22 is connected to a modulating amplifier 26.
The inputs to the amplifier include a modulating input 28 with bias input 30 and a negative feedback 32.
The modulating input has a frequency F1 for modulating the transmitted 1300 nm light at that frequency.
The laser 24 is likewise associated with an amplifier 33 with a modulating input 34, a bias input 36 and a negative feedback 38.
The frequency of the modulating signal F2 is different from the modulating frequency F1.
The lasers are mated to a wave division multiplex coupler 40.
The coupler combines the light output of the lasers and divides the light energy equally to N output ports.
A monitored fibre is connected to each of the output ports.
Consequently, the single pair of lasers provides optical power to several monitored fibres or cables.
At the receiving end of the fibre is an optical receiver 20 (FIG.
7) that intercepts and detects the laser light from each monitored fibre on a PIN detector diode 42.
The light is converted by the diode to an electrical signal that is amplified by preamplifier 44.
The electrical signal contains frequency components at F1 and F2, which are separated by band pass filters 46 and 50 and measured by measuring circuits 48 and 52
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Communications 
