Optics: measuring and testing – For optical fiber or waveguide inspection
Patent
1998-01-09
2000-04-04
Font, Frank G.
Optics: measuring and testing
For optical fiber or waveguide inspection
G01N 2100
Patent
active
060467975
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to an optical time domain reflectomer (OTDR) and a method of fault or break detection by optical line domain reflectometry.
Optical communication systems are increasingly relying on the use of optical amplifiers to boost the optical signals so that communication at high bit rates over long ranges can be achieved. The cable and amplifiers for such systems are often located in inaccessible places, so a method of fault finding remotely from the ends of the system is required.
2. Description of the Prior Art
One method for measuring faults in fiber spans which has been used extensively is the optical time domain reflectometer. This injects an optical pulse into the fiber and then measures the relative magnitude and delay of the back scatter from the pulse as it propagates through the fiber. Generally optical amplifiers used in commercial applications include an optical isolator which prevents the backscattered light from passing back through the amplifier. The isolator therefore precludes the use of this type of OTDR on fiber spans employing optical amplifiers.
Several methods have been proposed for fault location on a bidirectional optically amplified line e.g. R. Jensen, C. Davidson, D. Wilson and J. Lyons. "Novel Technique for Monitoring Long Haul Undersea Optical Amplifier Systems", Conference on Optical Fiber Communications 1994, Paper No. ThR3. The most convenient is the inclusion of cross couplers between the send and return amplifiers within a repeater as illustrated in FIG. 1. Note: Amplifiers A & C are an amplifier pair in one repeater, amplifiers B & C are an amplifier pair in the next repeater in the line.
Conventional OTDR's using an amplitude modulation pulse stream are not suitable for use in a system employing optical amplifiers since the amplifiers require a constant mean power with respect to the erbium fiber time constant. To overcome this it has been proposed by Y. Horiuchi, S. Ryu, K. Mochizuki and H. Wakabayashi, in "Novel Coherent Hetrodyne Optical Time Domain Reflectometry for Fault Localization of Optical Amplifier Submarine Cable Systems". IEEE Photonics Technology Letters, Vol 2, No. 4 (April 1990), that the pulses used are in the form of frequency shift keying (FSK) of an optical carrier (as in FSK RADAR).
Such systems suffer from degradation due to polarization effects and the present invention seeks to reduce the effect of polarization.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved optical time domain reflectometer.
According to the invention there is provided an optical time domain reflectometer comprising means for generating three different wavelengths and means for alternating between one of the wavelengths .lambda..sub.1 and the other two wavelengths in response to a signal generator for providing a test signal for transmission on an optical fiber transmission cable. By employing a different wavelength for alternate signal portions, polarization reflection states would be different in each case so that polarization sensitivity is reduced.
The three different wavelengths may be derived from a common narrow linewidth laser source by means of one or more optical modulators responsive to an RF signal which is triggered to provide the alternation between one of the wavelengths being the narrow linewidth, and the other two wavelengths. In one embodiment of the invention, a single acousto-optical modulator is coupled to the laser source and is provided with oppositely disposed modulating inputs driven alternately from an RF generator in response to a pulse signal generator. The modulator may have individual outputs for the three wavelengths, which outputs are combined onto a single line by optical coupling means to provide the test signal for transmission.
In another embodiment first, second and third acousto-optical modulators may be provided, all optically coupled to the output of the laser source, the first and second of which modulators are coupled to an RF generator which pro
REFERENCES:
patent: 4551019 (1985-11-01), Vella et al.
patent: 5118931 (1992-06-01), Udd et al.
patent: 5179420 (1993-01-01), So et al.
patent: 5428635 (1995-06-01), Zhiglinsky et al.
"Novel technique for monitoring long-haul undersea optical-amplifier systems" by R.A. Jensen et al., Conference on Optical Fiber Communications Technical Digest, 1994, pp. 256-257.
"Novel coherent hetrodyne optical time domain reflectometry for fault localization of optical amplifier submarine cable systems" by Y. Horiuchi et al., IEEE Photonics Technology Letters, vol. 2, No. 4, Apr. 1990.
Butler David
Spencer Geoffrey
Alcatel
Font Frank G.
Nguyen Tu T.
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