Optical time domain reflectometer with improved sidelobe suppres

Details Optical time domain reflectometer with improved sidelobe suppres Optical time domain reflectometer with improved sidelobe suppres

1992-06-24

1994-04-26

McGraw, Vincent P.

Optics: measuring and testing

For optical fiber or waveguide inspection

G01N 2188

Patent

active

053071409

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to an optical time domain reflectometer
2. Discussion of Prior Art
An optical time domain reflectometer is a known and commercially available device for determining the transmission properties of optical fibres. The device employs short laser pulses applied to a fibre input, and light returned to that input by reflection or backscattering within the fibre is analysed. Reflection occurs at fibre splices and major flaws such as cracks. Rayleigh scattering occurs due to microscopic flaws and refractive index changes in the fibre. Major flaws give rise to substantial reflections producing peaks in the return signal displayed as a function of time. The position of such a flaw is given by:
The factor of 2 in Equation (1) appears because the optical pulse covers the distance from fibre input to flaw and returns to the input before detection.
For any optical time domain reflectometer device, there is a fundamental limit to the length of optical fibre which may be investigated. This limit is set by the point at which the return signal becomes dominated by noise. The return signal becomes steadily weaker as the pulse time of flight and length of fibre increase due to the cumulative effects of absorption, reflection and scattering. A typical attenuation per unit length in a good quality optical fibre is 1 dB/km. Since an optical time domain reflectometer employs a double transit of a fibre, it will detect 2 dB/km attenuation in a good quality, splice-free fibre. As the fibre length increases to that required for long-haul communications, the attenuation becomes too severe for time domain measurement purposes.
In order to increase the length of optical fibre over which time domain measurements may be made, the laser pulse power may be increased. This is however undesirable because it increases the laser cost. Moreover, the input end of a fibre is a major source of reflection, and this may cause safety problems if high pulse powers are used. A further consideration is that power absorbed within a fibre at a flaw may damage the fibre, or may cause optical nonlinearity and measurement error. Alternatively, the pulse power may be kept constant and the pulse length (time duration) may be increased. This increases the energy in the pulse and the signal to noise ratio. However, it reduces the distance resolution (accuracy) for flaw detection in direct proportion to increase in pulse length. For example, with constant pulse intensity, increasing the duration of a pulse from 1 nanosecond to 1 microsecond increases the pulse energy and signal to noise ratio one thousandfold. However, a 1 nanosecond pulse has a physical length of 30 cm in free space and 20 cm in an optical fibre having a core refractive index of 1.5. Such a pulse will allow flaw position location to .+-.50 cm. For a 1 microsecond pulse, equivalent location accuracy is .+-.50 m. This implies that 100 meters of optical fibre would require physical inspection to locate a fault detected using a one microsecond pulse. Furthermore, multiple defects which are within 100 meters of one another would be unresolved.
Digital pulse correlation techniques have been used in attempts to overcome the foregoing problems. Examples of such techniques are described by K. Okada, K. Hashimoto, T. Shibata and Y. Nagaki in Electronics Letters, Jul. 31, 1980, Vol. 16, No. 16, pages 629-630 and by P. Healey in Proceedings 7th European Conference on Optical Communications, Copenhagen, 1981 pages 5.2. These techniques suffer several disadvantages. Firstly, in order to carry out correlation, two pulses are required. A received pulse must be synchronised with a delayed transmitted reference pulse waveform at the correlator. Testing over a range of pulse times of flight requires a series of correlations of return pulses with transmitted pulse waveforms with stepped delays. For example, in the case of a reflectometer with a resolution of 10 m and a fiber under test which is 20 km long, then 2000 correlations would be

REFERENCES:
Patent Abstracts of Japan, vol. 6, No. 217 (P-152) (1095), Oct. 30, 1982, & JP, A 57120836 (Nippon Denshin Denwa Kosha) Jul. 28, 1982.
Journal of Lightwave Technology, vol. 7, No. 8, Aug. 1989, IEEE, (New York USA) M. Tateda et al: "Advances in Optical Time-Domain Reflectometry" (Con't) pp. 1217-1224.
International Search Report.

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