Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1996-09-27
1998-12-01
Le, Que
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
25022712, H01J 516
Patent
active
058442359
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical-fiber inspection device and more particularly to an improvement in the extension of its measurable region and an enhancement of its resolution in measuring the distance to a fault point.
2. Description of the Prior Art
There are two well-known optical-fiber inspection device thus far: the Optical Time Domain Reflectometer (OTDR) and Optical Frequency Domain Reflectometer (OFDR). The present invention relates to an optical-fiber inspection device of an OFDR scheme which identifies a fault point in a section of optical fiber by frequency information obtained by detecting the reflected light from the fault point above.
FIG. 1 shows an example of a theoretical configuration drawing of an OFDR using a fiber interferometer in the prior art. A laser beam emitted from laser diode (LD) module 1 is controlled by LD driver 2 to sweep its frequency. The laser beam is equally divided into two by optical coupler 3; one is propagated through a path 3a and incident to an optical detector, photo-diode (PD) 5. The other is incident to a section of optical fiber 4, a device under test (DUT), and reflected at a fault point 4a (e.g., fracture) in the optical fiber. This reflected light is propagated through a path 3b via optical coupler 3 and incident to PD 5. PD 5 detects interference between these two light beams (the interfering means is omitted).
The output of the PD is analyzed with spectrum analyzer 6 employing a method, e.g., Fast Fourier Transform (FFT), and the frequency and intensity of the interference light. In addition, LD driver 2 gives a trigger to spectrum analyzer 6 for the timing of the measurement.
In this configuration, the oscillation frequency of LD 1 is swept linearly so that the sweeping is .DELTA.f per unit time (.DELTA.t). If a time difference t1 exists between the light directly incident to PD 5 via the optical coupler and the light incident to PD 5 after being reflected in the DUT, the frequency difference between these two light beams f1 is expressed as shown below.
This time difference is proportional to the distance from the light-incident end of the optical fiber to a fault (fault point 4a) where reflection occurs within the DUT and, for example, expressed in the next expression (the lengths of paths 3a and 3b must be the same).
where L is the distance up to a fault point. (As light advances and returns the same distance, 2.times.L is used).
V is the speed of light within the DUT. (Let the speed of light in a vacuum be c, and the refractive index of the DUT be n; then, V=c
.)
From the above equations, if the sweeping rate (.DELTA.f/.DELTA.t) of the oscillation frequency is kept constant (already known), the distance to the fault point (position of the fault point) in the DUT can be known from the frequency of the interference signal at the PD, and the reflected amount at the fault point can be known from the magnitude of the interference signal. This enables an inspection device for an optical fiber or the like to be configured.
In this case, the measurable distance is limited by the coherence length of the light source used. The coherence length Lc is given by the following equation in a simplified form:
where c is the speed of light in a vacuum
For example, if an optical fiber where n=1.5 and an LD with a line width of 1 MHz are used, the coherence length Lc is 200 m, that is, up to a distance of 100 m or so can be measured for the one-way distance. To extend the measurable distance, a light source with a narrower line width is required. However, it is not easy to provide a light source whose light emitting frequency can be swept and whose line width is narrow.
As a method to increase the measuring distance, there is a method of extending the path of the reference light. As shown in FIG. 2, it is a method to connect an extended optical fiber 3c with a length h. This makes measurement of a fault point possible in a range from the forward and return length h within a DUT to the coherence length Lc
REFERENCES:
patent: 4708471 (1987-11-01), Beckmann et al.
patent: 4732469 (1988-03-01), Souma
patent: 5453826 (1995-09-01), Sugimoto et al.
Arihara Mamoru
Hirata Takaaki
Komiyama Makoto
Sampei Yoshihiro
Suzuki Yasuyuki
Kojima Moonray
Le Que
Yokogawa Electric Corporation
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