Optical pulse tester capable of displaying Raman or Rayleigh scattered light

An optical pulse tester capable of displaying Raman scattered light is disclosed. An optical pulse with the specified wavelength is injected into an optical fiber to be measured via a directional coupler and an optical demultiplexer/multiplexer. The demultiplexer/multiplexer demultiplexes returning light from the optical fiber into Raman scattered light different from the optical pulse in wavelength. The directional coupler divides returning light from the optical fiber into Fresnel reflection light and Rayleigh scattered light with the specified wavelength. The switcher supplies the Rayleigh scattered light to a display In the case of long distance measurement or precise loss measurement of the optical fiber. Therefore, the display displays the characteristic waveform of the optical fiber with the Rayleigh scattered light. In contrast, the switcher supplies the Raman scattered light to the display in the case of the measurement of the near-end part of the optical fiber or the connecting point between optical fibers without a dead area. Therefore, the display displays the characteristic waveform of the optical fiber with the Raman scattered light.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical pulse testers, and more 
particularly, to optical pulse testers which inject optical pulses into an 
optical fiber and demultiplexes returning light from the optical fiber 
into Raman scattered light, and which displays the Raman scattered light. 
2. Background Art 
An example of the structure of a conventional optical pulse tester will be 
explained with reference to FIG. 8. As shown in FIG. 3, an optical pulse 
generator 1, a directional coupler 2, a light receiver 8, an amplifier 4, 
a display 5, and an optical fiber 10, are provided. In FIG. 8, the optical 
pulse generator 1 emits optical pulse 11 with wavelength .lambda.a. The 
optical fiber 10 is to be measured and comprises an optical fiber 10B 
connected to an optical fiber 10A with an connector (not shown). 
The optical pulse 11 with wavelength .lambda.a from the optical pulse 
generator 1 is injected into the optical fiber 10 via the directional 
coupler 2. The optical pulse 11 travels through the optical fiber 10 with 
the generation of Rayleigh scattered light 14 and Raman scattered light 
(not shown). Furthermore, Fresnel reflection light 18 is generated at the 
connecting point of the connector and the like. The parts of the scattered 
light generated in the optical fiber 10 return to an injected end of the 
optical fiber 10 as back-scattered light, that is, returning light 12. The 
returning light 12 from the optical fiber 10 is divided into the Fresnel 
scattered light 13 and the Rayleigh scattered light 14 in the directional 
coupler 2 and the Fresnel scattered light 13 and the Rayleigh scattered 
light 14 are received by the light receiver 3. The Fresnel scattered light 
13 and the Rayleigh scattered light 14 in the returning light 12 are 
detected by the light receiver 3. The outpost light from the light 
receiver 3 is amplified by the amplifier 4. The output light from the 
amplifier 4 is displayed on the display 5. 
Next, an example of the displayed waveform on the display 5 shown in FIG. 3 
will be explained with reference to FIG. 4. FIG. 4 shows an example of the 
characteristic waveform of the optical fibers 10A and 10B in the case of 
connecting the optical fiber 10B to the optical fiber 10A with the 
connector. In FIG. 4, a vertical axis indicates loss of the optical fibers 
10A and 10B and a horizontal axis indicates distance from the injected end 
of the optical fiber 10A. The vertical axis of FIG. 4, for example, 
indicates 40 dB with full scale. In FIG. 4, a peak 13A indicates the 
Fresnel reflection light 13 at the injected end of the optical fiber 10A. 
A straight line 14A indicates the Rayleigh scattered light 14 in the 
optical fiber 10A decreases exponentially with distance from the injected 
end of the optical fiber 10A (to the right of the horizontal axis in FIG. 
4) and is logarithmically transformed. In FIG. 4, a peak 13B indicates the 
Fresnel reflection light 13 at the connecting point where the optical 
fiber 10B is connected to the optical fiber 10A. A straight line 14B 
indicates the Rayleigh scattered light 14 in the optical fiber 10B 
decreases exponentially with distance from the injected end of the optical 
fiber 10A and is logarithmically transformed. A peak 13C indicates the 
Fresnel reflection light 13 at an outgoing end of the optical fiber 10B. 
In FIG. 4, the right side area of the peak 13C indicates noise. 
In FIG. 4, the Fresnel reflection light 13A, 13B, and 13C and the Rayleigh 
scattered light 14A and 14B have the same wavelengths as the optical pulse 
11. Accordingly, when the Rayleigh scattered light 14A and 14B are 
measured, since the Fresnel reflection light is incomparably larger than 
the Rayleigh scattered light, the amplifier 4 shown in FIG. 3 may be 
supersaturated by the Fresnel reflection light 13A, 13B, and 13C. When the 
amplifier 4 becomes supersaturated by the Fresnel reflection light 13A, 
13B, and 13C, a dead area is generated in the amplifier 4 until a circuit 
of the amplifier 4 becomes stable. In the dead area, the state of the 
optical fiber 10A and 10B are not clear for a constant time. In FIG. 4, 
numerical reference 21 and 22 indicate the dead area caused by the Fresnel 
reflection light 13A and 13B, respectively. In the part of the dead area 
21 and 22, the state of the near-end part of the optical fiber 10A and 10B 
cannot be exactly measured. An optical switch operating at high speed may 
be inserted between the directional coupler 2 and the light receiver 3 
shown in FIG. 3 so as not to inject the Fresnel reflection light 13 in the 
light receiver 3, so that it is possible not to supersaturate the input 
light in the amplifier 4. However, since the switching speed of the 
optical switch and the time corresponding to pulse width cannot be 
measured, the dead area cannot be eliminated. Accordingly, in the 
conventional art, a short pulse is used as the optical pulse 11 or the 
response speed of the circuit is improved so as to reduce the dead area. 
SUMMARY OF THE INVENTION 
In consideration of the above problems, it is an object of the present 
invention to provide an optical pulse tester capable of displaying Raman 
scattered light which is not influenced by Fresnel reflection light when 
the near-end of an optical fiber or the connecting point between optical 
fibers is measured. 
To satisfy this object, the present invention provides an optical pulse 
tester capable of displaying Raman scattered light comprising: an optical 
pulse generator for generating an optical pulse with the specified 
wavelength; a directional coupler to which the optical pulse Is supplied, 
for dividing returning light from an optical fiber to be measured into 
Fresnel reflection light and Rayleigh scattered light with the specified 
wavelength; a demultiplexer/multiplexer to which the optical pulse via the 
directional coupler is supplied, for injecting an output light into the 
optical fiber and for demultiplexing returning light from the optical 
fiber into Raman scattered light different from the optical pulse in 
wavelength; a display for displaying a characteristic waveform of the 
optical fiber; and a switcher for supplying the Rayleigh scattered light 
to the display in the case of long distance measurement or precise loss 
measurement of the optical fiber, and for supplying the Raman scattered 
light to the display in the case of the measurement of the near-end part 
of the optical fiber or the connecting point between optical fibers 
without a dead area. 
According to the present invention, a positive effect is that Rayleigh 
scattered light can be used in the case of long distance measurement and 
precise loss measurement of an optical fiber and Raman scattered light can 
be used in the case of the measurement of the near-end of an optical fiber 
or the connecting point between optical fibers without a dead area. 
Accordingly, influence by Fresnel reflection light can be reduced when the 
near-end of the optical fiber or the connecting point between the optical 
fibers is measured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Hereinafter, a preferred embodiment of the present invention will be 
explained with reference to the figures. FIG. 1 shows a block diagram of 
the structure of an optical pulse tester based on the preferred embodiment 
of the present invention. In FIG. 1, components which correspond to 
components in the conventional optical pulse tester shown in FIG. 3 will 
retain the original identifying numeral, and their description will not 
herein be repeated. In FIG. 1, an optical demultiplexer/multiplexer 6, a 
light receiver 7, an amplifier 8, and a switcher 9 are added to the 
conventional optical pulse tester shown in FIG. 3. The optical fiber 10 
comprises the optical fiber 10B connected to the optical fiber 10A with 
the connector (not shown). 
In FIG. 1, the optical pulse 11 with wavelength .lambda.a from the optical 
pulse generator 1 is injected Into the optical fiber 10 via the 
directional coupler 2 and the optical demultiplexer/multiplexer 6. In the 
optical fiber 10, the Rayleigh scattered light 14 with wavelength 
.lambda.a and the Raman scattered light 15 with wavelength .lambda.b are 
simultaneously generated. Furthermore, the Fresnel reflection light 13 is 
generated at the connecting point of the connector and the like. The parts 
of the scattered light generated in the optical fiber 10 return to the 
injected end of the optical fiber 10 as back-scattered light, that is, 
returning light 12. 
The Fresnel reflection light 13 and the Rayleigh scattered light 14 have 
the same wavelength .lambda.a as the optical pulse 11. However, the 
wavelength .lambda.b of the Raman scattered light 15 is different from the 
wavelength .lambda.a of the optical pulse 11. For example, when the 
wavelength .lambda.a of the optical pulse 11 is 1.55 .mu.m, the wavelength 
.lambda.b of the Raman scattered light 15 is 1.65 .mu.m. 
The level of the Raman scattered light 15 is lower by 2 or 3 orders of 
magnitude than one of the Fresnel reflection light 13 and the Rayleigh 
scattered light 14. The optical demultiplexer/multiplexer 6 is provided 
between the directional coupler 2 and the optical fiber 10, which 
demultiplexes the returning light 12 into the Raman scattered light 15 
with wavelength .lambda.b and supplies the Raman scattered light 15 to the 
light receiver 7. By the wavelength-selecting characteristics of the 
demultiplexer/multiplexer 6, the Fresnel reflection light 13 is not 
injected in the light receiver 7. The Raman scattered light 15 with 
wavelength .lambda.b is detected by the light receiver 7. The output light 
from the light receiver 7 is amplified to the required level by the 
amplifier 8. The Fresnel reflection light 13 and the Rayleigh scattered 
light 14 with wavelength .lambda.a among the returning light 12 are 
divided in the directional coupler 2 and are received by the light 
receiver 3. The Fresnel scattered light 13 and the Rayleigh scattered 
light 14 are detected by the light receiver 3. The output light from the 
light receiver 3 is amplified to the required level by the amplifier 4. 
The switcher 9 selects either the Rayleigh scattered light 14 via the 
amplifier 4 or the Raman scattered light 15 via the amplifier 8 and 
delivers an output light. In the case of long distance measurement and 
precise loss measurement of the optical fiber 10, the switcher 9 selects 
an output light from the amplifier 4. In contrast, in the case of the 
measurement of the near-end part of the optical fiber 10 and the 
connecting point of the connector without a dead area, the switcher 9 
selects an output light from the amplifier 8. An output light from the 
switcher 9 is supplied to the display 5. 
Next, an example of the displayed waveform on the display 5 shown in FIG. 1 
will be explained with reference to FIG. 2. FIG. 2 shows an example of the 
loss characteristic waveform of the optical fiber 10 based on the Raman 
scattered light 15. In FIG. 2, a vertical axis, a horizontal axis and the 
structure of the optical fibers 10A and 10B are the same as FIG. 4. In 
FIG. 2., numerical reference 23 indicates a dead area corresponding to an 
optical pulse width at the injected end of the optical fiber 10A. A 
straight line 15A indicates the Raman scattered light 15 generated in the 
optical fiber 10A decreases exponentially with distance from the injected 
end of the optical fiber 10A (to the right of the time axis in FIG. 2) and 
is logarithmically transformed. In FIG. 2, numerical reference 24 
indicates a dead area corresponding to an optical pulse width at a point 
where the optical fiber 10B is connected to the optical fiber 10A with the 
connector. A straight line 15B indicates the Raman scattered light 15 
generated in the optical fiber 10B decreases exponentially with distance 
from the injected end of the optical fiber 10A and is logarithmically 
transformed. In FIG. 2, the right side area of the straight line 15B 
indicates noise at the outgoing end of the optical fiber 10B. 
In a comparison of FIG. 2 with FIG. 4, the Fresnel reflection lights 13A, 
13B, and 13C respectively shown in FIG. 4 are not drawn in FIG. 2. Since 
the Raman scattered light 15A and 15B are different From the Fresnel 
reflection light 13A, 13B, and 13C in wavelength, the Raman scattered 
light 15A and 15B are demultiplexed the returning light 12 into by the 
demultiplexer/multiplexer 6 shown in FIG. 1, but the Fresnel reflection 
light 13A, 13B, and 13C are not demultiplexed the returning light 12 into 
by the demultiplexer/multiplexer 6. In FIG. 2 which the Raman scattered 
light 15A and 15B are displayed, the Raman scattered light 15A and 15B are 
not influenced by the dead area 21 and 22 based on the Fresnel reflection 
light 13A, 13B, and 13C shown in FIG. 4.