U.S. patent number RE34,972 [Application Number 08/026,156] was granted by the patent office on 1995-06-20 for optical fiber evaluation method and system.
This patent grant is currently assigned to Nippon Telegraph & Telephone Corporation. Invention is credited to Tsuneo Horiguchi, Mitsuhiro Tateda.
United States Patent |
RE34,972 |
Horiguchi , et al. |
June 20, 1995 |
Optical fiber evaluation method and system
Abstract
Use is made of a non linear interaction between a first
modulated signal light from a first light source and a second
signal light from a second light source which propagate in an
optical fiber to be examined in opposite directions, i.e.,
so-called Brillouin light amplification, to analyze the signal
waveform of the second signal light, which is influenced by the
action of such light amplification.
Inventors: |
Horiguchi; Tsuneo (Mito,
JP), Tateda; Mitsuhiro (Mito, JP) |
Assignee: |
Nippon Telegraph & Telephone
Corporation (Tokyo, JP)
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Family
ID: |
15592771 |
Appl.
No.: |
08/026,156 |
Filed: |
March 3, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
370220 |
Jun 21, 1989 |
04997277 |
Mar 5, 1991 |
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Foreign Application Priority Data
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Jun 24, 1988 [JP] |
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63-154828 |
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Current U.S.
Class: |
356/73.1 |
Current CPC
Class: |
G01M
11/319 (20130101); G01M 11/39 (20130101) |
Current International
Class: |
G01M
11/00 (20060101); G01N 021/88 (); G01N
021/00 () |
Field of
Search: |
;356/73.1,223,326,328,301,345,350
;250/227.12,227.23,227.19,227.21,227.17 ;359/615 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
M K. Barnoski et al.; "Optical Time Domain Reflectometer", Appl.
Opt., 1977, vol. 16, pp. 2375-2379. .
F. P. Kapron et al.; "Aspect of Optical Frequency-Domain
Reflectrometry", Tech. Digest of 100C'81, 1981, p. 106. .
M. C. Farries et al.; "Distributed Sensing Using Stimulated Raman
Interaction in a Monomode Optical Fiber", Symposium of Optical
Fiber Sensing, '84, pp. 121-132..
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Primary Examiner: McGraw; Vincent P.
Attorney, Agent or Firm: Spencer, Frank & Schneider
Claims
What is claimed is:
1. A method for evaluating properties of an optical fiber,
comprising the steps of:
making incident a first signal light in the form of modulated light
from a first light source upon an optical fiber to be measured;
propagating a second signal light from a second light source in
said optical fiber to be measured in the direction opposite to that
of the first signal light;
controlling the frequency of at least one of said first and second
light sources;
setting the frequency difference .DELTA.f between said first signal
light and said second signal light at a predetermined value;
producing an optical nonlinear interaction between said first
signal light and said second signal light;
detecting said second signal light after it undergoes said optical
nonlinear interaction;
determining the time dependent signal power W of the detected
second signal light; and
evaluating the properties of said optical fiber to be measured on
the basis of said time dependent signal power W,
wherein said optical nonlinear interaction is Brillouin light
amplification, and
wherein the loss .alpha. (dB/km) of a given section <a, b> of
said optical fiber is evaluated from the equation
where v represents the velocity of light in said optical fiber
(m/s), Wa and Wb represent the detected time-dependent signal power
of said second signal light at the respective ends a and b of said
section of said optical fiber to be measured, and t.sub.ab
represents the time required for the first or second signal light
to go back and forth between said section <a, b>.
2. A method for evaluating properties of an optical fiber as
claimed in claim 1, wherein the oscillation linewidth of at least
one of said first and second light sources is wider than the
bandwidth of Brillouin light amplification of said optical fiber to
be measured.
3. A method for evaluating properties of an optical fiber as
claimed in claim 1, wherein the step of determining said time
dependent signal power W of said second signal light is
accomplished by calculating in accordance with the following
equation: ##EQU2## where Wk is the time dependent signal power of
said second signal light which undergoes said Brillouin
amplification and is detected at the instance when the frequency
difference between said first signal light and said second signal
light is .DELTA.fk (k=1, 2, . . . , N, N being a positive
integer).
4. A method for evaluating properties of an optical fiber as
claimed in claim 1, wherein the step of determining the time
dependent signal power W of said second signal light comprises the
steps of maintaining the polarization of one of said first signal
light and said second signal light; producing time dependent
signals by changing the polarization state of the other of said
first signal light and said second signal light to alternatively
take one of a pair of orthogonal polarization states; detecting the
time dependent signal powers Ws and Wp of said time dependent
signals corresponding to said orthogonal polarization states; and
calculating said time dependent signal power W as the arithmetic
mean expressed by (Ws+Wp)/2.
5. A method for evaluating properties of an optical fiber as
claimed in claim 1, wherein the step of determining the time
dependent signal power W of the detected second signal light
comprises the steps of:
assigning f1 and f2 to the respective optical frequencies of said
first signal light and said second signal light so that the
frequencies f1 and f2 satisfy the relationship f1-f2-fB (wherein fB
is the Brillouin frequency shift);
detecting said second signal light, which is
Brillouin-light-amplified, to obtain the time dependent power WA of
the detected second signal light;
assigning f1' and f2' to the respective optical frequencies of said
first signal light and said second signal light so that the
frequencies f1' and f2' satisfy the relationship f2'-f1'=fB;
detecting said second signal light which is
Brillouin-light-amplified to obtain the time dependent power WB of
the detected second signal light; and
calculating the arithmetic means (WA+WB)/2 or the geometric mean
root .sqroot.WA.times.WB, and specifying said arithmetic mean or
said geometric means as the time dependent signal power W.
6. A method for evaluating properties of an optical fiber,
comprising the steps of:
making incident a first signal light in the form of modulated light
from a first light source upon an optical fiber to be measured;
propagating a second signal light from a second light source in
said optical fiber to be measured in the direction opposite to that
of the first signal light;
controlling the frequency of at least one of said first and second
light source;
setting the frequency difference .DELTA.f between said first signal
light and said second signal light at a predetermined value;
producing an optical nonlinear interaction between said first
signal light and said second signal light;
detecting said second signal light after it undergoes said optical
nonlinear interaction;
determining the time dependent signal power W of the detected
second signal light; and
evaluating the properties of said optical fiber to be measured on
the basis of said time dependent signal power W,
wherein said optical nonlinear interaction is Brillouin light
amplification, and
wherein said step of evaluating the properties of said optical
fiber includes evaluation of the relative refractive index
difference distribution, temperature distribution, or stress
distribution in said optical fiber to be measured from the
frequency difference .DELTA.f (where .DELTA.f=fB, the Brillouin
frequency shift) that makes the amplitude of said time dependent
signal power W maximum.
7. A method for evaluating properties of an optical fiber as
claimed in claim 6, wherein the step of determining the time
dependent signal power W of said second signal light comprises the
steps of maintaining the polarization of one of said first signal
light and said second signal light; producing time dependent
signals by changing the polarization state of the other said first
signal light and said second signal light to alternately take one
of a pair of orthogonal polarization states; detecting the time
dependent signal powers Ws and Wp of said time dependent signals
corresponding to said orthogonal polarization states; and
calculating said time dependent signal power W as the arithmetic
mean expressed by (Ws+Wp)/2.
8. A method for evaluating properties of an optical fiber as
claimed in claim 6, wherein the step of determining the time
dependent signal power W of the detected second signal light
comprises the steps of:
assigning f1 and f2 to the respective optical frequencies of said
first signal light and said second signal light so that the
frequencies fi and f2 satisfy the relationship f1-f2=fB (where fB
is the Brillouin frequency shift);
detecting said second signal light which is
Brillouin-light-amplified to obtain the time dependent power WA of
the detected second signal light;
assigning f1' and f2' to the respective optical frequencies of said
first signal light and said second signal light so that the
frequencies f1' and f2' satisfy the relationship f2'-f1'=fB;
detecting said second signal light which is
Brillouin-light-amplified to obtain the time dependent power WB of
the detected second signal light; and
calculating the arithmetic means (WA+WB)/2 or the geometric mean
root .sqroot.WA.times.WB, and specifying said arithmetic mean or
said geometric mean as the time dependent signal power W.
9. A system for evaluating properties of an optical fiber,
comprising:
a first light source means for emitting a first signal light in the
form of modulated light;
a second light source means for emitting a second signal light;
an optical frequency controlling device means for controlling the
optical frequency difference between said first signal light and
said second signal light;
means for launching said second signal light into an end of said
optical fiber to be measured;
an optical multi-demultiplexing means for launching said first
signal light onto the other end of said optical fiber to be
measured, and for extracting said second signal light at the other
end of said optical fiber;
.[.an optical frequency filter which receives light from said
multi-demultiplexing means and passes said second signal light but
not said first signal light;.].
a photo detecting means for convening .[.the light passed by said
optical frequency filter.]. .Iadd.said second signal light
extracted by said multi-demultiplexing .Iaddend.into an electric
signal; and
a signal processing means for processing and analyzing the time
dependent waveform of said electric signal from said photo
detecting means,
wherein at least one of said first light source means and said
second light source means is an optical frequency variable type
longitudinal single mode oscillation laser, and
wherein the optical frequency difference between said first signal
light and said second signal light is equal to or approximately
equal to the Brillouin frequency shift of said optical fiber to be
measured.
10. A system for evaluating properties of an optical fiber as
claimed in claim 9, further comprising means for changing the
polarization state of at least one of said first signal light and
said second signal light.
11. A system for evaluating properties of an optical fiber as
claimed in claim 9, wherein said first signal light emitted by said
first light source means is modulated by a pseudo-random
modulation.
12. A system for evaluating properties of an optical fiber as
claimed in claim 11, wherein said pseudo-random modulation is a
binary ASK (Amplitude Shift Keying).
13. A system for evaluating properties of an optical fiber as
claimed in claim 11, wherein said pseudo-random modulation is an
FSK (Frequency Shift Keying), and the optical frequency difference
.DELTA.f between said first signal light and second signal light is
equal to or approximately equal to the Brillouin frequency shift
when said FSK takes a mark signal, and is separated from the
Brillouin frequency shift by more than the Brillouin gain bandwidth
in the optical fiber to be measured when said FSK takes a space
signal.
14. A system for evaluating properties of an optical fiber as
claimed in claim 13, wherein said FSK is binary,
15. A system for evaluating properties of an optical fiber as
claimed in claim 9, wherein the oscillation linewidth of at least
one of said tint light source means and said second light source
means is greater than the Brillouin gain bandwidth of said optical
fiber to be measured. .Iadd.16. A system for evaluating properties
of an optical fiber of claim 9, further comprising an optical
frequency filter which receives light from said
multi-demultiplexing means and passes said second signal light but
not said first signal light, to said photo detecting means.
.Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for
evaluating the distribution of various properties of an optical
fiber along its longitudinal direction, such as optical loss in the
optical fiber (optical attenuation), the relative refractive index
difference between the core and cladding, core diameter, variation
in stresses applied to the optical fiber and influence of
temperature change on the optical fiber.
2. Prior Art
As means for determining the distribution of the loss of an optical
fiber along its longitudinal direction, there has been proposed
optical time domain reflectometry (hereinafter referred to as
"OTDR") in which Rayleigh back-scattered light generated in an
optical fiber is detected (see, for instance, M K, Barnoski et at.,
Appl. Opt., 1977, Vol. 16, pp. 2375-2379, "Optical time domain
reflectometer"). This method has been improved over 10 years since
it was proposed and at present becomes an indispensable technique
for the installation of an optical communication network and for
the maintenance thereof. There has also been proposed an optical
frequency-domain reflectometry (hereinafter referred to as "OFDR")
in which Rayleigh back-scattered light is analyzed in the frequency
domain, while the OTDR method analyzes it in time domain (see, for
instance, F P Kapron et al., Tech. Digest of IOOC'81, 1981, p. 106,
"Aspect of optical frequency-domain reflectometry"). These OTDR and
OFDR methods have almost reached the stage of completion.
However, since the strength of the back-scattered light is very
weak, it is difficult to increase the precision of its measurement
even if the signal to noise ratio is improved with a device such as
an averaging device.
A typical value of measurable one-way optical loss of commercially
available OTDR apparatuses is about 20 dB when the input optical
power within the test fiber is about 1 mW; the distance resolution
is 100 m (optical pulse width=1 microsecond) and the average
processing time is on the order of one minute. In order to extend
the dynamic range (i,e., measurable maximum optical attenuation of
an optical fiber), it is required to employ large-scale high power
lasers, represented by YAG lasers, and/or to make a sacrifice of
the spatial resolution and/or measurement time.
Furthermore, the amount of light reflected at the light input
endface of the test fiber or connectors in the optical transmission
line is greater than the Rayleigh scattered light by 3 or 4 orders
of magnitude and this results in saturation of a photo detector.
For this reason, a back-scattered light cannot be measured over a
certain distance ahead of the reflection point and thus a so-called
dead zone is formed.
An attempt for determining stresses in an optical fiber was
reported by M. C. Farries et ak., where they observed Raman
scattering amplification in an optical fiber when two lights having
different wavelengths are counterpropagated in the fiber (Tech
Digest in OFS (Symposium of Optical Fiber Sensing) '84, pp.
121-132, entitled "DISTRIBUTED SENSING USING STIMULATED RAMAN
INTERACTION IN A MONOMODE OPTICAL FIBRE") by M. C. Farries et al.
Although, in this report, the signal waveform change due to stress
is recognized, there is no quantitative relationship between the
stress and the waveform change and it failed to locate the position
at which the stress is applied. This is because the technique in
the above mentioned report utilizes the polarization effect on
Raman optical amplification gain, but it is difficult to interpret
a double-pass integrated polarization state in terms of the stress
applied to the fiber.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a
method and an apparatus for evaluating properties of an optical
fiber which makes it possible to determine with high precision the
distribution of various properties of the optical fiber along its
longitudinal direction, such as optical loss in the optical fiber,
the relative refractive index difference between the core and
cladding, core diameter, variation in stresses applied to the
optical fiber and influence of temperature change on the optical
fiber.
Another object of the present invention is to provide a method and
an apparatus for evaluating optical fiber attenuation
characteristics, whose dynamic range is greater than that achieved
by the conventional OTDR technique by 10 dB or more even using
lasers having only a few mW light power.
To achieve the objects, the present invention makes use of a
non-linear interaction between a first modulated signal light from
a first light source and a second signal light from a second light
source which counter propagate in an optical fiber to be examined,
i.e., so called Brillouin light amplification, and analyzes the
signal waveform of the second signal light which is influenced by
Brillouin light amplification.
The principle of Brillouin light amplification is as follows. When
a light is made incident upon a substance such as an optical fiber,
an acoustic wave is induced by the incident light itself, and
simultaneously, the acoustic wave generates light (Stokes light)
having a frequency lower than that of the incident light. If the
strength of the incident light is further increased, strong Stokes
light having a coherent phase is scattered in the direction
opposite to the incident light. This is the phenomenon of
stimulated Brillouin light amplification.
Accordingly, if the optical frequency of either the first or the
second signal light is equal to the optical frequency of the Stokes
light, the signal light can be amplified by the stimulated
Brillouin scattering mentioned above.
As explained above, in the present invention, an optical fiber is
characterized by the signal light waveform change due to the
non-linear effect of Brillouin light amplification and, therefore,
the present invention is quite different from the conventional
techniques such as OTDR in which the optical loss distribution in
an optical fiber along its longitudinal direction is evaluated by
measuring the Rayleigh back-scattered light, which is generated
through a linear scattering process.
Moreover, the present invention completely differs from the
technique described in the aforementioned Farries' report. Farries'
technique utilizes the polarization effects on Raman gain, while
the present invention utilizes Brillouin gain averaged over
polarization states as well as the change of Brillouin frequency
shift due to fiber properties.
In a first aspect of the present invention, a method for evaluating
properties of an optical fiber, comprises the steps of:
making incident a ftrst signal light in the form of modulated light
from a first light source upon an optical fiber to be measured;
propagating a second signal light from a second light source in the
optical fiber in the direction opposite to that of the first signal
light, while controlling at least one of the first and second light
sources, so that Brillouin light amplification is caused, by
providing a specific frequency difference between the first signal
light and the second signal light;
detecting the second signal light which is
Brillouin-light-amplified; and
evaluating the properties of the optical fiber based on the
detected second signal light.
Here, in the detection of the second signal light which is
Brillouin-light-amplified, the evaluation of the properties of the
optical fiber may be performed by changing the polarization state
of at least one of the first signal light and the second signal
light during one or a plurality of measurements to detect the
second signal light which is Brillouin-light-amplified with respect
to the average relative polarization state of the first signal
light and the second signal light and then to analyze change of a
waveform in time or amplitude and phase changes of the second
signal light.
The method may further comprise the steps of: assigning f1 and f2
to the respective optical frequencies of the first signal light and
the second signal light so that the frequencies f1 and f2 satisfy
the relation f1-f2=fB (wherein fB is the Brillouin frequency
shift);
detecting the second signal light which is
Brillouin-light-amplified to obtain a time dependent waveform WA of
the detected second signal light;
assigning f1' and f2' to optical frequencies of the first signal
light and the second signal light so that the frequencies f1' and
f2' satisfy the relation f2'-f1'=fB;
detecting the second signal light which is
Brillouin-light-amplified to obtain a time dependent waveform WB of
the detected second signal light; and
analyzing the arithmetic or geometric mean of the waveforms WA and
WB to evaluate the properties of the optical fiber.
The first signal light from the first light source may be
amplitude-modulated or frequency-modulated in the form of pulses
and the second signal light from the second light source has a
constant amplitude and frequency without being modulated and
wherein the loss distribution properties and the core diameter
distribution properties of the optical fiber is evaluated as a
slope distribution of the logarithmical expression of an amplitude
change in time of the detected second signal light which is
Brillouin-light-amplified. The method may comprise the steps
of:
fixing the frequency difference fB=.vertline.f1-f2.vertline.
between the optical frequencies fl and f2 of the lights from the
first light source and the second light source to a plurality of
values fB1, fB2;
detecting, for each fixed value, a time dependent waveform of the
second signal light which is Brillouin-light-amplified; finding
out, at each position of the optical fiber to be measured, the
optical frequency difference (maximum gain Brillouin frequency
shift) at which the Brillouin light amplification effect becomes
maximum from the plurality of the time dependent waveforms; and
evaluating the stress distribution, temperature distribution and
relative refractive index difference distribution in the optical
fiber to be measured from the position-dependence of the maximum
gain Brillouin frequency shift in the optical fiber. The loss
distribution and core diameter distribution in the optical fiber
may be evaluated by expanding an oscillation linewidth of at least
one of the first and second light sources so as to be wider than
that of Brillouin light amplification bandwidth of the optical
fiber to be measured.
The first signal light from the first light source may be
amplitude-modulated or frequency-modulated in the form of pulses
and the second signal light from the second light source has a
constant amplitude and frequency without being modulated and
wherein the loss distribution properties and core diameter
distribution properties of the optical fiber are evaluated as a
slope distribution of the logarithmical expression of an amplitude
change in time of the detected second signal light which is
Brillouin-light-amplified.
The method may comprise the steps of:
fixing the frequency difference fB=.vertline.f1-f2.vertline.
between the optical frequencies f1 and f2 of the lights from the
first light source and the second light source to a plurality of
values fB1, fB2;
detecting, for each fixed value, a time dependent waveform of the
second signal light which is Brillouin-light-amplified;
finding out, at each position of the optical fiber to be measured,
the optical frequency difference (maximum gain Brillouin frequency
shift) at which the Brillouin light amplification effect becomes
maximum from the plurality of the time dependent waveforms; and
evaluating the stress distribution, temperature and relative
refractive index difference distribution in the optical fiber to be
measured from the position-dependence of the maximum gain Brillouin
frequency shift in the optical fiber.
The loss distribution and core and diameter distribution in the
optical fiber may be evaluated by expanding an oscillation
linewidth of at least one of the first and second light sources so
as to be wider than that of Brillouin light amplification bandwidth
of the optical fiber to be measured.
Here, the first signal light from the first light source may be
amplitude-modulated or frequency-modulated in the form of pulses
and the second signal light from the second light source has a
constant amplitude and frequency without being modulated and
wherein the loss distribution properties and core diameter
distribution properties of the optical fiber are evaluated as a
slope distribution of the logarithmical expression of an amplitude
change in time of the detected second signal light which is
Brillouin-light-amplified. The loss distribution and core diameter
distribution in the optical fiber may be evaluated by expanding an
oscillation linewidth of at least one of the first and second light
sources so as to be wider than that of Brillouin light
amplification bandwidth of the optical fiber to be measured.
The method may comprise the steps of:
fixing the frequency difference fB=.vertline.f1-f2.vertline.
between the optical frequencies f1 and f2 of the lights from the
first light source and the second light source, to a plurality of
values fB1, fB2;
detecting, for each fixed value, a time dependent waveform of the
second signal light which is Brillouin-light-amplified; finding
out, at each position of the optical fiber to be measured, the
optical frequency difference (maximum gain Brillouin frequency
shift) at which the Brillouin light amplification effect becomes
maximum from the plurality of the time dependent waveforms; and
evaluating the stress distribution, temperature distribution and
relative refractive index difference distribution in the optical
fiber to be measured from the position-dependence of the maximum
gain Brillouin frequency shift in the optical fiber.
The loss distribution and core diameter distribution in the optical
fiber may be evaluated by expanding a spectral linewidth of at
least one of the first and second light sources so as to be wider
than that of Brillouin light amplification bandwidth of the optical
fiber to be measured.
In a second aspect of the present invention, a system for
evaluating the properties of an optical fiber, may comprise:
a first light source for emitting a first signal light in the form
of modulated light;
a second light source for emitting a second signal light which
propagates in the optical fiber to be measured in the direction
opposite to that of the first signal light;
an optical frequency controlling means for controlling the
frequency difference between the first signal light and the second
signal light:
an optical multi-demultiplexing means for making the first signal
light incident upon the optical fiber to be measured and for
extracting a Brillouin-light-amplified second signal light which is
amplified through Brillouin light amplification induced by the
first signal light and the second signal light;
a photo detecting means for converting the
Brillouin-light-amplified second signal light from the optical
multi-demultiplexing means, into an electric signal; and
a signal processing means for processing and analyzing the time
dependent waveform or amplitude and phase of the electric signal
from the photo detecting means
Here, the optical frequency controlling means may be a two-output
stabilized voltage source which provides a constant voltage E1 and
a variable voltage E2=E1+.DELTA.E or a two-output stabilized
current source which provides a constant current I1 and a variable
current I2=I1+.DELTA.I and the frequency difference between the
first and second light sources is controlled by supplying E1 or I1
to one of the first and second light sources and E2 and I2 to the
other of the first and second light sources.
The optical frequency controlling means may have two independent (a
first and a second) optical frequency controlling portions, the
first optical frequency controlling portion being a variable
stabilized voltage source or a variable stabilized current source
for changing the voltage or current to be supplied to the first
light source so as to maximize or minimize the signal level
detected by the photo detecting means and the second optical
frequency controlling portion being a constant voltage source or a
constant current source for stabilizing the frequency of the second
light source independently of the state of the first light
source.
The optical frequency controlling means may have two independent (a
first and a second) optical frequency controlling portions, the
first optical frequency controlling portion being a variable
stabilized voltage source or a variable stabilized current source
for detecting a beat signal based on the frequency difference
between the first and second light sources among signals detected
by the photo detecting means and for changing the voltage or
current to be supplied to the first light source so that a
frequency difference between a predetermined reference beat
frequency and the beat frequency actually detected becomes zero,
and the second optical frequency controlling portion being a
constant voltage source or a constant current source for
stabilizing the frequency of the second light source independently
of the state of the first light source.
In a third aspect of the present invention, a system for evaluating
the properties of an optical fiber, may comprise:
a first light source for emitting a first signal light in the form
of modulated light;
a second light source for emitting a second signal light which
propagates in the optical fiber to be measured in the direction
opposite to that of the first signal light;
an optical frequency controlling means for controlling the
frequency difference between the first signal light and the second
signal light:
an optical multi-demultiplexing means for making the first signal
light incident upon the optical fiber to be measured and for
extracting a Brillouin-light-amplified second signal light which is
amplified through Brillouin light amplification induced by the
first signal light and the second signal light;
an optical frequency filter receiving the Brillouin-light-amplified
second signal light from the optical muiti-demultiplexing means and
for filtering the Brillouin-light-amplified second signal light in
such a way that light having the frequency of the
Brillouin-light-amplified second signal light is passed but light
having the frequency of the first signal light is interrupted;
a photo detecting means for converting the output light from the
optical frequency filter into an electric signal; and
a signal processing means for processing and analyzing the time
dependent waveform or amplitude and phase of the electric signal
from the photo detecting means.
Here, the optical frequency controlling means may be a two-output
stabilized voltage source which provides a constant voltage E1 and
a variable voltage E2=E1+.DELTA.E or a two-output stabilized
current source which provides a constant current I1 and a variable
current I2=I1+.DELTA.I and the frequency difference between the
first and second light sources is controlled by supplying E1 or I1
to one of the first and second light sources and E2 and I2 to the
other of the first and second light sources.
The optical frequency controlling means may have two independent (a
first and a second) optical frequency controlling portions, the
first optical frequency controlling portion being a variable
stabilized voltage source or a variable stabilized current source
for changing the voltage or current to be supplied to the first
light source so as to maximize or minimize the signal level
detected by the photo detecting means and the second optical
frequency controlling portion being a constant voltage source or a
constant current source for stabilizing the frequency of the second
light source independently of the state of the first light
source.
In a fourth aspect of the present invention, a system for
evaluating properties of an optical fiber, may comprise:
a first light source for emitting a first signal light in the form
of modulated light;
a second light source for emitting a second signal light which
propagates in the optical fiber to be measured in the direction
opposite to that of the first signal light;
an optical frequency controlling means for controlling the
frequency difference between the first signal light and the second
signal light;
an optical multi-demultiplexing means for making the first signal
light incident upon the optical fiber to be measured and for
extracting a Brillouin-light-amplified second signal light which is
amplified through Brillouin light amplification induced by the
first signal light and the second signal light;
an optical frequency filter receiving the Brillouin-light-amplified
second signal light from the optical multi-demultiplexing means and
for filtering the Brillouin-light-amplified second signal light in
such away that light having the frequency of the
Brillouin-light-amplified second signal light is passed by light
having the frequency of the first signal light is interrupted;
a photo detecting means for converting the output light from the
optical frequency filter into an electric signal;
a signal processing means for processing and analyzing the time
dependent waveform or amplitude and phase of the electric signal
from the photo detecting means; and
means for changing the polarization state of at least one of the
first signal light and the second signal light.
The optical frequency controlling means may be a two-output
stabilized voltage source which provides a constant voltage E1 and
a variable voltage E2=E1+.DELTA.E or a two-output stabilized
current source which provides a constant current 11 and a variable
current I2=I1+.DELTA.I and the frequency difference between the
first and second light sources is controlled by supplying E1 or I1
to one of the first and second light sources and E2 and I2 to the
other of the first and second light sources.
The optical frequency controlling means may have two independent (a
first and a second) optical frequency controlling portions, the
first optical frequency controlling portion being a variable
stabilized voltage source or a variable stabilized current source
for changing the voltage or current to be supplied to the first
light source so as to maximize or minimize the signal level
detected by the photo detecting means and the second optical
frequency controlling portion being a constant voltage source or a
constant current source for stabilizing the frequency of the second
light source independently of the state of the first light
source.
The above and other objects, effects, features and advantages of
the present invention will become more apparent from the following
description of embodiments thereof taken in conjunction with the
accompanying drawings .
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram showing an arrangement of Embodiment
1 according to the present invention;
FIG. 2 is a waveform diagram illustrating an example of waveform
which is measured according to the present invention;
FIG. 3 is a block diagram showing an embodiment of the signal
processing apparatus shown in FIG. 1;
FIG. 4 is a circuit diagram showing an embodiment of the optical
frequency controlling apparatus shown in FIG. 1;
FIG. 5 is a schematic diagram showing an arrangement of Embodiment
2 according to the present invention;
FIGS. 6 and 7 are block diagrams showing two embodiments of the
optical frequency controlling apparatus;
FIG. 8 is a schematic diagram showing an arrangement of Embodiment
3 according to the present invention;
FIG. 9 is a schematic diagram showing an arrangement of Embodiment
4 according to the present invention;
FIGS. 10A-10C are explanatory diagrams for explaining the
polarization effect on waveforms to be measured;
FIG. 11 is a waveform diagram illustrating an example of waveforms
observed when the optical frequency difference .DELTA.f between the
first and second signal lights are varied;
FIG. 12 is a characteristic graph illustrating the relationship
between a position z is an optical fiber and the optical frequency
difference .DELTA.f between the first and second signal lights
(corresponding to the relative refractive index difference between
the core and cladding of the optical fiber) at which the maximum
optical signal level is obtained;
FIG. 13 is a characteristic graph illustrating a measured waveform
W(z) and its optimum approximation curve W'(z) in an optical fiber
where the core diameter varies along the longitudinal
direction;
FIGS. 14A-14F are waveform diagrams showing various modulation
waveforms of the first signal light;
FIG. 15 is a characteristic graph illustrating the stress
dependence of Brillouin frequency shift;
FIG. 16 is a schematic diagram showing an example of the
construction of a Mach-Zehnder interferometer;
FIGS. 17A and 17B are characteristics graphs showing the
transmission characteristics of Mach-Zehnder interferometer;
and
FIG. 18 is a waveform diagram showing an example of the waveform
which is measured according to the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiment 1
FIG. 1 shows Embodiment 1 of an evaluation apparatus in accordance
with the present invention. In FIG. 1, reference numeral 1 denotes
the first light source emitting a pulsed light which is
amplitude-modulated or frequency-modulated in pulse-like form and
the light source 1 emits a light having a narrow spectrum
linewidth. The first light source 1 can be arranged as follows.
That is, a light from a single longitudinal mode laser, such as a
CW operation YAG laser or a CW operation DFB laser, is
amplitude-modulated in the pulse-like form by driving an
acoustooptic modulator. Alternatively, the pulsed light can be
realized by injecting a current into a single longitudinal mode
laser such as a CW operation DFB laser. Reference numeral 2 denotes
the second light source of CW operation having a narrow spectrum
linewidth such as a YAG laser or a frequency tunable DFB laser.
Reference numeral 3 denotes an optical fiber to be examined or
measured, 4 an optical multi-demultiplexer, 5 a photo detector such
as a Ge-APD (Avalanche Photo Diode). Reference numeral 6 denotes a
signal processing unit which receives the electric signal from the
photo detector 5 and processes it to analyze the time-dependence,
the amplitude and/or the phase of the signal. Reference number 7
denotes an optical frequency controlling unit for controlling the
frequency difference between the lights from the first and second
light sources 1 and 2 to a desired value. Details of the units 6
and 7 will be described below.
The evaluation apparatus operates as follows:
A pulse-like light emitted from the first light source 1, i.e., the
first signal light S1, is coupled to the optical fiber 3 to be
examined through the optical multi-demultiplexer 4. On the other
hand, the light emitted from the second light source 2 i.e., the
second signal light S2, is launched from the end 3-2 of the optical
fiber 3 to be examined, which is opposite to the end 3-1 of the
optical fiber 3 at which the first signal light S1 is launched.
At this stage, the frequency f1 of the first signal light and the
frequency f2 of the second signal light are controlled by the
optical frequency controlling unit 7 so as to fulfill the following
relation:
Here, fB is the Brillouin frequency shift inherent to the optical
fiber 3 to be examined. For instance, for a silica fiber, fB is
about 13 GHz at a wavelength of 1.3 micron meters. When equation
(1) is satisfied, the second signal light S2 is amplified by the
first signal light S1 through the Brillouin light amplification
process. The waveform of the second signal light S2 detected by the
photo detector 5 can be derived as follows.
The gain g of the second signal light S2 due to the Brillouin light
amplification at a position z is expressed by the following
equation when g-1<1:
Here, A is a proportionality factor, and z represents the position
at which the first signal light S1 (a pulsed signal) propagating
through the optical fiber 3 to be examined is located, wherein the
end 3-1 of the optical fiber 3 is chosen as the reference point.
P1(z) represents the light power of the first signal light S1 at
the position z. Likewise, P2(z) is the light power of the second
signal light S2 at the position z. The optical loss in the optical
fiber 3 and its length are denoted as .alpha.(neper/m) and L (m),
respectively. We can assume that the optical losses for the first
signal light S1 and the second signal light S2 are identical, since
the frequency difference between these signal lights is quite
small. Under this condition, P1(z) and P2(z) can be expressed as
follows, respectively:
Considering equations (2) and (3) together with the attenuation
factor exp (-.alpha.z) of the second signal light S2 due to the
optical fiber loss between the points O and z, the power P2(z) of
the second signal light S2 detected by the photo detector 5 is
expressed by the following relation:
Here, the optical loss of the optical multi-demultiplexer 4 is
neglected. The first term on the right side of equation (4) is a DC
component and the second term corresponds to a component increased
due to the Brillouin light amplification. A time t at which the
second signal light S2 Brillouin-amplified at the position z is
detected by the photo detector 5, is expressed by the following
relation,
where V denotes the velocity of light. The time at which the signal
light S1 is injected in the fiber 3 is chosen as the time origin.
Therefore, the waveform of the received light signal level becomes
as shown in FIG. 2.
In FIG. 2, the optical loss .alpha. of the optical fiber 3 to be
examined can be obtained from the attenuation factor of the
Brillouin amplified component or from the difference between the
received light signal levels at arbitrary points. Comparing the
present invention with the conventional OTDR, it is easily
understood that they differ from each other in the following two
points:
(1) The attenuation factor of the waveform according to the
conventional OTDR is exp (-2.alpha.z) and thus the signal level
rapidly decreases, while that of the present invention is exp
(-.alpha.z) and thus the attenuation factor of the signal level is
small. (2) As seen from equation (4), in the present invention, the
received light signal level can be increased by increasing the
light power of the second light source 2.
Therefore, according to the present invention, the received light
signal level can be greatly increased and thus the measurement
accuracy can be improved, compared with the conventional OTDR.
In the foregoing explanation, the optical loss .alpha. in the
optical fiber 3 is assumed to be uniform throughout the fiber.
However, even if the loss in the optical fiber 3 along its
longitudinal direction is not uniform, the distribution of the loss
in the optical fiber 3 to be examined along its longitudinal
direction can be determined by utilizing the measuring system shown
in FIG. 1. More specifically, such a loss distribution can be
determined according to the foregoing procedures by, for instance,
separately measuring every interval of such an optical fiber, in
which interval the optical loss is uniform.
Referring now to FIG. 3, there is depicted an embodiment of the
signal processing unit 6 shown in FIG. 1. In FIG. 3, reference
numeral 61 represents an A/D converter for converting the analog
output from the photo detector 5 into a digital signal, 62
represents a CPU for processing the output from the A/D converter
61 as will be explained below, 63 represents an output unit such as
a CRT display or a printer and 64 represents a memory.
The measured waveform obtained in one measurement by the photo
detector 5, is shown in FIG. 2. If we denote the received light
signal level as W, this waveform in one measurement can be
expressed as W(z), since the time t has the following relation with
the position coordinate z along the optical fiber 3 to be examined,
as shown in equation (5). Numbers of waveforms can be successively
obtained by repeatedly launching light pulses into the optical
fiber 3 to be examined from the first light source 1. Thus, the
measured waveform of the k-th pulse can be expressed by WK(z) where
k represents the serial number of pulse. Moreover, the measured
waveform can be defined as a function of the operation parameters
of the whole evaluation apparatus of this invention. In other
words, the waveform is a function of the frequency difference
.DELTA.f (Hz) between the first and second light sources 1 and 2
and their relative polarizing angle .theta. (rad). Therefore, the
general form of the measured waveform will be expressed as Wk(z,
.DELTA.f, .theta.) hereinafter.
The measured waveform Wk(z, .DELTA.f, .theta.) obtained at the
photo detector 5 is converted into a digital signal in the A/D
convertor 61 and then inputted to the CPU 62. In the CPU 62,
various processings are performed according to the properties of
the optical fiber 3 to be evaluated. For instance, an averaging
processing on a number of measured waveforms whose .DELTA.f and
.theta. are fixed is performed in order to improve the signal to
noise ratio. This processing is, for instance, defined by the
following relation: ##EQU1##
In order to obtain the loss distribution from this waveform,
logarithmic conversion and differential processing are performed in
the CPU 61. In other words, this processing can be expressed by the
following formula:
These processings are almost the same as those in the conventional
OTDER. For instance, the signal processing apparatus employed in an
OTDER apparatus "OF 152 FIBER OPTIC TDR" manufactured by Tektronix
can be used as the signal processing unit 6.
Moreover, the processing for compensating the Brillouin gain
fluctuation dependent on the relative polarizing angle between the
first and second light sources 1 and 2 can be expressed by
In addition, at every position z in the optical fiber 3 to be
measured, a relative frequency .DELTA.f(z) which provides the
maximum wave height is obtained. That is, .DELTA.f(z) which
satisfies the following relation is obtained:
The physical meanings and an explanation of these processings will
be explained in the following embodiments.
An embodiment of the optical frequency controlling unit 7 will be
explained in detail.
In general, the frequency of a laser light can be controlled by
applying a voltage to the laser device or injecting a current to
the laser device, utilizing thermal effects or optoelectric effects
such as plasma effects or interactions therebetween.
FIG. 4 shows an embodiment of the frequency controlling unit 7 in
which the frequency is adjusted by controlling the voltage applied
to the laser device. In FIG. 4, a voltage E1 is applied to the
first light source 1 by a constant power supply 71 and a voltage E2
(=E1+.DELTA.E) is applied to the second light source 2 by the
constant voltage power supply 71 and a variable stabilized power
supply 72. By changing .DELTA.E through the control of the power
supply 72, the relative frequency .DELTA.f=f1-f2 between the first
and second light sources 1 and 2 can be controlled to a desired
value. Here, .DELTA.f is proportional to .DELTA.E within a
practical range. For instance, in the case of a Ring YAG laser
Model 102 manufactured by Lightwave Electronics Co.,
.DELTA.f/.DELTA.E=1.2 GHz/V.
Embodiment 2
While in the foregoing Embodiment 1, the frequencies of the first
and second light sources 1 and 2 are controlled by the single
frequency controlling unit 7, the respective frequencies of the
first and second light sources 1 and 2 can also be separately
controlled by using two frequency controlling units.
FIG. 5 is a diagram showing the second embodiment of the present
invention. In this embodiment, the first light source 1 and the
second light source 2 are controlled by independently arranged
optical frequency controlling units 7-1 and 7-2, respectively. The
optical frequency controlling unit 7-1 is the same as that used in
Embodiment 1 except that it is controlled by an output from the
photo detector 5. In addition, the second light source 2 is
voltage- or current-controlled by the optical frequency controlling
unit 7-2 so that the deviation of the frequency f2 from a
predetermined frequency f20, i.e., .vertline.f2-f20.vertline. is
within a predetermined set value.
On the other hand, the frequency of the first light source 1 is
controlled by the light frequency controlling unit 7-1 in
accordance with the output from the photo detector 5, so that the
frequency difference between the first and second light sources 1
and 2 coincides with the desired value. More specifically, there
are two useful methods for controlling the frequency. In one method
the output signal level of the photo detector 5 is maximized, and
in the other method, the frequency of the beat component is
adjusted to a predetermined value.
FIG. 6 shows a specific embodiment of an arrangement of the optical
frequency controlling unit 7-1 for maximizing the output signal
level of the photo detector 5. The unit 7-1 controls the output
voltage E(k) of the variable stabilized power supply 74 by the
signal level I(k) obtained at a signal level detector 73, to which
the light intensity signal is fed from the photo detector 5. As was
mentioned above, when the frequency difference .DELTA.f=f1-f2
between the first and second light sources 1 and 2 coincides with
the Brillouin frequency shift fB inherent to the optical fiber to
be measured, the probe light from the second light source 2 is
Brillouin-amplifled. Therefore, if the condition for maximizing the
output from the photo detector 5 can be maintained, the intended
measurement of the present invention can be performed without
correctly determining the frequency value fB. In a specific
example, when the voltage applied to the first light source 1 is
E1(k) and the detected signal level of the signal level detector 73
is I(k) at time t(k), an output voltage E1(k+1) of the variable
stabilized power supply 74, i.e., a voltage applied to the first
light source 1 at time t(k+1), is determined by the following
relation:
wherein A represents an appropriate positive proportionality
factor. As will be explained below, even if the frequencies of the
first and second light sources 1 and 2 are so determined that
f2-f1=fB is established, the optical loss can be measured. In this
case, since the second signal light loses its energy through the
Brillouin-interaction with the first signal light, the signal level
detected by the photo detector 5 is minimized, contrary to the
foregoing case (f1-f2=fB). Therefore, in such a case, the optical
frequency controlling device 7-1 is controlled so that the signal
level detected by the photo detector 5 is minimized. In this case,
a negative value is selected as the foregoing proportionality
factor A.
FIG. 7 shows a specific embodiment of the optical frequency
controlling unit 7-1 for adjusting the frequency of the beat
component of the output from the photo detector 5 to a
predetermined value.
As seen from the arrangement shown in FIG. 1, the Rayleigh
scattered light due to a pulsed light emitted from the first light
source 1 is detected by the photo detector 5 together with the
probe light emitted from the second light source 2. At this time, a
beat signal is generated in the photo detector 5, whose frequency
is equal to the frequency difference between the first and second
light sources 1 and 2.
Then, in the arrangement shown in FIG. 7, the output signal from
the photo detector 5 is supplied to a beat frequency detector 75 to
detect the beat frequency f(k) at time t(k). Reference numeral 76
denotes a reference beat frequency generator which provides a
reference beat frequency of. The beat frequency f(k) and the
reference beat frequency fo are supplied to a subtractor 77 to
obtain a frequency difference .delta.f=f(k)-fo. The frequency
difference .delta.f is supplied to a variable stabilized power
supply 78 to generate an output voltage E1(k)=E1(k-1)+A'.delta.f.
This output voltage E1(k) is applied to the first light source 1 so
that the first light source 1 is controlled to satisfy a condition
of: .delta.f=0 (wherein A' is a constant for feedback).
While the present invention has been explained with reference to
the voltage-control type optical frequency controlling units 7-1
and 7-2, they may be arranged as a current-control type. In this
case, the operations thereof are substantially identical with those
of the voltage-control type, except the circuit arrangement.
Embodiment 3
FIG. 8 shows Embodiment 3 of the present invention which is
identical with Embodiment 1 of the present invention as shown in
FIG. 1 except that an optical frequency filter 8 is disposed at the
front stage of the photo detector 5. The optical frequency filter 8
passes the second signal light S2 of frequency f2, but shuts off
the first signal light S1 of frequency f1. The operations of this
Embodiment 3 are almost identical to Embodiment 1. Thus, the
following effects are achieved: (1) The filter 8 prevents the
incidence of the Rayleigh back-scattered light (which is measured
by the conventional OTDR) generated while the first signal light S1
propagates along the optical fiber 3 to be measured into the photo
detector 5 and thus prevents the scattered light from being a noise
against the signal light S2. (2) The filter 8 prevents the strong
Fresnel reflection of the first signal light S1 reflected at the
incident end or the emitting end of the optical fiber 3 to be
measured or at the connector points in the optical fiber 3 from
entering the photo detector 5. Usually, when an extremely strong
signal is detected, any weak signals immediately after the strong
signal cannot be detected because of the trailing or the overshoot
behavior for the strong signal due to saturation of the detection
system. This results in the problem of a dead zone. However,
according to Embodiment 3 of this invention, the problem of such a
dead zone associated with the conventional OTDR can effectively be
solved by the insertion of the filter 8.
Embodiment 4
FIG. 9 shows Embodiment 4 of this invention. In FIG. 9, reference
numeral 9 denotes a polarization controlling device which controls
the polarization state of the second signal light S2. The device 9
is disposed between the second light source 2 and the optical fiber
3 to be measured. In Embodiments 1 and 2 shown in FIGS. 1 and 3,
respectively, it is tacitly assumed that the relation between the
polarization states of the first and second light sources 1 and 2
is constant throughout the optical fiber 3 to be measured. However,
such a condition is satisfied only in a special optical fiber such
as polarization-maintaining optical fiber or a multi-mode optical
fiber in which the polarization state is randomized. On the other
hand, the gain g due to the Brillouin light amplification exhibits
a polarization dependence such that it has a maximum value when the
polarization directions of the signal lights S1 and S2 coincide
with one another and such that it becomes unity when they are
perpendicular to one another. Therefore, when such a measurement is
performed on a typical single mode optical fiber, the polarization
controlling device 9 as shown in FIG. 9 needs to be arranged. The
gain g can easily be obtained and expressed by the following
equation:
Here, 1+G max represents the maximum gain obtained when the
polarization directions of the signal lights S1 and S2 coincide
with one another. B and .phi. are amounts depending on z and B
fulfills the relation: -1.ltoreq.B.ltoreq.1. .theta. represents the
polarization direction of the second signal light S2 when it enters
the optical fiber 3 to be measured.
Typical optical fibers have a slight modal birefringency.
Therefore, the polarization state of a signal light propagating
through the fiber varies depending on the position z. Thus, the
signal waveforms obtained when the typical optical fiber is
examined by the measurement systems of Embodiment 1 or 3 shown in
FIG. 1 or 8, are not as shown in FIG. 2 in practice, fluctuate as
shown in FIG. 10A. In FIGS. 10A to 10C, DC components are omitted.
Moreover, the abscissa axis expresses time in terms of a position
in the optical fiber in accordance with equation (5). This
fluctuation reflects the polarization state of the signal lights S1
and S2 at each position z in the optical fiber 3.
The polarization controlling device 9 can be formed by, for
instance, a polarizer for converting the second signal light S2
from the light source 2 into a linearly polarized light and a half
wave plate. In this case the second signal light S2 can be made
incident upon the optical fiber 3 in any polarization direction, by
rotating the half wave plate. FIG. 10A shows a waveform obtained
when the second signal light S2 is linearly polarized in a
direction .theta. and is made incident upon the optical fiber 3 to
be measured in the measuring system shown in FIG. 9. FIG. 10B shows
a waveform obtained when the second signal light S2 is linearly
polarized in the direction (.theta.+.pi./2), which is perpendicular
to the direction .theta., and is made incident upon the optical
fiber 3 to be measured.
As seen from equation (6), in FIGS. 10A and 10B the polarities of
the fluctuations are reversed. Therefore, if the signal levels
shown in FIG. 10A and 10B are averaged by signal processing unit 6
an output waveform having a level as shown in FIG. 10 C is
obtained. It is found that fluctuation is eliminated in the
waveform shown in FIG. 10C and that a smooth waveform which
reflects the optical loss of the optical fiber 3 to be examined is
obtained like in the case shown in FIG. 2. Moreover, it was
confirmed by experiments that the signal level was about 100 times
higher than that obtained by the conventional OTDR.
In the foregoing explanation, in order to obtain the wave forms
shown in FIG. 10A and 10B, the measurement is performed twice in
total by rotating the half wave plate which constitutes the
polarization controlling device 9 to change the polarization state
of the second signal light S2. Alternatively, the waveform shown in
FIG. 10C can directly be obtained by rapidly changing the
polarization state of the second signal light S2.
To this end, it is sufficient to use a polarization controlling
device 9 for changing the polarization state of the second signal
light S2, which is composed of an optical fiber (preferably, an
optical fiber such as polarization-maintaining optical fiber which
has a large mode birefringence) wound around a piezoelectric device
such as PZT and which experiences periodic or random elongation and
side pressure strain.
Alternatively, electro-optical effect type elements such as LiNbO3
are also preferable as a fast polarization controlling device
9.
In additional in the above explanation, the polarization
controlling device 9 is disposed between the optical fiber 3 to be
measured and the second light source 2 to change the polarization
state of the second signal light S2. However, the same effect can
of course be expected by changing the polarization state of the
first signal light S1.
Thus, the polarization controlling device 9 may be disposed between
the first light source 1 and the optical multi-demultiplexer 4 or
between the optical multi-demultiplexer 4 and the optical fiber 3
to be measured. Alternatively, the polarization controlling device
9 may be disposed in the middle of the optical fiber 3 to be
measured so that the polarized states of both signal lights S1 and
S2 can be changed.
Embodiment 5
Embodiment 5 is an embodiment of the method of this invention for
determining the distribution of the relative refractive index
difference between the core and cladding of an optical fiber along
its longitudinal direction. The Brillouin frequency shift fB varies
according to the material of an optical fiber. For instance, silica
optical fibers have an fB of about 13 GHz at a wavelength of 1.3
.mu.m. In optical fibers whose cores are doped with GeO2, their fB
decreases by 120 MHz per 0.1% increase of relative refractive index
difference between the core and cladding.
Therefore, the distribution of the relative refractive index
difference between the core and cladding in the longitudinal
direction of the optical fiber can be determined by measuring, as a
function of each position z, the difference between the optical
frequencies of the first signal light S1 and the second signal
light S2, (.DELTA.f=f1-f2) when the received light signal level
obtained by using the measurement system of the present invention
shown in FIG. 1, FIG. 8 or FIG. 9 reaches its maximum value.
More specifically, measurements are performed while repeatedly
changing the difference between the optical frequencies of the
first signal light S1 and second signal light S2, (.DELTA.f=f1-f2)
by the optical frequency controlling unit 7, and the resultant
various measured waveforms W(z, .DELTA.f) are stored with the help
of the signal processing unit 6 in a memory 64 disposed in the
signal processing unit 6 as shown in FIG. 11.
In FIG. 11, broken lines show the .DELTA.f-dependency of the
received light signal W at positions z1, z2, z3 and z4. As seen
from FIG. 11, this clearly shows that the values of .DELTA.f which
provide maximum values of W vary according to position z (such a
value is defined to be ".DELTA.f"). Thus, waveform analysis is
performed based on the data stored in the memory 64 of the signal
processing unit 6, and the optical frequency difference .DELTA.f
between the first signal light S1 and the second signal light S2,
which provides a maximum value of W at each position z, is thus
obtained.
The value .DELTA.f thus obtained is schematically shown in FIG. 12.
The longitudinal direction-dependency of the relative refractive
index difference between the core and cladding can be obtained from
the aforementioned relation between the value fB and the relative
refractive index difference between the core and cladding.
A method for measuring the distribution of the core diameter in the
longitudinal direction of the optical fiber to be measured,
according to the present invention, will now be explained. If the
spectral linewidths .DELTA.f1 and .DELTA.f2 of the first light
source 1 and the second light source 2 are identical with or
narrower than the Brillouin light amplification bandwidth,
.DELTA.fB, of the optical fiber 3 to be measured, the waveform W(z,
.DELTA.f) measured according to the present invention substantially
depends on the optical frequency difference between the first
signal light S1 and the second signal light S2, (.DELTA.f=f1-f2)
corresponding to the distribution of the relative refractive index
difference between the core and cladding in the longitudinal
direction of the optical fiber 3 as is shown in FIG. 11. Thus, in
order to determine the distribution of the core diameter in its
longitudinal direction, it is necessary to eliminate the influence
on the waveform W of the distribution of the relative refractive
index difference between the core and cladding in the longitudinal
direction. For that purpose, it is sufficient to use the first
light source 1 or the second light source 2 whose spectral
linewidth is wider than the Brillouin light amplification bandwidth
.DELTA.fB of the optical fiber 3 to be measured. The waveform W(z)
thus obtained is averaged with respect to .DELTA.f and thus does
not depend upon the distribution of the relative refractive index
difference between the core and cladding of the optical fiber in
its longitudinal direction. Moreover, the waveform W(z) may
alternatively be obtained by calculating the average value of the
measured waveform W(z, .DELTA.f) while changing the value of
.DELTA.f, i.e.,
using the signal processing unit 6, instead of using the first
light source 1 and/or the second light source 2 having such a wide
spectral linewidth.
On the other hand, in the measurement system shown in FIG. 9,
measurements can be performed without being influenced by the
polarization state of the signal light as already explained above.
The signal gain due to the Brillouin light amplification at this
time is given by
from equation (6). Strictly speaking, Gmax is a function of the
relative refractive index difference between the core and cladding
and the core diameter of the optical fiber, but it can in general
be assumed that it depends on only the core diameter and thus Gmax
is in inverse proportion to the square of the core diameter.
Therefore, it is assumed that the optical loss of the optical fiber
3 to be measured in its longitudinal direction is uniform, the
normalized waveform difference, {W'(z)-W(z))}/W'(z), represents the
distribution of the core diameter, 2.delta.a(z)/a, in the
longitudinal direction of the fiber, wherein W(z) is the waveform
shown in FIG. 13 as solid line, measured by the measurement system
shown in FIG. 9 according to the foregoing procedures, and W'(z) is
the waveform shown in FIG. 13 as a dotted line, which is the
least-square-approximated exponential function representing the
loss of the optical fiber obtained from the waveform W(z) and
.delta.a(z) is the variation in the core diameter and a represents
the average value of the core diameter.
In the foregoing explanation, the core diameter distribution in the
longitudinal direction of the fiber is determined based on W(z),
but the core diameter distribution in the longitudinal direction
may be obtained by compensating for the influence of the relative
refractive index difference distribution in the longitudinal
direction on the measured waveform W(z, .DELTA.f), if the relative
refractive index difference distribution in the longitudinal
direction is known, as a matter of course.
In the foregoing explanation, it is assumed that the intensity of
the first signal light S1 is modulated in the form of pulses, but
the first signal light S1 may be a single pulse or repetitive
pulses having a period of tc, as shown in FIG. 14A. In such a case,
the signal to noise ratio can be improved by an averaging-process
by the signal processing unit 6 and thus more precise measurements
can be carried out.
Furthermore, the first signal light S1 may be modulated by a pseudo
random code (such as M series code) as shown in FIG. 14B. In this
case, if the detected second signal light S2 is subjected to a
correlation processing by the signal processing unit 6 on the basis
of the principle of the correlation technique, the signal to noise
ratio is highly improved in proportion to the length of the code
when compared with the case shown in FIG. 14A (see, for instance,
K. Okada et at,, "Optical cable fault location using correlation
technique", Electron. Lett., vol. 16, p. 629, 1980). In the
conventional OTDR, if weak back-scattered light and strong Fresnel
reflection light coexist, the problem of linearity of the
measurement system arises and, therefore, it is impossible to use a
sufficiently large code length. In contrast, a strong signal such
as Fresnel reflection light can be eliminated by the use of the
optical frequency filter 8 in the present invention as already
explained above and thus we can make the best use of the
correlation technique.
Moreover, the same measurement as that in the time-domain explained
above can be performed in the frequency-domain as follows. That is,
the first signal light S1 can be amplitude-modulated by a frequency
F and by changing the frequency F, and frequency characteristics of
the amplitude and the phase of the second signal light S2 is
obtained, as shown in FIG. 14C. In this case, the problem of
linearity in the detection system does not arise and highly precise
measurements can be performed for the same reasons as in the
correlation technique.
In the foregoing explanation, the first signal light S1 has been
explained to be the amplitude-modulated light in every case, but it
is also possible to use frequency-modulated light as the first
signal light as shown in FIGS. 14D, 14E and 14F, which correspond
to FIGS. 14A, 14B and 14C respectively.
In FIGS. 14D-14F, it is assumed that the frequency f1 of the first
signal light S1 in the hatched portion is controlled by the optical
frequency controlling unit 7 so that .vertline.f1-f2-fB.vertline.
falls within the Brillouin light amplification bandwidth .DELTA.fs
of the optical fiber 3 to be measured. Moreover, it is also assumed
that the frequency f1' of the first signal light S1 in the portions
other than the hatched portions does not satisfy the foregoing
relation.
When the frequency is f1 (hatched portions), the conditions do not
change from those in the amplitude-modulation. Accordingly, the
first signal light S1 causes the Brillouin light amplification of
the second signal light S2, like the previous case. On the other
hand, when the frequency of the first signal light S1 is f1' (the
portions other than the hatched portions), although the first
signal light S1 propagates through the optical fiber 3 to be
measured, the same result as in the case of a space state, in the
intensity-modulation in which the first signal light S1 is absent,
is obtained and thus the Brillouin light amplification does not
occur since the signal light S1 and the signal light S2 do not
satisfy the requirements for the Brillouin light amplification.
That is, the result obtained by intensity-modulation is identical
with that obtained by frequency-modulation. An advantage attained
by frequency-modulation is that easy and fast modulation can be
performed by slightly changing the injection current without using
an external modulator when a semiconductor laser such as a DFB
laser or a DBR laser is used as the light source.
In the ease of a silica optical fiber, .DELTA.fB is about 100 MHz.
Therefore, it is sufficient to select the foregoing frequency f1'
so as to fulfill the requirement: .vertline.f1'-f1.vertline.>100
MHz. In this case, the injection current change of the
semiconductor laser can be small, on the order of about 0.1 mA.
The present invention has been explained with particular examples
concerning the measurements of the distributions of optical loss,
the relative refractive index difference between the core and
cladding and the core diameter of an optical fiber in its
longitudinal direction. However, it is also known that the
Brillouin frequency shift fB is dependent upon the change in stress
applied to the optical fiber and the change in temperature of the
optical fiber.
FIG. 15 is a graph showing an example of experimental results on
the relation between the stress applied to the optical fiber and
the Brillouin frequency shift. As seen from FIG. 15, the stress is
proportional to the Brillouin frequency shift change. The
proportionality factor depends on the material of the optical
fiber, but the factors for silica optical fibers are approximately
constant irrespective of the kind of dopant and the amount thereof.
Therefore, it is possible to measure the changes in stress applied
to the optical fiber along the longitudinal direction of the fiber
and changes in temperature of the optical fiber along the
longitudinal direction of the fiber by measuring the mount of
change in the Brillouin frequency shift fB wherein the received
light level of the second signal light S2 becomes maximum. As is
clear from the foregoing explanation, the amount of change in the
Brillouin frequency shift fB can be measured from the amount of
change in the frequencies of the signal lights S1 and S2 of the
first light source 1 and the second light source 2 which are
changed so that the received level of the second signal light S2
becomes maximum.
It is to be understood that changes and modifications can be made
to the preferred embodiments described above, to which the present
invention is not limited, without departing from the true spirit of
the present invention defined by the appended claims.
For instance, in the embodiment of the present invention shown in
FIG., 1, 8 or 9, the apparatus is equipped with a branch type
optical multi-demultiplexer 4, but this can be replaced by a Mach
Zehnder interferometer 10 shown in FIG. 16 as will be described
below.
In FIG. 16, the Mach Zehnder interferometer 10 has, for instance, a
port P1 connected to the first light source 1, a port P3 connected
to the photo detector 5 and a port P2 connected to a terminal
followed by the optical fiber 3 to be measured. In this case, the
port P4 is not used. Since the interferometer 10 has the
transmission characteristics as shown in FIG. 17A, the first signal
light S1 (having frequency f1) arrives at the optical fiber 3 to be
measured without any transmission loss in the interferometer 10
(between the ports P1 and P2) while the second signal light S2
(having frequency f2) is also made incident upon the photo detector
5 without any transmission loss in the interferometer 10 (between
the ports P2 and P3). This leads to a substantial reduction in the
insertion loss compared with those observed when branch type
optical multi-demultiplexers 4 shown in FIGS. 1, 8 and 9 are
used.
In this case, the Fresnel reflection light and the Rayleigh
back-scattered light due to the first signal light S1 having a
frequency of f1 do not enter the photo detector 5, even if the
optical frequency filter 8 is not used, and thus only the second
signal light S2 having a frequency f2 is detected.
Moreover, the characteristics of the interferometer 10 can be
switched from the state shown in FIG. 17A to the state shown in
FIG. 17B, when the optical path difference between the optical
paths I and II of the interferometer 10 is varied by, for instance,
applying a thermal stress to one of the optical paths. In this
case, the interferometer 10 may be deemed to be equivalent to a 3
dB branch type optical multi-demultiplexer, with respect to the
first signal light S1 having a frequency of f1.
Alternatively, in FIG. 17A. the same effect can be obtained even by
convening the frequency f1 of the first signal light S1 to, for
instance, (f1+f2)/2. In this case, if the second light source 2 is
not used, the apparatus for evaluating properties of an optical
fiber according to the present invention can be changed to a
conventional OTDR for measuring the Rayleigh back-scattered
light.
In all the embodiments of the present invention explained above, it
has been assumed that the optical frequency f1 of the first signal
light S1 at the time of mark satisfies the relation expressed by
equation (b 1). However, even if the second signal light S2 is
substituted for the first signal light S1 and the latter is
Brillouin-light-amplified by the second signal light S2, i.e.,
all the measurements of the present invention explained above can
likewise be performed. In this case, the second signal light S2
loses its light power through the Brillouin light amplification of
the first signal light S1 and, therefore, the waveform of the
second signal light S2 detected by the photo detector 5 shows a
form depressed from the DC component as shown in FIG. 18.
In the foregoing explanation, it has been assumed that the power of
the first signal light S1 is not changed even if there is a
non-linear interaction , i.e., the Brillouin light amplification
between the first and second signal lights S1 and S2. However, when
the gain of the Brillouin light amplification is large, the change
in the power of the first signal light S1 is not negligible and in
turn the properties of the optical fiber such as optical loss
cannot be correctly evaluated from the measured signal
waveform.
When the condition of equation (1) is established, the second
signal light S2 is amplified, while the power of the first signal
light S1 is reduced. By a simple analysis, the second term on the
right side of equation (4) is expressed by the following
relation:
The term exp(-Q(z)) represents the effect due to the reduction in
the power of the first signal light S1 and this leads to errors in
the measurement of the loss in the optical fiber. On the other
hand, if the requirement represented by equation (9) is
established, the power of the first signal light S1 increases and
thus the second term in the right side of equation (4) is expressed
by
In this way, it is impossible to correctly evaluate the loss in the
optical fiber from only one measurement of WA(z) or WB(z).
However, as seen from equations (10) and (11), a highly precise
measurement can be executed by measuring two waveforms i.e., WA(z)
and WB(z), since the term Q(z) can be eliminated by calculating
{WA(z)+WB(z)}/2 when Q(z)<<1 or WA(z).multidot.WB(z) when
Q(z)>1.
Thus, by comparing and analyzing the two results obtained by
replacing the pump and probe lights for Brillouin light
amplification, more precise evaluation of the characteristics can
be performed.
In all the embodiments of the present invention explained above, it
is assumed that the second light source 2 emits CW light, but the
second light source 2 may be a modulated light source like the
first light source 1. In such a case, the Brillouin light
amplification takes place only at a position (for instance, zo )
where the first signal light S1 (e.g., a light pulse) emitted from
the first light source 1 encounters with the second signal light S2
(e.g., a light pulse) emitted from the second light source 2.
Therefore, this method is quite suitable for obtaining information
such as change in the optical loss in an optical fiber, temperature
change, stress change and so on at a particular point (the
foregoing point zo) in the optical fiber 3. The point zo may be
selected at any desired point by controlling the relative time
interval between the emission of the first signal light S1 and that
of the second signal light S2.
As explained above, according to the present invention, the
characteristics of an optical fiber to be measured can be evaluated
by making use of the Brillouin light amplification effect between
the modulated signal light propagating through the optical fiber,
i.e., the first signal light and the second signal light which
propagates in the fiber in the direction opposite to the first
signal light and by analyzing the change in the waveform of the
second signal light resulting from the Brillouin light
amplification effect therebetween. In this respect, the signal
level of the change in the waveform is 100 times or more larger
than that obtained in the conventional OTDR, and the attenuation
rate of the signal level obtained according to the present
invention in proportion to the length of the optical fiber (in
other words, fiber loss) is 1/2 time that of the conventional OTDR.
Therefore, the following advantages can be attained according to
the present invention:
(1) Even if a light source of low power such as a semiconductor
laser is used, there can be provided an apparatus for evaluating
the properties of an optical fiber, which can provide a signal to
noise ratio that is relatively high compared with the conventional
OTDR and which can achieve highly precise measurements.
(2) Since the frequency of the first signal light differs from that
of the second signal light, it is possible to prevent strong
Fresnel reflection light due to the tint signal light from being
incident upon the photo detector by using an optical frequency
filter.
(3) To date, the measurement at a point immediately after a
connector junction at which a strong Fresnel reflection pulse is
generated has been impossible and thus a region where such
measurement is impossible . (i.e., dead zone) is formed. However,
such a dead zone can be eliminated in accordance with the present
invention and thus a measurement can be made at a point immediately
after the connector junction.
(4) The problem of linearity of the detection system does not arise
The correlation processing or the measurement in a frequency region
are easily performed and thus the signal to noise ratio can
substantially be improved.
(5) Moreover, the amount of change in the waveform of the second
signal light which is measured according to the present invention
depends on the optical frequency difference between the first
signal light and the second signal light and becomes maximum when
the difference coincides with the Brillouin frequency shift which
is determined by the kinds of materials of the optical fiber and
the relative refractive index difference between the core and
cladding of the optical fiber. Thus, the distribution of the
relative refractive index difference between the core and cladding
in the longitudinal direction of the optical fiber can be measured
by performing measurements while changing the optical frequency
difference between the first signal light and the second signal
light. In this respect, the conventional OTDR technique cannot
separately determine the distribution of the relative refraction
index difference between the core and cladding and that of the core
diameter, but such measurements can be performed according to the
present invention.
(6) Since the amount of change in the waveform of the second signal
light measured according to the present invention varies depending
upon the optical loss in the optical fiber, its core diameter as
well as the stress applied to the fiber, its temperature and so on,
the amounts of physical properties such as optical loss, core
diameter, stress and temperature in the longitudinal direction of
the optical fiber, can be determined according to the
invention.
As seen from the foregoing description, the present invention can
effectively be applied not only to evaluation of the
characteristics of an optical fiber when manufacturing optical
fibers but also to distributed remote measurements in which
variations in the stress applied to the fiber and the temperature
change thereof are utilized.
The invention has been described in detail with respect to
preferred embodiments, and it will now be apparent from the
foregoing to those skilled in the art that changes and
modifications may be made without departing from the invention in
its broader aspects, and it is the intention, therefore, in the
appended claims to cover all such changes and modifications as fall
within the true spirit of the invention.
* * * * *