JPH0979941A - Instrument and method for measuring transmission characteristic of optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To measure the spectral distribution of a light soliton pulse at a scattering point by time-resolving back scattered light which is generated by the Reyleight scattering of the pulse while the pulse is transmitted through an optical fiber and analyzing the spectral distribution of the pulse. SOLUTION: An optical pulse generator 2 is driven by means of a synchronizing signal from a synchronizing signal generating circuit 1 and a generated optical pulse is made incident to an optical fiber 20 to be measured through an optical branching device 3 and a connecting device 4 for connecting optical fiber to be measured. The optical pulse continuously generates back scattered light at each point while the pulse is propagated through the fiber 20 and the scattered light is inputted to an optical switch 6 through the devices 4 and 3. The synchronizing signal from the circuit 1 is delayed by a prescribed delay time by means of a delaying device 5 and allows the scattered light to pass through during the pulse width time only by driving the switch 6. A spectroscope 7 analyzes the spectral distribution of the scattered light and a photodetector 8 detects the light intensity and displays the spectral distribution on a wavelength axis display 9 synchronized to the wavelength sweep of the spectroscope 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber transmission characteristic measuring apparatus and measuring method for measuring chromatic dispersion and nonlinear optical constant of an optical fiber transmitting an optical soliton pulse.

[0002]

2. Description of the Related Art Changes in the spectral distribution of an optical soliton pulse propagating in an optical fiber are measured at the output end of the optical fiber from the spectral distribution after the optical fiber has propagated. The property of the optical soliton pulse propagating in the optical fiber can be determined from the change of the spectrum with respect to the propagation distance, and thereby the chromatic dispersion and the nonlinear optical constant of the optical fiber can be measured. Conventionally, the measurement of the spectral distribution according to the propagation distance has been performed by using optical fibers having different lengths or by sequentially cutting long optical fibers.

[0003]

In order to measure the spectral distribution of an optical soliton pulse propagating in an optical fiber, it has conventionally been necessary to change the length of the optical fiber. In the case of an already installed optical fiber, the length must be changed. The only way to change the optical fiber was to cut the optical fiber. Therefore, in order to determine the optimum optical soliton transmission conditions for the currently installed optical fiber, the peak power and pulse width of the incident optical soliton pulse are changed by trial and error to measure the spectral distribution at the output end. The method was taken. But,
It was not easy to change the peak power and pulse width of the optical soliton pulse, and it was impossible to continuously measure the spectral distribution of the actually propagating optical soliton pulse.

The present invention continuously measures the change in the spectral distribution during propagation of an optical fiber by using an actually used optical soliton pulse without cutting the optical fiber after being laid, and measuring the wavelength of the optical fiber. An object of the present invention is to provide an optical fiber transmission characteristic measuring device and a measuring method capable of determining the optimum optical soliton transmission conditions by measuring dispersion and nonlinear optical constants.

[0005]

The optical fiber transmission characteristic measuring apparatus and method of the present invention time-resolve backscattered light generated by Rayleigh scattering of an optical soliton pulse propagating in an optical fiber and analyze its spectral distribution. This is characterized by measuring the spectral distribution of the optical soliton pulse at the scattering point.

In the wavelength range where the value of the group velocity dispersion of the optical fiber is negative (abnormal dispersion range), the backscattered light generated by Rayleigh scattering of the optical soliton pulse propagating in the optical fiber is propagated at the scattering point. It agrees with the time waveform and spectral distribution of the optical soliton pulse. However, since the scattering intensity is weak, the property as an optical soliton is lost and the light propagates as an ordinary optical pulse. The pulse width of this backscattered light is widened by the dispersion of the optical fiber as in the case of the propagation of a normal optical pulse, but the spectral component does not change. Therefore, if the spectral component of this backscattered light is time-resolved, the scattering point in the optical fiber can be determined from the round-trip propagation time of the optical pulse, and the spectral distribution of the optical soliton pulse at that point can be measured.

Now, the optical pulse incident on the optical fiber is N
If the basic soliton of = 1 is set, the time waveform (pulse waveform) and the spectral distribution propagate in the optical fiber without change, so that the spectral distribution of the backscattered light also remains unchanged. Therefore, the incident condition of the basic soliton can be determined by measuring the spectral distribution of the backscattered light that is invariant to the propagation distance.

Further, when high-order solitons of N = 2, 3, ... Are made incident, the time waveform and the spectral distribution change intricately during the propagation of the optical fiber. Return to time waveform and spectral distribution. If this propagation period and soliton period Z _0, the pulse width for the input pulse is narrowest, the propagation distance opposite the spectral distribution is the most spread, in the N = 2 for second order _{soliton, mZ 0 + Z 0/2} ... (1), and the third-order soliton N = 3, the respective _{mZ 0 + Z 0/3 ...} (2) mZ 0 + 2Z 0/3 ... (3). Here, m is an integer.

Now, assuming that the optical soliton pulse is a sech type, the peak power at the time of incidence of a basic soliton of N = 1 is P
_If the peak power at the time of incidence of _1st and Nth-order solitons is P _N , the following relationship is established: P _N / P ₁ = N ² (4) Therefore, each peak power P
By measuring ₁ and P _N , the order of the soliton in the propagation can be obtained. Further, the order of the optical soliton pulse under an arbitrary incident condition can be determined from the change of the spectral distribution during propagation.

If the soliton period Z ₀ is measured using a higher-order soliton, and then the propagation of the basic soliton is confirmed,
The wavelength dispersion D of the optical fiber and the nonlinear optical constant n ₂ can be obtained. Assuming that the full width at half maximum of the time waveform of the pulse intensity of the basic soliton is τ ₀ , the central wavelength is λ _0, and the speed of light in vacuum is C, the chromatic dispersion D of the optical fiber is D = K ₁ π ² Cτ ₀ ² / (Z ₀ λ ₀ ² ) ... (5) Further, the nonlinear optical constant n ₂ / A _eff standardized by the effective area A _eff of the optical fiber is n ₂ / A _eff = K ₁ K ₂ λ ₀ / (P ₁ Z ₀ ) ... (6). Here, K ₁ and K ₂ are constants, and assuming that the incident light soliton waveform is a sech type, K ₁ = 0.322, K ₂ = 0.77
6

By the way, in the wavelength range (normal dispersion range) where the value of the group velocity dispersion of the optical fiber is positive, the spectrum width widens due to the self-phase modulation effect due to the nonlinear optical constant n ₂ of the optical fiber. Therefore, the non-linear optical constant n ₂ of the optical fiber can be measured even by using a normal optical pulse. Hereinafter, the measuring method will be described. Now, ignore the chromatic dispersion and set the peak power of the incident pulse to P
₀ , the central wavelength is λ ₀ , the spectral distribution is Gaussian,
Assuming that half width is Δω ₀ , the spectral width Δω after propagating the distance Z is Δω / Δω ₀ = {1 + K ₃ ² (2πn ₂ P ₀ Z / (λ ₀ A _eff )) ² } ^1/2 ... It is represented by (7). Therefore, the effective area A of the optical fiber is
nonlinear optical constant n ₂ / A _eff is normalized by _{eff, n 2 / A eff =} {(Δω / Δω 0) 2 -1} 1/2 / (2πK 3 P 0 Z / λ 0) ... (8) Becomes Here, K ₃ is a constant, and K ₃ = 0.877 when the incident light pulse waveform has a Gaussian shape.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment) FIG. 1 shows a first embodiment of the optical fiber transmission characteristic measuring apparatus of the present invention. In the figure, the optical fiber transmission characteristic measuring device of the present embodiment includes a synchronizing signal generating circuit 1, an optical pulse generator 2, an optical branching device 3, an optical fiber connecting device 4, a delay device 5, an optical switch 6, and a spectroscopic device. 7, a photodetector 8 and a wavelength axis indicator 9. Reference numeral 20 is a measured optical fiber connected to the measured optical fiber connecting device 4. Here, the synchronization signal generating circuit 1 and the optical pulse generator 2 correspond to the optical pulse generating means. Spectroscopic device 7, optical detector 8 and wavelength axis display 9
Corresponds to the optical spectrum detecting means.

The optical pulse generator 2 is a synchronization signal generation circuit 1
The optical pulse generated by being driven by the synchronizing signal output from the optical path passes through the optical branching device 3 and enters the optical fiber 20 to be measured via the optical fiber connecting device 4 to be measured. The optical pulse propagating through the optical fiber 20 to be measured continuously generates backscattered light at each point of the optical fiber. This backscattered light is input to the optical switch 6 via the measured optical fiber connection device 4 and the optical branching device 3. On the other hand, the synchronizing signal output from the synchronizing signal generating circuit 1 is given a predetermined delay time by the delay device 5. The optical switch 6 is driven by this synchronizing signal and allows the backscattered light to pass for the time corresponding to its pulse width. The spectral distribution of the backscattered light is analyzed by the spectroscopic device 7, the light intensity is detected by the photodetector 8, and the spectral distribution is displayed on the wavelength axis display 9 synchronized with the wavelength sweep of the spectroscopic device 7.

Here, the delay time set by the delay device 5 is the time required for the optical pulse to make a round trip to the measured point of the optical fiber 20 to be measured, and the backscattered light passes for a time equivalent to the half width of the optical pulse. If you control the optical switch 6 like this,
The measured spectral distribution shows the spectral distribution of the optical pulse being propagated at the measured point. The gate time of the optical switch 6 determines the distance resolution of the measured point.

(Second Embodiment) FIG. 2 shows a second embodiment of the optical fiber transmission characteristic measuring apparatus of the present invention. It should be noted that those having the same functions as those of the configuration shown in FIG. A feature of this embodiment is that an electric switch 10 is inserted between the photodetector 8 and the wavelength axis display 9 in place of the optical switch 6 of the first embodiment. The electric switch 10 allows the output signal of the photodetector 8 to pass for the time corresponding to the pulse width of the synchronization signal output from the delay device 5.

In this configuration, the backscattered light from the optical fiber 20 to be measured is analyzed for spectral distribution by the spectroscopic device 7,
After the light intensity is detected by the photodetector 8, the intensity signal passes through the electric switch 10 controlled with the same delay time as in the first embodiment. As a result, like the first embodiment, the wavelength axis display 9 synchronized with the wavelength sweep of the spectroscopic device 7 is displayed.
The spectrum distribution is displayed on. The electric switch 1
A gate time of 0 determines the distance resolution of the measured point.

(Third Embodiment) FIG. 3 shows a third embodiment of the optical fiber transmission characteristic measuring apparatus of the present invention. It should be noted that those having the same functions as those shown in FIG. 1 or FIG.
The feature of this embodiment is that a time axis indicator 11 of intensity is used instead of the delay device 5, the optical switch 6, and the wavelength axis indicator 9 of the first embodiment. The time axis indicator 11 receives a sync signal from the sync signal generation circuit 1.

In this configuration, the backscattered light from the optical fiber 20 to be measured passes through the spectroscopic device 7 having a fixed passing wavelength, the light intensity is detected by the photodetector 8, and the intensity is displayed on the time axis display. 11 is displayed. When the passing wavelength of the spectroscopic device 7 is fixed to the center wavelength of the backscattered light, the time change of the detected intensity shows the change of the center wavelength of the optical pulse propagating in the optical fiber 20 to be measured.

(Fourth Embodiment) FIG. 4 shows a fourth embodiment of the optical fiber transmission characteristic measuring apparatus of the present invention. In addition, what has the same function as the configuration shown in FIGS.
The same name and the same reference numeral are given and description thereof is omitted. The feature of this embodiment is that the synchronization signal generating circuit 1 of the first embodiment is used.
Instead of the optical pulse generator 2 and the optical pulse generator 2, an optical pulse generator 12 that itself generates an optical pulse having a predetermined pulse width, and a synchronization signal extraction circuit 1 that extracts a synchronization signal from the output optical pulse
3 is used.

In this configuration, the sync signal extraction circuit 13 extracts the sync signal from the optical pulse and uses it as a signal corresponding to the sync signal output from the sync signal generation circuit 1 of the first embodiment. This synchronization signal is given to the optical switch 6 via the delay device 5, and the backscattered light is passed for the time corresponding to the pulse width. The subsequent operation is similar to that of the first embodiment. The configuration of this embodiment can be applied to the second and third embodiments.

(Fifth Embodiment) FIG. 5 shows a fifth embodiment of the optical fiber transmission characteristic measuring apparatus of the present invention. In addition, what has the same function as the configuration shown in FIGS.
The same name and the same reference numeral are given and description thereof is omitted. The feature of this embodiment is that the synchronization signal extraction circuit 1 of the fourth embodiment is used.
3 between the pulse signal generator 14 for generating a synchronization signal having a predetermined pulse width, the optical pulse generator 12 and the optical splitter 3, It is provided with an optical switch 15 for passing an optical pulse only for the time of the pulse width. Instead of the optical pulse generator 12, the synchronization signal generating circuit 1 and the optical pulse generator 2 may be used as in the first to third embodiments.
The pulse signal output from the pulse signal generator 14 is given to the optical switch 6 via the delay device 5, and allows the backscattered light to pass for the time corresponding to the pulse width.

When the repetition period of the optical pulse is shorter than the propagation time of the optical pulse traveling back and forth in the optical fiber 20 to be measured, a large number of optical pulses exist in the optical fiber, and the backscattered light thereof is a large number of these optical pulses. Since it is a composite, it becomes difficult to determine the reflection point. In the present configuration, the repetition period of the pulse signal generated by the pulse signal generator 14 is made longer than the round-trip propagation time of the optical pulse, and the optical pulse is temporally selected and transmitted with the predetermined pulse width. Solve a problem. However, there is a trade-off between the effect that a large number of optical pulses are transmitted within the gate time of the optical switch 15 to increase the average power and improve the detection sensitivity, and the distance resolution determined by the gate time of the optical switches 6 and 15. Have a relationship. The configuration of this embodiment can be applied to the second and third embodiments.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific configuration examples of each part shown in FIGS. The combination of the synchronization signal generating circuit 1 and the optical pulse generator 2 is an active mode-locking type optical pulse generator, and a semiconductor laser, a mode-locked YAG laser or the like can be used. The optical pulse generator 12 is a passive mode-locking type optical pulse generator, and a collision pulse mode-locked laser can be used. The sync signal extraction circuit 13
It is possible to use a circuit that detects a part of the output optical pulse with an optical detector.

The optical branching device 3 sends an optical pulse to the optical fiber 20 to be measured through the optical fiber connecting device 4 to be measured,
The backscattered light returning from the optical fiber 20 to be measured is branched, and a half mirror, a polarization beam splitter, a non-polarization beam splitter, an optical directional coupler, an optical circulator, or the like can be used. An electro-optical modulator, an acousto-optic modulator, or the like can be used for the optical switch 15 that allows the light pulse to pass for a predetermined time and the optical switch 6 that allows the backscattered light to pass for a predetermined time.

The spectroscopic device 7 analyzes the spectrum of the backscattered light, and includes a dispersion prism, a diffraction grating type spectroscope, a Fabry-Perot type spectroscope, a wavelength filter such as a dielectric multilayer film filter, a colored glass filter and the like. Can be used. Instead of the spectroscopic device 7, the optical detector 8 and the wavelength axis display 9, an integrated optical spectrum analyzer may be used. Further, in the configuration shown in FIG. 3, an optical spectrum analyzer in which the spectroscopic device 7 and the photodetector 8 are integrated may be used and the detection output thereof may be used.

The time axis display 11 displays the spectral distribution of the backscattered light on the time axis, and an oscilloscope or the like can be used. (Experimental Result) FIG. 6 shows an experimental result according to the first embodiment. In the figure, the right horizontal axis is wavelength, the left horizontal axis is optical fiber propagation distance, and the vertical axis is spectrum intensity.
It is a three-dimensional display. In this experiment, a third-order optical soliton pulse having a center wavelength λ ₀ = 1.45 μm, a pulse width τ ₀ = 9 ps, and an average power P = 1 mW was used for an optical fiber to be measured having a length of 1.5 km. It can be seen that as the optical fiber propagation distance increases, the intensity of the central wavelength decreases and the spectrum width widens. When the propagation distance is further increased, the central intensity of the spectrum is recovered and its spread is narrowed. This measurement result shows that the measured optical fiber has 0.43 times the soliton period for the incident light soliton pulse.

Similar results can be obtained in the second and fourth embodiments. In the third embodiment, when the incident light pulse of the optical fiber 20 to be measured is a third-order soliton, its intensity has a maximum value for each round trip time of the soliton cycle Z ₀ , and is determined by the equations (2) and (3). The minimum value is shown every hour. This state is a sectional view in the propagation distance direction at the central wavelength λ ₀ of the spectral distribution in FIG. further,
If the passing wavelength of the spectroscopic device 7 is swept and the intensity of each wavelength is measured and three-dimensionally displayed, the same result as in FIG. 6 can be obtained.

FIG. 7 shows the experimental results according to the fifth embodiment. In the figure, the right horizontal axis is the optical fiber propagation distance, the left horizontal axis is the wavelength, and the vertical axis is the spectrum intensity.
It is a three-dimensional display. In this experiment, a third-order optical soliton pulse was propagated through the measured optical fiber having a length of 5 km. Light pulses are sent out with a pulse interval of 56 μs and a pulse width of 1
It was controlled by a pulse signal of μ second. The same pulse signal was used to control the passage of backscattered light. Since the repetition period of the optical pulse generator 12 used here is 10 nsec,
There are about 100 optical soliton pulses in the transmitted optical pulse, which improves the photosensitivity. As shown in the figure, the propagation distance
At 3.5 km, the central wavelength component becomes maximum and the spectral distribution of the input optical pulse is reproduced. Moreover, it can be seen that the change in the spectral distribution of the optical soliton pulse during the propagation of the optical fiber can be accurately measured.

(Procedure for Measuring Optical Fiber Transmission Characteristics) A procedure for obtaining a dispersion constant and a non-linear optical constant in an anomalous dispersion region of an optical fiber using an optical soliton pulse will be described. The spectral distribution and the central wavelength λ ₀ during the optical fiber propagation by an arbitrary higher-order optical soliton pulse having the peak power P _N are measured. From the measured spectral distribution, the soliton period Z ₀ , which is the propagation period of the maximum point of the central wavelength intensity, and the order N of the optical soliton pulse are determined using equations (1), (2), and (3). The incident power of the basic optical soliton pulse with N = 1 (4)
Obtained using the equation, set the propagation conditions of the basic optical soliton pulse. We confirm the propagation of a fundamental optical soliton pulse whose spectral distribution does not change with the propagation distance. The dispersion constant D is calculated using the equation (5). The non-linear optical constant n ₂ is calculated using the equation (6).

Next, the procedure for obtaining the non-linear optical constant of the optical fiber using a normal optical pulse will be described. In a normal optical pulse, the spectrum width is widened by the self-phase modulation effect due to the nonlinear optical constant n ₂ of the optical fiber, so that the nonlinear optical constant n ₂ can be measured by measuring this spectral width. Peak power P ₀ of incident light pulse, central wavelength λ ₀ ,
The spectral half width Δω ₀ is measured. The spectral width Δω at the propagation distance Z is measured. The non-linear optical constant n ₂ is calculated using the equation (8).

By the way, as the propagation distance Z becomes longer, the spectrum width further widens due to the effect of dispersion of the optical fiber. However, in the configuration for observing the backscattered light of the present invention, only the broadening of the spectrum width due to the self-phase modulation effect can be measured at a short distance, so the non-linear optical constant n ₂ is measured without cutting a long optical fiber. be able to.

[0032]

As described above, according to the optical fiber transmission characteristic measuring apparatus and measuring method of the present invention, the spectral distribution of the optical pulse during propagation of the optical fiber can be measured, so that the incident condition of the basic soliton is determined. It becomes possible to do. In addition, by injecting a high-order soliton and measuring its soliton period, the order of the optical soliton pulse, the dispersion constant, and the nonlinear optical constant of the optical fiber propagating in the optical fiber can be measured without cutting the installed optical fiber. can do.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical fiber transmission characteristic measuring device of the present invention.

FIG. 2 is a block diagram showing a second embodiment of the optical fiber transmission characteristic measuring device of the present invention.

FIG. 3 is a block diagram showing a third embodiment of the optical fiber transmission characteristic measuring device of the present invention.

FIG. 4 is a block diagram showing a fourth embodiment of the optical fiber transmission characteristic measuring device of the present invention.

FIG. 5 is a block diagram showing a fifth embodiment of the optical fiber transmission characteristic measuring device of the present invention.

FIG. 6 is a diagram showing an experimental result according to the first embodiment.

FIG. 7 is a diagram showing an experimental result according to a fifth embodiment.

[Explanation of symbols]

 1 Synchronous signal generation circuit 2 Optical pulse generator 3 Optical branching device 4 Optical fiber connection device under test 5 Delay device 6 Optical switch 7 Spectroscopic device 8 Optical detector 9 Wavelength axis indicator 10 Electric switch 11 Time axis indicator 12 Optical pulse Generator 13 Synchronous signal extraction circuit 14 Pulse signal generator 15 Optical switch 20 Optical fiber under test

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Amikam Lebanon 391 Ishida, Isehara City, Kanagawa Prefecture English Heights No. 203

Claims (7)

[Claims] 1. An optical pulse generating means for generating an optical pulse synchronized with a synchronizing signal generating circuit, the optical pulse being guided to an optical fiber to be measured, and a rear portion generated in the optical fiber to be measured by the propagation of the optical pulse. An optical branching device for branching scattered light, a delay device for giving a predetermined delay time to the synchronization signal output from the synchronization signal generation circuit, and the backscattered light temporally by the synchronization signal given the delay time. An optical fiber transmission characteristic measuring device comprising: an optical switch for selecting and outputting, and an optical spectrum detecting means for spectrally analyzing output light of the optical switch and displaying a light intensity distribution on a wavelength axis. 2. An optical pulse generating means for generating an optical pulse synchronized with a synchronizing signal generating circuit, and guiding the optical pulse to an optical fiber to be measured, and rearward generated in the optical fiber to be measured by the propagation of the optical pulse. An optical splitter that splits scattered light, a spectroscope that analyzes the wavelength component of the backscattered light that is split by the optical splitter, an optical detector that detects the intensity of the output light of the spectroscope, and the synchronization signal. A delay device that gives a predetermined delay time to the synchronization signal output from the generation circuit, an electrical switch that temporally selects the output of the photodetector by the synchronization signal given the delay time, the spectroscopic device, and the An optical fiber transmission characteristic measuring device, comprising: a wavelength axis indicator for inputting an output signal of an electric switch and displaying a light intensity distribution on a wavelength axis. 3. An optical pulse generating means for generating an optical pulse synchronized with a synchronizing signal generating circuit, and guiding the optical pulse to an optical fiber to be measured, and causing the optical pulse to propagate backward in the optical fiber to be measured. An optical branching device for branching scattered light, an optical spectrum detecting means for detecting a light intensity corresponding to each wavelength of the backscattered light branched by the optical branching device, a synchronization signal output from the synchronization signal generating circuit, and the An optical fiber transmission characteristic measuring device comprising: a time axis display for inputting a light intensity signal from an optical spectrum detecting means and displaying a light intensity distribution on a time axis. 4. An optical pulse generating means for generating an optical pulse, and a synchronous signal extracting circuit for extracting the synchronous signal of the optical pulse and applying the synchronous signal to a delay device or a time axis display. The optical fiber transmission characteristic measuring device according to any one of claims 1 to 3. 5. An optical pulse generating means for generating an optical pulse, a pulse signal generator for generating a pulse signal having a predetermined pulse width and giving it as a synchronization signal to a delay device or a time axis display, and the optical pulse generation. 4. An optical switch which is inserted between the means and the optical branching device, and which temporally selects and outputs the optical pulse in synchronization with the pulse signal. The optical fiber transmission characteristic measuring device according to any one of the claims. 6. A predetermined optical pulse is made incident on the optical fiber to be measured, the backscattered light thereof is observed, the backscattered light is spectrally analyzed on the time axis, and the backscattered light at a time position corresponding to the scattering point of the backscattered light is analyzed. An optical fiber transmission characteristic measuring method characterized by measuring the transmission characteristic of an optical fiber to be measured from a spectral distribution. 7. The incident light pulse is an optical soliton pulse,
The optical fiber transmission characteristic measuring method according to claim 6, wherein the order and the soliton period of the optical soliton pulse during the propagation of the optical fiber are determined from the spectral distribution of the backscattered light with respect to the propagation distance.