JP2003337081A - Method of measuring distribution of zero dispersion wavelength of optical fiber and device for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and a device for measuring the zero dispersion wavelength in the length direction of an optical fiber to be measured. <P>SOLUTION: An exploring optical pulse PD and two exciting light beams P<SB>1</SB>and P<SB>2</SB>with prescribed two wavelength are made incident on an optical fiber 6 to be measured, and a wave form of a back scattered light P<SB>b</SB>representing power distribution of light pulse in the length direction of the optical fiber is observed. From the wave form of the back scattered light P<SB>b</SB>, a power gain generating part of the exploring light pulse PD is detected, wherein the power gain is caused by modulation instability caused by mutual phase modulation introduced by the exciting light beams P<SB>1</SB>and P<SB>2</SB>of two wave lengths λ<SB>1</SB>and λ<SB>2</SB>. Furthermore, the zero dispersion wavelength of the gain generating part of the optical fiber 6 is determined as (λ<SB>1</SB>+λ<SB>2</SB>)/2 which is the intermediate wavelength of the two exciting light beams P<SB>1</SB>and P<SB>2</SB>, the wavelengths of which are λ<SB>1</SB>and λ<SB>2</SB>respectively. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a measuring method and a measuring apparatus for a zero dispersion wavelength distribution of an optical fiber. Especially, it is devised so that the distribution of the zero dispersion wavelength can be grasped over the entire length of the optical fiber in the longitudinal direction.

[0002]

2. Description of the Related Art An example of a method for measuring the distribution of dispersion value of an optical fiber is described in M. Ohasi, et al. (Electron. Lett, 199).
3, 29, pp. 426-428) and C. Unger, et al. (Tech.
DigestSymp. On Optical Fiber Measurement, Boulder
(1994), pp.65-68). In these methods, the distribution of the mode field diameter is estimated from the OTDR (exploration light pulse) waveform measured from both ends of the optical fiber, and the structural dispersion is estimated from the distribution.

Japanese Patent Application No. 7-109767 (Japanese Unexamined Patent Publication No. 8-21783, "Measuring method and measuring apparatus for zero-dispersion wavelength distribution of optical fiber") is also an example of a method for measuring the distribution of dispersion value of an optical fiber. It is shown. in this way,
An optical pulse and excitation light of a specified wavelength are injected into the optical fiber to be measured, and the waveform of the backscattered light of the optical pulse showing the power distribution of the optical pulse in the length direction of the optical fiber to be measured is induced by the excitation light. The gain generation part of the optical fiber under measurement, which indicates that the optical pulse power is amplified by the modulation instability, is detected, and the zero dispersion wavelength of the gain generation part of the measured optical fiber is determined from the wavelength of the pump light.

[0004]

[Problems to be Solved by the Invention] However, M. Ohasi, et.
The method disclosed by al. and C. Unger, et al. has a problem that access at both ends of the optical fiber to be measured is required.

In the method disclosed in Japanese Patent Application No. 7-109767 (Japanese Patent Laid-Open No. 8-21783), the wavelength is swept while the wavelength difference between the wavelength of the pumping light and the wavelength of the optical pulse is kept constant. You have to take a complicated structure. Furthermore, if the pumping light wavelength is in the anomalous dispersion region, a gain due to modulation instability occurs, so that the zero dispersion wavelength cannot be determined with high accuracy.

In view of the above-mentioned conventional technique, the present invention does not require access at both ends of the fiber and can measure the distribution of zero-dispersion wavelength in the longitudinal direction of the optical fiber to be measured with high accuracy. An object of the present invention is to provide a measuring method and a measuring device.

[0007]

The structure of the method for measuring the zero-dispersion wavelength distribution of an optical fiber according to the present invention for solving the above-mentioned problems is as follows.
The probe light pulse and the excitation light of two wavelengths are made to enter the optical fiber to be measured, and the waveform of the backscattered light of the probe optical pulse showing the power distribution of the probe optical pulse over the length direction of the optical fiber to be measured is observed. From the waveform of the backscattered light observed,
The gain generation part of the measured optical fiber is detected by detecting the gain generation part of the measured optical fiber, which is the part where the power of the probe optical pulse is amplified due to the modulation instability due to the cross phase modulation in the measured optical fiber. The zero dispersion wavelength of the part is determined from the wavelength of the excitation light.

Further, according to the present invention, in the above measuring method, the wavelength of the pumping light is swept, and when the gain generating portion is detected in each portion of the measured optical fiber, the pumping light is
The mid-wavelength of the wavelength is determined as the zero-dispersion wavelength of the gain generating portion, or the waveform slope becomes larger than the slope of the waveform when only fiber loss occurs in the backscattered light waveform. When there is a waveform portion, the measured optical fiber portion corresponding to this waveform portion is detected as a gain generating portion.

Further, the structure of the measuring apparatus for the zero-dispersion wavelength distribution of the optical fiber according to the present invention comprises a light source means for generating a probe light pulse, a pump light source means for generating a pump light of two wavelengths, the probe light pulse and the probe light pulse. A multiplexer that combines the excitation light, and combines the combined probe light and excitation light into the measured optical fiber,
Light receiving means for receiving the backscattered light of the probe light pulse showing the power distribution of the probe light pulse over the length of the optical fiber to be measured, and waveform generating means for generating the waveform of the backscattered light received by the light receiver means. And having.

Further, according to the present invention, in the above-mentioned measuring apparatus, there is provided a display means for displaying the waveform of the backscattered light generated, or the cross-phase modulation in the optical fiber to be measured from the waveform of the backscattered light generated. Is a portion where the power of the exploration light pulse is amplified by modulation instability caused by, and detects the gain generation portion of the measured optical fiber, and the zero dispersion wavelength of the gain generation portion of the measured optical fiber, The calculation means has a calculation means for determining from the wavelength of light, or the calculation means has a larger waveform slope than that of the waveform of the backscattered light when only fiber loss occurs. When there is a waveform portion, the measured optical fiber portion corresponding to this waveform portion is detected as a gain generating portion, and excitation when the gain generating portion is detected The second intermediate wavelengths of, or determined as the zero dispersion wavelength of said gain generation portion, and having a display means for displaying the generated backscattered light waveform.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

In the embodiment of the present invention, the probe light pulse and the excitation light of two predetermined wavelengths are made to enter the optical fiber to be measured,
Observe from the backscattered light waveform how the power of the exploration light pulse is amplified due to the modulation instability caused by the cross-phase modulation induced by the two wavelengths of excitation light, and the zero-dispersion wavelength from the excitation light wavelength. To decide.

That is, the zero-dispersion wavelength distribution of the optical fiber to be measured is measured by setting the wavelengths of the pumping light of two wavelengths to λ ₁ and λ _2, and an intermediate wavelength of 2 wavelengths ((λ ₁ + λ ₂ ) / 2). The backscatter observed with an optical pulse tester shows the phenomenon that the gain band and the gain of the modulation instability caused by the cross-phase modulation by the pumping light become large only when is equal to the zero-dispersion wavelength of the optical fiber under test. It is detected from the waveform of light. The waveform of the backscattered light shows the power distribution of the probe light pulse over the length direction of the optical fiber to be measured.

FIG. 1 shows the basic construction of a measuring apparatus for zero-dispersion wavelength distribution of an optical fiber. In FIG. 1, 1 is an optical pulse tester (OTDR), 2 is a pumping light source, 3 is a variable wavelength pumping light source, 4 and 5 are multiplexers, 6 is an optical fiber to be measured, and 7 is a wavelength filter.

The multiplexer 4 has a wavelength λ ₁ of pumping light P ₁ emitted from the pumping light source ₂ and a wavelength λ _{2 of} light emitted from the pumping light source 3 (however, this wavelength is swept and changed as described later). Of the excitation light P ₂ is combined. The multiplexer 5 is a probe optical pulse (“OTDR”) emitted from the optical pulse tester 1.
Pulse ”) P _D and the pumping light P _{1 and} P ₂ emitted from the pumping light source 2 and the pumping light source 3 are combined to form the optical fiber 6 to be measured.
Incident on. The wavelength filter 7 transmits only the light having the same wavelength as the exploration light pulse P _D among the backscattered light P _b generated in the optical fiber 6 to be measured, and the light having the same wavelength as the excitation lights P _{1 and} P ₂ It has an optical filter characteristic of not transmitting.

In the apparatus for measuring the zero-dispersion wavelength distribution of the optical fiber having such a structure, the optical pulse tester 1 emits the probe optical pulse P _D. This exploration light pulse P _D is
The two wavelengths of excitation light P _{1 and} P ₂ are incident on the measured optical fiber 6. Then, when the search light pulse P _D propagates in the optical fiber 6 to be measured, backscattered light P _b is generated at each portion along the length direction thereof. This back scattered light P _b
Is the optical fiber 6 to be measured toward the optical pulse tester 1 side.
By propagating through the inside and transmitting through the wavelength filter 7, only the light having the same wavelength component as the wavelength of the probe light pulse P _{D in} the back scattered light P _b is received by the optical pulse tester 1.

The optical pulse tester 1 has a light receiving portion, a waveform generating portion, and a display portion. The light receiving unit, for receiving the backscattered light P _b which has returned to occur in each part of the measured optical fiber 6. The waveform generation unit generates a waveform signal corresponding to the waveform of the received backscattered light P _b (electrical signal). In the display unit, based on the generated waveform signal, to display the waveform of the backscattered light P _b.

The power of the backscattered light P _b is proportional to the power of the exploration light pulse P _D in the generation section,
_D reaches the respective portions and is attenuated due to the loss of the optical fiber 6 to be measured, and the backscattered light P _b also receives the same amount of loss in the process of returning to the incident end of the search light pulse P _D.
The backscattered light P _b is exploration light pulse P _D is a light returned continuously from each part in the propagation, the backscattered light P _b from more distant longer time to return to end the incident.

Therefore, as compared with the exploration light pulse P _D incident on the optical fiber 6 to be measured, the backscattered light P returning is returned.
The waveform of _b remarkably spreads, and by observing the waveform, the distribution of the power of the exploration light pulse P _D in the optical fiber 6 to be measured in the length direction is found. Further, the slope of the change in the waveform of the backscattered light P _{b with} respect to that distance (this corresponds to the slope of the change in the power of the probe light pulse P _D ) represents the loss of the optical fiber 6 to be measured. .

Then, the backscattered light P displayed on the display unit is displayed.
_From the waveform of _b , to detect the gain generation part of the measured optical fiber 6, which is the part where the power of the probe optical pulse P _D is amplified due to the modulation instability due to the cross-phase modulation in the measured optical fiber 6. You can That is, the slope of the change in the waveform of the backscattered light P _{b with} respect to the distance is determined by the measured optical fiber 6
Will represent the loss of. Therefore, the "gain generation part"
Corresponds to a portion where the loss of the optical fiber 6 to be measured decreases, which is indicated by the inclination of the change of the backscattered light P _b . This means that the loss originally possessed by the measured optical fiber 6 itself does not change, so that in the portion where the loss detected from the slope of the waveform of the backscattered light P _b decreases, the gain is generated in that portion. . Therefore, in the waveform of the backscattered light, there was a waveform portion in which the slope of the waveform was large with respect to the slope of the waveform (the waveform that became a straight line that descended to the right) when only fiber loss occurred. In this case, the measured optical fiber part corresponding to this waveform part is
It is detected as a gain generating part.

For example, when the wavelength of the excitation light P ₁ is λ ₁ and the wavelength of the excitation light P ₂ is λ ₂ , backscattered light P _b as shown in FIG. 2 is obtained. Then, by observing the waveform of the backscattered light P _b, you are possible to determine the gain generation portion. That is, the back scattered light P _b
Of the waveforms of, the slope of the waveform when only fiber loss occurs (waveform that is a straight line that descends to the right,
In FIG. 2, the waveform at 10 km to 20 km),
The waveform part where the slope of the waveform becomes large is 0 km to 10 km.
Since it is in the vicinity, it is possible to determine the portion in the vicinity of 0 to 10 km as the gain generating portion. Then, the zero dispersion wavelength of this gain generating portion can be calculated as (λ ₁ + λ ₂ ) / 2.

Furthermore, when began to sweep the wavelength lambda ₂ of the pumping light P _2, When a portion the inclination of the waveform of the backscattered light P _b increases were observed, likewise, the backscattered light P _b of the waveform The part where the slope of is large is determined to be the part where the gain is generated, and the wavelength λ ₁ (this wavelength is fixed) and the wavelength λ at that time are fixed.
By using ₂ , the zero-dispersion wavelength of the newly determined gain generating portion can be calculated as (λ ₁ + λ ₂ ) / 2.

Here, the principle of measurement of the zero-dispersion wavelength distribution by this device configuration will be described. Modulation instability is one of the nonlinear phenomena of an optical fiber, and is a phenomenon in which when high-intensity light is incident on the optical fiber, gain is generated in the sidebands on the short wavelength side and the long wavelength side of the wavelength of the high-intensity light. . The dispersion relation of the modulation instability in the case where two wavelengths of pumping light are incident is as follows: the wavelength of the pumping light j (j = 1, 2) is λ _j , the dispersion value at the wavelength λ _j is D _j , and the nonlinear refractive index coefficient is where n _{2 is} the effective core area, A _{eff is} the group velocity, v _{gj is} the group velocity, c is the speed of light in vacuum, P _{j is} the power of the pump light _j , and Ω is the frequency shift from the pump light frequency. Can be expressed as

[0024]

[Equation 1]

K obtained from the equation (1) becomes a complex number, and 0
If there is an imaginary part (= ImK) that is not
A gain G is generated due to modulation instability, and the gain is given by G = 2ImK (2).

FIG. 3 shows Δv_g ^-1Values of 0, -0.2, -0.
When set to 4, -0.8, -10 (ps / km) (1)
The frequency characteristics of G calculated by the equations and (2) (excitation
Luminous P₁And exploration light pulse P_DFrequency difference). However
Then λ₁= 1550 nm, P ₁= P₂= 100 mW, γ
₁= 2.14 (W^-1/ Km).

[0027] For example, while monitoring the loss of the optical fiber to be measured 6 in the OTDR 1, when in the excited light P ₂ of the wavelength lambda ₂ is gradually varied to the long wavelength side lambda ₂ ⁽¹⁾ It is assumed that the loss of a certain portion of the optical fiber 6 to be measured is reduced for the first time. That is, it is assumed that a local gain is generated in that portion. In this case, it can be estimated that the zero-dispersion wavelength of the gain generating portion is (λ ₁ + λ ₂ ⁽¹⁾ ) / 2. Furthermore, when in the excited light wavelength lambda ₂ of P ₂ is varied to the long wavelength side lambda ₂ ^(2), if the first time loss in other parts is reduced, the zero dispersion wavelength of the portion (lambda _It can be estimated that it is ₁ + λ ₂ ⁽²⁾ / 2. If the zero-dispersion wavelength is sequentially determined in this way, the zero-dispersion wavelength distribution can be measured over the entire length of the optical fiber 6 to be measured.

[0028] The wavelength lambda ₂ wavelength lambda ₁ and the pumping light P ₂ of the pumping light P _1, the wavelength lambda _pr exploration light pulse P _D is arbitrarily selected, exploration light pulse P _D as shown in FIG. 4 Excitation light P
_It must be set to receive the gain due to the modulation instability induced by ₁ . That is, it is necessary to set λ _pr within a range where a gain due to modulation instability occurs. Therefore, the pumping light source 3 (wavelength λ ₂ ) is tuned to be variable in wavelength. Instead of the variable wavelength light source, a broadband light source and a variable wavelength filter that cuts out a part thereof may be used.

As shown in FIG. 3, the gain of modulation instability is
Excitation light P₁And excitation light P₂The absolute value of the group velocity difference of
However, the gain bandwidth becomes wider. Excitation as shown in FIG.
Light P ₁And exploration light pulse P_DFrequency difference of 380G
Hz (3 nm when converted to wavelength spacing (for wavelength 1.55 μm
Set)), Δv_g ^-1Is less than 0.4 ps / km
It becomes possible to measure with high accuracy. This is the resulting zero variance
The wavelength accuracy is 0.4 nm (when the wavelength is 1.55 μm,
It means that it is possible to measure

FIG. 5 shows the backscattered light P of the probe light pulse P _D.
The waveform of _b is shown. Here, the wavelength λ _pr of the search light pulse P _D
Is 1550nm, the wavelength λ ₁ of the excitation light P ₁ 1547nm
The zero-dispersion wavelength distribution of the measured optical fiber 6 is 1534 nm and 1536 nm in the order of sections A and B from the incident end side. However, the lengths of sections A and B are both 10 km
Is.

The wavelength solid line of the excitation light P ₂ in FIG. 5 lambda ₂
= 1519 nm, broken line is λ ₂ = 1521 nm, dotted line is λ
₂ = 1523 nm, dashed line indicates the backscattered light waveform in the case of lambda ₂ = 1525 nm. Section A in FIG.
Then, (λ ₁ + λ ₂ ) / 2 = 1534 nm and λ ₂ = 15
Only in the case of 21 nm, and in the interval B, (λ ₁ + λ ₂ ) / 2
It can be seen that the gain occurs only when λ ₂ = 1525 nm, which is = 1536 nm.

As described above, the wavelength λ ₂ of the pumping light P ₂ is swept from the short wavelength side to the long wavelength side to detect the presence or absence of gain, thereby determining the zero dispersion wavelength over the entire section of the optical fiber to be measured. be able to.

FIG. 6 shows another embodiment of the apparatus for measuring the zero dispersion wavelength distribution of an optical fiber. In this embodiment, the arithmetic unit 10 is added to the embodiment shown in FIG. The calculation unit 10 has a wavelength λ ₁ of the pumping light P ₁ output from the pumping light source 2 and a pumping light P ₁ output from the pumping light source 3.
₂ with wavelength lambda ₂ is inputted, the waveform signal showing waveforms of the backscattered light P _b from the waveform generator of the OTDR 1 is inputted.

The operating section 10, when being swept wavelength lambda ₂ of the pumping light P _2, are automatically observed waveform of the backscattered light P _b, the inclination increased portion of the change in the waveform When it exists, it is determined that the part is a gain generating part.
Then, the intermediate wavelength of the wavelengths λ _{1 and} λ ₂ at this time is automatically calculated as the zero dispersion wavelength of the gain generating portion.

[0035] In the embodiment described above, advance to secure the wavelength lambda _pr exploration light pulse P _D to the wavelength lambda ₁ of the pumping light P _1, but began to sweep the wavelength lambda ₂ of the pumping light P ₂ , Wavelength λ
₂ previously fixed, the wavelength lambda _1, while maintaining the wavelength interval of lambda _pr, 2 two wavelengths lambda _1, or sweeping the lambda _pr simultaneously, wavelength lambda _1, lambda _2, keeping the wavelength interval of lambda _pr As it is, the three wavelengths λ ₁ , λ ₂ , and λ _pr may be swept simultaneously.

[0036]

As described above, the present invention can measure the zero dispersion wavelength distribution over the entire length of the optical fiber to be measured. This makes it possible to specifically design a WDM transmission line or a non-linear transmission line in which the local dispersion value is an important parameter in design. If the distribution of the zero-dispersion wavelength in the longitudinal direction is obtained, the dispersion slope parameter can be used to estimate the distribution of the dispersion value in the longitudinal direction for wavelengths near the zero-dispersion wavelength.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an apparatus for measuring zero-dispersion wavelength distribution of an optical fiber.

FIG. 2 Backscattered light P _b generated by the probe light pulse P _D
It is a characteristic view showing an example of the waveform of.

FIG. 3 shows excitation light P ₂ and It is a characteristic diagram showing the relationship between the gain of the frequency difference and per unit length of the probe optical pulse P _D.

FIG. 4 is a characteristic diagram showing the relationship between the wavelengths λ _{1 and} λ _{2 of the} excitation lights P _{1 and} P ₂ and the wavelength λ _pr of the search light pulse P _D.

FIG. 5: Backscattered light P _b generated by the probe light pulse P _D
It is a characteristic view showing a waveform of.

FIG. 6 is a block diagram showing another embodiment of the measuring apparatus for the zero-dispersion wavelength distribution of an optical fiber.

[Explanation of symbols]

1 Optical pulse tester (OTDR) 2 excitation light source 3 Wavelength variable excitation light source 4 multiplexer 5 multiplexer 6 Optical fiber under test 7 wavelength filter 10 Operation part

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Masaharu Ohashi             2-3-1, Otemachi, Chiyoda-ku, Tokyo             Inside Telegraph and Telephone Corporation

Claims (8)

[Claims] 1. A probe light pulse and 2 are provided in an optical fiber to be measured.
Incident wavelength excitation light, observing the waveform of the backscattered light of the probe light pulse showing the power distribution of the probe light pulse over the length direction of the measured optical fiber, from the waveform of the backscattered light observed, The gain generation part of the measured optical fiber is detected, which is the part in which the power of the exploration light pulse is amplified by the modulation instability due to the mutual phase modulation in the measured optical fiber, and the gain generation part of the measured optical fiber is detected. The zero-dispersion wavelength of the part,
A method for measuring a zero-dispersion wavelength distribution of an optical fiber, which is determined from the wavelength of the excitation light. 2. The method for measuring the zero-dispersion wavelength distribution of an optical fiber according to claim 1, wherein the wavelength of the pumping light is swept, and the gain-generating portion is detected in each portion of the optical fiber under test. A method for measuring a zero-dispersion wavelength distribution of an optical fiber, characterized in that an intermediate wavelength between two wavelengths of pumping light is determined as a zero-dispersion wavelength of the gain generating portion. 3. The method for measuring the zero-dispersion wavelength distribution of an optical fiber according to claim 1, wherein the waveform of the backscattered light has a slope of a waveform when only a fiber loss occurs. On the other hand, when there is a waveform portion where the slope of the waveform becomes large, the measured optical fiber portion corresponding to this waveform portion is detected as the gain generation portion, and the zero-dispersion wavelength distribution of the optical fiber is characterized. Measuring method. 4. A light source means for generating an exploration light pulse, an excitation light source means for generating excitation light of two wavelengths, the exploration light pulse and the excitation light are combined, and the exploration light pulse and excitation are combined. A multiplexer for injecting light into the optical fiber to be measured, a light receiving means for receiving the backscattered light of the probe optical pulse showing the power distribution of the probe optical pulse over the length direction of the optical fiber to be measured, and the light receiving means And a waveform generating means for generating a waveform of the backscattered light received by the optical fiber. 5. A light source means for generating an exploration light pulse, an excitation light source means for generating excitation light of two wavelengths, the exploration light pulse and the excitation light are combined, and the exploration light pulse and the excitation are combined. A multiplexer for injecting light into the optical fiber to be measured, a light receiving means for receiving the backscattered light of the probe optical pulse showing the power distribution of the probe optical pulse over the length direction of the optical fiber to be measured, and the light receiving means 1. An apparatus for measuring zero-dispersion wavelength distribution of an optical fiber, comprising: a waveform generating means for generating a waveform of the backscattered light received by the received light; and a display means for displaying the waveform of the generated backscattered light. 6. A light source means for generating an exploration light pulse, an excitation light source means for generating excitation light of two wavelengths, the exploration light pulse and the excitation light are combined, and the exploration light pulse and excitation are combined. A multiplexer for injecting light into the optical fiber to be measured, a light receiving means for receiving the backscattered light of the probe optical pulse showing the power distribution of the probe optical pulse over the length direction of the optical fiber to be measured, and the light receiving means The waveform generating means for generating the waveform of the backscattered light received by the received light and the waveform of the generated backscattered light amplify the power of the exploration light pulse due to modulation instability due to cross-phase modulation in the optical fiber under test. And a zero-dispersion wavelength of the gain-generating portion of the optical fiber to be measured, and a calculating unit for determining the zero-dispersion wavelength of the gain-generating portion of the optical fiber to be measured from the wavelength of the pumping light. You Measuring apparatus of the zero-dispersion wavelength distribution of the optical fiber. 7. The calculating unit according to claim 6, wherein the waveform of the backscattered light has a waveform portion in which a waveform slope is larger than a waveform slope when only fiber loss occurs. In this case, the measured optical fiber portion corresponding to this waveform portion is detected as the gain generating portion, and the intermediate wavelength of the two wavelengths of the pumping light when the gain generating portion is detected is determined as the gain generating portion. A device for measuring the zero-dispersion wavelength distribution of an optical fiber, which is characterized by determining the zero-dispersion wavelength of the optical fiber. 8. An apparatus for measuring zero-dispersion wavelength distribution of an optical fiber according to claim 6 or 7, further comprising display means for displaying a waveform of the backscattered light generated.