JP2009031313A - Optical fiber characteristic measuring device and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method capable of measuring accurately the characteristic (a distribution of the magnitude of birefringence) of the optical fiber. <P>SOLUTION: This optical fiber characteristic measuring device is provided by improving an optical fiber characteristic measuring device wherein pulsed light is input into an optical fiber to be measured, and back-scattering light from the optical fiber to be measured relative to the pulsed light is detected by a light detection part to obtain a Stokes vector, and birefringence in the longitudinal direction is measured. The device is characterized by being provided with a birefringence operation part obtaining the magnitude of a linearly polarized light component and the magnitude of a circularly polarized light component of a birefringence vector based on the Stokes vector by at least three positions, and obtaining the magnitude of the birefringence. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical fiber characteristic measuring device and an optical fiber characteristic measuring method for measuring characteristics in a longitudinal direction of an optical fiber to be measured (distribution of polarization mode dispersion, distribution of magnitude of birefringence). The present invention relates to an optical fiber characteristic apparatus and an optical fiber characteristic measurement method that can measure the characteristic of an optical fiber.

  In recent years, in optical communication, there has been an increasing demand for higher transmission rates, and transmission rates of 10 [Gbps] and 40 [Gbps] have begun to be realized. However, since dispersion (material dispersion, waveguide dispersion, multimode dispersion, polarization mode dispersion) exists in an optical fiber that is a transmission medium, waveform degradation caused by this dispersion is highlighted as a cause of failure. When a single mode optical fiber is used as the optical fiber, chromatic dispersion (added material dispersion and waveguide dispersion) and polarization mode dispersion become problems. Among them, chromatic dispersion is relatively easy with dispersion compensating fiber (DCF), reverse dispersion fiber (RDF) with reverse characteristics of chromatic dispersion of single mode optical fiber, chromatic dispersion compensator, etc. Compensation is possible, and many solutions using these have been proposed and generalized.

  On the other hand, polarization mode dispersion has various causes. For example, structural defects in the optical fiber itself, core ovalization, bending, stress, and twist caused by manufacturing, installation conditions, and usage environment. And so on. As a result, birefringence occurs in the optical fiber and polarization mode dispersion occurs, but it exists randomly in the optical fiber, and the fluctuation is also severe. Therefore, it is difficult to compensate with passive components.

  In this way, it is difficult to compensate by passive components, so in the existing optical fiber, fatal defective parts are detected and removed, or defective parts are detected in the manufacturing process to prevent outflow to the market, or When manufacturing a fiber, it is required to feed back the measurement result of polarization mode dispersion to the manufacturing process and reduce the proportion of defective parts.

  Therefore, in order to realize detection of a defective portion or the like, it is important to measure the characteristics in the longitudinal direction of the optical fiber, and an optical fiber characteristic measuring device is used. In particular, an optical fiber characteristic device that measures the characteristics of a polarization mode dispersion value is called a polarization mode dispersion measurement device. In order to measure the polarization mode dispersion, for example, light components having orthogonal polarization modes (polarization states) are transmitted at a predetermined distance, and a time difference Δτ between the light components generated by the transmission is obtained.

Next, a method for measuring polarization mode dispersion will be described with reference to FIG. FIG. 6 is a diagram illustrating the principle of conventional polarization mode dispersion measurement. In FIG. 6, an optical fiber 100 is, for example, a single mode optical fiber, which is a measured optical fiber. Then, pulse light having an angular frequency ω ₁ , ω ₂ (ω ₁ ≠ ω ₂ and slightly different) is input from one end (input side) of the optical fiber 100, and the pulse light transmitted through the optical fiber 100 is changed to other ones. Light is received at the end (output side). On the input side, the polarization state of the pulsed light is polarized into different polarization states (for example, 0 ° and 45 ° with respect to the reference axis) and input to the optical fiber 100.

  On the output side, the polarization state of the pulsed light from the optical fiber 100 is separated into four directions (for example, 0 °, 45 °, 90 °, and circularly polarized light), and the light intensity in each direction is detected. Further, Stokes vector components (S0, S1, S2, S3) are obtained from the light intensity in each direction. Further, transmission of the optical fiber 100 is generally expressed by a Mueller matrix R (a 3 × 3 orthogonal matrix), and a Stokes vector and a Mueller matrix are expressed by the following equations.

Since the light intensity input to the optical fiber 100 is known, the Mueller matrix R can be obtained from the Stokes vector S ₀ of the input light on the input side and the Stokes vector S of the output light obtained on the output side. Of course, the Mueller matrix R is obtained for each of the angular frequencies ω ₁ and ω ₂ .

The Stokes vector S of the output light after passing through the optical fiber 100 having birefringence changes due to the influence of polarization mode dispersion with respect to changes in the angular frequencies ω ₁ and ω ₂ of the input light. This change is represented by using a vector in a polarization state space generally named as a polarization dispersion vector Ω, and the magnitude of the polarization dispersion vector Ω is equal to the polarization mode dispersion. Therefore, the change in the polarization state of the output light due to the change in the angular frequencies ω1 and ω2 is a well-known relational expression as shown in the following formula (1) using the polarization dispersion vector Ω and the Mueller matrix R ( For example, refer nonpatent literature 1.).

Further, Ω ₁ and Ω ₂ are different linearly polarized light components, and Ω ₃ is a circularly polarized light component. Note that the Stokes vector S of the output light is a four-component vector composed of (S0, S1, S2, S3), and the corresponding Mueller matrix R also has 4 rows and 4 columns. However, the S0 component is the total power of the light including the non-polarized component, and the polarization mode dispersion is negligible for the change of the optical power, and only the variation of the polarized component needs to be handled. Therefore, the Mueller matrix R is a matrix that represents the conversion of the Stokes vector when the polarization component is normalized (normalized) and expressed in a Poincare sphere, and the S0 component is omitted and becomes 3 rows and 3 columns. In addition, the measurement is performed in four independent azimuth polarization states, and the S0 component is subtracted.

  However, with the configuration shown in FIG. 6, only polarization mode dispersion of only the output end of the optical fiber 100 can be measured. Therefore, in order to measure the distribution in the longitudinal direction, there is a unidirectional measurement using a known technique, optical time domain reflectometry (hereinafter abbreviated as OTDR) (for example, Patent Document 1, Patent). Reference 2 and non-patent reference 2). The OTDR measures the characteristics of the optical fiber by inputting short pulse light and measuring the backscattered light with respect to the pulsed light, and also measures the reflection position according to the time until the backscattered light returns.

Next, FIG. 7 and FIG. 8 are configuration diagrams of a conventional optical fiber characteristic measuring apparatus. Here, the same components as those in FIG. 7 and 8, the light source unit 10 includes a wavelength tunable light source 11 and a pulse generation unit 12, and outputs pulsed light having angular frequencies ω ₁ and ω ₂ . Variable wavelength light source 11 is a continuous light output unit, the angular frequency omega _1, the omega ₂ is variably controlled to output the desired angular frequency omega _1, omega ₂ of the continuous light. The pulse generator 12 converts the continuous light from the wavelength tunable light source 11 into pulse light having a desired pulse width and outputs it.

The polarization variable section 20 arbitrarily variably polarizes the polarization state of each pulsed light from the light source section 10 (polarizes it into at least two different polarization states) and outputs it. The directional coupler 30 outputs the pulsed light polarized by the polarization variable unit 20 to the optical fiber 100 and the return light from the optical fiber 100, that is, the backscattered light. The light detection unit 40 detects the backscattered light from the directional coupler 30 by separating the light intensity of the backscattered light into at least four azimuth polarization states in synchronization with the pulsed light output from the light source unit 10, A normalized Stokes vector is obtained for each of the angular frequencies ω ₁ and ω ₂ .

The calculation unit 50 includes a matrix calculation unit 51, a polarization dispersion vector calculation unit 52, a linear polarization calculation unit 53, and a dispersion value calculation unit 54. Based on the Stokes vector obtained by the light detection unit 40, the calculation unit 50 generates a deviation due to backscattered light. A wave dispersion vector Ω _B is calculated, and using this, a linearly polarized component of the unidirectional polarization dispersion vector Ω is obtained, and further, polarization mode dispersion is obtained.

The matrix calculation means 51 obtains a Mueller matrix from the normalized Stokes vector for each of the angular frequencies ω ₁ and ω ₂ . The polarization dispersion vector computing means 52 obtains the polarization dispersion vector Ω _B due to the backscattered light at the angular frequency from the Mueller matrix obtained by the matrix computing means 51. The linear polarization calculation means 53 is the magnitude of the linear polarization component (Ω ₁ , Ω ₂ ) of the unidirectional polarization dispersion vector Ω from the polarization dispersion vector Ω _B by the backscattered light obtained by the polarization dispersion vector calculation means 52. I ask for it. The dispersion value calculation means 53 obtains a polarization mode dispersion value from the magnitude of the linearly polarized light component.

The control unit 60 instructs the light source unit 10 to indicate the angular frequencies ω ₁ and ω ₂ of the pulsed light, the pulse width and the pulse interval, instructs the polarization variable unit 20 to indicate the polarization state, and instructs the light detection unit 40 to indicate the polarization state to be detected. Synchronizes with the pulsed light, and instructs the calculation unit 50 to calculate.

The operation of such an apparatus will be described.
The control unit 60 causes the wavelength variable light source 11 to output continuous light at an angular frequency ω ₁ and causes the pulse generation unit 12 to output pulsed light with a desired pulse width and pulse interval. Further, the control unit 60 causes the polarization variable unit 20 to change the polarization state of the pulsed light to, for example, 0 ° and output the measured optical fiber 100 via the directional coupler 30.

  Then, backscattered light that is return light from the optical fiber 100 to be measured is input to the light detection unit 40 via the directional coupler 30. The light detection unit 40 detects the light intensity in four directions (for example, 0 °, 45 °, 90 °, circularly polarized light) according to an instruction from the control unit 60.

Similarly, in accordance with an instruction from the control section 60, the polarization state 45 ° at the angular frequency omega _1, the polarization state 0 ° at the angular frequency omega _2, with respect to 45 °, respectively, for detecting the light intensity of the four directions. Then, the light detection unit 40 obtains a Stokes vector S _B for each of the angular frequencies ω ₁ and ω ₂ .

When the stove vector S _B is obtained, the control unit 60 instructs the calculation unit 50 to calculate the polarization mode dispersion. Accordingly, the arithmetic unit 50 reads out the Stokes vector S _B from the light detection unit 40, the matrix calculating unit 51 calculates the Mueller matrix R _B of the back scattered light from the Stokes vector S _B.

The polarization dispersion vector calculating unit 52 calculates the polarization dispersion vector Omega _B by back scattered light from the Mueller matrix R _B. That is, a relationship of when measuring the light intensity of the backscattered light from the measured optical fiber 100, the Mueller matrix R _B, similarly the following formula in the formula (1) from Stokes vector S _B (2).

If the polarization dispersion matrix Ω _B representing the polarization dispersion vector Ω _B by the backscattered light is used, the following equation (3) is obtained from the equations (1) and (2).

In this way, the polarization dispersion vector calculation means 52 obtains the polarization dispersion vector Ω _B.
Further, the linear polarization calculation means 53 obtains the magnitude of the linear polarization component of the unidirectional polarization dispersion vector Ω. That is, the relationship of the Mueller matrix R _B by Mueller matrix R and backscattered light, using the matrix M, it is generally known that represented by the following formula (4).

Therefore, by substituting equation (4) into equation (3). The polarization dispersion vector Ω _{B of} the backscattered light, the linear polarization component vector Ω _L of the unidirectional Mueller matrix R and the unidirectional polarization dispersion vector Ω is expressed by the following equation (5).

Thus, when obtaining the magnitude (Δτ _B ) of the polarization dispersion vector Ω _B due to the backscattered light, since the matrix M and the Mueller matrix R are orthogonal matrices, the MR (transpose matrix) on the right side of the equation (5) is a vector. Does not change the size. Therefore, the following equation is obtained.

In this manner, the linear polarization calculation means 53 obtains the magnitude of the linear polarization components (Ω ₁ , Ω ₂ ) of the unidirectional polarization dispersion vector Ω. Furthermore, the distribution of each component Ω _{1 to} Ω ₃ of the polarization dispersion vector Ω is assumed to be a Gaussian distribution, and the magnitude (Δτ _B ) of the polarization dispersion vector Ω _B due to the backscattered light is calculated. The relationship with the magnitude (Δτ) of the polarization dispersion vector Ω, which is wave mode dispersion, is expressed by the following equation.

Note that <Δτ _B > is a statistical average value of values measured many times under various conditions.
Therefore, the dispersion value calculation means 54 obtains the polarization mode dispersion value from the polarization dispersion vector Ω _B obtained from the backscattered light (see, for example, Non-Patent Document 3).

GJFoschini, CDPoole, “Statistical theory of polarization dispersion in single mode fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, (USA), Laser & Electro-Optics Society (LEOS), November 1991, vol.9, (No.11 ), Pp.1439-1456 A.J.Rogers, “Polarization optical time domain reflectometry”, Electronics letters, (UK), The Institution of Electrical Engineers (IEE), 1980, Vol. 16, No. 13, pp. 489-490 Fabrizio Corsi, Andrea Galtarossa, Luca Palmieri, “Polarization Mode Dispersion Characterization of Single-Mode Optical Fiber Using Backscattering Technique”, JOURNAL OF LIGHTWAVE TECHNOLOGY, (USA), Laser & Electro-Optics Society (LEOS), October 1998, VOL .16, No.10, pp.1832-1843 JP 2003-106942 (paragraph numbers 0024-0066, FIG. 1-9) Special Table 2000-510246

As described above, the magnitude (Δτ _B ) of the polarization dispersion vector Ω _B is obtained from the backscattered light with respect to the pulse light of two wavelengths (angular frequencies ω ₁ , ω ₂ ), and the polarization mode dispersion (Δτ) is measured. I do.

However, in the apparatus shown in FIGS. 7 and 8, JNRoss, “Birefringence measurement in optical fibers by polarization-optical time-domain reflectometry”, Applied Optics, (USA), Optical Society of America (OSA), October 1982, As shown in Vol. 21, No. 19, pp. 3489-3495, there is a problem that the birefringent circularly polarized component in the optical fiber 100 to be measured cannot be easily and directly detected. As apparent from the equation (5), only the effect of the linearly polarized light components (Ω ₁ , Ω ₂ ) is obtained, and the effect of the circularly polarized light component Ω ₃ is eliminated. Therefore, when the magnitude of the polarization dispersion vector Ω _B (Δτ _B ) due to the backscattered light and the polarization mode dispersion (Δτ) are compared, the following equation is obtained.

Naturally, the polarization dispersion vector Ω _B obtained from the backscattered light includes only the linearly polarized components (Ω ₁ , Ω ₂ ) of the polarization dispersion vector Ω in a single direction. Further, although the OTDR measures the light traveling back and forth through the optical fiber 100 to be measured, if it is simply obtained as Δτ = Δτ _B / 2, the polarization mode dispersion (Δτ) cannot be obtained with high accuracy. For example, if the circularly polarized light component Ω ₃ increases due to torsion of the measured fiber 100 or the like, the polarization mode dispersion (Δτ) may become a smaller value.

Further, as shown in the above formula, the average <Δτ _B > of the polarization dispersion vector Ω _B obtained by the backscattered light is 0.64 (≈2 / π, and this value is, for example, F Curti, B.Daino, Q.Mao, F.Matera, CGSomeda, “Concatenation of Polarization Dispersion in Single-Mode Fibers”, Electronics letters, (UK), The Institution of Electrical Engineers (IEE), February 1989, Vol.25, No.4, pp.290-292) is generally used, but since this is a value obtained from many numerical simulations and statistics, all measured values are measured. There is a problem that it is not effective for the optical fiber 100, and it is difficult to accurately obtain polarization mode dispersion, that is, characteristics of the optical fiber.

  Accordingly, an object of the present invention is to realize an optical fiber characteristic measuring apparatus and an optical fiber characteristic measuring method for accurately measuring characteristics of an optical fiber to be measured.

The invention described in claim 1
An optical fiber characteristic measuring device that inputs pulsed light into the optical fiber to be measured, detects the backscattered light from the optical fiber to be measured with respect to the pulsed light, obtains the Stokes vector, and measures the birefringence in the longitudinal direction In
A light source unit that outputs the pulsed light;
A birefringence calculation unit is provided for determining the magnitude of the birefringence by obtaining the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the birefringence vector based on the Stokes vectors obtained from at least three positions. To do.

The invention according to claim 2
In an optical fiber characteristic measuring apparatus that measures the magnitude of birefringence in the longitudinal direction of an optical fiber to be measured,
A light source unit that outputs pulsed light;
A polarization variable section that polarizes and outputs each pulsed light output from the light source section into at least two different polarization states;
A directional coupler that outputs the pulsed light polarized by the polarization variable unit to the optical fiber to be measured, and receives backscattered light with respect to the output pulsed light;
The light intensity of the backscattered light from at least three positions with respect to the longitudinal direction of the optical fiber to be measured is detected from the directional coupler, and at least four different polarization states are synchronized with the pulsed light. A light detector that detects the light intensity of the azimuth and obtains a normalized Stokes vector;
A birefringence calculation unit for obtaining the magnitude of the birefringence by obtaining the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the birefringence vector based on the normalized Stokes vector obtained by the light detection unit is provided. It is a feature.

The invention according to claim 3 is the invention according to claim 1 or 2 ,
The light source
A continuous light output unit that outputs continuous light;
And a pulse generation unit for converting the continuous light output unit to a desired pulse width of continuous light.

The invention according to claim 4 is the invention according to any one of claims 1 to 3 ,
Birefringence calculating unit,
Matrix computing means for obtaining a Mueller matrix from the normalized Stokes vector for each position;
From the Mueller matrix obtained by the matrix computing means, birefringence vector computing means for obtaining a birefringence vector due to backscattered light by each position;
Linear polarization calculation means for obtaining the magnitude of the linearly polarized light component of the birefringence vector from the birefringence vector by the backscattered light obtained by the birefringence vector calculation means;
Circular polarization calculation means for obtaining the size of the circularly polarized component of the birefringence vector from the difference of the birefringence vector due to the backscattered light obtained by the birefringence vector calculation means;
Birefringence value calculating means for obtaining the magnitude of birefringence from the size of the circularly polarized light component of the circularly polarized light calculating means and the size of the linearly polarized light component of the linearly polarized light calculating means is provided.

The invention according to claim 5
In the optical fiber characteristic measurement method for measuring the birefringence in the longitudinal direction of the optical fiber to be measured,
The light source unit outputs pulse light;
A procedure of polarizing the pulsed light output from the light source unit into at least two different polarization states and inputting the polarized light to the optical fiber to be measured;
In synchronization with the pulsed light output from the light source unit, the light intensity of the backscattered light with respect to the pulsed light input to the optical fiber is detected separately in at least four directions of polarization, and the longitudinal direction of the optical fiber to be measured A procedure for obtaining normalized Stokes vectors at least at three positions with respect to
A procedure for obtaining a linearly polarized component and a circularly polarized component of a birefringence vector based on the normalized Stokes vector for each position obtained by the light detection unit,
A procedure for obtaining birefringence from the magnitude of the linearly polarized light component and the magnitude of the circularly polarized light component of the birefringence vector is provided.

As is apparent from the above description, the present invention has the following effects.
According to claims 1, 3, and 4 ,
A light detection unit obtains a normalized Stokes vector from backscattered light from each of at least three different positions. The calculation unit, because on the basis of the normalized Stokes vector determining the magnitude of the size and circularly polarized light component of the linearly polarized light component of the birefringence vector, using the results obtained from numerical simulation and statistics, the magnitude of birefringence There is no need to calculate. Thereby, birefringence can be accurately measured.

According to claims 2 to 4 ,
With respect to the pulsed light output from the light source unit, the backscattered light from each of at least three different positions is detected and separated into at least four azimuth polarization states to obtain a normalized Stokes vector. Then, the calculation unit calculates the magnitude of the linearly polarized light component and the circularly polarized light component of the birefringence vector based on the normalized Stokes vector, so that the birefringence is calculated using the results obtained from numerical simulation and statistics. There is no need. Thereby, birefringence can be accurately measured.

According to claim 5,
For the pulsed light output from the light source unit, backscattered light from each of at least three different positions is separated and detected in at least four azimuth polarization states to obtain a normalized Stokes vector. Then, since the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the birefringence vector are obtained based on the normalized Stokes vector, it is not necessary to calculate birefringence using the results obtained from numerical simulation and statistics. . Thereby, birefringence can be accurately measured.

Embodiments of the present invention will be described below with reference to the drawings.
[First embodiment]
1 and 2 are block diagrams showing a first embodiment of the present invention. Here, the same components as those in FIGS. 7 and 8 are denoted by the same reference numerals, and description thereof is omitted. In FIG. 1, a light source unit 70 is provided instead of the light source unit 10. Further, a calculation unit 80 is provided instead of the calculation unit 50.

Light source unit 70, the wavelength tunable light source 71 has a pulse generator 72, at least three different angular frequency of the pulsed light, for example, the angular frequencies _{ω 1, ω 2, ω 3} ( where, omega _1, omega _2, The angular frequency of ω ₃ is slightly different from each other, and three types of pulsed light with an angular frequency interval Δω) are output. Variable wavelength light source 71 is a continuous light output unit, the angular frequency ω _1, ω _2, ω ₃ were variably controlled, desired angular frequency omega _1, omega _2, and outputs the omega ₃ continuous light. The pulse generator 72 converts the continuous light from the wavelength variable light source 71 into pulse light having a desired pulse width and outputs the pulse light to the polarization variable unit 20.

In FIG. 2, the calculation unit 80 includes a matrix calculation unit 81, a polarization dispersion vector calculation unit 82, a linear polarization calculation unit 83, a circular polarization calculation unit 84, and a dispersion value calculation unit 85, which are obtained by the light detection unit 40. calculating the polarization dispersion vector Omega _B by back scattered light on the basis of the Stokes vector, obtains the magnitude of the size and circularly polarized light component of the linearly polarized light component in a single direction of polarization dispersion vector Omega with this, these size Then, polarization mode dispersion is obtained.

The matrix calculation means 81 obtains a Mueller matrix from the normalized Stokes vector for each of the angular frequencies ω ₁ , ω ₂ , and ω ₃ . Polarization dispersion vector calculating means 82, the Mueller matrix matrix calculating unit 81 is determined, determining the polarization dispersion vector Omega _B by back scattered light by the angular frequency. The linearly polarized light calculating means 83 calculates the magnitude of the linearly polarized component (Ω ₁ , Ω ₂ ) of the unidirectional polarization dispersion vector Ω from the polarization dispersion vector Ω _B by the backscattered light obtained by the polarization dispersion vector calculating means 82. I ask for it. The circular polarization calculation means 84 calculates the magnitude of the circular polarization component (Ω ₃ ) of the unidirectional polarization dispersion vector Ω from the difference of the polarization dispersion vector Ω _B by the backscattered light obtained by the polarization dispersion vector calculation means 82. Ask. The dispersion value calculation means 85 obtains the polarization mode dispersion value from the magnitude of the circular polarization component of the circular polarization calculation means 84 and the magnitude of the linear polarization component of the linear polarization calculation means 83.

The operation of such an apparatus will be described.
FIG. 3 is a flowchart showing the operation of the apparatus shown in FIGS.
The control unit 60 causes the variable wavelength light source 71 of the light source unit 70 to output continuous light at the angular frequency ω ₁ and causes the pulse generation unit 72 to output pulsed light with a desired pulse width and pulse interval (S10). Further, the control unit 60 causes the polarization variable unit 20 to change the polarization state of the pulsed light to, for example, 0 ° and output it to the measured optical fiber 100 via the directional coupler 30 (S11).

  Then, backscattered light that is return light from the optical fiber 100 to be measured is input to the light detection unit 40 via the directional coupler 30. The light detection unit 40 detects the light intensity in four directions (for example, 0 °, 45 °, 90 °, circularly polarized light) in synchronization with the pulsed light from the light source unit 70 according to an instruction from the control unit 60. For example, the light intensity is detected by separating the backscattered light into four azimuth polarization states by combining a polarizing element such as a half-wave plate, a phase element such as a quarter-wave plate, and a light receiving element. (S12).

When only one polarization state 0 ° is measured with respect to one angular frequency ω ₁ , the control unit 60 sets the polarization state of the pulsed light to 45 °, for example, to the polarization variable unit 20 and directivity is set. The light intensity is detected by outputting to the optical fiber 100 to be measured through the coupler 30 (S13, S11, S12).

Further, when the detection of different polarization states 0 ° and 45 ° is completed for one angular frequency ω ₁ , the light intensity is similarly detected and measured for the undetected angular frequencies ω ₂ and ω ₃ . (S14, S11 to S13). That is, the types of pulsed light input to the optical fiber 100 to be measured are the polarization states of 0 ° and 45 ° at the angular frequency ω ₁ , the polarization states of 0 ° and 45 ° at the angular frequency ω ₂ , and the polarization state at the angular frequency ω _3. There are six types of 0 ° and 45 °.

Furthermore ,, if the detection of the light intensity of all the angular frequency ω ₁ ~ω ₃ ended, the light detector 40, the angular frequency omega _1, omega _2, determine the Stokes vector S _B per omega ₃ (S14, S15 ).

When the stove vector S _B is obtained, the control unit 60 instructs the calculation unit 80 to calculate the polarization mode dispersion. Accordingly, the arithmetic unit 80 reads the Stokes vector S _B from the light detection unit 40, the matrix calculating unit 81 calculates the Mueller matrix R _B of the back scattered light from the Stokes vector S _B as well as the matrix calculating unit 51 (S16) . The optical detector 40 is the angular frequency omega _1, omega _2, after obtaining all omega ₃ Stokes vector S _B, the matrix is calculating unit 81 is seeking each Mueller matrix R _B, the angular frequency omega _1, Each time the Stokes vectors S _B of ω ₂ and ω ₃ are obtained, the matrix calculation means 81 may obtain the Mueller matrix R _B.

Then, similarly to the polarization dispersion vector calculation means 52, the polarization dispersion vector calculation means 82 obtains the polarization dispersion vector Ω _B due to the backscattered light from the Mueller matrix R _B by the angular frequencies ω1 to ω3. For example, each is obtained by a combination of ω ₁ and ω _{2 and} a combination of ω ₂ and ω ₃ . (S17). Further, like the linear polarization calculation means 53, the linear polarization calculation means 83 obtains the magnitude of the linear polarization components (Ω ₁ , Ω ₂ ) of the unidirectional polarization dispersion vector Ω (S18).

Then, the circular polarization calculation means 84 calculates the magnitude of the circular polarization component (Ω ₃ ) of the unidirectional polarization dispersion vector Ω from the difference of the polarization dispersion vector Ω _B by the backscattered light obtained by the polarization dispersion vector calculation means 82. I ask for it. That is, the polarization dispersion vector Omega _B by backscattered light obtained in the general measurement by way of using the OTDR is represented by the formula (5) as described above, differentiating the equation (5) Then, it is expressed by the following formula.

In addition, for the above equation, it can be assumed that the unidirectional polarization dispersion vector Ω or the linear polarization component vector Ω _L is constant for a small interval of ω. Therefore, the following equation is obtained.

  Therefore, the following relational expression (6) can be obtained.

  On the other hand, the polarization dispersion matrix Ω representing the polarization dispersion vector Ω and the Mueller matrix R are expressed by the equation (1), and the Mueller matrix R is an orthogonal matrix, so the following equation is obtained.

  Further, since the polarization dispersion matrix Ω is an opposite symmetric matrix, the above equation becomes the following equation.

Can be written. Therefore, equation (6) can be written by the following equation from the circularly polarized component vector Ω _C of the polarization dispersion vector Ω.

  Therefore, when the equation (5) is taken into account, the following equation is clearly expressed.

From the above, the polarization dispersion vector Ω _B is expressed by the following equation.

That is, the magnitude | Ω ₃ | of the circularly polarized light component is expressed by the following formula (7).

As described above, the circular polarization calculation means 84 further differentiates the polarization dispersion vector Ω _B obtained by the polarization dispersion vector calculation means 82, and divides the differentiated magnitude by the magnitude of the polarization dispersion vector Ω _B. Thus, the magnitude of the circular polarization component in the polarization mode dispersion is obtained (S19). Then, the dispersion value calculation means 85 obtains the value of the polarization mode dispersion (Δτ) from the magnitude of the circular polarization component of the circular polarization calculation means 84 and the magnitude of the linear polarization component of the linear polarization calculation means 83 (S20). Furthermore, the polarization mode dispersion may be obtained at each position in the longitudinal direction of the optical fiber 100 to be measured, and the distribution of the polarization mode dispersion in the longitudinal direction of the optical fiber 100 to be measured may be obtained.

As described above, the detection unit 40 detects the backscattered light with respect to each of the pulse lights having the angular frequencies ω _{1 to} ω ₃ output from the light source unit 70 and separates them into polarization states in at least four directions, and calculates the normalized Stokes vector. Ask. Then, since the calculation unit 80 obtains the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the polarization dispersion vector Ω based on the Stokes vector, the polarization mode dispersion is obtained using the results obtained from numerical simulation and statistics. There is no need to calculate. Thereby, the polarization mode dispersion can be accurately measured.

[Second Embodiment]
In the apparatus shown in FIG. 1 and FIG. 2, the embodiment for measuring the polarization mode dispersion has been described. However, the polarization mode dispersion is caused by birefringence in the optical fiber. That is, measurement of the magnitude of birefringence, which is one of the characteristics in the longitudinal direction of the optical fiber 100 to be measured, is also important, and FIG. 4 is a configuration diagram showing a second embodiment of the present invention. Here, the same components as those in FIG.

  In FIG. 4, a birefringence calculation unit 90 is newly provided, and the birefringence calculation unit 90 includes at least three positions z1, z2, and z3 with respect to the longitudinal direction of the optical fiber 100 to be measured. (However, the positions of z1, z2, and z3 are slightly different, and the light intensity from the minute position interval Δz) is detected, and the birefringence vector due to the backscattered light is calculated based on the normalized Stokes vector obtained for each position. The magnitude of the linearly polarized light component and the magnitude of the circularly polarized light component of the birefringence vector are obtained by using this, and the magnitude of the birefringence in the longitudinal direction of the measured optical fiber 100 is obtained from these magnitudes.

FIG. 5 shows a configuration diagram of the birefringence calculation unit 90. In FIG. 5, the birefringence calculating unit 90
It has a matrix calculation means 91, a birefringence vector calculation means 92, a linear polarization calculation means 93, a circular polarization calculation means 94, and a birefringence value calculation means 95. The matrix calculation means 91 obtains a Mueller matrix from the normalized Stokes vector for each position. The birefringence vector computing unit 92 obtains the birefringence vector due to the backscattered light according to the position from the Mueller matrix obtained by the matrix computing unit 91. The linearly polarized light calculating means 93 obtains the magnitude of the linearly polarized light component of the unidirectional birefringence vector from the birefringence vector by the backscattered light obtained by the birefringence vector computing means 92. The circularly polarized light calculating means 94 obtains the magnitude of the circularly polarized light component of the unidirectional birefringent vector from the difference between the birefringent vectors by the backscattered light obtained by the birefringent vector computing means 92. The birefringence value calculating means 95 obtains the magnitude of unidirectional birefringence from the size of the circularly polarized light component of the circularly polarized light calculating means 94 and the size of the linearly polarized light component of the linearly polarized light calculating means 93.

The operation of such an apparatus will be described.
In response to an instruction from the control unit 60, the light detection unit 40 synchronizes with the pulsed light of the light source unit 70 (for example, the angular frequency ω1 among the angular frequencies ω1 to ω3), and the light intensity of the backscattered light with respect to the pulsed light Are detected by separating them into polarization states of at least four directions (for example, 0 °, 45 °, 90 °, circularly polarized light). The light intensity is detected for each of the backscattered light from at least three different positions z1, z2, and z3 with respect to the longitudinal direction of the optical fiber 100 to be measured. Then, a normalized Stokes vector at each position z1 to z3 is obtained.

Then, the control unit 60 instructs the birefringence calculation unit 90 to calculate birefringence. Accordingly, the birefringence calculating unit 90 reads the Stokes vector S _B from the light detection unit 40, matrix calculation means 91 calculates the Mueller matrix R _B of the back scattered light from the Stokes vector of the Stokes vector S _B. Further, the birefringence vector calculating unit 92 obtains a birefringence vector beta _B of the back scattered light from the Mueller matrix R _B. That is, considering the change in the Stokes vector S as the change in the length direction of the optical fiber 100 to be measured, it can be expressed by the following equation (8), and thus the birefringence vector β _B is obtained in the same manner as the polarization dispersion vector Ω _B.

Further, the linear polarization calculation means 93 obtains the magnitude of the linear polarization component of the unidirectional birefringence vector β from the birefringence vector β _B by the backscattered light obtained by the birefringence vector calculation means 92. That is, the birefringence vector β in Equation (8) represents local birefringence in the polarization state space. Therefore, as is clear from the comparison between the equations (1) and (8), the birefringence vector β can be handled in the same way as the polarization dispersion vector Ω, for example, Fabrizio Corsi, Andrea Galtarossa, Luca Palmieri , “Beat Length Characterization Based on Backscattering Analysis in Randomly Perturbed Single-Mode Fibers”, JOURNAL OF LIGHTWAVE TECHNOLOGY, (USA), Laser & Electro-Optics Society (LEOS), July 1999, VOL.17, NO.7, As described in pp.1172-1178, the birefringence vector β _B obtained by the backscattered light and the linearly polarized component vector β _{L of the} unidirectional birefringence vector β are the same as in the equation (5). It is represented by the following formula.

In this way, the linear polarization calculation means 93 obtains the magnitude of the linear polarization component (β ₁ , β ₂ ) of the unidirectional birefringence vector β. Of course, as in the case of the polarization dispersion vector Ω _B , the effect of the circular polarization component β ₃ has disappeared. Therefore, the circular polarization calculating means 94 obtains the magnitude of the circularly polarized component of the unidirectional birefringence vector β from the birefringence vector β _B by the backscattered light obtained by the birefringence vector computing means 92. That is, the magnitude of the circularly polarized light component can be expressed by the following equation (9), similarly to the calculation of equation (7) performed for the polarization dispersion vector Ω _B.

  The operation until the pulse light from the light source unit 70 is detected by the light detection unit 40 and the operation of the calculation unit 80 are the same as those in the apparatus shown in FIG.

As described above, the circular polarization calculation means 94 further differentiates the birefringence vector β _B obtained by the birefringence vector calculation means 92, and divides the differentiated magnitude by the magnitude of the birefringence vector β _B. The magnitude | size by the circularly polarized light component of birefringence is calculated | required. Then, the birefringence value calculating means 95 calculates birefringence from the magnitude of the circularly polarized light component of the circularly polarized light calculating means 94 and the magnitude of the linearly polarized light component of the linearly polarized light calculating means 93. Further, the birefringence at the desired position is obtained from the light intensity at three points (z1, z2, and z3) around the desired position in the longitudinal direction of the optical fiber 100 to be measured, and the birefringence in the longitudinal direction of the optical fiber 100 to be measured is obtained. A refraction distribution may be obtained.

As described above, the detection unit 40 detects the backscattered light from each of the different positions z1, z2, and z3 with respect to the pulsed light having the angular frequency ω ₁ output from the light source unit 70 into at least four azimuth polarization states. Then, a normalized Stokes vector is obtained. Then, since the calculation unit 90 obtains the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the birefringence vector β based on the Stokes vector, the birefringence is calculated using the results obtained from numerical simulation and statistics. There is no need. Thereby, birefringence can be accurately measured.

In addition, this invention is not limited to this, The following may be sufficient.
In the apparatus shown in FIG. 4, the configuration in which the calculation unit 80 and the birefringence calculation unit 90 are provided is shown. However, when only the birefringence is obtained, the calculation unit 80 is not necessary. In this case, the light source unit 70 may output only one angular frequency.

Further, FIG. 1, in the apparatus shown in FIG. 4, the wavelength tunable light source 71, is shown sequentially varied outputs constituting the angular frequency omega ₁ ～Omega _3, provided three wavelength tunable light source 71, the angular frequency omega ₁ ˜ω ₃ may be output simultaneously. In this case, a multiplexer may be provided between the wavelength tunable light source 71 and the pulse generator 72, and a duplexer may be provided between the directional coupler 30 and the light detector 40.

  Further, although the configuration in which the wavelength variable light source 71 outputs continuous light having different angular frequencies (at least three types) is shown, at least three light sources that output a fixed wavelength may be provided instead of the wavelength variable light source 71.

  Furthermore, although the polarization variable unit 20 has been configured to polarize the pulsed light in two types of polarization states, the pulsed light may be polarized in a plurality of types of polarization states.

It is the block diagram which showed the 1st Example of this invention. It is the figure which showed the structure of the calculating part 80 in the apparatus shown in FIG. It is the flowchart which showed operation | movement of the apparatus shown in FIG. It is the block diagram which showed the 2nd Example of this invention. It is the figure which showed the structure of the birefringence calculating part 90 in the apparatus shown in FIG. It is the figure which showed the 1st structure of the conventional optical fiber characteristic measuring apparatus. It is the figure which showed the 2nd structure of the conventional optical fiber characteristic measuring apparatus. It is the figure which showed the structure of the calculating part 50 in the apparatus shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 20 Polarization variable part 30 Directional coupler 40 Light detection part 70 Light source part 71 Wavelength variable light source 72 Pulse generation part 80 Calculation part 81, 91 Matrix calculation means 82 Polarization dispersion vector calculation means 83, 93 Linear polarization calculation means 84, 94 Circular polarization calculation means 85 Dispersion value calculation means 90 Birefringence calculation section 92 Birefringence vector calculation means 95 Birefringence calculation means

Claims (5)

An optical fiber characteristic measuring device that inputs pulsed light into the optical fiber to be measured, detects the backscattered light from the optical fiber to be measured with respect to the pulsed light, obtains the Stokes vector, and measures the birefringence in the longitudinal direction In
A light source unit that outputs the pulsed light;
A birefringence calculation unit is provided for determining the magnitude of the birefringence by obtaining the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the birefringence vector based on the Stokes vectors obtained from at least three positions. Optical fiber characteristic measuring device. In an optical fiber characteristic measuring apparatus that measures the magnitude of birefringence in the longitudinal direction of an optical fiber to be measured,
A light source unit that outputs pulsed light;
A polarization variable section that polarizes and outputs each pulsed light output from the light source section into at least two different polarization states;
A directional coupler that outputs the pulsed light polarized by the polarization variable unit to the optical fiber to be measured, and receives backscattered light with respect to the output pulsed light;
The light intensity of the backscattered light from at least three positions with respect to the longitudinal direction of the optical fiber to be measured is detected from the directional coupler, and at least four different polarization states are synchronized with the pulsed light. A light detector that detects the light intensity of the azimuth and obtains a normalized Stokes vector;
A birefringence calculation unit for obtaining the magnitude of the birefringence by obtaining the magnitude of the linearly polarized component and the magnitude of the circularly polarized component of the birefringence vector based on the normalized Stokes vector obtained by the light detection unit is provided. A characteristic optical fiber characteristic measuring device. The light source
A continuous light output unit that outputs continuous light;
3. The optical fiber characteristic measuring apparatus according to claim 1, further comprising a pulse generating unit for converting the continuous light output unit into a desired pulse width of continuous light. The birefringence calculator is
Matrix computing means for obtaining a Mueller matrix from the normalized Stokes vector for each position;
From the Mueller matrix obtained by the matrix computing means, birefringence vector computing means for obtaining a birefringence vector due to backscattered light by each position;
Linear polarization calculation means for obtaining the magnitude of the linearly polarized light component of the birefringence vector from the birefringence vector by the backscattered light obtained by the birefringence vector calculation means;
Circular polarization calculation means for obtaining the size of the circularly polarized component of the birefringence vector from the difference of the birefringence vector due to the backscattered light obtained by the birefringence vector calculation means;
The birefringence value calculating means for obtaining the birefringence magnitude from the size of the circularly polarized light component of the circularly polarized light calculating means and the size of the linearly polarized light component of the linearly polarized light calculating means is provided . 4. The optical fiber characteristic measuring device according to any one of 3 above. In the optical fiber characteristic measurement method for measuring the birefringence in the longitudinal direction of the optical fiber to be measured,
The light source unit outputs pulse light;
A procedure of polarizing the pulsed light output from the light source unit into at least two different polarization states and inputting the polarized light to the optical fiber to be measured;
In synchronization with the pulsed light output from the light source unit, the light intensity of the backscattered light with respect to the pulsed light input to the optical fiber is detected separately in at least four directions of polarization, and the longitudinal direction of the optical fiber to be measured A procedure for obtaining normalized Stokes vectors at least at three positions with respect to
A procedure for obtaining a linearly polarized component and a circularly polarized component of a birefringence vector based on the normalized Stokes vector for each position obtained by the light detection unit,
A method for measuring optical fiber characteristics, comprising: a step of obtaining birefringence from a magnitude of a linearly polarized component and a magnitude of a circularly polarized component of the birefringence vector.

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