JP5313079B2 - Optical fiber characterization method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method to confirm that effectual single mode conditions are satisfied at a specific wavelength in an arbitrary optical fiber. <P>SOLUTION: An optical pulse with a wavelength of &lambda;a is emitted to one end 23a of an objective optical fiber 23, and an optical pulse tester 21 for measuring back scattered light is arranged. At the same time, a mode converter 24 for converting a transmission mode LP01 with a wavelength of &lambda;<SB>a</SB>to a higher-order mode LP11 is arranged between the optical fiber 23 and the optical pulse tester 21 to measure a loss &alpha;<SB>11</SB>of the LP11 in the optical fiber 23. The optical pulse tester 21 is connected to the other end 23a of the optical fiber 23 to measure a loss &alpha;<SB>01</SB>of the LP01 in the optical fiber 23. A loss ratio k=&alpha;<SB>11</SB>-&alpha;<SB>01</SB>(dB) of the loss &alpha;<SB>11</SB>of the LP11 to that &alpha;<SB>01</SB>of the LP01 is computed, and when the loss ratio k is a predetermined threshold Q(dB) or above, the optical fiber is judged to be single mode transmission. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

The present invention related to the characteristic evaluation method of the optical fiber, in particular, various optical fibers constructing the optical fiber communication network, whether it is possible to single mode transmission for light of a specific wavelength determining, or about the way to measure and evaluate the effective cutoff wavelength for various optical fiber.

  In an optical fiber, a wavelength region in which single mode transmission is possible (single mode wavelength region) is generally determined by the core diameter and the relative refractive index difference between the core and the clad. Since higher-order modes do not propagate in the optical fiber in the single mode wavelength region, signal degradation due to multimode dispersion does not occur, and high-speed signal transmission is possible.

Therefore, the cutoff wavelength (cut-off wavelength) as a parameter that defines the single mode wavelength region, and its measurement evaluation method are important. The cutoff wavelength is roughly classified as follows: 1. theoretical cutoff wavelength λ _c ; There are two effective cutoff wavelengths λ _ce , and only the fundamental propagation mode LP01 exists at wavelengths longer than the theoretical cutoff wavelength λ _c . On the other hand, even effective cutoff wavelength lambda _ce above, if the wavelength of less than the theoretical cutoff wavelength lambda _c, is the primary order mode LP11 can be present in the optical fiber. However, in the above wavelength range, the bending loss of LP11 is large, and the optical power of LP11 mode hardly reaches the end of the optical fiber, so that single mode transmission of only LP01 can be realized effectively. Accordingly, as shown in FIG. 14 is a graph showing the relationship between the fiber length and the effective cut-off wavelength lambda _ce, effective cutoff wavelength lambda _ce has a fiber length dependence, the fiber length of lambda _ce when the 0 It is considered that the limit value corresponds to the theoretical cutoff wavelength λ _c .

Conventionally, as a method for measuring the effective cutoff wavelength _λce , for example, as described in Non-Patent Document 1, a bending loss method (prior art 1) and a multimode excitation method (prior art 2) are known.

  The measurement system used for these methods is shown in FIGS. In the measurement system of the effective cutoff wavelength by the bending loss method, the optical fiber 103 to be measured is disposed between the white light source 101 and the light receiver 102 as shown in FIG. One end 103 a of the optical fiber 103 to be measured is connected to the spectrometer 105, and the spectrometer 105 is connected to the white light source 101. The other end 103 b of the optical fiber 103 to be measured is connected to the light receiver 102. A computer 106 is connected to the light receiver 102 and the spectroscope 105. In this measurement system, the optical fiber 103 to be measured is provided with a first bent portion 103c and a second bent portion 103d. The diameters of the first and second bent portions 103c and 103d are 280 mm and 60 mm, respectively.

  On the other hand, in an effective cutoff wavelength measurement system using a multimode excitation method, a multimode fiber 203 and a measured optical fiber 204 are disposed between a white light source 201 and a light receiver 202, as shown in FIG. The multimode fiber 203 and the measured optical fiber 204 are arranged in series, and one end 203 b of the multimode fiber 203 and one end 204 a of the measured optical fiber 204 are connected. The other end 203 a of the multimode fiber is connected to the spectroscope 205, and the spectroscope 205 is connected to the white light source 201. The other end 204 b of the optical fiber 204 to be measured is connected to the light receiver 202. A computer 206 is connected to the light receiver 202 and the spectroscope 205. In this measurement system, the optical fiber 204 to be measured is provided with a bent portion 204c. The diameter of the bent portion 204c is 280 mm.

  The fiber bending diameter and the measured optical fiber length (2 m) in FIGS. 15 and 16 are values standardized by ITU-T (International Telecommunication Union Telecommunication Standardization Sector). The bending loss method shown in FIG. 15 utilizes the difference in bending loss between the propagation mode LP01 and the first-order higher-order mode LP11. The multimode excitation method shown in FIG. It utilizes the fact that it changes greatly in the wavelength region where it changes from multimode to single mode. In addition, the value of the effective cutoff wavelength when measuring the length of the fiber to be measured by the bending loss method is 22 m is also widely used as the cable cutoff wavelength.

Edited by Koichi Matsumoto, “Optical Measuring Instruments Guide Full Revised”, Optronics, June 2004, pp.176-178 Takashi Matsui, Kunihiro Toge, Toshio Kurashima, Shigeru Hamada, Masatoshi Shimizu, “Examination of Contrast Technology Using Chirped Grating”, 2009 IEICE General Conference, B-13-13, p.501

  However, in recent years, research and development of optical fibers with very small bending loss has been promoted by devising dopant distribution and adding holes. In such optical fibers, the bending loss of the first-order higher-order mode LP11 is also low. Since it is smaller than the conventional optical fiber, measurement by the bending loss method is very difficult. The multi-mode excitation method is also the mode excitation state of the multi-mode fiber, the connection state between the multi-mode fiber and the optical fiber to be measured, and the loss characteristics of the optical fiber to be measured, such as OH-based absorption loss (around 1400 nm). When the value is large, an error is likely to occur.

Further, as shown in FIGS. 15 and 16, in the prior arts 1 and 2, the length of the optical fiber to be measured is designated, that is, the length of the optical fiber to be measured is adjusted in advance, The effective cutoff wavelength λ _ce needs to be determined as a value corresponding to the length (2 m in FIGS. 15 and 16), and the fiber length of λ _ce as shown in FIG. In order to obtain the dependence, it is necessary to change the length of the fiber to be measured, and to perform a plurality of measurements, which requires a large amount of operation.

On the other hand, as a practical countermeasure, in a normal optical fiber such as a single mode optical fiber (SMF) or a dispersion shifted optical fiber (DSF), the specific value of the cutoff wavelength must be determined by various methods. However, if it is possible to guarantee that the target optical fiber satisfies the effective single-mode condition at _a certain wavelength λ _a , single-mode transmission is possible at the wavelength of λ _a or more. The problem can be avoided if the shortest transmission signal wavelength is set to λ _a or more. If such a check can be easily performed, it is considered that, for example, the used wavelength region of the already installed optical fiber transmission line can be expanded to the short wavelength side.

  On the other hand, a photonic crystal fiber (PCF), which is a kind of hole-type optical fiber, which has been rapidly researched and developed in recent years, has a single mode or a full wavelength range depending on its structure. Since characteristics such as two modes or less can be realized, the cutoff wavelength may not actually exist.

Therefore, in consideration of the above situation, it can be said that it is very useful if it can be confirmed that the target optical fiber is operating in a single mode at _a certain wavelength λa.

However, a simple measurement system, and a method of evaluating the effective cut-off wavelength lambda _ce of small or large optical fiber extremely bend loss, and a method of evaluating the fiber length dependence of lambda _ce, effective in a certain wavelength lambda _a Until now, there has been no way to confirm that a typical single mode condition is satisfied.

  On the other hand, as the introduction of an optical fiber having a very small bending loss proceeds, it becomes necessary for an operator to distinguish a target core wire from a plurality of core wires at the laying or connection work place. At this time, the conventionally used method of applying bending to the core wire to leak the propagation mode LP01 cannot be applied because the bending loss of LP01 is too small. Therefore, a method is proposed in which an optical fiber long-period grating is formed by applying periodic stress to a target core wire, and LP01 mode light is converted into higher-order mode light and then leaked from the core wire. Effectiveness has been demonstrated (for example, refer nonpatent literature 2).

  However, even in this method, the necessary stress can be applied to the core wire in the cord because the cord coating is thick on the optical fiber cord used in the user's house or communication center building. Therefore, it is difficult to distinguish the target core line because light does not leak.

In view of the above, the present invention has been made to solve the above-described problems, and a method for evaluating the effective cutoff wavelength λ _ce of an optical fiber having extremely small or large bending loss, and a fiber of λ _ce It is an object of the present invention to provide a method for evaluating long dependence and a method for confirming that an effective single mode condition is satisfied at a specific wavelength in an arbitrary optical fiber.

In the present invention, an optical pulse tester (OTDR) for making an optical pulse of a specific wavelength enter one end of an optical fiber of interest, and mode conversion means for converting the propagation mode LP01 of the wavelength into a higher-order mode LP11 And use. Thus, if an appropriate fiber length is set at the time of measurement, the transmission loss α ₁₁ of LP ₁₁ at a specific wavelength and its length dependency can be obtained with high accuracy from the measured waveform of OTDR. The transmission loss α _{01 of} LP01 and its length dependency can be easily obtained by conventional OTDR measurement. This makes it possible to confirm effective single mode conditions and to evaluate the cutoff wavelength.

  In addition, since the bending loss of the higher-order mode LP11 is generally larger than that of the fundamental propagation mode LP01, it is possible to bend the LP11 mode light even in an optical fiber that is resistant to bending by providing an appropriate wavelength and an appropriate bending diameter. Loss can be detected from the OTDR waveform and used to control the optical fiber core.

That is, a method for confirming single mode transmission of an optical fiber according to the first invention for solving the above-described problem is as follows.
An optical pulse tester that emits an optical pulse of wavelength λa and measures backscattered light is disposed at one end of the target optical fiber, and the wavelength λ _{a is} interposed between the optical fiber and the optical pulse tester. the propagation mode LP01 to place mode conversion means for converting the higher-order mode LP11, measured loss alpha ₁₁ for LP11 in the optical fiber,
An optical pulse tester is connected to one end of the optical fiber, and a loss α ₀₁ of LP01 in the optical fiber is measured.
Calculating a loss ratio k = α ₁₁ −α ₀₁ (dB) between the loss α ₁₁ of the LP ₁₁ and the loss α ₀₁ of the LP ₀₁ ,
When the loss ratio k is equal to or greater than a set threshold value Q (dB), the optical fiber is determined to be single mode transmission.

A method for confirming single mode transmission of an optical fiber according to the second invention for solving the above-described problem is as follows.
A method for confirming single mode transmission of an optical fiber according to a first invention, comprising:
At the time of measuring and evaluating the loss α ₀₁ of LP01, the object optical fiber is subjected to a single bending with a diameter of 60 mm.

A method for confirming single mode transmission of an optical fiber according to the third invention for solving the above-described problem is as follows.
A method for confirming single mode transmission of an optical fiber according to the first or second invention, comprising:
As the mode conversion means, a long period grating is used.

A method for confirming single mode transmission of an optical fiber according to a fourth invention for solving the above-described problem is as follows.
A method for confirming single mode transmission of an optical fiber according to any one of the first to third inventions,
The threshold value Q is 16.3 dB or more and 22.3 dB or less.

A method for measuring a cutoff wavelength of an optical fiber according to a fifth invention for solving the above-described problem is as follows.
An optical pulse tester that is arranged on one end side of the target optical fiber, emits different light pulses in the wavelength range of λ ₁ to λ _n , and measures backscattered light,
Using a long period grating having a plurality of different grating periods as a mode conversion means for converting a propagation mode LP01 having a different wavelength in the range of λ ₁ to λ _n to a higher order mode LP11,
The ratio k = α ₁₁ −α ₀₁ (dB) between the loss α ₁₁ of LP ₁₁ and the loss α _{01 of} LP ₀₁ in the optical fiber at each wavelength is the value of the set threshold value Q (dB). The matching wavelength λ _m is determined as the cutoff wavelength of the LP11 mode.

A method for measuring a cutoff wavelength of an optical fiber according to a sixth invention for solving the above-described problem is as follows.
A method for measuring a cutoff wavelength of an optical fiber according to a fifth invention,
At the time of measurement and evaluation of the loss α ₀₁ of LP01, the object optical fiber is subjected to one bending with a diameter of 60 mm.

A method for measuring a cutoff wavelength of an optical fiber according to a seventh invention for solving the above-described problem is as follows.
A method for measuring a cutoff wavelength of an optical fiber according to a fifth or sixth invention,
The threshold value Q is 16.3 dB or more and 22.3 dB or less.

According to the present invention, an effective single mode condition at a specific wavelength can be confirmed by evaluating loss values of the LP01 and LP11 modes using a single wavelength optical pulse tester (OTDR). Further, the effective cutoff wavelength can be evaluated by using an optical pulse tester that oscillates light of a plurality of wavelengths or a wide wavelength range .

  Further, these effects can be realized by a very simple configuration including only an optical pulse tester and mode conversion means for converting the LP01 mode into the higher order mode LP11.

It is a figure for demonstrating the measuring system of the effective cutoff wavelength (lambda) _ce by a bending loss method, FIG. 1 (a) shows the schematic structure, FIG.1 (b) shows an example of the evaluation result. FIG. 2 is a diagram for explaining a method for confirming single-mode transmission of an optical fiber according to the first embodiment of the present invention, and FIG. 2A shows a case where a loss α ₁₁ of LP11 is measured; (B) shows a case where the loss α ₀₁ of the propagation mode LP01 is measured. It is a figure which shows an example of the measurement waveform by an optical pulse tester. It is a figure which shows an example of the modulation structure of the core refractive index of a long period optical fiber grating (LPG). It is a figure for demonstrating the confirmation method of the single mode transmission of the optical fiber which concerns on 2nd Example of this invention, Comprising: FIG. 5 (a) shows an example of the structure of the stress-applying LPG of the period Λ, FIG. 5B shows an example of the structure of a stress-applying LPG having a period Λ ₁ to a period Λ _N. It is a figure for demonstrating the confirmation method of the effective single mode condition of the optical fiber which concerns on 3rd Example of this invention. It is a figure which shows an example of the measurement waveform by an optical pulse tester. FIG. 8A is a diagram for explaining a method for measuring an effective cutoff wavelength of an optical fiber according to a fourth embodiment of the present invention, and FIG. 8A shows a case where a loss α ₁₁ of LP11 is measured; b) shows a case where the loss α ₀₁ of the propagation mode LP01 is measured. It is a graph showing the relationship between the wavelength λ and (α ₁₁ -α _01). It is a figure for demonstrating the contrast method of the optical fiber core wire which concerns on the reference example of this invention. It is a figure which shows an example of the wavelength dependence of the transmission loss of LP01 mode and LP11 mode. It is a figure which shows an example of the bending radius R dependence of the bending loss of LP01 mode and LP11 mode. It is a figure which shows an example of the measurement waveform by the optical pulse tester when bending with three types of bending radii was given. It is a figure which shows an example of fiber length dependence of effective cut-off wavelength (lambda) _ce (micrometer). It is a block diagram for demonstrating the measuring system of the effective cutoff wavelength by the conventional bending loss method. It is a block diagram for demonstrating the measuring system of the effective cutoff wavelength by the conventional multimode excitation method.

  The method for confirming single mode transmission of an optical fiber, the method for measuring the cut-off wavelength, and the method for comparing the optical fiber core wires according to the present invention will be described in detail in each embodiment.

A method for confirming single-mode transmission of an optical fiber according to the first embodiment of the present invention will be specifically described with reference to FIGS.
In FIG. 1B, the horizontal axis represents wavelength (μm), and the vertical axis represents optical loss (dB). Here, the optical loss (dB) is a standard method in which the LP11 mode component is completely attenuated and does not affect the LP01 mode component. The transmission power ratio 10 log (P (λ) / Pr (λ)) before and after the bending is given.

First, the principle correspondence between the present invention and the bending loss method (FIG. 15), which is the conventional method, will be described. By this method, confirmation of single mode transmission of an optical fiber at the wavelength λ _a , that is, It shows that it is possible to confirm whether or not one mode condition is satisfied with high accuracy.

  In the measurement system of the effective cutoff wavelength by the bending loss method, the optical fiber to be measured 13 is disposed between the white light source 11 and the light receiver 12 as shown in FIG. One end 13 a of the optical fiber 13 to be measured is connected to the white light source 11. The other end 13 b of the measured optical fiber 13 is connected to the light receiver 12. As required, the optical fiber 13 to be measured is provided with a bent portion 13c having a radius R.

The effective cutoff wavelength λ _ce is defined as a wavelength that satisfies the following formula (1). Here, P (λ) is the transmitted light power before bending the fiber, and Pr (λ) is the transmitted light power when bending the radius R. By transforming equation (1), equation (2) shown below is obtained. It can be understood that the expression (2) is an expression representing the result of the LP11 component contained in P (λ _ce ) being completely attenuated by giving a bend of radius R.

Therefore, when the LP01 component is P ₀₁ (mW) and the LP11 component is P ₁₁ (mW) in the light mode state at the input end of the optical fiber, the LP11 component is completely attenuated with the bending of the radius R. Assuming that the transmission loss (dB) of the LP01 and LP11 modes at the wavelength _λce at the total fiber length L is α ₀₁ and α ₁₁ , respectively, the following relational expressions (3), ( 4) is established. Here, as an approximation, the effects of mode conversion and mode coupling are ignored, but it is appropriate when L is in the range of several kilometers.

Although the value of P ₁₁ / P ₀₁ varies depending on the excitation condition at the time of fiber input from the light source, it is a value near 1 under normal experimental conditions, and 0.25 <P even considering variations due to various factors. ₁₁ / P ₀₁ <4. Therefore, at the effective cutoff wavelength λ _ce obtained by the bending loss method, the value of the ratio of LP01 and LP11 mode transmission loss (α ₁₁ −α ₀₁ ) is about 16.3 dB on the average and about 22.3 dB on the maximum. It can be said that

From the above consideration, in order to confirm whether the optical fiber having the total length L satisfies the effective single mode condition at the specific wavelength λ _a , the transmission loss of each of the LP01 and LP11 modes is measured, What is necessary is just to check whether the value of the ratio (α ₁₁ −α ₀₁ ) is equal to or greater than the threshold value Q (dB), and the value of Q may be 16.3 dB or 22.3 dB. For example, Q = 16.3 dB may be used as an average criterion, and Q = 22.3 dB may be used when strict worst-case evaluation is desired.

Accordingly, in the present invention, the configuration is a preferable shown in FIG. 2 in confirms single mode transmission optical fiber at a wavelength lambda _a, With this configuration, the transmission loss of the confirmation (LP01 mode and LP11 mode of the above principles Measurement).

That is, in the method for confirming single mode transmission in the optical fiber of this embodiment, as shown in FIG. 2, an optical pulse tester (OTDR) 21 and A mode converter (mode conversion means) 24 is arranged. One end 23a of the optical fiber 23 and the mode converter 24 are connected, and the mode converter 24 and the optical pulse tester 21 are connected. OTDR 21 emits a pulse of wavelength lambda _a, an instrument that measures the backscattered light. Mode converter 24 is a device for converting a propagation mode LP01 wavelength lambda _a to higher order modes LP11. Thus, the mode converter 24, as shown in FIG. 2 (a), used only in measuring loss alpha ₁₁ for LP11. As shown in FIG. 2B, when measuring the loss α ₀₁ of the propagation mode LP01, the optical pulse tester 21 and the device under test are measured using a normal optical connector, fusion, a normal single mode optical fiber cord, or the like. The optical fiber 23 may be connected. In this case, if the occurrence of the LP11 mode is concerned, in order to remove this, as shown in FIG. 2 (b), an appropriate bending, for example, a bending portion that is a single bending at a diameter of 60 mm. By providing 23b, only the LP11 mode can be attenuated, and α ₀₁ can be measured with high accuracy.

Here, FIG. 3 shows an example of a waveform measured by the optical pulse tester measured with the configuration shown in FIG. In FIG. 3, the horizontal axis indicates the position Z (m), and the vertical axis indicates the backscattered light power (dB). Evaluation of α ₀₁ and α ₁₁ is a measurement point in a range where a sufficient dynamic range above the noise level is obtained, and from the level difference of backscattered light (dB) at both ends of the same section length (= fiber length L1). , Each can be estimated. As a result, values of α ₀₁ and α _{11 at} the length L1 are obtained, and an effective single mode at the length L1 depends on whether the value of (α ₁₁ -α ₀₁ ) is equal to or greater than the threshold value Q (dB). It can be judged whether the condition is satisfied. L1 may be set as necessary within a range in which measurement evaluation is possible. For example, if L1 is 2 m, it is possible to match the conventional technique (standard method) shown in FIGS. 15 and 16. . If L1 is 22 m, matching with a standard method (cable cutoff wavelength) in the cable state can be achieved.

When the dynamic range is small or when the influence of reflected light noise or the like is considered to be large, a section of the fiber length L2 where these influences do not reach is selected, and waveform slopes S01 and S11 in the section are obtained, and α The value of (α ₁₁ −α ₀₁ ) may be determined from ₁₁ −α ₀₁ = (S ₁₁ −S ₀₁ ) × L 2. In estimating S01 and S11, a plurality of measurement points may be determined by fitting with a linear expression using a least square method or the like. In actual measurement, since the conversion efficiency from the LP01 mode to the LP11 mode is not about 100%, the waveform inclination may vary depending on the selected section, but an appropriate section that can be approximated by a straight line is selected and S11 is determined. Just do it. In this case, the fiber length L2 can be set to an arbitrary value. For example, if L2 is set to 2 m, it is possible to achieve matching with the conventional technique (standard method) shown in FIGS. If L2 is set to 22 m, matching with a standard method (cable cutoff wavelength) in a cable state can be achieved.

A method for confirming single mode transmission of an optical fiber according to a second embodiment of the present invention will be described with reference to FIGS.
In this embodiment, as an example of a mode converter that converts the propagation mode LP01 described in the first embodiment to a higher-order mode LP11, a long-period optical fiber grating using a periodic change in refractive index is used. It was.

A long-period optical fiber grating (LPG) can be used as a mode converter that converts the propagation mode LP01 to the higher-order mode LP11. As shown in FIG. 4, the long-period optical fiber grating has a core region 31 in the center of the axis and a clad portion 32 surrounding the core region 31, and the core region 31 has a period Λ in the longitudinal direction of the fiber. A plurality of grating portions 31a having different refractive indexes are formed. That is, a refractive index modulation structure is provided in the longitudinal direction of the core region 31 of the optical fiber. For example, as described in Non-Patent Document 2, as shown in FIG. 5A, it is realized by pressing a stress-applying LPG 40, which is a metal plate having irregularities with a period Λ, against the target optical fiber 45. it can. The stress applying type LPG 40 is provided with a fixing base 41 to which the optical fiber 45 to be measured is fixed and a plurality of protrusions 43, and a stress applying tool (metal) in which tips 43a of adjacent protrusions 43 are arranged with a period Λ. Plate) 42. An optical fiber 45 to be measured is arranged on the fixed base 41, arranged so that a plurality of projections 43 are in contact with the optical fiber to be measured 45, and a load F is applied to the stress applying tool 42 from above, thereby measuring the optical fiber to be measured. A long-period optical fiber grating can be realized with a period Λ with respect to the optical fiber 45. Since such LPG 40 can be easily attached and detached by detaching the stress applying tool 42, the stress applying tool 43 is attached to the measured fiber 45 during the measurement of α ₁₁ in FIG. _When measuring _01, the stress applying tool 43 is removed from the optical fiber 45 to be measured, so that it can be used as the mode converter described in the first embodiment (stress applying LPG). Also, a general optical fiber cord type LPG (general type LPG) in which such a modulation structure is permanently provided to the core by irradiation of ultraviolet rays can be used as a mode converter only at the time of measuring α ₁₁ . In this case, various optical fiber connectors and fusion splicing can be used for connection between the mode converter and the measured fiber 45.

In order for the above LPG to operate at _a specific wavelength λ _a , it is necessary to have a refractive index modulation period Λ satisfying the following equation (5). Here, n ₀₁ and n ₁₁ are effective refractive indexes of the LP01 mode and the LP11 mode, respectively.

  Since the period Λ is uniquely determined for each optical fiber, when the general type LPG is used as a mode converter, a general type LPG (optical fiber cord) in which the period Λ is set to an optimum value by the equation (5). Should be used.

However, for example, even in the same type of optical fiber, for example, SMF (1.3 micron band zero dispersion single mode fiber), there are subtle structural variations of the optical fiber, so that n ₀₁ and n ₁₁ are subtle. However, the optimum value of the period Λ is slightly different for each optical fiber. Therefore, when using the stress-applying LPG as a mode converter, an LPG having a chirp structure in which the value of the period Λ varies in the longitudinal direction according to the type of the optical fiber to be applied, for example, FIG. As shown, it is preferable to use a stress-applying LPG 50 that is a metal plate having irregularities with periods Λ _{1 to} Λ _N. The stress applying type LPG 50 is provided with a fixed base 51 to which the optical fiber 55 to be measured is fixed and a plurality of projections 53, and the tip 53a of the adjacent projection 53 has a period Λ ₁ at _one end 52b. The other end portion 52c includes a stress applying tool (metal plate) 52 disposed so as to gradually increase so as to have a period Λ _N (> Λ ₁ ). An optical fiber to be measured 55 is arranged on the fixed base 51, arranged so that a plurality of protrusions 53 are in contact with the optical fiber to be measured 55, and a load F is applied to the stress applying tool 52 from above, thereby measuring the optical fiber to be measured. A long-period optical fiber grating can be realized with respect to the optical fiber 55 in the range of periods Λ ₁ to Λ _N.

On the other hand, in order to secure a dynamic range when estimating α ₁₁ from the OTDR waveform, ideally, the conversion efficiency of LP01 → LP11 is desirably about 90% or more, but this lengthens the total length of the LPG. Although it is necessary to increase the refractive index periodically arranged, even if it is 90% or less, there is no problem if a conversion efficiency that does not hinder the estimation of the OTDR waveform is used.

A method for confirming an effective single mode condition of an optical fiber according to a third embodiment of the present invention will be described with reference to FIGS.
In this embodiment, a long-period optical fiber grating LPG is used as a mode converter for converting the propagation mode LP01 to the higher-order mode LP11.

In the confirmation of the effective single mode condition, as shown in FIG. 6, as an example, an optical pulse tester (OTDR) 61 and mode conversion arranged on one end 63a side of the optical fiber 63 to be measured. It is possible to use a configuration provided with a device (mode conversion means) 64. A mode converter 64 is connected to one end 63 a of the optical fiber 63 to be measured, and an optical pulse tester 61 is connected to the mode converter 64. Here, the measured fiber 63 is a pure silica core optical fiber having an effective cutoff wavelength of about 1500 nm at a fiber length L = 2 m, the OTDR wavelength λ _a is 1490 nm, and the mode converter 64 is the second embodiment described above. The stress applying type LPG described in the example was used.

  The measurement results are shown in FIG. Immediately after the position of the stress-applying LPG, the waveform inclination becomes steep and then becomes gentle again. This means that the LP11 component that has undergone mode conversion by LPG has been lost almost at the portion where the waveform inclination is steep, and has been lost without mode conversion at the portion where the waveform inclination is gentle thereafter. It means that the LP01 component is lost. Therefore, the slope of the steep waveform slope corresponds to S11 = 2.9 dB / m. The measured waveform slope when LPG is not used corresponds to S01 = 0.2 dB / km = 0.0002 dB / m.

Therefore, when converted to the fiber length L = 2 m (when evaluating the fiber cutoff wavelength) and 22 m (when evaluating the cable cutoff wavelength) in the standard measurement method, the value of (α ₁₁ −α ₀₁ ) is 5.8 dB ( 2m) and 63.8 dB (22 m). Therefore, when the threshold value Q = 22.3 dB, the effective single mode condition is not satisfied at the fiber length of 2 m, but the effective single mode condition is satisfied at the fiber length of 22 m from the magnitude relationship with Q. Can be determined.

A method for measuring the effective cutoff wavelength according to the fourth embodiment of the present invention will be described with reference to FIGS.
In this embodiment, a long-period optical fiber grating is used as a mode converter for converting the propagation mode LP01 to the higher-order mode LP11.

In the measurement method of the effective cutoff wavelength of the present embodiment, the configuration example shown in FIG. 8 is suitable, and an optical pulse tester (OTDR) 71 and a mode converter (on the one end 73a side of the optical fiber 73 are provided. (Mode conversion means) 74 is arranged. A mode converter 74 is connected to one end 73 a of the optical fiber 73, and an optical pulse tester 71 is connected to the mode converter 74. The optical pulse tester 71 is an instrument that can oscillate optical pulses having different wavelengths λ ₁ to λ _n or continuous wide wavelength widths from λ ₁ to λ _n and measure backscattered light. The mode converter 74 is a device capable of converting the propagation mode LP01 to the higher-order mode LP11 at each of the swept wavelengths λ ₁ to λ _n . As the mode converter 74, for example, as described in the second embodiment, a plurality of general type LPGs and stress applying type LPGs operating at respective wavelengths from λ ₁ to λ _n are arranged in tandem in multiple stages. A general LPG or a stress-applying LPG having a chirp structure in which the value of the period Λ changes in a certain range may be used.

Then, when the value of (α ₁₁ −α ₀₁ ) from λ ₁ to λ _n is obtained for each wavelength in the fixed fiber length L by the procedure described in the first embodiment described above, (α ₁₁ The relationship between -α ₀₁ ) and wavelength λ changes as indicated by the black circles shown in FIG. By fitting these n measurement points with a polynomial or the like and obtaining a wavelength λ _m that satisfies (α ₁₁ −α ₀₁ ) = Q (threshold), this wavelength λ _m corresponds to the effective cutoff wavelength λ _ce . From the figure, it is desirable to increase the evaluation accuracy when the wavelength range of the measurement wavelengths λ ₁ to λ _n is wider and the number of wavelengths is larger. However, since the value of (α ₁₁ −α ₀₁ ) changes abruptly before and after λ _ce , the measurement wavelength range and the number of wavelengths may be determined in consideration of the required evaluation accuracy. The Q value may be 16.3 dB or 22.3 dB.

Reference example

An optical fiber core wire comparison method according to a reference example of the present invention will be described with reference to FIGS. 10 and 11.
In this reference example, using a mode converter for converting the optical pulse tester that oscillates light pulses of the wavelength lambda _b and (OTDR) the propagation mode LP01 into higher order mode LP11.

In the reference method of the optical fiber core wire of the present reference example , the configuration shown in FIG. 10 is suitable, and the optical pulse tester 81 is provided on the one end 83a side of the optical fiber core wire 83 with extremely small bending loss. A mode converter (mode conversion means) 84 is disposed. A mode converter 84 is connected to one end 83 a of the optical fiber core 83, and an optical pulse tester 81 is connected to the mode converter 84. OTDR 81 can oscillate light pulses of the wavelength lambda _b, it is a device that measures the backscattered light. Mode converter 84 at a wavelength lambda _b, is a device capable of converting a propagation mode LP01 into higher order mode LP11. The mode converter 84, for example, as described in the second embodiment described above, may be used general-type LPG or stress-imparting LPG operating at lambda _b.

Here, FIG. 11 shows an example of the wavelength dependence of transmission losses A01 (dB / km) and A11 (dB / km) in the LP01 mode and the LP11 mode. In general, the value of A11 (dB / km) in the vicinity of the effective cut-off wavelength lambda _ce increases rapidly, in the present reference example, LP11 mode when the use wavelength lambda _b are in close wavelengths in the vicinity of lambda _ce sharply attenuated Thus, there is a possibility that the comparison operation position where the bending of the radius R is applied is not reached and the comparison operation cannot be performed. Therefore, although it is necessary to consider the distance from the optical pulse tester 81 to the reference working position 83b, typically, λ _b is a wavelength shorter by about 0.1 to 0.3 μm than λ _{ce at a} fiber length of 2 m. Should be set.

  As for the value of the bending radius R, it is necessary to consider the range in which the bending loss of the propagation mode LP01 does not occur. However, there is already an optical fiber having a negligible bending loss of the LP01 mode even with a radius of about 10 mm. It has been put into practical use.

  Specifically, as shown in an example in FIG. 12, the bending loss value of an optical fiber of the same type as the optical fiber used in the optical fiber cord to be compared is evaluated in advance, and the bending loss of the LP01 mode is evaluated. Is sufficiently small, and the bending radius at which the loss of the LP11 mode reaches T (dB) which is the threshold value for the control determination may be R. Further, the value of the threshold value T (dB) may be sufficiently larger than the noise level of the OTDR waveform, for example, 1 dB.

  The target operation may be performed by changing a plurality of R values in consideration of variations in characteristics of the optical fiber. As an example, FIG. 13 shows changes in the OTDR waveform when bending is performed with three bending radii of R1, R2, and R3 (R1 <R2 <R3). In addition, as a bending shape, it is good also as a 1 time or multiple rotation winding to a cylinder etc., and good also as U-shaped bending (semicircle winding) which is utilized for the conventional core wire contrast device.

  At the time of control work, when the control worker and the person monitoring the OTDR communicate with each other by telephone or the like, when the worker gives a bend, the control judgment is made based on whether or not the waveform change exceeding T (dB) occurs. Just do it.

According to the present invention, an effective single mode condition at a specific wavelength can be confirmed and an effective cutoff wavelength can be evaluated with a very simple configuration, which is useful for the optical communication industry.

DESCRIPTION OF SYMBOLS 11 White light source 12 Light receiver 13 Optical fiber 21 to be measured 21 Optical pulse tester 23 Optical fiber 24 to be measured 24 Mode converter 30 Long period optical fiber grating 40, 50 Stress application type LPG
43, 53 Protrusions 61 Optical pulse tester 63 Optical fiber to be measured 64 Mode converter 71 Optical pulse tester 73 Optical fiber to be measured 74 Mode converter 81 Optical pulse tester 83 Optical fiber to be measured 84 Mode converter

Claims (7)

An optical pulse tester that emits an optical pulse of wavelength λa and measures backscattered light is disposed at one end of the target optical fiber, and the wavelength λ _{a is} interposed between the optical fiber and the optical pulse tester. the propagation mode LP01 to place mode conversion means for converting the higher-order mode LP11, measured loss alpha ₁₁ for LP11 in the optical fiber,
An optical pulse tester is connected to one end of the optical fiber, and a loss α ₀₁ of LP01 in the optical fiber is measured.
Calculating a loss ratio k = α ₁₁ −α ₀₁ (dB) between the loss α ₁₁ of the LP ₁₁ and the loss α ₀₁ of the LP ₀₁ ,
A method for confirming single-mode transmission of an optical fiber, wherein the optical fiber is determined to be single-mode transmission when the loss ratio k is equal to or greater than a set threshold value Q (dB). A method for confirming single mode transmission of an optical fiber according to claim 1,
A method for confirming single-mode transmission of an optical fiber, wherein a one-time bend with a diameter of 60 mm is applied to a target optical fiber when measuring and evaluating the loss α ₀₁ of LP01. A method for confirming single-mode transmission of an optical fiber according to claim 1 or 2,
A method for confirming single-mode transmission of an optical fiber, wherein a long-period grating is used as the mode conversion means. A method for confirming single mode transmission of an optical fiber according to any one of claims 1 to 3,
The threshold value Q is 16.3 dB or more and 22.3 dB or less, and the method for confirming single-mode transmission of an optical fiber. An optical pulse tester that is arranged on one end side of the target optical fiber, emits different light pulses in the wavelength range of λ ₁ to λ _n , and measures backscattered light,
Using a long period grating having a plurality of different grating periods as a mode conversion means for converting a propagation mode LP01 having a different wavelength in the range of λ ₁ to λ _n to a higher order mode LP11,
The ratio k = α ₁₁ −α ₀₁ (dB) between the loss α ₁₁ of LP ₁₁ and the loss α _{01 of} LP ₀₁ in the optical fiber at each wavelength is the value of the set threshold value Q (dB). A method for measuring a cutoff wavelength of an optical fiber, wherein the matching wavelength λ _m is determined as a cutoff wavelength of the LP11 mode. A method for measuring the cutoff wavelength of an optical fiber according to claim 5,
A method for measuring a cutoff wavelength of an optical fiber, wherein a bending of a diameter of 60 mm is applied to a target optical fiber at the time of measurement and evaluation of a loss α ₀₁ of LP01. A method for measuring a cutoff wavelength of an optical fiber according to claim 5 or 6,
The method for measuring a cutoff wavelength of an optical fiber, wherein the threshold value Q is 16.3 dB or more and 22.3 dB or less.

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