JP4690690B2 - Optical fiber chromatic dispersion value and nonlinear constant measurement method, optical fiber chromatic dispersion value and nonlinear constant measurement device, fiber manufacturing method, dispersion distribution measurement method, measurement error compensation method, measurement condition identification method - Google Patents

Description

  The present invention relates to a method for evaluating nonlinear constants and chromatic dispersion values of optical fibers used mainly, an apparatus therefor, and a fiber manufacturing method using them.

  With the liberalization of the information communication field and the development of the information society, the amount of communication information tends to increase dramatically. With the practical application of erbium-doped fiber amplifier (EDFA) and Raman fiber amplifier that directly amplifies light, optical signals with very high intensity can be obtained in the 1.55 μm band. It was. This compensates for optical fiber transmission loss and enables repeaterless transmission over thousands of kilometers.

Using such an optical amplification technique, a wavelength division multiplexing (WDM) communication system and a time division multiplexing (TDM) communication system have been studied. In constructing an optical repeater system using an optical amplification technique, it is important to suppress signal waveform deterioration as much as possible when an optical signal propagates through an optical fiber. Causes of optical signal waveform degradation include the spread of optical pulses due to dispersion in the optical fiber, and nonlinear effects that occur in proportion to the magnitude of the optical signal intensity.

Non-linear optical effects that occur in optical fibers include self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). These nonlinear phenomena adversely affect transmission quality. Effect. Since the magnitude of such a nonlinear effect is determined by the nonlinear constant of the optical fiber, a method for measuring the value with high accuracy is required for designing the transmission line. The nonlinear constant is expressed by γ = 2π · n ₂ / λ · A _eff for light of wavelength λ, and the value of γ or n ₂ / A _eff described above is used for evaluating the nonlinear effect of the optical fiber. . Note that n ₂ is a nonlinear refractive index, and A _eff is an effective core area.

  As a method for measuring the nonlinear constant, a method has been reported in which an optical pulse is incident on an optical fiber using a pulsed light source and phase modulation is measured to determine the nonlinear constant from a change in power spectrum waveform caused by self-phase modulation ( RHStolen and Chinlon Lin, Physical Review A, Vol. 17, No. 4, pp. 1448-1453 (1992)). This method is generally called a self-phase modulation method (hereinafter referred to as SPM method).

  In addition, two light sources that generate optical signals of two different wavelengths are used, and a beat pulse generated by combining them is incident on the optical fiber to be measured, and nonlinearity of the optical fiber is detected from changes in the optical spectrum caused by the self-phase modulation effect. A constant can be obtained. This method is generally called the CW-SPM method.

  Furthermore, the probe light and pump light that is sinusoidally modulated at a specific frequency are made incident on the optical fiber, and the probe light is subjected to delayed self-heterodyne detection to obtain the phase modulation that the pump light brings to the probe light and to obtain a nonlinear constant. A calculation method has also been reported (A. Wada et al., ECOC92, p.42 (1992)). This method is generally called a cross phase modulation method (hereinafter referred to as XPM method).

  A non-linear phenomenon that is particularly problematic for wavelength division multiplexing is FWM. When the sideband generated by FWM overlaps with other transmission signal wavelengths in WDM communication, it causes noise and causes crosstalk. It is known that this FWM is likely to occur in an optical fiber having a large nonlinear effect, and is particularly likely to occur when the signal light is in the vicinity of the zero dispersion wavelength of the optical fiber.

  On the other hand, there is a problem that the request to suppress the FWM with respect to the chromatic dispersion value in the optical transmission line conflicts with the condition of waveform degradation due to chromatic dispersion. In order to solve such problems, the local chromatic dispersion value of the optical transmission line is not zero, but an optical fiber having a positive chromatic dispersion value at the transmission signal wavelength and an optical fiber having a negative chromatic dispersion value are combined. Thus, a dispersion compensation system has been devised that makes the chromatic dispersion amount of the entire optical transmission line close to zero.

  When designing and constructing such a dispersion compensation system, it is necessary to design the nonlinear characteristics and dispersion characteristics of the optical transmission line in a more optimal form. In designing, an optimum optical fiber combination is often obtained from the average chromatic dispersion value and the length of the optical fiber.

  However, the chromatic dispersion value of the optical fiber actually used is not necessarily uniform in the longitudinal direction of the fiber with respect to a certain wavelength. This is due to variations in optical fiber and cable manufacturing. For this reason, in high-speed, high-density, large-capacity transmission that is more restrictive, the theoretical design may not match the transmission state in the optical fiber actually used.

  Therefore, in such a case, it is necessary to accurately grasp the distribution value of the chromatic dispersion value in the fiber longitudinal direction of the optical fiber used in the dispersion compensation system and reflect it in the design.

From such circumstances, methods for measuring the distribution value of the chromatic dispersion value in the longitudinal direction of the optical fiber have been studied. At present, methods such as total reflection OTDR using linear phenomena and FWM, which is one of nonlinear optical phenomena, have been proposed. Note that the currently widely used measurement method is disclosed by LFMollenauer, et.al., which measures the backscattered light of idler light generated by FWM in an optical fiber and disperses it from its intensity fluctuation period. Is called non-linear OTDR (see Patent Document 1 and Non-Patent Document 1).
JP-A-8-21783 Optics Letters 1996, 21, pp.1724-1726

With the recent increase in speed and increase in WDM capacity, more strict control is required for optical fiber transmission line design. Therefore, from the optical transmission line design based on the average chromatic dispersion value and nonlinear constant for each fiber constant length, which is mainly performed at present, a more realistic chromatic dispersion value in the longitudinal direction of the optical fiber. There is an urgent need for a design that takes into account these fluctuations. At the same time, when the nonlinear constant of the optical fiber also varies in the longitudinal direction of the fiber, the variation of the nonlinear constant also affects the transmission pulse. Therefore, in order to design in more detail, it is necessary to measure not only the distribution value of the chromatic dispersion value but also the distribution value of the nonlinear constant.

  However, at present, there is no effective means for measuring fluctuations in the longitudinal direction of the nonlinear constant. For this reason, it is unknown how much the fluctuation of the nonlinear constant is in any currently used optical fiber and how much the fluctuation affects the optical transmission characteristics. Therefore, there has been no example in which the influence of fluctuations in the longitudinal direction of the nonlinear constant on the design of the optical transmission line and other optical transmission analysis has been studied.

  However, the nonlinear constant of the optical fiber is an important parameter characterizing the optical transmission characteristics, and it is of great significance to investigate the fluctuation of the nonlinear constant in the longitudinal direction of the fiber. Considering these developments, a method for measuring the longitudinal dependence of fiber characteristics (dispersion and nonlinear characteristics) that can contribute to the development of next-generation optical transmission lines, fiber devices that require adjustment of chromatic dispersion values and nonlinear constants, and The development of measuring instruments is an issue.

  In addition, a change in the glass diameter at the time of drawing is considered as a factor that causes a change in the chromatic dispersion value in the fiber longitudinal direction in the fiber manufacturing process. Since the refractive index distribution of the optical fiber preform to be drawn can be measured by a preform analyzer, drawing is performed with a glass diameter that provides a target chromatic dispersion value based on the measurement result. However, there are errors in the processing accuracy of the optical fiber preform and measurement errors in the preform analyzer, and variations in the longitudinal direction of the fiber occur. Therefore, from the viewpoint of fiber production, there is a demand for a method for producing an optical fiber that is more stable and has a good yield, and the development of a fiber characteristic evaluation method therefor is demanded.

  The present invention has been made in order to solve the above-described problems, and the object thereof is to distribute both the chromatic dispersion value and the nonlinear constant in the longitudinal direction of the fiber by utilizing the nonlinear optical effect in the optical fiber to be measured. To be able to measure the value.

Moreover, when evaluating using the above-described chromatic dispersion distribution measurement method of an optical fiber using the nonlinear OTDR method, other problems include development of a method of measuring with higher accuracy and an accurate analysis method.

In order to obtain an idler return light observation waveform with little variation in the nonlinear OTDR method, it is desirable that the intensity of the idler return light is strong. This is particularly important when measuring a transmission line optical fiber having a long length to the end with high accuracy. For this purpose, it is necessary to enter pulsed light having a higher peak intensity.

However, when the intensity of the pulse peak incident on the optical fiber to be measured is increased, stimulated Brillouin scattering (SBS) and MI (Modulation Instability) may occur in an anomalous dispersion fiber. There is.

First, the occurrence of SBS will be described. The generation of SBS is a phenomenon that occurs in an optical fiber when a pulse having a certain intensity or more is input. Backscattered light having a wavelength different from that of input light is generated, and all input power is not transmitted to the output end. The occurrence of SBS is a phenomenon that is more likely to occur when the fiber length is long or when the optical fiber has high nonlinearity. When the power of input light is high, it is difficult to completely suppress the occurrence of SBS.

Furthermore, since the input pump light and the probe light require a strong peak intensity, it is common to amplify the light from the LD with an optical amplifier such as an EDFA. At this time, the pump light and the probe light that are amplified by the optical amplifier are pulses that repeat in a cycle that reciprocates the entire length of the optical fiber to be measured, and the duty ratio (the ratio of the pulse time width to the pulse repetition time interval) is 0.1. About several percent (10% or less).

For example, in an optical fiber having a length of 10 km, the pulse repetition time interval is 98 μs, and the pulse width is 500 ns. When a pulse having such a low duty ratio and a long repetition period is amplified by an optical amplifier, a phenomenon occurs in which the front end of the pulse is rapidly amplified and the pulse shape is deformed (a pulse surge is generated). This is because, in the erbium-doped fiber used in the optical amplifier, the erbium ions are excited to the upper energy level by the excitation light while the pulse is not transmitted, and then suddenly from the upper level when the pulse is transmitted. This is because a laser transition occurs.

Thus, a pulse in which only the leading part of the pulse is deformed induces SBS at the leading part of the pulse during propagation through the optical fiber. In order to prevent such a phenomenon, there is a method in which the pulse is not amplified in the optical amplifier, and the continuous light from the LD is amplified by the optical amplifier, and then the pulse is shaped by the light intensity modulator.

However, in this case, there is a problem that the intensity of the pulse amplified by the optical amplifier is lowered due to the transmission loss of the optical intensity modulator. In addition, if excessive intensity light is input to the light intensity modulator, there is also a problem that the light intensity modulator is damaged. Further, in this chromatic dispersion distribution measurement method, the intensity of the pump light and the probe light is set so that the intensity ratio is 1: 2, as shown in the number (6) described later. However, it is difficult to set the intensity ratio to 2: 1 over the entire pulse width of a pulse in which a surge occurs at the tip and the peak intensity changes within the pulse width.

As shown in number (6), the extraordinary nonlinear effect is canceled by maintaining the intensity ratio at 2: 1. However, if this condition breaks down, the influence of the nonlinear effect that is not canceled out causes the number (12 An error occurs in the theoretical relationship shown in ().

Since the backscatter idler light intensity at an arbitrary point z in the longitudinal direction of the fiber is proportional to the pulse width and the peak intensity of the pump light and the probe light as shown in the equation (12), the vibration of the backscatter idler light The waveform appears as an effect integrated by the pulse width. Therefore, it is most desirable that the intensity is uniform over the entire width of the pulse, that is, a rectangular pulse shape satisfies the condition of the number (6) in the entire pulse width.

Next, the occurrence of MI will be described. It is known that the phenomenon of MI generation is caused by nonlinear interaction between signal light and noise of surrounding wavelengths, and occurs in an anomalous dispersion optical fiber. In particular, when an EDFA is used to obtain a high output, it often occurs due to a correlation between signal light and ASE noise. In this chromatic dispersion distribution measuring method, the intensity of two wavelengths having different wavelengths is set so that the intensity ratio is 2: 1 as shown in the equation (6).

However, when MI occurs in the optical fiber, the energy spreads as a peripheral wavelength component, and even if the intensity ratio is set to 2: 1 as an input condition, the intensity ratio of light of two wavelengths is 2: 1 at the output end. It will be far away. In such a state, similarly to the problem in the SBS, an error occurs in the theoretical relationship expressed by the equation (12) due to the influence of the non-cancelled nonlinear effect. Since MI is more generated as the length of the optical fiber is longer, the measurement result of the chromatic dispersion distribution deteriorates as the measurement progresses from the pulse input side to the end, and the error increases.

FIG. 26 shows a spectrum at the time of input to and transmission through the optical fiber to be measured when MI occurs during the measurement of an anomalous dispersion optical fiber in the nonlinear OTDR method. In the spectrum at the time of input, although the center wavelength does not change at the time of output with respect to the pump light wavelength 1553.4 nm and the probe light wavelength 1555.4 nm, the wavelength spectrum is broadened. Further, the idler light observed at this time is located at 1551.4 nm. When MI occurs, an error occurs in the theoretical relational expression (12), and the noise level at the observed idler wavelength increases as shown in the transmission spectrum of FIG. 26, and the idler light peak is buried in the noise level. It becomes a state that cannot be identified.

The occurrence of MI depends on the peak intensity of the pump light or probe light. Therefore, in order to suppress the influence, it is necessary to reduce the peak intensity of both or one. However, if the peak intensity is lowered, the generation of idler light to be observed is weakened and the SN ratio is deteriorated, resulting in an increase in measurement error.

When measuring the chromatic dispersion distribution of the anomalous dispersion optical fiber, the above-mentioned problem has been a problem proposal in the paper, measuring method to avoid this problem, Miguel Gonzalez Herraez and Luc Thevenaz, 29 th European Conference of Optical Proposed in Communication 2003, PD Th4.1.5. However, when the method described in this paper is used, in order to avoid the occurrence of MI, a more complicated measurement system configuration is required and there is a problem that workability is poor.

  The present invention has been made to solve the above-mentioned problems, and its purpose is to be applicable to all kinds of optical fibers, and to accurately measure the distribution characteristics in the longitudinal direction of the chromatic dispersion value of the optical fibers. Is to be able to do it.

  In order to solve the above-mentioned problems, a first aspect of the present invention is to provide linearly polarized probe pulse light and pump pulse light having a wavelength different from that of the probe light and having the same polarization state in an optical fiber to be measured. Intensity oscillation of backscattered light of the probe pulse light generated by Rayleigh scattering in the optical fiber to be measured or intensity vibration of backscattered light of idler light generated by a nonlinear phenomenon generated in the optical fiber to be measured. Measure at least one of them, determine the instantaneous frequency of the measured intensity vibration, determine the dependence of the instantaneous frequency on the intensity vibration of the pump light in the longitudinal direction of the optical fiber to be measured, and determine the dependence on the instantaneous frequency. The amount of change in the fiber longitudinal direction between the phase mismatch condition and the nonlinear constant of the optical fiber to be measured is obtained, and the measurement object is based on the amount of change. It is a method for measuring wavelength dispersion value and nonlinearity constant of the optical fiber to identify the fiber longitudinal direction of the distribution for at least one of the wavelength dispersion and the nonlinear coefficient of the fiber.

  According to a second aspect of the present invention, the chromatic dispersion value in the longitudinal direction of the optical fiber to be measured is determined to be positive or negative using the conditions relating to the wavelength and intensity of the pump pulse light and probe pulse light input to the optical fiber to be measured. Is a method for measuring the chromatic dispersion value and the nonlinear constant of the optical fiber.

  According to a third aspect of the present invention, an average value of the intensity vibration of the idler light is measured on the pulse emission side of the optical fiber to be measured, and the idler light with respect to the intensity vibration of the input pump pulse light is measured based on the measurement result. By measuring the dependence of the conversion efficiency and performing a regression analysis on the conversion efficiency using a theoretical function representing the conversion efficiency of idler light that depends on the intensity vibration of the pump light based on the measurement result, the measured optical fiber This is a method for measuring the chromatic dispersion value and nonlinear constant of an optical fiber, which simultaneously specifies the entire average chromatic dispersion value and average nonlinear constant.

  According to a fourth aspect of the present invention, for each end of the measured optical fiber, the distribution value in the fiber longitudinal direction of the chromatic dispersion value and the nonlinear constant is measured, and the two distribution values thus measured are compared. This is a method for measuring the chromatic dispersion value and nonlinear constant of an optical fiber that eliminates the influence of polarization fluctuations in the optical fiber to be measured.

  In a fifth aspect of the present invention, a predetermined amount of data in the vicinity of both ends of the measured optical fiber is deleted from the intensity vibration data of the idler light or probe pulse light, and the intensity vibration data after the deletion is analyzed. This is a method for measuring the chromatic dispersion value and nonlinear constant of an optical fiber.

  According to a sixth aspect of the present invention, there is provided a pump light source that generates pump light, a probe light source that generates probe light, a modulator that pulses the pump light and / or probe light, and the pulsed pump light. And the pulsed probe light to be combined and input to the optical fiber to be measured, and after the pump pulse light and the probe pulse light are input to the optical fiber to be measured, The wavelength of either the backscattered light of the probe pulse light generated in the measured optical fiber or the backscattered light of the idler light generated in the measured optical fiber is selected, and the waveform of the selected scattered light is represented. An optical fiber comprising: a light receiving means for outputting an electric signal; and a calculating means for calculating a distribution value in the fiber longitudinal direction of both the chromatic dispersion value and the nonlinear constant of the optical fiber to be measured from the electric signal waveform. A measuring device for wavelength dispersion and the nonlinear coefficient of the bar.

  A seventh aspect of the present invention is an apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber provided with an optical amplifying means for amplifying the pump pulse light.

  According to an eighth aspect of the present invention, there is provided a branching unit that branches the probe light generated from the probe light source into two lights, and a wave number shift by pulsing the first light of the two branched probe lights. The light receiving means multiplexes the back-scattered light of the probe pulse light output from the acousto-optic modulator and the second light branched by the branching means. For measuring the chromatic dispersion value and nonlinear constant of an optical fiber equipped with a multiplexer.

  According to a ninth aspect of the present invention, the pump pulse light and other light having a wavelength different from that of the probe pulse light are incident on both ends or any one end of the optical fiber to be measured, and the pump in the optical fiber to be measured It is a measuring device for chromatic dispersion value and nonlinear constant of an optical fiber provided with Raman amplification means for Raman amplification of pulsed light, probe pulsed light and idler light.

  According to a tenth aspect of the present invention, there is provided an apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber provided with an intensity measuring device for measuring intensity vibration of idler light on the pulse emission side of the optical fiber to be measured.

  An eleventh aspect of the present invention is an apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber in which a nonlinear optical medium is provided in an optical transmission line between the first multiplexer and the optical fiber to be measured. .

  According to a twelfth aspect of the present invention, the pump light source and the probe light source each generate light having linearly polarized light parallel to each other, and the first multiplexer has two linearly polarized lights parallel to each other. This is a device for measuring the chromatic dispersion value and nonlinear constant of an optical fiber that multiplexes two lights so that their polarization planes of polarization coincide.

  In a thirteenth aspect of the present invention, the pump light source and the probe light source each generate light having linearly polarized light parallel to each other, and the first multiplexer has two linearly polarized lights parallel to each other. Two lights are combined so that their polarization polarization planes coincide with each other, and the second multiplexer includes the back scattered light of the probe light and the back scattered light of the probe pulse light output from the acousto-optic modulator. And a chromatic dispersion value and a nonlinear constant of an optical fiber that multiplexes the second light branched by the branching means in a polarization state parallel to each other.

  In a fourteenth aspect of the present invention, the chromatic dispersion value of the drawn optical fiber and the distribution state of the nonlinear constant in the fiber longitudinal direction are measured using the measurement apparatus described above, and based on the measurement result, the chromatic dispersion value is measured. And a method of manufacturing an optical fiber by cutting out the optical fiber so that either or both of the nonlinear constants are distributed in the longitudinal direction of the fiber within ± 5% of the average value of the total length of the optical fiber.

  In a fifteenth aspect of the present invention, linearly polarized probe light and pump light having a wavelength different from that of the probe light and having the same polarization state are optically amplified independently, and the optical signal waveform after amplification is obtained. It is pulsed by the pulse modulator, and noise components of the pulsed probe pulse light and pump pulse light are removed by an optical filter, and then the probe pulse light and pump pulse light are combined and incident on the optical fiber to be measured. Measure the intensity vibration of backscattered light of idler light caused by nonlinear phenomenon in the measurement optical fiber, determine the instantaneous frequency of the measured intensity vibration, and the amount of change in the phase mismatch condition in the fiber longitudinal direction of the optical fiber to be measured And determining the dependence of the fiber longitudinal direction on the chromatic dispersion value of the optical fiber to be measured based on the amount of change. Method.

  In a sixteenth aspect of the present invention, the intensity of pump pulse light and probe pulse light incident on the optical fiber to be measured and the intensity of pump pulse light and probe pulse light transmitted from the optical fiber to be measured are measured and incident. The intensity of the probe pulse light and the pump pulse light at the time of transmission and the time of transmission are compared, and the intensity ratio of both pulse lights at the time of transmission is within 10% of the intensity ratio of both pulses at the time of incidence. This is a method for measuring the chromatic dispersion value of an optical fiber that adjusts the intensity of pump pulse light and probe pulse light.

  According to a seventeenth aspect of the present invention, the back-scattered light of the pump pulse light and the probe pulse light is observed, and the longitudinal chromatic dispersion value is determined by using the propagation strength in the longitudinal direction of both pulse lights in the optical fiber to be measured. This is a method for measuring the chromatic dispersion value of an optical fiber that performs analysis correction of the distribution.

  According to an eighteenth aspect of the present invention, there is provided a pump light source for generating pump light, pulse modulation means for pulsing the pump light, an optical amplifier for amplifying the intensity of the pump pulse light, and light for removing noise of the pump pulse light. A filter, a probe light source for generating probe light, pulse modulation means for pulsing the probe light, an optical amplifier for amplifying the intensity of the probe pulse light, an optical filter for removing noise of the probe pulse light, and the pump pulse A first multiplexer that combines the light and the probe pulse light and inputs the light to the optical fiber to be measured; and after the combined pump pulse light and the probe pulse light are input to the optical fiber to be measured A light receiving means for selecting a wavelength of backscattered light of idler light generated in the optical fiber to be measured and outputting an electric signal representing a waveform of the selected scattered light; Wherein the electrical signal waveform is a wavelength dispersion measuring apparatus of an optical fiber and a calculation means for calculating the longitudinal distribution value of the chromatic dispersion value of the optical fiber under test.

  In a nineteenth aspect of the present invention, the pulse modulation means for pulsing the pump light and the probe light includes an electric signal generator capable of generating an arbitrary electric signal pulse waveform. It is a chromatic dispersion value measuring apparatus of the described optical fiber.

  According to a twentieth aspect of the present invention, another pump having a wavelength different from that of the pump pulse light and the probe pulse light is incident on both ends or any one end of the optical fiber to be measured, and the pump in the optical fiber to be measured It is a chromatic dispersion value measuring apparatus for an optical fiber provided with Raman amplification means for Raman-amplifying light, probe light and idler light.

  In a twenty-first aspect of the present invention, the light receiving means includes a Fabry-Perot filter for selecting a wavelength of backscattered light of idler light generated in the optical fiber to be measured, and the Fabry-Perot filter is Finesse 2000 or more, This is a chromatic dispersion value measuring device for an optical fiber having a transmission full width at half maximum of 5 GHz or less and a characteristic in which an intensity ratio between transmitted light and blocked light is 40 dB or more.

  According to a twenty-second aspect of the present invention, there is provided a dispersion distribution measuring method for measuring a distribution state of a chromatic dispersion value in a longitudinal direction of an optical fiber to be measured, wherein the waveform data of an optical signal propagating in the optical fiber is divided into a plurality of regions This is a dispersion distribution measurement method for measuring the dispersion state by extracting each waveform data, nonlinearly fitting each of the extracted waveform data into a sine function, and calculating the dispersion value of each extracted waveform data.

  According to a twenty-third aspect of the present invention, there is provided a method for compensating for a measurement error that occurs when measuring a distribution state of chromatic dispersion values in the longitudinal direction of an optical fiber to be measured, the vibration intensity of input pump light, and input signal light. Is a measurement error compensation method that compensates for a measurement error using the vibration intensity, the measurement condition of the wavelength interval of the two input lights, the nonlinear constant of the optical fiber to be measured, and the vibration intensity of the idler light to be measured. .

  According to a twenty-fourth aspect of the present invention, there is provided a measurement condition specifying method for specifying a measurement condition to be set when measuring a distribution state of chromatic dispersion values in a fiber longitudinal direction of the optical fiber to be measured. The measurement condition specifying method is characterized in that the measurement condition is specified based on a chromatic dispersion value of a fiber and a nonlinear constant.

  In a twenty-fifth aspect of the present invention, the distribution state in the longitudinal direction of the chromatic dispersion value of the drawn optical fiber is measured using the measurement apparatus described above, and the distribution is performed in the longitudinal direction of the optical fiber based on the measurement result. In this optical fiber manufacturing method, the optical fiber is cut so that the chromatic dispersion value is within ± 5% of the average value of the total length of the optical fiber.

An object of the present invention is to make it possible to measure the distribution values of both the chromatic dispersion value and the nonlinear constant in the longitudinal direction of the optical fiber by utilizing the nonlinear optical effect in the measured optical fiber.

  Further, the present invention can accurately measure the distribution characteristic in the longitudinal direction of the chromatic dispersion value of the optical fiber. Furthermore, since an appropriate analysis method can be adopted for fibers having various dispersion characteristics in the 1.55 μm band, it is possible to cope with various dispersion fibers. For this reason, a great contribution is expected in designing a transmission line such as a dispersion compensation line and optical soliton communication used for repeaterless long-distance transmission with strong light intensity using an erbium-doped fiber amplifier, and its utility value is great.

  First, the measurement concept of the nonlinear OTDR method performed using a nonlinear coupled mode equation indicating the FWM generation process will be described.

In general, nonlinear coupled mode equations (Equation 1) to (Equation 3) are used as equations for describing degenerated four wave mixing (DFWM).

なお、周波数的に位相整合条件は、

を満たしているものとする（ω_p、ω_s、ω_cはそれぞれポンプ光、プローブ光、アイドラー光の角周波数）。 Here, p, s, and c represent pump light, probe light, and idler light, respectively. α represents the degree of loss of the optical fiber, gamma nonlinear constant _{(γ = 2π / λ p ·} n 2 / A eff, n 2; nonlinear refractive index, A _eff; effective core area) is. Δβ is a phase mismatch condition of the propagation constant and is shown in Equation (4).

The phase matching condition in terms of frequency is

(Ω _p , ω _s , and ω _c are angular frequencies of pump light, probe light, and idler light, respectively).

この数（6）の条件を保つことにより、SPMやXPMの効果を打ち消し、上記条件（2）の仮定を成立させることになる。 Since it is difficult to obtain an exact solution of the nonlinear coupled mode equations of the numbers (1) to (3), the analysis solution conditions (1) and (2) are approximated as follows.
(1) Transmission loss is not affected in the FWM generation process. (2) The effect of SPM and XPM is not considered. At this time, input pump light intensity P _p (z = 0) and input probe light intensity P _s (z = 0) are It is necessary to maintain the relationship as the following number (6).

By maintaining the condition of this number (6), the effect of SPM and XPM is canceled and the assumption of the condition (2) is established.

ここで、Ｅp；波長λｐ（ポンプ光）電界、Ｅｓ；波長λｓ（プローブ光）電界、Ｅｃ；アイドラー光電界、Ｐ_ｐ ^０；入力波長λｐ強度、Ｐ_ｓ ^０；入力波長λｓ強度、Δλ；入力二波長間隔である。 The derivation of the equation considering the above conditions (1) and (2) is as follows.

Here, Ep: wavelength λp (pump light) electric field, Es: wavelength λs (probe light) electric field, Ec: idler light electric field, P _p ⁰ ; input wavelength λp intensity, P _s ⁰ ; input wavelength λs intensity, Δλ: input Two wavelength intervals.

Accordingly, the electric field and light intensity of idler light at the position in the longitudinal direction z are as follows.

なお、Ｒ；レーリー散乱係数、α；損失係数、Ｄ；波長分散値である。 Here, considering the transmission loss of the pump light, the probe light, and the idler light, the intensity of the backscattered light of the idler light that is propagated through the fiber distance z and received is expressed by the following equation.

Here, R: Rayleigh scattering coefficient, α: loss coefficient, D: chromatic dispersion value.

ここで、λ=2πc／ωを用いると、数（13）は、

と表すことができる。そして得られる波長分散値Ｄは、数（15）に示すものとなる。

なお、f(t)；tにおけるアイドラー光の後方散乱光の波形の瞬時周波数である。 Here, the relationship between the phase mismatch condition Δβ and the chromatic dispersion value D at the pump light wavelength is expressed by the following equation.

Here, using λ = 2πc / ω, the number (13) becomes

It can be expressed as. The obtained chromatic dispersion value D is shown in the equation (15).

Note that f (t); is the instantaneous frequency of the waveform of the backscattered light of idler light at t.

In the conventional nonlinear OTDR method described above, the influence of nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM) generated in an optical fiber is not considered. In this embodiment, in order to obtain the nonlinear constant of the optical fiber, a measurement system that gives these nonlinear effects is constructed, the influence of the nonlinear effect in the longitudinal direction of the fiber in the measurement system is measured, and the longitudinal direction of the fiber is determined from the measurement result. The chromatic dispersion value and the nonlinear constant distribution value for are simultaneously obtained.

Here, a solution derived by Stolen and Bjorkholm (hereinafter referred to as the SB solution, R. Stolen and JEBjorkholm, J. Ouantum) is used as a solution when the nonlinear effects of the pump light on SPM and XPM probe light and idler light cannot be ignored. Electron., QE-18, p.1062, 1982 for details). A characteristic of the SB solution is that nonlinear effects such as SPM and XPM by pump light are incorporated. This SB solution holds when the pump light is very strong.

However, this SB solution does not consider the transmission loss of the optical fiber. The basic equations for the SB solution are shown below in Equations (16) to (18).

ここで、

である。なお、P_s ⁰は入射プローブ光強度を、P_p ⁰は入射ポンプ光パワーを表す。 Further, the idler light P _c (z) after propagation of the distance z in this case is expressed as follows.
(Case A)
When Δβ <0 (anomalous dispersion) and P _p ⁰ > −Δβ / 4γ, the idler light is expressed by Equation (19) and the probe light is expressed by Equation (20).

here,

It is. P _s ⁰ represents the incident probe light intensity, and P _p ⁰ represents the incident pump light power.

ここで

である。 (Case B)
In the case of Δβ <0 (abnormal dispersion) and P _p ⁰ <−Δβ / 4γ, or Δβ ≧ 0 (normal dispersion), the idler light is shown in Equation (22) and the probe light is shown in Equation (23). It is.

here

It is.

According to the numbers (19), (20), (22), and (23), the intensity Pc (z) and Ps (z) of idler light and probe light propagating in the longitudinal direction of the fiber are Δβ, γ, and pump light. It can be seen that g _a and g _b change depending on the intensity Pp. Therefore, as Mollenaure et al. Found Δβ from g _a and g _b by focusing on g _a and g _b of idler backscattered light in number (12), numbers (19), (20), and numbers (22), ( In 23), when the intensity of the backscattered light of the idler light or the probe light is measured and g _a and g _b with respect to the distance z are obtained, information on the dispersion value and the nonlinear constant can be obtained from the g _a and g _b .

By the way, the transmission loss of the optical fiber is not considered in the SB solution. Since g _a and g _b of the probe light or idler light depend on the intensity of the pump light, the transmission losses g _a and g _b change. Therefore, transmission loss must be considered when trying to obtain accurate values of Δβ and γ. The following two methods are conceivable as methods for incorporating the effect of transmission loss into the numbers (19) to (24).

で表される。
（Ｃ２のケース）Δβ＜０（異常分散）かつP_p ⁰＜−Δβ/4γの場合、またはΔβ≧0（正常分散）の場合、

で表される。 (Case C)
The pump light intensity at each point in the longitudinal direction of the fiber is calculated from the input pump light intensity and the fiber loss coefficient d, and the values are taken to calculate g _a and g _b . In this case, in each of the above two cases,
(C1 case) When Δβ <0 (abnormal dispersion) and P _p ⁰ > −Δβ / 4γ,

It is represented by
(C2 case) When Δβ <0 (abnormal dispersion) and P _p ⁰ <−Δβ / 4γ, or when Δβ ≧ 0 (normal dispersion),

It is represented by

で表される。
（Ｄ2のケース）Δβ＜０（異常分散）かつP_p ⁰＜−Δβ/4γの場合、またはΔβ≧0（正常分散）の場合、

で表される。 (Case D)
The pump light intensity P _p (z) at each point in the longitudinal direction of the fiber is measured by measuring the backscattered light of the pump light, and the values are taken to calculate g _a and g _b . Also in this case, it is shown as follows due to the difference between the two conditions as in the case C.
(D1 case) When Δβ <0 (abnormal dispersion) and P _p ⁰ > −Δβ / 4γ,

It is represented by
(D2 case) When Δβ <0 (abnormal dispersion) and P _p ⁰ <−Δβ / 4γ, or when Δβ ≧ 0 (normal dispersion),

It is represented by

  In the case of an optical fiber having a large loss factor α or a long strip length and a large total loss over the entire length, it is effective to amplify pump light, probe light, and idler light by Raman amplification from the front and rear. . In such a case, since the intensity change in the longitudinal direction of the pump light is not a decrease characteristic proportional to the fiber loss coefficient, it is effective to use the measured value as in the case D.

Two solutions of the numbers (19), (20), or the numbers (22), (23) can be obtained from the conditions for Δβ and P _p , any of which may be used. However, the numbers (22) and (23) are effective for evaluating the chromatic dispersion value. This is because the dependence of g _b on the intensity vibration of pump light differs between the case of anomalous dispersion (Δβ <0) and the case of normal dispersion (Δβ> 0). In other words, because of the difference between positive and negative Δβ, the change in g _b observed when the pump light intensity is increased is decreased for abnormal dispersion (Δβ <0) and increased for normal dispersion (Δβ> 0). Therefore, by measuring the increase / decrease in g _b with respect to the intensity of the pump light, it is possible to determine whether the chromatic dispersion value of the optical fiber is positive or negative. This advantage cannot be obtained with existing measurement methods.

  Next, with reference to FIG. 1, the measuring apparatus 100 in a present Example is demonstrated. The measuring apparatus 100 is for simultaneously measuring the chromatic dispersion value and the nonlinear constant distribution value in the longitudinal direction of the optical fiber to be measured.

  The light emitted from the pump light source 1 is phase-modulated by the phase modulator 2. A sine wave electric signal having a frequency of around 100 MHz output from the signal generator 3 is input to the phase modulator 2 via the electric signal amplifier 4. The phase modulation is performed to suppress stimulated Brillouin scattering (SBS) generated in the optical fiber 5 to be measured. If the intensity of the pump light incident on the optical fiber 5 to be measured is lower than the SBS threshold, there is no need for phase modulation.

Next, it is pulsed by the intensity modulator 6. The period f of the pulse applied to the intensity modulator 6 is determined by the time taken by the propagating pulse to reciprocate the length of the optical fiber 5 to be measured, and f = c / 2nL (L: fiber length, n: fiber refraction) Rate). The duty ratio of the pulse is about several percent. The optical pulse output from the intensity modulator 6 is amplified by the EDFA 7, and the amplified pump light passes through the band pass filter 8 and the polarization controller 9 and enters the coupler 10.

On the other hand, the probe light generated from the probe light source 11 is pulsed by the intensity modulator 12 similarly to the pump light, and is combined with the pump light by the coupler 10. At this time, adjustment is performed using the delay line 13 and the polarization controller 14 in order to match the timing of the two pulses (pulses of the pump light and the probe light) with the polarization state. The combined light passes through the circulator 15 and enters the optical fiber 5 to be measured. Here, the end of the optical fiber 5 to be measured is connected to the non-reflection end 16.

Probe light or idler light backscattered light from the optical fiber 5 to be measured passes through the circulator 15 and is converted into an electrical signal by the O / E converter 20 via the optical filter 17, EDFA 18, and optical filter 19. This electric signal is measured by the oscilloscope 21. The oscilloscope 21 receives a trigger signal output from the pulse generator 22. The intensity of the incident pump light and probe light is measured by the optical power meter 131 at the output port of the circulator 15.

The electric pulse signal necessary for pulsing the pump light and the probe light is generated by the pulse generator 22, branched into two, amplified by the electric signal amplifiers 23 and 24, and further supplied by the DC power source. The DC voltage components output from 25 and 26 are combined and input to the two intensity modulators 12 and 6, respectively. The electrical signal input to the oscilloscope 21 is digitized and input to the computer 124.

  Here, the pump light wavelength λp, its input intensity, and the probe light wavelength λs are appropriately set according to the characteristics of the optical fiber 5 to be measured. Details of this wavelength setting will be described below. The reason why this wavelength setting is necessary is that the above-mentioned measurement / analysis conditions (SB solution conditions) must be satisfied.

  Here, a specific measurement procedure using the measuring apparatus 100 will be shown. Here, an analysis method using the numbers (22), (23) and (24) from which positive / negative information of the chromatic dispersion value can be obtained will be described. An important point in this measurement is that the measurement conditions satisfying the above-described conditional expressions are set in accordance with the chromatic dispersion value and the nonlinear constant of the optical fiber 5 to be measured. Moreover, when setting such measurement conditions, it is also essential that the measurement conditions fall within the capability range of the measurement device 100 that is actually used.

First, the flow from setting the measurement conditions to finally calculating the analysis data is shown below.
(Measurement 1) Measurement conditions (see below) are set in consideration of the characteristics of the optical fiber 5 to be measured and the measuring instrument 100.
(Measurement 2) The backscattered light of the pump light is measured using the measuring device 100, and the intensity of the pump light in the fiber longitudinal direction is acquired.
(Measurement 3) The waveform of each backscattered light of the probe light or idler light is measured using the measuring device 100.
(Measurement 4) The processes of the above measurements 2 and 3 are repeated by changing the intensity of the input pump light a plurality of times.
(Measurement 5) using the measuring instrument 100, obtains respectively g _b in the longitudinal fiber direction points from the waveform of the probe light or idler light.

Then, each g _b value (a plurality of g _b values) with respect to the intensity of the pump light at each point in the fiber longitudinal direction obtained in the measurement 5 is substituted into the number (28) to obtain Δβ and γ.

Details of measurement 1 will be described below.
The setting of measurement conditions varies greatly depending on the characteristics of the optical fiber to be measured. Therefore, first, the setting of measurement conditions corresponding to the type of the optical fiber 5 to be measured will be examined.

  In setting the measurement conditions, it is assumed that the values of the length of the optical fiber 5 to be measured, the average dispersion value, and the nonlinear constant (average value over the entire length) are known in advance. The approximate values of the nonlinear constant and the average chromatic dispersion value are generally known depending on the type of optical fiber to be measured.

Therefore, the minimum information necessary for setting the measurement conditions is the type of the measured optical fiber 5 and the value of the length of the measured optical fiber 5. As an example, Table 1 shows typical types of currently known 1.55 μm band single mode optical fibers, their chromatic dispersion values, and general characteristics related to nonlinear constants.

  The measurement conditions differ depending on whether Δβ is positive or negative (whether the chromatic dispersion value is normal or abnormal). Therefore, a procedure for setting the measurement condition for each positive and negative of Δβ is illustrated.

Condition setting procedure 1: Anomalous chromatic dispersion value (Δβ <0).
1-1. Setting of distance resolution Determine the frequency per 1km of the measurement waveform (number of waves; g _b (km ^-1 ) / π) as 2 to 10 (when P _p = 0W).
At Pp = 0, from Δβ = −2g _b , Δβ (km ⁻¹ ) = − 4π to −20π (≡Δβ _max )

1-2. Setting the upper limit of input pump light intensity The intensity P _pmax that satisfies the critical condition of pump light is obtained from the nonlinear constant γ.
P _pmax = −Δβ _max / 4γ
Compare Ppmax with the upper limit value P _pmeasure of the measuring device.
a) P _pmeasure ≤ P _pmax ⇒ Setting upper limit value P _phigh = P _pmeasure
b) P _pmeasure > P _pmax ⇒ Setting upper limit value P _phigh = P _pmax

1-3. Determination of Δβ Using the number (13), the two-wavelength interval Δλ between the pump light and the probe light with respect to Δβ obtained in 1-1. Above is obtained.
a) Δλ ≦ Δλ _max (; measuring device limit) ⇒Δλ determination
b) Δλ> Δλ _max ⇒ Δλ = Δλ _max (Δβ is newly determined)

1-4. Determination of the input pump light intensity variable region The vibration amplitude component A of the lobe light or idler light; A = 4γ ² P _p ² / Δβ (Δβ + 4γP _p ), and the pump light region that is a measurable value is determined. To do.

The measurable limit value A _lim is determined depending on the reception sensitivity of the measuring instrument used. If the pump light intensity at which A is A _lim is P _pmin , the pump light intensity variable region is
P _pmin ≦ P _p ≦ P _phigh
It becomes.

1-5. Confirmation of g _b for probe light or idler light to be measured Using the number (23), obtain g _b of the probe light or idler light waveform in the pump light region obtained in 1-4 above.
g _b (P _p = P _phigh ) ≦ g _b ≦ g _b (P _p = P _pmin )

Condition setting procedure 2: Normal chromatic dispersion value (Δβ> 0).
2-1. Setting of distance resolution The frequency (number of waves; g _b (km ^-1 ) / π) per 1 km of the measured waveform is defined as 2 to 10 (when P _p = 0 W).
Δβ = 4π ~ 20π (≡Δβ _max )

2-2. Determination of Δβ The two-wavelength interval Δλ between the pump light and the probe light is obtained from Δβ determined using the number (13).
a) Δλ ≦ Δλ _max (; measuring device limit) ⇒Δλ determination
b) Δλ> Δλ _max ⇒ Δλ = Δλ _max

2-3. Determination of Variable Input Pump Light Intensity Area A pump light area in which the vibration amplitude component A of probe light or idler light; A = 4γ ² P _p ² / Δβ (Δβ + 4γP _p ) is measurable is determined.

The measurable limit value A _lim is determined depending on the reception sensitivity of the measuring instrument used.
If the pump light intensity at which A is A _lim is P _pmin , the pump light intensity variable region is
P _pmin ≦ P _p ≦ P _phigh
It becomes.

2-4. Using g _b confirmation number (23) in the measured probe light or idler light, obtains a g _b of the probe light or idler light wave in the pumping light region obtained above 2-3..
g _b (P _p = P _pmin ) ≦ g _b ≦ g _b (P _p = P _phigh )

Condition setting procedure 3: For optical fibers where the chromatic dispersion value cannot be determined, especially because the average chromatic dispersion value is close to zero, such as DSF optical fibers, etc. In the case of an optical fiber that changes in the longitudinal direction, or in the case of an optical fiber that is intentionally adjusted in the longitudinal direction of the optical fiber (for example, a dispersion-decreasing optical fiber), the positive / negative of the chromatic dispersion value should be It cannot be decided clearly.

In such a case, it is considered effective to set the measurement conditions on the assumption that the optical fiber is an anomalous dispersion optical fiber and correct the set value during the experiment as shown below. This is because, in the case of an anomalous dispersion optical fiber, the critical factor (P _p (0) <− Δβ / 4γ) of the pump light exists, so that the limiting factor increases.

On the other hand, it is not necessarily the case that the normal dispersion optical fiber is easier to set measurement conditions than the anomalous dispersion optical fiber. In this case, the vibration amplitude components of the probe light and idler light have the same absolute value of the chromatic dispersion value and when compared with anomalous dispersion optical fibers having different signs. Becomes smaller and weaker. Therefore, it is necessary to correct the value of Δβ (Δλ) to be smaller from the condition obtained by assuming an anomalous dispersion optical fiber, and increase the vibration amplitude component of the probe light and idler light to widen the measurable area. come. Hereinafter, the procedure for setting the measurement conditions in the case of an optical fiber in which the chromatic dispersion value cannot be determined will be described.

3-1. Setting of distance resolution The frequency per 1 km of the measurement waveform (number of waves; g _b (km ^-1 ) / π) is defined as 2 to 10 (when P _p = 0W).
Δβ = 4π ~ 20π (≡Δβmax)

3-2. Setting the upper limit of input pump light intensity The intensity P _pmax that satisfies the critical condition of pump light is obtained from the nonlinear constant γ.
P _pmax = -Δβ _max / 4γ
Compare P _pmax with the upper limit value P _pmeasure of the measuring device.
a) P _pmeasure ≤ P _pmax ⇒ Set upper limit value P _phigh = P _pmeasure
b) P _pmeasure > P _pmax ⇒ Setting upper limit value P _phigh = P _pmax

3-3. Determination of Δβ Using the number (13), the two-wavelength interval Δλ between the pump light and the probe light with respect to Δβ obtained from the above 3-1.
a) Δλ ≦ Δλ _max (; measuring device limit) ⇒Δλ determination
b) Δλ> Δλ _max ⇒Δλ = Δλ _max (Δβ is newly determined)

3-4. Determination of Input Pump Light Intensity Variable Region A pump light region where the vibration amplitude component A of probe light or idler light; A = 4γ ² P _p ² / Δβ (Δβ + 4γP _p ) is a measurable value is determined.

The measurable limit value A _lim is determined depending on the reception sensitivity of the measuring instrument used. If the pump light intensity at which A is A _lim is P _pmin , the pump light intensity variable region is
P _pmin ≦ P _p ≦ P _phigh
It becomes.

3-5. Confirmation of g _b in probe light or idler light to be measured Using the number (23), obtain g _b of probe light or idler light in the pump light region obtained in 3-3.
For anomalous dispersion g _b (P _p = P _phigh ) ≦ g _b ≦ g _b (P _p = P _pmin )
For normal dispersion g _b (P _p = P _pmin ) ≦ g _b ≦ g _b (P _p = P _phigh )

The measurement conditions for the optical fiber 5 to be measured are set by the measurement condition setting procedures 1 to 3 described above. Tables 2 and 3 show the results. FIGS. 8 and 9 show the measurable region for RDF (Reversed Dispersion Fiber) and HNLF (High NonLinearity Fiber) (D = 4 ps / nm / km). Here, the reception sensitivity of the measuring device 100 is shown as A _lim = 0.1. Since A _lim is determined according to the device characteristics of the measuring instrument 100, the above measurement conditions do not limit this patent.

When setting the maximum input intensity of pump light, it is necessary to consider the effect of stimulated Brillouin scattering (SBS). When SBS occurs, even if the input intensity of the pump light is increased, the light intensity when propagating through the optical fiber is limited, and it is difficult to accurately measure the intensity dependence of the pump light. Therefore, it is necessary to set the input intensity below the SBS threshold value of the optical fiber.

Since the SBS threshold varies depending on the characteristics (nonlinear characteristics, etc.) of the optical fiber 5 to be measured and the length of the fiber, it is necessary to confirm that no SBS is generated in advance on the optical fiber 5 to be measured. . In order to increase the SBS threshold and increase the input intensity of the pump light, it is effective to widen the spectral line width of the pump light by a method such as phase modulation.

After the measurements 1 to 4, seek g _b at each point in the fiber longitudinal direction of the optical fiber to be measured from the intensity-waveforms of the backscattered light resulting probe light or idler light (measurement 5), the value Δβ and γ are obtained from g _b and the intensity of the pump light.

First, g _b is obtained. The sinusoidal waveform g _b can be obtained by a method similar to the method shown by LFMollenauer, et.al. In this method, first, band-pass filter processing is performed on the acquired measurement data, and frequency components other than signal frequency components are removed. Next, an imaginary component of the measurement data is calculated by performing FFT (Fast Fourier Transform) and further performing IFFT (Inverse Fast Fourier Transform) by replacing the negative frequency with zero.

Subsequently, the phase angle at each point in the fiber longitudinal direction is calculated from the real component and the imaginary component. The phase angle is a value corresponding to g _b. In this way, the value of g _b at each point in the fiber longitudinal direction for each input pump light intensity is obtained.

Next, Δβ and γ are obtained using the plurality of g _b obtained above. First, the value of 4 g _b ² is plotted against the pump light intensity at each point in the fiber longitudinal direction. Fitting this plot with a straight line, find the intercept and slope. As shown in Formula (24), said sections represent [Delta] [beta] ^2. At this time, if the slope of the straight line is positive, the dispersion characteristic of the optical fiber to be measured at that point is normal dispersion, and conversely, if the slope is negative, it is abnormal dispersion. Since the absolute value of the slope is 4Δβγ, γ can be obtained by using the value of Δβ obtained from the intercept. By performing this operation over the entire fiber longitudinal direction, it is possible to know the chromatic dispersion value and the distribution state of both positive and negative and nonlinear constants in the longitudinal direction.

  Further, the backscattered light waveforms of the pump light, the probe light, and the idler light to be measured may be reflected at the connecting portions at both ends of the optical fiber to be measured, and a peak may be generated. Furthermore, when measuring idler light, the waveform distortion at the rising edge of the waveform on the incident side has an effect of causing an error during FFT analysis. For this reason, the calculation error can be suppressed by deleting a certain portion at both ends of the obtained waveform.

  Here, FIG. 10 shows an example of a waveform of backscattered light of idler light measured when the intensity of pump light input to the fiber is changed when a highly nonlinear fiber is used as the optical fiber to be measured. 11 and 12 show the chromatic dispersion value calculated using the above analysis method from the values shown in FIG. 10 and the distribution value of the nonlinear constant in the fiber longitudinal direction.

  Next, an optical fiber manufacturing method for manufacturing an optical fiber based on a measurement method for the chromatic dispersion value and nonlinear constant in the fiber longitudinal direction using the above-described measuring apparatus 100 will be described with reference to FIG.

  First, the refractive index distribution of the preform of the prepared optical fiber (step S1) is measured with a preform analyzer (step S2). From the result, the drawing conditions (drawing force, speed, temperature of the melted part, etc.) are determined (step S3). Then, the base material is drawn, and the obtained optical fiber is measured for the chromatic dispersion value and the non-linear constant distribution value in the fiber longitudinal direction using the measuring apparatus 100 described above (step S4).

Depending on the evaluation results, either the nonlinear constant, the chromatic dispersion value, or the distribution state of the both in the longitudinal direction of the fiber is partially included in the fluctuation within ± 5% of the average value over the entire length of the optical fiber. Cut out (step S5). As described above, in this embodiment, the chromatic dispersion value and the nonlinear constant of the optical fiber are measured in the longitudinal direction of the fiber, and the step of cutting out the portion where the nonlinear constant and the chromatic dispersion value have the optimum values is performed. In addition, an optical fiber having a chromatic dispersion value and a nonlinear constant more uniform and stable than the conventional one is manufactured (step S6).

  As described above, according to the measuring apparatus 100 in the present embodiment, it is possible to simultaneously measure both the nonlinear constant and the chromatic dispersion value of the optical fiber in the fiber longitudinal direction. Further, since an appropriate analysis method is used for optical fibers having various dispersion values in the 1.55 μm band, it is possible to cope with optical fibers having various wavelength dispersion values. Furthermore, by using the above measuring method in the fiber manufacturing process, an optical fiber having uniform and stable characteristics in the fiber longitudinal direction can be obtained. Therefore, a great contribution is expected in designing a transmission line such as a dispersion compensation line and optical soliton communication used for repeaterless long distance transmission with strong light intensity using an erbium-doped fiber amplifier, and its utility value is great.

<About other embodiments>
Next, instead of the measuring device 100, other measuring devices may be used to measure the distribution values of the chromatic dispersion value and the nonlinear constant in the fiber longitudinal direction of the optical fiber 5 to be measured. Hereinafter, other measurement apparatuses 101 to 106 other than the measurement apparatus 100 will be described with reference to FIGS.

  First, the measuring apparatus 101 shown in FIG. 2 will be described. The measuring apparatus 101 has a configuration for receiving probe light using a heterodyne detection method in order to improve sensitivity on the light receiving unit side. In this case, the probe light output from the probe light source 11 is bifurcated by a tap coupler 125, and one light is pulsed by an AO (acousto-optic) modulator 126. At this time, the probe light is frequency-shifted by the drive frequency.

The other light is input to the 3 dB coupler 28 installed on the light receiving unit side via the polarization controller 27. Thereafter, backscattered light of idler light from the measured optical fiber 5 is input to the 3 dB coupler 28 via the circulator 15, the optical filter 17, and the polarization controller 128. Therefore, in the 3 dB coupler 28, the light bifurcated by the tap coupler 125 and the backscattered light of idler light are combined.

At this time, the polarization states of the two lights are adjusted by the polarization controllers 27 and 128 so as to coincide with each other. The combined light is bifurcated and received by the double balanced photodetector 29. The electric signal output from the double-balanced photodetector 29 is amplified by the electric amplifier 30 and then the baseband is a frequency signal corresponding to the drive frequency of the AO modulator 126 output from the signal generator 32 by the mixer 31. And sent to the oscilloscope 21 through the low-pass filter 33. The oscilloscope 21 digitizes the input electrical signal and inputs it to the computer 124. The computer 124 performs a mean square process on the input digital data.

Next, the measuring apparatus 102 shown in FIG. 3 will be described. The measuring device 102 makes the pump light, the probe light, and the probe light within the measured optical fiber 5 by making third light having a wavelength different from that of the pump light and the probe light enter from both ends or any one end of the measured optical fiber 5. It has a structure for Raman amplification of the intensity of idler light. Raman amplification is performed by Raman-amplified light that is input to both ends of the optical fiber 5 to be measured from two Raman amplification light sources 35 and 36 via two WDM couplers 37 and 38 (here, WDM coupler 38). May be a circulator). When pump light and probe light are used near 1550 nm, light having a wavelength near 1450 nm is incident. Such Raman amplification is effective when the length of the optical fiber to be measured is long and when the transmission loss is large.

Next, the measuring apparatus 103 shown in FIG. 4 will be described. The measuring apparatus 103 includes an optical spectrum analyzer 39 on the pulse emission side of the optical fiber 5 to be measured. With this optical spectrum analyzer 39, the light intensity Pc (z) of idler light after passing through the optical fiber 5 to be measured can be measured. The idler conversion efficiency G _C (= Pc (z) / Ps (0)) is obtained from the measured value of the light intensity Pc (z). Considering the same case as the measurement condition, this conversion efficiency is expressed by the following equation for each of cases 1 and 2 below.

ケース２：Δβ＜０（異常分散）かつP_p ⁰＜−Δβ/4γの場合、またはΔβ≧0（正常分散）の場合、数(30)で表される。

Case 1: In the case of Δβ <0 (anomalous dispersion) and P _p ⁰ > −Δβ / 4γ, it is expressed by the number (29).

Case 2: When Δβ <0 (abnormal dispersion) and P _p ⁰ <−Δβ / 4γ, or Δβ ≧ 0 (normal dispersion), it is expressed by the number (30).

The number (29), will change depending As can be seen from (30), the conversion efficiency G _C idler light in the light intensity P _P of the pumping light. Δβ and γ contribute to the change. When determining the change of conversion efficiency G _C idler light after the measured optical fiber 5 transmission when began to strongly light intensity of the input pump beam at Therefore experiments, the wavelength dispersion value and nonlinearity constant of the measured value Will be obtained. Therefore, two parameters Δβ and γ can be determined by performing regression analysis using the numbers (29) and (30) as a setting that satisfies the conditions of the above cases 1 and 2. From the values of Δβ and γ, the average value of the chromatic dispersion value and the nonlinear constant over the entire length of the optical fiber 5 to be measured is obtained.

  In general, polarization mode dispersion exists in an optical fiber. Therefore, even if the polarization states of the pump light and the probe light match at the light incident end of the optical fiber 5 to be measured connected to the circulator 15, both light beams are transmitted in the process of propagating through the optical fiber 5 to be measured. The polarization state changes. Therefore, when measuring the optical fiber 5 to be measured having a long strip length or the optical fiber 5 to be measured having a large polarization mode dispersion value, the intensity of the measured idler light and probe light is measured near the fiber end of the optical fiber 5 to be measured. An error occurs between Pc (z) and Ps (z) and the functions shown in the numbers (19), (20) and the numbers (22), (23).

In such a case, one end of the optical fiber 5 to be measured is connected to a circulator, and after measuring the chromatic dispersion value and the distribution value of the nonlinear constant in the fiber longitudinal direction, the optical fiber to be measured that was terminated at this time The other fiber end of 5 is reconnected to the circulator 15, and the chromatic dispersion value and the non-linear constant distribution value in the fiber longitudinal direction are measured in the same manner. In this way, the pump light and the probe light are incident from both ends of the measured optical fiber 5, the same measurement is performed, and the two measurement results are compared. Thereby, data with less influence of polarization mode dispersion can be selected, and the influence of polarization fluctuation in the optical fiber can be removed.

  Next, the measuring apparatus 104 shown in FIG. 5 will be described. The measuring device 104 includes a nonlinear optical medium 40 between the coupler 10 that combines the pump light pulse and the probe light pulse and the optical fiber 5 to be measured. As described above, particularly when measuring the backscattered light of idler light, the intensity Pc (z) (z is near zero) of the idler light suddenly rises and is measured at the pulsed light incident part of the optical fiber 5 to be measured. Further, there is a large error between Pc (z) (z is near zero) and the analytical solution Pc (z) (z is near zero) represented by the numbers (19) and (22). Therefore, by providing a nonlinear optical medium 40 that generates weak idler light by a moderate nonlinear effect before entering the optical fiber 5 to be measured, an optical waveform with less error is obtained at the pulsed light incident portion of the optical fiber 5 to be measured. It is done. As the nonlinear optical medium 40, it is effective to use a highly nonlinear optical fiber at a short distance.

  Next, the measuring apparatus 105 shown in FIG. 6 will be described. The measuring device 105 is an optical transmission path of two pulse lights of pump light and probe light (in the case of pump light, the pump light source 1, the phase modulator 2, the intensity modulator 6, the EDFA 7, the band pass filter 8, the coupler 10, and the circulator. In the probe light, the polarization state of each light pulse in the probe light source 11, the intensity modulator 12, the delay line 13, the coupler 10, and the circulator 15). Are all polarized so that the phase modulator 2, the intensity modulator 6, the EDFA 7, the band pass filter 8, the intensity modulator 12, the delay line 13, the coupler 10, the circulator 15, etc. Components having wave holding characteristics (in particular, phase modulator 2a, intensity modulator 6a, EDFA 7a, bandpass filter 8 Configured to include the intensity modulator 12a, the delay line 13a, the coupler 10a, optical fiber transmission path connecting expressed as circulators 15a) and these (optical fiber connecting the respective units), and the like.

From the pulse light source unit (pump light source 1 and probe light source 11) of this measuring device 105, two-wavelength pulsed light is emitted so as to coincide with the polarization plane of polarization with linear polarization, and is incident on the measured optical fiber 5. With such a configuration, it is possible to prevent a decrease in the effect of FWM due to polarization fluctuations.

  Next, the measuring apparatus 106 shown in FIG. 7 will be described. The measuring device 106 uses an optical heterodyne detection system on the receiving side (see FIG. 2), and further, an optical transmission path of two pulse lights of pump light and probe light (in the pump light, the pump light source 1 and the phase modulator). 2, an optical transmission path through the intensity modulator 6, the EDFA 7, the band pass filter 8, and the coupler 10. In the probe light, the probe light source 11, the AO modulator 126, the delay line 13, the coupler 10, the optical filter 17, Phase modulator 2 and intensity modulation so as to keep the polarization state of each optical pulse as linearly polarized light having parallel polarization planes in a 3 dB coupler 28, tap coupler 125, circulator 15, etc. 6, EDFA 7, band pass filter 8, coupler 10, AO modulator 126, delay line 13, coupler 10, circulator 15, The filter 17, the 3dB coupler 28, and the tap coupler 125 are all components having polarization maintaining characteristics (in particular, the phase modulator 2a, the intensity modulator 6a, the EDFA 7a, the bandpass filter 8a, the coupler 10a, the AO modulator 126a, the delay A line 13a, a circulator 15a, an optical filter 17a, a 3dB coupler 28a, a tap coupler 125a, and the like), and an optical fiber transmission line (optical fiber connecting the above-described parts) connecting them.

Therefore, on the light receiving unit side using the optical heterodyne detection method, the polarization state of the light generated from the two pulse light source units (probe light source 11) and the backscattered light from the optical fiber to be measured always matches. Reception can be realized.

<About other embodiments>
Hereinafter, other embodiments of the present invention will be described with reference to the drawings. In particular, the present invention relates to a chromatic dispersion value measuring method, a measuring apparatus, and a data analysis method for an optical fiber using a nonlinear optical effect in an optical fiber to be measured. It is a feature. In this measurement method, the noise of two-wavelength probe pulse light and pump pulse light incident on the optical fiber to be measured is removed, and both pulse lights are shaped into a rectangular wave, so that the SNR of the idler light to be observed is reduced. Improvement, suppression of SBS generation, and suppression of modulation instability (MI generation) in anomalous dispersion optical fiber enable measurement with higher accuracy than before. Further, when used in the fiber manufacturing process, it is possible to obtain an optical fiber having uniform and stable characteristics in the longitudinal direction. Therefore, a great contribution is expected in designing a transmission line such as a dispersion compensation line and optical soliton communication used for repeaterless long distance transmission with strong light intensity using an optical amplifier. The embodiment of the present invention will be described below with reference to FIG.

FIG. 16 shows the configuration of an embodiment of a chromatic dispersion value measuring apparatus 200 for an optical fiber according to the present invention. In order to control the pump light, the apparatus 200 of this embodiment includes a pump light source 201, an optical phase modulator 206, a pulse modulator 210, an optical amplifier 212, an optical attenuator 214, an optical bandpass filter 216, and the like. The polarization controller 218 and the polarizer 220 are connected in order, and the optical signal modulator 203 and the electric signal amplifier 204 are sequentially connected to the optical phase modulator 206 in order to control the applied electric signal intensity and frequency. It is connected. Note that a pulse pattern generator 236 that supplies an electric signal to be applied is connected to the pulse modulator 210, and an oscilloscope 235 that receives a trigger signal from the pulse pattern generator 236 is connected to the pulse pattern generator 236.

In order to control the probe light, the probe light source 202, the optical phase modulator 207, the pulse modulator 211, the optical amplifier 213, the optical attenuator 215, the optical bandpass filter 217, and the polarization controller 219 are used. And an optical signal modulator 203 and an electric signal amplifier 205 are sequentially connected to the optical phase modulator 207. Note that a pulse pattern generator 236 and a delay line 238 are sequentially connected to the pulse modulation unit 211 in order to control the applied electric signal intensity, frequency, and pulse propagation time. An oscilloscope 235 that receives a trigger signal from the pulse pattern generator 236 is connected to the pulse pattern generator 236.

The polarizer 220 and the polarizer 221 are connected to a 3 dB coupler 222 that couples the controlled pump light and the probe light. The 3 dB coupler 222 is connected to a three-port optical circulator 223, an optical fiber 224 to be measured, The non-reflective end 226 is installed in order. That is, of the three ports of the optical circulator 223, the first port is coupled to the 3 dB coupler 222, the second port is coupled to the measured optical fiber 224, and the other third port is coupled to the optical filter 229 described later. Yes. Note that an oscilloscope 235, an OE converter 234, and an optical attenuator 228 are sequentially connected to the 3 dB coupler 222 in order to observe an optical pulse waveform.

An optical power meter 227 for measuring the light intensity before input is disposed on the input side of the optical fiber 224 to be measured, and output to the non-reflection end 226 side on the output side of the optical fiber 224 to be measured. An optical spectrum analyzer 225 for measuring the state of the light is disposed.

At the third port of the optical circulator 223, an optical filter 229, an optical amplifier 230, an optical filter 231, an APD (Abelanger Photodiode) 232, an electric signal amplifier 233, and an oscilloscope 235 are connected in order. Note that a control computer 237 is connected to the oscilloscope 235.

Next, a method for measuring the chromatic dispersion value of the optical fiber by the above-described measuring apparatus 200 will be described.
First, the pump light output from the pump light source 201 is phase-modulated by the optical phase modulator 206. A part of a sine wave electric signal having a frequency around 100 MHz outputted from the electric signal generator 203 is inputted to the optical phase modulator 206 via the electric signal amplifier 204. The phase modulation is performed to suppress stimulated Brillouin scattering (SBS) generated in the optical fiber 224 to be measured. For this reason, there is no need for phase modulation when the intensity of the pump light incident on the measured optical fiber 224 is lower than the SBS threshold.

Next, the pump light is pulsed by the pulse modulation unit 210. The period f of the pulse applied to the pulse modulator 210 is determined by the time that the propagating pulse spends in the reciprocation of the length of the optical fiber 224 to be measured, and f = c / 2nL (L: fiber length, n: fiber refraction) Rate). The duty ratio of the pulse is about several percent.

The pump light output from the pulse modulation unit 210 is amplified by the optical amplifier 212, and the amplified pump light passes through the optical attenuator 214, the optical bandpass filter 216, the polarization controller 218, and the polarizer 220 in this order, and then a 3 dB coupler. Incident to 222.

On the other hand, the probe light output from the probe light source 202 is phase-modulated by the optical phase modulator 207 in the same manner as the pump light. A part of the sine wave electric signal having a frequency of around 100 MHz outputted from the electric signal generator 203 is branched into the optical phase modulator 207 and inputted through the electric signal amplifier 205.

The light output from the optical phase modulator 207 is pulsed by the pulse modulation unit 211 and then amplified by the optical amplifier 213. The amplified probe light includes the optical attenuator 215, the optical bandpass filter 217, and the polarization controller. After passing through the polarizer 221 in order, the light enters the 3 dB coupler 222.

As described above, the pump light and the probe light are incident on the 3 dB coupler 222, and both are combined. Since it is preferable to match the polarization states of the two pulses (pump light and probe light) when both are combined, the polarization controllers 218 and 219 are used to preliminarily depolarize both. Adjust the wave condition.

Further, the transmission polarization axes of the polarizers 220 and 221 are adjusted so that the polarization axes of the pump light and the probe light linearly match when combined by the 3 dB coupler 222. Further, the 3 dB coupler 222, the optical circulator 223, and the optical fiber connecting them, and the optical fiber connecting the 3 dB coupler 222 and the polarizers 220, 221 use those that maintain the polarization state as necessary. It is preferable to do.

The light obtained by combining the pump light and the probe light is input to the 1 port of the optical circulator 223, is output from the 2 port, and enters the optical fiber 224 to be measured. Here, the end of the optical fiber 224 to be measured is connected to the non-reflection end 226. The non-reflection end 226 may be disposed as necessary.

In the measured optical fiber 224, when the pump light and the probe light propagate, a nonlinear phenomenon occurs, and idler light is generated. Backscattered light of idler light from the measured optical fiber 224 passes through the optical circulator 223 and is converted into an electrical signal by the APD 232 via the optical filter 229, the optical amplifier 230, and the optical filter 231. This electric signal is amplified by the electric signal amplifier 233 and then measured by the oscilloscope 235. The electrical signal input to the oscilloscope 235 is digitized and input to the control computer 237. The optical filter 229, the optical amplifier 230, and the optical filter 231 may be arranged as necessary.

A trigger signal output from the pulse pattern generator 236 is input to the oscilloscope 235. The intensity of the input pump light and probe light is measured by the optical power meter 227 at the output port (2 ports) of the optical circulator 223. In addition, the electric pulse signal necessary for pulsing the pump light and the probe light is generated by the pulse pattern generator 236, branched into two, and then input to the pulse modulators 210 and 211, respectively.

At this time, either one of the electric signal pulses generated from the pulse pattern generator 236 and bifurcated so that the generation timing of the pump light pulse and the probe light pulse combined by the 3 dB coupler 222 match, Delay is provided on the delay line 238 as necessary. In order to confirm the coincidence of the pulse timings of the pump light and the probe light, a part of the light output from the output port 222a of the 3 dB coupler 222 is input to the OE converter 234 via the optical attenuator 228, and the electric signal After being converted into a pulse, it is input to the oscilloscope 235.

At this time, the delay time between pulses can be grasped by observing the time waveform of both pulse lights in synchronization with the trigger signal from the pulse pattern generator 236. In accordance with this delay time, the delay time amount is adjusted by the delay line 238 to match the timings of the two pulses.

Next, a specific measurement procedure using the measurement apparatus and the measurement method will be described. This measurement is characterized in that measurement conditions are set according to the chromatic dispersion value and length of the optical fiber to be measured. Needless to say, when setting the measurement conditions, the functions and capabilities of the measuring device such as the light source and the light receiver actually used are taken into consideration. First, the flow from setting the measurement conditions to finally calculating the analysis data is shown below.

The chromatic dispersion value of the optical fiber to be measured first determines the measurement conditions (input pump light wavelength / intensity, input probe light wavelength / intensity) in consideration of the optical fiber to be measured and the characteristics of the measuring instrument. The procedure is to observe the backscattered light waveform, subsequently obtain the instantaneous frequency at each point in the longitudinal direction from the idler light waveform, and finally obtain the chromatic dispersion value D from Δβ at each point in the longitudinal direction. Each step will be described below.

First, setting of measurement conditions (input pump light wavelength / intensity, input probe light wavelength / intensity) will be described. This setting varies greatly depending on the characteristics of the optical fiber to be measured, but in general, when measuring an optical fiber, information on what kind of chromatic dispersion characteristic it has depending on the type of optical fiber is known in advance. It is normal. Therefore, when setting the measurement conditions, it is assumed that the length of the optical fiber to be measured and the value of the average chromatic dispersion value are known in advance, and the conditions are set using these values. The average chromatic dispersion value is generally known depending on the type of optical fiber. Therefore, when setting the measurement conditions, the minimum information necessary for the optical fiber to be measured is the type of optical fiber and the value of the length of the optical fiber. Table 4 shows typical types of 1.55 μm band single-mode optical fibers that are currently known and their general characteristics regarding chromatic dispersion values. Next, a procedure for setting measurement conditions will be described.

In setting the measurement conditions, first, the measurement wavelength (pump light wavelength) is set. The chromatic dispersion value obtained by this measurement is a value at the pump light wavelength. Therefore, the pump light wavelength is set to a desired wavelength. Next, the distance resolution is set. The number of vibrations per 1 km (number of waves; Δβ / 2π) of the observed waveform is determined. As the frequency of the idler light waveform per unit length is larger, the change with respect to the distance of the chromatic dispersion value can be obtained more finely, so Δβ / 2π is a measure of the distance resolution.

Next, Δβ is set. Δβ is determined by the chromatic dispersion value D of the optical fiber, the probe light wavelength, and the wavelength interval Δλ between the pump light and the probe light, as shown in the equation (14). Here, the average chromatic dispersion value of the optical fiber to be measured is substituted into the dispersion value D. Then, the two-wavelength interval Δλ between the pump light and the probe light at Δβ set by the distance resolution is obtained. This Δλ is compared with the wavelength interval limit value (Δλmax) of the measuring device. In this comparison, Δλ is determined as the two-wavelength interval when Δλ ≦ Δλ _max (measurement limit), and when Δλ> Δλ _max , Δλ that satisfies Δλ = Δλ _max (Δβ is newly determined) is two. Determined as wavelength interval.

When Δλ is larger than Δλmax, the interval between two wavelengths is Δλmax, and Δβ at that time is newly determined. For example, in the case of Δλ _max = 10 nm, the setting conditions for main fibers that can be considered as objects to be measured are generally set to the values shown in Table 5.

Next, the pulse peak intensity of the pump light and the probe light is determined. The intensity relationship between the pump light and the probe light is shown by the number (6). As can be seen from the equation (6), if the intensity of the probe light is determined, the pump light intensity is uniquely determined. When setting the maximum input intensity of the probe light, it is necessary to eliminate the influence of SBS. Even if the input intensity of the probe light is increased when SBS occurs, the light intensity propagating through the optical fiber is limited, and the intensity relationship between the pump light and the probe light determined by the input cannot be maintained. For this reason, in setting the input intensity, it is necessary to set it below the SBS threshold value of the optical fiber. Note that the SBS threshold varies depending on the characteristics (nonlinear characteristics, etc.) of the optical fiber and the length of the strip. Although the input intensity is set in consideration of the SBS threshold as described above, it is also necessary to confirm in advance that no SBS has occurred for each optical fiber to be measured during actual measurement. It is.

Further, when the optical fiber to be measured is an anomalous dispersion fiber, it is necessary to consider the influence of MI. Since the generation efficiency of MI is proportional to the peak intensity, the degree of generation differs between pump light and probe light having different intensities. Therefore, even if the optimum intensity ratio is adjusted at the input section, the intensity ratio gradually collapses during propagation of the optical fiber.

FIG. 17 is a flowchart showing a procedure for setting the optimum input intensity condition by observing the input light intensity to the measured optical fiber and the output light intensity from the optical fiber and comparing the intensity ratios. First, the intensity ratio between pump light and probe light input to the optical fiber and the intensity ratio between pump light and probe light output from the optical fiber are measured. The intensities of the pump light and the probe light can be examined at each wavelength by using an optical spectrum analyzer or the like.

When the intensity ratio of the transmitted light from the optical fiber changes by 10% or more compared to the intensity ratio of the input light, it is considered that the influence of SBS or MI is large. In such a case, the input intensity to the optical fiber is lowered while maintaining the intensity ratio between the pump light and the probe light. Then, the intensity ratio of the transmitted light from the optical fiber is measured, and this operation is repeated so that the change is within 10% with respect to the input light intensity ratio, thereby adjusting the intensity. By determining the input light intensity conditions of the pump light and the probe light as described above, the measurement error due to MI at the time of measuring the SBS or the anomalous dispersion fiber can be suppressed to a desired range.

Next, pulse modulation for adjusting the pulse shape of the pump light and the probe light in which the input intensity is set will be described. FIG. 18 shows the configuration of the pulse modulators 210 and 211 for adjusting the pulse shapes of the pump light and probe light input to the optical fiber 224 to be measured. The pulse modulation unit 210 is configured by connecting a function generator 239, a bias tee 241, a DC power source 243, and a light intensity modulator 245 to a pulse pattern generator 236 in order. The pulse modulation unit 211 is configured by connecting a function generator 240, a bias tee 242, a DC power source 244, and a light intensity modulator 246 to a pulse pattern generator 236 in order. The function in each configuration will be described below.

First, the pulse pattern generator 236 generates an electric pulse signal having a period corresponding to the length of the optical fiber. The electric pulse signal from the pulse pattern generator 236 is input to the two function generators 239 and 240 as an external trigger signal. The function generators 239 and 240 can generate electric signal pulses having an arbitrary shape. The electric signal pulses from the function generators 239 and 240 are combined with the DC voltage generated from the DC power sources 243 and 244 by the bias tees 241 and 242 and applied to the light intensity modulators 245 and 246. The light transmitted through the light intensity modulators 245 and 246 is converted into a waveform equivalent to the electric pulse waveform applied to the light intensity modulators 245 and 246.

The optical pulses output from the optical intensity modulators 245 and 246 are amplified by optical amplifiers 212 and 213 as shown in FIG. Here, the amplified optical pulse output from the optical amplifiers 212 and 213 is converted from the output port 222a of the 3 dB coupler 222 into an electric signal by the OE converter 234 via the optical attenuator 228, and the pulse shape is observed by the oscilloscope 235. To do. Then, the output electric pulse shape from the function generators 239 and 240 is changed so that the pulse shape becomes rectangular. In this manner, the high-intensity light pulse output from the optical amplifiers 212 and 213 can be made into an ideal rectangular wave. As a result, the generation of SBS in the optical fiber can be prevented and the vibration waveform of idler light can be stabilized.

FIG. 19 shows optical pulse waveforms output from optical amplifiers 212 and 213 such as an EDFA when rectangular electric pulses are applied to the optical intensity modulators 245 and 246 without performing pulse shaping. It can be seen that the intensity has a large surge at the beginning of the pulse. FIG. 20 shows output optical pulse waveforms from the optical amplifiers 212 and 213 when pulse shaping is performed. It can be seen that the pulse shape is rectangular and no surge occurs. For this reason, SBS is avoided.

The measurement is performed according to the input conditions of the pump light and the probe light determined as described above, and the backscattered light of the idler light is observed. Next, the instantaneous frequency at each point is obtained from the waveform of the backscattered light of the obtained idler light, and Δβ is obtained. The instantaneous frequency of the sinusoidal vibration waveform can be obtained by the method shown by L.F. Mollenauer, et.al. In this method, first, band-pass filter processing is performed on the acquired measurement data, and frequency components other than signal frequency components are removed.

Next, FFT (Fast Fourier Transform) is performed, the negative frequency is replaced with zero, and IFFT (Inverse Fast Fourier Transform) is performed to derive the imaginary component of the measurement data. Subsequently, the phase angle at each distance is calculated from the real number component and the imaginary number component. This phase angle is a value corresponding to the instantaneous frequency. In this way, the value of Δβ at each point of the length of the optical fiber for each input pump light intensity is obtained.

FIG. 21 is a vibration waveform of backscattered light of idler light observed when an optical fiber having a length of 25 km having a characteristic of an average chromatic dispersion value of 15.92 ps / nm / km at a wavelength of 1552 nm is used as the optical fiber to be measured. is there. FIG. 22 is a longitudinal distribution of chromatic dispersion calculated from the values in FIG. 21 using the above analysis method.

  In the present embodiment, FFT and IFFT are used to obtain Δβ. However, the present invention is not limited to this, and the sine wave parameter (frequency) that is located most in the observed waveform is not limited to this. From this, the frequency (Δβ) at each point can be obtained.

この式に非線形フィッティングを行う。ここで、zは位置、a2が分散に関するパラメータである。a1は振幅パラメータ、a3は位相のオフセット、a4は強度のオフセットであり、これらは必要に応じてフィッティングパラメータとする。
パラメータa2を使用すると、波長分散値は、

で求められる。 That is, by extracting some continuous data points from the idler backscattered light waveform and performing a non-linear fitting to a sine function to obtain a local dispersion value, by sequentially changing the data range to be extracted and performing fitting The distribution of chromatic dispersion values in the fiber longitudinal direction can be obtained. FIG. 14 shows a measurement waveform having a total of 3743 data points. For example, the 1800th to 2000th points which are 201 continuous points near the center are extracted. Assuming that the extracted 200 points have a single period, the change in intensity can be expressed by Equation 31,

Nonlinear fitting is performed on this equation. Here, z is a position, and a2 is a parameter relating to dispersion. a1 is an amplitude parameter, a3 is a phase offset, and a4 is an intensity offset. These are fitting parameters as required.
Using parameter a2, the chromatic dispersion value is

Is required.

The obtained chromatic dispersion value is set as the center position of the extracted data range, that is, the 1900th chromatic dispersion value. The initial value of the parameter a2 is the period obtained by Fourier transform without a filter, but the initial value may be any value near the variance to be obtained, and the prediction method is not limited.

  Next, the data range to be extracted is changed, for example, the data from the 1801st point to the 2001th point is extracted and the chromatic dispersion value is similarly obtained by nonlinear fitting, and the obtained chromatic dispersion value is changed to the 1901th point. The chromatic dispersion value. As the initial value of the parameter a2 after changing the data range to be extracted, the calculation result before changing the data range was used. Similarly, by determining the local chromatic dispersion value by nonlinear fitting while changing the data range to be extracted, the chromatic dispersion value at each position in the longitudinal direction of the fiber can be obtained. Distribution is obtained.

FIG. 15 shows a distribution of chromatic dispersion values calculated directly from the waveform of FIG. 14 by the fitting method, and the chromatic dispersion values are obtained with high accuracy.
The data range to be extracted may be an appropriate range in view of the balance between position resolution and accuracy. Due to the nature of the fitting, the data range to be extracted is sufficiently accurate when the intensity vibration is about 1 to 10 cycles.

を用いればよい。 Further, the number (31) is particularly effective when the chromatic dispersion value can be approximated to be constant within the fitting data range to be extracted, but when the chromatic dispersion value greatly changes in the fiber longitudinal direction, It is better to use a mathematical formula in which the chromatic dispersion value is distributed in the first order of the distance within the fitting data range to be extracted. Furthermore, when the amplitude of the intensity vibration also changes within the fitting data range to be extracted, it may be better to add a primary parameter of the amplitude. For the first-order parameters, b2 is related to the chromatic dispersion value, b1 is related to the amplitude,

May be used.

  Next, means for correcting the chromatic dispersion value calculated from the vibration period of the measurement data using the approximation error of the approximate expression used for measurement will be described.

  Although it is assumed that the idler backscattered light waveform is represented by the number (31), the number (31) is derived from a differential equation using an approximation that the idler light generated by the FWM is sufficiently small with respect to the input light. Therefore, the chromatic dispersion value D and the parameter a2 regarding the period of the intensity vibration of the idler light do not strictly satisfy the number (2). In particular, in systems where the nonlinear phenomenon is strong, such as when the input light is strong or the nonlinear constant of the optical fiber to be measured is large, the intensity of the idler light to be observed is large and easy to observe, but the error due to approximation increases.

と定義すると、当該A値が10以上の場合に、波長分散値の誤差は、

となる。数（35）により、波長分散値の真値と、アイドラー後方散乱光波形から算出される波長分散値との誤差が求められるので、測定値から誤差を算出し、求められた誤差分を補償することにより波長分散値の真値を求めることが可能となる。 Chromatic dispersion using the chromatic dispersion value calculated by nonlinear fitting using the numbers (31) and (32) from the intensity change data obtained by the simulation that solves the exact differential equation with the predetermined chromatic dispersion value. By comparing with the value, a relational expression between the error and each parameter is obtained. For example, when the ratio of the input pump light amount and the input probe light amount is 1: 2,

When the A value is 10 or more, the error of the chromatic dispersion value is

It becomes. Since the error between the true value of the chromatic dispersion value and the chromatic dispersion value calculated from the idler backscattered light waveform is obtained from the number (35), the error is calculated from the measured value and the obtained error is compensated. Thus, the true value of the chromatic dispersion value can be obtained.

  For example, input pump light intensity is Pp = 200mW, input probe intensity is Ps = 400mW, wavelength interval of two input lights is 2.298nm, chromatic dispersion value is 2.00ps / nm / km, nonlinear constant is When an optical fiber with γ = 2.025 / km / W is measured, the error is 0.07 ps / nm / km, that is, the chromatic dispersion value is calculated from the measured waveform using the numbers (31) and (32) to obtain 2.07 ps / nm / km. It becomes. Therefore, when the measured chromatic dispersion value is 2.07 ps / nm / km, the error value 0.07 ps / nm / km is compensated from the measured chromatic dispersion value 2.07 ps / nm / km including the error, An accurate chromatic dispersion value of 2.00 ps / nm can be calculated.

のようにも表すこともできる。 In Equation (35), an error is obtained from the nonlinear constant, the two input optical powers, and the wavelength interval between the two input optical powers. The intensity amplitude of the idler light when the Rayleigh scattering coefficient is 10 ⁻⁴ is Pc, If the loss of the optical fiber is 0,

It can also be expressed as

For this reason, it is possible to compensate for errors in the measured chromatic dispersion value using the nonlinear constant, the observed amplitude amplitude of the idler light, and the wavelength interval between the two input optical powers, and the input pump optical power It is also possible to compensate for the error of the chromatic dispersion value measured only from the intensity amplitude of the observed idler light. In particular, the use of the number (37) is useful because the error can be compensated without knowing the nonlinear constant. In other words, if the number (37) is used, the intensity amplitude of the idler light is ⁴ × 10 ⁻⁴ mW under the same measurement conditions as above, so that the chromatic dispersion value is 2.0 ps / nm / km without using the nonlinear constant in the calculation. In this case, the error 0.07 ps / nm / km can be calculated using the intensity amplitude Pp = 200 mW of the input pump light.

  Conversely, if the ratio of the input pump light quantity to the input probe light quantity is 1: 2, the wavelength of the optical fiber with a chromatic dispersion value of 2.0 ps / nm / km and a nonlinear constant of γ = 2.025 / km / W. When measuring the dispersion value, it can be seen that the error is reduced by increasing the wavelength interval or reducing the intensity vibration of the input pump light. The approximation error is inversely proportional to the fourth power of the wavelength interval and proportional to the square of the input pump light power. However, since the generated idler light becomes smaller in proportion to the square of the input pump light power, the intensity vibration of the input pump light is determined within a range in which the idler light can be measured.

  Therefore, by directly performing nonlinear fitting with a sine function in order to obtain the local intensity vibration period of the measurement data, it can be made less susceptible to noise components, and analysis accuracy can be improved. In addition, the analysis method that directly performs the non-linear fitting in this way does not require a filter process or is simple, and thus the analysis becomes easier. Further, the accuracy of the analysis can be improved by compensating the error due to approximation by back calculation. Further, the measurement error can be reduced by selecting an assumption condition with a small approximation error, and the measurement accuracy is improved.

Next, as another embodiment, the back scattered light of the pump light and the probe light is observed, and the longitudinal chromatic dispersion value is measured using the measurement result of the propagation intensity in the longitudinal direction of both lights in the measured optical fiber. A means for performing distribution analysis correction will be described.

It is assumed that the waveform of the idler light is expressed by the equation (11), but the equation (11) is an approximation that the transmission loss is not affected in the FWM generation process, and the effect of SPM and XPM is not considered. Since it is derived from the differential equation using the assumption, when the intensity relationship between the pump light and the idler light deviates from the number (6), the chromatic dispersion value D does not strictly satisfy the number (14). LFMollenauer, et.al. expresses the error with respect to Δβ under such influence by the number (38) of the following equation.

When the number (38) is taken into consideration, the relational expression between Δβ and the chromatic dispersion value D is as the following number (39).

As described above, the error of Δβ is greatly affected by the error, particularly when the nonlinear constant γ of the optical fiber to be measured is large. Recently, a highly nonlinear fiber having a nonlinear constant γ of 5 times or more than that of a normal fiber has been developed, and the problem of error becomes particularly large when evaluating such an optical fiber. Therefore, the measurement procedure to remove the influence of number (38) is shown below.

First, the instantaneous frequency at each point in the longitudinal direction is obtained from the waveform of the backscattered light of idler light by the above method, and Δβ is obtained using the relational expression (12). Next, the back scattered light of the pump light and the probe light is observed, and the longitudinal distribution of the propagation intensity of each light in the optical fiber is obtained. The backscattered light of the pump light and the probe light is adjusted by an optical filter so that light of each wavelength of the pump light and the probe light is selected by the light receiving unit, and the waveform thereof is observed using an oscilloscope. Further, the nonlinear constant γ can be obtained by an existing measurement method. For example, RHStolen and Chinlon Lin, Physical Review A, Vol.17, No.4, pp.1448-1453 (1992), ^{or, A.Wada et al., 18 th} European Conference of Optical Communication 1992, Technical digest p. 42 (1992)) can be used.

Next, Δβ _NL represented by the number (38) at each point in the longitudinal direction is obtained from the result of the longitudinal distribution of the propagation intensity of each light in the optical fiber. By subtracting Δβ _NL from Δβ, only the component depending on the chromatic dispersion value D can be obtained.

The longitudinal chromatic dispersion value D determined in this way is a value obtained by removing the influence of the pump light and the probe light due to a change in intensity due to a nonlinear effect or the like in the optical fiber, and is considered to have a higher accuracy. be able to.

  Next, another embodiment will be described with reference to FIG. FIG. 23 shows that pump light and probe are measured in the optical fiber 224 to be measured by injecting third light having a wavelength different from that of the pump light and probe light from both ends of the optical fiber 224 to be measured or one of the ends. A measurement apparatus configuration having a configuration for Raman amplification of the intensity of light and idler light is used.

Raman amplification is performed by Raman amplified light input from both of the two Raman amplification light sources 247 and 249 to both ends of the measured optical fiber 224 via the two WDM couplers 248 and 250. The WDM coupler 250 may use a circulator. When pump light and probe light are used near 1550 nm, light having a wavelength near 1450 nm is incident. Such Raman amplification is effective when the length of the optical fiber 224 to be measured is long and when the transmission loss is large. Since the transmission loss in the optical fiber is compensated by performing the Raman amplification, the SN ratio of the idler light can be maintained even when the input pump light and probe light are lowered in intensity. This leads to a further reduction in the influence of MI in the optical fiber, and is also effective in reducing measurement errors.

Furthermore, as another embodiment, in the configuration of the measurement system shown in FIGS. 16 and 23, as an element for selecting the wavelength of the backscattered light of idler light, Finesse 2000 or more, full width at half maximum of 5 GHz, transmission wavelength in filter output light A Fabry-Perot filter having an intensity ratio between light and cut-off wavelength light of 40 dB or more may be used.

Since the intensity of the idler light is about 30 to 40 dB smaller than the intensity of the pump light and the probe light, the transmission width is narrow as a characteristic of the wavelength selection element to observe the vibration waveform of the idler light, and the contrast ratio ( A large propagation loss) between the transmission wavelength and the cut-off wavelength band is desired. As a filter satisfying such conditions, a general filter can be used. For example, Finesse 2000, a filter having a full width at half maximum of 2.5 GHz (chromoux, model TFM-2000) corresponds to this. This filter consists of a Fabry-Perot resonator made of MEMS (Micro Electrical Mechanical System). Compared to a conventional Fabry-Perot resonator, the Finesse is large, the full width at half maximum is narrow, and the transmission loss is about 3 dB. Low. In addition, the contrast ratio is 40 dB or more, and this value is also a great difference compared to the 20 dB of a commercially available Fabry-Perot etalon filter. By using such a narrow band filter, mixing of pump light and probe light can be prevented, and the SN ratio of idler light can be improved. In order to further improve the wavelength selection characteristics of idler light, as shown in FIG. 24, the number of optical amplifiers 251 and optical filters 252 is increased, wavelength selection / intensity amplification of idler light is repeated, and wavelength selection is performed. It can also improve the performance.

Next, an optical fiber manufacturing method for manufacturing an optical fiber based on the measurement method for the chromatic dispersion value in the fiber longitudinal direction using the above-described measuring apparatus will be described with reference to FIG.

  First, the refractive index distribution of the preform of the prepared optical fiber is measured with a preform analyzer. The drawing conditions (drawing force, speed, temperature of the melted part, etc.) are determined from the results. Then, the base material is drawn, and the obtained optical fiber is evaluated for the distribution of the chromatic dispersion value in the longitudinal direction of the optical fiber using the chromatic dispersion distribution measuring apparatus described above. Based on the evaluation result, the distribution state of the chromatic dispersion value in the fiber longitudinal direction is partially cut out so as to be included in the fluctuation within ± 5% with respect to the average value in the entire length of the optical fiber. As described above, in this embodiment, the chromatic dispersion value of the optical fiber is evaluated in the fiber longitudinal direction, and the step of cutting out the portion having the optimum value of the chromatic dispersion value is performed. An optical fiber having a uniform and stable wavelength dispersion value can be manufactured.

  As described above, in the chromatic dispersion distribution measuring method of the optical fiber according to the present invention, the two wavelength pulse lights (pump pulse light and probe pulse light) input to the optical fiber at the transmitter are independently optically amplified, A narrow band optical band pass filter is transmitted. By independently amplifying each pulsed light, ASE noise generated in the optical amplifier can be cut.

In the present invention, the delay time is adjusted so that at least one of the pump pulse light and the probe pulse light overlaps in time, and is multiplexed by the coupler. Next, the combined light is input to the optical fiber to be measured. In the optical fiber, idler light is generated by a non-linear effect. When the measured optical fiber is an anomalous dispersion optical fiber, MI occurs in the optical fiber, but the ASE noise is removed from the pump pulse light and probe pulse light input to the measured optical fiber by an optical filter. Therefore, the SN ratio of the pump pulse light and the probe pulse light is improved, and the MI generation efficiency can be reduced.

Therefore, an increase in noise due to MI can be suppressed even at the wavelength of idler light, and the SN ratio of backscattered idler light to be observed can be improved. In addition, since the noise level of idler light has decreased, the sensitivity of the light receiving unit that observes the return light has been improved, and the intensity of the input pump pulse light and probe pulse light can be further reduced, further affecting the influence of MI. Reduced measurement can be performed.

In addition, the pulse modulation unit of the pump light and the probe light performs pulse shaping so that the output pulse waveform from the optical amplifier becomes a rectangular wave, thereby suppressing generation of a pulse surge by the optical amplifier, and SBS in the optical fiber. Can be suppressed.

Further, the intensity ratio between the pump pulse light and the probe pulse light input to the optical fiber to be measured and the intensity ratio between the pump pulse light and the probe pulse light transmitted through the optical fiber to be measured are compared and input so as to fall within 10%. By determining the peak intensities of the pump pulse light and the probe pulse light, the influence of MI in the optical fiber can be suppressed within a desired range.

Furthermore, since the S / N ratio of idler light is improved, it becomes possible to observe idler light with a weaker intensity. For this reason, the accuracy of the dispersion value with respect to the position can be improved by widening the wavelength interval between the pump light and the probe light and increasing the repetition frequency of the idler light to be observed. Further, in a state where the wavelength interval between the pump light and the probe light is not widened, weak light from a farther place can be observed, so that the measurable distance is extended.

In addition, the backscattered light of the pump pulse light and probe pulse light is observed, and the longitudinal chromatic dispersion value distribution is analyzed and corrected using the propagation intensity distribution values in the longitudinal direction of both lights in the measured optical fiber. Thus, a more accurate chromatic dispersion value can be obtained.

In the above-described measurement, the intensity of the idler light to be observed is amplified by using Raman amplification for the pump light, the probe light, and the idler light. Therefore, even if the input pump light and the probe light intensity to the fiber are further reduced. It becomes possible to observe with the same signal-to-noise ratio, and the influence of MI can be further reduced.

Furthermore, as a wavelength selection element for idler backscattered light in the light receiving section, a narrowband Fabry-Perot with Finesse 2000 or higher, full width at half maximum of 5 GHz or lower, and an intensity ratio between transmitted wavelength light and cutoff wavelength light in the filter output light of 40 dB or higher is used. Thus, mixing of pump light and probe light can be prevented and only idler light can be observed.

Thus, by reducing noise, suppressing SBS generation by pulse shaping, and improving the idler light SN ratio, measurement can be performed with lower input light intensity, and measurement errors can be reduced and an accurate chromatic dispersion value can be obtained. This effect is particularly prominent in the long end portion of a fiber having a long length or an anomalous dispersion fiber.

Further, by using such a measuring apparatus, the chromatic dispersion distribution state in the longitudinal direction of the drawn optical fiber is measured, and the chromatic dispersion value distributed in the longitudinal direction of the fiber is ± 5 with respect to the average value of the total length of the optical fiber. By cutting out the optical fiber so as to be within%, it is possible to obtain an optical fiber having a more uniform chromatic dispersion value over its entire length.

It is a figure which shows the structure of the measuring apparatus in this Embodiment. It is a figure which shows the structure of the measuring apparatus in another Example. It is a figure which shows the structure of the measuring apparatus in another Example. It is a figure which shows the structure of the measuring apparatus in another Example. It is a figure which shows the structure of the measuring apparatus in another Example. It is a figure which shows the structure of the measuring apparatus which has a polarization-maintaining function in another Example. It is a figure which shows the structure of the measuring apparatus which has the polarization maintaining function using the optical heterodyne detection system in another Example. It is a measurable area | region figure in RDF. It is a measurable area | region figure in HNLF (D = 4ps / nm / km). It is the backscattered light waveform of the idler light measured when the input pump light intensity was changed. It is a chromatic dispersion distribution diagram in the longitudinal direction of the fiber obtained as a result of analysis. It is a nonlinear coefficient distribution figure of the fiber longitudinal direction obtained as a result of analysis. It is a flowchart explaining the process of the manufacturing method of the optical fiber in this Embodiment. It is a figure which shows an example of an OTDR waveform. This is a dispersion distribution obtained by direct fitting. It is a figure which shows the structure of the measuring apparatus in another Example. It is a flowchart explaining the process of the input conditions to the to-be-measured optical fiber in this Embodiment. It is a figure which shows the structure of the pulse modulation part in another Example. It is an output pulse waveform from the EDFA when pulse shaping is not performed in the pulse modulation unit. It is the output pulse waveform from EDFA at the time of performing pulse shaping in a pulse modulation part. It is the backscattered light waveform of the observed idler light. It is a longitudinal direction wavelength dispersion distribution map obtained as a result of analysis. It is a figure which shows the structure of the measuring apparatus which performs a Raman amplification from the both ends or one one end of the to-be-measured optical fiber in another Example. It is a figure which shows the structure of the measuring apparatus which increased the optical amplifier and the wavelength selection filter in the wavelength selection part of the idler light in another Example. It is a flowchart explaining the process of the manufacturing method of the optical fiber in another Example. It is a measured optical fiber input light and transmitted light spectrum in a state where MI is generated in the nonlinear OTDR method.

Explanation of symbols

100, 101, 102, 103, 104, 105, 106, 200 Measuring device 1 Pump light source 2, 2a Phase modulator 3 Signal generator 4, 23, 24 Electric signal amplifier 5 Optical fiber to be measured 6, 6a, 12, 12a Intensity modulator 7, 7a, 18 EDFA
8, 8a Band pass filter 9, 14, 27, 128 Polarization controller 10, 10a Coupler 11 Probe light source 13, 13a Delay line 15, 15a Circulator 16 Non-reflective end 17, 17a, 19 Optical filter 20 O / E converter 21 Oscilloscope 22 Pulse generator 25, 26 DC power supply 28, 28a 3 dB coupler 29 Photo detector 30 Electric amplifier 31 Mixer 32 Signal generator 33 Low pass filter 35, 36 Light source for Raman amplification 37, 38 WDM coupler 39 Optical spectrum analyzer 40 Nonlinear optical medium 124 Computer 125, 125a Tap coupler 126, 126a AO modulator 131 Optical power meter 200 Measuring device 201 Pump light source 202 Probe light source 203 Electric signal generator 204, 2 5 Electrical signal amplifiers 206 and 207 Optical phase modulators 208 and 209 Polarization controllers 210 and 211 Pulse modulation units 212 and 213 Optical amplifiers 214 and 215 Optical attenuators 216 and 217 Optical bandpass filters 218 and 219 Polarization controllers 220 and 221 Polarization Sub unit 222 3 dB coupler 222 a 3 dB coupler output port 223 Optical circulator 224 Optical fiber 225 to be measured Optical spectrum analyzer 226 Non-reflection end 227 Optical power meter 228 Optical attenuator 229 Optical filter 230 Optical amplifier 231 Optical filter 232 APD
233 Electric signal amplifier 234 OE converter 235 Oscilloscope 236 Pulse pattern generator 237 Control computer 238 Delay line 239, 240 Function generator 241, 242 Bias 243, 244 DC power supply 245, 246 Light intensity modulator 247, 249 For Raman amplification Light source 248 WDM coupler 250 WDM coupler 251 Optical amplifier 252 Optical filter

Claims (22)

Linearly polarized probe pulse light and pump pulse light having a different wavelength from the probe pulse light and having the same polarization state are incident on the optical fiber to be measured , and the intensity of the pump pulse light is changed multiple times. incident to,該被intensity of backscattered light measured optical fiber within the pump pulse light caused by Rayleigh scattering in and thereto, the intensity of the backscattered light of the probe pulse light, or the generation in the measured optical fiber The change in at least one of the fiber longitudinal directions of the backscattered light intensity of the idler light caused by the nonlinear phenomenon is measured with respect to each pump pulse light incident intensity condition , and the measured probe pulse backscattered light or idler backscattered light local vibration circumference in the fiber longitudinal direction each point from the sinusoidal intensity oscillation distribution for the longitudinal fiber direction We obtain the instantaneous frequency, determined at least one of the wavelength dispersion and the nonlinear coefficient of the optical fiber to be measured from the pump power dependence of the instantaneous time frequency at each point of the fiber longitudinal direction, of a series of processes Fiber A method for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber, wherein the distribution in the longitudinal direction of the fiber with respect to at least one of the chromatic dispersion value and the nonlinear constant is specified by repeating at each point in the longitudinal direction.   The chromatic dispersion value in the fiber longitudinal direction of the optical fiber to be measured is discriminated using the conditions relating to the wavelength and intensity of the pump pulse light and probe pulse light input to the optical fiber to be measured. Item 2. A method for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to Item 1.   Measuring the average value of the intensity vibration of the idler light on the pulse emission side of the optical fiber to be measured, and measuring the dependence of the conversion efficiency of the idler light on the intensity vibration of the input pump pulsed light based on the measurement result; By performing regression analysis of the conversion efficiency using a theoretical function representing the conversion efficiency of idler light depending on the intensity vibration of the pump light based on the measurement result, the average chromatic dispersion value and the average nonlinearity of the entire optical fiber to be measured The method for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to claim 2, wherein the constant is specified simultaneously.   For each end of the optical fiber to be measured, the distribution value in the longitudinal direction of the chromatic dispersion value and the nonlinear constant is measured, and the two measured distribution values are compared to determine the polarization in the optical fiber to be measured. 2. The method for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to claim 1, wherein the influence of fluctuation is removed.   2. The intensity vibration data of the idler light or probe pulse light is deleted by a predetermined amount of data near both ends of the optical fiber to be measured, and the intensity vibration data after the deletion is analyzed. 3. A method for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to 2. A pump light source that generates pump light;
A probe light source for generating probe light;
A modulator for pulsing the pump light and / or probe light;
A first multiplexer that combines the pulsed pump light and the pulsed probe light and inputs the multiplexed light to the optical fiber to be measured;
After the pump pulse light and the probe pulse light are input to the measured optical fiber, the back scattered light of the probe pulse light generated in the measured optical fiber, or the back of the idler light generated in the measured optical fiber A light receiving means for selecting any wavelength of the scattered light and outputting an electrical signal representing the waveform of the selected scattered light;
Calculating means for calculating both the chromatic dispersion value and the nonlinear constant of the measured optical fiber in the longitudinal direction of the fiber from the electrical signal waveform, wherein the chromatic dispersion value and the nonlinear constant of the optical fiber are calculated. measuring device.   7. The apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to claim 6, further comprising an optical amplifying means for amplifying the pump pulse light. Branching means for branching the probe light generated from the probe light source into two lights;
An acoustooptic modulator that pulsates the first light of the two branched probe lights and shifts the wave number;
The light receiving means includes a second multiplexer that multiplexes the backscattered light of the probe pulse light output from the acousto-optic modulator and the second light branched by the branching means. The apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to claim 6 or 7.   The pump pulse light, the probe pulse light, and the idler in the optical fiber to be measured are incident on both ends or any one end of the optical fiber to be measured, and other light having a wavelength different from that of the pump pulse light and the probe pulse light. The apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to claim 6 or 7, further comprising a Raman amplifying unit for Raman-amplifying light.   The apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to claim 6 or 7, further comprising an intensity measuring device for measuring intensity vibration of idler light on a pulse emission side of the optical fiber to be measured.   8. The chromatic dispersion value and nonlinear constant of the optical fiber according to claim 6, wherein a nonlinear optical medium is provided in an optical transmission path between the first multiplexer and the optical fiber to be measured. measuring device. The pump light source and the probe light source each generate light having linearly polarized light parallel to each other,
The wavelength of the optical fiber according to claim 6 or 7, wherein the first multiplexer multiplexes the two lights having linearly polarized light parallel to each other so that polarization polarization planes coincide with each other. A device for measuring dispersion values and nonlinear constants. The pump light source and the probe light source each generate light having linearly polarized light parallel to each other,
The first multiplexer multiplexes the two lights having linearly polarized light parallel to each other so that polarization polarization planes coincide with each other,
The second multiplexer includes backscattered light of the probe light;
9. The backscattered light of the probe pulse light output from the acousto-optic modulator and the second light branched by the branching means are multiplexed in a polarization state parallel to each other. 2. An apparatus for measuring a chromatic dispersion value and a nonlinear constant of an optical fiber according to 1.   A chromatic dispersion value of a drawn optical fiber and a distribution state of a nonlinear constant in a fiber longitudinal direction are measured using the measuring device according to any one of claims 6 to 13, and the chromatic dispersion is based on the measurement result. The optical fiber is manufactured by cutting out the optical fiber so that one or both of the value and the nonlinear constant are distributed in the longitudinal direction of the fiber within ± 5% of the average value of the total length of the optical fiber. Method. The linearly polarized probe light and the pump light having a wavelength different from that of the probe light and having the same polarization state are pulsed independently by the pulse modulation unit, and then optically amplified independently, and the amplified probe The noise components of the pulsed light and pump pulsed light are removed by individual optical filters, and then the probe pulsed light and pump pulsed light are combined and incident on the optical fiber to be measured. Due to the nonlinear phenomenon in the optical fiber to be measured The change in the intensity of the backscattered light of the generated idler light with respect to the longitudinal direction of the fiber is measured, and the instantaneous frequency, which is a local vibration period at each point in the longitudinal direction of the fiber, from the measured sinusoidal intensity vibration distribution with respect to the longitudinal direction of the fiber. look, determine the local chromatic dispersion value from instantaneous time frequency in the fiber longitudinal direction each point, the longitudinal direction of each feature of the series of processes fiber It makes the wavelength dispersion measuring method of an optical fiber, characterized in that identifying the fiber longitudinal dependence on the wavelength dispersion value of the optical fiber under test be repeated.   The intensity of pump pulse light and probe pulse light incident on the optical fiber to be measured and the intensity of pump pulse light and probe pulse light transmitted from the optical fiber to be measured are observed, and the probe pulse light at the time of incidence and transmission And the intensity of the pump pulse light at the time of incidence so that the intensity ratio of the two pulse lights at the time of transmission is within 10% of the intensity ratio of the two pulses at the time of incidence. The method for measuring a chromatic dispersion value of an optical fiber according to claim 15, wherein:   The back-scattered light of the pump pulse light and probe pulse light is observed, and analysis correction of the chromatic dispersion value distribution in the longitudinal direction is performed using the propagation intensity in the longitudinal direction of both pulsed light in the optical fiber to be measured. The method for measuring a chromatic dispersion value of an optical fiber according to claim 15 or 16. First light removing a pump light source for generating a pump light, a first pulse modulating means for pulsing the pump light, a first optical amplifier for amplifying the intensity of the pump pulse light, the noise of the pump pulse light Filters,
A probe light source for generating a probe light, a second pulse modulating means for pulsing the probe light, a second optical amplifier for amplifying the intensity of the probe pulse light, the second light that removes noise of the probe pulse light Filters,
A first multiplexer that combines the pump pulse light and the probe pulse light and inputs the multiplexed light to the optical fiber to be measured;
After the combined pump pulse light and probe pulse light are input to the optical fiber to be measured, the wavelength of the backscattered light of idler light generated in the optical fiber to be measured is selected, and the selected scattering A light receiving means for outputting an electrical signal representing a light waveform;
A chromatic dispersion value measuring apparatus for an optical fiber, comprising: calculating means for calculating a longitudinal distribution of a chromatic dispersion value of the optical fiber to be measured from the electrical signal waveform.   19. The chromatic dispersion value measurement of an optical fiber according to claim 18, wherein the pulse modulation means for pulsing the pump light and the probe light includes an electric signal generator capable of generating an arbitrary electric signal pulse waveform. apparatus.   Other light having a wavelength different from that of the pump pulse light and the probe pulse light is incident on both ends or any one end of the optical fiber to be measured, and the pump light, the probe light, and the idler light in the optical fiber to be measured 20. The optical fiber chromatic dispersion value measuring apparatus according to claim 18, further comprising Raman amplification means for performing Raman amplification.   The light receiving means is provided with a Fabry-Perot filter for selecting the wavelength of the backscattered light of idler light generated in the optical fiber to be measured. The Fabry-Perot filter is transmitted through Finesse 2000 or more and transmission full width at half maximum of 5 GHz or less. 21. The chromatic dispersion value measuring apparatus for an optical fiber according to claim 18, wherein the intensity ratio between the wavelength light and the blocked wavelength light is 40 dB or more. Using the measurement method according to any one of claims 15 to 17, a longitudinal distribution state of a chromatic dispersion value of a drawn optical fiber is measured, and the longitudinal direction of the optical fiber is measured based on the measurement result. A method of manufacturing an optical fiber, comprising: cutting out the optical fiber so that a chromatic dispersion value distributed in the range is within ± 5% of an average value of the total length of the optical fiber.

Images