JP4568177B2 - Method and apparatus for measuring chromatic dispersion of optical fiber - Google Patents

Description

  The present invention relates to a chromatic dispersion measuring method and apparatus for measuring a distribution in a length direction of chromatic dispersion of an optical fiber.

  In optical communication systems using optical fibers and optical waveguides, knowing the dispersion of optical fibers, etc. by measurement, because dispersion of optical fibers has a significant effect on the characteristics of optical communication systems, such as causing distortion of signal waveforms during propagation. Is an indispensable item in communication system design. In particular, during actual system operation and testing, it is often necessary to measure the distribution in the length direction of the dispersion of the optical fiber with the optical cable installed. As described above, as a method for measuring the distribution in the length direction of dispersion of an optical fiber in a non-destructive manner, there is a method using four-wave mixing in an optical fiber, which is conventionally described in Non-Patent Document 1, for example. . However, since this conventional method needs to use the nonlinear optical effect in the optical fiber, it requires a high-intensity laser beam (in Non-Patent Document 1, the measurement system is based on a laser beam of about 1 W). Have been built), and so on.

L.F.Mollenauer et al. “Method for facile and accurate measurement of optical fiber dispersion maps”, OPTICS LETTERS, VOL.21, NO.21, pp.1724-1726 (November 1996)

  The present invention has been made in view of the above circumstances. By observing Rayleigh backscattered light and determining the chromatic dispersion of the optical fiber as a function of distance in a non-destructive manner, a high-power light source is not required, and cost performance is improved. An object of the present invention is to provide an optical fiber wavelength dispersion measuring method and apparatus excellent in the above.

  In order to achieve the above object, the present invention is a chromatic dispersion measuring method for measuring the distribution of the chromatic dispersion in the length direction of an optical fiber, wherein a light pulse is incident from one end of the optical fiber and the Rayleigh backscattered light intensity is measured. A Rayleigh backscattered light intensity observation step observed as a function of the distance from the input end, and an average for calculating a statistical average and variance at each distance of the Rayleigh backscattered light intensity observed in the Rayleigh backscattered light intensity observation step, and A dispersion calculating step, and a chromatic dispersion calculating step for calculating chromatic dispersion at each distance of the optical fiber from a statistical average and dispersion at each distance of the Rayleigh backscattered light intensity calculated in the average and dispersion calculating step. It is characterized by.

  The present invention is also a method for measuring the chromatic dispersion of an optical fiber as described above, and calculates cumulative chromatic dispersion from a statistical average and dispersion at each distance of Rayleigh backscattered light intensity to a predetermined distance of the optical fiber. A cumulative chromatic dispersion calculating step, and a differential step of obtaining chromatic dispersion at each distance by differentiating the cumulative chromatic dispersion calculated in the cumulative chromatic dispersion calculating step with respect to the distance. To do.

  The present invention is also a chromatic dispersion measuring apparatus for measuring the distribution of the chromatic dispersion in the length direction of an optical fiber, in which an optical pulse is incident from one end of the optical fiber, and the Rayleigh backscattered light intensity is measured as a distance from the input end. Rayleigh backscattered light intensity observing means for observing as a function, mean and dispersion calculating means for calculating a statistical mean and variance at each distance of Rayleigh backscattered light intensity observed by the Rayleigh backscattered light intensity observing means, And chromatic dispersion calculating means for calculating chromatic dispersion at each distance of the optical fiber from a statistical average and dispersion at each distance of the Rayleigh backscattered light intensity calculated by the average and dispersion calculating means. is there.

  Further, the present invention is an optical fiber chromatic dispersion measuring apparatus as described above, which calculates cumulative chromatic dispersion up to a predetermined distance of an optical fiber from a statistical average and dispersion at each distance of Rayleigh backscattered light intensity. A cumulative chromatic dispersion calculating means, and a differential means for obtaining chromatic dispersion at each distance by differentiating the cumulative chromatic dispersion calculated by the cumulative chromatic dispersion calculating means with respect to the distance. To do.

  The present invention is based on the OTDR (Optical Time Domain Reflectometry) technique which is a known technique. That is, an optical pulse is input to an optical fiber, and Rayleigh scattered light (reflected light) generated in the optical fiber is observed. The observed Rayleigh scattered light has statistical intensity fluctuations determined by the characteristics of the material of the optical fiber, the input optical pulse width, and the like. The present invention observes statistical fluctuations of Rayleigh scattered light by quantitatively clarifying the relationship between this statistical intensity fluctuation and the amount of chromatic dispersion experienced after an optical pulse is input to an optical fiber. Thus, a method for determining the chromatic dispersion distribution of the optical fiber is provided.

  The present invention can determine the chromatic dispersion of an optical fiber as a function of distance by observing Rayleigh backscattered light. Whereas the conventional method of generating four-wave mixing in an optical fiber requires an optical input of about 1 W, the present invention generally observes even an input light of several mW to several tens of mW. It is possible to provide a dispersion measuring method and apparatus excellent in cost performance that do not require a high-output light source.

  The present invention newly provides a technique for measuring a distribution in the length direction of dispersion of an optical fiber in a non-destructive manner by a method fundamentally different from the conventional method.

Embodiments of the present invention will be described below in detail with reference to the drawings.
FIG. 1 is an explanatory diagram showing a configuration of an optical fiber chromatic dispersion measuring apparatus according to an embodiment of the present invention. In Figure 1, 1 generates a coherent light pulses comprising a pulse width T ₀ is the optical pulse generator. For example, an optical pulse with T ₀ = 50 ps (picosecond), wavelength λ = 1.55 μm, and spectral spread of about 0.2 nm is generated. There are a number of devices that can be specifically used as the optical pulse generator 1, and can be realized by, for example, an optical pulse generated by a mode-locked laser, or can be realized by modulating a semiconductor laser beam that is continuously transmitted. It is. The optical pulse generator 1 is controlled by the trigger generator 10 and generates one optical pulse each time a trigger arrives. Reference numeral 2 denotes an optical fiber to be measured for measuring chromatic dispersion by the apparatus of this embodiment. An optical circulator 3 separates Rayleigh backscattered light (reflected light traveling in a direction opposite to the input light pulse) generated in the measured optical fiber 2 and guides it to the photodetector 4. The photodetector 4 has a band equivalent to the optical pulse width T _0, and outputs a current proportional to the square of the electric field amplitude of Rayleigh backscattered light at the instantaneous time. The optical circulator 3 can be simply replaced with an optical coupler as long as the optical loss in this portion is allowed. 5 is an amplifier for amplifying an output electrical signal from the photodetector 4, 6 is a band-pass filter having an appropriate bandwidth, for example, the optical pulse width T ₀ is in the order of band 20GHz when about 50 ps (picoseconds) is there. 5 and 6 are not essential components of the present invention, but are preferably selected so that the signal-to-noise ratio of the electrical signal is appropriate.

A data sampling device 7 is synchronized with the optical pulse generator 1 by the trigger generator 10. That is, the data sampling device 7 starts the sampling operation from the time when the optical pulse generated by the optical pulse generator 1 reaches the incident end of the optical fiber 2 to be measured, and determines the Rayleigh backscattered light intensity at the instantaneous time as a function of time. Are sampled and stored as data. The time t from the time when the optical pulse reaches the incident end of the optical fiber 2 to be measured is given by the distance L from the incident end of the optical fiber 2 to be measured, where v is the speed of light in the optical fiber 2 to be measured. Therefore, the data sampling device 7 tabulates and retains the Rayleigh backscattered light intensity as a function of the distance from the incident end of the optical fiber 2 to be measured. The sampling period in the data sampling unit 7 at this time, increase than the same level as the optical pulse width T _0. Intensity of the Rayleigh backscattered light because not vary faster than theoretically optical pulse width T _0, because there is no point to be smaller than the light pulse width T ₀ the sampling period.

  The above observations are repeated. That is, the trigger generator 10 waits for the optical pulse to reach the other end of the optical fiber 2 to be measured, then repeatedly generates a trigger, repeatedly performs the same observation, and the data sampling device 7 The Rayleigh backscattered light intensity is tabulated and stored as a function of the distance from the input end of the optical fiber 2 to be measured.

  8 is a calculation device for obtaining the statistical average and variance at each distance of the intensity data of Rayleigh backscattered light collected by the observation.

  9 is a calculation device that calculates the cumulative chromatic dispersion up to a predetermined distance of the optical fiber 2 to be measured using the statistical average and dispersion at each distance of the Rayleigh backscattered light intensity obtained by the calculation device 8. .

  Reference numeral 11 denotes a calculation device that obtains the chromatic dispersion of the measured optical fiber 2 at each distance by differentiating the cumulative chromatic dispersion up to a predetermined distance obtained by the calculation device 9 with respect to the distance.

  In order to obtain the chromatic dispersion at each distance of the optical fiber from the intensity information of the Rayleigh backscattered light collected in this way, the following knowledge is required.

を光ファイバに入射した場合に、入射端の近くで観測されるレイリー後方散乱光の電界振幅の平均μ並びに分散σ^２は次式で与えられる。（単に「分散」と表記した場合は、統計学上において標準偏差の２乗として定義される量を表し、「波長分散」とは区別する。）

The first finding relates to the statistical properties of Rayleigh backscattered light. An optical pulse with a pulse width (full width at half maximum) of T ₀ and an amplitude of 1 at the peak

Is incident on the optical fiber, the mean μ of the electric field amplitude of the Rayleigh backscattered light observed near the incident end and the dispersion σ ² are given by the following equations. (When simply described as “dispersion”, it represents an amount defined as the square of the standard deviation in statistics, and is distinguished from “chromatic dispersion”.)

と以下の関係にある。

The second finding relates to the fact that when the chromatic dispersion of the optical fiber is non-zero, the optical pulse width increases as the optical pulse travels from the incident end. The pulse width T (L) at this time is the second derivative of the propagation constant β of the optical fiber at the distance x.

And has the following relationship.

なる関係があるので、式（４）は以下のようになる。

β ₂ (x) and chromatic dispersion D (x) are

Therefore, equation (4) is as follows.

The statistical dispersion of the Rayleigh backscattered light intensity in this case is expressed as follows.

From Equation (2) and Equation (6),

であるから、式（７），式（８）より、

At L = 0

Therefore, from Equation (7) and Equation (8),

について解くことにより、

を得る。以上により、レイリー後方散乱光強度に関する統計的なパラメータである平均強度や分散と、距離Ｌにおける光ファイバの累積波長分散

との関係が明らかとなった。 Substituting equation (5) into equation (9)

By solving for

Get. As described above, the average intensity and dispersion, which are statistical parameters regarding the Rayleigh backscattered light intensity, and the accumulated chromatic dispersion of the optical fiber at the distance L

The relationship was revealed.

  The above relational expression is approximately established when the input light pulse has a Gaussian waveform as given by Expression (1), and the chromatic dispersion of the optical fiber can be calculated with a certain accuracy. Although it can be used in many cases even when the optical pulse is not a strict Gaussian waveform, it is possible to derive a more accurate calculation formula for an arbitrary optical pulse waveform by more rigorous mathematical considerations. is there.

  From the above knowledge, the wavelength dispersion at each distance can be obtained by taking the following steps using the Rayleigh backscattered light intensity corresponding to each distance in the optical fiber collected by the data sampling device 7.

(1) Step of calculating the average and dispersion of Rayleigh backscattered light intensity corresponding to each distance (2) Step of calculating cumulative chromatic dispersion up to the distance L using the average and dispersion calculated at the distance L (3) ) Determining chromatic dispersion at distance L by differentiating accumulated chromatic dispersion up to distance L with respect to L

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the above embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiment examples may be appropriately combined.

1 is a configuration explanatory view showing an optical fiber chromatic dispersion measuring apparatus according to an embodiment of the present invention. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Optical pulse generator, 2 ... Optical fiber to be measured, 3 ... Optical circulator, 4 ... Photo detector, 5 ... Amplifier, 6 ... Band pass filter, 7 ... Data sampling device, 8 ... Statistical average and dispersion | distribution in each distance 9... Calculation device for calculating cumulative chromatic dispersion, 10... Trigger generating device, 11.

Claims (2)

A chromatic dispersion measuring method for measuring a distribution in a length direction of chromatic dispersion of an optical fiber,
Full width at half maximum of the pulses expressed from one end of the optical fiber by the formula (1) is repeatedly incident optical pulses T _0, and sampled each time Rayleigh backscattered light intensity as a function of time, the Rayleigh backscattered light the sampled Rayleigh backscattered light intensity observation step for maintaining the intensity as a function of distance from the input end of the optical fiber ;
Rayleigh backscattered light observed in the Rayleigh backscattered light intensity observation step, where a ₁ and a ₂ are constants specific to the material of the optical fiber, α is a loss factor of the optical fiber, and E ² is energy of the optical pulse. An average and dispersion calculating step for calculating a statistical average μ and dispersion σ ² at a distance L from the input end of the optical fiber using the equations (2) and (3), respectively ;
From the statistical mean μ and dispersion σ ² at each distance of the Rayleigh backscattered light intensity calculated in the average and dispersion calculation step, λ is the wavelength, and chromatic dispersion D (x (x) at the distance x from the input end of the optical fiber) ) Cumulative value up to the distance L

A cumulative chromatic dispersion calculating step of calculating
An optical fiber comprising: a differentiation step of obtaining chromatic dispersion at each distance by differentiating the cumulative chromatic dispersion calculated in the cumulative chromatic dispersion calculating step with respect to the distance L. Chromatic dispersion measurement method.
However,

A chromatic dispersion measuring device for measuring a distribution of a chromatic dispersion in a length direction of an optical fiber,
Full width at half maximum of the pulses expressed from one end of the optical fiber by the formula (1) is repeatedly incident optical pulses T _0, and sampled each time Rayleigh backscattered light intensity as a function of time, the Rayleigh backscattered light the sampled Rayleigh backscattered light intensity observation means for maintaining the intensity as a function of the distance from the input end of the optical fiber ;
Rayleigh backscattered light observed by the Rayleigh backscattered light intensity observation means, where a ₁ and a ₂ are constants specific to the material of the optical fiber, α is a loss factor of the optical fiber, and E ² is energy of the optical pulse. Mean and dispersion calculating means for calculating the statistical mean μ and dispersion σ ² at the distance L from the input end of the optical fiber using the equations (2) and (3), respectively ,
From the statistical mean μ and dispersion σ ² at each distance of the Rayleigh backscattered light intensity calculated by the average and dispersion calculating means, λ is the wavelength, and chromatic dispersion D (x (x) at the distance x from the input end of the optical fiber) ) Cumulative value up to the distance L

Cumulative chromatic dispersion calculating means for calculating using the equation (10),
Differentiating means for obtaining chromatic dispersion at each distance by differentiating the cumulative chromatic dispersion calculated by the cumulative chromatic dispersion calculating means with respect to the distance L. Wavelength dispersion measuring device.
However,

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