JP2014206517A - Evaluation method for crosstalk characteristic of multicore optical fiber, and system thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an evaluation method for crosstalk characteristics of a multicore optical fiber that is easy and good in reproducibility, and a system thereof.SOLUTION: When measurement light generated by a light source 1 is made incident in a single-mode operation state on one arbitrary core at one end of a multicore optical fiber (FUT) 10 to be measured through a wavelength scrambler (PS) 2 and an SMF 3 and outgoing light from the other end of the FUT 10 is received by a power meter 7 through an SMF 4 so as to measure light intensity thereof, the phase state of the measurement light is changed by applying tension to the FUT 10 by changing the interval between micromotion bases 5 and 6 to which the one end of the FUT 10 and a region at a predetermined distance from the one end are fixed, a light intensity distribution in a time region accompanying the phase change of the measurement light is acquired while the polarization state of the measurement light is changed by the PS 2, and an average electric power coupling coefficient "h" of the FUT 10 is derived by using a light intensity deviation ΔP corresponding to a triple of a standard deviation of the light intensity distribution.

Description

  The present invention relates to a technique for evaluating characteristics of an optical fiber used for optical signal propagation.

  In recent years, research and development of multi-core optical fibers having a plurality of cores in the same cladding have been actively conducted for the purpose of expanding space utilization efficiency. A multi-core optical fiber can directly increase the transmission capacity of one optical fiber by using each core as an independent optical communication transmission line. Further, by obtaining the external characteristics of the optical fiber as a spatial distribution in the cross section or as interference of signal light between a plurality of cores, an effect of improving sensitivity in optical fiber sensing can be expected.

  In application of multi-core optical fiber to optical communication, optical fiber sensing, etc., it is necessary to grasp in detail the crosstalk characteristics between cores in the multi-core optical fiber. Crosstalk characteristics can be evaluated using a light source and a power meter to measure the ratio of the emitted light intensity of the core facing the light source and its adjacent core, or the backscattered light intensity of the reference core and its adjacent core. An OTDR method for measuring (see Non-Patent Document 1) and the like are known.

  However, in the transmission method and the OTDR method described above, there are problems in that the measurement is complicated and time-consuming because the intensity of emitted light or the intensity of backscattered light of a plurality of cores is measured. In particular, the transmission method has a problem in that the convergence of the emitted light intensity is poor due to the temporal instability of the crosstalk characteristics, and sufficient measurement reproducibility cannot be obtained.

  The present invention has been made in view of the above background, and an object of the present invention is to provide a method for evaluating the crosstalk characteristics of a multi-core optical fiber that is simple and has good reproducibility, and a system thereof.

  In the present invention, an arbitrary one core in a multi-core optical fiber is used, and the light intensity distribution in the time domain accompanying the phase change of the measurement light or the light intensity distribution in the wavelength domain accompanying the wavelength change of the measurement light is measured with the measurement light. By obtaining the average power coupling coefficient h from the light intensity deviation ΔP corresponding to three times the standard deviation σ of the light intensity distribution, it is possible to obtain a simple and reproducible crosstalk characteristic. It is a means to realize the evaluation method.

  At this time, the average power coupling coefficient h is expressed by the relational expression (1).

但し、Ｌは多コア光ファイバの長さ、ｅは自然対数の底を表す。
に光強度偏差ΔＰを代入することより求めることができる。

Where L is the length of the multi-core optical fiber and e is the base of the natural logarithm.
Can be obtained by substituting the light intensity deviation ΔP into.

  According to the present invention, since the crosstalk characteristic of the core can be evaluated by measuring the emitted light intensity in any one core in the time domain or the wavelength domain, the conventional evaluation for measuring the light intensity of a plurality of cores is possible. Compared to technology, it has the effect of being able to perform measurements easily. Further, since the emission light intensity is acquired while changing the polarization state of the measurement light, it is possible to dramatically reduce the measurement uncertainty due to the temporal variation of the crosstalk characteristics. Furthermore, since evaluation can be performed using a general-purpose variable wavelength light source, the wavelength dependency of the crosstalk characteristics of the multi-core optical fiber can be easily evaluated.

It is a block diagram which shows an example of the system which implements the evaluation method of the crosstalk characteristic of this invention. It is a figure which shows the measurement example of the emitted light intensity distribution in the wavelength range measured in the wavelength 1550nm band. It is a figure which shows the evaluation result of the 2 core optical fiber by the evaluation method of the cross talk characteristic of the time domain of this invention. It is a figure which shows the evaluation result in the time domain and the wavelength domain by the evaluation method of the crosstalk characteristic of this invention.

  Hereinafter, an embodiment of a method for evaluating a crosstalk characteristic of the present invention will be described with reference to the drawings.

  FIG. 1 shows an example of a crosstalk characteristic evaluation system that implements the crosstalk characteristic evaluation method of the present invention. This system includes a light source 1, a polarization scrambler (PS) 2, a single mode fiber ( SMF) 3, 4, fine movement table (MS) 5, 6 and power meter (PM) 7.

  The light source 1 is for generating measurement light having a predetermined measurement wavelength, and the measurement light preferably has a line width of at least several hundred MHz, and a general-purpose distributed feedback laser, wavelength variable light source, or the like is used. it can. The polarization scrambler 2 is for changing the polarization state of measurement light from the light source 1, and the light source 1 and the polarization scrambler 2 are connected using an optical system using an optical fiber or a lens.

  The SMFs 3 and 4 are for making the measurement light input / output to / from the measured multi-core optical fiber (FUT) 10 enter a single mode operation state, and each has a length of about 5 m. Specifically, the SMF 3 is disposed between the polarization scrambler 2 and one end of the multi-core optical fiber 10 to be measured (measurement light incident end), and the SMF 4 is the other end of the multi-core optical fiber 10 to be measured ( It is disposed between the measurement light emission end) and the power meter 7.

  The fine movement bases 5 and 6 are for entering measurement light into any one core at one end of the measured multi-core optical fiber 10 and for applying tension to the measured multi-core optical fiber 10. Specifically, the fine adjustment base 5 is opposed to the end face of the SMF 3 and the end face of one end of the multicore optical fiber 10 to be measured with a slight space therebetween, and the measurement light emitted from the SMF 3 is one end of the multicore optical fiber 10 to be measured. One end of each of the SMF 3 and the measured multi-core optical fiber 10 is fixed so as to be incident on any one of the cores, and the fine movement base 6 has a predetermined length from one end of the measured multi-core optical fiber 10. Measurement is performed by changing the interval between the fine movement bases 5 and 6 by fixing a part (usually several meters) apart and further operating one or both of the fine movement bases 5 and 6 (usually the fine movement base 6). It is possible to apply tension to the multi-core optical fiber 10. Note that the other end of the multi-core optical fiber 10 to be measured and the SMF 4 are usually fusion-connected.

  The power meter 7 receives light emitted from the other end of the multi-core optical fiber 10 to be measured via the SMF 4 and measures the light intensity.

  In the above configuration, the measurement light from the light source 1 is made into an arbitrary polarization state by the polarization scrambler 2 and is incident on the measurement target core of the measured multi-core optical fiber 10 via the SMF 3. The outgoing light from the multi-core optical fiber 10 to be measured is received by the power meter 7 via the SMF 4 and the light intensity is measured.

  Here, the SMFs 3 and 4 connected to both ends of the multicore optical fiber 10 to be measured need only be long enough to realize single mode operation at the measurement wavelength, and the length is not limited to 5 m. If the measured multi-core optical fiber 10 is not bent between the fine movement bases 5 and 6, the length from the one end of the measured multi-core optical fiber 10 to the fixed portion (interval between the fine movement bases 5 and 6) is An arbitrary length can be set. In addition, the power meter 7 does not perform the averaging process, and the sampling speed (time) is preferably sufficiently fast (short) as compared with the displacement speed (time) of the polarization state of the polarization scrambler 2. Furthermore, automatic measurement can be performed by controlling the light source 1, the polarization scrambler 2, the fine movement bases 5, 6 and the power meter 7 with a computer (PC) 8.

  When measurement light is incident on a desired core of the multi-core optical fiber 10 to be measured using the measurement system of FIG. 1, temporal variation in the intensity of the emitted light is observed at the exit end of the core of the multi-core optical fiber 10 to be measured. The Here, the temporal variation of the emitted light intensity occurs due to fluctuations in the propagation direction of the light wave coupling characteristics between adjacent cores in the multi-core optical fiber 10 to be measured and its dependence on the polarization state.

  Therefore, in the present invention, by operating the fine movement base 6 and applying tension to the multi-core optical fiber 10 to be measured, the phase state of the measurement light is intentionally changed, and the time of the emitted light intensity (movement of the fine movement base) is changed. Detect the histogram on the (amount) axis. Furthermore, by sampling the emitted light intensity while changing the polarization state of the measurement light by the polarization scrambler 2, the change in the emitted light intensity due to the crosstalk characteristics in the measured multi-core optical fiber 10 can be performed at high speed. It can be easily detected. It is desirable that the polarization state is set to at least 50 different states with respect to the movement amount of the fine movement table 6 and the emitted light intensity is sampled.

  Here, in FIG. 1, a variable wavelength light source is used as the light source 1, and the histogram of the emitted light intensity on the wavelength axis is observed by changing the wavelength of the measurement light, and the crosstalk in the measured multi-core optical fiber 10. It also becomes possible to detect a change in the intensity of the emitted light due to the characteristics at high speed and easily. When evaluating a wavelength region using a wavelength tunable light source, it is preferable that the line width of the wavelength tunable light source is several MHz or less and the measurement wavelength can be controlled with an accuracy of 0.02 nm or less. Moreover, when evaluating the wavelength region using a wavelength variable light source, it is not necessary to install two fine movement tables.

  FIG. 2 shows a histogram of the emitted light intensity in the wavelength region measured in the wavelength 1550 nm band. In this measurement example, the wavelength is swept at intervals of 0.02 nm, and the emitted light intensity is sampled 100 times while changing the polarization state at each wavelength. FIG. 2 shows that the light intensity distribution has a Gaussian probability distribution. Therefore, the statistical deviation ΔP of the received light intensity can be considered as three times the standard deviation σ.

  By obtaining ΔP = 3σ (unit: dB) from the standard deviation σ of the received light intensity distribution (histogram) in the measurement wavelength region (or time region), it depended on the crosstalk characteristics in the multicore optical fiber 10 to be measured. Statistical light intensity changes can be extracted more accurately. It should be noted that by deriving ΔP as the maximum deviation from the average value of the received light intensity distribution, it is possible to evaluate the crosstalk characteristic of the multicore optical fiber 10 to be measured with a smaller number of received light intensity samplings.

Here, if the power coupling coefficient between parallel waveguides of length L (unit: m) is h (unit: m ⁻¹ ), the light intensity change ΔXT (unit: dB) due to crosstalk in the parallel waveguide. Is given by the following equation (2).

従って、上述のΔＰとΔＸＴの関係から次式（３）が導かれる。

Therefore, the following equation (3) is derived from the relationship between ΔP and ΔXT described above.

（３）式をｈについて解くと、次式（４）が得られる。

When the equation (3) is solved for h, the following equation (4) is obtained.

但し、ｅは自然対数の底である。

Where e is the base of the natural logarithm.

Therefore, the average power coupling coefficient h (unit: m ⁻¹ ) at the measurement wavelength of the multi-core optical fiber 10 to be measured is obtained by substituting the light intensity deviation ΔP into the relational expression (4).

  Further, by substituting the average power coupling coefficient h into the following relational expression (5), the crosstalk XT (unit: dB) at the measurement wavelength of the multicore optical fiber 10 to be measured can be evaluated.

表１にその特性を示す２種類の２コア光ファイバ（ＴＣＦ）を用い、本発明の時間領域の評価手法によりクロストーク特性を測定した。

Using two types of two-core optical fibers (TCF) whose characteristics are shown in Table 1, crosstalk characteristics were measured by the time domain evaluation method of the present invention.

図３に本発明の時間領域のクロストーク特性の評価方法による上記２種類のＴＣＦの評価結果を示す。図中の２本の実線は表１に示した諸元から数値計算により求めたクロストーク特性を示す。また、図中のプロットは本発明の評価方法による測定結果を示している。図３から本発明の評価方法による測定結果は、計算結果と良く一致しており、被測定ＴＣＦのクロストーク特性が、その波長依存性を含め良好に評価出来ていることが分かる。ここで、従来の透過法により、波長1600ｎｍにおけるＴＣＦ２のクロストーク特性を評価した場合、その最大偏差は５ｄＢ以上であった。一方、本発明の評価方法における最大偏差は２ｄＢ以下であり、本発明のクロストーク特性の評価方法が、従来の評価技術よりも優れた再現性を有することが分かる。

FIG. 3 shows the evaluation results of the two types of TCFs according to the method for evaluating the crosstalk characteristics in the time domain of the present invention. Two solid lines in the figure indicate crosstalk characteristics obtained by numerical calculation from the specifications shown in Table 1. The plots in the figure show the measurement results obtained by the evaluation method of the present invention. FIG. 3 shows that the measurement result by the evaluation method of the present invention agrees well with the calculation result, and that the crosstalk characteristic of the TCF to be measured can be evaluated well including its wavelength dependency. Here, when the crosstalk characteristic of TCF2 at a wavelength of 1600 nm was evaluated by a conventional transmission method, the maximum deviation was 5 dB or more. On the other hand, the maximum deviation in the evaluation method of the present invention is 2 dB or less, and it can be seen that the evaluation method of the crosstalk characteristic of the present invention has reproducibility superior to that of the conventional evaluation technique.

  FIG. 4 shows the evaluation results of the time domain and the wavelength domain by the crosstalk characteristic evaluation method of the present invention. The plot in the figure shows the light intensity deviation ΔP determined by the time domain and wavelength domain evaluation methods. It can be confirmed from FIG. 4 that the evaluation results in the time domain and the wavelength domain are in good agreement.

  As described above, according to the present invention, the power coupling coefficient h is derived using the following relational expression (6) using the light source, the polarization scrambler, and the power meter. Makes it possible to evaluate crosstalk characteristics that are simple and excellent in reproducibility.

  1: Light source, 2: Polarization scrambler (PS), 3, 4: Single mode fiber (SMF), 5, 6: Fine motion table, 7: Power meter (PM), 8: Computer (PC), 10: Covered Measuring multi-core optical fiber.

M. Nakazawa et al., Opt.Express, vol.20, no.11, p.12530-12540, 2012.

Claims (7)

A method for evaluating crosstalk characteristics between cores of a multi-core optical fiber,
Measurement light of a predetermined wavelength generated by a light source is incident on one arbitrary core of one end of the multi-core optical fiber to be measured in a single mode operation state, and emitted light from the other end of the multi-core optical fiber to be measured When measuring the intensity of light received by a power meter,
Tension is applied to the multi-core optical fiber to be measured by the tension applying means to change the phase state of the measurement light, the light intensity distribution in the time domain accompanying the phase change of the measurement light, and the polarization state of the measurement light by the polarization scrambler Crosstalk characteristics obtained by obtaining an average power coupling coefficient h of a multi-core optical fiber to be measured using a light intensity deviation ΔP corresponding to three times the standard deviation of the light intensity distribution. Evaluation method. A method for evaluating crosstalk characteristics between cores of a multi-core optical fiber,
The measurement light generated by the light source is incident on one arbitrary core of one end of the multicore optical fiber to be measured in a single mode operation state, and the output light from the other end of the multicore optical fiber to be measured is received by the power meter. When measuring the light intensity,
Using a tunable light source as the light source, the wavelength of the measurement light is changed, and the light intensity distribution in the wavelength region associated with the change in the wavelength of the measurement light is acquired while changing the polarization state of the measurement light using a polarization scrambler. A method for evaluating crosstalk characteristics, wherein an average power coupling coefficient h of a multicore optical fiber to be measured is derived using a light intensity deviation ΔP corresponding to three times a standard deviation of an intensity distribution. The light intensity deviation ΔP is expressed by the following relational expression

Where L is the length of the multi-core optical fiber (unit: m), and e is the base of the natural logarithm.
The crosstalk characteristic evaluation method according to claim 1, wherein an average power coupling coefficient h of the multicore optical fiber to be measured is obtained by substituting into. A system for evaluating crosstalk characteristics between cores of a multi-core optical fiber,
A light source that generates measurement light of a predetermined wavelength;
A polarization scrambler that changes the polarization state of the measurement light from the light source;
Single mode operation state applying means for injecting measurement light that has passed through the polarization scrambler into any one core of one end of the multicore optical fiber to be measured in a single mode operation state;
Tension applying means for applying tension to the multi-core optical fiber to be measured;
A power meter that receives light emitted from the other end of the multi-core optical fiber to be measured and measures its light intensity;
Tension is applied to the multi-core optical fiber to be measured by the tension applying means to change the phase state of the measurement light, the light intensity distribution in the time domain accompanying the phase change of the measurement light, and the polarization state of the measurement light by the polarization scrambler Crosstalk characteristics obtained by obtaining an average power coupling coefficient h of a multi-core optical fiber to be measured using a light intensity deviation ΔP corresponding to three times the standard deviation of the light intensity distribution. Evaluation system. 5. The crosstalk characteristic of claim 4, wherein the tension applying means includes two fine movement bases each fixing one end of the multi-core optical fiber to be measured and a portion separated from the one end by a predetermined length. Evaluation system. A system for evaluating crosstalk characteristics between cores of a multi-core optical fiber,
A wavelength tunable light source that generates measurement light whose wavelength changes;
A polarization scrambler that changes the polarization state of the measurement light from the variable wavelength light source;
Single mode operation state applying means for injecting measurement light that has passed through the polarization scrambler into any one core of one end of the multicore optical fiber to be measured in a single mode operation state;
A power meter that receives light emitted from the other end of the multi-core optical fiber to be measured and measures its light intensity;
The wavelength of the measurement light is changed by the wavelength tunable light source, and the light intensity distribution in the wavelength region accompanying the change in the wavelength of the measurement light is acquired while changing the polarization state of the measurement light by the polarization scrambler. A crosstalk characteristic evaluation system, wherein an average power coupling coefficient h of a measured multi-core optical fiber is derived using a light intensity deviation ΔP corresponding to three times the deviation. The light intensity deviation ΔP is expressed by the following relational expression

Where L is the length of the multi-core optical fiber (unit: m), and e is the base of the natural logarithm.
The crosstalk characteristic evaluation system according to any one of claims 4 to 6, wherein an average power coupling coefficient h of the multicore optical fiber to be measured is obtained by substituting

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