JP6751362B2 - Propagation delay time difference measurement apparatus between spatial channels and propagation delay time difference measurement method between spatial channels - Google Patents

Description

The present invention relates to a technique for measuring a propagation delay time difference between spatial channels in an optical fiber for spatial multiplexing transmission such as a multimode optical fiber or a multicore optical fiber or an optical device.

As a technique for expanding the transmission capacity per optical fiber in optical communication, there is a spatial multiplexing transmission technique using a multimode optical fiber or a multicore optical fiber. In spatial multiplexing transmission, the transmission capacity is expanded by multiplexing signals in spatially different transmission channels such as modes and cores. However, if the delay time related to signal transmission differs between spatial channels, there is a problem that the load of signal processing required for signal restoration increases. Therefore, researches for reducing the delay time difference in the spatial multiplexing optical fiber have been actively conducted (for example, refer to Non-Patent Document 1), and accordingly, a technique capable of measuring the delay time difference with high resolution is required.

As a technique for measuring a delay time difference between spatial channels, there is, for example, a frequency swept optical interference method (FMCW method) (for example, see Non-Patent Document 2). The measurement principle of the FMCW method will be described below. FIG. 1 is an example of an apparatus configuration used in the FMCW method. Note that, here, a two-mode single-core fiber is used as the measured fiber, and a single-mode single-core fiber is used except for the measured fiber.

ここで、
ｉ_１、ｉ_２、ｉ_３及びφ_１、φ_２、φ_３は振幅及び位相の定数、
φ_{ｎｏｉｓｅ１}（ｔ）、φ_{ｎｏｉｓｅ２}（ｔ）、φ_{ｎｏｉｓｅ３}（ｔ）はそれぞれθ_１（ｔ）、θ_２（ｔ）、θ_３（ｔ）に含まれる位相雑音、
γは周波数掃引速度、
τ_１及びτ_２はそれぞれモード１とモード２の被測定ファイバ中の伝搬に係る遅延時間
である。 A light source having a frequency sweeping means is used as the light source, and continuous light whose frequency is swept linearly with respect to time is emitted. The emitted light is split into two by an optical demultiplexing element, one of which is incident as a test light on the fiber to be measured, and the other of which is incident as a local light on a delay fiber that gives a delay time τ _lo . In order to reduce the influence of the phase noise of the light source, the delay fiber length here is set so that the optical path length difference with respect to the measured fiber is sufficiently shorter than the coherence length of the light source. The fiber to be measured is axially connected on the incident side so that the test light propagates in the fiber to be measured in two modes. After the test light and the local light after propagating through the fiber under measurement are combined by the combining element, the beat signal resulting from the combination is converted into an electric signal by the light receiver. At this time, the beat signal I(t) detected by the light receiver is described by the following equation.

here,
i ₁ , i ₂ , i ₃ and φ ₁ , φ ₂ , φ ₃ are amplitude and phase constants,
φ _noise1 (t), φ _noise2 (t), and φ _noise3 (t) are phase noises included in θ ₁ (t), θ ₂ (t), and θ ₃ (t), respectively.
γ is the frequency sweep speed,
τ ₁ and τ ₂ are delay times related to propagation in the fiber under measurement of mode 1 and mode 2, respectively.

As shown in the equations (1) to (4), I(t) has three frequencies of frequencies γ(τ ₁ −τ _lo ), γ(τ ₂ −τ _lo ), and γ(τ ₁ −τ ₂ ). There are components, and these frequencies are determined by the delay time of each mode.

The frequency component of I(t) is analyzed by Fourier transform. The result of the Fourier transform of I(t) is shown in FIG. From the comparison of the peak positions in FIG. 2, the frequency difference Δf _DMD between θ ₂ (t) and θ ₁ (t) is obtained, and the delay time difference Δτ _DMD between mode 2 and mode 1 is calculated by the following equation.

T. Sakamoto et al. , "Differential mode delay managed transmission line for WDM-MIMO system using multi-step index fiber", J. Org. Lightw. Technol. , Vol. 30, No. 17, pp. 2783-2787, 2012. T. Ahn et al. , "High-resolution differential mode delay measurement for a multimodal optical fiber using a modified optical frequency replenishment dome reflex. Express, Vol. 13, No. 20, pp. 8256-8262, 2005. J. Lee et al. , "Fourier-domain low-coherence interferometry for differential mode delay analysis of an optical fiber", Opt. Lett. , Vol. 31, No. 16, pp. 2396-2398. 2006. T. Ahn et al. , "Optical frequency-domain chromatic dispersion dispersion measurement method for higher-order modes in an optical fiber", Opt. Express, Vol. 13, No. 25, pp. 10040-10048, 2005.

ここで
ΔｆはＩ（ｔ）のフーリエ変換における周波数分解能、
Ｔは測定時間、
ΔＦは周波数掃引幅
である。 Ignoring the phase noise, the measurement resolution Δτ of the delay time difference of the conventional FMCW method is described by the following equation.

Where Δf is the frequency resolution in the Fourier transform of I(t),
T is the measurement time,
ΔF is a frequency sweep width.

As shown in Expression (6), the delay time difference resolution can be improved by expanding the frequency sweep width of the test light. However, in practice, there is a limit to the frequency sweep width that can be used for measurement due to the performance of the light source and the band limitation of the element used. Generally, a wavelength tunable semiconductor laser with a wide frequency sweep width is used as a light source, but since the coherence length is short, the delay fiber length of the local light is measured to reduce the effect of phase noise and obtain the desired resolution. It is necessary to set strictly according to the length. In other words, there is the first problem that the accuracy of the length of the delay fiber must be increased in order to increase the measurement resolution of the device that measures the delay time difference between spatial channels with the FMCW.

In addition, the delay time difference between spatial channels generally has wavelength dependence. For this reason, when the frequency sweep width is expanded, the measured delay time difference has a spread, and it may be difficult to accurately evaluate the delay time difference. Therefore, it is required to measure the delay time difference between spatial channels with a limited frequency sweep width. That is, in the device for measuring the delay time difference between the spatial channels by the FMCW, it is necessary to realize high resolution with a limited frequency sweep width in order to evaluate the wavelength dependence of the delay time difference between the spatial channels. There was a problem.

In addition, as a delay time difference measurement method other than the FMCW method, there are a low coherence interferometry method (Non-Patent Document 3) and an impulse response used in Non-Patent Document 1. However, the resolution in the low coherence interferometry has the same relationship with the optical band as in the equation (6), and the impulse response also needs to be expanded in order to reduce the pulse width in order to improve the resolution. It has the same problem as above.

In order to solve the above problems, an object of the present invention is to provide a spatial channel propagation delay time difference measuring apparatus and a spatial channel propagation delay time difference measuring method having high resolution in a limited optical band.

In order to achieve the above object, the inter-spatial channel propagation delay time difference measuring device and the inter-spatial channel propagation delay time difference measuring method according to the present invention sweep a light frequency of a light source for each spatial channel to obtain a beat signal with local light. It was decided to measure the propagation delay time difference between spatial channels by dividing the beat frequency difference generated from the beat signal and analyzed from the time change rate of the phase difference of the beat signal by the frequency sweep speed.

Specifically, the inter-spatial channel propagation delay time difference measuring device according to the present invention is
A light source unit that emits continuous light whose frequency is swept temporally,
An optical demultiplexing device that splits continuous light from the light source unit into two;
A delay optical fiber for delaying one of the continuous lights branched by the optical demultiplexing element as local light,
A selective excitation unit that selectively enters the other of the continuous light branched by the optical demultiplexing element as a test light into a specific spatial channel of the DUT,
A selective separation unit for selectively separating the specific spatial channel of the test light transmitted through the DUT,
An optical multiplexing element that multiplexes the local light passing through the delay optical fiber and the test light of the specific spatial channel separated by the selective separation unit into a beat signal,
The two beat signals based on the test lights of the two different specific spatial channels are acquired from the optical multiplexing device, and the two specific spaces in the DUT are obtained from the time change rate of the phase difference between the beat signals. An arithmetic processing device for calculating a propagation delay time difference between channels,
Equipped with.

Further, the method for measuring the propagation delay time difference between spatial channels according to the present invention,
Continuous light whose frequency has been swept temporally is split into two, one of which is used as a local light to delay with a delay optical fiber, and the other of which is used as a test light to selectively enter a specific spatial channel of the DUT. Procedure and
A beat signal generation procedure for selectively separating the specific spatial channel from the test light transmitted through the device under test, and combining with the local light via the delay optical fiber to form a beat signal,
The two beat signals based on the test lights of the two specific spatial channels that are different in the beat signal generation procedure are acquired, and the two specific beat signals in the DUT are obtained from the time change rate of the phase difference between the beat signals. An arithmetic processing procedure for calculating a propagation delay time difference between spatial channels,
I do.

In the present invention, an element that selectively excites/separates a spatial channel is applied to the conventional FMCW device configuration, the beat signal is measured for each channel, and the beat frequency difference is determined from the time change of the phase difference between the beat signals. Thus, the delay time difference is measured with higher resolution than before. If the phases of the beat signal of mode 1 and the beat signal of mode 2 are θ ₁ (t) and θ ₂ (t), respectively, the phase difference Δθ(t) between them is as follows.

An example of the measurement result of Δθ(t) is shown in FIG. If the phase noise is ignored, Δθ(t) changes linearly with a slope of 2πΔf _DMD with respect to time. Therefore, Δf _DMD can be analyzed by linear fitting and Δτ _DMD can be obtained by using Expression (5). Since the conventional FMCW method analyzes the frequency from the Fourier transform, a small Δf _DMD such that Δθ(t) does not occur by 2π or more within the measurement time cannot be analyzed due to the frequency resolution of the Fourier transform, whereas the present invention uses Δθ( Since Δf _DMD is directly obtained from t), analysis can be performed even if Δθ(t) is 2π or less.

σ＜２πであることから、式（６）と式（８）の比較より本発明による方法は従来の遅延時間差測定法に比べて高分解能となることがわかる。 Actually, phase fluctuations and disturbances of the light source cause phase noise in Δθ(t) as shown in FIG. 3, so the delay time difference resolution in the present invention is determined by the phase noise. Assuming that the phase noise follows a normal distribution with a standard deviation σ, it can be analyzed if σ<2πΔf _DMD T (T is a measurement time). Therefore, the delay time difference resolution Δτ in the present invention is described by the following equation.

Since σ<2π, it can be seen from the comparison between the equations (6) and (8) that the method according to the present invention has a higher resolution than the conventional delay time difference measuring method.

Therefore, the present invention can provide a propagation delay time difference measuring apparatus between spatial channels and a propagation delay time difference measuring method between spatial channels having a high resolution in a limited optical band.

The arithmetic processing unit of the inter-spatial channel propagation delay time difference measuring apparatus according to the present invention is characterized by acquiring the beat signals for the same spatial channel a plurality of times and averaging the phases of the plurality of the beat signals. Further, the method for measuring the propagation delay time difference between spatial channels according to the present invention is characterized in that, in the arithmetic processing procedure, the beat signals for the same spatial channel are acquired a plurality of times, and the phases of the plurality of beat signals are arithmetically averaged. And

In the present invention, the phase noise can be reduced by measuring the phase a plurality of times and averaging them. That is, the delay time resolution can be improved by averaging. When the arithmetic mean is performed N times, the standard deviation of the phase noise is σ/√N, so the resolution is improved by √N times.

The arithmetic processing device of the arithmetic processing device of the inter-spatial channel propagation delay time difference measuring device according to the present invention is a beat signal acquisition start time at which the light source unit starts acquiring the beat signal from a frequency sweep start time at which frequency sweep is started. It is characterized in that the beat signals for the same spatial channel are acquired a plurality of times at different times, and the phases of the plurality of beat signals are arithmetically averaged. Further, the method for measuring the propagation delay time difference between spatial channels according to the present invention, in the arithmetic processing procedure, from the frequency sweep start time to start sweeping the frequency in the light incidence procedure to the beat signal acquisition start time to start acquiring the beat signal. The beat signals for the same spatial channel are acquired a plurality of times at different times, and the phases of the plurality of beat signals are arithmetically averaged.

Phase fluctuations due to mode coupling can be reduced by changing the initial frequency of continuous light that sweeps the frequency to generate a beat signal and averaging the phases. Therefore, the present invention can measure the propagation delay time difference between spatial channels even when coupling occurs between spatial channels.

The light source unit of the inter-spatial channel propagation delay time difference measuring apparatus according to the present invention is characterized in that an external frequency modulator or an external phase modulator is used as an external modulator.

The ability to improve resolution without increasing the frequency sweep width also means expanding the choice of light sources used for measurement. For example, a sufficient resolution can be realized even by using a laser having a small phase noise such as a fiber laser and performing a frequency sweep in a narrow band of about 10 GHz by an external modulator such as a single sideband modulator.

The light source unit of the inter-spatial channel propagation delay time difference measuring apparatus according to the present invention includes a central wavelength changing unit that changes the central wavelength of the continuous light,
The arithmetic processing unit,
Calculating the propagation delay time difference between the spatial channels for each different center wavelength of the continuous light, measuring the wavelength dependence of the propagation delay time difference between the spatial channels in the DUT, of the propagation delay time difference between the spatial channels. It is characterized in that wavelength differentiation is performed with respect to the wavelength dependence, and the wavelength dispersion difference between the spatial channels is calculated by dividing by the length of the DUT.

Further, the method for measuring the propagation delay time difference between spatial channels according to the present invention,
In the light incident procedure, changing the central wavelength of the continuous light,
In the arithmetic processing procedure,
Calculate the propagation delay time difference between the spatial channels for each different center wavelength of the continuous light, to measure the wavelength dependence of the propagation delay time difference between the spatial channels in the DUT,
The wavelength dependence of the propagation delay time difference between the spatial channels is differentiated by wavelength, and the wavelength dispersion difference between the spatial channels is calculated by dividing by the length of the DUT.

As described above, according to the present invention, the resolution can be improved without widening the frequency sweep width, so that measurement can be performed even when the wavelength dependence of the propagation delay time difference between spatial channels is large.

INDUSTRIAL APPLICABILITY The present invention can provide a spatial channel propagation delay time difference measuring device and a spatial channel propagation delay time difference measuring method having high resolution within a limited optical band.

It is a block diagram showing an example of the device composition used for the FMCW method. It is a conceptual diagram of the Fourier transform result of the beat signal in the FMCW method. It is an example of the measurement result of the phase difference of a beat signal measured in the present invention. It is a flow chart which shows a flow of measurement in an embodiment of the present invention. FIG. 1 is a block diagram showing an example of a device configuration used in an embodiment of the present invention.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In this specification and the drawings, constituent elements having the same reference numerals indicate the same elements.

(Embodiment 1)
Here, as an example, a case will be described in which a two-mode single-core fiber is used as the measured fiber and the delay time difference between the two modes is measured. The coupling between spatial channels is assumed to be negligible.
FIG. 5 is a configuration diagram illustrating a propagation delay time difference measuring apparatus 301 between spatial channels (hereinafter, also referred to as measuring apparatus 301) of the present embodiment. The measuring device 301 is
A light source unit 11 for emitting continuous light whose frequency is swept,
An optical demultiplexing element 12 that splits continuous light from the light source unit 11 into two,
A delay optical fiber 13 for delaying one of the continuous lights branched by the optical demultiplexer 12 as local light,
A selective excitation unit 14 that selectively injects the other of the continuous light split by the optical demultiplexing element 12 as a test light into a specific spatial channel of the DUT;
A selective separation unit 15 that selectively separates the specific spatial channel of the test light transmitted through the object to be measured;
An optical multiplexing element 16 that multiplexes the local light that has passed through the delay optical fiber 13 and the test light of the specific spatial channel that has been separated by the selective separation unit 15 into a beat signal,
The two beat signals based on the test lights of the two different specific spatial channels are acquired from the optical multiplexing element 16, and the two specific spaces in the DUT are obtained from the time change rate of the phase difference between the beat signals. An arithmetic processing unit 17 for calculating a propagation delay time difference between channels;
Equipped with.
The arithmetic processing device 17 includes a light receiver 21 that converts a beat signal into an electric signal, an A/D converter 22 that converts the electric signal from analog to digital, and an arithmetic processing unit that performs arithmetic processing.

Here, the measured object measured by the measuring device 301 in FIG. 5 is an optical fiber, and is shown as the measured fiber 100 in FIG. The measured fiber 100 is not included in the measuring device 301. The spatial channel in the fiber under test 100 is the propagation mode. In addition, in FIG. 5, except for the fiber 100 to be measured, it is assumed that the continuous light from the light source unit 100 is composed of a single core fiber that propagates in a single mode.

The measuring device 301 is
Continuous light whose frequency has been swept temporally is split into two, one of which is used as a local light to delay with a delay optical fiber, and the other of which is used as a test light to selectively enter a specific spatial channel of the DUT. Procedure and
A beat signal generation procedure for selectively separating the specific spatial channel from the test light transmitted through the device under test, and combining with the local light via the delay optical fiber to form a beat signal,
The two beat signals based on the two different test lights of the specific spatial channels are acquired in the beat signal generation procedure, and the two specific signals in the DUT are acquired from the time change rate of the phase difference between the beat signals. An arithmetic processing procedure for calculating a propagation delay time difference between spatial channels,
The propagation delay time difference between spatial channels is acquired by the propagation delay time difference measurement method between spatial channels.

FIG. 4 is a flowchart illustrating a calculation processing procedure performed by the calculation processing device 17. First, in step S01, the beat signal is measured for mode 1. A light source having a frequency sweeping unit is used as the light source unit 11 and emits continuous light whose frequency is swept linearly with respect to time. Since the measurement apparatus 301 can obtain a high delay time resolution even with a narrow frequency sweep width, a laser having a small phase noise such as a fiber laser is used as the light source unit 11 and the frequency is swept by the external modulator. The external modulator here may be either a frequency modulator or a phase modulator.

The frequency-swept continuous light is split into two by an optical demultiplexing element 12, one of which is incident as a local light on a delay fiber 13 which gives a delay time τ _lo, and the other of which is used as a test light using a selective pumping section 14 for selecting a mode. And enters the measured fiber 100 in mode 1. In this embodiment, since a laser with high coherence (coherence length of fiber laser: coherence length to several tens of kilometers) is used as the light source, the delay fiber length here is as long as the optical path length difference with respect to the measured fiber is within the coherence length. It does not need to be set strictly.

ただし、θ_１，１（ｔ）は次式で表される。

The test light of mode 1 after propagating through the fiber 100 to be measured is extracted by the selective separation unit 15 and is combined with the local light by the optical combining element 16. The beat signal I 1 — _{0, 1} (t) _{resulting from the} multiplexing is described by the following equation.

However, θ _1,1 (t) is expressed by the following equation.

I 1 — _0,1 (t) is converted into an electric signal by the light receiver 21, and converted into a digital signal by the A/D converter 22.

In step S02, the arithmetic processing unit _{23, I 1_0,1} signals _{I 1_π / 2,1} (t) of the phase of the delayed [pi / 2 in _(t), is calculated by the Hilbert transform of _{I 1_0,1} (t) .. I _{1 — π/2, 1} (t) is described by the following equation.

In step S03, I 1 — ₀ , 1 (t) and I _{1 — π/2, 1} (t) are used to calculate θ _1,1 (t) by the following equation.

ここで＜φ_{ｎｏｉｓｅ１、ｊ}（ｔ）＞及び＜φ_１、ｊ＞はそれぞれφ_{ｎｏｉｓｅ１、ｊ}（ｔ）とφ_１、ｊの加算平均を表す。
なお、ステップ４の加算平均は位相雑音を低減し遅延時間差分解能を向上させるために行うものであり、所望の分解能に対して位相雑音が小さい場合、Ｎ＝１として加算平均を省略し、θ_１（ｔ）＝θ_１、１（ｔ）としてもよい。 In step S04, the above steps S01 to S03 are performed N times, and N θ _1,j (t) (j=1 to N) are arithmetically averaged by the following equation to obtain θ ₁ (t).

Here, <φ _{noise1, j} (t)> and <φ _{1, j} > represent the arithmetic mean of φ _{noise1, j} (t) and φ _{1, j} , respectively.
Note that averaging of step 4 is to carry out in order to improve the time difference resolution delay to reduce phase noise, if the phase noise is small, averaging the N = 1 is omitted for the desired resolution, theta ₁ (T)=θ _1,1 (t).

The steps S01 to S04 are also performed for the mode 2 to calculate the phase θ ₂ (t) of the beat signal of the mode 2.

ここで、Δｆ_ＤＭＤは次式で表される。

式（１４）で得られたΔθ（ｔ）に線形フィッティングを行い、傾きからΔｆ_ＤＭＤを求める。 In step S05, the phase difference Δθ(t) between the mode 2 beat signal and the mode 1 beat signal is calculated. The calculated Δθ(t) is given by the following equation.

Here, Δf _DMD is expressed by the following equation.

Linear fitting is performed on Δθ(t) obtained by the equation (14), and Δf _DMD is obtained from the slope.

σ＜２πであるから、式（６）と式（１６）の比較より、本実施形態の空間チャネル間伝搬遅延時間差測定方法は非特許文献２の方法に比べて高分解能で遅延時間差を測定できる。 Finally, in step S06, the delay time difference Δτ _DMD between mode 1 and mode 2 is calculated by Δτ _DMD =Δf _DMD /γ. At this time, _assuming that φ _noise2,j (t)−φ _noise1,k (t) (j≠k) follows a normal distribution with standard deviation σ, the delay time difference is measured with the resolution Δτ of the following equation.

Since σ<2π, a comparison between equations (6) and (16) shows that the method for measuring the propagation delay time difference between spatial channels of the present embodiment can measure the delay time difference with higher resolution than the method of Non-Patent Document 2. ..

The arithmetic processing unit 17 is characterized by acquiring the beat signals for the same spatial channel a plurality of times and averaging the phases of the plurality of the beat signals. Since the arithmetic processing unit 17 calculates the phase for each of the plurality of beat signals, the phase noise reduction by the averaging is effective. For example, when the beat signal of the conventional FMCW method represented by the equation (1) is added and averaged, the phase noise and the phase constant term are different at each measurement, and therefore the addition converges to 0. Also, when the waveforms after the Fourier transform shown in FIG. 2 are added and averaged, the delay time difference resolution is determined by the frequency resolution of the Fourier transform, and therefore the resolution is not improved as compared with the equation (6). The measuring apparatus 301 is significantly different from the method of Non-Patent Document 2 in that the resolution can be improved by averaging the phases.

(Embodiment 2)
The present embodiment will be performed in the case where coupling occurs between spatial channels. The configuration of the inter-spatial-channel propagation delay time difference measuring apparatus of this embodiment is the same as that of the measuring apparatus 301 of FIG. In the present embodiment, steps S01 to S03 in the flowchart shown in FIG. 4 are performed N times while changing the time from the frequency sweep start time of the test light to the beat signal measurement start time, and N beat signals having different measurement start times are performed. This is different from the first embodiment in that the phase of is added and averaged in step S04. In addition, here, the case where the fiber to be measured is a two-mode single-core fiber and mode coupling occurs at the incident end of the test light and at one position in the fiber to be measured will be described.

ここで
φ_ｘ１は位相定数、
ｚ_ｘは結合が生じた距離地点、
Ｌは被測定ファイバ長、
αは結合の強さを示す定数でありα＜＜１である。
式（１７）において第４項がモード結合による影響を表し、時間に対して線形でない変化を与える。
ステップＳ０１〜Ｓ０３において、ｊ＝１では式（１７）に示されるθ_１，１（ｔ）を測定する。 When coupling occurs between modes, the phase θ _1,1 (t) of the beat signal of mode 1 is described by the following equation.

Where φ _x1 is the phase constant,
z _x is the distance point where the bond occurs,
L is the measured fiber length,
α is a constant indicating the strength of the bond and α<<1.
In Equation (17), the fourth term represents the effect of mode coupling, and gives a non-linear change with time.
In steps S01 to S03, when j=1, θ _1,1 (t) shown in equation (17) is measured.

ここでＴはＮ回測定のトータルの時間シフトである。 When j>1, the beat signal is measured by changing the time from the start of the frequency sweep of the test light to the start of the beat signal measurement in step S01. The measured phase θ _1,j (t) is described by the following equation.

Here, T is the total time shift of N measurements.

In step S04, the above steps S01 to S03 are performed N times, and N θ _1,j (t) (j=1 to N) are arithmetically averaged by the following equation to obtain θ ₁ (t).

The steps S01 to S04 are also performed for the mode 2 to calculate the phase θ ₂ (t) of the beat signal of the mode 2. θ ₂ (t) is described by the following equation.

After step S05, the phase difference Δθ(t)=θ ₂ (t)−θ ₁ (t) between the phase of mode 1 and the phase of mode 2 is calculated as in the first embodiment, and is calculated from the time change rate of Δθ(t). Calculate the propagation delay time difference.

Ｔ＝（２ｎ−１）／２γ（τ_１−τ_ｘ１）（ｎは１以上の整数）のとき式（２３）はゼロとなり、モード結合による位相揺らぎが打ち消される。一方、Ｔ＝ｎ／γ（τ_１−τ_ｘ１）のときに平均化の効果が最も小さいが、その場合位相揺らぎは平均化前と同じであり、本実施形態の処理で揺らぎ幅が増加することはない。また、式（２０）または（２１）より、Ｔ／Ｎ＝ｎ／γ（τ_１−τ_ｘ１）の場合も平均後の位相揺らぎは平均化前と等しくなる。したがって、本実施形態ではＴ≠ｎ／γ（τ_ｉ−τ_ｘｉ）かつＴ／Ｎ≠ｎ／γ（τ_ｉ−τ_ｘｉ）（ｉ＝１，２）であることを条件として、モード結合による位相揺らぎを低減することができる。 The fact that the effect of mode coupling can be reduced by the averaging of phases performed in the present embodiment is apparent when considered in the case of N=2 in Expression (20) or (21). If N=2 in the equation (20), the fourth term representing the influence of mode coupling is as follows.

When T=(2n−1)/2γ(τ ₁ −τ _x1 ) (n is an integer of 1 or more), the equation (23) becomes zero, and the phase fluctuation due to mode coupling is canceled. On the other hand, when T=n/γ(τ ₁ −τ _x1 ), the effect of averaging is the smallest, but in that case the phase fluctuation is the same as before averaging, and the fluctuation width increases by the processing of this embodiment. There is no such thing. Further, according to the equation (20) or (21), also in the case of T/N=n/γ(τ ₁ −τ _x1 ), the phase fluctuation after averaging becomes equal to that before averaging. Therefore, in the present embodiment, mode coupling is performed under the condition that T≠n/γ(τ _i −τ _xi ) and T/N≠n/γ(τ _i −τ _xi ) (i=1, 2). Phase fluctuation can be reduced.

In the above embodiment, a plurality of beat signals are measured by changing the measurement start time, but changing the measurement start time is synonymous with changing the initial frequency of the frequency sweep of the test light. Therefore, the present invention is not limited to the above-mentioned form, and a plurality of beat signals may be measured with different initial frequencies and their phases may be averaged.

(Embodiment 3)
The configuration of the inter-spatial-channel propagation delay time difference measuring apparatus of this embodiment is the same as that of the measuring apparatus 301 of FIG. In the case of this embodiment,
The light source unit 11
The center wavelength changing means for changing the center wavelength of the continuous light is provided,
The processor 17
Calculate the propagation delay time difference between the spatial channels for each different center wavelength of the continuous light, to measure the wavelength dependence of the propagation delay time difference between the spatial channels in the DUT,
The wavelength dependence of the propagation delay time difference between the spatial channels is differentiated by wavelength, and the wavelength dispersion difference between the spatial channels is calculated by dividing by the length of the DUT.

ここでＬは被測定ファイバの長さである。 The inter-spatial-channel propagation delay time difference measuring device of this embodiment can measure the chromatic dispersion difference between the spatial channels. The inter-spatial-channel propagation delay time difference measuring apparatus of this embodiment performs the measurement of Embodiment 1 or 2 a plurality of times by changing the central wavelength of the test light, and measures the wavelength dependence Δτ _DMD (λ) of the delay time difference. The chromatic dispersion difference D(λ) between mode 1 and mode 2 is calculated by the following equation.

Here, L is the length of the fiber to be measured.

(Other embodiments)
In the first and second embodiments, the single-mode single-core delay fiber is used in the optical path of the local light, but the present invention is not limited thereto, and the transmitted light is multiplexed between different channels of the spatial multiplexing optical fiber to obtain the beat signal. May be observed. In that case, since the phase of the beat signal changes at the time change rate of 2πΔf _DMD , it is possible to measure the delay time difference from the time change rate of the phase as in the above embodiment.

Even when the number of spatial channels to be measured is more than 2, the beat signals of the transmitted light and the local light of the individual spatial channels are observed using the corresponding spatial channel selective pumping section and spatial channel selective demultiplexing section, and their positions are measured. By calculating the phase difference, the delay time difference between each channel can be measured.

Although the measurement target is an optical fiber in the above embodiment, the delay time difference between spatial channels and the chromatic dispersion difference of the optical device can be similarly measured even if the measured fiber is changed to an optical device such as an optical filter.

(effect)
By using the inter-spatial-channel propagation delay time difference measuring apparatus of this embodiment, not only the spatial multiplexing optical fiber but also a small delay time difference at the level of an optical device such as a mode multiplexer/demultiplexer can be measured. Further, the inter-spatial-channel propagation delay time difference measuring apparatus of the present embodiment can evaluate the delay time difference with high resolution even in a narrow optical band, so that the delay time difference can be measured by changing the central wavelength of the test light a plurality of times. The wavelength dependence can also be evaluated. If the wavelength dependence of the delay time difference can be known, the chromatic dispersion difference between the spatial channels can be analyzed (Non-Patent Document 4), and the present invention can also be implemented as a chromatic dispersion difference evaluation method.

11: Light source unit 12: Optical demultiplexing device 13: Delay fiber 14: Selective pumping unit 15: Selective demultiplexing unit 16: Optical multiplexing device 17: Arithmetic processing device 21: Light receiver 22: A/D converter 23: Arithmetic processing unit 100: Fiber under measurement 301: Propagation delay time difference measuring device between spatial channels

Claims (9)

A light source unit that emits continuous light whose frequency is swept temporally,
An optical demultiplexing device that splits continuous light from the light source unit into two;
A delay optical fiber for delaying one of the continuous lights branched by the optical demultiplexing element as local light,
A selective excitation unit that selectively enters the other of the continuous light branched by the optical demultiplexing element as a test light into a specific spatial channel of the DUT,
A selective separation unit for selectively separating the specific spatial channel of the test light transmitted through the DUT,
An optical multiplexing element that multiplexes the local light passing through the delay optical fiber and the test light of the specific spatial channel separated by the selective separation unit into a beat signal,
The two beat signals based on the test lights of the two different specific spatial channels are acquired from the optical multiplexing element, and the two specific spaces in the DUT are obtained from the time change rate of the phase difference between the beat signals. An arithmetic processing device for calculating a propagation delay time difference between channels,
An apparatus for measuring a propagation delay time difference between spatial channels comprising: The arithmetic processing unit,
The propagation delay time difference measuring apparatus according to claim 1, wherein the beat signals for the same spatial channel are acquired a plurality of times, and the phases of the plurality of beat signals are added and averaged. The arithmetic processing unit,
The light source unit acquires the beat signal for the same spatial channel a plurality of times by changing the time from the frequency sweep start time when the frequency sweep starts to start the beat signal acquisition start time when the beat signal starts to be acquired, and the plurality of beats are acquired. 2. The propagation delay time difference measuring apparatus between spatial channels according to claim 1, wherein the phases of the signals are added and averaged. The light source unit,
An external frequency modulator or an external phase modulator is used as the external modulator, and the inter-spatial channel propagation delay time difference measuring device according to claim 1 to 3, wherein: The light source unit,
The center wavelength changing means for changing the center wavelength of the continuous light is provided,
The arithmetic processing unit,
Calculate the propagation delay time difference between the spatial channels for each different center wavelength of the continuous light, to measure the wavelength dependence of the propagation delay time difference between the spatial channels in the DUT,
5. The wavelength difference of the wavelength dependence of the propagation delay time difference between the spatial channels is calculated, and the wavelength dispersion difference between the spatial channels is calculated by dividing by the length of the DUT. A propagation delay time difference measuring apparatus between spatial channels according to any one of the above. Continuous light whose frequency has been swept temporally is split into two, one of which is used as a local light to delay with a delay optical fiber, and the other of which is used as a test light to selectively enter a specific spatial channel of the DUT. Procedure and
A beat signal generation procedure for selectively separating the specific spatial channel from the test light transmitted through the device under test, and combining with the local light via the delay optical fiber to form a beat signal,
The two beat signals based on the test lights of the two specific spatial channels that are different in the beat signal generation procedure are acquired, and the two specific beat signals in the DUT are obtained from the time change rate of the phase difference between the beat signals. An arithmetic processing procedure for calculating a propagation delay time difference between spatial channels,
Propagation delay time difference measurement method between spatial channels. The propagation delay time difference measuring method between spatial channels according to claim 6, wherein the beat signals for the same spatial channel are acquired a plurality of times in the arithmetic processing procedure, and the phases of the plurality of beat signals are arithmetically averaged. .. In the arithmetic processing procedure,
Obtaining the beat signal for the same spatial channel a plurality of times by changing the time from the frequency sweep start time to start sweeping the frequency in the light incident procedure to the beat signal acquisition start time to start obtaining the beat signal, 7. The propagation delay time difference measuring method between spatial channels according to claim 6, wherein the phases of the beat signals are added and averaged. In the light incident procedure, changing the central wavelength of the continuous light,
In the arithmetic processing procedure,
Calculate the propagation delay time difference between the spatial channels for each different center wavelength of the continuous light, to measure the wavelength dependence of the propagation delay time difference between the spatial channels in the DUT,
The wavelength dependence of the propagation delay time difference between the spatial channels is differentiated by wavelength, and the wavelength dispersion difference between the spatial channels is calculated by dividing by the length of the DUT. A method for measuring a propagation delay time difference between spatial channels as described.

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