JP5582473B2 - Optical frequency domain reflection measurement method and optical frequency domain reflection measurement apparatus - Google Patents

Description

  The present invention relates to an optical frequency domain reflection measurement method capable of measuring reflected light or backscattered light from an optical component or an optical transmission line with high spatial resolution, and an optical frequency domain reflection measurement apparatus using this method.

  As a technique capable of measuring reflected light and backscattered light from an optical component or an optical transmission line with high spatial resolution, optical frequency domain reflection (C-OFDR: using coherent light as shown in Non-Patent Document 1). Coherent Optical Frequency Domain Reflectometry). In this C-OFDR measurement method, coherent light frequency-swept is incident on a measurement object, reflected light and backscattered light from the measurement object, and pre-branched reference light are coherently detected, and measurement obtained thereby This is a technique that enables the identification of the loss distribution and the failure point of the measurement object by obtaining the reflected light and the backscattered light intensity at an arbitrary position in the measurement object by performing frequency analysis of the beat signal.

  Furthermore, as shown in Non-Patent Document 2, by monitoring the coherence characteristics of the light source using a reference interferometer and resampling the measurement beat signal using it as a clock, even in a region exceeding the laser coherence length, It is said that a highly accurate measurement result can be obtained by compensating for the spread of the reflection spectrum due to phase fluctuation (light source phase noise).

  Further, in measurement using coherent detection like C-OFDR, noise called fading noise appears on the measurement waveform. Fading noise is caused by non-uniformity of the backscattered light electric field at different positions, and is superimposed on the measurement waveform as fluctuations in Rayleigh backscattered light intensity. Thereby, the measurement accuracy of C-OFDR is greatly deteriorated.

  Therefore, it is known that a frequency shift averaging method (FSAV) as shown in Non-Patent Document 3 is effective for reducing fading noise. This FSAV is a method of reducing fading noise by performing a measurement process by changing the frequency (wavelength) of the test light a plurality of times for each measurement and averaging the waveforms obtained for each measurement.

  The effect of FSAV increases as the wavelength of the test light used for each measurement is different. In order to use FSAV effectively in C-OFDR, it is necessary to change the wavelength of the test light so that the frequency sweep range used in a single C-OFDR measurement does not overlap in each measurement for FSAV. There is. Therefore, in C-OFDR, as described above, a wide frequency sweep is necessary for high-resolution measurement, and more than one measurement is performed by changing the frequency of the test light further to reduce fading noise. Need to do. Thus, in order to realize measurement with high resolution, long distance, and reduced fading noise in C-OFDR, extremely broadband test light is used.

  However, in the existing technology, when such a broadband test light is used, the position of the reflection point obtained in each measurement is shifted due to the chromatic dispersion (CD: Chromatic Dispersion) of the measurement object. When averaging is performed, the waveform becomes wider, fading noise is reduced, but the resolution is degraded.

W. Eickhoff and R. Ulrich, Applied physics Letters, vol. 39, no 9, pp. 693-695, 1981. X.Fan 、 Y.Koshikiya and F.Ito, Optics Letters, vol.32.no.22, pp.3227-3229,2007 K. Shimizu, T. Horiguchi, and Y. Koyamada, IEEE / OSA J. Lightwave Technol. Vol. 10, No. 10, pp. 982-987, 1992

  As described above, in the conventional C-OFDR measurement method, in order to realize measurement with high resolution, long distance, and fading noise reduction, an extremely wide band test light is used. When such a broadband test light is used, the position of the reflection point obtained by each measurement shifts due to the chromatic dispersion of the measurement object, and when they are added and averaged, the waveform spreads and fading noise occurs. However, there is a problem that the resolution is degraded.

  The present invention has been made in view of the above circumstances, and can compensate for the deviation of the reflection point due to chromatic dispersion when using a wide-band test light, and achieves both low fading noise and high resolution. An object of the present invention is to provide an optical frequency domain reflection measurement method capable of performing the above and an optical frequency domain reflection measurement apparatus using this method.

In order to achieve the above object, the optical frequency domain reflection measurement method according to the present invention has the following configuration.
(1) The output light from the light source that sweeps the optical frequency with respect to time is bifurcated and is incident on the reference interferometer and the measurement interferometer, respectively. A measurement beat signal of backscattered light and reflected light obtained by being incident on a measurement object is detected, and the reference interferometer has the same wavelength dispersion coefficient as that of the measurement object for the output light from the light source and the output light. A monitor beat signal with the light delayed by the delay means is detected, and is selected from sampling data of the monitor beat signal waveform within a range of greater than 0 and less than or equal to π in the monitor beat signal waveform Each time a phase change occurs, the time is obtained, and from the sampling data of the waveform of the measurement beat signal, the time obtained every time the constant phase change of the measurement beat signal occurs Seeking takes values as sequence, Fourier-transform with respect to the sequence, measuring the reflectance of the light wave propagation direction in the measurement object.

Moreover, the optical frequency domain reflection measuring apparatus according to the present invention has the following configuration.
(2) A light source that sweeps the optical frequency with respect to time, backscattered light and reflected light obtained by making the output light from the light source incident, and making the incident output light and output light incident on the measurement object; A measurement interferometer for detecting the measurement beat signal, and an output light from the light source incident thereon, and the incident output light and the light delayed by the delay means having the same wavelength dispersion coefficient as the measurement object A reference interferometer for detecting the monitor beat signal, sampling means for sampling the monitor beat signal and the measurement beat signal, and the monitor beat signal from the sampling data of the waveform of the monitor beat signal sampled by the sampling means Each time a constant phase change arbitrarily selected in the range of greater than 0 and less than or equal to π occurs in the waveform of The value at the time obtained every time the constant phase change of the monitor beat signal occurs is obtained as a sequence from the sampling data of the waveform of the monitor signal, Fourier transform is applied to the sequence, and the light wave propagation direction in the measurement target is determined. Analyzing means for measuring the reflectance.

  (3) An optical fiber is used for the delay means.

As described above, the present invention compensates for the deviation of the reflection point due to chromatic dispersion when using broadband test light in C-OFDR measurement. Specifically, the reflection point due to chromatic dispersion is obtained by resampling the measurement signal using a reference interferometer comprising a delay means (eg, optical fiber) having the same chromatic dispersion coefficient as the object to be measured (eg optical fiber) It compensates for the position shift (the reflection waveform spread after addition averaging). This makes it possible to perform C-OFDR that achieves both low fading noise and high resolution.

  Therefore, according to the present invention, it is possible to compensate for the deviation of the reflection point due to the chromatic dispersion when using the broadband test light, and to perform the measurement that achieves both low fading noise and high resolution. A frequency domain reflection measurement method and an optical frequency domain reflection measurement apparatus using the method can be provided.

The block diagram which shows one Embodiment of the measuring apparatus which employ | adopted the optical frequency domain reflection measuring method of this invention. The block diagram which shows the structure of the normal C-OFDR measurement part in the measuring apparatus shown in FIG. Schematic which shows the difference in the position of the reflection spectrum obtained when measuring the to-be-measured object which has CD at a different wavelength. The figure which shows the relationship between the delay time at the time of considering wavelength dispersion, and the beat frequency in the optical frequency domain reflection measuring method of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of an embodiment of a C-OFDR measurement apparatus using a C-OFDR measurement method according to the present invention. In FIG. 1, the output light from the wavelength tunable light source 11 is branched into two by a first-stage optical directional coupler 12. One output light is incident on the measurement interferometer. In this measurement interferometer, it is branched into two by the optical directional coupler 13 at the next stage, one is used as local light, and the other is incident on the measurement object 14 as test light.

  The reflected light backscattered inside the measurement object 14 is extracted by the optical directional coupler 13, combined with the local light by the optical directional coupler 15, and then detected by the optical receiver 16. At this time, the interference beat signal generated by the interference of the two light waves is sampled by the sampling device 17, and the measured data is analyzed by the optical frequency analysis device 18, thereby measuring the backscattered light intensity distribution in the measurement target 14. Is done.

  On the other hand, the output light branched by the first-stage optical directional coupler 12 enters the reference interferometer. In this reference interferometer, the optical directional coupler 19 is branched into two. Among these, one output light is sent to the optical directional coupler 20 as reference light, and the other output light is passed through a delay fiber 21 having a CD coefficient equivalent to that of the object 14 to be measured. Are combined with the reference light by the optical receiver 22 and then detected by the optical receiver 22. At this time, the sampling device 17 samples the interference beat signal generated by the interference of the two light waves, the measured data is analyzed by the optical frequency analysis device 18, and the measurement result of the backscattered light intensity distribution in the measurement object 14 is measured. To be compared.

In the above embodiment, referring to FIG. 2 to FIG. 4, normally, when measurement is performed with test light having different wavelengths, the reflected light from the same point is observed with the CD. The fact that the position of the reflected waveform is shifted will be described.
FIG. 2 shows a configuration after the measurement interferometer of the measurement apparatus shown in FIG. 1, that is, the optical directional coupler 13 at the next stage. Here, one reflection in which the output light from the wavelength tunable light source 11 is the test light. Assume that the light is incident on a measurement target 14 having a point, and _two measurements are performed at different wavelengths (λ ₁ and λ ₂ ). Incidentally, in FIG. 2, a delay time due to CD in the measurement wavelength λ ₁ τCDFUTλ1, b is measured CD of the delay time tau _CDFUTramuda2 at the wavelength lambda _2, c beat frequency f _Biramuda1 is obtained at a measurement wavelength lambda _1, d Indicates the beat frequency f _bλ2 obtained at the measurement wavelength λ ₂ .

  In the C-OFDR, the wavelength variable light source 11 sweeps the optical frequency of the output light as shown in FIG. 3, and the optical directional coupler 13 branches to the local light and the test light incident on the measurement target 14. Then, the backscattered reflected light in the measurement target 14 is taken out by the optical directional coupler 13, the local light and the reflected light are combined by the optical directional coupler 15, and detected by the optical receiver 16. At this time, a beat signal is generated in the detection output by the combination of the local light and the reflected light. After sampling the beat signal in the time direction and performing frequency analysis, the frequency is converted into a measurement distance to obtain a reflectance distribution.

Here, when the object 14 to be measured has a CD, as shown in FIG. 4, the delay time until it is multiplexed differs depending on the measurement wavelength λ. This means that even if the reflected light from the same point is observed, the beat frequencies f _bλ1 and f _bλ2 obtained by the measurement wavelength λ are different. That is, when the measurement wavelengths are different, the reflection position is shifted despite the same reflection point being observed, and the waveform is broadened by averaging the reflection positions.

The above problem will be described in detail using mathematical expressions.
The electric field E (t) of the light wave swept in the C-OFDR measurement is expressed as follows.

ここで、Ｅ₀は光波の電界複素振幅、ω₀は初期角周波数、γは光周波数掃引率、θ(ｔ)時刻ｔにおける光波のランダムな位相である。この光波は二分岐されたローカル光及び被測定対象（ここでは被測定光ファイバ(Fiber under test)を想定する。以下、ＦＵＴ）１４に入射する試験光となる。

Here, E ₀ is the electric field complex amplitude of the light wave, ω ₀ is the initial angular frequency, γ is the optical frequency sweep rate, and θ (t) is the random phase of the light wave at time t. This light wave is assumed to be test light incident on a bifurcated local light and an object to be measured (here, an optical fiber to be measured (Fiber under test), hereinafter referred to as FUT) 14.

Next, let us consider a beat signal of reflected light and local light from a certain point in the FUT 14. At this time, if the reflected light receives the delay τ _CDFUT due to the CD of the _FUT 14 separately from the round-trip time τ _FUT from the _{FUT 14} , the beat signal is expressed by the following equation.

ここで、Ｒは反射率、Ｃ_FUTは定数である。また、ＦＵＴ１４のＣＤ係数Ｄ_FUTはスロープ（Ｄ_FUTの波長依存性）を持たず、かつ、一度のＣ−ＯＦＤＲ測定中においてはτ_CDFUTが一定であると仮定すると、τ_CDFUTは以下のように表される。

Here, R is a reflectance, and C _FUT is a constant. Assuming that the CD coefficient D _FUT of the _FUT 14 does not have a slope (D _FUT wavelength dependency) and that τ _CDFUT is constant during one C-OFDR measurement, τ _CDFUT is as follows: expressed.

ここで、Δλは試験光の波長差、ｖ_gは群速度である。式(2)、(3)より、得られるビート周波数ｆ_bFUTは以下のように表される。

Here, [Delta] [lambda] the wavelength difference of the test light, v _g is the group velocity. From the equations (2) and (3), the beat frequency f _bFUT obtained is expressed as follows.

式(4)よりビート周波数はΔλに依存することは明らかである。これは、フェーディング雑音低減のために異なる波長で複数回の測定を実施した際に、同一地点からの反射光を観測したにも関わらず、得られるビート信号周波数が異なってしまうことを意味する。従って、Ｃ−ＯＦＤＲでは、ビート周波数が距離に対応するため、反射点の位置がずれて測定され、それらを加算平均すると波形の広がりとなり、分解能の劣化を引き起こす。

From equation (4), it is clear that the beat frequency depends on Δλ. This means that when multiple measurements are performed at different wavelengths to reduce fading noise, the beat signal frequency obtained differs even though the reflected light from the same point is observed. . Therefore, in C-OFDR, since the beat frequency corresponds to the distance, the positions of the reflection points are measured with a shift, and when these are added and averaged, the waveform spreads, causing deterioration in resolution.

As described above, in the normal C-OFDR, when the FSAV is used to reduce the fading noise, the reflection waveform is widened by the CD.
Next, it will be described using mathematical formulas that a beat frequency shift due to CD is compensated when C-OFDR measurement is performed using a reference interferometer.

  Consider performing C-OFDR measurement with a configuration in which a reference interferometer is added to normal C-OFDR as shown in FIG. The beat signal obtained by the reference interferometer is expressed as follows.

ここで、τ_AUXは参照干渉計での遅延ファイバによる遅延時間、τ_CDAUXは遅延ファイバのＣＤによる遅延時間であり、以下の式で表される。

Here, τ _AUX is the delay time due to the delay fiber in the reference interferometer, and τ _CDAUX is the delay time due to the CD of the delay fiber, and is expressed by the following equation.

式(5)において、Ｍを自然数とし、例えば位相がπ（ただし、０より大きくπ以下であれば任意に選択してよい）変化する毎の時間ｔ_Mを考えると、以下のようになる。

In Equation (5), when M is a natural number, for example, a time t _M every time the phase changes by π (however, it may be arbitrarily selected if it is greater than 0 and less than or equal to π) is as follows.

上記の時間ｔ_Mにて式(2)に示したビート信号をリサンプリングすると、得られる信号波形Ｉ_FUT(ｔ_M)は以下のように表される。

When the beat signal shown in the equation (2) is resampled at the above time t _M , the signal waveform I _FUT (t _M ) obtained is expressed as follows.

ここで、Φ(ｔ_M)はランダムな位相の揺らぎであるため、Φ(ｔ)と同等であり、以下の式で表される。

Here, since Φ (t _M ) is a random phase fluctuation, it is equivalent to Φ (t) and is represented by the following equation.

また、得られるビート周波数ｆ_bFUTは以下の式で表される。

The obtained beat frequency f _bFUT is expressed by the following equation.

ここで、式(10)に式(3)、(6)を代入すると、得られるビート周波数ｆ_bFUTは以下の式で表される。

Here, when the equations (3) and (6) are substituted into the equation (10), the beat frequency f _bFUT obtained is expressed by the following equation.

式(11)において、Ｄ_FUT＝Ｄ_AUXのとき、Δλに依存する項は消失し、同一地点からの反射光におけるビート周波数は測定波長に依存せず一定となる。つまり、ＦＵＴと同等のＣＤ係数を有する遅延ファイバ２１を備える参照干渉計の測定信号をリサンプリングすることで、ＦＳＡＶを適用するために異なる波長での測定を実施した場合においても、ＣＤによる反射点位置のずれ（加算平均後の反射波形広がり）を補償することが可能となり、分解能を維持したままフェーディング雑音を低減することが可能となる。

In equation (11), when D _FUT = D _AUX , the term depending on Δλ disappears, and the beat frequency in the reflected light from the same point becomes constant without depending on the measurement wavelength. That is, even if measurement is performed at a different wavelength in order to apply FSAV by re-sampling the measurement signal of the reference interferometer including the delay fiber 21 having the CD coefficient equivalent to that of the FUT, the reflection point by the CD It is possible to compensate for a position shift (reflection waveform spread after addition averaging), and it is possible to reduce fading noise while maintaining the resolution.

Therefore, according to the C-OFDR measurement apparatus according to the present embodiment, it is possible to compensate for the deviation of the reflection point due to chromatic dispersion when using a wide-band test light, and to achieve both low fading noise and high resolution. It can be performed.
In the above embodiment, the optical fiber is taken as an example of the delay means of the reference interferometer and the object to be measured. However, if the CD coefficients of the object to be measured and the delay means of the reference interferometer are the same, the optical fiber is limited. It is obvious that the above effect can be obtained. Further, it is obvious that this method can be applied even when the reference interferometer is used as the light source phase noise compensation means as described in Non-Patent Document 2.

  In addition, 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. Moreover, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. In addition, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine the component covering different embodiment suitably.

   DESCRIPTION OF SYMBOLS 11 ... Variable wavelength light source, 12 ... First stage optical directional coupler, 13 ... Next stage optical directional coupler, 14 ... Object to be measured (FUT), 15 ... Optical directional coupler, 16 ... Optical receiver, 17 ... Sampling device, 18 ... optical frequency analysis device, 19, 20 ... optical directional coupler, 21 ... delay fiber, 22 ... optical receiver.

Claims (3)

Bifurcate the output light from the light source that sweeps the optical frequency with respect to time, and enter the reference interferometer and the measurement interferometer, respectively.
The measurement interferometer detects a measurement beat signal of output light from the light source and backscattered light and reflected light obtained by making the output light incident on a measurement target,
The reference interferometer detects a monitor beat signal between the output light from the light source and the light obtained by delaying the output light by a delay unit having the same wavelength dispersion coefficient as the measurement target,
From the sampling data of the waveform of the monitor beat signal, the time is obtained each time a constant phase change arbitrarily selected in the range of greater than 0 and less than or equal to π occurs in the waveform of the monitor beat signal,
Obtain a value at a time determined every time the constant phase change of the measurement beat signal occurs from the sampling data of the waveform of the measurement beat signal as a sequence,
An optical frequency domain reflection measurement method comprising performing Fourier transform on the sequence and measuring a reflectance in a light wave propagation direction in a measurement target. A light source that sweeps the optical frequency over time;
A measurement interferometer for detecting a measurement beat signal of backscattered light and reflected light obtained by making the output light from the light source incident and making the incident output light and output light incident on a measurement object;
A reference interferometer for detecting a monitor beat signal of the output light from the light source and the incident output light and the light obtained by delaying the output light by a delay unit having the same wavelength dispersion coefficient as the measurement target; ,
Sampling means for sampling the monitor beat signal and the measurement beat signal,
From the sampling data of the waveform of the monitor beat signal sampled by the sampling means, every time a constant phase change arbitrarily selected in the range of greater than 0 and less than π occurs in the waveform of the monitor beat signal, the time is obtained, From the sampling data of the waveform of the measurement beat signal, a value at a time obtained each time the constant phase change of the monitor beat signal occurs is obtained as a number sequence, Fourier transform is performed on the number sequence, and the light wave propagation direction in the measurement target is calculated. An optical frequency domain reflection measuring apparatus comprising: an analyzing means for measuring reflectance.   3. The optical frequency domain reflectometry apparatus according to claim 2, wherein an optical fiber is used as the delay means.

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