JP5159255B2 - Optical frequency domain reflection measurement method and apparatus - Google Patents

Description

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

  An optical frequency domain reflection measurement (C-OFDR) method using coherent light is a method capable of measuring reflected light and backscattered light from an optical component or an optical transmission line with high spatial resolution. In this optical frequency domain reflection measurement method, frequency-swept coherent light is incident on the object to be measured, and the reflected light and backscattered light from the object to be measured are combined with the previously branched reference light (coherent detection). By analyzing the frequency of the interference beat signal obtained in this way, it is possible to obtain the reflected light and backscattered light intensity at any position within the measured object, and to specify the loss distribution and failure point of the measured object Technology.

  As conventional techniques for realizing the C-OFDR method, Non-Patent Document 1 and Patent Document 1 perform frequency sweep of a coherent light source by using a primary modulation sideband generated by an external modulator, and a higher order generated at that time. A method is shown in which a beat signal in the modulation sideband and a beat signal in the primary modulation sideband are separated on the frequency axis by inserting delay means.

  However, in the C-OFDR method, as shown in Non-Patent Document 2, when a strong reflection point such as Fresnel reflection exists in the object to be measured, information in the vicinity of the reflection point is caused by phase noise at the strong reflection point. There was a problem that the information was not obtained.

Tsuji, Shimizu, Horiguchi, Koyamada, IEICE Technical Report OPE94-117, LQE94-96 (1995-02)

S. Venkatesh and W. V. Sorin, J. Lightwave Techol., Vol. 11, No. 10, pp. 1694-1700, 1993

JP 08-145846 A (Patent No. 3243774)

As described above, in the conventional optical frequency domain reflection measurement method, when a strong reflection point such as Fresnel reflection exists in the object to be measured, information near the reflection point is buried by phase noise at the strong reflection point. Therefore, there was a problem that the information could not be obtained.
The present invention has been made to solve the above problems, and can adjust the intensity of reflected light from an arbitrary position to be measured, and can perform high-resolution measurement even in the vicinity of a strong reflection point. An object of the present invention is to provide an optical frequency domain reflection measurement method and apparatus.

In order to achieve the above object, an optical frequency domain reflection measurement method according to the present invention is configured as follows.
(1) Optical modulation is performed on the coherent light to generate a modulation sideband, and the transmission light obtained by sweeping the frequency of the modulation sideband linearly with respect to time is branched into two, one for reference light and the other for signal light The signal light is incident on the object to be measured, and the signal light reflected or backscattered at an arbitrary position in the object to be measured and the reference light are combined to generate an interference beat signal, which is received. In the optical frequency domain measurement method for measuring at least one of reflectance and loss at an arbitrary position in the device under test by performing frequency analysis, the signal light is subjected to double sideband modulation as the optical modulation. And a delay time is adjusted in at least one of the optical paths of the reference light.

  (2) In (1), the adjustment of the delay time changes the superposition state between the beat signal waveforms in the + Nth order and −Nth order modulation sidebands (N is a natural number) generated by the double sideband modulation. By doing so, the reflected light intensity from an arbitrary position in the object to be measured is changed.

The optical frequency domain reflection measuring apparatus according to the present invention is configured as follows.
(3) a coherent light source, a light modulating unit that receives coherent light output from the coherent light source and generates modulation sidebands on both sides thereof, and sets both side modulation sidebands to the light modulating unit over time. Frequency sweeping means for linearly sweeping the frequency on the axis, an optical splitter that splits the output light of the optical modulation means into two parts, one of which is the reference light, and the other is the signal light, and the signal light is incident on the part to be measured Then, measured light extraction means for extracting signal light reflected or backscattered at an arbitrary position of the measured object as measured light, and combining the measured light and the reference light to generate interference beat signal light An optical multiplexer for receiving, an optical receiver for receiving an interference beat signal light obtained by the optical multiplexer and outputting it as an electrical signal, and a frequency analysis of the interference beat signal obtained by the optical receiver for measuring the object to be measured. In any position within Reflectivity that, characterized by comprising a frequency analyzer for measuring at least one of loss, the reference light, and a variable delay means for adjusting one of the delay time at least one of the signal light.

  (4) In (3), the variable delay means overlaps the interference beat signal waveforms in the optical combiner in the + Nth order and −Nth order modulation sidebands (N is a natural number) generated by the double sideband modulation. The reflected light intensity from an arbitrary position in the object to be measured is changed by changing the alignment state.

(5) In (3), the optical modulation means is a carrier wave suppression type double sideband modulator (DSB-SC modulator).
(6) In (3), the optical modulation means includes a double-sideband modulator (DSB modulator) that performs double-sideband modulation on the coherent light, and a delay fiber disposed immediately before the object to be measured. It is characterized by providing.

  That is, in the optical frequency domain reflection measurement method and apparatus configured as described above, when performing optical frequency domain reflection measurement (C-OFDR) using coherent light, a carrier-suppressed double sideband (DSB-SC) modulator is used. The reflected light of the strong reflection point measured by sweeping the frequency of the output light from the coherent light source and adding a delay time to at least one of the optical paths of the reference light in the signal light and adjusting the delay time The intensity is attenuated to reduce the phase noise level, and the measurement is performed again in this state.

  In short, according to the present invention, there is provided an optical frequency domain reflection measurement method and apparatus capable of adjusting the intensity of reflected light from an arbitrary position to be measured and capable of performing high-resolution measurement even in the vicinity of a strong reflection point. can do.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a block configuration diagram showing a first embodiment of a measuring apparatus based on an optical frequency domain reflection measuring method according to the present invention. In FIG. 1, coherent light output from a coherent light source 11 is input to a DSB-SC (carrier wave suppression double sideband) modulator 12. As shown in FIG. 2, the DSB-SC modulator 12 generates + N-order and −N-order modulation sidebands (N is a natural number) on both sides of coherent light by an RF signal output from the drive circuit 13. Therefore, by sweeping the RF signal frequency in the drive circuit 13, the optical frequency of the modulation sideband output from the DSB-SC modulator 12 is swept.

The DSB-SC modulator 12 can suppress sidebands and carrier waves of N = 2 or more, and can be ignored here because it can suppress 20 dB or more.
The output light of the DSB-SC modulator 12 is branched into two by the first optical directional coupler 14, and one of them is incident on the device under test 16 via the second optical directional coupler 15 as signal light, The other is output as reference light. The signal light reflected or backscattered in the device under test 16 is taken out by the second optical directional coupler 15, and together with the reference light, the third optical directional coupler is passed through the variable delay devices 17 and 18, respectively. After being combined with the reference light, it is received and detected by the optical receiver 20.

  When the signal light and the reference light are combined in this way, beat signal light is generated due to the interference. The interference beat signal light is received and detected by the optical receiver 20, and the interference beat signal of the detection output is subjected to frequency analysis by the frequency analysis device 22 after unnecessary frequency components are removed by the low-pass filter 21, and the device under test 16 The intensity of the reflected light and the backscattered light from each position are measured.

  At this time, as shown in FIG. 3A, when the reflection in the DUT 16 is weak, the reflected light from the reflection point can be taken out as signal light, but as shown in FIG. In addition, when there is a strong reflection point in the DUT 16, the reflected light in the vicinity of the point is obscured by the phase noise of the reflected light from the strong reflection point. Therefore, when a strong reflection point is observed in the measurement result, the delay time between the reference light and the signal light is adjusted by the variable delay devices 17 and 18 inserted in the optical path of the reference light or the optical path of the signal light, The measurement is performed again by attenuating the reflected light intensity of the strong reflection point to be measured to reduce the phase noise level.

From here, the principle of the reflected light intensity reduction will be described quantitatively.
A light source spectrum after DSB-SC modulation is considered using a model as shown in FIG. In FIG. 4, the frequencies f ₁ (t) and f ₂ (t) of the + 1st order sideband and the −1st order sideband are expressed as follows.

ここで、ｆ₀はコヒーレント光源の中心周波数、ｆ_mは駆動回路１３の初期出力ＲＦ信号周波数、γは周波数掃引速度であり、周波数掃引ΔＦおよび周波数掃引時間Ｔ_swpを用いてγ＝ΔＦ／Ｔ_swpと表される。
（１）式を用いると、＋１次および−１次側波帯の電場Ｅ₁(ｔ)、Ｅ₂(ｔ)はそれぞれ以下のようになる。

Here, f ₀ is the center frequency of the coherent light source, f _m is the initial output RF signal frequency of the drive circuit 13, and γ is the frequency sweep speed, and γ = ΔF / T using the frequency sweep ΔF and the frequency sweep time T _{swp. Expressed as swp} .
Using equation (1), the electric fields E ₁ (t) and E ₂ (t) of the + 1st order and −1st order sidebands are as follows.

ここで、Ｅ₀は電界振幅である。この電場に対して時間τだけ遅延した電場（これは、信号光の電場を意味する）は以下のように表される。

Here, E ₀ is the electric field amplitude. An electric field delayed by time τ with respect to this electric field (which means an electric field of signal light) is expressed as follows.

ＤＳＢ−ＳＣ変調においては、（２）式及び（３）式に表されるような電場が存在し、これらのビート信号によって生じる光電流は以下のように表される。

In DSB-SC modulation, there is an electric field represented by the equations (2) and (3), and the photocurrent generated by these beat signals is represented as follows.

Ｅ₁(ｔ)及びＥ₂(ｔ−τ)間、Ｅ₂(ｔ)及びＥ₁(ｔ−τ)間で生じるビート信号は、その周波数が（４）式で生じるビート信号の周波数と比較してはるかに大きいため、ローパスフィルタ２１にて除去することができるので、ここでは考慮する必要はない。

The beat signal generated between E ₁ (t) and E ₂ (t−τ) and between E ₂ (t) and E ₁ (t−τ) is compared with the frequency of the beat signal generated by the equation (4). Since it is much larger and can be removed by the low-pass filter 21, it is not necessary to consider it here.

  In the C-OFDR measurement using the DSB-SC modulator 12, two photocurrents resulting from the + 1st order and the −1st order sidebands are thus generated. Since these are measured on the same time axis, the superposition of these two waves becomes the final output photocurrent i. If the DC component is ignored, the beat signal power P obtained by the output photocurrent i is expressed as follows.

（５）式の振幅項cos²(２πｆ₀τ)から、ビート信号パワーの振幅は遅延時間τに対して周期ｆ₀で振動することがわかる。この時、強反射点からの信号の参照光に対する遅延時間をτ₀とし、信号光の光路中に挿入した可変遅延器１８によって新たに付与される遅延時間をτ₁とすると、（５）式は以下のように表される。

From the amplitude term cos ² (2πf ₀ τ) in the equation (5), it can be seen that the amplitude of the beat signal power oscillates with a period f ₀ with respect to the delay time τ. At this time, if the delay time of the signal from the strong reflection point with respect to the reference light is τ ₀ and the delay time newly given by the variable delay device 18 inserted in the optical path of the signal light is τ ₁ , Equation (5) Is expressed as follows.

ここで、（６）式の振幅項cos²[２πｆ₀(τ₀＋τ₁)]が０となる条件は、Ｍを０を含まない自然数とすると、

Here, the condition that the amplitude term cos ² [2πf ₀ (τ ₀ + τ ₁ )] in equation (6) is 0 is that M is a natural number not including 0:

となり、（７）式を満たすようにτ₁を決定することで、観測される強反射点からの信号パワーのみを０にすることができ、強反射点の位相雑音による影響を除去できる。
一方、参照光の光路中に挿入した可変遅延器１７によって新たに付与される遅延時間をτ₂とすると、同様にして、振幅項が０となる条件は、

Thus, by determining τ ₁ so as to satisfy the expression (7), only the signal power from the observed strong reflection point can be reduced to 0, and the influence of the phase noise at the strong reflection point can be eliminated.
On the other hand, when the delay time newly given by the variable delay device 17 inserted in the optical path of the reference light is τ ₂ , the condition that the amplitude term becomes 0 similarly is

であり、上記（８）式を満たすようにτ₂を決定することで、強反射点の位相雑音による影響を除去することができる。
信号光の光路及び参照光の光路の両方に挿入した可変遅延器１７，１８によって新たに付与される遅延時間をそれぞれτ₁，τ₂とすると、

By determining τ ₂ so as to satisfy the above equation (8), the influence of the phase noise at the strong reflection point can be removed.
If the delay times newly given by the variable delay devices 17 and 18 inserted in both the optical path of the signal light and the optical path of the reference light are τ ₁ and τ ₂ , respectively.

を満たすようにτ₁，τ₂を設定すれば、強反射点の位相雑音による影響を除去することができる。
一例として、（５）式の振幅項についてｆ₀＝２００ＴＨzとした場合の計算結果を図５に示す。図５より、時間に依らず遅延時間τが０．２５×１０^-14秒変化する毎に振幅が０となる点が生じることがわかる。これは参照光の光路及び信号光の光路間に０．５μｍの光路長差が生じる毎に振幅が０となることを示している。

If τ ₁ and τ ₂ are set so as to satisfy the above, the influence of the phase noise at the strong reflection point can be eliminated.
As an example, FIG. 5 shows a calculation result when f ₀ = 200 THz for the amplitude term of the equation (5). From FIG. 5, it can be seen that a point where the amplitude becomes zero occurs every time the delay time τ changes by 0.25 × 10 ⁻¹⁴ seconds regardless of the time. This indicates that the amplitude becomes zero every time a 0.5 μm optical path length difference is generated between the optical path of the reference light and the optical path of the signal light.

Therefore, when there is a strong reflection point in the DUT 16, under this condition, the optical path length difference is intentionally adjusted by a maximum of 0.5 μm (the delay time is adjusted by 0.25 × 10 ⁻¹⁴ seconds). Equivalent), the position of the reflected light to be measured can be moved to a point where the amplitude becomes weak, and the reflected light intensity can be weakened. Thereby, phase noise is reduced, and information in the vicinity of the strong reflection point can be obtained.

  The variable delay devices 17 and 18 as mechanisms capable of adjusting the delay time have a structure in which an optical fiber as shown in FIG. 6A is wound around a piezoelectric element, or a light as shown in FIG. It can also be realized by fixing two points of the fiber and moving one or both of the fixed points. Alternatively, as shown in FIG. 6C, the optical path length can be adjusted by an operating mirror structure. Further, the same effect can be obtained by adjusting the phase of the light wave with an optical phase modulator as shown in FIG. Note that the variable delay devices 17 and 18 for adjusting the delay time may be installed in one or both of the optical path of the reference light and the optical path of the signal light.

In the above-described embodiment, the case where only the + 1st order and −1st order sidebands are generated is described. Is possible.
(Second Embodiment)
FIG. 7 is a block diagram showing a second embodiment of the measuring apparatus based on the optical frequency domain reflection measuring method according to the present invention. However, in FIG. 7, the same parts as those in FIG. 1 are denoted by the same reference numerals, and different parts will be mainly described here.

  FIG. 7 shows a configuration in the case where a DSB modulator is used as the external modulator 12 in the measurement apparatus. Even with such a configuration, the reflected light intensity can be reduced on the same principle as in the first embodiment. However, in the DSB modulation, since the carrier wave is not suppressed as shown in FIG. 8, a light source phase noise spectrum is generated around the DC (direct current component) by the beat signal between the carrier waves. Therefore, a delay element 22 such as a dummy fiber is inserted immediately before the device under test 16 so as to separate the frequency region around the DC affected by the phase noise and the frequency region occupied by the beat signal from the object to be measured. It is good to.

  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 components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different example embodiments may be combined as appropriate.

The block block diagram which shows 1st Embodiment of the measuring apparatus based on the optical frequency domain reflection measuring method which concerns on this invention. FIG. 2 is a characteristic diagram showing an outline of a light source spectrum after DSB-SC modulation in the apparatus of FIG. 1. In the apparatus of FIG. 1, the wave form diagram which compares and shows the case where the reflection in a to-be-measured part is weak with the relationship between distance and a reflectance. FIG. 2 is a characteristic diagram showing a light source spectrum model after DSB-SC modulation in the apparatus of FIG. 1. The figure which shows the delay time dependence of beat signal power in the apparatus of FIG. The figure which shows the specific mechanism which can adjust delay time in the apparatus of FIG. The block block diagram which shows 2nd Embodiment of the measuring apparatus based on the optical frequency domain reflection measuring method which concerns on this invention. The characteristic diagram which shows the outline of the light source spectrum after DSB-SC modulation in the apparatus of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Coherent light source, 12 ... DSB-SC modulator, 13 ... Drive circuit, 14, 15, 19 ... Optical directional coupler, 16 ... Device under test, 17, 18 ... Variable delay device, 20 ... Optical receiver, 21 ... Low-pass filter, 22 ... Frequency analysis device, 23 ... Delay element.

Claims (4)

Optical modulation is performed on the coherent light to generate a modulation sideband, and the transmission light obtained by frequency sweeping the modulation sideband linearly with respect to time is split into two, one being a reference light and the other being a signal light, The signal light is incident on the object to be measured, and the signal light reflected or backscattered at an arbitrary position in the object to be measured and the reference light are combined to generate an interference beat signal, which is received and frequency is received. By analyzing, in an optical frequency domain measurement method for measuring at least one of reflectance and loss at an arbitrary position in the object to be measured,
Double-sideband modulation is performed as the optical modulation, and at least one of the signal light and the reference light is delayed in the optical path, and + Nth order and −Nth order modulation sidebands (N Is a natural number) by changing the superposition state between the beat signal waveforms , adjusting the delay time in the optical path, and attenuating the reflected light intensity from the strong reflection point to be measured, and the phase noise level due to the reflected light An optical frequency reflection measurement method, wherein the measurement is performed again so as to reduce the frequency. A coherent light source,
A light modulation means for entering coherent light output from the coherent light source and generating modulation sidebands on both sides thereof; and
Frequency sweeping means for linearly sweeping the both-side modulation sidebands on the time axis with respect to the optical modulation means;
An optical branching device that splits the output light of the light modulating means into two parts, one being a reference light and the other being a signal light;
Measured light extraction means for extracting the signal light that is incident on the measured part and reflected or backscattered at an arbitrary position of the measured object as measured light;
An optical multiplexer that combines the light to be measured and the reference light to generate an interference beat signal light;
An optical receiver that receives the interference beat signal light obtained by the optical multiplexer and outputs it as an electrical signal;
A frequency analysis device that performs frequency analysis of an interference beat signal obtained by the optical receiver and measures at least one of reflectance and loss at an arbitrary position in the object to be measured;
Variable delay means for delaying at least one of the reference light and signal light in an optical path for an arbitrary time ;
The variable delay means is a superposition state between interference beat signal waveforms in the optical combiner in the + Nth order and −Nth order modulation sidebands (N is a natural number) generated in the double sideband modulation due to the delay in the optical path. Is changed, the delay time in the optical path is adjusted, the reflected light intensity from the strong reflection point to be measured is attenuated, and the measurement state in which the phase noise level due to the reflected light is reduced is formed. An optical frequency reflection measuring device characterized by the above. 3. The optical frequency domain reflection measuring apparatus according to claim 2 , wherein the optical modulation means is a carrier wave suppression type double sideband modulator (DSB-SC modulator). It said light modulating means, claims, characterized in that it comprises double sideband modulator performing double side band modulation in the coherent light and (DSB modulator), and a delay fiber disposed immediately before the object to be measured 2. The optical frequency domain reflection measuring apparatus according to 2.

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