JP2011038839A - Optical frequency domain reflectometric method and optical frequency domain reflectometry - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To extend a sweep span to be acquired when a frequency is swept by using an external modulator. <P>SOLUTION: Modulation sidebands are generated by causing the output light of a light source 11 to pass through series-connected MZ light modulators 12 and 13, and its frequency is swept linearly with respect to time. The output light of the MZ light modulator is made to pass through an optical filter 23 and branched into two by an optical coupler 24. One and the other of the branched rays of light are made incident on a measuring object 25 as reference light and signal light, respectively. An interference beat signal is generated by combining the signal light reflected or scattered back at each spot inside the measuring object and the reference light, and received by a receiver 27, and its frequency components are analyzed by a frequency analyzer 28. By adjusting a DC voltage supplied to one 12 of the optical modulators to make a phase difference between light waves 0 rad, and by adjusting a DC voltage supplied to the other optical modulator to make a phase difference between light waves π rad, the carrier components of input light waves are suppressed, ± third order modulation sidebands are generated more intensely than the other sidebands, and the frequency sweep span by the MZ optical modulators is trebled. Thereby, a spatial resolution is reduced to 1/3. <P>COPYRIGHT: (C)2011,JPO&INPIT

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, an optical frequency domain reflection measurement method using coherent light (C -OFDR method). In this C-OFDR method, coherent light frequency-swept is incident on a measurement object, reflected light and backscattered light from the measurement object, and reference light branched in advance are coherently detected, and a measurement beat obtained thereby is obtained. 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 signal.

By the way, as a conventional technique of the C-OFDR method, there is a method for realizing frequency sweep of a coherent light source by using a primary modulation sideband generated by an external modulator (see Non-Patent Document 2). In this method, the spatial resolution ΔZ _min in the C-OFDR is expressed by assuming that the speed of light in the measurement object is v _g and the frequency sweep width of the primary modulation sideband is ΔF _1st.

で表され、高空間分解能を実現するためには１次変調側波帯の周波数掃引幅ΔF_1stを大きくすることが必要である。ここで、１次変調側波帯の周波数掃引幅は、変調器に入力する電気変調信号の周波数掃引幅ΔFと同一となる。
しかしながら、上記のような従来のＣ−ＯＦＤＲ法では、１次変調側波帯の周波数掃引幅ΔF_1stが変調器に入力される電気変調信号の周波数掃引幅ΔFと同一であるため、ΔF_1stは変調器に入力可能な変調信号の帯域によって制限されてしまい、通常20GHz程度（空間分解能約5mmに相当）が限界である。

In order to realize high spatial resolution, it is necessary to increase the frequency sweep width ΔF _1st of the primary modulation sideband. Here, the frequency sweep width of the primary modulation sideband is the same as the frequency sweep width ΔF of the electrical modulation signal input to the modulator.
However, in the conventional C-OFDR method as described above, since the frequency sweep width ΔF _1st of the primary modulation sideband is the same as the frequency sweep width ΔF of the electric modulation signal input to the modulator, ΔF _1st is This is limited by the bandwidth of the modulation signal that can be input to the modulator, and is usually about 20 GHz (corresponding to a spatial resolution of about 5 mm).

W. Eickhoff and R. Ulrich, Applied Physics Letters, vol. 39, no. 9, pp. 693-695, Nov. 1981. Y. Koshikiya, X. Fan, and F. Ito, “Long range and cm-level spatial resolution measurement using coherent optical frequency domain reflectmetry with SSB-SC modulator and narrow linewidth fiber laser”, IEEE / OSA J. Lightwave Technol. Vol 26, No. 18, pp. 3287-3294 (2008).

As described above, in the conventional C-OFDR method, the frequency sweep using the external modulator has a problem that the spatial resolution is limited by the bandwidth of the modulator.
The present invention has been made in view of the above circumstances, and an optical frequency domain reflection measurement method capable of increasing spatial resolution without being limited to the band of a modulator even when frequency sweeping is performed using an external modulation method. An object of the present invention is to provide an optical frequency domain reflection measuring 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) Output light from a coherent light source is subjected to external light modulation processing to generate a modulation sideband, and the generated modulation sideband is frequency-swept linearly with respect to time, and the frequency-swept external modulation processing is performed. The output light is split into two, one is used as the reference light, the other as the signal light, is incident on the object to be measured, and the signal light reflected or backscattered at each point of the object to be measured is combined with the reference light to interfere. A method of measuring a reflectance or a loss at each point in the object to be measured by generating a beat signal, receiving it, and performing frequency analysis, wherein the first light modulation is performed as the external light modulation processing. The second modulation process is performed sequentially, the phase difference of the light wave of one light modulation process is set to 0 rad, the phase difference of the light wave of the other light modulation process is set to π rad, and the carrier component of the input light wave is suppressed + 3rd order and -3rd order modulation sidebands It is configured to generate stronger than the other sidebands, and select one of the + 3rd order and −3rd order modulation sidebands to be used for generating an interference beat signal.

Moreover, the optical frequency domain reflection measuring apparatus according to the present invention has the following configuration.
(2) A light source that generates coherent light, an external light modulation unit that generates a modulated waveband upon incidence of output light from the light source, and a modulation sideband generated by the rotator light modulation unit with respect to time. Frequency sweeping means for linearly sweeping the frequency, optical branching means for branching the output light from the frequency-swept external modulation unit into two, one branched by the optical branching means as reference light, and the other as signal light Obtained by the combining means, which combines the signal light incident on the object to be measured and reflected or backscattered at each point of the object to be measured and the reference light to generate an interference beat signal. Light receiving means for receiving the interference beat signal and converting it into an electrical signal, and frequency analysis means for analyzing the frequency of the electrical signal obtained by the light receiving means, and from the result of the frequency analysis, Reflectance or loss at each point The MHF-Zehnder interferometer type first and second optical modulators connected in series with each other as the external optical modulator, and the first optical modulator A modulation signal generator for generating a modulation signal for sweeping an optical frequency with respect to the first and second optical modulators, and a modulation signal from the modulation signal generator to the first and second optical modulators; A distributor for distributing; variable delay means for adjusting a delay time between modulation signals input to the first and second optical modulators; and output light waves in the first and second optical modulators, respectively. A first and second DC voltage source for generating first and second control voltages for controlling the phase, and adjusting a DC voltage supplied to the one optical modulator to adjust the phase difference of the light wave Is adjusted to 0 rad, and the DC voltage supplied to the other optical modulator is adjusted. And πrad the phase difference of light waves by, suppresses carrier components in the input lightwave, a configuration for generating stronger than +3 order and -3 order modulation sidebands other sideband.

(3) In the configuration of (2), an optical filter is disposed after the first and second optical modulators connected in series.
(4) In the optical frequency domain reflection measurement apparatus according to (2) or (3), an acousto-optic frequency shifter is disposed on the optical path of the reference light.

  As described above, in the present invention, two Mach-Zehnder interferometer type optical modulators are used in the external modulation unit to strongly generate only the third-order modulation sideband, and the frequency sweep width by the modulator is tripled. To. This means that the spatial resolution becomes 1/3 (3 times improvement). Therefore, according to the present invention, even when the frequency is swept using an external modulation method, the optical frequency domain reflection measurement method capable of enhancing the spatial resolution without being limited to the band of the modulator, and the light using this method A frequency domain reflection measurement device can be provided.

The block diagram which shows 1st Embodiment of the measuring apparatus which employ | adopted the optical frequency domain reflection measuring method of this invention. The block diagram which shows 2nd Embodiment of the measuring apparatus which employ | adopted the optical frequency domain reflection measuring method of this invention. The figure which shows the outline | summary of the frequency sweep in the said 2nd Embodiment. In the said 2nd Embodiment, the figure which shows the relationship between the interference beat signal frequency and optical path length difference (DELTA) L which are obtained by sweeping the +/- 3rd order modulation sideband when AOM is not arrange | positioned on a reference optical path. The figure which shows the relationship between the interference beat signal frequency and optical path length difference (DELTA) L obtained by sweeping the +/- 3rd order modulation sideband, when AOM is arrange | positioned on a reference optical path in the said 2nd Embodiment. The figure which shows the area | region which the interference beat signal frequency obtained by the sweep of the +/- 3rd order modulation sideband occupies when AOM is arrange | positioned on a reference optical path in the said 2nd Embodiment.

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 employing the C-OFDR method according to the present invention. In FIG. 1, coherent light output from a coherent light source 11 is incident on a _first Mach-Zehnder interferometer type (hereinafter referred to as MZ) optical modulator (MZ ₁ ) 12, and the output light is second MZ light modulated. Is incident on the device (MZ ₂ ) 13. If ω is the angular frequency of the incident light, Ω is the angular frequency of the modulation signal, and φ is the modulation degree, the electric field of the light wave input to the first MZ optical modulator 12 is expressed as exp (iωt), and the first MZ the modulated signal input port RF ₁ of the optical modulator 12 is supplied modulated signal φ ₁ sin (Ωt) is the modulated signal input port RF ₂ of the second MZ optical modulator 13 modulates the signal φ ₂ sin (Ωt) Is supplied. These modulation signals are generated by the modulation signal generator 14, amplified by the amplifier 15, divided into two by the distributor 16, arbitrarily delayed by the variable RF delays 17 and 18, and attenuators 19 and 20, respectively, and subjected to amplitude control. The

Since the first and second MZ optical modulators 12 and 13 are connected to each other in series, the optical modulators 12 and 13 need to be synchronized. Specifically, N is a positive integer including 0, and the time from when the light wave is modulated by the first MZ light modulator 12 until it is modulated again by the second MZ light modulator 13 is represented by τ _0. The variable RF delays 17 and 18 are adjusted so that τ ₀ = τ _E + 2πN / Ω, where τ _E is the delay time between the modulation signals input to the first and second MZ optical modulators 12 and 13. . Further, the first MZ optical modulator 12 applies the DC voltage generated by the DC voltage source 21 to the input port DC ₁ so that the phase difference between the light waves passing through both the arms becomes zero, and the second MZ optical modulator 12 In the MZ optical modulator 13, a DC voltage generated by the DC voltage source 22 is applied to the input port DC ₂ so that the phase difference between the light waves passing through both arms becomes π. At this time, the second MZ optical modulator 13 obtains a modulated signal in a ± third-order sideband (details will be described later).

  An optical filter 23 is installed at the subsequent stage of the two optical modulators 12 and 13 to block either the +3 or −3 sideband. The output light from the optical filter 23 is branched by the optical directional coupler 24, one of which enters the measurement object 25 as signal light, and the other is used as reference light. The signal light reflected or backscattered in the measurement object 25 is extracted by the optical directional coupler 24, combined with the reference light by the optical directional coupler 26, and received and detected by the receiver 27. At this time, the frequency analysis device 28 analyzes the frequency of an interference beat signal generated by the interference between the signal light and the reference light. In this way, the reflected light and backscattered light intensity distribution from each position in the measuring object 25 is measured.

In the above configuration, the measurement method will be specifically described below.
First, considering the fourth sideband, since the electric fields applied between the arms of the first MZ optical modulator 12 are opposite to each other, the light wave output from the first MZ optical modulator 12 The electric field E ₁ (t) is expressed as follows.

ここでJ_n(φ)は一次のベッセル関数である。
上記電場E₁(t)は第２のＭＺ光変調器１３に入射されて変調を加えられ、さらに第２のＭＺ光変調器１３の両アーム間で位相差πを付与することで、第２のＭＺ光変調器１３からの出力光の電場E₂(t)は以下のようになる。

Here, J _n (φ) is a linear Bessel function.
The electric field E ₁ (t) is incident on the second MZ light modulator 13 to be modulated, and a phase difference π is given between both arms of the second MZ light modulator 13 to obtain the second electric field E ₁ (t). The electric field E ₂ (t) of the output light from the MZ light modulator 13 is as follows.

したがって、J₀(φ₁)−J₂(φ₁)＝0 （φ₁はおよそ1.84）とすることで、以下のように、±３次の側波帯に影響を与えずに±１次の側波帯を抑圧することができる。

Therefore, by setting J ₀ (φ ₁ ) −J ₂ (φ ₁ ) = 0 (φ ₁ is approximately 1.84), the ± first order without affecting the third order sidebands as follows: Can be suppressed.

この時、±５次側波帯は残存するが、φ₁ ＝1.84のとき、J₄(φ₁)／{J₂(φ₁)−J₄(φ₁)}＝0.08であり、±３次側波帯と比較して±５次側波帯は十分小さいため無視することができる。このように±３次側波帯を他の側波帯や搬送波よりも強く発生させることが可能となる。

At this time, the ± 5th order sideband remains, but when φ ₁ = 1.84, J ₄ (φ ₁ ) / {J ₂ (φ ₁ ) −J ₄ (φ ₁ )} = 0.08, and ± 3 The ± 5th order sidebands are sufficiently small compared to the next sidebands and can be ignored. In this way, it is possible to generate ± 3rd order sidebands stronger than other sidebands and carrier waves.

Here, by sweeping the modulation signal frequency from the modulation signal generator 14 with respect to time, the + 3rd order and the −3rd order sidebands can be swept. This means that Ω in the equation (4) is swept with respect to time. For this reason, the frequency sweep width of ± 3rd order sideband is ΔF _3rd = 3ΔF = 3ΔF _1st . Improvements can be realized.

  When the C-OFDR measurement is performed by sweeping the frequency of sidebands having the same order of absolute values, the obtained interference beat signal is intensity-modulated, and an accurate waveform cannot be obtained. In order to avoid this, either the +3 or the −3 order sideband is blocked by the optical filter 23 arranged at the subsequent stage of the two optical modulators 12 and 13. For example, if the -3rd order sideband is cut off, the output light from the optical filter 23 will be only the + 3rd order sideband, branched by the optical directional coupler 24, and one of them will enter the measurement object 25 as signal light. The other is used as reference light.

  The signal light reflected or backscattered in the measurement object 25 is extracted by the optical directional coupler 24, combined with the reference light by the optical directional coupler 26, and received and detected by the receiver 27. At this time, an interference beat signal is generated by the interference between the signal light and the reference light. By analyzing the frequency of the interference beat signal by the frequency analyzer 28, the reflected light and the backscattered light intensity distribution from each position in the measuring object 25 are measured.

(Second Embodiment)
FIG. 2 is a block diagram showing a second embodiment of the measuring apparatus employing the C-OFDR method according to the present invention. 2, the same parts as those in FIG. 1 are denoted by the same reference numerals, and different parts will be mainly described here.
The measuring apparatus according to the second embodiment shown in FIG. 2 includes an acoustooptic frequency shifter (AOM) 29 that shifts the optical frequency of the lightwave by the acoustooptic effect on the reference optical path. By separating the interference beat signal generated by the band on the frequency axis, it is an optimal configuration for avoiding the above-described intensity modulation of the interference beat signal and acquiring a correct interference beat signal. The configuration is the same as that of the first embodiment except that the AOM 29 is arranged on the reference optical path instead of the optical filter 23 arranged on the signal optical path in FIG.

As shown in FIG. 3, when the frequency sweep width of the + third-order sideband is ΔF _3rd and the sweep time is T, the optical frequency sweep speed γ _3rd of the + third-order sideband is

と表される。また、−３次側波帯の光周波数掃引速度γ_-3rdはγ_-3rd=−γ_3rdで表される。
図４はＡＯＭ２９を参照光路上に配置しない場合の±３次の変調側波帯における光路長差ΔLと得られる干渉ビート信号の関係を表した図である。±３次変調側波帯による干渉ビート信号周波数ｆ_bは参照光路と信号光路の光路長差をΔLとすると以下のように表される。

It is expressed. Further, the optical frequency sweep speed γ _-3rd of the _-3rd order sideband is represented by γ _-3rd = -γ _3rd .
FIG. 4 is a diagram showing the relationship between the optical path length difference ΔL in the ± 3rd order modulation sideband and the obtained interference beat signal when the AOM 29 is not arranged on the reference optical path. The interference beat signal frequency f _b due to the ± 3rd order modulation sideband is expressed as follows when the optical path length difference between the reference optical path and the signal optical path is ΔL.

それぞれの変調側波帯における干渉ビート信号は、光路長差ΔLが０から測定対象長Ｌ_tの２になるまでの周波数範囲で発生する。受信器で受光された際に負のビート信号は絶対値を取ることになるので、＋３次と−３次の干渉ビート信号は周波数軸上で重なってしまい、上述したように強度変調を受けて正しい周波数分布を得られない。一方、参照光がＡＯＭ２９にて周波数シフトを受ける場合は、図５に示すようなビート信号周波数が得られる。±３次変調側波帯による干渉ビート信号周波数はＡＯＭ２９による周波数シフトをＦ_AOMとすると、以下のように表される。

Interference beat signal in each of the modulation sidebands are generated in the frequency range up to the optical path length difference ΔL is 2 to be measured length L _t 0. Since the negative beat signal takes an absolute value when it is received by the receiver, the + 3rd order and -3rd order interference beat signals overlap on the frequency axis, and are subjected to intensity modulation as described above. The correct frequency distribution cannot be obtained. On the other hand, when the reference light undergoes a frequency shift in the AOM 29, a beat signal frequency as shown in FIG. 5 is obtained. The interference beat signal frequency by the ± 3rd order modulation sideband is expressed as follows, where the frequency shift by the _AOM 29 is F _AOM .

この時、図６に示すように負のビート信号周波数は絶対値で検出されるため、測定信号として使用しない−３次の干渉ビート信号を、測定信号として利用する＋３次の干渉ビート信号から分離するためには、式(9)が(ΔL,ｆ_b)=(L_t,0)を満足するようにF_AOMの値を決定すればよい。したがって、分離に必要なＡＯＭ２９による周波数シフトF_AOMの値は以下のように表される。

At this time, since the negative beat signal frequency is detected as an absolute value as shown in FIG. 6, the third-order interference beat signal not used as the measurement signal is separated from the + third-order interference beat signal used as the measurement signal. In order to achieve this, the value of F _AOM may be determined so that Equation (9) satisfies (ΔL, f _b ) = (L _t , 0). Therefore, the value of the frequency shift F _AOM required for the separation by the _AOM 29 is expressed as follows.

このように、参照光の光路にＡＯＭ２９を配置し、式(10)に従ってＡＯＭ２９の周波数シフト量を決定することで、図１の光フィルタ２３を用いずに＋３次側波帯による干渉ビート信号とそれ以外の変調側波帯による干渉ビート信号を周波数軸上で分離することができ、これによって強度変調を伴わない正しい干渉ビート信号を得ることができる。

In this way, by arranging the AOM 29 in the optical path of the reference light and determining the frequency shift amount of the AOM 29 according to the equation (10), the interference beat signal by the + third-order sideband and the optical filter 23 of FIG. Interference beat signals due to other modulation sidebands can be separated on the frequency axis, whereby a correct interference beat signal without intensity modulation can be obtained.

  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. Moreover, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, the optical filter 23 of the first embodiment and the AOM 29 of the second embodiment may be combined. 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.

11 ... coherent light source, 12 ... first MZ optical modulator (MZ _1), 13 ... second MZ optical modulator (MZ _2), 14 ... modulation signal generator, 15 ... amplifier, 16 ... distributor, 17 , 18 ... Variable RF delay, 19, 20 ... Attenuator, 21, 22 ... DC voltage source, 23 ... Optical filter, 24 ... Optical directional coupler, 25 ... Measurement object, 26 ... Optical directional coupler, 27 ... Reception 28, frequency analysis device, 29 ... acousto-optic frequency shifter (AOM).

Claims (4)

The output light from the coherent light source is subjected to external light modulation processing to generate modulation sidebands, the generated modulation sidebands are linearly swept with respect to time, and the frequency-swept output light of external modulation processing is used. The signal is split into two, one is used as the reference light, the other is used as the signal light, enters the object to be measured, and the signal light reflected or backscattered at each point of the object to be measured is combined with the reference light to generate an interference beat signal. An optical frequency domain measurement method for measuring reflectance or loss at each point in the object to be measured,
As the external light modulation process, a first light modulation process and a second modulation process are sequentially performed,
The phase difference of the light wave of one light modulation process is set to 0 rad, the phase difference of the light wave of the other light modulation process is set to π rad, the carrier component of the input light wave is suppressed, and the + 3rd and -3rd order modulation sidebands Is generated more strongly than other sidebands,
An optical frequency domain reflection measurement method, wherein one of the + 3rd order and −3rd order modulation sidebands is selected and used to generate an interference beat signal. A light source that generates coherent light;
An external light modulation unit that generates a modulated waveband by entering the output light of the light source;
A frequency sweeping means for linearly sweeping the frequency of the modulation sideband generated by the rotator optical modulation unit with respect to time;
Optical branching means for branching the output light from the externally modulated frequency swept in two;
One of the light branched by the light branching unit is used as a reference light and the other is input as a signal light to the object to be measured, and the signal light reflected or backscattered at each point of the object to be measured is combined with the reference light. A multiplexing means for generating an interference beat signal;
A light receiving means for receiving the interference beat signal obtained by the multiplexing means and converting it into an electrical signal;
Frequency analysis means for frequency analysis of the electrical signal obtained by the light receiving means,
An optical frequency domain measuring device characterized by measuring reflectance or loss at each point in the object to be measured from the result of the frequency analysis,
Mach-Zehnder interferometer type first and second optical modulators connected in series with each other as the external optical modulator,
A modulation signal generator for generating a modulation signal for sweeping an optical frequency with respect to the first and second optical modulators;
A distributor for distributing a modulation signal from the modulation signal generator to the first and second optical modulators;
Variable delay means for adjusting a delay time between modulation signals input to the first and second optical modulators;
First and second DC voltage sources for generating first and second control voltages for controlling the phase of the output light wave in each of the first and second optical modulators,
By adjusting the DC voltage supplied to the one optical modulator, the phase difference of the light wave is set to 0 rad, and by adjusting the DC voltage supplied to the other optical modulator, the phase difference of the light wave is set to π rad and input. An optical frequency domain reflection measuring apparatus that suppresses a carrier wave component of an optical wave and generates + 3rd order and −3rd order modulation sidebands more strongly than other sidebands.   3. The optical frequency domain reflectometry apparatus according to claim 2, wherein an optical filter is disposed downstream of the first and second optical modulators connected in series.   3. The optical frequency domain reflection measurement apparatus according to claim 1, wherein an acoustooptic frequency shifter is disposed on the optical path of the reference light.

Images