JP5412209B2 - Optical frequency domain reflection measurement method and optical frequency domain reflection measurement apparatus - Google Patents

Optical frequency domain reflection measurement method and optical frequency domain reflection measurement apparatus Download PDF

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JP5412209B2
JP5412209B2 JP2009184871A JP2009184871A JP5412209B2 JP 5412209 B2 JP5412209 B2 JP 5412209B2 JP 2009184871 A JP2009184871 A JP 2009184871A JP 2009184871 A JP2009184871 A JP 2009184871A JP 5412209 B2 JP5412209 B2 JP 5412209B2
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優介 古敷谷
文彦 伊藤
ファン・シンユー
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本発明は、光部品や光伝送路からの反射光あるいは後方散乱光を高空間分解能で測定することが可能な光周波数領域反射測定方法及びこの方法を利用した光周波数領域反射測定装置に関する。   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.

光部品や光伝送路からの反射光および後方散乱光を高空間分解能で測定することが可能な手法として、非特許文献1に示されるようなコヒーレント光を用いた光周波数領域反射測定法(C−OFDR法)がある。このC−OFDR法は、測定対象に周波数掃引されたコヒーレント光を入射し、測定対象からの反射光および後方散乱光と、予め分岐された参照光をコヒーレント検波し、これによって得られた測定ビート信号を周波数解析することで、測定対象内の任意の位置での反射光および後方散乱光強度を得て、測定対象の損失分布や故障点の特定を可能にする技術である。   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 that has been swept in frequency is incident on a measurement target, reflected light and backscattered light from the measurement target, and pre-branched reference light are coherently detected, and measurement beats obtained thereby are measured. 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.

ところで、C−OFDR法の従来の技術として、コヒーレント光源の周波数掃引を外部変調器によって生じる1次変調側波帯を用いて実現する方法がある(非特許文献2参照)。この方法において、C−OFDRにおける空間分解能ΔZminは測定対象内での光速をvg、1次変調側波帯の周波数掃引幅をΔF1stとすると 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.

Figure 0005412209
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ΔF1st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ΔF1st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、ΔF1st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。
Figure 0005412209
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.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).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).

以上のように、従来のC−OFDR法において、外部変調器を用いた周波数掃引では、空間分解能が変調器の帯域によって制限されるといった課題があった。
本発明は、上記の事情を鑑みてなされたもので、外部変調方式を用いて周波数掃引する場合でも、変調器の帯域に制限されることなく空間分解能を高めることのできる光周波数領域反射測定方法及びこの方法を用いた光周波数領域反射測定装置を提供することを目的とする。
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.

上記目的を達成するために本発明に係る光周波数領域反射測定方法は以下のような態様の構成とする。
(1)コヒーレント光源からの出力光を外部光変調処理を施して変調側波帯を発生させ、発生させた変調側波帯を時間に対して線形に周波数掃引し、周波数掃引した外部変調処理の出力光を2分岐し、一方を参照光とし、他方を信号光として被測定物に入射し、被測定物の各地点で反射または後方散乱された信号光と前記参照光を合波させて干渉ビート信号を生じさせ、これを受光して周波数解析することで、前記被測定物内の各地点における反射率または損失を測定する方法であって、前記外部光変調処理として、第1の光変調処理と第2の変調処理を順次行うようにし、一方の光変調処理の光波の位相差を0 radとし、他方の光変調処理の光波の位相差をπradとして、入力した光波の搬送波成分を抑圧し、+3次および−3次の変調側波帯を他の側波帯よりも強く発生させ、前記+3次および−3次の変調側波帯のいずれか一方を選択して干渉ビート信号の生成に用いる構成とする。
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.

また、本発明に係る光周波数領域反射測定装置は以下のような態様の構成とする。
(2) コヒーレント光を発生する光源と、前記光源の出力光を入射して変調測波帯を発生する外部光変調部と、前記部光変調部で発生させる変調側波帯を時間に対して線形に周波数掃引する周波数掃引手段と、前記周波数掃引された外部変調部からの出力光を2分岐する光分岐手段と、前記光分岐手段で分岐された一方を参照光とし、他方を信号光として被測定物に入射し、被測定物の各地点で反射または後方散乱された信号光と前記参照光を合波させて干渉ビート信号を生じさせる合波手段と、前記合波手段で得られた干渉ビート信号を受光して電気信号に変換する受光手段と、前記受光手段で得られた電気信号を周波数解析する周波数解析手段とを具備し、前記周波数解析の結果から前記被測定物内の各地点における反射率または損失を測定することを特徴とする光周波数領域測定装置であって、前記外部光変調部として、互いに直列に接続されるマッハ・ツェンダ干渉計型の第1及び第2の光変調器と、前記第1及び第2の光変調器に対して光周波数を掃引するための変調信号を発生する変調信号発生器と、前記変調信号発生器からの変調信号を前記第1及び第2の光変調器に分配するための分配器と、前記第1及び第2の光変調器に入力する変調信号間の遅延時間を調節する可変遅延手段と、前記第1及び第2の光変調器それぞれにおける出力光波の位相を制御するための第1及び第2の制御電圧を発生する第1及び第2の直流電圧源とを備え、前記一方の光変調器に供給する直流電圧を調整することで光波の位相差を0 radとし、他方の光変調器に供給する直流電圧を調整することで光波の位相差をπradとし、入力した光波の搬送波成分を抑圧し、+3次および−3次の変調側波帯を他の側波帯よりも強く発生させる構成とする。
Moreover, the optical frequency domain reflection measuring apparatus according to the present invention has the following configuration.
(2) a light source for generating coherent light, an external light modulator for generating a modulation sidebands incident output light of the light source relative to the outer portion light modulation sidebands be generated in the modulation unit 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)(2)の構成において、直列に接続された前記第1及び第2の光変調器の後段に光フィルタを配置する。
(4)(2)または(3)に記載の光周波数領域反射測定装置において、前記参照光の光路上に音響光学周波数シフタを配置する。
(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.

以上のように、本発明では、外部変調部に2台のマッハ・ツェンダ干渉計型光変調器を用いて3次の変調側波帯のみを強く発生させ、変調器による周波数掃引幅を3倍にする。これは、空間分解能が1/3になる(3倍の改善)ことを意味する。したがって、本発明によれば、外部変調方式を用いて周波数掃引する場合でも、変調器の帯域に制限されることなく空間分解能を高めることのできる光周波数領域反射測定方法及びこの方法を用いた光周波数領域反射測定装置を提供することができる。   As described above, in the present invention, only the third-order modulation sideband is generated strongly by using two Mach-Zehnder interferometer type optical modulators in the external modulator, 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.

本発明の光周波数領域反射測定方法を採用した測定装置の第1の実施形態を示す構成図。The block diagram which shows 1st Embodiment of the measuring apparatus which employ | adopted the optical frequency domain reflection measuring method of this invention. 本発明の光周波数領域反射測定方法を採用した測定装置の第2の実施形態を示す構成図。The block diagram which shows 2nd Embodiment of the measuring apparatus which employ | adopted the optical frequency domain reflection measuring method of this invention. 上記第2の実施形態における周波数掃引の概要を示す図。The figure which shows the outline | summary of the frequency sweep in the said 2nd Embodiment. 上記第2の実施形態において、AOMを参照光路上に配置しない場合に±3次の変調側波帯の掃引にて得られる干渉ビート信号周波数と光路長差ΔLの関係を示す図。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. 上記第2の実施形態において、AOMを参照光路上に配置した場合に±3次の変調側波帯の掃引にて得られる干渉ビート信号周波数と光路長差ΔLの関係を示す図。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. 上記第2の実施形態において、AOMを参照光路上に配置した場合に±3次の変調側波帯の掃引にて得られる干渉ビート信号周波数が占める領域を示す図。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 said 2nd Embodiment.

以下、図面を参照して本発明の実施の形態を詳細に説明する。
(第1の実施形態)
図1は、本発明に係るC−OFDR法を採用した測定装置の第1の実施形態を示すブロック構成図である。図1において、コヒーレント光源11から出力されたコヒーレント光は第1のマッハ・ツェンダ干渉計型(以下、MZ)光変調器(MZ1)12に入射され、その出力光は第2のMZ光変調器(MZ2)13に入射される。ωを入射光の角周波数、Ωを変調信号の角周波数、φを変調度とすると、第1のMZ光変調器12に入力する光波の電場はexp(iωt)と表され、第1のMZ光変調器12の変調信号入力ポートRF1には変調信号φ1sin(Ωt)が供給され、第2のMZ光変調器13の変調信号入力ポートRF2には変調信号φ2sin(Ωt)が供給される。これらの変調信号は変調信号発生器14で発生され、アンプ15で増幅され、分配器16で2分配されて、それぞれ可変RFディレイ17,18、アッテネータ19,20で任意に遅延され、振幅制御される。
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) light 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 a modulation signal generator 14, amplified by an amplifier 15, divided by 2 by a distributor 16, and arbitrarily delayed by variable RF delays 17 and 18 and attenuators 19 and 20, respectively, and subjected to amplitude control. The

上記第1、第2のMZ光変調器12,13は、互いに直列に接続されていることから、光変調器12,13間で同期を取る必要がある。具体的には、Nを0を含む正の整数、光波が第1のMZ光変調器12にて変調を受けてから第2のMZ光変調器13で再度変調されるまでの時間をτ0、第1、第2のMZ光変調器12,13に入力される変調信号間の遅延時間をτEとすると、τ0 =τE +2πN/Ωとなるよう可変RFディレイ17,18を調整する。また、第1のMZ光変調器12ではその両アームを通過する光波間での位相差が0となるように入力ポートDC1に直流電圧源21で発生される直流電圧を印加し、第2のMZ光変調器13ではその両アームを通過する光波間での位相差がπとなるように入力ポートDC2に直流電圧源22で発生される直流電圧を印加する。このとき、第2のMZ光変調器13からは、±3次側波帯(詳細は後述する)の変調信号が得られる。 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).

2台の光変調器12,13の後段には光フィルタ23が設置され、+3または−3次側波帯のどちらか一方が遮断される。光フィルタ23からの出力光は、光方向性結合器24によって分岐され、一方は信号光として測定対象25に入射され、他方は参照光として用いられる。測定対象25内で反射または後方散乱された信号光は光方向性結合器24により取り出され、光方向性結合器26により参照光と合波されて、受信器27によって受信検波される。この時、信号光と参照光の干渉によって生じる干渉ビート信号を周波数解析装置28によって周波数解析する。このようにして測定対象25内の各位置からの反射光および後方散乱光強度分布を測定する。   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.

上記構成において、以下、その測定方法について具体的に説明する。
まず、4次側波帯まで考慮し、第1のMZ光変調器12の両アーム間に付与される電場は互いに逆向きであることから、第1のMZ光変調器12から出力される光波の電場E1(t)は以下のように表される。
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.

Figure 0005412209
ここでJn(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。
Figure 0005412209
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.

Figure 0005412209
したがって、J01)−J21)=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。
Figure 0005412209
Therefore, by setting J 01 ) −J 21 ) = 0 (φ 1 is approximately 1.84), the ± first order without affecting the third order sidebands as follows: Can be suppressed.

Figure 0005412209
この時、±5次側波帯は残存するが、φ1 =1.84のとき、J41)/{J21)−J41)}=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。
Figure 0005412209
At this time, the ± 5th order sideband remains, but when φ 1 = 1.84, J 41 ) / {J 21 ) −J 41 )} = 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.

ここで、変調信号発生器14からの変調信号周波数を時間に対して掃引することで、+3次および−3次側波帯を掃引することができる。これは式(4)のΩを時間に対して掃引することを意味する。このため、±3次側波帯の周波数掃引幅はΔF3rd=3ΔF=3ΔF1stとなり、1次側波帯を利用した周波数掃引と比較して3倍の周波数掃引幅および3倍の空間分解能の改善を実現することができる。 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 the ± 3rd order sideband is ΔF 3rd = 3ΔF = 3ΔF 1st , which is 3 times the frequency sweep width and 3 times the spatial resolution compared to the frequency sweep using the primary sideband. Improvements can be realized.

絶対値が同次数の側波帯を周波数掃引してC−OFDR測定を実施した場合、得られる干渉ビート信号は強度変調され、正確な波形が得られなくなる。これを回避するために、2台の光変調器12,13の後段に配置した光フィルタ23によって、+3または−3次側波帯のどちらか一方を遮断する。例えば、−3次側波帯を遮断したとすると、光フィルタ23からの出力光は+3次側波帯のみとなり、光方向性結合器24によって分岐され、一方は信号光として測定対象25に入射され、他方は参照光として用いられる。   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.

測定対象25内で反射または後方散乱された信号光は光方向性結合器24により取り出され、光方向性結合器26により参照光と合波されて、受信器27によって受信検波される。この時、信号光と参照光の干渉によって干渉ビート信号が生じる。この干渉ビート信号を周波数解析装置28によって周波数解析することで、測定対象25内の各位置からの反射光および後方散乱光強度分布が測定される。   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.

(第2の実施形態)
図2は、本発明に係るC−OFDR法を採用した測定装置の第2の実施形態を示すブロック構成図である。尚、図2において、図1と同一部分には同一符号を付して示し、ここでは異なる部分を中心に説明する。
図2に示す第2の実施形態の測定装置は、音響光学効果によって光波の光周波数をシフトさせる音響光学周波数シフタ(AOM)29を参照光路上に配置したもので、+3および−3次側波帯によって生じる干渉ビート信号を周波数軸上で分離することで、上述した干渉ビート信号の強度変調を回避し、正しい干渉ビート信号を取得するための最適な構成である。図1の信号光路上に配置した光フィルタ23の代わりに、参照光路上にAOM29を配置する点以外は第1の実施形態と同様の構成である。
(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.

図3に示すように、+3次側帯波の周波数掃引幅をΔF3rd、掃引時間をTとしたとき+3次側波帯の光周波数掃引速度γ3rdAs 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

Figure 0005412209
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=−γ3rd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fb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。
Figure 0005412209
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.

Figure 0005412209
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Lt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FAOMとすると、以下のように表される。
Figure 0005412209
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 .

Figure 0005412209
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,fb)=(Lt,0)を満足するようにFAOM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FAOMの値は以下のように表される。
Figure 0005412209
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.

Figure 0005412209
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。
Figure 0005412209
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.

尚、本発明は、上記実施形態そのままに限定されるものではなく、実施段階ではその要旨を逸脱しない範囲で構成要素を変形して具体化できる。また、上記実施形態に開示されている複数の構成要素の適宜な組合せにより種々の発明を形成できる。例えば、第1の実施形態の光フィルタ23と第2の実施形態のAOM29とを組み合わせて構成してもよい。また、実施形態に示される全構成要素からいくつかの構成要素を削除しても良い。更に、異なる実施形態に亘る構成要素を適宜組み合わせてもよい。   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…コヒーレント光源、12…第1のMZ光変調器(MZ1)、13…第2のMZ光変調器(MZ2)、14…変調信号発生器、15…アンプ、16…分配器、17,18…可変RFディレイ、19,20…アッテネータ、21,22…直流電圧源、23…光フィルタ、24…光方向性結合器、25…測定対象、26…光方向性結合器、27…受信器、28…周波数解析装置、29…音響光学周波数シフタ(AOM)。 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)

コヒーレント光源からの出力光を外部光変調処理を施して変調側波帯を発生させ、発生させた変調側波帯を時間に対して線形に周波数掃引し、周波数掃引した外部変調処理の出力光を2分岐し、一方を参照光とし、他方を信号光として被測定物に入射し、被測定物の各地点で反射または後方散乱された信号光と前記参照光を合波させて干渉ビート信号を生じさせ、これを受光して周波数解析することで、前記被測定物内の各地点における反射率または損失を測定する光周波数領域測定方法であって、
前記外部光変調処理として、第1の光変調処理と第2の変調処理を順次行うようにし、
一方の光変調処理の光波の位相差を0 radとし、他方の光変調処理の光波の位相差をπradとして、入力した光波の搬送波成分を抑圧し、+3次および−3次の変調側波帯を他の側波帯よりも強く発生させ、
前記+3次および−3次の変調側波帯のいずれか一方を選択して干渉ビート信号の生成に用いることを特徴とする光周波数領域反射測定方法。
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.
コヒーレント光を発生する光源と、
前記光源の出力光を入射して変調測波帯を発生する外部光変調部と、
前記部光変調部で発生させる変調側波帯を時間に対して線形に周波数掃引する周波数掃引手段と、
前記周波数掃引された外部変調部からの出力光を2分岐する光分岐手段と、
前記光分岐手段で分岐された一方を参照光とし、他方を信号光として被測定物に入射し、被測定物の各地点で反射または後方散乱された信号光と前記参照光を合波させて干渉ビート信号を生じさせる合波手段と、
前記合波手段で得られた干渉ビート信号を受光して電気信号に変換する受光手段と、
前記受光手段で得られた電気信号を周波数解析する周波数解析手段とを具備し、
前記周波数解析の結果から前記被測定物内の各地点における反射率または損失を測定することを特徴とする光周波数領域測定装置であって、
前記外部光変調部として、互いに直列に接続されるマッハ・ツェンダ干渉計型の第1及び第2の光変調器と、
前記第1及び第2の光変調器に対して光周波数を掃引するための変調信号を発生する変調信号発生器と、
前記変調信号発生器からの変調信号を前記第1及び第2の光変調器に分配するための分配器と、
前記第1及び第2の光変調器に入力する変調信号間の遅延時間を調節する可変遅延手段と、
前記第1及び第2の光変調器それぞれにおける出力光波の位相を制御するための第1及び第2の制御電圧を発生する第1及び第2の直流電圧源とを備え、
前記一方の光変調器に供給する直流電圧を調整することで光波の位相差を0 radとし、
他方の光変調器に供給する直流電圧を調整することで光波の位相差をπradとし、入力した光波の搬送波成分を抑圧し、+3次および−3次の変調側波帯を他の側波帯よりも強く発生させることを特徴とする光周波数領域反射測定装置。
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 sweep means for frequency sweep linearly modulated sidebands be generated in the outer portion light modulator 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 0 rad,
By adjusting the DC voltage supplied to the other optical modulator, the phase difference of the light wave is set to π rad, the carrier component of the input light wave is suppressed, and the + 3rd order and −3rd order modulation sidebands are changed to other sidebands. An optical frequency domain reflection measuring apparatus characterized by generating the intensity more strongly than the other.
請求項2に記載の光周波数領域反射測定装置において、直列に接続された前記第1及び第2の光変調器の後段に光フィルタを配置することを特徴とする光周波数領域反射測定装置。   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. 請求項1または2に記載の光周波数領域反射測定装置において、前記参照光の光路上に音響光学周波数シフタを配置することを特徴とする光周波数領域反射測定装置。   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.
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