JP4364780B2

JP4364780B2 - Optical fiber characteristic evaluation method and apparatus

Info

Publication number: JP4364780B2
Application number: JP2004359342A
Authority: JP
Inventors: 邦弘戸毛; 和男保苅
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2004-12-13
Filing date: 2004-12-13
Publication date: 2009-11-18
Anticipated expiration: 2024-12-13
Also published as: JP2006170629A

Description

本発明は、ラマン光増幅という光ファイバのもつ非線形効果により生じた信号光の時間的波形変化を解析し、光ファイバの長手方向にわたるラマン光増幅特性等の分布を評価する方法および、そのための装置に関するものである。 The present invention analyzes a temporal waveform change of signal light caused by a nonlinear effect of an optical fiber called Raman light amplification, and evaluates the distribution of Raman light amplification characteristics and the like over the longitudinal direction of the optical fiber, and an apparatus therefor It is about.

ブロードバンドサービスの増加による伝送容量拡大の需要に伴い、ラマン光増幅効果を用いて信号品質の劣化を抑制し、光信号を長距離伝送するシステムが検討されている。ラマン光増幅は励起する光の波長を変化させることで、増幅する波長帯を自由に選択することができるため、複数の波長を用いて光信号を伝送するＷＤＭ（波長多重）システムにこの増幅効果を適用することが検討されている。これをＷＤＭシステムに適用するためには光線路の波長に対する光ファイバのラマン利得特性を評価することが重要となる。このラマン光増幅による利得特性を測定する方法として、下記の特許文献１「光ファイバの特性評価方法および装置」および特許文献２「光ファイバの特性評価方法および装置」にて、波長に対してラマン利得特性の光ファイバ長手方向にわたる分布を測定する方法および装置が示されている。 Along with the demand for an increase in transmission capacity due to an increase in broadband services, a system for transmitting optical signals over long distances by suppressing degradation of signal quality using the Raman light amplification effect is being studied. Since Raman light amplification can freely select the wavelength band to be amplified by changing the wavelength of pumping light, this amplification effect is applied to WDM (wavelength multiplexing) systems that transmit optical signals using multiple wavelengths. Is being considered. In order to apply this to a WDM system, it is important to evaluate the Raman gain characteristics of the optical fiber with respect to the wavelength of the optical line. As a method of measuring the gain characteristic by the Raman light amplification, the following Patent Document 1 “Optical Fiber Characteristic Evaluation Method and Device” and Patent Document 2 “Optical Fiber Characteristic Evaluation Method and Device” are described with respect to the wavelength. A method and apparatus for measuring the distribution of gain characteristics over the length of an optical fiber is shown.

特許文献１および特許文献２に述べられている特性評価装置の構成を図５に、測定波形の模式図を図６に示す。この特性評価装置では、第１光源１からのパルス光を被測定光ファイバ３の第１端末８−１から被測定光ファイバ３へ入射させ、被測定光ファイバ３のもう一端である第２端末８−２から第２光源２からのＣＷ光（連続光）を入射させて、前記被測定光ファイバ中で前記ＣＷ光を前記パルス光と対向させることにより、ラマン光増幅を生じさせる。そして、前記ラマン光増幅を受けた第２光源２からのＣＷ光は光カプラ４を介して光検出器５にて電気信号に変換され、この電気信号が信号処理装置７で信号処理される。このラマン光増幅された第２光源２からのＣＷ光パワーの時間的波形変化は図６のようになり、これを解析することにより被測定光ファイバ３の持つラマン利得特性の分布が測定できる。 FIG. 5 shows a configuration of the characteristic evaluation apparatus described in Patent Document 1 and Patent Document 2, and FIG. 6 shows a schematic diagram of a measurement waveform. In this characteristic evaluation apparatus, pulsed light from the first light source 1 is incident on the measured optical fiber 3 from the first terminal 8-1 of the measured optical fiber 3, and the second terminal that is the other end of the measured optical fiber 3. The CW light (continuous light) from the second light source 2 is entered from 8-2, and the CW light is made to face the pulsed light in the optical fiber to be measured, thereby causing Raman light amplification. Then, the CW light from the second light source 2 subjected to the Raman light amplification is converted into an electric signal by the photodetector 5 through the optical coupler 4, and this electric signal is signal-processed by the signal processing device 7. The temporal waveform change of the CW light power from the Raman light amplified second light source 2 is as shown in FIG. 6, and by analyzing this, the distribution of the Raman gain characteristic of the measured optical fiber 3 can be measured.

なお、特許文献１と特許文献２の違いは、特許文献１では図６に示す増幅成分および直流成分と被測定光ファイバ３に入射したパルス光のパワー、さらに被測定光ファイバ３の損失分布の個々の測定項目を測定し、ラマン利得特性の分布を測定する方法であるのに対して、特許文献２ではラマン利得特性が既知である参照光ファイバを被測定光ファイバ３と接続し、参照光ファイバと被測定光ファイバ３のラマン利得特性の比率から被測定光ファイバ３のラマン利得特性の分布を算出することで、増幅成分のみを測定すれば良く、上記の測定項目を削減し、簡易化された点にある。 The difference between Patent Document 1 and Patent Document 2 is that, in Patent Document 1, the amplification component and DC component shown in FIG. 6 and the power of the pulsed light incident on the optical fiber 3 to be measured and the loss distribution of the optical fiber 3 to be measured are shown. In contrast to the method of measuring individual measurement items and measuring the distribution of Raman gain characteristics, in Patent Document 2, a reference optical fiber whose Raman gain characteristics are known is connected to the optical fiber 3 to be measured, and the reference light By calculating the Raman gain characteristic distribution of the measured optical fiber 3 from the ratio of the Raman gain characteristics of the fiber and the measured optical fiber 3, it is only necessary to measure the amplification component, and the above measurement items can be reduced and simplified. The point is.

特開２００３−１５６４１０号公報JP 2003-156410 A 特開２００３−２７９４４７号公報JP 2003-279447 A

しかし、特許文献１および特許文献２に述べられている測定方法および装置構成では被測定光ファイバ３の両端にそれぞれ第１光源１、第２光源２を設置する必要があり、測定作業が煩雑になり、かつ多くの労力を必要とするという課題がある。 However, in the measurement method and apparatus configuration described in Patent Document 1 and Patent Document 2, it is necessary to install the first light source 1 and the second light source 2 at both ends of the optical fiber 3 to be measured, respectively, and the measurement work is complicated. However, there is a problem of requiring a lot of labor.

そこで本発明の目的は、光源、光カプラ、光波長フィルタまたは分光器、光検出器および信号処理装置の全ての装置を被測定光ファイバの一方の端末側に設置することで、前記課題を解決し、利便性が高い測定方法および装置構成を提供することにある。 Therefore, the object of the present invention is to solve the above-mentioned problem by installing all devices of a light source, an optical coupler, an optical wavelength filter or spectroscope, a photodetector and a signal processing device on one terminal side of an optical fiber to be measured. It is another object of the present invention to provide a highly convenient measurement method and apparatus configuration.

本発明は前記課題を解決するために、第１光源からのパルス光と、前記第１光源からのパルス光に対して時間遅延を設けた第２光源からのパルス光とを用いて、被測定光ファイバ中で前記第１光源からのパルス光の後方散乱光とそれと対向する前記第２光源からのパルス光の相互作用によってラマン光増幅した第１光源からのパルス光の後方散乱光を検出し、この検出信号を信号処理した波形の時間的変化を解析することで被測定光ファイバのラマン利得特性の分布測定を可能とする方法および装置である。 In order to solve the above problems, the present invention uses a pulsed light from a first light source and a pulsed light from a second light source provided with a time delay with respect to the pulsed light from the first light source. In the optical fiber, the back scattered light of the pulse light from the first light source amplified by Raman light is detected by the interaction of the back scattered light of the pulse light from the first light source and the pulse light from the second light source facing it. This is a method and apparatus that enables the distribution measurement of the Raman gain characteristics of the optical fiber to be measured by analyzing the temporal change of the waveform obtained by signal processing of the detection signal.

即ち、第１発明の光ファイバの特性評価方法は、第１光源からのパルス光と、前記第１光源からのパルス光と波長が異なり、かつ前記第１光源からのパルス光に対して時間遅延を設けた第２光源からのパルス光とを、第１光カプラにて合波し、被測定光ファイバの一端から前記被測定光ファイバへ入射させ、前記被測定光ファイバ中で前記第１光源からのパルス光の後方散乱光と前記第２光源からのパルス光との間の相互作用によりラマン光増幅を生じさせ、第２光カプラと、前記被測定光ファイバ中で前記第２光源からのパルス光により発生する後方散乱光を除去し前記第１光源からのパルス光の後方散乱光のみを透過させる光波長フィルタとを介して、前記ラマン光増幅された前記第１光源からのパルス光の後方散乱光のパワーを光検出器にて検出し、この検出信号を信号処理して波形を解析することにより、前記被測定光ファイバの長さ方向のラマン利得特性を測定することを特徴とする。 In other words, the optical fiber characteristic evaluation method according to the first aspect of the present invention is different in wavelength from the pulsed light from the first light source and the pulsed light from the first light source, and has a time delay with respect to the pulsed light from the first light source. The first light coupler combines the pulsed light from the second light source provided with the first optical coupler, enters the measured optical fiber from one end of the measured optical fiber, and the first light source in the measured optical fiber. The Raman light amplification is caused by the interaction between the backscattered light of the pulsed light from the second light source and the pulsed light from the second light source, and the second optical coupler and the second optical source from the second light source are measured in the optical fiber to be measured. The pulsed light from the first light source amplified by the Raman light is passed through an optical wavelength filter that removes the backscattered light generated by the pulsed light and transmits only the backscattered light of the pulsed light from the first light source. Light detection of backscattered light power Detected by, by analyzing the waveform of the detection signal signal processing to, and measuring the Raman gain characteristics in the length direction of the optical fiber to be measured.

また、第２発明の光ファイバの特性評価方法は、第１発明の方法において、前記被測定光ファイバの代わりに、ラマン利得特性について既知の光ファイバを参照光ファイバとして一本または複数本の被測定光ファイバと接続させた光ファイバを用いて波形を測定し、参照光ファイバのラマン利得特性と被測定光ファイバのラマン利得特性の比率を求めることにより、被測定光ファイバの長さ方向のラマン利得特性を算出することを特徴とする。 The optical fiber characteristic evaluation method according to the second aspect of the invention is the method of the first aspect of the invention, wherein one or a plurality of optical fibers having a known Raman gain characteristic is used as a reference optical fiber instead of the optical fiber to be measured. The waveform is measured using an optical fiber connected to the measurement optical fiber, and the ratio of the Raman gain characteristic of the reference optical fiber to the Raman gain characteristic of the optical fiber to be measured is obtained. The gain characteristic is calculated.

また、第３発明の光ファイバの特性評価装置は、光パルスを送出する第１光源と、前記第１光源からの光パルスと波長が異なり、かつ前記第１光源からのパルス光に対して時間遅延を設けた光パルスを送出する第２光源と、第１光カプラ及び第２光カプラと、光波長フィルタまたは分光器と、光検出器と、信号処理装置とを被測定光ファイバの一端側に配置した構成であって、前記第１光源からのパルス光と前記第２光源からのパルス光とを前記第１光カプラにて合波し、前記被測定光ファイバの一端から前記被測定光ファイバに入射させ、前記被測定光ファイバ中で前記第１光源からのパルス光の後方散乱光と前記第２光源からのパルス光との間の相互作用により、ラマン光増幅を生じさせ、前記第２光カプラと、前記被測定光ファイバ中で第２光源からのパルス光により発生する後方散乱光を遮断するための光波長フィルタまたは分光器とを介して、前記ラマン光増幅された前記第１光源からのパルス光の後方散乱光を検出する前記光検出器および前記信号処理装置を配置したことを特徴とする。 According to a third aspect of the present invention, there is provided the optical fiber characteristic evaluation apparatus, wherein the wavelength of the first light source that transmits the light pulse is different from that of the light pulse from the first light source, and the time of the pulse light from the first light source is long. One end side of the optical fiber to be measured includes a second light source that transmits an optical pulse with a delay, a first optical coupler and a second optical coupler, an optical wavelength filter or spectrometer, a photodetector, and a signal processing device. The pulsed light from the first light source and the pulsed light from the second light source are combined by the first optical coupler, and the measured light is transmitted from one end of the measured optical fiber. The light is incident on a fiber, and Raman light amplification is caused by the interaction between the backscattered light of the pulsed light from the first light source and the pulsed light from the second light source in the optical fiber to be measured. Two optical couplers in the optical fiber to be measured A backscattered light of the pulsed light from the first light source amplified by the Raman light is detected through an optical wavelength filter or a spectroscope for blocking backscattered light generated by the pulsed light from the second light source. The photodetector and the signal processing device are arranged.

また、第４発明の光ファイバの特性評価装置は、第３発明の装置において、前記被測定光ファイバの代わりに、ラマン利得特性について既知の光ファイバを参照光ファイバとして一本または複数本の被測定光ファイバと接続させた光ファイバを用いたことを特徴とする。 An optical fiber characteristic evaluation apparatus according to a fourth aspect of the invention is the apparatus according to the third aspect of the invention, wherein one or a plurality of optical fibers having a known Raman gain characteristic is used as a reference optical fiber instead of the optical fiber to be measured. An optical fiber connected to the measurement optical fiber is used.

本発明による測定方法および測定装置を用いることで、被測定光ファイバの片端側に全ての測定装置を設置することができ、被測定光ファイバの長さ方向にわたるラマン利得特性を測定する上で、測定稼動を削減し、利便性を向上させることができる。特に被測定光ファイバが敷設された長距離の光ファイバ伝送路の場合、光ファイバ伝送路の両側に測定装置を設置する必要が無く、簡単にラマン利得特性を測定できるため、より一層の利便性の向上を図ることができ、有効である。 By using the measuring method and measuring device according to the present invention, all measuring devices can be installed on one end side of the optical fiber to be measured, and when measuring the Raman gain characteristics over the length direction of the optical fiber to be measured, Measurement operation can be reduced and convenience can be improved. In particular, in the case of a long-distance optical fiber transmission line in which the optical fiber to be measured is laid, there is no need to install measuring devices on both sides of the optical fiber transmission line, and Raman gain characteristics can be measured easily, making it even more convenient Can be improved and is effective.

以下、図面を参照しながら本発明の実施例を詳細に説明する。 Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

［実施例］
図１には本発明の実施例に係る光ファイバの特性評価装置の構成を示す。図１に示すように、本特性評価装置は、光パルスを送出する第１光源１と、第１光源１からの光パルスと波長が異なり、かつ第１光源１からのパルス光に対して時間遅延を設け光パルスを送出する第２光源２と、第１光カプラ４−１及び第２光カプラ４−２と、光波長フィルタ５と、光検出器６と、表示装置を備えた平均化処理装置７とを被測定光ファイバの一端側（片端側）に配置している。即ち、特性評価を行うための（特性評価装置を構成するための）全て測定装置が、被測定光ファイバ３の片端側にのみ配置されている。 [Example]
FIG. 1 shows the configuration of an optical fiber characteristic evaluation apparatus according to an embodiment of the present invention. As shown in FIG. 1, this characteristic evaluation apparatus is different in wavelength from the first light source 1 that transmits an optical pulse, and the optical pulse from the first light source 1, and has a time relative to the pulsed light from the first light source 1. Averaging provided with a second light source 2 for providing an optical pulse with a delay, a first optical coupler 4-1 and a second optical coupler 4-2, an optical wavelength filter 5, a photodetector 6, and a display device. The processing device 7 is disposed on one end side (one end side) of the optical fiber to be measured. That is, all the measuring devices for performing the characteristic evaluation (for configuring the characteristic evaluation device) are arranged only on one end side of the optical fiber 3 to be measured.

第１光源１と第２光源２は第１光カプラ４−１の入力側に接続され、第１光カプラ４−１の出力側と被測定光ファイバ３の入射端末８−１との間には第２光カプラ４−２が介設され、第２光カプラ４−２の出力側に波長フィルタ５と光検出器６と平均化処理装置７とが順に接続されている。そして、第１光源１からのパルス光と第２光源２からのパルス光とを第１光カプラ４−２にて合波し、被測定光ファイバ３の一端の入射端末８−１から被測定光ファイバ３に入射させ、被測定光ファイバ３で第１光源１からのパルス光の後方散乱光と第２光源２からのパルス光との間の相互作用により、ラマン光増幅を生じさせ、第２光カプラ４−２と、被測定光ファイバ３で第２光源２からのパルス光により発生する後方散乱光を遮断するための光波長フィルタ６とを介して、前記ラマン光増幅された第１光源１からのパルス光の後方散乱光を光検出器６で検出し、この検出信号を信号処理装置７で信号処理して波形を解析することにより、被測定光ファイバ３の長さ方向のラマン利得特性を測定する。また、信号処理装置７で得られたゲイン分布の波形などは信号処理装置７に備えた表示装置に表示される。 The first light source 1 and the second light source 2 are connected to the input side of the first optical coupler 4-1, and between the output side of the first optical coupler 4-1 and the incident terminal 8-1 of the measured optical fiber 3. The second optical coupler 4-2 is interposed, and the wavelength filter 5, the photodetector 6, and the averaging processing device 7 are connected in order to the output side of the second optical coupler 4-2. Then, the pulsed light from the first light source 1 and the pulsed light from the second light source 2 are combined by the first optical coupler 4-2 and measured from the incident terminal 8-1 at one end of the measured optical fiber 3. The light is incident on the optical fiber 3, and Raman light amplification is caused by the interaction between the backscattered light of the pulsed light from the first light source 1 and the pulsed light from the second light source 2 in the optical fiber 3 to be measured. The first Raman light amplified through the two optical coupler 4-2 and the optical wavelength filter 6 for blocking the backscattered light generated by the pulsed light from the second light source 2 in the measured optical fiber 3. The backscattered light of the pulsed light from the light source 1 is detected by the photodetector 6, the detected signal is processed by the signal processing device 7, and the waveform is analyzed, whereby the Raman in the length direction of the optical fiber 3 to be measured is analyzed. Measure gain characteristics. Further, the waveform of the gain distribution obtained by the signal processing device 7 is displayed on a display device provided in the signal processing device 7.

なお、第２光源からのパルス光の後方散乱光を遮断する手段としては光波長フィルタ６に限定するものではなく、分光器を用いてもよい。この場合には分光器によって第１光源１からのパルス光の後方散乱光と第２光源２からのパルス光の後方散乱光とを分光して（即ち第２光源からのパルス光の後方散乱光を遮断して）、第１光源からの後方散乱光のみを光検出器６へ送出する。 The means for blocking the back scattered light of the pulsed light from the second light source is not limited to the optical wavelength filter 6, and a spectroscope may be used. In this case, the spectroscope splits the backscattered light of the pulsed light from the first light source 1 and the backscattered light of the pulsed light from the second light source 2 (that is, the backscattered light of the pulsed light from the second light source 2). Only the backscattered light from the first light source is sent to the photodetector 6.

また、信号処理装置７では平均化処理も行う。即ち、第１光源１及び第２光源２から送出される１回のパルス光に基づいて光検出器６で得られる検出信号の波形は変動の多い（ギザギザとした）ものであり、これでは精度のよい特性評価を行うことができない。そこで、第１光源１及び第２光源２からはパルス光を所定の周期で複数回送出し、信号処理装置７では、これら複数回のパルス光に基づいて光検出器６で得られる複数の検出信号を平均化処理（加算平均）して得た波形を解析することにより、精度のよい被測定光ファイバ３の長さ方向のラマン利得特性の測定を行う。 The signal processing device 7 also performs an averaging process. That is, the waveform of the detection signal obtained by the photodetector 6 based on a single pulse of light transmitted from the first light source 1 and the second light source 2 has a lot of fluctuations (jagged). It is not possible to perform good characterization. Therefore, the first light source 1 and the second light source 2 send pulsed light a plurality of times at a predetermined cycle, and the signal processing device 7 uses a plurality of detection signals obtained by the photodetector 6 based on the plurality of times of pulsed light. By analyzing the waveform obtained by averaging (addition averaging), the Raman gain characteristic in the length direction of the optical fiber 3 to be measured is accurately measured.

ここで本評価装置（評価方法）について更に詳述すると、第１光源１からのパルス光と、第１光源１からのパルス光と波長が異なり、かつ第１光源１からのパルス光に対して時間遅延を設けた第２光源２からのパルス光は、第１光カプラ４−１にて合波され、入射端末８−１から被測定光ファイバ３に入射し、被測定光ファイバ３中を同一方向に伝搬する。第１光源１と第２光源２からそれぞれ出射するパルス光に時間遅延を設けるためには、（１）第１光源１および第２光源２に時間遅延を設けたパルス駆動電流を加える電気段（電気的手段）による方法と、（２）第１光源１または第２光源２の出射端に任意の長さの光ファイバを接続して光路長を変化させる（即ち第１光源１から第１光カプラ４−１までの光路長と第２光源２から第１光カプラ４−１までの光路長とに差を設ける）光段（光学的手段）による方法がある。 Here, the evaluation apparatus (evaluation method) will be described in more detail. The pulse light from the first light source 1 is different from the pulse light from the first light source 1 in wavelength, and the pulse light from the first light source 1 is different. The pulsed light from the second light source 2 provided with the time delay is multiplexed by the first optical coupler 4-1, enters the measured optical fiber 3 from the incident terminal 8-1, and passes through the measured optical fiber 3. Propagate in the same direction. In order to provide a time delay for the pulsed light emitted from each of the first light source 1 and the second light source 2, (1) an electric stage for applying a pulse drive current having a time delay to the first light source 1 and the second light source 2 ( (2) An optical fiber having an arbitrary length is connected to the emission end of the first light source 1 or the second light source 2 to change the optical path length (ie, from the first light source 1 to the first light). There is a method using an optical stage (optical means) that provides a difference between the optical path length to the coupler 4-1 and the optical path length from the second light source 2 to the first optical coupler 4-1.

ここで被測定光ファイバ３の入射端末８−１から出射端末８−２にむけてｚ軸座標とする。ｖを被測定光ファイバ３中の光の伝搬速度、ｔを第１光源１からのパルス光が入射端末８−１から被測定光ファイバ３に入射した時から第２光源２からのパルス光が入射する時までの遅延時間とすると、被測定光ファイバ３の任意の位置ｚで発生した第１光源１から出射されたパルス光の後方散乱光は、遅れて出射した第２光源２からのパルス光と対向して被測定光ファイバ３中を伝搬し、被測定光ファイバ３の位置ｚ−ｖｔ／２において第２光源２からのパルス光との間の相互作用によりラマン光増幅される。このラマン光増幅された第１光源１から出射されたパルス光の後方散乱光が、第２光カプラ４−２および第２光源２からのパルス光の後方散乱光を除去するための光波長フィルタ５または分光器を介して、光検出器６にて検出される時の光パワーＰ_s（ｚ）は以下の式（１）で表される。

Here, it is set as z-axis coordinates from the incident terminal 8-1 to the emission terminal 8-2 of the optical fiber 3 to be measured. v is the propagation speed of light in the optical fiber 3 to be measured, and t is the pulse light from the second light source 2 from when the pulsed light from the first light source 1 enters the optical fiber 3 to be measured from the incident terminal 8-1. Assuming the delay time until the incidence, the backscattered light of the pulsed light emitted from the first light source 1 generated at an arbitrary position z of the optical fiber 3 to be measured is a pulse from the second light source 2 emitted late. It propagates through the optical fiber 3 to be measured in opposition to the light, and Raman light is amplified by the interaction with the pulsed light from the second light source 2 at the position z-vt / 2 of the optical fiber 3 to be measured. An optical wavelength filter for removing the back scattered light of the pulsed light emitted from the first light source 1 amplified by Raman light from the second optical coupler 4-2 and the second light source 2. The optical power P _s (z) when detected by the photodetector 6 via 5 or a spectroscope is expressed by the following formula (1).

ここで、Ｐ_s（０）は入射端末８−１における第１光源１から出射されたパルス光のパワー、Ｒ（ｚ）は後方散乱係数、Ｗ_sは第１光源１から出射されたパルス光のパルス幅、α_s（ｚ）は第１光源１から出射されたパルス光の波長に対するファイバ損失分布、Ｃは第２光カプラ４−２および光波長フィルタ５の透過率、Ｇ（ｚ）はラマン光増幅によるゲイン分布を示す。一方、第２光源２からのパルス光が無い時、式（１）は以下の式（２）のように与えられる。

Here, P _s (0) is the power of the pulsed light emitted from the first light source 1 at the incident terminal 8-1, R (z) is the backscattering coefficient, and W _s is the pulsed light emitted from the first light source 1. pulse width, α _s (z) is fiber loss distribution with respect to the wavelength of the pulsed light emitted from the first light source 1, C is the transmittance of the second optical coupler 4-2, and an optical wavelength filter 5, G (z) is The gain distribution by Raman light amplification is shown. On the other hand, when there is no pulsed light from the second light source 2, the expression (1) is given as the following expression (2).

すなわち、Ｇ（ｚ）は式（１）と式（２）の比、すなわち、第２光源２からのパルス光がある時とない時における光検出器６にて検出されるパワーの比で与えられる。ｖｔ／２≦ｚ≦Ｌの場合、Ｇ（ｚ）は以下の式（３）で表される。

That is, G (z) is given by the ratio of the formulas (1) and (2), that is, the ratio of the power detected by the photodetector 6 when there is pulse light from the second light source 2 and when there is no pulse light. It is done. In the case of vt / 2 ≦ z ≦ L, G (z) is represented by the following formula (3).

ここで、ｋは位置ｚに依存しない定数、γ（ｚ＋ｖｔ／２）およびＰ_p（ｚ＋ｖｔ／２）は、それぞれ位置ｚ＋ｖｔ／２における被測定光ファイバ３のラマン利得率および第２光源２からのパルス光のパワーを示す。図２は本実施例におけるＧ（ｚ）の測定波形の模式図を示す。本発明では、第１光源１からのパルス光と第２光源２のパルス光の間には、被測定光ファイバ３に入射する時間差が設けられているため、ラマン光増幅は第１光源１からのパルス光と第２光源２からのパルス光の間の位置で生じる。したがって、図２に示すように、位置ｚからｖｔ／２だけずれた位置の被測定光ファイバ３のラマン利得率および第２光源２からのパルス光のパワーを反映したゲイン分布Ｇ（ｚ）が、信号処理装置７の表示装置に表示される。ここで式（３）をｚ軸上でｖｔ／２だけ平行移動すると、０≦ｚ≦Ｌ−ｖｔ／２の範囲において以下の式（４）のようになる。

Here, k is a constant independent of the position z, γ (z + vt / 2) and P _p (z + vt / 2) are the Raman gain factor of the optical fiber 3 to be measured and the second light source 2 from the position z + vt / 2, respectively. Indicates the power of pulsed light. FIG. 2 shows a schematic diagram of a measured waveform of G (z) in the present embodiment. In the present invention, there is a time difference between the pulsed light from the first light source 1 and the pulsed light from the second light source 2 that is incident on the optical fiber 3 to be measured. Is generated at a position between the pulse light of the second light source 2 and the pulse light from the second light source 2. Therefore, as shown in FIG. 2, the gain distribution G (z) reflecting the Raman gain factor of the optical fiber 3 to be measured at a position shifted by vt / 2 from the position z and the power of the pulsed light from the second light source 2 is obtained. And displayed on the display device of the signal processing device 7. Here, when the expression (3) is translated by vt / 2 on the z-axis, the following expression (4) is obtained in the range of 0 ≦ z ≦ L−vt / 2.

ここで、α_p（ｚ）は第２光源２から出射されたパルス光の波長に対するファイバ損失分布を示す。したがって、位置ｚにおける被測定光ファイバ３のラマン利得率は以下の式（５）で与えられる。式（５）において、ゲイン分布Ｇ（ｚ）は前述のように第２光源２からのパルス光がある時とない時における光検出器７にて検出される第１光源１からのパルス光の後方散乱光パワーの比で与えられる。第２光源２からのパルス光の波長に対する損失分布α_p（ｚ）は第２光源２と同じ波長のパルス光源を持つＯＴＤＲ（optical time domain reflectmeter）を用いて容易に測定することができる。また、入射端末８−１（ｚ＝０）における第２光源２からのパルス光のパワーＰ_p（０）は、出射端末８−２における第２光源２からのパルス光のパワーＰ_p（Ｌ）（このパワーはパワーメータで容易に測定することができる）と、第２光源２からのパルス光の波長に対する損失分布α_p（ｚ）とに基づいて下式（６）から求めることができる。

Here, α _p (z) represents a fiber loss distribution with respect to the wavelength of the pulsed light emitted from the second light source 2. Therefore, the Raman gain factor of the measured optical fiber 3 at the position z is given by the following equation (5). In the equation (5), the gain distribution G (z) is the pulse light from the first light source 1 detected by the photodetector 7 with and without the pulse light from the second light source 2 as described above. It is given by the ratio of the backscattered light power. The loss distribution α _p (z) with respect to the wavelength of the pulsed light from the second light source 2 can be easily measured using an OTDR (optical time domain reflectmeter) having a pulsed light source having the same wavelength as the second light source 2. The power P _p (0) of the pulsed light from the second light source 2 at the incident terminal 8-1 (z = 0) is the power P _p (L) of the pulsed light from the second light source 2 at the output terminal 8-2. ) (This power can be easily measured with a power meter) and the loss distribution α _p (z) with respect to the wavelength of the pulsed light from the second light source 2 can be obtained from the following equation (6). .

更に、ここで０≦ｚ≦Ｌ−ｖｔ／２の範囲において、任意の一点ｚ＝ｚ_oにおけるラマン利得率γ（ｚ_o）が既知であると仮定し、γ（ｚ_o）で式（５）を正規化すると、以下の式（７）で与えられる。

Further, in this case 0 ≦ z ≦ L-vt / 2 range, the Raman gain factor at any point _{_{z = z o γ (z o}} ) is assumed to be known, gamma (z _o) in equation (5 ) Is normalized by the following equation (7).

ここで、ｍは位置ｚに依存しない定数である。式（７）より、正規化されたラマン利得率は、Ｇ（ｚ）とα_p（ｚ）、すなわちラマン光増幅によるゲイン分布と第２光源２からのパルス光の波長に対する損失分布で与えられる。前述のように、ゲイン分布Ｇ（ｚ）は第２光源２からのパルス光がある時とない時における光検出器７にて検出される第１光源１からのパルス光の後方散乱光パワーの比で与えられる。また、第２光源２からのパルス光の波長に対する損失分布α_p（ｚ）は第２光源２と同じ波長のパルス光源を持つＯＴＤＲを用いて容易に測定することができる。そして得られた正規化されたラマン利得率より、任意の一点ｚ_oにおけるラマン利得率γ（ｚ_o）が既知であるので、その他の位置ｚにおけるラマン利得率を算出することが可能である。 Here, m is a constant independent of the position z. From Expression (7), the normalized Raman gain factor is given by G (z) and α _p (z), that is, gain distribution due to Raman light amplification and loss distribution with respect to the wavelength of the pulsed light from the second light source 2. . As described above, the gain distribution G (z) indicates the backscattered light power of the pulsed light from the first light source 1 detected by the photodetector 7 with and without the pulsed light from the second light source 2. Given in ratio. Further, the loss distribution α _p (z) with respect to the wavelength of the pulsed light from the second light source 2 can be easily measured using an OTDR having a pulsed light source having the same wavelength as the second light source 2. Since the Raman gain factor γ (z _o ) at an arbitrary point z _o is known from the obtained normalized Raman gain factor, it is possible to calculate the Raman gain factors at other positions z.

図３はラマン利得率が既知である参照光ファイバ９を用いた特性評価装置の構成を示す。このような構成において、少なくとも任意の一点においてラマン利得率が既知である参照光ファイバ９と接続された一本または複数本（直列接続）の被測定光ファイバ３のラマン利得率の分布を測定することが可能である。 FIG. 3 shows a configuration of a characteristic evaluation apparatus using a reference optical fiber 9 having a known Raman gain factor. In such a configuration, the distribution of the Raman gain factor of one or a plurality of (in series connection) optical fibers 3 to be measured connected to the reference optical fiber 9 whose Raman gain factor is known at at least one arbitrary point is measured. It is possible.

以下では本発明の実施例として、１ｋｍの分散シフト光ファイバ（ＤＳＦ）を参照光ファイバ９として被測定光ファイバ３としての２ｋｍの分散補償ファイバ（ＤＣＦ）と接続した、２本の光ファイバにより構成される合計３ｋｍの長さを持つ被測定対象の分布特性評価を行った例を示す。ここでは、第１光源１からのパルス光の波長およびパルス幅を１．５５μｍ、４×１０^-6秒とし、第２光源２からのパルス光の波長およびパルス幅を１．４５μｍ、２×１０^-6秒とした。また、第１光源１からのパルス光が前記被測定対象の入射端末８−１から前記被測定対象に入射した時から第２光源２からのパルス光が入射する時までの遅延時間を５×１０^-6秒と設定した。 In the following, as an embodiment of the present invention, a 1 km dispersion-shifted optical fiber (DSF) is constituted by two optical fibers connected as a reference optical fiber 9 and a 2 km dispersion compensating fiber (DCF) as an optical fiber 3 to be measured. The example which performed distribution characteristic evaluation of the to-be-measured object with a total length of 3 km is shown. Here, the wavelength and pulse width of the pulsed light from the first light source 1 are 1.55 μm and 4 × 10 ⁻⁶ seconds, and the wavelength and pulse width of the pulsed light from the second light source 2 are 1.45 μm and 2 × 10 2. ^-6 seconds. In addition, a delay time from when the pulsed light from the first light source 1 is incident on the measurement target from the incident terminal 8-1 of the measurement target to when the pulsed light from the second light source 2 is incident is 5 ×. 10 ^-6 seconds was set.

図４にその測定結果を示す。図４に示すように被測定光ファイバ３であるＤＣＦは、参照光ファイバ９であるＤＳＦに対して約２．８倍のラマン利得率を持ち、光ファイバ内でほぼ一定に分布していることが分かる。この結果は、従来技術である特許文献２に示す方法を用いて、同じ被測定対象の測定を行った結果と良い一致を示していた。しかも、本実施例の特性評価装置では被測定光ファイバの片端側に全ての測定装置を設置することができるため、測定作業が簡単で利便性が向上し、特に長距離伝送光ファイバの測定を行う場合においては利便性が飛躍的に向上する。なお、前述のように、ラマン利得率の分布は０≦ｚ≦Ｌ−ｖｔ／２の範囲において得られるため、図４中に示されるように位置Ｌ−ｖｔ／２＝２．５ｋｍ以上（光ファイバ中の光速を２×１０⁸ｍ／ｓと仮定）がデッドゾーンになっていることが分かる。これについては、第１光源１からのパルス光のパルス幅、また第１光源１からのパルス光が入射端末８−１から被測定対象に入射した時から第２光源２からのパルス光が入射する時までの遅延時間を小さく設定することでデッドゾーンを小さくすることが可能である。 FIG. 4 shows the measurement results. As shown in FIG. 4, the DCF that is the optical fiber to be measured 3 has a Raman gain factor of about 2.8 times that of the DSF that is the reference optical fiber 9, and is distributed almost uniformly in the optical fiber. I understand. This result was in good agreement with the result of measuring the same object to be measured using the method shown in Patent Document 2 which is the prior art. Moreover, in the characteristic evaluation apparatus of this embodiment, since all the measuring apparatuses can be installed on one end side of the optical fiber to be measured, the measurement work is simple and the convenience is improved. In the case of doing so, the convenience is greatly improved. Since the Raman gain factor distribution is obtained in the range of 0 ≦ z ≦ L−vt / 2 as described above, the position L−vt / 2 = 2.5 km or more as shown in FIG. It can be seen that the speed of light in the fiber is assumed to be 2 × 10 ⁸ m / s). For this, the pulse width of the pulsed light from the first light source 1 and the pulsed light from the second light source 2 from the time when the pulsed light from the first light source 1 enters the measurement target from the incident terminal 8-1. It is possible to reduce the dead zone by setting the delay time up to the time to be small.

本発明の実施例に係る光ファイバの特性評価装置の構成図である。It is a block diagram of the characteristic evaluation apparatus of the optical fiber which concerns on the Example of this invention. 前記特性評価装置で得られるゲイン分布の測定波形の模式図である。It is a schematic diagram of a measurement waveform of gain distribution obtained by the characteristic evaluation apparatus. 本発明の実施例に係る光ファイバの特性評価装置であって参照光ファイバを用いた場合の特性評価装置の構成図である。It is a block diagram of the characteristic evaluation apparatus at the time of using the optical fiber characteristic evaluation apparatus which concerns on the Example of this invention, and uses a reference optical fiber. 前記特性評価装置における測定結果である。It is a measurement result in the said characteristic evaluation apparatus. 特許文献１および特許文献２に記載されている従来の光ファイバの特性評価装置の構成図である。It is a block diagram of the characteristic evaluation apparatus of the conventional optical fiber described in patent document 1 and patent document 2. FIG. 前記特性評価装置の測定波形の模式図である。It is a schematic diagram of the measurement waveform of the said characteristic evaluation apparatus.

Explanation of symbols

１第１光源
２第２光源
３被測定光ファイバ
４−１第１光カプラ
４−２第２光カプラ
５波長フィルタ
６光検出器
７信号処理装置（表示装置）
８−１入射端末
８−２出射端末
９参照光ファイバ DESCRIPTION OF SYMBOLS 1 1st light source 2 2nd light source 3 Optical fiber to be measured 4-1 1st optical coupler 4-2 2nd optical coupler 5 Wavelength filter 6 Photodetector 7 Signal processing apparatus (display apparatus)
8-1 Incident terminal 8-2 Emission terminal 9 Reference optical fiber

Claims

A pulsed light from a first light source and a pulsed light from a second light source having a wavelength different from that of the pulsed light from the first light source and having a time delay with respect to the pulsed light from the first light source; The light is combined by one optical coupler, is incident on the optical fiber to be measured from one end of the optical fiber to be measured, and the back scattered light of the pulse light from the first light source and the second light source in the optical fiber to be measured Raman light amplification is caused by the interaction with the pulsed light of the second optical fiber, and the backscattered light generated by the pulsed light from the second light source in the second optical coupler and the optical fiber to be measured is removed to remove the first light. The power of the backscattered light of the pulsed light from the first light source amplified by the Raman light is detected by a photodetector through an optical wavelength filter or a spectroscope that transmits only the backscattered light of the pulsed light from the light source. Detect and signal this detection signal By analyzing the waveform by physical, characteristics evaluation method of the optical fiber, characterized by measuring the Raman gain characteristics in the length direction of the optical fiber to be measured.

2. The method according to claim 1, wherein, instead of the optical fiber to be measured, an optical fiber having a Raman gain characteristic known as a reference optical fiber is connected to one or a plurality of optical fibers to be measured. And calculating the ratio of the Raman gain characteristic of the reference optical fiber and the Raman gain characteristic of the optical fiber to be measured to calculate the Raman gain characteristic in the length direction of the optical fiber to be measured. Characterization method.

A first light source for transmitting an optical pulse, a second light source for transmitting an optical pulse having a wavelength different from that of the optical pulse from the first light source and having a time delay with respect to the pulsed light from the first light source, A first optical coupler and a second optical coupler, an optical wavelength filter or spectroscope, a photodetector, and a signal processing device are arranged on one end side of the optical fiber to be measured. The pulsed light and the pulsed light from the second light source are combined by the first optical coupler, and are incident on the measured optical fiber from one end of the measured optical fiber. The interaction between the backscattered light of the pulsed light from one light source and the pulsed light from the second light source causes Raman light amplification, and the second optical coupler and the second optical fiber to be measured are Backscatter caused by pulsed light from the light source The photodetector and the signal processing device for detecting backscattered light of the pulsed light from the first light source amplified by the Raman light are arranged via an optical wavelength filter or a spectroscope for blocking light. An optical fiber characteristic evaluation apparatus.

4. The apparatus according to claim 3, wherein an optical fiber in which one or a plurality of optical fibers to be measured is used as a reference optical fiber instead of the optical fiber to be measured is used as a reference optical fiber. An optical fiber characteristic evaluation apparatus characterized by the above.