JP2007085754A - Optical pulse tester and optical fiber longitudinal direction characteristic test method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pulse tester capable of measuring the characteristic in the longitudinal direction of an optical fiber by a precise wavelength, and a test method using the optical pulse tester. <P>SOLUTION: An optical fiber laser for generating light wherein a plurality of longitudinal modes are oscillated and the wavelength width of the whole spectrum is narrow because mode intervals are very narrow is used as a light source of the optical pulse tester. A wavelength dependent characteristic can be measured accurately by the light source, and a coherence noise is reduced. The optical pulse tester is effective for bidirectional OTDR measurement requiring an especially narrow measuring wavelength width and polarized wave OTDR measurement, and reduction of a measurement error is realized. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention mainly relates to an optical pulse tester for evaluating characteristics in the longitudinal direction of an optical fiber and a test method using the optical pulse tester.

    The longitudinal characteristics of the optical fiber include loss, chromatic dispersion, and polarization mode dispersion (PMD), but these characteristics may vary for some reason in the manufacturing process. In the longitudinal characteristic test of the optical fiber, these characteristics are measured.

Usually, an optical pulse tester (OTDR: Optical Time Domain Reflectmeter) is used to measure the loss variation in the longitudinal direction of the optical fiber. This measuring instrument makes an optical pulse incident on the optical fiber to be measured, measures the temporal change in the backscattered light intensity due to Rayleigh scattering in the fiber, and determines the longitudinal direction from the change in the backscattered light intensity with respect to the distance of the optical fiber to be measured. The change in direction loss is obtained.

A light source of a general optical pulse tester uses a Fabry-Perot LD having a center wavelength of 1.55 μm or 1.31 μm and an optical spectrum width extending from several nm to less than 10 nm. The Fabry-Perot LD has an effect of reducing coherent noise when Rayleigh scattering is performed by having a plurality of oscillation longitudinal modes.

With the recent increase in optical fiber communication speed, it has become clear that not only optical fiber loss but also chromatic dispersion and PMD longitudinal fluctuations affect the quality of optical pulses propagating in the optical fiber. Yes. Therefore, as an inspection of the optical fiber in the laying transmission line and the optical fiber manufacturing process, the chromatic dispersion distribution and PMD distribution in the longitudinal direction are also measured, and several measurement methods have been proposed.

  As a method for measuring the chromatic dispersion distribution, a nonlinear OTDR method (Non-Patent Document 1 and Patent Document 1) using a nonlinear effect (four-wave mixing) in an optical fiber, or an OTDR waveform measured from both ends of an optical fiber to be measured is used. There is a bidirectional OTDR method (Non-Patent Document 2) calculated from an average. Bidirectional OTDR can use a commercially available optical pulse tester, and can be performed more easily than the nonlinear OTDR method.

Hereinafter, the principle of the bidirectional OTDR method will be described.
The backscattered light waveform measured by OTDR is given as the sum of a component dependent on loss and a component dependent on structural change in which the core diameter, relative refractive index difference, and the like change in the longitudinal direction. Therefore, the backscattered light waveform is measured by OTDR from both ends of the optical fiber to be measured, and the arithmetic average of both is calculated. Thereby, the component depending on the loss is canceled out, and the change of only the component depending on the structural change can be obtained.

λ：測定波長
L：被測定光ファイバの長さ
S_f(λ、z)：前記一端（距離z=0）から光を入射して測定した後方散乱光強度
S_b(λ、L-z)：他端（距離z=L）から光を入射して測定した後方散乱光強度
α_s(z)：距離zにおける局所的散乱係数
2W(λ、z)：波長λ、距離zにおけるモードフィールド径（ＭＦＤ）
A：距離zに依存しない定数 When the intensity of backscattered light generated at a point z from the end of the optical fiber is S (z) [dB], the structural change component I (λ, z) of the optical fiber is expressed by the following equation.

λ: Measurement wavelength
L: Length of optical fiber to be measured
S _f (λ, z): Backscattered light intensity measured by entering light from the one end (distance z = 0)
S _b (λ, Lz): Backscattered light intensity measured with light incident from the other end (distance z = L)
α _s (z): Local scattering coefficient at distance z
2W (λ, z): Mode field diameter (MFD) at wavelength λ and distance z
A: Constant independent of distance z

を基準値として（1）式を規格化すると、下記の（2）式が得られる。

（2）式より、長手方向のＭＦＤの分布は次式で表される。

Value at any distance z ₀

By normalizing equation (1) using as a reference value, the following equation (2) is obtained.

From the formula (2), the distribution of the MFD in the longitudinal direction is expressed by the following formula.

として（３）式を計算する方法が行われている。 As described above, the MFD distribution in the longitudinal direction can be calculated from Equation (3) if the MFD at an arbitrary distance z ₀ is determined at the wavelength λ. Therefore, normally, a reference optical fiber having a length z ₀ whose MFD value is known in advance by another method is used, and this is connected to the optical fiber to be measured, and measurement is performed by OTDR as described above. From the measurement results, the MFD of the reference optical fiber

(3) is calculated.

c：光速度
n：コア屈折率 Next, the chromatic dispersion of the optical fiber is expressed as the sum of material dispersion and waveguide (structure) dispersion. The material dispersion can be obtained from the Selmeier relational expression using the relative refractive index difference of the optical fiber. Here, the material dispersion is considered to be uniform in the longitudinal direction. The waveguide (structure) dispersion σ _w is expressed by the following equation using a relational expression of wavelength differentiation of MFD.

c: speed of light
n: Core refractive index

The distribution of the waveguide (structure) dispersion σ _{w in} the longitudinal direction can be obtained by measuring the MFD distribution at any two wavelengths using the equation (3) and using the relationship of the equation (4). From the above, the chromatic dispersion distribution can be obtained by adding the value of the material dispersion and the value of the waveguide (structure) dispersion obtained by the equation (4).

Furthermore, as a method for measuring the polarization mode dispersion (PMD) distribution, a method using a conventional device combining an OTDR and a polarizer has been proposed as a simpler method (Non-Patent Document 3). About the method of a nonpatent literature 3, the measurement principle is shown below.

F(ω)：光源のフーリエスペクトル In the measurement principle of Non-Patent Document 3, a state where there is no inter-polarization mode coupling in the optical fiber to be measured is considered. If the loss of the optical fiber is ignored, the electric field E _i (t) (i = x or y) at the distance z of orthogonal eigenlinearly polarized modes is expressed by the following equation, where β _i is the propagation constant of each mode. The

F (ω): Fourier spectrum of the light source

｜γ(Dω₀z)｜：コヒーレンス度
D：単位長さ当たりの群遅延時間差（Differential mode delay;ＤＧＤ）
ω：光源の中心周波数
コヒーレンス度γは、光源スペクトルが理想的な単一周波数成分のときに1となる。 The light intensity I (z) passing through the polarizer becomes interference between both orthogonal polarization modes, and can be expressed by the following equation when the angle formed by the birefringence axis of the polarizer is 45 °.

| Γ (Dω ₀ z) |: Degree of coherence
D: Differential group delay (DGD) per unit length
ω: The center frequency coherence degree γ of the light source is 1 when the light source spectrum is an ideal single frequency component.

（7）式より後方散乱光波形の強度変化の周期を読み取ることにより、長手方向各地点でのＤＧＤを求めることができる。 Here, considering the backscattered light in the optical fiber having the length L, the backscattered light intensity R (z) at the distance z is as follows.

By reading the intensity change period of the backscattered light waveform from the equation (7), DGD at each point in the longitudinal direction can be obtained.

Japanese Patent Laid-Open No. 8-21783 L.F.Mollenauer, P.V.Mamyshev and M.J.Neubelt “Method for facile and accurate measurement of optical fiber dispersion maps” Optics Letters 1996, 21, pp.1724-1726 K. Nakajima, M. Ohashi and M. Tateda, “Chromatic dispersion distribution measurement along a single-mode optical fiber” J. Lightwave Technology, vol. 15, pp. 1095-1101, July 1997. Tanikawa Shoji, Goto Ryuichiro, Matsuo Shoichiro, Himeno Kunihiko, “Evaluation Method of Fiber Longitudinal PMD Using Polarized OTDR”, 2003 IEICE Autumn Meeting B-825, IEICE OFT 2003-17 P. 41

However, the conventional optical fiber measurement method using OTDR has the following problems.
The measurement principle described above is based on the measurement of single-wavelength backscattered light. However, the conventional OTDR normally uses a Fabry-Perot LD having a wide optical spectrum as a light source in order to suppress coherent noise.

Therefore, when the chromatic dispersion distribution is measured by the bidirectional OTDR method using a Fabry-Perot LD as a light source, although the coherent noise is suppressed, the measurement wavelength cannot be set clearly, and the obtained results are averaged over the used wavelength band. Can only be obtained. Similarly, even when measuring the polarization mode dispersion distribution, a clear measurement wavelength cannot be set, and the obtained measurement value also includes the wavelength dependence of the polarization mode dispersion, and is averaged with the light source wavelength. Value.

In order to set the measurement wavelength more accurately, for example, when a DFB-LD oscillating in a single longitudinal mode is used as a light source, coherent noise is increased due to good coherence, and an accurate value can be obtained. It will disappear. In particular, since PMD changes randomly depending on the wavelength, it is strongly desired to measure at a single wavelength while suppressing coherent noise. Therefore, a method of controlling the temperature of a single mode oscillation laser such as a DFB-LD to slightly change the oscillation wavelength with time is used. However, such temperature control has a problem that the configuration becomes complicated.

Therefore, the present invention has been made to solve these problems, and an optical pulse tester capable of measuring characteristics in the longitudinal direction of an optical fiber with a precise wavelength, and a test method using the optical pulse tester. The purpose is to provide.

According to a first aspect of the vehicle power supply system of the present invention, there is provided an optical fiber laser as a light source, an optical pulse controller for pulsing output light from the optical fiber laser, and light output from the optical pulse controller. A measurement unit that measures backscattered light generated by inputting a pulse into the optical fiber to be measured, and the optical fiber laser includes a rare earth metal element-doped optical fiber as a laser amplification medium, and a half value of an envelope of an oscillation spectrum. A resonator that has a wavelength selection element for making the total width 1 nm or less and a laser oscillation longitudinal mode interval is 1 GHz or less is formed, and a plurality of laser longitudinal modes are oscillated by the resonator to generate the output light. This is an optical pulse tester.

A second aspect is an optical pulse tester characterized in that the wavelength selection element is a wavelength tunable filter capable of changing a transmission center wavelength.

A third aspect is an optical pulse tester characterized in that the optical fiber laser has an absolute value of chromatic dispersion in one round of the resonator of 1 ps / nm or more at the center wavelength of the oscillation light spectrum.

A fourth aspect is an optical pulse tester characterized in that the optical fiber laser can change the oscillation center wavelength of the output light by changing the wavelength selection characteristic of the wavelength selection element.

A fifth aspect is an optical pulse tester characterized in that the wavelength selection element can change the wavelength measurement characteristic during measurement of backscattered light from the optical fiber to be measured.

A sixth aspect is the addition of a polarization controller that inputs the optical pulse output from the optical pulse controller and controls the polarization state thereof, and the optical pulse whose polarization state is adjusted by the polarization controller Is input to the optical fiber to be measured.

In a seventh aspect, a light pulse whose polarization state is adjusted by the polarization controller is input and a polarizer that passes only a predetermined linearly polarized light is added, and the light pulse that has been linearly changed by the polarizer is added. An optical pulse tester characterized by being inputted to the optical fiber to be measured.

In an eighth aspect, a predetermined optical pulse is generated based on laser light having a plurality of longitudinal modes whose full width at half maximum of the envelope of the oscillation spectrum is 1 nm or less and whose interval is 1 GHz or less. An optical fiber longitudinal characteristic test method comprising: inputting into a measurement optical fiber to generate backscattered light; and inputting the backscattered light to measure a longitudinal characteristic of the optical fiber to be measured.

A ninth aspect is the optical fiber longitudinal characteristic test method, wherein the center wavelength of the laser light is changed during measurement of the backscattered light from the optical fiber to be measured.

In a tenth aspect, the light pulse is converted into predetermined linearly polarized light, the linearly polarized light is input to the optical fiber to be measured to generate backscattered light, and the backscattered light is input to change the polarization state. It is an optical fiber longitudinal characteristic test method characterized by measuring.

In an eleventh aspect, one end of each of a predetermined reference optical fiber and the measured optical fiber is connected, and the optical pulse is transmitted from two directions from the open end of the reference optical fiber and the open end of the measured optical fiber. The intensity distribution waveform of the backscattered light generated in the reference optical fiber and the optical fiber to be measured is measured from the two directions, the average value of both is calculated, and the wavelength of the optical pulse is set to a predetermined wavelength. The average value is calculated a plurality of times by changing the predetermined number of times two or more times within the range, and based on the MFD value of the reference optical fiber, the average value of the optical fiber to be measured is determined from the average value of the predetermined number of times. An MFD distribution and a chromatic dispersion distribution are calculated.

As described above, by using the optical pulse tester according to the present invention, it is possible to measure loss at a precise wavelength, polarization mode dispersion, MFD, and longitudinal distribution characteristics of chromatic dispersion with low noise. It is possible to contribute to the transmission design of the fiber transmission line and the quality stabilization at the time of manufacturing the optical fiber.

A configuration of an optical pulse tester and a test method using the optical pulse tester in a preferred embodiment of the present invention will be described in detail with reference to the drawings. In addition, about each structural part which has the same function, the same code | symbol is attached | subjected and shown for simplification of illustration and description.

The optical pulse tester of the present invention used for measuring the longitudinal characteristics of an optical fiber is characterized in that a wavelength-tunable multi-longitudinal mode oscillation optical fiber laser is used as a light source. When a ring resonator is used as the resonator of the optical fiber laser, the resonance frequency f (Hz) of the laser oscillation mode satisfies the following condition when the resonator length is L (m).

q：整数
L：共振器長（m）
c：光速度
n：コア屈折率
このとき、隣り合う発振モード間の周波数差Δfは

となる。

q: integer
L: Resonator length (m)
c: speed of light
n: Core refractive index At this time, the frequency difference Δf between adjacent oscillation modes is

It becomes.

When, for example, a typical erbium-doped optical fiber (EDF) is used as a rare earth metal element laser amplification medium constituting the optical fiber laser, the resonator length L is a distance on the order of meters. For example, in the case of a ring-type resonator with L = 1 (m), if n = 1.47 and λ = 1.55 μm, Δf is 2.04 × 10 ⁸ (Hz) from equation (9).

The gain wavelength band of EDF has a wide band from around 1530 nm to around 1560 nm. Therefore, a large number of modes having a frequency difference Δf can be laser-oscillated within the gain wavelength band of the EDF that is a laser amplification medium. A state in which these many modes are oscillated simultaneously is a multi-longitudinal mode laser oscillation state.

Furthermore, it becomes possible to control the oscillation wavelength by inserting a wavelength selection element into the resonator of the optical fiber laser. For example, when a bandpass filter with a center wavelength of 1550 nm and a transmission full width at half maximum of 1 nm is inserted in the resonator as a wavelength selection element, the gain wavelength band for laser oscillation is limited to the transmission wavelength band of the bandpass filter, Laser oscillation is performed only at wavelengths near 1549.5 nm to 1550.5 nm.

In the above case, when the resonator length L = 1 (m), when the envelope of the oscillation spectrum is measured, a spectrum having a center wavelength of 1550 nm and a full width at half maximum of about 1 nm is obtained, and Δf = 2.04 × 10 ⁸ (Hz ) There are many longitudinal modes at intervals. That is, about 612 longitudinal modes exist within a spectral width of 1 nm.

In contrast, a conventional OTDR uses a Fabry-Perot type semiconductor laser as a light source. The Fabry-Perot semiconductor laser has a resonator length of 300 μm and a refractive index of about 3.5 (InGaAsP), and the frequency interval Δf of the oscillation mode is about 100 GHz. In addition, because of the gain spread of the laser medium used, the envelope of the oscillation spectrum usually has a spread of about several nm.

As described above, the output light from the optical fiber laser, which is the light source of the optical pulse tester of the present invention, is limited in the envelope of the oscillation light spectrum within a certain wavelength as compared with the conventional OTDR light source. Regardless, it has a large number of oscillation modes.

    Thus, when light having a large number of oscillation modes is used as the light source of the optical pulse tester, there is an effect that coherent noise is reduced. Moreover, since the spread of the entire oscillation spectrum is suppressed below the transmission wavelength band of the wavelength selection element, it is possible to improve the measurement wavelength setting accuracy and measure the backscattered light waveform with reduced noise. It becomes possible to do.

Further, by using a wavelength tunable filter capable of changing the center wavelength as the wavelength selection element, it is possible to measure characteristics at each wavelength within the gain band of the laser medium.

The multi-longitudinal mode oscillation optical fiber laser used in the optical pulse tester of the present invention is set so that the absolute value of the chromatic dispersion value at the oscillation center wavelength is 1 ps / nm or more per round of the resonator. If so, stable multimode oscillation can be realized.

Furthermore, it is possible to further reduce coherent noise by changing the setting of the transmission center wavelength of the wavelength selection element (wavelength tunable filter) inserted in the resonator when measuring the backscattered light.

The configuration of the optical pulse tester according to the first embodiment of the present invention is shown in FIG. The optical pulse tester 1 of this embodiment includes an optical fiber laser 2, an isolator 3, an optical pulse control unit 4, a circulator 5, an electric pulse generator 6, an electric signal amplifier 7, a photodetector 8, an electric signal amplifier 9, and an oscilloscope 10. It is comprised.

In the optical pulse tester 1 of this embodiment, an optical fiber laser 2 is used as a light source, which is the above-described multi-longitudinal mode optical fiber laser. As a resonator configuration of the optical fiber laser 2, for example, a ring type shown in FIG. 2 and a reflection type shown in FIG. 3 are conceivable.

An optical fiber laser 21 having a ring resonator configuration shown in FIG. 2 includes a pumping light source 22, a WDM coupler 23, an erbium-doped optical fiber (EDF) 24, an isolator 25, a wavelength selection element 26, and a coupler 27. The excitation light source 22 outputs excitation light having a wavelength of 980 nm or 1480 nm. In FIG. 2, a ring composed of a WDM coupler 23, an EDF 24, an isolator 25, a wavelength selection element 26, and a coupler 27 forms a resonator.

3 includes a pumping light source 22, an EDF 24, a wavelength selection element 26, and a coupler 27, and reflection mirrors (half mirrors) or fibers at both ends of the EDF 24. A grating 32 is provided. In the optical fiber laser 31, a resonator is formed between two reflection mirrors (half mirrors) or fiber gratings 32.

The optical fiber laser 2 shown in FIG. 2 or the optical fiber laser 31 shown in FIG. 3 is applied to the optical fiber laser 2 that is the light source of the optical pulse tester 1. As a laser amplification medium of the optical fiber laser 2, for example, an erbium-doped optical fiber (EDF) can be used. As the wavelength selection element 26, an optical filter having a transmission wavelength bandwidth (FWMH) of 1 nm and a wavelength variable region of 1520 nm to 1570 nm can be used.

For example, when the optical fiber laser 21 is used as the optical fiber laser 2, the total length (resonator length) of the fiber in the resonator portion is 283 m. The oscillation longitudinal mode interval of the optical fiber laser 21 is inversely proportional to the resonator length as shown in the equation (9). When the resonator length L = 283 (m) in the equation (9), the oscillation longitudinal mode interval Δf is about 720 (kHz).

FIG. 4 shows the result of measuring the laser oscillation spectrum when the center wavelength of the wavelength selection element 26 is set to 1555 nm with the optical spectrum analyzer in the optical fiber laser 21 having the above configuration. It can be seen from FIG. 4 that the full width at half maximum of the laser oscillation light spectrum is 0.18 nm. This laser oscillation light spectrum is an envelope of a number of longitudinal mode laser oscillation states.

The laser oscillation light spectrum cannot measure each longitudinal mode with the optical spectrum analyzer having a wavelength resolution of about 0.01 nm. Therefore, this was measured using a delayed self-heterodyne interferometer. The measured spectrum waveform is shown in FIG. From the figure, it can be seen that a plurality of modes exist at a frequency interval of 720 kHz.

In order to stabilize the oscillation of the multi-longitudinal mode laser, the absolute value of the chromatic dispersion value in the resonator can be set to 1 ps / nm or more per round of the resonator. That is, in the resonator, optical fibers having different wavelength dispersion characteristics, such as SMF (1.3 μm band single mode optical fiber, dispersion value D = 17 ps / nm / km @ 1550 nm) or DCF (dispersion compensation fiber, dispersion value D = − By inserting 120 ps / nm / km @ 1550 nm) with an appropriate length, it is possible to control the chromatic dispersion value per round of the resonator.

The configuration of the optical pulse tester according to the second embodiment of the present invention is shown in FIG. In this embodiment, the optical pulse tester is configured using OTDR which is a conventional optical pulse tester. The optical pulse tester 41 of this embodiment includes an OTDR 43 in addition to the optical fiber laser 2, isolator 3, optical pulse controller 4, circulator 5, photodetector 8, electric signal amplifier 7, and another circulator 42. . Also in the optical pulse tester 41 of this embodiment, the optical fiber laser 2 used in the first embodiment can be applied.

The resonator configuration of the optical fiber laser is not limited to that shown in FIGS. 2 and 3. For example, a configuration using a plurality of pumping light sources and a configuration in which constituent optical components are arranged different from those shown in FIGS. Is possible.

  Next, a measurement method for measuring the chromatic dispersion characteristic in the longitudinal direction of the optical fiber using the optical pulse tester 1 according to the first embodiment of the present invention shown in FIG. 1 will be described below.

An optical fiber 11 to be measured, which is an optical fiber to be measured, is connected to a port 5b of a circulator 5 from which the optical pulse tester 1 of the present embodiment emits an optical pulse. In order to emit an optical pulse from the optical pulse tester 1 to the optical fiber 11 to be measured, first, in the optical fiber laser 2, the excitation light is output from the excitation light source 22 and is oscillated in a multi-longitudinal mode. Is sent out.

The output light transmitted from the coupler 27 to the outside of the optical fiber laser 2 reaches the optical pulse controller 4 via the isolator 3. On the other hand, the electric pulse generated by the electric pulse generator 6 is amplified by the electric signal amplifier 7 and then input to the optical pulse controller 4.

The optical pulse controller 4 shapes the optical pulse based on the pulse shape and repetition period of the electric pulse. The pulse width and repetition period of the electric pulse output from the electric pulse generator 6 are appropriately adjusted and set according to the length of the optical fiber 11 to be measured and the required distance resolution. As a specific configuration of the optical pulse controller 6, for example, an AO modulator (acousto-optic modulator) or LN modulator and an electric amplifier that amplifies an electric pulse are combined.

The optical pulse output from the optical pulse controller 4 is input to the port 5a of the circulator 5, and is emitted to the outside from the port 15b. That is, the optical pulse is incident on the optical fiber 11 to be measured from the port 5b.

As the light pulse propagates through the measured optical fiber 11, a backscattering phenomenon due to Rayleigh scattering occurs at each point in the longitudinal direction of the measured optical fiber 11. The back-scattered light (back-scattered light) travels in the opposite direction to the incident light pulse and returns to the port 5b of the circulator 5 again.

The backscattered light that travels backward through the optical fiber 11 to be measured and reaches the circulator 5 is input to the port 5b, then output from the port 5c, transmitted to the photodetector 8, and converted into an electrical signal. The converted electric signal is amplified by the electric signal amplifier 9 and input to the oscilloscope 10.

The oscilloscope 10 inputs a predetermined electrical signal from the electrical pulse generator 6 as a trigger signal, and displays the time-varying waveform of the backscattered light according to the trigger signal. FIG. 7 shows the result of measuring the time-varying waveform of the backscattered light on the optical fiber 11 to be measured having a length of 12 km using the optical pulse tester 1 of the present embodiment. In the figure, as a comparison, the result of measurement using a conventional DFB-LD as a light source is also displayed as 52.

Comparing both waveforms of the measurement results 51 and 52, the measurement result 51 using the optical fiber laser 2 of the present invention as the light source has reduced coherent noise and light compared to the measurement result 52 using the conventional light source. It can be seen that the fiber loss change is clearly measured. FIG. 7 shows the result of measurement using the first embodiment of the present invention. Even when the optical pulse tester 41 of the second embodiment of the present invention shown in FIG. Similar measurement results are obtained.

Further, in the method for measuring the time-varying waveform of the backscattered light for the optical fiber 11 to be measured as described above, the transmission center wavelength of the wavelength selection element 26 installed in the optical fiber laser 2 that is a light source is varied. By doing so, it is possible to reduce coherent noise. This can be realized, for example, by giving a periodic change of ± 1 nm to the initially set transmission center wavelength.

Next, a measurement method regarding the polarization mode dispersion distribution in the longitudinal direction of the optical fiber 11 to be measured will be described below.
In FIG. 8, the structure of the optical pulse tester 61 which is 3rd embodiment of this invention is shown. In the optical pulse tester 61 of this embodiment, a polarization controller 62 and a polarizer 63 are added to the optical pulse tester 1 of the first embodiment shown in FIG.

FIG. 9 shows another embodiment for measuring the polarization mode dispersion distribution in the longitudinal direction of the optical fiber 11 to be measured. FIG. 9 shows a configuration of an optical pulse tester 64 according to the fourth embodiment of the present invention. The optical pulse tester 64 of the present embodiment is configured using a conventional OTDR, similarly to the optical pulse tester 41 of the second embodiment shown in FIG. Also in this embodiment, a polarization controller 62 and a polarizer 63 are added to the optical pulse tester 41 of the second embodiment shown in FIG.

Hereinafter, as one embodiment, a measurement method of the present invention for measuring the longitudinal polarization mode dispersion distribution of the optical fiber 11 to be measured using the optical pulse tester 61 shown in FIG. 8 will be described.

In FIG. 8, the light output from the optical fiber laser 2 passes through the isolator 3 and is sent to the optical pulse controller 4 where it is pulsed. The optical pulse shaped by the optical pulse controller 4 is controlled in the polarization state by the polarization controller 62 and then passes through the polarizer 63 through the circulator 5. The polarization controller 62 is adjusted so that the output intensity of the polarizer 63 is maximized.

The light pulse transmitted through the polarizer 63 is incident on the optical fiber 11 to be measured, and generates backscattered light inside the optical fiber 11 to be measured. The generated backscattered light enters the polarizer 63 and the circulator 5 from the opposite direction, enters the photodetector 8 from the port 5C of the circulator 5, and is converted into an electric signal.

The electrical signal converted by the photodetector 8 is amplified by the electrical signal amplifier 9 and input to the oscilloscope 10. The oscilloscope 10 displays the time-varying waveform of the backscattered light using a predetermined electrical signal from the electrical pulse generator 6 as a trigger. Further, using this time-varying waveform, the polarization mode dispersion (DGD: Differential Group Delay) distribution characteristic in the longitudinal direction of the optical fiber 11 to be measured is obtained based on the measurement principle described above.

FIG. 10 shows an example in which the longitudinal polarization mode dispersion distribution of the optical fiber 11 to be measured is measured using the optical pulse tester according to the third embodiment of the present invention shown in FIG. A similar measurement can be performed using the optical pulse tester 64 of the fourth embodiment of the present invention shown in FIG.

Next, an optical pulse tester according to another embodiment of the present invention for measuring the longitudinal MFD and chromatic dispersion value distribution of the optical fiber 11 to be measured will be described with reference to FIG. FIG. 11 is a diagram showing a configuration of an optical pulse tester 71 according to the fifth embodiment of the present invention. In this embodiment, a polarization controller 62 and a reference optical fiber 72 are added to the optical pulse tester 1 of the first embodiment of the present invention shown in FIG.

FIG. 12 shows an optical pulse tester 73 according to the sixth embodiment of the present invention for measuring the longitudinal MFD and chromatic dispersion value distribution of the optical fiber 11 to be measured. The present embodiment is configured by using an OTDR that is a conventional optical pulse tester, similarly to the second embodiment and the fourth embodiment. In this embodiment, a polarization controller 62 and a reference optical fiber 72 are added to the optical pulse tester 41 of the second embodiment of the present invention shown in FIG.

The measurement method of the present invention for measuring the longitudinal MFD and chromatic dispersion value distribution of the measured optical fiber 11 using the optical pulse tester 71 shown in FIG. 11 will be described below.
The wavelength of the light output from the optical fiber laser 2 is set to a predetermined value by the wavelength selection element 26 provided in the optical fiber laser 2. The optical output from the optical fiber laser 2 passes through the isolator 3 and is sent to the optical pulse controller 4 where it is shaped into an optical pulse.

The optical pulse output from the optical pulse controller 4 passes through the polarization controller 62 and the circulator 5 and enters the reference optical fiber 72. The reference optical fiber 72 is connected to the measured optical fiber 11, and the light pulse incident on the reference optical fiber 72 passes through the reference optical fiber 72 and the measured optical fiber 11.

In the above, the polarization controller 62 constantly changes the polarization state of the optical pulse so that the polarization state of the optical pulse incident on the reference optical fiber 72 and the optical fiber 11 to be measured is random in time. ing.

The light backscattered in the reference optical fiber 72 and the measured optical fiber 11 passes through the circulator 5 and enters the photodetector 8 to be converted into an electric signal. The electric signal is amplified by the electric signal amplifier 9 and input to the oscilloscope 10. The oscilloscope 10 displays the time-varying waveform of the backscattered light using a predetermined electrical signal from the pulse generator 6 as a trigger.

When measuring the backscattered light, the coherent noise can be further reduced by changing the transmission center wavelength of the wavelength selection element 26. For example, it is preferable to give a periodic wavelength change of ± 0.5 nm from the center wavelength.

Following the measurement of the backscattered light from the reference optical fiber 72 and the measured optical fiber 11, the same measurement is performed from the opposite direction of the connected reference optical fiber 72 and the measured optical fiber 11. Then, an arithmetic average of two waveforms obtained by measurement from both directions is calculated.

The measurement from both directions of the connected reference optical fiber 72 and the measured optical fiber 11 as described above is performed a plurality of times by changing the wavelength of the output light of the optical fiber laser 2. The adjustment of the wavelength of the output light of the optical fiber laser 2 is performed by adjusting the wavelength selection element 26 provided in the optical fiber laser 2. Based on the arithmetic mean of longitudinal fluctuations at the respective wavelengths calculated from the measurement results from both directions, the longitudinal distribution of MFD and chromatic dispersion can be obtained based on the aforementioned measurement principle.

FIG. 13 and FIG. 14 show examples of longitudinal distributions of MFD and chromatic dispersion values obtained by using the above measurement method. The above measurement method can also be applied to the optical pulse tester 73 according to the sixth embodiment of the present invention shown in FIG. 12, and the same measurement results as those shown in FIGS.

As described above, in the polarization mode distribution measurement using the polarizer 63 and the chromatic dispersion distribution measurement by the bidirectional OTDR method, the measurement can be performed with a more accurate wavelength under the measurement conditions close to the theory. The wavelength dependence of the distribution and polarization mode distribution can be measured with high accuracy.

The description in this embodiment shows an example of an optical pulse tester according to the present invention, and the present invention is not limited to this. The detailed configuration and detailed operation of the optical pulse tester in the present embodiment can be appropriately changed without departing from the spirit of the present invention.

FIG. 1 is a configuration diagram of an optical pulse tester 1 according to a first embodiment of the present invention using a fiber laser as a light source. FIG. 2 is a configuration diagram of a ring resonator of the optical fiber laser 2 used in the optical pulse tester of the present invention. FIG. 3 is a configuration diagram of a reflection type resonator of the optical fiber laser 2 used in the optical pulse tester of the present invention. FIG. 4 is an example of an output light spectrum of the optical fiber laser 2 measured by an optical spectrum analyzer. FIG. 5 is an example of the output spectrum of the optical fiber laser 2 measured with a delayed self-heterodyne interferometer. FIG. 6 is a configuration diagram of an optical pulse tester 41 according to a second embodiment of the present invention using a conventional OTDR. FIG. 7 is an example of a backscattered light fluctuation waveform when an optical fiber having a length of 12 km is measured. FIG. 8 is a configuration diagram of an optical pulse tester 61 according to the third embodiment of the present invention for PMD distribution measurement using a wavelength tunable optical fiber laser. FIG. 9 is a configuration diagram of an optical pulse tester 64 according to a fourth embodiment of the present invention for PMD distribution measurement using a tunable optical fiber laser and a conventional OTDR. FIG. 10 is a graph showing an example of the measurement result of the DGD distribution. FIG. 11 is a configuration diagram of an optical pulse tester 71 according to a fifth embodiment of the present invention for bidirectional OTDR measurement using a wavelength tunable optical fiber laser. FIG. 12 is a configuration diagram of an optical pulse tester 73 according to a sixth embodiment of the present invention for bidirectional OTDR measurement using a wavelength tunable optical fiber laser and a conventional OTDR. FIG. 13 shows an example of the longitudinal MFD diameter distribution characteristic of the optical fiber measured using the optical pulse tester 71 according to the fifth embodiment of the present invention. FIG. 14 is an example of the chromatic dispersion distribution characteristic in the longitudinal direction of the optical fiber measured using the optical pulse tester 71 of the fifth embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 41, 61, 64, 71, 73 ... Optical pulse tester 2, 21, 31 ... Optical fiber laser 3, 25 ... Isolator 4 ... Optical pulse control part 5, 42 ... Circulator 6 ... Electric pulse generator 7, 9 ... Electric signal amplifier 8 ... Photo detector 10 ... Oscilloscope 11 ... Optical fiber to be measured 22 ... Excitation light source 23 ... WDM coupler 24. .... Erbium-doped optical fiber 26 ... Wavelength selection element 27 ... Coupler 32 ... Half mirror or fiber grating 43 ... OTDR
51, 52 ... Time-varying waveform of backscattered light 62 ... Polarization controller 63 ... Polarizer 72 ... Reference optical fiber

Claims (11)

An optical fiber laser as a light source;
An optical pulse controller for pulsing output light from the optical fiber laser;
A measuring unit for measuring the backscattered light generated by inputting the optical pulse output from the optical pulse control unit into the optical fiber to be measured; and
The optical fiber laser includes a rare earth metal element-doped optical fiber as a laser amplification medium and a wavelength selection element for setting the full width at half maximum of the envelope of the oscillation spectrum to 1 nm or less, and the lasing longitudinal mode interval is 1 GHz or less. An optical pulse tester characterized in that a resonator is formed and a plurality of laser longitudinal modes are oscillated by the resonator to generate the output light. The wavelength selection element is:
2. The optical pulse tester according to claim 1, wherein the optical pulse tester is a tunable filter capable of changing a transmission center wavelength. The optical fiber laser is
3. The optical pulse tester according to claim 1, wherein an absolute value of chromatic dispersion in one round of the resonator is 1 ps / nm or more at a center wavelength of an oscillation light spectrum. The optical fiber laser is
4. The optical pulse tester according to claim 1, wherein an oscillation center wavelength of the output light can be changed by changing a wavelength selection characteristic of the wavelength selection element. 5. The wavelength selection element is:
5. The optical pulse tester according to claim 4, wherein the wavelength measurement characteristic can be changed during measurement of backscattered light from the optical fiber to be measured. Add a polarization controller to control the polarization state by inputting the optical pulse output from the optical pulse control unit,
6. The optical pulse tester according to claim 1, wherein an optical pulse whose polarization state is adjusted by the polarization controller is input to the optical fiber to be measured. Add a polarizer that passes only a predetermined linearly polarized light by inputting an optical pulse whose polarization state is adjusted by the polarization controller,
The optical pulse tester according to claim 6, wherein the optical pulse linearly changed by the polarizer is input to the optical fiber to be measured. A predetermined light pulse is generated based on laser light having a plurality of longitudinal modes whose full width at half maximum of the envelope of the oscillation spectrum is 1 nm or less and whose interval is 1 GHz or less,
The optical pulse is input to the optical fiber to be measured to generate backscattered light, and the backscattered light is input to measure the longitudinal characteristic of the optical fiber to be measured. Test method. 9. The optical fiber longitudinal characteristic test method according to claim 8, wherein a center wavelength of the laser beam is changed during measurement of backscattered light from the optical fiber to be measured. The light pulse is converted into predetermined linearly polarized light, the linearly polarized light is input to the optical fiber to be measured to generate backscattered light,
The optical fiber longitudinal characteristic test method according to claim 8, wherein the polarization state is measured by inputting the backscattered light. Connecting one end of each of the predetermined reference optical fiber and the measured optical fiber;
The optical pulse is input from two directions from the open end of the reference optical fiber and from the open end of the measured optical fiber,
An intensity distribution waveform of the backscattered light generated in the reference optical fiber and the optical fiber to be measured is measured from the two directions, and an average value of both is calculated.
The average value is calculated a plurality of times by changing the wavelength of the light pulse within a predetermined wavelength range and changing it a predetermined number of times twice or more,
9. The optical fiber longitudinal direction according to claim 8, wherein an MFD distribution and a chromatic dispersion distribution in the longitudinal direction of the optical fiber to be measured are calculated from the average value of the predetermined number of times based on the MFD value of the reference optical fiber. Characteristic test method.

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