JP2010025725A - Wavelength dispersion measuring device and method for measuring wavelength dispersion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength dispersion measuring device capable of precisely measuring a wavelength dispersion without being affected by an α parameter, a modulation index and a loss in a measuring system of a modulator each being a cause that increases a measuring error. <P>SOLUTION: After a phase modulation of an optional modulation index is applied to continuous light output from a light source 10 by a phase modulator 12, the light is incident on a dispersion medium 13. The phase modulator 12 is driven by means of a sine wave signal output from a frequency variable oscillator 11. Output light of the dispersion medium 13 is converted into an electric signal by a light receiver 14 and an alternate current component of the electric signal is observed by means of a spectrum analyzer 15. While phase modulation-intensity modulation conversion occurs in the dispersion medium 13, when a phase modulation frequency is sweeped, a time waveform of the output light of the dispersion medium 13 does not have an intensity modulation component, that is, there is a modulation frequency becoming continuous light. The wavelength dispersion of the dispersion medium is precisely and directly derived from the value of the modulation frequency. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a chromatic dispersion measuring apparatus and a chromatic dispersion measuring method for measuring a chromatic dispersion value which is one of basic characteristics of optical fibers and optical components.

  Chromatic dispersion, which is one of the basic characteristics of optical fibers and optical components that make up an optical communication system, greatly affects the transmission characteristics of optical signals in an optical communication system. In order to suppress the characteristic deterioration of the optical communication system due to the chromatic dispersion, a technique for accurately measuring the chromatic dispersion of the optical fiber or the optical component is first required. On the other hand, due to the growing demand for expansion of communication capacity, an attempt to newly cultivate a wavelength band other than the communication wavelength band that has been used so far, such as the 1.0 μm band, has been advanced (see Non-Patent Document 1 below). ). It is desirable to establish a chromatic dispersion measurement technique for such a new wavelength band.

  As a method for measuring chromatic dispersion, the measurement wavelength is stepped like a phase shift method (see the following non-patent document 2), an optical pulse method (see the following non-patent document 3), or an interference method (see the following non-patent document 4). There is a method of measuring chromatic dispersion by measuring the group delay time characteristic by sweeping and then differentiating it by wavelength. However, in these measurement methods, a light source capable of wavelength sweeping or a light source having a plurality of wavelengths is required. This is a wavelength band in which technical development has not been sufficiently performed, such as the above-described 1.0 μm band. In some cases, it may not be easy to implement.

As a wavelength dispersion measurement method that does not require a wavelength-swept light source or a light source having a plurality of wavelengths, there is a baseband AM response measurement method (see Non-Patent Document 5 and Non-Patent Document 6 below).
The configuration of chromatic dispersion measurement by this method shown in Non-Patent Document 6 below is shown in FIG.

As shown in FIG. 14, in this method, the phase relationship of the modulation sidebands generated when the light output from the light source 10 is intensity-modulated by the intensity modulator 40 changes from the chromatic dispersion in the dispersion medium 13. As a result, the fact that the time waveform of light changes is utilized. When the intensity modulation frequency is changed, an intensity modulation frequency at which the intensity modulation component of the time waveform of light takes a minimum value periodically exists. The frequency is observed using the light receiver 14 and the network analyzer 41. The frequency f _{m at} which the intensity modulation component takes a minimum value is given by Equation (1).

  Here, n is the order of the minimum value (1, 2, 3,...), D is the chromatic dispersion value per unit length of the dispersion medium, L is the dispersion medium length, and λ is the measurement wavelength (light source wavelength). It is. The chirp parameter (α parameter) of the intensity modulator 40 is assumed to be zero. The frequency at which the intensity modulation component takes a minimum value is measured, and the chromatic dispersion value DL can be obtained from Equation (1).

  On the other hand, a method using a phase modulator has been proposed as another wavelength dispersion measurement method that does not require a wavelength-swept light source or a light source having a plurality of wavelengths (see Non-Patent Document 7 and Patent Document below). In this method, light from a light source is incident on a dispersion medium after being subjected to phase modulation at a specific frequency. In the dispersion medium, the phase modulation component is converted into the intensity modulation component, and the chromatic dispersion of the dispersion medium is derived using the fact that the conversion ratio depends on the chromatic dispersion.

FIG. 15 shows the configuration of a chromatic dispersion measuring apparatus shown as an example in Patent Document 1 below.
As shown in FIG. 15, the light from the light source 10 is modulated at a specific frequency by the external phase modulator 12. The phase modulator 12 is driven by an oscillator 50 that operates at a fixed frequency. Further, a part of the output light of the phase modulator 12 is demultiplexed by the demultiplexer 51, and the optical spectrum is observed by the optical spectrum analyzer 52 to obtain the modulation index of phase modulation.

  A part of the phase-modulated light after passing through the dispersion medium 13 is converted into an intensity-modulated component according to the dispersion. Therefore, the AC component and the DC component included in the output of the light receiver 14 are measured by the AC voltmeter 53 and the DC voltmeter 54, respectively, and the intensity ratio is obtained by the comparator 58 to know the wavelength dispersion of the dispersion medium 13. Is possible. A band pass filter 55 is inserted in front of the AC voltmeter 53.

  In Patent Document 1 below, the intensity of phase-modulated light incident on the dispersion medium 13 is changed by setting the optical amplifier 56 or the variable optical attenuator 57 to change the effect of self-phase modulation in the dispersion medium. Proposes a method for identifying the chromatic dispersion sign of a dispersion medium. Further, as described in the following Patent Document 1, in the chromatic dispersion measuring method proposed in the following Patent Document 1, chromatic dispersion measurement is performed with the phase modulation frequency fixed.

Japanese Patent No. 3237684 Kenji Kurokawa, outside five, "High Capacity WDM Transmission in 1.0μm Band over Low Loss PCF Using Supercontinuum Source", 2008 Optical Fiber Communication Conference and Exhibition / National Fiber Optic Engineers Conference (OFC / NFOEC2008), 2008, OMH5 Bruno Costa, 3 others, “Phase Shift Technology for the Measurement of Chromatical Dispersion in Optical Fibers Using LED's”, IEEE JOURNAL QE-18, no. 10, October 1982, p. 1509-1515 L. G. Cohen, 1 other person, “Pulse delay measurements in zero material dispersal wave length region for optical fibers”, APPLIED OPTICs, Vol. 16, no. 12, December 1977, p. 3136-3139 Mitshiro Tateda, two others, “Interferometric method for Chromatic Dispersion Measurement in a Single-Optical Fiber”, IEEE JOURNAL OF QUALANT QE-17, no. 3, March 1981, p. 404-407 B. Christensen, two others, "SIMPLE DISPERSION MEASUREMENT TECHNIQUE WITH HIGH RESOLUTION", ELECTRONICS LETTERS, Vol. 29, no. 1, January 7, 1993, p. 132-134 Nobuyoshi Namihira, "DWDM optical measurement technology", 1st edition, Optronics, Inc., March 10, 2001, p. 42-43 A. R. Craplyvy, three others, “PHASE MODULATION TO AMPLITUDE MODULATION CONVERSION OF CW LASER LIGHT IN OPTICAL FIBRES”, ELECTRONICS LETTERS, Vol. 22, no. 8, April 10, 1986, p. 409-411

  In the chromatic dispersion measurement method (baseband AM response measurement method) using the intensity modulator 40 shown in FIG. 14, an equation (2) is obtained from only two measurement values of the wavelength of the light source 10 and the frequency at which the intensity modulation component takes a minimum value. The chromatic dispersion can be derived from 1). In this case, since these two parameters can be measured with high accuracy, chromatic dispersion can be accurately derived.

  However, equation (1) assumes that the chirp parameter (α parameter) of the intensity modulator 40 is zero. An actual intensity modulator has a finite α parameter, and it is necessary to derive chromatic dispersion in consideration of this influence. However, it is not easy to accurately obtain the α parameter of the intensity modulator 40, and as a result, the actually measured chromatic dispersion value includes an error.

  In the chromatic dispersion measurement method using the phase modulator 12 shown in FIG. 15, it is necessary to accurately measure the phase modulation index in order to derive the chromatic dispersion. This leads to measurement errors. Further, in this method, it is necessary to divide the output signal of the light receiver 14 into two to determine the ratio between the AC component and the DC component. This leads to a measurement error of the chromatic dispersion value.

  The present invention has been made in view of the above-described circumstances, and in a chromatic dispersion measuring device using a light source that outputs continuous light and an optical modulator, the α parameter of the modulator, which is a factor that increases measurement error, and modulation It is an object of the present invention to provide a chromatic dispersion measuring apparatus that does not include the influence of the index and the loss of the measurement system and enables highly accurate chromatic dispersion measurement.

The chromatic dispersion measuring apparatus according to the first invention for solving the above-mentioned problems is
In the chromatic dispersion measuring device for obtaining the chromatic dispersion value of the measured medium,
A light source that generates continuous light;
An electrical signal generator for generating a frequency sweepable sinusoidal signal;
A phase modulator driven by a sine wave signal output from the electrical signal generator and performing phase modulation on the output light of the light source;
A light receiver that converts the output light output from the phase modulator into an electrical signal when the light is incident on the measured medium; and
AC component observation means for observing the intensity of the AC component of the electrical signal output from the light receiver,
A frequency at which the AC component takes a minimum value is obtained by the AC component observation means, and a chromatic dispersion value of the medium to be measured is obtained from this frequency.

The chromatic dispersion measuring apparatus according to the second invention for solving the above-mentioned problems is
In the chromatic dispersion measuring device for obtaining the chromatic dispersion value of the measured medium,
A light source that generates continuous light;
A noise generator that generates a wide-band electrical noise component;
A phase modulator driven by an electrical noise component output from the noise generator and performing phase modulation on the output light of the light source;
A light receiver for converting the output light output from the phase modulator into an electric signal when the output light is incident on the measured medium;
AC component observation means for observing the intensity of the AC component of the electrical signal output from the light receiver,
A frequency at which the AC component takes a minimum value is obtained by the AC component observation means, and a chromatic dispersion value of the medium to be measured is obtained from this frequency.

A chromatic dispersion measuring apparatus according to a third invention for solving the above-mentioned problems is the chromatic dispersion measuring apparatus according to the first invention or the second invention.
A dispersion medium having a known chromatic dispersion value is provided after the phase modulator,
When the output light output from the phase modulator passes through the dispersion medium whose chromatic dispersion value is known and then enters the measured medium, and the output light output from the phase modulator is the measured medium The frequency at which the AC component takes a minimum value is obtained by the AC component observation means for the two cases of direct incidence on the light, and the chromatic dispersion value of the measured medium including the sign is obtained from these frequencies. .

A chromatic dispersion measuring apparatus according to a fourth invention for solving the above-mentioned problems is the chromatic dispersion measuring apparatus according to any one of the first to third inventions,
The light source is a wavelength variable light source in which the wavelength of the continuous light is variable;
The wavelength dependence of the wavelength dispersion value of the measured medium is measured.

A chromatic dispersion measuring method according to a fifth invention for solving the above-mentioned problems is as follows.
In the chromatic dispersion measuring method for obtaining the chromatic dispersion value of the measured medium,
A process of generating continuous light;
Generating a frequency sweepable sinusoidal signal;
Performing phase modulation on the continuous light using the sine wave signal;
Converting the output light when the phase-modulated continuous light is incident on the measured medium into an electrical signal;
Observing the intensity of the alternating current component of the electrical signal;
Obtaining a frequency at which the AC component takes a minimum value, and obtaining a chromatic dispersion value of the medium to be measured from this frequency.

A chromatic dispersion measuring method according to a sixth invention for solving the above-described problems is as follows.
In the chromatic dispersion measuring method for obtaining the chromatic dispersion value of the measured medium,
A step of generating continuous light;
Generating a wide-band electrical noise component;
Performing phase modulation on the continuous light using the electrical noise component;
Converting the output light when the phase-modulated continuous light is incident on the measured medium into an electrical signal;
Observing the intensity of the alternating current component of the electrical signal;
Obtaining a frequency at which the AC component takes a minimum value, and obtaining a chromatic dispersion value of the medium to be measured from this frequency.

A chromatic dispersion measuring method according to a seventh invention for solving the above-mentioned problems is the chromatic dispersion measuring method according to the fifth invention or the sixth invention,
Passing the phase-modulated continuous light through a dispersion medium having a known chromatic dispersion value;
The AC component is divided into two types: a case where the phase-modulated continuous light passes through the dispersion medium and then enters the medium to be measured, and a case where the phase-modulated continuous light directly enters the medium to be measured. And obtaining a frequency having a minimum value and obtaining a chromatic dispersion value of the measured medium including a sign from these frequencies.

A chromatic dispersion measuring method according to an eighth invention for solving the above-described problem is the chromatic dispersion measuring method according to any one of the fifth to sixth inventions,
The continuous light has a variable wavelength,
The wavelength dependence of the wavelength dispersion value of the measured medium is measured.

  According to the present invention, there is provided a chromatic dispersion measuring apparatus capable of deriving chromatic dispersion with high accuracy only from a modulation frequency without including the influence of factors that increase measurement errors such as an α parameter of a modulator, a modulation index, and a measurement system loss. Is possible.

  Hereinafter, embodiments of a wavelength dispersion measuring apparatus and a wavelength dispersion measuring method according to the present invention will be described in detail with reference to the drawings.

A first embodiment of a wavelength dispersion measuring apparatus and a wavelength dispersion measuring method according to the present invention will be described.
FIG. 1 is a block diagram showing an example of a chromatic dispersion measuring apparatus according to the present embodiment. FIG. 2 shows the phase of each modulation sideband immediately after phase modulation and under the condition that the phase modulated light has no intensity modulation component at the output of the dispersion medium. FIG. 3 is an explanatory diagram schematically showing the correlation, FIG. 3 is an explanatory diagram schematically showing the phase correlation of each modulation sideband under the condition that the phase modulated light has no intensity modulation component at the output of the dispersion medium, and FIG. 5 is a graph showing the calculation result of the time waveform of light at the medium output, FIG. 5 is a graph showing the calculation result of the correlation between the AC component intensity and the frequency at the receiver output, and FIG. 6 is the procedure of the wavelength dispersion method according to this embodiment. It is a flowchart to explain.

  As shown in FIG. 1, in the chromatic dispersion measuring apparatus according to the present embodiment, a sine wave phase modulation having an arbitrary modulation index is first performed by a phase modulator 12 on continuous light having a single wavelength output from a light source 10. Then, the light enters the dispersion medium 13. The phase modulator 12 is driven by a sine wave signal output from the frequency variable oscillator 11.

  The output of the dispersion medium 13 is converted into an electric signal by the light receiver 14, and the AC component is observed by the spectrum analyzer 15. The light immediately after the phase modulator 12 is a continuous light chirped into a sine wave, but an intensity modulation component is generated in the dispersion medium 13 by phase modulation-intensity modulation conversion.

  Here, when the sinusoidal signal frequency output from the variable frequency oscillator 11 is swept, there is a modulation frequency at which the time waveform of the dispersion medium output light has no intensity modulation component, that is, becomes continuous light. The chromatic dispersion of the dispersion medium 13 can be directly derived from only the value of the modulation frequency.

The above operations will be described in order. The optical frequency and electric field amplitude of the continuous light output from the light source 10 are assumed to be ω ₀ and E ₀ , respectively. At this time, the electric field of continuous light is expressed by equation (2).

ここで、ｆ_mは位相変調周波数、Δθは位相変調の変調指数、Ｊ_nは第１種ベッセル関数である。また、ｎは整数であり、変調側波帯の番号を示す。
すなわち、式（３）はＪ_n（Δθ）で表わされる振幅を有する変調側波帯の電界が角周波数間隔２πｆ_mで発生することを示している。ｎの絶対値は元の連続光から離れるにしたがって大きくなり、その符号はもとの連続光周波数より高周波側が正となる。 When sinusoidal phase modulation is performed on the continuous light by the phase modulator 12, the electric field of the light at the output of the phase modulator 12 is expressed by Expression (3).

Here, f _m is a phase modulation frequency, Δθ is a modulation index of phase modulation, and J _n is a first type Bessel function. Further, n is an integer indicating the modulation sideband number.
That is, Equation (3) shows that the electric field of modulation sidebands having an amplitude represented by J _n ([Delta] [theta]) is generated at the angular frequency interval 2 [pi] f _m. The absolute value of n increases as the distance from the original continuous light increases, and the sign is positive on the higher frequency side than the original continuous light frequency.

FIG. 2 schematically shows the phase correlation of the modulation sideband generated by the phase modulation when t = 0. E _n in FIG. 2 represents the n-th modulation sidebands. The actual amplitude of the modulation sideband is given by E ₀ · J _n (Δθ), and they have different sizes, but in FIG. 2, this amplitude deviation is ignored for simplification. is doing.

Angle from Re {E} axis E _n in the Im {E} -Re {E} plane 2 is shows the relative phase of E _n. When n is a negative odd number, J _n (Δθ) takes a negative value, and for other values of n, J _n (Δθ) takes a positive value. For this reason, as shown in FIG. 2, the phase of the modulation sideband when n is a negative odd number is shifted by π from the other modulation sideband.

When the light output from the phase modulator 12 is incident on the dispersion medium 13, each modulation sideband receives a phase change due to chromatic dispersion. As a result, phase modulation-intensity modulation conversion occurs, and an intensity modulation component is generated. Assuming that the group velocity dispersion value and length of the dispersion medium 13 are β ₂ and L, respectively, the electric field of light after the dispersion medium 13 is expressed by Expression (4).

Considering the case of t = 0, from Equation (4), the phase change of each modulation sideband caused by passing through the dispersion medium 13 having the group velocity dispersion of β ₂ L is Im {E} − in FIG. This corresponds to rotating the Re {E} plane by 2π ² n ² (f _m ² β ₂ L). That is, the relative amount of phase change received by each modulation sideband due to group velocity dispersion is proportional to n ² . Here, in the case of n = ± 1, that is, when the modulation sideband of E ± 1 is considered, the amount of phase change that the modulation sideband receives after passing through the dispersion medium 13 having the group velocity dispersion of β ₂ L is 2π. ² a ^{_{(f m 2 β 2 L)}} .

すなわち、この場合、分散媒体１３後の光は強度変調成分を持たない連続光に戻ることになる。 When the amount of phase change is an even multiple of π, that is, when the following equation (5) is satisfied with k as a natural number, the phase of each modulation sideband returns to the correlation as shown in FIG.

That is, in this case, the light after the dispersion medium 13 returns to continuous light having no intensity modulation component.

Here, by solving equation (5) f _m, of the formula (6) below is obtained.

この場合、図２と図３に示される位相関係は、元の連続光の光周波数を中心として対称的な関係になっている。したがって、式（７）が成り立つ場合においても、分散媒体１３後の光は強度変調成分を持たない連続光に戻ることになる。 On the other hand, when the phase change amount 2π ² (f _m ² β ₂ L) is an odd multiple of π, that is, when k is a natural number and the following equation (7) holds, the phase of each modulation sideband is shown in FIG. The correlation is as shown.

In this case, the phase relationships shown in FIGS. 2 and 3 are symmetrical with respect to the optical frequency of the original continuous light. Therefore, even when Expression (7) is satisfied, the light after the dispersion medium 13 returns to continuous light having no intensity modulation component.

Here, by solving equation (7) f _m, and becomes the following equation (8).

ただし、ｋは自然数とする。なお、式（９）は位相変調指数Δθを含まないため、式（９）が成立する場合に位相変調光が連続光に戻るという振舞いは、いかなる位相変調指数Δθについても成立する。 Summarizing Equation (6) and Equation (8), when the phase modulation frequency satisfies the following Equation (9), the phase modulated light at the output of the dispersion medium 13 returns to continuous light having no intensity modulation component. .

However, k is a natural number. Since equation (9) does not include the phase modulation index Δθ, the behavior that the phase-modulated light returns to continuous light when equation (9) is satisfied holds for any phase modulation index Δθ.

ただし、ｃは真空中の光速度である。 The relationship between the group velocity dispersion β _{2 at} the wavelength λ and the chromatic dispersion D per unit length is expressed by the following equation (10).

Where c is the speed of light in vacuum.

Here, the following equation (11) is obtained by substituting equation (10) into equation (9).

ただし、ｋは自然数とする。 Further, when the equation (11) is solved for the total chromatic dispersion amount | D | L of the dispersion medium 13, the following equation (12) is obtained.

However, k is a natural number.

In this way, by sweeping the modulation frequency of the sine wave phase modulation and obtaining the modulation frequency in which the time waveform of the light at the output of the dispersion medium 13 does not have an intensity modulation component, that is, continuous light, dispersion is performed from the equation (12). The total chromatic dispersion amount | D | L of the medium 13 can be obtained. As can be seen from the equation (12), in the dispersion measurement in the wavelength dispersion measuring apparatus and the wavelength dispersion measuring method according to the present embodiment, the wavelength λ of the output light of the light source 10 used and the light at the output of the dispersion medium 13 are continuous light. It is possible to accurately derive chromatic dispersion from only two parameters of the modulation frequency f _m . Both of these two parameters can be measured with high accuracy. As a result, the obtained chromatic dispersion value also has high accuracy.

Here, the dispersion medium 13 is a fiber having a chromatic dispersion D of 17 [ps / (nm · km)] and a length L of 80 [km], and the operation check of the above-described chromatic dispersion measurement method was performed by numerical calculation. Results are shown.
The wavelength of the light source was 1550 [nm]. When these conditions are put into the equation (11), the following equation (13) is obtained.

In the case of k = 0.5, 1, 1.5, 2 in equation (13), that is, for the case of f _m = 6.77, 9.58, 11.73, 13.55 [GHz] FIG. 4 shows a result obtained by numerical calculation of a time waveform of light at a fiber output having a length of 80 [km].
Here, calculation was performed for two cases of modulation index 0.07π and 0.19π of phase modulation. In any phase modulation index, when k is a natural number, that is, when f _m = 9.58, 13.55 [GHz], the time waveform of light is continuous light having no intensity modulation component, otherwise It can be seen that the light is intensity modulated.

Further, FIG. 5 shows the result of the numerical calculation of the intensity of the modulation frequency component in the output of the light receiver when the phase modulation frequency is changed from 0.5 [GHz] to 20 [GHz] in the same system as described above. Show.
In FIG. 5, it can be seen that the AC component intensity has a minimum value in each frequency component of the modulation frequency f _m = 9.6, 13.6, 16.6, 19.2 [GHz]. These four frequencies correspond to the cases of k = 1, 2, 3, 4 in equation (12). From these values, the value 1360 [ps / nm] of | D | L of the measured medium 13 is derived.

Here, the procedure of the wavelength dispersion method according to the present embodiment will be described.
As shown in FIG. 6, first, in step S10, the light source 10, the phase modulator 12, the oscillator 11, the dispersion medium (measuring medium) 13, the light receiver 14, and the spectrum analyzer 15 are connected.

Next, in step S11, the frequency of the oscillator 11 is swept, and the AC component intensity at each modulation frequency is measured by the spectrum analyzer 15.
Next, in step S12, obtains the modulation frequency f _m of the AC component intensity takes a minimum value.
Finally, in step S13, the total chromatic dispersion value | D | L of the dispersion medium (measuring medium) 13 is obtained from Expression (12).

A second embodiment of the chromatic dispersion measuring apparatus and chromatic dispersion measuring method according to the present invention will be described.
FIG. 7 is a block diagram showing an example of a chromatic dispersion measuring apparatus according to the present embodiment, FIG. 8 is a graph showing a calculation result of the correlation between the AC component intensity and the frequency at the receiver output, and FIG. 9 is a chromatic dispersion according to the present embodiment. It is a flowchart explaining the procedure of a method.

  As shown in FIG. 7, in this embodiment, unlike the first embodiment shown in FIG. 1, a noise generator 20 is used instead of the frequency variable oscillator 11. The broadband white noise signal output from the noise generator 20 is input to the phase modulator 12, phase-modulated to the light from the light source 10, and then incident on the dispersion medium 13.

  The output of the dispersion medium 13 is converted into an electric signal by the light receiver 14, and the AC component within the white noise signal frequency range is observed by the spectrum analyzer 15. In the dispersion medium 13, an intensity modulation component is generated by phase modulation-intensity modulation conversion for each frequency component. However, since the intensity modulation component becomes 0 for the frequency satisfying Equation (11), the AC component intensity takes a minimum value for this frequency. It is possible to directly derive the chromatic dispersion of the dispersion medium from Equation (12) from the frequency that takes this minimum value.

Operation confirmation of the chromatic dispersion measuring method in this embodiment using a fiber having a chromatic dispersion D of 17 [ps / (nm · km)] and a length L of 80 [km] as in the first embodiment as a measured medium 13. FIG. 8 shows the result obtained by numerical calculation.
In FIG. 8, the AC component intensity has a minimum value in each frequency component of the modulation frequency f _m = 9.6, 13.6, 16.6, 19.2 [GHz]. These four frequencies correspond to the cases of k = 1, 2, 3, 4 in equation (12). From these values, the value 1360 [ps / nm] of | D | L of the measured medium 13 is derived.

Here, the procedure of the wavelength dispersion method according to the present embodiment will be described.
As shown in FIG. 9, first, in step S20, the light source 10, the phase modulator 12, the noise generator 20, the dispersion medium (measuring medium) 13, the light receiver 14, and the spectrum analyzer 15 are connected.

Next, in step S21, the spectrum analyzer 15 measures the modulation frequency dependence of the AC component intensity.
Next, in step S22, obtains the modulation frequency f _m of the AC component intensity takes a minimum value.
Finally, in step S23, the total chromatic dispersion value | D | L of the dispersion medium (measuring medium) 13 is obtained from Expression (12).

A third embodiment of the wavelength dispersion measuring apparatus and wavelength dispersion measuring method according to the present invention will be described.
FIG. 10 is a configuration diagram illustrating an example of a chromatic dispersion measuring apparatus according to the present embodiment, and FIG. 11 is a flowchart illustrating a procedure of a chromatic dispersion method according to the present embodiment.
As shown in FIG. 10, in this embodiment, in addition to the configuration of the first embodiment shown in FIG. 1, a dispersion medium 30 whose chromatic dispersion value is known is inserted after the phase modulator 12. Yes. The dispersion measurement is performed twice for the state in which the dispersion medium 30 is inserted and the state in which the dispersion medium 30 is removed, so that the value of the product DL of the wavelength dispersion value and the length of the medium to be measured 13 includes the positive and negative signs. Can be derived. The measurement principle is shown below.

For a dispersion medium 30 with a known chromatic dispersion value, the chromatic dispersion value and length are set to D ₀ and L ₀ , respectively. On the other hand, with respect to the measured medium 13, the chromatic dispersion value and the length are set to D and L, respectively. In the case where phase modulation light is incident after connecting these two fibers, the minimum value of the frequency at which the intensity of the intensity modulation frequency component takes the minimum value (that is, the frequency when k = 1 in Equation (11)). _{Is set} to f _m1 . In this case, the following formula (14) is established.

Subsequently, when the dispersion medium 30 having a known chromatic dispersion value is removed and the phase-modulated light is incident only on the measured medium 13, the minimum value of the frequency at which the intensity of the intensity modulation frequency component takes the minimum value (that is, In Equation (11), f _m2 is the frequency when k = 1. In this case, the following formula (15) is established.

式（１６）から分かるように、本実施例に係る波長分散測定装置及び波長分散測定方法により、被測定媒体１３の波長分散値と長さの積ＤＬの値を、正負の符号も含めて導出することが可能である。 By subtracting the square of both sides of equation (14) from the square of both sides of equation (15), the following equation (16) is obtained.

As can be seen from Expression (16), the value of the product DL of the chromatic dispersion value and the length of the measured medium 13 including the positive and negative signs is derived by the chromatic dispersion measuring apparatus and the chromatic dispersion measuring method according to the present embodiment. Is possible.

Here, the procedure of the wavelength dispersion method according to the present embodiment will be described.
As shown in FIG. 11, first, in step S30, the light source 10, the phase modulator 12, the oscillator 11, the dispersion medium (dispersion amount known) 30, the dispersion medium (measuring medium) 13, the light receiver 14, and the spectrum analyzer 15 are displayed. Connecting.

Next, in step S31, the frequency of the oscillator 11 is swept, and the AC component intensity at each modulation frequency is measured by the spectrum analyzer 15.
Next, in step S32, a modulation frequency f _{m1 at} which the AC component intensity takes a minimum value is obtained.
Next, in step S33, the dispersion medium (dispersion amount known) 30 is removed, and the output of the phase modulator 12 is connected to the dispersion medium (measuring medium) 13.

Next, in step S34, the frequency of the oscillator 11 is swept, and the AC component intensity at each modulation frequency is measured by the spectrum analyzer 15.
Next, in step S35, it obtains the modulation frequency f _m2 where the AC component intensity takes a minimum value.
Finally, in step S36, the total chromatic dispersion value DL of the dispersion medium (measuring medium) 13 is obtained from equation (16).

A fourth embodiment of the chromatic dispersion measuring apparatus and chromatic dispersion measuring method according to the present invention will be described.
FIG. 12 is a block diagram showing an example of a chromatic dispersion measuring apparatus according to this embodiment, and FIG. 13 is a flowchart for explaining the procedure of the chromatic dispersion method according to this embodiment.
As shown in FIG. 12, this embodiment is characterized in that the light source used is a wavelength variable light source 31. As a result, dispersion measurement can be performed within a wavelength range that can be output from the wavelength tunable light source 31.

Here, the procedure of the wavelength dispersion method according to the present embodiment will be described.
As shown in FIG. 13, first, in step S40, the wavelength tunable light source 31, the phase modulator 12, the noise generator 20, the dispersion medium (measuring medium) 13, the light receiver 14, and the spectrum analyzer 15 are connected.

Next, in step S41, the spectrum analyzer 15 measures the modulation frequency dependence of the AC component intensity.
Next, in step S42, obtains the modulation frequency f _m of the AC component intensity takes a minimum value.
Next, in step S43, the total chromatic dispersion value | D | L of the dispersion medium (measuring medium) 13 is obtained from Expression (12).

Next, in step S44, the set wavelength of the wavelength tunable light source is changed. After changing the set wavelength of the wavelength tunable light source, steps S41 to S43 are executed again. Then, after changing the set wavelength within the wavelength range that can be output from the wavelength tunable light source 31, or within the range that requires chromatic dispersion measurement, the process proceeds to step S45.
Finally, in step S45, the total chromatic dispersion value | D | L of the dispersion medium (measuring medium) 13 is obtained.

[Other Examples]
The preferred embodiments of the chromatic dispersion measuring apparatus and the chromatic dispersion measuring method according to the present invention have been illustrated and described above, but the embodiments of the present invention are not limited to the above-described embodiments, and the scope of the present invention. It is within the scope of the present invention to replace, change, add, increase / decrease the number of components, change the shape design, change the setting parameters, and the like.

  In the chromatic dispersion measuring devices of Non-Patent Document 5 and Non-Patent Document 6, an error in the value of the α parameter of the intensity modulator leads to a measurement error of chromatic dispersion. Further, in the chromatic dispersion measuring apparatus of Patent Document 1, it is necessary to prepare a measurement system with a known phase modulation index or to separately measure the phase modulation index in order to derive chromatic dispersion. Further, in the chromatic dispersion measuring apparatus of Patent Document 1, a measurement error of the intensity ratio of the direct current component to the alternating current component at the light receiver output leads to a measurement error of chromatic dispersion.

  On the other hand, the chromatic dispersion measuring device and the chromatic dispersion measuring method according to the present invention include the effects of the α parameter of the phase modulator 12, the modulation index, and the loss of the measurement system, which are factors that increase the measurement error. It is possible to derive a highly accurate chromatic dispersion value from only the frequency value of the AC signal.

  The present invention can be used in a chromatic dispersion measuring apparatus that measures a chromatic dispersion value, which is one of the basic characteristics of optical fibers and optical components.

It is a block diagram which shows an example of the chromatic dispersion measuring apparatus which concerns on a 1st Example. It is explanatory drawing which showed typically the phase correlation of each modulation sideband in the conditions which phase modulation light does not have an intensity | strength modulation component in a dispersion medium output immediately after phase modulation. It is explanatory drawing which showed typically the phase correlation of each modulation sideband in the conditions from which a phase modulation light does not have an intensity | strength modulation component in a dispersion medium output. It is a graph which shows the calculation result of the time waveform of light in a dispersion medium output. It is a graph which shows the calculation result of the correlation of the alternating current component intensity in an optical receiver output, and a frequency. It is a flowchart explaining the procedure of the wavelength dispersion method which concerns on a 1st Example. It is a block diagram which shows an example of the chromatic dispersion measuring apparatus which concerns on a 2nd Example. It is a graph which shows the calculation result of the correlation of the alternating current component intensity in an optical receiver output, and a frequency. It is a flowchart explaining the procedure of the wavelength dispersion method which concerns on a 2nd Example. It is a block diagram which shows an example of the chromatic dispersion measuring apparatus which concerns on a 3rd Example. It is a flowchart explaining the procedure of the wavelength dispersion method which concerns on a 3rd Example. It is a block diagram which shows an example of the chromatic dispersion measuring apparatus which concerns on a 4th Example. It is a flowchart explaining the procedure of the wavelength dispersion method which concerns on a 4th Example. It is a figure which shows the 1st structural example of the conventional chromatic dispersion measuring apparatus. It is a figure which shows the 2nd structural example of the conventional chromatic dispersion measuring apparatus.

Explanation of symbols

10 Light source 11 Phase modulator 12 Oscillator (frequency variable)
13 Dispersion medium (measuring medium)
14 Photoreceiver 15 Spectrum analyzer 20 Noise generator 30 Dispersion medium 31 with known chromatic dispersion amount Wavelength variable light source 40 Intensity modulator 41 Network analyzer 50 Oscillator (frequency fixed)
51 Splitter 52 Optical Spectrum Analyzer 53 AC Voltmeter 54 DC Voltmeter 55 Bandpass Filter 56 Optical Amplifier 57 Variable Optical Attenuator 58 Comparator

Claims (8)

In the chromatic dispersion measuring device for obtaining the chromatic dispersion value of the measured medium,
A light source that generates continuous light;
An electrical signal generator for generating a frequency sweepable sinusoidal signal;
A phase modulator driven by a sine wave signal output from the electrical signal generator and performing phase modulation on the output light of the light source;
A light receiver that converts the output light output from the phase modulator into an electrical signal when the light is incident on the measured medium; and
AC component observation means for observing the intensity of the AC component of the electrical signal output from the light receiver,
A chromatic dispersion measuring apparatus characterized in that a frequency at which an AC component takes a minimum value is obtained by the AC component observation means, and a chromatic dispersion value of a medium to be measured is obtained from this frequency. In the chromatic dispersion measuring device for obtaining the chromatic dispersion value of the measured medium,
A light source that generates continuous light;
A noise generator that generates a wide-band electrical noise component;
A phase modulator driven by an electrical noise component output from the noise generator and performing phase modulation on the output light of the light source;
A light receiver for converting the output light output from the phase modulator into an electric signal when the output light is incident on the measured medium;
AC component observation means for observing the intensity of the AC component of the electrical signal output from the light receiver,
A chromatic dispersion measuring apparatus characterized in that a frequency at which an AC component takes a minimum value is obtained by the AC component observation means, and a chromatic dispersion value of a medium to be measured is obtained from this frequency. A dispersion medium having a known chromatic dispersion value is provided after the phase modulator,
When the output light output from the phase modulator passes through the dispersion medium whose chromatic dispersion value is known and then enters the measured medium, and the output light output from the phase modulator is the measured medium The frequency at which the AC component takes a minimum value is obtained by the AC component observation means for the two cases of direct incidence on the light, and the chromatic dispersion value of the measured medium including the sign is obtained from these frequencies. The chromatic dispersion measuring apparatus according to claim 1 or 2. The light source is a wavelength variable light source in which the wavelength of the continuous light is variable;
The wavelength dispersion measuring apparatus according to any one of claims 1 to 3, wherein wavelength dependency of a wavelength dispersion value of the medium to be measured is measured. In the chromatic dispersion measuring method for obtaining the chromatic dispersion value of the measured medium,
A process of generating continuous light;
Generating a frequency sweepable sinusoidal signal;
Performing phase modulation on the continuous light using the sine wave signal;
Converting the output light when the phase-modulated continuous light is incident on the measured medium into an electrical signal;
Observing the intensity of the alternating current component of the electrical signal;
And a step of obtaining a frequency at which the AC component takes a minimum value, and obtaining a chromatic dispersion value of the medium to be measured from the frequency. In the chromatic dispersion measuring method for obtaining the chromatic dispersion value of the measured medium,
A step of generating continuous light;
Generating a wide-band electrical noise component;
Performing phase modulation on the continuous light using the electrical noise component;
Converting the output light when the phase-modulated continuous light is incident on the measured medium into an electrical signal;
Observing the intensity of the alternating current component of the electrical signal;
And a step of obtaining a frequency at which the AC component takes a minimum value, and obtaining a chromatic dispersion value of the medium to be measured from the frequency. Passing the phase-modulated continuous light through a dispersion medium having a known chromatic dispersion value;
The AC component is divided into two types: a case where the phase-modulated continuous light passes through the dispersion medium and then enters the medium to be measured, and a case where the phase-modulated continuous light directly enters the medium to be measured. 7. A method for measuring chromatic dispersion according to claim 5 or 6, further comprising: obtaining a frequency that takes a minimum value, and obtaining a chromatic dispersion value of the measured medium including a sign from these frequencies. . The continuous light has a variable wavelength,
The chromatic dispersion measuring method according to claim 5, wherein wavelength dependency of a chromatic dispersion value of the medium to be measured is measured.

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