JP3250586B2 - Chromatic dispersion measurement device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring the chromatic dispersion of a dispersion medium such as an optical fiber, and more particularly, to a method for transmitting a phase (or frequency) modulated signal through an optical transmission line composed of an optical fiber. Waveform distortion (intensity modulation component)
The present invention relates to a chromatic dispersion measuring device that measures chromatic dispersion of the optical transmission line by measuring the chromatic dispersion.

[0002]

2. Description of the Related Art In general, quartz (silica) glass, multi-component glass, PM
An optical fiber using a material such as MA (polymethyl methacrylate) is given. Above all, silica-based single-mode optical fibers have remarkably reduced loss, and their application to large-capacity and long-distance transmission has been studied and put into practical use. Since the propagation speed of the optical signal in the optical fiber varies depending on the wavelength because the group velocity of the optical signal varies depending on the wavelength, the waveform of the optical signal is distorted while the optical signal propagates through the optical fiber, and the deterioration of the signal deteriorates. It becomes a factor.

In order to minimize such signal degradation due to chromatic dispersion, it is necessary to make the carrier of the optical signal coincide with the zero dispersion wavelength (wavelength at which chromatic dispersion becomes zero) of the optical fiber.
Therefore, accurately measuring the chromatic dispersion of an optical fiber is an important technique for designing an optical communication system.

Here, a typical chromatic dispersion measuring apparatus which has been put into practical use (for details, see “Y. Horiuchi, Y. Nami
hira, and H. Wakabayashi, IEEE Photonics Technology
Letters. Vol.1 pp458-460 (1989) ”).
This will be described with reference to FIG. A conventional chromatic dispersion measuring apparatus 1 shown in FIG. 6 includes a semiconductor laser 2, an external resonator of the semiconductor laser 2, a diffraction grating 3 capable of changing a diffraction angle, and a sine applied to the semiconductor laser 2. An electric signal source 4 for supplying an electric signal of a wave, an isolator 5, a directional coupler 6 for branching an optical signal supplied via the isolator 5 into two, and one branch of the directional coupler 6 A wavelength meter 7 coupled to the directional coupler 6; a directional coupler 8 coupled to the other branch 6b of the directional coupler 6; and a reference light coupled to one branch 8a of the directional coupler 8. A fiber 9, a measured optical fiber 10 coupled to the other branch 8b of the directional coupler 8, a photoelectric converter 11 for converting an optical signal propagated by the reference optical fiber 9 into an electric signal, and a Propagated by the measuring optical fiber 10 And a photoelectric converter 12 for converting an optical signal into an electric signal, a photoelectric converter 1
1, a multiplier 14 coupled to the photoelectric converter 12, an electric signal source 15 for applying an electric signal to the multipliers 13, 14, and a band coupled to the multiplier 13. It comprises a pass filter 16, a band pass filter 17 coupled to the multiplier 14, and a phase meter 18 coupled to these band pass filters 16, 17.

According to the above configuration, first, the semiconductor laser 2
Is directly modulated by the sine wave signal output from the electric signal source 4, and an optical signal whose intensity is sine wave modulated is supplied to the isolator 5. After passing through the isolator 5, this optical signal is branched in two directions by the directional coupler 6, and one optical signal is input to the wavelength meter 7 after passing through the branch 6a, where the oscillation wavelength of the optical signal is changed. Measured.

The optical signal passing through the other branch 6b is further branched in two directions by the directional coupler 8,
One is input to the reference optical fiber, and the other is input to the measured optical fiber 10, respectively. The optical signals input to the reference optical fiber 9 and the measured optical fiber 10 are respectively
Each of the photoelectric converters 11 and 12 converts the electric signal into an electric signal, and generates a light receiving current proportional to the square of the electric field of the optical signal.

The electric signal output from the photoelectric converter 11 is multiplied by the sine wave generated by the electric signal source 15 by the multiplier 13 and converted into a low-frequency electric signal. The electric signal converted here passes through the band-pass filter 16 and is then input to the phase meter 18. On the other hand, the electric signal output from the photoelectric converter 12 is multiplied by the multiplier 14 with a sine wave generated by the electric signal source 15 and converted into a low-frequency electric signal. The electric signal converted here is
After passing through the band pass filter 17, it is input to the phase meter 18.

The phase difference between the electric signal propagated by the optical fiber under test 10 and the electric signal propagated by the reference optical fiber 9 is measured by the phase meter 18, and the optical fiber 10 under test is measured from the phase difference. The propagation time difference between the optical signal and the reference optical fiber 9 is measured. Furthermore, in the chromatic dispersion measuring apparatus 1 described above, the oscillation wavelength of the semiconductor laser 2 can be changed by changing the angle (diffraction angle) of the diffraction grating 3, so that the wavelength dependence of the propagation time difference can be measured. it can.

Here, the wavelength dependence of the propagation time difference is given by τ (λ)
Then, τ (λ) can be approximated as in the following equation.

(Equation 1) The values of the coefficients A, B, and C can be obtained from the measured propagation time differences by the least square method.

Thus, chromatic dispersion can be obtained by differentiating τ (λ) with wavelength “λ”,

(Equation 2) The zero-dispersion wavelength “λ ₀ ” is D (λ) in equation (2).
Is set to 0. Therefore, the chromatic dispersion of the measured optical fiber 10 can be obtained from the wavelength dependence of the propagation time of the intensity modulation signal, and the zero-dispersion wavelength “λ ₀ ” can be obtained from the chromatic dispersion.

When measuring the chromatic dispersion of a long optical fiber, a measurement error may occur in the propagation time difference due to an effective change in the fiber length due to a change in temperature or the like. For this reason, a method has been proposed in which the reference optical signal is also propagated through the optical fiber to be measured at the same time to cancel the measurement error. Generally, this type of method is called a difference method, and today, a measurement accuracy of a delay time difference of about 0.1 ps is obtained by the difference method. However, in the difference method, when measuring the chromatic dispersion value at a certain wavelength, it is necessary to measure the propagation time difference in a certain wavelength range including that wavelength.

In recent years, erbium-doped fiber optical amplifiers have made it possible to realize simple optical amplification in the 1.5 μm wavelength band used for optical communication, and to provide multi-relay by 1R relay that does not require a regenerative repeater. The feasibility of optical communication systems is increasing. Here, in order to increase the transmission distance, a technique for reducing deterioration of reception sensitivity due to beat noise between spontaneously emitted light generated by an optical repeater (optical amplifier) is required. Conventionally, as this kind of technology, an optical bandpass filter of about 1 nm is inserted into each repeater,
There is a method of removing spontaneous emission optical noise outside this band.
However, when trying to measure the total chromatic dispersion of the transmission line of such a long-distance multi-repeater optical communication system, the wavelength range in which a signal can be transmitted is limited by an optical bandpass filter inserted in the middle, so that the conventional difference can be measured. It is difficult to measure chromatic dispersion by the method.

In order to solve this problem, the present applicant has
By transmitting a phase or frequency modulated optical signal through an optical fiber and measuring the distortion of the signal waveform after transmission,
A “zero-dispersion wavelength measuring device” for measuring a chromatic dispersion value has already been proposed (reference: JP-A-5-87684). In the method applied to this zero dispersion wavelength measuring device,
Since the propagation delay between the sideband components of the optical phase modulation signal is observed, there is no need to measure the delay time difference by changing the wavelength of the signal (carrier). In addition, this method does not require a reference optical fiber.

The principle of the above method will be described below with reference to FIG. Zero dispersion wavelength measuring device 41 shown in FIG.
Is a wavelength tunable light source 22, an optical branching unit 23, a wavelength measuring unit 24, an optical phase modulating unit 25, and an electric signal source 26.
, A photoelectric conversion unit 27, an amplification unit 28, and a spectrum intensity measurement unit 29. In such a configuration, the optical signal phase-modulated by the optical phase modulating means 25 is expressed by the following equation.

(Equation 3) In the above equation (3), ω _c is the angular frequency of the carrier wave, ω _m
Represents a modulation frequency, and β represents a phase modulation index.

In the configuration shown in FIG. 7, the phase-modulated optical signal represented by the equation (3) is input to the optical fiber 31 to be measured. In the CW (continuous wave) optical signal propagated in the measured optical fiber 31, an intensity modulation component is generated according to the chromatic dispersion of the measured optical fiber 31. Therefore, the CW optical signal is expressed by the following equation.

(Equation 4) However, in the above equation (4), K represents a propagation constant, and K
_n is a propagation constant at ω _c + ω _m . The K _n is,
The expansion is performed as shown in the following equation centering on ω _c .

(Equation 5)

The CW optical signal represented by the equation (4) is square-detected by the photoelectric converter 27 and converted into an electric signal.
The electric signal after the photoelectric conversion is represented by the following equation.

(Equation 6) However, in the above equation (6), i ₀ is the average received light current and is represented by the following equation.

(Equation 7) Here, in the above equation (7), α is an attenuation constant representing the loss of the optical fiber.

Here, of the received light current i (t), the component of the frequency ω _m is normalized by the average received light current i ₀ and is expressed by the following equation.

(Equation 8) Also, near the zero dispersion wavelength,

(Equation 9) Equation (8) is approximated as

(Equation 10) It can be.

The value of .delta.i / i ₀ in the above equation (10), for example, obtained by using a spectrum analyzer (spectral intensity measuring means 29), measuring the relative intensity to the average photodetector current of the spectrum of frequencies omega _m Can be The chromatic dispersion D (ps / km / nm) is
From equation (9),

[Equation 11] Given by

[0019]

As described above, the conventional chromatic dispersion measurement method using the difference method measures the propagation delay time difference of an optical signal by changing the wavelength, and determines the wavelength dependence of the obtained delay time difference. Is a method of knowing the chromatic dispersion value from the above, but when measuring the total amount of chromatic dispersion of the transmission line of the multi-relay system, the wavelength band in which the signal can be transmitted is limited by the optical bandpass filter inserted in the repeater. Therefore, it may not be applicable.

On the other hand, in the zero-dispersion wavelength measuring device 41 already proposed by the present applicant, the phase (or frequency)
In the modulated optical signal, the intensity modulation component generated after propagating through the optical fiber is measured, and the chromatic dispersion value is measured from the intensity modulation component, so that the chromatic dispersion value can be measured without changing the wavelength of the signal. Can be measured. Therefore, it can be said that the present invention is also useful when measuring the total amount of chromatic dispersion of a multi-repeater system. However, in the zero-dispersion wavelength measuring device 41, of the photoelectric conversion signal component, it means that measure the spectral intensity of the frequency omega _m, the obtained is the absolute value of the wavelength dispersion values, wavelength dispersion value There is a drawback that the sign of can not be known.

The present invention has been made in view of the above circumstances, and has as its object to provide a chromatic dispersion measuring apparatus capable of measuring not only the absolute value of the chromatic dispersion value but also the sign. I do.

[0022]

According to a first aspect of the present invention, there is provided a chromatic dispersion measuring apparatus which measures a waveform distortion of an optical signal transmitted through an optical transmission line by measuring a waveform distortion of the optical signal. An apparatus for measuring chromatic dispersion, comprising: a light source that outputs coherent light; a light branching unit that branches a plurality of lights output from the light source; and a wavelength of the light that is branched by the light branching unit. Wavelength measuring means, optical phase modulating means for modulating the phase of the light branched by the optical branching means, and light for intensity-modulating an optical signal output from the optical phase modulating means in synchronization with the optical phase modulating means Intensity modulation means;
An electric signal source for generating an electric signal to be inputted to the light intensity modulating means and the optical phase modulating means; and a variable attenuation which variably controls the amplitude of the electric signal outputted from the electric signal source and inputted to the light intensity modulating means. Means, a polarity inversion circuit for changing the polarity of the electric signal output from the variable attenuation means,
A photoelectric conversion unit that converts an optical signal that has propagated through the optical transmission path into an electric signal; and a spectrum intensity measurement unit that measures the spectrum intensity of the electric signal output from the photoelectric conversion unit. I have. Claim 2
The chromatic dispersion measuring apparatus described in (1) is characterized in that, in the above configuration, a variable wavelength light source is used as the light source. Furthermore, the chromatic dispersion measuring apparatus according to claim 3 is provided between the optical transmission path and the photoelectric conversion unit in the configuration according to claim 1 or 2, and propagates through the optical transmission path. It is characterized by comprising optical amplification means for amplifying an optical signal. According to a fourth aspect of the present invention, there is provided a chromatic dispersion measuring apparatus according to any one of the first to third aspects, further including a band-pass filter and an electric signal intensity measuring unit as the spectrum intensity measuring unit. It is characterized by.

[0023]

The light splitting means splits the coherent light output from the light source into a plurality of lights, and the wavelength measuring means measures the wavelength of the light split by the light splitting means. The optical phase modulator modulates the phase of the light split by the optical splitter, and the light intensity modulator modulates the intensity of the optical signal output from the optical phase modulator in synchronization with the phase modulator. Here, the variable attenuating means and the polarity reversing means change the amplitude and polarity of the electric signal input from the electric signal source to the light intensity modulating means. Then, the photoelectric conversion unit converts the optical signal propagated through the optical transmission line into an electric signal, and the spectrum intensity measuring unit measures the spectrum intensity of the electric signal.

In an optical signal, waveform distortion caused by intensity modulation given before transmission and waveform distortion caused by phase modulation and chromatic dispersion of a fiber are added to each other when they are in phase, and mutually different when they are out of phase. Cancel each other out. Therefore, by changing the polarity of intensity modulation before transmission, the magnitude (absolute value) of waveform distortion after transmission changes. Since the phase of the waveform distortion after transmission is inverted by the sign of the chromatic dispersion and the magnitude relationship of the waveform distortion after transmission according to the inversion of the polarity of the intensity modulation is reversed, the sign of the chromatic dispersion can be determined. That is, the absolute value of the chromatic dispersion value and its sign are measured.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention are based on the basic principle of the present invention. First, the basic principle will be described. In the phase-modulated (or frequency-modulated) signal, the phase of the waveform distortion generated after propagating through the optical fiber is inverted according to the sign of the chromatic dispersion. Therefore, the intensity of the phase-modulated signal is further modulated before transmission with a modulation degree (extinction ratio) of about half of the waveform distortion generated after transmitting the phase-modulated signal, and the magnitude of the waveform distortion after transmission is measured.

The waveform distortion caused by the intensity modulation given before the transmission and the waveform distortion caused by the phase modulation and the chromatic dispersion of the optical fiber are added to each other when they are in phase, and cancel each other when they are out of phase. Therefore, if the polarity of the intensity modulation before transmission is changed, the magnitude (absolute value) of the waveform distortion after transmission changes. The sign of the chromatic dispersion can be determined because the phase of the waveform distortion after transmission is inverted by the sign of the chromatic dispersion and the magnitude relationship of the waveform distortion after transmission according to the inversion of the polarity of the intensity modulation is reversed. .

An embodiment of the present invention based on the above basic principle will be described with reference to the drawings. First,
With reference to FIG. 1, a chromatic dispersion measuring device 21 according to a first embodiment of the present invention will be described. The chromatic dispersion measuring device 21 shown in FIG.
, Wavelength measuring means 24, optical phase modulating means 25, electric signal source 26, variable attenuation circuit 27, and polarity inverting circuit 28
, Light intensity modulating means 29, photoelectric conversion means 31, amplifying means 32, and spectrum intensity measuring means 33.

An optical fiber to be measured (optical transmission line) is provided between the light intensity modulating means 29 and the photoelectric conversion means 31.
30 has been inserted. The measured optical fiber 31 is
It is optically coupled to the light intensity modulation means 29 and the photoelectric conversion means 31. The tunable wavelength light source 22 outputs coherent CW (continuous wave) light and can change the waveform of the CW light. In an actual device,
A multi-electrode distributed feedback (DBR) laser or a semiconductor laser with an external resonator is preferably used. Of course, if one wants to know only the chromatic dispersion of a specific wavelength, a single wavelength light source that oscillates only that wavelength may be used.

The light branching means 23 is provided with the wavelength tunable light source 22.
In the actual device, a directional coupler, an optical branch circuit (Y-branch circuit), or the like is preferably used. The wavelength measuring means (waveform meter) 24 measures the wavelength of the light split by the light splitting means 23. Further, the optical phase modulation means 2
Numeral 5 modulates the phase of the light split by the light splitting means 23. In an actual device, LiNb
An O ₃ optical phase modulator, a semiconductor electroabsorption type optical phase modulator and the like are preferably used. The light intensity modulating means 29 is for further modulating the intensity of the optical signal that has been subjected to the optical phase modulation. In an actual device, a LiNbO ₃ Mach-Zehnder light intensity modulator, a semiconductor electroabsorption type light intensity modulator, or the like is used. Is preferably used.

The electric signal source 26 is provided with the optical phase modulation means 2.
5 to generate an electric signal to be input to the control unit 5. The variable attenuator 27 is a circuit for changing the amplitude of the electric signal output from the electric signal source 26 and input to the light intensity modulator 29, and the polarity inverting circuit 28 is connected to the light intensity converter 29. This is for inverting the polarity of the input electric signal.

The photoelectric conversion means 31 is provided for the optical fiber 3 to be measured.
The optical signal that propagates 0 is converted into an electric signal,
In an actual device, a photoelectric (O / O) having a property of generating a light-receiving current proportional to the square of the electric field strength of an optical signal.
E) A converter is preferably used. The amplifying unit 32 amplifies the electric signal output from the photoelectric conversion unit 31 to a predetermined magnitude. Further, the spectrum intensity measuring means 33 measures the spectrum intensity of the electric signal output from the amplifying means 32. In an actual apparatus, a spectrum analyzer or the like is suitably used.

Next, the operation and operation principle of the chromatic dispersion measuring apparatus having the above-described configuration will be described with reference to FIGS. The coherent CW optical signal output from the wavelength tunable light source 22 is branched in two directions by the optical branching unit 23, and one CW light is transmitted through the branch 23a.
The wavelength is input to the wavelength measuring means 24, where the oscillation wavelength of the CW optical signal is measured. The other CW light branched by the light branching unit 23 is input to the optical phase modulation unit 25 after passing through the other branch 23b.

The optical phase modulation means 25 includes an electric signal source 26
Is input, and the optical phase modulating means 25 modulates the phase of the CW optical signal. The modulated optical signal is incident on the measured optical fiber 30. CW propagated in the measured optical fiber 30
The optical signal is converted into an electric signal by the photoelectric conversion unit 30. This electric signal is amplified by the amplifying means 32 and input to the spectrum intensity measuring means 33. The spectrum intensity measuring means 33 measures the spectrum intensity of the electric signal, and measures the chromatic dispersion value from the obtained spectrum intensity.

FIG. 5 shows a phase modulated signal (a phase modulated optical signal) before being input to the optical fiber 30 to be measured.
As shown in (a), the electric field strength (power of optical signal)
Is constant over time. Optical phase modulation means 2
If the electric signal input to 5 is a sine wave, the phase change of the optical signal becomes sinusoidal as shown in FIG. At this time, since the time change of the optical frequency is given by the time derivative of the phase change, it also becomes sinusoidal as shown in FIG.

When this optical signal propagates through the measured optical fiber 30, the propagation speed varies depending on the wavelength, that is, the frequency, so that a relative delay occurs in the direction indicated by the arrow shown in FIG. 5C. Here, since the sign of chromatic dispersion is different between the shorter wavelength side and the longer wavelength side with respect to the zero dispersion wavelength, they are referred to as a normal dispersion region and an abnormal dispersion region, respectively. The direction of the relative delay of the optical signal in each of the normal dispersion region and the abnormal dispersion region is a direction indicated by a solid line and a broken line in FIG. Therefore, after propagating through the optical fiber 30 to be measured, as shown in FIG. 5E, the electric field intensity which was constant before the propagation has irregularities, that is, waveform distortion (intensity modulation component).

The fundamental frequency of the intensity modulation component coincides with the frequency of the optical signal input to the optical phase modulator 25.
In the normal dispersion region and the abnormal dispersion region, the phases are inverted, and each has a waveform represented by a solid line and a broken line as shown in FIG. Therefore, as described above, the magnitude of the waveform distortion, if measured as spectral intensity component of the frequency omega _m by the spectral intensity measuring means 25,
From this measured value, the chromatic dispersion value can be known.

Next, a method for measuring the sign of chromatic dispersion will be described. At the same time that the optical signal before transmission is phase-modulated by the optical phase modulator 25, the intensity is modulated by the light intensity modulator 29 in synchronization with the phase modulation. On this occasion,
In the variable attenuation circuit 27, the light intensity modulating means 29 is controlled so that the modulation degree (extinction ratio) of the intensity modulation is about half of the magnitude of the waveform distortion after transmission obtained earlier for the phase modulation signal.
Adjust the input signal strength.

At this time, the electric field strength of the optical signal before transmission changes sinusoidally as shown in FIG. Therefore,
The electric field strength after propagating through the optical fiber under test 30 includes waveform distortion caused by intensity modulation before transmission (see FIG. 5D) and waveform distortion caused by dispersion of the optical fiber from the phase modulation component (FIG. 5 ( e)) and a waveform as shown in FIG. 5 (f). As is clear from this figure, in the normal dispersion region, the intensity modulation component before transmission and the waveform distortion caused by dispersion after transmission are in phase with each other, and both are added to each other.
In the anomalous dispersion region, the phases are opposite to each other and therefore cancel each other.

Therefore, the polarity of the intensity modulation is inverted by the polarity inverting circuit 28, the magnitude of the waveform distortion after transmission is measured, and the magnitude relationship is compared, whereby the sign of the chromatic dispersion can be determined. This means that when the intensity-modulated signal before transmission is captured as a pulse, the sign of the chirping of the pulse is changed, the pulse spread after transmission is measured, and the sign of the chromatic dispersion is determined. It is.

FIG. 2 is a diagram showing a configuration of a chromatic dispersion measuring apparatus 21 according to a second embodiment of the present invention. The chromatic dispersion measuring apparatus 21 shown in FIG. The optical signal attenuated by the
To improve the measurement accuracy. As the optical amplifier 34, an erbium-doped optical amplifier, a semiconductor optical amplifier, or the like is preferably used.

FIG. 3 is a diagram showing a configuration of a chromatic dispersion measuring device 21 according to a third embodiment of the present invention. The chromatic dispersion measuring device 21 shown in FIG.
Instead of using (see FIGS. 1 and 2) the spectrum intensity of the electric signal after photoelectric conversion, a band-pass filter 3 is used.
In step 5, a desired signal spectrum component is extracted, and its intensity is measured by the electric signal intensity measuring means 36.

FIG. 4 is a diagram showing a configuration of a chromatic dispersion measuring apparatus 21 according to a fourth embodiment of the present invention. The chromatic dispersion measuring apparatus 21 shown in FIG. The measuring device 21 (see FIG. 3) amplifies the optical signal attenuated by propagating through the optical fiber 30 to be measured by the optical amplifying means 34 to improve the measurement accuracy. As the optical amplifying means 34, an erbium-doped optical amplifier, a semiconductor optical amplifier, or the like is preferably used as in the second embodiment. In addition, the above-mentioned first to fourth
In this embodiment, the operation principle is common, and the actual operation is the same. Therefore, the description of the operation in the second to fourth embodiments is omitted.

[0043]

As described above, according to the chromatic dispersion measuring apparatus of the present invention, the optical phase modulating means modulates the phase of the light branched by the optical branching means, and the light intensity modulating means modulates the optical phase. The optical signal output from the modulation means is intensity-modulated in synchronization with the phase modulation means. Here, the variable attenuation means and the polarity inversion means change the amplitude and the polarity of the electric signal input from the electric signal source to the light intensity modulation means, so that the spectrum intensity measurement means (band-pass filter and electric signal intensity measurement means) In addition to the absolute value of the chromatic dispersion value, the sign can be measured from the spectral intensity of the optical signal measured by the method.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of a chromatic dispersion measuring apparatus 21 according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a schematic configuration of a chromatic dispersion measuring device 21 according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a schematic configuration of a chromatic dispersion measuring device 21 according to a third embodiment of the present invention.

FIG. 4 is a diagram showing a schematic configuration of a chromatic dispersion measuring apparatus 21 according to a fourth embodiment of the present invention.

FIG. 5 is a diagram for explaining the operation principle of the chromatic dispersion measuring device 21 according to the first to fourth embodiments of the present invention;
(A) is the electric field strength before transmission, (b) is the phase change before transmission, (c) is the overall delay due to the optical frequency, (d) is the waveform distortion due to intensity modulation before transmission, and (e) is the received signal. Waveform distortion due to dispersion of the measurement optical fiber 30, and (f) represents waveform distortion after transmission.

FIG. 6 is a diagram showing a schematic configuration of a chromatic dispersion measuring apparatus 1 according to a conventional difference method.

FIG. 7 is a diagram showing a schematic configuration of a conventional zero-dispersion wavelength measuring device 41.

[Explanation of symbols]

 Reference Signs List 21 chromatic dispersion measuring device 22 variable wavelength light source 23 optical branching means 24 wavelength measuring means 25 optical phase modulation means 26 electric signal source 27 variable attenuation circuit (variable attenuation means) 28 polarity inversion circuit 29 light intensity modulation means 30 optical fiber to be measured ( (Optical transmission path) 31 photoelectric conversion means 32 amplification means 33 spectrum intensity measurement means 34 optical amplification means 35 bandpass filter 36 electric signal intensity measurement means

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-5-87684 (JP, A) JP-A-61-105439 (JP, A) JP-A-5-87683 (JP, A) JP-A 61-105 105440 (JP, A) JP-A-55-15081 (JP, A) JP-A-3-115939 (JP, A) JP-A-4-285836 (JP, A) JP-A-62-207927 (JP, A) JP-A-61-91537 (JP, A) JP-A-61-65132 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01M 11/00-11/08

Claims (4)

(57) [Claims] An apparatus for measuring chromatic dispersion of an optical transmission line by measuring waveform distortion of an optical signal propagated through the optical transmission line, comprising: a light source that outputs coherent light; Light splitting means for splitting the split light into a plurality of light, wavelength measuring means for measuring the wavelength of the light split by the light splitting means, and optical phase modulation means for modulating the phase of the light split by the light splitting means. A light intensity modulator for intensity-modulating an optical signal output from the optical phase modulator in synchronization with the optical phase modulator; and generating an electric signal to be input to the light intensity modulator and the optical phase modulator. An electrical signal source; a variable attenuator for variably controlling the amplitude of the electrical signal output from the electrical signal source and input to the light intensity modulator; a polarity for changing the polarity of the electrical signal output from the variable attenuator A conversion circuit, a photoelectric conversion unit that converts an optical signal that has propagated through the optical transmission line into an electric signal, and a spectrum intensity measurement unit that measures the spectrum intensity of the electric signal output from the photoelectric conversion unit. A chromatic dispersion measuring device characterized by the above-mentioned. 2. The chromatic dispersion measuring apparatus according to claim 1, wherein a variable wavelength light source is used as said light source. 3. An optical amplifying means provided between the optical transmission path and the photoelectric conversion means, for amplifying an optical signal propagated through the optical transmission path. 3. The chromatic dispersion measuring device according to any one of 2. 4. The chromatic dispersion measuring apparatus according to claim 1, further comprising a band-pass filter and an electric signal intensity measuring unit as the spectrum intensity measuring unit.