JP3250587B2 - 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 chromatic dispersion measuring apparatus for measuring the position dependence of the chromatic dispersion in an optical fiber.

[0002]

2. Description of the Related Art Conventionally, a silica-based single mode optical fiber has been remarkably reduced in loss, and has been used as an optical transmission line in large-capacity, long-distance transmission. By the way, ordinary single-mode optical fibers have the property that chromatic dispersion, that is, the group velocity of an optical signal differs depending on the wavelength, so that the signal waveform is distorted while the optical signal propagates through the optical fiber, and the signal quality is reduced. It causes deterioration.

Further, it has been pointed out that when the input light intensity to the optical fiber is high, the signal quality is degraded due to the nonlinear optical effect in which the refractive index of the optical fiber changes according to the signal intensity. For example, in an optical communication system of intensity modulation and direct detection, a phenomenon in which the frequency of a carrier changes (so-called self-phase modulation effect) occurs due to a change in the refractive index of an optical fiber in accordance with a change in the intensity of an optical signal. When such an optical signal propagates through an optical fiber having chromatic dispersion, distortion occurs in the signal waveform.

[0004] The principle that the chromatic dispersion of an optical fiber affects the signal quality can be classified into the above two types. On the other hand, the signal waveform is distorted due to the relative transmission delay due to the chromatic dispersion of the frequency component of the signal, causing signal quality deterioration. In this case, the influence of the chromatic dispersion depends on the total chromatic dispersion of the signal transmission path. Therefore, when designing a transmission line, it is possible to take a sufficient measure by measuring the total amount of chromatic dispersion of the transmission line for each relay section.

The other is signal quality degradation caused by a combination of chromatic dispersion and nonlinear optical effects. This signal degradation is caused not only by the average value of the chromatic dispersion of the optical fiber but also by the chromatic dispersion of each element of the transmission line. Occurs depending on the distribution of the value from the average value. In general, since the chromatic dispersion value of an optical fiber depends on its distance, when constructing a long-span optical transmission system considering the nonlinear optical effect, it is important to measure the position distribution of the chromatic dispersion of the optical fiber. Become.

As a method for measuring the chromatic dispersion value of an optical fiber, a method called a difference method has conventionally been generally used. This is a method of measuring the wavelength dependence of the relative propagation delay time of an optical signal, approximating the measurement result with a power series function of the wavelength, and differentiating the obtained function with the wavelength to obtain a chromatic dispersion value.

[0007]

However, it has been difficult to measure the position dependence of the chromatic dispersion of an optical fiber using the above-described difference method. Therefore, as a method of measuring the position distribution of the chromatic dispersion of an optical fiber, the present applicant measures the distance dependence of the transmission line loss from the time dependence of the reflected light intensity due to backward Rayleigh scattering generated in the optical fiber. It was considered effective to apply the OTDR (Optical Time Domain Reflectometry) technology.

However, it is difficult to measure the position dependence of the chromatic dispersion of an optical fiber by combining the conventional chromatic dispersion measuring technique by the difference method with the OTDR as it is. This is because the difference method determines the wavelength dispersion value from the wavelength dependence of the signal propagation time, but the OTDR determines the reflection position from the arrival time of the optical signal reflected in the optical fiber. This is because it is difficult to distinguish between the delay and the delay depending on the distance to the reflection point. Although slight unevenness (distortion) occurs in the distance dependence of the reflected signal intensity due to wavelength dispersion, high-precision measurement cannot be performed unless the loss of the measured optical fiber is uniform. Further, in order to measure the position dependence of chromatic dispersion at a certain wavelength, it is necessary to perform the same measurement in a wide wavelength region including that wavelength.

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 the position dependence of chromatic dispersion in an optical fiber (optical transmission line) by OTDR. And

[0010]

According to a first aspect of the present invention, there is provided a chromatic dispersion measuring apparatus for measuring a position dependence of chromatic dispersion of an optical transmission line, wherein the light source outputs a coherent optical signal. An optical branching unit that branches an optical signal output from the light source, a wavelength measuring unit that measures a wavelength of one output optical signal of the optical branching unit, a signal source that generates a predetermined electric signal, An optical phase modulating means for modulating the phase of the other output optical signal of the optical branching means in accordance with the electric signal generated by the signal source; and a variable attenuation circuit variably controlling the amplitude of the electric signal generated by the signal source. A polarity inversion circuit for variably controlling the polarity of the output electric signal of the variable attenuation circuit; and a first light for modulating the light intensity of the output optical signal of the optical phase modulation means in accordance with the output electric signal of the polarity inversion circuit. Intensity modulation means and a predetermined pal A pulse signal source for generating a signal, a second modulating light intensity in response to the first optical intensity modulating means output optical signal said pulse signal source in generating pulse signals
Light intensity modulating means, connected to one end of the optical transmission path,
A directional coupling circuit for inputting an optical signal output from the second optical intensity modulation means to the optical transmission line and outputting a reflected optical signal from the optical transmission line; and a photoelectric converter for converting the reflected optical signal into an electric signal A converter, an amplifier circuit for amplifying an output electric signal of the photoelectric converter, spectrum intensity measuring means for measuring the spectrum intensity of the output electric signal of the amplifier circuit, and an analog-to-digital converter connected to the spectrum intensity measuring means. And a digital signal processing circuit connected to the analog-to-digital conversion circuit.

According to a second aspect of the present invention, there is provided a chromatic dispersion measuring apparatus according to the first aspect, wherein an optical amplifier is provided in a stage preceding the photoelectric converter. Furthermore, the chromatic dispersion measuring apparatus according to claim 3 is the chromatic dispersion measuring apparatus according to claim 1 or 2, wherein the spectrum intensity measuring means is
Particularly, it is characterized in that it comprises a bandpass filter and a power meter.

[0012]

According to the above configuration, the phase (or frequency) modulated optical signal is intensity-modulated by the second light intensity modulating means in accordance with a predetermined pulse signal, and is incident on the optical transmission line. Then, the magnitude of waveform distortion after propagation through the optical fiber is measured by the spectrum intensity measuring means, and the absolute value of chromatic dispersion is measured. Further, the first light intensity modulating means superimposes an intensity modulation signal of about half the magnitude of the waveform distortion on the phase modulation signal before transmission, and the intensity modulation component after transmission when the polarity of the intensity modulation component is inverted. Is compared in the digital signal processing circuit, the sign of the chromatic dispersion is measured.

Further, the optical signal is converted into a pulse signal having a period and a duty ratio according to the length of the optical transmission line and the required distance resolution, and a component reflected by backward Rayleigh scattering in the middle of the optical transmission line is observed in the digital signal processing circuit. Then, the total amount of chromatic dispersion up to the reflection point is measured. Since the thus obtained chromatic dispersion value is twice the total chromatic dispersion amount from the connection terminal of the optical fiber to be measured to the reflection point, the position dependence of the chromatic dispersion is expressed as follows. Obtained by differentiating. As described above, the present invention measures the relative intensity of the spectrum of the reflected light signal relative to the average light signal intensity instead of measuring the intensity of the reflected light in the measurement of the loss distribution of the transmission line by the conventional OTDR. It has become.

According to the second aspect of the present invention, since the optical amplifier amplifies the intensity of the reflected light signal and inputs the amplified signal to the photoelectric converter, the signal-to-noise intensity ratio (S / N ratio) is increased.
However, it is further improved. Further, according to the chromatic dispersion measuring apparatus of the third aspect, since a band-pass filter and a wattmeter are particularly used as the spectrum measuring means, the manufacturing cost of the entire apparatus is reduced.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. First, a chromatic dispersion measuring apparatus according to a first embodiment of the present invention will be described. FIG. 1 is a diagram showing a schematic configuration of a chromatic dispersion measuring apparatus according to the present embodiment. In this figure, reference numeral 1 denotes a light source capable of generating coherent light and changing a wavelength. In this case, a distributed feedback (DFB) semiconductor laser, a semiconductor laser with an external resonator, or the like is preferably used.

Reference numeral 2 denotes an optical branching unit. In an actual device, a directional coupler, an optical branching circuit, or the like is preferably used. Reference numeral 3 denotes a signal source for generating an electric signal, and 4 denotes an optical phase modulation means. The optical signal supplied from the light source 1 via the optical branching means 2 is phase-modulated according to the electric signal generated by the signal source 3. In an actual apparatus, a LiNbO3 optical phase modulator, a semiconductor electroabsorption type optical phase modulator, or the like is preferably used as the optical phase modulator 4.

Reference numeral 6 denotes a variable attenuator for adjusting the intensity of the electric signal generated by the signal source 3 and outputting the adjusted signal. Reference numeral 7 denotes a polarity inverting circuit for changing the polarity of the electric signal output from the variable attenuator 6. , A pulse signal source for generating a pulse signal, 5,
Reference numeral 9 denotes light intensity modulating means. The light intensity modulating means 5 (first light intensity modulating means) converts an optical signal output from the optical phase modulating means 4 according to an electric signal supplied from the polarity inversion circuit 7. The intensity is modulated, and the light intensity modulating means 9 (second light intensity modulating means) modulates the intensity of the optical signal output from the light intensity modulating means 5 according to the electric signal generated by the pulse signal source 8.
In an actual device, the light intensity modulating means 5, 9
A LiNbO3 Mach-Zehnder light intensity modulator, a semiconductor electroabsorption type light intensity modulator or the like is preferably used.

A directional coupler 10 and a photoelectric converter 14 generate a photocurrent proportional to the square of the electric field of the optical signal. Reference numeral 15 denotes an amplifier circuit that amplifies the electric signal photoelectrically converted by the photoelectric converter 14. Reference numeral 16 denotes a spectrum intensity measuring unit which measures the spectrum intensity of the electric signal supplied via the amplifier circuit 15. Reference numeral 17 denotes an analog-to-digital conversion circuit, and reference numeral 18 denotes a digital signal processing circuit. The digital signal converted by the analog-to-digital conversion circuit 17 is averaged to improve a signal-to-noise (S / N) intensity ratio. Reference numeral 13 denotes a wavelength measuring unit that measures the wavelength of an optical signal supplied from the light source 1 via the optical branching unit 2. 11 is a terminal to which the optical fiber to be measured is to be connected;
Reference numeral 12 denotes an optical fiber to be measured.

Next, the operation of the chromatic dispersion measuring apparatus having the above configuration will be described with reference to FIG. 1 and FIGS. 5 (a) to 5 (f). The coherent optical signal output from the light source 1 is split by the optical splitting means 2, and one of the split optical signals is input to the wavelength measuring means 13, and its oscillation wavelength is measured. The other branched optical signal is input to the optical phase modulator 4, where it is phase-modulated according to the electric signal generated by the signal source 3.

The phase-modulated optical signal is further converted into a pulse waveform as shown in FIG. Here, the pulse waveform output from the light intensity modulating means 9 is set to a period and a pulse width according to the length and the distance measurement resolution of the measured optical fiber 12. The reflection point is identified based on the time when such a pulse signal reaches the photoelectric converter 14.
Uses TDR technology.

Assuming that the length of the optical fiber under test 12 is L, the pulse period T in FIG.

(Equation 1) Is set to be Where Vg is the group velocity of the optical signal,

(Equation 2) Given by Here, c represents the speed of light, and n represents the relative refractive index of the core of the optical fiber. The measurement distance resolution is
Determined by the pulse width Δτ,

(Equation 3) Given by

The optical signal reflected by Rayleigh scattering in the optical fiber passes through the directional coupler 10 and is converted by the photoelectric converter 14
Is input to Since the photoelectric converter 14 has a property of generating a light receiving current proportional to the square of the electric field of the optical signal, the magnitude of the waveform distortion of the optical signal after transmission depends on the signal source in the electrical signal after photoelectric conversion. 3 can be obtained by measuring the relative intensity of the spectrum component equal to the frequency of the signal output from 3 to the average received light current.

Since the arrival time of the signal to the photoelectric converter 14 varies depending on the position of the reflection point in the optical fiber 12 to be measured, the time dependence of the measured value is measured to obtain the reflection point of the measurement result. You can know the distance to.
Normally, since the light intensity of the reflected light is extremely weak, in the present embodiment, the digital signal processing circuit 18 averages the measurement results (that is, obtains the average of a plurality of measured values) to obtain a signal-to-noise intensity. The ratio has been improved.

Here, the principle of measuring chromatic dispersion by the chromatic dispersion measuring apparatus of the present invention will be described with reference to FIGS. 4 (a) to 4 (f). The phase-modulated optical signal is shown in FIG.
As shown in FIG. When this optical signal propagates through the optical fiber, it undergoes a relative delay indicated by an arrow in FIG. 4C, and a waveform is formed into an electric field intensity (see FIG. 4A) that was temporally constant before transmission. Distortion occurs (see FIG. 4E). Here, the phase of the waveform distortion differs between the normal dispersion region and the abnormal dispersion region by π, and the waveforms are indicated by solid lines and broken lines in the figure, respectively.

The optical signal reflected by the backward Rayleigh scattering in the middle of the optical fiber and reaching the photoelectric converter 14 corresponds to the chromatic dispersion of the reciprocating transmission line from the terminal 11 to the reflection point in the optical fiber 12 to be measured. Waveform distortion occurs. Since the spectrum intensity of the signal due to the waveform distortion does not depend on the sign of the chromatic dispersion, only the absolute value of the chromatic dispersion is measured here.

Therefore, the light intensity modulating means 5 further modulates the intensity of the phase modulation signal with a modulation degree (extinction ratio) of about half the magnitude of the waveform distortion caused by the chromatic dispersion obtained in FIG. The magnitude of the transmitted intensity modulated signal obtained by inverting the sign of the intensity modulated signal using the polarity inversion circuit 7 is compared. The intensity modulation signal has a waveform as shown in FIG. 4D in synchronization with the phase modulation signal. In the optical signal, the waveform distortion due to the intensity modulation component superimposed before transmission and the waveform distortion due to the dispersion of the fiber are added to each other when they are in phase, and are canceled when they are out of phase. Since the magnitude relationship of the waveform distortion (intensity modulation component) after transmission is inverted depending on the sign of the sign of the chromatic dispersion, it is possible to determine the sign of the chromatic dispersion of the optical signal reflected at each reflection point of the optical transmission line. it can.

Next, referring to FIG. 5, the operation of the chromatic dispersion measuring apparatus according to the present embodiment will be described with reference to FIG. 1 and FIGS.
This will be described with reference to FIG. Here, consider the measured optical fiber 12 whose chromatic dispersion characteristic is as shown in FIG. The intensity of the optical signal reflected by the measured optical fiber 12 depends on the loss of the measured optical fiber 12, as shown in FIG.
Has a time dependency as shown in FIG.

On the other hand, the measurement result of the signal waveform distortion is shown in FIG.
The characteristics shown in FIG. Here, when the intensity modulated signal is superimposed on the optical signal before transmission, the waveform distortion of the optical signal after transmission is indicated by a broken line and a dashed line in FIG. Shown respectively. The resulting total chromatic dispersion in the reciprocating path from the reflection point to the terminal 11 is as shown in FIG. 5D.

The result of this measurement is expressed by the following equation using the wavelength dispersion value D (λ, z) at the wavelength λ and the distance z.

(Equation 4) Here, the coefficient “2” means that the dispersion is superimposed by the reciprocation of light. The chromatic dispersion value at the position of the distance z is given by the following equation obtained by differentiating the equation (4).

(Equation 5) As described above, according to the chromatic dispersion measuring apparatus according to the first embodiment of the present invention, it is possible to measure the distance dependency of the chromatic dispersion of the transmission path.

Next, a chromatic dispersion measuring apparatus according to a second embodiment of the present invention will be described with reference to FIG. The chromatic dispersion measuring apparatus shown in FIG. 2 is different from the chromatic dispersion measuring apparatus according to the first embodiment shown in FIG. 1 in that an optical amplifier 19 is provided in front of the photoelectric converter 14 so that the light input to the photoelectric converter 14 can be changed. The point is to amplify the signal. Thereby, the signal-to-noise intensity ratio can be improved.

Next, a chromatic dispersion measuring apparatus according to a third embodiment of the present invention will be described with reference to FIG. The difference between the chromatic dispersion measuring apparatus shown in FIG. 3 and the chromatic dispersion measuring apparatus according to the first embodiment shown in FIG. 1 is that the signal output from the signal source 3 is particularly used as the spectrum intensity measuring means 16 (see FIG. 1). And a power meter 21 for measuring the intensity of a signal transmitted through the band-pass filter 21.
Therefore, it is possible to configure the chromatic dispersion measuring apparatus using components that can be easily obtained in consideration of cost and the like.
Since the power meter 21 is capable of digital output, the analog-to-digital converter 17 (see FIG. 1) is unnecessary.

The principle of operation of each of the chromatic dispersion measuring apparatuses according to the second and third embodiments is the same as that of the first embodiment, and the operation is the same.

[0033]

As described above, according to the present invention,
The phase (or frequency) modulated optical signal is intensity-modulated by the second optical intensity modulating means according to a predetermined pulse signal, and is incident on the optical transmission line. Then, the magnitude of waveform distortion after propagation through the optical fiber is measured by the spectrum intensity measuring means, and the absolute value of chromatic dispersion is measured. Also,
The first light intensity modulation means superimposes an intensity modulation signal of about half the magnitude of the waveform distortion on the phase modulation signal before transmission,
The magnitude of the intensity modulation component after transmission when the polarity of the intensity modulation component is inverted is compared in the digital signal processing circuit, whereby the sign of the chromatic dispersion is measured.

Further, the optical signal is converted into a pulse signal having a period and a duty ratio corresponding to the length of the optical transmission line and the required distance resolution, and a component reflected by backward Rayleigh scattering in the middle of the optical transmission line is observed in the digital signal processing circuit. Then, the total amount of chromatic dispersion up to the reflection point is measured. Since the thus obtained chromatic dispersion value is twice the total chromatic dispersion amount from the connection terminal of the optical fiber to be measured to the reflection point, the position dependence of the chromatic dispersion is expressed as follows. Obtained by differentiating.

As described above, according to the present invention, instead of measuring the intensity of the reflected light, the relative intensity of the spectrum of the reflected optical signal with respect to the average optical signal intensity is measured instead of measuring the intensity of the reflected light. It is designed to measure. Therefore, there is an effect that the OTDR can measure the position dependence of the chromatic dispersion in the optical fiber.

[Brief description of the drawings]

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

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

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

FIGS. 4A and 4B are schematic diagrams for explaining the principle of measuring chromatic dispersion by the chromatic dispersion measuring apparatus according to the first embodiment of the present invention, wherein FIG. 4A shows the electric field strength before transmission, and FIG. Phase change, (c) overall delay due to optical frequency, (d) waveform distortion due to intensity modulation before transmission, (e) waveform distortion due to dispersion of measured optical fiber 12, (f) after transmission The waveform distortion is represented.

5A and 5B are schematic diagrams for explaining the operation of the chromatic dispersion measuring apparatus according to the first embodiment of the present invention, wherein FIG. 5A is a waveform after intensity modulation according to a pulse signal, and FIG. (C) shows the measurement result of the waveform distortion, (d) shows the total amount of chromatic dispersion in the reciprocating path, and (e) shows the chromatic dispersion characteristics of the optical fiber 12 to be measured.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 light source 2 light branching means 3 signal source 4 optical phase modulation means 5 light intensity modulation means (first light intensity conversion means) 6 variable attenuation circuit 7 polarity inversion circuit 8 pulse signal source 9 light intensity modulation means (second light Intensity conversion means) 10 directional coupler 11 terminal 12 optical fiber to be measured (optical transmission line) 13 wavelength measurement means 14 photoelectric converter 15 amplifier circuit 16 spectrum intensity measurement means 17 analog-to-digital conversion circuit 18 digital signal processing circuit 19 optical amplifier 20 bandpass filter 21 wattmeter

Claims (3)

(57) [Claims] 1. A chromatic dispersion measuring device for measuring the position dependence of chromatic dispersion of an optical transmission line, comprising: a light source for outputting a coherent optical signal; and an optical branching means for branching the optical signal output from the light source. A wavelength measuring means for measuring a wavelength of one output optical signal of the optical branching means; a signal source for generating a predetermined electrical signal; and a phase of the other output optical signal of the optical branching means by the signal source. Optical phase modulation means for modulating according to the generated electric signal; a variable attenuation circuit for variably controlling the amplitude of the electric signal generated by the signal source; and variably controlling the polarity of the output electric signal of the variable attenuation circuit. A polarity inverting circuit, first light intensity modulating means for modulating the light intensity of the output optical signal of the optical phase modulating means in accordance with the output electric signal of the polarity inverting circuit, and a pulse signal source for generating a predetermined pulse signal The first light Second light intensity modulating means for modulating the light intensity of the output light signal of the degree modulating means in accordance with a pulse signal generated by the pulse signal source; and a second light intensity modulating means connected to one end of the optical transmission line. A directional coupling circuit for inputting an optical signal output from a modulating unit to the optical transmission line and outputting a reflected optical signal from the optical transmission line; a photoelectric converter for converting the reflected optical signal into an electric signal; An amplifier circuit for amplifying an output electric signal of the converter; a spectrum intensity measuring unit for measuring a spectrum intensity of the output electric signal of the amplifier circuit; an analog-to-digital conversion circuit connected to the spectrum intensity measuring unit; A chromatic dispersion measuring device comprising: a digital signal processing circuit connected to a conversion circuit. 2. The chromatic dispersion measuring apparatus according to claim 1, wherein an optical amplifier is provided in a stage preceding the photoelectric converter. 3. The apparatus according to claim 2, wherein said spectrum intensity measuring means comprises:
3. The chromatic dispersion measuring apparatus according to claim 1, further comprising a bandpass filter and a power meter.