JP5487068B2 - Chromatic dispersion measuring apparatus and chromatic dispersion measuring method using the same - Google Patents

Description

  The present invention relates to a technical field such as a chromatic dispersion measuring device for measuring chromatic dispersion of an optical pulse, and more particularly to an optical pulse propagating through an optical transmission line of an optical fiber network in a high-speed optical communication system having a transmission rate of several tens of Gbit / s. The present invention relates to a chromatic dispersion measuring apparatus for measuring chromatic dispersion and a chromatic dispersion measuring method using the same.

  In recent years, data communication has been shifting to one via an optical fiber, and along with this, the data transmission speed has been dramatically increased. In the near future, in such a high-speed optical communication system via an optical fiber, it is considered to use ultrashort optical pulses and communicate at a transmission rate of several tens of Gbit / s or higher, which is faster than the current transmission rate. Has been.

By the way, when performing data communication in a high-speed optical communication system, there is a problem that crosstalk and transmission errors always occur.
However, as the data transmission speed becomes higher, the width of each light pulse and the interval between the light pulses before and after each other become narrower, and when performing the above-described reliable data communication with the above-described crosstalk and transmission error, This is a very important issue as explained below.

The speed at which light travels through the material is determined by the refractive index of the material, and the higher the refractive index, the slower the light speed. In materials such as glass, semiconductors, and optical crystals, the refractive index changes depending on the frequency of light (wavelength in air), so the speed of light depends on the wavelength. It is known that due to the wavelength dependence of the refractive index, the waveform of the light pulse is distorted while the light pulse travels through the material, and the time width of the light pulse is increased. Furthermore, in an optical waveguide represented by an optical fiber, the effective refractive index of the optical waveguide is determined according to the shape and size of each of the core and the cladding, and the speed of light depends on the wavelength. Therefore, the structure of the optical waveguide is also a factor that increases the time width of the optical pulse.
Thus, the characteristic that the speed of light differs according to the wavelength of light is hereinafter referred to as wavelength dispersion or simply dispersion.

As described above, while traveling through the optical fiber, the waveform of the optical pulse is distorted or the time width of the optical pulse is widened due to the above-described wavelength dispersion. Since this interval is wider than chromatic dispersion, it is not a big problem.
However, if the data transmission speed is higher than several tens of Gbit / s, the chromatic dispersion becomes wider than the interval between the front and rear optical pulses, and the front and rear optical pulses interfere with each other, causing crosstalk and transmission errors. End up. For this reason, simply trying to increase the transmission speed with the current technology cannot realize data communication with higher speed and higher reliability.

In order to remove (or control) chromatic dispersion in the high-speed optical communication system described above, first, chromatic dispersion of various optical components used in the high-speed communication system is measured, and the characteristics of chromatic dispersion of each member are grasped. There is a need.
For example, there is a wavelength dispersion measuring apparatus using a spectrum shearing interferometer using a frequency shifter that measures the spectrum phase of various components in order to obtain chromatic dispersion from the change of the spectrum phase (see, for example, Patent Document 1).
In this spectrum shearing interferometer, in order to uniquely measure the spectrum phase, the cos component and the sin component of the optical pulse are converted into a horizontal polarization component and a vertical polarization component, respectively, and polarization separation is performed, so that two orthogonal components are obtained. Therefore, an interferometer is configured using a spatial optical system.

In a spectrum shearing interferometer, an optical pulse is propagated by linearly polarized light in an optical fiber constituting a part of the interferometer.
In this spectrum shearing interferometer, in order to generate the orthogonal two components of the cos component and the sin component, it is necessary to convert linearly polarized light into circularly polarized light.
This circularly polarized light is formed by superimposing two orthogonally polarized lights, ie, horizontally polarized light and vertically polarized light orthogonal to each other in the vertical and horizontal directions. There is a 90 ° phase difference between horizontally polarized light and vertically polarized light.
Therefore, by using a polarization beam splitter to spatially separate circularly polarized light into horizontal polarized light and vertical polarized light, two orthogonal components of a cos component and a sin component can be obtained.

As described above, for measurement of chromatic dispersion, it is necessary to obtain two orthogonal components of a cos component and a sin component in a plurality of wavelength bands.
In contrast, in an optical fiber, only light of a specific wavelength corresponding to the optical length of the optical fiber is propagated without changing from circularly polarized light to elliptically polarized light, and light of other wavelengths is transmitted from circularly polarized light to elliptically polarized light. The orthogonal two components cannot be maintained in a stable state, and the orthogonal two components cannot be obtained with high accuracy.

  For this reason, by using a spatial optical system in the optical path related to the separation of the orthogonal two components so that the circularly polarized light does not change to elliptically polarized light, the circularly polarized light can be stably propagated to the light of all applicable wavelengths, and cos An orthogonal two component of a component and a sin component is generated with high accuracy.

JP 2007-85981 A

However, although the chromatic dispersion measuring apparatus of Patent Document 1 can accurately obtain two orthogonal components of an optical pulse, since it uses a spatial optical system, input / output of light between the optical fiber and the spatial optical system. As a result, optical loss occurs. Due to this light loss, there is a problem that the intensity of light is lowered and the measurement sensitivity is reduced.
Further, since the chromatic dispersion measuring apparatus of Patent Document 1 uses a spatial optical system, there is a problem that the configuration of the apparatus is complicated and the size cannot be reduced due to the necessity of arranging components necessary for the spatial optical system.

  Therefore, the present invention has been made in view of the above problems, and provides a chromatic dispersion measuring device and the like that can reliably and highly stably measure the chromatic dispersion of an optical pulse, which can reduce the size of the device. With the goal.

In order to solve the above problem, the invention according to claim 1 is an incident path (incident optical fiber 1) configured of an optical fiber having polarization maintaining characteristics that propagates an optical signal to be measured incident from a measurement target. And the measured optical signal is incident from the first incident end connected to the incident path, and the measured optical signal incident from the incident end is converted into the first measured optical signal and the first measured optical signal. On the other hand, it is separated into two of the second measured optical signals having the same polarization direction, the first measured optical signal is emitted from the first emission end, and the second measured optical signal is emitted from the second emission end. And an optical branching unit (optical branching unit 2) that generates a frequency difference between the first optical signal to be measured and the second optical signal to be measured when the optical signal is output, and the first optical output terminal. 1st optical path for propagating an optical signal to be measured and comprising an optical fiber having polarization maintaining characteristics (First optical fiber 3) and a second branch path (second optical fiber) that is connected to the second emission end and propagates the second optical signal to be measured and has polarization maintaining characteristics. 4) and the phase of the optical measurement optical signal propagating through the provided branch path provided in any one of the first branch path and the second branch path alternately with binary values of 0 degree and 90 degrees An optical phase shifter (optical phase shift unit 8) to be changed to, a first optical signal to be measured incident from a second incident end connected to the first branch path, and a second branch path The optical phase shifter obtained by combining the second measured optical signal incident from the third incident end and interfering with the first measured optical signal and the second measured optical signal is its own. Phase between the measurement optical signal of the provided branch path and the measurement optical signal of the other branch path The phase difference between the measurement optical signal of the branch path where the optical phase shifter is provided and the measurement optical signal of the other branch path. The second optical component (sin component) interference element in the case of 90 degrees is used as an optical signal to be measured as an optical signal to be measured. And a coupling path (coupling optical fiber 6) composed of an optical fiber for propagating the combined optical signal to be measured, and the combined measured signal from a fourth incident end connected to the coupling path. Increasing an optical signal, sweeping a frequency range (bandpass frequency width) through which the combined measured optical signal passes, and performing frequency decomposition to extract a spectral component of the frequency range from the combined measured optical signal, The result of frequency decomposition An optical frequency sweep unit (optical frequency sweep unit 9) that emits from the fourth output end as a signal, and an output optical path that is connected to the fourth output end and includes an optical fiber that propagates the component measured optical signal ( The component optical signal to be measured is incident from an outgoing optical fiber 10) and a fifth incident end connected to the outgoing optical path, the component optical signal to be measured is converted into an electric signal, and the conversion result is converted into an interference signal. And a control unit (control unit 12) that acquires the interference signals of the first light component and the second light component in time series in synchronization with a change in the phase of the optical phase shifter. ).

  In order to solve the above problem, the invention according to claim 2 is provided in any one of the first branch path and the second branch path, and the optical path length difference between the first branch path and the second branch path. It further has an optical delay unit (optical delay unit 7) for adjusting the light intensity.

  In order to solve the above problem, the invention according to claim 3 is that the optical delay unit is provided in one of the first branch path and the second branch path, and the optical phase shifter is provided in the other. Features.

  In order to solve the above problem, the invention according to claim 4 is characterized in that the optical delay unit and the optical phase shifter are integrally provided in one of the first branch path and the second branch path. Features.

  In order to solve the above-mentioned problem, the invention according to claim 5 is characterized in that the control unit receives the interference signal of the first light component and the first reception unit that receives the interference signal of the first light component. And a second receiving unit.

  In order to solve the above-described problem, in the invention according to claim 6, the control unit performs time series using the first light component and the second light component as a data pair as a measurement unit for each measurement frequency sweep in the measurement range. It is characterized by acquiring.

  In order to solve the above-mentioned problem, the invention according to claim 7 is characterized in that, in the first light component and the second light component constituting the data pair, a time for performing the phase change of the light component measured first, It is characterized in that it is set short with respect to the time for changing the phase of the light component to be measured later.

  In order to solve the above-mentioned problem, the invention according to claim 8 is that the control unit performs either one of the first light component or the second light component in one sweep in the measurement frequency sweep in the measurement range. The process of acquiring only one interference signal is performed alternately and repeatedly for the first light component and the second light component.

  In order to solve the above-described problem, the invention according to claim 9 is configured such that the coupling path is configured by an optical fiber having polarization maintaining characteristics, and the polarization direction of the combined optical signal to be measured is provided in the subsequent stage of the coupling path. And a controller for controlling the optical frequency, and the polarization controller and the optical frequency sweep unit are connected by an optical fiber having polarization maintaining characteristics.

  In order to solve the above problem, the invention according to claim 10 is characterized in that a calibration light source (calibration light source 103) that emits calibration light and the combined optical signal that is output from the output end of the optical multiplexing unit are sixth. A light input switching unit (light input switching unit 101) that enters from the incident end, enters the calibration light from the seventh incident end, selects either the combined light signal or the calibration light, and emits the light from the fifth emission end. ), The optical input switching unit is interposed between the optical multiplexing unit and the optical frequency sweeping unit, and the combined optical signal or the calibration emitted from the optical input switching unit Any one of the lights is incident on the fourth incident end of the frequency sweep unit via the second optical fiber (connection optical fiber 104).

  In order to solve the above-mentioned problem, the invention according to claim 11 is a chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 10, A branching unit is provided in a portion for evaluating the chromatic dispersion of the optical transmission line to be measured, and the polarization control unit controls the polarization of the optical signal to be measured obtained from the branching unit to a linearly polarized wave. Aligned with the polarization axis propagating in the measuring apparatus, the optical signal to be measured is incident on the wavelength dispersion measuring apparatus via the incident path, and the optical signal to be measured is obtained from the interference signals of the first and second light components A change in the spectral phase of the signal is obtained, and chromatic dispersion in the measurement object is evaluated.

  In order to solve the above problem, the invention according to claim 12 is a chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 10, An optical signal subjected to polarization control is incident on the incident end of the measurement target for evaluating chromatic dispersion, and the polarization of the measured optical signal emitted from the output end of the measurement target is controlled to be linearly polarized, Aligned with the polarization axis propagating in the chromatic dispersion measuring device, the measured optical signal is incident on the chromatic dispersion measuring device via the incident path, and the interference signal of the first light component and the second light component is A change in spectral phase of the optical signal to be measured is obtained, and chromatic dispersion in the measurement object is evaluated.

  In order to solve the above-mentioned problem, the invention according to claim 13 is a chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 10, An optical signal subjected to polarization control is incident on the incident end of the measurement target for evaluating chromatic dispersion, and the polarization of the measured optical signal emitted from the output end of the measurement target is controlled to be linearly polarized, Aligned with the polarization axis propagating in the chromatic dispersion measuring device, and when the measurement object is multiplexed with a plurality of wavelength channels, an optical signal of a single wavelength channel is extracted from the measured signal as the measured optical wavelength signal In the chromatic dispersion measuring device, the chromatic dispersion of the measurement object for each wavelength channel is evaluated for the measured optical wavelength signal.

According to the present invention, since the interferometer of the chromatic dispersion measuring device is configured by the first branch path and the second branch path of the optical fiber having the polarization maintaining characteristic without using the spatial optical system, the conventional example As described above, light loss due to input / output of light between the optical fiber and the spatial optical system does not occur, and the light intensity of the optical signal to be measured is not reduced.
Further, according to the present invention, the first branch path and the second branch constituting the interferometer are propagated through the apparatus while maintaining the polarization of the optical signal to be measured by the optical fiber having the polarization maintaining characteristic. By switching the phase difference of the second measured optical signal propagated to the second branch path to 0 degree and 90 degrees in time series for the first measured optical signal propagated to the first branched path in the path, The first optical component (cos component) from the first and second optical signals to be measured in the same polarization state which are stable, and the second optical component (sin component) having a phase different by 90 ° from the first optical component. Each interference element can be extracted, and the chromatic dispersion of the optical pulse can be measured with higher accuracy and higher sensitivity than in the past.
Further, according to the present invention, since the spatial optical system is not used, the configuration of the apparatus is simplified, and there is no need to arrange necessary parts as in the spatial optical system, and the apparatus can be miniaturized. Become.

It is a figure for demonstrating the measurement of variation | change_quantity (DELTA) phi ((nu)) of a spectral phase from an optical pulse. It is a block diagram which shows the structural example of the chromatic dispersion measuring apparatus by 1st Embodiment. In the first embodiment, the frequency sweep operation of the optical frequency sweep unit 9, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the sampling of the electrical signal from the light detection unit 11 in the control unit 12 It is a wave form diagram which shows the timing of operation | movement. In the second embodiment, the frequency sweep operation of the optical frequency sweep unit 9, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the sampling of the electrical signal from the light detection unit 11 in the control unit 12 It is a wave form diagram which shows the timing of operation | movement. It is a block diagram which shows the structural example of the chromatic dispersion measuring apparatus by 4th Embodiment. The frequency sweep operation of the optical frequency sweep unit 9 in the fifth embodiment, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the operation of the electrical signal sampling from the light detection unit 11 in the control unit 12 It is a wave form diagram which shows the timing of. It is a figure explaining the measuring method which measures the dispersion parameter of the optical pulse which propagates an optical fiber transmission line using the chromatic dispersion measuring apparatus of this invention. It is a figure explaining the measuring method which measures the dispersion parameter of the optical pulse which propagates an optical component using the wavelength dispersion measuring apparatus of this invention. It is a figure which shows the structural example for adjusting so that the polarization direction of the light which injects into the optical frequency sweep part 84 may correspond with the single polarization axis (for example, slow axis) of the optical fiber which comprises a band pass optical filter. . It is a figure explaining the measurement of the dispersion parameter of the optical pulse which generate | occur | produced with the optical signal generator using the chromatic dispersion measuring apparatus in any one of 1st Embodiment to 5th Embodiment. It is a block diagram which shows the structural example of the chromatic dispersion measuring apparatus by 10th Embodiment.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings. In this embodiment, for example, a main optical fiber transmission line between Tokyo and Osaka, a metro optical fiber network in an urban area, etc., and an optical pulse propagating through an optical fiber transmission line such as an optical network using wavelength division multiplexing transmission. It is an embodiment when the present invention is applied to a chromatic dispersion measuring apparatus for evaluating chromatic dispersion characteristics and a chromatic dispersion measuring method using the chromatic dispersion measuring apparatus.

[Chromatic dispersion]
First, chromatic dispersion measured by the chromatic dispersion measuring apparatus in the present embodiment will be described.

  In this embodiment, as described above, the spectral phase of an optical pulse propagating through an optical fiber transmission line is measured, and the characteristics of chromatic dispersion generated in the optical fiber transmission line are evaluated. In particular, this embodiment will be described with reference to an embodiment suitable for evaluating the chromatic dispersion characteristics of an optical fiber transmission line used in a high-speed optical communication system of about 10 GBit / s to 40 GBit / s.

  In order to evaluate the chromatic dispersion of the optical fiber transmission line, the frequency-wave number relationship, that is, the dispersion relationship is important. From this relationship, the speed at which light propagates through the optical fiber transmission line can be obtained. This speed refers to the speed at which the center of gravity of the light pulse moves, and is called “group speed”. The wavelength dependence of the group velocity represents chromatic dispersion.

  This group velocity is given as the slope (derivative coefficient) of the frequency-wavenumber characteristic curve. In vacuum and air, the frequency-wavenumber characteristic is a straight line, and the group velocity is constant regardless of the frequency. In a substance such as a metal, the frequency-wave number characteristic is not a straight line, and the group velocity changes according to the frequency. Since the optical fiber transmission line through which the optical pulse propagates is mainly made of glass, chromatic dispersion occurs according to the characteristics of the glass, and chromatic dispersion occurs according to the shape and dimensions of the core and cladding. The group velocity will change according to the frequency (which may be referred to as wavelength).

  Here, since the optical pulse includes not only a single wavelength but also various wavelength components, if the group velocity depends on the wavelength, the width of the optical pulse increases as it propagates in the optical fiber transmission line, and the optical pulse The waveform of the pulse is distorted, and the signals are overlapped by the front and rear optical pulses to cause crosstalk, resulting in an error.

Since chromatic dispersion increases in proportion to the length of a propagating optical fiber or other medium, the optical network becomes more widespread, and optical pulse distortion increases as the length of the path composed of optical fibers and optical components increases. Becomes a serious problem.
Therefore, compensating for chromatic dispersion is an important issue when constructing and operating an optical network. In order to compensate for the chromatic dispersion, it is necessary to evaluate the degree of the chromatic dispersion.

A dispersion parameter D, which is a chromatic dispersion characteristic of the optical fiber transmission line, is expressed by the following equation (1). The unit of the dispersion parameter is, for example, ps / nm / km. In this equation (1), Δτ _g is a group delay time difference, L is a distance through which light propagates, and Δλ is a wavelength difference.

In the present embodiment, the dispersion parameter D is, for example, the length of an optical fiber transmission line or an optical component. Since the distance L through which light propagates is known, the dispersion parameter D can be calculated if the group delay time difference with respect to the wavelength difference is obtained.
The wavelength difference is expressed as the following equation (2) by the frequency difference Δν. In equation (2), ν is the frequency and c is the speed of light.

  By substituting equation (2) into equation (1), equation (3) shown below is obtained.

By the way, the frequency of light is very high, and it is impossible to measure the vibration of the electric field of the light by electrical measurement, for example, the frequency of light having a wavelength of 1500 nm corresponds to 200 THz (terahertz). . Therefore, an interferometer is used as means for measuring the phase of light.
In this interferometer, incident light is split in two directions by a beam splitter, and each light passes through an independent path and is then combined again. The phase difference due to the divided light propagating through each path can be measured as the intensity of the interference light after the combination.

Therefore, in the present embodiment, a part of the optical pulse itself propagating through the optical fiber transmission line is extracted, and a part of the extracted optical pulse is Δν ₀ by a frequency shifter (AOFS: acousto-optic frequency shifter). Using the optical pulse shifted in frequency, the interference fringe obtained by interfering with the original optical pulse is polar-coordinated into intensity and phase, so that the frequency derivative of the phase of the original optical pulse can be obtained. Delay time can be measured.

Hereinafter, it demonstrates more concretely using figures.
FIG. 1A is a diagram showing the waveform of an optical pulse propagating through an optical fiber transmission line, and is a time waveform of an optical pulse with the horizontal axis representing time t and the vertical axis representing signal intensity I. In the example shown in the figure, it is assumed that the optical pulse repeats ON and OFF every 25 ps (40 Gbit / s). Assuming that the spectral phase after propagation through the optical fiber transmission line or optical component is φ (ν), and the change in spectral phase with respect to the frequency difference of Δν is Δφ (ν), the group delay time difference Δτ _g is expressed by the following equation ( 4). Here, the spectral phase is generally called optical pulse chirp, and describes how the phase changes as a function of frequency. Here, the phase generated by the optical fiber transmission line is described. It shows a change.

  By substituting Equation (4) into Equation (3), a relational expression represented by Equation (5) shown below is derived.

  The dispersion parameter D in the optical fiber transmission line or the optical component is obtained by calculating the change in the spectral phase with respect to the frequency difference Δν with respect to the optical pulse propagated in the optical fiber transmission line or the optical component by the equation (5). Thus, chromatic dispersion can be evaluated. Since higher-order chromatic dispersion such as dispersion slope is expressed as the frequency dependence of the dispersion parameter, chromatic dispersion of all orders can be evaluated by equation (5).

FIG. 1B shows the dependence characteristic of the phase ν of the optical pulse on the frequency ν. The optical pulse of the optical fiber transmission line to be measured has a second-order dispersion, and the phase changes as shown by a parabola having an upward convex shape as shown in FIG. When the frequency is slightly shifted by Δν _{0, the} spectral phase also changes by Δφ due to this shift, as shown by the dotted line in the figure.
The value of the change Δφ in the spectral phase is equivalent to a value obtained by differentiating the original optical pulse (solid line in the figure) that has not been frequency shifted. Therefore, the group delay time difference Δτ _g can be obtained by dividing the spectral phase change Δφ by the frequency shift amount Δν ₀ as shown in the equation (4).

[Spectral shearing interferometer]
In a spectrum shearing interferometer, an input optical pulse is branched into two with the same polarization direction maintained by two branch paths provided in the interferometer, and propagates through one of the branch paths. Is given a frequency shift.
FIG. 1C shows an intensity spectrum waveform of an optical pulse. This figure shows the dependence of the frequency ν on the electric field R. As shown by the solid line, the original optical pulse that has not been frequency shifted has a spectrum in which the electric field R is maximum (peak) at the center frequency ν _0. I understand. On the other hand, when the frequency is slightly shifted by Δν ₀ (illustrated by a dotted line), the peak of the intensity spectrum is shifted, but the waveform does not change. That is, since the value of the electric field R does not change even if the frequency is slightly shifted, the power spectrum of the original optical pulse indicated by the absolute value is approximated by the power spectrum of the optical pulse whose frequency is slightly shifted by Δν _0. be able to.

For this reason, the spectrum of the interference fringe caused by this frequency shift is acquired, and the change Δφ (ν) of the spectral phase with respect to the frequency difference Δν shown in FIG. 1B, that is, a value equivalent to the value obtained by differentiating the optical pulse. Can be measured as As a result, the dispersion parameter can be calculated by substituting the obtained spectral phase change Δφ (ν) into the equation (5) to evaluate the chromatic dispersion. For more information on this spectral shearing interferometer, see References (OPTICS LETTERS Vol.19, No.4, pp.287-289, February 15, 1994, "Analysis of ultrashort pulse-shape measurement using linear interferferometers"), VictorWong and Ian Walmsley).
Hereinafter, in addition to the configuration shown in the above-mentioned reference, the basic principle of chromatic dispersion evaluation by the spectrum shearing interferometer according to the present embodiment including unambiguous phase measurement by detecting orthogonal two components will be described. .

An optical pulse that has propagated through an optical fiber transmission line or an optical component to be evaluated for chromatic dispersion is used as an optical signal to be measured, and the optical signal to be measured is kept in the same polarization direction by the optical branching unit. Separate into branch paths. The optical signal under measurement propagating to one of these two branch paths is phase-shifted by π / 2, and the first optical signal to be measured propagating through two branch paths, for example, the first branch path and the second optical path propagating through the second branch path. The interferometer is configured to multiplex the two optical signals to be measured.
The time waveform of the electric field of the first optical signal to be measured propagating through the first branch path is expressed by the following equation (6). In this equation (6), t is time and ν ₀ is the center frequency of the signal under measurement.

In this equation (6), the time waveform of the first optical signal to be measured representing the orthogonal two components is shown, the upper row is the cos component which is one of the orthogonal two components, and the lower row is the orthogonal two components. The sin component which is the other component of is shown. Each of | E1 cos (t) | and | E1 sin (t) | represents the absolute value of the envelope of the electric field. Here, the cos component, which is the first optical component in the first optical signal to be measured, and the sin component, which is the second optical component having a phase different by π / 2 with respect to the cos component, have the same polarization direction.
Ψ is a phase in time domain notation and includes a term related to chromatic dispersion. In the equation (6), the phase component depending on the signal modulation format is omitted.
In the spectrum interferometer, the time waveform of the electric field of Expression (6) is dispersed, that is, Fourier transformed, and converted to a spectrum with the center frequency ν ₀ as the origin, and the following Expression (7) is obtained.

Next, a frequency shift of Δν ₀ is given to the second measured optical signal propagating through the second branch path with respect to the center frequency ν ₀ . This frequency shift Δν ₀ is very small, there is the relationship of ν ₀ »Δν _0. The time waveform of the electric field of the second optical signal to be measured propagating through the second branch path is expressed by the following equation (8). Corresponding to the first measured optical signal, the phase difference of the cos component in the second measured optical signal is 0 degree and the phase difference of the sin component is 90 degrees, and the sin component has a phase difference of π / 2. Is added. Here, in order to obtain the power spectrum of the interference component (interference fringe) by the sin component, a phase difference of π / 2 is given to the second optical signal to be measured, and the sin component is used. Further, the cos component, which is the first light component in the second optical signal to be measured, and the sin component, which is the second light component having a phase different by π / 2 with respect to the cos component, have the same polarization direction.

When the above equation (8) is Fourier transformed into a spectrum with the center frequency ν ₀ as the origin, the following equation (9) is obtained.

As described above, since the frequency shift Δν ₀ is very small, the following approximate expression of Expression (10) is applied to each of the cos component and the sin component in Expression (9).

  The first optical signal to be measured propagating on the first branch path and the second optical signal to be measured propagating on the second branch path are combined again by the coupling unit, and the combined optical signal to be measured is optically detected after the multiplexing. When detected by the unit, a power spectrum due to interference between the first measurement optical signal and the second measured optical signal is obtained. The power spectra of the interference components of the cos component and the sin component after recombination are represented as | Ecos (ν) | and | Esin (ν) |, respectively, and the polarization directions in the electric field spectra of the equations (7) and (9) are the same. The cos components of the first optical signal to be measured and the second optical signal to be measured, and the sin components are overlapped to obtain the square of the absolute value, thereby obtaining the interference component of the cos component and the interference component of the sin component. A power spectrum is calculated | required like the following formula | equation (11).

In the above equation (11), assuming that the frequency shift Δν ₀ is equal to the frequency difference Δν in the equations (4) and (5), the term representing the phase difference as in the following equation (12) is the frequency difference Δν. It is assumed that it is equal to the change Δφ (ν) of the spectral phase with respect to.

  Next, the equation (11) is modified to obtain the terms of the cos component and the sin component corresponding to the change amount Δφ (ν) of the spectrum phase as shown in the following equation (13). Here, when performing the recombination, the polarization directions of the first measured optical signal and the second measured optical signal are the same, and the cos component and the sin in the first measured optical signal and the second measured optical signal The polarization direction of the component is also the same.

The interference fringe component in the interference between the cos component and the sin component in the first measured optical signal and the second measured optical signal is included in | Ecos (ν) | and | Esin (ν) | in Expression (13). .
Further, in this embodiment, when recombining the first measured optical signal and the second measured optical signal to obtain the above equation (13), the first measured optical signal and the second measured light One of the signals is alternately shifted in phase between 0 degrees and 90 degrees with respect to the other. This phase shift is performed by alternately applying a phase shift voltage, which is a voltage for controlling the amount of phase shift, to the optical phase shift unit that is an optical phase shifter. Even after this phase shift, the polarization directions of the first measured optical signal and the second measured optical signal are the same.
As a result, the cos component (upper stage) and the sin component (lower stage) in the signal under measurement of Expression (8) are obtained alternately. In this embodiment, the optical phase shift unit is provided in the second branch path, and as described above, the phase shift voltage is applied to the phase of the first optical signal to be measured that propagates through the first branch path. The phase difference of the second optical signal to be measured propagating through the second branch path is alternately changed by two values of 0 degree (cos component acquisition) and 90 degrees (sin component acquisition). As a result, in the multiplexing of the first measured optical signal and the second measured optical signal, the cos component interference component and the sin component interference component are alternately obtained. Then, by substituting the power spectrum of the interference component of the pair of cos component and sin component obtained alternately into the following equation (14), the change amount Δφ (ν) of the spectrum phase with respect to the frequency difference Δν is It is uniquely determined in the range of 0 to 2π by the tan-1 of the valence function. Here, the pair of the cos component and the sin component is a unit for obtaining the change Δφ (ν) of the spectrum phase for each frequency.

  In Expression (14), in order to obtain the rightmost expression, the cos component in each of the first measured optical signal that propagates through the first branch path and the second measured optical signal that propagates through the second branch path, and The following equation (15) was applied assuming that the power of the sin component was equal.

The spectral phase change Δφ (ν) is periodically folded in the range of 0 to 2π, and is unfolded by unwarp processing to release the phase folding.
As described above, the spectral phase change Δφ (ν) with respect to the frequency difference Δν is measured using the spectrum shearing interferometer, and the dispersion parameter D is calculated by substituting it into the equation (5) to transmit the optical fiber. Evaluation of chromatic dispersion characteristics in the road.

On the other hand, in the measurement of the dispersion parameter D using a spectrum interferometer, the dispersion parameter D can be calculated by obtaining a group delay time by differentiating the measured spectrum phase φ (ν). However, when this frequency differentiation is performed, the measurement noise in the spectrum phase is also differentiated at the same time, and sharp spike noise obtained by differentiating the measurement noise is superimposed on the group delay time, thereby reducing the accuracy in calculating the dispersion parameter. It will be.
As described above, the spectrum interferometer has a drawback that the dispersion parameter cannot be accurately detected. For this reason, in the present invention, the spectral dispersion is measured with high accuracy by measuring the change Δφ (ν) of the spectral phase with respect to the frequency difference Δν using a spectrum shearing interferometer.

[Configuration and Function of Chromatic Dispersion Measuring Apparatus] <First Embodiment>
Next, the configuration and function of the chromatic dispersion measuring apparatus according to the present embodiment will be described with reference to FIG. FIG. 2 is a block diagram illustrating a configuration example of the chromatic dispersion measuring apparatus according to the present embodiment.
The chromatic dispersion measuring apparatus includes an incident optical fiber 1 as an incident path, an optical branching unit 2, a first optical fiber 3 as a first optical branching path, a second optical fiber 4 as a second optical branching path, and an optical coupling unit 5. , A coupling optical fiber 6 as a coupling path, an optical delay unit 7, an optical phase shift unit 8 as an optical phase shifter, an optical frequency sweep unit 9, an outgoing optical fiber 10 as an outgoing light path, a light detection unit 11, a control Unit 12, phase control line 13, frequency control line 14, and detection control line 15. The optical path length of the first optical fiber 3 and the second optical fiber 4 is the interferometer length of the spectrum shearing interferometer.

  The incident optical fiber 1 has one end that receives an optical pulse from an optical fiber transmission line or an optical component that is an evaluation target for evaluating chromatic dispersion, and the other end that is connected to the incident end (first incident end) of the optical branching unit 2. ing. Here, an optical pulse propagated through an optical fiber transmission line or an optical component to be evaluated for evaluating chromatic dispersion is made incident through the incident optical fiber 1, and the incident optical pulse is used as an optical signal to be measured.

The optical branching unit 2 branches the optical signal to be measured input from the incident end in two directions, with respect to the first optical fiber 3 having one end connected to one outgoing end (first outgoing end). The light beam is guided as a first measured light signal, and the other light beam is guided as a second measured light signal to the second optical fiber 4 having one end connected to the other emitting end (second emitting end). Here, the first optical signal to be measured has a time waveform shown in Expression (6) and has a frequency spectrum shown in Expression (7). The second optical signal to be measured has a time waveform shown in Expression (8) and has a frequency spectrum shown in Expression (9).
Further, the optical branching unit 2 includes a first measured optical signal that is emitted from one emission end to the first optical fiber 3 and a second measured optical signal that is emitted from the other emission end to the second optical fiber 4. In the meantime, a carrier frequency difference is generated.

In the present embodiment, as a result of the occurrence of the carrier frequency difference, for example, it is emitted to the second optical fiber 4 so as to have a frequency different from the frequency of the first optical signal to be measured emitted to the first optical fiber 3. A frequency shift Δν ₀ is given as a carrier frequency difference to the second optical signal to be measured. On the other hand, the first measured optical signal emitted to the first optical fiber 3 has no frequency change.

The optical branching unit 2 uses, for example, an acousto-optic frequency shifter. A zero-order light output port of the acousto-optic frequency shifter is connected to one end of the first optical fiber 3, and a primary light output port is connected to one end of the second optical fiber 3. Acousto-optic frequency shifter, when the high frequency of .DELTA..nu ₀ is supplied, outputs a first measured optical signal from the zero-order light output ports are not frequency-shifted, whereas, frequency by frequency .DELTA..nu ₀ from the primary light output port The shifted second optical signal to be measured is output. Since the optical branching unit 2 recombines in an optical coupling unit 5 described later to acquire an interference component, the optical branching unit 2 emits the first and second measured optical signals with the same polarization direction.

The optical coupling unit 5 has one incident end (second incident end) connected to the other end of the first optical fiber 3 and the other input end (third incident end) connected to the other end of the second optical fiber 4. Has been. Further, the optical coupling unit 5 has an output end (third output end) connected to the coupling optical fiber 6.
The optical coupling unit 5 combines the first measured optical signal input from one incident end and the second measured optical signal input from the other incident end, and combines the combined measured signals. An optical signal is emitted from the emission end to the coupling optical fiber 6.

Further, an optical delay unit 7 is inserted in the path of the first optical fiber 3. The optical delay unit 7 is provided in an optical fiber having a shorter optical path length than the other for the purpose of making the optical path length difference between the first optical fiber 3 and the second optical fiber 4 the same. A delay for adjustment to be eliminated is given to the optical signal under measurement.
However, by providing the optical delay unit 7 to eliminate the optical path length difference, fluctuations in the optical path length occurring between the first optical fiber 3 and the second optical fiber 4 can be reduced. The measurement accuracy of the change Δφ (ν) in the spectral phase at can be improved.
It is not necessary to provide the optical path length difference between the first optical fiber 3 and the second optical fiber 4 as long as it does not affect the measurement accuracy.

  An optical phase shift unit 8 is interposed in the path of the second optical fiber 4. The optical phase shift unit 8 alternately shifts the phase of the second optical signal to be measured propagating through the second optical fiber 4 to 0 degrees and 90 degrees in a constant first period. That is, the optical phase shift unit 8 sets the phase difference between the first measured optical signal propagating through the first optical fiber 3 and the second measured optical signal propagating through the second optical fiber 4 to a constant first. It is alternately switched between 0 degrees and 90 degrees in one cycle. Here, the optical phase shift unit 8 shifts the phase of the second optical signal to be measured with respect to the first optical signal to be measured, but the polarization direction of the second optical signal to be measured is changed to the first optical signal to be measured even after the shift. It is emitted as the same optical signal.

As a result, when the phase difference between the first measured optical signal propagating through the first optical fiber 3 and the second measured optical signal propagating through the second optical fiber 4 is 0 degree, the cos component detection mode, 90 The case of the degree can be set to the sin component detection mode. A cos component and a sin component in the first optical signal to be measured are expressed by Expression (6). Further, the second measured optical signal having a phase difference of 0 degree is defined as a cos component, and the signal having a phase difference of 90 degrees is defined as a sin component, which are represented by an upper stage and a lower stage of Equation (8), respectively.
Here, as described above, the polarization directions of the first measured optical signal and the second measured optical signal emitted from the optical branching unit 2 are the same, and the optical phase with respect to the first measured optical signal. The polarization direction of the second optical signal to be measured whose phase difference is 0 degree or 90 degrees by the shift unit 8 is also the same. Therefore, by switching the phase difference generated in the optical phase shift unit 8 between 0 degrees and 90 degrees, it is possible to select which of the cos component and the sin component in the two orthogonal components is detected. In the present embodiment, when the second measured optical signal is shifted by 0 degree with respect to the first measured optical signal, interference of the cos component of the first measured optical signal and the second measured optical signal occurs. When the second measured optical signal is shifted by 90 degrees with respect to the first measured optical signal, interference of the sine components of the first measured optical signal and the second measured optical signal occurs. That is, when the phase shift difference of the second measured optical signal is 0 degree, the optical coupling unit 5 uses the interference component in the cos component of the first measured optical signal and the second measured optical signal as the combined measured optical signal. When the phase shift difference is 90 degrees, the interference component in the sine component of the first measured optical signal and the second measured optical signal is output as a combined measured optical signal.

For example, a phase shifter using an electro-optic crystal (for example, LiNb03) can be used for the optical phase shift unit 8, and the phase shift voltage (V ₀ , V ₉₀ , which will be described later) is changed to change the phase. Can be switched between 0 degrees and 90 degrees.
In this embodiment, the optical delay unit 7 is connected to the first optical fiber 3 and the optical phase shift unit 8 is connected to the second optical fiber 4. The optical delay unit 7 is inserted into an optical fiber having a short optical path length among the fiber 3 and the second optical fiber 4, and the optical phase shift unit 8 is connected to the other.
As described above, by inserting each of the optical delay unit 7 and the optical phase shift unit 8 into the optical paths of different optical fibers, residual reflected light is transmitted between the optical delay unit 7 and the optical phase shift unit 8. Can be prevented from reciprocating. For this reason, it is possible to remove the spectrum ripple generated by the reciprocal resonance of the residual reflected light.

  The optical frequency sweep unit 9 has an incident end (fourth incident end) connected to the other end of the coupling optical fiber 6, and an emission end (fourth emission end) connected to one end of the emission optical fiber 10. The optical frequency sweep unit 9 is, for example, a tunable bandpass filter, and a predetermined measurement frequency range is generated by a trigger signal indicating the start of a frequency sweep period (a frequency sweep period within a set measurement frequency range). A sweep is performed to change the frequency at. The optical frequency sweep unit 9 changes the center frequency of the band pass frequency width to pass through in a time series within the range of the measurement frequency. Further, the optical frequency sweep unit 9 performs a process of extracting an interference element having a frequency corresponding to the bandpass frequency width from the combined optical signal to be measured incident from the coupling optical fiber 6, that is, the combined optical signal to be measured. Perform frequency resolution. The optical frequency sweep unit 9 emits a component-measured optical signal after frequency decomposition (a signal indicating a spectral intensity for each frequency) to the outgoing optical fiber 10 from the outgoing end. The frequency used for frequency number resolution of the combined optical signal to be measured that is obtained by combining the first optical signal to be measured and the second optical signal to be measured is the center frequency in the bandpass frequency range. By the frequency decomposition, an interference element (interference component) of a cos component or a sin component for each frequency is detected.

The light detection unit 11 has an incident end (fifth incident end) connected to the other end of the outgoing optical fiber 10.
The light detection unit 11 converts the component measured optical signal incident from the output optical fiber 10 into an electrical signal, and outputs the conversion result as an interference signal to the control unit 12.
Here, when the optical phase shift unit 8 sets the phase shift amount to 0 degree, the component measured optical signal is an interference element of the cos component of the corresponding frequency, and the optical phase shift unit 8 sets the phase shift amount. In the case of 90 degrees, it is an interference component of the sin component of the corresponding frequency.

  In the present embodiment, each of the incident optical fiber 1, the first optical fiber 3, and the second optical fiber 4 uses a polarization maintaining optical fiber (PMF) having polarization maintaining characteristics. . The polarization axes of the incident optical fiber 1, the first optical fiber 3, and the second optical fiber 4 are all aligned in the same direction, and the polarization directions of the first measured optical signal and the second measured optical signal are changed. It is made the same and it is made to inject into the optical coupling part 5. FIG. For this reason, the optical signal to be measured uses a polarization controller (not shown), and the polarization of the optical signal to be measured immediately after propagation through the optical fiber transmission line to be evaluated for chromatic dispersion is linearly polarized and its polarization axis is incident. After being aligned with the polarization axis (for example, the slow axis) of the optical fiber 1, the light enters the incident optical fiber 1. Also, a polarization maintaining optical fiber may be used for the coupling optical fiber 6 and the outgoing optical fiber 10.

The control unit 12 is applied to the optical phase shift unit 8 every first cycle in synchronization with a trigger signal indicating the start point of the frequency sweep cycle input from the optical frequency sweep unit 9 via the frequency control line 14. The phase shift voltage is supplied to the optical phase shift unit 8 via the phase control line 13 in order to change the phase shift voltage alternately to perform the phase shift.
That is, when the measurement frequency is n points, the cos component and the sin component are paired with respect to one frequency, so that the voltage is alternately switched every first cycle obtained by dividing the frequency sweep cycle by 2n. Is performed in synchronization with the trigger signal.
In addition, the control unit 12 alternately receives cos component and sin component interference signals from the light detection unit 11 via the detection signal line 15 in synchronization with the first period. Then, the control unit 12 uses the cos component and sin component interference elements acquired in time series as a pair and uses them as power spectrum data for calculating dispersion parameters at each frequency.

Next, the control unit 12 uses the level of the input interference signal as the power spectrum, obtains the square of the power spectrum of the cos component and the power spectrum of the sin component, and obtains the power spectrum of Expression (11).
Further, the control unit 12 obtains a power spectrum of a pair of cos components and sin components for each frequency by using the equation (13) obtained by modifying the equation (11). Δφ (ν) can be obtained. Then, the control unit 12 calculates the dispersion parameter for each frequency by substituting this phase change Δφ (ν) into the equation (5).
As described above, in this embodiment, the start of the frequency sweep in the measurement frequency range is notified by the trigger signal supplied from the optical frequency sweep unit 9.

Further, the control unit 12 sets 2n times the first cycle as a frequency sweep cycle, generates a trigger signal indicating the start of this frequency sweep cycle, outputs a trigger signal to the optical frequency sweep unit 9, and in the range of measurement frequencies, A configuration for controlling frequency sweeping may be employed.
In addition, by using the trigger signals for the start point and end point of the frequency sweep cycle without making the frequency sweep cycle continuous, it is not necessary to set the start point and end point at the same time. During this period, a time for returning the frequency to the initial value of the sweep can be provided. For this reason, as in the case where the end point and the start point are the same, in order to change from the start point to the initial value, the first first period in the frequency sweep period may be shortened depending on the frequency change time. As a result, the measurement time can be controlled with higher accuracy.

Next, the operation of measuring the optical signal under measurement of the chromatic dispersion measuring apparatus shown in FIG. 1 in the present embodiment will be described with reference to FIG. FIG. 3 shows the timing of the frequency sweep operation of the optical frequency sweep unit 9, the phase shift operation of the optical phase shift unit 8 corresponding thereto, and the sampling of the interference signal from the light detection unit 11 in the control unit 12. FIG.
That is, FIG. 3A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 9 with the vertical axis representing voltage and the horizontal axis representing time. In FIG. 3A, the H level (V _H ) and L level (V _L ) of the trigger signal output from the optical frequency sweep unit 9 are controlled using TTL control (TTL (Transistor Transistor Logic) interface). ).

In FIG. 3B, the vertical axis represents frequency, the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 9. In FIG. 3B, ν ₁ is a sweep start frequency (the lowest frequency in the measurement frequency range), and ν ₂ is a sweep stop frequency (the maximum frequency in the measurement frequency range). For this reason, the frequency ν ₁ to the frequency ν ₂ are in the measurement frequency range, that is, the frequency sweep range.
FIG. 3C is a diagram illustrating a waveform of a phase shift voltage in which the vertical axis is voltage, the horizontal axis is time, and the phase difference applied to the optical phase shift unit 8 is changed in the first period. The phase shift voltage V ₀ is a voltage when the phase difference is 0 degree (cos component), and the phase shift voltage V ₉₀ is a voltage when the phase difference is 90 degrees (sin component). The time for applying the phase shift voltage of the cos component and the sin component is the same period, that is, the first period Δt.
FIG. 3D is a diagram illustrating a sampling cycle timing at which the vertical axis represents voltage, the horizontal axis represents time, and the control unit 12 receives an interference signal from the light detection unit 11 as time-series data.
3 (c) and 3 (d), in order to clearly describe the first period, the horizontal time scale of FIG. 3 (a) and FIG. Is shown.

The optical frequency sweep unit 9 generates a trigger signal and outputs the trigger signal to the control unit 12 in the frequency sweep period of time “T _{1 + 1} −T ₁ ” and performs frequency decomposition of the combined optical signal to be measured. Therefore, a sweep process for linearly increasing the frequency from the frequency ν ₁ to the frequency ν ₂ is started. Here, if the user measures the sweep change before the actual measurement and finds that the linearity of the swept frequency over time is not achieved, the frequency of the sweep is calibrated and the frequency sweep nonlinearity is corrected To do. In this embodiment, the frequency is swept from the low frequency side to the high frequency side. However, the frequency may be swept from the high frequency side to the low frequency side. The timing control is not limited to TTL control, and for example, a CMOS (Metal Oxide Semiconductor) interface may be used.

When the trigger signal is supplied, the control unit 12 alternately outputs the phase shift voltage V ₀ and the phase shift voltage V ₉₀ to the optical phase shift unit 8 every first period Δt in synchronization with the trigger signal. Start processing. In the present embodiment, the voltage is supplied from the phase shift voltage V _0, but it may be supplied from the phase shift voltage V ₉₀ .
As a result, the optical phase shift unit 8 sets the phase of the second measured optical signal propagating through the second optical fiber 4 to 0 degree or 90 degrees by the supplied phase shift voltage V ₀ and phase shift voltage V _90. Change.
For each first period Δt, the light detection unit 11 alternately uses a component measured optical signal having a cos component interference element and a component measured optical signal having a sin component interference element at each frequency as an interference signal. Supply to the control unit 12.

Then, in synchronization with the first period Δt, the control unit 12 samples the component measured optical signal at the center portion of the first period Δt, for example, so that the component having the cos component interference element at each frequency alternately. A pair of interference signals of the optical signal to be measured and the component optical signal to be measured having a sin component interference element can be obtained. That is, by changing the phase of the second optical signal to be measured from 0 degrees to 90 degrees, a pair of cos component and sin component interference elements can be obtained. As a result, n cos component and sin component interference element pairs are obtained from the 2n first period Δt within the measurement frequency range.
As a result, as described above, the control unit 12 calculates the phase change Δφ (ν), and calculates the dispersion parameter using the equation (14) based on the phase change Δφ (ν). .

As described above, the control unit 12 corresponds to the discrete data, that is, the first period Δt, with respect to the change in the spectrum power and the spectrum phase at each frequency of the component optical signal to be measured, according to the sampling period shown in FIG. The component measured signal at the frequency output from the optical frequency sweeping unit 9 is acquired at the timing as a sampled interference signal.
At this time, since the optical frequency sweeping unit 9 sweeps by gradually changing the amount of change in the frequency with respect to the first period Δt for switching the phase shift voltage, the frequency generated between the pair of measurements of the cos component and the sin component. The change is slight, and it can be considered that the frequency is constant between a pair of data.
For example, when 1000 points are measured alternately for each of the cos component and the sin component, the frequency change in the measurement frequency range accompanying switching of the phase shift voltage is only 1/2000 of the entire frequency sweep range. If the frequency sweep cycle T _{1 + 1} -T ₁ is 1 s (seconds), the first cycle Δt, which is the phase shift voltage switching time, is 0.5 ms.
The control unit 12 sets the sampling period of the component optical signal to be measured to 0.5 ms similarly to the first period, and synchronizes the data sampling timing with the switching of the phase shift voltage. Here, the sampling timing is delayed by, for example, Δt / 2 with respect to the change timing of the phase shift voltage so that the component measured optical signal can be reliably sampled.

Sampling of the 2000 component optical signals to be measured is alternately assigned to sampling of the cos component and the sin component by 1000 points. As a result, the sampling interval of two orthogonal components (sampling interval = first period × 2) is 1/1000 of the entire frequency sweep range (frequency measurement range). When the number of sampling points is fixed, the phase shift voltage output from the optical phase shift unit 8 and the sampling period of the component measured optical signal are proportional to the frequency sweep period. That is, the first period Δt is obtained by dividing the frequency sweep period by the number of frequencies corresponding to the resolution of the frequency to be measured.
Therefore, if the frequency sweep period is shortened, the sampling period is correspondingly shortened. When the spectrum shape is complicated, it is necessary to further increase the number of sampling points and improve the resolution. Even when this resolution is improved, the sampling period is shortened in the case of the same frequency sweep period.

Further, it is assumed that the optical signal to be measured follows an ITU (International Telecommunication Union) grid with an interval of 100 GHz, and the frequency sweep range ν ₂ −ν ₁ is 100 GHz.
For example, when the measured optical signal is assigned to the C band 31st channel of the ITU grid, the frequency ν ₁ and the frequency ν ₂ are 193.05 THz and 193.15 THz, respectively.
Each of the cos component and the sin component is acquired at 1000 points, and is set to 100 MHz as a sampling interval of orthogonal two components, that is, a cycle twice the first cycle Δt.
In order to improve the measurement accuracy of the change Δφ (ν) in the spectral phase, it is necessary to obtain the phase at each sampling point with high accuracy. For this reason, it is preferable that the band pass frequency width in the optical frequency sweep unit 9 be narrower than half the sampling interval.
The narrower the bandpass frequency width, the higher the resolution of frequency resolution, and the change Δφ (ν) of the spectrum phase with respect to the frequency can be measured with high accuracy.
However, by reducing the bandpass frequency width, the amount of light incident on the light detection unit 11 is reduced, and the influence of measurement noise is increased.

In this embodiment, the bandpass frequency width is set to 1/4 of the sampling frequency, that is, 25 MHz. At this time, the finesse of the bandpass optical filter used for the optical frequency sweep unit 9 is a value obtained by dividing the peak interval by the full width at half maximum of the transmission peak, that is, 4000.
When phase fluctuation is measured using a heterodyne spectrum interferometer, the time constant required for the phase to fluctuate about 180 degrees is about 5 s when the length of the optical fiber used for the path of the interferometer is about 1 m. Therefore, when the frequency sweep period is set to 1 s, it is expected that the influence of the phase fluctuation of the interferometer is small.
When it is necessary to further reduce the phase fluctuation in order to improve the measurement accuracy, the frequency sweep cycle may be further shortened. For example, by reducing the frequency sweep period to about 0.1 s, the influence of phase fluctuation in frequency dispersion can be ignored.

In the present embodiment, the optical frequency sweep unit 9 has been described as linearly sweeping the frequency. However, the sweep is stopped and the quadrature two components are measured during the sampling of the pair of cos component and sin component. You may make it perform a sweep in steps so that it may become the same frequency during a period.
With this configuration, there is no frequency fluctuation between the pair of cos component and sin component, the measurement accuracy of the interference signal can be improved, and the phase change Δφ (ν) can be obtained with high accuracy.

According to the above-described configuration, an interferometer is configured using an optical fiber having polarization maintaining characteristics, and the orthogonal two components are maintained in a stable state. Therefore, it is not necessary to spatially arrange necessary parts, and the structure can be simplified and the apparatus can be downsized.
Furthermore, since the interferometer is configured without using a spatial optical system, there is no loss of light in the input / output of light between the optical fiber and the spatial optical system, and a decrease in light intensity is suppressed. The chromatic dispersion can be measured while maintaining the measurement sensitivity.

<Second Embodiment>
Next, a chromatic dispersion measuring apparatus according to the second embodiment will be described. The second embodiment has the same configuration as the first embodiment, but in the configuration of FIG. 2, the cos component and the sin component are received in parallel by the two reception ports provided in parallel to the control unit 12. It has the composition to do.
FIG. 4 shows the timing of the frequency sweep operation of the optical frequency sweep unit 9, the phase shift operation of the optical phase shift unit 8 corresponding thereto, and the sampling of the interference signal from the light detection unit 11 in the control unit 12. FIG.

FIG. 4A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 9, where the vertical axis is voltage and the horizontal axis is time. In FIG. 4A, the H level and L level of the trigger signal output from the optical frequency sweep unit 9 are set so as to conform to the TTL control.
In FIG. 4B, the vertical axis represents frequency, the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 9. In FIG. 4B, ν ₁ is a sweep start frequency (the lowest frequency in the measurement frequency range), and ν ₂ is a sweep stop frequency (the maximum frequency in the measurement frequency range).
FIG. 4C is a diagram illustrating a phase shift voltage waveform in which the vertical axis is voltage, the horizontal axis is time, and the phase difference applied to the optical phase shift unit 8 is changed in the first period. The phase shift voltage V ₀ is a voltage when the phase difference is 0 degree (cos component detection mode), and the phase shift voltage V ₉₀ is a voltage when the phase difference is 90 degrees (sin component detection mode). The time for applying the phase shift voltage of the cos component and the sin component is the same period, that is, the first period Δt.
In FIG. 4D, the vertical axis represents voltage, the horizontal axis represents time, and the control unit 12 receives the interference signal from the light detection unit 11 as time-series data in parallel from the reception port P1 and the reception port P2. It is a figure which shows the timing of the sampling period to perform. In the present embodiment, the reception port P1 receives the component measured optical signal of the cos component, and the reception port P2 receives the component measured optical signal of the sin component.
4 (c) and 4 (d), in order to clearly describe the first period, the horizontal time scale of FIG. 4 (a) and FIG. 4 (b) is expanded, and only a part of the time range is displayed. Is shown.

Control unit 12, while outputting the phase-shifted voltage V _0, when the photodetecting section 11, which receives the component measured optical signal by the receiving port P1, and outputs a phase shift voltage V _90, photodetector The component measured optical signal is received from the unit 11 through the reception port P2.
In the second embodiment, as in the first embodiment, the frequency sweep period is acquired at 1000 points for each of the cos component and the sin component. The frequency sweep cycle is 1 s as in the first embodiment, and the first cycle Δt, which is the phase shift voltage switching time, is 0.5 ms. The sampling period of each of the reception port P1 and the reception port P2 is 1 ms, and the sampling timing is shifted by 0.5 ms between the reception port P1 and the reception port P2.

The control unit 12 performs A / D (analog / digital) conversion, and acquires the voltage level of the interference signal from the light detection unit 11 as digital data.
Therefore, the operation speed of the A / D conversion circuit in the control unit 12 may be a limiting factor when it is desired to shorten the sampling period. However, in this embodiment, by adopting two systems of parallel reception of the reception port P1 and the reception port P2, the sampling rate of each reception port is half that of reception by only one port, so A / D conversion The limit on the operating speed of the circuit can be increased by a factor of two.
In addition, since it is not necessary to alternately distribute the measurement timings of the cos component and the sin component in one reception port, the data processing program is simplified and the data processing speed can be improved.

<Third Embodiment>
Next, a chromatic dispersion measuring apparatus according to the third embodiment will be described. The third embodiment has the same configuration as that of the first embodiment or the second embodiment, except that the period during which the phase shift voltage is supplied (first period) is measured with the cosine component, the sin component, The length of time is different in the case of measuring.
That is, in the measurement of the component optical signal to be measured, it is not necessary to measure the cos component and the sin component that determine the phase change Δφ (ν) with the same time width in the sampling period.
When the control unit 12 acquires interference signals of two orthogonal components from the light detection unit 11 in the order of the cos component and the sin component, the pair of sampling periods of the cos component and the sin component in the first embodiment are left as they are, The time width for applying the phase shift voltage supplied to the optical phase shift unit 8 is shortened for the measurement time of the cos component, and this reduced time is added to the measurement time for the sin component.
Thereby, the measurement interval between the paired cos component and sin component can be shortened, the amount of change in frequency associated with the time series switching of the cos component and sin component can be reduced, and the frequency accuracy of measurement can be improved.
Therefore, it is possible to improve the measurement accuracy of the phase change Δφ (ν) measured in frequency units and obtain the dispersion parameter with high accuracy.
Here, the time for changing the phase of the light component measured before either the cos component or the sin component (that is, the time for applying the phase shift voltage) is the phase of the other light component measured later. Set it shorter than the time to change.

In the waveform diagrams of FIG. 3 (d) in the first embodiment and FIG. 4 (d) in the second embodiment, the sampling period for obtaining a pair of cos and sin components of two orthogonal components is 2Δt. . In the measurement of the cos component, the time during which the control unit 12 applies the phase shift voltage V ₀ to the optical phase shift unit 8 is Δt−δ, and the time during which the phase shift voltage V ₉₀ is applied is Δt + δ. As a result, the sampling period for acquiring the cos component and the sin component as a pair of orthogonal two components is 2Δt, which is the same as in the first and second embodiments. In addition, the time from when the phase shift voltage is applied until the control unit 12 performs sampling, that is, the time from the start of the measurement time to the time when the control unit 12 samples the interference signal is the time δ0 for both the cos component and the sin component. And Here, with respect to Δt−δ, Δt + δ, and δ0, the relationship shown in the following equation (16) is established.

Therefore, the sum of the time for applying the phase shift voltage to the pair of cos and sin components of the quadrature two components is 2Δt, and the sampling period of the quadrature two components is the same as in the first and second embodiments. It becomes.
As to how to set δ and δ0, the sampling condition of how much the frequency resolution is set, the response time from when the phase shift voltage of the optical phase shift unit 8 is changed to when the phase is actually changed, and This is determined by the operating frequency of the A / D conversion circuit that samples the voltage signal from the light detection unit 11.
Note that, under the above-described sampling conditions (frequency sweep period 1S, orthogonal two component data points 1000), shortening the switching time from the cos component to the sine component to 1/10 or less is possible with a commercially available phase shifter, A / D It is possible to cope with this by using a conversion circuit.

<Fourth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the fourth embodiment will be described. FIG. 5 is a block diagram illustrating a configuration example of the fourth embodiment. The same components as those in the first embodiment are denoted by the same reference numerals, and components different from those in the first embodiment will be described below.
In the case of using the optical delay unit 7 that eliminates the optical path length difference between the first optical fiber 3 and the second optical fiber 4 and the optical phase shift unit 8 that shifts the phase of the optical signal to be measured, The optical delay unit 7 and the optical phase shift unit 8 are connected to different optical fibers.

As can be seen from the configuration of the first embodiment shown in FIG. 2, it is not necessary to provide each of the optical delay unit 7 and the optical phase shift unit 8 in different optical fibers. Therefore, in the fourth embodiment, the optical delay unit 7 and the optical phase shift unit 8 are combined and integrated to delay the light propagation of the optical delay unit 7 and the optical delay / light having a function of shifting the phase difference of the light of the optical phase shift unit 8. The phase shift unit 47 is provided in either one of the first optical fiber 3 and the second optical fiber 4. When either one of the first optical fiber 3 and the second optical fiber 4 has an optical path length shorter than the other, an optical delay / optical phase shift unit 47 is provided on one side to correct the optical path length difference with respect to the other.
Thereby, the apparatus can be further downsized as compared with the first embodiment by using the optical delay / optical phase shift unit 47 in which the optical delay unit 7 and the optical phase shift unit 8 are integrated.
Further, by combining the optical delay unit 7 and the optical phase shift unit 8 and integrating them, compared to the case where the optical delay unit 7 and the optical phase shift unit 8 are separated and provided in the same optical fiber, Residual reflected light generated in the optical fiber can be reduced. For this reason, the spectral ripple due to the resonance of the residual reflected light shown in the first embodiment can be suppressed to a lower level.

<Fifth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the fifth embodiment will be described. The chromatic dispersion measuring apparatus according to the fifth embodiment has the same configuration as that of the first embodiment shown in FIG.
The fifth embodiment is different from the first embodiment in that the first embodiment performs sampling of interference signals from the light detection unit 11, and sets a pair of cos component and sin component in the same frequency sweep period. In contrast, the fifth embodiment uses two frequency sweep periods to sample the cos component interference signal in one frequency sweep period and the sin component interference signal in the other frequency sweep period. This is a configuration for performing sampling.

Accordingly, the control unit 12 shifts the phase with respect to the optical phase shift unit 8 during the frequency sweep period for measuring the cos component in synchronization with the trigger signal output from the optical frequency sweep unit 9 in the two frequency sweep periods. A voltage V ₀ is supplied, and a phase shift voltage V ₉₀ is supplied to the optical phase shift unit 8 during a frequency sweep period in which a sin component is measured. Here, the length of the frequency sweep period for detecting the cos component and the sin component is the same. Therefore, the period during which the control unit 12 supplies the phase shift voltage V ₀ to the optical phase shift unit 8 and the period during which the phase shift voltage V ₉₀ is supplied are the same as the first period Δt.
In the present embodiment, as described above, the first period Δt to which the same phase shift voltage is applied is equal to the frequency sweep period. That is, T _{l + 1} −T _l = T _{l + 2} −T _{l + 1} = Δt.

FIG. 6 shows the timing of the frequency sweep operation of the optical frequency sweep unit 9, the phase shift operation of the optical phase shift unit 8 corresponding thereto, and the sampling of the interference signal from the light detection unit 11 in the control unit 12. FIG.
FIG. 6A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 9, where the vertical axis represents voltage and the horizontal axis represents time. In FIG. 6A, the H level and L level of the trigger signal output from the optical frequency sweep unit 9 are set so as to conform to the TTL control.

In FIG. 6B, the vertical axis represents frequency, the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 9. In FIG. 6B, ν ₁ is a sweep start frequency (the lowest frequency in the measurement frequency range), and ν ₂ is a sweep stop frequency (the maximum frequency in the measurement frequency range).
FIG. 6C is a diagram illustrating a waveform of the phase shift voltage in which the vertical axis is voltage, the horizontal axis is time, and the phase difference applied to the optical phase shift unit 8 is changed in the first period. The phase shift voltage V0 is a voltage when the phase difference is 0 degree (cos component detection mode), and the phase shift voltage V90 is a voltage when the phase difference is 90 degrees (sin component detection mode). The time for applying the phase shift voltage of the cos component and the sin component is the same period, that is, the first period Δt. In the present embodiment, the frequency sweep period (T _{1 + 1} −T ₁ , T _{1 + 2} −T _{1 + 1} ) and the first period have the same length.

In FIG. 6D, the vertical axis represents voltage, the horizontal axis represents time, the control unit 12 generates an interference signal from the light detection unit 11, and the control unit 12 performs cos component and sin component for each frequency sweep period. It is a figure which shows the timing of the sampling period which receives any one of these interference signals. In this embodiment, in two frequency sweep periods, the cos component is sampled in the first frequency sweep period, the sin component is sampled in the subsequent frequency sweep period, and the interference signal of the same frequency is used as a pair of orthogonal two-component data. Here, the control unit 12 uses the same measurement frequency (corresponding to the frequency resolution) in each of the frequency sweep cycle for performing the frequency decomposition of the interference component of the cos component and the frequency sweep cycle for performing the frequency decomposition of the interference component of the sin component. Is set in advance as a measurement cycle (sampling cycle), and the cos component and the sin component are sampled in the set measurement cycle.
6 (c) and 6 (d), in order to clearly describe the first period, the horizontal time scale of FIGS. 6 (a) and 6 (b) is expanded, and only a part of the time range is displayed. Is shown.

In the present embodiment, since each of the cos component and the sin component is measured with the same sampling period in different frequency sweep periods, the phase difference in the optical phase shift unit 8 is switched as in the first embodiment. Therefore, the cos component and sin component interference signals can be obtained at the same frequency, and the measurement accuracy of the measured phase change φ (ν) can be improved.
However, the optical path length difference between the first optical fiber 3 and the second optical fiber 4 fluctuates every frequency sweep by causing the first optical fiber 3 and the second optical fiber 4 to slightly expand and contract with the vibration of the apparatus. Then, the phase difference between the first optical fiber 3 and the second optical fiber 4 deviates from 0 degrees or 90 degrees. As a result, the orthogonality of the cos component and the sin component is lowered, and the measurement accuracy of the phase change φ (ν) is lowered. For this reason, it is necessary to increase the stability against vibration of the first optical fiber 3 and the second optical fiber 4 so that the optical path length between the first optical fiber 3 and the second optical fiber 4 does not change with time.

<Sixth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the sixth embodiment will be described. FIG. 7 is a diagram for explaining a measurement method for measuring a dispersion parameter of an optical pulse propagating through an optical fiber transmission line by using the chromatic dispersion measurement device according to any one of the first to fifth embodiments. . In this figure, a chromatic dispersion measuring device 66 is a chromatic dispersion measuring device according to any of the first to fifth embodiments.
The monitoring optical branching unit 62 is arranged at a position for evaluating chromatic dispersion in the path of the optical fiber transmission path 61, extracts an optical pulse propagating to the optical fiber transmission path 61 as a signal under measurement, and monitors the optical fiber. The light is emitted to the polarization controller 64 through 63.

  At this time, the monitoring optical branching unit 62 extracts a part of the power of the optical pulse propagating in the optical fiber transmission line 61 so as not to attenuate to such an extent that the propagation in the optical fiber transmission line 61 is affected. In the present embodiment, the monitoring optical branching unit 62 branches a part of the power of the optical pulse propagated to the optical fiber transmission line 61, for example, 10%, to the monitoring optical fiber 63 as a measured optical signal. That is, due to the branching of the monitoring optical branching unit 62, the power branching ratio of the optical pulse propagating through the optical fiber transmission line 61 and the optical signal under measurement propagating through the monitoring optical fiber 63 becomes 9: 1. For the monitoring optical fiber 63, for example, a standard dispersion single mode optical fiber is used.

The polarization controller 64 sets the polarization state of the optical signal to be measured to linear polarization, and the polarization axis thereof is the polarization axis (for example, the slow axis) of the incident optical fiber 65 (the incident optical fiber 1 in FIG. 2 or 5). Then, the measured optical signal is emitted to the incident optical fiber 65.
As the incident optical fiber 65, a polarization maintaining optical fiber is used, and the polarization axis is a polarization axis with respect to the polarization maintaining optical fibers (first optical fiber 3 and second optical fiber 4) inside the wavelength dispersion measuring device 66. Align.

As described above, in the present embodiment, when measuring the chromatic dispersion in the optical fiber transmission line 61, the chromatic dispersion is measured by using the optical pulse that actually propagates through the optical fiber transmission line 61. A dedicated light source is prepared, a measurement light pulse is incident from the light source on the incident end of the optical fiber transmission line 61, a measurement light pulse output from the output end of the optical fiber transmission line 61 is taken out, and this measurement is performed. Therefore, it is not necessary to take out a light pulse for measurement and measure chromatic dispersion.
Not only the measurement of chromatic dispersion in the entire optical fiber transmission line 61 but also the chromatic dispersion of the distance to that position can be measured at any position of the optical fiber transmission line 61, and the measurement position of chromatic dispersion can be freely set. The degree can be improved.
In addition, since the spatial optical system is not used, the device itself can be reduced in size, using an optical pulse that carries the device and propagates in an arbitrary position in any optical fiber transmission path and transmits information. In order to monitor this optical pulse, the chromatic dispersion of the optical fiber transmission line 61 is evaluated by measuring the change Δφ (ν) of the spectral phase with respect to a minute frequency shift without requiring a measurement light source. be able to.

<Seventh Embodiment>
Next, a chromatic dispersion measuring apparatus according to the seventh embodiment will be described. FIG. 8 is a diagram for explaining a measurement method for measuring a dispersion parameter of an optical pulse propagating through an optical component using the chromatic dispersion measurement device according to any one of the first to fifth embodiments. In this figure, a chromatic dispersion measuring device 78 is a chromatic dispersion measuring device according to any one of the first to fifth embodiments.
In this embodiment, when measuring the chromatic dispersion of the optical component to be measured, a special light source for measurement is not prepared, but usually an optical pulse transmitted for information transmission is output to the optical fiber transmission line. An optical transmitter is used as the light source 71. As described above, the light source 71 is a light source used in an optical fiber transmission line, and in this embodiment, an optical transceiver that converts an interference signal into an optical pulse is used. The light source 71 emits an optical pulse as an optical signal to be measured to the incident optical fiber 72.

The incident optical fiber 72 is configured by an optical fiber similar to that used in an optical fiber transmission line in which an optical component as the measurement target 74 is arranged. An incident light control unit 73 is inserted in the incident optical fiber 72.
The incident light control unit 73 controls the power and polarization state of the measured optical signal propagating through the incident optical fiber 72, and transmits the controlled measured optical signal to the measured object 74 via the incident optical fiber 72. The light is emitted to the incident end.
By controlling the power of the optical signal under measurement, it is possible to measure and evaluate the power dependency of chromatic dispersion in the object under measurement 74, that is, the relationship between the degree of chromatic dispersion and the power.
Further, by controlling the polarization state of the optical signal to be measured, it is possible to measure and evaluate the dependency of chromatic dispersion on the polarization state, that is, the relationship between the polarization state and the degree of chromatic dispersion.

One end of the output optical fiber 75 is connected to the measurement target 74 at the output end, and the measurement target optical signal incident from the input end is output from the output end to the output optical fiber 75.
The outgoing optical fiber 75 is configured by an optical fiber similar to that used in an optical fiber transmission line in which an optical component as the measurement target 74 is arranged.
In the polarization controller 76, the other end of the outgoing optical fiber 75 is connected to the incident end, and the measured optical signal from the measured target 74 enters. The polarization controller 76 has one end of an incident optical fiber 77 connected to the output end. The incident optical fiber 77 uses a polarization maintaining optical fiber, and the polarization axis is polarized with the polarization maintaining optical fibers (the first optical fiber 3 and the second optical fiber 4) inside the wavelength dispersion measuring device 78. Align with the axis.

The polarization controller 76 sets the polarization state of the optical signal to be measured to linear polarization, and the polarization axis thereof is the polarization axis (for example, the slow axis) of the incident optical fiber 77 (the incident optical fiber 1 in FIG. 2 or FIG. 5). Then, the measured optical signal is emitted to the incident optical fiber 77.
Further, the measurement object 74 may be a reflective optical component. In the case of a reflective optical component, the incident end and the exit end of the measurement target 74 are the same, and the incident optical fiber 72 to the measurement target 74 and the emission optical fiber 75 from the measurement target 74 are the same. It will be connected via a circulator.

  As described above, the device can be miniaturized without using a spatial optical system, and the optical signal propagated through the optical component to be measured can be measured at any place with the device being carried. By measuring the change Δφ (ν) of the spectral phase with respect to a minute frequency shift as an optical signal, it is possible to evaluate the chromatic dispersion of the optical component to be measured using the optical pulse that actually propagates in the optical component. it can.

<Eighth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the eighth embodiment will be described. The chromatic dispersion measuring apparatus according to the eighth embodiment has the same configuration as that of any of the first to fifth embodiments.
In the eighth embodiment, the polarization direction of the combined optical signal to be measured incident on the optical frequency sweep unit 9 in FIG. 2 is controlled to coincide with the polarization axis of the bandpass optical filter in the optical frequency sweep unit 9. The spectral resolution characteristics are improved. In the following description, only the configuration different from the first to fifth embodiments will be described.
The optical frequency sweep unit 9 uses a bandpass optical filter having a narrow bandpass frequency width and a high finesse in order to improve the frequency resolution.
As an example of this bandpass optical filter, there is an optical element configured by providing a resonator having a high Q value in an optical fiber.

As described above, when the bandpass optical filter itself is formed of an optical fiber, it is advantageous to further reduce the size and weight of the configuration of the chromatic dispersion measuring apparatus according to the first to fifth embodiments already described.
On the other hand, in a bandpass optical filter, when a polarization-maintaining optical fiber (hereinafter referred to as an optical fiber) is used without using a polarization-maintaining optical fiber, when strain is applied to the optical fiber due to a fixed state or the like, Polarization dependence with a different refractive index occurs with respect to the polarization direction of incident light. As a result, in the bandpass optical filter, there arises a problem that the bandpass frequency width is expanded depending on the polarization direction, and there are a plurality of bandpass frequency bands depending on the polarization direction instead of a single bandpass frequency band.

In order to solve the problem of polarization dependence occurring in this bandpass optical filter, a polarization controller is provided in front of the optical frequency sweep unit composed of the bandpass optical filter, and the polarization direction of the incident light is changed to the bandpass optical filter. The optical fiber constituting the optical fiber may be adjusted to coincide with a specific polarization axis so as to eliminate the polarization dependence. Here, it is assumed that the polarization controller matches the polarization direction of the incoming combined optical signal to be measured with the slow axis of the optical fiber used for the bandpass optical filter.
FIG. 9 shows a configuration example for adjusting the polarization direction of incident light so as to coincide with a single polarization axis (for example, slow axis) of an optical fiber constituting the bandpass optical filter. The optical fiber 81 in FIG. 9 corresponds to the coupling optical fiber 6 in FIG. 2, the optical frequency sweep unit 84 in FIG. 9 corresponds to the optical frequency sweep unit 9 in FIG. 2, and the optical fiber 85 in FIG. The holding optical fiber corresponds to the outgoing optical fiber 10 of FIG.

In the present embodiment, for the purpose of controlling polarization, the incident side optical fiber 81 and the connecting optical fiber 83 in FIG. 9 are polarization maintaining optical fibers. In the polarization controller 82, the exit end of the optical fiber 81 is connected to the entrance end, and the entrance end of the connection optical fiber 83 is connected to the exit end. In addition, the polarization controller 82 matches the polarization direction of the combined optical signal to be measured incident via the optical fiber 81 with the slow axis of the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84. The light is emitted to the connection optical fiber 83.
As a result, the optical frequency sweep unit 84 transmits the combined optical signal to be measured, whose polarization characteristics coincide with the slow axis of the optical fiber constituting the internal bandpass optical filter, from the optical coupling unit 5 via the connection optical fiber 83. Incident.
Then, the optical frequency sweeping unit 84 frequency-resolves the combined measured optical signal by sweeping the bandpass frequency width in the measurement frequency range, and transmits the optical signal via the optical fiber 85 as the component measured optical signal. The light is emitted to the detection unit 11.

  As described above, the polarization controller 82 matches the polarization direction of the combined optical signal to be measured with the polarization axis of the optical fiber constituting the band-pass optical filter in the optical frequency sweep unit 84, so that the spectral resolution characteristic depending on the polarization direction is obtained. Degradation can be prevented, and the accuracy of frequency resolution characteristics can be improved.

When the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84 is a polarization maintaining optical fiber, the polarization controller 82 can be omitted by using the optical fiber 81 as a polarization maintaining optical fiber. it can. That is, the polarization direction of the incident-side optical fiber 81 and the polarization direction of the optical fiber constituting the bandpass optical filter in the optical frequency sweep unit 84 may be aligned and directly connected.
As described above, in the case where all the light transmission paths of the chromatic dispersion measuring apparatus are configured by optical components based on optical fibers, it is possible to prevent deterioration in accuracy of frequency resolution (spectral resolution characteristics) in the optical frequency sweep unit 84. it can.

<Ninth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the ninth embodiment will be described. The chromatic dispersion measuring apparatus according to the ninth embodiment uses the chromatic dispersion measuring apparatus according to any one of the first to fifth embodiments to measure the dispersion parameter of the optical pulse generated by the optical signal generator. FIG. 10 measures the dispersion parameter of the optical pulse generated by the optical signal generator 91 propagating through the optical pulse transmission path, using the chromatic dispersion measuring apparatus according to any one of the first to fifth embodiments. It is a figure explaining a method.
In FIG. 10, a chromatic dispersion measuring device 97 is the chromatic dispersion measuring device according to any one of the first to fifth embodiments.

10 includes an optical signal generator 91, an optical fiber transmission line 92, a polarization controller 93, an optical fiber 94, a wavelength channel selection filter 95, an incident optical fiber 96, and a chromatic dispersion measuring device 97. It is composed of As the optical fiber 94 and the incident optical fiber 96, polarization maintaining optical fibers are used.
The optical signal generator 91 includes an optical transponder / optical transceiver and a wavelength multiplexer, and functions as an optical signal generator that generates an optical pulse in an optical transmission device used in an optical network.

The optical signal generator 91 has an optical output end connected to one end of the optical fiber transmission line 92.
The optical fiber transmission line 92 is a basic element of an optical fiber cable used for an optical network, and the total length is determined according to the optical network to be used. For example, in the case of a submarine optical fiber network, the total length of the optical fiber transmission line 92 may be 1000 km or more.
Further, in order to improve the attenuation of the optical pulse due to the increase in the total length of the optical fiber transmission line 92, an optical repeater may be inserted in the middle of the transmission line. When this optical repeater is inserted, it is necessary to evaluate the dispersion parameter of the optical pulse that has passed through the optical repeater. Even in this evaluation, an optical amplifier such as an optical amplifier is included in the optical fiber transmission line 92. Devices used for the relay device are inserted.

The optical signal generator 91 according to the present embodiment is configured such that an optical transponder / optical transceiver as a component encodes, for example, a semiconductor laser that continuously oscillates and codes the output light of the semiconductor laser in accordance with data to be transmitted. Have a container. This optical modulator uses, for example, 10 Gbps-NRZ (non-return to zero) intensity modulation or 40 Gbps-DQPSK (differential quadrature phase-shift keying) modulation as a data modulation format.
Therefore, the optical signal under measurement to be measured in this embodiment includes an optical signal of a modulation format such as 10 Gbps-NRZ or 40 Gbps-DQPSK described above. However, the optical signal under measurement to be measured in the present embodiment is not limited to these modulation formats, and includes other modulation formats that are in the research and development stage such as QAM (quadture amplitude modulation) and will be put to practical use in the future. .

The other end of the optical fiber transmission line 92 is connected to the optical input end of the polarization controller 93.
The polarization controller 93 converts the polarization state of the optical signal to be measured incident on the light incident end from the optical fiber transmission path 92 into linearly polarized light, and changes the polarization axis of the optical signal to be measured to the incident optical fiber 96 (see FIG. 2 or the incident optical fiber 1) of FIG. 5 is aligned with the polarization axis (for example, the slow axis), and then the optical signal to be measured is output to the optical fiber 94 having one end connected to the light output end.

In the wavelength channel selection filter 95, the other end of the optical fiber 94 is connected to the light incident end, and an optical signal to be measured is incident from the polarization controller 93 via the optical fiber 94.
The wavelength channel selection filter 95 selectively transmits the optical signal of the wavelength channel to be evaluated in the chromatic dispersion measuring device 97 from the incident measured optical signal, and the measured optical signal to the chromatic dispersion measuring device 97. As shown in FIG.

In addition, the wavelength channel selection filter 95 is disposed at the subsequent stage of the polarization controller 92 in FIG.
That is, when the polarization state of the optical signal under measurement whose polarization axis is aligned with the incident optical fiber 96 is significantly changed by the wavelength channel selection filter 95, after selecting the wavelength channel as described above, A configuration in which the polarization controller 93 is disposed after the wavelength channel selection filter 95 so that the process of aligning the polarization axes of the optical signals of the channels is performed. In this case, a wavelength channel selection filter 95 is provided between the optical fiber transmission line 92 and the optical fiber 94, and a polarization controller 93 is provided between the optical fiber 94 and the incident optical fiber 96. Further, in the configuration in which the polarization controller 93 is disposed after the wavelength channel selection filter 95, the optical fiber 94 does not need to be a polarization maintaining optical fiber.

  According to the chromatic dispersion measuring apparatus according to the present embodiment described above, the chirp characteristics of the optical transponder / optical transceiver constituting the optical signal generator 91 are evaluated by changing the frequency channel that the wavelength channel selection filter 95 transmits. Can do. When this chirp characteristic (frequency chirp amount) is evaluated, an optical fiber patch cord having a shorter length (for example, several meters) may be used instead of the optical fiber transmission line 92. The optical fiber patch cord is composed of an optical fiber such as a standard dispersion single mode optical fiber or a dispersion shifted optical fiber.

The wavelength channel selection filter 95 used in the present embodiment is used for selecting any frequency of the multiplexed wavelength channels used in the wavelength division multiplexing transmission, and also in other embodiments of the present invention. Can be used.
For example, in the chromatic dispersion measuring apparatus according to the sixth embodiment, a selection is made by providing a wavelength channel selection filter 95 after the monitoring branching unit 62, that is, between the monitoring branching unit 62 and the chromatic dispersion measuring device 66. Corresponding to the wavelength of the filtered filter, it is possible to extract the measured optical signal of any one of the multiplexed wavelength channels and evaluate the chromatic dispersion characteristic of this one wavelength channel.

<Tenth Embodiment>
Next, the configuration and function of the chromatic dispersion measuring apparatus according to the tenth embodiment will be described with reference to FIG. FIG. 11 is a block diagram illustrating a configuration example of a chromatic dispersion measuring apparatus according to the tenth embodiment. The same components as those in the first embodiment are denoted by the same reference numerals, and different configurations and operations from those in the first embodiment will be described below.
11, in the chromatic dispersion measuring apparatus of the present embodiment, an optical input switching unit 101 is interposed between the optical coupling unit 5 and the optical frequency sweep unit 9 of the chromatic dispersion apparatus of the first embodiment of FIG. It has a configuration. The chromatic dispersion measuring apparatus according to the present embodiment is provided with a calibration light source 103.

For this reason, in this embodiment, the coupling optical fiber 6 which is a coupling path in the chromatic dispersion measuring apparatus according to the first embodiment of FIG. The light source 103 for calibration and the connecting optical fiber 104 are replaced.
In other words, the optical coupling unit 5 has one end of the connection optical fiber 107 connected to the light emitting end (third emitting end). The light input switching unit 101 emits one of the optical signals incident from the two light incident ends from the light emitting end. The light input switching unit 101 has a light emission end (fifth emission end) connected to one end of the connection optical fiber 104. In the light input switching unit 101, one of the two light incident ends (sixth incident end) is connected to the other end of the connection optical fiber 107, and the other of the two light incident ends (seventh incident end) is connected light. It is connected to one end of the fiber 102. The other end of the optical fiber 104 is connected to the incident end of the optical frequency sweep unit 9. The light input switching unit 101 is supplied with a control signal from the control unit 12 via the light input switching control line 105 as to whether one of the optical signals incident from the two light incident ends is emitted from the light emitting end. Is done.
The calibration power source 103 has a light emitting end connected to the other end of the connection optical fiber 102 and one end of a calibration light source control line 106 to which a control signal for frequency control from the control unit 12 is supplied. The other end of the calibration light source control line 106 is connected to the control unit 12.

In the chromatic dispersion measuring apparatus described above, the control unit 12 includes a measured optical signal incident from the optical coupling unit 5 via the connection optical fiber 107 and a calibration incident from the calibration light source 103 via the connection optical fiber 102. An electrical signal (control signal) for controlling which light is emitted to the light incident end of the optical frequency sweep unit 9 is output to the optical input switching unit 101 via the optical input switching control line 105.
When the control signal indicating that the calibration light incident from the connection optical fiber 102 is output is supplied from the control unit 12, the light input switching unit 101 emits the calibration light incident from the connection optical fiber 102. The light is emitted from the end to the light incident end of the optical frequency sweep unit 9 via the connection optical fiber 104.

Here, the optical frequency sweep unit 9 performs the same processing as the frequency sweep on the optical signal to be measured already described in the first embodiment with respect to the calibration light incident from the connection optical fiber 104, and the outgoing optical fiber 10. Then, the component measured optical signal obtained by the sweep is emitted to the light detection unit 11.
The light detection unit 11 converts the component optical signal to be measured incident from the outgoing optical fiber 10 into an electrical signal, and outputs the converted electrical signal to the control unit 12.
The control unit 12 can calibrate the optical frequency swept by the frequency sweep unit 9 by changing the optical frequency of the calibration light emitted from the calibration light source 103 and measuring the input electrical signal. it can.

For example, a tunable optical filter using an optical fiber etalon is used for the optical frequency sweep unit 9. In this tunable optical filter, the transmitted light frequency is generally variable by expanding and contracting the optical fiber with a piezoelectric element such as PZT (preparation of lead zirconate titanate). This tunable optical filter is commercially available from Micron Optics, Inc. as filter FFP-TF2 and filter controller FFP-C.
In the tunable optical filter using this piezoelectric element, the sweeping light frequency fluctuates due to fluctuations in the driving voltage of the piezoelectric element or voltage movement of the controller. Therefore, in order to eliminate the fluctuation of the frequency of the component measurement optical signal due to the fluctuation of the sweep optical frequency, the sweep optical frequency (transmission frequency) of the tunable optical filter is set to the frequency of the calibration light of the calibration light source 103. It is necessary to calibrate the deviation.

The calibration light source 103 is emitted to the connection optical fiber 102 by a control signal (control command or control electrical signal) transmitted from the control unit 12 via the calibration light source control line 106 via the calibration light source control line 106. Control the frequency of calibration light.
The control unit 12 controls whether the optical signal to be measured or the calibration light is emitted from the optical input switching unit 101, controls the frequency sweep in the optical frequency sweep unit 9, and controls the optical frequency of the calibration light of the calibration light source 103. , The wavelength dispersion measurement of the optical signal to be measured and the calibration of the frequency of the optical signal to be measured can be performed alternately. Even if the optical frequency transmitted through the optical frequency sweep unit 9 is shifted, the calibration is performed. The frequency shift can be calibrated with light, and the dispersion parameter for each frequency can be accurately measured.

The control unit 12 indicates that the frequency band transmitted by the optical frequency sweep unit 9 varies with each sweep, and that the intensity is maximized when the calibration light output from the calibration light source 103 is transmitted from the optical frequency sweep unit 9. Detection is performed by determining the wavelength.
Further, when the deviation of the optical frequency transmitted through the optical frequency sweeping unit 9 is not large, the frequency of the optical signal to be measured may be calibrated every predetermined time.

As a configuration of the calibration light source 103, wavelength calibration and wavelength stabilization control are performed. For example, any one of the following five types of configurations may be used.
(1) Configuration consisting of laser elements that oscillate in a single longitudinal mode (2) Use a plurality of laser elements that oscillate in a single longitudinal mode and have different oscillation wavelengths, and use any of the different oscillation wavelengths as calibration light Configuration in which the optical switch is selected (here, switching of the optical switch is controlled by the control unit 12)
(3) A tunable laser element that oscillates in a single longitudinal mode and whose oscillation wavelength is variable, and has a configuration in which the oscillation wavelength is varied according to a command (control command) or an electrical control signal from the control unit 12 Is control data indicating the oscillation wavelength, and the electrical control signal is, for example, a voltage value corresponding to the oscillation wavelength)
(4) A configuration in which an optical signal from a broadband light source is transmitted through an optical filter, and a peak wavelength or frequency width of the transmitted light is varied according to a command from the control unit 12 or an electric control signal. (5) The longitudinal mode is a multimode. A laser element or an optical frequency comb light source that varies the frequency period of the peak wavelength in accordance with a command from the control unit 12 or an electric control signal

Further, the configuration of the configuration light source 103 in this embodiment is not limited to the configurations (1) to (5) described above.
Further, the configuration for adding the configuration function of the optical frequency in the description of the present embodiment has been described based on the chromatic dispersion measuring device according to the first embodiment, but other configurations than the first embodiment of the present invention. The present invention can also be easily applied to the chromatic dispersion measuring apparatus according to the embodiment.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes design and the like within a scope not departing from the gist of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Incident optical fiber, 2 ... Optical branching part, 3 ... 1st optical fiber, 4 ... 2nd optical fiber, 5 ... Optical coupling part, 6 ... Optical fiber for coupling, 7 ... Optical delay part, 8 ... Optical phase shift , 9, 84 ... optical frequency sweep unit, 10 ... outgoing optical fiber, 11 ... light detection unit, 12 ... control unit, 13 ... phase control line, 14 ... frequency control line, 15 ... detection signal line, 47 ... optical delay Optical phase shift unit 61: Optical fiber transmission line 62: Monitoring optical branching unit 63: Monitoring optical fiber 64, 76, 82, 93: Polarization controller 65, 72, 77, 96: For incidence Optical fiber, 66, 78 ... wavelength dispersion measuring device, 71 ... light source, 73 ... incident light control unit, 74 ... measurement object, 75 ... outgoing optical fiber, 81, 85 ... optical fiber, 83, 102, 104, 107 ... Connection light 91: Optical signal generator, 92 ... Optical fiber transmission line, 94 ... Optical fiber, 95 ... Wavelength channel selection filter, 97 ... Wavelength dispersion measuring device, 101 ... Optical input switching unit, 103 ... Light source for calibration, 105 ... Light input switching control line 106: Light source control line for calibration

Claims (13)

An incident path composed of an optical fiber having polarization maintaining characteristics, which propagates an optical signal to be measured incident from a measurement target;
A measured optical signal is incident from a first incident end connected to the incident path, and the measured optical signal incident from the incident end is separated into a first measured optical signal and a second measured optical signal. The first measured optical signal is emitted from the first emission end, and the second measured optical signal having the same polarization direction as that of the first measured optical signal is emitted from the second emission end, And an optical branching unit that generates a frequency difference between the first measured optical signal and the second measured optical signal when emitted,
A first branch path connected to the first output end, propagating the first optical signal to be measured, and configured by an optical fiber having polarization maintaining characteristics;
A second branch path that is connected to the second emission end, propagates the second optical signal to be measured, and includes an optical fiber having polarization maintaining characteristics;
Light that is provided in one of the first branch path and the second branch path, and alternately changes the phase of the optical measurement optical signal propagating through the provided branch path by binary values of 0 degrees and 90 degrees. A phase shifter;
The first measured optical signal incident from the second incident end connected to the first branch path and the second measured optical signal incident from the third incident end connected to the second branch path And the optical phase shifter obtained by the interference between the first optical signal to be measured and the second optical signal to be measured, and the optical signal of the branch path on which the optical phase shifter is provided and the other optical path When the phase difference between the measurement optical signals is 0 degree, the interference element of the first optical component, and the optical phase shifter between the measurement optical signal of the branch path where the optical phase shifter is provided and the measurement optical signal of the other branch path When the phase difference is 90 degrees, the interference element of the second light component is the combined optical signal to be measured, and the optical combining unit that outputs from the third output end,
A coupling path connected to the third exit end and configured by an optical fiber that propagates the combined optical signal to be measured;
The combined optical signal to be measured is incident from the fourth incident end connected to the coupling path, the frequency range through which the optical signal to be combined passes is swept, and the frequency from the combined optical signal to be measured is swept. An optical frequency sweeping unit that performs frequency decomposition to extract a spectral component of the range, and outputs the result of frequency decomposition as a component measured optical signal from the fourth output end;
An output light path configured by an optical fiber connected to the fourth output end and propagating the component measured optical signal;
A light detector that enters the component measured optical signal from a fifth incident end connected to the outgoing light path, converts the component measured optical signal into an electrical signal, and converts the conversion result into an interference signal;
A chromatic dispersion measuring apparatus, comprising: a control unit that acquires the interference signals of the first optical component and the second optical component in time series in synchronization with a change in phase of the optical phase shifter.   The optical delay unit is provided in any one of the first branch path and the second branch path, and further includes an optical delay unit that adjusts an optical path length difference between the first branch path and the second branch path. Item 2. The chromatic dispersion measuring apparatus according to Item 1.   The chromatic dispersion measurement apparatus according to claim 2, wherein the optical delay unit is provided in one of the first branch path and the second branch path, and the optical phase shifter is provided in the other.   3. The chromatic dispersion measuring apparatus according to claim 2, wherein the optical delay unit and the optical phase shifter are integrally provided in one of the first branch path and the second branch path. 4.   The control unit includes a first receiving unit that receives the interference signal of the first light component, and a second receiving unit that receives the interference signal of the second light component. The chromatic dispersion measuring device according to any one of claims 1 to 4.   The said control part acquires the said 1st light component and said 2nd light component as a data unit in time series as a data unit for every sweep of the measurement frequency in a measurement range, The Claim 1 to Claim 5 characterized by the above-mentioned. The chromatic dispersion measuring device according to any one of the above.   In the first light component and the second light component constituting the data pair, the time for changing the phase of the light component measured earlier is set to the time for changing the phase to the light component measured later. The chromatic dispersion measuring device according to claim 6, wherein the chromatic dispersion measuring device is set short.   In the sweep of the measurement frequency in the measurement range, the control unit acquires the interference signal of only one of the first light component and the second light component in one sweep. The chromatic dispersion measuring apparatus according to claim 1, wherein the chromatic dispersion measurement apparatus is alternately and repeatedly performed on a component and the second light component.   The coupling path is composed of an optical fiber having polarization maintaining characteristics, and a controller for controlling the polarization direction of the combined optical signal to be measured is inserted in the subsequent stage of the coupling path, and the polarization controller and the light 9. The chromatic dispersion measuring apparatus according to claim 1, wherein the chromatic dispersion measuring apparatus is connected to the frequency sweeping unit by an optical fiber having polarization maintaining characteristics. A calibration light source that emits calibration light;
The combined optical signal output from the output end of the optical combining unit is incident from a sixth incident end, the calibration light is incident from a seventh incident end, and either the combined optical signal or the calibration light is selected. And a light input switching unit that emits from the fifth exit end,
The optical input switching unit is interposed between the optical multiplexing unit and the optical frequency sweep unit;
The combined optical signal or the calibration light emitted from the optical input switching unit is incident on the fourth incident end of the frequency sweep unit via a second optical fiber. The chromatic dispersion measuring device according to any one of claims 1 to 9. A chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 10,
A branching unit is provided in a portion for evaluating the chromatic dispersion of the optical transmission line to be measured, and the polarization control unit controls the polarization of the optical signal to be measured obtained from the branching unit to a linearly polarized wave. Aligned with the polarization axis propagating in the measuring apparatus, the optical signal to be measured is incident on the wavelength dispersion measuring apparatus via the incident path, and the optical signal to be measured is obtained from the interference signals of the first light component and the second light component. A method for measuring chromatic dispersion, comprising: obtaining a change in spectral phase of a signal and evaluating chromatic dispersion in the measurement object. A chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 10,
An optical signal subjected to polarization control is incident on the incident end of the measurement target for evaluating chromatic dispersion, and the polarization of the measured optical signal emitted from the output end of the measurement target is controlled to be linearly polarized, Aligned with the polarization axis propagating in the chromatic dispersion measuring device, the measured optical signal is incident on the chromatic dispersion measuring device via the incident path, and the interference signal of the first light component and the second light component is A chromatic dispersion measuring method characterized by obtaining a change in spectral phase of an optical signal under measurement and evaluating chromatic dispersion in the measurement object. A chromatic dispersion measuring method for obtaining chromatic dispersion of a measurement object using the chromatic dispersion measuring apparatus according to any one of claims 1 to 10,
An optical signal subjected to polarization control is incident on the incident end of the measurement target for evaluating chromatic dispersion, and the polarization of the measured optical signal emitted from the output end of the measurement target is controlled to be linearly polarized, Aligned with the polarization axis propagating in the chromatic dispersion measuring device, and when the measurement object is multiplexed with a plurality of wavelength channels, an optical signal of a single wavelength channel is extracted from the measured signal as the measured optical wavelength signal And measuring the chromatic dispersion of the measurement object for each wavelength channel in the chromatic dispersion measuring device using the measured optical wavelength signal.

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