JP2011179918A - Wavelength dispersion measuring device and wavelength dispersion measuring method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength dispersion measuring device and the like which can be miniaturized, and measures the wavelength dispersion of an optical pulse reliably and highly stably. <P>SOLUTION: The device includes: a light branching and multiplexing part for separating a light signal to be measured to a first and second light signals to be measured, multiplexing the first and second light signals to be measured reflected at a termination part to be mentilned later, emitting the multiplexed light signal to be measured, and generating a frequency difference between the individual signals; a first branch path composed of polarization maintaining optical fibers for propagating the first light signal to be measured, and having a termination part at its end; a second branch path composed of polarization maintaining optical fibers for propagating the second light signal to be measured, and having a termination part at its end; an optical phase shifter which is provided in either the first or second branch path, and changes the phase of the signal propagating through the branch path to 0° and 45° alternately; an optical-frequency sweep part for performing frequency resolution of the multiplexed light signal to be measured by interference of the first and second light signals to be measured, while being synchronized with the changes in the phase; and a control part for acquiring the wavelength dispersion from an acquired interference component. <P>COPYRIGHT: (C)2011,JPO&INPIT

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 a light signal to be measured is incident from a first incident end connected to the incident path, the light signal to be measured is emitted from the first incident / exit end, and is multiplexed from the first incident / exit end. A first optical circuit comprising an optical circulation section (optical circulation section 2) for outputting a measured optical signal from the first output end, and an optical fiber having polarization maintaining characteristics, one end of which is connected to the first input / output end. From the connection path (optical fiber 3) and the second input / output end connected to the other end of the first connection path, the measured optical signal is divided into two signals, a first measured optical signal and a second measured optical signal. And the first optical signal to be measured is emitted from a third input / output end, and the first measured optical signal is The second measured optical signal having the same polarization direction as the optical signal and shifted by a preset frequency with respect to the first measured optical signal is emitted from a fourth incident / exit end, The first measured reflected light signal incident from the third incident / exit end and the second measured reflected light signal incident from the fourth incident / exit end are divided by the frequency with respect to the first measured reflected light signal. An optical branching and coupling unit (optical branching and coupling unit 4) that outputs the combined optical signal to be measured from the second input / output end as a result of interference by combining the shifted signal and one end A first branch path (optical fiber 5) composed of an optical fiber having polarization maintaining characteristics connected to the third input / output end, and a polarization maintaining end connected to the fourth input / output end A second branch path (optical fiber 6) composed of an optical fiber having characteristics; The first measured optical signal incident from the fifth incident / exit end connected to the other end of the branch path is totally reflected, and the first branched path from the fifth incident / exit end as the first measured reflected light signal. The second optical signal to be measured that is incident from the first terminal portion (first terminal portion 11) exiting to the second end and the sixth input / output end connected to the other end of the second branch path is totally reflected, and 2 a second termination portion (second termination portion 12) that emits from the sixth input / output end to the second branch path as a reflected light signal to be measured, and one of the first branch path and the second branch path An optical phase shifter (optical phase shift) that alternately changes the phase of each of the optical signal to be measured and the optical signal to be measured that propagates through the provided branch path by a binary value of 0 degree and 45 degrees Part 8) and an optical fiber connected to the first output end for propagating the combined optical signal to be measured The combined optical signal to be measured is incident from the measurement coupling path (the outgoing light introducing optical fiber 13) configured by the above and the second incident end connected to the measurement coupling path, and the combined optical signal to be measured The frequency range that passes through, the frequency decomposition that extracts the spectral component of the frequency range from the combined optical signal to be measured, and the light that is output from the second emission end as the component optical signal to be measured as a result of the frequency decomposition An outgoing light path (outgoing optical fiber 15) configured by a frequency sweeping unit (optical frequency sweeping unit 14), an optical fiber connected to the second outgoing end and propagating the component measured optical signal, and the outgoing light Photodetection unit (photodetection unit 16) that receives the component measured optical signal from a third incident end connected to the path, converts the component measured optical signal into an electrical signal, and uses the conversion result as an interference signal And the position of the optical phase shifter When the first light component of the first measured reflected light signal and the second measured reflected light signal and the phase difference are 45 degrees when the phase difference is set to 0 in synchronization with the change in the difference And a control unit (control unit 17) for acquiring an interference signal of a second light component between the first measured reflected light signal and the second measured reflected light signal in time series.

  In order to solve the above-described problem, the invention according to claim 2 is characterized in that the reflectance of either one or both of the first termination unit and the second termination unit is variable, and the control unit is configured to transmit the light. The difference in light intensity between the first measured reflected light signal and the second measured reflected light signal incident on the branch coupling unit is obtained by calculating the difference between the interference signal of the first light component and the interference signal of the second light component. When the voltage value of the direct current component in the waveform is detected and the voltage value exceeds a preset threshold value, the reflectivity of the terminal portion having variable reflectivity is controlled so that the voltage value is equal to or less than the threshold value. It is characterized by.

  In order to solve the above problem, the invention according to claim 3 is characterized in that a light intensity adjusting unit is inserted in either or both of the first branch path and the second branch path, and the control unit is configured to transmit the light. The difference in light intensity between the first measured reflected light signal and the second measured reflected light signal incident on the branch coupling unit is obtained by calculating the difference between the interference signal of the first light component and the interference signal of the second light component. When the voltage value of the waveform is detected and the voltage value exceeds a preset threshold value, the light intensity adjustment unit causes the intensity of the reflectance in the first measured reflected light signal and the second measured reflected light signal to be reflected. The voltage value, which is the difference between the two, is controlled to be equal to or less than the threshold value.

  In order to solve the above problem, the invention according to claim 4 is provided in any one of the first branch path and the second branch path, and the optical path length between the first branch path and the second branch path. The optical delay unit further adjusts the difference.

  In order to solve the above-mentioned problem, in the invention according to claim 5, 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. It is characterized by.

  In order to solve the above-mentioned problem, in the invention described in claim 6, the optical delay unit and the optical phase shifter are integrally provided in one of the first branch path and the second branch path. It is characterized by.

  In order to solve the above-mentioned problem, the invention according to claim 7 is characterized in that the control unit receives the interference signal of the first optical component and the interference signal of the second optical component. And a second receiving unit.

  In order to solve the above-mentioned problem, according to an eighth aspect of the present invention, in the control unit, the first optical component and the second optical component are measured for each interference signal as a measurement unit for each measurement frequency sweep in the measurement range. The chromatic dispersion measuring apparatus according to claim 1, wherein the data is acquired in time series as a data pair.

  In order to solve the above-mentioned problem, the invention according to claim 9 is directed to the phase of the optical component measured earlier in the first optical component and the second optical component constituting the data pair for each interference signal. The time for performing the change is set shorter than the time for performing the phase change to the polarization component to be measured later.

  In order to solve the above-described problem, the invention according to claim 10 is characterized in that the control unit performs either one of the first light component and the second light component in a single sweep in the measurement frequency sweep in the measurement range. The process of acquiring only one of the interference signals is alternately and repeatedly performed on the first light component and the second light component.

  In order to solve the above-described problem, the invention according to claim 11 is configured such that the measurement coupling path is configured by an optical fiber having polarization maintaining characteristics, and the combined optical signal to be measured is provided in the subsequent stage of the measurement coupling path. A polarization controller for controlling the polarization direction of the light is inserted, 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-mentioned 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 11. A branching portion is provided in a portion for evaluating the chromatic dispersion of the optical transmission line to be measured, and the polarization controller controls the polarization of the optical signal to be measured obtained from the branching portion to a linearly polarized wave, and the wavelength Aligned with the polarization axis propagating in the dispersion measuring apparatus, the measured optical signal is incident on the chromatic dispersion measuring apparatus via the incident path, and the measured signal is obtained from the interference signals of the first light component and the second light component. A change amount of a spectral phase of the optical signal is obtained, and chromatic dispersion in the measurement object is evaluated.

  In order to solve the above problems, 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 11. The optical signal subjected to polarization control is incident on the incident end of the measurement target to be evaluated for chromatic dispersion, and the polarization of the optical signal to be measured 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 input. From this, a change in spectral phase is obtained, and chromatic dispersion in the measurement object is evaluated.

According to this 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.

In addition, 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 parts required in the spatial optical system, and the apparatus can be downsized. It becomes possible.
Further, according to the present invention, in the optical path of the interferometer, the optical signal to be measured is subjected to frequency shift processing twice in the forward path and the return path, so that the frequency given in one direction using the conventional frequency shifter A frequency shift that is double the shift can be easily provided, and the frequency resolution can be easily improved by enhancing the separation characteristic on the frequency axis between the optical signal to be measured that has been frequency-shifted and the optical signal that has not been frequency-shifted. Can be improved.

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 figure which shows the waveform of the 1st measured optical signal which is not frequency-shifted, and the 2nd measured optical signal which was frequency-shifted. It is a block diagram which shows the structural example of the chromatic dispersion measuring apparatus by 1st Embodiment. It is a figure which shows the other structural example of the optical circulation part. FIG. 5 is a diagram illustrating another configuration example of the optical branching and coupling unit 4. In the first embodiment, the frequency sweep operation of the optical frequency sweep unit 14, the phase shift operation of the optical phase shift unit 8 corresponding thereto, and the sampling of the electrical signal from the light detection unit 16 in the control unit 17 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 14, 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 16 in the control unit 17 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 14 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 16 in the control unit 17 It is a wave form diagram which shows the timing of. It is a block diagram which shows the structural example of the optical frequency sweep part 84 by 6th Embodiment. 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.

  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 light by electrical measurement, for example, the frequency of light with a wavelength of 1500 nm is 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 split into two by maintaining the same polarization direction 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 OPTICS LETTERS Vol.19, No.4, pp.287-289, February 15, 1994, "Analysis of ultrashort pulse-shape measurement using linear interferferometers", Victor Wong. 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. 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. Here, in the present embodiment, as described in detail later, the configuration of a reflection type spectrum shearing interferometer is targeted. The terminal part of the interferometer is configured to reciprocate a path that causes interference by reflecting the measurement light. For this reason, the measurement light propagates twice in the same path in the case of one direction. That is, the first measured optical signal propagates in the first branch path in the forward direction toward the terminal portion, is reflected at the terminal portion, and propagates in the return path through the first branch path as the first measured reflected light signal. Propagate. Similarly, the second optical signal to be measured propagates along the second branch path in the direction of the terminal part on the forward path, reflects off the terminal part, and propagates along the second branch path on the return path as the second optical signal to be measured. Propagate twice. Accordingly, the π / 2 phase shift applied to either the first measured optical signal or the second measured optical signal is given by π / 4 in each of the forward path and the backward path, and becomes π / 2 in the round trip. It is a given configuration. As a result, the phase is changed to 0 degree and 45 degrees in one direction, so that the applied voltage for changing the phase can be reduced to about half compared to 0 degree to 90 degrees at a time. Power saving can be realized.
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 formula (6), t is time and ν ₀ is the center frequency of the optical signal to be measured.

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 | E ₁ ^cos (t) | and | E ₁ ^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. Here, in the present embodiment, as described above, since the configuration is a reflection type spectrum shearing interferometer, the second optical signal to be measured propagates back and forth along the second branch path. Therefore, the present embodiment, the second optical signal to be measured gives the frequency shift .DELTA..nu ₀ but, like the phase shift, given by the frequency shift .DELTA..nu _0/2 in the forward and backward propagation of the respective one-way, round-trip Thus, Δν _{0 is given} to the second optical signal to be measured. By using this reflection type spectrum shearing interferometer, the amount of frequency shift can be easily doubled as compared with a transmission type spectrum shearing interferometer that generates an interference signal only by propagation in one direction.

FIG. 2 is a waveform diagram showing signal intensities of the first optical signal to be measured that has not been frequency-shifted and the second optical signal to be measured that has been frequency-shifted. The horizontal axis represents the frequency and the vertical axis represents the optical signal. Indicates strength. In contrast to FIG. 2A in which the frequency shift is 200 MHz, FIG. In the case of FIG. 2 (a), there is a portion (Q) that causes interference due to overlapping of tails of an optical signal that has been given a frequency shift and an optical signal that has not been given a frequency shift. For this reason, separation between the optical signal given the frequency shift and the optical signal not given the frequency shift in FIG. 1B becomes insufficient, and the spectral phase Δφ may not be measured accurately. On the other hand, if the frequency shift amount is doubled by increasing the frequency shift amount, for example, as shown in FIG. 2B, the tails of the optical signal to which the frequency shift is applied and the optical signal to which the frequency shift is not applied overlap. This can be suppressed.
At this time, it is conceivable that the amount of phase shift is simply doubled from 200 MHz to 400 MHz. However, if such processing is performed, the efficiency of the signal intensity that is output after phase shift decreases, and the measured interference The intensity of the signal is lowered, and the measurement accuracy is lowered.
Further, although it is conceivable to improve the resolution of frequency separation, it is necessary to generate a new bandpass filter in order to improve the resolution of frequency separation.

On the other hand, in the present embodiment, the frequency shift of the second measured optical signal can be easily doubled by reciprocating and passing the configuration for frequency shifting the measured light. 2 Signal separation is performed without giving attenuation of signal intensity to the optical signal under measurement. In addition, since the frequency shift of the second optical signal to be measured is doubled, the characteristics of the optical signal separation on the frequency axis are improved, and the bandpass filter in the current optical frequency insertion / extraction unit is used to substantially reduce the frequency. The resolution can be improved.
The time waveform of the electric field of the second optical signal to be measured propagating through the second branch path after propagating back and forth along the optical path of the interferometer as described above 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. (90 degrees) 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 formula (8) is Fourier transformed into a spectrum with the center frequency ν ₀ as the origin, the following formula (9) is obtained.

As described above, the frequency shift .DELTA..nu ₀ (forward .DELTA..nu _0/2, a backward .DELTA..nu _0/2, reciprocating .DELTA..nu ₀ in) because it was a fine, to each of the cos component and the sin component in the formula (9) On the other hand, the approximate expression of the following expression (10) was applied.

  After the first optical signal under measurement propagating through the first branch path and the second optical signal under measurement propagating through the second branch path propagate back and forth along the optical path of the interferometer, 2 When the signal to be measured is combined again by the branching and coupling unit, and the combined optical signal to be measured is detected by the optical detection unit, it is caused by interference between the first optical signal to be measured and the second optical signal to be measured. A power spectrum is obtained.

The power spectra of the interference components of the cos component and the sin component after recombination are expressed as | E ^cos (ν) | and | E ^sin (ν) |, respectively, and the polarization directions in the electric field spectra of the equations (7) and (9) Are obtained by superimposing the cos components of the first optical signal to be measured and the second optical signal to be measured, and the sin components 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. Each 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 reflected light signal and the second measured reflected light signal are the same, and the first measured reflected light signal and the second measured reflected light signal are the same. The cos component and the sin component also have the same polarization direction.

The interference fringe component in the interference between the cos component and the sin component in the first measured reflected light signal and the second measured reflected light signal is expressed by | E ^cos (ν) | and | E ^sin (ν) | include.
In the present embodiment, when recombining the first measured reflected light signal and the second measured reflected light signal to obtain the above equation (13), as described above, the first measured reflected light is used. Either one of the optical signal and the second measured reflected optical signal is alternately shifted in phase from 0 degree to 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 is performed in each of the reciprocations of the optical path of the interferometer, the polarization directions of the first measured reflected light signal and the second measured reflected light signal are the same.

As a result, the cos component (upper stage) and the sin component (lower stage) in the optical 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 in the forward path and the return path is alternately changed by two values of 0 degree (cos component acquisition) and 45 degrees (sin component acquisition). As a result, in the multiplexing of the first measured reflected light signal and the second measured reflected light signal, the phase difference between the first measured reflected light signal and the second measured reflected light signal alternates between 0 degrees and 90 degrees. Therefore, 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 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 expression on the rightmost side, the first measured optical signal (and the first measured reflected light signal) propagating through the first branch path and the second measured optical signal propagating through the second branch path. The following formula (15) was applied, assuming that the power of the cos component and the sin component in each of the measurement light signal (and the second measured reflected light signal) are 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 wavelength 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. 3 is a block diagram illustrating a configuration example of the chromatic dispersion measuring apparatus according to the present embodiment.
In FIG. 3, the chromatic dispersion measuring apparatus includes an incident optical fiber 1 as an incident path, an optical circulation unit 2, a connecting optical fiber 3, an optical branching and coupling unit 4, a first optical fiber 5 as a first optical branching path, A second optical fiber 6 as an optical branch path, an optical delay unit 7, an optical phase shift unit 8 as an optical phase shifter, a termination unit 11, a termination unit 12, an outgoing light introducing optical fiber 13, an optical frequency sweep unit 14, and an output An outgoing optical fiber 15, a light detection unit 16, a control unit 17, a phase control line 18, a sweep frequency control line 19, and a detection control line 20 are provided as an emission path. Here, the distance of the light propagation path in the interferometer is the distance of the reciprocating optical path from the entrance / exit end to the end portion of the optical branching and coupling unit 4 and from the end portion to the entrance / exit end.

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 incident on the chromatic dispersion measuring device via the incident optical fiber 1, and the incident optical pulse is used as an optical signal to be measured. Then, the light is emitted to the optical circulation unit 2 through the connection optical fiber 3 having one end connected to the light incident / incident end (first light incident / exit end) (arrow S1).
Further, the optical signal under measurement propagates back and forth from the optical circulation unit 2 to the first termination unit 11 of the first optical fiber 5 and from the optical circulation unit 2 to the second termination unit of the second optical fiber 6. Become. Here, the direction of the forward path in which the light to be measured propagates from the optical circulation unit 2 to the termination unit (the termination unit 11 and the termination unit 12) via the optical branching and coupling unit 4 is a traveling direction, and the optical branching and coupling unit from the termination unit to the traveling direction. The return path returning to the optical circulation unit 2 via 4 will be described as the reflection direction (retreat direction). Therefore, with respect to the first measured optical signal traveling from the optical branching and coupling unit 4 to the termination unit 11, the light reflected by the termination unit 11 and returning to the optical branching and coupling unit 4 is defined as the first measured reflected light signal. On the other hand, with respect to the second measured optical signal traveling from the optical branching and coupling unit 4 to the termination unit 12, the light reflected by the termination unit 12 and returning to the optical branching and coupling unit 4 will be described as the second measured reflected light signal. .

  When the optical circulating unit 2 receives a measured optical signal from the first incident end (first incident end) connected to the other end of the connection optical fiber 3 via the connection optical fiber 3 in the traveling direction. The incident optical signal to be measured is emitted to the connection optical fiber 3 having one end connected to the first input / output end (first input / output end) (arrow S2), and connected light in the reflection direction. A combined optical signal to be measured that enters the first input / output end from the one end of the fiber 3 is output to the output light introducing optical fiber 13 connected to the first output end (first output end) ( Arrow S12). As an optical component used for this optical circulation part 2, there exists an optical fiber circulator, for example.

  FIG. 4 is a diagram illustrating a configuration example of the light circulation unit 2 and illustrates a configuration example using an optical component other than the optical circulator. 4 has an optical isolator 21 and a 2 × 1 optical coupler 22. The 2 × 1 optical coupler 22 emits the measured optical signal incident from the incident end corresponding to the first incident end from the terminal as the first input / output end, and enters from the terminal as the first entrance / exit end. The combined optical signal to be measured is output from the terminal serving as the first output end to the output light introducing optical fiber 13. In addition, an optical isolator 21 is disposed at a terminal corresponding to the first input / output end so as not to return the combined optical signal to be measured as reflected light to the light source of the optical light to be measured. Further, since the circulator has a narrow frequency band for propagation, the configuration shown in FIG. 4 can cover a wide band with respect to the optical circulator when the band for measurement is very wide.

  The optical branching and coupling unit 4 has a first input / output end (second input / output end) connected to the other end of the connection optical fiber 3 and branches the optical signal to be measured incident from the connection optical fiber 3 into two parts, One branched light beam is a first measured light signal, and the other branched light beam is a second measured light signal. Further, the optical branching / coupling unit 4 emits the first optical signal to be measured from the second incident / exit end (third incident / exit end) to the first optical fiber 5 (arrow S3), while the second measured signal is measured. An optical signal is emitted from the third incident / exit end (fourth incident / exit end) to the second optical fiber 6 (arrow S4). 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). In the optical branching and coupling unit 4, the other end of the optical fiber 5 is connected to the second entrance / exit end, and the other end of the second optical fiber 6 is connected to the third entrance / exit end.

Further, the optical branching and coupling unit 4 shifts the frequency between the first measured optical signal and the second measured optical signal so that the first measured optical signal and the second measured optical signal have different frequencies. Make a difference. For example, in this embodiment, the optical branching and coupling unit 4 includes the first optical signal to be measured that is emitted from the second incident / exit end to the first optical fiber 5 and the second optical fiber from the other incident end / third incident / exit end. between the second measured light signal emitted to 6, to generate a carrier frequency difference .DELTA..nu _0/2. At this time, the light branching and coupling section 4 does not perform the frequency shift relative to the first optical signal to be measured, giving a carrier frequency difference .DELTA..nu _0/2 with respect to the second optical signal to be measured. At this time, the first measured optical signal and the second measured optical signal have the same polarization direction.

The optical branching and coupling unit 4 uses, for example, an AOFS (acousto-optic frequency shifter). The AOFS zero-order light output port is connected to one end of the first optical fiber 5, and the primary light output port is connected to one end of the second optical fiber 6. AOFS, when high frequency of .DELTA..nu _0/2 is supplied, outputs a first measured optical signal from the zero-order light output ports are not frequency-shifted, whereas the frequency .DELTA..nu from the primary light output port _0/2 by the frequency The shifted second optical signal to be measured is output.
Further, the optical branching and coupling unit 4 includes the first measured reflected light signal (arrow S <b> 9) reflected by the first terminal unit 11 and incident from the second input / output end, and the first reflected light signal reflected from the second terminal unit 12. Polarization of the first measured optical signal and the second measured optical signal in order to recombine the second measured optical signal (arrow S10) incident from the 3 entrance / exit ends and acquire the interference component by recombination. The light is emitted with the same direction (arrow S12).
The first measured reflected light signal is simply incident on the second incident / exit of the optical branching / coupling unit 4 via the first optical fiber 5, and the second measured reflected light signal is incident on the optical branching / coupling unit via the second optical fiber 6. 4 is incident on the third entrance / exit end.

An optical delay unit 7 is inserted in the path of the first optical fiber 5. This 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 5 and the second optical fiber 6 the same. A delay for adjustment to be eliminated is given to the optical signal under measurement.
Thus, 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 5 and the second optical fiber 6 can be reduced. The measurement accuracy of the change Δφ (ν) in the spectral phase in 14) can be improved.
If the optical path length difference between the first optical fiber 5 and the second optical fiber 6 does not affect the measurement accuracy, there is no need to provide the optical delay unit 7.

  The first termination unit 11 is a total reflection mirror provided at the other end (termination) of the first optical fiber 5, and totally reflects the first optical signal to be measured (arrow S <b> 5) that has propagated through the first optical fiber 5. As a first reflected signal to be measured, the propagation direction is reversed and emitted to the first optical fiber 5 in the reflection direction (arrow S7).

  An optical phase shift unit 8 is interposed in the path of the second optical fiber 6. The optical phase shift unit 8 alternately shifts the phases of the second measured optical signal and the second measured reflected optical signal propagating through the second optical fiber 6 to 0 degrees and 45 degrees in a fixed first period. Shift. That is, the optical phase shift unit 8 has a constant phase difference given between the first measured optical signal propagating through the first optical fiber 5 and the second measured optical signal propagating through the second optical fiber 6. Second measured reflected light propagating through the second optical fiber 6 with respect to the first measured reflected light signal propagating through the first optical fiber 5 and alternately switching between 0 degree and 45 degrees in the first period The phase difference applied to the signal is alternately switched between 0 degrees and 45 degrees in a constant first period. As a result, when the first measured reflected light signal and the second measured reflected light signal are incident on the optical branching and coupling unit 4 in the reflection direction, a phase difference of 0 degrees and 45 degrees is given in the forward path, and in the return path. Since the phase difference of 0 degree and 45 degree is given, the phase difference between the second measured light reflected signal is between 0 degree and 90 degrees by the sum of the phase differences given in the round trip with respect to the first measured reflected light signal. Will be shifted. Here, the optical phase shift unit 8 shifts the phase of the second measured optical signal with respect to the first measured optical signal, but the polarization direction of the second measured optical signal is changed to the first measured optical signal even after the shift. Output as the same signal.

  As a result, the first measured reflected light signal propagating through the first optical fiber 5 and the second measured reflected light signal propagating through the second optical fiber 6 when entering the optical branching and coupling section 4 in the reflection direction. When the phase difference between them is 0 degree, the cos component detection mode can be set, and when the phase difference is 90 degrees, the sin component detection mode can be set. A cos component and a sin component in the first measured reflected light signal are expressed by Expression (6). Further, the second measured reflected light 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.

Here, as described above, the polarization directions of the first measured optical signal and the second measured optical signal emitted from the optical branching and coupling unit 2 are the same, and light is emitted with respect to the first measured optical signal. The polarization direction of the second measured optical signal whose phase difference is 0 degree or 45 degrees by the phase shift unit 8 is also the same, and the optical phase shift unit 8 performs the phase difference with respect to the first measured reflected light signal. The polarization direction of the second measured reflected light signal set to 0 degree or 45 degrees is also the same.
Therefore, by switching the phase difference generated in the optical phase shift unit 8 between 0 degree and 45 degrees, the phase difference of the second measured reflected light signal with respect to the first measured reflected light signal is reciprocated. In the first cycle, 0 degrees and 90 degrees are set, and it is possible to select which of the cos component and the sin component in the orthogonal two components is detected.
In the present embodiment, when the phase of the second measured reflected light signal is shifted by 0 degree with respect to the phase of the first measured reflected light signal, the first measured reflected light signal and the second measured reflected light signal When the phase of the second measured reflected light signal is shifted by 90 degrees with respect to the phase of the first measured reflected light signal, the first measured reflected light signal and the second measured reflected light are interfered. Interference of the sin component of the optical signal occurs. In other words, when the phase shift difference between the first measured reflected light signal and the second measured reflected light signal is 0 degree, the optical branching and coupling unit 4 determines the first measured reflected light signal and the second measured reflected light signal. When the interference component in the cos component is output as a combined optical signal to be measured and the phase shift difference is 90 degrees, the interference component in the sine component of the first measured reflected light signal and the second measured reflected light signal is combined. Output as measurement optical signal.

For example, a phase shifter using an electro-optic crystal (for example, LiNb0 ₃ ) can be used for the optical phase shift unit 8, and by changing applied phase shift voltages (V ₀ and V ₄₅ described later), The amount of phase shift can be switched between 0 degrees and 45 degrees.
Thereby, the optical phase shift unit 8 shifts the phase of the second measured optical signal propagating in the traveling direction through the second optical fiber 6 by 0 degree or 45 degrees in the forward path with respect to the first measured optical signal, Further, the phase of the second measured reflected light signal propagating in the reflection direction through the second optical fiber 6 is shifted by 0 degree or 45 degrees in the return path with respect to the first measured reflected light signal. As a result, the optical phase shift unit 8 causes the first measured reflected light signal to be emitted from the optical branching / coupling unit 4 in the traveling direction when it enters the optical branching / coupling unit 4 in the round trip return path. With respect to the optical signal to be measured, the phase is alternately shifted between 0 degree and 90 degrees in the first cycle. Here, the optical phase shift unit 8 sets the phase of the second measured optical signal with respect to the first measured optical signal and the phase of the second measured reflected optical signal with respect to the first measured reflected optical signal. Although shifted, the light is emitted with the same polarization direction.

Further, the configuration shown in FIG. FIG. 5 is a diagram illustrating another configuration example of the optical branching and coupling unit 4.
In FIG. 5, the optical branching and coupling unit 4 includes a 2 × 1 optical coupler 41 and a SAW (surface acoustic wave) filter 42. A SAW filter 42 is inserted into one of the two terminals from which the branched light of the 2 × 1 optical coupler 41 is emitted so as to generate a frequency shift. Then, the SAW filter 42, the second to the second optical signal to be measured with respect to the optical signal to be measured, and the second second frequency .DELTA..nu _0/2 to the measured reflected light signal to the measurement reflected light signal Give a shift.

In the present embodiment, the optical delay unit 7 is inserted into the first optical fiber 5 and the optical phase shift unit 8 is inserted into the second optical fiber 6. An optical delay unit 7 is inserted into one of the first optical fiber 5 and the second optical fiber 6 with a short optical path length, and an optical phase shift unit 8 is inserted into 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 14 has an incident end (second incident end) connected to the other end of the outgoing light introducing optical fiber 13 and an outgoing end (second outgoing end) connected to one end of the outgoing optical fiber 15. The optical frequency sweep unit 14 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. Here, the optical frequency sweeping unit 14 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 14 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 outgoing light introducing optical fiber 13, that is, the combined optical signal to be measured. Perform frequency decomposition (spectral decomposition). The optical frequency sweep unit 14 emits the component-measured optical signal after frequency decomposition (a signal indicating the spectral intensity for each frequency) from the emission end to the emission optical fiber 15. The frequency used for frequency resolution of the combined optical signal to be measured, which is the combined 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. One end of the outgoing optical fiber 15 is connected to the outgoing end of the optical frequency insertion / retraction unit 14.

The second termination portion 12 is a total reflection mirror provided at the other end (termination) of the second optical fiber 6, like the first termination portion 11, and has been propagated through the second optical fiber 6. The optical signal (arrow S6) is totally reflected, and as a second measured reflected signal, the propagation direction is reversed and emitted to the second optical fiber 6 in the reflection direction (arrow S8).
The first termination portion 11 and the second termination portion 12 are, for example, a metal film such as Al (aluminum), Ag (silver), Au (gold), or a dielectric film (single phase or multilayer) on a silicon substrate or a glass substrate. A reflecting mirror coated with the above may be used. Alternatively, the end surfaces at the other ends of the first optical fiber 5 and the second optical fiber 6 may be coated with a metal to form a reflection surface, and the reflection surfaces may be used as the first termination portion 11 and the second termination portion 12. good.

The light detector 16 has an incident end (third incident end) connected to the other end of the outgoing optical fiber 15. The light detection unit 16 converts the component optical signal to be measured incident from the outgoing optical fiber 15 into an electric signal, and outputs the conversion result as an interference signal to the control unit 17 via the detection control line 20.
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 45 degrees, since the phase difference is 90 degrees in the round trip, the interference component is a sin component of the corresponding frequency.

In the present embodiment, each of the incident optical fiber 1, the connecting optical fiber 3, the first optical fiber 5, and the second optical fiber 6 is a polarization maintaining optical fiber (PMF) having polarization maintaining characteristics. Is used. The incident optical fiber 1, the connecting optical fiber 3, the first optical fiber 5, and the second optical fiber 6 are all aligned in the same direction, and the first optical signal to be measured and the second optical light to be measured The optical branching and coupling unit 4 for combining the first measured reflected light signal and the second measured reflected light signal so that the polarization directions of the signal, the first measured reflected light signal, and the second measured reflected light signal are the same. It is made to enter.
Therefore, the polarization directions of the first measured reflected light signal and the second measured reflected light signal that are multiplexed by the optical branching and coupling unit 4 are aligned. 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 outgoing light introducing optical fiber 13 and the outgoing optical fiber 15.

The control unit 17 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 14 via the frequency control line 19. This phase shift voltage (V ₀ , V ₄₅ ) is alternately changed to supply the phase shift voltage to the optical phase shift unit 8 via the phase control line 18 in order 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 17 alternately receives cos component and sin component interference signals from the light detection unit 16 via the detection control line 20 in synchronization with the first period. Then, the control unit 17 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 the dispersion parameter at each frequency.

Next, the control unit 17 obtains the power spectrum of the cos component and the power spectrum of the sine component by using the level of the interference signal that is an input electric signal as the power spectrum, and obtains the power spectrum of Expression (11). .
Further, the control unit 17 obtains a power spectrum of a pair of cos component and sin component for each frequency by using the equation (13) obtained by modifying the equation (11), and by using the equation (14), the change in phase for each frequency. Δφ (ν) can be obtained.
Then, the control unit 17 calculates the dispersion parameter of the frequency ν (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 14.

In addition, the control unit 17 sets 2n times the first cycle as a frequency sweep cycle, generates a trigger signal indicating the start of the frequency sweep cycle, outputs a trigger signal to the optical frequency sweep unit 14, and in a measurement frequency range, 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. 3 in the present embodiment will be described with reference to FIG. FIG. 6 shows the frequency sweep operation of the optical frequency sweep unit 14, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the sampling of an interference signal that is an electrical signal from the light detection unit 16 in the control unit 17. It is a wave form diagram which shows the timing of operation | movement.
That is, FIG. 6A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 14 with the vertical axis representing voltage and the horizontal axis representing time. In FIG. 6A, the H level (V _H ) and L level (V _L ) of the trigger signal output from the optical frequency sweep unit 14 are controlled using TTL control (TTL (Transistor Transistor Logic) interface). ).

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 14. 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). 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. 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 V ₀ is a voltage when the phase difference is 0 degree (cos component), and the phase shift voltage V ₄₅ is a voltage when the phase difference is 45 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.
In FIG. 6D, the vertical axis represents voltage, the horizontal axis represents time, and the timing of the sampling period at which the control unit 17 receives an interference signal, which is an electrical signal from the light detection unit 16, as time-series data. FIG.
6 (c) and 6 (d), in order to clearly describe the first period, the horizontal time scale of each of FIGS. 6 (a) and 6 (b) is expanded, and a part of time Only the range is shown.

The optical frequency sweep unit 14 generates a trigger signal and outputs the trigger signal to the control unit 17 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 ν1 to the frequency ν2 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 17 alternately outputs the phase shift voltage V ₀ and the phase shift voltage V ₄₅ 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 may be supplied from the phase shift voltage V ₄₅ .
As a result, the optical phase shift unit 8 uses the phase shift voltage V ₀ and the phase shift voltage V ₄₅ to be supplied with respect to the phase of the first measured optical signal propagating in the traveling direction through the first optical fiber 5. The phase of the second measured optical signal propagating through the two optical fibers 6 in the traveling direction is shifted by 0 degree or 45 degrees, and the phase of the first measured reflected optical signal propagating through the first optical fiber 5 in the reflecting direction The phase of the second measured reflected light signal propagating in the reflection direction through the second optical fiber 6 is shifted by 0 degree or 45 degrees.
For each first period Δt, the light detection unit 16 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 controller 17.
For each first period Δt, the light detection unit 16 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 controller 17.

And the control part 17 is a component which has the interference element of a cos component in each frequency alternately by sampling the said component to-be-measured optical signal in the center part of 1st period (DELTA) t synchronizing with 1st period (DELTA) t, for example. 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, in the reciprocation of the optical path of the interferometer, the phase of the second measured reflected light signal is changed from 0 degrees to 90 degrees with respect to the phase of the first measured reflected light signal, whereby a pair of cos component and sin component Interference elements can be obtained. As a result, a pair of interference elements of n cos components and sin components of each frequency ν and specific frequency ν is obtained from the 2n first period Δt within the measurement frequency range.
As a result, as described above, the control unit 17 obtains the phase change Δ (ν ₀ ) of the specific frequency ν ₀ measured in the same first period as the phase change Δφ (ν). Then, the control unit 17 calculates the dispersion parameter by substituting the change Δφ (ν) of the spectrum phase into the equation (5).

As described above, the control unit 17 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 measured optical signal by the sampling period shown in FIG. An electrical signal obtained by sampling the component measured optical signal at the frequency ν output from the optical frequency sweep unit 14 at the timing is acquired as an interference signal.
At this time, since the optical frequency sweeping unit 14 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 a 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 17 sets the sampling period of the component optical signal to be measured to 0.5 ms as in 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.

The optical signal to be measured follows an ITU grid at 100 GHz intervals, 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 for each interference signal, which is an electrical signal, is acquired at 1000 points, and the sampling interval of two orthogonal components, that is, 100 MHz is set as 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 14 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 16 is reduced, and the influence of measurement noise is increased.
However, in the reflection type spectrum shearing interferometer in the present embodiment, the frequency shift amount can be easily doubled as compared with the transmission type spectrum shearing interferometer that advances the measured optical signal only in one direction. Since the first measured reflected light signal that has not been frequency-shifted and the second measured reflected light signal that has been frequency-shifted can be sufficiently separated as shown in FIG. This effectively increases the frequency resolution. Therefore, according to the present embodiment, it is possible to improve the frequency resolution as compared with the transmission type spectrum shearing interferometer without reducing the amount of light.

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 14 is a value obtained by dividing the peak interval by the full width at half maximum of the transmission peak, that is, 4000. Here, supplies high frequency of the (Δν _0/2) and 200MHz to light branching and coupling section 4. For this reason, the frequency shift (Δν ₀ ) of the frequency of the second measured reflected light signal with respect to the frequency of the first measured reflected light signal is 400 MHz in the round trip of the optical path of the interferometer.
When the 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 sec, the influence of the phase fluctuation of the interferometer is expected to be 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 sec, the influence of phase fluctuation in frequency dispersion can be ignored.

In the present embodiment, the optical frequency sweep unit 14 is described as linearly sweeping the frequency. However, the sweep is stopped and the quadrature two components are measured during the sampling period of a 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 change Δφ (ν) of the spectrum phase 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, there is no need to spatially arrange components required in the spatial optical system, and the structure can be simplified and the apparatus can be miniaturized.
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.

In addition, according to the present embodiment, as already described, since the reflection type spectrum shearing interferometer is used, each branch path of the first optical fiber 5 and the second optical fiber 6 in the interferometer is measured. Since the optical signal propagates back and forth in the traveling direction (first measured optical signal, second measured optical signal) and reflection direction (first measured reflected optical signal, second measured reflected optical signal), the frequency shift The amount can be easily doubled compared to the conventional method, and the overlap on the frequency axis of the measured reflected light signal that has not been frequency-shifted and the measured reflected light signal that has been frequency-shifted is suppressed, and at the time of frequency decomposition Since the waveform separation characteristics on the frequency axis are improved, the frequency resolution can be improved.
In addition, according to the present embodiment, in order to obtain a constant frequency resolution, the high frequency supplied to the optical branching and coupling unit 4 can be halved, and power saving can be achieved compared to the case of supplying a double frequency. can do.
Furthermore, according to the present embodiment, the amount of phase shift in the optical phase shift unit 8 can be halved to 45 degrees in one direction so as to be 90 degrees in the reciprocation, so that the applied voltage is reduced. Therefore, the voltage can be lowered with the simplification of the apparatus.

In the present embodiment, the reflectance of either one or both of the first termination portion 11 and the second termination portion 12 may be variable.
Then, the control unit 17 determines the difference in light intensity between the first measured reflected light signal and the second measured reflected light signal incident on the optical branching and coupling unit 4 as the interference signal of the first light component and the first light component. Detection is based on an interference signal of two light components. At this time, when the detected difference exceeds a preset threshold value, the control unit 17 controls the reflectance of the terminal portion having a variable reflectance so that the difference is equal to or less than the threshold value.
For example, as a reflector, the end portion with variable reflectivity is made of metal so that the change in thickness has an inclination so that the thickness varies depending on the position so that the thickness gradually changes as the position changes. Alternatively, a dielectric film is coated on the substrate, and the reflectance is controlled depending on the position of the film used.

In the measurement of light intensity, when the control unit 17 uses a photodetector when detecting an interference signal, the interference component is detected as an AC voltage component as an output voltage value, while components having different light intensity are DC components. Will be detected as. Therefore, the control part 17 performs control which changes the position used for reflection in a reflecting plate so that a direct-current component is no longer detected. At this time, a detector capable of detecting an optical signal from alternating current to direct current is used for the light detection section 16, and a mechanism capable of moving the reflecting plate at the terminal section capable of changing the reflectance by a control signal from the control section 17. It is necessary to provide it.
The reflectance changing process described above is a configuration for controlling in real time when the balance of the light intensity is lost due to a temperature change.
Further, in order to adjust the balance of the light intensity after the creation, the operator may adjust the reflectivity while measuring the interference signal by the control unit 17.

Further, as described above, as a control of the light intensity balance, instead of changing the reflectance of the terminal portion, the light intensity adjusting unit is provided in either the first optical fiber 5 or the second optical fiber 6 or both. By means of this light intensity adjusting unit, the first measured optical signal and the first measured reflected light signal propagating through the first optical fiber 5, the second measured optical signal propagating through the second optical fiber 6, and the first You may comprise so that the control part 17 may adjust so that the light intensity with two to-be-measured reflected light signals may become the same.
The measurement of the light intensity is the same as that in the case of changing the reflectance of the terminal portion described above, and an attenuator or the like is used as the light intensity adjustment unit.
Then, the control unit 17 determines the difference in light intensity between the first measured reflected light signal and the second measured reflected light signal incident on the optical branching and coupling unit 4 as the interference signal of the first light component and the first light component. Detection is based on an interference signal of two light components. At this time, when the detected difference exceeds a preset threshold value, the control unit 17 controls the attenuation rate of light in the attenuator so that the difference becomes equal to or less than the threshold value.

<Second Embodiment>
Next, a chromatic dispersion measuring apparatus according to the second embodiment will be described. The second embodiment has the same configuration as that of the first embodiment, but in the configuration of FIG. 3, the cos component and the sin component are received in parallel by two reception ports provided in parallel to the control unit 17. It has the composition to do.
FIG. 7 shows the frequency sweep operation of the optical frequency sweep unit 14, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the interference signal that is an electrical signal output from the light detection unit 16 in the control unit 17. It is a wave form diagram which shows the timing of the operation | movement which performs this sampling.

FIG. 7A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 14 with the vertical axis representing voltage and the horizontal axis representing time. In FIG. 7A, the H level and L level of the trigger signal output from the optical frequency sweep unit 14 are set so as to conform to the TTL control.
In FIG. 7B, 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 14. In FIG. 7B, 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. 7C is a diagram illustrating a waveform of a phase shift voltage that changes the phase difference applied to the optical phase shift unit 8 in the first period, with the vertical axis representing voltage and the horizontal axis representing time. 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 ₄₅ is a voltage when the phase difference is 45 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. 7D, the vertical axis represents voltage, the horizontal axis represents time, and the control unit 17 parallelly transmits an interference signal, which is an electrical signal output from the light detection unit 16, from the reception port P1 and the reception port P2. It is a figure which shows the timing of the sampling period received as time series data. 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.
7 (c) and 7 (d), in order to clearly describe the first period, the horizontal time scale of FIGS. 7 (a) and 7 (b) is expanded, and only a part of the time range is displayed. Is shown.

When the control unit 17 outputs the phase shift voltage V ₀ from the light detection unit 16, the control unit 17 receives the component measured optical signal of the cos component through the reception port P 1 and outputs the phase shift voltage V _45. The component measured optical signal of the sin component is received by 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 period is 1 s as in the first embodiment, and the first period Δt, which is the phase shift voltage switching time, is 0.5 msec. The sampling period of each of the reception port P1 and the reception port P2 is 1 msec, and the sampling timing is shifted by 0.5 msec between the reception port P1 and the reception port P2.

The control unit 17 performs A / D (analog / digital) conversion, and acquires the voltage level of the interference signal, which is an electrical signal, from the light detection unit 16 as digital data. The control unit 17 includes an A / D conversion circuit that performs A / D (analog / digital) conversion processing on an interference signal that is an electrical signal.
Therefore, the operation speed of the A / D conversion circuit that performs A / D conversion of the interference signal in the control unit 17 may be a limiting factor when it is desired to shorten the sampling period. However, in this embodiment, by adopting dual 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. The operating speed limit can be increased by a factor of two. In this case, the control unit 17 performs an A / D conversion process on the interference signal input from the reception port P1, and an A / D conversion process on the interference signal input from the reception port P2. It has two A / D conversion circuits with the conversion circuit. The reception port P1 and the reception port P2 are configured by one system of interference signals, that is, one reception port.
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, It is set to a different length of time when measuring.
That is, in the measurement of the component optical signal to be measured output from the optical frequency sweep unit 14, the electrical signal is a pair for obtaining the change Δφ (ν) of the spectral phase input from the light detection unit 16 to the control unit 17. It is not necessary to measure the cos component and the sin component in a certain interference signal in the same time width in the sampling period.

When the control unit 17 acquires interference signals of two orthogonal components from the light detection unit 16 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.
This shortens the measurement interval between the cos component and sin component that form a pair of interference signals, reduces the amount of change in frequency associated with the time series switching of the cos component and sin component, and improves the measurement frequency accuracy. Can do.
Therefore, it is possible to improve the measurement accuracy of the change Δφ (ν) of the spectrum phase measured in the frequency unit and obtain the dispersion parameter with high accuracy.

In the waveform diagrams of FIG. 6D in the first embodiment and FIG. 7D in the second embodiment, the sampling period for acquiring the cos component and the sin component that are a pair of orthogonal two components of the interference signal is 2Δt. In the cos component measurement, the time for which the control unit 17 applies the phase shift voltage V ₀ to the optical phase shift unit 8 is Δt−δ, and the time for which the phase shift voltage V ₄₅ 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. Further, after applying the phase shift voltage, the time until the control unit 17 performs sampling, that is, the time from the start of the measurement time to the time when the control unit 17 samples the interference signal, which is an electrical signal, is expressed as a cos component and a sin component. In both cases, time δ _{0 is} assumed. Here, Δt-δ, with respect to Delta] t + [delta] and [delta] _0, the relationship shown in the following equation (16).

Therefore, the sum of the time for applying the phase shift voltage to the cos component and the sin component of the quadrature two components of the interference signal that is an electric signal is 2Δt, and the sampling period of the quadrature two components is the first implementation. It becomes the same as that of a form and 2nd Embodiment.
As to how to set δ and δ ₀ , the sampling conditions for how much the frequency resolution is set, the response time from the change of the phase shift voltage of the optical phase shift unit 8 to the actual change of the phase And it is determined by the operating frequency of the A / D conversion circuit that samples the interference signal from the light detection unit 16.
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. 8 is a block diagram showing 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 5 and the second optical fiber 6 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 comparison with the configuration of the first embodiment shown in FIG. 3, 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, Light having the function of combining the optical delay unit 7 and the optical phase shift unit 8 and integrating them to delay the propagation of light of the optical delay unit 7 and the function of shifting the phase difference of the light of the optical phase shift unit 8 The delay / optical phase shift unit 47 is provided on either the first optical fiber 5 or the second optical fiber 6. When either one of the first optical fiber 5 and the second optical fiber 6 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 can be implemented in any configuration of the first embodiment shown in FIG. 3 or the fourth embodiment shown in FIG. Will be described.
The fifth embodiment is different from the first embodiment in that the first embodiment performs sampling of an interference signal that is an electric signal from the light detection unit 16 and sweeps a pair of cos component and sin component with the same frequency. In contrast to the time series in the period, in the fifth embodiment, using the two frequency sweep periods, the interference signal that is an electrical signal of the cos component is sampled in one frequency sweep period, and the other This is a configuration in which an interference signal that is an electric signal of a sin component is sampled in the frequency sweep cycle.

Accordingly, the control unit 17 performs phase shift with respect to the optical phase shift unit 8 during the frequency sweep cycle for measuring the cos component in synchronization with the trigger signal output from the optical frequency sweep unit 14 in two frequency sweep cycles. 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 lengths of the frequency sweep periods for detecting the cos component and sin component interference signals are 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. 9 shows the frequency sweep operation of the optical frequency sweep unit 14, the phase shift operation of the optical phase shift unit 8 corresponding to this, and the sampling of interference signals that are electrical signals from the light detection unit 16 in the control unit 17. It is a wave form diagram which shows the timing of operation | movement.
FIG. 9A is a diagram illustrating the output timing of the trigger signal output from the optical frequency sweep unit 14, where the vertical axis is voltage and the horizontal axis is time. In FIG. 9A, the H level and L level of the trigger signal output from the optical frequency sweep unit 14 are set so as to conform to the TTL control.

In FIG. 9B, the vertical axis represents frequency and the horizontal axis represents time, and shows the time change of the resonance frequency output in the sweep of the optical frequency sweep unit 14. In FIG. 9B, 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. 9C is a diagram illustrating a waveform of the phase shift voltage that changes the phase difference applied to the optical phase shift unit 8 in the first period, with the vertical axis representing voltage and the horizontal axis representing time. 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 ₄₅ is a voltage when the phase difference is 45 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. 9D, the vertical axis represents voltage, the horizontal axis represents time, the control unit 17 indicates an interference signal from the light detection unit 16, and the control unit 17 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 17 uses the same measurement frequency (corresponding to the frequency resolution) in each of the frequency sweep cycle in which the frequency decomposition of the interference component of the cos component is performed and the frequency sweep cycle in which the frequency decomposition of the interference component of the sin component is performed. 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.
9 (c) and 9 (d), in order to clearly describe the first period, the horizontal time scale of FIGS. 9 (a) and 9 (b) is expanded, and only a part of the time range is displayed. Is shown.

In the present embodiment, since the interference signals of the cos component and the sin component are measured with the same sampling period in different frequency sweep periods, the mutual phase difference in the optical phase shift unit 8 is measured as in the first embodiment. The frequency change due to the switching does not occur, the cos component and sin component interference signals can be obtained at the same frequency, and the measurement accuracy of the measured spectral phase change Δφ (ν) can be improved.
However, the optical path length difference between the first optical fiber 5 and the second optical fiber 6 fluctuates every frequency sweep by causing the first optical fiber 5 and the second optical fiber 6 to slightly expand and contract with the vibration of the apparatus. Then, the phase difference between the first optical fiber 5 and the second optical fiber 6 deviates from 0 degree or 45 degrees. As a result, the orthogonality of the cos component and the sin component is lowered, and the measurement accuracy of the change Δφ (ν) in the spectral phase is lowered. For this reason, it is necessary to increase the stability against vibration of the first optical fiber 5 and the second optical fiber 6 so that the optical path length between the first optical fiber 5 and the second optical fiber 6 does not change with time.

<Sixth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the sixth embodiment will be described. The chromatic dispersion measuring apparatus according to the sixth embodiment can be implemented in any configuration of the first to fifth embodiments, and will be described below with reference to FIG.
In the sixth embodiment, the polarization direction of the combined optical signal to be measured incident on the optical frequency sweep unit 14 in FIG. 3 is controlled to coincide with the polarization axis of the bandpass optical filter in the optical frequency sweep unit 14, 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 14 uses a bandpass optical filter having a narrow bandpass frequency width and high finesse in order to improve the resolution with respect to frequency.
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 dependency occurring in the bandpass optical filter, a polarization controller is provided in front of the optical frequency sweep unit 14 composed of the bandpass optical filter, and the polarization direction of the incident light is determined to be bandpass light. Adjustment may be made so as to coincide with a specific polarization axis of the optical fiber constituting the filter to eliminate the polarization dependency. 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. 10 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 the optical fiber constituting the bandpass optical filter. The optical fiber 81 in FIG. 10 corresponds to the outgoing light introducing optical fiber 13 in FIG. 3. When 84 in FIG. 10 is the optical frequency sweep unit, it corresponds to the optical frequency sweep unit 14 in FIG. The fiber 85 is a polarization non-maintaining optical fiber and corresponds to the outgoing optical fiber 15 in 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. 10 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 via the connection optical fiber 83. The light enters from the optical branching and coupling part 4 via the.
When 84 is an optical frequency sweeping unit, the optical frequency sweeping unit 84 frequency-decomposes the combined measured optical signal by sweeping the bandpass frequency width in the measurement frequency range, and the component measured optical signal. The light is emitted to the light detection unit 16 through the optical fiber 85.

  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.

<Seventh Embodiment>
Next, a chromatic dispersion measuring apparatus according to the seventh embodiment will be described. FIG. 11 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 apparatus according to any one of the first to sixth embodiments. . In this figure, a chromatic dispersion measuring device 66 is a chromatic dispersion measuring device according to any one of the first to sixth 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 an optical signal to be measured, and monitors light. The light is emitted to the polarization controller 64 through the fiber 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. 3 or FIG. 8). Then, the measured optical signal is emitted to the incident optical fiber 65.
The incident optical fiber 65 is a polarization maintaining optical fiber, and the polarization axis is the polarization axis with the polarization maintaining optical fibers (the first optical fiber 5 and the second optical fiber 6) 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.

<Eighth Embodiment>
Next, a chromatic dispersion measuring apparatus according to the eighth embodiment will be described. FIG. 12 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 apparatus according to any one of the first to sixth embodiments. In this figure, a chromatic dispersion measuring device 78 is a chromatic dispersion measuring device according to any one of the first to sixth 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 is a polarization maintaining optical fiber, and the polarization axis is polarized with the polarization maintaining optical fibers (the first optical fiber 5 and the second optical fiber 6) inside the chromatic 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. 3 or FIG. 8). 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.

  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 circulation part 3,83 ... Connection optical fiber 4 ... Optical branching coupling part 5 ... 1st optical fiber 6 ... 2nd optical fiber 7 ... Optical delay part 8 ... Optical phase shift part 11 ... 1st Termination unit 12 ... second termination unit 13 ... outgoing light introduction optical fiber 14, 84 ... optical frequency sweep unit 15 ... outgoing optical fiber 16 ... light detection unit 17 ... control unit 18 ... phase control line 19 ... frequency control line 20 ... detection Control line 47 ... Optical delay / optical phase shift part 61 ... Optical fiber transmission line 62 ... Monitoring optical branching part 63 ... Monitoring optical fiber 64, 76, 82 ... Polarization controller, 65, 72, 77 ... Incident optical fiber 66, 78 ... wavelength dispersion measuring device 71 ... light source 73 ... incident light control unit 74 ... measurement object 75 ... emitting optical fiber 81, 85 ... optical fiber

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;
An optical signal to be measured is incident from a first incident end connected to the incident path, the optical signal to be measured is emitted from the first incident / exit end, and is measured from the first incident / exit end. An optical circulation unit (optical circulation unit 2) that outputs an optical signal from the first emission end;
A first connection path (optical fiber 3) composed of an optical fiber having polarization maintaining characteristics, one end connected to the first input / output end;
The first optical signal to be measured is separated into two optical signals, ie, a first optical signal to be measured and a second optical signal to be measured, from a second input / output end connected to the other end of the first connection path. The optical signal is emitted from a third input / output end, has the same polarization direction as the first optical signal to be measured, and is shifted by a preset frequency with respect to the first optical signal to be measured. Two measured optical signals are emitted from the fourth incident / exit end, while the first measured reflected optical signal incident from the third incident / exit end and the second incident from the fourth incident / exit end A signal obtained by shifting the measured reflected light signal with respect to the first measured reflected light signal by the frequency is combined, and the combined measured optical signal is output to the second input / output as an interference result by the combining. A light branching and coupling part (light branching and coupling part 4) emitted from the end;
A first branch path (optical fiber 5) composed of an optical fiber having polarization maintaining characteristics, one end of which is connected to the third input / output end;
A second branch path (optical fiber 6) composed of an optical fiber having polarization maintaining characteristics, one end connected to the fourth input / output end;
The first measured optical signal incident from the fifth incident / exit end connected to the other end of the first branch path is totally reflected, and the first measured reflected light signal is reflected from the fifth incident / exit end to the first A first termination portion (first termination portion 11) that emits to one branch path;
The second measured optical signal incident from a sixth incident / exit end connected to the other end of the second branch path is totally reflected, and the second measured reflected light signal is reflected from the sixth incident / exit end as the second measured reflected light signal. A second termination portion (second termination portion 12) that exits to the second branch path;
The phase of each of the optical signal to be measured and the optical signal to be measured, which is provided on one of the first branch path and the second branch path and propagates through the provided branch path, is set to 0 degree and 45 degrees. An optical phase shifter (optical phase shift unit 8) that alternately changes in binary;
A measurement coupling path (outgoing light introducing optical fiber 13) composed of an optical fiber connected to the first emitting end and propagating the combined optical signal to be measured;
The combined optical signal to be measured is incident from a second incident end connected to the measurement coupling path, the frequency range through which the optical signal to be combined passes is swept, and the combined optical signal to be measured is An optical frequency sweep unit (optical frequency sweep unit 14) that performs frequency decomposition to extract spectral components in the frequency range, and outputs the result of frequency decomposition as a component measured optical signal from the second output end;
An outgoing light path (outgoing optical fiber 15) composed of an optical fiber connected to the second outgoing end and propagating the component measured optical signal;
A light detection unit (light detection) that enters the component measured light signal from a third incident end connected to the outgoing light path, converts the component measured light signal into an electrical signal, and uses the conversion result as an interference signal. Part 16),
An interference signal of a first optical component between the first measured reflected light signal and the second measured reflected light signal when the phase difference is set to 0 in synchronization with a change in the phase difference of the optical phase shifter; and A control unit (control unit 17) for acquiring an interference signal of a second optical component between the first measured reflected light signal and the second measured reflected light signal in the case where the phase difference is 45 degrees; A chromatic dispersion measuring apparatus comprising: The reflectivity of either the first terminal part or the second terminal part or both terminal parts is variable,
The control unit calculates a difference in light intensity between the first measured reflected light signal and the second measured reflected light signal incident on the optical branching and coupling unit, an interference signal of the first light component, and a second When the voltage value of the direct current component in the waveform with the interference signal of the light component is detected and the voltage value exceeds a preset threshold value, the reflectance of the terminal portion with variable reflectivity is less than the threshold value. The chromatic dispersion measuring apparatus according to claim 1, wherein control is performed so that A light intensity adjusting unit is inserted in either or both of the first branch path and the second branch path,
The control unit calculates a difference in light intensity between the first measured reflected light signal and the second measured reflected light signal incident on the optical branching and coupling unit, an interference signal of the first light component, and a second When the voltage value of the waveform with the interference signal of the light component is detected and the voltage value exceeds a preset threshold value, the first measured reflected light signal and the second measured reflected light are detected by the light intensity adjusting unit. The chromatic dispersion measuring apparatus according to claim 1, wherein the voltage value, which is a difference in reflectance intensity with respect to a signal, is controlled to be equal to or less than the threshold value.   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. The chromatic dispersion measuring apparatus according to any one of claims 1 to 3.   5. The chromatic dispersion measuring apparatus according to claim 4, 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.   5. The chromatic dispersion measuring apparatus according to claim 4, 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.   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 6.   The control unit acquires the first light component and the second light component in a time series as a data pair for each interference signal as a measurement unit for each measurement frequency sweep in a measurement range. The chromatic dispersion measuring apparatus according to claim 1.   In the first light component and the second light component constituting the data pair for each interference signal, the time for changing the phase of the light component measured earlier is changed to the polarization component measured later. The chromatic dispersion measuring apparatus according to claim 8, wherein the chromatic dispersion measuring apparatus is set to be short with respect to a time for performing the measurement.   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 measuring apparatus is alternately and repeatedly performed on the light component and the second light component.   The measurement coupling path is composed of an optical fiber having a polarization maintaining characteristic, and a polarization controller for controlling the polarization direction of the optical signal to be combined is inserted in the subsequent stage of the measurement coupling path, and the polarization The chromatic dispersion measuring apparatus according to claim 1, wherein the controller and the optical frequency sweeping unit are connected by an optical fiber having polarization maintaining characteristics. 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 11,
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 11,
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 method for measuring chromatic dispersion, comprising determining a change in spectral phase and evaluating chromatic dispersion in the measurement target.

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