Polarization characteristics measuring device and optical spectrum analyzer

Abstract

To enable wide dynamic range spectroscopy of signal light and measurement of polarization states.SOLUTION: The present disclosure relates to a polarization characteristics measuring device and an optical spectrum analyzer characterized by having: a diffraction grating that spectrally separates light being measured in a predetermined wavelength; and a Stokes parameter measurement unit (12) that branches the light being measured for each wavelength spectrally separated by the diffraction grating into four beams of light, and measures a Stokes parameter from each branched beam of light, on the basis of the power I0 of a linearly polarized light component of azimuth angle 0 degree, power I90 of a linearly polarized light component of azimuth angle 90 degrees, power I45 of a linearly polarized light component of azimuth angle 45 degrees, and power Iq45 of a circularly polarized light component detected using a 1 / 4 wavelength plate and a polarizer, and also characterized in that either the fast axis or the slow axis of the 1 / 4 wavelength plate forms an angle of 45 degrees with respect to the rulings direction of the diffraction grating.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a polarization characteristic measuring device and an optical spectrum analyzer. [Background technology]

[0002] An optical spectrum analyzer has been proposed that includes a Stokes parameter measurement unit and is capable of measuring the optical spectrum and polarization state of signal light such as a WDM (Wavelength Division Multiplexing) signal (see, for example, Patent Document 1). As described in paragraph 0017 of Patent Document 1, a spectrometer that separates signals for each wavelength is preferably one that has little polarization dependency, such as an etalon spectrometer.

[0003] However, etalon spectrometers have difficulty in dispersing light over a wide dynamic range. In contrast, spectrometers using diffraction gratings can disperse light over a wide dynamic range. However, the diffraction efficiency of diffraction gratings differs greatly between P-polarized and S-polarized light, which makes it difficult to accurately measure the polarization state of the signal light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-002190 Summary of the Invention [Problem to be solved by the invention]

[0005] The present disclosure aims to enable wide dynamic range spectroscopy and measurement of the polarization state of signal light. [Means for solving the problem]

[0006] The polarization characteristic measuring device of the present disclosure includes: a diffraction grating (11g) for separating the light to be measured into predetermined wavelengths; The light to be measured is split into four beams for each wavelength separated by a diffraction grating, and the power I0 of the linearly polarized component at an azimuth angle of 0 degrees and the power I of the linearly polarized component at an azimuth angle of 90 degrees are measured from each of the split beams. 90 and the power I of the linearly polarized component at an azimuth angle of 45 degrees 45 and the power of the circularly polarized component detected using a quarter-wave plate and a polarizer I q45 a Stokes parameter measurement unit (12) for measuring Stokes parameters based on the above; Equipped with One of the fast and slow axes of the quarter-wave plate is at a 45-degree angle with respect to the ruling direction of the diffraction grating.

[0007] The optical spectrum analyzer of the present disclosure comprises: A polarization characteristic measuring device (91) according to the present disclosure; a rotating unit (15) that rotates the diffraction grating; a wavelength control unit (16) that controls the rotating unit to select a wavelength to be measured; a storage unit (18) that stores the Stokes parameters for each wavelength acquired by the polarization characteristic measurement device; a display unit (19) that displays the Stokes parameters for each wavelength stored in the storage unit; Equipped with.

[0008] The polarization characteristic measuring method of the present disclosure includes: A diffraction grating (11g) separates the light to be measured into predetermined wavelengths, The Stokes parameter measurement unit (12) splits the light to be measured into four beams for each wavelength split by the diffraction grating, and measures the power I0 of the linearly polarized component at an azimuth angle of 0 degrees and the power I of the linearly polarized component at an azimuth angle of 90 degrees from each of the split beams. 90 and the power I of the linearly polarized component at an azimuth angle of 45 degrees 45 and the power of the circularly polarized component detected using a quarter-wave plate and a polarizer I q45 and measuring the Stokes parameters based on A polarization characteristic measuring method, comprising: One of the fast axis and the slow axis of the quarter-wave plate is at an angle of 45 degrees to the ruling direction of the diffraction grating.

[0009] The above disclosures can be combined as much as possible. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to perform wide dynamic range spectroscopy and measurement of the polarization state of signal light. [Brief explanation of the drawings]

[0011] [Figure 1] 1 shows an example of the configuration of a polarization characteristic measuring device according to the present disclosure. [Figure 2] 1 illustrates an example embodiment of an optical spectrum analyzer of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. Note that the present disclosure is not limited to the embodiments shown below. These implementation examples are merely illustrative, and the present disclosure can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art. Note that components with the same reference numerals in this specification and drawings indicate the same components.

[0013] FIG. 1 shows an example of the configuration of a polarization characteristic measurement device according to the present disclosure. A polarization characteristic measurement device 91 according to this embodiment includes a spectroscopic unit 11 and a Stokes parameter measurement unit 12. The spectroscopic unit 11 separates the light under measurement into predetermined wavelengths. The light under measurement can be any signal light, such as a WDM signal. The Stokes parameter measurement unit 12 measures each polarization component required for the Stokes parameters for each wavelength separated by the spectroscopic unit 11.

[0014] The Stokes parameter measurement unit 12 calculates Stokes parameters using the obtained polarization components, and calculates the SOP (State of Polarization) and OSNR (Optical Signal to Noise Ratio) of the light under measurement using the Stokes parameters.

[0015] The polarization characteristics measurement device 91 of this embodiment executes the polarization characteristics measurement method of the present disclosure. In the polarization characteristics measurement method of this embodiment, a diffraction grating provided in the spectroscopic unit 11 separates the light to be measured into predetermined wavelengths, and a Stokes parameter measurement unit 12 measures the Stokes parameters for each wavelength separated by the diffraction grating. This Stokes parameter measurement unit 12 splits the light into four beams and measures the power I0 of the linearly polarized component with an azimuth angle of 0 degrees and the power I 90 and the power of the linearly polarized component at an azimuth angle of 45°. 45 and the power of the circularly polarized component using a quarter-wave plate and a polarizer. q45 The Stokes parameters are detected based on the above formula, and either the fast axis or the slow axis of the quarter-wave plate is at an angle of 45 degrees with respect to the ruling direction of the diffraction grating.

[0016] Fig. 2 shows an embodiment of an optical spectrum analyzer equipped with a polarization characteristics measurement device 91. The optical spectrum analyzer 1 comprises a spectroscopic unit 11, a Stokes parameter measurement unit 12, a rotation unit 15, a wavelength control unit 16, a control unit 17, a storage unit 18, and a display unit 19. The spectroscopic unit 11 and the Stokes parameter measurement unit 12 are provided in the polarization characteristics measurement device 91. In the embodiment of Fig. 2, the optical spectrum analyzer is configured by further comprising the following components in addition to the polarization characteristics measurement device 91.

[0017] The rotating unit 15 rotates the diffraction grating 11g using a motor and an encoder. The rotating unit 15 is a means for rotating the diffraction grating 11g, and can be configured, for example, using an encoder for controlling the rotation angle of the diffraction grating 11g and a motor for rotating the encoder. In this embodiment, the rotating unit 15 rotates the diffraction grating 11g about a rotation axis parallel to the ruling direction of the diffraction grating 11g. By rotating the diffraction grating 11g using this mechanism, light of a desired wavelength is extracted from the exit slit 11s. The wavelength control unit 16 controls the motor and encoder provided in the rotating unit 15 to continuously sweep a specified wavelength range.

[0018] The Stokes parameter measurement unit 12 includes a polarization component measurement unit 12A and a calculation processing unit 12B. The polarization component measurement unit 12A measures the powers I0 and I 90 ,I 45 ,I q45 The calculation processing unit 12B measures the power I0, I 90 ,I 45 ,I q45 The Stokes parameters are calculated based on the above. As a result, the Stokes parameters for each wavelength are measured. The Stokes parameters for each wavelength acquired by the polarization characteristic measuring device 91 are stored in the memory unit 18, and the Stokes parameters for each wavelength are displayed on the display unit 19 together with the waveform of the optical spectrum. The control unit 17 controls the measurement range and the contents of the measurement results to be displayed on the display unit 19 in accordance with external instructions.

[0019] The spectroscopic unit 11 includes a collimator 11c that converts the light to be measured into parallel light, a diffraction grating 11g that receives the parallel light and outputs diffracted light, a condenser 11d that condenses the diffracted light from the diffraction grating 11g, and an exit slit 11s that passes the diffracted light condensed by the condenser 11d. As described above, in the present disclosure, the spectroscopic unit 11 performs spectroscopy using the diffraction grating 11g.

[0020] The Stokes parameter measurement unit 12 includes a collimator lens 121 that converts the light that has passed through the exit slit 11s into parallel light, and beam splitters 122 and 123 that split the parallel light from the collimator lens 121 into four beams. The Stokes parameter measurement unit 12 measures the Stokes parameters (S0, S1, S2, S3) using these four parallel lights.

[0021] In this embodiment, when measuring the Stokes parameters (S0, S1, S2, S3), the optical power I0 of the 0-degree linearly polarized component and the optical power I of the 90-degree linearly polarized component are measured. 90 , the optical power of the 45-degree linearly polarized component I 45 , the optical power of the circularly polarized component I q45 An example of measuring is shown below.

[0022] Specifically, the first light beam split by the beam splitter 122 is split by the polarizing beam splitter 124 into a 0-degree polarized component (x-polarized) and a 90-degree polarized component (y-polarized), and one of the lights is received by the photodetector 131. As a result, the optical power I of the 0-degree polarized component (x-polarized) is m0 The other light split by the polarizing beam splitter 124 is received by the photodetector 132. This allows the optical power I of the 90-degree polarized component (y-polarized light) to be measured. m90 can be detected.

[0023] The second light beam split by the beam splitter 122 is split by the beam splitter 123, and one of the split lights is passed through the polarizer 125 with an azimuth angle of 45° and received by the photodetector 133. As a result, the optical power I of the 45° polarized component (45° polarization) is m45 The polarizer 125 may be referred to as a first polarizer.

[0024] The second light beam split by the beam splitter 122 is passed through a quarter-wave plate 127 and a polarizer 126 with an azimuth angle of 45, and is received by a photodetector 134. As a result, the optical power I of the circularly polarized component is mq45 The polarizer 126 may be referred to as a second polarizer.

[0025] The calculation processing unit 12B calculates the diffraction efficiency E of the diffraction grating 11g for P-polarized light at each wavelength. P and the diffraction efficiency of S-polarized light E S The calculation processing unit 12B is provided with a memory for storing the optical power (I m0 ,I m90 ,I m45 ,I mq45 ) to the diffraction efficiencies E P and E S and corrected value (I0,I 90 ,I 45 ,I q45 ) to calculate the Stokes parameters (S0, S1, S2, S3) of the light under measurement.

[0026] The Stokes parameters (S0, S1, S2, S3) are the optical powers I0, I 90 , I 45 , I q45 It is expressed by the following equation using (Number 1) S0=I0+I 90 (1) S1=2I0-S0(2) S2=2I 45 -S0(3) S3=2I q45 -S0(4) Here, S0 is the total amount of light, S1 is the difference between the amount of x-polarized light and the amount of y-polarized light, S2 is the difference between the amount of 45-degree polarized light and the amount of 135-degree polarized light, and S3 is the difference between the amount of right-handed circularly polarized light and the amount of left-handed circularly polarized light. Completely polarized light is displayed as a polarization state on the Poincaré sphere with S1, S2, and S3 as the coordinate axes.

[0027] In this embodiment, the polarization axis of the polarizing beam splitter 124 is set to be parallel to the ruling direction of the diffraction grating 11g. As a result, the polarizing beam splitter 124 separates the light into a polarized component in the ruling direction of the diffraction grating 11g and a polarized component perpendicular to the ruling direction. Therefore, in this embodiment, the diffraction efficiency E of P polarized light for each wavelength in the diffraction grating 11g is calculated in advance. P and the diffraction efficiency of S-polarized light E S is stored, and the optical power I detected by the optical receivers 131 and 132 is m0 and I m90 Correct the following.

[0028] Specifically, the calculation processing unit 12B calculates the optical power I m0 and I m90 can be corrected. (Number 5) I0=I m0 / E S (5) I 90 =I m90 / E P (6)

[0029] The polarizer 125 has its polarization axis set at 45 degrees to the direction of the lines on the diffraction grating 11g. In this case, the optical power I detected by the photodetector 133 ism45 is a diffraction efficiency according to the ratio of the P-polarized component to the S-polarized component. Therefore, the calculation processing unit 12B calculates the optical power I m45 can be corrected. (Number 7) I 45 =I m45 ·{E S I0 / (I0+I 90 )+E P I 90 / (I0+I 90 ) (7)

[0030] In this embodiment, either the fast axis or the slow axis of the quarter-wave plate 127 forms an angle of 45 degrees with respect to the ruling direction of the diffraction grating 11g. Therefore, the fast-axis light and the slow-axis light are equally separated into P-polarized light and S-polarized light, and input to the polarizer 126. In this embodiment, the polarization axis of the polarizer 126 is set to be parallel or perpendicular to the ruling direction of the diffraction grating 11g. Therefore, when the light to be measured is clockwise circularly polarized, all of the light passes through the polarizer 126, and when it is counterclockwise circularly polarized, all of the light is blocked by the polarizer 126. Light linearly polarized in the ruling direction or perpendicular to the ruling direction is converted to circularly polarized light, and the optical receiver 134 detects half the optical power.

[0031] The optical power I detected by the optical receiver 134 mq45 is a diffraction efficiency according to the ratio of the P-polarized component to the S-polarized component. Therefore, the calculation processing unit 12B calculates the optical power I mq45 can be corrected. (Number 8) I q45 =I mq45 ·{E S I0 / (I0+I 90 )+E P I 90 / (I0+I 90 ) (8)

[0032] In this embodiment, the influence of the diffraction efficiency of the diffraction grating 11g can be eliminated by correcting the optical power using equations (5) to (8). Therefore, the polarization characteristic measuring device 91 according to this embodiment can perform both wide dynamic range spectroscopy and measurement of the polarization state.

[0033] The polarization characteristic measuring device 91 of the present disclosure measures the Stokes parameters at a given wavelength at a constant cycle, and thereby the arithmetic processing unit 12B can measure the time change of the state of polarization (SOP) based on the Stokes parameters.

[0034] When signal light of an arbitrary wavelength is separated from an optical signal propagating through a WDM transmission line with an optical fiber amplifier, the total light amount S0 includes the optical power I of the measured light. signal and ASE (Amplified Spontaneous Emission) noise power I ase Therefore, the following equation holds: (Number 9) S0=I total =I signal +I ase (9)

[0035] In addition, in the present disclosure, either the fast axis or the slow axis of quarter wave plate 127 forms an angle of 45 degrees with the ruling direction of diffraction grating 11g. Therefore, in the present disclosure, the following equation holds: (Number 10) S1=2I0-I total =2{I0(signal)+I ase / 2}-{I signal +I ase} =2I0(signal)-I signal (10) (Number 11) S2=2I 45 -I total =2{I 45 (signal)+I ase / 2}-{I signal +I ase} =2I 45 (signal)-I signal (11) (Number 12) S3=2I q45 -I total =2{I q45 (signal)+I ase / 2}-{I signal +I ase} =2I q45 (signal)-I signal (12)

[0036] From the above equation, I ase Since is canceled, the optical power I of the light under test is calculated using the following equation: signal can be calculated.

number

[0037] I ase can be calculated using the following formula:

number

[0038] The degree of polarization (DOP) is an index that indicates the ratio of polarized optical power to total optical power, and can be calculated using the following formula:

number

[0039] The polarization characteristics measurement device of the present disclosure measures the spectrum of the light under measurement and the Stokes parameters at any spectral peak, and can measure the OSNR corresponding to the peak wavelength or center wavelength of any WDM signal based on the Stokes parameters. The OSNR of the light under measurement can be calculated using the following equation: (Number 16) OSNR = S0 × DOP / {S0(1-DOP)} (16) =DOP / (1-DOP)

[0040] As described above, the present disclosure can measure the Stokes parameters at any wavelength at a constant cycle and measure the time change of the state of polarization (SOP) based on the Stokes parameters. In addition, one revolution on the Poincaré sphere (for example, one revolution around the equator) corresponds to a phase difference of 360° (2π), and measuring the SOPs of multiple wavelengths can also determine PMD (Polarization Mode Dispersion).

[0041] The arithmetic processing unit 12B of the present disclosure can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network. [Explanation of symbols]

[0042] 1: Optical spectrum analyzer 11: Spectroscopic section 11c: Collimator 11g: Diffraction grating 11d: Concentrator 11s: Exit slit 12: Stokes parameter measurement unit 121: Collimator lens 122, 123: Beam splitter 124: Polarizing beam splitter 125, 126: Polarizer 127:1 / 4 wave plate 131, 132, 133, 134: Receiver 12A: Polarization component measurement unit 12B: Processing unit 15: Rotating part 16: Wavelength control section 17: Control unit 18: Storage part 19: Display section 91: Polarization characteristic measuring device

Claims

1. a diffraction grating (11g) for separating the light to be measured into predetermined wavelengths; The light to be measured is split into four beams for each wavelength separated by the diffraction grating, and the power I of the linearly polarized component at an azimuth angle of 0 degrees is calculated from each of the split beams. ０ and the power I of the linearly polarized component at an azimuth angle of 90 degrees ９０ and the power I of the linearly polarized component at an azimuth angle of 45 degrees ４５ and the power I of the circularly polarized component detected using a quarter-wave plate and a polarizer. ｑ４５ a Stokes parameter measurement unit (12) for measuring Stokes parameters based on the above; Equipped with One of the fast axis and the slow axis of the quarter wave plate forms an angle of 45 degrees with respect to the ruling direction of the diffraction grating. Polarization characteristics measurement device.

2. The polarization characteristic measuring device (91) of claim 1; A rotating unit (15) that rotates the diffraction grating; a wavelength control unit (16) that controls the rotating unit to select a wavelength to be measured; a storage unit (18) for storing the Stokes parameters for each wavelength acquired by the polarization characteristic measuring device; a display unit (19) that displays the Stokes parameters for each wavelength stored in the storage unit; Optical spectrum analyzer with

3. A diffraction grating (11g) separates the light to be measured into predetermined wavelengths, A Stokes parameter measurement unit (12) splits the light to be measured into four beams for each wavelength split by the diffraction grating, and measures the power I of a linearly polarized component at an azimuth angle of 0 degrees from each of the split beams. ０ and the power I of the linearly polarized component at an azimuth angle of 90 degrees ９０ and the power I of the linearly polarized component at an azimuth angle of 45 degrees ４５ and the power I of the circularly polarized component detected using a quarter-wave plate and a polarizer. ｑ４５ and measuring the Stokes parameters based on A polarization characteristic measuring method, comprising: One of the fast axis and the slow axis of the quarter wave plate forms an angle of 45 degrees with respect to the ruling direction of the diffraction grating. Polarization characteristics measurement method.

Images