JP2006042234A - Osnr measuring method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To present an OSNR measuring method and apparatus capable of measuring an optical signal noise ratio (OSNR). <P>SOLUTION: An optical band pass filter (OBPF) 20 extracts a waveform of a target to be measured or a signal light component of a channel. A beam splitter 22 equally splits an output beam of the OBPF 20 into seven beams. Light intensities P<SB>1</SB>, P<SB>2</SB>, P<SB>3</SB>, P<SB>4</SB>of respective linearly polarized components in a horizontal direction, a vertical direction, +45° direction and -45° direction are obtained by polarizers 24, 28, 32, 36 and photo-diodes 26, 30, 34, 38. Light intensities P<SB>5</SB>, P<SB>6</SB>of respective turning-right and turning-left circularly polarized components are obtained by quarter-wave plates 40, 46, polarizers 42, 48 and photo-diodes 44, 50. A light intensity S<SB>0</SB>of signal light as a target to be measured is obtained by a photo-diode 52. Subtracters 54, 58, 62 calculate S<SB>1</SB>(=P<SB>1</SB>-P<SB>2</SB>), S<SB>2</SB>(=P<SB>3</SB>-P<SB>4</SB>) and S<SB>3</SB>(=P<SB>5</SB>-P<SB>6</SB>), respectively. An arithmetic unit 68 calculates a degree of polarization (DOP) from S<SB>1</SB>, S<SB>2</SB>, S<SB>3</SB>, S<SB>0</SB>and a converter 70 converts DOP into OSNR. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an OSNR measurement method and apparatus for measuring an optical signal-to-noise ratio (OSNR), and more specifically to an OSNR measurement method and apparatus for measuring an optical signal-to-noise ratio (OSNR) in an all optical network. .

  In an optical network, particularly an optical network in which an optical path can be selected, OSNR management is very important. In a wavelength division multiplexing (WDM) optical transmission system, since an optical path can be set for each wavelength, management of the OSNR for each wavelength is also important. The wavelength path processing of the optical node includes wavelength separation, cross connect (add / drop), optical regeneration, and wavelength multiplexing, and the OSNR may fluctuate greatly due to these.

  In the conventional method of measuring the OSNR for each wavelength in the WDM optical transmission system, the peak power of each signal wavelength and the intensity of the floors on both sides are measured by an optical spectrum analyzer, and the ratio is defined as OSNR.

  As shown in FIG. 3, noise light is also mixed in the signal band in the one-channel optical signal separated from the WDM signal light. The noise light is ASE light generated by an optical amplifier or the like.

  As another method for separating the noise light component in the signal band, after separating the target wavelength or channel component from the WDM signal light, the signal is orthogonal to the polarization component of the signal by the polarization controller and polarization beam splitter (PBS). There is known a method of measuring the OSNR separately from the polarization component. In this method, since the optical noise light component is divided into two by the PBS, the polarization component of the signal includes half of the noise component in addition to the signal component, and the orthogonal polarization component includes the remaining half of the noise component. Since the noise component in the signal band can be measured quantitatively, the OSNR can be measured with high accuracy.

  In the former method, in the WDM optical transmission system, the floor strength increases due to leakage from an adjacent channel. In addition, it is difficult to estimate the optical noise component existing in the signal band. For these reasons, it has been difficult to accurately evaluate the OSNR with the conventional method.

  The latter method requires a polarization controller that controls the polarization direction of the signal light after wavelength separation in a predetermined direction according to the output light of the polarization beam splitter. The polarization controller needs to be feedback-controlled infinitely following the polarization fluctuation of the signal light, resulting in a complicated and large device configuration.

  An object of the present invention is to provide an OSNR measurement method and apparatus capable of measuring OSNR with higher accuracy with a simple configuration.

  According to the OSNR measurement method of the present invention, as described in claim 1, a polarization degree measurement step for measuring the polarization degree of the input signal light, and the polarization degree measured in the polarization degree measurement step are used as the input signal. And an OSNR conversion step for converting the optical signal-to-noise ratio (OSNR) of light into a representative value.

  The OSNR measurement apparatus according to the present invention includes a polarization degree measurement apparatus that measures the degree of polarization of input signal light, and the polarization degree measured by the polarization degree measurement apparatus. And an OSNR conversion device that converts an optical signal-to-noise ratio (OSNR) of light into a representative value.

  As can be easily understood from the above description, according to the present invention, the OSNR can be measured with a very simple configuration. Since an optical noise component in the signal band is also taken into account, an accurate OSNR can be measured.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows a schematic block diagram of an embodiment of the present invention. A solid line indicates an optical signal, and a broken line indicates an electrical signal. The inventors of the present application have discovered that the degree of polarization (DOP) is correlated with OSNR. In this embodiment, the degree of polarization (DOP) of the signal light to be measured is measured by the Stokes parameter, and the OSNR of the target signal light is determined from the obtained DOP.

  The optical transmission terminal station 10 outputs WDM signal light composed of linearly polarized signal light having wavelengths λ1 to λn. The WDM signal light propagates through the optical fiber transmission line 12 and enters the OSNR measurement device 14. The OSNR measurement device 14 is installed in, for example, an optical receiving terminal station.

  In the measurement device 14, the optical bandpass filter (OBPF) 20 extracts the signal light component of the wavelength or channel to be measured from the WDM signal light input from the optical fiber transmission line 12. The OSNR of each wavelength λ1 to λn can be measured by changing the pass wavelength of the optical bandpass filter 20 to a desired wavelength of λ1 to λn.

  The optical splitter 22 divides the output light of the optical bandpass filter 20 into seven equal parts.

The first light component output from the optical splitter 22 is input to the photodiode 26 via the horizontal polarizer 24. Thus, measuring the light intensity P ₁ in the horizontal direction of the linearly polarized light component.

The second light component output from the optical splitter 22 is input to the photodiode 30 via the vertical polarizer 28. Thus, measuring the light intensity P ₂ in the vertical direction of the linear polarized light component.

The third light component output from the optical splitter 22 is input to the photodiode 34 via the +45 degree polarizer 32. Thus, measuring the light intensity P ₃ of the linearly polarized light component of + 45 °.

The fourth light component output from the optical splitter 22 is input to the photodiode 38 via the −45 degree polarizer 36. Thus, measuring the light intensity P ₄ of the linearly polarized light component of -45 degrees.

The fifth light component output from the optical splitter 22 is input to the photodiode 44 via the quarter wavelength plate 40 and the +45 degree polarizer 42. The clockwise circular polarized light component can be extracted by the quarter wavelength plate 40 and the +42 degree polarizer 42. Thus, measuring the light intensity P ₅ of right-handed circularly polarized light component.

The sixth light component output from the optical splitter 22 is input to the photodiode 50 via the quarter-wave plate 46 and the −48 degree polarizer 48. The counterclockwise circularly polarized light component can be extracted by the ¼ wavelength plate 46 and the −45 degree polarizer 48. Thus, measuring the light intensity P ₆ of the left-handed circularly polarized light component.

The seventh light component output from the optical splitter 22 is input to the photodiode 52. Thus, measuring the light intensity S ₀ of the signal light to be measured.

The subtractor 54 subtracts the output P _{2 of the} photodiode 30 from the output P _{1 of the} photodiode 26 and outputs S ₁ (= P ₁ −P ₂ ) as a result to the A / D converter 56. A / D converter 56 converts the output _{S 1} of the subtractor 54 into a digital signal.

The subtractor 58 subtracts the output P _{4 of the} photodiode 38 from the output P _{3 of the} photodiode 34 and outputs the result S ₂ (= P ₃ −P ₄ ) to the A / D converter 60. A / D converter 60 converts the output _{S 2} of the subtractor 58 into a digital signal.

Subtractor 62 subtracts the output _{P 6} of the photodiode 50 from the output _{P 5} of the photodiode 44, and outputs the result _S 3 a ₍₌ P 5 -P ₆₎ to the A / D converter 64. A / D converter 64 converts the output _{S 3} of the subtractor 62 into a digital signal.

S ₁ , S ₂ , and S ₃ are so-called Stokes parameters.

A / D converter 66 converts the output _{S 0} of the photodiode 52 into a digital signal.

The outputs of the A / D converters 56, 60, 64, 66 are input to the arithmetic unit 68. The arithmetic unit 68 uses the outputs of the A / D converters 56, 60, 64, and 66 to express the following equation:
(S ₁ ² + S ₂ ² + S ₃ ² ) ^1/2 / S ₀
Calculate This calculation result indicates the degree of polarization (DOP) of the signal light extracted by the optical bandpass filter 20. Since S ₁ , S ₂ , and S ₃ depend on the incident light intensity, the above expression is normalized by the incident light intensity S ₀ . Therefore, the DOP value obtained by the above equation does not depend on the incident light intensity.

  FIG. 2 shows the result of actual measurement of the correlation between the optical signal-to-noise ratio OSNR and the polarization degree DOP. The horizontal axis represents OSNR (dB), and the vertical axis represents DOP (%).

  The conversion device 70 compares the DOP value output from the arithmetic device 68 with the correspondence shown in FIG. 2 and converts it to the OSNR value. The conversion device 70 is composed of, for example, a lookup table having correspondence data shown in FIG.

  The signal component has a degree of polarization of almost 100%, while the ASE noise component has a degree of polarization of almost 0%. The incident signal light at the receiving terminal station is obtained by superposing ASE noise light with a polarization degree of 0% on signal light with a polarization degree of 100%. Therefore, the ASE noise component can be quantitatively grasped by measuring the degree of polarization of incident light. The ASE noise component grasped here includes, of course, a noise component within the signal wavelength band. Therefore, in this embodiment, the OSNR can be measured in consideration of noise components in the signal band.

  In the above embodiment, the DOP measurement method by measuring the Stokes parameters is adopted. However, it is obvious that the OSNR can be measured similarly when the DOP is measured by other methods.

It is a schematic block diagram of one Example of this invention. It is a figure which shows the measurement result of the relationship between OSNR and DOP. It is a schematic diagram which shows the noise light component mixed in a signal band.

Explanation of symbols

10: Optical transmitting terminal 12: Optical fiber transmission line 14: OSNR measuring device 20: Optical bandpass filter (OBPF)
22: optical splitter 24: polarizer 26: photodiode 28: polarizer 30: photodiode 32: polarizer 34: photodiode 36: polarizer 38: photodiode 40: 1/4 wavelength plate 42: polarizer 44: photo Diode 46: 1/4 wavelength plate 48: Polarizer 50: Photodiode 52: Photodiode 54: Subtractor 56: A / D converter 58: Subtractor 60: A / D converter 62: Subtractor 64: A / D converter 66: A / D converter 68: arithmetic device 70: conversion device

Claims (8)

A degree of polarization measurement step for measuring the degree of polarization of the input signal light;
An OSNR measurement method comprising: an OSNR conversion step of converting the polarization degree measured in the polarization degree measurement step into a value representative of an optical signal-to-noise ratio (OSNR) of the input signal light. The polarization degree measuring step is
A dividing step of dividing the input signal light into seven components;
From the seven components divided in the division step, horizontal linear polarization component, vertical linear polarization component, +45 linear polarization component, -45 degree linear polarization component, clockwise rotation polarization component, and counterclockwise rotation polarization An intensity measurement step for measuring each light intensity of the component, and the light intensity of the input signal light;
Difference between light intensity of horizontal linear polarization component and light intensity of vertical linear polarization component, difference of light intensity of +45 linear polarization component and light intensity of -45 degree linear polarization component, light of clockwise rotation polarization component A difference calculating step for calculating the difference between the intensity and the light intensity of the left-handed rotational polarization component;
2. The OSNR measurement method according to claim 1, further comprising: a normalization step of normalizing a square root of a sum of squares of the differences calculated in the difference calculation step with a light intensity of the input signal light. .   3. The OSNR measurement method according to claim 1, wherein the OSNR conversion step includes an OSNR determination step of collating the polarization degree measured in the polarization degree measurement step with a correspondence relationship between the polarization degree and the OSNR.   The OSNR measurement method according to any one of claims 1 to 3, further comprising an extraction step of extracting signal light having a wavelength to be measured from a wavelength division multiplexed optical signal input from an optical transmission line. A degree of polarization measuring device (22-68) for measuring the degree of polarization of the input signal light;
OSNR conversion device (70) for converting the polarization degree measured by the polarization degree measurement device into a value representative of the optical signal-to-noise ratio (OSNR) of the input signal light.
An OSNR measurement apparatus comprising: The polarization degree measuring device is
An optical splitter (22) for dividing the input signal light into seven components;
From the seven components divided by the optical splitter, horizontal linear polarization component, vertical linear polarization component, +45 linear polarization component, -45 degree linear polarization component, clockwise rotation polarization component, and counterclockwise rotation polarization Intensity measuring devices (24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52) that measure the light intensity of each component and the light intensity of the input signal light. )When,
Horizontal linear polarization component of the light intensity and the light intensity of the perpendicular linear polarization components differential (S _1), + 45 of the light intensity of the light intensity and -45 degree linearly polarized component of the linearly polarized component difference (S _2), A difference calculation device (54, 58, 62) for calculating a difference (S ₃ ) between the light intensity of the clockwise rotation polarization component and the light intensity of the left rotation polarization component;
The square root of the sum of the squares of the differences (S ₁ , S ₂ , S ₃ ) calculated by the difference calculation device (54, 58, 62) is normalized by the light intensity (S ₀ ) of the input signal light. Arithmetic unit (56, 60, 64, 66, 68)
The OSNR measurement apparatus according to claim 5, comprising:   The OSNR conversion device (70) includes a table indicating the correspondence between the degree of polarization and the OSNR, and the degree of polarization measured by the degree of polarization measurement device is prepared in the table, and the optical signal pair of the input signal light is combined. The OSNR measurement apparatus according to claim 5 or 6, wherein the OSNR is determined.   And a wavelength extraction device (20) for extracting signal light having a wavelength to be measured from the wavelength division multiplexed optical signal input from the optical transmission line and supplying the extracted component as the input signal light to the polarization degree measuring device. The OSNR measurement apparatus according to any one of claims 5 to 7.

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