KR20050101539A - Osnr monitoring method and apparatus using tunable optical bandpass filter and polarization nulling method - Google Patents

Abstract

The present invention provides an apparatus and method for monitoring an optical signal-to-noise ratio using a polarization extinction method and a wavelength-variable bandpass optical filter having a variable central wavelength in a passband in a wavelength division multiplex optical transmission system. The present invention provides an apparatus and method for monitoring a more accurate optical signal-to-noise ratio by considering in real time the finite polarization extinction ratio, polarization mode dispersion and nonlinear birefringence of optical elements that affect optical signal-to-noise ratio measurements. In addition, the wavelength-variable bandpass optical filter is controlled to filter all wavelength bands of the wavelength division multiplexed signal, thereby eliminating the problem of implementing an optical signal-to-noise ratio monitoring device for each demultiplexed optical signal, thereby providing a cost reduction effect. do.

Description

Optical signal-to-noise ratio monitoring method and apparatus using wavelength-variable bandpass optical filter and polarization extinction method {OSNR Monitoring Method and Apparatus Using Tunable Optical Bandpass Filter and Polarization Nulling Method}

The present invention relates to a method and apparatus for monitoring an optical signal-to-noise ratio in an optical communication network, and more particularly, using a wavelength-variable bandpass filter and a polarization extinction method in a wavelength division multiplexing (WDM) optical transmission system. A method and apparatus for automatically monitoring the optical signal to noise ratio of each channel.

It is essential to understand the transmission performance of the optical transmission system in order to reliably operate and manage the wavelength division multiplexed ultra-high capacity optical communication network. Optical signal to noise ratio (OSNR) is defined as the ratio of optical signal power to noise power included in an optical signal band. By accurately measuring the optical signal to noise ratio, the performance of the optical transmission system can be determined.

As a widely used optical signal-to-noise ratio measurement method, a method of measuring optical signal-to-noise ratio by linearly predicting the intensity of ASE noise of an optical signal band from the ASE (Amplified Spontaneous Emission) noise intensity of an optical signal outer band is provided. (H. suzuki and N. Takachino, Electronics Letter, vol. 35, pp. 836-837, 1999). The linear prediction method is a method of predicting ASE noise of an optical signal band through a dotted line extending from a constant ASE noise of an optical signal band as shown in FIG. 1, and using the predicted ASE noise to calculate an optical signal-to-noise ratio. It can be measured.

However, in a wavelength division multiplex optical transmission system in which each optical signal often passes through different paths and passes through a different number of Erbium-doped fiber amplifiers (EDFAs), the ASE noise included in each optical signal band is included. This may be different as shown in FIG. 2. Therefore, since the ASE noise linearly predicted from the constant ASE noise of the optical signal outer band is different from the ASE noise included in the actual optical signal band, the accurate optical signal-to-noise ratio cannot be measured by the linear prediction method.

To solve this problem, Korean Patent No. 341825, "Optical Signal-to-Noise Ratio Monitoring Method and Apparatus Using Polarization Extinction Method," measures the optical signal-to-noise ratio by the polarization extinction method using the polarization characteristics of the optical signal and the ASE noise. . Referring to FIG. 3, a conventional optical signal-to-noise ratio measurement method using a polarization extinction method is performed. The wavelength division multiplexed optical signal including ASE noise is demultiplexed by the waveguide-type rotating grating 11. The optical signal with any polarization passes through a quarter-wave plate 12 and a linear polarizer 13. For example, when the quarter wave plate 12 rotates at a speed of 15 Hz and the linear polarizer 13 rotates at a speed of 0.1 Hz, the minimum power and the maximum power of the optical signal are changed by the polarization characteristic of the optical signal including ASE noise. Can be measured. When the polarization state of the linear polarizer 13 and the polarization state of the optical signal output from the quarterwave plate 12 coincide, the power of the optical signal to be measured becomes maximum. On the other hand, when the polarization state of the linear polarizer 13 and the polarization state of the optical signal output from the quarter wave plate 12 are perpendicular to each other, pure optical signal components are removed and only ASE noise is output, and the power of the optical signal measured is Minimum.

The optical signal passing through the linear polarizer 13 is converted into a voltage by the photodetector 14, amplified by the log amplifier 15 and shown in the oscilloscope 16. The computer 17 calculates the minimum power and the maximum power from the voltage shown in the oscilloscope, and calculates the optical signal power and ASE noise from the optical signal to noise ratio.

However, the conventional optical signal-to-noise ratio monitoring method using only the polarization extinction method does not consider the case where the polarization extinction ratio of the linear polarizer 13 is finite by assuming that the polarization extinction ratio of the linear polarizer 13 is ideal. In addition, the conventional optical signal-to-noise ratio monitoring method is limited in its use range by not considering the phenomenon that the degree of polarization of the optical signal is lowered due to the polarization mode dispersion and nonlinear birefringence of the optical fiber.

The polarization mode dispersion refers to a time difference between signal components traveling in two polarization axes perpendicular to each other according to the polarization characteristics of an optical fiber or an optical element when an optical signal is transmitted through an optical path. The polarization mode dispersion is changed over time because it is sensitive to changes in the surrounding environment such as temperature or external pressure.

In addition, non-linear birefringence is a birefringence caused by the refractive index of the optical fiber is changed by the intensity of the optical signal, and when several optical signals are transmitted through a single optical fiber at the same time, the polarization state of the adjacent channel changes very quickly. Since the change of polarization varies depending on the polarization state between each channel, the influence of nonlinear birefringence also changes with time.

Due to the polarization mode dispersion and nonlinear birefringence, an unpredictable error occurs in the conventional optical signal-to-noise ratio monitoring method using only the polarization extinction method. That is, due to the finite polarization extinction ratio of the linear polarizer 13, the polarization mode dispersion of the optical fiber, and the nonlinear birefringence, optical signals of polarization components perpendicular to the polarization state of the linear polarizer 13 are completely removed while passing through the linear polarizer 13. It doesn't work. However, the conventional optical signal-to-noise ratio monitoring method using only the polarization extinction method has a problem in that the accuracy of optical signal-to-noise ratio measurement is deteriorated by not considering such an effect.

In addition, the conventional optical signal-to-noise ratio monitoring method using only the polarization extinction method should include a waveguide type diffraction grating 11 for demultiplexing, and provide an optical signal-to-noise ratio monitoring apparatus for each of the demultiplexed optical signals. Therefore, there is a problem that requires a high cost.

1 is a diagram showing ASE noise of an optical signal band by conventional linear prediction;

2 shows an example of ASE noise included in each optical signal band passing through different paths and different numbers of erbium-doped fiber amplifiers;

3 is a block diagram of an optical signal to noise ratio monitoring apparatus using a conventional polarization extinction method;

4 illustrates the principle of an optical signal to noise ratio monitoring method according to the present invention;

5 shows a first embodiment of an optical signal to noise ratio monitoring apparatus according to the present invention;

6 shows a second embodiment of an optical signal to noise ratio monitoring apparatus according to the present invention;

7 is an experimental configuration for verifying the effectiveness of the optical signal to noise ratio monitoring apparatus according to the present invention;

8 is a graph showing an optical signal to noise ratio measured according to the experimental configuration of FIG.

9 shows a third embodiment of an optical signal to noise ratio monitoring apparatus according to the present invention; And

10 is a view showing a fourth embodiment of the optical signal to noise ratio monitoring apparatus according to the present invention.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problem. to provide.

The present invention provides an apparatus and method for monitoring a more accurate optical signal-to-noise ratio by considering in real time the finite polarization extinction ratio, polarization mode dispersion and nonlinear birefringence of optical elements that affect optical signal-to-noise ratio measurements.

In the present invention, the center wavelength is λ ₁ ,. , λ _i ,… A device for monitoring the optical signal-to-noise ratio (OSNR) of a wavelength division multiplexed optical signal with λ _n (i = 1,2, ..., n), comprising: a polarization controller for adjusting the polarization state of the wavelength division multiplexed optical signal ; A variable wavelength bandpass optical filter for varying the center wavelength lambda _v and filtering an optical signal having a center wavelength lambda _i for measuring the optical signal-to-noise ratio among the wavelength division multiplexed optical signals; Optical signal separation means for separating the filtered optical signal into a first optical signal and a second optical signal; A linear polarizer for linearly polarizing the second optical signal; And OSNR measuring means for measuring the power of the first optical signal and the second optical signal passing through the linear polarizer to calculate the optical signal to noise ratio of the optical signal.

The OSNR measuring means obtains the power P ₁ of the first optical signal when λ _v coincides with λ _i, and obtains the power P ₂ that minimizes the power of the second optical signal passing through the linear polarizer by adjusting the polarization controller. . In addition, the OSNR measuring means obtains the power P ₃ of the first optical signal when λ _v is shifted from λ _i by a predetermined amount, and adjusts the polarization regulator to minimize the power of the second optical signal passing through the linear polarizer. seek to P _4, the power _{P 1, P 2, P 3} , P ₄ is obtained by using the optical signal-to-noise ratio. When the polarization state of the optical signal passing through the polarization controller is perpendicular to the polarization state of the linear polarizer, the power of the second optical signal passing through the linear polarizer is minimal. The predetermined amount in which λ _v deviates from λ _i is determined within a range in which the wavelength variable bandpass optical filter can measure a part of an optical signal having a central wavelength of λ _i .

Preferably, the optical signal to noise ratio monitoring apparatus further comprises a polarization control signal generator for generating a predetermined polarization control signal to enable the polarization controller to adjust the polarization state of the optical signal.

Preferably, the optical signal-to-noise ratio monitoring device, to generate a predetermined wavelength of the control signal the wavelength variable band center wavelength of the pass optical filter λ _v is the wavelength control to a variable in the range from λ _n at λ ₁ It further comprises a signal generator.

In another embodiment of the present invention, the center wavelength is λ ₁ ,. , λ _i ,. A device for monitoring an optical signal-to-noise ratio (OSNR) of a wavelength division multiplexed optical signal having a wavelength λ _n (i = 1,2, ..., n), comprising: a polarization controller for adjusting a polarization state of a wavelength division multiplexed optical signal ; A variable wavelength bandpass optical filter for varying the center wavelength lambda _v and filtering an optical signal having a center wavelength lambda _i for measuring the optical signal-to-noise ratio among the wavelength division multiplexed optical signals; A polarization separator that separates the filtered optical signal into two optical signals whose polarization states are perpendicular to each other; And OSNR measuring means for calculating an optical signal-to-noise ratio using the power of two optical signals having mutually perpendicular polarization states.

When λ _v coincides with λ _i, the OSNR measuring means adjusts the polarization controller to adjust the power P ₁ of the optical signal having the polarization component that matches the polarization state of the polarization controller and the polarization component perpendicular to the polarization state of the polarization controller. Find the power P ₂ of the optical signal. The OSNR measuring means also adjusts the polarization controller when λ _v deviates by λ _i by a predetermined amount, so that the power P ₃ of the optical signal having the polarization component corresponding to the polarization state of the polarization controller and the polarization state of the polarization controller are mutually different. The power P ₄ of the optical signal having the vertical polarization component is obtained, and the optical signal-to-noise ratio is obtained using the waves P ₁ , P ₂ , P ₃ , and P ₄ . The predetermined amount in which λ _v deviates from λ _i is determined within a range in which the wavelength variable bandpass optical filter can measure a part of the optical signal having a central wavelength of λ _i .

Preferably, the optical signal to noise ratio monitoring apparatus further comprises a polarization control signal generator for generating a predetermined polarization control signal to enable the polarization controller to adjust the polarization state of the optical signal.

Preferably, the optical signal-to-noise ratio monitoring device, to generate a predetermined wavelength of the control signal the wavelength variable band center wavelength of the pass optical filter λ _v is the wavelength control to a variable in the range from λ _n at λ ₁ It further comprises a signal generator.

Hereinafter, with reference to the accompanying drawings will be described the configuration and operation according to a preferred embodiment of the present invention. The same reference numerals among the drawings refer to components having the same function.

4 is a view showing the principle of the optical signal to noise ratio monitoring method according to the present invention, Figure 5 is a view showing a first embodiment of the optical signal to noise ratio monitoring apparatus according to the present invention.

A portion of the optical signal including the ASE noise transmitted through the wavelength division multiplex optical network is separated by the 99: 1 directional coupler 20 and then used for the measurement of the optical signal to noise ratio. The 99: 1 directional coupler 20 extracts a part of the optical signal corresponding to 1/100 of the optical signal intensity including the ASE noise and inputs it to the polarization controller 22.

The polarization controller 22 receives the polarization control signal from the polarization control signal generator 38 and adjusts the polarization state of the optical signal such that the intensity of the optical signal including the ASE noise output from the linear polarizer 26 is the smallest. . The polarization controller 22 performs a polarization extinction method in which the optical signal input to the linear polarizer 26 is removed while passing through the linear polarizer 26 when the optical signal having the polarization state perpendicular to the polarization state of the linear polarizer 26 is perpendicular to each other. When the polarization state of the linearly polarized light signal passing through the polarization controller 22 is perpendicular to the polarization state of the new polarizer 26, the ASE noise output from the linear polarizer 26 by the polarization extinction method may be included. The intensity of the optical signal is the smallest.

The optical signal is linearly polarized when passing through the polarization controller 22, while the ASE noise over the wideband is not linearly polarized even when passing through the polarization controller 22. Although both the optical signal and the ASE noise pass through the polarization controller 22, there is no change in intensity.

In addition, the linearly polarized optical signal must be completely removed while passing through the linear polarizer 26 when the polarization state is exactly perpendicular to the polarization state of the linear polarizer 26. However, as described above, the finite of the linear polarizer 26 It is not completely eliminated by the polarization extinction ratio, polarization mode dispersion of the optical fiber and nonlinear birefringence. ASE noise is linearly polarized as it passes through the linear polarizer 26 and the power is reduced by half.

The linear polarizer 26 may be configured to have a polarization state fixed as an optical fiber based optical element. Therefore, by controlling the polarization controller 22, the polarization state of the optical signal can be adjusted to be perpendicular to the polarization state of the polarization controller 22. The polarization controller 22 may be implemented using an optical fiber and a piezoelectric element. As will be appreciated by those skilled in the art, the linear polarizer 26 and the polarization controller 22 are not limited to the above-described forms, and may be implemented using various conventional devices.

The optical signal including the ASE noise passing through the polarization controller 22 is input to the wavelength variable bandpass optical filter 24. The passband of the wavelength variable bandpass optical filter 24 is configured to pass only one optical signal without the influence of adjacent channels, as shown in FIG. The wavelength variable bandpass optical filter 24 receives the wavelength control signal from the wavelength control signal generator 40 to change the center wavelength λ _v of the pass band. The wavelength variable bandpass optical filter 24 may be controlled to filter all wavelength bands of the wavelength division multiplexed signal according to the wavelength control signal of the wavelength control signal generator 40. Accordingly, the present invention can measure both optical signal-to-noise ratios of wavelength division multiplexed signals without using a demultiplexer such as a waveguide diffraction grating.

In addition, the wavelength variable bandpass optical filter 24 has a predetermined range in the center wavelength λ _i of the optical signal to measure the optical signal noise ratio as shown in FIG. 4 according to the wavelength control signal of the wavelength control signal generator 40. It can be controlled to filter the misaligned optical signal. Wavelength-variable bandpass optical filter (24) using MEMS-based half symmetric cavity resonator, Fabry-Perot variable filter, integrated optical device including grating, multilayer thin film device, acoustic optical Filters and the like can be used. In view of the accuracy of optical signal-to-noise ratio measurement, narrow-band wavelength-variable optical filters suitable for passing bands narrower than those of optical signals are preferable, and filters using an antisymmetric resonator based on MEMS are preferred as current technologies.

Referring to the principle of the optical signal-to-noise ratio monitoring method according to the present invention, the center wavelength λ _v of the wavelength variable bandpass optical filter 24 measures the optical signal-to-noise ratio by the wavelength control signal of the wavelength control signal generator 40. Adjusted to match the center wavelength λ _i of the optical signal to be intended. The optical signal extracted by the 99: 1 directional coupler 20 passes through the polarization controller 22 and the wavelength-variable bandpass optical filter 24, and then the first optical signal and the first optical signal are separated by the 1: 1 directional coupler 28. It is branched into two light signals. The first optical signal, which is one of the branched optical signals, is an optical signal including ASE noise, and is converted into a digital signal by the analog-to-digital converter 32a after photoelectric conversion by the photodetector 30a along path 1. The power calculator 34 calculates the power P ₁ of the first optical signal including the ASE noise.

After that, while the center wavelength λ _v of the wavelength variable bandpass optical filter 24 is maintained to coincide with the center wavelength λ _i of the optical signal, the polarization controller 22 according to the polarization control signal of the polarization control signal generator 38. The polarization state of the optical signal passing through is adjusted. The second optical signal branched by the 1: 1 directional coupler 28 is an optical signal containing ASE noise, and is converted to photoelectric by the photodetector 30b through the linear polarizer 26 along the path 2, followed by the analog-to-digital converter 32b. Is converted into a digital signal. The power calculator 34 calculates the power of the second optical signal including the linearly polarized ASE noise.

The polarization state of the polarization regulator 22 is scanned within a certain range and finds the minimum value among the calculated powers. As described above, the power of the optical signal including the ASE noise calculated by the power calculator 34 when the polarization state of the optical signal passing through the polarization controller 22 is perpendicular to the polarization state of the linear polarizer 26. Becomes the minimum. The power P ₂ of the optical signal including the ASE noise at this time is obtained.

Next, according to the wavelength control signal of the wavelength control signal generator 40, the center wavelength λ _v of the wavelength variable bandpass optical filter 24 is shifted by a predetermined amount from the center wavelength λ _i of the optical signal, as shown in FIG. 4. Adjusted. The predetermined amount is determined within a range in which the wavelength variable bandpass optical filter 24 can partially pass the optical signal for which the optical signal to noise ratio is to be measured. The predetermined amount is preferably 0.3 to 3 R GHz when the transmission speed of the optical signal is R Gbps in terms of the measured optical signal to noise ratio accuracy. For example, when the transmission speed of the optical signal is 10 Gbps, the center wavelength λ _v of the wavelength variable bandpass optical filter 24 is adjusted to deviate from the center wavelength λ _i of the optical signal within a range of 3 to 30 GHz. According to the wavelength control signal of the wavelength control signal generator 40, the center wavelength λ _v of the wavelength variable bandpass optical filter 24 may be adjusted by a predetermined amount to the left or the right of the center wavelength λ _i of the optical signal.

When the passband of the wavelength variable bandpass optical filter 24 is changed, the intensity of the optical signal is measured to be smaller than before changing the passband due to the transmission characteristics of the filter 24. However, since ASE noise is constant over the wideband, the strength of the ASE noise has almost the same value. The first optical signal including the ASE noise branched by the 1: 1 directional coupler 28 is converted into a digital signal by the analog-to-digital converter 32a after photoelectric conversion by the photodetector 30a along path 1. The power calculator 34 calculates the power P ₃ of the first optical signal including the ASE noise.

After that, while the center wavelength λ _v of the wavelength variable bandpass optical filter 24 is maintained to be shifted from the center wavelength λ _i of the optical signal by a predetermined amount, the polarization control signal of the polarization control signal generator 38 polarizes the light according to the polarization control signal. The polarization state of the optical signal passing through the regulator 22 is adjusted. The second optical signal including the ASE noise branched by the 1: 1 directional coupler 28 is photoelectrically converted by the photodetector 30b via the linear polarizer 26 along path 2 and then by the analog-to-digital converter 32b. Is converted into a digital signal. The power calculator 34 calculates the power of the second optical signal including the linearly polarized ASE noise.

The polarization state of the polarization regulator 22 is scanned within a certain range and finds the minimum value among the calculated powers. As described above, the power of the optical signal including the ASE noise calculated by the power calculator 34 when the polarization state of the optical signal passing through the polarization controller 22 is perpendicular to the polarization state of the linear polarizer 26. Becomes the minimum. The power P ₄ of the optical signal including the ASE noise at this time is obtained.

P ₁ , P ₂ , P ₃ , and P ₄ described above are given by the following equation.

Here, P _S1 is an intensity watt of the optical signal passing through the wavelength variable bandpass optical filter 24 when λ _v coincides with λ _i , excluding the intensity of ASE noise. P _S2 is an intensity (watt) of the optical signal passing through the wavelength variable bandpass optical filter 24 when λ _v is shifted from λ _i by a predetermined amount, and ASE noise is excluded. S _ASE is the power density (watt / nm) of the ASE noise, and B ₀ represents the bandwidth (nm) of the wavelength variable bandpass optical filter 24. ε represents the polarization extinction ratio of the linear polarizer 26.

Therefore, S _ASE B ₀ refers to the power of the ASE noise passing through the wavelength variable bandpass optical filter 24. P _S1 ε and P _S2 ε are the intensities of the optical signals measured even though the polarization states of the optical signals passing through the polarization controller 22 are perpendicular to the polarization states of the linear polarizer 26 due to the finite polarization extinction ratio. If ε is an ideal case, the photodetector 30b would only measure ASE noise for P ₂ and P ₄ . And P ₂ is the ASE noise intensity at P ₄ one-half of the ASE noise intensity in the P ₁ and P ₃ P ₂ P _4, and if the ASE noise is linear and passes through the linear polarizer 26 along the path 2 This is because the power is reduced to 1/2 by polarization.

From Equation 1, ε, P _S1 , S _ASE , and OSNR are obtained as follows.

Where B _r (nm) is the resolution, which is the reference bandwidth that determines the power of ASE noise in OSNR calculations.

The OSNR calculation unit 36 calculates the OSNR by using the calculation formula (2) from P ₁ , P ₂ , P ₃ , and P ₄ calculated by the power calculation unit 34.

On the other hand, [epsilon] value can be generated by the polarization mode dispersion and nonlinear birefringence of the optical transmission system in addition to the finite polarization extinction ratio inherent in the linear polarizer 26. Since the polarization state changes with time due to polarization mode dispersion and nonlinear birefringence, ε may also change continuously with time. In the present invention, even if the value of ε changes over time, as shown in Equation 2, by considering the effect of the change of the value on the optical signal to noise ratio in real time, it is possible to increase the accuracy of the optical signal to noise ratio measurement.

In order to measure the optical signal-to-noise ratio more accurately, P ₂ and P ₄ should be measured after adjusting the polarization controller 22 to minimize the value of ε. This means that the intensity of the optical signal passing through the linear polarizer 26 having a finite polarization extinction ratio should be minimized. That is, the amount of optical signal components P _S1 ε and P _S2 ε included in P ₂ and P ₄ is smaller than or equal to the intensity of the amplified natural emission optical noise (0.5S _AES B ₀ ). By adjusting 22, it is possible to measure the intensity of the ASE noise and the optical signal-to-noise ratio more accurately.

Those skilled in the art to measure the power of the optical signal in the order of P ₁ , P ₂ , P ₃ , P ₄ in measuring the optical signal to noise ratio in the first embodiment, It will be appreciated that it is not necessarily limited to this order. Since only the order of measuring the power of the optical signal is different, the detailed description is omitted.

6 is a view showing a second embodiment of the optical signal to noise ratio monitoring apparatus according to the present invention.

The 99: 1 directional coupler 20 extracts a part of the optical signal corresponding to 1/100 of the optical signal intensity including the ASE noise and inputs it to the polarization controller 22.

The polarization controller 22 receives the polarization control signal from the polarization control signal generator 38 and operates to separate the optical signal output from the polarization separator 42 into an optical signal having two mutually perpendicular polarization components. That is, the optical signal having the same polarization component as the polarization state of the polarization controller 22 proceeds along path 3, and the optical signal having the polarization components perpendicular to the polarization state of the polarization controller 22 travels along path 4 do.

In general, the optical signal components perpendicular to the polarization state of the optical signal passing through the polarization controller of the optical signal passing through the polarization separator should be completely removed, but the finite polarization extinction ratio and polarization mode dispersion and nonlinear birefringence Are not completely removed. ASE noise also linearly polarizes through the polarization splitter, reducing power by half.

The optical signal including the ASE noise passing through the polarization controller 22 is input to the wavelength variable bandpass optical filter 24. The configuration and operation characteristics of the wavelength tunable bandpass optical filter 24 are the same as those in the first embodiment described with reference to FIG.

Referring to the principle of the optical signal to noise ratio monitoring method according to the present invention with reference to Figures 4 and 6, the center wavelength λ of the wavelength variable band pass optical filter 24 by the wavelength control signal of the wavelength control signal generator 40 _v is adjusted to match the center wavelength λ _i of the optical signal for which the optical signal-to-noise ratio is to be measured. Thereafter, the polarization state of the optical signal passing through the polarization controller 22 is adjusted according to the polarization control signal of the polarization control signal generator 38.

The optical signal having the polarization component coinciding with the polarization state of the polarization controller 22 is converted into a digital signal by the analog-to-digital converter 32a after photoelectric conversion by the photodetector 30a along path 3 together with the ASE noise. The power calculator 34 calculates the power P ₁ of the optical signal including the linearly polarized ASE noise.

An optical signal having a polarization component perpendicular to the polarization state of the polarization controller 22 is converted into a digital signal by the analog-to-digital converter 32b after photoelectric conversion by the photodetector 30b along path 4. The power calculator 34 calculates the power P ₂ of the optical signal including the linearly polarized ASE noise. The polarization state of the polarization controller 22 is scanned within a predetermined range, and finds the minimum value among the optical signal powers detected by the photodetector 30b. When an optical signal having a polarization component perpendicular to the polarization state of the polarization controller 22 is extracted along the path 4, the power of the optical signal including the ASE noise calculated by the power calculator 34 is minimized. The power of the optical signal including the ASE noise at this time is P ₂ . Since the optical signal is separated by the polarization separator 42 into optical signals of polarization components that are perpendicular to each other, P ₁ and P ₂ are measured at once.

Next, according to the wavelength control signal of the wavelength control signal generator 40, the center wavelength λ _v of the wavelength variable bandpass optical filter 24 is shifted by a predetermined amount from the center wavelength λ _i of the optical signal, as shown in FIG. 4. Adjusted. The predetermined amount is determined within the range in which the wavelength variable bandpass optical filter 24 can pass at least part of the optical signal for which the optical signal to noise ratio is to be measured, as in the first embodiment. The predetermined amount is preferably 0.3 to 3 R GHz when the transmission speed of the optical signal is R Gbps in terms of the measured optical signal to noise ratio accuracy.

Thereafter, the polarization state of the optical signal passing through the polarization controller 22 is adjusted according to the polarization control signal of the polarization control signal generator 38. The optical signal having the polarization component coinciding with the polarization state of the polarization controller 22 is converted into a digital signal by the analog-to-digital converter 32a after photoelectric conversion by the photodetector 30a along path 3 together with the ASE noise. The power calculator 34 calculates the power P ₃ of the optical signal including the linearly polarized ASE noise.

An optical signal having a polarization component perpendicular to the polarization state of the polarization controller 22 is converted into a digital signal by the analog-to-digital converter 32b after photoelectric conversion by the photodetector 30b along path 4. The power calculator 34 calculates the power P ₄ of the optical signal including the linearly polarized ASE noise. The polarization state of the polarization controller 22 is scanned within a predetermined range, and finds the minimum value among the optical signal powers detected by the photodetector 30b. When an optical signal having a polarization component perpendicular to the polarization state of the polarization controller 22 is extracted along the path 4, the power of the optical signal including the ASE noise calculated by the power calculator 34 is minimized. The power of the optical signal including the ASE noise at this time is P ₄ . Since the optical signal is separated by the polarization separator 42 into optical signals of polarization components that are perpendicular to each other, P ₃ and P ₄ are measured at once.

P ₁ , P ₂ , P ₃ , and P ₄ described above are given by the following equation.

Here, the definitions of P _S1 , P _S2 , S _ASE , and B _O are the same as in Equation 1. However, ε represents the polarization extinction ratio of the polarization separator 42.

In Equation 3, unlike Equation 1, when measuring P1 and P3, since the optical signal including the ASE noise passes through the polarization separator 42, only the linear polarization component of the ASE noise is detected and the ASE noise power is reduced to 1/2. .

From Equation 3,?, P _S1 , S _ASE , and OSNR are obtained as follows.

Where B _r (nm) is the resolution, which is the reference bandwidth that determines the power of ASE noise in OSNR calculations.

The OSNR calculation unit 36 calculates the OSNR from P ₁ , P ₂ , P ₃ , and P ₄ calculated by the power calculation unit 34 by the equation (4).

7 is an experimental configuration for verifying the effectiveness of the optical signal to noise ratio monitoring apparatus according to the present invention. The wavelength variable laser 52 is used to provide an optical signal, and the erbium-doped fiber amplifier is used as the ASE light source 54 to provide ASE noise. The optical signal coupled by the 1: 1 directional coupler 60 and the ASE noise are again divided into two parts by the 1: 1 directional coupler 62.

One part is input to the Optical Spectrum Analyzer (64) and the optical signal-to-noise ratio is measured by linear prediction. In this case, since the optical signal band is narrow and the ASE noise band is considerably wide and flat, the optical signal-to-noise ratio measured by linear prediction is accurate. The other part is input to the OSNR monitoring device A so that the optical signal to noise ratio is measured according to the present invention. Optical variable attenuators 56 and 58 installed in the wavelength variable laser 52 and the ASE light source 54 are used to increase and decrease the power of the optical signal and the amount of ASE noise to change the optical signal to noise ratio.

8 shows the optical signal to noise ratio measured according to the experimental configuration of FIG. An optical device using an optical fiber and a piezoelectric element as the polarization regulator 22, a filter using a half symmetric cavity resonator based on a MEMS as the wavelength variable bandpass optical filter 24, an optical fiber based as the linear polarizer 26 An optical element is used. The optical signal is a 10 Gb / s Non-Return-to-zero (NRZ) signal, with a wavelength of 1550 nm and an optical signal intensity of -20 dBm. For comparison of the experimental results, the optical signal to noise ratio measured using the optical spectrum analyzer 64 is 21 dB. As can be seen from the results, even when measuring more than 14 hours, the optical signal-to-noise ratio measured according to the present invention is within an error range of approximately 0.1 dB compared to the optical signal-to-noise ratio measured using the optical spectrum analyzer 64. Is measured.

Those skilled in the art to which the present invention pertains will be capable of various substitutions, modifications and changes without departing from the spirit of the present invention. For example, as shown in FIG. 9, the third embodiment according to the present invention may be configured by changing the positions of the polarization controller 22 and the wavelength variable bandpass optical filter 24 of FIG. 5. In the case of the third embodiment, the optical signal-to-noise ratio can be obtained by using Equations 1 and 2 as in the first embodiment. In addition, as shown in FIG. 10, a fourth embodiment according to the present invention may be configured by changing the positions of the polarization controller 22 and the wavelength variable bandpass optical filter 24 of FIG. 6. In the case of the fourth embodiment, the optical signal-to-noise ratio can be obtained by using Equations 3 and 4 as in the second embodiment.

Accordingly, the present invention is not limited to the above-described embodiments and the accompanying drawings, and such changes, etc., should be considered to be within the scope of the following claims.

The present invention is an apparatus and method for monitoring an optical signal-to-noise ratio in a wavelength division multiplexing optical transmission system. By taking into account, accurate optical signal to noise ratio can be measured. In addition, the wavelength-variable bandpass optical filter is controlled to filter all wavelength bands of the wavelength division multiplexed signal, thereby eliminating the problem of implementing an optical signal-to-noise ratio monitoring device for each demultiplexed optical signal, thereby providing a cost reduction effect. do.

Claims (23)

The center wavelength is λ ₁ ,. , lambda _i ,... An apparatus for monitoring an optical signal-to-noise ratio (OSNR) of a wavelength division multiplexed optical signal with λ _n (i = 1, 2, ..., n), A polarization controller for controlling a polarization state of the wavelength division multiplexed optical signal; A variable wavelength bandpass optical filter for varying the center wavelength lambda _v and filtering an optical signal having a center wavelength lambda _i for measuring the optical signal-to-noise ratio among the wavelength division multiplexed optical signals; Optical signal separation means for separating the filtered optical signal into a first optical signal and a second optical signal; A linear polarizer for linearly polarizing the second optical signal; And When λ _v coincides with λ _i , the power P ₁ of the first optical signal is obtained, and the power P ₂ at which the power of the second optical signal passing through the linear polarizer is minimized is obtained. λ _v is moved by λ _i to obtain a power P ₃ of the first optical signal, and a power P ₄ at which the power of the second optical signal passing through the linear polarizer is minimized is obtained. OSNR measuring means for obtaining the optical signal to noise ratio by using the power P ₁ , P ₂ , P ₃ , P ₄ , The optical signal of the wavelength division multiplex optical transmission system, wherein the power of the second optical signal passing through the linear polarizer is minimized when the polarization state of the second optical signal is perpendicular to the polarization state of the linear polarizer by the polarization controller. Noise-to-noise monitoring device. The optical signal-to-noise ratio monitoring apparatus of the wavelength division multiplex optical transmission system according to claim 1, wherein the predetermined amount of shifting λ _v from λ _i is 0.3-3 R GHz when the transmission speed of the optical signal is R Gbps. 2. The apparatus of claim 1, wherein the optical signal separation means is a 1: 1 directional coupler. 4. The method of claim 3, wherein B _r (nm) is the reference bandwidth for determining the power of ASE (Amplified Spontaneous Emission) noise during OSNR calculation as resolution and B _O is the bandwidth (nm) of the tunable bandpass optical filter. The optical signal-to-noise ratio monitoring apparatus of the wavelength division multiplex optical transmission system for obtaining the optical signal-to-noise ratio according to the following equation. The apparatus of claim 1, wherein the bandwidth of the wavelength variable bandpass optical filter is a narrowband filter smaller than the bandwidth of the optical signal for which the optical signal to noise ratio is to be measured. 2. The apparatus of claim 1, wherein said OSNR measuring means comprises: a first photodetector for detecting said first optical signal and converting it into an electrical signal; A second photodetector for detecting a second optical signal passing through the linear polarizer and converting the second optical signal into an electrical signal; A first analog to digital converter for converting an electrical signal of the first photodetector into a digital signal; A second analog to digital converter for converting an electrical signal of the second photodetector into a digital signal; A power calculator configured to calculate power P ₁ , P ₂ , P ₃ , and P ₄ using the converted digital signal; And And an OSNR calculator for calculating an optical signal-to-noise ratio using P ₁ , P ₂ , P ₃ , and P ₄ calculated by the power calculator. The apparatus of claim 1, further comprising a polarization control signal generator configured to generate a predetermined polarization control signal to enable the polarization controller to adjust the polarization state of the optical signal. The wavelength control signal generator of claim 1, further comprising a wavelength control signal generator configured to generate a predetermined wavelength control signal and to vary the central wavelength λ _v of the wavelength variable bandpass optical filter within a range from λ ₁ to λ _n . Optical Signal-to-Noise Ratio Monitoring System of Division Multiplex Optical Transmission System. The apparatus of claim 1, wherein the wavelength variable bandpass optical filter is an optical filter using a MEMS-based antisymmetric resonator. The center wavelength is λ ₁ ,. , λ _i ,… An apparatus for monitoring an optical signal-to-noise ratio (OSNR) of a wavelength division multiplexed optical signal with λ _n (i = 1, 2, ..., n), A polarization controller for controlling a polarization state of the wavelength division multiplexed optical signal; A variable wavelength bandpass optical filter for varying the center wavelength lambda _v and filtering an optical signal having a center wavelength lambda _i for measuring the optical signal-to-noise ratio among the wavelength division multiplexed optical signals; A polarization separator that separates the filtered optical signal into a first optical signal and a second optical signal in which polarization states are perpendicular to each other; When λ _v coincides with λ _i , the power P ₁ of the first optical signal having the polarization state coinciding with the polarization state of the polarization regulator and the second optical signal having the polarization state perpendicular to the polarization state of the polarization regulator Find the power P ₂ of λ _v is moved by λ _i by a predetermined amount so that the power P ₃ of the first optical signal having a polarization state coinciding with the polarization state of the polarization regulator and the polarization state perpendicular to the polarization state of the polarization regulator Find the power P ₄ of 2 light signal, And an OSNR measurement means for obtaining an optical signal-to-noise ratio using the powers P ₁ , P ₂ , P ₃ , and P ₄ . The optical signal-to-noise ratio monitoring apparatus of the wavelength division multiplex optical transmission system according to claim 10, wherein the predetermined amount of shifting λ _v from λ _i is 0.3 to 3 R Ghz when the transmission speed of the optical signal is R Gbps. 11. The optical signal according to claim 10, wherein B _r (nm) is a reference bandwidth for determining the power of ASE noise in calculating OSNR as a resolution and B _O is a bandwidth (nm) of a wavelength variable bandpass optical filter. Optical signal-to-noise ratio monitoring device of wavelength division multiplex optical transmission system for obtaining signal-to-noise ratio. The optical signal to noise ratio monitoring apparatus of claim 10, wherein the bandwidth of the wavelength variable bandpass optical filter is a narrow band filter smaller than the bandwidth of the optical signal to measure the optical signal to noise ratio. 11. The apparatus of claim 10, wherein said OSNR measuring means comprises: a first photodetector for detecting said first optical signal and converting it into an electrical signal; A second photodetector for detecting the second optical signal and converting the second optical signal into an electrical signal; A first analog to digital converter for converting an electrical signal of the first photodetector into a digital signal; A second analog to digital converter for converting an electrical signal of the second photodetector into a digital signal; A power calculator for calculating power P ₁ , P ₂ , P ₃ , and P ₄ using the converted digital signal; And And an OSNR calculator for calculating an optical signal-to-noise ratio using P ₁ , P ₂ , P ₃ , and P ₄ calculated by the power calculator. The apparatus of claim 10, further comprising a polarization control signal generator configured to generate a predetermined polarization control signal to enable the polarization controller to adjust the polarization state of the optical signal. The wavelength control signal generator of claim 10, further comprising: a wavelength control signal generator configured to generate a predetermined wavelength control signal so that the center wavelength λ _v of the wavelength variable bandpass optical filter is variable within a range from λ ₁ to A _n . Optical Signal-to-Noise Ratio Monitoring System of Division Multiplex Optical Transmission System. The apparatus of claim 10, wherein the wavelength variable bandpass optical filter is an optical filter using a MEMS-based antisymmetric resonator. The center wavelength is λ ₁ ,. , λ _i ,… A method for monitoring the optical signal-to-noise ratio (OSNR) of a wavelength division multiplexed optical signal with λ _n (i = 1, 2, ..., n), Separating the optical signal passing through the polarization controller and the wavelength variable bandpass optical filter into a first optical signal and a second optical signal by a directional coupler; The power P of the first optical signal when the center wavelength λ _v of the wavelength variable bandpass optical filter is varied to match the center wavelength λ _i of the optical signal to measure the optical signal to noise ratio among the wavelength division multiplexed optical signals ₁ is obtained, and the power P ₂ at which the power of the optical signal measured after passing the second optical signal through the linear polarizer is minimized is obtained; λ _v is moved by λ _i by a predetermined amount to obtain the power P ₃ of the first optical signal, and the power P of the optical signal measured after passing the second optical signal through the linear polarizer is minimum. Find ₄ , The optical signal-to-noise ratio monitoring method of the wavelength division multiplex optical transmission system for obtaining the optical signal-to-noise ratio using the power P ₁ , P ₂ , P ₃ , P ₄ . 19. The method according to claim 18, wherein the predetermined amount of shifting λ _v from λ _i is 0.3 to 3 R GHz when the transmission speed of the optical signal is R Gbps. 19. The system of claim 18, wherein the first and second optical signals are separated by a 1: 1 directional coupler, where B _r (nm) is the reference bandwidth for determining the power of ASE noise in OSNR calculation as resolution and B _0. A method for monitoring an optical signal-to-noise ratio of a wavelength division multiplex optical transmission system in which the optical signal-to-noise ratio is obtained by the following equation when is the bandwidth of the wavelength-variable bandpass optical filter. The center wavelength is λ ₁ ,. , λ _i ,… A method for monitoring the optical signal-to-noise ratio (OSNR) of a wavelength division multiplexed optical signal with λ _n (i = 1, 2, ..., n), Separating the optical signal passing through the polarization controller and the wavelength variable bandpass optical filter into a first optical signal and a second optical signal which are perpendicular to each other by a polarization separator; When the center wavelength λ _v of the wavelength variable bandpass optical filter is variable and coincides with the center wavelength λ _i of the optical signal to measure the optical signal-to-noise ratio among the wavelength division multiplexed optical signals, the polarization state of the polarization controller is consistent. Obtaining power P ₁ of the first optical signal having a polarization state and power P ₂ of the second optical signal having a polarization state perpendicular to the polarization state of the polarization controller; When λ _v is varied and shifted by λ _i by a predetermined amount, the polarization state perpendicular to the power P ₃ of the first optical signal having a polarization state coinciding with the polarization state of the polarization controller and the polarization state of the polarization controller Obtain a power P ₄ of the second optical signal having? The optical signal-to-noise ratio monitoring method of the wavelength division multiplex optical transmission system for obtaining the optical signal-to-noise ratio using the power P ₁ , P ₂ , P ₃ , P ₄ . 22. The method of claim 21, wherein the predetermined amount of shifting λ _v from λ _i is 0.3 to 3 R GHz when the transmission speed of the optical signal is R Gbps. 22. The method according to claim 21, wherein when B _r (nm) is a reference bandwidth for determining the power of ASE noise in OSNR calculation as the resolution and B _O is a bandwidth (nm) of the wavelength variable bandpass optical filter, Optical signal-to-noise ratio monitoring method of wavelength division multiplex optical transmission system for obtaining signal-to-noise ratio.