WO2005025095A1

WO2005025095A1 - Osnr monitoring apparatus for wdm optical transmission system compensating for pmd and using a polarization nulling method

Info

Publication number: WO2005025095A1
Application number: PCT/KR2004/000972
Authority: WO
Inventors: Yun Chur Chung; Seung Kyun Shin; Chul Han Kim
Original assignee: Teralink Communications Co., Ltd.
Priority date: 2003-09-08
Filing date: 2004-04-28
Publication date: 2005-03-17
Also published as: KR20050025700A; KR100613744B1

Abstract

Disclosed herein is an Optical Signal to a Noise Ratio (OSNR) monitoring system for a Wavelength Division Multiplexing (WDM) optical transmission system. The apparatus includes a Polarization Mode Dispersion (PMD) compensating means, a wavelength variable bandpass optical filter, a first polarization controller, a second polarization controller, an optical signal separating means, a linear polarizer and an OSNR measuring means. The PMD compensating means compensates for PMD of the wavelength division multiplexed optical signal. The wavelength variable bandpass optical filter filters out an optical signal having a center wavelength λ, whose OSNR will be measured, from the wavelength division multiplexed optical signal, other channels’ OSNR will be measured by varying a center wavelength λ thereof. The first polarization controller controls a polarization state of the filtered optical signal. The optical signal separating means separates the polarization-controlled optical signal into first and second optical signals. The linear polarizer linearly polarizes the second optical signal.

Description

Description OSNR MONITORING APPARATUS FOR WDM OPTICAL TRANSMISSION SYSTEM COMPENSATING FOR PMD AND USING A POLARIZATION NULLING METHOD Technical Field

[1] The present invention relates generally to an optical performance monitoring apparatus on an optical communication network, and more particularly to an optical signal to noise ratio monitoring apparatus, which can automatically monitor optical signal to noise ratios in an wavelength division multiplexing optical transmission system using a polarization nulling method and a polarization mode dispersion compensating method. Background Art

[2] In order to reliably operate and manage Wavelength Division Multiplexing (WDM) ultrahigh capacity optical communication networks, it is essential to ascertain the transmission performance of an optical transmission system. An Optical Signal to Noise Ratio (OSNR) is defined as the ratio of an optical signal power to a noise power included in an optical signal band. By measuring the OSNR accurately, the transmission performance of an optical transmission system can be ascertained.

[3] Of conventional OSNR measuring methods, there is a method of measuring an OSNR by linearly interpolating the intensity of Amplified Spontaneous Emission (ASE) noise on the basis of the intensity of ASE noise in interbands other than the optical signal band (H. Suzuki and N. Takachino, Electronics Letter Vol. 35, pp. 836-837, 1999). The linear interpolation method is a method of estimating the intensity of ASE noise in an optical signal band through the use of dotted lines by linearly extending from the position of the intensity of ASE noise in interbands other than the optical signal band as shown in FIG. 1, thus measuring an OSNR. Disclosure of Invention Technical Problem

[4] However, in a WDM optical transmission system in which each optical signal may pass through different paths and different numbers of Erbium-Doped Fiber Amplifiers (EDFAs), the intensities of ASE noise include in each optical signal level may be different as shown in FIG. 2 (The third dotted line level must be better positioned at different level than of the first dotted line, perhaps at higher position). Accordingly, the intensity of ASE noise linearly interpolated using the intensity of ASE noise interbands other than an optical signal band is significantly different from the intensity of ASE noise in an actual optical band, so a precise OSNR cannot be obtained using the linear interpolation method.

[5] In order to solve the above-described problem, Korean Patent No. 341825 entitled "Method and apparatus for monitoring OSNR using polarization nulling method" discloses a technology in which an OSNR is measured using a polarization nulling method that utilizes the polarization characteristics of an optical signal and ASE noise. With reference to FIG. 3, a conventional apparatus for measuring an OSNR using the polarization nulling method is described below. A wavelength division multiplexed optical signal including ASE noise is demultiplexed by a waveguide diffraction grating 11. An optical signal having a certain polarization passes through a quarter- wave plate 12 used as a polarization controller and a linear polarizer 13. For example, if the quarter-wave plate 12 and the linear polarizer 13 are rotated at speeds of 15 Hz and 0.1 Hz, respectively, the minimum power and the maximum power of the optical signal can be measured using the polarization characteristics of the optical signal including ASE noise. When the polarization state of the linear polarizer 13 is identical with that of an optical signal output from the quarter- wave plate 12, the power of the measured optical signal is maximized. In contrast, when the polarization direction of the linear polarizer 13 is orthogonal to that of an optical signal output from the quarter- wave plate 12, only 1/2 of ASE noise power become the output with a pure optical signal eliminated, and the power of the measured optical signal is minimized.

[6] An optical signal having passed through the linear polarizer 13 is converted into an electrical signal by an optical detector 14 amplified by a log amplifier 15 and displayed on an oscilloscope 16. A computer 17 obtains maximum and minimum power based on a voltage displayed on the oscilloscope 16 and calculates the power of an optical signal and ASE noise using the maximum and the minimum power, thus measuring an OSNR.

[7] However, the conventional apparatus for monitoring an OSNR using only a polarization nulling method is limited to certain uses because it does not consider the phenomenon that the degree of polarization of an optical signal is decreased due to Polarization Mode Dispersion (PMD) and nonlinear birefringence. In particular, in the case of PMD, the accumulated influences of PMD caused by optical fiber and various optical elements situated on an optical line are exhibited, which also become a principal cause of the deteriorated performance of ultra high speed optical networks.

[8] PMD is related to a time difference when an optical signal travels along two po- larization axes orthogonal to each other.

[9] The light traveling along one polarization axis moves slower or faster than the light polarized along the other axis. The corresponding difference in propagation time is called the differential group delay (DGD), expressed in picoseconds. The differential group delay and the orthogonal polarization modes are the fundamental manifestations of the common first-order form of polarization mode dispersion. Accordingly, when PMD occurs, it is almost impossible to control the respective frequency components of an optical signal to have a single polarization state.

[10] FIG's. 4 through 7 show optical spectra that are measured by inputting optical signals having undergone PMD of 20ps into a polarization controller and a linear polarizer and performing a polarization nulling method so as to investigate the influence of the PMD on a polarization nulling method. FIG. 4 shows optical spectra in the case where the polarization controller controls the state of the polarization to be identical with that of the linear polarizer, FIGs. 5 and 6 show optical spectra in the case where the polarization controller controls the state of the polarization to be partially matched with that of the linear polarizer, and FIG. 7 shows optical spectra in the case where the polarization controller controls the state of the polarization to be orthogonal to that of the linear polarizer. As apparent from the spectra shown in the drawings, the frequency components of the optical signal do not have the same polarization state, but have different polarization states. In such case, the intensity of an optical signal, as shown in FIG. 4 is measured by adjusting the polarization axis of the polarizer to correspond to the polarization state of the frequency component having maximum intensity.

[11] When PMD occurs, however, the intensity of an optical signal would be measured to be lower than actual intensity and, further, parts of the optical signal would be measured as ASE noise, causing error in the measurement of an OSNR.

[12] Among these two causes of generating error in the measurement of an OSNR, that is, the cases where the intensity of an optical signal is measured as being lower than actual intensity or the case where the intensity of ASE noise is measured to be higher than actual intensity, error generated by the latter is larger than that of the former, for ASE noise is normally lower than an optical signal by 10-30 dB and thus, even small portions of the optical signal may be greater than actual ASE noise.

[13] Accordingly, in order to correctly measure an OSNR, it is required to appropriately compensate PMD before applying a polarization nulling method.

[14] Additionally, the conventional OSNR monitoring scheme using only the po- larization nulling method is problematic in that it requires the waveguide type refraction grating 11 to perform demultiplexing and one OSNR monitoring apparatus is required for each demultiplexed optical signal thus incurring great expense. Technical Solution

[15] Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an OSNR monitoring apparatus for a WDM optical transmission system, in which PMD is compensated for, and a polarization nulling method and a wavelength variable bandpass filter, in which the center wavelength of a pass band is varied, are employed.

[16] In order to accomplish the above object, the present invention provides an apparatus for monitoring Optical Signal to Noise Ratio (OSNR) of wavelength division multiplexed optical signals having center wavelengths of λ , ... λ , ..., λ (i=l, 2, ..., n) 1 i n in a Wavelength Division Multiplexing (WDM) optical transmission system, comprising: Polarization Mode Dispersion (PMD) compensating means for compensating for PMD of the wavelength division multiplexed optical signal; a wavelength variable bandpass optical filter for filtering out an optical signal having a center wavelength λ , whose OSNR is to be measured, from the wavelength division multiplexed optical signal by varying a center wavelength λ thereof; a first po- larization controller for controlling a polarization state of the filtered optical signal; optical signal separating means for separating the polarization-controlled 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. [17] Preferably, the OSNR measuring means may obtain a power P of the first optical signal when λ is identical with λ , a minimal power P of the second optical signal v l 2 having passed through the linear polarizer by allowing λ to be shifted from λ by a pre- determined distance, and the OSNR using the powers P and P . 1 2

[18] Preferably, the predetermined distance may be 0.3-3R GHz when a transmission rate of the optical signal is R Gbps.

[19] Preferably, the PMD compensating means may comprise a second polarization controller for controlling a polarization state of an input optical signal and a polarization maintaining optical fiber.

[20] Preferably, the OSNR monitoring apparatus may former comprise a polarization control signal generating unit for controlling the first polarization controller using a first polarization control signal and the second polarization controller using a second polarization control signal so that a power of the second optical signal obtained by the OSNR measuring means is minimized. [21] Preferably, the OSNR monitoring apparatus may former comprise a wavelength control signal generating unit for generating a wavelength control signal so that the center wavelength λ of the wavelength variable bandpass optical filter is varied within a range of λ to λ . 1 n

[22] In addition, the present invention provides an apparatus for monitoring OSNR of wavelength division multiplexed optical signals having center wavelengths of λ , ..., λ , 1 i ..., λ (i=l, 2, ..., n) in a WDM optical transmission system, comprising: PMD com- n pensating means for compensating for PMD of the wavelength division multiplexed optical signals; a wavelength variable bandpass optical filter for filtering out an optical signal having a center wavelength λ , whose OSNR is to be measured, from the wavelength division multiplexed optical signals by varying a center wavelength λ thereof; a first polarization controller for controlling a polarization state of the filtered optical signal; a polarization beam splitter for separating the polarization-controlled optical signal into a first and a second optical signals having polarization states orthogonal to each other, respectively; and OSNR measuring means. [23] Preferably, the OSNR measuring means may obtain a power P of the first optical signal having a polarization state identical with a polarization state of the first polarization controller when λ is identical with λ , a power P of the second optical signal v l 2 having a polarization state orthogonal to the polarization state of the first polarization controller by allowing λ to be shifted from λ by a predetermined distance, and the OSNR using the powers P and P . 1 2

[24] Preferably, the predetermined diatance may be 0.3-3R GHz when a transmission rate of the optical signal is R Gbps.

[25] Preferably, the PMD compensating means may comprise a second polarization controller for controlling a polarization state of an input optical signal and a polarization maintaining optical fiber.

[26] Preferably, the OSNR monitoring apparatus may former comprise a polarization control signal generating unit for controlling the first polarization controller using a first polarization control signal and the second polarization controller using a second polarization control signal so that a power of the second optical signal obtained by the OSNR measuring means is minimized.

[27] Preferably, the OSNR monitoring apparatus may former comprise a wavelength control signal generating unit for generating a wavelength control signal so that the center wavelength λ of the wavelength variable bandpass optical filter is varied within a range of λ to λ . 1 n Description of Drawings

[28] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[29] FIG. 1 is a view showing the ASE noise of an optical signal band obtained by a linear interpolation method;

[30] FIG. 2 is a view showing an example of ASE noise included in each of the bands of optical signals having passed through different paths and different numbers of EDFAs;

[31] FIG. 3 is a block diagram of a conventional OSNR monitoring apparatus using a polarization nulling method;

[32] FIGs. 4 to 7 are views showing optical spectra measured by applying a polarization nulling method to optical signals having undergone PMD;

[33] FIG. 8 is a diagram schematically showing a method of monitoring an OSNR in accordance with the present invention;

[34] FIG. 9 is a block diagram of an OSNR monitoring apparatus in accordance with a first embodiment of the present invention;

[35] FIG. 10 is a block diagram of another OSNR monitoring apparatus in accordance with a second embodiment of the present invention;

[36] FIGs. 11 to 13 are eye diagrams of optical signals modulated to 10 Gb/s;

[37] FIG. 14 is a block diagram of an experimental setup used to verify the effectiveness of the OSNR monitoring apparatus of the present invention; and

[38] FIG. 15 is a graph showing OSNRs measured by the experimental setup of FIG. 14. Mode for Invention

[39] Reference should now be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar elements.

[40] FIG. 8 is a diagram schematically showing a method of monitoring an OSNR in accordance with the present invention. FIG. 9 is a block diagram of an OSNR monitoring apparatus in accordance with a first embodiment of the present invention.

[41] A portion of an optical signal including ASE noise is separated by a 99: 1 directional coupler 22 from a WDM optical communication network and then is used to measure an OSNR. The 99: 1 directional coupler 22 extracts a portion of the optical signal, which has a 1/100 intensity of the optical signal and inputs the extracted portion of the optical signal to a second polarization controller 32 of PMD compensation apparatus 30.

[42] The PMD compensation apparatus 30 includes the second polarization controller 32 and a polarization maintaining optical fiber 34 and fonctions to compensate for the PMD of an optical signal. The second polarization controller 32 controls the polarization state of an optical signal input to the polarization maintaining optical fiber 34 in response to a second polarization control signal from a polarization control signal generating unit 28 to thereby compensate for a desired amount of PMD. The second polarization controller 32 can be implemented by using optical fiber and a piezoelectric element.

[43] The optical signal, whose polarization has been determined by the second polarization controller 32, is compensated for PMD, while passing through the polarization maintaining optical fiber 34. The polarization maintaining optical fiber 34 is an optical fiber in which the polarization planes of lightwaves launched into the fiber are maintained during propagation with little or no cross-coupling of optical power between the polarization modes. The polarization maintaining optical fiber 34 may have eqiivalently different PMD values depending upon the polarization state of an input optical signal.

[44] For example, in the case where a polarization maintaining optical fiber 34 with a total PMD value of 20 ps is employed, an output optical signal will have a maximum PMD value of 20 ps by adjusting the polarization state of the input optical signal to have an angle of 45 degrees with respect to the two polarization axes of the polarization maintaining optical fiber 34. If the polarization state of the input optical signal is adjusted to coincide with the two polarization axes of the polarization maintaining optical fiber 34 the output optical signal may have a PMD value eqiivalently identical with 0 ps.

[45] After having passed through the polarization maintaining optical fiber 34 optical signal including ASE noise is input to a wavelength variable bandpass optical filter 26. The pass band of the wavelength variable bandpass optical filter 26 as shown in FIG. 8, is designed to pass an optical signal channel therethrough without affecting neighboring channels. The wavelength variable bandpass optical filter 26 receives a wavelength control signal from the wavelength control signal generating unit 24 and varies the center wavelength λ of the pass band. The wavelength variable bandpass optical filter 26 may be controlled to filter out all the wavelength bands of a wavelength division multiplexed signal in response to the wavelength control signal from the wavelength control signal generating unit 24. Accordingly, the present invention enables all the OSNRs of wavelength division multiplexed signals to be measured even without using a demultiplexer, such as a waveguide diffraction grating. [46] Further, the wavelength variable bandpass optical filter 26 as shown in FIG. 8, may be controlled to filter out an optical signal with a shift of a predetermined distance from a center wavelength λ of the optical signal whose OSNR is to be measured, in response to a wavelength control signal from the wavelength control signal generating unit 24. The distance that the center wavelength λ of the wavelength variable bandpass optical filter 26 can be shifted from the center wavelength λ of the optical signal is determined within a range in which the wavelength variable optical bandpass filter 26 can pass at least a portion of the optical signal whose OSNR is to be measured. Considering the accuracy of an OSNR measurement, the distance of shift is preferably 0.3-3 R GHz when a transmission rate is R Gbps. For example, when the transmission rate of an optical signal is 10 Gbps, the center wavelength λ of the wavelength variable bandpass optical filter 26 is adjusted to be shifted from the center wavelength λ of the optical signal by 3 - 30 GHz. The center wavelength λ of the wavelength variable bandpass optical filter 26 can be adjusted to be shifted from the center wavelength λ of the optical signal to the right or left by a predetermined distance in response to a wavelength control signal from the wavelength control signal generating unit 24.

[47] For the wavelength variable bandpass optical filter 26 there may be employed a filter using a MicroElectroMechanical Systems (MEMS) based half symmetric cavity resonator, a Fabry-Perot variable filter, an integrated optical device including a grating, a multilayered thin film device, an acoustic optical filter, etc. To achieve OSNR measurement accuracy, a narrow band wavelength variable optical filter that is adequate to pass a band narrower than the band of an optical signal is preferable and this filter is better to be fabricated using the MEMS based half symmetric cavity resonator.

[48] An optical signal having passed through the wavelength variable bandpass filter 26 is input to a polarization separating device 40. The polarization separating device 40 includes a first polarization controller 42, a 1 : 1 directional coupler 44 and a linear polarizer 46. The first polarization controller 42 receives a first polarization control signal from the polarization control signal generating unit 28, and controls the polarization state of an optical signal so that the intensity of the optical signal, including ASE noise, output from the linear polarizer 46 is minimized. In the case where an optical signal input to the linear polarizer 46 has a polarization state that is orthogonal to the polarization state of the linear polarizer 46 the first polarization controller 42 performs a polarization nulling method in which polarization is nulled while the input optical signal passes through the linear polarizer 42. When the polarization state of an optical signal having passed through the first polarization controller 42 is orthogonal to that of the linear polarizer 46 the intensity of an optical signal including ASE noise output from the linear polarizer 46 is minimized by the polarization nulling method.

[49] The linear polarizer 46 is an optical fiber based optical device and constructed to fix the polarization state thereof. Accordingly, the polarization state of an optical signal may be adjusted to be orthogonal to that of the linear polarizer 46 by controlling the first polarization controller 42. The first polarization controller 42 may be implemented using an optical fiber and a piezoelectric device in the same manner as the second polarization controller 32. Those skilled in the art can appreciate that the linear polarizer 4 the second polarization controller 32 and the first polarization controller 42 are not limited to the above-described construction but may be implemented using various general devices.

[50] An OSNR calculating apparatus 50 includes optical detectors 52a and 52b, analog to digital converters 54a and 54b, a power calculating unit 56 and an OSNR calculating unit 58, and detects an input optical signal, converts the input optical signal to a digital signal and calculates the power of the optical signal and an OSNR.

[51] The method of monitoring an OSNR in accordance with to the present invention is described below. A portion of a wavelength division multiplexed optical signal having center wavelengths of λ , ..., λ , ..., λ (i=l, 2, ..., n) is extracted by the 99:1 directional 1 i n coupler 22, and input to the second polarization controller 32. The second polarization controller 32 compensates for PMD by controlling the polarization state of the extracted optical signal in response to a second polarization control signal from the polarization control signal generating unit 28. An output optical signal of the second polarization controller 32 is input to the polarization maintaining optical fiber 34. An output optical signal of the polarization maintaining optical fiber 34 in which PMD is compensated for is input to the wavelength variable bandpass optical filter 26. [52] The center wavelength λ of the wavelength bandpass optical filter 26 is controlled to coincide with the center wavelength λ of an optical signal to be measured, in response to a wavelength control signal from the wavelength control signal generating unit 24. An optical signal having passed the wavelength variable bandpass optical filter 26 passes through the first polarization controller 42, and is branched into first and second optical signals by the 1:1 directional coupler 44. While traveling along a path 1, the first optical signal is photoelectrically converted into an electric signal by the optical detector 52a and converted into a digital signal by the analog to digital converter 54a. The power calculating means 56 calculates power P . The power P obtained as described above contains a pure optical signal component occupying the most part of an optical signal and an ASE noise component occupying the minute part of the optical signal. [53] Thereafter, the center wavelength λ of the wavelength bandpass optical filter 26 as shown in FIG. 8, is controlled to be shifted from the center wavelength λ of an optical signal by a predetermined amount. In the case where the center wavelength λ is identical with the center wavelength λ , it is difficult to measure ASE noise using the polarization nulling method since the intensity of an optical signal passing through the linear polarizer 46 is relatively high (due to imperfection of polarization extinction ratio of linear polarizer, PMD or nonlinear birefringence). [54] Accordingly, in the present invention, the polarization nulling method is performed after the center wavelength λ is controlled to be shifted from the center wavelength λ by an amount of 0.3-3 R GHz. That is, after the pass band of the wavelength variable bandpass optical filter 26 is changed, the intensity of ASE noise of a polarization component orthogonal to an optical signal input to the linear polarizer 46 is measured by adjusting the first polarization controller 42 in response to a first polarization control signal from the polarization control signal generating unit 28. When the pass band of the wavelength variable bandpass optical filter 26 is changed, the intensity of a measured optical signal becomes lower than before the pass band is changed due to the transmission characteristics of the optical filter 26. However, the intensity of ASE noise is almost the same as before the pass band is changed because the ASE noise has substantially constant value across a wide band. Therefore, it is easier than before the band is changed to implement the polarization nulling method. [55] While traveling along a path 2, the second optical signal, to which the polarization nulling method is applied, passes through the linear polarizer 4 is photoelectrically converted into an electric signal by the optical detector 52b, and is converted into a digital signal by the analog to digital converter 54b. The power calculating means 56 calculates the power P of the second optical signal including linearly polarized ASE 2 noise. [56] The obtainment of ASE noise using the polarization nulling method may be implemented in such a way that the polarization state of the first polarization controller 42 is scanned within a certain range and a minimum value is found among the calculated powers. When the polarization state of an optical signal passing through the first polarization controller 42 is orthogonal to that of the linear polarizer 4 the power of the optical signal including ASE noise which is calculated at the power calculating unit 56 is minimized.

[57] In the meantime, in order to compensate for PMD by controlling the polarization state of an optical signal input to the polarization maintaining optical fiber 34 the polarization control signal generating unit 28 receives a feedback signal and generates an appropriate polarization control signal corresponding to the received feedback signal. For the feedback signal, there may be employed the Degree Of Polarization (DOP) measured by a DOP meter 27 that was introduced in a thesis by C. Francia et al. ("Simple dynamic polarization mode dispersion compensator," IEE Electron Lett., Vol. 35, No. 5, pp. 414-415, 1999). That a DOP value is maximized means that the PMD of an optical signal is maximally compensated for. Therefore, the polarization control signal generating unit 28 controls the second polarization controller 32 in such a manner that a DOP value is maximized, using the DOP value of the optical signal having passed through the polarization maintaining optical fiber 34 as the feedback signal.

[58] In addition to the scheme of using the DOP value as the feedback signal, the present invention presents a scheme of measuring a particular frequency component in the spectrum of an optical signal and using the measured frequency component as the feedback signal used to compensate for PMD. When the polarization nulling method is performed after the center wavelength λ is adjusted to be shifted from the center wavelength λ by 0.3-3R GHz, only ASE noise component can be measured if PMD does not occur. However, if PMD occurs, a portion of an optical signal is measured together with ASE noise component. Accordingly, by controlling the second polarization controller 32 so that the intensity of the particular frequency component is minimized (that is, the intensity of the second optical signal is minimized), the influence of PMD may be minimized.

[59] The OSNR calculating unit 58 obtains an OSNR by using the intensity of an optical signal obtained from the first optical signal and the intensity of ASE noise obtained from the second optical signal.

[60] FIG. 10 is a block diagram showing another OSNR monitoring apparatus in accordance with a second embodiment of the present invention.

[61] In the first embodiment, the polarization separating means 40 includes the first polarization controller 42, the 1 : 1 directional coupler 44 and the linear polarizer 4 while in the second embodiment, a polarization separating means 40 includes a first polarization controller 42 and a polarization beam splitter 48. The first polarization controller 42 receives a polarization control signal from a polarization control signal generating unit 28, and separates an optical signal output from the polarization beam splitter 48 into two optical signals having polarization states orthogonal to each other. As a result, a first optical signal having a polarization state identical with the polarization state of the first polarization controller 42 travels along a path 3, and a second optical signal having a polarization state orthogonal to the polarization state of the first polarization controller 42 travels along a path 4. Since those skilled in the art can appreciate that an OSNR can be measured in the same method as in the first embodiment, a detailed description of the measurement is omitted here.

[62] FIGs. 11 to 13 are eye diagrams of optical signals modulated to 10 Gb/s. In the case where a first-order PMD value, that is, a DGD value, is 0 ps as shown in FIG. 11, eyes are widely opened. In the case where a DGD value is 50 ps as shown in FIG. 12, eyes are distorted, so an error may occur in the measurement of an OSNR. However, when an optical signal having a DGD value of 50 ps is compensated for PMD, eyes may be restored as shown in FIG. 13, so an OSNR may be correctly measured.

[63] FIG. 14 is a block diagram of an experimental setup that was used to verify the effectiveness of the OSNR monitoring apparatus of the present invention. In the experimental setup, a wavelength variable laser 62 was used to provide an optical signal and an EDFA was used as an ASE light source 64 to provide ASE noise. PMD was generated using a PMD emulator 76. An optical signal and ASE noise having been coupled to each other by a 1:1 directional coupler 70 were separated into two components by another 1 : 1 directional coupler 72.

[64] One component was input to an Optical Spectrum Analyzer (OSA) 74 and the OSNR of this portion was measured by a linear interpolation method. In this case, since the optical signal band is considerably narrow and the ASE noise is very broad and even an OSNR measured by the linear interpolation method can be considered to be exact. The other component was input to an OSNR monitoring device A and the OSNR of this component was measured according to the present invention. The OSNR was varied by changing the power of the optical signal and the intensity of ASE noise using optical variable attenuators 66 and 68 connected to the PMD emulator 76 and the ASE light source 64 respectively.

[65] FIG. 15 is a graph showing OSNRs measured by the experimental setup of FIG. 14. In this measurement, optical devices including optical fibers and piezoelectric devices were used as polarization controllers 32 and 42, and a filter having a MEMS based half symmetric cavity resonator was used as the wavelength variable bandpass optical filter 26. And an optical fiber based optical device was used as the linear polarizer 46. The optical signal was a 10 Gb/s Non-Return-to-Zero (NRZ) signal at a wavelength of 1550 nm. Optical signal having the same intensity at each of two polarization axes by controlling the input polarization of the PMD emulator 76 were provided as an input ( γ=0.5, γ designates the ratio of the intensities of optical signal between two polarization axes). The polarization maintaining fiber 34 having 20 ps was used in the OSNR monitoring device A to compensate for PMD.

[66] As apparent from the results, even in the case where a first-order PMD value of 20 ps occurred, an OSNR was measured within a measurement error range of 1 dB. However, in the case where a first-order PMD value was greater than 20 ps, a measurement error started to occur when an OSNR greater than 20 dB was measured, as shown in FIG. 15 (in the case of DGD=30 ps). Through these measurements, the influence of PMD on the OSNR measuring method using the polarization nulling method could be ascertained. However, this result was achieved because the polarization maintaining fiber 34 for compensating for PMD of 20 ps was used in the OSNR monitoring apparatus A. In the case where a PMD value to be compensated for is greater than a PMD value that is compensated for by the polarization maintaining fiber 34 (for example, DGD>30 ps, or the PMD value is 30% or more of the transmission speed of an optical signal), or the influence of high-order PMD exists, an OSNR can be accurately measured by constructing a PMD generating means using two or more polarization controllers and two or more polarization maintaining fibers.

[67] Those skilled in the art can perform various substitutions, modifications and additions. For example, a third embodiment can be constructed by interchanging positions between the first polarization controller 42 and the wavelength variable bandpass optical filter 26 of FIG. 9. In this third embodiment, an OSNR can be obtained in the same manner as in the first embodiment. Further, a fourth embodiment can be constructed by interchanging positions between the first polarization controller 42 and the wavelength variable bandpass optical filter 26 of FIG. 10. In this fourth embodiment, an OSNR can be obtained in the same manner as in the second embodiment. Industrial Applicability

[68] As described above, the present invention provides an OSNR monitoring apparatus for WDM optical transmission system, which can accurately measure an OSNR by compensating for PMD and utilizing a polarization nulling method. Additionally, in the present invention, the wavelength variable bandpass optical filter is controlled to filter all the wavelength bands of a wavelength division multiplexed signal, so a problem that an OSNR device is provided for each of demultiplexed optical signals is eliminated, thus reducing expenses. Further, the present invention uses the polarization nulling method in the state where the center wavelength of the pass band of the wavelength variable bandpass optical filter is made to be shifted by a predetermined amount, so an OSNR can be easily measured. [69] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

[1] An apparatus for monitoring Optical Signal to Noise Ratio (OSNR) of wavelength division multiplexed optical signals having center wavelengths of λ , ..., λ , ..., λ (i=l, 2, ..., n) in a Wavelength Division Multiplexing (WDM) optical i n transmission system, comprising: Polarization Mode Dispersion (PMD) compensating apparatus for compensating for PMD of the wavelength division multiplexed optical signal; a wavelength variable bandpass optical filter for passing an optical signal having a center wavelength λ , whose OSNR is to be measured, from the wavelength division multiplexed optical signal by varying a center wavelength λ thereof; a first polarization controller for controlling a polarization state of the filtered optical signal; optical signal separating apparatus for separating the polarization-controlled 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 apparatus for obtaining a power P of the first optical signal when λ coincides with λ , a minimal power P of the second optical signal v l 2 having passed through the linear polarizer by allowing λ to be shifted from λ by a predetermined distance, and the OSNR from the values of the powers P and P 1 2

[2] The OSNR monitoring apparatus as set forth in claim 1, wherein the predetermined distance is 0.3-3R GHz when a transmission rate of the optical signal is R Gbps.

[3] The OSNR monitoring apparatus as set forth in claim 1, wherein the PMD compensating means comprises a second polarization controller for controlling a polarization state of an input optical signal and a polarization maintaining optical fiber.

[4] The OSNR monitoring apparatus as set forth in claim 3, further comprising a polarization control signal generating unit for controlling the first polarization controller using a first polarization control signal and the second polarization controller using a second polarization control signal so that a power of the second optical signal obtained by the OSNR measuring means is minimized.

[5] The OSNR monitoring apparatus as set forth in claim 3, further comprising a Degree Of Polarization (DOP) meter for measuring a DOP of an optical signal having passed through the polarization maintaining optical fiber.

[6] The OSNR monitoring apparatus as set forth in claim 5, further comprising a polarization control signal generating unit for controlling the first polarization controller using a first polarization control signal so that a power of the second optical signal obtained by the OSNR measuring means is minimized, and the second polarization controller using a second polarization control signal so that a DOP value measured by the DOP meter is maximized.

[7] The OSNR monitoring apparatus as set forth in claim 1, wherein the OSNR measuring means comprises: a first optical detector for detecting the first optical signal and converting the first optical signal into an electrical signal; a second optical detector for detecting the second optical signal having passed through the linear polarizer and converting the second optical signal into an electrical signal; a first analog to digital converter for converting the analog electrical signal output from the first optical signal into a digital signal; a second analog to digital converter for converting the analog electrical signal output from the second optical signal into a digital signal; a power calculating unit for calculating the powers P and P using the converted 1 2 digital signals; and an OSNR calculating unit for calculating the OSNR using the powers P and P 1 2 calculated in the power calculating unit.

[8] The OSNR monitoring apparatus as set forth in claim 1, wherein the optical signal separating means is a 1 : 1 directional coupler.

[9] The OSNR monitoring apparatus as set forth in claim 3, wherein the PMD compensating means comprises two or more second polarization controllers and two or more polarization maintaining optical fibers for controlling a polarization state of the input optical signals when a PMD value is 30% or more of a transmission rate of the optical signal or high-order PMD exists.

[10] The OSNR monitoring apparatus as set forth in claim 1, further comprising a wavelength control signal generating unit for generating a wavelength control signal so that the center wavelength λ of the wavelength variable bandpass optical filter is varied within a range of λ to λ . 1 n

[11] The OSNR monitoring apparatus as set forth in claim 1, wherein the wavelength bandpass optical filter is an optical filter using a half symmetric cavity resonator constructed as Micro Electro Mechanical System (MEMS). [12] An apparatus for monitoring OSNR of wavelength division multiplexed optical signals having center wavelengths of λ , ..., λ , ..., λ (i=l, 2, ..., n) in a WDM 1 i n optical transmission system, comprising: PMD compensating apparatus for compensating for PMD of the wavelength division multiplexed optical signals; a wavelength variable bandpass optical filter for passing an optical signal having a center wavelength λ , whose OSNR is to be measured, from the wavelength division multiplexed optical signals by varying a center wavelength λ thereof; a first polarization controller for controlling a polarization state of the filtered optical signal; a polarization beam splitter for separating the polarization-controlled optical signal into a first and a second optical signals having polarization states orthogonal to each other, respectively; and OSNR measuring apparatus for obtaining a power P of the first optical signal having a polarization state identical with a polarization state of the first polarization controller when λ is identical with λ , a power P of the second optical v l 2 signal having a polarization state orthogonal to the polarization state of the first polarization controller by allowing λ to be shifted from λ by a predetermined distance, and the OSNR from the powers P and P .

1 2

[13] The OSNR monitoring apparatus as set forth in claim 12, wherein the predetermined distance is 0.3-3R GHz when a transmission rate of the optical signal is R Gbps.

[14] The OSNR monitoring apparatus as set forth in claim 12, wherein the PMD compensating means comprises a second polarization controller for controlling a polarization state of an input optical signal and a polarization maintaining optical fiber.

[15] The OSNR monitoring apparatus as set forth in claim 14 forther comprising a polarization control signal generating unit for controlling the first polarization controller using a first polarization control signal and the second polarization controller using a second polarization control signal so that a power of the second optical signal obtained by the OSNR measuring means is minimized.

[16] The OSNR monitoring apparatus as set forth in claim 14 forther comprising a Degree Of Polarization (DOP) meter for measuring a DOP of an optical signal having passed through the polarization maintaining optical fiber.

[17] The OSNR monitoring apparatus as set forth in claim 1 further comprising a polarization control signal generating unit for controlling the first polarization controller using a first polarization control signal so that a power of the second optical signal obtained by the OSNR measuring means is minimized, and the second polarization controller using a second polarization control signal so that a DOP value measured by the DOP meter is maximized.

[18] The OSNR monitoring apparatus as set forth in claim 12, wherein the OSNR measuring means comprises: a first optical detector for detecting the first optical signal and converting the first optical signal into an electrical signal; a second optical detector for detecting the second optical signal having passed through the linear polarizer and converting the second optical signal into an electrical signal; a first analog to digital converter for converting the analog electrical signal output from the first optical signal into a digital signal; a second analog to digital converter for converting the analog electrical signal output from the second optical signal into a digital signal; a power calculating unit for calculating the powers P and P using the converted 1 2 digital signals; and an OSNR calculating unit for calculating the OSNR using the powers P and P 1 2 calculated in the power calculating unit.

[19] The OSNR monitoring apparatus as set forth in claim 14 wherein the PMD compensating means comprises two or more second polarization controllers and two or more polarization maintaining optical fibers for controlling a polarization state of the input optical signal when a PMD value is 30% or more of a transmission rate of the optical signal or high-order PMD exists.

[20] The OSNR monitoring apparatus as set forth in claim 12, further comprising a wavelength control signal generating unit for generating a wavelength control signal so that the center wavelength λ of the wavelength variable bandpass optical filter is varied within a range of λ to λ . 1 n

[21] The OSNR monitoring apparatus as set forth in claim 12, wherein the wavelength bandpass optical filter is an optical filter using a half symmetric cavity resonator constructed as MEMS.