DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0017] In the present invention, there is provided an optical performance monitoring apparatus for measuring an optical signal-to-noise ratio for each channel by measuring power and ASE (Amplified Spontaneous Emission) noise for each channel of a WDM (wavelength Division Multiplexing) signal to monitor the optical performance of the WDM optical communication system.

[0018] FIG. 2 is an exemplary diagram of a WDM optical communication system to which the present invention can be applied.

[0019] The WDM optical communication system to which the present invention is applied comprises an optical multiplexer 210 for multiplexing N−1 channel signals, an optical booster amplifier 230 for amplifying WDM signals multiplexed by the multiplexer 210 , an optical line amplifier 240 for amplifying the WDM signal transferred through an optical transmission line, an optical pre-amplifier 250 , an optical demultiplexer 220 for demultiplexing the WDM signal, transferred through the optical transmission line, depending on each channel wavelength, and an optical performance monitoring apparatus 200 for monitoring optical performance for each channel of the WDM optical communication system. As shown in FIG. 2 , the optical performance monitoring apparatus 200 is disposed at input stage and output stage of each amplifier to perform performance monitoring for each channel of the WDM signal transferred through the optical transmission line. Inputs of the optical performance monitoring apparatus 200 are obtained by tapping a part of the WDM signal from the optical transmission line by using optical tap couplers 260 .

[0020] FIG. 3 is a diagram of an embodiment of an optical performance monitoring apparatus for a WDM optical communication system in accordance with the present invention.

[0021] That is, FIG. 3 shows detailed configuration of the optical performance monitoring apparatus 200 of FIG. 2 according to the present invention.

[0022] The optical performance monitoring apparatus 200 in accordance with the present invention includes an 1×N optical coupler 300 for distributing the WDM signal tapped from the optical transmission line to output, a plurality of 1×2 optical couplers 310 receiving the distributed optical signal from the 1×N optical coupler 300 , a plurality of optical fiber Bragg gratings 320 , each for selecting an optical signal having a corresponding wavelength in the optical signal from the corresponding one of the 1×2 optical couplers 310 to output the selected optical signal to the 1×2 optical coupler 310 , a plurality of optical detectors 330 , each receiving the optical signal selected by the optical fiber Bragg gratings 320 from the corresponding one of the 1×2 optical couplers 310 to detect optical signal power for each channel of a selected wavelength, an optical detector 331 receiving the optical signal distributed from the 1×N optical coupler 300 directly to detect total power of the received optical signal, a channel multiplexer 340 for outputting sequentially the optical signal detected by the plurality of the optical detectors 330 and the optical detector 331 , an analog/digital converter 350 for converting power of analog optical signals sequentially outputted from the channel multiplexer 340 to digital values, and a signal processor 360 for measuring power for each channel and total ASE noise power of the WDM signal by using the digital values from the analog/digital converter 350 and for measuring the optical signal-to-noise ratio for each channel.

[0023] Next, it will be described in detail for the operation of the WDM optical communication system in accordance with the present invention.

[0024] The WDM signal inputted to the 1×N optical coupler 300 is obtained by tapping 270 a part of the optical signal from the optical transmission line. The input WDM signal including not only N−1 channel WDM signals (λ ₁ , λ ₂ , . . . , λ _N−1 ) but also the ASE noise generated by the optical amplifier is inputted to the input port of the 1×N optical coupler 300 through an input optical transmission line. The 1×N optical coupler 300 of the present invention is a typical star type coupler that distributes the input signal to each output port with a predetermined insertion loss. The optical signal outputted from the output port of the 1×N optical coupler 300 is applied to the optical fiber Bragg gratings 320 through the 1×2 optical couplers 310 .

[0025] Each of the optical fiber Bragg gratings 320 is a wavelength selecting filter that reflects only a channel signal having a center wavelength identical to a corresponding Bragg wavelength and transmits remaining signals. Therefore, in order to measure the optical power for each channel of the input WDM signal, the Bragg wavelength of each of the optical fiber Bragg gratings 320 is designed to be identical to the center wavelength (λ ₁ , λ ₂ , . . . , λ _N−1 ) of the WDM signal inputted from the optical transmission line for each channel.

[0026] In one embodiment of the present invention, the optical fiber Bragg gratings are used to discriminate the input WDM signal for each channel, but any wavelength selecting filter capable of discriminating the input WDM signal for each channel can be used in place of the optical fiber Bragg gratings.

[0027] The channel signals discriminated by the optical fiber Bragg gratings 320 are applied to the optical detectors 330 through the 1×2 optical couplers 310 .

[0028] Here, the 1×2 optical couplers 310 are used to apply the WDM signal to the optical fiber Bragg gratings 320 and to combine the channel signals reflected from the optical fiber Bragg gratings 320 at the optical detectors 330 . Provided that an optical circulator is used instead, the total insertion loss of the optical performance monitoring apparatus 200 can be reduced. That is, when the number of the channels of the input WDM signal is small, a simple cost effective 1×2 optical coupler can be used. On the contrary, when the number of the channels of the input WDM signal is large, the optical circulator is more beneficial than the 1×2 optical coupler 310 because the channel power of the WDM signal at the input stage of the optical detectors 330 and 331 could be smaller than sensitivity of the optical detectors 330 and 331 due to increase of the insertion loss. Each of the channel powers of the WDM signals detected by the optical detectors 330 and 331 are applied to the analog/digital converter 350 through the channel multiplexer 340 . The channel multiplexer 340 sequentially applies the outputs of the optical detectors 330 and 331 to the analog/digital converter 350 . The analog/digital converter 350 converts an analog value of the power applied from the channel multiplexer 340 to a digital value. The signal processor 360 computes the optical signal-to-noise ratio for each channel of the WDM signal by using the digital value for the power of each channel converted at the analog/digital converter 350 , the total input power, and the ASE noise profile for each wavelength of the optical amplifier pre-stored at the memory.

[0029] Hereinafter, functional description of the present invention will be provided.

[0030] The power for each channel of the input WDM signal can be obtained by measuring the power of each of the channel signals reflected from the optical fiber Bragg gratings 320 at the optical detectors 330 . Since the optical fiber Bragg gratings 320 have Bragg wavelengths identical to the center wavelength (λ ₁ , λ ₂ , . . . , λ _N−1 ) of the input WDM signal, each of the optical fiber Bragg gratings 320 reflects the channel signal having the wavelength identical to its Bragg wavelength and transmits remaining signals. In order to prevent the signals transmitted from the optical fiber Bragg gratings 320 from being recombined, outputs of the optical fiber Bragg gratings 320 are terminated.

[0031] And, a last output port among the output ports of the 1×N optical coupler 300 is coupled to neither the 1×2 optical coupler 310 nor the optical fiber Bragg gratings 320 . Therefore, at the optical detector 331 coupled to the last output port, total power of the input WDM signal (WDM signal power+ASE noise power) is measured. Accordingly, the ASE noise power included in the input WDM signal can be obtained as follows 1 $\begin{array}{cc}{V}_{total,ASE}=V_N-\sum _{i=1}^{N-1}V_{{\lambda }_i}& \mathrm{Eq}.\left(1\right)\end{array}$

[0032] In Eq. (1), V _total,ASE represents the total ASE noise power of the input WDM signal, V _N represents the total power of the WDM signal measured at the optical detector 331 , and V _λi represents the i-th channel power of the WDM signal measured at the corresponding one of the optical detectors 330 . The ASE noise power for each channel can be obtained by using the total ASE noise power measured by Eq.(1), the channel power of the WDM signal, and the ASE noise profile for each channel to input power level of the optical amplifier under normal conditions pre-stored at the memory in the signal processor 360 .

[0033] FIG. 4 is a graph of the optical signal-to-noise ratio for each channel of the WDM signal in accordance with the present invention.

[0034] The graph of FIG. 4 shows a measurement result of the optical signal-to-noise ratio for each channel of the WDM optical communication system in an embodiment of the present invention, which is compared to a result measured by a commercially available optical spectrum analyzer having b 0 . 1 nm resolution.

[0035] In the present invention, the total ASE noise power of the optical amplifier can be measured as well as the optical signal-to-noise ratio for each channel of the WDM signal in the WDM optical communication system. Therefore, when the measured total ASE noise power is larger than upper limit of the total ASE noise power of the optical amplifier stored at the signal processor 360 , an alerting signal transferred to an optical amplifier control circuit so as to use it for performance monitoring for the WDM optical communication system including the optical amplifier.

[0036] As described above, extra wavelength selecting filter or extra output port of the wavelength selecting filter to obtain the ASE noise sample is not required in the present invention. Therefore, the optical performance monitoring apparatus can be constructed simply and cost effectively. Furthermore, Not only the optical signal-to-noise ratio for each channel of the WDM signal but also the total ASE noise power of the optical amplifier can be measured in the present invention.

[0037] While the present invention has been shown and described with respect to the particular embodiments, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.