Abstract:
An apparatus monitoring frequency of WDM signals using pilot tone and two interleaved solid etalon filters for efficient management and maintenance of wavelength division multiplexing optical transmission network is provided. Desirably, the apparatus for optical frequency monitoring includes splitting means, a plurality of optical filtering means, optical detecting means, and frequency detecting means. The splitting means splits optical signal and the optical signal includes pilot tone. The plurality of optical filtering means filters an output of the splitting means with particular passband. The optical detecting means converts an output of the optical filtering means into electrical signal. The frequency detecting means detects an intensity of the pilot tone from an output of the optical detecting means and calculates optical frequency.

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for monitoring frequency of WDM signals. In particular, the present invention relates to an apparatus monitoring frequency of WDM signals using pilot tone and two interleaved solid etalon filters for efficient management and maintenance of wavelength division multiplexing optical transmission network. 
     BACKGROUND OF THE INVENTION 
     Optical transmission networks with wavelength division multiplexing employ a number of lasers of different wavelengths in order to increase transmission capacity of each optical fiber. 
     In Optical transmission networks with wavelength division multiplexing scheme, even though lasers operate on low transmission speed, transmission capacity per optical fiber can be dramatically increased. Therefore, conventional optical fibers that are already installed don&#39;t need to be replaced. 
     However, since multiplexers and demultiplexers are sensitive to frequency of each channel and output level of each channel is heavily related with overall performance of system in wavelength division multiplexing optical transmission network, the frequency monitoring equipment is necessary for efficient management of optical transmission network. 
     Conventional methods to monitor frequency of optical signals employ two optical filters with interleaved transmission characteristics and intensities of optical signals passed the optical filters are compared. 
     The conventional methods can be used with single channel systems but cannot be applied to multiple channel systems like systems with wavelength division multiplexing technique. 
     Conventional frequency monitoring methods for multiple channel systems include a method using channel crossover characteristic of AWG (arrayed-waveguide grating) and a method using AOTF (accusto-optic tunable filter). 
     However, the method using channel selection characteristic of AWG requires expensive ANG and the method using AOTF is not able to monitor frequency and optical output at systems with 100 GHz channel since bandwidth of AOTF is bigger than 1 nm. 
     SUMMARY OF THE INVENTION 
     An apparatus monitoring frequency of WDM signals using pilot tone and two interleaved solid etalon filters for efficient management and maintenance of wavelength division multiplexing optical transmission network is provided. 
     Desirably, the apparatus for optical frequency monitoring includes splitting means, a plurality of optical filtering means, optical detecting means, and frequency detecting means. The splitting means splits optical signal and the optical signal includes a pilot tone. The plurality of optical filtering means filters an output of the splitting means with particular passbands. The optical detecting means converts an output of the optical filtering means into an electrical signal. The frequency detecting means detects an intensity of the pilot tone from an output of the optical detecting means and calculates optical frequency. 
     Desirably, the frequency detecting means includes analog-digital converting means, fast Fourier transforming means, and frequency calculating means. The analog-digital converting means converts the output of the optical detecting means into a digital signal. The fast Fourier transforming means performs fast Fourier transform on the digital signal. The frequency calculating means measures the intensity of the pilot tone from an output of the fast Fourier transforming means and calculates frequency from the intensities of the pilot tone. 
     Desirably, the frequency detecting means includes electrical filtering means, power detecting means, and frequency calculating means. The electrical filtering means passes the corresponding pilot tone of optical signal from the output of the optical detecting means. The power detecting means converts intensity of the pilot tone from an output of the electrical filtering means into the voltage. The frequency calculating means calculates frequency from the voltage from the power detecting means. 
     Desirably, the frequency calculating means calculates frequency by using the ratio of the intensities of the pilot tone. 
     Desirably, frequency calculating calculates frequency by using difference of the intensities of the pilot tone. 
     Desirably, the optical filtering means is a plurality of filters having interleaved transmission characteristics. 
     Desirably, the optical filtering means is a solid Fabry-Perot etalon filter and transmission characteristics of the Fabry-Perot etalon filter have periodical characteristics in response to wavelength of optical signal. 
     Desirably, the optical filtering means is an optical fiber Fabry-Perot etalon filter and transmission characteristics of the optical fiber Fabry-Perot etalon filter have periodical characteristics in response to wavelength of optical signal. 
     Desirably, the optical filtering means is an arrayed-waveguide grating and resonant frequency of the arrayed-waveguide grating is similar to channel of wavelength division multiplexed signal. 
     Desirably, the optical filtering means is an optical filter and the optical filter operates with frequency range of wavelength division multiplexed optical signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The embodiments of the present invention will be explained with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagram illustrating an apparatus for monitoring frequency using optical filters of interleaved transmission characteristics in accordance with an embodiment of the present invention; 
     FIG. 2 is a graph illustrating characteristics of optical filters that have interleaved transmission characteristics; 
     FIG. 3 is a diagram illustrating an apparatus for monitoring frequency using optical filters of interleaved transmission characteristics in accordance with an embodiment of the present invention; 
     FIG. 4 shows graphs to explain operating principles of the present invention; 
     FIG. 5 is a graph illustrating the ratio of the intensity of pilot tone with respect to frequency of optical signal, the optical signal measured with optical filters of interleaved transmission characteristics; 
     FIG. 6 is a graph illustrating the process to extract frequency by inverse function of FIG. 5; and 
     FIG. 7 is a graph illustrating experimental result of frequency measurement in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a diagram illustrating an apparatus for monitoring frequency using optical filters of interleaved transmission characteristics in accordance with an embodiment of the present invention. 
     The frequency monitoring device in accordance with an embodiment of the present invention includes a first star coupler ill, a second star coupler  112 , a first optical filter  113 , a second optical filter  114 , a first photodetector  115 , a second photodetector  116 , an analog-digital converter  117 , and a frequency detector  120 . 
     The first star coupler  111  tabs optical signals supplied from optical path. The second star coupler  112  splits the optical signals supplied from the first star coupler  111 . The first optical filter  113  and the second optical filter  114  have the interleaved transmission characteristics and perform optical signal filtering. The first photodetector  115  and the second photodetector  116  convert the optical signals supplied from the first optical filter  113  and the second optical filter  114  into the electrical signals. The analog-digital converter  117  converts the electrical signals into digital signals. The frequency detector  120  receives the digital signals and detects frequency. 
     The frequency detector  120  includes fast Fourier transformer  121  and frequency calculator  122 . 
     The fast Fourier transformer  121  performs Fourier transform on signals supplied from the analog-digital converter  117 . The frequency calculator  122  detects frequency from the signal supplied from the fast Fourier transformer  121 . 
     The optical frequency monitor in accordance with an embodiment of the present invention operates as follows. 
     The first star coupler  111  is connected with optical path and tabs a part of optical signal, which includes pilot tone allocated for each optical channel. 
     The second star coupler  112  receives output of the first star coupler  111  and splits it. 
     That is, the first star coupler  111  tabs only 1% of optical signal advancing through optical path and passes the rest 99%. The second star coupler  112  evenly splits the optical signal from the first star coupler. 
     The evenly split optical signals are sent to the first optical filter  113  and the second optical filter  114 . Desirably, two optical filters with interleaved transmission characteristics can implement the optical filters  113 ,  114 . 
     Also, two solid Fabry-Perot etalon filters can implement the optical filters  113 ,  114 . The two solid Fabry-Perot etalon filters have periodical transmission characteristics on the basis of wavelength of optical signal. 
     Also, two optical fiber Fabry-Perot etalon filters can implement the optical filters  113 ,  114 . The two optical fiber Fabry-Perot etalon filters have periodical transmission characteristics on the basis of wavelength of optical signal. 
     Also, AWGs can implement the optical filters  113 ,  114 . The crossover frequency of the neighboring channels of the AWGs is similar to the frequency of the wavelength division multiplexed signal. 
     Also, optical filters can implement the optical filters  113 ,  114 . The optical filters operate with frequency range of the wavelength division multiplexed optical signal. 
     FIG. 2 is a graph illustrating characteristics of optical filters that have interleaved transmission characteristics. 
     The optical frequency at the maximum transmittance of the first optical filter is identical with the optical frequency at the minimum transmittance of the second optical filter. 
     Therefore, the intensities of the optical signal passed through the first optical filter and the second optical filter are changed in accordance with frequency of the optical signal. As the intensities of the optical signal decrease, the intensities of pilot tone decrease. 
     The etalon filter can be implemented by silica glass and both sides of the silica glass are coated. Since characteristics of the etalon filter changes in accordance with temperature, temperature of the etalon filter is controlled to maintain a certain value by thermoelectric cooler and thermistor. 
     The first photodetector  115  and the second photodetector  116  detect the intensities of optical signals supplied from the first optical filter  113  and the second optical filter  114 , respectively. 
     The analog-digital converter  117  receives analog signal and converts the analog signal into digital signal. The frequency detector  120  received the digital signal and detects frequency of the digital signal. 
     That is, the frequency detector  120  performs fast Fourier transform using the fast Fourier transformer  121  and then the frequency calculator  122  measures the to calculate intensities of pilot tone of optical signal optical frequency. 
     The frequency calculator  122  can measure the frequency of optical signal either by the ratio or the difference of intensities of pilot tone of optical signals. 
     FIG. 3 is a diagram illustrating an apparatus for monitoring frequency using optical filters of interleaved transmission characteristics in accordance with an embodiment of the present invention. The optical frequency monitor of FIG. 3 includes first star coupler  311 , second star coupler  312 , first optical filter  313 , second optical filter  314 , first photodetector  315 , second photodetector  316 , electrical filters  301 ,  302 , power detector  303 , and frequency calculator  322 . 
     The first star coupler  311  tabs optical signals supplied from optical path. The second star coupler  312  splits the optical signals supplied from the first star coupler  311 . The first optical filter  313  and the second optical filter  314  have interleaved transmission characteristics and perform optical signal filtering. The first photodetector  315  and the second photodetector  316  detect intensities of optical signals supplied from the first optical filter  313  and the second optical filter  314 . The electrical filter arrays  301  and  302  extract each pilot tone from the outputs of the photodetectors  315 ,  316 . The power detector arrays  303  measure intensities of the pilot tones. The frequency calculator  322  calculates frequency on the basis of the intensity of the pilot tone. 
     The operations of the first star coupler  311 , the second star coupler  312 , the first optical filter  313 , the second optical filter  314 , the first photodetector  315 , and the second photodetector  316  are same as operations of those devices previously described with an embodiment shown in FIG.  1 . 
     The electrical filter arrays  301  and  302  filter off the other pilot tones except the corresponding pilot tone of optical signal from the output of the photodetectors  315  and  316 . The power detector arrays  303  convert the intensities of the pilot tone into the voltages. 
     The frequency calculator  322  calculates optical frequency on the basis of the intensity of the pilot tone. The frequency calculator  322  can measure the frequency of optical signal either by ratio of measured optical power or by difference of measured optical powers. 
     FIG. 4 shows graphs to explain operating principles of the present invention. 
     Since optical signals pass through optical filters  313 ,  314  with interleaved transmission characteristics, optical signals of A, B, C, and D have different intensities. 
     If difference in intensity between two signal passed through the first and second optical filters  313  and  314  are known, it can be also known where the signal is located in frequency among A, B, C, and D. 
     However, frequency of multiple optical signals cannot be obtained from the intensity information of optical signals and therefore pilot tone is used to distinguish each optical signal which has different optical frequency. 
     As intensity of optical signals passed through the first optical filter  313  and the second optical filter  314  changes, intensity of corresponding pilot tone also changes. Therefore, frequency information of the optical signal can be obtained from the intensity of pilot tone. The pilot tone is detected after analog-digital conversion and fast Fourier transform are applied to the optical signal. 
     If etalon filters are used, the first optical filter  313  and the second optical filter  314  have same characteristics of period. Therefore, each period can be applied to each optical signal of different wavelength and frequency of multiple optical signals can be measured at the same time. 
     Frequency range of measurement is half of the period of the etalon filter. 
     That is, if the period of the etalon filter is 100 GHz, the frequency range is 50 GHz, which is half of the period of optical filters. 
     FIG. 5 is a graph illustrating the ratio of intensity of pilot tone with respect to optical frequency of the signal, which is passed through the two interleaved optical filters. The frequency of optical signal superimposed by pilot tone is changed from 193.14 THz to 193.06 THz. Intensity of the optical signal passed through optical filters is measured by comparing intensity of pilot tone. 
     It is shown that ratio of pilot tone intensity is linearly decreased with the frequency from 193.08 THz 30 to 193.13 lHz in FIG.  5 . 
     FIG. 6 is a graph illustrating the process to extract frequency by inverse function of FIG.  5 . 
     Therefore, frequency of input optical signal can be known by the ratio of pilot tone intensity. 
     The frequency of optical signals can be calculated from the ratio of pilot tone intensity by following mathematical equation  1 . 
     
       
           F =193.09877−9.8372×10 −4   P +4.20339×10 −6   P   2 +8.1668×10 −7   P   3 −1.87299×10 −8   P   4 −4.04682×10 −9   P   5   [Equation 1] 
       
     
     F: frequency of optical signal in THz 
     P: intensity of pilot tone in dBm 
     FIG. 7 is a graph illustrating experimental result of frequency measurement in accordance with the present invention. FIG. 7 shows error between frequency of optical signals measured by the optical frequency monitor and frequency of optical signals measured by optical frequency meter. 
     FIG. 7 shows that it is possible to measure within ± 1 GHz under 50 dB range with input variance from −15 dBm to −25 dBm. 
     In wavelength division multiplexing optical transmission system, the optical frequency monitor in accordance with the present invention employs etalon filters whose transmission characteristic is contrary to pilot tone. Therefore, the optical frequency monitor detects frequency of a number of optical signals simultaneously. The optical frequency monitor doesn&#39;t have dynamic part and measures accurate frequency with simple algorithm and consequently, economical frequency monitoring is possible in wavelength division multiplexing optical transmission network. 
     Although representative embodiments of the present invention have been disclosed for illustrative purpose, those who are skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit of the present invention as defined in the accompanying claims.