Abstract:
An apparatus for measuring the wavelength, optical power, and an optical signal-to-noise ratio (OSNR) of each optical signal in wavelength-division-multiplexing optical communication includes: elements for splitting a part of wavelength-division-multiplexed (WDM) signals, elements for amplifying the WDM signals and generating spontaneous emission light simultaneously, elements for reflecting a predetermined section of the spontaneous emission light and generating an optical reference signal, and elements for combining the optical reference signal with the part of the WDM signals split by the splitting elements and generating a combined light. The apparatus has components for filtering the combined light at a fixed temperature and generating a waveform which is the same as an optical spectrum of the combined light in the time domain. The apparatus includes elements for converting the waveform into an electrical signal and components for signal processing that measure the wavelength, the optical power, and the OSNR of the WDM signals.

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus for measuring the wavelength, optical power, and optical signal-to-noise ratio (OSNR) of each optical signal in wavelength-division multiplexing (WDM) optical communication using optical reference signals for reference wavelengths and an optical tunable band-pass filter. 
     BACKGROUND OF THE INVENTION 
     WDM technologies allow many optical signals with different wavelengths to travel together along a single fiber and increase the transmission capacity. To use such WDM technologies in communication, the wavelength, optical power, and OSNR of each optical signal should be measured for the communication administration. 
     FIG. 1 is a screen image of the optical spectrum illustrating the wavelengths, optical powers, and OSNRs of four optical signals when we measure optical signals a, b, c, and d by a conventional optical spectrum analyzer. The conventional optical spectrum analyzer has a rotating diffraction grating and a fixed photo-diode to measure the wavelength, optical power, and OSNR. In conventional methods, the optical power and OSNR of each optical signal is measured by using the photo-diode in the optical spectrum analyzer. In addition, the OSNR of optical signal is defined as the ratio of its optical power to its noise power. 
     Although such an optical spectrum analyzer has advantages of wide range and precision measurement, it is bulky and mechanically unstable. 
     To compensate these drawbacks, three methods were proposed. 
     FIG. 2 is a diagram illustrating an apparatus using a fixed diffraction grating  206  and a separate photo-diode array  205  for the measurement. This method was disclosed by U.S. Pat. No. 5,796,479, “Signal Monitoring apparatus for Wavelength Division Multiplexed Optical Communication”, D. Derickson, R. L. Jungerman. 
     Wavelength-division-multiplexed optical signals are supplied from the optical fiber  201  and collimated by the lens  202 . The halves of the collimated optical signals go through the half mirror  203  and then are diffracted by the fixed diffraction grating  206 . The optical signals diffracted by the diffraction grating  206  go through the polarization compensator  207  to reduce the polarization dependence of the measurement. Then, all of the optical signals are reflected upon the flat mirror  208  and go through the polarization compensator  207  again. The diffraction grating  206  diffracts the optical signals again. The halves of the diffracted wavelength-division optical signals are reflected right angle by the half mirror  203  and focused to the photo-diode array  205  by the lens  204 . 
     The photo-diode array  205  consists of separated photo-diodes. The separated photo-diodes are a pair of the photo-diodes that are slightly separated. Each of them is spatially located at the position where the optical signal with the wavelength of ITU-T standard grid is irradiated. 
     Accordingly, if the wavelength of the irradiated optical signal is the same as ITU-T standard grid, the electric outputs of the separated photo-diodes are equal. However, if the wavelength of the irradiated optical signal is not the same as ITU-T standard grid, the electric outputs of the separated photo-diodes are not equal. Consequently, the wavelength of each of the wavelength-division-multiplexed signals can be estimated on the basis of ratio of the electric outputs. 
     The optical power of each optical signal is measured by using the total power of the separated photo-diodes. In addition, the optical power measured by the photo-diode of DN, located between the separated photo-diodes as shown in FIG. 2, is used to approximate its noise power. However, this conventional method is disadvantageous in that the optical fiber  201 , the diffraction grating  206 , and the photo-diode array  205  must be exactly aligned in free space. 
     FIG. 3A is a diagram illustrating an apparatus with a fixed diffraction grating and a photo-diode array for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to conventional methods. This method was disclosed by “A high-performance optical spectrum monitor with high-speed measuring time for WDM”, K. Otsuka et al. at 1997 European Conference on Optical Communication. 
     Wavelength-division-multiplexed signals supplied by optical fiber  301  are polarization-compensated at polarization compensator  302 . The compensated signals are collimated by a lens  303  and diffracted by the fixed diffraction grating  304 . The diffracted signals are focused by a lens  305  and flat mirror  307  and irradiated to the photo-diode array  308 . 
     In this apparatus, the wavelength, optical power, and OSNR of each of the wavelength-division-multiplexed signals are obtained on the basis of Gaussian approximation using the result of spatially discrete measurement. FIG. 3B shows an example of Gaussian approximation, which is based on discrete values measured by the photo-diode in FIG.  3 A. 
     Same process is applied to other optical signals of different wavelengths. 
     However, as illustrated at FIG. 3 a,  the optical fiber  301 , the diffraction grating  304 , and the photo-diode array  308  are spatially separated and therefore complicated free-space alignment among those devices is required for accurate measurement. Also, Gaussian approximation is an overhead. 
     FIG. 4 is a diagram illustrating an apparatus with a blazed bragg grating for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the conventional methods. This method was disclosed by “High Resolution Fiber Grating Optical Network Monitor”, Chris Koeppen et al. at National Fiber Optic Engineers Conference 1998. 
     A blazed bragg grating  401  is inscribed on optical fiber  402 . A part of wavelength-division-multiplexed signals are reflected on the blazed bragg grating  401  and irradiated to the photo-diode array  403  through the glass block  404 . The methods for measuring the wavelength, optical power, and OSNR of each optical signal are the same as those of the apparatus shown in FIG.  3 A. 
     However, the photo-diode array and the blazed bragg grating are spatially separated and therefore complicated free-space alignment among those devices is required for accurate measurement. 
     SUMMARY OF THE INVENTION 
     An apparatus for measuring the wavelength and optical power and optical signal-to-noise ratio of each optical signal in WDM optical communication is provided. 
     The apparatus includes the following means. The splitting means splits a part of wavelength-division-multiplexed optical signals. The amplifying means amplifies said wavelength-division-multiplexed optical signals and generates spontaneous emission light simultaneously. The reflection means reflects a predetermined section of said spontaneous emission light and generates an optical reference signal for reference wavelength. The combining means combines said optical reference signal with said wavelength-division-multiplexed signals and generates the combined light. The wavelength-division-multiplexed signals are split by said splitting means. The filtering means filters said combined light at fixed temperature and generates the same waveform as the optical spectrum of said combined light in time domain. The converting means converts said waveform into an electric signal. And, the signal-processing means measures said wavelength, said optical power, and said optical signal-to-noise ratio of each of said wavelength-division-multiplexed optical signals by employing said electric signal. 
     Preferably, the apparatus further includes signal-generating means for generating control signal. The control signal controls said filtering means and said signal-processing means. 
     Preferably, the apparatus further includes optically isolating means for passing the optical signal passing through said reflecting means in uni-direction. 
     An apparatus for measuring the wavelength and optical power and OSNR of each optical signal in WDM optical communication according to another embodiment of the present invention is provided. 
     The apparatus includes the following means. The splitting means splits a part of wavelength-division-multiplexed optical signals. A spontaneous emission light source generates spontaneous emission light. The reflecting means reflects a predetermined section of said spontaneous emission light and generates an optical reference signal for reference wavelength. The combining means combines said optical reference signal with wavelength-division-multiplexed signals and generates the combined light. The wavelength-division-multiplexed signals are split by said splitting means. The filtering means filters said combined light at fixed temperature and generates the same waveform as the optical spectrum of said combined light in time domain. The converting means converts said waveform into an electric signal. And, The signal-processing means measures said wavelength, said optical power, and said optical signal-to-noise ratio of each of said wavelength-division-multiplexed optical signals by employing said electric signal. 
     Preferably, the apparatus further includes signal-generating means for generating control signal. The control signal controls said filtering means and said signal-processing means. 
     Preferably, the apparatus further includes optically terminating means for terminating spontaneous emission light without any reflection of said spontaneous emission light passing through said means for reflecting. 
     More preferably, the spontaneous emission light source is a light-emitting diode (LED). 
     More preferably, the optical reference signal is discerned by applying a driving current into said LED, which is composed of a constant current and an alternating current with specific frequency. 
     An apparatus for measuring the wavelength and optical power and OSNR of each optical signal in WDM optical communication according to another embodiment of the present invention is provided. 
     The apparatus includes the following means. The amplifying means amplifies wavelength-division-multiplexed optical signals and generates spontaneous emission light simultaneously. The reflecting means reflects a predetermined portion of spontaneous emission light and generates an optical reference signal for reference wavelength. The combining means combines said optical reference signal with said wavelength-division-multiplexed signals and generates the combined light. The wavelength-division-multiplexed signals pass through said reflecting means. The filtering means filters said combined light at fixed temperature and generates the same waveform as the optical spectrum of said combined light in time domain. The converting means converts said waveform into an electric signal. And, signal-processing means measures said wavelength, said optical power, and said OSNR of each of said wavelength-division-multiplexed optical signals by employing said electric signal. 
     Preferably, the apparatus further includes signal-generating means for generating control signal. The control signal controls said filtering means and said signal-processing means. 
     Preferably, the amplifying means is one of an erbium-doped fiber amplifier and a semiconductor optical amplifier. 
     Preferably, the reflecting means is one of a fiber bragg grating and an integrated optical device including grating. 
     Preferably, the filtering means is one of a tunable Fabry-Perot filter, an integrated optical device including grating, and a multi-layer thin film device. 
    
    
     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 screen image of the optical spectrum illustrating the wavelengths, optical powers, and OSNRs of optical signals a, b, c, and d when measuring them by a conventional optical spectrum analyzer; 
     FIG. 2 is a diagram illustrating an apparatus with a fixed diffraction grating and a separate photodiode array for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the conventional methods; 
     FIG. 3A is a diagram illustrating an apparatus with a fixed diffraction grating and a photo-diode array for measuring the wavelength, optical power, and OSNR of each optical signal in a WDM optical communication according to the conventional methods; 
     FIG. 3B is a graph illustrating Gaussian approximation for spatially discrete measurement using the apparatus with a fixed diffraction grating and a photodiode array in FIG. 3A; 
     FIG. 4 is a diagram illustrating an apparatus with a blazed bragg grating for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the conventional methods; 
     FIG. 5 is a diagram illustrating an apparatus for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the first embodiment of the present invention; 
     FIG. 6 is a diagram illustrating an apparatus for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the second embodiment of the present invention; 
     FIG. 7 is a diagram illustrating an apparatus for measuring wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the third embodiment of the present invention; 
     FIG. 8A is a screen image of the conventional optical spectrum analyzer illustrating the optical spectrum of seven optical signals used in the first embodiment of the present invention; and 
     FIG. 8B is a graph illustrating the measured wavelengths, optical power, and OSNR of seven optical signals according to the first embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 5 is a diagram illustrating an apparatus for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the first embodiment of the present invention. 
     The apparatus includes two 2×1 optical couplers  503 ,  506 , an 1×2 optical coupler  501 , two fiber bragg gratings  504 ,  505 , an optical isolator  511 , an optical bandpass tunable filter  507 , a photo-diode  508 , a signal processor  510 , an optical amplifier  502 , and a control signal generator  509 . 
     Each of the fiber bragg gratings  504 ,  505  reflects only the light at the specific wavelength that is determined by its own grating period. 
     The optical amplifier  502  is an erbium-doped fiber amplifier or a semiconductor optical amplifier. Besides the fiber bragg grating, an integrated optical device with grating can be used as a reflecting device. 
     A Tunable Fabry-Perot filter, an integrated optical device including grating, or a multi-layer thin film device implements the optical bandpass tunable filter  507 . 
     The apparatus according to the first embodiment of the present invention operates as follows. 
     A part of wavelength-division-multiplexed optical signals are split by the 1×2 optical coupler  501  and supplied to the 2×1 optical coupler  506 . The optical amplifier  502  amplifies the optical signals that are not split by the 1×2 optical coupler  501  and generates spontaneous emission light simultaneously. Both of the amplified signals and spontaneous emission light are supplied to the concatenated fiber bragg gratings  504 ,  505  through the 2×1 optical coupler  503 . As the reflection wavelength of each of fiber bragg gratings  504 ,  505  is predetermined to be outside the wavelengths of said wavelength-division-multiplexed optical signals, only the section of spontaneous emission light at the same wavelength as the reflection wavelength of each fiber bragg grating is reflected backward. Since the reflected light has the same wavelength as that of the fiber bragg grating, it can be used as the optical reference signal for reference wavelength in wavelength measurement. 
     The optical signals passing through fiber bragg gratings  504 ,  505  are supplied to the optical isolator  511 . Since the optical isolator  511  passes optical signals in uni-direction, errors caused by Rayleigh back scattering or optical reflection at non-continuous points can be prevented. 
     The optical reference signals reflected by the fiber bragg gratings  504 ,  505  are split by the 2×1 optical coupler  503  and sent to the 2×1 optical coupler  506 . The 2×1 optical coupler  506  combines said optical reference signals with said wavelength-division-multiplexed optical signals in asymmetric coupling ratio to make the optical reference signals remarkable in optical spectrum, and sends the combined light to the optical bandpass tunable filter  507 . The wavelength-division-multiplexed optical signals are the signals split by the 1×2 optical coupler  501 . 
     Passband of the optical bandpass tunable filter  507  is controlled by the control signal issued from the control signal generator  509 . The passband of the optical bandpass tunable filter in the embodiment of the present invention gets increasing in proportion to the amplitude of the control signal. The control signal generator  509  sends the control signal of ramp shape to the optical bandpass tunable filter  507 . Consequently, the optical bandpass tunable filter  507  generates the same waveform as the optical spectrum of the combined light in which said optical reference signals generated by fiber bragg gratings  504 ,  505  and said wavelength-division-multiplexed optical signals are included. The photo-diode  508  converts said waveform into an electric signal. 
     The output electric signal of the photo-diode  508  and the control signal of ramp shape are sent to the signal processor  510 . The signal processor  510  measures the optical power and OSNR of each of the wavelength-division-multiplexed optical signals by using the conventional methods. 
     To measure the wavelength of each of the wavelength-division-multiplexed optical signals, the signal processor  510  discerns the optical reference signals in said waveform. Since the optical spectrum shapes of said optical reference signals are determined by the optical spectrum of spontaneous emission light of the optical amplifier  502  and reflection characteristics of the fiber bragg gratings  504 ,  505 , the optical reference signals are easily discerned by analyzing the waveform converted by said photo-diode. 
     Since the wavelengths of the optical reference signals are the same as the reflection wavelengths of the fiber bragg gratings, the wavelength of each optical signal is measured on the basis of the location information of the optical signals and the optical reference signals in time domain. 
     FIG. 6 is a diagram illustrating an apparatus for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the second embodiment of the present invention. 
     The apparatus includes an 1×2 optical coupler  601 , two 2×1 optical couplers  602 ,  608 , two fiber bragg gratings  609 ,  610 , an optical terminator  611 , a spontaneous emission light source  607 , an optical bandpass tunable filter  603 , a photo-diode  604 , a signal processor  605 , and a control signal generator  606 . 
     Each of the fiber bragg gratings  609 ,  610  reflects only the light at the specific wavelength that is determined by its own grating period. 
     The apparatus according to the second embodiment of the present invention operates as follows. 
     First, a part of wavelength-division-multiplexed optical signals are split by the 1×2 optical coupler  601  and supplied to the 2×1 optical coupler  602 . Meanwhile, a spontaneous emission light source  607  generates the broadband spontaneous emission light whose optical spectrum is roughly shown within FIG.  6 . 
     The spontaneous emission light is supplied to the concatenated fiber bragg gratings  609 ,  610  through the 2×1 optical coupler  608 . 
     As the reflection wavelength by each of fiber bragg gratings  609 ,  610  is predetermined to be outside the wavelengths of said wavelength-division-multiplexed optical signals, only the section of spontaneous emission light at the same wavelength as the reflection wavelength of each fiber bragg grating is reflected backward. 
     Because the optical terminator absolutely does not reflect any optical signal, spontaneous emission light passing through fiber bragg gratings  609 ,  610  is terminated at the optical terminator  611  and only the spontaneous emission signals for wavelength optical reference are generated. 
     Since the reflected light has the same wavelength as that of the fiber bragg grating, it can be used as the optical reference signal for reference wavelength in wavelength measurement. 
     The optical reference signals generated by the fiber bragg gratings  609 ,  610  are split by the 2×1 optical coupler  608  and sent to the 2×1 optical coupler  602 . 
     The 2×1 optical coupler  602  combines said optical reference signals with said wavelength-division-multiplexed optical signals in asymmetric coupling ratio to make the optical reference signals remarkable in optical spectrum, and sends the combined light to the optical bandpass tunable filter  603 . The wavelength-division-multiplexed optical signals are split by the 1×2 optical coupler  601 . 
     Passband of the optical bandpass tunable filter  603  is controlled by the control signal issued from the control signal generator  606 . The passband of the optical bandpass tunable filter in the embodiment of the present invention gets increasing in proportion to the amplitude of the control signal. The control signal generator  606  sends control signals of ramp shape to the optical bandpass tunable filter  603 . Consequently, the optical bandpass tunable filter  603  generates the same waveform as the optical spectrum of the combined light in which the optical reference signals and said wavelength-division-multiplexed signals are included. The wavelength-division-multiplexed signals are split by the 1×2 optical coupler  601 . The photo-diode  604  converts said waveform into an electric signal. 
     The output electric signal of the photo-diode  604  and the control signal of ramp shape are sent to the signal processor  605 . The signal processor  605  measures the optical power and OSNR of each of the wavelength-division-multiplexed optical signals by using the conventional methods. 
     To measure the wavelength of each of the wavelength-division-multiplexed optical signals, the signal processor  605  discerns the optical reference signals in said waveform. 
     Since the optical spectrum shapes of said optical reference signals are determined by the optical spectrum of the spontaneous emission light of the spontaneous emission light source  607  and reflection characteristics of the fiber bragg gratings  609 ,  610 , the optical reference signals are easily discerned by analyzing the waveform converted by said photo-diode. 
     Since the wavelengths of the optical reference signals are the same as the reflection wavelengths of the fiber bragg gratings, the wavelength of each optical signal is measured on the basis of the location information of the optical signals and the optical reference signals in time domain. 
     Another method of discerning the optical reference signal is to modulating the optical power of the spontaneous emission light. 
     For example, an LED is used as the spontaneous emission light source. The driving current that is composed of a constant current and an alternating current with specific frequency is applied to the LED. Then the optical power of the optical reference signal reflected by the fiber bragg grating is modulated at the same frequency as the driving current of the LED. Consequently, the optical reference signal is easily discerned. 
     FIG. 7 is a diagram illustrating an apparatus for measuring the wavelength, optical power, and OSNR of each optical signal according to the third embodiment of the present invention. 
     The apparatus includes an optical amplifier  701 , an 1×2 optical coupler  702 , two 2×1 optical couplers  703 ,  707 , two fiber bragg gratings  704 ,  706 , an optical bandpass tunable filter  708 , a photo-diode  709 , a signal processor  710 , and a control signal generator  711 . 
     Since the optical amplifier  701  not only amplifies the wavelength-division-multiplexed optical signals, but also generates the spontaneous emission light simultaneously, the optical amplifier also performs a function of the spontaneous emission light source. 
     Each of the fiber bragg gratings  704 ,  706  reflects only the light of the specific wavelength that is determined by its own period of the gratings. 
     The apparatus according to the third embodiment of the present invention operates as follows. 
     First, the wavelength-division-multiplexed optical signals are supplied to the optical amplifier  701 . The optical amplifier amplifies the wavelength-division-multiplexed optical signals and generates spontaneous emission light. 
     Both of the amplified optical signals and the spontaneous emission light are split by the 1×2 optical coupler  702  and supplied to the concatenated fiber bragg gratings  704 ,  706  through the 2×1 optical coupler  703 . 
     As the reflection wavelength of each of fiber bragg gratings  704 ,  706  is predetermined to be outside the wavelengths of said wavelength-division-multiplexed optical signals, only the section of spontaneous emission light at the same wavelength as the reflection wavelength of each fiber bragg grating is reflected backward. Since the reflected light has the same wavelength as that of the fiber bragg grating, it can be used as the optical reference signal for reference wavelength in wavelength measurement. 
     The optical reference signals are split by the 2×1 optical coupler  703  and sent to the 2×1 optical coupler  707 . Meanwhile, the optical signals passing through the fiber bragg gratings  704 ,  706  are also sent to the 2×1 optical coupler  707 . The 2×1 optical coupler  707  combines said optical reference signals with said wavelength-division-multiplexed optical signals in asymmetric coupling ratio to make the optical reference signals remarkable in optical spectrum, and sends the combined light to the optical bandpass tunable filter  708 . 
     Passband of the optical bandpass tunable filter  708  is controlled by the control signal issued from the control signal generator  711 . The passband of the optical bandpass tunable filter in the embodiment of the present invention gets increasing in proportion to the amplitude of the control signal. The control signal generator  711  sends the control signal of ramp shape to the optical bandpass tunable filter  708 . Consequently, the optical bandpass tunable filter  708  generates the same waveform as the optical spectrum of the combined light in which said optical reference signals generated by fiber bragg gratings  704 ,  706  and said wavelength-division-multiplexed optical signals are included. The photo-diode  709  converts said waveform into an electric signal. 
     The output electric signal of the photo-diode  709  and the control signal of ramp shape are sent to the signal processor  710 . The signal processor  710  measures the optical power and OSNR of each of the wavelength-division-multiplexed optical signals by using the conventional methods. 
     The methods for measuring the wavelength, optical power, and OSNR of each of said wavelength-division-multiplexed optical signals using this apparatus is the same as those of the first and second embodiments of the invention as described above. 
     FIG. 8A is a screen image on the conventional optical spectrum analyzer illustrating the optical spectrum of seven optical signals used in the first embodiment of the present invention. FIG. 8B is a graph illustrating the measured wavelengths, optical powers, and OSNRs of said seven optical signals according to the first embodiment of the present invention. 
     Since the apparatus according to the present invention doesn&#39;t require free-space alignment, problems in the conventional methods can be solved. In addition, complex Gaussian approximation based on the spatially discrete measurement is not performed. The wavelength measurement is not affected by the environmental changes such as temperature and humidity because the fiber bragg grating is very stable against them. 
     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.