Abstract:
The inventor proposes herein a novel optical monitor requiring only a single fiber-coupled photodetector. In one embodiment of the present invention, the optical monitor further includes an optical coupler for tapping a portion of an optical signal, a tunable filter for filtering the tapped optical signal at a predetermined frequency, and a Faraday rotator mirror for removing any polarization dependence of the tapped optical signal and for reflecting the filtered optical signal back through the tunable filter and the coupler. Subsequently, the photodetector of the optical monitor measures the power of the filtered optical signal. The optical spectrum of the optical signal is thus measured by scanning the tunable filter across the band of the optical signal and measuring the power of the optical signal as a function of the optical frequency of the tunable filter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This patent application is related to U.S. patent application Ser. No. 10/378,411 entitled “Low-Loss Integrated Optical Coupler and Optical Switch” filed Mar. 3, 2003, which is herein incorporated by reference in its entirety. 
   FIELD OF THE INVENTION 
   This invention relates to the field of Wavelength Division Multiplexing (WDM) systems and, more specifically, to an optical signal monitor for use with such systems. 
   BACKGROUND OF THE INVENTION 
   Progress in WDM networks is placing new requirements for optical monitoring of system performance and channel characteristics. Conventional grating spectrometers, wavemeters and scanning Fabry-Perot filters have been proposed for optical monitoring, but often are unsuitable because of cost, size, performance or reliability. 
   SUMMARY OF THE INVENTION 
   The present invention advantageously provides a novel optical monitor requiring only a single fiber-coupled photodetector. 
   In one embodiment of the present invention, an optical monitor includes an optical coupler for tapping a portion of an optical signal, a tunable filter for filtering the tapped optical signal at a predetermined frequency, a Faraday rotator mirror for removing any polarization dependence of the tapped optical signal and for reflecting the filtered optical signal back through the tunable filter, and a single photodetector, for measuring the power of the filtered optical signal. To measure the optical spectrum of the optical signal, the tunable filter is scanned across the band of the optical signal and the power measured by the photodetector is recorded as a function of optical frequency of the tunable filter. 
   In an alternate embodiment of the present invention, a method of optical monitoring includes a) tapping an optical signal, b) filtering the tapped optical signal at a predetermined frequency, c) eliminating the polarization dependence of the tapped optical signal, d) filtering again, the filtered optical signal, e) determining the power of the filtered optical signal, and f) repeating steps b) through e) for each frequency throughout the band of the optical signal to determine an optical spectrum of the optical signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a high level block diagram of an embodiment of a novel optical monitor in accordance with the present invention; 
       FIG. 2   a  depicts a high level block diagram of an embodiment of a scanning tunable filter suitable for use in the optical monitor of  FIG. 1 ; 
       FIG. 2   b  graphically depicts the principle of operation of the tunable filter of  FIG. 2   a;    
       FIG. 3  depicts a high level block diagram of an embodiment of a control unit suitable for use in the optical monitor of  FIG. 1 ; 
       FIG. 4  depicts a high level block diagram of an embodiment of an optical monitor in silica waveguide planar lightwave technology; 
       FIG. 5   a  graphically depicts a spectrum measured by a conventional spectrum analyzer; and 
       FIG. 5   b  graphically depicts an optical spectrum measured by the optical monitor of  FIG. 4 . 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  depicts a high level block diagram of an embodiment of a novel optical monitor in accordance with the present invention. The optical monitor  100  of  FIG. 1  illustratively comprises an optical coupler (illustratively a 5/95 optical coupler)  110 , a scanning tunable filter  120 , a control unit  130 , a mirror (illustratively a Faraday rotator mirror (FRM))  140  and a photodetector  150 . Although in  FIG. 1 , the optical coupler  110  is illustratively depicted as a 5/95 optical coupler, optical couplers with various other splitting ratios (e.g., 1/99) may also be implemented within an optical monitor in accordance with the present invention. Furthermore, although in  FIG. 1 , the mirror  140  is illustratively depicted as a Faraday rotator mirror, if the polarization sensitivity of the tunable filter is sufficiently low, the FRM may be replaced by a plain mirror. Furthermore, other means of directing (reflecting) light, such as a Sagnac loop, may be implemented in place of the mirror in a novel optical monitor in accordance with the present invention. Even further, other means of substantially eliminating the polarization sensitivity of an optical signal, such as a quarter-wave plate, may be implemented in place of the FRM in a novel optical monitor in accordance with the present invention. Although in  FIG. 1 , the control unit  130  is depicted as being an included component of the optical monitor  100  in accordance with the present invention, a control unit (such as a computer) may be provided by a user and as such the control unit would not be an included component of an optical monitor of the present invention. 
     FIG. 2   a  depicts a high level block diagram of an embodiment of a scanning tunable filter suitable for use in the optical monitor  100  of  FIG. 1 . The scanning tunable filter  120  of  FIG. 2   a  illustratively comprises three coupled thermooptically-tuned Mach-Zehnder Interferometer (MZI) filters  210   1 – 210   3  (collectively MZI filters  210 ). Each of the MZI filters illustratively comprise a phase shifter  220   1 – 220   3  for providing tuning capability to the tunable filter  120 . The MZI filters  210  have an exponential distribution of free-spectral range from 200 to 12800 GHz. When the filter  120  is tuned to a particular optical frequency, the peaks of all transmissivity spectra of all of the MZI filters  210  are aligned at that frequency, as depicted in  FIG. 2   b.    
     FIG. 2   b  graphically depicts the principle of operation of the tunable filter  120  of  FIG. 2   a . In  FIG. 2   b , the transmissivity of the filter  120  is graphed as a function of frequency. As previously noted and evident from  FIG. 2   b , when the filter  120  is tuned to a particular optical frequency, the peaks of all transmissivity spectra of all of the MZI filters  210  are aligned at that frequency. Although in  FIG. 2   a , the tunable filter  200  is illustrated as comprising three MZI filters, various other numbers of MZI filters may also be implemented within a tunable filter in accordance with the present invention. 
     FIG. 3  depicts a high level block diagram of an embodiment of a control unit suitable for use with the optical monitor  100  of  FIG. 1 . The control unit  130  of  FIG. 3  illustratively comprises a processor  310  as well as a memory  320  for storing information and control programs. The processor  310  cooperates with conventional support circuitry  330  such as power supplies, clock circuits, cache memory and the like as well as circuits that assist in executing the software routines stored in the memory  320 . As such, it is contemplated that some of the process steps discussed herein as software processes may be implemented within hardware, for example, as circuitry that cooperates with the processor  310  to perform various steps. The control unit  130  also contains input-output circuitry  340  that forms an interface between the various functional elements communicating with the control unit  130 . For example, in the embodiment of  FIG. 1 , the control unit  130  communicates with photodetector  150  via a signal path S 1  and to the scanning tunable filter  120  via signal path O 1 . 
   Although the control unit  130  of  FIG. 3  is depicted as a general purpose computer that is programmed to perform various control functions in accordance with the present invention, the invention can be implemented in hardware, for example, as an application specified integrated circuit (ASIC). As such, the process steps described herein are intended to be broadly interpreted as being equivalently performed by software, hardware, or a combination thereof. 
   Referring back to  FIG. 1  and assuming a 5/95 splitting ratio, 5% of the spectrum of an optical signal is tapped off of, for example, a WDM system by the coupler  110  and communicated to the scanning tunable filter  120 . The scanning tunable filter  120  filters the input WDM optical signal and communicates the filtered optical signal to the FRM  140 . One function of the FRM  140  is to eliminate any polarization sensitivity in the tunable filter  120 . The FRM  140  accomplishes this by causing the reflected optical signal to be orthogonally polarized to the polarization of the incoming optical signal. As previously noted though, if the polarization sensitivity of the tunable filter  120  is sufficiently low, then the FRM  140  may be replaced by a plain mirror. In addition and as previously noted, the FRM  140  of  FIG. 1  may be replaced by another means of eliminating the polarization sensitivity of an optical signal, such as a quarter-wave plate. 
   The optical signal is reflected by the FRM  140  and again passes through the scanning tunable filter  120 . Once again and as previously noted, the optical signal of the present invention may be directed back to the scanning tunable filter  120  by means, such as a Sagnac loop, other than the FRM  140  of  FIG. 1 . The second pass of the optical signal through the scanning tunable filter  120  significantly improves the resolution, extinction ratio, and polarization dependence of the optical signal. The optical signal from the scanning tunable filter again passes through the coupler  110  and is communicated to the photodetector  150 . The power of the optical signal measured by the photodetector  150  is recorded by the control unit  130  as a function of the optical frequency that the scanning tunable filter  120  was configured to filter. To measure the optical spectrum of the WDM optical signal tapped by the coupler  110 , the control unit  130  scans the scanning tunable filter  120  across the band of the WDM optical signal, each time recording the power measured by the photodetector as a function of the optical frequency of that the scanning tunable filter  120  was configured to filter. 
   To make the optical monitor  100  of  FIG. 1  more resistant to wavelength, polarization, and fabrication (WPF) changes and to reduce the loss of the optical monitor  100 , the coupler  110  may comprise a novel multi-section optical coupler as described in U.S. patent application Ser. No. 10/378,411 entitled “Low-Loss Integrated Optical Coupler and Optical Switch” filed Mar. 3, 2003, which is herein incorporated by reference in its entirety. Such a multi-section optical coupler is comprised of a plurality of optical couplers substantially equal in length and in one embodiment each comprising a nominal 90° phase shift between their local eigenmodes. As such, and because the plurality of optical couplers are substantially similar, the couplers change in substantially the same manner in the presence of WPF changes, thereby giving the multi-section optical coupler high WPF tolerance. Furthermore, a desired power splitting ratio for the multi-section optical coupler is obtained by adjusting the relative phases of the optical couplers by, in one embodiment, varying the relative path lengths between the waveguides of the waveguides interconnecting the couplers. 
   In an experiment, the inventor constructed an optical monitor in accordance with the present invention in a silica waveguide planar lightwave circuit (PLC) on a silicon substrate.  FIG. 4  depicts a high level block diagram of an embodiment of an optical monitor of the present invention in silica waveguide planar lightwave technology. The optical monitor was integrated along with a band multiplexer  420  and a band demultiplexer  430  on a single PLC chip. The waveguide index step of the PLC chip of  FIG. 4  is 0.80%. The tunable filter of  FIG. 4  consists of seven coupled thermooptically-tuned MZI filters with an exponential distribution of free-spectral range from 200 to 12800 GHz. To test the monitor three laser lines were launched (two spaced by 100 GHz and one with a considerably larger spacing) into a wavelength add-drop node, constructed using the band demultiplexer  430  and multiplexer  420 . The output spectrum of the wavelength add-drop node was then measured with a conventional optical spectrum analyzer and with the embodiment of the optical monitor of the present invention depicted in  FIG. 4 . 
     FIG. 5   a  graphically depicts the spectrum measured by a conventional spectrum analyzer for the experimental parameters described above. In  FIG. 5   a , the optical powers of the three laser lines are plotted (in dBm) as a function of the wavelength of the laser lines (in nm).  FIG. 5   b  graphically depicts the spectrum measured by the optical monitor of  FIG. 4  for the experimental parameters described above. Once again in  FIG. 5   b , the optical powers of the three laser lines are plotted (in dBm) as a function of the wavelength of the laser lines (in nm). As evident from  FIG. 5   b , an optical monitor of the present invention is capable of resolving the channel powers on at least a 100-GHz grid. 
   One advantage of the proposed optical monitor of the present invention is that it is capable of being easily constructed in a planar lightwave circuit (PLC), such as silica waveguides with thermooptic phase shifters. More specifically, the optical coupler and the tunable filter of an optical monitor of the present invention may be integrated into the PLC with the photodetector and the FRM connected either directly to the PLC or by a short piece of fiber. In addition, an optical monitor in accordance with the present invention may be integrated with other PLC-based components, such as a band multiplexer, saving cost, loss, and physical size. The proposed optical monitor is especially suitable for implementation in a PLC because PLCs often have polarization-dependence problems, which are solved in the proposed optical monitor by an FRM. 
   While the forgoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.