Abstract:
An optical performance monitor having a tunable fiber grating sandwiched between the first and second reflective surfaces of a fiber etalon provides high sensitivity, while simultaneously monitoring an optical data signal. Individual channels within the optical data signal are monitored by tuning the fiber grating over the wavelength range of the optical data signal to measure an intensity of each channel. As the tunable grating scans the wavelength range of interest, it simultaneously changes the resonant cavity length of the etalon, thus allowing the etalon to effectively monitor the tunable grating with a wavelength outside the wavelength range of the optical data signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. patent application Ser. No. 60/283,931 filed Apr. 17, 2001. 
    
    
     TECHNICAL FIELD 
     This present invention relates to optical performance monitors for use in optical communication systems. 
     BACKGROUND OF THE INVENTION 
     In wavelength division multiplexed (WDM) optical communication systems, many different wavelength channels are simultaneously carried by a single optical waveguide. In dense wavelength division multiplexed (DWDM) systems, up to 80 channels with a 50 GHz (0.4 nm) channel spacing can be accommodated in the 1525 nm to 1575 nm wavelength range (C band). 
     Performance of these systems is verified with an optical performance monitor (OPM) by monitoring the wavelength, power, and signal-to-noise ratio (SNR) of each of the DWDM channel signals. For example, an OPM located at the receiver line terminal is important for network commissioning and long term monitoring. An OPM located at the transmitter end can perform the added task of wavelength locking. Between the terminals, an OPM can measure deviations from the predefined channel wavelengths due to drifts or instabilities. For example, the OPM can be used to measure the successful reconfiguration of a tunable add/drop multiplexer or the imbalances in a recombined DWDM signal resulting from the individual channels traveling disparate paths throughout the network. 
     Traditionally, optical performance monitoring has been accomplished with optical spectrum analyzers, which for example, achieve high optical resolution by using a monochromator. In general, these optical spectrum analyzers are limited by the slow scanning speeds necessary for achieving high resolution and sensitivity. 
     An example of an optical spectrum analyzer with a faster response time is based on a tunable Fabry-Perot etalon. A Fabry-Perot etalon has two partially reflective mirrors, or surfaces, facing each other and separated by a predetermined gap that forms a cavity. The etalon has a periodic response to a multi-wavelength input signal; namely, it only transmits certain wavelengths for which the cavity is said to be in resonance. The spacing between the certain wavelengths, or fringes as they are commonly called, is referred to as the free spectral range (FSR) of the cavity and is a function of the reflectivity and the spacing between the mirrors. Typically, the etalon is tuned by varying the spacing between the mirrors. 
     However, optical spectrum analyzers based on tunable Fabry-Perot etalons are limited by the periodic response of the etalon, which makes it difficult to uniquely determine the wavelength of interest, particularly in the presence of multiple wavelengths. A second disadvantage relates to the fact that a Fabry-Perot filter with high resolution is limited in free spectral range, i.e. it cannot be tuned over a wide range of wavelengths at a high resolution. In fact, the mechanical and optical requirements imposed on a tunable Fabry-Perot filter for achieving the required rejection, stability, and wavelength setting accuracy make such devices excessively costly. 
     It is an object of the instant invention to provide an optical performance monitor that obviates the above disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an optical performance monitor having a tunable grating sandwiched between the first and second reflective surfaces of an etalon. Individual channels within the optical data signal are monitored by tuning the grating over the wavelength range of the optical data signal to measure an intensity of each channel. As the tunable grating scans the wavelength range of interest, it simultaneously changes the resonant cavity length of the etalon, thus allowing the etalon to effectively monitor the tunable grating with a wavelength outside the wavelength range of the optical data signal wavelength. Advantageously, this optical performance monitor provides high sensitivity, while simultaneously monitoring an optical data signal. 
     Accordingly, the present invention relates to an optical performance monitor comprising: 
     a first port for launching an optical data signal including a plurality of wavelength channels within a wavelength range; 
     a second port for launching a reference signal having a wavelength λ; 
     an etalon for receiving the optical data signal and the reference signal, the etalon having a free spectral range (FSR) and including a first partially reflective surface and a second partially reflective surface spaced apart by a length l for forming a resonant cavity for a signal having a wavelength substantially equal to λ; 
     a tunable grating optically disposed between the first and second reflective surfaces of the etalon for successively reflecting one channel at a time from the optical data signal; 
     a first detector for measuring an output of the tunable grating; and 
     a second detector for measuring an output of the etalon. 
     Advantageously, the OPM of the instant invention has significantly lower cost considerations than the high quality, tunable etalons found in prior art OPMs. 
     Furthermore, relatively little splitting of the optical signal is required, thus requiring lower tap splits in the network. Since there is relatively low loss, a device in accordance with the present invention is optionally multi-functional. For example, the light extracted with a OPM in accordance with the instant invention may also be analyzed to determine polarization dispersion loss, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the present invention will now be described in conjunction with the accompanying drawing in which: 
     FIG. 1 is a schematic diagram of an optical performance monitor. 
    
    
     DETAILED DESCRIPTION 
     With reference to FIG. 1, an optical performance monitor  10  according with an embodiment of the present invention includes a tunable fiber grating  12  optically disposed between a first fixed fiber grating  14  and a second fixed fiber grating  16 . Preferably, the tunable fiber grating  12  has a slit type function and is tuned by applying a strain, such as compression, to the fiber grating. The first fixed fiber grating  14  and the second fixed fiber grating  16  form an etalon  18 , i.e. a Fabry-Perot etalon, having a cavity length l. Preferably, the first and second fiber gratings  14  and  16  are fiber Bragg gratings having a reflectivity between 10% and 80%. It is also preferred that the tunable fiber grating  12  be a tunable fiber Bragg grating. The fiber Bragg gratings are measurable in either transmission or reflection. 
     A three port circulator  20  is provided for transmitting optical signals to and from the etalon  18  and the tunable grating  12 . More specifically, optical signals launched into a first port  22  of the circulator  20  are circulated to a second port  24  where they are transmitted to the tunable grating  12  and the etalon  18 . The optical signals reflected from the tunable grating  12  or the etalon  18  are transmitted to the second port  24  of the circulator  20  where they are circulated to a third port  26 . A first wavelength division multiplexer (WDM)  30  is optically coupled to the first port  22  for multiplexing an optical data signal  40  to be analyzed and a reference laser signal  42 . A second WDM  32  is optically coupled to the third port  26  for demultiplexing the reflected signals into first and second sub-signals. Conveniently, the first and second detectors  50  and  52  are disposed to receive the first and second sub-signals, respectively. 
     Typically, the optical data signal  40  is a multiplexed data signal, including a plurality of wavelength channels, transmitted over a range of wavelengths, e.g. from 1525 nm to 1575 nm, while the laser signal  42  is out of this range, e.g. at 1300 nm. However, other wavelengths and wavelength ranges are also within the scope of the present invention. For example, the laser signal  42  may be just outside or overlapping an end of the wavelength range of the optical data signal  40 . Preferably, the tunable grating  12  is tunable over the entire wavelength range of the optical data signal  40 , while the fiber gratings  14  and  16  forming the etalon  18  reflect light having the wavelength corresponding to the laser signal  42 . 
     For example, if the tunable grating  12  is tuned over a 50 nm range spanning from 1525 to 1575 nm, the spectral information of the optical signal in the range of 1525 to 1575 nm can be monitored by the first detector  50 . 
     Similarly, if the fiber gratings  14  and  16  forming the etalon  18  reflect light having the wavelength corresponding to the laser signal  42 , e.g. 1300 nm, then the output of the etalon  18  can be monitored at 1300 nm at the second detector  52 . Since the output signal of the etalon  18  is dependent upon the cavity length l of the etalon  18 , which in turn is dependent upon the degree to which the tunable grating  12  is tuned, the tunable grating  12  is effectively monitored with the etalon  18 . In other words, if the laser source  42  corresponds to 1300 nm, then the spectral response of the etalon  18  at 1300 nm will vary with the applied strain to the tunable grating  12 . 
     In general, the effective optical path length change Δd of a tunable fiber grating, is related to the length of the fiber grating d, the wavelength of the fiber grating λ G , and the tunable range of the fiber grating Δλ G , by:            Δ                   λ   G         λ   G       =       Δ                 d     d                            
     If the tunable grating  12  is tunable over 50 nm (Δλ G ) at 1550 nm (λ G ), and the length of the tunable grating is about 30 mm, the effective optical length change Δd will be about 1 mm or 1000 μm. 
     The change in resonant wavelength of the etalon Δλ FP  is related to the change in length of the etalon Δl, the length of the etalon cavity l, and the wavelength of the etalon λ FP , by:            Δ                   λ   FP         λ   FP       =       Δ                 l     l                            
     If the etalon wavelength λ FP  is about 1300 nm, the effective optical length change Δl is about 1 mm or 1000 μm, and the cavity length l is about 6 cm, then the change in resonant wavelength Δλ FP  will be about 21 nm (3700 GHz). 
     The spacing between wavelengths in the reference signal, or FSR of the etalon is given by:        FSR   =       λ   FP   2       2      nl                              
     Wherein n is the refractive index, l is the length of the cavity, and λ FP  is the wavelength of interest. For a wavelength of 1300 nm, a refractive index of about 1.5, and a cavity length of about 6 cm (6×10 7  nm), the FSR will be as high as 0.009 nm (1.6 GHz). If the length of the cavity l is kept much larger than the maximum change in length Δl, e.g. 30 to 75 times larger, the FSR can be assumed to be constant throughout a small range of cavity lengths. Alternatively, various methods can be included for compensating for the small differences in fringe spacing. 
     Accordingly, the number of fringes of the resonant signal that are observed in the second detector  52  as the grating is tuned, is as high as 2000. Since the optical power of the monitor (1300 nm) is adjustable to high levels, this fringe counting is interpolated to provide wavelength accuracy better than 10 pm. 
     In operation, the optical data signal  40  and the laser signal  42  are coupled into the WDM  30  and are transmitted to the first port  22  of the circulator  20 , where they are circulated and launched out of the second port  24 . Both signals propagate to the etalon  18  and the tunable grating  12 . 
     More specifically, the optical data signal will propagate through the first fixed fiber grating  14  unaffected and will be transmitted to the tunable fiber grating  12 . If the tunable fiber grating  12  is tuned to reflect a predetermined channel of the optical data, that channel will be reflected by the grating  12  back to the second port  24  of the circulator  20 , where it is circulated to the third port  26  and transmitted to the WDM  32 . The WDM  32  substantially transmits the reflected signal to the first detector  50  where the intensity is measured (wavelength is unknown). Channels not reflected by the tunable grating  12  are transmitted through the second fixed fiber grating  16  and output an output port  54  of the monitor  10 . 
     The reference laser signal  42  will also be transmitted to the first fiber grating  14 . Although some of the laser signal  42  will be reflected and transmitted by the first and second fiber gratings  14  and  16 , respectively, a significant portion may resonate within the etalon cavity  18 , if the length of resonant cavity l is a multiple of λ FP /2. A resonant signal transmitted from the etalon  18  through the first fiber grating  14  propagates back to the second port  24  of the circulator  20 , where it is circulated to the third port  26  and transmitted to the WDM  32 . The WDM  32  substantially transmits the resonant signal to the second detector  52  where the intensity at about 1300 nm is measured. 
     As the tunable fiber grating  12  is tuned over the wavelength range of the optical data signal  40 , the first detector  50  will observe a plurality of peaks corresponding to the individual channels, e.g. I.T.U. channels, of the optical data signal, while the second detector  52  observes a much larger number of peaks corresponding to the fringes of the resonant signal. More specifically, as the phase of the etalon  18  changes, the second detector  52  observes a fringe, or a portion of a fringe, for each cavity length l proportional to λ/2. Knowing the cavity length of the etalon l, these fringes are counted to determine the relative wavelength(s) of the individual channels of the optical data signal  40 . An absolute wavelength measuring device  56  can be included to determine a base wavelength, from which all of the gathered information can be used to construct an accurate spectral response. 
     Advantageously, the instant invention provides high sensitivity with respect to wavelength monitoring. 
     Of course, numerous other embodiments can be envisaged without departing from the spirit and scope of the invention.