Abstract:
The specification describes optical performance monitors which are simplified by coupling single tunable optical filters to multiple channels being monitored. Optical measurements for more than one channel may be made simultaneously. The optical system architecture is preferably an optical performance monitor for a WDM system. In a system designed according to the invention n channels may be monitored using n photodetectors, n optical splitters, but only n/2 tunable optical filters. Additional system simplification may be obtained using optical switching elements coupled to the optical splitters.

Description:
FIELD OF THE INVENTION 
     The invention relates to improvements in optical systems based on tunable optical filters. 
     BACKGROUND OF THE INVENTION 
     There exists a well known category of optical devices that perform optical filtering and can be tuned to select a narrow band of wavelengths from a wider wavelength spectrum. These devices are used in a variety of optical systems. Of specific interest are wavelength division multiplexed systems that operate typically over wavelength bands of tens of nanometers. These systems require optical performance monitoring (OPM) to ensure that signal power, signal wavelength, and signal to noise ratios (OSNR) are within specified limits. Other applications for tunable optical filters, inter alia, are for optical noise filtering, noise suppression, and wavelength division multiplexing. 
     For the purpose of describing this invention the focus will be OPM systems, more specifically, OPM systems for wavelength division multiplexed (WDM) systems. It will be understood that the invention is not so limited. 
     In WDM systems, basic system design assumes wavelength stability. However, a variety of dynamic changes occur due to temperature changes, component aging, electrical power variations, etc. For optimum system performance it is necessary to monitor these changes and adjust system parameters to account for them. To accomplish this, optical channel monitors (OCMs), also known as optical performance monitors (OPMs), may be used to measure critical information for the various channels in the WDM system. OPMs may monitor signal dynamics, determine system functionality, identify performance change, etc. In each case they typically provide feedback for controlling network elements to optimize operational performance. More specifically, these tunable optical filters scan the C-, L- and/or C+L-band wavelength range and precisely measure channel wavelength, power, and optical signal-to-noise ratio (OSNR). 
     Performance parameters for tunable optical filters are likewise important for the effectiveness of OPMs. These include adjacent channel isolation and non-adjacent channel isolation. Adjacent channel isolation is the difference between the minimum point in the pass channel and the maximum point in the adjacent channels over all relevant polarization states and over the temperature range of the specification. Non-adjacent channel isolation is the difference between the minimum point in the pass channel and the maximum point of non-adjacent channels. It is also useful for tunable optical filters used in these monitors to have very narrow bandwidth. That produces more information as the signal band is scanned by the tunable optical filter. On the other hand, for measuring optical power in a selected channel over a wider bandwidth, a tunable filter with a correspondingly wider bandwidth makes that measurement simpler. This is among several trade-offs encountered in OPM design. There is also the ubiquitous trade-off of cost. 
     In a multichannel system, the monitor for each channel has several operating elements. Recognizing that each of these elements are multiplied many times over in assessing the overall system cost, an apparently small cost efficiency in the design of the monitor is likewise multiplied to reach the overall cost impact. In some cases increasing the complexity of the monitors by adding elements may result in a system cost reduction depending on the relative costs of the elements. 
     SUMMARY OF THE INVENTION 
     I have designed a cost effective tunable optical filter system for OPM in which individual tunable optical filter elements are used to monitor more than one channel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The description of the invention below may be more easily understood when considered in conjunction with the drawing, in which: 
         FIG. 1  is a schematic representation of a conventional WDM/OPM system wherein each WDM channel is monitored using a tunable optical filter; 
         FIG. 2  is a schematic view of the basic elements in the WDM/OPM system of  FIG. 1 ; 
         FIG. 3  is a schematic similar to that of  FIG. 1  of a prior art system wherein each tunable optical filter is used to monitor more than one WDM channel; 
         FIG. 4  is a schematic representation of an alternative arrangement to that shown in  FIG. 3 ; 
         FIG. 5  is a schematic representation of a first embodiment of the invention wherein tunable optical filters perform dual use; 
         FIG. 6  shows an embodiment of the invention in which the unit shown in  FIG. 5  is scaled to serve a multichannel system with eight inputs; and 
         FIG. 7  shows an alternative embodiment of the invention using combinations of dual use tunable optical filters and optical switches. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a conventional approach to OPM in a WDM system is illustrated. As mentioned earlier, the description of the invention is focused on WDM systems as but one example of an application in which tunable optical filters are used for OPM, and the use of the invention to analyze and correct for wavelength drift etc. in the respective channels of the WDM system. For simplicity,  FIG. 1  shows three channels  11 ,  12 , and  13 . However, it is understood that typical WDM systems may have many more channels.  FIG. 1  shows transmission lines  15   a ,  15   b , and  15   c  between multiplexer  16  at a sending site and demultiplexer  17  at the receiver. In one embodiment of OPM, the multiplexed signals are tapped, via taps  18   a ,  18   b  and  18   c , and the tapped signals are optically coupled to tunable optical filters  19   a ,  19   b  and  19   c , for analysis of the WDM signal in each channel, and detection of channel degradation. The tunable optical filters may sweep across selected wavelengths near the specified channel wavelength, or they may sweep across the WDM band. The output of the tunable optical filters reveals, for example, power changes in the individual channels of the signal. The power spectrum is measured by photodiodes  20   a ,  20   b , and  20   c . Results are fed back via feedback loops  21   a ,  21   b , and  21   c  to the input stage for adjusting signal parameters of the individual channels to correct errors. 
     A generalized schematic of the tunable optical filter/detector subassembly is represented by  FIG. 2 , where the tapped signal  25  is conducted via optical fiber link  26  to tunable optical filter  27 . The filtered signal is conducted via optical fiber link  28  to the detector. In the embodiment shown the detector  29  is a photodiode. The detector may comprise other known means for measuring the properties of the filtered light signal. 
     It has been recognized that economies in the OPM system may be realized by using the tunable optical filters for more than one channel in the WDM system. A known approach for doing this is represented in  FIG. 3 , where the OPM system monitors eight channels, inputs  1 - 8 , using only  4  tunable filters  27   a - 27   d . Inputs  1  and  2 , for example, are coupled to a one-by-two optical switch  25   a . The output of the switch is coupled via waveguide  26   a  to tunable optical filter  27   a  and the optical power output, for example, from the tunable optical filter is coupled via waveguide  28   a  to photodiode  29   a . With the switches  25   a - 25   d  in the position shown, inputs  1 ,  3 ,  5 , and  7  may be monitored. By switching optical switches  25   a - 25   d , inputs  2 ,  4 ,  6 , and  8  may be monitored. In each case the tunable optical filters are swept across the wavelength bands being monitored. It may be as convenient to sweep the tunable optical filters across the entire WDM band. However, some economies may result from using tunable optical filters with tuning ranges less than the entire WDM band, and tailored to a smaller band that includes the channel wavelengths being monitored. The system in  FIG. 3 , compared with that in  FIG. 1 , requires half the number of photodiodes, and half the number of tunable optical filters, but adds four one-by-two optical switches. Note that in this system all eight channels cannot be monitored simultaneously, as with a system constructed of OPM units represented in  FIG. 1 , where each channel OPM has a dedicated tunable optical filter. 
     A modification of the system of  FIG. 3  is represented in  FIG. 4 , where eight inputs are monitored using two four-by-one optical switches that comprise switching elements  31   a ,  32   a ,  33   a , and  31   b ,  32   b , and  33   b . In this system, the number of tunable optical filters and the number of photodetectors is again reduced, with a corresponding addition of two optical switches. For example, comparing the systems represented by  FIGS. 3 and 4 , the system monitoring eight channels based on modules represented in  FIG. 4  requires two tunable optical filters, two photodetectors, and six one-by-two optical switches, compared with four tunable optical filters, four photodetectors, and four one-by-two optical switches for the OPM system of  FIG. 3 . Note again that the switching matrix for the OPM system of  FIG. 4  has four separate switching states, meaning that only two of eight channels can be monitored simultaneously. 
     A more useful and cost effective approach is shown in  FIG. 5 , a preferred embodiment of the invention. In the OPM of  FIG. 5 , two inputs, input  1  and input  2 , are monitored by directing the inputs each through an optical splitter  51 ,  54 , then both are directed through tunable optical filter  52 , then back through the splitters. Light from input  1  is coupled through splitter  51  to the input “a” of tunable optical filter  52  and exits from output “b”. Output “b” is coupled back through splitter  54  to photodetector  53 . Light from input  2  is coupled through splitter  54  to the input “b” of tunable optical filter  52  and exits from output “a”. Output “a” is coupled back through splitter  51  to photodetector  55 . The tunable optical filter  52  effectively has two inputs and two outputs. In both cases, light from an input will arrive at both photodetectors. The light in the selected band is measured for the monitoring function. Light from the other band is “noise” and is ignored. 
     In the OPM system of  FIG. 5 , two channels, i.e. input  1  and input  2 , may be monitored using two splitters, two photodetectors and one tunable optical filter. In the OPM system represented by  FIG. 3 , two channels are monitored by units each having one tunable filter, one photodiode, and one one-by-two optical switch. It would appear that the units in the OPM system of  FIG. 5  are more complex than the corresponding units of  FIG. 3 , requiring five optical elements instead of four. But there are two important advantages of the system of  FIG. 5  over the system represented in  FIG. 3 . One, while there is an added element in each unit, the combined cost of the five elements in the two channel monitor of  FIG. 5  is still substantially less than the cost of the four optical elements for the two channel monitor of  FIG. 3 . This is due partly to the high cost of the one-by-two switch element, e.g.,  25   a  in  FIG. 3 . The second advantage is an important operational one. With the OPM represented by  FIG. 5 , both channels in each unit may be monitored simultaneously. 
     For simultaneous monitoring of more than one channel, the wavelengths being swept by the tunable optical filter and the light being measured at the photodetector elements should be coordinated. This requires that the data acquisition from the photodetectors be synchronized with the tuning element of the tunable optical filter. The synchronizing loop is represented by  56  in  FIG. 5 . 
     The optical splitters described above may split the input signal in any suitable ratio. Typically the ratio will be approximately 50-50 but considerations may arise that favor other ratios. 
     A typical tunable optical filter is a reciprocal device with at least two ports. Thus port “a” in  FIG. 5 , normally the input port becomes an input/output port for the operation of the tunable optical filter in systems constructed according to the invention. Port “b” in  FIG. 5 , normally the output port, is also an input/output port. 
     It will be recognized that the optical splitter in the systems shown comprises a 1:2 optical splitter used backwards. Typically a 1:2 optical splitter is used to divide an optical signal that is fed to the input side of the optical splitter, i.e. the “one” side, with the two divided parts of the signal exiting the optical splitter on the “two” side. In the operation of the splitter according to the invention, the input signal is coupled to one of the waveguides on the “two” side. The return signal from the double pass tunable optical filter is coupled to the waveguide on the “one” side of the splitter and is directed to the photodetector through the second waveguide on the “two” side. Thus in the context of the arrangement of the invention the optical splitter has an input and an output on the “two” side, and an input/output on the “one” side, and is referred to here as a 2:1 optical splitter. 
     The splitter may be a fused optical fiber splitter, or other equivalent element performing this function. While a 2:1 optical splitter is shown here for this function, alternative coupling and/or routing elements may be used. For example, a 2:2 optical splitter could be used with an added output used to measure other parameters. Furthermore, a circulator could be substituted for the simple 2:1 splitter shown in  FIG. 5 . In this case, the input light enters the first port of the circulator, exits the second port, returns into the second port after passing through the tunable filter, and exits the third port, whereupon it is measured using the photodetector. Thus the circulator has two inputs and one output, equivalent to a simple one-by-two optical splitter. Both options are considered optical splitters or as functional equivalents for optical splitting. The optical splitters functioning in the systems of the invention characteristically have one port that serves as an input, another port that serves as an output, and a third port that serves as both an input and output. These devices can be referred to as “three port splitters”. As indicated a three port splitter may have additional ports for added functionality. 
       FIG. 6  shows a system with inputs  1 - 8  based on the module of  FIG. 5 . Each of the  1 - 8  inputs is coupled to the “two” side of a 2:1 splitter, i.e., splitters  61   a - 61   h . The “one” sides of the eight splitters are coupled to tunable optical filters as shown. Each of the tunable optical filters serves two inputs. Tunable optical filter  62   ab  serves inputs  1 ,  2 , tunable optical filter  62   cd  serves inputs  3 ,  4 , tunable optical filter  62   ef  serves inputs  5 ,  6 , and tunable optical filter  62   gh  serves inputs  7 ,  8 . The other side of each of the splitters  61   a - 61   h  is coupled to a photodetector, i.e., photodiodes  63   a - 63   h . Thus in this embodiment, eight channel OPM requires eight splitters and eight photodetectors, but only four tunable optical filters. For simplicity and clarity in  FIG. 6 , the synchronization between the photodetectors and the tunable optical filters is not shown. 
     The operation of these OPMs is as described in connection with  FIG. 5 . All eight channels represented by inputs  1 - 8  may be monitored simultaneously. 
     Another embodiment showing OPM system simplification based on multiple inputs sharing tunable optical filters is shown in  FIG. 7 . This embodiment uses 1×2 optical switches  70   a - 70   d  in the manner shown in  FIG. 3 . It uses four optical splitters  71   a - 71   d , and four photodetectors  73   a - 73   d . However, in contrast with the OPM embodiment of  FIG. 3 , the embodiment of  FIG. 7  has two fewer tunable optical filters. Tunable optical filter  72   ab  serves inputs  1 - 4  and tunable optical filter  72   cd  serves inputs  5 - 8 . Only half of the inputs may be measured simultaneously. 
     The tunable optical filter may be one of a variety of designs. An example of this form of device is described in U.S. Pat. No. 5,917,626, issued Jun. 29, 1999. This tunable optical filter is based on controlling the distance between an input optical path and the axis of a GRIN lens, and using the lens to transmit the beam to an interference filter. The filter passes spectral components within the characteristic wavelength band and reflects spectral components outside the characteristic wavelength band. The pass and rejection bands of the filter can be easily tuned to form a tunable optical filter useful in WDM multiplexers and demultiplexers. The wavelength band varies with the angle of incidence of light to the normal direction to the filter. The filter has means for directing the optical signal along an input optical path substantially parallel to the axis of the GRIN lens at a distance from the axis, and adjusting the distance so that a spectral component in the first input optical signal transmitted by the lens is passed or reflected by the filter. More details on this device may be found in the cited patent, which is incorporated herein by reference. 
     Another suitable category of tunable optical filters is MEMS filters. An example of this type of tunable optical filter is described in U.S. Pat. No. 6,373,632, issued Apr. 16, 2002, also incorporated herein by reference. More information on this category of devices is available through:
         http://www.axsun.com/html/products_omx_telecom.htm       

     Several known tunable optical filter designs include a photodetector element integrated with the tunable optical filter. This physically restricts access to the tunable optical filter in such a way as may prevent a convenient means for sharing the tunable optical filter between multiple inputs as described above. In most such cases it is only necessary, in order to implement this invention, to disintegrate the tunable optical filter and the photodetector and couple the output to one of the multiple inputs. 
     In general, a tunable optical filter is an optical filter that can be tuned over a wavelength range of at least 10 nm. A typical tunable optical filter will filter an input optical band of, for example 1550 nm to 1580 nm, to channels of one or a few nm over that optical band. Tuning may be effected by changing an electrical operating parameter of the tunable optical filter (e.g. voltage or current), by mechanically changing the physical structure of the device, by heating or cooling the device, etc. 
     In the preferred embodiment, the optical signals that are delivered to the input port of the tunable optical filter are signals tapped from the WDM system being monitored. The tapped WDM signal may be modulated, i.e. tapped after the modulator, or unmodulated, i.e. tapped before the modulator. 
     In the preferred embodiments of the invention the OPM is implemented using optical fiber assemblies and components. However, one or more elements and steps of the OPM system and method may involve other forms of waveguides. For example, an optical integrated circuit may be used to route the optical signals through the tunable optical filter. 
     The term “coupled” in the context of the invention means optically coupled in any suitable manner. 
     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.