Abstract:
An apparatus for selective fiber optical channel monitoring and channel replication of wavelength division multiplexed (WDM) signals is constructed by combining a dispersive element, such as an arrayed waveguide grating (AWG), with a multiplicity of simple 1×1 optical switches and either an optical splitter/combiner or a second AWG. In operation, each optical channel in a WDM group may be sequentially monitored or replicated. When this apparatus is preceded by an N×1 optical switch, any optical channel on any one of N input optical fibers to the switch may be selected for monitoring or replication.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of (1) U.S. Provisional Patent Application 61/851,968 filed Mar. 13, 2013, (2) U.S. Provisional Patent Application Ser. No. 61/854,402 filed Apr. 22, 2013, and (3) U.S. Non-Provisional application Ser. No. 14/072,528 filed Nov. 5, 2013, the contents of which are hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION 
       [0002]    The present invention relates to an apparatus for selective fiber optical channel monitoring and channel replication of wavelength division multiplexed (WDM) signals in modern fiber optical communication systems. 
       BACKGROUND OF INVENTION 
       [0003]    Reference (1) includes a discussion of the broad reach of fiber optical communications systems into the modern world and the specific need to monitor individual fiber optic links in these systems for fiber breakage so that failed links may be quickly be identified and automatically corrected by quickly switching a spare optical fiber into service. The inventive contribution made in Reverence (1) was a cost effective optical switching apparatus that could sequentially direct tapped optical signals from fiber optic transmission links to monitoring equipment that could detect fiber breaks in several seconds or less. This same switching apparatus could also be used to replicate optical signals for broadcast and multimedia applications. 
         [0004]    In many cases, there is a need not only to monitor optical fibers used in modern communication systems for continuity (lack of breaks) but also to monitor the performance of each of a multiplicity of optical channels carried by each individual fiber in a wavelength division multiplexed (WDM) format. For example, it is important to detect and identify when a laser diode that serves as the optical source for one of many WDM channels on a fiber has failed or is approaching failure due to a declining power output. 
         [0005]    WDM is a well established method for increasing the information carrying capacity of individual fibers. Depending on the system application, the number of optical channels that are wavelength division multiplexed range from several channels, referred to as Coarse WDM (or CWDM), to 160 channels or more, referred to as Dense WDM (or DWDM). In either case, WDM leads to a beneficial reduction in the total number of optical fibers required to convey some specified amount of information or data. However, in order for WDM to be a cost effective strategy, the cost for monitoring the individual channels multiplexed onto a fiber must be reasonably modest. While the state-of-the-art for monitoring has reached this threshold using MEMS technology previously discussed in Reference (1), further improvements to reduce the complexity and expense of the monitoring function would be welcomed. 
         [0006]    The state-of-the-art for channel monitoring is to direct the output of a WDM optical fiber into a dispersive Planar Lightwave Circuit (PLC) such as an Arrayed Waveguide Grating (AWG), that is well known in the art, to split up a multiplicity of WDM channels (say, N channels) carried on a single input fiber into a group of single channels carried on a multiplicity N of individual output fibers with one channel on each fiber. Then, an optical switch, similar to the MEMS switch described in Reference (1), is used to sequentially direct the channel outputs to an optical monitoring device (test set). 
         [0007]    Clearly, it would be desirable to make this switching function more cost effective. It would also be desirable if this cost effective alternative switching apparatus could also be used for other applications such as optical signal replication. 
         [0008]    Also see U.S. Pat. No. 7,330,620 B2 and, U.S. Patent Application Publication Nos. 2009/0232447 A1. 2012/0087668 A1, and 2012/0230690 A1. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention relates to an improved method for monitoring WDM optical channels or replicating individual optical channels that are multiplexed on a single fiber. Broadly speaking, the concept is to replace a rather expensive MEMS switch that is presently used for this function (see WHITE PAPER from JDS Uniphase Corporation “A Performance Comparison of WSS [Wavelenght Selective Switching] Switch Engine Technology”, Document 30162724, May 2009 on www.jdsu.com.) with a group of 1×1 optical switches in conjunction with an optical combiner device such as a splitter/combiner or AWG in a cost effective manner. 
         [0010]    The present approach employs a dispersive component such as an Arrayed Waveguide Grating (AWG) to split up a multiplicity of WDM channels on a single input fiber into a group of single channels on a multiplicity of individual output waveguides with one channel on each waveguide. In this case, a waveguide may be either an optical fiber or a guided pathway in Planar Lightwave Circuit (PLC) (See “Planar lightwave circuit devices for optical communications present and future” by Hiroshi Takahashi et al, Proceedings of the SPIE, Vol. 5246 (2003) pp 520-530.). At present, the use of an optical fiber is usually the most economic choice for the waveguide. However, PLC technology is expected to become preferred for this waveguide in the future because these circuits are expected to become less expensive and to fit into smaller package sizes. 
         [0011]    Rather than using an expensive MEMS optical switch to sequentially direct the split channel outputs to an optical monitoring test set, the present invention employs a series of inexpensive 1×1 switches (that is, simple on/off switches) to turn off all but one selected channel. Then the single remaining active optical channel is passed through either an optical splitter/combiner, operating in the combiner mode, or through another AWG operating in the multiplexer mode to a single output fiber that is directed to the monitoring device. 
         [0012]    Alternatively, the single optical channel on the output fiber may be replicated using a second optical splitter/combiner, operating in the splitter mode, and be broadcast (multicast) over a multiplicity of output fibers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows an optical circuit diagram for an optical demultiplexer employing an AWG, an array of 1×1 optical switches and an optical splitter/combiner that can be used to select a single optical channel from a WDM group of channels for purposes of monitoring. 
           [0014]      FIG. 2  is similar to  FIG. 1  except that the splitter/combiner has been replaced by a second AWG. 
           [0015]      FIG. 3  shows an optical circuit diagram for an optical demultiplexer employing an AWG, an array of 1×1 optical switches and an optical splitter/combiner that can be used to select a single optical channel from a WDM group of channels for purposes of replicating or multicasting. 
           [0016]      FIG. 4  is similar to  FIG. 3  except that the splitter combiner has been replaced by a second AWG. 
           [0017]      FIG. 5  is an optical circuit diagram that shows how two or more (say, p) optical demultiplexers can be connected through a p×m optical switch to produced m groups of replicated outputs using multiple splitter/combiners. 
           [0018]      FIG. 6  is similar to  FIG. 5  but the splitter/combiners associated with optical demultiplexing are replaced by AWGs. 
           [0019]      FIG. 7  shows how the optical circuits shown in  FIG. 1  or  2  can be connected to an N×1 optical fiber switching circuit and housed in a common enclosure so that any single optical channel on any of a multiplicity of N input fibers carrying assorted WDM channels can be directed to an external monitoring test set. The selected channel can be changed in time so all channels on all fibers may be monitored in any sequence that is desired. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    With reference to the attached drawings, embodiments of the present invention will be described below. 
         [0021]    Two useful functions that are included in many modern-day fiber optical communication systems are (1) monitoring individual optical channels that are multiplexed by WDM on a single optical fiber, and (2) replication of an optical signal on a single optical fiber onto a multiplicity of optical fibers. Both of these functions can be accomplished using the optical circuits and equipment disclosed in this invention. With reference to the attached drawings, embodiments of the present invention will be described below. 
         [0022]      FIG. 1  shows an optical circuit diagram for a specialized optical demultiplexer employing an AWG, an array of 1×1 optical switches, and an optical splitter/combiner that can be used to select a single optical channel from a WDM group of channels for purposes of monitoring. A single input fiber  1  on the left carrying n wavelength division multiplexed (WDM) optical channels is directed to an AWG  2  which causes the WDM channels to split apart so that only a single channel is carried on each of n output optical waveguides,  3   a  to  3   n , that are either optical fibers or planar optical waveguides. Each of these n output optical waveguides are connected using an optical connectors  4   a  to  4   n  that connect to a group of dedicated 1×1 optical switches  5   a  to  5   n . These switches have dedicated optical waveguide entry ports  6   a  to  6   n  and exit ports  7   a  to  7   n . The exit ports are connected to dedicated optical waveguides  9   a  to  9   n  associated with the splitter/combiner  11  using optical connectors  8   a  to  8   n . Optical waveguides  9   a  to  9   n  may be optical fibers or planar optical waveguides. All of these optical waveguides,  9   a  to  9   n , are introduced into optical splitter/combiner  11  and the output of this splitter/combiner is an optical fiber  12  that carries only a single optical channel. This fiber is directed to an optical channel monitor device (test set). 
         [0023]    In operation, the AWG  2  spatially disperses the n WDM optical channels carried by optical fiber  1  in close analogy to the way a prism disperses a white light beam into a rainbow of discrete colors. Carrying the analogy further, an incident white light beam may be properly thought of as a wavelength division multiplex of all of the colors that are demultiplexed by the prism into spatially separated color channels that form the rainbow. In the case of fiber optical communication systems, the multiplexed light beam is usually comprised in a multiplicity of closely spaced infrared wavelengths, lambda-a through lambda-n that each carries information in a modulated format. All but one of the n 1×1 switches  5   a  through  5   n  are turned off. The only switch that remains turned on is  5   i  which passes lambda-i, and is indicated by  10  in  FIG. 1 . This is only this optical channel that reaches the splitter/combiner  11 . This channel passes through the splitter/combiner  11 , (operating in a combiner mode) and is subsequently directed by the output optical fiber  12  to a monitoring device. 
         [0024]    In practice, there are two principal types of splitter/combiners fabricated as Planar Lightwave Circuits (PLCs) that are most economical to employ on modern systems. The first type has cachinnated stages of “Y” shaped optical waveguides. The first stage splits the incident beam into 2 equal parts while the second stage splits these two optical beams into four beams and the j-th stage splits the optical beam into 2′ output beams (see “Silica-on-silicon base 1×N optical splitter: Design, fabrication, and characterization”, Indian Journal of Engineering and Materials Sciences, Vol. 12, February 2005, pp. 12-16). This type of optical splitter has the desirable characteristic that it tends to exhibit low optical insertion loss when operated in the combiner mode. However, it is not as commercially available as a more common “funnel” optical splitter/combiner (see U.S. Pat. No. 7,330,620) that has substantially higher optical insertion loss when operated in a combiner mode that is required for the present application. 
         [0025]    In applications where only the funnel optical splitters are available to operate with all n wavelengths required by a system, it may be necessary to add an optical amplifier to increase the power level of the optical channel carried by fiber  12  in order to compensate for the attenuation introduced by a free-space splitter/combiner. Alternatively, the free-space splitter/combiner may be replaced by an AWG as shown in  FIG. 2  and discussed next. 
         [0026]      FIG. 2  has only one component that is different from the optical circuit shown in  FIG. 1 . The optical splitter/combiner  11  shown in  FIG. 1  is replaced by a second AWG  13  operating in a multiplexing mode. This is done to take advantage of the well known fact that the optical insertion loss for an AWG is low when operated in either the demultiplexing or multiplexing modes. In order to ensure low insertion loss, the second AWG  13  used in the multiplexer mode must be matched in characteristics to the first AWG  2  that is operated in the demultiplexer mode. In addition, it is essential that all output wavelength ports remain precisely aligned so that each output wavelength, lambda-i from the first AWG  2  goes into the corresponding input port for AWG  13 . Otherwise, the optical insertion loss could become excessive. 
         [0027]    It may appear strange that only a single optical channel  14  of wavelength, lambda-i, passes through AWG  13  in its multiplexing mode, since this is not what one would usually consider to be multiplexing. (Multiplexing usually relates to simultaneously combining more than one channel.) However, in operation of the optical circuit shown in  FIG. 2 , many different wavelengths pass through AWG  13 , but only one at a time. In order for all of these wavelengths to experience a similar low optical insertion loss, it is necessary that they converge onto the same output fiber  12 . This can happen in the multiplexing mode for any number of optical channels from 1 up to n. However, this patent application is directed to the special case where only one optical channel at a time is directed to the monitoring device. 
         [0028]      FIG. 3  shows a different application for the optical circuit shown in  FIG. 1 . Rather than directing the output of optical fiber  12  to a monitoring device, this output may be split by and optical splitter/combiner  15 , into a multiplicity, k, of output channels on output fibers  16   a  to  16   k . The optical circuit shown in  FIG. 3  is useful for applications that require replicating and rebroadcasting single optical channels. 
         [0029]      FIG. 4  shows a different application for the optical circuit shown in  FIG. 2 . Rather than directing the output of optical fiber  12  to a monitoring device, this output may be split by and optical splitter/combiner  15 , into a multiplicity, k, of output channels on output fibers  16   a  to  16   k . The optical circuit shown in  FIG. 4  is applicable to applications that require replicating and rebroadcasting single optical channels. 
         [0030]      FIG. 5  shows how two or more optical circuits shown in  FIG. 1  can be used in a switched mode to rebroadcast groups of single output channels that are derived from multiple input WDM optical fibers  1   a  to  1   p . Rather than directing the outputs of optical fibers  12   a  to  12   p  to monitoring devices, these outputs may be redirected to an optical p×m switch  21  that has m output fibers each carrying a single optical channel that can be split for rebroadcasting by splitter/combiners  22   a  to  22   m . The optical circuit shown in  FIG. 5  is suitable for applications that require replicating and rebroadcasting a multiplicity q of single optical channels on output fibers  23   a   1  to  23   aq  and  23   m   1  to  23   mq . Optional optical amplifiers  20  may be used to increase the optical signal level if needed. 
         [0031]      FIG. 6  shows how two or more optical circuits shown in  FIG. 2  can be used in a switched mode to rebroadcast groups of singe output channels that are derived from multiple input WDM optical fiber  1   a  to  1   p . Rather than directing the outputs of optical fibers  12   a  to  12   p  to a monitoring device, these outputs may be redirected to a p×m optical switch  21  that has m output fibers each carrying a single optical channel that can be split for rebroadcasting by splitter/combiners  22   a  to  22   m . The optical circuit shown in  FIG. 6  is applicable to applications that require replicating or rebroadcasting a multiplicity of single optical channels. Optional optical amplifiers  20  may be used to increase the optical signal level if needed. 
         [0032]      FIG. 7  Shows how an N×1 fiber switching device  30  described in Reference (1) can be connected to the optical circuit  40 , as shown in either  FIG. 1  or  FIG. 2 , so that any single optical channel on any one of the N WDM input fibers  29   a  to  29 N can be directed to a monitoring device. All of these components may be contained in a single apparatus enclosure  50  that may be rack mounted and electrically driven by a common electronic controller similar to the one described in References (1) and (2). Alternatively the components may be in separate enclosures depending on the number of optical fibers required for various applications. In either case, the electric controller would normally be interconnected to an graphical interface unit (GUI) located outside of the apparatus enclosure through electrical cables using any of a number of convenient interface protocols such as HTML 5 for fast response. However, if a multiplicity of apparatuses are each similar to the one in  FIG. 7 , it is possible, for reasons of economy, to make only one of the enclosures contain the primary control electronics and all of the electronics need to interface with the external GUI. 
         [0033]    While the above discussion of the preferred embodiments of this invention are representative, other combinations of similar optical circuit elements and enclosure designs to accomplish selective fiber optical channel monitoring and channel replication of WDM signals should be considered to be within the scope of this invention.