Optical channel adding/dropping filter
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. application Ser. No. 08/019,961 filed with the United States Patent and Trademark Office on Feb. 19, 1993, entitled &#34;Rapidly Tunable Integrated Optical Filter&#34;. 
     This application is related to U.S. application Ser. No. 08/019,957 filed with the United States Patent and Trademark Office on Feb. 19, 1993, entitled &#34;Rapidly Tunable Wideband Integrated Optical Filter&#34;. 
     This application is related to U.S. application Ser. No. 08/019,951 filed with the United States Patent and Trademark Office on Feb. 19, 1993, entitled &#34;Rapidly Tunable Wideband Integrated Laser&#34;. 
     This application is related to U.S. application Ser. No. 08/019,952 filed with the United States Patent and Trademark Office on Feb. 19, 1993, entitled &#34;Rapidly Tunable Integrated Laser&#34;. 
     This application is related to U.S. application Ser. No. 08/138,659 filed with the United States Patent and Trademark Office on Oct. 18, 1993, entitled &#34;Optical Passband Filter&#34;. 
     TECHNICAL FIELD 
     This invention relates to optical communications systems. More particularly, this invention relates to optical channel adding/dropping filters used in optical communications systems. 
     BACKGROUND 
     The capacity and speed of communications systems may be increased by transmitting information in optical form over networks composed of optically transmissive nodes, fibers, and the like. High capacity optical communications systems require that many optical signals be frequency division multiplexed in the components of an optical network. Receivers must be configured such that a desired optical frequency or band of frequencies may be retrieved from a group of many optical frequencies. This necessitates the provision of suitable optical filters such as an optical channel adding/dropping filter for dropping a frequency component from a multiplexed signal composed of a large number of frequencies or, alternatively, substituting a frequency component that has the same carrier frequency as the original component but a different modulation. While U.S. Pat. No. 5,461,685 discloses an optical filter that employs two frequency routing devices, until now there has been no convenient approach to filtering the individual components from a multiplexed series of optical frequencies with only a single frequency routing device. 
     SUMMARY 
     In accordance with this invention, an optical channel adding/dropping filter has been developed. The optical filter in accordance with this invention may be based upon photonic integrated circuitry which can provide a very sharp transition between the various passbands which can be realized at a cost lower than that associated with implementing prior optical filters. 
     In one example of the invention, Applicants have realized that a single integrated optical multiplexer/demultiplexer disclosed, for example, in U.S. Pat. Nos. 5,002,350 and 5,136,671 may be used to create a monolithic optical channel adding/dropping filter. In specific terms, this example of the invention comprises an optical frequency routing device having N inputs and N outputs, where N&gt;1. A plurality of waveguides each couple one of the outputs to one of the inputs of the frequency routing device in a consecutive order to form a series of optically transmissive feedback loops. 
     As a result of this arrangement, an input signal composed of a plurality of multiplexed optical frequencies directed to an input of the routing device is first demultiplexed to yield a sequence of demultiplexed frequency components which are directed to the outputs of the routing device. The demultiplexed frequencies may be rerouted back to the inputs of the routing device via the optical feedback loops between the outputs and the inputs. A remaining output that is not incorporated into one of the feedback loops is provided for receiving the resulting multiplexed output signal. Rather than rerouting the demultiplexed frequency components back through the routing device via optical loops, one or more of the loops may be opened up to either drop, add, or drop and add a frequency component. If a preselected frequency component or components is prevented from returning to the input of the routing device by opening its corresponding loop, this frequency component can be dropped so that the resulting multiplexed output signal corresponds to the input signal except that one frequency component has been removed. Similarly, rather than removing the frequency component, it may be replaced by another frequency component which has the same carrier frequency but which contains a different modulated signal. Additionally, a frequency component absent from the input signal may be added to the resulting multiplexed output signal. 
     In one particular example of the invention, the N×N frequency routing device is provided with N-1 waveguides that couple N-1 outputs of the outputs to N-1 of the inputs. A first end of the waveguides is coupled to the respective outputs in a sequential order and a second end of the waveguides is coupled to the respective inputs in a sequential order opposite to the sequential order of the first ends. 
     This is only an example of the invention. The full scope of the invention entitled to an exclusionary right is set forth in the claims at the end of this application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of an example of an optical channel adding/dropping filter in accordance with this invention. 
     FIGS. 2(a) and 2(b) are diagrams of the frequency routing device employed in the optical filter of this invention. 
     FIG. 3 is a diagram of an alternative example of an optical channel adding/dropping filter in accordance with this invention. 
     FIG. 4 is a diagram illustrating the details of the frequency routing devices shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows an example of the optical channel adding/dropping filter of the present invention. It is composed of one frequency routing device 30 and a plurality of waveguides. These structures may be monolithically integrated on a semiconductive wafer and they may be created by known photolithographic techniques. 
     The frequency routing devices employed in the present invention operate as multiplexers and demultiplexers of optical frequencies. The details of their construction and operation are more fully described in U.S. Pat. Nos. 5,002,350 and 5,136,671 to Dragone, the entire contents of which are hereby incorporated by reference into this application. Before the optical passband filter of the present invention is described, the pertinent features of the frequency routing devices will be reviewed. 
     FIGS. 2(a) and (b) show a functional illustration of a routing device 12 of the type employed in the present invention. In general, the routing device may have N input waveguides and M output waveguides to form an N×M frequency routing device. For example, the routing device 12 shown in FIG. 2(a) has four input waveguides 16 1 , 16 2 , 16 3 , 16 4 , and four output waveguides 18 1 , 18 2 , 18 3 , 18 4 . In general, for a frequency routing device of a given design, the output waveguide to which a particular frequency component will be directed is a function of the frequency and the particular input waveguide to which the frequency component is directed. The routing device 12 can be configured to demultiplex preselected input optical frequencies directed to any of its input waveguides. For example, as seen in FIG. 2(a), if a multiplexed signal composed of preselected frequencies F 1 , F 2 , F 3 , F 4 , is directed to the first input waveguide 16 1  of the routing device 12, frequency F 1  will be directed to output waveguide 18 1 , frequency F 2  will be directed to output waveguide 18 2 , frequency F 2  will be directed to output waveguide 18 3 , and frequency F 4  will be directed to output waveguide 18 4 . The details of constructing an appropriate frequency routing device for demultiplexing and multiplexing a series of preselected frequency components is disclosed in the above-referenced patents. 
     FIG. 2(b) illustrates the manner in which the optical signal is demultiplexed by the frequency routing device seen in FIG. 2(a) if it is directed to the second input waveguide 16 2  rather than the first input waveguide 16 1 . In this case frequency F 1  will be directed to output waveguide 18 2 , frequency F 2  will be directed to output waveguide 18 3 , frequency F 3  will be directed to output waveguide 18 4 , and frequency F 4  will be directed back up to output waveguide 18 1  since the routing device 12 only has four output waveguides. If the routing device 12 is provided with a fifth output waveguide 18 5  (shown in dashed lines in FIG. 2(b) ) to form a 4×5 routing device, frequency F 4  will appear on this fifth output waveguide instead of on the first output waveguide 18 1 . 
     The routing devices discussed above are symmetrical in operation. Therefore, if in connection with the routing device shown in FIG. 2(a) frequencies F 1 , F 2 , F 3 , F 4  are directed to waveguides 18 1 , 18 2 , 18 4 , and 18 5 , respectively (which now function as input waveguides), frequencies F 1  and F 2  will be output on waveguide 16 1  and frequencies F 3  and F 4  will be output on waveguide 16 2 . 
     A general property of the frequency routing device seen in FIG. 2, which is configured to demultiplex and multiplex a given set of frequency components, is suggested by FIGS. 1 and 2. Specifically, if an optical signal is directed to any selected input waveguide 16 i , the first frequency component of the signal will appear on output waveguide 18 i , the second frequency component will appear on output waveguide 18.sub.(i+1), the third frequency component will appear on output waveguide 18.sub.(i+2), etc. Alternatively, the frequency routing device may be configured so that the first frequency component appears on output waveguide 18.sub.(i+1) rather than output waveguide 18 i , in which case all the subsequent frequency components will be shifted by one output waveguide relative to the waveguide they would appear on if the first frequency component was directed to output waveguide 18 i . After a given frequency component is directed to the last output waveguide (i.e., the Nth output waveguide), the next frequency component will be directed back up to the first output waveguide, as seen, for example, in the case of frequency F 4  of FIG. 2(b). In other words, the N output waveguides may be considered a repeating series of waveguides having a period N, where, for example, the ith output waveguide serves not only as the ith output waveguide, but also as the(i+N)th output waveguide. The invention employs this property of the frequency routing device in the manner described below. 
     Referring again to FIG. 1, the N×N frequency routing device 30 of the invention has an input 16 1  on which is directed an input signal composed of (N-1) individual frequency components. In contrast to the frequency routing device shown in FIG. 2, this frequency routing device is preconfigured to direct the first frequency to the second output waveguide 18 2  and not the first output waveguide 18 1  as in FIG. 2(a). Accordingly, for the sake of clarity, the first frequency component of the signal will be designated F 2  and not F 1 . The subsequent frequency components will be designated, as usual, F 3 , F 4 , . . . F N , respectively. 
     The invention employs the frequency routing device 30 to form an optical filter by coupling the output waveguides back to the input waveguides in the manner shown in FIG. 1. In particular, beginning with the second output waveguide 18 2 , a series of optically transmissive feedback loops are formed by coupling the output waveguides 18 2 , 18 3 , . . . , 18 N  to a respective input waveguide 16 2 , 16 3 , . . . , 16 N  in a sequential order opposite to the sequential order in which the output waveguides are arranged. As seen in FIG. 1, output waveguide 18 2  is coupled to the input waveguide 16 N , output waveguide 18 3  is coupled to input waveguide 16.sub.(N-1), etc. Finally, output waveguide 18 N  is coupled to input waveguide 16 2 . Output waveguide 18 1  does not enter into a feedback loop so as to serve as an output for the resulting output signal. 
     The optical filter formed by coupling the outputs of a frequency routing device back to the inputs in the manner described above processes a multiplexed input signal as follows. First, the input signal is demultiplexed in accordance with the operation of the routing device shown in FIG. 2(a) so that frequency component F 2  is directed from the input waveguide 16 1  to output waveguide 18 2 , frequency component F 3  is directed from the input waveguide 16 1  to output waveguide 18 3 , etc. As a result of the coupling arrangement between the output waveguides and the input waveguides, the individual frequency components are fedback to the input waveguides in a sequential order that is opposite relative to the order in which they were output. Specifically, frequency F 2  is directed to input waveguide 16 N , frequency F 3  is directed to input waveguide 16.sub.(N-1), and so on, until finally, frequency F N  is fedback to input waveguide 16 2 . 
     These demultiplexed frequency components are next rerouted through the frequency routing device in accordance with the above-mentioned general property of a frequency routing device that is properly configured to serve as a multiplexer or demultiplexer (i.e., for an optical signal directed to any selected input waveguide 16 i , the second frequency component F 2  will appear on output waveguide 18 i+1 ), the third frequency component F 3  will appear on output waveguide 18.sub.(i+2), etc.). Accordingly, since in the case of frequency component F 2 , i=N (i.e., frequency component F 2  is input to input waveguide 16 N ), frequency F 2  is directed to the output waveguide corresponding to i+1=N+1, which in this case is output waveguide 18 1 . Similarly, since for frequency component F 3 , i=N-1 (i.e., frequency component F 3  is input to input waveguide 16.sub.(N-1), frequency F 3  is directed to the output waveguide corresponding to i+2=N+1, which in this case is again output waveguide 18 1 . This process continues in a similar manner for each frequency component until finally, frequency component F N , for which i=2, is directed to the output waveguide corresponding to i=i+(N-1)=N+1, which once again corresponds to output waveguide 18 1 . Accordingly, all the frequency components of the input signal will be directed to output waveguide 18 1 . 
     As described to this point, the optical filter of the invention simply routes a multiplexed input signal to a preselected output waveguide such as output waveguide 18 1  in the example of the invention shown in FIG. 1. In order to employ the device as a filter, one or more of the waveguide loops coupling the output waveguides back to the input waveguides must be opened up to either drop or add a frequency component. For example, in FIG. 1 the loop directing frequency component F 3  from the output back to the input of routing device 30 is opened at an arbitrary point. If desired, by simply preventing frequency component F 3  from returning to the input of the routing device this frequency component can be dropped so that it does not appear on the output waveguide 18 1  and hence the multiplexed output signal corresponds to the input signal except that one frequency component has been removed. Similarly, rather than removing frequency component F 3 , it may be replaced by frequency component F 3 , which has the same carrier frequency as F 3  but may contain a different modulated signal. Additionally, if a frequency component were absent from the input signal it may be added to the resulting multiplexed output signal. 
     FIG. 3 shows another example of the invention in which the rerouted multiplexed signal is directed to output waveguide 18 N  rather than output waveguide 18 1  as in the first example of the invention shown in FIG. 1. In this case the first output waveguide 18 1  is coupled to the last input waveguide 16 N  (in FIG. 1 the second output waveguide 18 2  was coupled to the last input waveguide 16 N ). The subsequent output waveguides 18 2 , 18 3 , . . . 18.sub.(N-1) are coupled to the input waveguides in sequential order similar to the configuration in FIG. 1. Output waveguide 18 N  does not form a feedback loop so that it may serve as the output waveguide for the resulting output multiplexed signal. As a consequence of this arrangement the input multiplexed signal will be directed to output waveguide 18 N . One or more of the waveguides 18 1 , 18 2 , . . . , 18.sub.(N-1) may be opened to add or drop a frequency component in the same manner discussed in connection with the first example of the invention. 
     One skilled in the art will recognize that the examples of the invention described herein can be generalized to direct the input signal to any desired output of the frequency routing device by choosing a suitable arrangement for the optical feedback loop between the outputs and the inputs of the routing device. 
     FIG. 4 shows the pertinent details of the frequency routing device employed in the filter of the invention. Each frequency routing device contains a plurality of input waveguides 26 connected to a free space region 28. A plurality of output waveguides 40 extends from the free space region 28 and is connected to an optical grating 42. The optical grating 42 comprises a plurality of unequal length waveguides which provides a predetermined amount of path length difference to a corresponding plurality of input waveguides 44 connected to another free space region 46. The free space region 46 is connected to a plurality of output waveguides 38. These frequency routing devices operate as multiplexers and demultiplexers of optical frequencies. As noted above, details of their construction and operation are more fully described in the U.S. Pat. Nos. 5,002,350 and 5,136,671 to Dragone. In the case of the frequency routing device seen in FIG. 1, the input waveguides 16 1 , 16 2 , . . . , 16 N  correspond to the input waveguides 26 shown in FIG. 4 and the output waveguides 18 1 , 18 2 , . . . , 18 N  correspond to output waveguide 38.