Abstract:
An improved dense wavelength division multiplexer for the separation of optical channels is provided. The dense wavelength division multiplexer includes the inputting of an optical signal with the optical signal containing a plurality of optical channels; the separating of one or more of the plurality of optical channels from the optical signal using separators at least partly arranged in a multi-stage parallel cascade configuration; and the outputting of the separated plurality of channels along a plurality of optical paths. The dense wavelength division multiplexer of the present invention provides for a lower insertion loss by requiring an optical signal to travel through fewer optical components in the separation process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 09/130,386, entitled “Fiber Optic Dense Wavelength Division Multiplexer Utilizing a Multi-Stage Parallel Cascade Method of Wavelength Separation,” filed on Aug. 6, 1998 now U.S. Pat. No. 6,263,126. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to fiber optic networks, and more particularly to fiber optic dense wavelength division multiplexers. 
     BACKGROUND OF THE INVENTION 
     Fiber optic networks are becoming increasingly popular for data transmission due to their high speed, high capacity capabilities. Multiple wavelengths may be transmitted along the same optic fiber. These wavelengths are combined to provide a single transmitted signal. A crucial feature of a fiber optic network is the separation of the optical signal into its component wavelengths, or “channels”, typically by a dense wavelength division multiplexer. This separation must occur in order for the exchange of wavelengths between signals on “loops” within networks to occur. The exchange occurs at connector points, or points where two or more loops intersect for the purpose of exchanging wavelengths. 
     Add/drop systems exist at the connector points for the management of the channel exchanges. The exchanging of data signals involves the exchanging of matching wavelengths from two different loops within an optical network. In other words, each signal drops a channel to the other loop while simultaneously adding the matching channel from the other loop. 
     FIG. 1 illustrates a simplified optical network  100 . A fiber optic network  100  could comprise a main loop  150  which connects primary locations, such as San Francisco and New York. In-between the primary locations is a local loop  110  which connect with loop  150  at connector point  140 . Thus, if local loop  110  is Sacramento, wavelengths at San Francisco are multiplexed into an optical signal which will travel from San Francisco, add and drop channels with Sacramento&#39;s signal at connector point  140 , and the new signal will travel forward to New York. Within loop  110 , optical signals would be transmitted to various locations within its loop, servicing the Sacramento area. Local receivers (not shown) would reside at various points within the local loop  110  to convert the optical signals into the electrical signals in the appropriate protocol format. 
     The separation of an optical signal into its component channels are typically performed by a dense wavelength division multiplexer. FIG. 2 illustrates add/drop systems  200  and  210  with dense wavelength division multiplexers  220  and  230 . An optical signal from Loop  110  (λ 1 -λ n ) enters its add/drop system  200  at node A ( 240 ). The signal is separated into its component channels by the dense wavelength division multiplexer  220 . Each channel is then outputted to its own path  250 - 1  through  250 -n. For example, λ 1  would travel along path  250 - 1 , λ 2  would travel along path  250 - 2 , etc. In the same manner, the signal from Loop  150  (λ 1 ′-λ n ′) enters its add/drop system  210  via node C ( 270 ). The signal is separated into its component channels by the wavelength division multiplexer  230 . Each channel is then outputted via its own path  280 - 1  through  280 -n. For example, λ 1 ′ would travel along path  280 - 1 , λ 2 ′ would travel along path  280 - 2 , etc. 
     In the performance of an add/drop function, for example, λ 1  is transferred to path  280 - 1 . It is combined with the others of Loop  150 &#39;s channels into a single new optical signal by the dense wavelength division multiplexer  230 . The new signal is then returned to Loop  150  via node D ( 290 ). At the same time, λ 1 ′ is transferred to path  250 - 1  from  280 - 1 . It is combined with the others of Loop  110 &#39;s channels into a single optical signal by the dense wavelength division multiplexer  220 . This new signal is then returned to Loop  110  via node B ( 260 ). In this manner, from Loop  110 &#39;s point of view, channel λ 1  of its own signal is dropped to Loop  150  while channel λ 1 ′ of the signal from Loop  150  is added to form part of its new signal. The opposite is true from Loop  150 &#39;s point of view. This is the add/drop function. 
     Conventional methods used by dense wavelength division multiplexers in separating an optical signal into its component channels includes the use of filters and fiber gratings as separators. A “separator,” as the term is used in this specification, is a unit of optical components which separates one or more channels from an optical signal. Filters allow a target channel to pass through while redirecting all other channels. Fiber gratings target a channel to be reflected while all other channels pass through. Both filters and fiber gratings are well known in the art and will not be discussed in further detail here. FIG. 3 illustrates a conventional multi-stage serial cascade configuration of separators in a dense wavelength division multiplexer  300 . In this conventional method, each separator targets only one channel to be filtered/reflected and sent along a path. For example, an optical signal containing channels λ 1 -λ n  is inputted into separator  310 A, which filters/reflects channel λ 1  and send it along its own path  320 - 1 . The remaining channels λ 2 -λ n  are sent to the next separator  310 B, which filters/reflects channel  2  and sends it along its own path  320 - 2 . This continues until each channel has been filtered/reflected and sent along its own path. Thus, with this method, for N channels there are N separators. 
     FIG. 4 illustrates a conventional single stage parallel configuration of separators in a dense wavelength division multiplexer  400 . In this conventional method, the original optical signal containing λ 1 -λ n  enters a signal splitter  410  which splits the signal onto N separate paths, each split signal containing channels λ 1 -λ n . Each of these split signals is sent along a separate path  420 - 1  through  420 -n. Each signal is then filtered or reflected by the separators  430 A- 430 N to output one particular channel. For example, a split signal containing channels λ 1 -λ n  exits the splitter  410  onto path  420 - 1 . The split signal enters separator  430 A which filters/reflects channel λ 1  and sends it along path  420 - 1 . Another split signal containing λ 1 -λ n  exits splitter  410  onto path  420 - 2  and enters separator  430 B. Separator  430 B filters/reflects channel λ 2  and sends it along path  420 - 2 . This process repeats to separate each channel. Thus for N channels, there must be N separators plus a signal splitter. 
     A problem with the conventional configurations of separators above is the resulting high insertion loss. Insertion loss is the attenuation of an optical signal caused by the insertion of an optical component, such as a connector, coupler, or filter. For the multi-stage serial cascade configuration illustrated in FIG. 3, each time the optical signal goes through a separator  310 A- 310 N an amount of insertion loss results. For example, if the optical signal in FIG. 3 has eight channels λ 1 -λ 8  and each component causes 1 dB of insertion loss. By the time λ 8  is separated, it would have passed through eight separators. As would thus suffer 8 dB of insertion loss. 
     The same problem exists for the single stage parallel configuration in FIG.  4 . Assume again that the optical signal contains eight channels and each component causes 1 dB of insertion loss. In splitting one signal onto eight paths, a 9 dB insert loss results. Another 1 dB of loss is added by the separator  430 A- 430 N. Thus, each channel suffers 10 dB of insertion loss. 
     Therefore, there exists a need for a dense wavelength division multiplexer with a method of separation which lowers insertion loss. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     An improved dense wavelength division multiplexer for the separation of optical channels is provided. The dense wavelength division multiplexer includes the inputting of an optical signal with the optical signal containing a plurality of optical channels; the separating of one or more of the plurality of optical channels from the optical signal using separators at least partly arranged in a multi-stage parallel cascade configuration; and the outputting of the separated plurality of channels along a plurality of optical paths. The dense wavelength division multiplexer of the present invention provides for a lower insertion loss by requiring an optical signal to travel through fewer optical components in the separation process. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is an illustration of a simplified optical network. 
     FIG. 2 is an illustration of conventional add/drop systems and dense wavelength division multiplexers. 
     FIG. 3 is an illustration of a conventional multi-stage serial cascade configuration of separators. 
     FIG. 4 is an illustration of a conventional single stage parallel configuration of separators. 
     FIGS. 5A and 5B are simple block diagrams of a first preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention. 
     FIG. 6 is an illustration of a second preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention. 
     FIG. 7 is an illustration of a third preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention. 
     FIG. 8 is an illustration of a fourth preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention. 
     FIG. 9 is a block diagram of a first embodiment of a separator which may be used with the present invention. 
     FIG. 10 is a block diagram of a second embodiment of separator which may be used with the present invention. 
     FIG. 11 is a block diagram of the first embodiment of a separator performing the add/drop function in accordance with the present invention. 
     FIG. 12 is a block diagram of the second embodiment of a separator performing the add/drop function in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention relates to an improvement in a dense wavelength division multiplexer. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     A dense wavelength division multiplexer (DWDM) in accordance with the present invention provides for a lower insertion loss by requiring an optical signal to travel through fewer optical components. To more particularly describe the features of the present invention, please refer to FIGS. 5A through 14 in conjunction with the discussion below. 
     FIG. 5A is a simple block diagram of a first preferred embodiment of a DWDM with a multi-stage parallel cascade configuration of separators in accordance with the present invention. An optic signal containing channels λ 1 -λ n  enters the DWDM  500  through node A ( 240 ). The signal passes through a separator  510 A. The separator  510 A divides the signal into two separate signals, one containing the odd channels (λ 1 , λ 3 , λ 5 , . . . ) ( 530 ) and the other containing the even channels (λ 2 , λ 4 , λ 6 , . . . ) ( 540 ), i.e., every other channel. These odd and even channels are each passed through another separator  510 B- 510 C which further divides them by every other channel. This division continues until only one channel is outputted to each optic fiber,  250 - 1  through  250 -n. 
     This multi-stage parallel cascade configuration of separators reduces the amount of insertion loss typically suffered with the conventional configurations because it reduces the number of components through which an optical signal must travel. For example, as illustrated in FIG. 5B, if an optical signal contains eight wavelengths λ 1 -λ 8 , only seven separators  510 A- 510 G are used. Assume that each separator causes 1 dB of insertion loss. Since each channel only goes through three separators, they only suffer 3 dB of insertion loss, much less than the 8 dB and 10 dB of the conventional multi-stage serial and single stage parallel configurations respectively. The relationship between the number of stage M and the number of separators N for the DWDM  500  of the present invention is N=2 M . M is much smaller than N, thus the DWDM  500  of the present invention has lower insertion loss than both conventional configurations. 
     FIG. 6 illustrates a second preferred embodiment of a DWDM in accordance with the present invention. This DWDM  600  has a hybrid parallel-serial cascade configuration. Certain stages of the DWDM uses a parallel cascade configuration of separators as described in conjunction with FIGS. 5A and 5B above. Along with these parallel cascade stages are stages which use a serial cascade configuration of separators. For example, stages 1 and 2 in the DWDM  600  uses a parallel cascade configuration while stage 3 uses a serial cascade configuration. Assume that an optical signal containing channels λ 1 -λ 16  is input into the DWDM  600 . Separator  610 A separates them into two signals, one containing the odd channels (λ 1 , λ 3 , . . . λ 15 ), the other containing the even channels (λ 2 , λ 4 , . . . λ 16 ). The odd channels are input into separator  610 B which separates them further into two sets. One set of signals (λ 1 , λ 5 , . . . λ 13 ) is input into separator  620 A, while the other set (λ 3 , λ 7 , . . . λ 15 ) is input into separator  620 B. The even channels are input into separator  610 C which separates them further into sets of signals. One set of signals (λ 2 , λ 6 , . . . λ 14 ) is input into separator  620 C, while the other set (λ 4 , λ 9 , . . . λ 16 ) input into separator  620 D. Separators  620 A- 620 D are in a serial cascade configuration which filters for each individual channel and outputs each onto separate paths. By using this hybrid configuration, a user has more flexibility in deciding how many separators will be used. This can be important when costs is a particular concern to a user. 
     FIG. 7 illustrates a third embodiment of a DWDM in accordance with the present invention. This DWDM  700  has a programmable router configuration which adds programmability to the parallel cascade configuration illustrated in FIGS. 5A and 5B. In this embodiment, the separators ( 710 A- 710 G) may be programmed to route particular channels to particular paths and therefore function as 1×2 switches. For example, assume that an optical signal containing channels λ 1 -λ 8  is input into the DWDM  700 . Separator  710 A is programmed to route the odd channels (λ 1 , λ 3 , λ 5 , λ 7 ) to separator  710 B and the even channels (λ 2 , λ 4 , λ 6 , λ 8 ) to separator  710 C, as with the embodiment illustrated in FIG.  5 B. Separator  710 B is programmed to route λ 1  and λ 5  to separator  710 D, and λ 3  and λ 7  to separator  710 F. However, separator  710 C is programmed to flip the route of the wavelengths, represented by the “1”, such that λ 6  and λ 8  are routed to  710 F instead of  710 G, and λ 2  and λ 4  are routed to  710 G instead of  710 F. Similarly, separators  710 D and  710 G are programmed not to flip the route of the wavelengths while separators  710 E and  710 F are, resulting in the outputs as shown. Comparing the outputs with the outputs in FIG. 5B, one can see the rerouting of λ 3 , λ 7 , λ 2 , λ 4 , and λ 8 . 
     FIG. 8 illustrates a fourth embodiment of a DWDM in accordance with the present invention. This DWDM  800  also contains separators which function as 2×2 switches, as with the programmable router configuration of FIG.  7 . However, in this embodiment, these separators are used to perform the add/drop function. For example, assume an optical signal containing wavelengths λ 1 -λ 8  in input into the DWDM  800 . Separator  810 A separates this signal into its odd (λ 1 , λ 3 , λ 5 , λ 7 ) and even (λ 2 , λ 4 , λ 6 , λ 8 ) channels. The odd channels are input into separator  810 B, which further separates them into two sets of channels, (λ 1 , λ 5 ) and (λ 3 , λ 7 ). The (λ 3 , λ 7 ) set of channels are input into separator  810 C which separates them into separate channels λ 3  and λ 7 . Channel λ 3  is then dropped. To be added is channel λ 3 ′ which is inputted into separator  810 C. Acting as a 2×2 switch as described with the second embodiment above, channel λ 3 ′ is then added to λ 7  by the separator  810 C. This signal is looped back as an input to separator  810 B, which adds λ 7  and λ 3 ′ to λ 1  and λ 5 . This combined signal is looped back as an input to separator  810 A, which adds channels λ 1 , λ 5 λ 7 , λ 3 ′ to channels λ 2 , λ 4 , λ 6 , λ 8 , resulting in one optical signal containing channels λ 1 , λ 3 ′, λ 4 , λ 5 , λ 6 , λ 7 , and λ 8 . This new signal is then the output of the DWDM  800 . Thus, in this manner, channel λ 3  is dropped while channel λ 3 ′ is added. For this embodiment, for every three stages, one channel may be dropped from a group of eight channels. More generally, for 2 n  channels and m stages, 2 n-m  channels may be dropped. 
     Separators which may be used with the multi-stage parallel cascade configuration of the present invention are disclosed in co-pending U.S. Patent Applications “Fiber Optic Dense Wavelength Division Multiplexer with a Phase Differential Method of Wavelength Separation Utilizing a Polarization Beam Splitter and a Nonlinear Interferometer”, Ser. No. 09/696,108, filed on Oct. 24, 2000, and in U.S. Pat. Nos. 6,130,971, 6,169,828, and 6,215,926 all assigned to the assignee of the present application. Applicant hereby incorporates these co-pending applications and U.S. Patents by reference. 
     FIG. 9 illustrates one embodiment of a separator which may be used with the present invention. This embodiment is disclosed in U.S. Pat. No. 6,215,926. The separator  900  comprises an input fiber  930  for inputting an optical signal, and two output fibers  940  and  960 . It also comprises two blocks of glass  910 A- 910 B, where the index of refraction for glass block  910 A is greater than the index of refraction for glass block  910 B, placed directly next to each other. Adjacent to one side of the blocks  910 A and  910 B is a nonlinear interferometer  950  which introduces a phase difference into the even channels while maintaining the same phase for the odd channels. At the place where the two blocks  910 A- 910 B meet, the glass is coated with a reflective coating  920  with a reflectivity, for example, of 50%. 
     The reflective coating  920  splits the optical signal containing λ1-λn into at least two portions  962 ,  964 . According to the general operation of beam splitters, when light travels through glass block  910 B and then is reflected from a surface of glass block  910 A (which has a greater index of refraction than glass block  910 B), the light undergoes a π phase shift. This π phase shift is indicated in FIG. 9 by the negative sign of the electric field (−E1) associated with signal  962  after it is reflected at the 50% reflective coating  920 . Otherwise, the light does not undergo a phase shift, as is indicated by the positive sign of the electric field (E2) associated with signal  964  after it is transmitted through the 50% reflective coating  920  in FIG.  9 . This reflection phase flip is very well known in the art and will not be further described here. In the preferred embodiment, the reflective coating  920  is polarization insensitive. The nonlinear interferometer  950  then introduces a π phase difference into the even channels while maintaining the phase of the odd channels. The two output fibers  940  and  960  are then aligned, or placed at a particular distance from the separator  900 , such that even channels are captured in phase in one fiber while the odd channels are captured in phase in the other. An example of a nonlinear interferometer which may be used with the separator  900  is disclosed in U.S. Pat. No. 6,169,604, assigned to the assignee of the present application. Applicant hereby incorporates this U.S. Patent by reference. 
     FIG. 10 is a simple block diagram of a second embodiment of a separator which may be used with the present invention. This embodiment is disclosed in U.S. Pat. Nos. 6,130,971 and 6,169,828, assigned the assignee of the present application. FIG. 10 shows a separator  1000  comprising an optic fiber  1010  for inputting an optical signal. The signal passes through a lens  1050 . It travels into a polarization beam splitter  1070  which splits the signal based on its polarization. The portion of the signal parallel to a plane in the splitter  1070  (S signal) is reflected toward an interferometer  1050 A. The portion of the signal perpendicular to the plane in the splitter  1070  (P signal) passes through toward an interferometer  1050 B. The interferometers  1050 A and  1050 B introduce phase differences in the even channels but not the odd channels. An example of interferometer  1050 A and  1050 B are also disclosed in U.S. Pat. Nos. 6,169,604 and 6,130,971. 
     FIGS. 11 and 12 illustrate the two embodiments of separators of FIGS. 9 and 10 respectively, performing the add/drop function as described in conjunction with the DWDM of FIGS. 7 and 8. In each embodiment illustrated in FIGS. 11 and 12, an additional input fiber ( 1110  of FIG. 11 and 1210 of FIG. 12) is added to input a second optical signal. These embodiments performing the add/drop function are also disclosed in their respective co-pending U.S. applications. 
     A dense wavelength division multiplexer with a multi-stage parallel cascade configuration of channel separators has been disclosed. This configuration provides for a lower insertion loss by requiring an optical signal to travel through fewer optical components. 
     Although the multistage parallel configuration of the present invention has been described with the specific embodiments of the separators, one of ordinary skill in the art will understand that other separators may be used with the configuration of the present invention without departing from the spirit and scope of the present invention. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.