Fiber optic dense wavelength division multiplexer utilizing a multi-stage parallel cascade method of wavelength separation

[0002] The present invention relates to fiber optic networks, and more particularly to fiber optic dense wavelength division multiplexers.

[0003] 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.

[0004] 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.

[0005] 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'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.

[0006] 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 (λ₁-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, λ₁would travel along path 250-1, λ₂would travel along path 250-2, etc. In the same manner, the signal from Loop 150 (λ₁′-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, λ₁′ would travel along path 280-1, λ₂′ would travel along path 280-2, etc.

[0007] In the performance of an add/drop function, for example, λ₁is transferred to path 280-1. It is combined with the others of Loop 150'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, λ₁′ is transferred to path 250-1 from 280-1. It is combined with the others of Loop 110'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's point of view, channel λ₁of its own signal is dropped to Loop 150 while channel λ₁′ of the signal from Loop 150 is added to form part of its new signal. The opposite is true from Loop 150's point of view. This is the add/drop function.

[0008] 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 λ₁-λ_nis inputted into separator 310A, which filters/reflects channel λ₁and send it along its own path 320-1. The remaining channels λ₂-λ_nare sent to the next separator 310B, which filters/reflects channel λ₂and 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.

[0009] 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 λ₁-λ_nenters a signal splitter 410 which splits the signal onto N separate paths, each split signal containing channels λ₁-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 430A-430N to output one particular channel. For example, a split signal containing channels λ₁-λ_nexits the splitter 410 onto path 420-1. The split signal enters separator 430A which filters/reflects channel λ₁and sends it along path 420-1. Another split signal containing λ₁-λ_nexits splitter 410 onto path 420-2 and enters separator 430B. Separator 430B filters/reflects channel λ₂and 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.

[0010] 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 310A-310N an amount of insertion loss results. For example, if the optical signal in FIG. 3 has eight channels λ₁-λ₈and each component causes 1 dB of insertion loss. By the time λ₈is separated, it would have passed through eight separators. λ₈would thus suffer 8 dB of insertion loss.

[0011] 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 430A-430N. Thus, each channel suffers 10 dB of insertion loss.

[0012] 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.

[0013] 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.

[0014] FIG. 1 is an illustration of a simplified optical network.

[0015] FIG. 2 is an illustration of conventional add/drop systems and dense wavelength division multiplexers.

[0016] FIG. 3 is an illustration of a conventional multi-stage serial cascade configuration of separators.

[0017] FIG. 4 is an illustration of a conventional single stage parallel configuration of separators.

[0018] 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.

[0019] FIG. 6 is an illustration of a second preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention.

[0020] FIG. 7 is an illustration of a third preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention.

[0021] FIG. 8 is an illustration of a fourth preferred embodiment of a dense wavelength division multiplexer in accordance with the present invention.

[0022] FIG. 9 is a block diagram of a first embodiment of a separator which may be used with the present invention.

[0023] FIG. 10 is a block diagram of a second embodiment of separator which may be used with the present invention.

[0024] FIG. 11 is a block diagram of the first embodiment of a separator performing the add/drop function in accordance with the present invention.

[0025] FIG. 12 is a block diagram of the second embodiment of a separator performing the add/drop function in accordance with the present invention.

[0026] 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.

[0027] 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.

[0028] 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 λ₁-λ_nenters the DWDM 500 through node A (240). The signal passes through a separator 510A. The separator 510A divides the signal into two separate signals, one containing the odd channels (λ₁, λ₃, λ₅, . . . ) (530) and the other containing the even channels (λ₂, λ₄, λ₆, . . . ) (540), i.e., every other channel. These odd and even channels are each passed through another separator 510B-510C 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.

[0029] 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 λ₁-λ₈, only seven separators 510A-510G 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.

[0030] 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 λ₁-λ₁₆is input into the DWDM 600. Separator 610A separates them into two signals, one containing the odd channels (λ₁, λ₃, . . . λ₁₅), the other containing the even channels (λ₂, λ₄, . . . λ₁₆). The odd channels are input into separator 610B which separates them further into two sets. One set of signals (λ₁, λ₅, . . . λ₁₃) is input into separator 620A, while the other set (λ₃, λ₇, . . . λ₁₅) is input into separator 620B. The even channels are input into separator 610C which separates them further into sets of signals. One set of signals (λ₂, λ₆, . . . λ₁₄) is input into separator 620C, while the other set (λ₄, λ₈, . . . λ₁₆) input into separator 620D. Separators 620A-620D 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.

[0031] 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 (710A-710G) may be programmed to route particular channels to particular paths. For example, assume that an optical signal containing channels λ₁-λ₈is input into the DWDM 700. Separator 710A is programmed to route the odd channels (λ₁, λ₃, λ₅, λ₇) to separator 710B and the even channels (λ₂, λ₄, λ₆, λ₈) to separator 710C, as with the embodiment illustrated in FIG. 5B. Separator 710B is programmed to route λ₁and λ₅to separator 710D, and λ₃and λ₇to separator 710E. However, separator 710C is programmed to flip the route of the wavelengths, represented by the “1”, such that λ₆and λ₇are routed to 710F instead of 710G, and λ₂and λ₄are routed to 710G instead of 710F. Similarly, separators 710D and 710G are programmed not to flip the route of the wavelengths while separators 710E and 710F are, resulting in the outputs as shown. Comparing the outputs with the outputs in FIG. 5B, one can see the rerouting of λ₃, λ₇, λ₂, λ₄, and λ₈.

[0032] 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 λ₁-λ₈in input into the DWDM 800. Separator 810A separates this signal into its odd (λ₁, λ₃, λ₅, λ₇) and even (λ₂, λ₄, λ₆, λ₈) channels. The odd channels are input into separator 810B, which further separates them into two sets of channels, (λ₁, λ₅) and (λ₃, λ₇). The (λ_{3 λ7}) set of channels are input into separator 810C which separates them into separate channels λ₃and λ₇. Channel λ₃is then dropped. To be added is channel λ₃′ which is inputted into separator 810C. Acting as a 2×2 switch as described with the second embodiment above, channel λ₃′ is then added to λ₇by the separator 810C. This signal is looped back as an input to separator 810B, which adds λ₇and λ₃′ to λ₁and λ₅. This combined signal is looped back as an input to separator 810A, which adds channels λ₁, λ₅, λ₇, λ₃′ to channels λ₂, λ₄, λ₆, λ₈, resulting in one optical signal containing channels λ₁, λ₃, λ₄, λ₅, λ₆, λ₇, and λ₈. This new signal is then the output of the DWDM 800. Thus, in this manner, channel λ₃is dropped while channel λ₃′ is added. For this embodiment, for every three stages, one channel may be dropped from a group of eight channels. More generally, for 2ⁿchannels and m stages, 2^n−mchannels may be dropped.

[0033] 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 entitled “Fiber Optic Dense Wavelength Division Multiplexer With a Phase Differential Method of Wavelength Separation Utilizing Glass Blocks and a Nonlinear Interferometer”, Ser. No. 09/248,021, filed on Feb. 10, 1999; and “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. No. 6,130,971 and U.S. Pat. No. 6,169,828, all assigned to the assignee of the present application. Applicant hereby incorporates these co-pending applications and U.S. Patents by reference.

[0034] FIG. 9 illustrates one embodiment of a separator which may be used with the present invention. This embodiment is disclosed in U.S. Patent Application entitled “Fiber Optic Dense Wavelength Division Multiplexer with a Phase Differential Method of Wavelengths Separation Utilizing Glass Blocks and a Nonlinear Interferometer”, Ser. No. 09/248,021. 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 910A-910B, where the index of refraction for glass block 910A is greater than the index of refraction for glass block 910B, placed directly next to each other. Adjacent to one side of the blocks 910A and 910B 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 910A-910B meet, the glass is coated with a reflective coating 920 with a reflectivity, for example, of 50%.

[0035] 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 910B and then is reflected from a surface of glass block 910A (which has a greater index of refraction than glass block 910B), 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.

[0036] 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. No. 6,130,971 and U.S. Pat. No. 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 1050A. The portion of the signal perpendicular to the plane in the splitter 1070 (P signal) passes through toward an interferometer 1050B. The interferometers 1050A and 1050B introduce phase differences in the even channels but not the odd channels. An example of interferometer 1050A and 1050B are also disclosed in U.S. Pat. No. 6,169,604 and U.S. Pat. No. 6,130,971.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.