OPTICAL ADD DROP AND DISPERSION COMPENSATION APPARATUS

Abstract: The present invention comprises a method and apparatus for dispersion compensation of WDM signals carried in transmission lines of a WDM system. Herein, selection and placement of dispersion compensation modules and add drop modules are selected according to the level of dispersion of signals carried in particular channels. The channels are separated using an optical add drop multiplexer so that the designer knows in advance which and how many modules to place on which branch waveguides. By this application, the output of each branch waveguide is made to be substantially similar. Likewise, by this design, a greater number of customers can be interconnected because of the advantageous use of add drop modules which do not require the intermediate step of converting optical signals into electrical signals during the add drop process.

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Placement of the compensation fiber on the transmission line also has an effect on suppression dispersion. Figure 2 depicts transmission line dispersion (y-axis units being in ps/n ) . Herein, a 20 km dispersion compensation fiber was applied to 100 km of transmission fiber. The resulting graph depicts the dispersion line 14 of the transmission line below the ideal transmission line 10 of 1700 (ps-n ) at upto 1540 nm and at above line 10 at above 1550 nm. In fact, upwards of 1560 nm, the dispersion will begin to have deleterious effects on the transmission integrity. A proposed prior solution is to space dispersion compensation fiber at select locations in the transmission line. Figure 3 depicts the placement of four dispersion compensation fiber modules thereby creating four transmission line segments, 16, 18, 20, 22. As shown, each of the segments has minimal dispersion off of the ideal transmission line 10. Long transmission line segments require large optical signal power to effect transmission. Large optical signal power inevitably causes non-linear effects and hence dispersion at the receiving end (e.g. self-phase modulation, cross phase modulation, and four wave mixing) . Self-phase modulation is especially problematic given that it causes a frequency shift at the pulse edges of the signal to be transmitted and thus results in an additional influence on the signal by the dispersion fiber. Likewise, in WDM systems, several signals are transmitted on different wavelengths. Fluctuations in signal power is problematic because the WDM system provides for a variable number of channels at a constant overall optical power. Where power is lost from use of dispersion compensation fibers, additional amplifiers are required. Optical amplifiers are expensive and therefore are of limited application. Likewise, with the boosted power, an increase in the fiber non-linear properties occurs, thereby leading to more dispersion. Finally, there is the ever present need in the art to route and interconnect as many customer lines as possible in order to maximize use and revenue from the optical transmission system. One proposed design solution is set out in US Patent 6,021,245. Herein a design solution to the above problem is the placement of fiber dispersion compensation modules (DK modules) before and after the optical amplifiers (pre and post compensation relative to the amplifier) . DK modules are well known to one skilled in the art and are a common design feature of optical transmission systems. The net effect of this design is to reduce dispersion to substantially zero. However, such designs require many amplifiers of high optical output necessary to overcome the power losses from the compensation modules making this design expensive and unattractive. Likewise, it does not address the need in the art to route and interconnect as many customer lines as possible. Another proposed design solution for overcoming non- linearities caused dispersion is the use of add drop modules in series with the DK modules. Add drop modules are ell known in the art and operate to selectively add and/or drop select wavelengths of select channels. In some cases entire channels can be dropped and regenerated. The effect of dropping and adding select wavelengths and/or channels is to replace dispersed wavelengths with non-dispersed wavelengths. Hence, dispersed optical power is selectively removed and regenerated non-dispersed. One such proposed designed is set out in Figure 4, which incorporates an optical, add drop multiplexer therein. 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As is known in the art, the bandwidth may depend on the data rate. Line segment 50 contains no dispersion compensation nor add drop means. Herein, the designer relies upon DK module 34 to provide any compensation necessary for channels carried in this line segment. Line segment 46 includes a DK module 52 having 10 kilometers of wound optical fiber. An add drop module 53 is placed in series with DK module 52. Add drop module 53 includes a drop module 54, 3R signal regenerator 56, and add module 58, the function and use of each being well known to one skilled in the art. Drop module 54 operates to selectively drop at least one (55) of a group of channels 51 transmitted in line 46. Typical add drop modules can add drop 5-50% of the channels being routed therethrough. As depicted, a first channel 55 is selectively dropped from the group of channels 51. Channel 55 is then regenerated by regenerator 56 having no dispersion. The regenerated channel is then carried via waveguide 57 to add module 58. The add module 58 recombines the group of channels 51 and transmits them along branch waveguide 46 to multiplexer 70. Multiplexer 70 may also comprise an interleaver and group filter. The output power of line 46 may lose 1 dB as a result of the dispersion compensation via DK module 52. This loss can be made up by booster amplifier 38. Branch waveguide 48 includes a DK module 60. As designed, module 60 includes 20 kilometers of wound optical fiber. Waveguide 48 includes no other compensation or add drop means thereby indicating that the designer intended that the channels being carried on this line segment require no regeneration nor add/dropping. The optical power is down about 2 dB, which may be made up by booster amplifier 38. Branch waveguide 50 includes a DK module 62 having 30 kilometers of wound optical fiber. The line segment further includes add drop module 63 which functions essentially the same as add drop module 53. Herein drop module 64 separates the channels being carried along this line segment into smaller parallel groups or individual channels (line segments) 71. A first channel (line segment) 72 of the smaller group is directed to regeneration means 66 which regenerates the first channel to then be carried along line segment 74 to add module 68. Add module 68 recombines the channels being carried on line segments 71 and routes them along line segment 50 to multiplexer 70. The optical power loss along this line segment is about 3 dB, which may be made up by booster amplifier 38. The above described prior art design has at least the disadvantage of including the use of 3R regeneration means (or their equivalents) in that such means require the optical/electrical/optical conversion of the signal for its regeneration. Such conversion is complicated and costly. Likewise, the need of customer connectivity is not addressed. The object of the present invention is to provide an optical transmission system wherein dispersion can be compensated for while costs due to regeneration and optical/electrical conversion is minimized and/or eliminated. The compensation is effected without the use of individual line generators such as the prior art 3R generator. Finally, the present invention facilitates a higher number of interconnections than the prior art apparatus, thereby adding value to the transmission system in which the present invention may be incorporated. These objects are achieved by means of an apparatus and method in which add/drop modules are designed into an optical add drop multiplexer such that channels are added/dropped without resorting to regeneration. Rather, optical signals are routed into the add portion of the module, the signals originating from outside multiplexer. The newly added optical signals may originate from other customers/sites. The newly added signals are not as dispersed as those being dropped, thereby compensating for dispersion while facilitating signal (re) routing. This object is further achieved by the conscious design and select implementation of dispersion compensators particular to the known dispersion of a branch waveguide. By this design, branch waveguides with higher dispersion will have a higher number of DK modules and visa versa. Likewise, the number of add drop modules implemented is made dependent upon known dispersion values. By this arrangement, the output optical power of each branch waveguides is made substantially similar. The figures represent only the component parts of the optical transmission system necessary for the understanding of the present invention. Missing components include signal processing means at receiving and transmitting ends, fiber connection means, etc. Figure 5 sets out an embodiment of the present invention. Herein, a first and second preamplifier 102, 103 oppose and are serially connected with a common DK module 104 along an optical transmission line 100 of a WDM system. By way of example, as depicted, DK module 104 includes 100 kilometers of wound optical fiber. An asymmetric optical add drop multiplexer 106 (OADM) is serially connected, via transmission line 100, downstream to the second preamplifier 103. A common booster amplifier 108 is serially connected downstream from the OADM 106. The OADM comprises opposing demultiplexer and multiplexer elements 110 and 160 respectively. The demultiplexer and multiplexer elements may comprise interleaver and group filters or other equivalent elements as known to one skilled in the art to provide access to desired channels and wavelengths which, as will be known in advance, will require replacement into a less dispersed form. The wavelengths are carried in channels for which dispersion as a whole compensation may also be required. Where no dispersion compensation and/or replacement is required, the channels can be designed to pass through the multiplexer 106 unchanged. Demultiplexer 110 divides the transmission signal, being carried on transmission line 100, into a select number of parallel subgroups, each comprising a select number of channels. The number of subgroups and channels therein is a matter of design choice within the scope of the invention. By way of example, 8 channels per group are depicted in Fig. 5, with 8 wavelengths per channel. However, the number of channels may range from at least 2-40 with a spacing of at least 50-200 GHz. Herein, as depicted, demultiplexer 110 divides the transmission signal into 4 parallel subgroups . Each of the subgroups is carried on one of four branch waveguides or line segments 120, 130, 140, 150. Each of the line segments includes a number of elements selected to optimize the operating optical power budget of the OADM 106. As is known in advance from the design of the OADM, signals carried by waveguide 120 will require little dispersion compensation. As such, for these signals, the apparatus relies upon the common OK module 104. Given the low dispersion requirements for this waveguide, the designer is afforded the opportunity to include a plurality of add drop modules. The plurality of add drop modules facilitates the interconnection with other waveguides thereby providing interconnectivity with other customer sites. The interconnectivity is enhanced over prior art designs, which relied upon regeneration rather than substitution of signals. The add drop modules further operate to attenuate the optical power of the signals in the channels being carried by the respective waveguide such that the output power of all of the OADM branch waveguides is made substantially similar. Having substantially similar output power facilitates multiplexing, amplification, and transmission. The plurality of add drop modules further enhances the instant designs benefit to the transmission system by providing enhanced connectivity. The enhancement is also of economic value to the transmission system operator in that more customers can be connected to his/her transmission system. As depicted, branch waveguide 120 includes four add drop modules 122, 124, 126, 128. Each module includes means (not shown) to connect with other waveguides such that a select number of channels may be dropped and added by design choice and/or customer needs. The modules are selected such that the combined dropped and added channels result in an output optical power of waveguide 120 being substantially similar to optical power of the other OADM waveguides and wherein the dispersion of the exiting signals is substantially minimized (e.g. zero dispersion) . Add drop modules 122, 124, 126, 128 differ from those previously used in optical add drop multiplexers in that the instant modules route optical signals without the intermediate step of converting the optical signals into electrical signals. The instant modules may comprise an optical circulator and programmable fiber Bragg gratings, and/or other wavelength based splitting arrangement known to one skilled in the art wherein, by way of example, 2 dB can be dropped per module. With an exemplary optical powerbudget of 9 dB, the instant design incorporates 4 add drop modules to drop upwards of 8 dB if so designed. Branch waveguide 120 does not include any DK modules because the system is designed to route signals through this waveguide having a dispersion level that is acceptable for transmission reception thereby requiring no DK module compensation. This design is facilitated by appropriate selection of demultiplexer 110. The output of branch waveguide 120 is directed to multiplexer 160 which serves to combine the waveguide output with other waveguides so as to provide a single set of signals to be transmitted along transmission line 100. Branch waveguide 130 includes a lesser number of add drop modules then waveguide 120. Herein, waveguide 130 includes three add drop modules 131, 132, 133. The add drop modules operate in essentially the same manner as the above add drop modules, 122, 124, 126 and 128. Herein a DK module 134 is serially connected with modules 131, 132, 133. The number of add drop and DK modules is selected so that the power output of the waveguide 130 is substantially similar to the power of the other OADM waveguides. The DK module operates 0 o P. cn cn CU x 0 Ω cn n_ Φ cn s: cn o 0 X 3 TJ Cn ■ 2 M rt t-1 TJ Hi μ» rt rt P. n rt 0 μ- ø cn ø P. Cu rt O μ- P- 0 U Cu ø T ø Cu O H 1 0 CU CU rt tr o o H O tr ø 4 φ ø rt cn tr tr P. rt M <Ω 0 P. rt <1 tr rt p. j P. 0 Ω <! < Φ 0 • s O 0 μ- cn tr 3 TJ TJ cn cn cn Φ Φ TJ ø 0 μ- φ cn μ- Φ ø Ω ø 4 φ Φ μ- rt 3 r+ cn μ- cn ø 0 φ rt ø r ιΩ ø Φ cu O cn sQ rt Ω σ sQ Φ *> sQ ιΩ cn o Φ H μ- ιΩ rr t- > rt H PJ Ω 0 ι-i 0 ø ø μ- Hi Hi ø Cu CU i 0 Φ ø 0 rt P. rr ø μ- X ø ø rt cn 0 tr ø o μ- ø cn cn *< ø μ- 0 μ- cn O ■ ζ μ- μ- ?v tr O tr Ω cn ø φ 0 μ- μ- μ- rt rt T p. rt ø ^ rt Ω rt 3 α Hi 0 p. P. ø Φ r 0 μ- H" 0 P. rt TJ cn O μ- μ- φ μ- rt Ω tr rt Φ μ- T O Φ μ- l φ Φ o H O ft cn ø rt Φ μ- ø ø tr 0 3 o μ- cu μ- Φ O cn Cυ O P. ø rt Φ cn 2. 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Φ ø S 0 cn > O 0 rt ø TJ 0 ø 4 Φ cn rt • s tr Φ tr φ OD ø μ- Ω rr K * cn 0 p. 9- CU μ- Ω ø μ- O TJ Φ 0 Φ Cu Φ μ- Φ l-S r-" cn rt Φ μ- μ» 0 rt o ø 4 rt P. < O cn rt 3 <i P J P» P. 0 0 TJ tr μ- O rt ji> cn p. μ- 1 μ- ø Φ Φ * 3 ø tr Φ cn φ Cn rt 0 TJ Φ 0 <l ø tr r ø φ O Hi H Φ 0 rt H μ- Q • o 0 Φ α o H rt μ- ø Φ Ω cn 0 rt ι-i μ> 0 P. μ- o cn 0 rt tr ≤. cn 0 ø 4 H tr O CO μ- 1 0 o 0 μ- ø 4 μ- o μ- o φ μ- «3 μ> o 0 3 o μ j ø ιΩ p. H3 Φ cn <i ø ø i-i o ,(=> TJ μ- ø O φ tr Φ ø 4 Φ Φ 0 O ø μ- cn cn Φ • 0 cn cn cn Optical power loss incurred in OADM 106 is made up for by common booster amplifier 108 positioned downstream of the OADM. As a design alternative, in certain embodiments a number of attenuators may be substituted for add drop modules since attenuators can be made to perform essentially similar functions to the add drop modules described in connection with the invention, namely, reducing the amplitude of a signal without appreciably distorting its waveform. Optical attenuators are generally passive devices requiring no intermediate step of converting the optical signal into an electrical signal. Likewise, the degree of attenuation may be fixed, continuously adjustable, or incrementally adjustable. Although the invention has been shown and described with respect to an exemplary embodiment thereof, it should be understood by those skilled in the art that the foregoing and various changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.

Claims: The invention claimed is: 1. An Add Drop apparatus attached to a transmission line of WDM optical transmission system, comprising: means for dividing a WDM-signal into different signal bands having different relative dispersions rates, said dividing means further passing said bands over a plurality of branch waveguides, said waveguides comprising optically communicating add-drop-modules and dispersion-compensating- modules; and means for combining said signal bands to a WDM signal, characterized in that: the waveguides carrying signal bands of higher dispersion, as compared to the remaining signal bands of the remaining waveguides, comprise dispersion compensation means of higher attenuation than the remaining waveguides; the waveguides carrying signal bands of higher dispersion, as compared to the remaining signal bands of the remaining waveguides, comprise a lesser number of add drop modules as compared to the number of add drop modules on the remaining waveguides; such that each waveguide power output level is substantially similar. 2. The apparatus according to claim 1, further characterized in that at least one of said waveguides contains no dispersion compensation means and dispersion compensation for said at least one of said waveguides is carried out by at least one common dispersion compensation module serially connected before said dividing means. 3. The apparatus according to claims 1-2, wherein said dispersion compensation means comprises at least one signal compensation module and at least one of said waveguides comprises an attenuation module. 4. The apparatus according to claims 1-3, wherein a number of add drop modules and signal compensation modules are selected such that the combined attenuation of all of said signal bands of each of said waveguides is substantially similar. 5. The apparatus according to claim 4, wherein said number of add drop modules comprise means for substituting a first number channels for a second number of channels wherein said first and second channels are of the same wavelength. 6. The apparatus according to claims 1-5, wherein each of said waveguides includes a different number of add drop modules and dispersion compensation modules. 7. The apparatus according to claims 1-6, wherein said dispersion compensation means for each waveguide and common dispersion compensation means each have an adopted dispersion value . 8. A method for dispersion compensation of WDM signals carried on a transmission line of a WDM optical transmission system, said method using an apparatus having means for dividing a WDM-signal into different signal bands to be carried on different waveguides, said signal bands having different dispersions values, said dividing means further passing said bands over said different waveguides, and means for combining said signal bands to a WDM signal to be carried over said transmission line, comprising the steps of: - placing dispersion compensation means of higher attenuation on signal lines carrying signal bands of higher dispersion than other dispersion compensation means, or no dispersion compensation means, than on other signal lines carrying signals of lower dispersion; - placing a higher number of add drop modules on waveguides carrying signals having lower dispersion values than other signals carried on other waveguides, than on waveguides having higher dispersion values; such that the output of each of said lines is substantially similar. 9. The method according to claim 8, further comprising the step of placing a common dispersion compensation module upstream of said dividing means such that said common dispersion compensation module provides dispersion compensation at least for said waveguide with no dispersion compensation means. 10. The method according to claims 8-9, wherein said add drop module further comprises means for substituting a first number channels for a second number of channels wherein said first and second channels are of the same wavelength and said step of placing dispersion compensation means further comprises the step of using dispersion compensation means for each signal line having an adopted dispersion value.

Inventors: LEISCHING PATRICK, LEISCHING, PATRICK
Applicants: LEISCHING PATRICK, SIEMENS AG, LEISCHING, PATRICK, SIEMENS AKTIENGESELLSCHAFT
Application year: 2000
Main classification: H04B10
IPC codes: H04B10