WO2002015441A2 - Dispersion compensation for high speed optical transmission systems - Google Patents

Abstract

A tuneable dispersion compensation module may be used to determine a desired dispersion compensation value for a transmission path. In one aspect, the tunable dispersion compensation module comprises at least one variable coupler that is capable of receiving one or more optical signals and providing the signals to a first path having a first dispersion, or second path having a second dispersion. Once the desired value is determined, a connectable compensation module is configured to provide the optimal dispersion compensation value determined using the tuneable module. The connectable module may include a reconfigurable connector that is capable of receiving one or more optical signals and providing the signals to a first path having a first dispersion and a second path having a second dispersion. As is the case with the tuneable module, multiple stages can be provided to the connectable module to achieve desired range and/or granularity of dispersion.

Description

DISPERSION COMPENSATION FOR HIGH SPEED OPTICAL TRANSMISSION SYSTEMS

TECHNICAL FIELD This invention relates generally to dispersion compensation, and in one aspect to dispersion compensation for high speed optical transmission systems.

BACKGROUND High speed optical transmission systems, particularly 10 Gb/s (gigabytes per second) and above, are very sensitive to optical dispersion. An optical signal typically undergoes an amount of optical dispersion as the signal travels through optic fiber or other media. Although there are a number of types of dispersion, a particularly troublesome type of dispersion has been chromatic dispersion (related to differences in signal propagation as a function of wavelength). Chromatic (and other types of) dispersion, or other signal degradation, can lead to broadening, smearing, or other types of distortions of waveforms, ultimately causing problems in communication systems such as detection and/or demodulation errors and the like.

A number of approaches have been proposed or attempted for reducing or eliminating dispersion or its effects. In one approach, an optical signal which has undergone an amount of chromatic dispersion is provided to a medium which provides an amount of opposite dispersion. Among the media or devices which can be used to provide opposite dispersion are a fiber Bragg grating and dispersion compensation fiber (DCF). Numerous devices or materials using DCF are commercially available, as will be known to those of skill in the art.

One shortcoming of these types of devices is the difficulty they present to achieving an optimal amount of dispersion compensation. Although DCF and these other materials and devices have been useful in optical systems, they only offer a single fixed compensation value (e.g., 20 picoseconds/nanometer (ps/nm)). Fixed-value compensators are often undesirable for at least the reason that it is difficult when designing or building an optical system to calculate or predict with precision the exact amount of dispersion that needs to be compensated. Accordingly, there is a significant chance that a dispersion compensator, such as DCF, selected and installed on the basis of design or prediction, will have a non-optimal value, and, often, one which cannot be readily changed.

For example, an optical network which is to involve a first optical transceiver coupled to a second optical transceiver by a transmission line comprised of optic fiber is being designed. During the design process, it is decided that the length of the optic fiber between the two transceivers is to be a certain value X. Based on this value, it is estimated that a certain amount Y of dispersion, particularly chromatic dispersion, will be introduced onto a signal propagating from the transceiver to transceiver. Using this estimated Y value, the amount of DCF that would need to be inserted between the two transceivers in order to compensate for the dispersion is determined. The network is then built according to these specifications.

Unfortunately, however, after the network is completed, it is discovered that the actual amount of dispersion caused by the optical fiber is less than Y, and thus, the DCF, in actuality, is introducing a net dispersion onto the signal. Thus, the network must now be altered to eliminate this net dispersion introduced by the excess DCF. However, altering the network to eliminate the net dispersion will be extremely difficult. The network will have to be shut down and some measure of DCF will have to be removed. Even so, it will be uncertain whether the amount removed was sufficient until the network is brought back on- line. Once the network is brought back on-line, it may be discovered that too much DCF was removed and the whole process must be repeated again, however, with DCF now being added, rather than removed. As can be seen, this process can continue ad infinitum without an optimal value ever being achieved.

In response, certain devices or approaches have been suggested for a module which can provide adjustable or tuneable amounts of dispersion compensation, even after the module has been installed into an optical network, to include the device disclosed in United States Patent No. 5,608,562 entitled "OPTICAL COMMUNICATIONS SYSTEM WITH ADJUSTABLE DISPERSION COMPENSATION" (hereinafter referred to as "the '562 patent."). However, at least one problem with these previous devices, particularly the device taught by '562 patent, is the signal or power losses that may result from these devices.

For example, the adjustable dispersion compensation device of the '562 patent employs optical switches, several pieces of fiber, a circulator and/or a mirror. Several of these items provide an amount of signal or power loss. For example, there can be splicing losses between the switch and the fiber and losses in the switches themselves. Splicing losses in these previous devices are typically large, e.g., because of mode mismatch. For instance, use of the mirror can double the splicing and switch losses and the circulator can provide additional loss. SUMMARY OF THE INVENTION The present invention is directed towards methods and apparatus for reducing or eliminating the amount of dispersion on optical signals propagating within an optical communication system(s). In one embodiment, a desired dispersion compensation value is determined, at least partially, using a tuneable compensation apparatus. This desired dispersion compensation amount is then provided to the optical communication system by a connectable compensation apparatus that has been configured to provide said desired compensation.

In at least one embodiment, the tuneable compensation apparatus is capable of adjustably coupling an optical signal or signals to none, one or a selected plurality of dispersion compensation devices, such as dispersion compensation fiber (DCF) devices. Furthermore, in at least one embodiment, the tuneable compensation apparatus determines the desired compensation value by coupling the optical signal to different combinations of compensation devices until a desired signal characteristic is achieved. In one embodiment, the connectable compensation apparatus may provide the desired compensation value by being configured so as to connect at least a first input path to at least one of a plurality of output compensation paths so as to route at least a first optical signal along one of a plurality of output compensation path options. Multiple stages can be provided to achieve desired range and/or granularity of dispersion compensation. Preferably, the connectable compensation apparatus can be used to compensate a plurality of WDM (wavelength division multiplex) channels.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows maybe better understood. Additional features and advantages of the invention will be described hereinafter which form the subj ect of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed maybe readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the present invention, and the advantages thereof, reference is made to the following descriptions taken in conjunction with the accompanying drawing, in which:

FIGURE 1 shows a block diagram depicting components of an exemplary embodiment of a tunable dispersion compensation fiber module;

FIGURE 2 shows a block diagram of an exemplary microbanding mechanism-based variable coupler useable in connection with embodiments of the present invention;

FIGURE 3 A shows an exemplary configuration of a system for mechanically moving fibers to provide a variable fiber coupler; FIGURE 3B shows a second exemplary configuration of a system for mechanically moving fibers to provide a variable fiber coupler;

FIGURE 3C shows a third exemplary configuration of a system for mechanically moving fibers to provide a variable fiber coupler;

FIGURE 4 shows a block diagram of an exemplary embodiment of a tuneable dispersion compensation fiber module used as an indicator of dispersion compensation value, e.g., in system deployment;

FIGURE 5 shows a block diagram depicting an exemplary application of an exemplary DC finder;

FIGURE 6A shows an exemplary embodiment of the exterior of a DC finder; FIGURE 6B shows a second exemplary embodiment of the exterior of a DC finder;

FIGURE 6C shows a third exemplary embodiment of the exterior of a DC finder; FIGURE 6D shows a fourth exemplary embodiment of the exterior of a DC finder; FIGURE 7 shows a block diagram depicting components of an exemplary embodiment of the interior of a DC finder; FIGURE 8A shows an exemplary embodiment of the exterior of a DCFM;

FIGURE 8B shows a second exemplary embodiment of the exterior of a DCFM; FIGURE 8C shows a third exemplary embodiment of the exterior of a DCFM; FIGURE 9 shows a block diagram depicting components of an exemplary embodiment of the interior of a DCFM;

FIGURE 10 shows a block diagram depicting components of an exemplary embodiment of the interior of a DCFM, the components being programmed to achieve an optimal dispersion valued determined, at least in part, using a DC finder.

DETAILED DESCRIPTION To overcome the earlier discussed problems associated with prior art adjustable dispersion compensation devices, the applicants for United States Application No. 09/639,615 ("the '615 application") entitled "TUNEABLE DISPERSION COMPENSATION MODULE," the contents of which are hereby incorporated by reference herein, developed a tuneable dispersion compensation fiber module ("TDCFM") which may achieve a substantially constant insertion loss, regardless of the amount of dispersion compensation being provided. An embodiment of the TDCFM of the '615 application is depicted in FIGURE 1.

In the embodiment depicted in FIGURE 1 (i.e., device 112), a first path-determining component 113 receives an optical signal on a first optical fiber 118. The input optical signal can include a number of different multiplexed channels (such as a WDM (wavelength division multiplexed) signal) or be a single wavelength stream. In one embodiment, the path- determining component 113 determines whether (and/or how much of) the incoming signal is routed along a first DCF path 122a or a second DCF path 122b. The two paths 122a and 122b may be configured to provide different amounts of dispersion compensation, such as by providing different lengths of DCF, as depicted.

Preferably, the path-determining component 113 is a low-loss device and preferably creates substantially no splicing losses, h one embodiment, the path-determining component 113 provides loss about equal to that of the fiber. Although it is possible to provide path- determining devices which can be operated (i.e., tuned or adjusted) partially or fully in a manual fashion (such as permitting a user to use a button, key or similar device to operate the path-determining device), in at least one embodiment, path-determining device 113 is fully or partially operable in response to a control signal (such as an electrical or optical control signal), e.g., using one or more relays or the like. hi the embodiment of FIGURE 1, in addition to path-determining component 113, device 112 also contains path-determining components 114b and 114c, each of which receives signals from either (or both) of two input paths and determines whether (or how much of) the signals received are provided to a first path (in the case of component 114b, path 124a, or in the case of component 114c, path 126a) or a second path (in the case of component 114b, path 124b, or in the case of component 114c, path 126b). The second and third path-determining devices 114b and 114c otherwise operate in a manner substantially similar to that of the first path-determining component 113. Furthermore, the difference in compensation between paths 124a and 124b may be different from that of paths 126a and 126b, as well as that of paths 122a and 122b, in order to enable device 112 to provide a relatively large range of compensation values. A combining component 116 receives signals from either (or both) of the last pair 126a, 126b and outputs an optical signal on the output fiber 128.

To illustrate the potentially large range of compensation values which can be provided by the device 112 of FIGURE 1, assume that, in a first stage 112a, the first path determining component 113 is coupled to two output paths 122a and 122b where the first path 122a provides a first unit or amount of dispersion compensation (1U) and the second path 122b provides substantially no dispersion compensation. Although the value of 1U will depend on the intended application of the dispersion compensation module, for purposes of illustration, a typical device might provide a compensation of 10 ps/nm.

Continuing on, assume that in a second stage 112b, a choice is provided between a dispersion compensation of twice that of the loop 122a (2U), this choice being loop 124a, or zero, this choice being loop 124b. Moreover, in a third stage 112c, a choice is provided between dispersion compensation four times that of loop 122a (4U), this choice being loop 126a, or zero, this being loop 126b. As seen in Table I, in this fashion, by using the path- determining devices 113, 114b, 114c to select one path or the other, in various combinations, it is possible for the device depicted in FIGURE 1 to provide any multiple of 1U dispersion compensation between zero and seven (with a granularity of 1U). TABLE 1

As will be understood by those of skill in the art, additional stages can be included to increase the range of dispersion compensation available and the value of 1U can be selected to provide the desired granularity. It is also possible to provide systems in which not all values in the range are available with a 1U granularity (e.g., if at least one stage has a value which is not a power-of-two multiple of 1U). Moreover, the compensation provided by the loop choices for a particular stage may be opposite in sign.

A number of devices can be used as path-determining components. In one embodiment, a path-determining component comprises an optical switch. In another embodiment, as depicted in FIGURE 2, a variable fiber coupler 212 (or variable fiber splitter) 212 used as the path-determining device provides a coupling ratio between the signals on the output fibers 124a, 124b, such as in response to a control signal 314. The variable fiber coupling of FIGURE 2 can be provided, e.g., using a microbanding mechanism. FIGURE 3 A-3 C depict a half-block mechanical fiber coupler, of a type which can be used as path-determining components 114b and 114c. In the device of FIGURES 3A-3C, alignment blocks 312a, 312b can be mechanically moved (such as in response to manual operation or in response to, e.g., a solenoid, controlled by a control signal). The blocks 312a and 312b can be moved among a first configuration (FIGURE 3 A) with fiber 122a aligned with fiber 124a and fiber 122b aligned with fiber 124b, a second configuration (FIGURE 3B) with fiber 122b aligned with fiber 124a and a third configuration (FIGURE 3C) with fiber 122a aligned with fiber 124b. hi this way, regardless of whether signals are received on the first fiber 122a or the second fiber 122b, they can be output onto either the first output fiber 124a or the second output fiber 124b. In yet another embodiment, a device (not shown) can be provided which uses commercially available variable splitters such as those available from Optics For Research, Inc. of Cauldwell, New Jersey. h addition to a number of different path-determining components, a number of types of DCF can be used in various embodiments of the device. Preferably, DCF is selected to provide desired dispersion and slope, so as to compensate a plurality of WDM channels. For example, conventional DCF can be used to compensate standard single-mode fiber (SSMF) while non-conventional DCF can be used for compensating, e.g., non-zero dispersion shifted fibers (NZDSF).

Furthermore, in some embodiments, attenuation elements (not shown) may be provided, e.g., in the shorter DCF paths (e.g., 122b, 124b, and 126b in FIGURE 1), to balance the amount of attenuation among the various path choices. A number of devices or methods can be used for providing attenuation elements (e.g., an off-center splice or a high attenuation fiber, such as that available from TNO, Inc.).

The tuneable dispersion compensation fiber module (TDCFM) of the '615 application can be used for a number of purposes, one of which is measuring or analyzing a system or a system component, particularly the amount of chromatic dispersion in the system or system component, as is depicted in FIGURE 4.

In FIGURE 4, a selected location along a transmission line 1122 connecting a transmitter 1155 to a receiver 1144, such as a location near the receiver, is tapped and provided to a tunable dispersion compensation fiber module 1112. In such an instance, TDCFM 1112 may provide an indication of the dispersion compensation value needed at the tap point and/or output information to other devices, such as information for generating eye diagram 1132, as well as other information useful in system deployment.

In at least some embodiments, changes or adjustments in the dispersion compensation provided by a TDCFM occur substantially autonomously in response to sensing certain parameters. Although it is possible to provide a control signal in response to various measurements or events, in at least some embodiments, data indicative of dispersion is collected and (preferably autonomously) analyzed by the TDCFM or the TDCFM communicating with an auxiliary electronic device(s) (e.g., a computer). The performance data which is collected can represent a direct measurement of residual dispersion, e.g., at the various receivers, or can be data indicative of performance or characteristics which are related to the presence and/or magnitude of dispersion, such as error rates and the like.

However, despite the advantages of the TDCFM described above, such an apparatus can be costly to produce and/or purchase. Therefore, in some instances, use of a TDCFM(s) to compensate for dispersion within an optical network, particularly if there are several transmission paths within the network for which compensation is necessary, may be commercially impractical.

To overcome these cost issues without sacrificing the accuracy attainable with tuneable measuring apparatuses, in an exemplary embodiment of the present invention, a desired amount of dispersion compensation is provided to a transmission path by a relatively low-cost apparatus. However, the desired compensation value is determined with the aid of a relatively more-expensive tuneable measuring apparatus. An illustration of an embodiment of such a measuring apparatus being used to aid in the determination of the optimal compensation value for a transmission path is provided in FIGURE 5. hi the embodiment of FIGURE 5, in a manner similar to that which is depicted in FIGURE 4, a selected location along a transmission line 530 of an exemplary optical communication network (in one embodiment, a location towards the middle point between a transceiver of the network (e.g., transceiver 510) and a receiver of the network (e.g., receiver 520), and, in at least one embodiment, at some point proximate to receiver 520) is tapped and provided to a tuneable measuring apparatus, referred to for purposes of this disclosure as dispersion compensation finder (DC finder) 500, embodiments of which are depicted in FIGURES 6A-6D and 7. h at least some embodiments, DC fmder 500 is similar to the TDCFM described earlier. Moreover, in at least one embodiment, DC finder 500 measures and analyzes the signals propagating along the transmission line 530 toward receiver 520 to determine the optimal amount of dispersion compensation to be provided to the signals in order to recover the waveforms of the signals as originally transmitted. As shown in FIGURE 5, DC finder 500 may also be coupled to electronic device(s) 540 (e.g., a computer, an oscilloscope, an eye diagram, bit error measurement devices, etc., or some combination thereof) that may aid the DC finder in its measurement and analysis of the received signals.

FIGURES 6A-6D depict embodiments of a portion of the exterior components of DC finder 500. In the embodiment of FIGURE 6A, exterior components of the DC finder include an input connection 610, an output connection 620, and an indicator 630. at least some embodiments, such as the embodiment depicted in FIGURE 6B, exterior components of the DC fmder also includes an increase/decrease mechanism 640. Likewise, in other embodiments, the exterior components include a serial control port 650 and/or a parallel control port 660, preferably to provide flexibility in control (as is depicted in FIGURE 6C). In yet other embodiments, such as the embodiment of FIGURE 6D, exterior components of DC fmder 500 includes a port 690 preferably for connection to measuring/analyzing devices (e.g., a computer, an eye diagram, a bit error rate measurement device, etc., or combinations thereof). It will be appreciated that the exterior components of DC indicator 500 may include any combination or hybrid of the above embodiments. Furthermore, the external features described herein are by way of example only, as different external features may be used.

Indicator 630 depicted in FIGURES 6A-6D preferably includes a display such as a liquid crystal display (LCD), a touch screen display, or any other means by which the dispersion compensation currently provided by DC finder 500 may be displayed. The indicator 630 may depict any indication of the current compensation value, to include displaying a particular number or a number along with units of measurement. Most preferably, the display of indicator 630 also signals when a desired characteristic has been achieved. This desired characteristic may be an optimal compensation value, a desired dispersion value, a desired bit error rate, etc. Such signaling may be done by numerous means to include the appearance of the phrase "OPTIMAL," "OPT," "MAX, " or "PEAK" within the display (such as phrase 670 in FIGURES 6A and 6C). The above may also be accomplished through a component, such as an LED, emitting a particular color of light when the optimal, peak, or otherwise desired value is achieved (e.g., red or green) (such as LED 680 in FIGURE 6B). Moreover, the above need not be accomplished by a visual indicator. For instance, the event may be signified by an audible signal such as a beep. Moreover, the event may be signified by some combination of the above.

Moreover, in at least some embodiments, the increase/decrease mechanism 640 may be used to increase or decrease the amount of dispersion compensation being provided by DC fmder 500. In one embodiment, the increase/decrease mechanism may consist of two raised button structures, one for increasing the dispersion compensation, the other for decreasing the compensation. In another embodiment, rather than being separate structures, the buttons may appear as part of a touch screen display of the indicator 630. In yet another embodiment, rather than buttons, the increase/decrease mechanism may be a compensation dial which may be rotated one way or another to increase or decrease dispersion. In still yet another embodiment, rather than a dial, the increase/decrease mechanism maybe a switch which may be toggled in a particular direction to increase or decrease compensation. An embodiment of a portion of the interior components of DC finder 500 is depicted in FIGURE 7. this embodiment, a portion of the interior components of DC finder 500 is similar to that of the TDCFM depicted in FIGURE 1. As can be seen, a first path- determining component 713 receives an optical signal on a first optical fiber 718 coupled to the input connection 610 (not shown in FIGURE 7). The input optical signal can include a number of different multiplexed channels (such as a WDM signal) or be a single wavelength stream. In one embodiment, the path-determining component 713 determines whether (and/or how much of) the incoming signal is routed along a first DCF path 722a or a second DCF path 722b. In at least one embodiment, the two paths 722a and 722b are configured to provide different amounts of dispersion compensation, such as by providing different lengths of DCF, as depicted. The path-determining component 713 and two paths 722a and 722b combined make up a first stage 730.

Preferably, path-determining component 713 is a low-loss device and preferably creates substantially no splicing losses, hi one embodiment, the path-determining component 713 provides loss about equal to that of the fiber. Moreover, although it is possible to provide path-determining components which can be operated partially or fully in a manual fashion (such as by manipulation of the increase/decrease mechanism 640), in at least one embodiment, path-determining component 713 is fully or partially operable in response to a control signal (such as an electrical or optical control signal), e.g., using one or more relays or the like. In the embodiment of FIGURE 7, DC finder 500 also contains path-determining components 714b and 714c which receive signals from either (or both) of two input paths and determine whether (or how much of) the signals received are provided to their respective first paths 724a and 726a or second paths 724b and 726b. Like paths 722a and 722b, each of the paths 724a, 724b, 726a, and 726b have a particular compensation value associated with that path. Moreover, as was the case with paths 722a and 722b, in at least one embodiment, there is a difference between the compensation provided by path 724a and the compensation provided by path 724b. The same holds true for paths 726a and 726b. Furthermore, path- determining device 714b and paths 724a and 724b comprise second stage 731. Likewise, path-determining device 714c and paths 726a and 726b comprise third stage 732.

Like the TDCFM of FIGURE 1 , a potentially large range of compensation values may be provided by DC fmder 500. The compensation provided by a particular path (e.g., 722a) may be of almost any magnitude (including zero) and of either sign (i.e., "+" or "-"). Moreover, as mentioned, in at least one embodiment, there is a difference in compensation between the paths of a particular stage (e.g., 724a vs. 724b and 726a vs. 726b). Similarly, in at least one embodiment, the difference in compensation between the paths of a particular stage is different from that of the other stages (in other words, in at least one embodiment, the compensation difference between paths 724a and 724b is not the same as the difference between 726a and 726b) in order to provide a relatively large range of compensation values. Furthermore, the difference in compensation values between the paths of a particular stage is preferably chosen to provide a desired granularity to the compensation values provided by the overall device. As will be understood by those of skill in the art, additional stages, path-determining components, and/or path choices can be included to increase the range of dispersion compensation available or the granularity of the compensation values (however, fewer stages, etc., may be used as well). In addition, there may be more than two path choices for a path determining component, as well as a different number of path choices for each component. A number of devices can be used as path-determining components including those path-determining components mentioned earlier (e.g., those depicted in FIGURES 2 and 3A-

3C). Moreover, a number of types of DCF can be used in various embodiments of the device.

Preferably, DCF is selected to provide desired dispersion and slope, so as to compensate a plurality of WDM channels. For example, conventional DCF can be used to compensate standard single-mode fiber (SSMF) while non-conventional DCF can be used for compensating, e.g., non-zero dispersion shifted fibers (NZDSF).

After third stage 732 of FIGURE 7, a combining component 716 receives signals from either (or both) of last pair 726a, 726b and outputs an optical signal on output fiber 728 coupled to output connection 620. In some embodiments, attenuation elements (not shown) maybe provided, e.g., in the shorter DCF paths (e.g., 722b, 724b, and 726b in the embodiment of FIGURE 7) to balance the amount of attenuation among the various path choices. A number of devices or methods can be used for providing attenuation elements (e.g., an off-center splice or a high attenuation fiber, such as that available from INO, Inc.).

Furthermore, also in at least some embodiments, DC finder 500 includes control circuitry (not shown) which provides control signals to the path-determining components guiding the path-determining components with respect to the apportionment of the signals among the various path choices. However, such control circuitry may be internal to the path- determining components. At least in some embodiments control signals maybe provided in response to manual input (e.g., manipulation of increase/decrease mechanism 640). In other embodiments, generation of the control signals is done autonomously by the control circuitry or some other component of DC finder 500 (e.g., the path determining component(s) in embodiments where the control circuitry is not internal thereto). In still other embodiments, rather the being provided by control circuitry internal to DC finder 500, the control signals are provided by electronic devices (e.g., computers) coupled to DC fmder 500, e.g., via ports 650 and 660. h at least some embodiments, the control signals are provided by some combination of the above. Preferably, the control circuitry also provides control signals to indicator 630 regarding the current dispersion compensation value.

In at least one embodiment, the above discussed control signals are provided in response to the collection, processing, and/or analysis of data indicative of signal performance. Such collection, processing, analysis and/or generation of a control signal(s) maybe performed by a component(s) of DC finder 500. For example, the above tasks maybe accomplished by the path determining component(s), some other element of DC finder 500 (e.g., control circuitry external to the path determining component(s)), or some combination thereof, at least one embodiment, at least a portion of the above tasks are performed by a sensor(s). Such a sensor(s) may be part of the path determining component(s) itself, some other element of DC fmder 500, or some combination thereof. The performance data which is collected, processed, analyzed, etc., may represent such things as a direct measurement of residual dispersion, e.g., at the path-determining components or at the various receivers, or can be data indicative of signal performance or signal characteristics which are related to the presence and/or magnitude of dispersion, such as error rates and the like. In at least some embodiments, such collection, processing, analysis, and/or generation of control signals is performed, at least in part, by electronic devices coupled to DC fmder 500, e.g., via port 690 (e.g., computers, eye diagram devices, bit error rate measurement devices, oscilloscopes, etc.). In addition, at least a portion of the sensor(s) discussed above, may be part of electronic devices coupled to DC fmder 500.

Furthermore, in at least one embodiment, DC finder 500 includes internal memory (not shown) for storing dispersion or compensation values and/or other signal data. For example, in one embodiment, DC fmder 500 includes a means by which the original shape (or form) of the transmitted signal(s) or the desired shape or other desired characteristics for the signal(s) may be inputted and stored in the internal memory. The control circuitry may then use the stored shape and or characteristic data in its determination of the appropriate compensation path for the input signals. The electronic device(s) that may be coupled to DC fmder 500 in some embodiments of the present invention (e.g., computers, eye diagram devices, bit error rate detectors, etc.) preferably are able to aid in the measurement and or analysis of the dispersion on the signal(s), other characteristics of the signal(s), and/or the compensation provided by DC fmder 500. As part of such, in some embodiments, these electronic devices and/or computers may also display a characteristic of the signal, such as the dispersion on the signal, or the compensation provided by DC finder 500. Also, in some embodiments, these devices and/or computers may provide control signals, as discussed earlier, to DC finder 500. For purposes of this invention, a computer may be any suitable processor-based device to include a PC, laptop, personal digital assistant, etc. In at least one embodiment, DC fmder 500 determines the desired compensation value for a particular transmission path (e.g., transmission path 530 in FIGURE 5) by adjusting the magnitude and/or the sign of the dispersion compensation provided by DC fmder 500 until a desired characteristic is achieved. Such adjustment of magnitude and/or sign is accomplished by altering the configuration of at least one path determining component of DC finder 500. This desired characteristic may include a desired eye diagram shape, a desired bit error rate, or the achievement of the original signal waveform or other desired wave shape. In at least one embodiment, detection of the achievement of the desired characteristic(s) may done by path determining components, by electronic devices (such as computers) coupled to DC finder 500, e.g., via port 670, by a user of DC fmder 500, by control circuitry, or some combination thereof. hi one embodiment, DC finder 500 can provide full or partial, manual or autonomous, adjustment of the compensation provided. Furthermore, in at least one embodiment, the adjustment in the dispersion compensation provided can be performed autonomously by control circuitry, path determining components, electronic devices (such as computers) coupled to DC finder 500, or some combination thereof. For example, control circuitry may provide control signals to the path deteraiining components instructing the path determining components to apportion a signal among the various path choices in a manner that adjusts the current value of the dispersion compensation provided by some increment.

In an embodiment where DC fmder 500 includes an increase/decrease mechanism 640, a user may manually adjust the compensation provided by DC finder 500 by manipulating the mechanism 640 to alter the compensation value. For example, in one such embodiment, a user increases the compensation provided through manipulation of mechanism 640 until the display of an eye diagram device coupled to DC finder 500 depicts the desired diagram shape.

As mentioned, in at least one embodiment, indicator 630 of DC finder 500 displays the current value of the dispersion compensation provided by DC fmder 500. Also as mentioned, in at least one embodiment, indicator 630 signals when a desired characteristic, such as an optimal dispersion compensation value, has been achieved or measured, i at least one embodiment, when the desired characteristic has been achieved or measured, the compensation value at which the desired characteristic was achieved, etc., is captured by DC finder 500. This may be done, for example, by freezing or locking the display screen of indicator 630 so as to preserve the compensation value at which the desired characteristic was achieved, or by locking the path determining components into the state at which the desired characteristic was achieved, such embodiments, the display or the path-determining components may be unlocked by the initiation of an override operation. In some embodiments, the compensation value is recorded within internal memory of DC finder 500. Once the dispersion compensation value that allows for the desired dispersion characteristic has been determined, a second apparatus, embodiments of which are depicted in FIGURES 8A-8C, 9, and 10, is configured to provide this dispersion compensation value preferably determined, in at least in part, using DC fmder 500. The second apparatus, for purposes of this disclosure, is referred to as a dispersion compensation fiber module (DCFM) (also referred to as a connecting compensation apparatus). In at least one embodiment, after the DC finder has served its purpose in the determination of the desired compensation value, the DC finder is removed from the transmission system. Then, in one embodiment, the DCFM, already having been configured to provide the desired compensation value, is inserted in its place (including all necessary connections). For example, in the embodiment of FIGURE 5, DC fmder 500 would be removed from the network (along with additional electronic devices 540) and a pre-configured DCFM would be inserted therefor. However, in an alternative embodiment, the DCFM may be configured after being inserted in place of the DC finder. An embodiment of a portion of exterior components of a DCFM 800 is shown in FIGURE 8A. In this embodiment, the portion of the exterior of DCFM 800 includes an input connection 810 and an output connection 820. In another embodiment, shown in FIGURE 8B, the exterior portion also includes exterior switches, button, keys, triggers and/or other similar devices S_t to S_n, each of which corresponds to one of the interior (in at least one embodiment, reconfigurable) path connectors, discussed further below. In such an instance, the interior path connectors are configured or set through the manipulation of the exterior switches SI to Sn to route the transmission signals along particular compensation paths within the module which provide for the desired dispersion compensation. In yet another embodiment, shown in FIGURE 8C, instead of exterior triggers, etc., the DCFM includes a port 830 whereby control signals from an external electronic device may be relayed to the path connectors, the path connectors being set in response to such control signals, hi one embodiment, the exterior of DCFM 800 does not include an equivalent of indicator 630.

An embodiment of a portion of the interior components of DCFM 800 is shown in FIGURE 9. As can be seen, the depicted portion of the interior of DCFM 800 shares some similarities to that of the portion of the interior of DC finder 500 depicted in FIGURE 7. For instance, similar to FIGURE 7, an optical signal is received on a first optical fiber 918 coupled to an input connection 810 (not shown in FIGURE 9). Moreover, the input optical signal can include a number of different multiplexed channels (such as a WDM signal) or be a single information stream. hi addition, the portion of the interior of DCFM depicted in FIGURE 9 comprises a first stage 930 which includes two path choices 922a and 922b, the two paths 922a and 922b, in at least one embodiment, being configured so as to provide different amounts of dispersion compensation, such as by providing different lengths of DCF. Furthermore, DCFM of FIGURE 9 also includes a second stage 931, which includes a first DCF path 924a and a second DCF path 924b. Like the first stage 930, the two paths 924a and 924b are, in at least one embodiment, configured to provide different amounts of dispersion compensation. Likewise, third stage 932 includes a first DCF path 926a and a second DCF path 926b, the two paths, in at least one embodiment, being configured to provide different amounts of dispersion compensation as well. Moreover, also in at least one embodiment, the difference in compensation between the paths of a particular stage is different from that of the other stages (e.g., the compensation difference between 924a and 924b is not the same as the difference between 926a and 926b) in order to provide a relatively large range of compensation values.

A number of types of DCF can be used in various embodiments of the device. Preferably, DCF is selected to provide desired dispersion and slope, so as to compensate a plurality of WDM channels. For example, conventional DCF can be used to compensate standard single-mode fiber (SSMF) while non-conventional DCF can be used for compensating, e.g., non-zero dispersion shifted fibers (NZDSF).

Also similar to FIGURE 7, after third stage 932 of FIGURE 9, a combining component 916 receives signals from either (or both) of compensation paths 926a, 926b and outputs an optical signal on the output fiber 928. This optical signal then propagates towards output connection 820.

However, at least one notable difference between the structure depicted in FIGURE 7 and the structure illustrated in FIGURE 9, is that in FIGURE 9, rather than including path- determining components 713, 714b, and 714c, DCFM 800 of FIGURE 9, instead, includes path connectors 913, 914b, and 914c. Path connectors 913, 914b, and 914c (each represented by three dots in FIGURE 9) may be any means by which two path choices may be (in at least embodiment, reconfigurably) connected to each other (e.g., fiber couplings, switches, etc.). In at least one embodiment, path connectors 913, 914b, and 914c are less intricate and/or less costly than path-determining components 713, 714b, and 714c. For instance, in one embodiment, path connectors 913, 914b, and 914c do not possess the sensing and/or intelligence capabilities which some embodiments of the path-determining components of FIGURE 7 possess. Instead, in one embodiment, path connectors 913, 914b, and 914c are only capable of (in at least one embodiment, reconfigurably) connecting certain dispersion compensation paths of DCFM 800 together. However, like path-determining components 713, 714b, and 714c, path connectors 913, 914b, and 914c preferably are low loss. h at least one embodiment, path connectors 913, 914b, and 914c are configured , connected, set, etc., such that signals received by DCFM 800 are routed along those compensation paths within DCFM 800 that provide for the desired compensation value determined, at least in part, using the DC finder. Such configuration of the path connectors may be accomplished manually or in response to control signals provided by an external device (e.g., an external electronic or optical device which, when connected to the DCFM, relays control signals to the path connectors, the connections within the path connectors being set in response to the signals), h at least one embodiment, the path connectors remain in this configuration until they are reconfigured manually or, in some embodiments, in response to a control signal (e.g., from an earlier mentioned external electronic or optical device).

As will be understood by those of skill in the art, additional stages, path connectors and/or path choices can be included in DCFM 800 to increase the range of dispersion compensation available (however, fewer stages, path connectors and or path choices maybe used as well). Moreover, additional (in at least one embodiment, reconfigurable) path connectors and/or path choices may also be included in DCFM 800 to tailor the granularity between the overall compensation values provide by the module. In addition, there may be more than two connection options for a single path connector, as well as a different number of connection options for each path connector within the DCFM. hi addition, in some embodiments of DCFM 800 of FIGURE 9, attenuation elements (not shown) may be provided in the shorter DCF paths (e.g., 922b, 924b, and 926b in the embodiment of FIGURE 9) to balance the amount of attenuation among the various path choices. A number of devices or methods can be used for providing attenuation elements

(e.g., an off-center splice or a high attenuation fiber, such as that available from INO, Inc.).

As mentioned, in at least one embodiment, DCFM 800 does not include any internal sensing, intelligence, or memory circuitry as is found in some embodiments of the DC finder.

However, regardless of the configuration of it's interior and exterior portions, DCFM 800 is, in at least some embodiments, less complex and less costly than the DC finder that is (at least partially) used to determine the desired compensation value which the DCFM is to be configured to provide.

An example of the configuring of DCFM 800 of the embodiment of FIGURE 9 to provide an optimal compensation value is provided below. Assume that first path 922a of DCFM 800 provides a first unit or amount of dispersion compensation (e.g., 10 ps/nm) and second path 922b provides a compensation that is equal in magnitude, but opposite in sign, when compared to that of the first path (i.e, -10 ps/nm). Moreover, assume that in second stage 931, loop 924a has a compensation value twice that of path 922a (that being 20 ps/nm) and path 924b provides a compensation value twice that of path 922b (that being -20 ps/nm). Furthermore, in third stage 932, assume that path 926a provides a dispersion compensation four times that of path 922a (that being 40 ps/nm) and path 926b provides a dispersion compensation four times that of path 922b (that being -40 ps/nm). As seen in Table II, in this fashion, depending upon how path connectors 913, 914b, and 914c are configured, it is possible for the DCFM to provide increments of 20 ps/nm dispersion compensation between 70 and -70 ps/nm.

TABLE II

hi this example, it was previously determined, at least in part, using a DC finder that the optimal dispersion compensation for a particular transmission line is 30 ps/nm. As can be seen from the table above, in order to provide this compensation value, DCFM 800 should be configured such that the first path connector 913 connects line 918 to path 922a. Moreover, the second path connector 914b should connect path 922a to path 924b. Finally, third path connector 914c should connect path 924b to 926a. FIGURE 10 depicts DCFM 800 of FIGURE 9 configured in this manner. Although in FIGURE 10, each of the path connectors are shown as com ecting a first path to a second path, the path connectors of the present invention may connect a first path to a plurality of other paths.

The methods and structures described herein overcome some of the cost issues associated with other methods and apparatuses by providing a relatively lower cost means to provide dispersion compensation without sacrificing the accuracy and precision associated with the relatively more expensive and complex tuneable compensation apparatus, such as the TDCFM.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiment of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or alter to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein maybe utilized according to steps.

Claims

WHAT IS CLAIMED IS:

1. A method for providing dispersion compensation in an optical communications system, the method comprising: determining a desired dispersion compensation value for an optical propagation path using, at least in part, a tuneable compensation apparatus; and inserting into the propagation path a connecting compensation apparatus, said connecting compensation apparatus configured to provide the desired dispersion compensation value determined using, at least in part, said tuneable compensation apparatus.

2. The method of claim 1, wherein said tunable compensation apparatus comprises at least a first path-determining component which is capable of receiving a first optical signal from said transmission path and which is tunable between at least first and second configurations, each of said at least first and second configurations providing a particular dispersion compensation to said first optical signal, said path-determining component, when in said first configuration, providing at least a first portion of said first signal to a first compensation path, and said path-determining component, when in said second configuration, providing said at least first portion of said first signal to a second compensation path.

3. The method of claim 2, wherein at least one of said first path and said second path includes dispersion compensation fiber.

4. The method of claim 3, wherein said dispersion compensation fiber has a loss and wherein said first path-determining component has an insertion loss not substantially greater than the dispersion compensation fiber.

5. The method of claim 2, wherein said first path-determining component, first compensation path and second compensation path comprise a first stage of said tuneable compensation apparatus and wherein said tuneable compensation apparatus further comprises a second stage having a second path-determining device coupled to a third compensation path and a fourth compensation path.

6. The method of claim 5, wherein said tuneable compensation apparatus includes more than two stages.

7. The method of claim 2, wherein said determining of said desired compensation value includes altering the configuration of said first path-determining component so as to adjust the compensation provided by said tuneable compensation apparatus to said first optical signal until a desired signal characteristic is achieved.

8. The method of claim 7, wherein said desired signal characteristic comprises a desired bit error rate.

9. The method of claim 7, wherein said desired signal characteristic comprises substantially zero dispersion on said first optical signal.

10. The method of claim 7, wherein said configuration of said first-path determining component is manually altered.

11. The method of claim 7, wherein said configuration of said first-path determining component is altered in response to a control signal.

12. The method of claim 11, wherein said tuneable compensation apparatus further comprises a sensor which senses at least a first parameter of at least a portion of said optical communication system and outputs said control signal in response.

13. The method of claim 12, wherein said at least a first parameter is related to residual dispersion.

14. The method of claim 7, wherein said tuneable compensation apparatus displays the current amount of compensation being provided to said first optical signal.

15. The method of claim 7, wherein said tuneable compensation apparatus signals when said desired signal characteristic is achieved.

16. The method of claim 7, wherein said tuneable compensation apparatus further comprises memory operable to store data indicating the amount of compensation provided by said tuneable compensation apparatus when said desired signal characteristic is achieved.

17. The method of claim 7, wherein said tuneable compensation apparatus further comprises memory operable to store at least one signal characteristic of said optical signal as transmitted, said stored at least one signal characteristic being used in determining said desired compensation value.

18. The method of claim 1, wherein said tuneable compensation apparatus is coupled to at least one electronic device used to determine said desired compensation value.

19. The method of claim 1 wherein said at least one electronic device is selected from the group consisting of an eye diagram device and a bit error rate detector.

20. The method of claim 1, wherein said connecting compensation apparatus comprises at least a first path connector configured to route an optical signal from at least a first input path to at least one of a plurality of output compensation paths.

21. The method of claim 20, wherein the plurality of output compensation paths comprise a first compensation path having a first dispersion compensation value and a second compensation path having a second dispersion compensation value.

22. The method of claim 21, wherein at least one of said at least a first input path, said first output compensation path, and said second output compensation path includes dispersion compensation fiber.

23. The method of claim 21 , wherein attenuation in said first output compensation path is substantially equal to attenuation in said second output compensation path.

24. The method of claim 21, wherein said second output compensation path provides dispersion compensation that is opposite in sign to the dispersion compensation provided by said first compensation path.

25. The method of claim 21 , wherein said second dispersion compensation value is substantially zero.

26. The method of claim 20, wherein said first path connector and the plurality of output compensation paths comprise a first stage of said connecting compensation apparatus, and wherein said connecting compensation apparatus further comprises a second stage comprising a second path connector configured to couple at least one of said output compensation paths of the first stage to at least one of a plurality of output compensation paths for the second stage.

27. The method of claim 26, wherein said first stage may provide an amount of dispersion compensation within a first group of compensation values and said second stage may provide an amount of dispersion compensation within a second group of compensation values, said first group of compensation values being different from said second group of compensation values.

28 The method of claim 26, wherein said comiecting compensation apparatus includes more than two stages.

29. The method of claim 20, wherein said first path connector may be reconfigured.

30. The method of claim 1, wherein said optical communication system is a wavelength division multiplexing (WDM) communication system.

31. A dispersion compensation apparatus for providing a desired compensation value in an optical communication system, the apparatus comprising: at least a first path connector which may be configured so as to connect at least a first input path to at least one of a plurality of output compensation paths; wherein said desired compensation value is determined, at least in part, using a tuneable compensation apparatus.

32. The apparatus of claim 31 , wherein the plurality of output compensation paths comprise a first compensation path having a first dispersion compensation value and a second compensation path having a second dispersion compensation value.

33. The dispersion compensation apparatus of claim 32, wherein at least one of said first output compensation path and said second output compensation path includes dispersion compensation fiber.

34. The dispersion compensation apparatus of claim 32, wherein attenuation in said first output compensation path is substantially equal to attenuation in said second output compensation path.

35. The dispersion compensation apparatus of claim 32, wherein said second output compensation path provides dispersion compensation that is opposite in sign to the dispersion compensation provided by said first output compensation path.

36. The dispersion compensation apparatus of claim 32, wherein the dispersion compensation provided by said second output compensation path is substantially zero.

37. The dispersion compensation apparatus of claim of claim 31, wherein said optical communication system includes a plurality of channels.

38. The dispersion compensation apparatus of claim 37, wherein said dispersion compensation apparatus provides dispersion compensation for substantially all of said plurality of channels.

39. The dispersion compensation apparatus of claim 37, wherein said tuneable dispersion compensation module provides compensation for only one of said channels.

40. The dispersion compensation apparatus of claim 31, wherein said first path connector and all output compensation paths which may be directly connected thereto comprise a first stage of said dispersion compensation apparatus, and wherein said dispersion compensation apparatus further comprises a second stage comprising a second path connector capable of connecting at least one of said output compensation paths of said first stage to at least one of a plurality of output compensation paths for the second stage.

41. The dispersion compensation apparatus of claim 40, wherein said first stage may provide an amount of dispersion compensation within a first group of compensation values and said second stage may provide an amount of dispersion compensation within a second group of compensation values, said first group of compensation values being different from said second group of compensation values.

42. The dispersion compensation apparatus of claim 40, wherein said dispersion compensation apparatus includes more than two stages.

43. The dispersion compensation apparatus of claim 31, wherein said connections made by said at least a first path connector may be reconfigured.

44. The dispersion compensation apparatus of claim 31 , wherein said tuneable compensation apparatus comprises: at least a first path-determining component which is capable of receiving at least a first optical signal from said transmission path and which is tunable between at least first and second configurations, each of said at least first and second configurations providing a particular dispersion compensation to said at least a first optical signal, said path-determining component, when in said first configuration, providing at least a first portion of said first optical signal to a first compensation path and said path-determining component, when in said second configuration, providing said at least first portion of said first optical signal to a second compensation path.

45. A connecting compensation apparatus for providing a desired dispersion compensation value to an optical communication system comprising: a first connector means for connecting at least a first input path means to at least one of a plurality of output compensation path means; wherein said desired compensation value is determined, at least in part, using a tuneable compensation apparatus.

46. The apparatus of claim 45, wherein the plurality of output compensation path means comprise a first compensation path means having a first dispersion compensation value and a second compensation path means having a second dispersion compensation value.

47. The apparatus of claim 46, wherein said second output compensation path means provides dispersion compensation that is opposite in sign to the dispersion compensation provided by said first output compensation path means.

48. The apparatus of claim 46, wherein the dispersion compensation provided by said second output compensation path means is substantially zero.

49. The apparatus of claim 46, wherein attenuation in said first output compensation path is substantially equal to attenuation in said second output compensation path.

50. The apparatus of claim 45, wherein said first connector means and all output compensation path means which may be directly connected thereto comprise a first stage of said dispersion compensation apparatus, and wherein said dispersion compensation apparatus further comprises a second stage comprising a second connector means capable of connecting at least one of said output compensation path means of said first stage to at least one of a plurality of output compensation path means for the second stage.

51. The apparatus of claim 50, wherein said first stage may provide an amount of dispersion compensation within a first group of compensation values and said second stage may provide an amount of dispersion compensation within a second group of compensation values, said first group of compensation values being different from said second group of compensation values.

52. The apparatus of claim 50, wherein said connectable compensation apparatus includes more than two stages.

53. The apparatus of claim 45, wherein said first connector means is reconfigurable.

54. The apparatus of claim 44, wherein said tuneable compensation apparatus comprises: a first optical path means having a first dispersion; a second optical path means having a second dispersion; a third means for receiving a first optical signal and providing at least a first portion of said first optical signal to one of said first path and said second path means; and a fourth means for receiving said at least a first portion of said first optical signal which has traveled along at least one of said first path and said second path and outputting an optical signal which includes said at least first portion of said first signal.

55. An optical communications system comprising: at least one transmitting device; at least one receiving device; a transmission path coupling at least one of said at least one transmitting device to at least one of said at least one receiving device; and a connecting compensation apparatus coupled to said transmission path to provide a desired dispersion compensation value, said desired compensation value having been determined, at least in part, using a tuneable compensation apparatus, and said connecting compensation apparatus comprising: at least a first path connector which may be configured to connect at least a first input path to at least one of a plurality of output compensation paths.

56. Dispersion compensation apparatus for use in a digital optical communication system comprising: at least a first path-determining component which receives at least a first digital optical signal and which is configurable between at least first and second configurations, said path-determining component, when in said first configuration, providing at least a first portion of said first signal to a first optical path and said path-determining component, when in said second configuration, providing said at least first portion of said first signal to a second path; and wherein said first path provides dispersion compensation having a first value and said second path provides dispersion compensation having a second value, different from said first value.

57. Apparatus, as claimed in Claim 56, wherein said path determining component is formed on said dispersion compensation fiber.

58. Apparatus, as claimed in Claim 56, wherein said path determining component is manually configurable between said first configuration and said second configuration.

59. Apparatus, as claimed in Claim 56, wherein said path-determining component is configurable between said first configuration and said second configuration in response to a control signal.

60. Apparatus, as claimed in Claim 59, further comprising a sensor which senses at least a first parameter of at least a portion of said optical communication system and outputs said control signal in response.

61. Apparatus, as claimed in Claim 60, wherein said first parameter is related to residual dispersion.

62. Apparatus, as claimed in Claim 56, wherein at least one of said first path and said second path includes dispersion compensation fiber.

63. Apparatus, as claimed in Claim 62, wherein said dispersion compensation fiber has a loss and wherein said path-determining component has an insertion loss not substantially greater than the loss of dispersion compensation fiber.

64. Apparatus, as claimed in Claim 56, wherein attenuation in said first path is substantially equal to attenuation in said second path.

65. Apparatus, as claimed in Claim 56, wherein said path-determining component, first path and second path comprise a first stage of said apparatus and wherein said apparatus further comprises a second stage having a second path-determining device coupled to a third path and a fourth path.

66. Apparatus, as claimed in Claim 65, wherein said first stage can be configured to provide a first magnitude of dispersion compensation and said second stage can be configured to provide a second magnitude of dispersion compensation, different from said first magnitude.

67. Apparatus, as claimed in Claim 65, wherein said apparatus includes more than two stages.

68. Apparatus, as claimed in Claim 56 wherein said second value of dispersion compensation is opposite in sign to said first value of dispersion compensation.

69. Apparatus, as claimed in Claim 56, wherein said second value of dispersion compensation is substantially zero.

70. Apparatus, as claimed in Claim 56, wherein said optical communication system includes a plurality of channels.

71. Apparatus, as claimed in Claim 70, wherein said tuneable dispersion compensation module provides dispersion compensation for substantially all of said plurality of channels.

72. Apparatus, as claimed in Claim 70, wherein said tuneable dispersion compensation module provides compensation for only one of said channels.

73. Apparatus, as claimed in Claim 56, wherein said path-determining component comprises a half-blocked fiber coupler.

74. Apparatus, as claimed in Claim 56, wherein said path-determining component comprises a microbanded component.