Abstract:
A stacked waveplate device that performs an optical wavelength filtering function is described which provides dispersion with a first magnitude and a first sign for a first optical path having a first output polarization and which provides a second dispersion with a substantially equal but oppositely-signed dispersion for a second optical path defining an output having an orthogonal polarization to the polarization of said first output path. Optical paths are configured to pass through first and second stacked waveplate devices sequentially with the optical dispersion of said second device having an approximately equal magnitude but opposite sign compared to the optical dispersion of the first optical stacked waveplate devices so as to provide canceling or compensation of optical dispersion. A device is configured to use cancellation or compensation of dispersion in sequential stacked waveplate devices to provide outputs with characteristics similar to outputs of previous stacked waveplate devices but with substantially reduced dispersion characteristics.

Description:
The present invention relates to a method and apparatus for compensating chromatic dispersion and, in particular, a method and apparatus introducing positive and negative dispersion in a fashion to cancel out total dispersion in wavelength selective optical devices. 
     BACKGROUND INFORMATION 
     Systems which use optical components, exclusively or partially, for communicating information (typically digitally), switching, routing, transmitting and the like, generally provide certain advantages over, e.g. fully-electronic networks (e.g. providing typically higher data rates, requiring less physical space, less susceptibility to electromagnetic interference, and the like) but also present their own set of issues. These issues include signal loss and signal dispersion, each of which can occur either during transmission along optical fiber cables (or other transmission lines) or in discrete equipment or components such as optical routers, switches, hubs, bridges, multiplexers and the like. Certain types of components, such as erbium doped fiber amplifiers (EDFA) can provide sufficient amplification to overcome some or all transmission line losses, thus providing a system in which the limiting factor tends to be dispersion. 
     In general, dispersion refers to change or degradation of the wave shape of an optical signal, such as an (ideally) square-edged pulse. In general, the fact that different wavelengths have different effective rates of transmission along an optical transmission line and/or different indices of refraction and reflection can lead to pulse (or other signal) degradation, e.g. such that an original signal comprising a sequential plurality of square-edged pulses will, as a result of so called chromatic dispersion be changed such that each pulse, rather than retaining a substantially square-edged shape will have a more rounded, Gaussian shape. Dispersion can lead to, e.g. partial overlap between successive pulses resulting in signal detection problems such as high bit error rates, decrease in spectral efficiency or other problems, especially when combined with signal loss (amplitude reduction). Accordingly, it would be useful to provide a method and apparatus for use in optical systems, which can compensate for and/or reduce the amount of dispersion effect. 
     The dispersion problems become even more severe for wavelength division multiplexing (WDM) systems. The dense wavelength-division multiplexing (DWDM) scheme is widely adapted as one of the optimal solution to improve the bandwidth usage on optical fibers. By multiplexing multiple signals on different optical wavelengths, bandwidth of a single fiber can be multiple folded. Key optical components in DWDM systems include those which perform wavelength combining (multiplexing) and separating (demultiplexing) functions. The spectral response of the multiplexers and demultiplexers for DWDM applications are generally accompanied by certain dispersion effects that are determined by the underlying filtering technology. For example, the dispersion characteristic of a fiber Bragg grating can be determined by Hilbert transforming its transmission spectral response (e.g. as generally described in “Dispersion Properties of Optical Filters for WDM Systems” G. Lenz, B. J. Eggleton, C. R. Giles, C. K. Madsen, and R. E. Slusher, IEEE Journal of Quantum Electronics, Vol. 34, No 8 Page 1390-1402). The dispersion effects of wavelength multiplexing and filtering are very different from those of optical fibers. Optical fiber generally shows a linear dependency of its dispersion characteristic versus wavelength. Wavelength filters, multiplexers and demultiplexers, on the other hands, generally show nonlinear dispersion properties, e.g. correlated to its amplitude (spectral) response within its passband window. Although the accumulated dispersion due to fiber span can be compensated by different methods, such as dispersion compensating fibers or dispersion compensating fiber chirped gratings, dispersions caused by multiplexers/demultiplexers are difficult to compensate by conventional approaches. At least in narrow wavelength channel spacing DWDM systems that carry high data-rate information, it would be advantageous to provide dispersion filters, multiplexers, and demultiplexers that introduce minimum dispersion onto the signals. 
     SUMMARY OF THE INVENTION 
     The present invention includes a recognition of the existence, nature and/or source of certain problems in previous approaches, including as described herein. In one embodiment, the present invention involves the recognition that the chromatic dispersion occurring in a propagation path where polarization is intact or unchanged is substantially opposite to the dispersion along a similar propagation path but in which polarization is changed. According to one embodiment, the multi-stage or multi-component device is configured such that dispersion introduced at two different stacked waveplate filters along the optical path substantially cancel one another out, such as by introducing roughly equal amounts of positive and negative dispersion. In this context, dispersion values are approximately equal in magnitude if the difference in magnitude is sufficiently small that, upon combining oppositely-signed signals the resultant signal has a dispersion, in at least a first wavelength band of interest (such as a 90-95% transmission wavelength band) which is sufficiently low to achieve desired signal dispersion goals such as being less than about 10 ps, preferably less than 5 ps, more preferably less than about 3 ps and even more preferably less than about 2 ps. In one embodiment, chromatic dispersion which would otherwise be caused by stacked waveplate filters is (at least partially) canceled or compensated by the manner of arranging the optical signal propagation path. 
     A stacked waveplate device is described which provides dispersion with a first magnitude and a first sign for a first optical path having a first output polarization and which provides a second dispersion with a substantially equal but oppositely-signed dispersion for a second optical path defining an output having an orthogonal polarization to the polarization of said first output path. Optical paths are configured to pass through first and second stacked waveplate devices sequentially with the optical dispersion of said second device having an approximately equal magnitude but opposite sign compared to the optical dispersion of the first optical stacked waveplate device so as to provide canceling or compensation of optical dispersion. A device is configured to use cancellation or compensation of dispersion in sequential stacked waveplate devices to provide outputs with characteristics similar to outputs of previous stacked waveplate devices but with substantially reduced dispersion characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram illustrating a stacked waveplate filter and an optical propagation path; 
     FIG. 2 is a graph illustrating simulated or modeled transmission and dispersion (group delay) characteristics of an optical propagation path with unchanged polarization; 
     FIG. 3 is a graph illustrating simulated or modeled transmission and dispersion characteristics of an optical propagation path with changed polarization; 
     FIG. 4 is a simplified block diagram illustrating an optical system according to an embodiment of the present invention. 
     FIG. 5 is a partial block diagram illustrating the phenomenon that some undesirable optical signals will not interfere with the desired optical signals at the output of an optical system as illustrated in FIG.  4 . 
     FIG. 6 is a partial block diagram illustrating the phenomenon that some undesirable optical signals will not interfere with the desired optical signals at the output of an optical system as illustrated in FIG.  4 . 
     FIG. 7 is a graph illustrating simulated or modeled transmission and group delay characteristics of a first optical propagation path of FIG. 4; 
     FIG. 8 is a graph illustrating simulated or modeled transmission and group delay characteristics of a second optical propagation path of FIG. 4; 
     FIG. 9 is a graph depicting transmission and group delay of a stacked waveplate filter without using the present invention; 
     FIG. 10 is a graph depicting transmission and group delay of a stacked waveplate filter system according to an embodiment of the present invention; and 
     FIG. 11 is a simplified block diagram illustrating an optical system in according to an embodiment of the present invention 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As depicted in FIG. 1, a stacked waveplate filter  1100  is made up of a plurality of substantially aligned individual waveplates  1106   a, b, c.  In one embodiment each waveplate is formed of a birefringent crystal, as will be understood by those of skill in the art. Fast and slow axes  1107   a, b, c    1108   a, b, c,  for each waveplate, are illustrated. In the illustrated example, an optical propagation path  1109  passes successively through the waveplates  1106   a, b, c.  The input,  1110 , which enters the first waveplate  1106   a  has, in this example, vertical polarization, denoted by a vertical arrow  1110   a.  After passing through the stacked waveplate filter  1100 , the input signal,  1110 , is decomposed into two components with different polarizations. One component,  1112 , has vertical polarization represented by a vertical arrow, and the other component,  1114 , has horizontal polarization, orthogonal to the first polarization, represented by a horizontal arrow. The configuration illustrated in FIG. 1 substantially provides two optical paths. The first optical path begins with the input  1110  having vertical polarization and output  1112  having vertical polarization (designated the Vertical-Vertical path). The second path has input  1110  with vertical polarization and output  1114  with horizontal polarization (designated the Vertical-Horizontal path). FIGS. 2 and 3 illustrate (modeled or simulated) transmission ( 1208 ) and group delay ( 1210 ) for the Vertical-Vertical path (FIG. 2) and for Vertical-Horizontal path (FIG.  3 ). FIGS. 2 and 3 illustrate that the stacked waveplate of FIG. 1 operates as a periodic optical filter, i.e. providing output defining a transmission curve which (as a function of wavelength) is periodic. Comparison of FIGS. 2 and 3 illustrates that chromatic dispersion characteristics within each transmission band ( 1212   a, b, c  for FIG. 2 and 1312 a, b  for FIG. 3) have substantially opposite shapes (i.e. have group delay peaks in the middle of transmission bands for FIG.  2  and have group delay troughs in the middle of the transmission bands for FIG.  3 ), i.e. that the periodicity of the two outputs from the stacked waveplates (respectively illustrated in FIGS.  2  and three) is complementary. In FIGS. 2 and 3, dispersion is expressed as group delay in units of picoseconds (ps). Dispersion is often characterized as picoseconds/km-nm. Transmission is shown in FIG.  3  and FIG. 2 as transmission loss ratio expressed in decibels (dB) (as a function of wavelength expressed in nanometers (nm)). One embodiment of the present invention involves the recognition of the opposite or inverted nature of the dispersion characteristics for these two different optical paths and/or a manner in which these features of the dispersion characteristics can be used to compensate the chromatic dispersion (i.e. to substantially reduce or eliminate chromatic dispersion) e.g. caused by a stacked waveplate filter. 
     FIG. 4 illustrates a simplified diagram of a double stage stacked-waveplate optical system according to one embodiment of the present invention. Many double stage stacked-waveplate systems such as described in U.S. Pat. Nos. 5,694,233 and 5,912,748 (incorporated herein by reference) are used e.g. to generate more desirable spectra or to provide better performance. However, without careful arrangement of the optical paths of these double stage systems, the resulting chromatic dispersion might be significant. The system presented in FIG. 4 shows that, with present invention, the resulting chromatic dispersion can be significantly reduced or compensated. 
     In the optical system described in FIG. 4, an incoming signal  400  passes through an optical fiber  800  and a collimator  810  to enter the system. The input optical signal  400  is then decomposed by a beam displacer  10  into two components: signal  410  with horizontal polarization (represented in FIG. 4 by a dot) and signal  420  with vertical polarization (represented in FIG. 4 by a vertical line). After passing though the beam displacer  10 , component  410  passes through a half-wave plate  20  so that its polarization is changed to vertical, the resulting signal being designated as  430 . Although optical signals  430  and  420  have the same polarization, they are spatially separated. Optical signals  430  and  420  then pass through a stacked waveplate filter  30  made up of a plurality of substantially aligned individual waveplates  30   a,    30   b,  and  30   c.  In one preferred embodiment, the stacked waveplate filter provides temperature compensation (reduces excursions from desired performance caused by changes in component temperature) such as by selecting two or more waveplates or waveplate components with thermal performance which cancel one another out, e.g. as described in U.S. patent application Ser. No. 09/020,706 titled Temperature Insensitive Polarization Filter, incorporated herein by reference. 
     As illustrated in FIG. 1, the output signals of stacked waveplate filter  30  (corresponding to incoming signals  430  and  420 ) are two sets of two signals with orthogonal polarizations. The output signals corresponding with signal  430  are signal  440  (with horizontal polarization) and signal  460  (with vertical polarization). The output signals corresponding to signal  420  are signal  450  (with horizontal polarization) and signal  470  (with vertical polarization). Two polarization beamsplitters  40  and  50  are then used to separate signals with different polarization. Signals with vertical polarization,  460  and  470  are separated from signals with horizontal polarization,  440  and  450  by these two polarization beamsplitters. Signals with the same polarization are spatially separated. 
     To combine two spatially separated signals with the same polarization together without energy loss, the polarization of one signal needs to be changed. Signal  470  with the vertical polarization passes through a half-waveplate  60 , and its polarization is changed to horizontal. The resulting signal with horizontal polarization is designated as  480 . Signals  460  and  480  pass through a beam displacer  90  and are combined into signal  510 . The signal  510  is then passed through the collimator  820  into optical fiber  840  to enter the next stage of the system. 
     To combine signals  440  and  450  with horizontal polarization together, a similar technique is used. Signal  440  passes through a half-waveplate  70  so that its polarization is changed to vertical. The signal  450  is passed through a glass  80  so that the index difference between the optical paths of signals  440  and  450  can be compensated. Signal  440  goes through the optical path  10 - 20 - 30 - 40 - 50 , and signal  450  goes through the optical path  10 - 30 - 40 - 50 . There is an index difference between these two optical paths; therefore, the glass  80  is provided. It is generally desirable to make the effective optical path length of the signals  440 ,  450  substantially equal, as they reach the beam diverter  100 . As can be seen from FIG. 4, the optical path of signal  440 , as it arrives at beam diverter  100 , includes passage through wave plates  20  and  70 . The glass  80  has proper optical properties (length, index of refraction, and the like) to increase the optical path length of signal  450  to match the optical path length of signal  440 . Similarly, glass  200  increases the path length of signal  660  to match the path length of signal  650 , in view of the passage of signal  650  through waveplates  170  and  190 , before reaching the beam diverter  210 . 
     After passing through the half-waveplate  70  and glass  80 , signals  440  and  450  become signal  490  (with vertical polarization) and signal  500  (with horizontal polarization). These two signals are then combined into signal  520  by the beam displacer  100 . The signal  520  is then passed through the collimator  830  into optical fiber  850  to enter the next stage of the system. 
     Signal  510  is made up of signals  460  and  470  whose polarization is unchanged by the waveplate filter  30 . On the other hand, signal  520  is made up of signals  440  and  450  whose polarization is changed by the waveplate filter  30 . To compensate for the dispersion induced by waveplate  30 , further manipulation is conducted. 
     After passing through collimator  820 , optical fiber  840 , and collimator  870 , signal  510  is first decomposed into two signals with different polarization by beam displacer  160 . After passing through beam displacer  160 , the incoming signal  510  is decomposed into signal  620  (with horizontal polarization) and signal  630  (with vertical polarization). Signal  620  is then passed through the half-waveplate  170 , and its polarization is changed to vertical. This resulting signal with vertical polarization is designated as  640 . Although signal  640  and  630  have the same polarization, they are spatially separated. Signals  640  and  630  then pass through a stacked waveplate filter  180  made up of a plurality of substantially aligned individual waveplates  180   a,    180   b,  and  180   c.  The output signals of stacked waveplate filter  180  corresponding with incoming signals  640  and  630  are two sets of two signals with different polarizations orthogonal to each other. The output signals corresponding with the input signal  640  are signal  650  (with horizontal polarization) and signal  670  (with vertical polarization). The output signals corresponding with the input signal  630  are signal  660  (with horizontal polarization) and signal  680  (with vertical polarization). 
     Since signal  510  is made up of signal  460  and  470  whose polarization is not changed by the waveplate filter  30 , signals  650  and  660  whose polarization is changed by the waveplate filter  180  are desired. This way, the chromatic dispersion caused by stacked waveplate filters  30  and  180  can be compensated. To combine signals  650  and  660  (which have horizontal polarization and are spatially separated) without energy loss, the polarization of one of them needs to be changed. Signal  650  is passed through half-waveplate  190 , and its polarization is then changed to vertical. The resulting signal is designated as  690 . Since signal  650  goes through the optical path  160 - 170 - 180  and signal  660  goes through optical path  160 - 180 , to compensate the index difference between these two paths, a glass  200  is used for signal  660 . The signal  660  (with vertical polarization) passes through the glass  200  without polarization change, and the resulting signal is designated as  700 . Signal  690  (with vertical polarization) and signal  700  (with horizontal polarization) are combined together in the beam displacer  210 . The resulting signal is designated as  710 . The signal  710  is then passed through collimator  890  to enter optical fiber, systems, or network. 
     Signal  670  and  680  (with vertical polarization) will diverge after they pass through  190 - 210  and  200 - 210  respectively as illustrated in FIG.  5 . The signal  670  (with vertical polarization) becomes signal  670   b  (with horizontal polarization) after it passes through the half-waveplate  190 . As shown in FIG. 5, the signal  680  (with vertical polarization) becomes signal  680   b  (with vertical polarization) after it passes through the glass  200 . As shown in FIGS. 4 and 5, the signals  670   b  and  680   b  will not converge with the signals  690  and  700  in the beam displacer  210 ; therefore, their effects are not taken into account here. 
     After passing through collimator  830  (FIG.  4 ), optical fiber  850 , and collimator  860 , signal  520  is first decomposed into two signals with different polarization by the beam displacer  110 . After passing through the beam displacer  110 , signal  520  is decomposed into signal  530  (with horizontal polarization) and signal  540  (with vertical polarization). Signal  530  is then passed through the half-waveplate  120 , and its polarization is then changed to vertical. The resulting signal (with vertical polarization) is designated as  550 . Although signals  540  and  550  have the same polarization, they are spatially separated. Signals  540  and  550  then pass through a stacked waveplate filter  130  made up of a plurality of substantially aligned individual waveplates  130   a,    130   b,  and  130   c.  The output signals of stacked waveplate filter  130  corresponding with incoming signals  540  and  550  are two sets of two signals with different polarization orthogonal to each other. The output signals corresponding with signal  550  are signal  560  (with horizontal polarization) and signal  580  (with vertical polarization). The output signals corresponding with signal  540  are signal  570  (with horizontal polarization) and the signal  590  (with vertical polarization). 
     Since signal  520  is made up of signal  440  and  450  whose polarization is changed by the waveplate filter  30 , signals  580  and  590  whose polarization is not changed by the waveplate filter  130  is desired. This way, the chromatic dispersion caused by stacked waveplate filters  30  and  130  can be compensated. To combine signals  580  and  590  (which have the vertical polarization and are spatially separated) without energy loss, the polarization of one of them needs to be changed. Signal  590  is passed through the half waveplate  140 , and its polarization is then changed into horizontal. The resulting signal is designated as  600 . Signal  580  (with vertical polarization) and signal  600  (with horizontal polarization) are then combined by the beam displacer  150  into signal  610 . The signal  610  is then passed through collimator  880  to enter optical fiber, systems, or network. 
     Signal  560  and  570  with horizontal polarization will diverge after they pass through  150  and  140 - 150  respectively as illustrated in FIG.  6 . Signal  570  (with horizontal polarization) becomes signal  570   b  (with vertical polarization) after it passes through the half-waveplate  140 . As shown in FIGS. 4 and 6, signals  560   b  and  570   b  will not converge with signals  580  and  600  in the beam displacer  150 ; therefore, their effects are not taken into account here. 
     FIG. 7 illustrates modeled chromatic dispersion characteristics and transmission of the first optical path  10 - 30 - 90 - 160 - 180 - 210  (Vertical-Vertical-Vertical-Horizontal) and FIG. 8 illustrates modeled chromatic dispersion characteristics and transmission of the second optical path  10 - 30 - 100 - 110 - 130 - 150  (Vertical-Horizontal-Vertical-Vertical). FIGS. 7 and 8 show transmission loss (expressed as decibels)  510 ,  610  and group delay  512   612  (simulated or modeled) as a function of wavelength (in nanometers). These simulation results illustrate that chromatic dispersion can be compensated significantly with the method and apparatus of the present invention. The small peaks shown in FIG.  7  and FIG. 8 are believed to be due to numerical error. 
     FIG. 9 illustrates the relatively high amount of chromatic dispersion (group delay)  710  occurring within a transmission band  712  of a typical stacked waveplate filter such as described in U.S. Pat. No. 5,694,233 (incorporated herein by reference) in the absence of the present invention. FIG. 9 illustrates that typical previous waveplate filters were subject to relatively high dispersion such as 5 to 9 ps (or more) in at least part of the transmission band  714 . In contrast, FIG. 10 illustrates that when a stacked waveplate filter apparatus is modified, e.g., as illustrated in FIG. 4, the group delay  810  within a high-transmission region  814  of the transmission curve  812  is substantially reduced (such as generally having a magnitude less than about 5 ps). The shape of the dispersion waveform which is achieved, as shown in FIG. 10, is particularly advantageous in that the dispersion is relatively flat over a relatively wide region of the passband (e.g. relatively flat from about 1555.975 nm to about 1556.3 nm, in the example of FIG.  10 ), in contrast to systems which provide only relatively narrow bandwidths in which the lowest (albeit possibly greater than about 5 ps) dispersion occurs. 
     FIG. 11 illustrates a simplified diagram of another double stage stacked-waveplate optical system according to one embodiment of the present invention. Reference numerals in FIG. 11 which are found in FIG. 4 refer to corresponding components. 
     In the system illustrated in FIG. 11, an incoming signal  400  passes through an optical fiber  800  and a collimator  810  to enter the system. The input signal  400  is then decomposed into two signal components: signal  410  (with horizontal polarization) and signal  420  (with vertical polarization) by the beam displacer  10 . Signal  410  is then passed through the half-waveplate  20 , and its polarization is changed to vertical. The resulting signal with vertical polarization is designated as  430 . Signals  420  and  430  then pass through the stacked waveplate filter  30  made up of a plurality of substantially aligned individual waveplates  30   a,    30   b,  and  30   c.  As illustrated in FIG. 1, the output signals of stacked waveplate filter  30  corresponding with incoming signals  430  and  420  are two sets of two signals with orthogonal polarizations. The output signals corresponding with input signal  430  are signal  440  (with horizontal polarization) and signal  460  (with vertical polarization). The output signals corresponding with input signal  420  are signal  450  (with horizontal polarization) and signal  470  (with vertical polarization). Two polarization beamsplitters  40  and  50  are used to separate signals with different polarizations. 
     The signals with vertical polarization,  460  and  470  are separated from signals with horizontal polarization,  440  and  450 , by the two polarization beamsplitters  40  and  50 . Signals with the same polarization are spatially separated. Signal  460  and  470  are then passed through stacked waveplate filters  960  made up of a plurality of substantially aligned individual waveplates  960   a,    960   b,  and  960   c.  As illustrated in FIG. 1, the output signals of stacked waveplate filter  960  corresponding with incoming signals  460  and  470  are two sets of two signals with different polarization orthogonal to each other respectively. The output signals corresponding with signal  460  are signal  1480  (with vertical polarization) and signal  1500  (with horizontal polarization). The output signals corresponding with the signal  470  are signal  1490  (with vertical polarization) and signal  1510  (with vertical polarization). 
     Since signals  460  and  470  have polarization which is not changed by the stacked waveplate filter  30 , signal  1500  and  1510  whose polarization is changed by stack waveplate filter  960  are desired. This way, the chromatic dispersion caused by the stacked waveplate filters  30  and  960  can be compensated. To combine signals  1500  and  1510  (which have the same polarization and are spatially separated) without energy loss, the polarization of one of them needs to be changed. Signal  1500  passes through the half-waveplate  980  and, its polarization is then changed to vertical polarization. The resulting signal is then designated as signal  1520 . Signal  1510  is passed through the glass  990  since there is index different between two optical paths  10 - 20 - 30 - 40 - 960  through which the signal  1500  goes and  10 - 30 - 40 - 960  through which the signal  1510  goes. After passing through the glass  990 , the polarization of  1510  is unchanged and the resulting signal is designated as  1530 . Signal  1520  (with vertical polarization) and signal  1530  (with horizontal polarization) are combined in the stacked waveplate filter  900 , and the resulting signal is designated  1540 . Signal  1540  is then passed through collimator  820  to enter optical fiber, systems, or network. Signals  1480  and  1490  will not interfere with signal  1500  and  1510  as demonstrated in FIG.  5 . Therefore, their effects are not taken into account. 
     The signals with horizontal polarization,  440  and  450  are separated from signals with horizontal polarization,  460  and  470  by the two polarization beamsplitters  40  and  50 . These signals have the same polarization and spatially separated. Signal  440  and  450  are then passed through stacked waveplate filters  970  made up of a plurality of substantially aligned individual waveplates  970   a,    970   b,  and  970   c.  As illustrated in FIG. 1, the output signals of stacked waveplate filter  970  corresponding with incoming signals  440  and  450  are two sets of two signals with orthogonal polarizations. The output signals corresponding with signal  440  are signal  1550  (with vertical polarization) and signal  1570  (with horizontal polarization). The output signals corresponding with signal  450  are signal  1560  (with vertical polarization) and signal  1580  (with horizontal polarization). Since the polarization of signals  440  and  450  is changed by the stacked waveplate filter  30 , signals  1570  and  1580  whose polarization is not changed by the stacked waveplate filter  970  are desirable. This way, the chromatic dispersion caused by the stacked waveplate filters  30  and  970  can be compensated. To combine signals  1570  and  1580  (which have the same polarization and are spatially separated) without energy loss, the polarization of one of them needs to be changed. Signal  1570  passes through the half-waveplate  910  and is changed into vertical polarization. The resulting signal is designated as signal  1590 . Signal  1580  passes through the glass  920  since there is an index difference between the two optical paths ( 10 - 20 - 30 - 40 - 50 - 970  through which the signal  1570  goes and  10 - 30 - 40 - 50 - 970  through which the signal  1580  goes). After passing through the glass  920 , the polarization of signal  1580  is unchanged and the resulting signal is designated as signal  1600 . Signal  1590  with vertical polarization and signal  1600  with horizontal polarization are combined in beam displacer  930 , and the resulting signal is designated as signal  1610 . The signal  1610  is then passed through collimator  830  to enter optical fiber, systems, or network. Signals  1550  and  1560  will not interfere with signal  1570  and  1580  after entering beam displacer  930  as demonstrated in FIG.  5 . Therefore, their effects are not taken into account. 
     Those of skill in the art will understand how to fabricate, select or provide components as described herein, including waveplates, beam displacer, polarization beam splitters, glasses and the like. 
     In summary, and with particular reference to FIG. 4, one embodiment of the present invention provides apparatus usable for providing reduced dispersion optical signals which includes a first device  810 ,  10 ,  20  for receiving a first optical signal  400  and outputting second and third spaced-apart optical signals  420 ,  430 ; a stacked waveplate filter device(s)  30  for receiving the second and third signals and outputting fourth and fifth spaced apart output pairs, the fourth output pair comprising sixth and seventh signals  440 ,  460  and eighth and ninth signals  450 ,  470 ; a second device(s)  40 ,  60 ,  90 ,  820 ,  840 ,  870 ,  160 ,  170  for directing the seventh and ninth signals, to a second stacked waveplate filter device(s)  180 , which outputs tenth  650 ,  670  and eleventh  660 ,  680  signal pairs; third device(s)  50 ,  70 ,  80 ,  100 ,  830 ,  850 ,  860 ,  110 ,  120  for directing the sixth and eighth signals, to a third stacked waveplate filter device(s)  130 , which outputs twelfth  560 ,  580  and thirteenth,  570 ,  590  signal pairs; fourth device(s)  190 ,  200 ,  210 ,  890  for combining a portion of each of the tenth and eleventh signal pairs to output a fourteenth signal  710 ; and fifth device(s)  140 ,  150 ,  880  for combining a portion of each of the twelfth and thirteenth signal pairs to output a fifteenth signal  610  ; wherein chromatic dispersion arising from the first and second stacked waveplate filter device(s) is substantially compensated and wherein chromatic dispersion arising from the first and third stacked waveplate filter device(s) is substantially compensated. 
     With particular reference to FIG. 11, one embodiment of the present invention provides apparatus usable for providing reduced dispersion optical signals including first device(s)  810 ,  10 ,  20  for receiving a first optical signal  400  and outputting second and third spaced-apart optical signals  420 ,  430 ; a stacked waveplate filter device(s)  30  for receiving the second and third signals and outputting fourth and fifth spaced apart output pairs, the fourth output pair comprising sixth and seventh signals  440 ,  460  and eighth and ninth signals  450 ,  470 ; second device(s) ( 40 ) for directing the seventh and ninth signals, to a second stacked waveplate filter device(s) ( 960 ) which outputs tenth ( 1480 ,  1500 ) and eleventh ( 1490 ,  1510 ) signal pairs; third device(s) ( 50 ) for directing the sixth and eighth signals, to a third stacked waveplate filter device(s), ( 970 ) which outputs a twelfth ( 1550 ,  1570 ) and thirteenth, ( 1560 ,  1580 ) signal pairs; fourth device(s) ( 980 ,  990 ,  900 ) for combining a portion of each of the tenth and eleventh signal pairs to output a fourteenth signal ( 1540 ); and fifth device(s) ( 910 ,  920 ,  930 ) for combining a portion of each of the twelfth and thirteenth signal pairs to output a fifteenth signal ( 1610 ); wherein chromatic dispersion arising from the first and second stacked waveplate filter device(s) is substantially compensated and wherein chromatic dispersion arising from the first and third stacked waveplate filter device(s) is substantially compensated. 
     With particular reference to FIG. 11, one embodiment of the present invention provides apparatus for reducing optical signal dispersion which includes device(s)  810 ,  10 ,  30 ,  40 ,  50  for receiving at least a first optical signal and outputting at least second  460 ,  470  and third  440 ,  450  optical signals, the device(s) for receiving including first optical filter device(s)  30  defining substantially periodic transmissions of the second and third optical signals, as a function of wavelength, the first optical filter device(s) imparting a first dispersion; device(s)  960 ,  980 ,  990 ,  900 ,  820  for receiving the second optical signal and outputting a fourth signal  1540 , the device(s) for receiving the second optical signal comprising a second periodic optical filter  960 , the second periodic optical filter imparting a second dispersion which substantially compensates the first dispersion; and device(s)  970 ,  910 ,  920 ,  930 ,  830  for receiving the third optical signal and outputting a fifth signal  1610 , the device(s) for receiving the third optical signal comprising a third periodic optical filter  970 , the third periodic optical filter imparting a third dispersion which substantially compensates the first dispersion. 
     In light of the above description, a number of advantages of the present invention can be seen. The present invention can achieve a substantial reduction in dispersion along an optical path. Preferably, the present invention provides for a reduction in dispersion of a discrete optical device such as a stacked waveplate filter, compared to the amount of dispersion which would occur in a typical stacked waveplate or similar device in the absence of using the present invention. The present invention can achieve such reduction in dispersion while providing the desired types of output (typically, output of two orthogonal polarized paths) and preferably can provide such reduction in dispersion using a combination of, and/or pathway through components, each one of which is typically readily available, including providing a pathway through a sequential plurality of stacked waveplate devices with appropriate polarization change or control. Accordingly, the present invention is able to achieve reduction in dispersion at relatively low cost. 
     A number of variations and modifications of the present invention can be used. Although the present invention can be embodied in an optical router, it is possible to use configurations of the present invention and other types of devices such as switches, hubs, bridges, multiplexers, demultiplexers and the like. Although the present invention is believed to be particularly useful in the context of WDM signals, it is also possible to implement the present invention for use with other types of signals including unmultiplexed signals. Although the present invention was illustrated in connection with particular polarization sequences (i.e. Vertical-Vertical-Vertical-Horizontal; Vertical-Horizontal-Vertical-Vertical) other polarization sequences can also be used. 
     The present invention, in various embodiments, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, subcombinations, and subsets thereof. Those of skills in the art will understand how to make and use the present invention after understanding the present disclosure. The present invention, in various embodiments, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiment hereof, including in the absence of such items as may have been used in previous devices or processes, e.g. for improving performance, achieving ease and/or reducing cost of implementation. The present invention includes items which are novel, and terminology adapted from previous and/or analogous technologies, for convenience in describing novel items or processes, do not necessarily retain all aspects of conventional usage of such terminology. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. Although the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e.g. as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.