Abstract:
A method and device is disclosed for dispersion compensation of an optical signal having periodic dispersion within a multi-channels system. For example interleaved optical channels often exhibit dispersion having a characteristic that is repeated in adjacent channels. By providing a periodic device that allows for polarization dependent routing of an interleaved signal to allow for multiple passes of said signal through a multi-cavity GT etalon, having a free-spectral range that corresponds to the channel spacing, the dispersion in the interleaved signal can be lessened and practically obviated or balanced to a desired level. This invention provides a device and method to achieve that end.

Description:
This application is a continuation-in-part of application Ser. No. 09/434,376 filed Nov. 5, 1999, a continuation-in-part of application Ser. No. 09/634,707 filed Aug. 7, 2000 and a continuation-in-part of application Ser. No. 09/702,861 filed Nov. 1, 2000. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to a device for compensation of chromatic dispersion in optical fiber communication systems and specifically to a multiple pass multi-cavity etalon dispersion compensating device. 
     BACKGROUND OF THE INVENTION 
     Most high-speed fiber optic communication systems today use externally modulated lasers to minimize laser ‘chirp’ and reduce the effects of chromatic dispersion in the fiber. Even with external modulation, there is a certain amount of ‘chirp’ or broadening of the laser spectrum, because any modulated signal must contain frequency ‘sidebands’ which are roughly as wide as the modulation rate. Higher bit rate transmission systems consequently have broader frequency sidebands, and at the same time can tolerate less phase delay because of the shorter bit period. Next-generation high bit rate systems are consequently very sensitive to chromatic dispersion of the optical fiber and any components such as WDM&#39;s within the system. 
     Chromatic dispersion of optical fiber is roughly constant over the 1550 nm communication window, and can be compensated by several techniques including dispersion compensating fiber, fiber Bragg gratings, etc. However, certain wavelength filtering components such as WDM&#39;s can have significant dispersion characteristics due to a fundamental Kramers-Kronig type relationship between transmission spectrum and dispersion characteristics. This type of dispersion characteristic typically varies substantially over the narrow WDM passband, and therefore is difficult to compensate using conventional techniques such as dispersion compensating fiber. It is one objective of the present invention to compensate for the dispersion from WDM devices, including multiplexers, demultiplexers, and interleavers. Conventional laser systems are known to utilize directly modulated semiconductor lasers. In combination with chromatic dispersion characteristics of single-mode optical fiber, chirping of these lasers contributes to the spread of optical pulses and results in intersymbol interference and overall degradation in transmission. Intersymbol interference in a digital transmission system is the distortion of the received signal by overlap of individual pulses to the degree that the receiver cannot reliably distinguish between groupings of pulses. 
     Current and “next-generation” high speed systems employ transmitters which use narrow linewidth lasers and external modulators in a window or range of wavelengths about 1550 nm. These external modulators generally have a very low chirp; some designs have a zero or negatively compensating chirp. Nevertheless, transmission distance is still dispersion-limited to about 80 kilometers at transmission rates of 10 Gb/s using conventional single mode fibers. 
     One solution to this problem is in the use of dispersion shifted fiber which has little dispersion in the 1550 nm window. However, replacing existing fiber with dispersion shifted fiber is costly. Other approaches have been proposed such as optical pulse shaping to reduce laser chirp, using a semiconductor laser amplifier to impose a chirp on the transmitted signal that counteracts the chirping of the directly modulated semiconductor laser. 
     Approaches that are more consistent with the teachings of this invention attempt to reduce the intersymbol interference at or near the receiver, or intermediate the transmitter and the receiver. Essentially any medium capable of providing a sufficient dispersion opposite to that of the optical fiber can serve as an optical pulse equalizer. For example it is known to use a special optical fiber having an equal chromatic dispersion at a required operating wavelength but opposite in sign to that of the transmitting fiber. Other methods include the use of fiber Bragg gratings, FBGs, as disclosed in U.S. Pat. No. 5,909,295 in the name of Li et al., and disclosed by Shigematsu et al., in U.S. Pat. No. 5,701,188 assigned to Sumitomo Electric Industries, Ltd., and the use of planar lightwave circuit (PLC) delay equalizers. Unfortunately, special compensating fiber has a very high insertion loss and in many applications is not a preferable choice. Fiber gratings are generally not preferred for some field applications due to their narrow bandwidth, and fixed wavelength. PLCs are also narrow band, although tunable devices; fabricating a PLC with large dispersion equalization remains to be difficult. Shigematsu et al. disclose a hybrid of both of these less preferred choices; dispersion compensating fiber with chirped Bragg gratings. 
     In a paper entitled “Optical Equalization to Combat the Effects of Laser Chirp and Fiber Dispersion” published in the Journal of Lightwave Technology. Vol. 8, No. 5, May 1990, Cimini L. J. et al. describe an optical equalizer capable of combating the effects of laser chirp and fiber chromatic dispersion on high-speed long-haul fiber-optic communications links at 1.55 μm. Also discussed is a control scheme for adaptively positioning the equalizer response frequency. Cimini et al. describe a device having only one common input/output port at a first partially reflective mirror and a second 100% reflective mirror together forming a cavity. The control scheme described attempts to track signal wavelength by obtaining feedback from a receiver. The amplitude response of the equalizer is essentially flat with wavelength at the input/output port, and thus, the proposed control scheme is somewhat complex requiring processing of high speed data at the optical receiver. As well, the proposed control method is stated to function with return to zero, RZ, signals but not with non-return to zero, NRZ, signals, a more commonly used data format. Although the equalizer described by Cimini et al. appears to perform its intended basic dispersion compensating function, there exists a need for an improved method of control of the position of the equalizer frequency response, and as well, there exists a need for an equalizer that will provide a sufficient time shift over a broader range of wavelengths. U.S. Pat. No. 5,023,947 in the name of Cimini et al., further describes this device. 
     A Fabry-Perot etalon having one substantially fully reflective end face and a partially reflective front face is known as a Gires-Tournois (GT) etalon. In a paper entitled “Multifunction optical filter with a Michelson-Gires-Tournois interferometer for wavelength-division-multiplexed network system applications”, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device is described which is hereafter termed the MGT device. The MGT device as exemplified in FIG. 1 serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror  16 . The etalon  10  used has a 99.9% reflective back reflector  12   r  and a front reflector  12   f  having a reflectivity of about 10%; hence an output signal from only the front reflector  12   f  is utilized. 
     In an article entitled “Optical All-Pass Filters for Phase Response Design with Applications for Dispersion Compensation” by C. K. Madsen and G. Lenz, published in  IEEE Photonics Letters , Vol. 10 No. 7, July 1998, a coupled reflective cavity architecture in optical fiber is described for providing dispersion compensation. Cavities are formed in the optical fiber between fiber Bragg grating reflectors. However a multi-cavity filter in fiber has a limited free spectral range (FSR) insufficient for a telecommunications system. For a typical 100 GHz FSR required in the telecommunications industry, the cavity length is about 1 mm. A Bragg grating reflector, if manufactured using commonly available grating-writing techniques, would need to be longer than 1 mm, and hence the two reflector cavity structure would be too long to achieve the necessary FSR. Another draw back to this prior art solution is the requirement for an expensive optical circulator to separate the input and output signals. 
     As of late, interleaving/de-interleaving circuits are being used more widely. These specialized multiplexor/demultiplexers serve the function of interleaving channels such that two data streams, for example a first stream consisting of channel 1, 3, 5, 7, and so on, is interleaved, or multiplexed with a second stream of channels, 2, 4, 6, 8, and so on, for forming a single signal consisting of channels 1, 2, 3, 4, 5, 6, 7, 8, and so on. Of course the circuit can be used oppositely, to de-interleave an already interleaved signal, into plural de-interleaved streams of channels. One such interleaver circuit is described in U.S. Pat. No. 6,125,220 issued in the name of Copner et al., and another is in U.S. Pat. No. 6,040,932 issued in the name of Colbourne et al. Although interleaver circuits perform a desired function, it has been discovered that some of these circuits suffer from unwanted periodic chromatic dispersion within each channel. It is this type of periodic dispersion that can be obviated or lessened by the instant invention. It should also be noted that in many instances it is not desirable to completely eliminate all chromatic dispersion; it is believed that a small amount of such dispersion can be useful in reducing non-linear effects in WDM systems; therefore, the instant invention can be used to lessen dispersion by a required amount. 
     Hence, it is an object of this invention to overcome some of the limitations of the prior art described above. Furthermore, it is an object of the invention to provide a device that will compensate for or lessen dispersion over a plurality of interspaced wavelength channels simultaneously. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention, there is provided a dispersion compensation device for compensating a dispersion of an optical input beam, the device comprising: 
     polarization dependent beam routing means having an input port, for routing a polarized optical beam launched into the input port along a first path in one of two directions in dependence upon the polarization state of the polarized optical beam, 
     at least one multi-cavity etalon defining at least two resonant cavities, optically coupled to the routing means, said at least one multi-cavity etalon for receiving at least one optical beam from the routing means and for directing at least one reflected optical beam back to the routing means for the reflected light beam to follow a second path in the routing means; and 
     at least one rotator for rotating the polarization of light in the optical path between the routing block and the etalon so that the at least one reflected light beam follows the second path in the routing means, 
     whereby the polarized light beam launched into the input port undergoes multiple reflections from the etalon to reduce dispersion of the optical input beam. 
     The device may also comprise beam directing means, optically coupled to the routing means, for receiving at least one beam from the routing means and for directing at least one beam back to the routing means. 
     The routing means may be exemplified by a birefringent crystal or by a polarizing beam splitter. 
     If the input beam is not polarized, a polarization diversity means should be provided to divide the optical input beam into two sub-beams having orthogonal polarizations before the sub-beams are passed into the routing means. After the multiple passes and reflections from the etalon, the two sub-beams may be re-combined to recover the full power of the dispersion-compensated optical beam. 
     In accordance with the another aspect of the invention, there is provided a dispersion compensation device for compensating a dispersion of an optical input beam comprising: 
     input beam splitting means for spatially separating the input beam into two orthogonally polarized beams; 
     first polarization rotating means, optically coupled to the input beam splitting means, said first polarization rotating means for rotating the polarization of one of the two beams such that the two beams have the same polarization; 
     polarization dependent beam routing means, optically coupled to the first polarization rotating means, said polarization dependent beam routing means for routing the two beams on a first path for the two beams having a first polarization and on a second path for the two beams having a second polarization, orthogonal to the first polarization; 
     second polarization rotating means, optically coupled to the polarization dependent beam routing means, said second polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization; 
     at least one multi-cavity etalon, optically coupled to the second polarization rotating means, said at least one multi-cavity etalon for receiving the two beams from the second polarization rotating means and for launching the two beams back to the second polarization rotating means, said multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one end face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonant cavities; 
     third polarization rotating means, optically coupled to the polarization dependent beam routing means, said third polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization; 
     beam directing means, optically coupled to the third polarization rotating means, said beam directing means for receiving the two beams from the third polarization rotating means and for directing the two beams back to the third polarization rotating means; 
     fourth polarization rotating means, optically coupled to the polarization dependent beam routing means, said fourth polarization rotating means for rotating the polarization of one of the two beams such that the two beams have orthogonal polarizations; and 
     output beam combining means, optically coupled to the fourth polarization rotating means, said output beam combining means for spatially combining the two orthogonally polarized beams into an output beam; 
     whereby the two beams undergo multiple passes through the at least one multi-cavity etalon and thereby the dispersion correction of the two beams is increased. 
     It is understood by those educated in this art that, for example, the splitting and combining means could be birefringent crystals with a walk-off axis orthogonal to the walk-off axis of the routing means which could be another birefringent crystal. Further, the walk-off axes of both of these elements are orthogonal to the optical path through these elements. 
     In accordance with the another aspect of the invention, there is provided a dispersion compensating system, containing at least two dispersion compensating devices, for compensating an overall dispersion of an optical input beam comprising: 
     input beam routing means for routing an input beam to a first dispersion compensating device, said first dispersion compensating device comprising: 
     input beam splitting means for spatially separating the input beam into two orthogonally polarized beams; 
     first polarization rotating means, optically coupled to the input beam splitting means, said first polarization rotating means for rotating the polarization of one of the two beams such that the two beams have the same polarization; 
     polarization dependent beam routing means, optically coupled to the first polarization rotating means, said polarization dependent beam routing means for routing the two beams on a first path for the two beams having a first polarization and on a second path for the two beams having a second polarization, orthogonal to the first polarization; 
     second polarization rotating means, optically coupled to the polarization dependent beam routing means, said second polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization; 
     at least one multi-cavity etalon, optically coupled to the second polarization rotating means, said at least one multi-cavity etalon for receiving the two beams from the second polarization rotating means and for launching the two beams back to the second polarization rotating means, said multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one end face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonant cavities; 
     third polarization rotating means, optically coupled to the polarization dependent beam routing means, said third polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization; 
     beam directing means, optically coupled to the third polarization rotating means, said beam directing means for receiving the two beams from the third polarization rotating means and for directing the two beams back to the third polarization rotating means; 
     fourth polarization rotating means, optically coupled to the polarization dependent beam routing means, said fourth polarization rotating means for rotating the polarization of one of the two beams such that the two beams have orthogonal polarizations; 
     output beam combining means, optically coupled to the fourth polarization rotating means, said output beam combining means for spatially combining the two orthogonally polarized beams into an output beam; 
     whereby the two beams undergo multiple passes through the at least one multi-cavity etalon and thereby the dispersion correction of the two beams is increased; 
     at least one intermediate beam routing means for routing an output beam, of at least a first dispersion compensating device, such that said output beam becomes an input beam of another dispersion compensating device, said another dispersion compensating device comprising: 
     input beam splitting means for spatially separating the input beam into two orthogonally polarized beams; 
     first polarization rotating means, optically coupled to the input beam splitting means, said first polarization rotating means for rotating the polarization of one of the two beams such that the two beams have the same polarization; 
     polarization dependent beam routing means, optically coupled to the first polarization rotating means, said polarization dependent beam routing means for routing the two beams on a first path for the two beams having a first polarization and on a second path for the two beams having a second polarization, orthogonal to the first polarization; 
     second polarization rotating means, optically coupled to the polarization dependent beam routing means, said second polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization; 
     at least one multi-cavity etalon, optically coupled to the second polarization rotating means, said at least one multi-cavity etalon for receiving the two beams from the second polarization rotating means and for launching the two beams back to the second polarization rotating means, said multi-cavity etalon having at least one end face that is highly reflective and substantially not transmissive to light and at least two other faces that are partly reflective and partly transmissive, the one end face and the at least two other faces being separated from one another by predetermined gaps, the at least three faces forming at least two resonant cavities; 
     third polarization rotating means, optically coupled to the polarization dependent beam routing means, said third polarization rotating means for rotating the polarization of the two beams such that the two beams have the same polarization; 
     beam directing means, optically coupled to the third polarization rotating means, said beam directing means for receiving the two beams from the third polarization rotating means and for directing the two beams back to the third polarization rotating means; 
     fourth polarization rotating means, optically coupled to the polarization dependent beam routing means, said fourth polarization rotating means for rotating the polarization of one of the two beams such that the two beams have orthogonal polarizations; 
     output beam combining means, optically coupled to the fourth polarization rotating means, said output beam combining means for spatially combining the two orthogonally polarized beams into an output beam; 
     whereby the two beams undergo multiple passes through the at least one multi-cavity etalon and thereby the dispersion correction of the two beams is increased; and 
     output beam routing means, optically coupled to the last dispersion compensating device, said output beam routing means for routing an output beam of the last dispersion compensating device to an output port; 
     whereby the beam undergoes dispersion correction at each dispersion compensating device that results in balancing the compensation of the overall dispersion of an optical input beam. 
     An additional embodiment of this invention is provided by a method of dispersion compensation for simultaneously compensating for dispersion present within individual channels in a multi-channel signal, the method comprising: 
     providing a polarization dependent beam routing and directing means for routing and directing the multi-channel signal in a polarization dependent manner and at least one multi-cavity etalon optically coupled to said polarization dependent beam routing means; and 
     launching a multi-channel signal into said polarization dependent beam routing means to allow for multiple passes through said beam routing means and said multi-cavity etalon, and capturing a dispersion compensated multi-channel signal from said polarization dependent beam routing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be described in conjunction with the drawings in which: 
     FIG. 1 is a schematic side view of a three-cavity GT etalon tuned to provide dispersion compensation for an output signal provided by an interleaver circuit; 
     FIG. 2 is a plot of time delay vs. wavelength for a dispersion compensator; 
     FIG. 3 is a plot of dispersion vs. wavelength for a three-cavity GT etalon dispersion compensator; 
     FIG. 4 is an exemplary plot of the dispersion resulting from utilizing two different multi-cavity systems to correct for the dispersion slope; 
     FIG. 5 is a schematic illustration of a dispersion compensator of the invention utilizing a three-cavity etalon and a beam routing block tuned to provide dispersion compensation for an output signal provided by an interleaver circuit; 
     FIG. 5 a  is a schematic side view of a polarization diversity (xBD) block of FIG. 5; 
     FIG. 6 is a schematic illustration of a dispersion compensator of the invention utilizing two 3-cavity GT etalons in another embodiment of the present invention to provide dispersion compensation for an output signal originated by an interleaver circuit; 
     FIG. 7 is a schematic illustration of a dispersion compensator of the invention utilizing a 3-cavity GT etalon in another embodiment of the present invention to provide dispersion compensation for an output signal originated by an interleaver circuit; 
     FIG. 8 is a schematic illustration of another embodiment of the device of the invention using a bi-directional input/output; 
     FIG. 9 is a schematic illustration of yet another embodiment of the device of the invention, and 
     FIG. 10 is a schematic illustration of still another embodiment of the invention using a beam splitter as a routing block. 
    
    
     DETAILED DESCRIPTION 
     The specifications of U.S. application Ser. Nos. 09/434,376; 09/634,707; and 09/702,861 of which the present application is a C-I-P, are being hereby incorporated by reference. 
     FIG. 1 shows a side view of a 3-cavity GT etalon tuned to provide dispersion compensation for an output signal from an interleaver circuit. The cavities are made of three blocks of silica providing a predetermined gap of 0.52 mm between reflective end faces. The device has a front input/output face having a reflectivity of 0.088%, a second face with a reflectivity of 3.6%, a third face with reflectivity of 42% and an outward (back) face that is substantially totally reflective having a reflectivity of practically 100%. The periodic nature of time delay versus wavelength is shown in FIG.  2 . 
     FIG. 3 illustrates dispersion vs. wavelength for a 3-cavity GT etalon dispersion compensator, wherein the periodic nature of the dispersion is evident from the plot. 
     FIG. 4 is a plot of dispersion resulting from utilizing two different multi-cavity systems to correct for dispersion. As a result, one can obtain a constant dispersion over a wide spectral region. By changing the relative phase of the two systems, one can obtain different dispersion. 
     The solid line in FIG. 4 is the dispersion from a first multi-cavity etalon, disp i N a  while the dotted line is the dispersion from a second multi-cavity etalon that does not necessarily have the same number of cavities as in the first multi-cavity etalon. The resulting dispersion after passing through both etalons with a flat line result for λ i  1.5499 to 1.5501 nm is a dashed line. This is an illustration of how the invention allows for the correction of dispersion in an optical fiber. Note that if the second multi-cavity etalon&#39;s dispersion characteristic is tuned to a slightly longer wavelength, the resulting dispersion for λ i  1.5499 to 1.5501 nm would be a flat line but with a dispersion value below zero. Similarly, if the second multi-cavity etalon&#39;s dispersion characteristic is tuned to a slightly shorter wavelength, the resulting dispersion would be a flat line but with a dispersion value above zero. The use of a multipass system allows for a greater range of change in each multi-cavity etalon since a multipass multi-cavity is a way of amplifying the effect of a single pass through the multi-cavity system. In fact, several multiple pass multi-cavity etalon systems can be cascaded to achieve a still greater range of dispersion effects. 
     In order to obtain a zero-dispersion (FIG.  4 ), the channels&#39; central wavelength should correspond. Notwithstanding, this feature can be used as an advantage of the invention. For example, by varying the optical path length between the etalon end faces of the GT dispersion compensator, the etalon&#39;s dispersion characteristics can be varied to a slightly higher or lower wavelength. This tuning can be effected by providing a change in temperature to the GT device, for example by providing a heating element adjacent the etalon to heat the block, or conversely by providing cooling to reduce the optical path length. In addition, both can be done simultaneously in the case of a system with two multiple pass multi-cavity etalons where one multiple pass multi-cavity etalon would be heated and the second multiple pass multi-cavity would be cooled. Further to this would be a solution where one multiple pass multi-cavity etalon would be heated or cooled by a greater amount than the second multiple pass multi-cavity etalon. Thus the flat but present dispersion present in the optical fiber within the system can be compensated for by providing a required flat dispersion compensation that is afforded by fine tuning the GT cavity optical path length. 
     FIG. 5 a  illustrates the functioning of an x-beam displacer  101  to separate the two orthogonally polarized beams vertically and rotate the polarization of one beam using a half waveplate HWP  121  so both beams  111  have the same o polarization. As shown in FIG. 5, the beams  111  then travel straight through a y-beam displacer  102 . The beams  111  then pass through a quarter waveplate QWP  103  resulting in a change in polarization to circular. The circular polarized light  111   c  passes into the multi-cavity etalon  109  and is reflected back and passes through QWP  103  that changes the polarization to extraordinary, e. The e-beams  112  are displaced by y-beam displacer  102  and reflect off of the QWP mirror assembly  107  that changes the polarization of the beams to ordinary, o  113 . The o-beams  113  travel straight through y-beam displacer  102 , impinge on QWP  103 , become circularly polarized  113   c  and travel into multi-cavity etalon  109  and are reflected back and impinge on QWP  103 . The beams undergo a polarization change to become e polarized beams  114 . As e-beams  114  pass through element  102  they are displaced by y-beam displacer  102  until the beams impinge QWP mirror assembly  107  that changes the polarization of the beams  115  to o and o-beams  115  travel straight through beam displacer  102  and through QWP  103  changing to circularly polarized light  115   c . As before, the beams  115   c  are reflected by multi-cavity etalon  109  and impinge on QWP  103 . QWP  103  changes the polarization of the beams to e  116  which are displaced by y-beam displacer  102  and exit the optical element. One of the e beams is passed through HWP  122  causing a change in polarization to an o beam and then beams  116  are recombined to a single beam by x-beam displacer  108  to form beam  117 . These embodiments allow reflecting from the multi-cavity etalon at near-normal incidence reducing degradation of the signal due to insertion loss and the dispersion properties of the etalon. 
     Now referring to FIG. 6, in the QWP mirror assembly the mirror has been replaced with an additional multi-cavity etalon  1110 . Beams  112  have the same polarization and conditions as in FIG. 5 until after these beams travel through the QWP  1071  becoming circularly polarized, impinge on multi-cavity etalon  1110 , are reflected back to QWP  1071  which changes the polarization to o, and then travel straight through y-beam displacer  102 . Each time the beams are redirected by the multi-cavity etalon  109  and pass through QWP  103  for the second time, the beams undergo a polarization change to e and y-beam displacer  102  displaces them. Each time the beams are redirected by multi-cavity etalon  1110  and pass through QWP  1071  for the second time the beams undergo a polarization change to o polarization which allows them to travel straight through the y-beam displacer  102 . Thus are the beams redirected until they are recombined to a single beam by x-beam recombiner  108  to form beam  117 . 
     Referring to FIG. 7, the beam displacer  102  in FIG. 6 is replaced with a block of polarizing beam splitters PBSs  302 . These beam splitters allow o polarized beam to pass straight through and deflect e polarized beams 90 degrees. 
     The “ordinary” beams  311 ,  313 ,  315 , and  317 , travel through the beam splitters to impinge first on the QWP  103  becoming circularly polarized and then on the multi-cavity etalon  109 . The beams that return from the multi-cavity etalon  109  again pass through the QWP  103  to become e polarized beams  312 ,  314 ,  316 , and  318 , and then impinge on the QWP mirror assembly  107 . The QWP mirror assembly  107  reflects the beams and changes the polarization to “o” which allows the beams to pass through the PBS. The result is eight reflections from the etalon as each beam  311 ,  313 ,  315 ,  317  is directed twice at the etalon. This process of reflection and transmission occurs until the final stage where the beams of  318  are recombined in x-beam combiner  308 . Note that the mirror of the QWP mirror assembly  107  can be replaced with a multi-cavity etalon which may have the same number of cavities as the first multi-cavity etalon, as illustrated in FIG. 6, but does not have to and the invention herein disclosed is not so restricted. 
     These systems of multiple pass tunable dispersion compensators can be used in combinations to set different dispersion corrections and result in an overall dispersion correction for a set of International Telecommunication Union (ITU) channels. Each separate multipass tunable dispersion compensator etalon system would solve a different dispersion problem. As shown in the illustrations in FIGS. 5,  6 , and  7 , there is no angle tuning of the individual systems. Rather, each multiple pass tunable compensator allows for the optical beam to enter and leave the etalon normal to the front surface, R 4 . This reduces ripple loss that is caused by the interaction of beams from the different surfaces when the incident beam is not normal to the front surface. Of course by having multiple passes this ripple loss would be increased on each pass. Therefore, for the multiple pass multi-cavity etalons, having the optical beam at a normal incidence is a requirement to keep losses low. The use of multiple passes through the same multi-cavity etalon allows the dispersion to be corrected without attaining losses due to fiber coupling the etalon at each surface and at each multi-cavity interface. Thus the multiple pass system takes the advantages of the multi-cavity etalon and improves the optical properties further. These systems are still temperature tunable as discussed earlier in this document. 
     In the embodiment of FIG. 8, the input and output port  330  of the device is the same and is coupled with a garnet/HWP block  332 , a yBD beam displacer  334 , a Wollaston prism  336 , a beam displacer  338  and a dual-fiber collimator  340  which has a GRIN lens  342  and a two-fiber tube  344 . The arrangement  332 - 344  functions as an isolator so that the single input/output port is sufficient. It will be noted that the mirror  107  extends to reflect the beam  116 , but the quarter waveplate  346  does not extend to the same degree. As a result, the beam  116  undergoes a reflection and goes back over the same path to the output port  330 . 
     In the embodiment of FIG. 9, the device has a circulator that employs, instead of a Wollaston prism as in FIG. 8, a PBS block coupled to collimators and beam displacers on one side and a garnet-HWP unit  332  on the other side. The collimator is coupled to the input/output port  330  as in the embodiment of FIG.  8 . 
     The arrangement of FIG. 10 has a similar circulator design as the embodiment of FIG.  8 . It will be seen, however, that the birefringent routing block  102  of FIGS. 5 and 8 is replaced by a polarizing beam splitter block  302  as in FIG.  7 . Because of the need to reroute the input beam from the input/output port  330  through the PBS and back to the input/output port, an additional mirror  352  is disposed in the path of the optical beam  354  exiting the PBS in order to reflect the beam  354  back into the PBS to follow a reverse route with multiple reflections from the etalon. It will be understood that the input beam in the embodiment of FIG. 10 will undergo eight passes through the etalon  109  (four on its way “forward” from the input to the mirror  352  and four on its way “back”). 
     The elements analogous to elements in other embodiments are not mentioned in detail. 
     Of course numerous other embodiments may be envisaged, without departing from the spirit and scope of the invention herein disclosed.