Abstract:
Interleavers, based on a Michelson interferometer with a Gires-Tournois (GT) etalon in each arm, are becoming popular in the filtering of light in the fiber optics telecommunications industry. As the channel spacing becomes closer together, e.g. 50 GHz or 25 GHz, dispersion compensation becomes an important factor in the choice and design of a system. The present invention solves the problem of increased chromatic dispersion by utilizing multi-cavity Gires-Tournois (MCGT) etalons, wherein the dispersion from one MCGT is used to compensate or cancel the dispersion from the other MCGT. In an optimum design for a dual cavity GT etalon, the dispersion profile of the first MCGT will have a similar amplitude and frequency as the dispersion profile of the second MCGT, only shifted by half the period so that the positive slopes of one profile are aligned with the negative slopes of the other profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. Patent Applications No. 60/293,985 filed May 30, 2001, and No. 60/312,469 filed Aug. 16, 2001. 
    
    
     TECHNICAL FIELD 
     The present application relates to an optical interferometer, and in particular to a multi-cavity etalon interferometer useful as a low dispersion optical interleaver filter. 
     BACKGROUND OF THE INVENTION 
     Optical interleavers are becoming a popular tool in dense wavelength division multiplexed (DWDM) communications networks as an interface between components designed for signals with a first wavelength channel spacing and components designed for signals with a second wavelength channel spacing. In the past 200 GHz channel spacing was the norm, but as the demand for increased bandwidth grew, 100 GHz channel spacing became the standard. In the next generation of communications networks 50 GHz channels spacing and even 25 GHz channel spacing will become common place. However, conventional de-multiplexing filters, e.g. dichroic filters, do not have the capability to separate channels that are so closely spaced. Accordingly, optical interleavers are used to separate the closely spaced channels into two sets of channels, which are twice as far apart. This process can continue until the channels are far enough apart for conventional multiplexing to be effective. 
     Interleavers can take several forms, including Birefringent Crystal Interleavers, Integrated Lattice Filter Interleavers, and Michelson Gires-Tournois (MGT) Interleavers. The present invention relates to Michelson Gires-Tournois Interleavers, such as those disclosed in U.S. Pat. No. 6,169,626 issued Jan. 2, 2001 in the name of Jye-Hong Chen et al, and U.S. Pat. No. 6,252,716 issued Jun. 26, 2001 in the name of Reza Paiam. Both of these references disclose the use of an interferometer, including a beamsplitter and two Gires-Tournois (GT) resonators, for interleaving/de-interleaving optical wavelength channels. Polarization-based versions of the MGT Interleavers are disclosed in U.S. Pat. No. 6,130,971 issued Oct. 10, 2000; U.S. Pat. Nos. 6,169,604 and 6,169,828 issued Jan. 2, 2001; and U.S. Pat. No. 6,215,926 issued Apr. 10, 2001 all in the name Simon Cao. The polarization-based interferometers typically include a polarization beam splitter (PBS) and two GT resonators each with a birefringent waveplate therein. GT etalons with a birefringent waveplate are referred to as BGTs. Single BGT versions of the invention are possible, since orthogonally polarized components of a single beam will effectively “see” different resonators, if an appropriate waveplate is provided in the resonator cavity. However, polarization diversity front ends are required in the single BGT versions. 
     The aforementioned conventional MGT Interleavers provide acceptable chromatic dispersion at 100 GHz; however, unacceptable chromatic dispersion is created at the 50 and 25 GHz level. 
     An object of the present invention is to overcome the shortcomings of the prior art by providing an optical interferometer for use as an interleaver, which displays relatively low dispersion. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention relates to an interferometer comprising: 
     a first port for launching a first input optical signal; 
     a beam splitter for dividing the first input optical signal into first and second sub-beams, and for directing the first and second sub-beams along first and second optical paths, respectively; 
     a first multi-cavity resonator in the first optical path for re-directing the first sub-beam to interfere with the second sub-beam, the first multi-cavity resonator including a first resonant cavity providing a first dispersion profile to the first sub-beam, and a second resonant cavity for modifying the first dispersion profile resulting in a second dispersion profile for the first sub-beam; 
     a second multi-cavity resonator in the second optical path for re-directing the second sub-beam to interfere with the first sub-beam forming first and second output beams, the second multi-cavity resonator including a third resonant cavity providing a third dispersion profile to the second sub-beam, and a fourth resonant cavity for modifying the third dispersion profile resulting in a fourth dispersion profile for the second sub-beam, wherein dispersion from the first multi-cavity resonator compensates for dispersion in the second multi-cavity resonator providing less overall dispersion to the first and second output beams; 
     a second port for outputting the first output signal; and 
     a third port for outputting the second output signal. 
     Another aspect of the present invention relates to an interferometer comprising: 
     a first port for launching a first input optical signal; 
     first phase-biasing means for introducing an initial phase difference between first and second orthogonally polarized components of the first input optical signal; 
     a multi-cavity resonator comprising first and second resonant cavities, each of the first and second resonant cavities including second phase biasing means for providing an additional phase difference between the first and second components of the input optical signal, whereby, when the first and second components are recombined, first and second output beams are formed; wherein the first resonant cavity provides first and second dispersion profiles to the first and second components, respectively, and the second resonant cavity modifies the first and second dispersion profiles resulting in third and fourth dispersion profiles, respectively, whereby the dispersion from the first component compensates for the dispersion from the second component, and the first and second output signals have less overall dispersion; 
     a second port for outputting the first output signal; and 
     a third port for outputting the second output signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
     FIG. 1 is a schematic representation of a conventional MGT interferometer; 
     FIG. 2 is a schematic representation of a multi-cavity MGT interferometer according to the present invention; 
     FIG. 3 a  is a schematic representation of a multi-cavity MGT interferometer with an optical path length difference between the two arms thereof according to another embodiment of the present invention; 
     FIGS. 3 b  to  3   d  are examples of dispersion profiles resulting from the multi-cavity etalon of FIG. 3 a , in which the front cavities have different front reflectivities; 
     FIG. 4 a  is a spectral response for a multi-cavity MGT interleaver with de-phased arms and a specific optical path length difference; 
     FIG. 4 b  is a chromatic dispersion profile for the interleaver of FIG. 4 a;    
     FIG. 4 c  illustrates the complementary dispersion profiles of first and second multi-cavity etalons of the interleaver of FIG. 4 a , as well as the resultant dispersion profile; 
     FIG. 5 is a schematic representation of a polarization-based multi-cavity BGT interferometer according to another embodiment of the present invention; 
     FIG. 6 is a schematic representation of a polarization-based multi-cavity BGT interferometer with an optical path length difference between the two arms thereof according to another embodiment of the present invention; 
     FIGS. 7 a  to  7   c  are response and dispersion curves for a conventional polarization-based 50 GHz BGT interleaver; 
     FIGS. 8 a  to  8   c  are response and dispersion curves for a polarization-based 50 GHz multi-cavity BGT interleaver according to the present invention; 
     FIG. 9 is a schematic representation of a polarization-based multi-cavity BGT interferometer comprising a single multi-cavity BGT resonator according to another embodiment of the present invention; 
     FIGS. 10 a  and  10   b  are side views of the ports from the interferometer of FIG. 9; 
     FIG. 11 is a schematic representation of a solid multi-cavity resonator for use with the embodiments of FIGS. 5 to  9 ; and 
     FIGS. 12 a  and  12   b  are response and dispersion profiles for a double passed 50 GHz BGT interleaver. 
    
    
     DETAILED DESCRIPTION 
     The conventional MGT interferometer, generally indicated at  1  in FIG. 1, includes a beam splitter  2  for separating an incoming light beam  3  into sub-beams  4  and  5 , and for directing the sub-beams  4  and  5  along separate arms  7  and  8  to resonators  9  and  10 . The resonators  9  and  10  are typically in the form of Gires-Tournois etalons; however, other forms of resonators are possible, such as ring resonators. Each resonator is comprised of a front partially reflective mirror  12  and at least one substantially fully reflective mirror  13 . The mirrors  12  can have the same reflectivity or different reflectivity&#39;s, depending on the desired response. Similarly, the arms  7  and  8  can have the same effective path length or there can be an optical path length difference. The resonators  9  and  10  provide both of the sub-beams  7  and  8  with a nonlinear response curve, and direct the sub-beams  7  and  8  back together causing interference, resulting in a pair of output beams  15  and  16  being formed. Accordingly, when the device is arranged as an interleaver/deinterleaver the output beams  15  and  16  comprise odd and even spaced wavelength channels, respectively. As an example, for a given free spectral range (FSR) the resonators  9  and  10  will have a cavity length 2 L defined by the equation: 
     
       
         2 L=c /(2× n×FSR× cos(θ))  
       
     
     in which: 
     c is the speed of light; 
     n is the index of refraction of the resonator cavity; and 
     θ is the angle from normal at which the sub-beams enter the resonators. 
     As well known in the art, the aforementioned assembly can also be used to interleave two sets of wavelength channels into a single set of closely spaced channels by launching the two sets along the paths taken by the output signals  15  and  16 . The resulting closely spaced set of wavelength channels will exit via the path taken by the incoming light beam  3 . 
     With reference to FIG. 2, a multi-cavity MGT interferometer according to the present invention is generally indicated at  21 , and includes a beam splitter  22 , a first port  23 , a second port  24 , a third port  25 , a fourth port  26 , a first arm  27  and a second arm  28 . The beam splitter  22  is preferably a  50 / 50  beam splitter comprised of two triangular prisms  22   a  and  22   b  with a partially reflective coating  29  therebetween. Each port  23 ,  24 ,  25  and  26  preferably includes an end of an optical waveguide  31  encased in a ferrule  32 , and optically coupled to a graded index (GRIN) lens  33 . Although the preferred embodiment of the present invention requires only three ports, a four port device is also within the scope of the invention. Moreover, all four ports are illustrated to show the flexibility of the device, i.e. light launched through either the first port  23  or the fourth port  26  will exit the second port  24  and/or the third port  25  and vice versa. 
     The first arm  27  preferably includes a first multi-cavity GT etalon  34 , which is comprised of a front resonant cavity  35   a , and a rear resonant cavity  36   a . The preferred embodiment comprises two resonant cavities; however, additional resonant cavities are within the scope of this invention. The front resonant cavity is defined by a front partially-reflective surface  37   a  and a middle partially-reflective surface  38   a . The rear resonant cavity  36   a  is defined by the middle partially-reflective surface  38   a  and a back substantially fully-reflective surface  39   a.    
     Similarly, the second arm  28  preferably includes a second multi-cavity GT etalon  44 , which is comprised of a front resonant cavity  35   b , and a rear resonant cavity  36   b . The front resonant cavity is defined by a front partially-reflective surface  37   b  and a middle partially-reflective surface  38   b . The rear resonant cavity  36   b  is defined by the middle partially-reflective surface  38   b  and a back substantially fully-reflective surface  39   b.    
     The front and rear resonant cavities  35   a ,  35   b ,  36   a  and  36   b  can be fabricated out of solid material with the reflective coatings applied thereto or they can be fabricated with air gaps defined by coated substrates separated by spacers, as is well known in the art. In the air gap version, the coated substrates are preferably wedge-shaped to prevent back reflections (see FIG.  3 ). 
     The reflectivity of the back surfaces  39   a  and  39   b  are as close to 100% as possible, and preferably within the range of 95% to 100%. The middle surfaces  38   a  and  38   b  preferably have a reflectivity ranging between 6% and 30%, while the front surfaces  37   a  and  37   b  have a relatively low reflectivity, preferably ranging from between 0% and 5%. The front resonant cavities are designed to reduce overall dispersion without perturbing the spectrum. 
     In particular, the front resonant cavities  35   a  and  35   b  are designed to modify the dispersion profiles produced by the rear resonant cavities  36   a  and  36   b , respectively, so that the overall dispersion profile of the first multi-cavity etalon  34  is complimentary to the dispersion profile of the second multi-cavity etalon  44 . Ideally, the front resonant cavities  35   a  and  35   b  adjust the dispersion profiles from the rear resonant cavities  36   a  and  36   b , respectively, providing the overall dispersion profiles with symmetrical peaks (see the discussion of FIGS. 3 b  to  3   d  below). In other words, because of the quasi-periodic nature of the chromatic dispersion resulting from the first and second multi-cavity etalons  34  and  44 , it is the object of the design process for the dispersion profile of the first multi-cavity etalon  34  to have similar periodicity and amplitude as the dispersion profile of the second multi-cavity etalon  44 , only shifted so that the positive slopes of one profile are aligned with the negative slopes of the other profile. Obviously, it is impossible to eliminate all dispersion, but a great deal of the dispersion can be compensated for by this arrangement, as evidenced by FIGS. 4 b  and  4   c.    
     FIG. 3 a  illustrates an interferometer  41 , similar to interferometer  21 , illustrated in FIG. 2, with an effective optical path length difference introduced between the first and second arms  27  and  28 . The optical path length difference can be introduced by providing an air gap  46  or by positioning a transparent spacer (not shown) between the multi-cavity etalon  44  and the beam splitter  22 . FIG. 3 a  also illustrates an alternative means for launching and outputting the signals into and from the interferometer  41 . In this embodiment only an input port  53  and two output ports  54  and  55  are required. As before, each port includes a ferrule  56  encompassing an end of an optical waveguide  57 , which is optically coupled to a GRIN lens  58 . The input beam of light is launched so that the beam splitter  22  will split the beam, and direct the sub-beams at angles normal to the first and second multi-cavity resonators  34  and  44 . After re-combining and interfering, one of the output beams will travel out via one of the output ports  55 , while the other output beam will travel towards the input port  53 , only to be re-routed by a circulator  59  to the other output port  54 . Obviously, the circulator  59  can be replaced by another equally effective device known in the art. 
     If we assume that each of the resonant cavities  35   a ,  35   b ,  36   a  and  36   b  have an optical cavity length of 2L (as defined above), effective dispersion compensation has been obtained when the optical path length difference is one half of the optical cavity length, i.e. L. Particularly effective dispersion compensation has been obtained when the optical path length difference is L+Δ, where Δ equals +/−(λ c /4) or a multiple thereof, and the cavity lengths of the resonant cavities in the shorter arm have been de-phased by Δ., i.e. the optical cavity length of the resonant cavities  35   a  and  36   a  is 2L+Δ. 
     FIGS. 3 b  to  3   d  illustrate the effect of changing the reflectivity of the front surfaces  37   a  and  37   b  in a 25 GHz interleaver with a middle surface  38   a  and  38   b  reflectivity of 2.2%. In FIG. 3 b  the front cavity is effectively eliminated by reducing the reflectivity to 0%, and the resulting dispersion profile has asymmetrical (“shark-fin”) peaks. If the front surfaces  37   a  and  37   b  are provided with a reflectivity of approximately 0.0125% (FIG. 3 c ) the peaks become substantially symmetrical. If the reflectivity is increased, as in FIG. 3 d , the dispersion profile becomes non-linear. Other relatively good results can be obtained by readjusting the reflectivity of the middle surfaces  38   a  and  38   b , and determining the best possible front surface reflectivity. 
     FIGS. 4 a  and  4   b  illustrate associated spectral response and dispersion profiles, respectively, for such a multi-cavity MGT interferometer with de-phased cavity lengths and an optical path length difference of L+Δ. For this example, the reflectivity of the front surfaces  37   a  and  37   b  is approximately 0.013% (or −39 dB), the reflectivity of the middle surfaces  38   a  and  38   b  is approximately 2.2%, and the reflectivity of the back surfaces  39   a  and  39   b  is approximately 99.5%. Due to the low reflectivity of the front surfaces  37   a  and  37   b , the middle surfaces  38   a  and  38   b  are wedged shaped to minimize back reflection. FIG. 4 c  illustrates complementary dispersion profiles  40  and  50  from the first and second multi-cavity etalons  34  and  44 , respectively, while line  60  represents the overall dispersion of an output signal. The plot in FIG. 4 c  clearly illustrates how the positive slopes of dispersion profile  40  are aligned with the negative slopes of dispersion profile  50  for reducing the overall dispersion profile  60 . The resultant chromatic dispersion, represented by line  60  is not the exact summation of the chromatic dispersions from the first and second multi-cavity etalons  34  and  43  (lines  40  and  50 , respectively). In fact, there is an interference effect between the electrical fields from the two multi-cavity etalons  34  and  44  that determines the overall dispersion. 
     A polarization-based version of the present invention is illustrated in FIG. 5, in which an interferometer  61  includes a polarization beam splitter (PBS)  62 , a first port  63 , a second port  64 , a third port  65 , a fourth port  66 , a first arm  67 , and a second arm  68 . The PBS  62  comprises two triangular prisms  62   a  and  62   b  with a polarization beam splitting coating  69  therebetween. Each of the ports  63 ,  64 ,  65  and  66  includes an optical waveguide  71 , encased in a ferrule tube  72 , and optically coupled to a GRIN lens  73 . The first arm  67  includes a multi-cavity BGT etalon  74  comprising a first resonant cavity  75   a  and a second resonant cavity  76   a . The first resonant cavity  75   a  is defined by a front partially-reflective surface  77   a  and a middle partially-reflective surface  78   a . The second resonant cavity  76   a  is defined by the middle partially-reflective surface  78   a  and a back substantially fully reflective surface  79   a . Similarly, the second arm  68  includes a multi-cavity BGT etalon  84  comprising a first resonant cavity  75   b  and a second resonant cavity  76   b . The first resonant cavity  75   b  is defined by a front partially-reflective surface  77   b  and a middle partially-reflective surface  78   b . The second resonant cavity is defined by the middle partially-reflective surface  78   b  and a back substantially fully reflective surface  79   b . The reflectivity of the front surfaces  77   a  and  77   b  is preferably between 0.3% and 1.2%, and ideally 0.7%. The reflectivity of the middle surfaces  78   a  and  78   b  is preferably between 6% and 22%, and ideally 14%. The back surfaces  79   a  and  79   b  are preferably as close to 100% as possible, but typically ranges between 95% and 100%. Each arm  67  and  68  includes a first phase shifting element  82 , preferably in the form of an ⅛ waveplate aligned at a 45° angle. Each resonant cavity  75   a ,  75   b ,  76   a  and  76   b  includes a second phase shifting element  83 , preferably in the form of a ¼ waveplate at a 45° angle. 
     The PBS  62  splits an input beam of light from the first port  63  into orthogonally polarized sub-beams  80  and  81 . The sub-beams  80  and  81  each pass through one of the first phase shifting element  82 , whereby an initial phase shift is introduced between the respective components thereof. As the light passes through each of the second phase shifting elements  83 , an additional phase shift is introduced between the components of the sub-beams  80  and  81 , whereby the polarization of every other wavelength channel is rotated by 90°. Accordingly, when the sub-beams  80  and  81  are recombined in the beam splitter  62 , the odd (or even) spaced wavelength channels with one polarization are directed to the second port  64 , while the even (or odd) spaced wavelength channels with the orthogonal polarization are directed to the third port  65 . 
     FIG. 6 illustrates another embodiment of the present invention, in which a polarization-based interferometer  91  introduces an optical path length difference between the components of the sub-beams. The interferometer  91  is very similar to the interferometer  61  of FIG. 5; however the first phase biasing elements  82  are replaced by wider birefringent delay sections  92 . The delay section  92  introduce a larger optical path length difference between the components of the sub-beams  80  and  81 , such as the previously discussed distance L or L+Δ. 
     FIGS. 7 a  to  7   c  illustrate spectral and dispersion profiles for a conventional single cavity 50 GHz BGT interleaver, while FIGS. 8 a  to  8   c  illustrate similar plots for a multi-cavity BGT according to FIG.  5 . In this example, the reflectivity of the front surfaces  77   a  and  77   b  is 0.7%, the reflectivity of the middle surfaces  78   a  and  78   b  is 14%, and the reflectivity of the back surfaces  79   a  and  79   b  is 99.5%. The chromatic dispersion is reduced from 75 ps/nm to less than 10 ps/nm with very little reduction in passband width. 
     A single BGT version of the present invention is illustrated in FIG. 9, in which an interferometer  101  includes an input port  102 , a first output port  103 , a second output port  104 , a first polarization beam splitter (PBS)  105 , a second PBS  106 , a non-reciprocal polarization rotator  107 , and a single multi-cavity BGT resonator  174 . 
     Each port includes an optical waveguide  111  encased in a ferrule tube  112  and optically coupled to a lens  113 . A birefringent beam splitter  114  is optically coupled to the lens  113  for separating input light into orthogonally polarized sub-beams and/or for combining like-polarized sub-beams of output light into a single beam. A ½-waveplate  116  is provided for rotating the polarization of one of the sub-beams parallel with the other. As seen from the side in FIGS. 10 a  and  10   b , the waveplate  116  is positioned at a different location at the input port  102  than at the output ports  103  and  104 , for reasons that will be explained below. 
     The first and second PBS  105  and  106  each include two triangular prisms  117  with a polarization beam splitting coating  118  therebetween. The non-reciprocal rotator  107  is comprised of a ¼ waveplate  121  and a Faraday rotator  122  arranged so that they have no resultant effect on the polarization of light traveling from the input port  102 , while rotating the polarization of light traveling from the second PBS  106  towards the input port  102  by 90°. 
     The multi-cavity BGT resonator  174  includes a front resonant cavity  175  and a rear resonant cavity  176 . A front partially reflective surface  177  and a middle partially reflective surface  178  define the front resonant cavity  175 , while the middle surface  178  and a back substantially fully reflective surface  179  define the rear resonant cavity  176 . Preferably, the substrate with the middle partially reflective surface  178  is wedge-shaped to prevent back reflections. The reflectivity of the front, middle and back surfaces  177 ,  178  and  179 , respectively, are similar to those of the corresponding surfaces from interferometers  61  and  91 . A first phase biasing element  182 , preferably in the form of a ⅛-waveplate, is positioned between the second PBS  106  and the multi-cavity etalon  174 . If a larger optical path length difference is desired, as in FIG. 6, the first phase biasing element  182  can be replaced by a wider birefringent delay section. A second phase biasing means  183 , preferably in the form of a ¼-waveplate, is positioned in each resonant cavity  175  and  176 . 
     With reference to FIGS. 9 and 10 a , a randomly polarized beam of light is launched through input port  102 , wherein the light is collimated by lens  113 , and separated into orthogonally polarized sub-beams by birefringent beam splitter  114 . The polarization of one of the sub-beams, e.g. the extraordinary sub-beam, is rotated by 90° by the waveplate  116 , so that both sub-beams have the same polarization, e.g. vertical. For the sake of convenience, since both sub-beams are identical, we will only discuss the behavior of one until they are output. The input sub-beam travels through the first PBS  105 , the non-reciprocal rotator  107 , and the second PBS  106  essentially unchanged. The first phase biasing element aligned at 45° to the input sub-beam introduces an initial phase difference between the sub-beam&#39;s two components. As the light travels through the multi-cavity etalon  174 , the second phase biasing elements  183  provide additional phase biasing, whereby the polarization of every other wavelength channel, e.g. the even channels, is rotated by 90° so that the wavelength channels in the sub-beam of light output from the multi-cavity etalon  174  have alternating polarizations. As a result, the wavelength channels, e.g. even channels, that have had their polarization rotated, e.g. to horizontal, will not travel through the second PBS  106 , but will be re-directed to the second output port  104 . The remaining wavelength channels, e.g. odd channels, travel through the second PBS  106 , but will subsequently have their polarization rotated, e.g. to horizontal, by the non-reciprocal rotator  107 , whereby the first PBS  105  will re-direct them to the first output port  103 . As seen in FIG. 10 b , because the polarization of both of the sub-beams is now orthogonal to their initial polarization (FIG. 10 a ) the waveplate  116  is positioned in a different path, e.g. the ordinary path, to ensure both sub-beams are recombined. 
     Typically the multi-cavity BGT etalons include an air gap for each of the resonant cavities, whereby the second phase biasing elements  83  can be angle tuned; however, as illustrated in FIG. 11, it is possible to provide a solid version of a multi-cavity BGT etalon. The solid multi-cavity BGT etalon  200  includes a first phase biasing element  201 , a front resonant cavity  202 , and a rear resonant cavity  203 . The first phase biasing element is preferably a conventional ⅛-waveplate oriented at a 45° angle to the incoming beam. Each resonant cavity has multi-layered sandwich arrangement. The front resonant cavity  202  includes a front partially reflective coating  204 , a ¼ wave liquid crystal plate  205 , a phase matching liquid crystal plate  206 , and a middle reflective coating  207 . These layers are spaced apart by first, second and third transparent blocks  208 ,  209  and  210 . Similarly, the rear resonant cavity  203  includes the middle reflective coating  207 , a ¼ wave liquid crystal plate  211 , a phase matching liquid crystal plate  212 , and a back reflective coating  213 . Each of these layers is spaced apart by fourth, fifth, and sixth transparent blocks  214 ,  215 , and  216 . 
     One method of altering the spectral response of a multi-cavity etalon interferometer is to double pass the sub-beams through their respective multi-cavity etalons. FIGS. 12 a  and  12   b  illustrate a spectral response and a dispersion profile, respectively, for a 50 GHz multi-cavity BGT in which the signals have been double passed through the multi-cavity etalons. In a comparison with FIGS. 8 a  and  8   c , we can conclude that increased isolation can be obtained by paying a penalty of increased dispersion. This provides the designer with the flexibility to provide a device with varying specifications depending upon the system requirements.