Abstract:
An air-density-tuned interferometer includes: a hermetically sealed enclosure; a front window coupled to the hermetically sealed enclosure; an interferometer residing within the hermetically sealed enclosure; and a pressure changing device coupled to the hermetically sealed enclosure. The interferometer within the enclosure includes: a first glass plate optically coupled to the front window; a first reflective coating coupled to the first glass plate; a second reflective coating optically coupled to the first reflective coating; a second glass plate coupled to the second reflective coating; and a plurality of spacers coupled to the first and second glass plates, forming an optical interometric cavity therein. The pressure changing device manipulates the gas pressure within the cavity. The air-density-tuned interferometer does not incorporate additional elements into the optical path within the interferometer and that does not disturb or move any of the optical components disposed within or associated with this optical path.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is claiming under 35 USC 119(e) the benefit of provisional patent application Ser. No. 60/347,196 filed on Jan. 8, 2002. 

   FIELD OF THE INVENTION 
   The present invention relates to optical switching, routing, filtering, dispersion compensating, multiplexing and de-multiplexing devices and methods. More particularly, the present invention relates to a tunable interferometer comprising parallel reflective surfaces whose tuning capability is utilized to perform the switching, routing, filtering, dispersion compensation, multiplexing and/or de-multiplexing. 
   BACKGROUND OF THE INVENTION 
   The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carrying capacity of existing fiber cable is Wavelength Division Multiplexing (WDM). In this method, multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light and each light-wave-propagated information channel corresponds to light within a specific wavelength range or “band.” 
   In this specification, these individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.” 
   Because of the increased network traffic resulting from the use of the WDM technique, there is an increasing need for optical switching and routing devices that can quickly route or re-route numerous channels amongst various optical communications lines. An optical interferometer is a device that produces a periodic phase modulation in a composite optical signal. When an optical interferometer is incorporated as a component within another optical apparatus, this periodic phase modulation may be utilized advantageously to produce periodic transmission, reflection, optical delay or polarization properties within optical signals or composite optical signals passed through the apparatus. Moreover, if the optical interferometer is tunable, the peak positions of the modulated functions may be controllably varied so as to align the peak positions to standard channel positions to maximize optical throughput or so as to switch or route channels amongst various outputs. 
   Various mechanical, thermo-optic, electro-optic or magneto-optic methods have been employed to provide tuning capabilities to optical interferometers. Although these methods provide adequate tuning capabilities, they invariably add additional optical components and/or electronic connections to the interferometer, thereby increasing the complexity and difficulty of fabricating and aligning the interferometer and potentially reducing the stability and optical throughput of the interferometer. Accordingly, there remains a need for an improved tunable interferometer that does not incorporate additional elements into the optical path within the interferometer and that does not disturb or move any of the optical components disposed within or associated with this optical path. The present invention addresses such a need. 
   SUMMARY OF THE INVENTION 
   The present invention addresses the above stated needs through the disclosure of an air-density-tuned interferometer and optical switching, routing and multiplexing devices and methods utilizing the air-density-tuned interferometer. A first preferred embodiment of an air-density-tuned interferometer in accordance with the present invention comprises two transparent plates separated by at least one spacer and comprising respective partially-reflecting parallel surfaces such that an optical cavity is formed between the surfaces, wherein the plates and at least one spacer are contained within a sealed enclosure that incorporates a pressure-changing mechanism. A second preferred embodiment of an air-density-tuned interferometer in accordance with the present invention comprises a first transparent plate comprising a partially-reflecting surface, a second plate comprising a 100% reflecting surface parallel to the partially reflecting surface, at least one spacer separating the plates such that an optical cavity is formed between the surfaces, wherein the plates and the at least one spacer are contained within a sealed enclosure that incorporates a pressure changing mechanism. A third preferred embodiment of an air-density-tuned interferometer in accordance with the present invention comprises a first transparent plate comprising a partially-reflecting surface, a second plate comprising a 100% reflecting surface parallel to the partially reflecting surface, at least one spacer separating the plates such that an optical cavity is formed between the surfaces, an internal birefringent waveplate optically coupled between the partially reflective coating and the 100% reflective coating and an external birefringent waveplate optically coupled to the first transparent plate outside the optical cavity, wherein the plates, the at least one spacer and the birefringent waveplates are contained within a sealed enclosure that incorporates a pressure changing mechanism. The reflective surfaces may be formed from reflective or partially reflective coatings disposed on the plates. 
   The optical cavity between the parallel reflective or partially reflective surfaces comprises an “air” gap that is filled with air or another gas. The air or gas within the air gap communicates with air or gas outside of the optical cavity but within the enclosure through a hole, opening, slit, slot, etc. in or between the plates or the at least one spacer. The pressure changing mechanism causes changes in the density of air or gas so as to control the refractive index of air or gas within the optical cavity. The change in refractive index produces small precise changes in the optical path length within the optical cavity such that maxima and minima in periodic curves of transmission, reflection or polarization rotation are shifted by precisely controlled amounts. These shifts are utilized, within devices incorporating the air-density-tuned interferometer, to adjust the positions of the transmission, reflection or polarization rotation into correspondence with standard optical channel positions, to provide variable chromatic dispersion for optical channels or to switch or route channels. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is an illustration of a first preferred embodiment of an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 2A  is an illustration of a second preferred embodiment of an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 2B  is an illustration of a tunable multiplexer and de-multiplexer that utilizes an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 2C  is an illustration of a tunable chromatic dispersion compensator that utilizes an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 2D  is a schematic illustration of a tunable chromatic dispersion compensator system that utilizes air-density-tuned interferometers in accordance with the present invention. 
       FIG. 3A  is an illustration of a third preferred embodiment of an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 3B  is an illustration of a first 2×2 tunable and switchable interleaved channel separator apparatus that utilizes air-density-tuned interferometers in accordance with the present invention. 
       FIGS. 3C-3D  are illustrations of a second 2×2 tunable and switchable interleaved channel separator apparatus that utilizes an air-density-tuned interferometer in accordance with the present invention illustrating the pathways of signal light rays therethrough. 
       FIGS. 3E-3F  are each a top view and a side view of a third 2×2 tunable and switchable interleaved channel separator apparatus that utilizes an air-density-tuned interferometer in accordance with the present invention illustrating the pathways of signal light rays therethrough. 
       FIG. 4A  is a functional signal routing diagram of two different switch states of a 2×2 tunable and switchable interleaved channel separator device. 
       FIG. 4B  is a pair of schematic graphs of the spectrum of the polarization-rotated light and the spectrum of polarization-non-rotated light reflected from an air-density-tuned non-linear interferometer in accordance with the present invention in two different operational states. 
       FIG. 4C  is an alternative functional signal routing diagram of two different switch states of a tunable and switchable interleaved channel separator device. 
       FIG. 4D  is a functional signal routing diagram of a tunable and switchable interleaved channel separator device utilizing asymmetric channel separation. 
       FIG. 5A  is an illustration of a first pressure-changing device comprising an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 5B  is an illustration of a second pressure-changing device comprising an air-density-tuned interferometer in accordance with the present invention. 
       FIG. 5C  is an illustration of a third pressure-changing device comprising an air-density-tuned non-linear interferometer in accordance with the present invention. 
       FIG. 6  is an illustration of a polarizing port as utilized within a tunable and switchable interleaved channel separator device. 
   

   DETAILED DESCRIPTION 
   The present invention provides an improved tunable interferometer apparatus as well as methods and apparatuses for utilizing the tunable interferometer within optical communications networks. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
   To more particularly describe the features of the present invention, please refer to  FIGS. 1 through 6  in conjunction with the discussion below. 
     FIG. 1  is an illustration of a first preferred embodiment of an air-density-tuned interferometer in accordance with the present invention. The air-density-tuned interferometer  100  ( FIG. 1 ) comprises a hermetically sealed enclosure  184  of substantially constant internal volume. The enclosure  184  carries a pressure changing device  186  and a first or front window  188   a  and a second or rear window  188   b . The air-density-tuned interferometer  100  further comprises a Fabry-Perot interferometer  101  contained within the enclosure  184 . The windows  188   a - 188   b  permit optical coupling to be maintained between the internal Fabry-Perot interferometer  101  and the external environment while maintaining a controlled air pressure within the enclosure  184 . 
   The Fabry-Perot interferometer  101  comprising the air-density-tuned interferometer  100  comprises two glass plates  180   a - 180   b  optically coupled to one another so as to form an optical interferometric cavity  110  between the plates, wherein the inside face (that is, the face that faces into the cavity) of the first glass plate  180   a  is coated with a first partially reflective coating  140  and the inside face of the second glass plate  180   b  is coated with a second partially reflective coating  120  wherein the two coatings are substantially parallel to one another. The two glass plates  180   a - 180   b  are separated by a spacer or spacers  112 , such that the interferometric cavity  110  between the first partially reflective coating  140  and the second partially reflective coating  120  maintains a constant width during any air-pressure changes. The spacers  112  preferably comprise a zero-thermal-expansion or low-thermal-expansion material. 
   The exact pressure within the constant volume enclosure  184  may be varied and controlled by the pressure-changing device  186 . Examples of preferred pressure-changing devices are illustrated in  FIGS. 5A-5C  and discussed in more detail below. The air pressure within the optical cavity  110  is equilibrated with that in the remainder of the enclosure  184  through an opening  190  which may be an orifice, crevice, slit or, simply, an open side. Since the internal volume of the enclosure  184  is maintained substantially constant, the controlled variations in gas pressure within the enclosure  184  cause corresponding changes in the density of air (or gas) within the enclosure  184 . These density changes, in turn, cause changes in the refractive index of air or gas within the enclosure  184  and, in particular, within the optical cavity  110  between the reflective coatings  120  and  140 . 
   Within the air-density-tuned interferometer  100 , the first glass plate  180   a  is optically coupled to the front window  188   a  and the second glass plate  180   b  is optically coupled to the rear window  188   b . These optical couplings permit an input composite optical signal  104  to enter into the Fabry-Perot interferometer  101  whilst permitting a reflected composite optical signal  105  and a transmitted composite optical signal  106  to exit the apparatus  100  via the front window  188   a  and rear window  188   b , respectively. 
   The mechanism for and magnitude of the air density tuning of the interferometer  100  are now described. The following analysis is specific to a Fabry-Perot type interferometer but may be extended, in straightforward fashion, to other air-density-tuned interferometer apparatuses in accordance with the present invention. As is well-known, the transmission function, τ, of a Fabry-Perot interferometer is given by the following equation 
             τ   =     1     1   +       (       4   ⁢   R       1   -     R   2         )     ⁢           ⁢     sin   2     ⁢           ⁢     (     φ   2     )                   Eq   .           ⁢   1             
 
wherein R is the reflectivity of the mirrors and φ is the phase change of a light ray for one round-trip between the mirrors. Under the assumptions that there are no phase changes at the mirror surfaces and that the path of the light ray is perpendicular to the parallel mirror surfaces, then, the phase change φ is given by 
             φ   =         (       2   ⁢           ⁢   π     λ     )     ⁢           ⁢   2   ⁢           ⁢   η   ⁢           ⁢   L     =       (       4   ⁢           ⁢   π   ⁢           ⁢   η   ⁢           ⁢   L     c     )     ⁢           ⁢   v               Eq   .           ⁢   2             
 
wherein η is the refractive index of the material between the mirrors, λ is the wavelength of light in vacuum, L is the distance between the two mirrors, c is the speed of light in vacuum and v is the frequency of light in vacuum. By combining Eq. 1 and Eq. 2, it is clear that the transmission, τ, is periodic in frequency, with the period or, in other words, the Free Spectral Range (FSR), given by 
             FSR   =     c     2   ⁢           ⁢   η   ⁢           ⁢   L               Eq   .           ⁢   3             
 
   It is further evident from Eq. 1 that, at a hypothetical null frequency, v 0 =0, the corresponding transmission τ 0  mathematically assumes its maximum value of unity, i.e., τ 0 =1, which is true for all values of L, R and η. Although the position of v 0  is fixed, the positions of other local maxima, i.e., “peaks”, in the transmission curve can vary depending upon η and L. The first peak at finite frequency occurs at a frequency v 1 =FSR=c/2ηL. The frequency position of this first peak shifts by a small amount as the refractive index η changes, with the rate of the frequency shift, dv 1 /dη, of the first peak in the transmission curve given by 
                 ⅆ     v   1         ⅆ   η       =         ⅆ     (   FSR   )         ⅆ   η       =         -   c       2   ⁢           ⁢     η   2     ⁢           ⁢   L       =       -     (           ⁢   FSR   )       η                 Eq   .           ⁢   4             
 
The rate of shift of the i th  transmission peak, v i , is given by 
                 ⅆ     v   i         ⅆ   η       =       i   ⁢           ⁢       ⅆ     (   FSR   )         ⅆ   η         =           -   i     ⁢           ⁢     (   FSR   )       η     ≈       -     v   i       η                 Eq   .           ⁢   5             
 
wherein the final approximate equality results from the fact that i≈v i /(FSR) since the change in FSR is negligible for small changes in refractive index. Finally, since the medium between the two mirror surfaces is assumed to be air with n≈1, 
                 dv   i       v   i       ≈       -   d     ⁢           ⁢   η             Eq   .           ⁢   6             
 
   Within the wavelength region utilized for fiber optic telecommunications-that is, from about 1500-1620 nm—an average value of v i  in Eq. 6 may be taken as v i =193.1 THz, which is a recommended anchor point for channel positions conforming to the International Telecommunications Union (ITU) grid. Also, the refractive index of air at 1 atm pressure within this wavelength range is virtually constant within a range 1.0002732-1.0002733. The quantity (η air −1) is closely proportional to the density of air, pair, with a proportionality constant, h, given by h≈6.68 cm 3 /mole. Written in differential form, this relationship becomes dη air =h dρ. Then, under the reasonable assumption that the gas density is sufficiently low that ideal gas behavior is obeyed, both this proportionality constant as Eq. 6 may be inserted into the ideal gas law to derive the change in pressure, dP, corresponding to a given peak shift, dv i  This is found to be 
             dP   =       RT   ⁢           ⁢   d   ⁢           ⁢   ρ     =         -     RT   h       ⁢           ⁢   d   ⁢           ⁢     η   air       =     ∼       RT   h     ⁢           ⁢       dv   i     v                     Eq   .           ⁢   7             
 
wherein R is the ideal gas constant (for instance, R=82.0575 atm-cm 3 -mole −1 -deg −1 ) and T is absolute temperature in Kelvin, assumed to be constant at 298 K.
 
As an example of the use of the equations given above, it is useful to calculate the pressure change required for a peak shift equivalent to one-half channel spacing. In effect, this represents the maximum channel shift that is required in a practical sense as a result of the periodicity of τ. The ITU grid defines a common standard channel spacing of 100 GHz. It is therefore assumed, in this example, that the interferometer FSR matches this grid spacing, that is, FSR=100 GHz. Therefore, a negative channel shift, dv i , of −50 GHz is required to shift the peak maxima by one-half of the channel spacing. This −50 GHz shift requires an increase in refractive index of air given by 
         d   ⁢           ⁢     η   air       ≈       50   ⁢           ⁢   GHz       193100   ⁢           ⁢   GHz       ≈     0.000259   .         
 
This corresponds to an approximate pressure change, in atmospheres, given by dP, wherein 
       dP   ≈         82.0575   ×   298     6.68     ×   0.000259     ≈     0.95   ⁢           ⁢     atm   .           
 
This can be accomplished by changing the internal pressure from 1.00 atm to 1.95 atm such that the refractive index changes from 1.000273 to 1.000532. Finally, from Eq. 4, 
           d   ⁢           ⁢     (   FSR   )         (   FSR   )       =       -       d   ⁢           ⁢   η     η       =         -   0.000259     1.000273     ≈     -     0.000259   .               
 
Therefore, although the transmission peak maxima shift by a significant amount relative to the channel spacing, the relative decrease in FSR is much less than 1% and will not be observable in most practical applications.
 
     FIG. 2A  is an illustration of a second preferred embodiment of a air-density-tuned interferometer in accordance with the present invention. The second air-density-tuned interferometer  200  ( FIG. 2A ) is similar to the first air-density-tuned interferometer  100  ( FIG. 1 ) except that the second reflective coating  120  is a 100% reflective coating within the air-density-tuned interferometer  200 . Accordingly, since there is no transmitted light beam through the second reflective coating  120 , only a front window  188   a  is required and the interferometer contained within the enclosure  184  is a Gires-Tournois interferometer  201 . The front window  188   a  permits an input composite optical signal  104  to enter the Gires-Tournois interferometer  201  and a reflected output composite optical signal  105  to exit the Gires-Tournois interferometer. Since the rear coating  120  reflects all the light input to the Gires-Tournois interferometer  201 , there is no transmitted beam and all the wavelengths input to the interferometer  201  in composite optical signal  104  are reflected out of the interferometer  201  in the output composite optical signal  105 . Instead of separating the paths of lights of various wavelengths, the Gires-Tournois interferometer  201  produces a periodic phase delay or phase shift, which is a function of wavelength, in the reflected composite optical signal  105 . 
     FIG. 2B  is an illustration of a tunable multiplexer and de-multiplexer that utilizes an air-density-tuned Gires-Tournois interferometer similar to that illustrated in FIG.  2 A. The tunable multiplexer and de-multiplexer  203  ( FIG. 2B ) comprises a beam splitter  214  that reflects 50% of an input light and transmits 50% of an input light optically coupled to a mirror  219  at a first side and to an air-density-tuned Gires-Tournois interferometer  200  at a second side. The tunable multiplexer and de-multiplexer  203  further comprises a first lens or lens assembly  212  optically coupled to the beam splitter  214  at a third one of its sides, a second lens or lens assembly  218  optically coupled to the beam splitter  214  at a fourth one of its sides, an input fiber  210  and a first output fiber  216   a  optically coupled to the first lens or lens assembly  212  and a second output fiber  216   b  optically coupled to the second lens or lens assembly  218 . In the drawing of  FIG. 2B , the first output fiber  216   a  is to be understood as being behind the input fiber  210 . In an alternative embodiment, the first output fiber  216   a  may be eliminated and the one fiber  210  used bi-directionally for both input and output. In another alternative embodiment, the mirror  219  may be replaced by a second air-density-tuned Gires-Tournois interferometer. 
   The multiplexer and de-multiplexer  203  shown in  FIG. 2B  operates similarly to a Michelson interferometer. An input composite optical signal  213  is collimated by the first lens or lens assembly  212  and then directed into the beam splitter  214 . The beam splitter  214  separates the composite optical signal  213  into a first portion  215   a  that is transmitted straight through the beam splitter  214  and a second portion  215   b  that is reflected at the beam splitter  214 . The first portion  215   a  travels through the beam splitter  214  to the mirror  219  (or the air-density tuned interferometer  200 ) and is reflected therefrom back into the beam splitter  214 . The second portion  215   b  travels from the beam splitter  214  to the air-density tuned interferometer  200  (or the mirror  219 ) and is reflected therefrom back into the beam splitter  214 . The two portions  215   a - 215   b  recombine within the beam splitter  214  and are passed to the first  216   a  or the second  216   b  output fiber depending upon whether the returning lights of the two portions  215   a - 215   b  are in-phase or out-of-phase at the position at which they recombine. 
   The recombination of the two portions  215   a - 215   b  separates the lights according to wavelength since alternating bands of wavelengths are either in-phase or out-of-phase upon recombining within the beam splitter  214 . Therefore, the apparatus  203  can operate as a de-multiplexer, or, alternatively, as a multiplexer if the light pathways are reversed. Further, the tuning of the air-density-tuned interferometer  200  controls the round trip phase delay incurred by the portion (either  215   a  or  215   b ) that is reflected by the interferometer  200  and therefore controls the routing of the wavelengths upon recombination of the two portions  215   a - 215   b  within the beam splitter  214 . Therefore, the apparatus  203  can also operate as a switch. Those wavelengths that are in-phase upon recombination are focused by lens  212  into the first output fiber  216   a  whilst those wavelengths that are out of phase are focused by lens  218  into the second output fiber  216   b.    
     FIG. 2C  is an illustration of a tunable chromatic dispersion compensator that utilizes an air-density-tuned interferometer in accordance with the present invention. The tunable chromatic dispersion compensator  205  shown in  FIG. 2C  is a modified version of an apparatus disclosed in U.S. Pat. No. 6,487,342, assigned to the assignee of the present application. Applicant incorporates this patent herein by reference in its entirety. 
   The tunable chromatic dispersion compensator  205  ( FIG. 2C ) comprises an input fiber  220 , an output fiber  222  disposed adjacent to the input fiber  220 , a lens or lens assembly  228  optically coupled to the fibers  220 - 222  and disposed substantially at its focal distance f from the end faces of the fibers  220 - 222  and an air-density-tuned interferometer  200  of the Gires-Tournois type ( FIG. 2A ) optically coupled to the lens  228 . The air-density-tuned interferometer  200  is disposed at a side of the lens  228  opposite from the fibers  220 - 222  within the chromatic dispersion compensator  205 . The two fibers  220 - 222  are equidistantly disposed about an axis  230  that is parallel to the long dimension of the fibers  220 - 222  and passes through the center of the lens  228  and the air-density-tuned Gires-Tournois interferometer  200 . 
   In operation, a composite optical signal  104  that comprises undesired chromatic dispersion is delivered to the tunable chromatic dispersion compensator  205  from the input fiber  220 , passes through the lens  228  and is collimated by this lens  228 . The collimated composite optical signal then interacts with and reflects from the air-density-tuned Gires-Tournois interferometer  200  within a region  232 . The air-density-tuned Gires-Tournois interferometer  200  adds a periodically oscillating chromatic dispersion to each channel of the composite optical signal. This added chromatic dispersion compensates for undesired chromatic dispersion in the composite optical signal  104  through algebraic cancellation. Stated differently, light wavelengths of each channel comprising undesired positive chromatic dispersion receive compensatory negative chromatic dispersion and vice versa. The reflected, dispersion compensated composite optical signal  105  then returns from the air-density-tuned Gires-Tournois interferometer  200  as a collimated beam that is focused by lens  228  into the output optical fiber  222 . Alternatively, the output fiber  222  could be eliminated and the one fiber  220 , disposed on the axis  230 , could be used bi-directionally for both input and output. 
     FIG. 2D  is a schematic illustration of a tunable chromatic dispersion compensator system that utilizes air-density-tuned interferometers in accordance with the present invention. The dispersion compensator system  206  shown in  FIG. 2D  is a modified version of a system disclosed in the aforementioned U.S. Pat. No. 6,487,342 as well as in a co-pending U.S. patent application assigned to the assignee of the present application entitled “Synthesis of Optical Dispersion Compensators and Methods Using a Series of Gires-Tournois Cavities,” Ser. No. 09/750,933, filed on Dec. 29, 2000. Applicant incorporates this patent application herein by reference in its entirety. The tunable chromatic dispersion compensator system  206  ( FIG. 2D ) comprises a set of m multiple sequentially arranged tunable chromatic dispersion compensator devices  205 . 1 - 205 . m , each utilizing an air-density tuned Gires-Tournois interferometer. The individual devices  205 . 1 ,  205 . 2 , . . . ,  205 . m  are optically coupled in sequence by the m−1 optical couplings  234 . 1 - 234 .( m− 1), each of which preferably comprises an optical fiber. 
   In the system  206  (FIG.  2 D), the various air-density-tuned Gires-Tournois interferometers  200  within the sequence of individual tunable chromatic dispersion compensator devices  205 . 1 , 205 . 2 , . . . ,  205 . m  comprise various sets of operational parameters that may vary from one individual device to another. These operational parameters associated with a particular one of the individual devices  205 . 1 ,  205 . 2 , . . . ,  205 . m  comprise the reflectivity r 1  of the front mirror  188   a  of the interferometer  200  and the optical path length L 0  of the interferometer  200 . Since L 0 =ηL, the optical path length is tunable by the air-density tuning previously described. The sequence of devices shown in  FIG. 2D  causes a summation of the chromatic dispersion provided by the various individual devices. Through this summation, the compensatory chromatic dispersion introduced by the system  206  may be made to conform to a particular desired form, as a function of wavelength. 
   In operation, a composite optical signal  104  that requires chromatic dispersion compensation and comprises the set of n channels λ c   1 -λ c   n  enters the first tunable chromatic dispersion compensator device  205 . 1  in the series of devices. The composite optical signal  104  is then directed to each one of the remaining individual devices  205 . 2 - 205 . m  in sequence by means of the optical couplings  234 . 1 - 234 .( m− 1). In each of the individual devices  205 . 1 - 205 . m , partial compensatory chromatic dispersion is introduced into each of the channels λ 1 -λ n  comprising the composite optical signal  104 . Finally, the composite optical signal  105  comprising the chromatic dispersion compensated channels λ c   1 -λ c   n  exits the system  206  from the last device  205 . m . The compensatory chromatic dispersion introduced into each of the channels comprises the algebraic sum of that introduced within each one of the individual devices  205 . 1 - 205 . m . Since each individual device  205 . 1 - 205 . m  introduces a periodic chromatic dispersion curve into the composite optical signal and since the period and magnitude of the introduced chromatic dispersion may vary one such device to another in a controlled fashion, very complicated periodic dispersion may be generated by (and thus compensated by) the tunable chromatic dispersion compensator system  206 . 
     FIG. 3A  is an illustration of a third preferred embodiment of an air-density-tuned interferometer in accordance with the present invention. The third air-density-tuned interferometer  300  ( FIG. 3A ) comprises all the same components as does the second air-density-tuned interferometer  200  (FIG.  2 A). Further, the apparatus  300  comprises an internal birefringent waveplate  195  within the optical cavity  110  optically coupled between the partially reflective coating  140  and the 100% reflective coating  120  and an external birefringent waveplate  182  optically coupled to the front plate  180   a  outside the optical cavity  110 . Therefore, the interferometer  301  contained within the enclosure  184  comprise a non-linear interferometer of the type disclosed in U.S. Pat. No. 6,169,604 and in U.S. Pat. No. 6,310,690. Both of these patents, which are assigned to the assignee of the present application, are incorporated herein by reference in their entirety. The operation of such an interferometer is described in more detail in these referenced patents. In brief, however, an input linearly polarized light  104  comprised of multiple channels is reflected as light  107 , wherein the polarization of a first set of channels comprising reflected light  107  is rotated and the polarization of a second set of channels comprising reflected light  107  and interleaved with the first set of channels is not rotated. The first and second sets of channels may comprise alternating “odd” and “even” channels as shown in FIG.  4 B and discussed in reference to that figure or else may comprise sets of bands comprising non-equivalent band widths as shown in FIG.  4 D and discussed in reference to that figure. The air-density tuning of the interferometer  301  shifts the wavelength bands comprising rotated polarization and non-rotated polarization to facilitate switching or channel grid alignment in the 2×2 tunable and switchable interleaved channel separator apparatuses described in the following. 
     FIG. 3B  is an illustration of a first 2×2 tunable and switchable interleaved channel separator apparatus that utilizes air-density-tuned interferometers  300   a - 300   b  in accordance with the present invention and as shown in FIG.  3 A. The 2×2 tunable and switchable interleaved channel separator device  302 , shown in  FIG. 3B , is a modified version of an apparatus disclosed in U.S. Pat. No. 6,130,971, that is assigned to the assignee of the present application and which is incorporated herein by reference in its entirety. The 2×2 tunable and switchable interleaved channel separator device  302  ( FIG. 3B ) comprises a first input optical fiber  310  and a second input optical fiber  340  for inputting optical signals and first  320  and second  330  output optical fibers for outputting optical signals. As an input composite optical signal leaves the first input optical fiber  310 , it diverges. A first lens  350  collimates the input composite optical signal and directs it toward a polarization beam splitter  370  which decomposes the composite optical signal into two sub-signals having mutually orthogonal polarizations. The s-polarized portion of the input composite optical signal polarized parallel to a plane in the polarization beam splitter  370  is reflected towards a first air-density-tuned non-linear interferometer  300   a . The p-polarized portion of the signal polarized perpendicularly to the plane in the polarization beam splitter  370  passes through towards a second air-density-tuned non-linear interferometer  300   b.    
   The set of channels comprising light whose polarization is rotated (the even channels, in this example) is directed to the second output optical fiber  330  and the light comprising the other set of channels (“odd channels”) is directed to the first output optical fiber  320 . When light is input from the second input optical fiber  340 , the set of channels comprising light whose polarization is rotated is directed to the first output optical fiber  320  and the light comprising the other set of channels is directed to the second output optical fiber  330 . 
   Each of the air-density-tuned non-linear interferometers  300   a - 300   b  comprising the 2×2 tunable and switchable interleaved channel separator device  302  may be tuned or varied by changing the density, and hence the refractive index, of air or gas within the interferometers. This tuning causes controlled variations or shifts of the wavelengths whose polarization plane is rotated and of the wavelengths whose polarization is not rotated. It is assumed that the two air-density-tuned non-linear interferometers  300   a - 300   b  are always both in the same state of tuning. Thus, for instance, by simultaneously tuning both of the air-density-tuned non-linear interferometers  300   a - 300   b  so as to cause shifts equivalent to one-half of their common FSR, the output pathways of odd and even channels may be reversed—and thereby switched—from those described above. 
     FIGS. 3C-3D  are illustrations of a second 2×2 tunable and switchable interleaved channel separator apparatus that utilizes a single air-density-tuned interferometer in accordance with the present invention and as shown in FIG.  3 A. The 2×2 tunable and switchable interleaved channel separator device  304 , shown in  FIGS. 3C-3D , is a modified version of an apparatus disclosed in a co-pending U.S. patent application assigned to the assignee of the present application entitled “Multi-Functional Optical Device Utilizing Multiple Polarization Beam Splitters and Non-Linear Interferometers,” Ser. No. 09/630,891, filed on Aug. 2, 2000. Applicant incorporates this patent application herein by reference in its entirety. 
   The 2×2 tunable and switchable interleaved channel separator device  304  ( FIGS. 3C-3D ) receives input from a first input optical port  416   a  ( FIG. 3C ) and separates the channels therein into a first set of channels and a second set of channels, wherein the first and second sets are interleaved with one another or are defined by interleaved pass bands, wherein the first set is output to a second output optical port  416   d  and the second set is output to a first output optical port  416   b . The 2×2 tunable and switchable interleaved channel separator device  304  further receives input from a second input optical port  416   c  ( FIG. 3D ) and separates the channels therein into a third set of channels and a fourth set of channels, wherein the third set comprises the same wavelengths as the first set and is output to the first output optical port  416   b  and wherein the fourth set comprises the same wavelengths as the second set and is output to the second output optical port  416   d.    
   All four ports  416   a - 416   d  comprising the apparatus  304  are polarizing ports of the type illustrated in FIG.  6 . The polarizing port  416  ( FIG. 6 ) comprises an optical fiber  680 , an optical collimator  682 , a birefringent walk-off plate  684  and a reciprocal optical rotator  686 . The optical collimator  682  is optically coupled to the optical fiber  680  and either receives input from or directs output to the fiber  680 . The birefringent walk-off plate  684  of the polarizing port  416  ( FIG. 6 ) is optically coupled to the collimator  682  at a side opposite to the fiber  680  and has the property of physically separating an unpolarized light beam received from collimator  682  into a deflected light beam  690  and an un-deflected light beam  688 . The deflected light  690  comprises an e-ray having a first linear polarization orientation and the un-deflected light  688  comprises an o-ray having a second linear polarization orientation perpendicular to that of the e-ray. The reciprocal optical rotator  686 , which is optically coupled to the birefringent walk-off plate  684  at a side opposite to the collimator  682 , is disposed so at to intercept the path of only one of the two beams  688 - 690 . The reciprocal optical rotator  686  rotates the polarization orientation of said intercepted beam by 90° so as to be parallel to that of the other beam. In the reverse light propagation direction, that is, when the polarizing port  416  is utilized as an output port, the reciprocal optical rotator  686  rotates the polarization orientation of only one of two beams so that said beams subsequently comprise mutually orthogonal polarization orientations and such that these two beams are subsequently combined upon passage through the birefringent walk-off plate  684 . The reciprocal optical rotator  686  may be disposed so as to intercept either the o-ray  688  or the e-ray  690  and preferably comprises a half-wave plate. 
   Referring once again to  FIGS. 3C-3D , the 2×2 tunable and switchable interleaved channel separator device  304  further comprises a first  402  and a second  404  polarization beam splitter (PBS) between which are disposed a first non-reciprocal optical rotator  406  and a first reciprocal optical rotator  408 . The first PBS  402  receives optical input from the input port  416   a  which is disposed adjacent to a side of the PBS  402  opposite to the first non-reciprocal optical rotator  406  and the first reciprocal optical rotator  408 . An air-density-tuned non-linear interferometer  300  is disposed adjacent to the second PBS  404  at a side opposite to the first non-reciprocal optical rotator  406  and first reciprocal optical rotator  408 . The input port  416   a , first PBS  402 , second PBS  404 , first non-reciprocal optical rotator  406 , first reciprocal optical rotator  408  and air-density-tuned non-linear interferometer  300  are disposed along a line which defines a main axis or dimension of the device  304 . 
   The 2×2 tunable and switchable interleaved channel separator device  304  further comprises a third PBS  422  disposed off the main axis and optically coupled to port  416   b  and to port  416   c  and a second non-reciprocal optical rotator  418  and a second reciprocal optical rotator  420  disposed between the third PBS  422  and the second PBS  404 . The 2×2 tunable and switchable interleaved channel separator device  304  may further comprise an optical reflector  412  disposed adjacent to a face of the first PBS  402  which does not intersect the main axis of the apparatus. The optical reflector  412  may comprise a right-angle prism, as shown, but could also comprise a mirror. If present, the optical reflector  412  is optically coupled between the first PBS  402  and port  416   d . If the optical reflector is not present, then the port  416   d  is directly optically coupled to the PBS  402 . Another optical reflector could likewise be optically coupled between the third PBS  422  and the port  416   c.    
   The first non-reciprocal optical rotator  406  and the second non-reciprocal optical rotator  418  within the 2×2 tunable and switchable interleaved channel separator  304  preferably are Faraday rotators. The first reciprocal optical rotator  408  and the second reciprocal optical rotator  420  are preferably half-wave plates. The first pair of optical rotators  406 - 408  has the property such that linearly polarized light passing completely therethrough from left to right does not incur polarization plane rotation whilst linearly polarized light passing completely therethrough from right to left does incur a 90° rotation of its polarization plane. Likewise, the second pair of optical rotators  418 - 420  has the property such that linearly polarized light passing completely therethrough from top to bottom does not incur polarization plane rotation whilst linearly polarized light passing completely therethrough from bottom to top does incur a 90° rotation of its polarization plane. The reciprocal and the non-reciprocal optical rotator comprising each such pair need not be disposed in the order shown. One of ordinary skill in the art will know how to configure a reciprocal and a non-reciprocal optical rotator so as to have the properties noted above. 
   The three PBS&#39;s  402 ,  404  and  422  comprising the 2×2 tunable and switchable interleaved channel separator device  304  each have the property of transmitting signal light comprising a first polarization (p-polarization) therethrough whilst simultaneously deflecting or reflecting signal light comprising a second polarization (s-polarization). The air-density-tuned non-linear interferometer  300  operates similarly to the air-density-tuned non-linear interferometers  300   a - 300   b  utilized in the 2×2 tunable and switchable interleaved channel separator  302  ( FIG. 3B ) and causes polarization rotation of linearly polarized light of the second set of channels and the fourth set of channels upon reflection therefrom while leaving the polarization orientation of linearly polarized light of the first and third sets of channels unchanged. 
   Further shown in FIGS.  3 C- 3 D), as well as in several following figures of this document, are the polarization orientations of various signal light rays. These polarization orientations are indicated by double barbed arrows and/or crosses inscribed within circles. Unless otherwise indicated, double barbed arrows indicate light polarization along the indicated direction within the plane of the illustration, and crosses indicate light polarization normal to the plane of the page. Superimposed arrows and crosses either indicate unpolarized or randomly polarized light or superimposed rays which, in projection, have mutually perpendicular polarization plane orientations. These polarization-indicating symbols are included for the convenience of the reader and are not to be construed as components of the invention. Each light pathway illustrated in  FIGS. 3C-3D  actually comprises two beams, both with the same linear polarization orientation, as shown in FIG.  6 . These two beams are offset from one another along a direction perpendicular to the plane of the drawing and therefore cannot be shown separately within  FIGS. 3C-3D . 
   Details of the operation of the 2×2 tunable and switchable interleaved channel separator device  304  are provided in the aforementioned co-pending U.S. patent application with Ser. No. 09/630,891.  FIG. 3C  illustrates the pathways of an input composite optical signal comprising the channels λ 1 -λ n  from the first input port  416   a  to the interferometer  300 , of a first set of channels (e.g. “odd” channels λ 1 , λ 3 , λ 5  . . . ) from the interferometer  300  to the second output port  416   d  and of a second set of channels (e.g. “even” channels λ 2 , λ 4 , λ 6  . . . ) from the interferometer  300  to the first output port  416   b .  FIG. 3D  illustrates the pathway of a second composite optical signal comprising the channels λ′ 1 -λ′ n  from the second input port  416   c  to the interferometer  300 , of a third set of channels (e.g. “odd” channels λ′ 1 , λ′ 3 , λ′ 5  . . . ) from the interferometer  300  to the first output port  416   b  and of a fourth set of channels (e.g. “even” channels λ′ 2 , λ′ 4 , λ′ 6  . . . ) from the interferometer to the second output port  416   d . These pathways may be verified by noting the polarization states the signal lights at various points along the pathways and by taking account of the operation of the PBS&#39;s, the operation of the pairs of optical rotators, the operation of the air-density-tuned non-linear interferometer and the operation of the polarizing ports. 
     FIGS. 3E-3F  are each a top view and a side view of a third 2×2 tunable and switchable interleaved channel separator apparatus that utilizes an air-density-tuned interferometer in accordance with the present invention illustrating the pathways of signal light rays therethrough. The 2×2 tunable and switchable interleaved channel separator device  306 , shown in  FIGS. 3E-3F  is a modified version of an apparatus disclosed in U.S. Pat. No. 6,396,629 and in a co-pending U.S. patent application entitled “Switchable Interleaved Optical Channel Separator And Isolator Device And Optical Systems Utilizing Same” Ser. No. 09/792,231, filed on Feb. 23, 2001. Both of these are assigned to the assignee of the present application and are incorporated herein by reference in their entirety. 
   The 2×2 tunable and switchable interleaved channel separator device  306  receives first input from a first input optical port  416   a  and receives second input from a second input optical port  416   c  (top drawing of FIG.  3 E). The device  306  then separates the channels received from the first optical port into a first set of channels and a second set of channels wherein the first and second sets of channels are interleaved with one another or are defined by interleaved pass bands, and separates the channels received from the second optical port into a third set of channels and a fourth set of channels, wherein the wavelengths of the third channels are the same as the wavelengths of the first channels and wherein the wavelengths of the fourth channels are the same as the wavelengths of the second channels. The first set of channels and the fourth set of channels are output to a second output optical port  416   d  whilst the second set channels and the third set of channels are output to a first output optical port  416   b  (FIG.  3 F). 
   As shown in  FIGS. 3E-3F , the 2×2 tunable and switchable interleaved channel separator device  306  comprises four optical ports  416   a - 416   d , a polarization beam splitter (PBS)  512  optically coupled to the two input ports  416   a  and  416   c , a first birefringent walk-off plate  502  (or, simply termed. “birefringent plate”) optically coupled to the two output ports  416   b  and  416   d  and the PBS  512 , a second birefringent plate  504 , a non-reciprocal optical rotator  406  and a reciprocal optical rotator  408  disposed between and optically coupled to the first and second birefringent plates, and an air-density-tuned non-linear interferometer  300  optically coupled to the second birefringent plate  504 . 
   All four ports  416   a - 416   d  comprising the 2×2 tunable and switchable interleaved channel separator device  306  are polarizing ports of the type illustrated in FIG.  6 . The first input port  416   a , PBS  512 , first birefringent plate  502 , second birefringent plate  504 , first non-reciprocal optical rotator  406 , first reciprocal optical rotator  408  and air-density-tuned non-linear interferometer  300  are disposed along a line which defines a main axis or dimension of the 2×2 tunable and switchable interleaved channel separator device  306 . The second input port  416   c  is at an angle to this axis (FIG.  3 E). The first output port  416   b  and the second output port  416   d  are disposed to either side of the input port  416   a  and are optically coupled to the first birefringent plate  502 . The pair of optical rotators  406 - 408  does not rotate the polarization plane of light passing therethrough from left to right but does rotate, by 90°, the polarization plane of light passing therethrough from right to left. 
   The PBS  512  receives p-polarized optical input from the first input port  416   a  and receives s-polarized light from the second input port  416   c  (FIG.  3 E). The signals delivered to the apparatus  306  from the first  416   a  and the second  416   c  input port have mutually perpendicular linear polarizations and are combined by the PBS  512 . The combined signals then pass from the PBS  512  into the first birefringent plate  502 . 
   The two birefringent plates  502 - 504  ( FIGS. 3E-3F ) each have the property of transmitting signal light comprising a first polarization (o-ray) therethrough substantially parallel to the main axis whilst simultaneously causing a deflection or offset of a signal light comprising a second polarization (e-ray). The path of the e-ray is deflected within either birefringent plate but is substantially parallel to (thereby offset from) that of the o-ray immediately upon exiting the plate. The optical axes of the two birefringent plates  502 - 504  are disposed such that, for e-rays passing through both such birefringent plates in a same direction, the offset of the e-ray immediately caused by passage through the second such birefringent plate is equal and opposite to the offset of the e-ray immediately caused by the passage through the first birefringent plate. As oriented in  FIGS. 3E-3F , the e-rays and α-rays are polarized vertically and horizontally, respectively, during their traverses through the birefringent plates  502 - 504 . 
   The pathways and polarization orientations of signal rays are shown in both top view (top diagram of each figure) and side view (lower diagram of each figure) in  FIGS. 3E-3F . The complete set of two input beams, as separated by either the first  416   a  or the second  416   c  input port, is only visible in the top view of each figure. The light of a first WDM composite optical signal, which, for illustration purposes only, is assumed to be comprised of a plurality of wavelength division multiplexed channels λ 1 -λ n , is input to the PBS  512  from the first input port  416   a  ( FIG. 3E ) such that the two separated input beams both comprise p-polarized light with respect to the PBS  512 . The light of a second WDM composite optical signal, which, for illustration purposes only, is assumed to be comprised of a plurality of wavelength division multiplexed channels λ′ 1 -λ′ n , is input to the PBS  512  from the second input port  416   c  ( FIG. 3E ) such that the two separated input beams both comprise s-polarized light with respect to the PBS  512 . The p-polarized channels λ 1 -λ n  are transmitted directly through the PBS  512  and the s-polarized channels λ′ 1 -λ′ n  are reflected within the PBS  512  such that these two sets of channels are spatially combined. 
   After being spatially combined by the PBS  512 , the channels λ 1 -λ n  and the channels λ′ 1 -λ′ n  enter the first birefringent plate  502 . The horizontally polarized channels λ 1 -λ n  comprise undeflected o-rays and the vertically polarized channels λ′ 1 -λ′ n  comprise deflected e-rays within the first birefringent plate  502  (FIG.  3 E). After emerging from the first birefringent plate  502 , channels λ 1 -λ n  pass through the non-reciprocal optical rotator  406 , the reciprocal optical rotator  408  and the second birefringent plate  504 . The elements  406 - 408  are disposed such that light passing through both from left to right does not experience polarization plane rotation. Thus, the channels λ 1 -λ n  and λ′ 1 -λ′ n  respectively propagate through the second birefringent plate  504  as an undeflected o-ray and as a deflected e-ray (FIG.  3 E). 
   The λ 1 -λ n  and λ′ 1 -λ′ n  channels all arrive at the points  515   a - 515   b  on the air-density-tuned non-linear interferometer  300 . The air-density-tuned non-linear interferometer  300  operates similarly to those utilized in the 2×2 tunable and switchable interleaved channel separators  302  ( FIG. 3B ) and  304  ( FIGS. 3C-3D ) and causes polarization rotation of linearly polarized light of the second set of channels and the fourth set of channels upon reflection therefrom while leaving the polarization orientation of linearly polarized light of the first and third sets of channels unchanged. 
     FIG. 3F  illustrates the return pathways of the first set of channels and the identical return pathways of the fourth set of channels through the 2×2 tunable and switchable interleaved channel separator  306 .  FIG. 3F  farther illustrates the return pathways of the second set of channels and the identical return pathways of the third set of channels through the 2×2 tunable and switchable interleaved channel separator  306 . The term “return pathway” herein refers to the pathway of a channel after its reflection from and interaction with the air-density-tuned non-linear interferometer  300 . In these examples, it is assumed that the first channels comprise the odd channels λ 1 , λ 3 , λ 5  . . . , that the second channels comprise the even channels λ 2 , λ 4 , λ 6  . . . , that the third channels comprise another set of odd channels λ′ 1 , λ′ 3 , λ′ 5  . . . and that the fourth channels comprise another set of even channels λ′ 2 , λ′ 4 , λ′ 6  . . . . 
   As illustrated in the lower diagram of  FIG. 3F , the light of the reflected first set of channels, whose polarization is not rotated by the air-density-tuned non-linear interferometer  300 , remains horizontally polarized upon re-entering the second birefringent plate  504 . Further, the light of the reflected fourth set of channels, whose polarization is rotated by 90° by the air-density-turned non-linear interferometer  300 , is also horizontally polarized upon re-entering the second birefringent plate  504 . As a result, on its return pathway, the light of the first set of channels λ 1 , λ 3 , λ 5  . . . and of the fourth set of channels λ′ 2 , λ′ 4 , λ′ 6  . . . comprises o-rays with respect to the second birefringent plate  504  and passes directly through the birefringent plate  504  without deflection towards the reciprocal optical rotator  408  and non-reciprocal optical rotator  406 . Simultaneously, the light of the reflected second set of channels λ′ 1 , λ′ 3 , λ′ 5  . . . whose polarization plane is rotated by 90° upon reflection from air-density-tuned non-linear interferometer  300 , and the light of the reflected third set of channels λ′ 1 , λ′ 3 , λ′ 5  . . . , whose polarization is not rotated upon reflection from air-density-tuned non-linear interferometer  300 , comprise e-rays with respect to the second birefringent plate  504  and are therefore deflected along their return pathway within the second birefringent plate  504 . 
   During passage from right-to-left through the pair of elements  406 - 408 , the polarization plane orientation of light is rotated by 90°. Because of this rotation, the light of the first set of channels and of the fourth set of channels thus becomes polarized as c-rays within the first birefringent plate  502  and the light of the second set of channels and of the third set of channels becomes polarized as o-rays within the first birefringent plate  502  (FIG.  3 F). The first birefringent plate  502  therefore deflects the light comprising the first and fourth sets of channels but allows the light comprising the second and third sets of channels to pass directly therethrough without deflection. The optic axes of the two birefringent plates  502 - 504  are symmetrically oriented with respect to one another about a vertical plane perpendicular to the axis of device  306 . Because of this disposition of the two optic axes, the offsets of channels polarized as e-rays in birefringent plate  502  and of channels polarized as e-rays in birefringent plate  504  are opposite to one another as illustrated by comparison of the pathways shown in the lower diagram of FIG.  3 F. 
   Subsequent to passing through the birefringent plate  502  in the return direction, the first and fourth sets of channels are directed to the second output port  416   d  and the second and third sets of channels are directed to the first output port  416   b  (FIG.  3 F). As described previously, since each port is a polarizing port, the two physically separate beams comprising each channel are recombined by each respective output port and focused into the respective fiber of the port. The second output port  416   d  (first output port  416   b ) is disposed so as to only receive vertically (horizontally) polarized light and thus receives the vertically (horizontally) polarized light of the first and fourth (second and third) sets of channels upon exit of this light from the birefringent plate  502 . In this fashion the apparatus  306  behaves as a 2×2 interleaved channel separator. The tuning and switching capability arises from the adjustment capability of the air-density-tuned non-linear interferometer  300  as previously described. 
     FIG. 4A  is a functional signal routing diagram of two different switch states of a 2×2 tunable and switchable-interleaved channel separator device, which may comprise any of the devices  302 - 306 . In the first switch state “0”, odd-channel signals input to the switchable interleaved channel separator device from Port A and from Port C are directed to Port B and to Port D, respectively, whereas even channel signals input to the tunable and switchable interleaved channel separator device from Port A and from Port C are directed to Port D and to Port B, respectively. For instance, as shown in the upper diagram of  FIG. 4A , in the state “0” the output at Port B consists of the odd channels (λ 1 , λ 3 , λ 5  . . . ) from the first composite optical signal λ 1 -λ n  input at Port A plus the even channels (λ′ 2 , λ′ 4 , λ′ 6  . . . ) from the second composite optical signal λ′ 1 -λ′ n  input at Port C. Further, in the state “0”, the output at Port D consists of the odd channels (λ′ 1 , λ′ 3 λ′ 5  . . . ) from the second composite optical signal plus the even channels (λ 2 , λ 4 , λ 6  . . . ) from the first composite optical signal. In the state “1”, the pathways of the odd channels and the even channels are reversed from those in the state “0” as shown in the lower diagram of FIG.  4 A. 
   The principle that permits the change of operational states of the 2×2 tunable and switchable interleaved channel separators  302 - 306  is illustrated in  FIG. 4B , which presents two schematic graphs of the spectrum  321  of pass bands of the polarization-rotated light and the spectrum  322  of pass bands of non-polarization-rotated light. The upper and lower graphs of  FIG. 4B  represent the spectra of pass bands reflected from an air-density-tuned non-linear interferometer in a first operational state “0” and in a second operational state “1”, respectively. The locations of “odd” channels  324  and of “even” channels  326  are also shown in the graphs of FIG.  4 B. As previously discussed, tuning of the air-density-tuned interferometers comprising the channel separator causes the spectra  321 - 322  of pass bands effectively “shift” to either the left or the right accordingly. A very slight change in the pass band widths also accompanies this shift, but this effect is negligible in regards to the operation of any of the air-density-tuned non-linear interferometers  302 - 306 . The effect of the shift of the spectra of pass bands is to cause the spectrum  321  of pass bands of polarization rotated light to either coincide with the locations of the even channels  326  or the odd channels  324 , depending upon the selected operational state, as may be seen be comparing the two graphs of FIG.  4 B. 
     FIG. 4C  is an alternative functional signal routing diagram of a tunable and switchable interleaved channel separator device. In this alternative routing scheme, the apparatus only comprises one input port (Port A). In the state “0”, the switchable interleaved channel separator device directs odd charnels to Port B and even channels to Port D. In the state “1”, the odd channels are directed to Port D and the even channels are directed to Port B. 
     FIG. 4D  is an alternative functional signal routing diagram of a tunable and switchable interleaved channel separator device. In this alternative routing scheme, channels input at either Port A or Port C are asymmetrically separated into two sets of bands shown in the lower drawing of FIG.  4 D. The input channels are thus divided into a set  202  of narrow bands and a set  204  of wide bands that is interleaved with the set of narrow bands. The set of wide bands from Port A is then combined with the set of narrow bands from Port C since these combined sets are both routed to Port B as shown in the upper diagram of FIG.  4 D. Further, the set of wide bands from Port C is combined with the set of narrow bands from Port A to be output together at Port D. 
   This separation of a composite optical signal into a first set of channels defined by a set of first band widths and into a second set of channels defined by a second set of band widths different from the first band widths is herein referred to as asymmetric interleaved channel separation. The relation between the band widths w 1  and w 2  is controlled by the retardance of the waveplates and the reflectance of the front mirror of an air-density-tuned non-linear interferometer ( FIG. 3A ) as is disclosed in U.S. Pat. No. 6,310,690. This patent is assigned to the assignee of the present application and is incorporated herein by reference in its entirety. The positions of the channels encompassed by the narrow bands and by the wide bands are indicated by stippled patterns and striped patterns, respectively, in the lowermost diagram of FIGS.  4 D. 
   In the particular switch state shown in  FIG. 4D , the signal channels denoted in stippled patterns are directed, in logical crosswise fashion, from Port A to Port D and from Port C to Port B within a 2×2 tunable and switchable interleaved channel separator. The signal channels denoted in striped patterns in  FIG. 4D  are directed from Port A to Port B and from Port C to Port D. The output at either Port B or Port D comprises the collection of channels routed from both Port A and Port C. For instance, as shown in the upper diagram, of  FIG. 4D , the output at Port B consists of the channels (λ 2 -λ 8 , λ 10 -λ 16 , λ 18 -λ 24  . . . ) from the first composite optical signal λ 1 -λ n  input at Port A plus the channels (λ 1 ′, λ 9 ′, λ 17 ′ . . . ) from the second composite optical signal λ 1 ′-λ n ′ input at Port C. The output at Port D consists of the channels (λ 1 , λ 9 , λ 17  . . . ) from the first composite optical signal plus the channels (λ′ 2 -λ′ 8 , λ′ 10 -λ′ 16 , λ′ 18 -λ′ 24  . . . ) from the second composite optical signal. 
   The exact pressure within the constant volume enclosure  184  comprising an air-density-tuned interferometer may be varied and controlled by the pressure-changing device  186 . Examples of preferred pressure-changing devices are illustrated in  FIGS. 5A-5C .  FIG. 5A  illustrates a first pressure-changing device  186 . 1  comprising a gas inlet tube  187  coupled to a gas reservoir or pump  189 . Gas may be either forced or pumped from the gas reservoir or pump  189  into the interior of the enclosure  184  through the gas inlet tube so as to increase the density of gas within the enclosure  184 . Alternatively, gas may be withdrawn from the enclosure  184  through either the inlet tube  187  or through an auxiliary outlet tube (not shown) so as to decrease the density of gas within the enclosure  184 . Increasing air density increases the refractive index and decreasing air density decreases the refractive index of air within the enclosure  184 . 
     FIG. 5B  illustrates a second pressure-changing device  186 . 2  comprising a plunger  194  disposed within a feed-through  192 . The plunger  194  forms a gas-tight seal with the feed-through so that air or gas cannot enter into or escape from the interior of the enclosure  184 . The mass of gas within the enclosure  184  therefore remains constant. However, the plunger  194  is free to slide within the feed-through  192  either closer to or further from the interior of the enclosure  184 . This sliding of the plunger directly changes the density of gas within the enclosure  184  by either compressing the gas or allowing the gas to expand. 
     FIG. 5C  illustrates a third pressure-changing device  186 . 3  that is suitable for use in situations in which only small refractive index adjustments are required. The device  186 . 3  comprises a plunger  198  that is disposed outside of the enclosure  184  and a flexible membrane portion  196  of the enclosure. Preferably, the membrane portion  196  comprises a thinned portion of the wall of the enclosure  184 . Alternatively, the membrane portion  196  may comprise a separate membrane affixed over a hole in the enclosure wall with a gas-tight seal. The plunger  198  either depresses or retracts from the flexible membrane portion  196 . The membrane portion  196  is flexed into the interior of the enclosure  184  when the plunger  198  is pushed against it, thereby slightly decreasing the gas volume and slightly increasing the pressure, density and refractive index of the constant mass of air or gas within the enclosure. When the plunger  198  is withdrawn, the membrane portion  196  may flex outward so as to equilibrate the pressure inside and outside of the enclosure  184 . This outward flexing decreases the pressure, density and refractive index of the air or gas within the enclosure  184 . Preferably, air or gas cannot enter into or escape from the interior of the enclosure  184  through the membrane portion  196 . 
   An improved tunable interferometer apparatus and methods and systems utilizing said apparatus in fiber optic communications networks have been disclosed. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.