birefringent or polarization filters are described in “ optical waves in crystals ”, yeh and yariv , referenced above . the transmission through a birefringent element sandwiched between parallel polarizers is periodic in frequency and is given by : i  ( λ ) := [ cos  [ π · ( n e - n 0 ) λ · l · 10 6 ] ] 2 eqn .  1 where i is the intensity , λ is the wavelength in nanometers , n e is the extraordinary index of refraction , n o is the ordinary index of refraction and l is the thickness of the crystal in mm . the crystal is oriented with its optic axis at 45 degrees to the input polarization . note that in this governing relationship the transmission is periodic in optical frequency ( i . e . inverse wavelength ) and the transmission is lossless at the peak . the present systems use this fundamental controlling eqn . 1 together with a number of others in providing shaped transmissivity characteristics with low insertion loss , polarization insensitivity and compensation for temperature variation . fig1 to 4 depict the arrangement and relationships of the components of a 1 × 2 passband flattened and temperature compensated interleaving filter 10 , the components of which are seated on a generally planar surface 13 of a stainless steel optical bench 12 ( see fig2 and 3 ). the optical bench 12 ( which alternatively can be of other materials such as silicon ) contains shaped recesses 14 formed by electron discharge machining ( edm ), in which recesses 14 in the planar surface 13 the polarizing components and crystals are mounted and precisely aligned and angled . a gradient index ( grin ) lens responsive to the wideband input stream from an optical fiber is a collimator 16 for the wdm channel stream . the collimator lens 16 is secured within a cylindrical metal housing 20 which is welded within a stainless steel clip 22 of general u - shape , where base legs 24 , 25 are welded to the optical bench 12 . the clip 22 design allows for precise and stable tilt and translation adjustment of the collimator lens during assembly . after collimation , the input beam is transmitted through a beam displacing polarizer 28 , here of yvo4 crystal , which splits the input beam into two parallel beams ( fig4 particularly ) with crossed polarizations that are shifted 1 mm with respect to the other . splitting the input beam into separate polarizations and then recombining after filtering results in a polarization independent device . the two beams are then incident on a first birefringent crystal stage comprised of a yttrium orthovanadate ( yvo4 ) crystal 30 and a lithium niobate ( linbo 3 crystal , 32 configured to form a first a temperature compensated pair . there are a number of different birefringent materials which can be used for temperature compensation , including yvo4 and linbo3 . yvo4 has high birefringence , δn = 0 . 2039 at 1550 nm , and is readably available commercially . linbo3 has a large thermooptic coefficient opposite in sign to yvo4 and is also readably available commercially . the required length ratio for temperature compensation of yvo4 to linbo3 is 6 . 60 : 1 . the lengths scale inversely with channel spacing , and 50 ghz and 25 ghz spacing are achievable . the optical ( c ) axis of the yvo4 crystal and the linbo3 crystal 32 are oriented at 45 ° relative to the input polarizer 28 . for a 100 ghz to 200 ghz channel spacing the yvo4 crystal 30 is 7 . 370 mm long and the lithium niobate crystal 32 is 1 . 116 mm long . the second stage , which is also temperature compensated but employed to flatten the peak of maximum transmissivity of the passband is comprised of another set of yvo4 36 and lithium niobate crystals 38 . the lengths of each of these are twice that of the like crystals used in the first stage . the optical axes of each separate crystal 36 , 38 of the second pair are oriented along a crystal edge and the crystals are tilted — 14 . 8 degrees with respect to the top surface 13 of the optical bench 13 . the edges of the lithium niobate crystals 32 and 38 are electrically shorted with a conductive coating 40 such as silver epoxy which conducts away charges built up due to the pyroelectric property of linbo 3 . such electric charges would otherwise tend to build up on the surface of crystals 32 , 38 as the temperature is cycled , and the result could be uncontrolled hysteresis of the index of refraction . such charges must be eliminated for linbo 3 to be used as a temperature compensating element . conductive silver epoxy 40 ( or metallization or anti static liquid ) of the +/− c faces of a crystal electrically shorts the crystal and dissipates charges . dopants such as mgo which reduce the electrical conductivity would also reduce the pyroelectric effects . the two beams after being transmitted through the birefringent crystals 30 , 32 , 36 , 38 are incident on another yvo4 beam displacing polarizer 41 . each input beam is split up into two beams with different polarizations , as best seen in fig4 . two of the beams with crossed polarization overlap and are coupled directly into one of two output grin lens collimators 47 , 48 . the two other beams are combined in part by using a single prism 42 to direct the s polarized beam to one side of a polarizing beam splitter cube , 43 with the p polarized beam being redirected off a pair of prisms 44 , 45 to an orthogonal side of the beam splitter cube 43 . the path lengths of the two combined beams are matched to better than 1 mm in order to minimize polarization mode dispersion ( pmd ). the resulting overlapping beams are then coupled into the second output collimator 48 . the output collimators 47 , 48 are laser welded to clips which are in turn laser welded to the optical bench 12 . referring specifically to fig2 the optical bench 12 is mounted inside a tray 50 with a fiber feedthrough 52 in the end wall 54 receiving an input optical fiber 18 in line with the input collimator 16 . although the output side is not shown in this view it differs in having only pairs of elements for delivering the two output beams . the bench 12 is attached to the base of the tray 50 with rtv adhesive or , as shown , a silicone sheet 58 can be used to provide cushioning from shock and vibration . the input fiber 18 and two output fibers are fed through the fiber feedthroughs and sealed with epoxy . a lid 58 is attached to the body of the tray 50 and the waist is sealed with epoxy in a dry nitrogen atmosphere . all of the optical surfaces , including the crystals , are antireflection coated to minimize optical loss . the experimentally measured transmission ( using an led and an optical spectrum analyzer ) of a fiber coupled , passband flattened 100 ghz interleaver is shown in fig5 . the unit uses two stages of lithium niobate and yvo4 crystals . this measured response is charted in fig5 . the spacing between transmissivity peaks is that of a 100 ghz channel spacing to 200 ghz channel spacing interleaving filter . wider passband flatness in a filter ( i . e . a broadening of the width of the transmissivity peaks ), prevents narrowing of the transmission spectra when filters are cascaded and reduces the required wavelength accuracy of the wdm source lasers . it also improves system performance by reducing the attenuation of the information content of a modulated signal . by adding additional birefringent elements , the passband of the interleaving filter is flattened to a selectable degree . as shown in fig6 which depicts response variations between one , two and three crystals in series , the passband of a single element birefringent filter is 0 . 35 nm wide at the − 0 . 5 db bandwidth by adding a second and third birefringent element , shown by dotted and dashed lines respectively , the maximum is progressively broadened . the first element is of length l and oriented with its c axis at 45 degrees . for a series of two , the second element is of length 2 l and has an orientation of — 14 . 8 degrees , substantially widening the amplitude at maximum without broadening the cutoff point . addition of a third element of length 2 l and orientation =+ 10 degrees broadens the maximum even further , but introduces intermediate dips of minor magnitude . for a 100 ghz / 200 ghz interleaver the bandwidth for a passband flatness of − 0 . 5 db is 0 . 35 nm for the single stage , 0 . 47 nm for the two stage , and 0 . 60 nm for the three stage design . even better flatness can be achieved by adding more elements ; however , this comes at the expense of additional cost and insertion loss . although the curves depict the results of measurements with crystals of only one type , they are equally valid for temperature compensated combinations using different crystal types . the polarization dependent loss ( pdl ) of the interleaving filter must be minimized to a value below 0 . 1 db . this is achieved during coupling of the two crossed polarization beams incident on each output grin lens collimator 46 or 48 . in order to minimize pdl the beams must be coupled into the output fiber with the same efficiency . this is not necessarily at the peak coupling efficiency of each beam . this can be determined by varying or switching the input polarization to the interleaving filter 10 until no variation of power on the output fibers is measured . in order to make a polarization independent fiber based interleaving filter , the signal is split up into two beams using the lossless beam displacing polarizer 28 . the two beams are transmitted through the birefringent elements 30 , 32 , 36 , 38 and recombined into two outputs using the additional lossless beam displacing polarizer 42 . the s polarized output of one of the beams is recombined with the p polarization of the other beam . every other channel of a wdm stream is thus separated into one of the two output collimators 47 , 48 and the output fibers to which they couple . devices typically operate over a 0 - 70 ° c . temperature range and should be passively temperature compensated . for a 100 ghz filter response the center wavelength drift should typically be less than +/− 0 . 0015 nm /° c . the use of different crystals with opposite signs of the birefringence or thermooptic coefficient in the manner described achieves this result . the retardance of two crystals in series is given by : γ 1 + γ 2 := 2 · π λ · ( l 1 · δ   n 1 + l 2 · δ   n 2 ) where γ is the phase retardance , l is the length of each crystal , and the birefringence is given by δn 1 = n e , 1 − n o , 1 for the first crystal . for the second crystal δn 2 = n e − n o if the crystal axis is parallel to that of the first crystal and is δn 2 = n o − n e if it is rotated 90 degrees . the change of retardance with temperature is given by :  ( γ 1 + γ 2 )   t := ( 2 · π λ ) · ( l 1 ·  ( δ   n 1 )  t + l 2 ·  ( δ   n 2 )  t + l 1 · α 1 · δ   n 1 + l 2 · α 2 · δ   n 2 ) where α is the thermal expansion coefficient . the condition for compensation is given by : l 1 l 2 := - (  ( δ   n 1 )  t + α 1 · δ   n 1 ) (  ( δ   n 2 )  t + α 2 · δ   n 2 ) usually the thermal expansion coefficient term can be neglected . with the optic axis of the crystals in alignment , compensation is achieved using two crystals with different signs of the thermooptic coefficient . if the crystals are rotated 90 degrees with respect to each other , materials can be used with the same sign of the thermooptic coefficient . during assembly , both the frequency period and absolute wavelength of the peaks must be adjusted . this can be controlled by tight tolerances of the thickness of the polished crystals . the crystals or the input beam angle can also be tilted to adjust the wavelength . another approach is to polish the crystal to form a slight wedge shape , with the beam - incident faces thus being non - orthogonal to beam direction . then the wavelength and period can be adjusted by translating the beam on the crystal . in order for both the parallel beams to see the same thickness of crystal , the wedge angle should be transverse to the plane of the two incident beams . two crystals with opposing wedges can be translated relative to each other to adjust the thickness and minimize any beam steering . another approach to tune the wavelength is to choose from a set of linbo3 crystals at slightly different thickness , and tuning by substituting for the best response . a spacing of 10 microns can allow for tuning while only slightly changing the temperature compensation condition . adjustment of the absolute wavelength peaks of the filter can also be achieved by using a zero order half wave plate after each of the stages . by rotating the waveplates additional birefringence is introduced which tunes the filter . zero order waveplates are used to minimize temperature dependence of the waveplate . the alignment and tolerance of the optical components are critical . both insertion loss and manufacturing assembly cost need to be minimized . an alternative to the stainless steel optical bench is to use a silicon bench as a platform to mount all of the components . precise v - grooves are etched onto the silicon substrate and components are dropped into them and attached with epoxy . 1 × 2 interleavers can be cascaded to split every nth channel into one of n output fibers . for the second stage a crystal of half the thickness of the first stage is required . in general the transmission through an n stage interleaver will have a transmittance at one of the output fibers given by : i  ( λ ) := ∏ i = 1 n   n  [ cos  [ π · ( n e - n 0 ) λ · l i · 10 6 ] ] 2 4 the other fibers will have the same wavelength dependent transmittance with the peaks shifted by a multiple of the input channel spacing . each stage of a birefringent filter is not limited to separating every other channel . more generally a single stage can group every nth channel onto a single fiber and the remaining ( n − 1 ) adjacent channels in each period onto a separate fiber . one approach is to use a solc type filter described in the yeh and yariv treatise referred to above . there are two designs , folded and fanned , which rely on a stack of rotated birefringent plates of equal thickness . another example of an arrangement in accordance with the invention , referring now to fig7 divides the input beam from an input collimater 60 into an s polarized beam and an orthogonal p polarized beam at a first beam splitting polarizer cube 62 . the p polarized beam is directed back into parallelism for compactness at a prism 63 , and both beams then pass separately through sheet polarizers 64 , 65 to temperature compensating birefringent crystal pairs 70 , 71 and 73 , 74 as described above , after which separate beam pairs are recombined . s polarized components are angled off a prism 80 to one face of a second polarizer cube 82 , which receives the p polarized beam at another face . from the second cube 82 two orthogonal beams merge , each combining s and p components , and the two combined beams are directed to first and second collimators 84 , 85 respectively . this arrangement simplifies beam recombination but , because of the characteristics of polarizing beam splitters , the cross - talk between adjacent channels is higher , even though reduced somewhat by the sheet polarizers . in addition , the costs of using separate crystals of equal lengths must be considered . other examples of arrangements in accordance with the invention , referring to fig8 and fig9 make use of different optical arrangements to recombine the s and p polarized beams into the second output fiber . in fig8 the optical layout from the input collimator through the birefringent crystals and the second beam displacing polarizer is the same as described previously . here , however , a prism 90 is used to pick off the center beam emerging from the second beam displacer 41 which contains both required polarizations . this beam is reflected with another prism 92 and coupled directly into an output collimator 47 . the other two beams emerging from the second beam displacer 41 are recombined within a third beam displacing polarizer 94 . the length of the last beam displacer 90 is twice that of the first two 28 , 41 due to the need for twice the displacement . since the path lengths of the two beams would not otherwise be matched , a compensating plate 96 is inserted in one of the beam paths to match the optical path length . a high index material such as lithium niobate , with the crystal axis aligned with the input polarization , is used in the s polarized beam . another approach to match the optical path lengths is shown in fig9 . the two beams emerging from the second beam displacer 41 are transmitted through a half wave plate 98 and then recombined using a third beam displacement polarizer 100 . the half wave plate 98 rotates the polarization 90 degrees , which ensures that the overall path lengths of the two beams are matched after going through the final beam displacer 100 . the half wave plate 98 is a zero order design to reduce the temperature dependence . the combined beams in the midregion of the second beam displacing polarizer 41 are angled off a common prism 102 to the second output collimator 48 . although a number of variants and alternatives have been described , the invention is not limited thereto but emcompasses all forms and modifications within the scope of the appended claims .