Interleaving optical filter

Abstract

An optical signal filter for providing a periodic transfer function in transmitting signals within a selected bandwidth, by which passbands are interleavered into groups of separate outputs. The filter employs the transmissivity characteristic of birefringent crystals in conjunction with splitting the input beam into orthogonal and separate components, while compensating for temperature variations by pairing crystals of different types. The transmissivity functions are independent of the polarization of the input beam, and are shaped to flatten transmissivity peaks by the use of cascaded stages of birefringent crystal pairs.

1 . An interleaving optical filter for wave energy, for providing a periodic low loss transmissivity characteristic in the rate of 25 GHz to 200 GHz spacing and operating with substantial polarization independence and with compensation for temperature variations, said filter comprising: 
 a support providing a generally planar surface extending substantially parallel to a principal optical axis for the filter;    an input collimator mounted on the support at an input region thereon to provide a collimated beam along the principal optical axis;    a first beam displacing polarizer mounted on the support to receive the collimated beam, the polarizer transmitting two beams of orthogonal polarization that are parallel to the principal optical axis;    a first pair of birefringent crystals receiving the two beams and being of different thermooptic coefficients and with lengths along the principal optical axis that are selected to compensate for temperature-induced phase retardation variations, the first pair being rotated 45° with respect to the planar surface about the principal optical axis;    a second pair of birefringent crystals of materials like the first pair but of different length, and being rotated with respect to the planar surface to provide transmissivity peaks that have passband flatness of −0.5 dB of about 0.47 nm and a center wavelength drift of less than ±0.0015 nm/° C.;    a second beam displacing polarizer receiving the two beams transmitted through the pairs of birefringent crystals for splitting each of the two beams into two beams with different polarizations;    a beam recombining unit receiving the beams from the beam displacing polarizer for combining the beams therefrom into two polarization independent beams in the component beams with less than one 1 mm path length difference.    
     
     
         2 . An interleaving optical filter as set forth in  claim 1  above, including output collimators coupled to transmit the different polarization independent beams and an interleaving optical filter as set forth in  claim 1  above, wherein the second beam displacing polarizer transmits a pair of orthogonally polarized individual beams and a combined beam having orthogonally polarized components, and the beam recombining means directs the combined beam as one of the outputs.  
     
     
         3 . An interleaving optical filter as set forth in  claim 1  above, wherein the second beam displacing polarizer transmits a pair of orthogonally polarized individual beams and a combined beam having orthogonally polarized components, and the beam recombining unit directs the combined beam as one of the outputs.  
     
     
         4 . An interleaving optical filter as set forth in  claim 3  above, wherein the beam recombining means includes a third beam displacing polarizer receiving the orthogonally polarized individual beams, and further includes a path length compensator in one of the beam paths to the third beam displacing polarizer.  
     
     
         5 . An interleaving optical filter as set forth in  claim 3  above, wherein the beam recombining means includes a third beam displacing polarizer receiving the orthogonally polarized individual beams, and further includes a half wave plate in both beam paths to the third beam displacing polarizer.  
     
     
         6 . An interleaving optical filter as set forth in  claim 3  above, wherein the beam recombining means comprises a polarizing beam splitter cube and prism means for directing the orthogonally polarized individual beams to different faces of the beam splitter cube.  
     
     
         7 . An interleaving optical filter as set forth in  claim 1  above, wherein the birefringent crystals are of opposite sign, and wherein the second pair of crystals have a negative angular rotation relative to the first pair.  
     
     
         8 . An interleaving optical filter as set forth in  claim 1  above, wherein the pairs of birefringent crystals each comprise a YV04 crystal and an LiNbO 3  crystal having length ratios of 6.60:1 and wherein the crystals of the second pair are twice the length of those in the final pair.  
     
     
         9 . An interleaver filter as set forth in  claim 1  above, wherein the first and second beam displacing polarizers are of YV04 crystal and the beam recombining unit comprises a prism and polarizing beam splitter cube.  
     
     
         10 . An interleaving optical filter as set forth in  claim 2  above, wherein the input collimator and output collimators comprise gradient index lenses, wherein the filter further includes housings attached to the collimators and the housings are attached to the support, wherein the collimators are disposed along the principal axis, and the filter further comprises input and output optical fibers in communication with the input and output collimators respectively.  
     
     
         11 . An optical assembly for retaining a number of birefringent elements, polarizing elements and collimating elements in precise axial and rotational positions along an optical axis, comprising: 
 an optical bench having a principal planar surface, the surface including inset recesses disposed at spaced locations along the length of the optical axis; the recesses having angles relative to the planar surface to define the rotational orientation of the birefringent elements;    adjustable attachment mechanisms attached to the optical bench at collimator positions along the optical axis;    collimator housings each supporting a different collimator and each attached to a different one of the attachment mechanisms;    and a containment housing encompassing the optical assembly and including optical fiber feedthroughs along the optical axis and in alignment with collimating elements therein.    
     
     
         12 . An optical assembly in accordance with  claim 11  above, wherein the optical bench is of stainless steel, and wherein in addition to the attachment mechanisms are laser welded to the optical bench and at least one of the birefringent elements are conductively coupled to the optical bench.  
     
     
         13 . An optical assembly in accordance with  claim 12  above, wherein the housing includes a tray and a lid in a sealed configuration, and a layer of resilient material supporting the optical bench in the tray.  
     
     
         14 . An interleaving optical filter for introducing a periodic transfer function with flattened apices within transfer in a wider band spectrum of an input optical beam, comprising: 
 a first polarizer in the path of the input optical beam;    at least two stages of pairs of birefringent crystals of opposite thermooptic coefficients, the crystals of each pair having a like length proportionality but the lengths of the crystals in the first pair having a selected ratio to the lengths of the crystals of the second pair, the longer pair being disposed closer to the first polarizer; and a second polarizer in the path of the optical beam subsequent to the two stages.    
     
     
         15 . An optical filter as set forth in  claim 14  above, wherein the like crystals within each stage have like optical axes and orientations relative to the polarizer direction and wherein the filter further includes beam displacing means between the first polarizer and the at least two stages for directing beams of orthogonal polarization through the at least two stages, and wherein the filter further includes an optical circuit for recombining the beams of orthogonal polarization into two beam sets, each including both polarizations.  
     
     
         16 . An optical filter as set forth in  claim 15  above, wherein the lengths of the crystals are selected to provide a selected periodicity in the transfer function, and wherein the optical circuit recombines the beams in the beam sets with equal path lengths.  
     
     
         17 . An optical filter as set forth in  claim 16  above, wherein each pair of crystals comprise a YVO4 crystal and an LiNbO 3  crystal, and wherein the C axes of the first pair are at 45° to the direction of the first polarizer and the C axes of the second pair are at □14.8°, and wherein the length proportionality of the crystals in each pair is 6.60:1 and the ratio between the two pairs is 1:2.  
     
     
         18 . An Optical filter for introducing an interleaving function into a signal beam occupying an optical band, comprising: 
 a polarizing beam splitting cube for dividing the signal beam into two orthogonally polarized beams;    first and second birefringent crystal sets, each including an initial polarizer sheet and two pairs of serially disposed birefringent crystals of opposite thermooptic sign; and    beam recombining means including a second polarizing beam splitting cube receiving the two orthogonally polarized beams and providing two output beams, each including both polarization components.    
     
     
         19 . A filter for separating an input band of optical signal frequencies into a number of periodicity varying transmissive frequency bands divided into at least two groups, comprising: 
 at least one pair of birefringent crystals disposed serially along an optical axis for receiving the optical signals, at least one of the crystals having a pyroelectric characteristic;    polarizing means disposed along the optical axis and bordering the at least one pair of optical crystals; and    means coupled to crystals having a pyroelectric characteristic for dissipating electric charges induced thereon.    
     
     
         20 . A filter as set forth in  claim 19  above, wherein the crystals of a pair have optical axes that are similarly aligned relative to the optical axis of the device, and wherein the polarizing means comprises a beam displacing polarizer for dividing the optical signals into two beams directed along the optical axis.  
     
     
         21 . A filter as set forth in  claim 20  above, wherein the at least one pair of birefringent crystals comprises two pairs, the second pair having alignments of their optical axes that are alike within the pair but different from the other pair, and have a selected length relationship to the crystals of the other pair.  
     
     
         22 . A filter as set forth in  claim 19  above, wherein the means for dissipating electric charges comprises conductive coating material disposed on selected surfaces of the crystals.  
     
     
         23 . An optical filter for transferring optical signals in either direction between a terminal at one side and a pair of terminals at the other side, in either direction, the optical signals periodically spaced in wavelength, the filter transferring the signals with a selected periodic passband function, comprising: 
 at least two serial stages of birefringent light propagating elements arranged to provide differential retardations between orthogonal polarization components of the optical beams, each stage being configured with at least two elements of different thermooptic characteristics to be substantially athermal over a selected temperature band, the differential retardation relationships being selectively varied between the stages to provide selected passband widths at the selected passband periodicity;    at least one polarization responsive beam path juncture device between the terminal at one side and the at least two stages and transferring optical signals therebetween, splitting the signals in the beam path in accordance with polarization in one direction, and combining the signals transferred in the other direction; and    at least two beam path juncture devices each between the terminal of a different one of the pair of terminals and the at least two stages for processing optical signals therebetween, said beam path juncture devices being polarization responsive and disposed in series, and including elements arranged to cross-combine signals of orthogonal polarization, such that the filter is polarization insensitive.    
     
     
         24 . A filter as set forth in  claim 23  above, wherein the light propagating elements are arranged in two sets of at least two stages disposed in separate paths, and like elements in the parallel paths are of matched lengths and thermooptic characteristics.  
     
     
         25 . A filter as set forth in  claim 23  above, wherein the filter further includes separate lens collimators at the outputs of the separate ones of the at least two beam path juncture device, and means associated with the at least one beam path juncture device for establishing the input polarizations of the beams to minimize the polarization dependent loss to below 0.1 dB.  
     
     
         26 . A filter as set forth in  claim 23  above, wherein the relative angles of the fast axes of the stages are selected to vary the retardation relationships, and wherein the filter includes elements for equalizing the path lengths of the separate optical beams to minimize PMD.  
     
     
         27 . A filter as set forth in  claim 23  above wherein the stages have an athermal characteristic such that the center wavelength drift is less than 0.0015 nm/° C.  
     
     
         28 . A filter as set forth in  claim 23  above, wherein the athermal compensation conditions is approximately in accordance with:  
       
         
           
             
               
                 
                   L 
                   1 
                 
                 
                   L 
                   2 
                 
               
               := 
               
                 
                   - 
                   
                     
                       ( 
                       
                         
                           
                              
                             
                               ( 
                               
                                 Δ 
                                  
                                 
                                     
                                 
                                  
                                 
                                   n 
                                   1 
                                 
                               
                               ) 
                             
                           
                           
                              
                             T 
                           
                         
                         + 
                         
                           
                             
                               α 
                               1 
                             
                             · 
                             Δ 
                           
                            
                           
                               
                           
                            
                           
                             n 
                             1 
                           
                         
                       
                       ) 
                     
                     n 
                   
                 
                 
                   ( 
                   
                     
                       
                          
                         
                           ( 
                           
                             Δ 
                              
                             
                                 
                             
                              
                             
                               n 
                               2 
                             
                           
                           ) 
                         
                       
                       
                          
                         T 
                       
                     
                     + 
                     
                       
                         
                           α 
                           2 
                         
                         · 
                         Δ 
                       
                        
                       
                           
                       
                        
                       
                         n 
                         2 
                       
                     
                   
                   ) 
                 
               
             
           
           
           
               
           
         
       
       where L is the length of each crystal, Δn is the birefringence for a crystal, and α is the thermal expansion coefficient.  
     
     
         29 . A multiplexer/demultiplexer functioning in accordance with an interleaver transfer function for processing interleaved, wavelength multiplexed signals of a first inter-channel wavelength periodicity present at one terminal and signals at half the first inter-channel wavelength periodicity present at a pair of terminals, comprising: 
 a pair of optical beam paths, each comprising at least two stages of optical delay elements disposed in series, each stage of each path including at least two optical elements in series, whose lengths and thermooptic characteristics are selected to provide a differential phase retardation between orthogonally polarized beam components that is athermal over a selected temperature range to define a periodic transmission characteristic of chosen periodicity, the lengths and fast axis orientations of each stage being selected to broaden the passbands of the transmission characteristic;    a first beam splitter/combiner in the optical communication path between the one terminal and the pair of beam paths for (1) directing and separating received signals of arbitrary state of polarization from the one terminal into the two beam paths, the signals on the two beam paths being orthogonally polarized in a fixed relation to one another, and for (2) combining orthogonally polarized optical beams from the beam paths into signals of arbitrary state of polarization at the said one terminal; and    a second and third beam splitter/combiner means in the optical communication path between the pair of beam paths and the pair of terminals, for (1) cross-combining orthogonally polarized beam components received from the beam paths and exhibiting periodic polarization characteristics at half the first wavelength periodicity into separate beams exhibiting periodic transmission characteristics at half the first wavelength periodicity and transferring them to the pair of terminals, and for (2) separating signals received at the pair of terminals, signals exhibiting periodic transmission characteristics at half the first periodicity, into orthogonally polarized signals, and transferring orthogonally polarized signals to the beam paths through stages as individual beams.    
     
     
         30 . A multiplexer/demultiplexer as set forth in  claim 29  above, wherein multiplexed signals applied to the terminals have arbitrary states of polarization and where the beam splitter/combiners are polarization sensitive and divide or combine beams in accordance with polarization and direction.