Liquid crystal filter with tunable rejection band

Abstract

An optical wavelength filter uses birefringence and polarization to discriminate and block a wavelength stop band. Birefringent elements, each having a fixed retarder and a liquid crystal, are placed along the signal path with the fast and slow axes in each pair being aligned and rotationally displaced from other pairs. The birefringence parts are arranged by their thickness, birefringence and progressive rotational orientation so that each pair contributes to achieving a specific polarization state for specific periodic wavelength discrimination bands. A selection polarizer at the output blocks bands that have this polarization state. The birefringence elements are tuned in a coordinated manner and thereby to select one or more wavelengths as the band stop discrimination band.

1 . A spectral band stop filter, comprising: 
 at least one selection polarizer and a plurality of birefringence elements coupled along a signal propagation path leading up to the selection polarizer,    wherein each of the birefringence elements in the plurality has a birefringence and a rotational orientation that complements a respective birefringence and rotational orientation of at least one other one of the remaining birefringence elements in said plurality, in that each of the birefringence elements imparts a predetermined polarization orientation to a band discrimination wavelength in a set of wavelengths at the input, said predetermined polarization orientation being different from an orientation of other wavelengths in the set of wavelengths,    wherein the selection polarizer is oriented orthogonal to said predetermined polarization orientation, thereby blocking transmission of said band discrimination wavelength at the selection polarizer and permitting the other wavelengths to be transmitted by the selection polarizer.    
   
   
       2 . The spectral band stop filter of  claim 1 , further comprising an input polarizer, wherein the input polarizer is optically oriented at 45° to said first of said plurality of birefringence elements along the signal propagation path, and the input polarizer determines said given orientation at the input.  
   
   
       3 . The spectral band stop filter of  claim 1 , comprising at least two stages of said birefringence elements, wherein the birefringence elements in said stages are tunable in unison to impart the predetermined polarization orientation to the discrimination wavelength.  
   
   
       4 . The spectral band stop filter of  claim 3 , further comprising at least one polarizer disposed between the stages.  
   
   
       5 . A spectral band stop filter, comprising: 
 at least one selection polarizer and at least two stages of birefringence elements, each of the stages comprising one or more birefringence elements coupled along a signal propagation path leading up to the selection polarizer,    wherein each birefringence element in each stage has a birefringence and a rotational orientation that complements a respective birefringence and rotational orientation of one or more remaining birefringence elements in said stage, in that each of the birefringence elements contributes part of a change in a polarization state of a band discrimination wavelength from a first of said birefringence elements up to the selection polarizer,    wherein the band discrimination wavelength emerges from the signal propagation path with a predetermined polarization orientation; and,    wherein the selection polarizer is oriented orthogonal to said predetermined polarization orientation, whereby the discrimination wavelength is stopped or reflected at the selection polarizer and other wavelengths substantially are transmitted; and,    wherein a stop band of each of the stages overlaps at the discrimination wavelength.    
   
   
       6 . The spectral band stop filter of  claim 1 , comprising at least two stages of said birefringence elements, wherein the birefringence elements in said stages are tunable independently and wherein a stop band of each of the stages is tunable to enlarge a bandwidth at the discrimination wavelength.  
   
   
       7 . The spectral band stop filter of  claim 1 , comprising at least two stages of said birefringence elements, wherein the birefringence elements in said stages are tunable independently and wherein a stop band of each of the stages is tunable to provide at least two tunable band discrimination wavelengths.  
   
   
       8 . The spectral band stop filter of  claim 1 , comprising at least two stages of said birefringence elements, wherein the birefringence elements in said stages are tunable in unison to the discrimination wavelength, wherein the tunable discrimination wavelength emerges from a last of the birefringence elements in a first stage at 45° orientation to fast and slow axes of a first of the birefringence elements in a next stage.  
   
   
       9 . The spectral band stop filter of  claim 8 , further comprising at least one polarizer between the stages, optically oriented at 135° to said fast and slow axes.  
   
   
       10 . The spectral band stop filter of  claim 1 , wherein the birefringence elements each comprise a pair with at least one fixed retarder and at least one tunable retarder, wherein fast and slow axes of the fixed and tunable retarders in the pair are rotationally aligned to one another to provide a birefringence for the pair that is determined by individual thicknesses and birefringences of the fixed and tunable retarders of said pair.  
   
   
       11 . The spectral band stop filter of  claim 10 , wherein the tunable retarder comprises a liquid crystal tunable birefringence element.  
   
   
       12 . The spectral band stop filter of  claim 11 , wherein the fixed retarder is index matched to at least one adjacent part of said filter.  
   
   
       13 . The spectral band stop filter of  claim 10 , wherein the birefringence elements in at least one stage are configured as one of a Lyot configuration filter, Lyot-Ohman configuration filter, Solc configuration filter and Evans configuration filter.  
   
   
       14 . The spectral band stop filter of  claim 10 , wherein the selection polarizer is substantially reflective.  
   
   
       15 . The spectral band stop filter of  claim 14 , wherein the selection polarizer comprises a wire grid polarizer.  
   
   
       16 . A spectral band stop filter, comprising: 
 at least one selection polarizer that reflects light components having a predetermined polarization orientation when the selection polarizer is oriented orthogonal thereto; and    a plurality of birefringence elements coupled along a signal propagation path leading up to the selection polarizer,    wherein each of the birefringence elements imparts the predetermined polarization orientation to a band discrimination wavelength in a set of wavelengths at the input, said predetermined polarization orientation being different from an orientation of other wavelengths in the set of wavelengths,    wherein the selection polarizer is oriented orthogonal to said predetermined polarization orientation, thereby being configured to reflect said band discrimination wavelength at the selection polarizer and permitting the other wavelengths to be transmitted by the selection polarizer.    
   
   
       17 . The filter of  claim 16 , wherein the band discrimination wavelength is a wavelength of a laser.  
   
   
       18 . The method of  claim 17 , wherein the selection polarizer is a wire grid type polarizer.  
   
   
       19 . The filter of  claim 17 , wherein the filter is positioned in a light path between a lens system and a light sensor array.  
   
   
       20 . The filter of  claim 19 , wherein at least one of the birefringence elements includes a tunable liquid crystal retarder, and the at least one tunable liquid crystal retarder includes first and second substrates having respective rubbing directions opposite to one another.  
   
   
       21 . The filter of  claim 16 , wherein the filter is configured to operate independently of the polarization of incoming light at the input.  
   
   
       22 . A method for wavelength filtering comprising: 
 placing a succession of birefringence elements along a signal propagation path, and arranging a thickness, birefringence and rotational orientation of the birefringence elements to provide a set of periodically related wavelengths wherein a wavelength component of each of the periodically related wavelengths, having a given polarization state at an input to the signal propagation path, emerges at an output of the signal propagation path at a distinct polarization state and distinct rotational orientation;    placing a selection filter at said output of the signal propagation path and configuring the selection filter orthogonal to the distinct polarization state and orientation, thereby blocking said signal components at said distinct polarization state; and,    wherein one of the periodically related wavelengths corresponds to a selected wavelength that emerges at the output at the distinct polarization state and orientation, and therefore is blocked by the selection filter.    
   
   
       23 . The method of  claim 22 , wherein the birefringence elements comprise pairs of fixed retarders and electrically controllable liquid crystals, and are placed such that fast and slow axes of the retarders and the liquid crystals are aligned with on another in each of said pairs.  
   
   
       24 . The method of  claim 22 , wherein the distinct polarization state is plane polarization at a predetermined rotational orientation and the selection filter comprises a polarizer oriented orthogonal to said rotational orientation.  
   
   
       25 . The method of  claim 22 , wherein the selection filter comprises a reflective polarizer configured to reflect said wavelength component, thereby stopping transmission thereof at the polarizer.  
   
   
       26 . The method of  claim 22 , comprising configuring the birefringence elements in at least one stage configured as one of a Lyot, Lyot-Ohman, Solc and Evans configuration filter.