Wavelength discrimination filter for infrared wavelengths

Abstract

An optical comb filter has polarizers placed before and after two or more birefringent elements, each having a fixed retarder and an aligned tunable liquid crystal. The birefringent elements are equal in birefringence and thickness and are tuned in unison. As few as two birefringent elements are provided, thereby minimizing transitions where infrared absorption occurs. The birefringent elements are in a symmetrical array of rotational angles, encompassing 90° of span in a bandpass configuration where the polarizers are parallel. Polarization components from the input polarizer or a given birefringent element are coupled in unequal proportions to the fast and slow axes of the next birefringent element, by using a relative angle that is distinctly different from 45°. The filter is tolerant of rotational alignment errors. A two element filter uses angles of 22.5 and 67.5°; a three element version has 12.5, 45 and 77.5°; and a four element filter has 7.5, 29.5, 60.5 and 82.5°; all these angles being ±5°.

1 . A spectral filter, comprising:
 a plurality of birefringent elements coupled along a signal propagation path leading from a predetermined polarization alignment up to a selection polarizer, each of the birefringent elements having a fast axis and a slow axis at a right angle to one another;   wherein a first of the birefringent elements along the signal propagation path is arranged with one of the respective fast and slow axes thereof at a rotational angle less than 45° to said polarization alignment, whereby a light component oriented at the polarization alignment is divided unequally into said fast and slow axes;   wherein at least one subsequent one of the birefringent elements is rotationally displaced relative to the first of the birefringent elements;   wherein the plurality of birefringent elements occupy a series of rotation angles up to the selection polarizer.   
     
     
         2 . The spectral filter of  claim 1 , further comprising an input polarizer, and wherein the input polarizer has an optical orientation parallel to said predetermined polarization alignment. 
     
     
         3 . The spectral filter of  claim 1 , wherein the plurality of birefringent elements consists of two rotational orientations. 
     
     
         4 . The spectral filter of  claim 3 , wherein the rotational orientations are substantially 22.5°±5° and 67.5°±5° relative to a zero axis defined by said polarization alignment. 
     
     
         5 . The spectral filter of  claim 1 , wherein the plurality of birefringent elements consists of three rotational orientations. 
     
     
         6 . The spectral filter of  claim 5 , wherein the rotational orientations are substantially 12.5°±5°; 45.5°+5°; and 77.5°±5° relative to a zero axis defined by said polarization alignment. 
     
     
         7 . The spectral filter of  claim 1 , wherein the plurality of birefringent elements consists of four rotational orientations. 
     
     
         8 . The spectral filter of  claim 7 , wherein the rotational orientations are substantially 7.5°±5°; 29.5°±5°; 60.5°±5°; and 82.5°*5° relative to a zero axis defined by said polarization alignment. 
     
     
         9 . The spectral filter of  claim 1 , wherein the tunable retarder comprises a liquid crystal tunable birefringence element. 
     
     
         10 . The spectral filter of  claim 1 , wherein the predetermined polarization alignment is determined by an input polarizer, wherein the selection polarizer has an optical axis parallel to the predetermined polarization alignment, and wherein the series of rotation angles up to the selection polarizer is a symmetrical distribution among: the input polarizer to the first said birefringent element; the first birefringent element each subsequent birefringent element; and, a last of the plurality of birefringent elements to the selection polarizer. 
     
     
         11 . The spectral filter of  claim 10 , wherein a rotational displacement from the input polarizer to the first birefringent element is equal to a rotational displacement from the last said birefringent element to the selection polarizer. 
     
     
         12 . The spectral filter of  claim 11 , wherein the symmetrical distribution totals 90°. 
     
     
         13 . The spectral filter of  claim 1 , wherein each of the birefringent elements comprises an electrically tunable liquid crystal abutted with a fixed birefringent retarder, the liquid crystal having fast and slow axes aligned parallel to respective fast and slow axes of the fixed birefringent retarder, thereby defining a total thickness and birefringence. 
     
     
         14 . The spectral filter of  claim 13 , wherein the thickness and birefringence of each of the plurality of birefringent elements is equal. 
     
     
         15 . The spectral filter of  claim 14 , wherein the liquid crystals of the plurality of birefringent elements are tuned to maintain equal retardation. 
     
     
         16 . A method for comb filtering a light signal, comprising:
 arraying a set of birefringent elements between an input signal polarized at a predetermined polarization orientation and a selection polarizer;   setting a first of the birefringent elements to an optical orientation that unequally couples a component of the input signal at said predetermined polarization orientation to fast and slow axes of said first birefringent element;   setting at least one subsequent birefringent element in the set to an optical orientation that is rotationally displaced from the optically orientation of the first of the birefringent elements, by an angle that symmetrically distributes the birefringent elements over a 90° angular span between the input signal and the selection polarizer.   
     
     
         17 . The method of  claim 16 , wherein the predetermined polarization is established by an input polarizer and wherein the input polarizer and the selection polarizer have optical axes that are parallel, and further comprising operating the set of birefringent elements and the input and selection polarizers as a spectral band pass filter. 
     
     
         18 . The method of  claim 16 , wherein the selection polarizer has an optical axis that is perpendicular to the predetermined polarization orientation, and further comprising operating the set of birefringent elements and the selection polarizer as a spectral band stop filter.