Etalon cavity with filler layer for thermal tuning

Abstract

A temperature-stable and temperature-tunable composite etalon with an increased field of view is constructed by partially filling the cavity with a transmissive parallel-plate filler. The coefficient of thermal expansion and the index of refraction of the filler material are selected so as to produce the desired rate of change of the optical length of the cavity as a function of temperature. The filler plate is preferably chosen to be significantly thicker than the remaining air gap in the etalon cavity. As a result, the fact that the filler plate occupies a large part of the cavity space increases the acceptance angle of the etalon of the invention in comparison with corresponding conventional air-spaced etalons.

1 . A method of increasing a field of view of an etalon filter that includes an optical cavity defined by two substantially parallel optical surfaces separated by at least one medium with a corresponding medium index of refraction, said index of refraction contributing to an effective cavity index of refraction, the method comprising the step of: 
 placing a transmissive filler plate within the optical cavity to establish a new effective cavity index of refraction, wherein the transmissive filler plate is made of a material having a higher index of refraction than said medium index of refraction, so that the new effective cavity index of refraction is greater than said effective cavity index of refraction.    
   
   
       2 . The method of  claim 1 , wherein each of said optical surfaces is a surface of an optical plate.  
   
   
       3 . The method of  claim 1 , wherein each of said optical surfaces is a coated surface of an optical plate.  
   
   
       4 . The method of  claim 1 , wherein said optical surfaces are separated by a spacer structure that includes a plurality of spacers peripherally distributed between the optical surfaces.  
   
   
       5 . The method of  claim 4 , further including a central foot spacer axially disposed between said optical surfaces through an aperture in said transmissive filler plate.  
   
   
       6 . The method of  claim 1 , wherein said optical surfaces are separated by a spacer structure that includes a tubular ring spacer between the optical surfaces.  
   
   
       7 . The method of  claim 6 , further including a central foot spacer axially disposed between said optical surfaces through an aperture in said transmissive filler plate.  
   
   
       8 . The method of  claim 1 , wherein said transmissive filler plate is in optical contact with one of said two optical surfaces.  
   
   
       9 . The method of  claim 1 , wherein said transmissive filler plate has a surface coated with an antireflective coating.  
   
   
       10 . The method of  claim 1 , wherein said optical surfaces are separated by a spacer structure that is made of a spacer material selected from the group consisting of fused silica, CLEARCERAM-HS and NEX-C; and said transmissive filler plate is made of a filler material selected from the group consisting of fused silica, CLEARCERAM-HS and NEX-C.  
   
   
       11 . The method of  claim 2 , wherein each of said optical surfaces is a coated surface of an optical plate, and said optical surfaces are separated by a spacer structure that includes a plurality of spacers peripherally distributed between said optical surfaces.  
   
   
       12 . An optical cavity with a field of view increased according to the method of  claim 1 .  
   
   
       13 . An optical cavity with a field of view increased according to the method of  claim 2 .  
   
   
       14 . An optical cavity with a field of view increased according to the method of  claim 3 .  
   
   
       15 . An optical cavity with a field of view increased according to the method of  claim 4 .  
   
   
       16 . An optical cavity with a field of view increased according to the method of  claim 5 .  
   
   
       17 . An optical cavity with a field of view increased according to the method of  claim 6 .  
   
   
       18 . An optical cavity with a field of view increased according to the method of  claim 7 .  
   
   
       19 . An optical cavity with a field of view increased according to the method of  claim 8 .  
   
   
       20 . An optical cavity with a field of view increased according to the method of  claim 9.