Optical transmission filter with extended out-of-band blocking

Abstract

In accordance with the invention, a filter is fabricated by a modified form of the process disclosed in U.S. Pat. No. 7,068,430. In particular, the method is modified to take into account the effect of absorption by filter material. The method is exemplified by the fabricating of an ultraviolet light transmission filter for transmitting a band within the range 230-320 nanometers. The resulting filter comprises plurality of hard-coating, thin-film layers of alternating high and low index of refraction. The improved filter provides high transmission, sharp edge slopes, and deep and extended out-of-band blocking. As compared with currently available filters, the filter provides transmission up to three or more times greater, edge slopes up to four times sharper, and deep extended out-of-band blocking extending further, even through the visible range.

1 . An optical bandpass filter for transmitting a passband of ultraviolet light comprising: 
 a substrate having one or more surfaces for supporting thin film coatings;    a plurality of alternating layers of at least two hard-coating materials of higher refractive index and lower refractive index overlying at least one of the surfaces to form a transmission filter wherein;    the passband of the filter is a band of ultraviolet light within the range of 230 nanometers to 320 nanometers, with an average transmission of light over the passband exceeding 40% and an average optical density at all wavelengths over the range 230 to 600 nanometers of at least 3.0.    
   
   
       2 . The bandpass filter of  claim 1  wherein the alternating layers form a long wave pass interference filter component and a short wave pass interference filter component.  
   
   
       3 . The bandpass filter of  claim 1  wherein the long wave pass filter component and the short wave pass filter component overlie opposing surfaces of the substrate.  
   
   
       4 . The bandpass filter of  claim 1  wherein the long wave pass filter component and the short wave pass filter component overlie the same surface of the substrate.  
   
   
       5 . The bandpass filter of  claim 1  wherein the plurality of alternating layers on at least one surface forms a multi-cavity Fabry-Perot transmission filter.  
   
   
       6 . The bandpass filter of  claim 1  wherein the optical density of the average transmission at all wavelengths over the range 320 to 700 nm is at least 4.0.  
   
   
       7 . The bandpass filter of  claim 2  wherein the long wave pass interference filter component comprises alternating layers of silica and hafnia.  
   
   
       8 . The bandpass filter of  claim 2  wherein the short wave pass interference filter component comprises alternating layers of silica and hafnia.  
   
   
       9 . The bandpass filter of  claim 2  wherein the interference filters are composed of hafnia (HfO2) material layers exhibiting a stoichiometric ratio of atomic oxygen to atomic hafnium given as N(O)/N(Hf)>2.  
   
   
       10 . The bandpass filter of  claim 2  wherein the interference filters are composed of hafnia (HfO2) material layers exhibiting a stoichiometric ratio of atomic oxygen to hydroxide (OH) as N(O)/N(OH)<1.6.  
   
   
       11 . In an optical analysis system comprising a source of excitation light for exciting a sample, an excitation filter between the source and the sample, and a collection light path from the sample; 
 the improvement wherein the light source comprises an ultraviolet light source and the excitation filter comprises a filter according to  claim 1 .    
   
   
       12 . A method of making an optical bandpass filter for transmitting a band of light with extended out-of-band blocking comprising: 
 providing a substrate having one or more surfaces;    forming on one or more of the surfaces a transmission filter each comprising alternating layers of at least two hard coating materials of higher refractive index and lower refractive index;    each of the layers deposited by a data processor controlled process comprising:    calculating with the data processor, a theoretical transmission T i  of light through the layer,    calculating with the data processor an expected deposition time t i  of the layer,    measuring during the deposition of the layer for a period of time less than t i  a measured transmission T m  of light through the layer,    determining with the data processor when deposition of the layer is to terminate based upon the theoretical transmission T i  and the measured transmission T m ; and    wherein the calculation of t i  and T i  accounts for absorption by the layer by the relation of Equations (9) thru (14) herein.    
   
   
       13 . The method of  claim 12  wherein the alternating layers form a long wave pass filter component and a short wave pass filter component.  
   
   
       14 . The method of  claim 12  wherein the alternating layers form a multi-cavity Fabry-Perot filter.  
   
   
       15 . The method of  claim 12  wherein the at least two deposited materials include silica and hafnia.  
   
   
       16 . The method of  claim 13  wherein the silica and hafnia are deposited by sputtering.  
   
   
       17 . The method of  claim 15  wherein the silica is deposited by ion beam assisted sputtering and the hafnia is deposited by sputtering without ion beam assistance.  
   
   
       18 . The method of  claim 15  wherein the sputtering is conducted in an O 2  ambient with the flow of O 2  chosen to reduce UV light absorption by the deposited layers.  
   
   
       19 . An optical bandpass filter for transmitting a passband of light comprising: 
 a substrate having one or more surfaces for supporting thin film coatings;    a first interference filter overlying one of the surfaces of the substrate comprising alternating layers of at least two materials of higher refractive index and lower refractive index to form a bandpass characteristic;    a second interference filter overlying one of the surfaces of the substrate comprising alternating layers of at least two materials of higher refractive index and lower refractive index to form blocking in addition to that provided by the first bandpass characteristic interference filter;    wherein: 
 the passband of the filter is a band of ultraviolet light within the range of 230 nanometers to 320 nanometers, with an average transmission of light over the passband exceeding 40%;  
 the average optical density at all wavelengths over the range 230 to 840 nm is at least 3.0;  
 the substrate is substantially transparent to ultraviolet light;  
 the first and second interference filters comprise hard coatings and non-metals that are substantially transparent to ultraviolet light and cooperate to substantially block light with wavelengths outside the passband over a range that includes visible wavelengths.