Fabry-perot filter, wavelength-selective infrared detector and infrared gas analyzer using the filter and detector

Abstract

A Fabry-Perot filter device for selectively transmitting three wavelength bands of infrared radiation, including a reference light band, wherein the filter device comprises a fixed mirror formed on a substrate; a movable mirror arranged opposite to the fixed mirror with a gap formed therebetween so that the movable mirror is displaced with respect to the fixed mirror by applying an external force; a fixed electrode formed on the fixed mirror; and a movable electrode formed on the movable mirror and arranged opposite to the fixed electrode wherein the movable electrode is displaced by applying a potential difference across the fixed and movable electrodes so that the width of the gap is varied in at least three steps, whereby at least three wavelength bands of infrared radiation are selectively transmitted through the filter device.

What is claimed is:  
     
         1 . A Fabry-Perot filter device for passing infrared radiation from a light source in a wavelength selective manner, said device comprising: 
 a fixed mirror formed on a substrate;    a movable mirror arranged opposite to said fixed mirror with a gap formed therebetween so that said movable mirro is displaced respect to said fixed mirror by applying an external force;    a fixed electrode formed on said fixed mirror; and    a movable electrode formed on said movable mirror and arranged opposite to said fixed electrode; wherein 
 said movable mirror is displaced by applying a potential difference across said fixed electrode and said movable electrode so that a width of said gap is varied in at least three steps, whereby at least three wavelength bands of said infrared radiation are selectively transmitted through said Fabry-Perot filter device.  
   
     
     
         2 . The device of  claim 1 , wherein said fixed mirror and said movable mirror comprise silicon.  
     
     
         3 . The device of  claim 1 , wherein said fixed electrode and said movable electrode comprise silicon with a high impurity concentration.  
     
     
         4 . An infrared gas analyzer for determining concentration of a gas being measured according to output of an infrared detector, said analyzer comprising: 
 a light source for emitting infrared radiation to said gas being measured;    a wavelength selective filter for passing said infrared radiation from said light source in a wavelength selective manner; and    an infrared detector for detecting infrared radiation passing through said wavelength selective filter; wherein said wavelength selective filter comprises a Fabry-Perot filter device comprising: 
 a fixed mirror formed on a substrate;  
 a movable mirror arranged opposite to said fixed mirror with a gap formed therebetween so that said movable mirror is displaced with respect to said fixed mirror by applying an external force;  
 a fixed electrode formed on said fixed mirror; and  
 a movable electrode formed on said movable mirror and arranged opposite to said fixed electrode; wherein 
 said movable mirror is displaced by applying a potential difference across said fixed electrode and said movable electrode so that a width of said gap is varied in at least three steps, whereby at least three wavelength bands of said infrared radiation are selectively transmitted through said Fabry-Perot filter device.  
 
   
     
     
         5 . The analyzer of  claim 4 , wherein said wavelength selective filter is located before said infrared detector in an optical path and comprises a wide bandpass filter for passing only a specific band of wavelengths.  
     
     
         6 . The analyzer of  claim 4  or  5 , wherein said fixed mirror and said movable mirror comprise silicon, and said fixed electrode and said movable electrode comprise silicon with high impurity concentration.  
     
     
         7 . A wavelength selective infrared detector comprising: 
 a substrate;    a wavelength selective filter formed on said substrate for passing infrared radiation from a light source in a wavelength selective manner; and    an infrared detector formed on said substrate for detecting infrared radiation passing through said wavelength selective filter.    
     
     
         8 . The detector of  claim 7 , wherein said infrared detector comprises elements located in a sealed cavity formed in said substrate and wherein said wavelength selective filter is formed on said sealed cavity so that said sealed cavity is vacuum sealed or sealed with an inert gas therein.  
     
     
         9 . A wavelength selective infrared detector comprising: 
 a wavelength selective filter formed on a first substrate to pass infrared radiation from a light source in a wavelength selective manner; and    an infrared detector formed on a second substrate and comprising detector elements for detecting infrared radiation passing through said wavelength selective filter, wherein said first substrate and said second substrate are joined together.    
     
     
         10 . The detector of  claim 9 , wherein said infrared detector elements are located within a grooved shaped part formed in said second substrate so as to form a sealed cavity and said first substrate and said second substrate are joined together in a vacuum or inert gas atmosphere so that said wavelength selective filter is positioned on said grooved shaped part to form said sealed cavity, whereby said sealed cavity is vacuum sealed or sealed with inert gas therein.  
     
     
         11 . The detector of any of claims  7 - 10 , wherein said wavelength selective filter is a Fabry-Perot filter device comprising a fixed mirror located on a substrate and a movable mirror arranged opposite to said fixed mirror so that a gap is formed therebetween and said movable mirror is displaced with respect to said fixed mirror by applying an external force.  
     
     
         12 . The detector of any of claims  7 - 10 , wherein said wavelength selective filter is a Fabry-Perot filter device comprising a fixed mirror located on a substrate, a movable mirror arranged opposite to said fixed mirror with a gap formed therebetween so that said movable mirror is displaced with respect to said fixed mirror by applying an external force, a fixed electrode formed on said fixed mirror, and a movable electrode formed on said movable mirror and arranged opposite to said fixed electrode, whereby said movable mirror is displaced by applying a potential difference across said fixed electrode and said movable electrode thereby to vary a width of said gap.  
     
     
         13 . The detector of any of claims  7 - 10 , wherein said wavelength selective filter comprises a Fabry-Perot filter device comprising a fixed mirror and a movable mirror with a gap therebetween which is varied in a plurality of steps by applying a plurality of voltages across said fixed mirror and said movable mirror whereby said Fabry-Perot filter device passes a plurality of wavelength bands of infrared radiation.  
     
     
         14 . The detector of any of claims  7 - 10 , wherein said infrared detector is an infrared bolometer.  
     
     
         15 . The detector of any of claims  7 - 10 , wherein a plurality of wavelength selective filters and a plurality of infrared detectors are horizontally arrange in arrays.  
     
     
         16 . An infrared gas analyzer for determining concentration of a gas being measured according to output of an infrared detector, said analyzer comprising: 
 a light source for emitting infrared radiation to said gas being measured;    a wavelength selective filter for passing infrared radiation from said light source in a wavelength selective manner; and    an infrared detector for detecting infrared radiation passing through said wavelength selective filter; wherein 
 said wavelength selective filter and said infrared detector are configured using a wavelength selective infrared detector comprising: 
 a substrate;  
 said wavelength selective filter being formed on said substrate; and  
 said infrared detector being formed on said substrate.  
 
   
     
     
         17 . An infrared gas analyzer for determining concentration of a gas being measured according to output of an infrared detector, said analyzer comprising 
 a light source for emitting infrared radiation to said gas being measured;    a wavelength selective filter for passing infrared radiation from said light source in a wavelength selective manner; and    an infrared detector for detecting infrared radiation passing through said wavelength selective filter; wherein 
 said wavelength selective filter is formed on a first substrate; and  
 said infrared detector is formed on a second substrate; wherein said first substrate and said second substrate are jointed together.  
   
     
     
         18 . The analyzer of  claim 16  or  17 , further comprising a wide bandpass filter disposed between said wavelength selective infrared detector and said light source so as to pass only a specific band of wavelength.  
     
     
         19 . The analyzer of any of claims  4 , 5 ,  16  and  17 , wherein said gas being measured contains carbon dioxide and water, and concentrations thereof are measured.  
     
     
         20 . The analyzer of any of claims  4 , 5 ,  16  and  17 , wherein said gas being measured contains carbon dioxide and carbon monoxide, and concentrations thereof are measured.  
     
     
         21 . The analyzer of any of claims  4 , 5   16  and  17 , wherein said gas being measured contains nitrogen oxide and sulfur oxide and concentrations thereof are measured.  
     
     
         22 . The analyzer of any of claims  4 ,  5 ,  16  and  17 , wherein said gas being measured contains carbon dioxide and nitrogen oxide and concentrations thereof are measured.  
     
     
         23 . The analyzer of any of claims  4 ,  5 ,  16  and  17 , wherein said gas being measured contains carbon dioxide and sulfur oxide, and concentrations thereof are measured.  
     
     
         24 . A Fabry-Perot filter device comprising: 
 a fixed mirror;    a movable mirror arranged opposite to said fixed mirror with a gap formed therebetween; wherein 
 width of said gap is varied by displacing said movable mirror with respect to said fixed mirror; and wherein 
 said movable mirror is formed using a multilayer optical thin film comprising at least one layer presenting tensile stress.  
 
   
     
     
         25 . The device of  claim 24 , wherein said movable mirror is formed using at least one layer which presents compressive stress.  
     
     
         26 . The device of  claim 24 , wherein said movable mirror is formed using at least two layers presenting different levels of tensile stress.  
     
     
         27 . The device of  claim 24 , wherein said thin film is a three layer optical thin film produced by laminating a high refractive index layer, a low refractive index layer, and another high refractive index layer.  
     
     
         28 . The device of  claim 24 , wherein said thin film is a three layer optical thin film having a structure selected from the group consisting of the following structures denoted (a)-(d): 
 (a) a structure wherein said thin film is produced by laminating a high refractive index layer presenting compressive stress, a low refractive index layer presenting tensile stress, and another high refractive index layer presenting compressive stress, in the foregoing order;    (b) a structure wherein said thin film is produced by laminating a high refractive index layer presenting tensile stress, a low refractive index layer presenting compressive stress, and another high refractive index layer presenting tensile stress, in the foregoing order;    (c) a structure wherein said thin film is produced by laminating a high refractive index layer presenting a low tensile stress, a low refractive index layer presenting a high tensile stress, and another high refractive index layer presenting a low tensile stress, in the foregoing order; and    (d) a structure wherein said thin film is produced by laminating a high refractive index layer presenting a high tensile stress, a low refractive index layer presenting a low tensile stress, and another high refractive index layer presenting a high tensile stress, in the foregoing order.    
     
     
         29 . The device of any of claims  24 - 28 , wherein thickness of said thin film is λ/4, wherein λ is the wavelength.  
     
     
         30 . The device of  claim 24 , further comprising a fixed electrode formed on said fixed mirror; and a movable electrode formed on said movable mirror; and an isolation layer disposed between said fixed electrode and said movable electrode.  
     
     
         31 . The device of  claim 30 , wherein said isolation layer is formed on said fixed electrode.  
     
     
         32 . The device of  claim 30  or  31 , wherein said isolation layer is made of silicon nitride or silicon oxide.  
     
     
         33 . The device of  claim 24 , wherein said fixed mirror is formed on a substrate, and wherein said gap is formed by depositing a sacrificial layer of predetermined shape and size between said fixed mirror and said movable mirror and then removing the sacrificial layer completely or partially by means of etching.  
     
     
         34 . The device of  claim 33 , wherein a vertical cross section of said sacrificial layer is approximately a trapezoid and electrode pads are formed in a region outside of said sacrificial layer.  
     
     
         35 . The device of  claim 33  or  34 , further comprising an antireflection layer formed on a backside of said substrate; and a metal aperture formed on said antireflection layer through a protection layer and having an optical area in part; wherein said sacrificial layer is first removed by etching and then a portion of said protection layer present in said optical area is removed.  
     
     
         36 . The device of  claim 33  or  34 , further comprising etching holes formed at a center and along a periphery of said movable mirror in order to etch said sacrificial layer.