Optical Sensor For Extreme Environments

Abstract

An optical sensing probe includes a tube having a tip portion configured for placement in an environment in which conditions are to be sensed and an etalon having a known characteristic disposed proximate the tip portion. The tube also includes a head portion remote from the tip portion containing a light directing element for directing light beams at the etalon and receiving reflected light beams from the etalon wherein the received reflected light beams are used for determining an environmental condition proximate the tip portion. A method for measuring a thickness of the etalon may include directing a light beams at different frequencies at the etalon and receiving the light beams from the etalon. The method may also include identifying conditions of the respective light beams condition received from the etalon and then calculating a first thickness of the etalon responsive to the respective conditions and the known characteristic.

1 . A method for measuring a thickness of an etalon comprising:
 directing a first light beam at a first frequency and a second light beam at a second frequency at a first portion of an etalon having a known characteristic;   receiving the first light beam and the second light beam from the etalon; and   identifying a first condition of the first light beam and a second condition of the second light beam received from the etalon; and   calculating a first thickness of the etalon responsive to the first condition, the second condition, and the known characteristic.   
   
   
       2 . The method of  claim 1 , wherein the first condition comprises at least one of an interference maximum and an interference minimum of the first light beam. 
   
   
       3 . The method of  claim 2 , wherein the second condition comprises at least one of an interference maximum and an interference minimum of the second light beam. 
   
   
       4 . The method of  claim 3 , further comprising determining respective refractive indices of the etalon for the at least one of the interference maximum and the interference minimum of the first light beam and for the at least one of the interference maximum and the interference minimum of the second light beam. 
   
   
       5 . The method of  claim 4 , wherein the respective refractive indices are determined according to a Sellmeier equation. 
   
   
       6 . The method of  claim 4 , wherein the first thickness is calculated according to the formula:
     t =(λ1*λ2)/(2*(λ2* n 1−λ1* n 2));   where t is the first thickness; λ1 is a wavelength of the first frequency, λ2 is a wavelength of the second frequency, n1 is the refractive index at the first frequency, and n2 is the refractive index at the second frequency.   
   
   
       7 . The method of  claim 3 , wherein the first condition and second condition correspond to adjacent at least one of the interference maximum and minimum of the first light beam and at least one of the interference maximum and minimum of the second light beam. 
   
   
       8 . The method of  claim 7 , further comprising:
 calculating at least a second thickness for a different first frequency and a different second frequency; and   calculating an average thickness corresponding to at least the first thickness and the second thicknesses.   
   
   
       9 . The method of  claim 1 , further comprising:
 moving the etalon relative to the first light beam and the second light beam to align the beams with a second portion of the etalon;   performing the steps of  claim 1  to calculate the thickness of a second portion of the etalon.   
   
   
       10 . The method of  claim 1 , further comprising:
 disposing the etalon in an environment in which conditions are to be sensed; and   using the calculated thickness to determine an environmental condition proximate the etalon.   
   
   
       11 . The method of  claim 10 , wherein the environmental conditions comprise at least one of a temperature and a pressure. 
   
   
       12 . The method of  claim 1 , wherein the light beams are directed to impinge upon a surface of the etalon at an angle normal to the surface. 
   
   
       13 . The method of  claim 1 , wherein the known characteristic comprises a relationship between a refractive index of the etalon and wavelength of light incident on the etalon. 
   
   
       14 . The method of  claim 13 , wherein the relationship comprises a Sellmeier equation for the etalon. 
   
   
       15 . The method of  claim 1 , wherein the etalon comprises silicon carbide. 
   
   
       16 . The method of  claim 1 , wherein the etalon comprises a single crystal silicon carbide. 
   
   
       17 . A system for measuring a thickness of an etalon comprising:
 a first light source for directing a first light beam having a first wavelength at a first portion of an etalon having a known characteristic;   a second light source for directing a second light beam having a second wavelength different from the first wavelength at the etalon;   an optical receiver for receiving the first light beam and the second light beam from the etalon and for providing a first power signal corresponding to the first light beam received from the etalon and a second power signal corresponding to the second light beam received from the etalon; and   a processor for identifying a first condition of the first power signal and a second condition of the second power signal received from optical receiver and calculating a thickness of the etalon responsive to the first condition, the second condition, and the known characteristic.   
   
   
       18 . The system of  claim 17 , wherein the first light beam and the second light beam are directed to impinge upon a surface of the etalon at an angle normal to the surface. 
   
   
       19 . The system of  claim 17 , wherein the first light source and the second light source comprise a single laser capable of selectively generating light at the first wavelength and the second wavelength. 
   
   
       20 . The system of  claim 17 , wherein the first light source and the second light source comprise a single broadband light source. 
   
   
       21 . The system of  claim 17 , wherein the light beams are directed at the etalon along a light path comprising at least a free space portion. 
   
   
       22 . The system of  claim 21 , wherein the light path comprises a single mode optical fiber and a collimator. 
   
   
       23 . The system of  claim 22 , wherein the collimator comprises at least one of a fiber collimating self imaging lens and a fiber imaging lens 
   
   
       24 . The system of  claim 21 , wherein the etalon is disposed at a minimum light beam waist location in the free space portion. 
   
   
       25 . The system of  claim 21 , further comprising a polarizer disposed in the light path in the free space portion. 
   
   
       26 . The system of  claim 17 , further comprising a circulator for separating the light beams directed at the etalon from the light beams received from the etalon. 
   
   
       27 . The system of  claim 22 , wherein the single mode fiber is configured to function as a pin hole for allowing optimization of a light beam incidence angle on the etalon. 
   
   
       28 . The system of  claim 17 , wherein the optical receiver comprises at least one of an optical detector and an optical spectrum analyzer. 
   
   
       29 . An optical sensing probe comprising a tube having a tip portion configured for placement in an environment in which conditions are to be sensed, an etalon having a known characteristic that changes responsive to an environmental condition disposed proximate the tip portion; and a head portion remote from the tip portion containing a light directing element for directing light beams at the etalon and receiving reflected light beams from the etalon wherein the received reflected light beams are used for determining an environmental condition proximate the tip portion. 
   
   
       30 . The probe of  claim 29 , further comprising a fiber bundle for conducting respective portions of the light beams between the head portion and the etalon. 
   
   
       31 . The probe of  claim 29 , further comprising a light source for providing a first light beam at a first frequency and a second light beam at a second frequency to the light directing element. 
   
   
       32 . The probe of  claim 29 , further comprising a processor for identifying a first condition of the first light beam reflected from the etalon and a second condition of the first light beam reflected from the etalon and calculating a thickness of the etalon responsive to the first condition, the second condition, and the known characteristic. 
   
   
       33 . The probe of  claim 29 , wherein the etalon is configured to seal an interior of the tube proximate the tip. 
   
   
       34 . The probe of  claim 33 , further comprising a window configured to seal an interior the tube proximate the head. 
   
   
       35 . The probe of  claim 34 , wherein the interior of the tube contains at least a partial vacuum. 
   
   
       36 . The probe of  claim 29 , further comprising a tip cage disposed around the tip portion proximate the etalon for providing protection of the etalon. 
   
   
       38 . The probe of  claim 29 , wherein the etalon comprises single crystal silicon carbide. 
   
   
       40 . The probe of  claim 29 , wherein the tip portion comprises a material having a coefficient of thermal expansion about the same as the etalon effective to limit heat induced stresses on the etalon. 
   
   
       42 . The probe of  claim 29 , wherein at least a portion of the tube comprises a material having a lower coefficient of thermal conductivity than the tip portion. 
   
   
       43 . The probe of  claim 29 , further comprising at least one telescoping portion between the head portion and the tip portion. 
   
   
       44 . The probe of  claim 29 , wherein the light directing element further comprises a mechanism for aiming the light beams at the etalon to achieve a desired light incidence angle with respect to a surface of the etalon. 
   
   
       45 . The probe of  claim 29 , wherein the light directing element further comprises a polarizer disposed in a light beam path of the light beams. 
   
   
       46 . The probe of  claim 29 , wherein the tip portion and the head portion comprise two separate elements configured for allowing passage of the light beams therethrough. 
   
   
       47 . The probe of  claim 29 , wherein a plurality of tubes are disposed within a tip housing around a least the respective tip portions of the tubes. 
   
   
       48 . The probe of  claim 47 , wherein the respective head portions corresponding to the plurality of tubes comprise separate elements disposed within a head housing around the respective head portions of the tubes. 
   
   
       49 . The probe of  claim 29 , wherein the etalon is attached to a rotating element and the tube is disposed relative to the rotating element for directing the light beams at the etalon and receiving the reflected light beams from the etalon when the etalon is positioned within a light path of the light beams as the rotating element moves the etalon into the light path. 
   
   
       50 . The probe of  claim 49 , wherein the rotating element comprises at least one of a wheel and a turbine blade. 
   
   
       51 . The probe of  claim 50 , wherein the tube is disposed proximate a support structure of the wheel. 
   
   
       52 . The probe of  claim 49 , further comprising a support element for attaching the etalon to the rotating element. 
   
   
       53 . The probe of  claim 52 , wherein the support element comprises a material having a coefficient of thermal expansion about the same as the etalon for limiting heat induced stresses on the etalon. 
   
   
       54 . The probe of  claim 52 , wherein the support element comprises a material having a lower coefficient of thermal conductivity than the etalon. 
   
   
       55 . The probe of  claim 29 , wherein the etalon is configured to deform responsive to a pressure differential on opposite surfaces of the etalon sufficiently for being sensed by the probe as a received light beam cross section difference from a transmitted light beam cross section. 
   
   
       56 . The probe of  claim 55 , further comprising at least one beam expansion lens disposed in a light path of the light beams for increasing the transmitted light beam cross section for impinging on a relatively larger surface of the etalon. 
   
   
       57 . The probe of  claim 29 , further comprising a two dimensional optical detector for generating an image responsive to the received beams. 
   
   
       58 . The probe of  claim 57 , further comprising an image processor in communication with the two dimensional optical detector for analyzing at least a portion of the image to determine a temperature of the etalon. 
   
   
       59 . The probe of  claim 57 , further comprising an image processor in communication with the two dimensional optical detector for analyzing the image to determine at least one of a pressure and a pressure distribution on the etalon. 
   
   
       60 . The probe of  claim 57 , further comprising a beam splitter disposed in a light path of the light beams for directing the received light beams to the two dimensional optical detector. 
   
   
       61 . The probe of  claim 29 , further comprising a point detector for receiving at least a portion of the received light beams for indicating an alignment condition of the light beams with respect to the etalon. 
   
   
       62 . An optical sensor comprising:
 a chamber having an inlet for receiving a fluid into the chamber, an aperture formed in a wall of the chamber, and an etalon having a known characteristic that changes responsive to an environmental condition sealing the aperture; and   a light directing element for directing light beams at the etalon and receiving reflected light beams from the etalon, wherein the reflected light beams are used for determining an environmental condition in the chamber.   
   
   
       63 . The sensor of  claim 62 , further comprising a light source for providing a first light beam at a first frequency and a second light beam at a second frequency to the light directing element. 
   
   
       64 . The sensor of  claim 62 , further comprising a processor for identifying a first condition of the first light beam reflected from the etalon and a second condition of the first light beam reflected from the etalon and calculating a thickness of the etalon responsive to the first condition, the second condition, and the known characteristic. 
   
   
       65 . The sensor of  claim 62 , further comprising a two dimensional optical detector for generating an image responsive to the received light beams. 
   
   
       66 . The sensor of  claim 65 , further comprising an image processor in communication with the two dimensional optical detector for analyzing the image to determine a pressure on the etalon. 
   
   
       67 . The sensor of  claim 62 , wherein the light directing element comprises a single mode fiber for emitting the light beams along a free space path to the etalon and receiving the reflected light beams from the free space path. 
   
   
       68 . The sensor of  claim 67 , further comprising a lens for focusing the emitted light beams and received light beams on a focused spot of the etalon. 
   
   
       69 . The sensor of  claim 67 , further comprising a lens for collimating the emitted light beams and received light beams on majority portion of the surface of the etalon. 
   
   
       70 . The sensor of  claim 62 , wherein the light directing element further comprises a mechanism for aiming the light beams at the etalon to achieve a desired light incidence angle with respect to a surface of the etalon. 
   
   
       71 . The sensor of  claim 70 , wherein the mechanism comprises a tilt element. 
   
   
       72 . The sensor of  claim 70 , wherein the mechanism comprises a translation element.