Optical temperature sensor using photoelastic waveguides

• US 4,756,627 A
• Filed: 08/17/1984
• Issued: 07/12/1988
• Est. Priority Date: 08/17/1984
• Status: Expired due to Fees

• Current Assignee: Sperry Corporation
• Original Assignee: Sperry Corporation
• Inventors: Nelson, Arthur R.
• Primary Examiner(s): Cuchlinski, Jr., William A.
• Assistant Examiner(s): Worth, W. Morris
• Application Number:
  US06/641,643
• Time in Patent Office: 1,425 Days
• Field of Search: 356/44, 374/130, 374/131, 374/161, 374/159, 350/96.14, 350/96.13, 350/150, 250/227, 73/705
• US Class Current: 374/159
• CPC Class Codes: G01K 5/52 with electrical conversion means for final indication

Abstract

An integrated-optic waveguide apparatus uses a photoelastic substrate for sensing temperature changes. Optical channels are formed in a photoelastic crystal substrate by depositing film stripes thereon to cause stress-induced changes in the relative refractive index responsive to changes in the substrate temperature. Absorption and coupling or switching of light waves between the channels provides a variable light intensity as a function of temperature. A single-mode temperature sensor of the interferometer type provides high sensitivity without requiring a secondary reference light source. Multimode temperature sensors are also described which exhibit a wide dynamic range useful up to several hundred degrees C. at a slight compromise in sensitivity.

Claims

1. An apparatus for optically sensing temperature changes, comprising a source of light energy, a light detector, a receiver coupled to said detector, and temperature sensing means interposed between said source and said detector, wherein said temperature sensing means comprises:

an optically transparent photoelastic substrate having a planar surface and an input region whereat light energy is coupled for guidance therein, an index of refraction responsive to stress applied thereto, a first thermal coefficient of expansion, and an output region for extracting at least a portion of said light energy,

a film having a second thermal coefficient of expansion deposited upon said planar surface of said substrate in a predetermined pattern extending from said input region to said output region,

said film defining first and second photoelastic waveguides proximate thereto and residing within said substrate, corresponding to regions of stress induced change in index of refraction responsive to said temperature changes, for coupling said portion of said light energy from said input region to said output region,

differences of said first and second thermal coefficients of expansion applying a stress to said substrate that varies said index of refraction corresponding to said waveguide, said variation in index of refraction varying said portion of light inversely according to said temperature changes,

said film having longitudinal portions spaced a predetermined distance therebetween for coupling said first and second waveguides, said coupling responsive to temperature changes of said film and said substrate,

means for coupling single-mode light rays from said source of light energy to one of said first and second waveguides, and

means for coupling said first and second waveguides, respectively, to said light detector,

said light detector further comprising first and second detector means coupled to said first and second waveguides, respectively.

2. Apparatus as set forth in claim 1, wherein said first and second detector means provide corresponding electrical signals indicative of light signals emitted from said first and second waveguides, respectively.

3. Apparatus as set forth in claim 2, said receiver further comprising means for providing electrical signals representative of algebraic sums and differences of said first and second electrical signals, and means for providing a further signal indicative of a quotient of said sums and differences.

4. An apparatus for optically sensing temperature changes, of the type having a source of light energy, a light detector, and temperature sensing means interposed between said source and said detector, wherein said temperature sensing means comprises:

an optically transparent photoelatic substrate having a planar surface and an input region whereat light energy is coupled for guidance therein, an index of refraction responsive to stress applied thereto, a first thermal coefficient of expansion, and an output region for extracting at least a portion of said light energy,

a film having a second thermal coefficient of expansion deposited upon said planar surface of said substrate in a predetermined pattern extending from said input region to said output region, said film defining a photoelastic waveguide proximate thereto and residing within said substrate, corresponding to a region of stress induced change in index of refraction responsive to said temperature changes, for coupling said portion of said light energy from said input region to said output region,

differences of said first and second thermal coefficients of expansion applying a stress to said substrate that varies said index of refraction corresponding to said waveguide, said variation in index of refraction varying said portion of light inversely according to said temperature changes,

an optical waveguide disposed in said substrate and essentially independent of temperature and coupled to said input region to provide a reference optical signal,

means for apportioning said light energy between said photoelastic waveguide and said temperature independent waveguide at said input region,

means for combining said reference optical signal from said temperature independent waveguide and a temperature dependent optical signal from said photoelastic waveguide to provide a temperature dependent output signal to said light detector,

said source of light energy, said waveguides, said means for apportioning light energy and said means for combining said optical signals comprising single-mode light propagation means.

5. An optical waveguide temperature sensor, comprising:

an optically transparent photoelastic substrate having a planar surface and an input region whereat light energy is coupled for guidance therein, an index of refraction responsive to stress applied thereto, and a first thermal coefficient of expansion,

a film having a second thermal coefficient of expansion deposited upon said planar surface of said substrate in a predetermined pattern extending from said input region to an output region,

said film defining at least one photoelastic waveguide proximate thereto and residing within said substrate, corresponding to a region of stress induced change in index of refraction responsive to said temperature changes, for coupling at least a portion of said light energy from said input region to said output region,

differences of said first and second thermal coefficients of expansion applying a stress to said substrate that varies said index of refraction corresponding to said waveguide, said variation in index of refraction varying said portion of light inversely according to said temperature changes,

an optical waveguide essentially independent of temperature, disposed in said substrate, a coupled to said input region and having a longitudinal axis extending from said input region to said output region for propagating light wave energy incident thereto,

said photoelastic waveguide disposed at a predetermined angle to said longitudinal axis, for propagating or reflecting portions of light wave energy incident on said temperature independent waveguide in accordance with changes in said index of refraction of said photoelastic waveguide.

6. Apparatus for optically sensing temperature changes, of the type having a source of light energy, a light detector, and temperature sensing means interposed between said source and said detector, wherein said temperature sensing means comprises:

an optically transparent photoelastic substrate having a planar surface and an input region whereat light energy is coupled for guidance therein, an index of refraction responsive to stress applied thereto, a first thermal coefficient of expansion, and an output region coincident with said input region for extracting at least a portion of said light energy,

a film having a second thermal coefficient of expansion deposited upon said planar surface of said substrate in a predetermined pattern extending from said input region to said output region, said film defining a plurality of photoelastic waveguides proximate thereto and residing within said substrate, corresponding to a region of stress induced change in index of refraction responsive to said temperature changes, for coupling said portion of said light energy from said input region to said output region,

differences of said first and second thermal coefficients of expansion applying a stress to said substrate that varies said index of refraction corresponding to said waveguide, said variation in index of refraction varying portions of light inversely according to said temperature changes,

said substrate further comprised of a substantially planar focal surface for receiving said light energy,

said substrate additionally including a circularly cylindrical surface opposite said focal surface and a circularly cylindrical reflector means opposite said focal surface at said cylindric surface,

said reflector means having a center of curvature lying substantially symmetrically at said planar focal surface,

further comprising optical fiber means having a first end for excitation by said source of light energy coupled to a second end in abutting relationship with said focal surface for illuminating a portion of said reflector means, and a third end also coupled to said focal surface for energizing said light detector, said first, second, and third ends coupled through bidirectional optical coupler means,

said film patterned to define a plurality of tapered elements having respective large ends disposed adjacent said cylindrical surface and respective pointed ends disposed adjacent said focal surface, said tapered elements being symmetrically disposed with respect to said center of curvature, pairs of said elements defining ones of said plurality of photoelastic waveguides therebetween,

so that light energy illuminating said reflector means is diverted from said center of curvature responsive to index of refraction changes of said photoelastic waveguides according to temperature changes of said substrate.