Methods for sensing temperature, pressure and liquid level and variable ratio fiber optic coupler sensors therefor

• US 5,026,984 A
• Filed: 01/16/1990
• Issued: 06/25/1991
• Est. Priority Date: 01/16/1990
• Status: Expired due to Term

• Current Assignee: Mid-America Commercialization Corporation
• Original Assignee: Sperry Marine, Inc. (Northrop Grumman Corporation)
• Inventors: Gerdt, David W.
• Primary Examiner(s): Nelms, Daivd C.
• Application Number:
  US07/465,710
• Time in Patent Office: 525 Days
• Field of Search: 250/227.14, 250/227.24, 250/227.21, 250/231.19, 350/96.29, 374/161, 374/162
• US Class Current: 250/227.21
• CPC Class Codes: G01D 5/35345 using Amplitude variations to detect the measured quantity
• CPC Class Codes: G01F 23/14 by measurement of pressure
• CPC Class Codes: G01F 23/18 Indicating, recording or alarm devices actuated electrically
• CPC Class Codes: G01F 23/292 Light , e.g. infrared or ultraviolet
• CPC Class Codes: G01K 5/52 with electrical conversion means for final indication
• CPC Class Codes: G01L 1/243 using means for applying force perpendicular to the fibre axis
• CPC Class Codes: G01L 11/025 using a pressure-sensitive optical fibre

Abstract

Temperature, pressure and liquid level parameters are sensed by the variable ratio fiber optic coupler sensor wherein the optical coupling ratio varies in accordance with stress applied to the coupler encapsulant resulting from application of the parameter being sensed. Such use of the coupler sensor is particularly advantageous in fuel tanks, which are subject to spark hazard. Vertical arrays of such sensors are utilized to determine levels of liquids in such fuel tanks. Various sensor configurations are disclosed.

Claims

1. A fiber optic coupler pressure sensor comprising

a housing having an aperture therein,

a fiber optic coupler encapsulated in said aperture having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers,

encapsulant material for encapsulating said common optical core in said aperture, said encapsulant material having a refractive index variable with stress applied thereto, such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said encapsulant material, and

compressible means cooperative with said encapsulant material so that pressure applied to said encapsulant material compresses said compressible means, so as to create stress in said encapsulant material in accordance with said applied pressure.

2. The sensor of claim 1 wherein said compressible means comprises a layer of compressible material disposed between said encapsulant material and said housing.

3. The sensor of claim 1 wherein said compressible means comprises air bubbles injected into said encapsulant material proximate said common optical core.

4. The sensor of claim 1 wherein said compressible means comprises compressible material encapsulated in said encapsulant material proximate said common optical core.

5. A fiber optic coupler pressure sensor comprising

a housing,

a pressure cell within said housing, bounded by said housing on three sides and by an elastic member deformable in accordance with pressure applied to said cell on a fourth side,

a fiber optic coupler having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers, and

encapsulant material encapsulating said common optical core, said encapsulating material exposed to environmental pressure on one side and coupled to said elastic member on a side opposite said one side, said encapsulant material having a refractive index variable with stress applied thereto, such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said encapsulant material,

said stress generated by deflections of said elastic member in response to pressure differences between said pressure cell and said environmental pressure.

6. A fiber optic coupler pressure sensor comprising

first and second pressure reservoirs, and

a fiber optic coupler sensor having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers, said common optical core being encapsulated in a membrane formed of encapsulant material having a refractive index variable with stress applied thereto such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said membrane,

said membrane being positioned between said first and second pressure reservoirs so that said membrane is deflected in accordance with pressures within said first and second reservoirs, said stress being generated in said encapsulant material in accordance with said pressures.

7. A fiber optic coupler temperature sensor comprising

a fiber optic coupler having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers,

a layer of encapsulant material for encapsulating said common optical core, said encapsulant material having a refractive index variable with stress applied thereto, such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said encapsulant material, and

a layer of further material fastened to said layer of encapsulant material, said further material and said encapsulant material having substantially different coefficients of thermal expansion with respect to each other so that temperature changes cause said layers to bend, so as to apply stress to said encapsulant material in accordance with said temperature changes.

8. A fiber optic coupler temperature sensor comprising

a fiber optic coupler having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers,

a layer of encapsulant material encapsulating said common optical core, said encapsulant material having a refractive index variable with stress applied thereto, such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said encapsulant material, and

a temperature responsive element cooperative with said layer of encapsulant material, said temperature responsive element changing shape in response to temperature variations, so as to apply stress to said encapsulant material in accordance with said temperature variations.

9. The temperature sensor of claim 8 wherein said temperature responsive element comprises a bi-metallic element fastened to a surface of said layer of encapsulant material.

10. The temperature sensor of claim 8 wherein said temperature responsive element comprises a bi-metallic element encapsulated in said encapsulant material.

11. The temperature sensor of claim 8 wherein said temperature responsive element comprises two bonded together layers of glass having different coefficients of thermal expansion with respect to each other fastened to a surface of said layer of encapsulant material.

12. The temperature sensor of claim 8 wherein said temperature responsive element comprises two bonded together layers of glass having different coefficients of thermal expansion with respect to each other encapsulated in said encapsulant material.

13. A fiber optic coupler temperature sensor comprising

a housing,

a fiber optic coupler having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers,

a layer of encapsulant material encapsulating said common optical core, said encapsulant material having a refractive index variable with stress applied thereto, such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said encapsulant material,

said layer of encapsulant material being disposed in said housing to form a cavity with respect to said housing, and

a layer of nematic liquid crystal material in said cavity having molecules appropriately oriented to stress said encapsulant material when said molecules change dimension in response to temperature changes,

said stress in said encapsulating material thereby being generated in response to temperature changes.

14. A method of sensing temperature utilizing a fiber optic coupler sensor having a plurality of input optical fibers each having a core, said cores of said optical fibers being merged and fused in a waist region to form a common optical core wherefrom a plurality of output optical fibers emerge, said fiber optic coupler distributing light energy incident to one of said input optical fibers between said plurality of output optical fibers, said waist region being encapsulated in an encapsulant material having a refractive index variable with stress applied thereto such that said incident light energy is distributed between said plurality of output optical fibers as a function of stress applied to said encapsulating material, said optical fibers and said encapsulant material having substantially different coefficients of thermal expansion with respect to each other;

comprising the steps of;

positioning said fiber optic coupler sensor to sense said temperature,

said stress being generated in said encapsulant material by said different coefficients of thermal expansion in response to changes in said temperature, and

converting said distributed light energy emerging from said output optical fibers into corresponding electrical signals, thereby sensing said temperature changes.