Fiber-optic pressure sensor, variants and method for producing a resilient membrane

Abstract

The inventions relate to measurements of a hydrostatic and/or fast-changing pressure by optical means, and are suitable for aerodynamic investigations of aircraft and small spacecraft, in robotics, including small force micro-clamps, in remote pressure monitoring (in wells, vessels, cylinders), in medicine and medical and biological investigations, hydroacoustics, security systems. The object is to improve the sensitivity and enhance the temperature and vibration stability of a pressure sensor. The pressure sensor is a low Q-factor Fabry-Perot cavity/interferometer 6 at the end of a single-mode optical fiber 1 with the 4% Fresnel reflection from the silica glass/air interface. Another movable mirror of the cavity is formed by an end face 10 of a short (1-3 mm) optical fiber length 9 having a small inertial mass and inserted into the center of a flexible membrane 9 of 500-700 μm in diameter, tightly sealing a small air volume of about 1-3 mm 3 inside a second capillary 3 with the external diameter of 0.5 to 0.9 mm. End faces 7 and 10 of the optical fibers 1 and 9 forming the Fabry-Perot cavity 6 with a gap of about 50 μm are placed inside the capillary 2 with the internal diameter of about 145 μm. The temperature stability of the sensor is ΔP/ΔT≈0.001 atm/° C.

What is claimed is:  
     
         1 . A fiber optic pressure sensor comprising a Fabry-Perot interferometer, one of reflective surfaces of the Fabry-Perot interferometer being an end face of a single-mode optical fiber which is mounted along the axis of a capillary and secured therein, characterized in that the sensor further comprises a second capillary and a flexible diaphragm attached to one end face of the second capillary, a surface of the diaphragm forming a second reflective surface of the Fabry-Perot interferometer, the capillary being mounted and secured along the axis of the second capillary on the side of the second end face of the second capillary.  
     
     
         2 . The sensor according to  claim 1  wherein said capillaries are of silica glass.  
     
     
         3 . The sensor according to  claim 1  wherein said flexible diaphragm is of a metal foil or a metallized polymer film.  
     
     
         4 . The sensor according to  claim 1  wherein a thickness of said flexible diaphragm is no less than 10 μm.  
     
     
         5 . The sensor according to  claim 1  wherein the internal diameter of the capillary is 0.8 to 4% greater than the diameter of the optical fiber.  
     
     
         6 . The sensor according to  claim 1  wherein a gap between the internal surface of the second capillary and the external surface of the capillary is from 5 to 20 μm.  
     
     
         7 . The sensor according to  claim 1  wherein said second capillary in inserted into the capillary at a depth of from 1 to 2 mm.  
     
     
         8 . The sensor according to  claim 1  wherein a length of said capillaries is from 2 to 4 mm.  
     
     
         9 . The sensor according to  claim 1  wherein an end of the optical fiber projects from the second capillary at a length from 0.5 to 1 mm.  
     
     
         10 . The sensor according to  claim 1  wherein a distance between the reflective surfaces of the Fabry-Perot interferometer is from 10 to 1000 μm.  
     
     
         11 . The sensor according to  claim 1  wherein the second capillary is secured in the capillary with an epoxy.  
     
     
         12 . The sensor according to  claim 1  wherein the optical fiber is secured in the capillary with an epoxy.  
     
     
         13 . A fiber optic pressure sensor comprising a Fabry-Perot interferometer, reflective surfaces of the Fabry-Perot interferometer being formed by end faces of optical fibers, one of the optical fibers being a single-mode optical fiber, ends of the optical fibers being arranged along an axis of a capillary, characterized in that the sensor further comprises a second capillary and a flexible diaphragm, a second optical fiber length is secured in the flexible membrane mounted on the side of one end face of the second capillary, the capillary being mounted and secured along an axis of the second capillary on the side of the second end face of the second capillary.  
     
     
         14 . The sensor according to  claim 13  wherein said capillaries are of silica glass.  
     
     
         15 . The sensor according to  claim 13  wherein said flexible diaphragm is of an organosilicon elastomer.  
     
     
         16 . The sensor according to  claim 13  wherein a thickness of the flexible diaphragm is from 100 to 400 μm.  
     
     
         17 . The sensor according to  claim 13  wherein the internal diameter of the capillary is from 10 to 40% greater than the diameter of the optical fiber.  
     
     
         18 . The sensor according to  claim 13  wherein a gap between the internal surface of the second capillary and the external surface of the capillary is from 5 to 20 μm.  
     
     
         19 . The sensor according to  claim 13  wherein said capillary is inserted into the second capillary at a depth of from 1 to 3 mm.  
     
     
         20 . The sensor according to  claim 13  wherein a length of said capillaries is from 2 to 4 mm.  
     
     
         21 . The sensor according to  claim 13  wherein an end of the first optical fiber is inserted into the capillary at a length from 300 to 500 mm.  
     
     
         22 . The sensor according to  claim 13  wherein an end of the second optical fiber is inserted into the capillary at a length from 250 to 400 mm  
     
     
         23 . The sensor according to  claim 13  wherein a distance between the end faces of the optical fibers is from 30 to 100 μm.  
     
     
         24 . The sensor according to  claim 13  wherein a free volume inside the first capillary is filled with a gaseous substance.  
     
     
         25 . The sensor according to  claim 13  wherein said capillary is secured in the second capillary with an epoxy.  
     
     
         26 . The sensor according to  claim 13  wherein said first optical fiber is secured in the capillary with an epoxy.  
     
     
         27 . A method for fabricating a flexible diaphragm, including attaching the diaphragm to an end face of a second capillary, characterized by inserting an optical fiber length into a capillary, then inputting a polymerizing liquid into the capillary on the side of an end face of the capillary, so that to wet the capillary end face and the optical fiber, an amount of the liquid being sufficient to form meniscuses on the internal surface of the second capillary and the optical fiber length, and a thin film of the liquid to tightly seal the entire end face of the second capillary.  
     
     
         28 . The method according to  claim 27  wherein a length of a multi-mode optical fiber is inserted into the capillary.  
     
     
         29 . The method according to  claim 27  wherein an end of the optical fiber length is inserted with a gap into the capillary inserted into the second capillary on the side of the end face opposite to the end face on the side of which the optical fiber is inserted.  
     
     
         30 . The method according to  claim 27  wherein said polymerizing liquid is an organosilicon compound which forms, upon polymerization, a flexible rubber-like film.  
     
     
         31 . The method according to  claim 27  wherein said polymerizing liquid is a silicone rubber, hermetic or aquaseal.  
     
     
         32 . The method according to  claim 27  wherein the internal diameter of the capillary is 8 to 36% greater than the diameter of the optical fiber inserted therein.  
     
     
         33 . The method according to  claim 27  wherein immediately after inputting the polymerizing liquid its excess is removed by a length of a dry optical fiber or a thin wire.  
     
     
         34 . The method according to  claim 27  wherein upon polymerization of the liquid, the end of the optical fiber projecting from the capillary is cut.  
     
     
         35 . The method according to  claim 27  wherein the end face of the optical fiber inserted into the capillary is made by shattering upon having been scribed and subsequent broken by stretching along the optical fiber axis.