Phase-shifted fiber bragg grating sensor and method for producing same

Abstract

T-phase-shifted fiber Bragg gratings in optical waveguides, and methods of formation thereof. Sensing apparatus comprising such gratings using femtosecond pulse duration lasers and specialized transmission diffraction elements or phase masks.

1 . A method for inscribing a phase shifted Bragg grating in an optical waveguide, comprising the steps of:
 providing the optical waveguide;   providing a diffractive optical element which incorporates a phase shift that, when exposed to a ultrashort laser pulse, creates an interference pattern on the optical waveguide which has a phase shift in its pattern; and   providing electromagnetic radiation from the ultrashort laser pulse, wherein the electromagnetic radiation has a pulse duration of less than or equal to 5 picoseconds, wherein the wavelength of the electromagnetic radiation has a characteristic wavelength in the wavelength range from 150 nm to 2.0 microns, and wherein the electromagnetic radiation step comprises irradiating a surface of the diffractive optical element with the electromagnetic radiation incident on the optical waveguide, from the diffractive optical element, and being sufficiently intense to cause the permanent change in index of refraction in the core of the optical waveguide when exposed to a minimum succession of laser pulses, to form a Bragg grating.   
     
     
         2 . The method according to  claim 1  further comprising a step of introducing a beam block to remove the effect of phase-shift splitting observed in grating structures written using phase shifted phase masks. 
     
     
         3 . The method according to  claim 1 , where the phase shifted Bragg grating is a thermally stable type II phase grating. 
     
     
         4 . The method according to  claim 3  where the birefringence of the thermally stable type II phase grating is controlled by varying the polarization of the inscription ultrafast laser with respect to the waveguide axis. 
     
     
         5 . A device comprising a Bragg grating Fabry-Perot structure in a fiber or waveguide where the first Bragg grating is shifted with respect to the second Bragg grating by N·A G +A G /2, where N is an integer number and A G  is the period of the Bragg gratings. 
     
     
         6 . The device of  claim 5  where the phase shifted grating is written into an optical fiber taper that is coated with a sensing layer. 
     
     
         7 . The method of  claim 1  where the optical waveguide is an actively doped optical fiber. 
     
     
         8 . The method according to  claim 2 , where the phase shifted Bragg grating is a thermally stable type II phase grating. 
     
     
         9 . The method of  claim 2  where the optical waveguide is an actively doped optical fiber.