Surrounding refractive index measurement method based on thin cladding long period fiber bragg grating polarization property

Abstract

The invention relates to a surrounding refractive index measurement method based on the thin cladding long period fiber bragg grating polarization property. The radius of a cladding is 20.75 micrometers, detection light waves are any complete polarized light ranging from 1690 nm to 1710 nm, the wavelength interval is 1 nm, and the measurement scope of the surrounding refractive index ranges from 1 to 1.46. When the known environment refractive index is nI, output polarization states P1, P2...PN of the equal-spaced light waves with the wavelengths Lambada1, Lambada2 ...LambadaN after the light waves pass through long period fiber bragg gratings are detected; spherical distance sequences l1, l2... lN-1 between corresponding points of the polarization states P1, P2...PN on a poincare sphere are calculated; the maximum value lmaxI in the spherical distance sequences l1, l2... lN-1 is taken, wherein the lmaxI is a measurement result under the known surrounding refractive index nI; a relation curve between the surrounding refractive index n and the lmax is drawn; after the lmaxI of the certain unknown environment refractive index is calculated, and a corresponding n value of the lmaxI can be obtained through the curve. By means of the surrounding refractive index measurement method, single-color tunable laser light source is adopted for measurement, so that measured values are changed in an approximately-linear mode along with changes of surrounding refractive index; the surrounding refractive index measurement method is relatively simple in technological process and suitable for actual measurement.

Description

technical field [0001] The invention belongs to the technical field of optical fiber sensing, in particular to a method for measuring the ambient refractive index based on the polarization characteristics of a thin-clad long-period optical fiber grating. Background technique [0002] A long period fiber grating is a passive device with a periodic refractive index profile in the axial direction, which couples light energy from the guided wave mode propagating in the core into the cladding mode in the cladding, resulting in a transmission spectrum A series of stop bands are generated at the response wavelength. [0003] Based on the characteristic that the resonant wavelength shifts with the changes to be measured, long-period fiber grating sensors have been widely used in various optical sensing devices, such as environmental refractive index, temperature, stress and other sensors. [0004] The earliest document that reported the use of the offset of the resonant wavelength ...

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Problems solved by technology

[0006] The purpose of the present invention is to solve the problem that the broadband light source needs to be used to detect long-period fiber gratings in the existing environmental refractive index measurement method, and the generated resonance wavelength offset has poor linearity with the change of the environmental refractive index, and provides a A relatively simple and easy-to-implement method for measuring the ambient refractive index based on the polarization characteristics of thin-clad long-period fiber gratings. The ambient refractive index measurement method can use a narrow-band light source to measure the ambient refractive index using the polarization state of the output light. The measured value changes approximately linearly with the change of the refractive index of the environment; the long-period fiber grating with thin cladding is used to improve the sensing sensitivity

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