Optical fiber refractometer based on cladding-mode Bragg grating

Abstract

We demonstrate an optical fiber refractometer based on a cladding-mode Bragg grating. It consists of a long-period grating (LPG) followed by a fiber Bragg grating (FBG). The LPG partially couples light from the core mode to a cladding mode, both of which are reflected by the FBG. Part of the cladding mode reflection is coupled back to the core mode through the original LPG and used for refractive index sensing. The core mode reflection is used to compensate for the temperature cross sensitivity of the refractometer. The sensors operate in the reflection mode and can be multiplexed on a fiber in wavelength domain.

Coupling between counterpropagating cladding modes in fiber Bragg gratings

“…Finally, we must point out that the operation principle of the refractometer presented in [10] is not based on the coupling between cladding modes as claimed in that work, but on the coupling between the core and cladding modes. As we have demonstrated here, if the spectral lines of cladding-to-cladding coupling had been observed, a stronger coupling line between the core and the cladding mode would have been observed as well.…”

“…Thus, a number of tools are available to transfer energy between modes; however, the development of an efficient mode engineering requires new ways to increase the ability to accurately control the excitation of specific modes and the amount of power that is transferred. There are already some examples in which in-fiber mode engineering achieved by combining FBG and LPG has demonstrated devices with new and interesting properties [8][9][10].…”

“…The operation principle is the interaction between core and cladding modes in the grating as demonstrated in [11,12]. These devices have been proposed as reflectors for large modal area lasers [13,14] and as sensors or actuators [8][9][10]15]. For these kinds of applications, there is a need to investigate the transfer of energy specifically between cladding modes in order to pave the way for future developments.…”

Multiplexed displacement fiber sensor using thin core fiber exciter

“…These attributes have led to the successful application of optical fiber sensors in structural monitoring and machine control. 1,2 Fiber grating-based sensors, including fiber Bragg gratings (FBGs), [3][4][5] long period fiber gratings (LPFGs), 6,7 and tilted fiber Bragg gratings (TFBG), 8 have been employed as displacement measurement devices, either by direct stretch, the use of cantilever beams and similar structures, or perpendicular displacement to the sensors. Unfortunately, the abovementioned displacement sensing technologies have a major limitation that the measurement dynamic range is restricted due to the glass nature of optical fiber.…”

Refractive index sensing measurement based on periodically tapered small core singlemode fibre

“…To date several types of optical fibre refractometers have been proposed for applications in growth areas for RI sensing [1][2][3][4] . The most common approaches are a fibre Bragg grating 1 , long period grating 2 , tapered microfibre 3 and singlemode fibre (SMF)-multimode fibre (MMF)-singlemode 4 based fibre refractometers.…”