Tapered Optical Fibre Sensors: Current Trends and Future Perspectives

Abstract

The development of reliable, affordable and efficient sensors is a key step in providing tools for efficient monitoring of critical environmental parameters. This review focuses on the use of tapered optical fibres as an environmental sensing platform. Tapered fibres allow access to the evanescent wave of the propagating mode, which can be exploited to facilitate chemical sensing by spectroscopic evaluation of the medium surrounding the optical fibre, by measurement of the refractive index of the medium, or by coupling to other waveguides formed of chemically sensitive materials. In addition, the reduced diameter of the tapered section of the optical fibre can offer benefits when measuring physical parameters such as strain and temperature. A review of the basic sensing platforms implemented using tapered optical fibres and their application for development of fibre-optic physical, chemical and bio-sensors is presented.

Keywords: evanescent wave spectroscopy; functional nano-thin coatings; modal interferometry; tapered optical fibre sensors; whispering gallery mode.

Figure 1

Schematic illustration of the flame approach used for the fabrication of tapered optical fibres.

Figure 2

Geometry of a (a) parabolic (b) linear and (c) exponential–linear taper profiles [23]. Reprinted with permission. Copyright 2003 Elsevier.

Figure 3

A schematic diagram of a waist enlarged tapered optical fibre.

Figure 4

(a) Transmission spectrum of a hard-clad multimode silica optical fibre with the plastic cladding removed (black line), and after (red line) immersion into a porphyrin dye compound; (b) absorption spectrum calculated from Figure 4a [37,42]; Reprinted from [37] under creative common free licence (http://creativecommons.org/licenses/by/4.0/).

Figure 5

(a) Schematic illustration of the structure of a tapered optical fibre; HE₁₁ fundamental core mode coupled to higher order modes HE₁₂ at the first taper transition region; higher order modes are then combined with the fundamental core mode at second transition region HE₁₁ + HE_12; and (b) a typical channelled transmission spectrum of the non-adiabatic taper with waist diameter of 10 μm fabricated in an optical fibre with a cut-off wavelength of 670 nm (Fibrecore SM670). Reprinted from [37] under creative common free licence (http://creativecommons.org/licenses/by/4.0/).

Figure 6

Schematic illustrations of: (a) a fibre Bragg grating (FBG) and (b) a long period grating (LPG). Reprinted from [37] under creative common free licence (http://creativecommons.org/licenses/by/4.0/).

Figure 7

Micrograph of a tapered long period grating TLPG with three micro-tapers. Taper 1 has a distorted profile. The fibre at position A is offset from the fibre axis compared to position B. The centre axis of the fibre is shown as a dashed red line. SM750 fibre was used (magnification × 10); Reprinted with permission from [55]. Copyright 2018 Elsevier.

Figure 8

(a) Schematic illustration of the surface plasmon resonance (SPR) sensor configuration employing prism for surface plasmon excitation; and (b) shift in resonance features (wavelength or incidence angle) caused by the binding event [59], (Reprinted with permission. Copyright 2007 Elsevier).

Figure 9

Transmission spectrum of LPG tapered fibre device; reprinted with permission from [66].

Figure 10

Schematic illustration of the fibre-taper cascaded LPG; arrows indicate coupling of the fundamental core mode (green arrow) to the cladding modes at LPG-1, their interaction with the surrounding medium at tapered region and recombination of higher order modes with the fundamental core mode at LPG-2 [67] (adapted from [67]).

Figure 11

Schematic illustration of a refractive index (RI) sensor based on a multimode fibre taper splice between two lengths of single mode fibre, each of which contain an FBG (adapted from [69]).

Figure 12

A schematic illustration of the inline double-pass Mach–Zehnder interferometer (MZI); arrows indicate excited cladding modes of the length of fibre separating the two tapers, creating different optical path lengths for higher order modes traveling in the cladding and fundamental core mode travelling in the core. Reprinted from [37] under creative common free licence (http://creativecommons.org/licenses/by/4.0/).

Figure 13

Photographs of (a) MZI with two micro-abrupt-tapers in a cladding-reduced Er/Yb codoped fibre; L_P, length of the spanning fibre (between the centres of the adjacent abrupt tapers); and L_D, lengths of the interferometer; L1, length of the first taper, L2 length of the second taper and d1 and d2 diameters of the taper one and two respectively; (b) cross-sectional views of the cladding-reduced Er/Yb co-doped fibre; and (c) 6.3-pL liquid drop is approaching the micro-abrupt-taper [72]. (© 2012 IEEE. Reprinted, with permission from [72]).

Figure 14

Schematic illustration of the WEBT-DSBT MZI; WEBT, waist-enlarged taper-pair; and DSBT, embedded down-stretching-bitaper (adapted from [21]).

Figure 15

Schematic illustration of the temperature-independent refractometer based on an in-fibre MZI, fabricated by sandwiching a tapered PCF between two standard single mode fibres (CorningSMF28) [80], Reprinted with permission. Copyright 2013 Elsevier.

Figure 16

(a) Schematic illustration of an optical gas sensor based on a PhC nanobeam cavity; and (b) of the PhC nanobeam cavity [85], © Astro Ltd. Reproduced by permission of IOP Publishing. All rights reserved.

Figure 17

(a,b) Image of S-tapered PCF; (c) sketch of the S-tapered PCF; and (d) cross sections of the S-tapered PCF [82].

Figure 18

Transmission spectra of a tapered single mode optical fibre (cut-off wavelength 670 nm) with a taper waist of diameter 10 μm and of length 25 mm, measured in: H₂O, black line, an aqueous solution of Tetrasodium Pyrophosphate Porhine (TSPP) of concentration 10 μM, red line. The absorption spectra of TSPP measured using a conventional UV-vis spectrometer is shown by the blue line [42].

Figure 19

(a) Schematic illustration of the experimental set-up; (b) image of a 2 μm polymer microsphere delivered and adhered to a 1 μm diameter taper; and (c) Raman spectra of 1 μm diameter poly styrene (PS) and Poly-methyl methacrylate (PMMA) microspheres attached to a fibre taper using ca. 760 nm pumping wavelength [31], © 2012 IEEE. Reprinted, with permission).

Figure 20

(a) Experimental configuration for nanoparticle detection using Rayleigh scattering, (b) the changes in the transmission of the tapered fibre as polystyrene nanoparticles bound to a taper of diameter 800 nm and length 33 mm (adapted from [101]. © 2012 IEEE. Reprinted, with permission from [101]).

Figure 21

Schematic illustration of the experimental set-up of the multi-tapered fibre optic SPR sensor (adapted from [94]).

Figure 22

Schematic illustration of the layer-by-layer (LBL) deposition process. Each bilayer deposition constitutes a full cycle of (a–c). (d) The SEM image of multilayer film [CS/PSS] at 1080× magnification [132]. Reprinted with permission. Copyright 2013 Elsevier).

Figure 23

Tapered fibre Michelson interferometer proposed fibre inclinometer. Inset: schematic diagram detail of the fibre-taper Michelson interferometer (adapted [142]).

Figure 24

Experimental set-up and configuration of the micro-displacement sensor [143]; IEEE copyright material, Reprinted under creative common free licence (http://creativecommons.org/licenses/by/4.0/).

Figure 25

Schematic illustration of an LPG composed of a tapered optical fibre with a uniform waist and a side-contacted metal grating (adapted from [57]).

Figure 26

(a) Sensing head geometry with FBG/chirped tapered FBG (CTFBG) structure; and (b) sensing head geometry using two CTFBGs in biconical taper structure [155]. © IOP Publishing. Reproduced by permission of IOP Publishing. All rights reserved).

Figure 27

(a) Schematic illustration of the a device consisting of tapered fibre with a lateral-shifted junction; (b) experimental setup with the left and right insets showing the optical micrographs of the fibre taper (P) and the lateral shifted junction (J) in the interferometer, respectively; and (c) attenuation spectra of asymmetric interferometer at different temperatures (adapted from [156]).

Figure 28

Microscopic image of the peanut-shape structure: (a) after the arc discharge treatment and (b) after the fusion splicing. Z₁, distance to ellipsoidal-structure section Z₂, length of ellipsoidal-structure section; R, radius of the ellipsoidal-structure section; (c) schematic diagram of the cascading two waist enlarged structures-MZI (adapted from [157]).

Figure 29

(a) Side view of the S-tapered fibre (SFT) in optical microscopy. (b) Top view of the SFT. (c) Transmission spectrum of the single SFT MZI in air. (Reprinted with permission from [162]).