A Review of Microfiber-Based Temperature Sensors

Abstract

Optical microfiber-based temperature sensors have been proposed for many applications in a variety of industrial uses, including biomedical, geological, automotive, and defense applications. This increasing demand for these micrometric devices is attributed to their large dynamic range, high sensitivity, fast-response, compactness and robustness. Additionally, they can perform in-situ measurements remotely and in harsh environments. This paper presents an overview of optical microfibers, with a focus on their applications in temperature sensing. This review broadly divides microfiber-based temperature sensors into two categories: resonant and non-resonant microfiber sensors. While the former includes microfiber loop, knot and coil resonators, the latter comprises sensors based on functionally coated/doped microfibers, microfiber couplers, optical gratings and interferometers. In the conclusions, a summary of reported performances is presented.

Keywords: fiber coupler; fiber resonator; microfibers; nanofibers; temperature sensor.

Figure 1

Schematic of an optical MF. A uniform waist region (center) is connected to single mode optical fiber pigtails (SMF 28) by conical transition regions.

Figure 2

The flame brushing technique: a flame travels under an optical fiber clamped at its extremity onto two computer-controlled stages which are moving apart.

Figure 3

Schematic of (a) taper profile evolution using the multi-sweep tapering method; (b) Experimental implementation of the single-sweep tapering method.

Figure 4

Modes evolution along the length of the tapered optical fiber.

Figure 5

Schematic diagram illustrating the local tapering angle Ω in the transition region.

Figure 6

Mode field intensity profile for the HE₁₁ mode at various MF radii r. The field discontinuity is presented as spikes at the silica-air interface.

Figure 7

Fraction of the power in the evanescent field against MF radius, for a Silica MF in air at λ = 1.55 μm.

Figure 8

Field intensity distribution of silica MFs in air with radii of (a) 1 μm, (b) 0.5 μm and (c) 0.2 μm.

Figure 9

Schematic of a functionalised sensor based on light confinement.

Figure 10

(a) Schematic diagram of isopropanol-sealed capillary. (b) Schematic diagram of the MF coated with the high refractive index Al₂O₃ nanofilm. (c) Dependence of the resonant wavelength shift of metal nanofilm coated MF for increasing temperatures. (d) Schematic diagram of graphene assisted MF (GAMF). (e) Transmissivity change of the GAMF for increasing temperatures.

Figure 11

(a) Schematic of a MF coupler: two supermodes are excited at the coupler waist region. (b) An output spectrum from the MF coupler.

Figure 12

Experimental setup for temperature measurement using a bi-conical MF coupler tip (MFCT).

Figure 13

(a) SEM image of the MFC waist region. (b) MFC output power spectrum. (c) MFC transmission spectrum at different applied temperatures.

Figure 14

(a) Schematic of the FIB inscribed grating at the MF tapered tip for temperature sensor; (b) The schematic of laser inscribed Bragg grating in MF; (c) The schematic of a MF placed on top of metal nanofilm Bragg grating.

Figure 15

(a) Silica thin film long period grating on MF; (b) Schematic of MF LPG based on periodic tapered waist; (c) Schematic diagram of mechanical induced MF LPG.

Figure 16

(a) Schematic diagram of the fabricated inline MF MZI; (b) Shift in the transmission wavelength dip of the inline MF-MZI for temperature increases; (c) Experimental scheme for the polymer coated MF MZI; (d) Spatial frequency shift ($∆$f) of the polymer overlay MF-MZI for increasing temperatures.

Figure 17

Schematic of loop resonator (LR) while arrows represent the direction of light propagated in LR.

Figure 18

Schematic of the original methodology used for manufacturing KR.

Figure 19

Modified technique to manufacture the KR.

Figure 20

Microcoil resonator (a) Schematic of the resonator: a MF coiled around a solid cylindrical rod; (b) Cross section.