Microfiber optical sensors: a review

Abstract

With diameter close to or below the wavelength of guided light and high index contrast between the fiber core and the surrounding, an optical microfiber shows a variety of interesting waveguiding properties, including widely tailorable optical confinement, evanescent fields and waveguide dispersion. Among various microfiber applications, optical sensing has been attracting increasing research interest due to its possibilities of realizing miniaturized fiber optic sensors with small footprint, high sensitivity, fast response, high flexibility and low optical power consumption. Here we review recent progress in microfiber optical sensors regarding their fabrication, waveguide properties and sensing applications. Typical microfiber-based sensing structures, including biconical tapers, optical gratings, circular cavities, Mach-Zehnder interferometers and functionally coated/doped microfibers, are summarized. Categorized by sensing structures, microfiber optical sensors for refractive index, concentration, temperature, humidity, strain and current measurement in gas or liquid environments are reviewed. Finally, we conclude with an outlook for challenges and opportunities of microfiber optical sensors.

Figure 1.

Index profile of an optical microfiber.

Figure 2.

Z-Components of the Poynting vectors (S_z) of the HE₁₁ mode of a (a) 200 and (b) 400 nm diameter silica microfibers at 325 nm wavelength [12].

Figure 3.

Fractional power of the fundamental mode outside the core of silica microfibers at 325 and 650 nm wavelength. Dashed lines: single-mode cutoff diameters [12].

Figure 4.

Z-direction Poynting vectors of a 400 nm diameter silica microfiber at 633 nm wavelength (a) in 3-D view; and (b) in 2-D view [3].

Figure 5.

Schematic diagram of taper-drawing technique.

Figure 6.

Illustration of direct drawing a polymer microfiber from a solution. Bulk polymer materials (a) is firstly dissolved in a certain solvent (b), and a droplet of polymer solution is picked up and placed upon a substrate and drawn by a tip after evaporation of the solvent to a certain degree (c), then the tip is withdrawn with a speed of 0.1–1 m/s to form a polymer microfiber (d).

Figure 7.

(a) SEM image of a 100 nm diameter tellurite glass microfiber [29]; (b) SEM image of a 15 mm diameter micro-ring made with a 520 nm diameter silica microfiber [9]; (c) A coiled 260 nm diameter silica microfiber with a total length of about 4 mm [9]; (d) SEM image of a 350 nm dameter PMMA microfiber. Scale bar: 50 μm; (e) A 280 nm diameter polystyrene (PS) microfiber doped with CdSe quantum dots [30]; (f) PAM microfiber doped with aligned GNRs [31].

Figure 8.

Microfiber absorption sensor [33]. (a) Biconical tapered fiber with a 900 nm diameter waist (microfiber). Scale bars: 125 μm; (b,c) Cartoon and optical micrographs of microfluidic chip based microfiber sensor fabrication procedures; (d) Optical micrograph of a 1.5 μm diameter microfiber guiding a laser with a wavelength of 473 nm embedded in a microchannel; (e) Optical micrograph of the fluorescence excited by evanescent field outside a 1.5 μm diameter microfiber. Scale bars: 125 μm; (f) Transmission spectra of different MB concentrations for the sensor with a 900 nm diameter microfiber. Inset: absorbance at 635 nm wavelength versus MB concentrations for two microfibers with diameters of 900 nm and 1.5 mm, respectively; (g) Cycling measurement with 500 pM MB solutions for a 900 nm diameter microfiber. The y-errors are determined from three repeated measures.

Figure 8.

Microfiber absorption sensor [33]. (a) Biconical tapered fiber with a 900 nm diameter waist (microfiber). Scale bars: 125 μm; (b,c) Cartoon and optical micrographs of microfluidic chip based microfiber sensor fabrication procedures; (d) Optical micrograph of a 1.5 μm diameter microfiber guiding a laser with a wavelength of 473 nm embedded in a microchannel; (e) Optical micrograph of the fluorescence excited by evanescent field outside a 1.5 μm diameter microfiber. Scale bars: 125 μm; (f) Transmission spectra of different MB concentrations for the sensor with a 900 nm diameter microfiber. Inset: absorbance at 635 nm wavelength versus MB concentrations for two microfibers with diameters of 900 nm and 1.5 mm, respectively; (g) Cycling measurement with 500 pM MB solutions for a 900 nm diameter microfiber. The y-errors are determined from three repeated measures.

Figure 9.

microfiber Bragg gratings (MFBG) [42]. (a) Scanning electron microscope (SEM) image of a MFBG inscribed on a 1.8 μm diameter silica microfiber; (b) Close-up view of the MFBG; (c) Transmission and reflection spectra of the MFBG. Inset is dependence of the reflection wavelength shift on the ambient RI (black dot line) and the corresponding RI sensitivity (red hollow dot line) of the MFBG used for measuring the RI of a glycerin solution.

Figure 10.

Optical microfiber resonators. Schematics of (a) loop; (b) knot; (c) multicoil resonators [3]; (d) SEM image of a loop resonator; (e) SEM image of knot resonator using a 520 nm diameter silica microfiber [9]; (f) Schematic of an optical-rod-wrapped microfiber multicoil resonator [48].

Figure 11.

Refractive-index sensor based on copper-rod-supported microfiber loops [51]. (a) Schematic side view of a copper-rod-supported microfiber loop; (b) Spectral shifts of a resonant peak caused by index change of the solution. The eight peaks are obtained by adding a 5 μL ethanol into a 500 μL water in steps. The loop is about 480 μm in diameter and is assembled with a 2.4 μm diameter microfiber; (c) Resonant wavelength as a function of the refractive index change. The black dots are resonant wavelengths extracted from (b), and the numerical fitting is obtained with a calculated slope of 17.8 (nm/RIU).

Figure 12.

Schematic illustration of microfiber knot resonator tied on a copper rod. The upward arrow in the figure indicates the direction of the electric current in the copper rod [68].

Figure 13.

Schematic diagram of (a) the silica microfiber sensing element; and (b) the proposed sensor with a Mach-Zehnder interferometer [12].

Figure 14.

Microfiber-based MZI. (a) Optical microscope image of a MZI assembled with two 480 nm diameter tellurite microfibers. White light from a supercontinuum source is launched into and picked up from the MZI by two silica fiber tapers. The white arrows indicate the direction of light propagation [79]; (b) Schematic of a hybrid photonic-plasmonic MZI. The structure in the dashed box represents the in-fiber return-signal plasmonic probe; (c) A closed-up view of the plasmonic probe; (d) Typical transmission spectrum of the hybrid MZI [82].

Figure 15.

(a) Schematic configuration of the MZI-based RI sensor; (b) Schematic diagram of the sensing arm. BBS, broadband light source; ODL, optical delay line; OSA, optical spectrum analyzer; MF, microfiber [80].

Figure 16.

Functionally activated silica microfibers. (a) Schematic illustration of a gelatin coated microfiber for RH sensing [84]; (b) TEM images of microfibers (black cylinder) decorated with PdAu nanoparticles for hydrogen sensing [85].

Figure 17.

Functionally activated polymer microfibers. (a) Typical light-emitting polymer microfibers excited by 355 nm light. The microfibers are doped with different fluorescent dyes to emit different colors of light. Scale bars: 50μm [90]; (b) A 280 nm diameter PS microfiber doped with CdSe quantum dots [30]; (c) Three PAM nanofibers doped with aligned GNRs [31].

Figure 18.

QD-doped microfiber optical sensor [30]. (a) Schematic illustration of a QD-doped PS microfiber optical sensor; (b) PL intensity of the microfiber exposed to ambient relative humidity (RH) ranging from 7% to 81%. Inset, optical microscopy image of the QD-doped microfiber sensing element. Scale bar: 50 μm; (c) Response of the MF sensor to alternately cycled 54% and 19% RH air; (d) Typical time-dependent integrated PL intensity of the microfiber reveals a response time of about 90 ms when RH jumps from 33% to 54%.