State-of-the-Art Optical Microfiber Coupler Sensors for Physical and Biochemical Sensing Applications

Abstract

An optical fiber coupler is a simple and fundamental component for fiber optic technologies that works by reducing the fiber diameter to hundred nanometers or several micrometers. The microfiber coupler (MFC) has regained interest in optical fiber sensing in recent years. The subwavelength diameter rationales vast refractive index (RI) contrast between microfiber "core" and surrounding "cladding", a large portion of energy transmits in the form of an evanescent wave over the fiber surface that determines the MFC ultrasensitive to local environmental changes. Consequently, MFC has the potential to develop as a sensor. With the merits of easy fabrication, low cost and compact size, numerous researches have been carried out on different microfiber coupler configurations for various sensing applications, such as refractive index (RI), temperature, humidity, magnetic field, gas, biomolecule, and so on. In this manuscript, the fabrication and operation principle of an MFC are elaborated and recent advances of MFC-based sensors for scientific and technological applications are comprehensively reviewed.

Keywords: biosensors; chemical sensors; microfiber coupler; optical fiber; physical sensors; sensors.

Figure 1

The schematic of (a) MFC, (b) MFC loop, and (c) MFC tip. The waist region and transition region are mentioned in (a).

Figure 2

The schematic diagram of Mach–Zehnder interferometer (MZI) made by two MFCs.

Figure 3

(a) Schematic of the polyethylene oxide (PEO) coated MFC. (b) Schematic diagram of the experimental setup for humidity sensing. (c) Dip wavelength shift of transmission spectrum in the relative humidity range from 73.3% to 77.8%. (d) Dip wavelength versus RH for the two samples. Copyright (2015) from Reference [61].

Figure 4

Schematic diagram of (a) the MFC and (b) PVA-coating technique. (c) U-shaped MFC and two capillaries. (d) The humidity-sensing process of PVA-coated U-shaped MFC.

Figure 5

Experiment setup of the proposed sensor.

Figure 6

(a) Schematic of the MFC-based magnetic field sensor with MF as the surrounding. (b) Schematic of the experimental measurement setup. (c)Transmission spectral responses to the magnetic field strength. (d) Dip wavelength shift as a function of magnetic field strength. Copyright (2015) from Reference [67].

Figure 7

Schematic diagram of the MFC-based microfluidic flowmeter.

Figure 8

(a) The fabrication setup of MFCs; (b) the fabricated MFC sensing unit; (c) the mesoporous silica coating on the surface of the MFC; (d) transmission spectral responses of the MFC before and after it is coated with mesoporous silica layer; (e) the cross-section of the fabricated MFC sensing unit. Copyright (2019) from Reference [74].

Figure 9

The experimental setup for AMC sensing. Copyright (2019) from Reference [74].

Figure 10

Schematic diagram of the embedded MFC and the experimental setup.

Figure 11

Schematic diagram of the process of waist surface modification and antibody immobilization.

Figure 12

(a) Transmission spectral responses near the turning point of effective group index difference (the spectra are offset for clarity). (b) Peak wavelength shift with different concentrations of cTnI (PBS, 2, 4, 6, 8, and 10 fg/mL, respectively). (c) Real-time response transmission spectrum with PBS, 2 and 4 fg/mL. Inset: corresponding wavelength shift. (d) Measured response to cTnI antigen and other non-specific protein (CRP, IgG, and PSA) with the same concentration of 10 fg/mL in PBS buffer. Copyright (2018) from Reference [77].