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.

Tapered Optical Fibre Sensors: Current Trends and Future Perspectives

A highly sensitive temperature sensor based on an all-fiber Sagnac loop interferometer combined with metal-filled side-hole photonic crystal fiber (PCF) is proposed and demonstrated. PCFs containing two side holes filled with metal offer a structure that can be modified to create a change in the birefringence of the fiber by the expansion of the filler metal. Bismuth and indium were used to examine the effect of filler metal on the temperature sensitivity of the fiber-optic temperature sensor. It was found from measurements that a very high temperature sensitivity of −9.0 nm/°C could be achieved with the indium-filled side-hole PCF. The experimental results are compared to numerical simulations with good agreement. It is shown that the high temperature sensitivity of the sensor is attributed to the fiber microstructure, which has a significant influence on the modulation of the birefringence caused by the expansion of the metal-filled holes.

Highly sensitive temperature sensor using a Sagnac loop interferometer based on a side-hole photonic crystal fiber filled with metal.

A Comprehensive Review of Optical Fiber Refractometers: Toward a Standard Comparative Criterion

A Mach-Zehnder interferometer (MZI) based fiberoptic refractive index (RI) sensor is constructed by uniformly tapering standard single mode fiber (SMF) for RI measurement. A custom flame-based tapering machine is used to fabricate microfiber MZI sensors directly from SMFs. The fabricated MZI device does not require any splicing of fibers and shows excellent RI sensitivity. The sensor with a cladding diameter of 35.5 µm and length of 20 mm exhibits RI sensitivity of 415 nm/RIU for RI range of 1.332 to 1.384, 1103 nm/RIU for RI range of 1.384 to 1.4204 and 4234 nm/RIU for RI range of 1.4204 to 1.4408, respectively. The sensor reveals a temperature sensitivity of 0.0097 nm/°C, which is relatively low in comparison to its ultra-high RI sensitivity. The proposed inexpensive and highly sensitive optical fiber RI sensors have numerous applications in chemical and biochemical sensing fields.

https://www.mdpi.com/1424-8220/19/7/1652/pdf

Tapered Fiber-Optic Mach-Zehnder Interferometer for Ultra-High Sensitivity Measurement of Refractive Index.

Simultaneous measurement of temperature and strain or temperature and curvature based on an optical fiber Mach–Zehnder interferometer

TCF-MMF-TCF fiber structure based interferometer for refractive index sensing

High temperature probe sensor with high sensitivity based on Michelson interferometer

A novel refractive-index (RI) Michelson interferometer based on a waist-enlarged taper is achieved. Such a device is fabricated by splicing a section of multimode fiber (MMF) at one end of single-mode fiber (SMF). Due to the fiber bitaper at the coupling point of SMF and MMF, the light is coupled into the MMF from lead in fiber core, and the intermodal interference will occur for the optical path difference between core mode and cladding mode. Then, the light will be reflected at the end of the fiber and recoupled back into the lead out fiber core by the fiber bitaper. When the lights return back to the lead out fiber, the intermodal interference will occur for the optical path difference between core mode and high order mode. The sensor has a linear response to RI with a sensitivity of −178.424 dB/RIU in the range of 1.351–1.4027 RIU. The temperature cross sensitivity is also analyzed, and the influence can be eliminated by simultaneous measurement of RI and temperature through the matrix equation. The proposed sensor features the advantages of easy fabrication, low cost, high mechanical strength, which make it a good candidate for bio-chemical measurement.

In-Fiber Quasi-Michelson Interferometer Based on Waist-Enlarged Fiber Taper for Refractive Index Sensing

A Fourier analysis applied to the Mach–Zehnder interferometer (MZI) transmission spectrum for simultaneous refractive index (RI) and temperature measurements is proposed and experimentally demonstrated in this Letter. In the fast Fourier transform (FFT) spectrum of the MZI transmission spectrum, several frequency components are generally observed, which means that the transmission spectrum of the MZI is formed by the superposition of some dual-mode interference (DMI) spectra, and each frequency component represents different core-cladding interferences. We can select some dominant frequency components in the FFT spectrum of the MZI transmission spectrum to take the inverse FFT (IFFT). Then, the corresponding DMI patterns can be obtained. Due to the shift of the wavelength of these DMI spectra with changes in the environmental parameters, we can use the coefficient matrix of these DMI spectra for multi-parameter sensing. In this Letter, two DMI patterns are separated from the resultant transmission spectrum of the MZI. As the RI and temperature change, the shifts of the two DMI patterns with respect to the RI and temperature will be observed. The sensitivities of the RI and temperature are −137.1806  nm/RIU (RI unit) and 0.0860  nm/°C, and −22.9955  nm/RIU and 0.0610  nm/°C for the two DMIs. Accordingly, it can be used to simultaneously measure RI and temperature changes. The approach can eliminate the influence of multiple interferences and improve the accuracy of the sensor.

Simultaneous refractive index and temperature measurements by using dual interference in an all-fiber Mach–Zehnder interferometer

An approach for surrounding refractive index (SRI) measurement is proposed and demonstrated using all fiber Mach–Zehnder interferometer (MZI) formed by fusion splicing a short section of thin-core fiber between two sections of single-mode fibers with lateral offsets. The transmission spectrum of the MZI is dominantly formed by the superposition of two cladding modes separately interference with the core mode, which correspond to two frequency components in its fast Fourier transform (FFT) spectrum, respectively. In this letter, the two frequency components are extracted to take the inverse FFT, respectively, thus we can obtain two dual-mode interference (DMI) patterns between the cladding modes and the core mode. As the SRI changes, the shifts of the two DMI patterns with SRI will be observed. Through this way, we can eliminate the influence of multiple interferences and improve the accuracy of the sensor. The approach can also be utilized with other modal interferometric sensors.

Xu Yan

Papers

TCF-MMF-TCF fiber structure based interferometer for refractive index sensing

High temperature probe sensor with high sensitivity based on Michelson interferometer

In-Fiber Quasi-Michelson Interferometer Based on Waist-Enlarged Fiber Taper for Refractive Index Sensing

Simultaneous refractive index and temperature measurements by using dual interference in an all-fiber Mach–Zehnder interferometer

Fourier Analysis Applied on MZI Transmission Spectrum for Refractive Index Measurement