High-Sensitive All-Fiber Fabry-Perot Interferometer Gas Refractive Index Sensor Based on Lateral offset Splicing and Vernier Effect

A compact and high sensitivity refractive index (RI) sensor has been theoretically and experimentally demonstrated based on dual-mode interferometer (DMI) in an eccentric core few-mode fiber (ECFMF). The DMI is fabricated by fusion splicing a piece of ECFMF etched by hydrofluoric acid (HF) and two single mode fibers (SMFs) with a lateral-offset. The interference is formed by LP01 and LP11 modes in the eccentric core of ECFMF. The etched ECFMF-DMI based on core-core mode interference exhibits a higher RI sensitivity than the DMI based on core-cladding mode interference. The sensitivity reaches up to 2565.2 nm/RIU around the RI of 1.4. Both of the etched and unetched ECFMF-DMIs have low temperature sensitivities of 9.6 pm/°C and 33.1 pm/°C, respectively. The etched ECFMF-DMI based on the core-core mode interference possesses tremendous superiority for RI measurement due to its high RI sensitivity and low temperature cross, therefore the proposed sensor has great potentials in chemical and biological fields.

This paper proposes and demonstrates an ultrasensitive refractive index (RI) sensor based on harmonic Vernier effect (HEV) and a cascaded Fabry-Perot interferometer (FPI). The sensor is fabricated by sandwiching a hollow-core fiber (HCF) segment between a lead-in single-mode fiber (SMF) pigtail and a reflection SMF segment with an offset of 37 µm between two fiber centers to form a cascaded FPI structure, where the HCF is the sensing FPI, and the reflection SMF is the reference FPI. To excite the HEV, the optical path of the reference FPI must be multiple times (>1) that of the sensing FPI. Several sensors have been made to conduct RI measurements of gas and liquid. The sensor's ultrahigh RI sensitivity of up to ∼378000 nm/RIU can be achieved by reducing the detuning ratio of the optical path and increasing the harmonic order. This paper also proved that the proposed sensor with a harmonic order of up to 12 can increase the fabricated tolerances while achieving high sensitivity. The large fabrication tolerances greatly increase the manufacturing repeatability, reduce production costs, and make it easier to achieve high sensitivity. In addition, the proposed RI sensor has advantages of ultrahigh sensitivity, compactness, low production cost (large fabrication tolerances), and capability to detect gas and liquid samples. This sensor has promising potentials for biochemical sensing, gas or liquid concentration sensing, and environmental monitoring.

In this paper, we report a sensitivity-controllable refractive index (RI) sensor based on a reflective θ-shaped microfiber resonator cooperated with Vernier effect. The θ-shaped microfiber resonator is a reflective all-fiber device with comb spectrum under weak coupling condition. By cascading it with a fiber Fabry-Perot interferometer, Vernier effect is generated to demodulate surrounding RI with enhanced sensitivity. Theoretical analysis reveals that RI sensitivity of the combined structure with Vernier effect is m times higher than the sensitivity of singular θ-shaped microfiber resonator. Moreover, by adjusting cavity length of the θ-shaped microfiber resonator, magnification factor M = (m + 1) can be tuned which enables the RI sensitivity to be controlled. Experimental result demonstrates that the RI sensitivity can be widely tuned from 311.77 nm/RIU (Reflective index unit) to 2460.07 nm/RIU when the cavity length of the θ-shaped microfiber resonator is adjusted from 9.4 mm to 8.7 mm. The θ-shaped microfiber resonator based all-fiber RI sensor featuring controllable sensitivity and compact size can be widely used for chemical and biological detections. The proposed scheme of generating Vernier effect also offers a universal idea to increase measurement sensitivity for optical fiber sensing structures with comb spectrum.

Intensity-modulated refractive index sensor based on the side modes of fiber Bragg grating

A microfiber-enabled Fabry-Perot interferometer (FPI) constructed by splicing a section of microfiber between two cleaved standard single-mode fibers (SMFs) with unique relative fiber cross section relationship has been proposed and experimentally demonstrated. The opening air cavity between the two SMF ends connected by the microfiber serves as an FP cavity and also a direct sensing head. The sensing characteristics of the FPIs with different cavity lengths and microfiber diameters have been studied. A force sensitivity as high as 167.41 nm/N (~200 pm/μe) and a high refractive index (RI) sensitivity of 1330.8 nm/RIU (around a RI of 1.33) have been achieved by using the microfiber-based FPI with ~21 μm cavity length and ~44 μm microfiber diameter. Such a device has several merits such as simple configuration, compactness and reliability in operation owing to the extremely low thermal cross-sensitivities.

Fiber optic refractive index sensing using an inline dual semi-distributed interferometer

In this paper, an all-fiber novel fabricated Mach-Zehnder interferometer (MZI) based on refractive index (RI) sensor is presented and simulated. This RI sensor is composed of a single-mode fiber (SMF) with a core diameter of 9 μm and a multimode fiber (MMF) with a core diameter of 105 μm. The microgroove can be etched by femtosecond laser or ground by grinding machine technology. It greatly reduces the limitations of structure fabrication. The RI response of this sensor is obtained by beam propagation method. In the range of 1.33-1.334 RIU, the RI sensitivity is -10373 nm/RIU and the linearity is 99.98% by wavelength demodulation. The intensity demodulation shows a nonlinear trend, and the sensitivity increases with the increase of RI . The maximum RI sensitivity is -9614 dB/RIU, and the fitting is 98.69%. The structure has the advantages of simple manufacture, high sensitivity and good linearity. It is very suitable for the practical applications of RI sensing with high sensitivity.

We propose a highly sensitive interferometric sensor based on a tapered fiber concatenated with a highly birefringent (HiBi) photonic crystal fiber (PCF) for the detection of physical as well as chemical parameters. The splicing of single mode fiber (SMF) and HiBi PCF is achieved without any offset. The tapered section is employed for curvature and temperature sensing while the HiBi PCF is used for refractive index sensing. Experimental results show that the proposed Mach–Zehnder interferometer has a curvature sensitivity of 14.77 nm/m−1 with a resolution of $6.77 \times 10^{\mathrm {\mathbf {-4}}}\,\,\text{m}^{\mathrm {\mathbf {-1}}}$ . Furthermore, the refractive index (RI) sensitivity is found to be 257 nm/RIU along with a resolution of $3\times 10^{\mathrm {\mathbf {-5}}}$ RIU, which is comparable to commercial refractometer. The proposed setup shows a temperature sensitivity of 311 pm/°C. The curvature, RI, and temperature sensitivity are significantly higher with linear response.

A high-sensitivity refractive index (RI) sensor based on the surface lattice resonance of periodic gold nanoparticle array is proposed. The influence of sensor structural parameters on the extinction spectrum is systematically investigated. The physical mechanism behind the extinction peak is discussed by the electric field simulations. The optimal structural parameters are determined by calculating and analyzing key sensing performance metrics, namely sensitivity and figure of merit (FOM). In the considered RI range, the optimal sensor exhibits a sensitivity of 562.42 nm RIU−1 and a FOM of 54.38 RIU−1. Notably, in a specific subrange, the sensitivity and FOM can reach maximum values of 600 nm RIU−1 and 60.61 RIU−1, respectively.

A compact and ultra high sensitive RI sensor using modal interference in an integrated optic waveguide with metal under-cladding

A novel highly sensitive airflow sensor based on Fabry–Perot interferometer (FPI) and Vernier effect is proposed and demonstrated. The sensor is fabricated by splicing a section of hollow-core fiber (HCF) between a lead-in single mode fiber (SMF) and a section of SMF, which is almost one-fifteenth the length of the HCF. Airflow changes the cavity length of the FPI, which leads to the shift of the reflection spectrum. The reflection beam from the last end of the SMF is utilized to generate the Vernier effect, which enlarges the shift of the spectrum for 9.57 times than that of a single FPI. According to the experimental results, when the airflow ranges from ${\text{0 to 7 m/s}}$ , the spectrum of the sensor shifts as high as 7.9 nm which is almost eight times more than that of the self-heating sensor based on silver-coated FBG. And the highest airflow velocity sensitivity of the sensor can reach ${\text{1.541 nm/(m/s)}}$ in the region of ${\text{3m/s} \sim {7m/s}}$ . Besides, the sensor has a series of advantages such as fast response, low cost, compact size, and reflection measurement.

High sensitivity refractive index sensor based on micro-fiber with micro-arched transition

An ultra high-sensitivity temperature and refractive index (RI) sensor based on surface plasmon resonance (SPR) effect in D-shape high birefringence photonic crystal fiber (PCF) is presented and discussed by the finite element method (FEM). The designed PCF is polished to enhance the interaction of the evanescent field with the external medium, ultimately improving the sensing performance. The influence of three different plasma materials, gold, silver and aluminum, on the sensing performance was investigated. Finally, the chemically stable plasma material gold is selected to coat on flat surface of D-shape PCF for SPR sensing, realizing the sensitive real-time monitoring of RI and temperature. The sensing performance was greatly improved, with a wide RI (1.43 to 1.50) and temperature (36 °C to 86 °C) detection range, and the corresponding maximum sensitivity was 44850 nm/RIU and -16.875 nm/°C, respectively. Furthermore, the sensitivity is not particularly sensitive to the sizes of the designed air holes, which leads to a sensor with high structure tolerance. Numerical results show that the coating thickness not only affects the sensitivity but also the detection range of the sensor, so we can tune the RI and temperature detection range by adjusting the gold film thickness to meet specific detection requirements. The sensor with enhanced sensitivity, tunable detection range, and high fabrication tolerance has a wide application prospect in the field of life science research, chemical production, and environmental monitoring.

We propose and experimentally demonstrate a refractive index (RI) sensor based on cascaded microfiber knot resonators (CMKRs) with Vernier effect. Deriving from high proportional evanescent field of microfiber and spectrum magnification function of Vernier effect, the RI sensor shows high sensitivity as well as high detection resolution. By using the method named "Drawing-Knotting-Assembling (DKA)", a compact CMKRs is fabricated for experimental demonstration. With the assistance of Lorentz fitting algorithm on the transmission spectrum, sensitivity of 6523nm/RIU and detection resolution up to 1.533 × 10(-7)RIU are obtained in the experiment which show good agreement with the numerical simulation. The proposed all-fiber RI sensor with high sensitivity, compact size and low cost can be widely used for chemical and biological detection, as well as the electronic/magnetic field measurement.

A high sensitive refractive index sensor based on the cladding etched photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI) is proposed, which is spliced a section of photonic crystal fiber between two single modes fibers (SMFs).The interference fringe of the MZI shifts with the variation of the ambient refractive index (RI). It is found that the RI sensitivity slightly decrease with an increase in the interference length. The sensitivities of MZI with 35 mm PCF, 40 mm PCF, and 45 mm PCF are 106.19 nm/RIU, 93.33 nm/RIU, and 73.64 nm/RIU, respectively, in the range of 1.333 to 1.381. After etched, the RI sensitivity of the MZI could be improved obviously. The RI sensitivities of the MZI with 35 mm PCF are up to 211.53 nm/RIU and 359.37 nm/RIU when the cladding diameter decreases to 112 μm and 91 μm, respectively. Moreover, the sensor is insensitive to temperature, and the measured sensitivity is only 9.21 pm/°C with the range from 20°C to 500°C. In addition, the sensor has advantage of simple fabrication, low cost, and high RI sensitivity.