All-fiber Mach-Zehnder interferometer based on two liquid infiltrations in a photonic crystal fiber

Hybrid sensor based on a hollow square core fiber for temperature independent refractive index detection

In this work, a hybrid sensor based on a section of hollow square core fiber (HSCF) spliced between two single mode fibers is proposed for the measurement of refractive index of liquids. The sensor, with a length of a few millimeters, operates in a transmission configuration. Due to the HSCF inner geometry, two different interferometers are generated. The first, a Mach-Zehnder interferometer, is insensitive to the external refractive index, and presents a sensitivity to temperature of (29.2 ± 1.1) pm/°C. The second one, a cladding modal interferometer, is highly sensitive to the external refractive index. An experimental resolution of 1.0 × 10^-4 was achieved for this component. Due to the different responses of each interferometer to the parameters under study, a compensation method was developed to attain refractive index measurements that are temperature independent. The proposed sensor can find applications in areas where refractive index measurements are required and the control of room temperature is a challenge, such as in the food and beverage industry, as well as in biochemical or biomedical industries.

Fabrication and Sensing Application of Phase Shifted Bragg Grating Sensors

As a special kind of Bragg grating, phase-shifted fiber Bragg grating (PS-FBG) has attracted extensive attention because of its extremely narrow transmission window and excellent sensing performance. The main purpose of this manuscript is to discuss the PS-FBG with special sensing characteristics and explore the influence of different inscription technologies on the sensing characteristics of PS-FBG by comparing the existing inscription methods. The sensing characteristics, advantages and disadvantages of PS-FBG with different structures are analyzed.

High-precision photonic crystal fiber-based pressure sensor with low-temperature sensitivity

A novel high-precision photonic crystal fiber-based pressure sensor with low-temperature sensitivity is proposed. The sensor is fabricated by fusion splicing a photonic crystal fiber with a hollow core fiber immersed in polydimethylsiloxane. Owing to the special structure of the photonic crystal fiber, the temperature cross-coupling effect can be minimized and the membrane shape can be controlled. Experimental results indicate that the pressure sensitivity of the FP pressure sensor is 2.47 nm/kPa, 5.37 times the temperature sensitivity of 0.46 nm/°C. The proposed FP pressure sensor has broad application prospects in chemical and biological detection for monitoring pressure in real time.

Fiber Mach-Zehnder-interferometer-based liquid crystal biosensor for detecting enzymatic reactions of penicillinase

A novel, to the best of our knowledge, liquid crystal (LC) biosensor, based on an optical fiber Mach-Zehnder interferometer (MZI), is proposed. The proposed optical fiber MZI consists of two single-mode fibers and a tapered photonic crystal fiber (PCF). The PCF is coated with 4^'-pentyl-biphenyl-4-carboxylic acid (PBA)-doped 4-cyano-4^'-pentylbiphenyl (5CB). Being a pH-sensitive material, PBA can manipulate LC molecules to different orientations according to their pH values. When the orientation of LC molecules changes with varying pH, the effective refractive index of the cladding modes also is accordingly affected. Enzymatic reactions of penicillinase can release H⁺, which causes the decrease of the pH. Therefore, the enzymatic reactions of penicillinase can be sensed by monitoring the peak shift in the interference spectrum. The effects of the tapered diameter on the sensitivity of the sensor were experimentally investigated as well.

Design and fabrication of a heterostructured cladding solid-core photonic bandgap fiber for construction of Mach-Zehnder interferometer and high sensitive curvature sensor

A heterostructured cladding solid-core photonic bandgap fiber (HCSC-PBGF) is designed and fabricated which supports strong core mode and cladding mode transmission in a wide bandgap. An in-line Mach-Zehnder interferometer (MZI) curvature sensor is constructed by splicing single mode fibers at both ends of a HCSC-PBGF. Theoretical analysis of this heterostructured cladding design has been implemented, and the simulation results are consistent with experiment results. Benefiting from the heterostructured cladding design, an enhanced curvature sensing sensitivity of 24.3 nm/m^-1 in the range of 0-1.75 m^-1 and a high quality interference spectrum with 20 dB fringe visibility are achieved. In order to eliminate the interference of longitudinal strain and transverse torsion on the result of the curvature sensing experiment, we measure the longitudinal strain and transverse torsion sensing properties of HCSC-PBGF, and the results show that the impact is negligible. It is obvious that this high-sensitivity and cost-effective all fiber sensor with a compact structure will have a promising application in fiber sensing.

Highly sensitive temperature sensor based on an ultra-compact Mach-Zehnder interferometer with side-opened channels

We demonstrated an ultra-compact and highly sensitive temperature sensor by using an in-fiber Mach-Zehnder interferometer (MZI) with side-opened channels. The MZI was constructed by off-center splicing a thin piece of microstructured optical fiber (MOF) between two single-mode fibers. Then, two side-opened channels were drilled through the MOF along the transverse direction by using a femtosecond laser to let the liquid with a high thermo-optic coefficient enter into the air cavity of the MZI. Due to the large effective refractive index (RI) difference between the core mode and the cavity mode excited by the offset splicing point, the scale of the MOF-based MZI can be reduced to several hundred micrometers. Experimental results show that the MOF-based MZI infiltrated with isopropanol has a temperature sensitivity up to 3.8 nm/°C with small strain cross-sensitivity of 0.0001 °C/μϵ in the temperature range of 22°C-34°C, which makes it a competitive fiber sensor in highly sensitive temperature sensing applications, including healthcare and consumer electronics.

High-quality Mach-Zehnder interferometer based on a microcavity in single-multi-single mode fiber structure for refractive index sensing

A fiber inline Mach-Zehnder interferometer (MZI) based on a microcavity with two symmetric openings in single-multi-single mode fiber (SMSF) structure is proposed. By using the finite difference beam propagation method (FD-BPM), the interference spectrum simulation result shows that the MZI can still have high-quality interference even if the microcavity deviates along the radial direction for 3 μm. Therefore, it allows a larger fabrication tolerance and tremendously decreases the fabrication difficulty. Then a microcavity with two symmetric openings in SMSF was fabricated by using femtosecond laser-induced water breakdown. The insertion loss of the microcavity immerged in water is only -8  dB, and the MZ interference peak contrast in the transmission spectrum reaches more than 30 dB. The MZI based on the microcavity in SMSF can be used as a practical liquid refractive index sensor as its high-quality interference spectrum, ultrahigh sensitivity (9756.75 nm/RIU), high refractive index resolution (2×10^-5  RIU), good linearity (99.93%), and low-temperature crosstalk (0.04 nm/°C).

Photonic crystal fiber in-line Mach-Zehnder interferometer for explosive detection

We report a photonic crystal fiber (PCF) in-line Mach-Zehnder interferometer used as a gas sensor device which exhibits high sensitivity to the explosive trinitrotoluene (TNT). The interferometric sensor head is formed by embedding a segment of large-mode-area/grapefruit PCF between standard single-mode fibers via butt coupling, which produces two small air gaps in between terminated fiber ends with ceramic ferrule connectors as coupling regions, which also serve as inlet/outlet for the gas. The spectral response of the interferometer is investigated in terms of its wavelength spectrum. The selectivity to TNT vapor is achieved by immobilizing a molecular recognition ployallylamine layer on the inner surface of the holey region of the PCF. The TNT-induced variations of the interference fringes are measured and the sensing capability of the proposed sensor is demonstrated experimentally.