Hollow-Core Photonic Crystal Fiber Mach-Zehnder Interferometer for Gas Sensing

Experimental Study of White Light Interferometry in Mach-Zehnder Interferometers Based on Standard Single Mode Fiber

In this work, we experimentally analyzed and demonstrated the performance of an in-line Mach-Zehnder interferometer in the visible region, with an LED light source. The different waist diameter taper and asymmetric core-offset interferometers proposed used a single-mode fiber (SMF). The visibility achieved was V = 0.14 with an FSR of 23 nm for the taper MZI structure and visibilities of V = 0.3, V = 0.27, and V = 0.34 with FSRs of 23 nm, 17 nm, and 8 nm and separation lengths L of 2.5 cm, 4.0 cm, and 5.0 cm between the core-offset structure, respectively. The experimental investigation of the response to the temperature sensor yielded values from 50 °C to 300 °C; the sensitivity obtained was 3.53 a.u./°C, with R2 of 0.99769 and 1% every 1 °C in the transmission. For a range of 50 °C to 150 °C, 20.3 pm/°C with a R2 of 0.96604 was obtained.

Computational analysis of a scalable optically homogeneous free-space interferometer

This study describes a scalable optically homogeneous free-space interferometer. Computationally modeled as an unbroken block of fused silica, the interferometer's six-sided design is simple and intuitive, exploiting total internal reflection and refraction to split and recombine a collimated input beam. During propagation, one portion of the split beam remains within the substrate to act as a reference beam. The second portion of the split beam is exposed to the surrounding environment, enabling real-world environment characterization in real time. Validation of the interferometer concept is performed using numerical and analytical techniques. Based on its scalability and robustness, the proposed interferometer design is primed for applications in atmospheric sensing, passive chemical detection, and spaceborne technologies.

Femtosecond-laser-inscribed Bragg grating in hollow-core fiber for highly sensitive optofluidic sensing

An optofluidic sensor based on a Bragg grating in hollow-core fiber (HCF) is experimentally demonstrated. The grating is inscribed into the HCF by femtosecond laser illumination through a phase mask. Periodic index modulation is introduced into the silica material surrounding the hollow core, causing cladding mode resonance, and multiple reflection peaks are observed in the grating spectrum. These reflection peaks later shift to longer wavelengths when high-index liquid is infiltrated into the HCF. The new reflection peak results from the backward coupling of the liquid core mode of the waveguide, the mode field of which overlaps with the grating modulation surrounding the liquid core. The resonant wavelength of the liquid-core fiber grating increases with the index value of the infiltrating liquid, and optofluidic refractive index sensing is realized with the device. The highest refractive index sensitivity, 1117 nm/RIU, is obtained experimentally in the index range of 1.476-1.54. The infiltrated hollow-core fiber Bragg grating also exhibits high temperature sensitivity due to the high thermal-optic coefficient of the liquid, and a sensitivity of -301 pm/°C is achieved in the temperature range of 25°C to 60°C.

Ultrasensitive refractive index fiber sensor based on high-order harmonic Vernier effect and a cascaded FPI

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.

Surface Plasmon Resonance-Based Gold-Coated Hollow-Core Negative Curvature Optical Fiber Sensor

The hollow-core fiber-based sensor has garnered high interest due to its simple structure and low transmission loss. A new hollow-core negative-curvature fiber (HC-NCF) sensor based on the surface plasmon resonance (SPR) technique is proposed in this work. The cladding region is composed of six circular silica tubes and two elliptical silica tubes to reduce fabrication complexity. Chemically stable gold is used as a plasmonic material on the inner wall of the sensor structure to induce the SPR effect. The proposed sensor detects a minor variation in the refractive indices (RIs) of the analyte placed in the hollow core. Numerical investigations are carried out using the finite element method (FEM). Through the optimization of structural parameters, the maximum wavelength sensitivity of 6000 nm/RIU and the highest resolution of 2.5 × 10^-5 RIU are achieved in the RI range of 1.31 to 1.36. In addition, an improved figure of merit (FOM) of 2000 RIU^-1 for Y-polarization and 857.1 RIU^-1 for X-polarization is obtained. Because of its simple structure, high sensitivity, high FOM, and low transmission loss, the proposed sensor can be used as a temperature sensor, a chemical sensor, and a biosensor.

Embedded racetrack microring resonator sensor based on GeSbSe glasses

In this article, a compact racetrack double microring resonator (MRR) sensor based on Ge₂₈Sb₁₂Se₆₀ (GeSbSe) is investigated. The sensor device consists of a racetrack microring, an embedded small microring, and a strip waveguide. Electron beam lithography (EBL) and dry etching are used to fabricate the device. The compact racetrack double MRR device are obtained with Q-factor equal to 7.17 × 10⁴ and FSR of 24 nm by measuring the transmission spectrum. By measuring different concentrations of glucose solutions, a sensitivity of 297 nm/RIU by linear fitting and an intrinsic limit of detection (iLOD) of 7.40 × 10^-5 are obtained. It paves the way for the application of chalcogenide glasses in the field of biosensing.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

A Review: Application and Implementation of Optic Fibre Sensors for Gas Detection

At the present time, there are major concerns regarding global warming and the possible catastrophic influence of greenhouse gases on climate change has spurred the research community to investigate and develop new gas-sensing methods and devices for remote and continuous sensing. Furthermore, there are a myriad of workplaces, such as petrochemical and pharmacological industries, where reliable remote gas tests are needed so that operatives have a safe working environment. The authors have concentrated their efforts on optical fibre sensing of gases, as we became aware of their increasing range of applications. Optical fibre gas sensors are capable of remote sensing, working in various environments, and have the potential to outperform conventional metal oxide semiconductor (MOS) gas sensors. Researchers are studying a number of configurations and mechanisms to detect specific gases and ways to enhance their performances. Evidence is growing that optical fibre gas sensors are superior in a number of ways, and are likely to replace MOS gas sensors in some application areas. All sensors use a transducer to produce chemical selectivity by means of an overlay coating material that yields a binding reaction. A number of different structural designs have been, and are, under investigation. Examples include tilted Bragg gratings and long period gratings embedded in optical fibres, as well as surface plasmon resonance and intra-cavity absorption. The authors believe that a review of optical fibre gas sensing is now timely and appropriate, as it will assist current researchers and encourage research into new photonic methods and techniques.

In-Fiber Interferometric-Based Sensors: Overview and Recent Advances

In-fiber interferometric-based sensors are a rapidly growing field, as these sensors exhibit many desirable characteristics compared to their regular fiber-optic counterparts and are being implemented in many promising devices. These sensors have the capability to make extremely accurate measurements on a variety of physical or chemical quantities such as refractive index, temperature, pressure, curvature, concentration, etc. This article is a comprehensive overview of the different types of in-fiber interferometric sensors that presents and discusses recent developments in the field. Basic configurations, a brief approach of the operating principle and recent applications are introduced for each interferometric architecture, making it easy to compare them and select the most appropriate one for the application at hand.

Dual-Core Fiber-Based Interferometer for Detection of Gas Refractive Index

We demonstrate a dual-core fiber-based Mach–Zehnder interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The fiber used here have a solid germanium-doped silica core and an air core that allows gases to flow through. Coherent laser beams are coupled to the two cores, respectively, and thus excite guiding modes thereby. Interferogram would be produced as the light transmitted from the dual cores interferes. Variations in refractive index of the hollow core lead to variations in phase difference between the modes in the two cores, thus shifting the interference fringes. The fringe shifts can be then interrogated by a photodiode together with a narrow slit in front. The resolution of the sensor was found to be ~1 × 10−8 RIU, that is comparable to the highest resolution obtained by other fiber sensors reported in previous literatures. Other advantages of our sensor include very low cost, high sensitivity, straightforward sensing mechanism, and ease of fabrication.