All fiber M-Z interferometer for high temperature sensing based on a hetero-structured cladding solid-core photonic bandgap fiber

Analysis and Design of Fiber Microprobe Displacement Sensors Including Collimated Type and Convergent Type for Ultra-Precision Displacement Measurement

In this paper, a fiber optic microprobe displacement sensor is proposed considering characteristics of micro-Michelson interference structure and its components. The principal error of micro Fabry-Perot interferometric structure is avoided, and high-precision interferometric displacement measurement is realized. The collimated microprobe and convergent microprobe are analyzed, simulated, and designed for the purposes of measuring long-distance displacement and small spot rough surface, respectively. The core parameters of the probes' internal components are mapped to coupling efficiency and contrast of the sensor measurements, which provides a basis for the probes' design. Finally, simulation and experimental testing of the two probes show that the collimated probe's working distance and converging probe's tolerance angle can reach 40 cm and ±0.5°, respectively. The designed probes are installed in the fiber laser interferometer, and a displacement resolution of 0.4 nm is achieved.

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.

Investigation of a Bragg Grating-Based Fabry–Perot Structure Inscribed Using Femtosecond Laser Micromachining in an Adiabatic Fiber Taper

This paper presents the fabrication of a fiber Bragg grating (FBG)-based Fabry–Perot (FP) structure (7 mm total length) in an adiabatic fiber taper, investigates its strain and temperature characteristics, and compares the sensing characteristics with a standard polyimide coated FBG sensor. Firstly, a simulation of the said structure is presented, followed by the fabrication of an adiabatic fiber taper having the outer diameter reduced to 70 μ m (core diameter to 4.7 μ m). Next, the sensing structure, composed of two identical uniform FBG spaced apart by a small gap, is directly inscribed point-by-point using infrared femtosecond laser (fs-laser) micromachining. Lastly, the strain and temperature behavior for a range up to 3400 μ ε and 225 ° C, respectively, are investigated for the fabricated sensor and the FBG, and compared. The fabricated sensor attains a higher strain sensitivity (2.32 pm/ μ ε ) than the FBG (0.73 pm/ μ ε ), while both the sensors experience similar sensitivity to temperature (8.85 pm/ ° C). The potential applications of such sensors include continuous health monitoring where precise strain detection is required.

Miniaturized Fabry-Perot probe utilizing PMPCF for high temperature measurement

We propose a miniaturized optical fiber Fabry-Perot probe for high temperature measurement (up to 1000°C). It is simply fabricated by fusion splicing a short section of polarization-maintaining photonic crystal fiber (PMPCF) with a single-mode fiber (SMF). The interface between the core of the SMF and air holes of the PMPCF, and the end face of the PMPCF work as the mirrors. The pure silica core of the PMPCF is employed as the sensing element. Experimental results show that the probe has a high thermal stability and the temperature sensitivity reaches up to 15.34 pm/°C, which is not affected by the length of the PMPCF. The linearity of temperature response is as high as 99.83%. The proposed sensor has promising prospects in practical applications due to simple fabrication process, low cost, compact size, and excellent repeatability.

High temperature Vernier probe utilizing photonic crystal fiber-based Fabry-Perot interferometers

This study proposes a highly sensitive and stable optical fiber probe based on Vernier effect for high temperature measurement (up to 1000 °C), utilizing photonic crystal fiber (PCF)-based Fabry-Perot interferometers (FPIs). The cascaded FPIs are fabricated by fusion splicing a section of polarization maintaining PCF to a lead-in single-mode fiber, and then a section of temperature-insensitive hollow core PCF is spliced between the PMPCF and a multi-mode fiber. The shift of the spectral envelope is monitored to measure the temperature variation. Experimental results show that the sensitivities of three fabricated probes are as high as 173.43 pm/ °C, 230.53 pm/ °C and 535.16 pm/ °C when operating from room temperature to 1000 °C, which are consistent with theoretical results. The sensitivities are magnified about 13, 19 and 45 times compared with the single FPI. The linearity of the temperature response is as high as 99.73%. The proposed probe has great application prospects due to compactness, high sensitivity and low cost.

Ultrafast All-Optical Signal Modulation Induced by Optical Kerr Effect in a Tellurite Photonic Bandgap Fiber

Ultrafast all-optical signal modulation induced by optical Kerr effect (OKE) was demonstrated in an all-solid tellurite photonic bandgap fiber (PBGF) which was designed and fabricated based on TeO2–Li2O–WO3–MoO3–Nb2O5 (TLWMN, high-index rods), TeO2–ZnO–Na2O–La2O3 (TZNL, background), and TeO2-ZnO-Li2O-K2O-Al2O3-P2O5 (TZLKAP, cladding) glasses. At the input of a control pulse with high intensity, OKE occurred in the tellurite PBGF and the transmission bands of the tellurite PBGF shifted. The signal at 1.57 μm transmitting in the fiber core can be ultrafast all-optically modulated by the ultrafast single pulse (200 kW, 200 fs) under OKE, where the modulation speed can reach 50 GHz, faster than some commercial LiNbO3 modulators. The results in this paper can be applied to multi-monitors, local area network, detectors, multi-sources, etc.

Temperature-Insensitive Refractive Index Sensor with Etched Microstructure Fiber

A Mach–Zehnder interferometer (MZI) based on an etched all-solid microstructure fiber (MOF) has been demonstrated. The MZI works on the basis of interference between the vibrant core and cladding modes in the MOF. The all-solid MOF has a heterostructure cladding composed of Ge-doped rod arrays and pure silica, and thus can support and propagate a vibrant cladding mode with a large mode area. When the outermost cladding of MOF is etched, the cladding mode becomes sensitive to the ambient refractive index (RI). The etched MOF can work as a sensing head for RI sensing. By comparing the interference spectra, the extinction ratio has remained stable at around 20 dB after the MOF was etched. The RI sensing characteristics of the MZI with an etched MOF have also been investigated. The results show that the RI sensitivity can reach up to 2183.6 nm/RIU with a low-temperature coefficient (<10 pm/°C).

In-fiber integrated quasi-distributed high temperature sensor array

An in-fiber integrated quasi-distributed high temperature sensor array is proposed and demonstrated. The sensor array consists of some weakly reflective joint surfaces which are welded by single mode fiber (SMF) and double-clad fiber (DCF). The characteristics of the reflected signal of the sensor array are analyzed, and the relationship between the signal intensity and the number of sensors is simulated for evaluating sensor multiplex capacity. Due to its all-silica structure, the sensor array could test temperature up to 1000°C for a long time. This sensor array is flexible and easy to be fabricated only by splicing without any connector, which will be beneficial to space constrained quasi-distributed high temperature sensing applications.

Manufacturing a Long-Period Grating with Periodic Thermal Diffusion Technology on High-NA Fiber and Its Application as a High-Temperature Sensor

We demonstrated a kind of long-period fiber grating (LPFG), which is manufactured with a thermal diffusion treatment. The LPFG was inscribed on an ultrahigh-numerical-aperture (UHNA) fiber, highly doped with Ge and P, which was able to easily diffuse at high temperatures within a few seconds. We analyzed how the elements diffused at a high temperature over 1300 °C in the UHNA fiber. Then we developed a periodically heated technology with a CO₂ laser, which was able to cause the diffusion of the elements to constitute the modulations of an LPFG. With this technology, there is little damage to the outer structure of the fiber, which is different from the traditional LPFG, as it is periodically tapered. Since the LPFG itself was manufactured under high temperature, it can withstand higher temperatures than traditional LPFGs. Furthermore, the LPFG presents a higher sensitivity to high temperature due to the large amount of Ge doping, which is approximately 100 pm/°C. In addition, the LPFG shows insensitivity to the changing of the environment’s refractive index and strain.

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.

High-Temperature Sensor Based on Fabry-Perot Interferometer in Microfiber Tip

A miniaturized tip Fabry-Perot interferometer (tip-FPI) is proposed for high-temperature sensing. It is simply fabricated for the first time by splicing a short length of microfiber (MF) to the cleaved end of a standard single mode fiber (SMF) with precise control of the relative cross section position. Such a MF acts as a Fabry-Perot (FP) cavity and serves as a tip sensor. A change in temperature modifies the length and refractive index of the FP cavity, and then a corresponding change in the reflected interference spectrum can be observed. High temperatures of up to 1000 °C are measured in the experiments, and a high sensitivity of 13.6 pm/°C is achieved. This compact sensor, with tip diameter and length both of tens of microns, is suitable for localized detection, especially in harsh environments.

Surface-plasmon-resonance-based optical-fiber temperature sensor with high sensitivity and high figure of merit

We propose and demonstrate a novel and compact optical-fiber temperature sensor with a high sensitivity and high figure of merit (FOM) based on surface plasmon resonance (SPR). The sensor is fabricated by employing a single-mode twin-core fiber (TCF), which is polished as a circular truncated cone and coated with a layer of gold film and a layer of polydimethylsiloxane (PDMS). Owing to the high refractive index sensitivity of SPR sensors and high thermo-optic coefficient of PDMS, the sensor realizes a high temperature sensitivity of -4.13  nm/°C to -2.07  nm/°C in the range from 20°C to 70°C, transcending most other types of optical-fiber temperature sensors. Owing to the fundamental mode beam transmitting in the TCF, the sensor realizes a high FOM of up to 0.034/°C, more than twice that of SPR sensors based on multimode fiber. The proposed temperature sensor is meaningful and will have potential applications in many fields, such as biomedical and biomaterial.

All-optical dynamic photonic bandgap control in an all-solid double-clad tellurite photonic bandgap fiber

All-optical dynamic photonic bandgap (PBG) control by an optical Kerr effect (OKE) is investigated in an all-solid double-clad tellurite photonic bandgap fiber (PBGF) which is fabricated based on TeO₂-Li₂O-WO₃-MoO₃-Nb₂O₅ (TLWMN, high-index rod) glass, TeO₂-ZnO-Na₂O-La₂O₃ (TZNL, inner cladding) glass, and TeO₂-ZnO-Li₂O-K₂O-Al₂O₃-P₂O₅ (TZLKAP, outer cladding) glass. To the best of our knowledge, this is the first demonstration of all-optical dynamic PBG control in optical fibers. This PBGF has a high nonlinear refractive index which can lead to a significant OKE and induce the generation of all-optical dynamic PBG control. The transmission spectrum is simulated with the pump peak power increasing from 0 to 300 kW, which shows an obvious PBG shift. Dynamic PBG control is demonstrated both numerically and experimentally at the pump peak power of 200 kW (ON or OFF) at the signal of 1570 nm.