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Silicon dioxide (SiO2), a new type of inorganic nonmetallic material, has been widely used in people's livelihood and military industry. For silicon dioxide-based applications, ultra-precision micromachining is a crucial component that impacts the performance of the finished products. This research reports the precise micromachining of silicon dioxide wafers by femtosecond laser. A prediction model for groove processing size is developed, and an experiment is carried out to investigate the silicon dioxide groove processing technology. The effects of processing parameters that include laser power, scanning speed, scanning repetitions, and defocus amount on the processed properties of grooves, the heat affected zone, and processed roughness are discussed. The variable defocus processing method is proposed, which can effectively improve the inclination angle of the groove wall and the quality of the machined groove. Moreover, scanning electron microscopy is used to analyze the groove morphology, allowing for the improvement of surface quality and processing parameter optimization.

Femtosecond (Fs) laser micromachining is the most effective and flexible method for edge-cutting or transforming the physical properties of various crystalline brittle materials. Fs-laser micro-machining produces slag on the residual surface of micro-structures that reduces the quality and processing efficiency of a machined residual surface. In order to overcome the challenges overlaid during the processing of brittle materials, Fs-laser assisted ultrasonic nitrogen jet micro-machining technique is proposed. The method was applied to quartz chips to investigate improvements in surface quality after laser processing. For conceptualizations, an ultrasonic nitrogen nozzle based on a piezoelectric transducer was designed and Finite Element Method (FEM) was employed to realize the transition of a flow field. An experiment was performed that differentiate the quality of micro-grooves into quartz chips, and the results promote the significance of Fs-laser-assisted ultrasonic nitrogen jet micromachining for the processing of brittle materials. Besides, the machining quality at residual surfaces of quartz chips after laser processing was substantially improved. The process provides an aid to break down the slag into further tiny nano-particles and prevent a recast layer, meanwhile, it enhances the surface quality and processing efficiency without implementing any extensive procedure.

We present a relative sensitivity of in-fiber inter-cross demodulation of a Mach–Zehnder interferometer (MZI) based on an etched multi-core fiber (eMCF). The sensor can measure the external refractive index (RI) and temperature with a large fringe visibility of 15 dB. It is tuned using a simple technique of slow chemical etching. When the outer cores of MCF will be exposed to the surrounding, a large difference of relative effective RI is observed, which enhances the sensitivity of the sensor. The sensor's wavelength and intensity responses have displayed that it can function with three different inter-cross-demodulation phenomena. A superior RI sensitivity of 178.20 dB/RIU in the range of 1.334 to 1.370, and temperature sensitivity of 66.73 pm/°C in the range of 30 to 80 °C are obtained, with an adequate linear response. Besides, it can readily resolve the issues of cross-sensitivity. Moreover, it has many advantages including easy fabrication, compact size, multiplex, repeatable, stable, and can easily differentiate RI and temperature, which lack others.

We propose and demonstrate Mach-Zehnder interferometer (MZI), which is the refractive index (RI) insensitive and temperature highly sensitive based on etched multi-core fiber (MCF) structure. The MCF and Fiber Bragg Grating (FBG) are used as hybrid sensing elements. The fabrication of the interferometer is provided a new taper-like structure by etching the MCF to further expose the side cores to the surroundings. The interferometer has produced a sensitivity of 103.2pm/°C within the ambient temperature up-to 70°C. Moreover, the superior temperature sensitivity is 89.19pm/°C, 66.64pm/°C, 56.42pm/°C in the range of 24°C to 130°C, and RI-insensitive in the range of 1.34 to 1.38, for different waists of etched seven-core fiber interferometers (E7CFIs) $\sim ~84.70\mu \text{m}$ , $93.10\mu \text{m}$ , $108.67\mu \text{m}$ , respectively. Compared with the conventional FBGs, the sensitivity of the interferometer is significantly improved by 8 times. E7CFI’s novel and advantageous features can easily be distinguished other devices. Besides, the proposed sensing architecture is compact, easy to fabricate, highly sensitive, easy to reproduce, and makes it an inexpensive fiber optic device.

A design of the highly non-linear and ultra-sensitive modified core of a photonic crystal fiber (PCF) sensor is reported for the detection of various liquid analytes. The sensor design comprises three rings of a hexagonal lattice around the liquid-infiltrated hollow core. A steric-shaped hollow core design in PCF effectively optimizes the sensor’s sensitivity and performance. The design is analyzed numerically by using the full vector finite element method with perfectly matched layers to find a rigorous solution. The proposed PCF sensor exhibits ultra-high relative sensitivity, a high power fraction, high non-linearity coefficient, low confinement loss, ultra-low chromatic dispersion, compact effective mode area, and single-mode operation in a wider wavelength range, i.e., within visible and infrared wavelength regimes. At an operating wavelength of 1.0 µm, optimum relative sensitivities of 96.37%, 97.40%, and 99.82% with high non-linear coefficients of 78.0 W − 1 k m − 1 , 66.09 W − 1 k m − 1 , and 62.57 W − 1 k m − 1 and extremely low confinement loss of ∼ 10 − 10 d B / m are obtained for water, ethanol, and benzene analytes, respectively. Further, waveguiding characteristics of the PCF sensor endorse its usage for a range of liquid analyte detections. It is a simple design and can be fabricated using existing manufacturing technologies.

A simple structured ultra-highly sensitive fiber optic gas pressure sensor with low temperature cross sensitivity is proposed and demonstrated. The sensor is composed of two cascaded Mach–Zehnder interferometers (MZIs), which consist of single mode-multimode-single mode fiber structure with a micro-machined air cavity in each multimode fiber. The micro air cavities are drilled by a femtosecond laser and act as a gas pressure sensing element. Interference happens between the beams transmitting along the air cavity and the remaining silica core of the multimode fiber. The difference between the fringe intervals of the two MZIs are small enough for generating Vernier effect in gas pressure measurement. The same configuration employed in the two MZIs is to obtain the same temperature response, so that preventing the Vernier effect from appearing in temperature changes. Therefore, the proposed sensor can achieve a high gas pressure sensitivity but is insensitive to temperature variation. Experimental results show that the sensitivity for gas pressure measurement is −82.131 nm/MPa.

Quartz material is widely used in the automotive industry, aerospace, optical and electronic fields and so on. But quartz is a hard and brittle material, which makes it very difficult to process. F...

A relative humidity (RH) sensor based on an integrated Mach–Zehnder interferometer (MZI) consisting of two off-axis twisted deformation points and an air channel passing through the core of a single-mode fiber (SMF) is proposed and experimentally demonstrated. The off-axis deformations are achieved by twisting the SMF under continuous arc discharge. The air channel is created between the two off-axis deformation points by femtosecond laser machining. Because the air channel is exposed to the environment, its refractive index is directly determined by the RH, which affects the optical path difference of the MZI and, in turn, leads to the shifts in the resonance wavelength. A prototype MZI demonstrates an RH sensitivity of approximately −0.144 nm/%RH without the use of any hygroscopic materials. More prominently, an ultrafast response time of about 68 ms is achieved. The proposed MZI structure is therefore demonstrated to be an excellent RH sensor candidate due to its compact size, outstanding performance, ease of fabrication, and all-fiber configuration.

A Mach–Zehnder interferometer based on double off-axis twisted deformation on single mode fiber (SMF) is proposed and experimentally demonstrated for gas pressure measurements. The off-axis twisted deformation is achieved by twisting a SMF under continuous arc discharging. An open air-gap was created on the cladding of the SMF between the two off-axis deformation points by using a femtosecond laser. The cladding modes transmit through the air-gap whose refractive index is directly determined by the air pressure. The interferometer is applied for high-sensitive pressure sensing with a sensitivity of −5.183 nm/MPa and temperature cross-sensitivity of 13.3 kPa/°C. Consequently, the open air-gap added MZI structure can therefore be considered an excellent candidate as a gas pressure sensor in harsh environments due to its low cost, compact size, ease of fabrication, and all-fiber configuration.

We present and demonstrate an ultra-sensitive temperature sensor based on an isopropanol-sealed Mach-Zehnder Interferometer (MZI). The structure is composed of a refractive index sensitive MZI that encapsulated in a capillary tube filled with isopropanol. The isopropanol with a high thermo-optic coefficient, which directly constitutes the sensing arm of the encapsulated MZI. When the temperature changes, the optical path difference between the two MZI’s interference arms changed greatly due to the drastic change in the refractive index of isopropanol, leading to wavelength shift significantly. Experimental results show an ultra-high sensitivity of 7.3819 nm/°C with good linearity. In addition, the temperature sensor is insensitive to the surrounding refractive index change. Excellent performance demonstrates it a good candidate for temperature measurement.

A customized fiber bragg grating (FBG) acceleration sensor is designed for non-intrusive low flow rate measuring, it has an average sensitivity of 245.2 pm/g. Experimental tests for water system shows the relationship between the standard deviation of vibration data and flow rate is quadratic, the 2 of quadratic curve fitting is 0.990.

In this paper, we present an enhanced sensitivity of temperature sensors based on thermo-coupled Multicore Fiber (MCF) structures. The sensors are all fabricated using a controlled arc power of a splicing device. Two different principles of a Mach-Zehnder interferometer (MZI) and Michelson interferometer (MI) have been observed experimentally. The MZI and MI structures exhibit temperature sensitivity as 136.67 pm/°C and 70.61 pm/°C, respectively, and found insensitive to the refractive index (RI). Also, its RI response can readily resolve the issues of cross-sensitivity.

Quartz tuning fork is a kind of widely used piezoelectric device. However, as a hard and brittle material, silicon dioxide crystal, which is the core component of tuning fork, is difficult to realize high precision microprocessing. In this paper, femtosecond laser is used to process silicon dioxide crystal. The laser processing parameters which affect quality of tuning fork sidewall are theoretically analyzed, and an experimental study is carried out to optimize the technological parameters and improve the quality of tuning fork crystal. In addition, a set of processing route of quartz tuning device is designed, The tuning fork sensor manufactured has the characteristics of low cost, compact structure, low power consumption and will have a wide application prospects in the future.

A numerical solution, using the general formulation of the transcendental equation and eigenmode values, is proposed to demonstrate number of strongly guided modes propagating in sapphire crystal fibers (SCF). The SCF is considered to have hexagonal geometry of a single crystal and multimode step-index profile. Several distinguished characteristics are naturally embedded in SCF compared to other ordinary optical fibers. The eigenmodes of the SCF are numerically determined and are a combination of transverse electric, transverse magnetic, and hybrid modes. The numerical solution for wave-guiding in ultra-violet (UV), visible infrared (VIS/IR) spectrum, is investigated by the number of modal propagation under strongly guided approximation. The cross-sectional energy distribution of fundamental mode (FM) and higher order modes (HoM) describe the variation of the effective mode index with respect to the change in the core radius. The proposed waveguide is of ~35micron radius, exhibit −0.077 dB/m confinement loss at 200 nm. The simulations in this study are performed by the COMSOL multi-physics® software.

&alpha, MoO3 has been used as a hydrogen sensing material due to its excellent properties and unique crystalline layer structure. However, the low repeatability of &alpha, MoO3 based hydrogen sensor restricts its practical application. In this paper, the effect of intercalated ion species and the amount in &alpha, MoO3 is experimentally investigated and discussed. It is concluded that the repeatability of the sensor depends on the radius of intercalated ions and amount of ionic bonds. The optimal ion species is Na+ and the optimal amount of precursor is 1 mmol.

The performance of spiral micro-structured fiber Bragg grating (FBG) and standard FBG hydrogen sensor with different morphologies of Pt-WO3 has been presented. Femtosecond laser was employed to fabricate spiral micro-sturctured on fiber cladding of FBG. WO3 nanorods, nanowires, and nanolamellae were synthesized by hydrothermal process and decorated with Pt nanoparticles by decomposing Pt(acca)2 precursor (molar ratio of Pt:W = 1:5). FBG hydrogen sensors with Pt-WO3 nanolamellae have the highest sensitivity (spiral micro-structured FBG sensor ~208 pm/%H, standard FBG sensor ~110 pm/%H), followed by nanorod and nanowire sensors. Moreover, the sensitivity of the spiral micro-structured FBG sensors are about 1.8 ~ 2.5 times than that of standard FBG sensors. The results show that the sensors with WO3 nanowires have the fastest response time.

Femtosecond laser is ideal tool for micro-machining of hard and brittle materials, such as quartz material. The basis of experiments on femtosecond laser polishing quartz shown that, when other pro...

This paper proposed and demonstrated fiber Bragg grating (FBG) hydrogen sensor by integrating optical fiber laser micro-machining and thin film technology. The spiral microstructure ablated sensor probe was sputtered with hydrogen sensitive material, palladium-silver (Pd-Ag) composite film, in which the atomic ratio of the two metals is controlled at Pd:Ag = 4:1. Experimental work was conducted at ambient temperature and relative humidity of 25 °C and 32% respectively. Double spiral microstructure at pitch 90 μm and sputtered with 520 nm Pd/Ag film recorded hydrogen sensitivity of 51.5 pm/%H responding to hydrogen gas in air. This translated to approximately 7.5 times higher than the unprocessed standard FBG. The sensor’s hydrogen response is reversible, has improved response time and could have potential for hydrogen gas monitoring.

A sensitivity enhanced gas pressure sensor with Vernier effect is proposed in this paper. The sensor is a cascade configuration which includes two Fabry–Perot interferometers (FPIs) with different free spectrum range (FSR). Each Fabry–Perot interferometer is fabricated by inserting a piece of hollow core fiber (HCF) in between two sections of single mode fiber (SMF). Femtosecond laser is applied for drilling an opening on the HCF of sensing FPI to make the air hole interact with outside environment, while the air hole of the reference FPI is kept closed. Gas pressure response is measured by monitoring the wavelength shift of the envelope extracted from the superimposed spectrum of the cascaded FPIs. Experimental results show that the sensitivity of the cascade configuration can attain -65.482nm/MPa, which is -13 times of single sensing FPI.

The near-infrared luminescence of Bi-doped aluminosilicate fiber within pump wavelengths between 710 and 990 nm and under varying pump powers and operation temperatures has been investigated systematically. It is revealed that there exist two emission bands peaking around 1120 and at 1300 nm, which are believed to be associated with two different aluminum-related bismuth active centers (BAC-Al1 and BAC-Al2); the BAC-Al2 is found to be more efficiently excited at a lower temperature within the spectral range of 790–850 nm. This is the first time, to the best of our knowledge, that two sub-bands of BAC-Al are identified. The thermal annealing effect on the luminescence of BACs is also studied under 830 nm pumping. In contrast to the quenching of BAC-Si, the luminescence intensity of BAC-Al1 is enhanced by 1.5 times after 500°C heating and subsequent cooling. The results demonstrate an effective strategy for tuning the emission scheme in the range of 1100–1400 nm by adjusting the excitation wavelength and operation temperature.

We have presented and demonstrated a fiber optic gas pressure sensor with ultra-high sensitivity based on Vernier effect. The sensor is composed of two integrated parallel Mach-Zehnder interferometers (MZIs) which are fabricated by fusion splicing a short section of dual side-hole fiber (DSHF) in between two short pieces of multimode fibers (MMFs). Femtosecond laser is applied for cutting off part of the MMF and drilling openings on one air hole of the DSHF to achieve magnified gas pressure measurement by Vernier effect. Experimental results show that the gas pressure sensitivity can be enhanced to about −60 nm/MPa in the range of 0-0.8 MPa. In addition, the structure possesses a low temperature cross-sensitivity of about 0.55 KPa/°C. This presented sensor has practically value in gas pressure detection, environmental monitoring and other industrial applications.

An extrinsic Fabry-Perot interferometer (EFPI) composed of double fiber FP cavities in a glass capillary tube to generate Vernier effect has been fabricated and employed for gas pressure sensing. A lead-in single-mode fiber (LSMF) and a reflective single-mode fiber (RSMF) were inserted into the capillary tube to form a FP cavity. Femtosecond (fs) laser was used to ablate openings on a capillary tube for gas passage to the FP cavity. A fusion hole was also drilled on the end face of a SMF by fs laser. The sensitivity of the sensor is enhanced due to Vernier effect. Experimental results show that the sensitivity was as high as 86.64 nm/MPa in the range of 0~0.6 MPa, which is 32.8 times larger than that of an open-cavity EFPI sensor without Vernier effect. The temperature cross-sensitivity of the sensor was measured to be about 5.18 KPa/&deg, C. The proposed sensor was characterized by its high sensitivity, compact structure and ease of fabrication, and would have extensive application prospects in gas sensing fields.

A time division multiplexing of 106 weak fibers Bragg gratings (FBGs) based on a ring resonant-cavity is demonstrated. A semiconductor optical amplifier is connected in the cavity to function as an amplifier as well as a switch. The 106 weak FBGs are written along a SMF-28 fiber in serial with peak reflectivity of about −30 dB and equal separations of 5 m. The crosstalk and spectral distortion are investigated through both theoretical analysis and experiments.

A field-programmable gate array (FPGA)-based large-capacity sensing network with ultra-weak fiber Bragg gratings (FBGs) is proposed and experimentally studied. The demodulation system is constructed to interrogate 1642 serial time-division-multiplexing FBGs with a peak reflectivity of about −40 dB and equal separations of 2.5 m. Two semiconductor optical amplifiers and an InGaAs linear sensor array controlled by an FPGA are introduced to the demodulation system to achieve fast, precise, and flexible interrogation. The low crosstalk and spectral distortion are investigated through both theoretical analysis and experiments.