Fiber End Face Research Articles

Optical Interferometric Force Sensor Based on a Buckled Beam

This paper reports a novel extrinsic Fabry-Perot interferometer (EFPI)-based fiber optic sensor for force measurement. The prototype force sensor consists of two EFPIs mounted on a stainless steel rectangular frame. The primary sensing element, i.e., the first EFPI, is formed between the endface of a horizontally-placed optical fiber and a stainless steel buckled beam. The second EFPI, fashioned between a longitudinally-placed optical fiber and a silver-coated glass beam, is arranged to demonstrate the amplification mechanism of the buckled beam structure. When the sensor is subjected to a tension force, the pre-buckled beam will deflect backward, resulting in a longitudinal/axial displacement of the pre-buckled beam. The axial displacement is further transferred and amplified to a horizontal/vertical deflection at the middle of the buckled beam, leading to a relatively-significant change in the Fabry-Perot cavity length. A force sensitivity of 796 nm/N (change in cavity length/Newton) is achieved with a low-temperature dependence of 0.005 N/°C. The stability of the sensor is also investigated with a standard deviation of ± 5 nm, corresponding to a measurement resolution of ±0.0064 N. A simulation is conducted to study the axial displacement and stress distribution of the sensor when it is subjected to a tension load of 250 N. It is demonstrated that the maximum stress of the sensor is tremendously reduced attributed to the buckled design, enabling a long service life cycle of the force sensor. The robust and simple-to-manufacture force sensor has great potential in structural health monitoring, robotics control, and oil/ gas refining systems.

Sapphire optical fiber high-temperature vibration sensor.

A sapphire fiber high-temperature vibration sensor with an extrinsic Fabry-Perot interferometer (EFPI) structure is proposed and experimentally demonstrated. The vibrating diaphragm of the sensor is a supported beam structure fabricated by etching a single-side polished sapphire wafer using a femtosecond laser. The FP cavity of the sensor is composed of the sapphire fiber end face and the polished surface of the vibrating diaphragm. The interference signal of the sensor is picked up by the sapphire fiber and transmitted to a laser interferometry demodulator through a multimode fiber. Experimental results show that the acceleration response is linear in the range of 0-10 g along with an acceleration sensitivity of 20.91 nm/g. The resonance frequency of the sensor is 2700 Hz, which is consistent with the ANSYS simulation results. The sensor can also work in the temperature range from room temperature to 1500 ℃, providing a feasible method for vibration measurements in high-temperature environments.

Acetone Gas Sensor Based on Two-Photon Polymerized Micropillar on End Face of Fiber

Acetone Gas Sensor Based on Two-Photon Polymerized Micropillar on End Face of Fiber

Resonance Modes and Influencing Factors of Micro Sensing Unit on Fiber End Face

Resonance Modes and Influencing Factors of Micro Sensing Unit on Fiber End Face

Theoretical calculation of fiber cavity coupling silicon carbide membrance

Single spin color centers in solid materials are one of the promising candidates for quantum information processing, and attract a great deal of interest. Nowadays, single spin color centers in silicon carbide, such as divacancies and silicon vacancies have been developed rapidly, because they not only have similar properties of the NV centers in diamond, but also possess infrared fluorescence that is more favorable for transmission in optical fiber. However, these centers possess week fluorescence with broad spectrum, which prevents some key technologies from being put into practical application, such as quantum key distribution, photon-spin entanglement, spin-spin entanglement and quantum sensing. Therefore, optical resonator is very suitable for coupling centers to filter their spectrum and enhance the fluorescence by Purcell effect. It is very advantageous to use the fiber end face as cavity mirrors, thereby the fiber can provide small cavity volume corresponding to a large enhancement in spin color centers, and collect the fluorescence in cavity simultaneously, which has no extra loss in comparison with other collection methods. In this work, the properties and performance of fiber Fabry-Perot cavity coupling silicon carbide membrane are mainly studied through theoretical calculation. Firstly, some parameters are optimized such as membrane roughness and mirror reflection by calculating the mode of the fiber cavity and enhancing the color centers coupling into the cavity, then analyzing the properties of different modes in cavity, the enhancement effect on cavity coupling color centers, and other relevant factors affecting the cavity coupling color centers. Next, the influences of dominated factor and vibration on the properties of the cavity, the enhancement and outcoupling of centers coupled into the cavity are investigated, and finally the optimal outcoupling efficiency corresponding to different vibration intensities is obtained. These results give direct guidance for the further experimental design and direction for optimization of the fiber cavity coupling color centers.

Humidity sensing and temperature response performance of polymer gel cold-spliced optical fiber Fabry-Perot interferometer

An extrinsic Fabry-Perot Interferometric (EFPI) fiber humidity probe has been proposed. This composite EFPI fiber humidity sensor combines the characteristics of various fiber structures, such as the cascaded spliced fibers, coating film on the fiber end-face and in-line fixed Fabry-Perot cavity in capillary. The polymethyl methacrylate (PMMA) was filled in between the air gap between single mode fiber and muti-modes fiber as the humidity sensitive material to establish the EFPI optical fiber humidity sensor. The best sensitivity has been experimentally demonstrated with 0.4172 nm/%RH in the relative humidity range of 10%RH-70%RH, where the corresponding fitting coefficient is 0.99939, and the response time is less than 14 min. Furthermore, the wavelength temperature sensitivity based on wavelength modulation is 0.06074 nm/℃, and the power responded sensitivity is 0.056 dBm/℃ in the temperature range of 20℃-65℃. The experimental results indicate that the proposed fiber probe can simultaneously measure humidity and temperature, or in other way, it will significantly improve the accurate measurement for humidity by analyzing and compensating the influencing factors from the environmental temperature.

Периодическая генерация затопленных струй при лазерном нагреве торца волокна

It has been experimentally found that when the end face of an optical fiber with an absorbing coating is heated by continuous laser radiation with a wavelength of 0.97 μm, submerged jets are periodically generated in water. Each such jet is formed as a result of the collapse of a gas-vapor bubble arising from the explosive boiling of water. The mechanisms of formation of bubbles and jets are discussed.

Rapid fabrication of sub-micron scale functional optical microstructures on the optical fiber end faces by DMD-based lithography.

The rapid development of optical fiber application systems puts forward higher requirements for the miniaturization and integration of optical fiber devices. One promising solution is to integrate diffractive optical microstructures on the end faces of optical fibers. However, rapid microfabrication on such tiny and irregular substrates is a challenge. In recent years, Femtosecond laser polymerization technology has become an effective solution to the challenge, which can be flexibly applied for the fabrication of complex 3D microstructures with ultra-high resolution. When the demand for the lithography resolution is not very high, other microfabrication methods with a lower technical threshold may be developed for achieving a balance between fabrication precision, cost and efficiency. In this paper, we report a Digital Micromirror Device (DMD) based lithography method dedicated to the fabrication of functional optical microstructures on the optical fiber end faces. Especially, it's also applicable to single-mode fibers (SMFs). By the projection via a 40x objective lens, the fabrication resolution of 0.405 μm was achieved within an exposure area of 209.92 μm × 157.44 μm. We evaluated the microfabrication results by the photomicrographs and the optical diffraction modulation effects of the functional optical microstructures. This method provides a new idea for fabricating both hybrid optical fiber devices and SMF devices, and it may be an alternative method for resolving the conflict between the precision, the cost and the efficiency.

Fabrication of Fresnel zone plate in chalcogenide glass and fiber end with femtosecond laser direct writing

Chalcogenide glass (ChG) has excellent optical properties in the mid-infrared range. The micro-optical fiber devices made of ChG play a strong part in various infrared sensing and detection fields. In this work, Fresnel zone plate (FZP) was fabricated on the Ge-As-S ChG surface and the end face of ChG fiber through femtosecond laser direct writing. FZP with high optical quality was obtained by optimizing the laser writing parameters. Clear focusing pattern can be observed in the visible and infrared bands with the prepared FZP. The full width at half maximum (FWHM) of laser beam with wavelength of 1550 nm is recorded to be 10.39 μm at the focus of FZP, and the numerical aperture (NA) is 0.122. With a well-designed FZP on the fiber end face, the ChG fiber can achieve a self-focusing effect without the collimating and focusing lens. The FWHM of the 1550 nm laser beam at the focus is measured to be 7.46 μm, and NA is 0.162. The combination of FZP and ChG fiber can make the structure of the optical fiber more compact and has potential application value in enhancing the interaction between light and material in the infrared band.

(INVITED)Miniaturized lenses integrated on optical fibers: Towards a new milestone along the lab-on-fiber technology roadmap

Optical fibers and lenses are key components in many optical systems. Lenses used to couple light into the fiber or to tailor the output beam coming out from the fiber itself typically involve bulk optical schemes and complex packaging that limit the compactness and flexibility of the overall system. In the last years, new technological solutions based on the lab-on-fiber technology have been proposed to directly integrate optical components on the fiber end-face with a monolithic approach. In this review, we provide a detailed state of the art of the miniaturized lenses integrated on the end-face of optical fibers reported so far. The proposed devices have been divided into three main categories (refractive, diffractive and resonant including metasurfaces) according to their working principle. A critical discussion on the different categories of lenses is given, also providing a comparison in terms of ease of design and fabrication, application flexibility, compactness, scientific and technological maturity, and time to market.

Laser-induced boiling of subcooled liquid: influence of the radiation power on the vapor bubble nucleation and growth

This paper presents the results of an experimental study of the effect of laser radiation power on the growth and collapse dynamics of vapor bubbles formed near the end-face of an optical fiber through which the radiation is transmitted into the liquid. It is shown that this influence is insignificant in the power range from 3 to 10 W. We propose a hypothesis that this effect is due to the fact that experiments with different radiation power have bubbles occurring and growing under similar temperature conditions. Our proposition has been confirmed by numerical simulations.

Machine learning identifies liquids employing a simple fiber-optic tip sensor.

We proposed an extremely simple fiber-optic tip sensor system to identify liquids by combining their corresponding droplet evaporation events with analyses using machine learning techniques. Pendant liquid droplets were suspended from the cleaved endface of a single-mode fiber during the experiment. The optical fiber-droplet interface and the droplet-air interface served as two partial reflectors of an extrinsic Fabry-Perot interferometer (EFPI) with a liquid droplet cavity. As the liquid pendant droplet evaporated, its length diminished. A light source can be used to observe the effective change in the net reflectivity of the optical fiber sensor system by observing the resulting optical interference phenomenon of the reflected waves. Using a single-wavelength probing light source, the entire evaporation event of the liquid droplet was precisely captured. The measured time transient response from the fiber-optic tip sensor to an evaporation event of a liquid droplet of interest was then transformed into image data using a continuous wavelet transform. The obtained image data was used to fine-tune pre-trained convolution neural networks (CNNs) for the given task. The results demonstrated that machine learning-based classification methods achieved greater than 98% accuracy in classifying different liquids based on their corresponding droplet evaporation processes, measured by the fiber-optic tip sensor.

Lignin-containing fibers extraction and hydrogel preparation for fiber-optic relative humidity sensor fabrication

Herein, a physicochemical pretreatment method, namely, a combination of recyclable acid hydrotrope (p-toluenesulfonic acid, p-TsOH) treatment and ultrasonic treatment (10 s), was employed to extract lignin-containing fibers (LFs) from wood material (poplar). The LFs were used to produce reinforced polyvinyl alcohol (PVA) hydrogels via a dissolution-regeneration process in LiBr⋅3H2O solution. After coating the produced hydrogel on the end face of a single-mode fiber (SMF), a fiber-optic relative humidity sensor (FORHS) was fabricated to monitor environmental humidity. A high sensitivity (4.7 dB) and good repeatability (R2 = 0.98, the confidence factor) were found within a humidity range from 45 % to 80 %. Therefore, this study provides a sustainable approach for valorization of lignocellulosic biomass.

Zernike mode analysis of an adaptive optics system for horizontal free-space laser communication

Free-space laser communication (FSLC) has attracted wide attention and developed rapidly due to its high bandwidth and strong capability to resist interception and interference. However, communication performance is severely affected by atmospheric turbulence. In this paper we consider the end face of the coupling fiber and adopted power-in-the-bucket (PIB) as the evaluation criterion instead of the traditionally used coupling efficiency. The relationship between FSLC performance (PIB) and adaptive optics parameters (corrected Zernike modes number) is derived for the first time. We simulated and conducted an 8.9 km horizontal FSLC experiment to analyze the effects of corrected modes on communication performance. The simulation and experiment results consistently show that, for turbulence of r0 = 7.51 cm, it is sufficient to correct the first 30 modes. The bit error rate (BER) and the PIB reached values of about 10−7 and 70%, respectively. For r0 = 4.43 cm correction of the first 22 modes, yielded BER and PIB values of around 10−5 and 55%, respectively. For r0 = 1.7 cm, no matter how many Zernike modes are corrected, the BER and PIB values are about 10−3 and 20% respectively. This experiment and its results provide an important reference for corrected modes on communication performance under different turbulence strengths.

Simulation of liquid gas and displacement optical fiber sensing system based on vortex beam

This paper attempts to propose an optical fiber sensing system based on the vortex beam, which can be applied to measure liquid refractive index (RI), gas concentration and displacement. Its sensing structure consists of a silver-coated microfiber for surface plasma resonance (SPR) sensors, a fiber aligned with it, and a interference structure for Gaussian beam interfering with the transmitted vortex beam. Due to SPR effect, when the RI of liquid changes, the wavelength of resonance spectrum will shift accordingly, thus the sensitivity of the RI can be achieved. When the gas refractive index n and distance L between two fiber end face change, phase difference will be generated between the transmitted vortex beam and the bypass Gaussian beam. Thereupon, the interference pattern of the two beams will rotate, the measurement of the gas RI and displacement can be achieved. In comparison with the traditional optical fiber SPR sensor, the simulation result shows that SPR excited by the vortex beam is more explicit with higher sensitivity. When the RI of the external environment is 1.385, the sensitivity of 4600 nm/RIU under the condition of Gaussian beam, the sensitivity of sensor based on vortex beam increases to 6800 nm/RIU, which is 47.8% higher. When the RI of the external environment is 1.365–1.385, the average sensitivity of the sensor increases from 3950 nm/RIU to 5100 nm/RIU. Meanwhile, the gas RI sensitivity is 36°/0.0001 RIU and the displacement sensitivity is 36°/ 100 nm. In terms of the CCD angle resolution, the theoretical resolution of gas RI measurement is 1.54e-10 RIU, and the resolution of displacement is 1.54e-4 nm.

Multicore fiber integrated beam shaping devices for long-range plasmonic trapping.

The multicore fiber beam shaping devices based on surface plasmon polaritons (SPPs) have been proposed and demonstrated. The gold film is covered on the end face of the optical fiber. An air slit is perforated in the center of each core and the gratings with a fixed period are designed on the gold film on one side of the slit to obtain a deflected beam. Multiple deflected beams based on the multicore fiber interfere and form a periodic field, where the period of the interference field is determined by the deflection angle of the beams and the spacing between the cores. The interference field of the multiple deflected beams can be used to trap the nanosphere. The Maxwell stress tensor method is used to calculate the transverse and longitudinal trapping forces on a nanosphere. The nanosphere can be stably trapped at 45 μm away from the end face of the fiber. Such an all-fiber trapping system is compact and flexible integration, and is promising for long-working-distance and multiple-particle trapping.

Highly precise in-plane displacement sensor based on an asymmetric fiber Fabry-Perot interferometer.

An in-plane displacement sensor based on an asymmetric extrinsic fiber Fabry-Perot interferometer (EFPI) is proposed and demonstrated. The asymmetric EFPI composed of a step-shaped external reflector and a cleaved fiber end face can be equivalent to two parallel FPIs with slightly different cavity lengths. By calculating the peak intensity difference of the two FPIs, the in-plane displacement can be demodulated with enhanced sensitivity and suppressed common mode noise. Both theoretical analyses and experimental results show that the sensitivity and the linear range of the in-plane displacement sensor are dependent on the cavity length. A displacement resolution of 5 nm and a linear range of ±7µm under the cavity length of 250 µm are achieved in the experiment. The proposed in-plane displacement sensor with a nanometric resolution and compact size can be widely used in the fields of metrology, accelerometers, and semiconductor manufacture.

Highly-Efficient Sulfonated UiO-66(Zr) Optical Fiber for Rapid Detection of Trace Levels of Pb2.

Lead detection for biological environments, aqueous resources, and medicinal compounds, rely mainly on either utilizing bulky lab equipment such as ICP-OES or ready-made sensors, which are based on colorimetry with some limitations including selectivity and low interference. Remote, rapid and efficient detection of heavy metals in aqueous solutions at ppm and sub-ppm levels have faced significant challenges that requires novel compounds with such ability. Here, a UiO-66(Zr) metal-organic framework (MOF) functionalized with SOH group (SOH-UiO-66(Zr)) is deposited on the end-face of an optical fiber to detect lead cations (Pb) in water at 25.2, 43.5 and 64.0 ppm levels. The SOH-UiO-66(Zr) system provides a Fabry–Perot sensor by which the lead ions are detected rapidly (milliseconds) at 25.2 ppm aqueous solution reflecting in the wavelength shifts in interference spectrum. The proposed removal mechanism is based on the adsorption of [Pb(OH)] in water on SOH-UiO-66(Zr) due to a strong affinity between functionalized MOF and lead. This is the first work that advances a multi-purpose optical fiber-coated functional MOF as an on-site remote chemical sensor for rapid detection of lead cations at extremely low concentrations in an aqueous system.

Fiber endface photoacoustic generator for quantitative photoacoustic tomography.

We demonstrate a novel fiber endface photoacoustic (PA) generator using infrared (IR) 144 laser dye dispersed within an ultraviolet adhesive. The generator provides a wide acoustic bandwidth in the transducer frequency range of 2-7 MHz, high thermal conversion efficiency (${\gt}90\%$), good PA signal controllability (well-controlled IR 144 concentration), and high feasibility (simple procedures). Through a series of experimental validations, we show that this fiber-based endface PA generator can be a useful tool for a broad range of biomedical applications such as calibrating the local absorption coefficient of biological tissue for quantitative PA tomography.

Self-powered and high-performance all-fiber integrated photodetector based on graphene/palladium diselenide heterostructures.

An all-fiber integrated photodetector is proposed and demonstrated by assembling a graphene/palladium diselenide (PdSe2) Van der Waals heterostructure onto the endface of a standard optical fiber. A gold film is covered on the heterostructure working as an electrode and a mirror, which reflects back the unabsorbed residual light for further reusage. Owing to the low bandgap of PdSe2, the all-fiber photodetector shows a broadband photoresponse from 650 to 1550 nm with a high photoresponsivity of 6.68×104 AW-1, enabling a low light detection of 42.5 pW. And the fastest temporal response is about 660 µs. Taking advantage of heterostructures, the photodetector can work in self-powered mode with the on/off ratio about 82. These findings provide new strategies for integrating two-dimensional materials into optical fibers to realize integrated all-fiber devices with multi-function applications.