Effects of a UV absorber in silica-loaded resin on DLP silica fiber preform fabrication.

3D printing technologies have distinguished advantages in manufacturing arbitrary shapes and complex structures that have attracted us to use digital light processing (DLP) technology for specialty silica optical fiber preforms. One of the main tasks is to develop an appropriate recipe for DLP resin that is UV sensitive and loaded with silica nanoparticles. In this work, the effects of a UV absorber in highly silica-loaded resin on DLP printing are experimentally investigated. Spot tests and DLP printing are carried out on resins with varying dosages of a typical UV absorber, Sudan Orange G. Based on the experimental results, the UV absorber can significantly improve the resolution of DLP printed green bodies while requiring a larger exposure dose.

Compact, remote optical waveguide magnetic field sensing using double-pass Faraday rotation-induced optical attenuation.

Compact, magnetic field, B sensing is proposed and demonstrated by combining the two Faraday rotation elements and beam displacement crystals within a micro-optical fiber circulator with a fiber reflector and ferromagnets to allow high contrast attenuation in an optical fiber arm. Low optical noise sensing is measured at λ=1550n m as a change in attenuation, α, of optical light propagating through the rotators and back. The circulator's double-pass configuration, using a gold mirror as a reflector, achieves a magnetic field sensitivity s=Δ α/Δ B=(0.26±0.02)d B/m T with a resolution of Δ B=0.01m T, over a detection range B=0-89m T. The circulator as a platform provides direct connectivity to the Internet, allowing remote sensing to occur. The method described here is amenable to multisensor combinations, including with other sensor technologies, particularly in future integrated waveguide Faraday optical circuits and devices, extending its utility beyond point magnetic field sensing applications.

Submonolayer biolasers for ultrasensitive biomarker detection.

Biomarker detection is key to identifying health risks. However, designing sensitive and single-use biosensors for early diagnosis remains a major challenge. Here, we report submonolayer lasers on optical fibers as ultrasensitive and disposable biosensors. Telecom optical fibers serve as distributed optical microcavities with high Q-factor, great repeatability, and ultralow cost, which enables whispering-gallery laser emission to detect biomarkers. It is found that the sensing performance strongly depends on the number of gain molecules. The submonolayer lasers obtained a six-order-of-magnitude improvement in the lower limit of detection (LOD) when compared to saturated monolayer lasers. We further achieve an ultrasensitive immunoassay for a Parkinson's disease biomarker, alpha-synuclein (α-syn), with a lower LOD of 0.32 pM in serum, which is three orders of magnitude lower than the α-syn concentration in the serum of Parkinson's disease patients. Our demonstration of submonolayer biolaser offers great potentials in high-throughput clinical diagnosis with ultimate sensitivity.

Voltage, thermal and magnetic field fiber sensors based on magnetic nanoparticles-doped photonic liquid crystal fibers.

In this article, highly sensitive voltage, thermal and magnetic field fiber sensors were obtained in magnetic nanoparticles-doped E7 liquid crystals filled into photonic crystal fibers (PLCF). The voltage and temperature sensitivity reached at 12.598 nm/V and -3.874 nm/°C, respectively. The minimum voltage response time is 48.2 ms. The phase transition temperature Tc of liquid crystal with magnetic dopant was reduced from 60 °C to 46 °C. The magnetic field sensor based on magnetic nanoparticles-doped PLCF were obtained with sensitivity of 118.2 pm/mT from 400 to 460 mT.

Low energy X-ray dosimeter based on LYSO:Ce fluorescent powder.

Cerium-doped lutetium yttrium orthosilicate (LYSO:Ce) powder has been synthesized by the co-precipitation method. The influence of the Ce3+ doping concentration on the lattice structure and luminescence characteristics of LYSO:Ce powder was investigated by X-ray diffraction (XRD) and photoluminescence (PL). The XRD measurement indicates that the lattice structure of LYSO:Ce powder was not changed by doping ions. PL results show that LYSO:Ce powder has better luminescence performance when the Ce doping concentration is 0.3 mol%. In addition, the fluorescence lifetime of the samples was measured, and the results show that LYSO:Ce has a short decay time. The radiation dosimeter was prepared by LYSO:Ce powder with a Ce doping concentration of 0.3 mol%. Radioluminescence properties of the radiation dosimeter also were studied under X-ray irradiation at doses from 0.03 to 0.76 Gy, with dose rate from 0.09 to 2.284 Gy/min. The results show that the dosimeter has a certain linear relationship response and stability. The radiation responses of the dosimeter at different energies were obtained under X-ray irradiation with X-ray tube voltages ranging from 20 to 80 kV. The results show that the dosimeter has a certain linear relationship response in the low energy range of radiotherapy. These results indicate the potential application of LYSO:Ce powder dosimeters in remote radiotherapy and online radiation monitoring.

Non-Intrusive Pipeline Flow Detection Based on Distributed Fiber Turbulent Vibration Sensing.

We demonstrate a non-intrusive dynamic monitoring method of oil well flow based on distributed optical fiber acoustic sensing (DAS) technology and the turbulent vibration. The quantitative measurement of the flow rate is theoretically acquired though the amplitude of the demodulated phase changes from DAS based on the flow impact in the tube on the pipe wall. The experimental results show that the relationships between the flow rate and the demodulated phase changes, in both a whole frequency region and in a sensitive-response frequency region, fit the quadratic equation well, with a max R2 of 0.997, which is consistent with the theoretical simulation results. The detectable flow rate is from 0.73 m3/h to 2.48 m3/h. The experiments verify the feasibility of DAS system flow monitoring and provide technical support for the practical application of the downhole flow measurement.

Finger Bending Sensing Based on Series-Connected Fiber Bragg Gratings.

Smart wearable devices are occupying an increasingly important position in scientific research and people's life fields. As an indispensable component of smart wearable devices, sensors play a crucial role in their sensing and feedback capabilities. In this paper, we investigate the bending gesture sensing for the most dexterous part of human anatomy, the finger. Based on series-connected fiber Bragg gratings (FBGs), recognition of finger bending posture is achieved by MATLAB modeling and the cubic spline interpolation.

Optical Fiber-Integrated Metasurfaces: An Emerging Platform for Multiple Optical Applications.

The advent of metasurface technology has revolutionized the field of optics and photonics in recent years due to its capability of engineering optical wavefronts with well-patterned nanostructures at subwavelength scale. Meanwhile, inspired and benefited from the tremendous success of the "lab-on-fiber" concept, the integration of metasurface with optical fibers has drawn particular interest in the last decade, which establishes a novel technological platform towards the development of "all-in-fiber" metasurface-based devices. Thereby, this review aims to present and summarize the optical fiber-integrated metasurfaces with the current state of the art. The application scenarios of the optical fiber metasurface-based devices are well classified and discussed accordingly, with a brief explanation of physical fundamentals and design methods. The key fabrication methods corresponding to various optical fiber metasurfaces are summarized and compared. Furthermore, the challenges and potential future research directions of optical fiber metasurfaces are addressed to further leverage the flexibility and versatility of meta-fiber-based devices. It is believed that the optical fiber metasurfaces, as a novel all-around technological platform, will be exploited for a large range of applications in telecommunication, sensing, imaging, and biomedicine.

Compact terahertz birefringent gratings for dispersion compensation.

Terahertz radiation as an upcoming carrier frequency for next-generation wireless communication systems has great potential to enable ultra-high-capacity transmissions with several tens of gigahertz bandwidths. Nevertheless, dispersion is one of the main impairments in achieving a higher bit rate. Here, we experimentally demonstrate a compact terahertz dispersion compensator based on subwavelength gratings. The gratings are fabricated from the low-loss cyclic olefin copolymer exploiting micro-machining fabrication techniques. With the strong index modulation introduced in the subwavelength grating, the high negative group velocity dispersion of -188 (-88) ps/mm/THz is achieved at 0.15 THz for x-polarization (y-polarization), i.e., 7.5 times increase compared to the state-of-the-art reported to date for terahertz. Such high negative dispersion is realized in a grating of 43 mm length. The asymmetric cross-section and periodic-structural modulation along propagation direction lead to considerable birefringence that maintains and filters two orthogonal polarization states, respectively. These polymer-based birefringent gratings can be integrated into terahertz communication systems for dispersion compensation of both long-haul wireless links and waveguide-based interconnect links.

Compact Tri-FFPI sensor for measurement of ultrahigh temperature, vibration acceleration, and strain.

As a high-precision fiber optic sensor, a single optical fiber Fabry Pérot interferometer (FFPI) sensor is often used to measure parameters such as temperature or strain. However, the use of combined FFPIs to measure multiple parameters simultaneously has rarely been reported. In this paper, a compact Tri-FFPI sensor consisting of three series-connected FFPIs is proposed to measure high temperature, high acceleration, and large strain. The total length and diameter of the sensing part are only 2558.9 µm and 250 µm, respectively. One of the FFPIs, FFPI-1, contains a cantilever beam structure to measure vibration acceleration. FFPI-2 is used to measure temperature and the temperature compensation of the strain measurement. FFPI-3 is used to measure strain. To ensure that the sensor has high measurement sensitivity, two demodulation methods are used: the light intensity demodulation method for vibration acceleration and the wavelength demodulation method for temperature and strain. The sensor is capable of withstanding ultrahigh temperatures up to 1000°C.

Adaptive high-precision demodulation method based on vector matching and cluster-competitive particle swarm optimization for multiplexed FPIs.

Multiplexed fiber optic Fabry-Perot interferometer (FPI) sensors are well known for their precision, simple construction, simpler wiring, and high sensing qualities. However, the limitations on existing demodulation methods degrade the measurement accuracy of multiplexed FPI sensors and necessitate large cavity length differences. In this paper, we propose an adaptive high-precision demodulation method based on vector matching and cluster-competitive particle swarm optimization (CCPSO), which transforms cavity length demodulation into searching for the global extreme. The proposed CCPSO, which uses agglomeration within clusters and competition between clusters simultaneously, enables the improvement of the global extreme search capabilities. The theoretical analysis and experimental results show that the proposed demodulation method decreases the lower limit of the needed cavity length differences to 22 µm, which is reduced by 76.9% compared with the fast Fourier transform-based method. An accuracy of 1.05 nm is achieved with a cavity length difference of 27.5 µm and a signal-to-noise ratio of 36.0 dB for the noise.

A sequentially bioconjugated optofluidic laser for wash-out-free and rapid biomolecular detection.

Microstructures can improve both sensitivity and assay time in heterogeneous assays (such as ELISA) for biochemical analysis; however, it remains a challenge to perform the essential wash process in those microstructure-based heterogeneous assays. Here, we propose a sequential bioconjugation protocol to solve this problem and demonstrate a new type of fiber optofluidic laser for biosensing. Except for acting as an optical microresonator and a microstructured substrate, the miniaturized hollow optical fiber (HOF) is used as a microfluidic channel for storing and transferring reagents thanks to its capability in length extension. Through the capillary action, different reagents were sequentially withdrawn into the fiber for specific binding and washing purposes. By using the sequentially bioconjugated FOFL, avidin molecules are detected based on competitive binding with a limit of detection of 9.5 pM, ranging from 10 pM to 100 nM. It is demonstrated that a short incubation time of 10 min is good enough to allow the biomolecules to conjugate on the inner surface of the HOF. Owing to its miniaturized size, only 589 nL of liquid is required for incubation, which reduces the sample consumption and cost for each test. This work provides a tool to exploit the potential of microstructured optical fibers in high-performance biosensing.

Characterization of YAG:Ce phosphor dosimeter by the co-precipitation method for radiotherapy.

Yttrium aluminum garnet (YAG) doped with Ce was synthesized via the co-precipitation method with NH4HCO3 as the precipitant. The spectroscopic properties and the effects of the Ce doping concentration and sintering atmosphere on the crystal phase were investigated. The dosimeter of YAG:Ce phosphor material was prepared to study the radioluminescence (RL) characteristics of a clinical linear accelerator. A satisfying linear relationship between the radiation dose and RL signal was obtained, which provided a reference for the YAG:Ce phosphor material used in radiotherapy and real-time remote radiation detection.

Joint-peaks demodulation method based on multireflection peaks of a few-mode fiber Bragg grating for reducing sensing error.

A few-mode fiber Bragg grating (FM-FBG) inscribed in a few-mode fiber (FMF) can maintain multiple reflection peaks due to the stable multiple modes in FMF. This paper studies the sensing characteristics of multiple reflection peaks for a four-mode FBG (4M-FBG) and innovatively proposes a joint-peak demodulation method based on one FM-FBG to reduce measurement error in temperature or strain sensing. This joint-peak demodulation method, theoretically explained and experimentally verified, provides the possibility of generating miniature sensors with high measurement accuracy and stable measurement performance. The potential of 4M-FBG for simultaneous measurement of strain and temperature is studied in this paper. By measuring the changes of wavelength and intensity of the reflection peaks, temperature and strain can be measured effectively.

Endless Single-Mode Photonics Crystal Fiber Metalens for Broadband and Efficient Focusing in Near-Infrared Range.

The advent of the 'lab-on-fiber' concept has boosted the prosperity of optical fiber-based platforms integrated with nanostructured metasurface technology which are capable of controlling the light at the nanoscale for multifunctional applications. Here, we propose an endless single-mode large-mode-area photonic crystal fiber (LMA-PCF) integrated metalens for broadband and efficient focusing from 800 to 1550 nm. In the present work, the optical properties of the substrate LMA-PCF were investigated, and the metalens, consisting of dielectric TiO2 nanorods with varying radii, was elaborately designed in the fiber core region with a diameter of 48 µm to cover the required phase profile for efficient focusing with a high transmission. The focusing characteristics of the designed metalens were also investigated in detail over a wide wavelength range. It is shown that the in-fiber metalens is capable of converging the incident beams into the bright, symmetric, and legible focal spots with a large focal length of 315-380 µm depending on the operating wavelength. A high and average focusing efficiency of 70% was also obtained with varying wavelengths. It is believed the proposed fiber metalens may show great potential in applications including fiber laser configuration, machining, and fiber communication.

Photo-induced bleaching and thermally stimulated recovery of BAC-P in Bi-doped phosphosilicate fibers.

The first results of the study on photobleaching and thermally induced recovery in Bi-doped phosphosilicate fiber have been presented. It was revealed that the rate of bleaching of phosphor-related Bi active center (BAC-P) becomes slower with the decrease of photon energy. The quadratic dependence of the bleaching rate of BAC-P on laser power is obtained under 532 nm laser irradiation. The effect of temperature on the bleaching dynamics of BAC-P is also investigated under 532 nm radiation, suggesting a thermally aggravated bleaching process upon heating at certain temperatures (≥300∘C). Furthermore, the thermal recovery of bleached Bi-doped silica-based fiber (BDF) is investigated and a 13% increase of luminescence is achieved upon thermal quenching for 5 min at 400ºC. The underlying mechanism of photobleaching and thermo-stimulated recovery process of BAC-P is also discussed.

Helical distributed feedback fiber Bragg gratings and rocking filters in a 3D printed preform-drawn fiber.

Using induced UV attenuation across a twisted fiber asymmetric core drawn from a 3D printed preform, linear fiber Bragg gratings (FBGs) are produced on one side of the core. By removing the twist, a helical grating with a period matching the twist rate is produced. Balancing the rate with the polarization beat length in a form birefringent fiber allows the production of a combined rocking filter and FBG device with tunable properties. Direct observation of the fiber grating dispersion within the rocking filter rejection band is possible.

High-power and high-energy Nd:YAG-Nd:YVO4 hybrid gain Raman yellow laser.

We present a simple and reliable method to successfully reconcile the average output power and pulse energy of the solid-state Raman yellow lasers. By virtue of the hybrid laser gain of Nd:YAG and Nd:YVO4 in an intracavity frequency-doubled Raman, much higher pumping is allowed and nearly linear polarized fundamental and Stokes waves can be delivered for efficient non-critical phase matching. 7.6 W of yellow output at 588 nm is obtained under incident pump power of 42.0 W at the pulse repetition frequency (PRF) of 110 kHz and the pulse energy reaches 0.41 mJ under the same incident pump power at the PRF of 10 kHz.

Impact of Al2O3 doping on Bi active center photobleaching in Bi/Er-codoped fibers.

In this Letter, the impact of Al2O3 doping on the Bi active center (BAC) photobleaching is investigated in Bi/Er-codoped fibers (BEDFs). By measuring the evolution of emission attributed to the BAC associated with silica (BAC-Si) at ∼1400nm, the linear relationship between the ratio of unbleached/bleached part (γUB/γB) and 830 nm irradiation intensity (P830) was revealed in the log-log plot. The experimental results demonstrate that Al2O3 doping or its induced defects could be one key factor exaggerating the BAC photobleaching in BEDFs.

Thermal bleaching of BACs in bismuth/erbium co-doped fiber fabricated through 3D silica lithography.

Bismuth/erbium co-doped optical fiber fabricated through 3D silica lithography is thermally treated with various conditions. Then the thermal treatment effect on bismuth active centers (BACs) in this fiber is investigated. The thermal bleaching of the BAC associated with Al and the BAC associated with Si is observed after thermal treatment at high temperatures (300°C-800°C). It is found that the absorption and luminescence of BACs dramatically decrease after the thermal treatment, even totally bleaching at 700°C. The results show that the temperature and dwell time have significant effects on the thermal bleaching and activation of BACs. The underlying mechanisms of these thermal-induced effects are further discussed.