Tilted Bragg grating in a glycerol-infiltrated specialty optical fiber for temperature and strain measurements.

In this Letter, we propose an in-line tilted fiber Bragg grating sensor for temperature and strain measurements. The grating is inscribed in a specialty optical fiber using tightly focused femtosecond laser pulses and the line-by-line direct writing method. Beside the central core in which the grating is produced, a hollow channel filled with glycerol aqueous solution significantly improves the sensitivity of the fiber cladding modes due to its high thermo-optic coefficient. We show that the temperature sensitivity of the core mode is 9.8 pm/°C, while the one of the cladding modes is strongly altered and can reach -24.3 pm/°C, in the investigated range of 20-40°C. For the strain measurement, sensitivities of the core mode and the cladding modes are similar (∼0.60 pm/µÎµ) between 0 and 2400 µÎµ. The significative difference of temperature sensitivity between the two modes facilitates the discrimination of the dual parameters in simultaneous measurements.

Signal processing integrated with fiber-optic Vernier effect for the simultaneous measurement of relative humidity and temperature.

A novel inline Fabry-Perot interferometer (FPI) for simultaneous relative humidity (RH) and temperature monitoring is proposed. The sensing probe consists of a section of hollow core Bragg fiber (HCBF) spliced with a single-mode fiber pigtail. The end-face of the HCBF is coated with Chitosan and ultraviolet optical adhesive (UVOA), forming two polymer layers using a well-designed fabrication process. The surfaces of the layers and splicing point will generate multiple-beam interference and form Vernier-effect (VE) related envelopes in the reflection spectrum. A signal processing (SP) method is proposed to demodulate the VE envelopes from a complicated superimposed raw spectrum. The principle of the SP algorithm is analyzed theoretically and verified experimentally. The sensor's RH and temperature response are studied, exhibiting a high sensitivity of about 0.437 nm/%RH and 0.29 nm/ ∘C, respectively. Using a matrix obtained from experiment results, the simultaneous RH and temperature measurement is achieved. Meanwhile, the simple fabrication process, compact size and potential for higher sensitivity makes our proposed structure integrated with the SP algorithm a promising sensor for practical RH and temperature monitoring.

Highly sensitive gas pressure sensor based on the hollow core Bragg fiber and harmonic Vernier effect.

A highly sensitive inline gas pressure sensor based on the hollow core Bragg fiber (HCBF) and harmonic Vernier effect (VE) is proposed and experimentally demonstrated. By sandwiching a segment of HCBF between the lead-in single-mode fiber (SMF) and the hollow core fiber (HCF), a cascaded Fabry-Perot interferometer is produced. The lengths of the HCBF and HCF are precisely optimized and controlled to generate the VE, achieving a high sensitivity of the sensor. Meanwhile, a digital signal processing (DSP) algorithm is proposed to research the mechanism of the VE envelope, thus providing an effective way to improve the sensor's dynamic range based on calibrating the order of the dip. Theoretical simulations are investigated and matched well with the experimental results. The proposed sensor exhibits a maximum gas pressure sensitivity of 150.02 nm/MPa with a low temperature cross talk of 0.00235 MPa/ ∘C. All these advantages highlight the sensor's enormous potential for gas pressure monitoring under various extreme conditions.

Dual Fabry-Perot interferometers gas pressure sensor in a parallel configuration based on a hollow core Bragg fiber and the harmonic Vernier effect.

An ultra-high sensitivity parallel-connected Fabry-Perot interferometers (FPIs) pressure sensor is proposed and demonstrated based on hollow core Bragg fiber (HCBF) and harmonic Vernier effect. The HCBF functions as a micro Fabry-Perot cavity and possesses low transmission loss. One FPI acts as the sensing unit while the other FPI is used as the reference unit to generate the Vernier effect. The sensing FPI was prepared by fusion splicing a section of HCBF between a single-mode fiber (SMF) and a hollow silica tube (HST), and the reference FPI was fabricated by sandwiching a piece of HCBF between two SMFs. Two FPIs with very different free spectral ranges (FSRs) in the fringe pattern were connected to the 2 × 2 coupler parallelly, which realizes the harmonic Vernier effect and ensures the stability of the interference fringe. Laboratory results exhibited that the pressure sensitivity can be enhanced to 119.3 nm/MPa within 0-0.5 MPa by the proposed sensor. Moreover, low-temperature crosstalk of 0.074 kPa/° was achieved. Compared with the traditional optical fiber gas pressure sensor, the advanced sensor features high sensitivity, stability, easy fabrication, and fast response, which can be a promising candidate for real-time and high-precision gas pressure monitoring.

Fabry-Perot interferometers for highly-sensitive multi-point relative humidity sensing based on Vernier effect and digital signal processing.

A highly sensitive relative humidity (RH) sensor based on Fabry-Perot interferometers (FPI) is proposed and experimentally demonstrated. The sensor is fabricated by splicing a segment of hollow core Bragg fiber (HCBF) with single mode fiber (SMF) and functionalized with chitosan and ultraviolet optical adhesive (UVOA) composite at the end of HCBF to form a hygroscopic polymer film. The reflection beams from the splicing point and the two surfaces of the polymer film generate the Vernier effect in the reflection spectrum, which significantly improves the humidity sensitivity of the sensor. To demodulate the envelope based on the Vernier effect and realize multi-point sensing, a digital signal processing (DSP) algorithm is proposed to process the reflection spectrum. The performance of the DSP algorithm is theoretically analyzed and experimentally verified. The proposed sensor demonstrates a high sensitivity of 1.45 nm/% RH for RH ranging from 45% RH to 90% RH. The compact size, high sensitivity and multiplexing capability make this sensor a promising candidate for RH monitoring. Furthermore, the proposed DSP can potentially be applied to other sensors based on the Vernier effect to analyze and extract valuable information from the interference spectrum.

Single-ring hollow-core anti-resonant fiber with a record low loss (4.3†dB/km) for high-power laser delivery at 1†µm.

We report a 7-tube single-ring hollow-core anti-resonant fiber (SR-ARF) with a record low transmission loss of 4.3 dB/km @1080 nm, which is almost half of the current lowest loss record of an SR-ARF (7.7 dB/km @750 nm). The 7-tube SR-ARF has a large core diameter of 43 µm and a wide low-loss transmission window exceeding 270 nm for the 3-dB bandwidth. Moreover, it exhibits an excellent beam quality with an M2 factor of 1.05 after 10-m-long transmission. The robust single-mode operation, ultralow loss, and wide bandwidth make the fiber an ideal candidate for short-distance Yb and Nd:YAG high-power laser delivery.

Ultrawide bandwidth single-mode polarization beam splitter based on dual-hollow-core antiresonant fiber.

An ultrawide bandwidth and single-mode polarization beam splitter (PBS) based on an air-gap type of dual-hollow-core antiresonant fiber (DHC-ARF) is proposed. Nested tubes are introduced into two cladding tubes between two cores to weaken the wavelength dependence of coupling length in DHC-ARF for obtaining ultrawide bandwidth. By tuning the cladding tube sizes, higher-order core modes with the lowest loss can be coupled with cladding tube modes, and thus, effectively, single-mode operation is achieved. Numerical results demonstrate that an 8.15 cm long DHC-ARF can be used to develop a PBS with an operating bandwidth of 370 nm ranging from 1.28 to 1.65 µm, where a polarization extinction ratio is below -20dB and a high-order mode extinction ratio exceeds 100.

Temperature and curvature insensitive all-fiber sensor used for human breath monitoring.

In this paper, an all-fiber sensor based on hollow core Bragg fiber (HCBF) is proposed and successfully manufactured, which can be used for human breath monitoring. Benefiting from the identical outer diameters of HCBF and single mode fibers (SMFs), the sensor can be directly constructed by sandwiching a segment of HCBF between two SMFs. Based on optical propagation properties of HCBF, the transmission light is sensitive to specific environmental change induced by human breath. Thus, the breath signals can be explicitly recorded by measuring the intensity of the transmitted laser. The sensor presents a rapid response time of ∼0.15 s and recovery time of ∼0.65 s. In addition, the HCBF-based sensor shows good insensitivity to the variation of temperature and curvature, which enables its reliable sensing performance in the dynamic and changeful environment.

Simultaneous measurement of magnetic field and temperature based on two anti-resonant modes in hollow core Bragg fiber.

A simple and compact magnetic field and temperature dual-parameter sensor is proposed, which is based on a sandwich structure consisting of a section of hollow core Bragg fiber (HCBF) filled with magnetic fluid (MF) and two sections of single-mode fiber (SMF). The corresponding relationship between the resonant dip with different periods in the transmission spectrum and specific anti-resonant (AR) mode in the HCBF is determined. The resonant dips based on different AR modes shift differently when the magnetic field intensity and temperature change. Then, the simultaneous measurement of the magnetic field intensity and temperature can be achieved by utilizing a cross matrix. The experimental results show that the maximum magnetic field sensitivity in the range of 0-12 mT is 86.43 pm/mT, and the maximum temperature sensitivity in the range of 20-60 ℃ is 17.8 pm/℃. The proposed sensor has the advantages of compact structure, easy fabrication and low cost, thus, it has great potential applications in the field of simultaneous sensing of magnetic field intensity and temperature in complex environments.

Design of hollow core step-index antiresonant fiber with stepped refractive indices cladding.

With the benefits of low latency, wide transmission bandwidth, and large mode field area, hollow-core antiresonant fiber (HC-ARF) has been a research hotspot in the past decade. In this paper, a hollow core step-index antiresonant fiber (HC-SARF), with stepped refractive indices cladding, is proposed and numerically demonstrated with the benefits of loss reduction and bending improvement. Glass-based capillaries with both high (n = 1.45) and low (as low as n = 1.36) refractive indices layers are introduced and formatted in the cladding air holes. Using the finite element method to perform numerical analysis of the designed fiber, results show that at the laser wavelengths of 980 and 1064 nm, the confinement loss is favorably reduced by about 6 dB/km compared with the conventional uniform cladding HC-ARF. The bending loss, around 15 cm bending radius of this fiber, is also reduced by 2 dB/km. The cladding air hole radius in this fiber is further investigated to optimize the confinement loss and the mode field diameter with single-mode transmission behavior. This proposed HC-SARF has great potential in optical fiber transmission and high energy delivery.

Simultaneous measurement of temperature and strain based on a hollow core Bragg fiber.

A novel, to the best of our knowledge, reflective sensor fabricated by simply sandwiching a homemade hollow core Bragg fiber (HCBF) between two single-mode fibers is proposed and demonstrated for the simultaneous measurement of the temperature and the strain. Different from traditional Fabry-Perot interferometer (FPI) sensors that can achieve only one-parameter sensing with inevitable cross-correspondence to other parameters, the proposed sensor based on the HCBF, which functions as an FPI-inducing FPI spectrum pattern and a weak waveguide confining light-inducing periodic envelope in reflection spectrum, ensures double-parameter sensing. For the HCBF-based reflective sensor, different sensing mechanisms lead to the various sensitivity values of temperature and strain (2.98 pm/°C, 19.4 pm/°C, 2.02 pm/µÎµ, -0.36pm/µÎµ), resulting in a different shift of the confining spectrum envelope and the FPI spectrum fringe. Experimental results indicate that our proposed sensor can measure temperature and strain simultaneously by utilizing a 2×2 matrix. Taking advantage of the compact size, easy fabrication, and low cost, this sensor has an applicable value in harsh environment for simultaneous strain and temperature sensing.

Bragg labeled wavelength calibrates interferometric sensors in hollow core fiber.

A Bragg labeled wavelength (BLW) employed to the sensitivity calibration in an interference pattern has been proposed and experimentally demonstrated. According to the critical condition of Fabry-Perot (FP) interference and the antiresonant (AR) effect, the length of hollow core fiber (HCF) is artificially controlled to form a FP microcavity by collapsed fusion splicing. Dual-spectral features of the BLW and inline multimode interference (IMMI) dominate the transmission spectrum of the collapsed Bragg HCF (BHCF). The location of the BLW remains unchanged once the air-core diameter is selected. Sensing performance is investigated to validate the calibration function of the proposed BHCF. In particular, the temperature sensitivity of the BLW and multimode interference are 12.8 pm/°C and 87.1 pm/°C, respectively, corresponding to the reference sensitivity induced by the Bragg structure and the measurement sensitivity of the IMMI. All these findings highlight the calibration of HCF-based interferometric sensors in practical applications.

Liquid-level sensing based on a hollow core Bragg fiber.

A novel optical fiber liquid level sensor based on a hollow core Bragg fiber (HCBF) was proposed and demonstrated. The HCBF was first designed and successfully fabricated with periodic transmission band in the spectrum and a transmission loss of ~3.48 dB/cm. An inline optical fiber liquid-level sensor was fabricated by simply sandwiching a piece of HCBF between two single mode fibers. The sensing performance was experimentally tested. A linear liquid-level sensitivity of ~1.1 dB/mm, and fast response time less than 3s was obtained by the intensity demodulation measurement. The temperature and refractive index cross-sensitivities were also investigated. The experimental results indicate that our proposed structure has tiny temperature and RI dependence, which makes it a promising liquid level sensing platform for different liquids.

Wideband and continuously-tunable fractional photonic Hilbert transformer based on a single high-birefringence planar Bragg grating.

We propose and experimentally demonstrate wideband and continuously tunable fractional-order photonic Hilbert transformers (FrHT). These are realized by a single apodized planar Bragg grating within a high-birefringence planar substrate. The fractional order of the FrHT is continuously tuned and precisely controlled by changing the polarization state of the input light. The experimental characterization demonstrates an operating bandwidth up to 120 GHz with amplitude ripples below 3 dB. The optical phase shift response is directly measured to verify the proposed tuning property, demonstrating transform orders of around 1, 0.7, and 0.5. This approach is simple, stable, and compact compared to other existing methods and has great potential in the fields of ultrafast all-optical signal processing.

Single-polarization single-mode double-ring hollow-core anti-resonant fiber.

A novel single-polarization single-mode double-ring hollow-core anti-resonant fiber with two single-polarization regions (1545-1553 nm and 1591-1596 nm) is proposed. Single-polarization guidance is achieved by coupling a polarized fundamental mode and silica mode by using different tube thicknesses. Specifically, when the wavelength is 1550 nm, only a single x-polarized fundamental mode with a low loss of 0.04 dB/m is propagated by a polarization extinction ratio of 17662 and minimum higher-order mode extinction ratio of 393 by optimizing the structural parameters. Furthermore, this fiber also exhibits high-performance bend resistance. The x-polarized FM loss is as low as 0.11 dB/m with single-polarization single-mode guidance when the proposed fiber was bent at a bend radius of 8 cm toward the x-direction.

Single-phase tunable white-light-emitting Sr3 La(PO4)3 : Eu2+, Mn2+ phosphor for white LEDs.

A novel, near-ultraviolet-excited white-light-emitting phosphor Sr3La(PO4)3:Eu2+, Mn2+ was synthesized by the solid-state method. Luminescence properties and the energy transfer mechanism were investigated in detail by photoluminescence spectra and decay curves. With the energy transfer between Eu2+ and Mn2+, a cold white light with chromaticity coordinates of (0.2790, 0.2273), correlated color temperature of 6501 K, Ra of 70, and external quantum efficiency of 35.5% was realized by changing the ratios of Eu2+ and Mn2+ in the Sr3La(PO4)3:Eu2+, Mn2+ phosphors. Resonant energy transfer from Eu2+ to Mn2+ ions has been demonstrated to be a dipole-dipole mechanism in Sr3La(PO4)3. The energy transfer efficiency increases with Mn2+ concentration increasing, and reaches a maximum of 55.6%.

Ultrabroadband polarization splitter based on three-core photonic crystal fiber with a modulation core.

We design an ultrabroadband polarization splitter based on three-core photonic crystal fiber (PCF). A modulation core and two fluorine-doped cores are introduced to achieve an ultrawide bandwidth. The properties of three-core PCF are modeled by using the full-vector finite element method along with the full-vector beam propagation method. Numerical results demonstrate that an ultrabroadband splitter with 320 nm bandwidth with an extinction ratio as low as -20 dB can be achieved by using 52.8 mm long three-core PCF. This splitter also has high compatibility with standard single-mode fibers as the input and output ports due to low splicing loss of 0.02 dB. All the air holes in the proposed structure are circular holes and arranged in a triangular lattice that makes it easy to fabricate.

Design of dual-core optical fibers with NEMS functionality.

An optical fiber with nano-electromechanical functionality is presented. The fiber exhibits a suspended dual-core structure that allows for control of the optical properties via nanometer-range mechanical movements. We investigate electrostatic actuation achieved by applying a voltage to specially designed electrodes integrated in the cladding. Numerical and analytical calculations are preformed to optimize the fiber and electrode design. Based on this geometry an all-fiber optical switch is investigated; we find that optical switching of light between the two cores can be achieved in a 10 cm fiber with an operating voltage of 35 V.

Nanomechanical Optical Fiber with Embedded Electrodes Actuated by Joule Heating.

Nanomechanical optical fibers with metal electrodes embedded in the jacket were fabricated by a multi-material co-draw technique. At the center of the fibers, two glass cores suspended by thin membranes and surrounded by air form a directional coupler that is highly temperature-dependent. We demonstrate optical switching between the two fiber cores by Joule heating of the electrodes with as little as 0.4 W electrical power, thereby demonstrating an electrically actuated all-fiber microelectromechanical system (MEMS). Simulations show that the main mechanism for optical switching is the transverse thermal expansion of the fiber structure.

Nanomechanical optical fiber.

Optical fibers are an excellent transmission medium for light and underpin the infrastructure of the Internet, but generally after fabrication their optical properties cannot be easily modified. Here, we explore the concept of nanomechanical optical fibers where, in addition to the fiber transmission capability, the internal core structure of the fiber can also be controlled through sub-micron mechanical movements. The nanomechanical functionality of such fibers is demonstrated in the form of dual core optical fibers, in which the cores are independently suspended within the fiber. The movement-based optical change is large compared with traditional electro-optical effects and we show that optical switching of light from one core to the other is achieved through moving one core by just 8 nm.