Core Of Single-mode Fiber Research Articles

Highly sensitive optical fiber bending sensor based on hollow core fiber and single mode fiber

We demonstrate a highly sensitive Fabry–Perot interferometer bending sensor based on vernier effect. The sensor device consists of two cascaded fabry-Perot cavities, one is an air-cavity formed by a section of hollow core fiber spliced with single-mode fiber, and the other is formed by two mirrors written in single-mode fiber by femtosecond laser. By accurately controlling the cavity length of the Fabry–Perot interferometer in single mode fiber, achieved by using femtosecond laser micromachining platform, a large and controllable amplification factor can be obtained. The bending sensitivity of our device is ∼52.877 nm/m−1. Such a device is simple in structure, easy in fabrication and stable in operation.

Identifications and classifications of human locomotion using Rayleigh-enhanced distributed fiber acoustic sensors with deep neural networks

This paper reports on the use of machine learning to delineate data harnessed by fiber-optic distributed acoustic sensors (DAS) using fiber with enhanced Rayleigh backscattering to recognize vibration events induced by human locomotion. The DAS used in this work is based on homodyne phase-sensitive optical time-domain reflectometry (φ-OTDR). The signal-to-noise ratio (SNR) of the DAS was enhanced using femtosecond laser-induced artificial Rayleigh scattering centers in single-mode fiber cores. Both supervised and unsupervised machine-learning algorithms were explored to identify people and specific events that produce acoustic signals. Using convolutional deep neural networks, the supervised machine learning scheme achieved over 76.25% accuracy in recognizing human identities. Conversely, the unsupervised machine learning scheme achieved over 77.65% accuracy in recognizing events and human identities through acoustic signals. Through integrated efforts on both sensor device innovation and machine learning data analytics, this paper shows that the DAS technique can be an effective security technology to detect and to identify highly similar acoustic events with high spatial resolution and high accuracies.

Single-mode output by controlling the spatiotemporal nonlinearities in mode-locked femtosecond multimode fiber lasers

The performance of fiber mode-locked lasers is limited due to the high nonlinearity induced by the spatial confinement of the single-mode fiber core. To massively increase the pulse energy of the femtosecond pulses, amplification is obtained outside the oscillator. Recently, spatiotemporal mode-locking has been proposed as a new path to fiber lasers. However, the beam quality was either highly multimode and/or with low pulse energy. Here we present an approach to reach high energy per pulse directly in the mode-locked multimode fiber oscillator with a near single-mode output beam. Our approach relies on spatial self-beam cleaning via the nonlinear Kerr effect and we demonstrate a multimode fiber oscillator with M2<1.13 beam profile, up to 24 nJ energy and sub-100 fs compressed duration. The reported approach is further power scalable with larger core sized fibers and could benefit applications that require high power ultrashort lasers with commercially available optical fibers.

Reel-to-Reel Fabrication of In-Fiber Low-Loss and High-Temperature Stable Rayleigh Scattering Centers for Distributed Sensing

This paper presents a method of reel-to-reel femtosecond laser direct writing that enables the continuous inscription of low-loss and high-temperature stable Rayleigh scattering centers inside the core of single-mode optical fibers for distributed temperature sensing up to 1000°C. By examining the correlation between the Rayleigh backscattering profile and the cross-section morphology of femtosecond laser-induced nanograting in fiber cores, this paper reveals the mechanisms that underlie the fabrication of high-temperature stable distributed fiber sensors with low loss. By fine-tuning laser exposure conditions, the femtosecond laser-fabricated Rayleigh scattering enhanced section could achieve an optimized propagation loss of 0.01 dB/cm with an increased signal-to-noise ratio of over 35 dB for meters of lengths. Long-term high-temperature stability of the Rayleigh scattering enhanced section was successfully demonstrated, with improved thermal stability and signal-to-noise ratio. The fabrication method presented here provides a promising technique to improve the performance of Optical Frequency-Domain Reflectometry based distributed sensing for harsh environment applications.

In-fiber wavelength-selective reflector based on Y-junction coupled whispering gallery mode resonator

An in-fiber wavelength-selective reflector based on a compact Y-junction coupled whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The Y-junction used to simultaneously excite and collect the WGM field through evanescent field coupling is fabricated by femtosecond laser ablation, where the common port of the Y-junction connects to the original single-mode fiber core. As a proof of concept, a microsphere functioning as a WGM cavity is embedded into a femtosecond laser ablated hole in the middle of the two branches of the Y-junction. A reflection passband with a full width at half maximum (FWHM) less than 0.1 nm is experimentally achieved, corresponding to a quality factor (Q-factor) on the order of 104. Such a highly integrated in-fiber WGM reflector holds the potential for remote sensing with high resolution or providing narrow band optical feedback to lasers.

Ultrahigh-sensitivity label-free optical fiber biosensor based on a tapered singlemode- no core-singlemode coupler for Staphylococcus aureus detection

An ultra-high sensitivity label-free optical fiber biosensor for inactivated Staphylococcus aureus (S. aureus) detection is proposed and investigated in this study, with additional advantages of robust and stability compared to traditional tapered fiber structure. The proposed fiber biosensor is based on a tapered singlemode- no core-singlemode fiber coupler (SNSFC) structure, where the no core fiber was tapered to small diameter (taper-waist diameter of about 10 μm) and functionalized with the pig immunoglobulin G (IgG) antibody for detection of S. aureus. The measured maximum wavelength shift of the sensor for an S. aureus concentration of 7 × 101 CFU/mL (colony forming unit per milliliter) is 2.04 nm, which is equivalent to a limit of detection (LOD) of 3.1 CFU/mL (a highest LOD reported so far for optical fiber biosensors), considering the maximum wavelength variation of the sensor in phosphate buffered saline (PBS) is ±0.03 nm over 40 min, where 3 times of maximum wavelength variation (3 × 0.03 = 0.09 nm) is defined as measurement limit. The response time of the developed fiber sensor is less than 30 min. The ultra-sensitive biosensor has potential to be widely applied to various areas such as disease, medical diagnostics and food safety inspection.

Novel Microfiber Sensor and Its Biosensing Application for Detection of hCG Based on a Singlemode-Tapered Hollow Core-Singlemode Fiber Structure

A novel microfiber sensor is proposed and demonstrated based on a singlemode-tapered hollow core-singlemode (STHS) fiber structure. Experimentally a STHS with taper waist diameter of $26.5~\mu \text{m}$ has been fabricated and RI sensitivity of 816, 1601.86, and 4775.5 nm/RIU has been achieved with RI ranges from 1.3335 to 1.3395, from 1.369 to 1.378, and from 1.409 to 1.4175 respectively, which agrees very well with simulated RI sensitivity of 885, 1517, and 4540 nm/RIU at RI ranges from 1.3335 to 1.337, from 1.37 to 1.374, and from 1.41 to 1.414. The taper waist diameter has impact on both temperature and strain sensitivity of the sensor structure: (1) the smaller the waist diameter, the higher the temperature sensitivity, and experimentally 26.82 pm/°C has been achieved with a taper waist diameter of $21.4~\mu \text{m}$ ; (2) as waist diameter decrease, strain sensitivity increase and 7.62 pm/ $\mu \varepsilon $ has been achieved with a taper waist diameter of $20.3~\mu \text{m}$ . The developed sensor was then functionalized for human chorionic gonadotropin (hCG) detection as an example for biosensing application. Experimentally for hCG concentration of 5 mIU/ml, the sensor has 0.5 nm wavelength shift, equivalent to limit of detection (LOD) of 0.6 mIU/ml by defining 3 times of the wavelength variation (0.06 nm) as measurement limit. The biosensor demonstrated relatively good reproducibility and specificity, which has potential for real medical diagnostics and other applications.

Polymer Microlens on Pillar Grown From Single-Mode Fiber Core for Silicon Photonics

A novel coupling device composed of a combination of microlens and pillar having an approximately $8~\mu \text{m}$ diameter and $20~\mu \text{m}$ height and grown from single-mode fiber (SMF) core end is proposed. This device was successfully fabricated by using a self-written waveguide and dipping method with UV-curable resin. Theoretical and experimental studies were done. A high coupling efficiency of more than −0.97dB was obtained for coupling to an ultrahigh numerical aperture (0.41) SMF with a spot diameter of $3.7~\mu \text{m}$ at a $1.55~\mu \text{m}$ wavelength in place of a silicon chip with an integrated spot-size converter. In contrast, it is only −4.0dB by a conventional SMF.

基于单模光纤传输的单模-无心-单模光纤型表面等离子体共振传感器（特邀）

基于单模光纤传输的单模-无心-单模光纤型表面等离子体共振传感器（特邀）

A Compact Refractometer With High Sensitivity Based on Multimode Fiber Embedded Single Mode-No Core-Single Mode Fiber Structure

A novel refractometer based on multimode fiber (MMF) embedded single mode-no core-single mode (ME-SNS) fiber structure is presented and investigated in detail. The ME-SNS structure has a higher refractive index (RI) sensitivity because of the stronger evanescent field generated by RI modulation. There is an obvious attenuation dip when 3 segment MMFs are embedded in single mode-no core-single mode (SNS) fiber structure. Total length of sensing unit is only 1.8 mm. Experimental results show that the maximal RI and the temperature sensitivity of the ME-SNS fiber structure are 567.158 nm/RIU and 0.009 nm/°C.

All-fiber-optic vector magnetic field sensor based on side-polished fiber and magnetic fluid.

A kind of compact all-fiber-optic vector magnetic sensor is proposed and demonstrated. The sensor consists of a side-polished-fiber (SPF)-integrated with singlemode-no core-singlemode (SNS) fiber structure. A section of side-polished fiber breaks the axially symmetry of the composite structure. The as-fabricated sensor supports vector sensing and has a magnetic field strength sensitivity of up to -2370 pm/mT over 2-6 mT range. The physical mechanism is that the modal interference is strongly influenced by the refractive index (RI) near the side-polished surface. The advantages of the proposed sensor lie in low cost, simple structure and easy manufacture, which make it attractive in the field of magnetic field vector sensing.

Yb3+ doped silica nanostructured core fiber laser.

We report development of ytterbium doped silica fiber with nanostructured core for laser applications. We study influence of non-continuous distributed Yb dopants on gain, beam quality, and fiber laser performance. The fiber core is composed of over 43 thousand nanorods with a central part doped with Yb. The diameter of each nanorod is 72 nm. With this method we obtained a flat refractive index profile with uniformity of 1.3 × 10-4 refractive index unit (RIU) despite the non-uniformity of 1.2 × 10-3 RIU in Yb doped preform rods used for the fiber development. We demonstrate a nanostructured core single-mode fiber laser with 61.8% of slope efficiency, and extremely low numerical aperture 0.027 of generated mode.

Expanded-Beam Backside Coupling Interface for Alignment-Tolerant Packaging of Silicon Photonics

We demonstrate an alignment-tolerant backside coupling interface in the O-band for silicon photonics by generating an optimized through-substrate (downward) directionality beam from a TE-mode grating coupler and hybrid integrating the chip with backside silicon microlenses to achieve expanded beam collimation. The key advantage of using such an expanded beam interface is an increased coupling tolerance to lateral and longitudinal misalignment. A 34 $\mu$ m beam diameter was achieved over a combined substrate thickness of 630 $\mu$ m which was then coupled to a thermally expanded core single-mode fiber to investigate the tolerances. A 1-dB fiber-to-microlens lateral alignment tolerance of 14 $\mu$ m and an angular alignment tolerance of $1^{\circ }$ was measured at a wavelength of 1310 nm. In addition, a large $\pm$ 2.5 $\mu$ m 1-dB backside alignment accuracy was measured for the placement of microlens with respect to the grating. The radius of curvature of Si microlens to achieve a collimated beam was 480 $\mu$ m, and a 1-dB longitudinal alignment tolerance of 700 $\mu$ m was measured for coupling to a single-mode expanded core fiber. The relaxation in alignment tolerances make the demonstrated coupling interface suitable for chip-to-package or chip-to-board coupling

A Mach–Zehnder Interferometer Based on a No-Core Fiber With In-Fiber Waveguides

A Mach-Zehnder interferometer constructed by femtosecond-laser written in-fiber waveguides in no-core fiber (NCF) is demonstrated. Such an interferometer device consists of a section of the NCF in which a waveguide has been inscribed, fusion spliced with single-mode fiber at its two ends. One of the interferometer arms is formed by the waveguide inscribed and the other follows the diverging light path inside the NCF before arriving at the end of single-mode fiber core. Such an internal structure-based device can be used not only for strain sensing but also for external refractive index measurement.

High Sensitivity Refractometer Based on a Tapered-Single Mode-No Core-Single Mode Fiber Structure.

We have proposed a novel tapered-single mode-no core-single mode (TSNS) fiber refractometer based on multimode interference. The TSNS structure exhibits a high contrast ratio (>15 dB) and a uniform interference fringe. The influence of different lengths and diameters of the TSNS on the refractive index unit (RIU) sensitivity was investigated. The experimental investigations indicated a maximum sensitivity of 1517.28 nm/RIU for a refractive index of 1.417 and low-temperature sensitivity (<10 pm/°C). The experimental and simulation results are also in good agreement.

Modeling optical fiber space division multiplexed quantum key distribution systems.

We report a model to use to evaluate the performance of multiple quantum key distribution (QKD) channel transmission using spatial division multiplexing (SDM) in multicore (MCF) and few-mode fibers (FMF). This model is then used to analyze the feasibility of QKD transmission in 7-core MCFs in two practical scenarios involving the (1) transmission of only QKD channels and (2) simultaneous transmission of QKD and classical channels. In the first case, standard homogeneous MCFs enable transmission distances per core compatible with transmission parameters (distance and net key rate) very close to those of single-core single-mode fibers. For the second case, heterogeneous MCFs must be employed to make this option feasible.

Theoretical and experimental study of artificially controlled backscattering fiber using femtosecond laser fabrication

An artificially controlled backscattering (ACBS) fiber is theoretically proposed and experimentally verified using a femtosecond (fs) laser micromachining system. The backscattering power of the ACBS fiber per unit length is similarly defined as the one of the single mode fiber (SMF). We have calculated the relative reflecting power per unit length from a fiber with a reflector fabricated on an SMF core at 5576 m by the fs laser beam. The impact of the pulse energy of the fs laser and the reflector position on the performance of the ACBS fiber are then compared via observing the backscattering trace. The results show that the proposed ACBS fiber can enhance the backscattering of the SMF. High backscattering fibers (HBSFs) can be achieved via appropriate selection of fabrication parameters, offering an efficient way to shorten the fiber length and form a random distributed feedback fiber laser.

Temperature-Independent Micro-Refractometer Based on Cascaded In-Fiber Air Cavities With Strain-Error Correction

We proposed a new fabrication technology of in-fiber air-cavity by splicing single-mode fiber and hollow core fiber (HCF) filled with the glycerol solution in this paper. A fiber-optic interferometer composed of two cascaded ellipsoidal in-fiber air cavities was fabricated by the above-mentioned method. The two air cavities are the mode splitter and the mode combiner of the transmission-type multimode interferometer (MMI), respectively. The collapsed HCF with the length of $973~\mu \text{m}$ between them can be used as the refractive index (RI) sensing area of the MMI, and the temperature sensitivity of the MMI can be ignored compared with the RI sensitivity. Owing to the RI insensitivity and temperature insensitivity, the air cavity with the minimum wall thickness of $6.2~\mu \text{m}$ is used as the reflection-type Fabry–Perot interferometer to compensate the strain of the MMI. Research shows that the sensor can eliminate the effects of temperature and strain in the measurement of RI.

Multiple off-axis fiber Bragg gratings for 3D shape sensing.

Point-by-point femtosecond laser processed fiber Bragg gratings are arranged around the edge of a standard single-mode optical fiber core. The relative amplitudes of at least three such fiber Bragg gratings are utilized to detect the central position of the mode field within the fiber core and calculate the local curvature of the fiber. An analytical approximation is given, and an experimental validation is performed.

A bend-resistant low bending loss and large mode area two-layer core single-mode fiber with gradient refractive index ring and multi-trench

Abstract We propose a bend-resistant large mode area single-mode fiber with low bending loss. The fiber consists of three parts, including a two-layer core, a gradient refractive index ring and multi-trench. Performance of the proposed fiber is investigated by using a finite element method at 1550 nm wavelength. Large mode area of 2622 μm2 at a bend radius of 20 cm can be achieved with the function of the two-layer core and the gradient refractive index ring. The gradient refractive index ring acts as a resonant ring, which also contributes to bend-resistant performance. Bending loss can be reduced to about 0.092 dB/m due to the multi-trench in the cladding, which is also beneficial to improve transmission efficiency. Moreover, the fiber can ensure single-mode operation by exploiting high suppression of the higher order modes. The bending direction has no effect on the performance of the fiber because of the symmetric design.