Core Of Single-mode Fiber Research Articles

In-fiber optofluidic michelson interferometer for detecting small volume and low concentration chemicals with a fiber ring cavity laser

An in-fiber optofluidic Michelson interferometer fabricated by femtosecond laser micromachining technology was presented. The interferometer structure was prepared by a special optical fiber with two cores and a microchannel. One core was connected to the core of a single-mode fiber (SMF) and functioned as a sensing arm on the microchannel inner wall, the other core worked as a reference arm and was connected to the optical waveguide introduced by femtosecond laser processing. By combining with a ring cavity laser, the detection limit (DL) of the interferometer could be improved. The laser sensor presented a high linear response with a refractive index (RI) sensitivity of 1039.77 nm/RIU and the calculated DL could reach as low as 1.675 × 10 −5 RIU. Then, the sensor measured different concentrations of vitamin C in tablets in the concentration range of 0–10 mg/mL with a detection limit of 0.06 mg/mL. Due to the advantages of optofluidic structure, the volume of the detection solution could be as low as few tens of nanoliters, and the time to replace and detect the solution only took a few seconds. In addition, the laser sensor realized label-free DNA hybridization detection through surface functionalization, and could detect 1 μM target DNA solution with high specificity. This sensing device had great potential for small volume and low concentration sample detection in the biochemical field. • An in-fiber optofluidic Michelson interferometer for detecting small volume and low concentration chemicals is presented. • The interferometer structure is prepared by a special microstructured optical fiber with two cores and a microchannel. • One core is connected with the SMF core as a sensing arm on the microchannel inner wall. • The other core is connected with an optical waveguide from the SMF core as a reference arm in the cladding. • Detection limit can reach as low as 0.06 mg/mL.

Optical fiber humidity sensor with C60-THAM as molecule receptors

This study proposed a high sensitivity optical fiber relative humidity sensor (OFHS). Besides, a fullerene derivative, C 60 -THAM, was synthesized and firstly reported as a molecule receptor in the OFHS. Compared with the widely researched carbon-based materials in OFHS, the C 60 -THAM film has abundant amino dangling bonds, hence emerges naturally as excellent receptors for water molecules. Besides, a cascaded Fabry-Perot interferometer (FPI) was fabricated in the core of single-mode fiber (SMF) without surface damage by femtosecond laser. In the OFHS, the relative humidity (RH) was measured by the Vernier effect. The working principle of the OFHS was analyzed through calculation and simulation. In the experiment, C 60 -THAM was coated on the plant fiber end. The sensitivity of the sensor is about 0.42 nm/RH as the relative humidity change from 30% to 65% while the optical-matter reaction length is only about 150 nm. Moreover, the sensor exhibited a stable performance as the RH was decreasing from 65% to 30%. The OFHS in this work is fast response and the response time for the rising and decay process was about 2.2 s and 3.2 s respectively. • A fullerene derivative, C 60 -THAM, was synthesized and its application in the fiber humidity sensor was researched. • The humidity sensor is fast response and the rising and decay time is about 2.2 s and 3.2 s respecitively. • The sensitivity is 0.42 nm/RH as the humidity change from 30% to 65% while the optical-matter reaction length is 150 nm.

Randomly-Coupled Multi-Core Fiber Technology

Randomly-coupled multi-core fiber (MCF) technology has come to attract lots of attention because of its strong applicability to long-haul transmission systems. Compared with weakly-coupled MCFs with independent cores, it can simultaneously realize higher spatial channel density and ultralow transmission loss using existing ultralow-loss single-mode fiber (SMF) core designs. The strong mode coupling characteristics of randomly-coupled MCFs can provide favorable optical properties, such as suppressed accumulation of modal dispersion (MD), mode-dependent loss (MDL), and nonlinear impairments. This article gives an overview of randomly-coupled MCF technology advancements. First, we describe the classification and design of randomly-coupled MCFs and explain what the randomly-coupled MCFs are and how they are designed. State-of-the-art randomly-coupled MCFs can accommodate four, seven, or 12 cores in a standard 125- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> cladding while achieving ultralow transmission loss and/or small MD, which are very promising for long-haul transmission media. Next, we present the methods to characterize the optical properties of randomly-coupled MCFs and the difference compared to conventional SMF measurements. We also show the low-loss low-MDL connectivity of this type of MCF and the cabling that can suppress MD. A field-deployed randomly-coupled MCF cable testbed is also presented, which confirmed the favorable optical properties of randomly-coupled MCFs after deployment. Then, multi-core amplifier technologies are briefly summarized, and finally, we discuss the performance improvements in the transmissions over randomly-coupled MCFs and suitable application areas.

Ultrahigh-sensitivity label-free singlemode- tapered no core-singlemode fiber immunosensor for Listeria monocytogenes detection

A challenge for optical fiber biosensor is to achieve ultrahigh sensitivity with narrow full width at half maximum (FWHM) of the spectrum. To address this challenge, an ultrahigh-sensitivity microfiber interferometer fiber ring laser (FRL) biosensor is proposed and investigated for Listeria monocytogenes (L. monocytogenes) detection. The fiber biosensor is composed of a singlemode- tapered no core-singlemode (STNS) fiber configuration, which is functionalized with the anti-L. monocytogenes antibodies. An Erbium Doped Fiber Amplifier is applied to the sensor to excite laser and thus reduce the FWHM of the spectrum, which significantly improved the limit of detection (LoD). The proposed STNS FRL biosensor has excellent reproducibility, specificity and sensitivity for L. monocytogenes. The developed STNS FRL biosensor can directly detect L. monocytogenes cells with LoD as low as 1.0 cell/mL, indicating the capability for detecting single cell of L. monocytogenes. Real lettuce and milk samples have been tested and test result in lettuce and milk samples has deviations within ± 30 % from that of Phosphate-buffered saline (PBS) for L. monocytogenes concentrations vary from 101 to 103 cells/mL(g). The developed STNS FRL biosensor has ultrahigh sensitivity, good stability, reproducibility, and specificity, which has potential applications in diseases/medical diagnostics.

A fiber sensor for different curvature measurement based on nano-erbium-ytterbium co-doped fiber cascade structure as both source and sensor

A fiber sensing system based on Nano-Erbium-Ytterbium co-doped fiber (Nano-EYDF) is proposed and demonstrated, where it works as both source and sensor. Especially, a fiber curvature sensor based on Mach-Zehnder interferometer is fabricated in a Nano-EYDF, which is a cascaded structure composed of single-mode fiber (SMF), Nano-EYDF and SMF. Based on the principle of evanescent wave coupling, the light transmitted in the SMF core can be coupled into the Nano-EYDF cladding. Therefore, the Nano-EYDF core mode and cladding mode that satisfy the phase matching conditions can couple evanescent waves to form a double-beam interference. When the external environment changes, the interference spectrum will change, so the curvature sensing can be realized by observing the wavelength shift. The experimental results show when the length of the Nano-EYDF is 20 mm and 25 mm, the transmission spectrum of the sensor appears blue shift in the downforce curvature range of 0–1.76 m−1, and the curvature sensitivity is −9.763 nm/m−1 and −12.787 nm/m−1 respectively. In the propulsion curvature range of 0–0.94 m−1, the transmission spectrum exhibits blue shift, and the curvature sensitivity can be up to −9.261 nm/m−1 and −9.411 nm/m−1, respectively. The sensor is easily fabricated and has high sensitivity, and demonstrates good application prospects in curvature measurement fields such as 3D shape reconstruction and geological monitoring.

Ultra-short phase-shifted fiber Bragg grating in a microprobe for refractive index sensor with temperature compensation

Ultra-short phase-shifted fiber Bragg grating (PS-FBG) composed of cascaded microchannels is proposed and demonstrated experimentally. The cascaded microchannels are fabricated based on femtosecond (fs) laser micromachining technology in the microprobe and a fiber Bragg grating (FBG) is written in the core of single-mode fiber (SMF) near the microprobe. Ultra-short PS-FBG is sensitive to both refractive index (RI) and temperature, while FBG is only sensitive to temperature, so FBG can be used to compensate temperature crosstalk. The length of the microprobe is less than 150 μm and its diameter is about 40 μm, and the total length of the ultra-short PS-FBG is less than 95 μm, which is about one fiftieth of the traditional FBG. Experimental results show that the RI sensitivity of the PS-FBG is 463.7953 nm/RIU in the range of 1.3323 ∼ 1.3797, the temperature sensitivity is about −28.9 pm/°C from 40 °C to 80 °C, and the temperature crosstalk can be well suppressed based on the transfer matrix method.

Mach-Zehnder-Sagnac interferometric curvature sensor based on polarization-maintaining multi-mode fiber using a specified core offset.

An interferometric bending sensor based on polarization-maintaining fiber (PMF) in combination with multi-mode fiber (MMF) is proposed and demonstrated experimentally. One end of MMF was fusion spliced to PMF, and the other was fusion spliced to the single mode fiber (SMF) pigtail of a 3 dB optical fiber coupler with a core offset. One stress zone in lead-out Panda type PMF was intentionally aligned and fusion spliced to the core of SMF to enhance mode coupling. A pronounced interference pattern with a fringe contrast of 28.1 dB was obtained, and a bending sensitivity of 18.66nm/m-1 was achieved experimentally. Such a bending sensor with easy fabrication could be a potential candidate for curvature measurements.

In-Fiber Integrated Quasi-Distributed Temperature Sensor Array With High Spatial Resolution for Silicon Nitride Igniter

As a key component in the engine system, silicon nitride igniter’s stable and reliable operation is a necessary condition for the proper functioning of the engine system. However, an online high temperature sensor of silicon nitride igniter inside the engine is absent. This paper presents and demonstrates a novel in-fiber integrated high temperature sensor array with high spatial resolution for online temperature field measurement on a silicon nitride igniter. The sensor array with a series of weak reflectors is written by a femtosecond laser in the core of single-mode fiber. The reflectivity of each reflector can be controlled from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−8</sup> by adjusting the written power of femtosecond laser. Two adjacent weak reflectors form a compact fiber sensor. The size of a single sensor can be as small as 3–5 mm, so it is able to measure the temperature fluctuation in a space down to several millimeters. The five-sensor-array, with the help of a white light interference demodulation system, can measure high temperature up to 1000°C. Due to the advantages of high reliability, compact construction and simple fabrication, this high temperature sensor array can be widely applied in high temperature sensing applications with space constrains.

Highly sensitive bending sensor based on Mach-Zehnder interferometer built in Sagnac loop with specified offset

A high sensitivity bending sensor based on Mach-Zehnder interferometer (MZI) built in Sagnac loop is proposed and experimentally demonstrated. One stress zone in Panda type polarization maintaining fiber (PMF) was intentionally collimated to the core of single mode fiber (SMF) and then the discharge was performed by fusion splicer. A commercial fiber Bragg grating (FBG) acting as phase shift monitor was fusion spliced to the PMF. The prepared fiber optical configuration was connected to a 3 dB coupler to construct a Sagnac loop. The spectral characteristics of the proposed bending sensor as a function of curvature were investigated experimentally. The bending sensitivity of 12.5 nm/m−1 was obtained experimentally. Furthermore, the π phase shift was detected when the curvature was 0.587 m−1. Such bending sensor with simple fabrication and cost-efficient could be a good candidate in the field of curvature measurements.

Numerical Simulation of the Self-Imaging at Different Cascaded Optical Fiber Specifications

Cascaded optical fiber single mode-no core-single mode fiber (SNS) attracted attention for being the base of various photonic devices. These devices are used in optical communication, fiber sensors, and fiber laser technology. The effect of variable NCF specifications, length, diameter, external refractive index (ERI), propagating wavelength on the self-imaging position, and the multimode interference (MMI) is studied. The study aims to simulate and analyze cascaded optical fiber by using the finite element beam envelope method (BEM). To the best of our knowledge, this is the first report that studied the self-imaging in cascaded optical fiber longitudinally by using BEM. The NCF length is important in determining the coupled out intensity and peak transmission wavelength. The field in the cascaded fiber is simulated for single and multi-wavelengths to evaluate the maximum transmission and study the structure's tunability. A tunable filter is simulated, where varying the length of the NCF about 0.6 mm produces a wavelength shift of about 40 nm. The BEM is effective in studying the field propagation in large guiding photonic devices

Filament-arrayed Bragg gratings for azimuthally resolved displacement sensing in single-mode fibers.

Filament arrays were inscribed off-axis in the core of standard single-mode telecommunication fiber, using femtosecond laser pulses. The flexible line-by-line writing formed uniform, parallel filaments, permitting Bragg grating sensing of the photoelastic response from inside of the narrow grating plane. Active monitoring of the Bragg resonance wavelength while driving a lateral fiber tip displacement directly informed on the fiber mechanics when coupled with opto-mechanical modelling. Overlaying of parallel and orthogonal gratings further provided a strongly contrasting azimuthal sensitivity, which paves the way for multi-dimensional displacement sensing with improved precision.

Long-Period Grating Based Coupler for Multi-Core Fiber Systems

In this work, we propose and demonstrate numerically a long-period grating (LPG) based technique to couple light from a single-mode fiber (SMF) to all cores of a multi-core fiber (MCF), i.e., an SMF to MCF coupler. The light launched into the SMF core is coupled to the SMF cladding due to the LPG inscribed in the core. Then, the optical power is transferred between fibers claddings, enhanced by the reduction of the SMF cladding radius. Finally, the MCF cladding optical power is distributed by all cores of the MCF due to identical LPGs inscribed in them. We use the coupled-mode theory to study and design the proposed device. We achieve a coupling efficiency of approximately 90% of the input power in a total length of 15.1 cm. We also study the coupler's sensitivity to the SMF radius and LPGs period. Results show that the proposed device can improve the pumping efficiency of MCF amplifiers.

Optical fiber fabry-perot sensor based on a singlemode-hollow core-singlemode fiber structure for direct detection of phase transition in n-octadecane

An optical fiber Fabry-Perot sensor to monitor the solid-liquid and liquid-solid phase changes in n-octadecane is investigated. The sensor probe is fabricated by splicing a short section of a hollow core fiber between two single-mode fibers. Light from a broadband source launched into the probe experiences multiple reflections as it travels towards the cleaved end of the probe immersed in the material sample and then back to the detector, creating an output interference spectrum with multiple interference dips. The phase change of the material sample in the vicinity of the fiber probe results in changes of various parameters acting upon the probe (such as temperature, strain or bending) which in turn cause changes in the output interference spectrum. By analyzing the changes in the output spectrum of the probe, such as spectral shift of a selected interference dip, the phase change within a material sample in the vicinity of the fiber probe can be accurately detected. This work is useful for providing a better understanding of the phase change mechanism in phase-change materials (PCMs) and for in-situ phase change detection in PCM-based energy storage systems.

Broadband mode-selective couplers based on tapered side-polished fibers.

We propose the broadband mode-selective coupler (MSC) formed with a side-polished six mode fiber (6MF) and a tapered side-polished small core single-mode fiber (SC-SMF) or an SMF. The MSCs are designed to allow the LP01 mode in the SC-SMF and SMF to completely couple to the LP01, LP11, LP21, LP02, LP31, LP12 modes in the 6MF over a broadband wavelength range. The phase-matching conditions of the MSCs are satisfied by tapering the SC-SMF and SMF to specific diameters. The tapered fibers are side-polished to designed residual fiber thickness using the wheel polishing technique. The effective indices of the side-polished fibers are measured with the prism coupling method. The MSCs provide high coupling ratio and high mode purity. High coupling efficiencies in excess of 81% for all the higher-order modes are obtained in the wavelength range 1530-1600 nm. For the LP01, LP11, LP21, LP02, LP31, LP12 MSCs at 1550 nm, the coupling ratios are 96.2%, 99.8%, 89.5%, 85.0%, 90.9%, 96.1%, respectively, and the mode purity of the MSCs is higher than 88.0%. The loss of the MSCs is lower than 1.8 dB in the wavelength range 1530-1600 nm. This device can be applied in broadband mode-division multiplexing transmission systems.

The effect of fusion current on thermally diffused expanded core of splicing ultra-low loss fiber and traditional single-mode fiber

Ultra-low loss (ULL) optical fiber is already being used at high altitude in optical communication due to the stable performance in long haul links. Whereas, traditional single-mode optical fiber (SMF) is still used in most general areas since the high price of ULL fiber. Accordingly, there will be an increasing number of junctions between SMF and ULL fibers. The main contents of this work are to study the influence of fusion current on the splice loss caused by thermally diffused expanded core (TEC) of splicing SMF and ULL fibers, and the variations on the diffusion of core dopants and fusion time are also taken into account. Furthermore, the analysis was verified by experiment and simulation. The results illustrate that the changes in fusion current will cause varying degrees of changes in the core and lead to different splice losses due to diffusion, which plays a guidance role in the current calibration of fusion splicer in the future when splicing SMF and ULL fibers. The significance of this work is to reduce the splice loss of SMF and ULL fibers. If the method is applied in a modern fusion splicer, the splice loss of SMF and ULL fibers will be deduced further.

Monolithic integration of microlenses on the backside of a silicon photonics chip for expanded beam coupling.

To increase the manufacturing throughput and lower the cost of silicon photonics packaging, an alignment tolerant approach is required to simplify the process of fiber-to-chip coupling. Here, we demonstrate an alignment-tolerant expanded beam backside coupling interface (in the O-band) for silicon photonics by monolithically integrating microlenses on the backside of the chip. After expanding the diffracted optical beam from a TE-mode grating through the bulk silicon substrate, the beam is collimated with the aid of microlenses resulting in an increased coupling tolerance to lateral and longitudinal misalignment. With an expanded beam diameter of 32 μm, a ±7 μm lateral and a ±0.6° angular fiber-to-microlens 1-dB alignment tolerance is demonstrated at the wavelength of 1310 nm. Also, a large 300 μm longitudinal alignment tolerance with a 0.2 dB drop in coupling efficiency is obtained when the collimated beam from the microlens is coupled into a thermally expanded core single-mode fiber.

Characterization of Multimode Interference Based Optical Fiber

In this paper, the fiber optic cascade based on multimode interference (MMI) is demonstrated and investigated via COMSOL Multiphysics software. Two different cascades were adopted in this work, namely, single mode–multimode–single mode (SMS) and single mode–No–Core–single mode fiber (SNS). The transmission spectrum tuning range depending on the geometrical dimensions of the midst fiber (MMF and NCF). According to the results, the achieved tuning range due to the diameter fiber changing is higher than that of the length changing. The high tunability about 171 nm is achieved within the SMS cascade for the diameter changes from 61 µm –to– 64 µm and about 340 nm is achieved within the SMS cascade for the length changes from 3.9 mm –to– 4.9 mm. In addition, the increasing in the midst fiber diameter caused a red shift in the peak wavelength of the transmission spectrum. While, the increasing in the fiber length caused a blue shift in the wavelength. The calculated lengths for the NCF show good agreement with results that achieved from the simulation software with a total deviation about 0.46 %. While its about 6.5 % for the MMF, since the cladding refractive index is not considered in the theoretical model.

Single-mode optical fibers coupling: Study of the field of view

Single mode Fiber (SMF) is commonly used in long distance applications as in optical communication. Coupling of the laser beam into SMF core to get high coupling efficiency is a challenging task in practice. The aim of our work is to study the impact of the field of view on the coupled light beam, taking into account well designed coupling lens to be sure that the focal spot has matched to the mode of propagation.Here in this article, a conical lens of 1.5 refractive index was used for light coupling process. The used criteria at HeNe laser of 0.5438 nm wavelength showed a degradation in the output signal due to generated distortion as the field of view increased from 0 to 6 degrees.

High sensitivity strain sensor based enhanced polarized mode coupling and FBG sensitization

A highly sensitive strain sensor based on enhanced polarized mode coupling is proposed and demonstrated experimentally. The proposed Sagnac interferometric strain sensor comprises a segment of Panda-type polarization maintaining fiber (PMF) serving as a sensing component and a commercial fiber Bragg gratings (FBG) serving as a sensitizing component. The sensitization effect of FBG has been analyzed and demonstrated experimentally. The core of single mode fiber is intentionally collimated and fusion spliced to one stress applied zone built-in Panda type PMF to excite effectively the polarized mode. The spectra shift towards long wavelength with increasing strain. The measured strain sensitivity reaches up to 176 pm/με. The proposed fiber strain sensor characterizing simple fabrication and high sensitivity can be a good candidate in the field of strain measurements.

Multiplexable intrinsic Fabry-Pérot interferometers inscribed by femtosecond laser for vibration measurement in high temperature environments

This paper reports fabrication technique and demodulation algorithm developments to use multiplexable intrinsic Fabry-Pérot interferometer (IFPI) fiber sensor array for distributed vibration measurements at high temperatures. Using femtosecond laser direct writing scheme, IFPI array were fabricated through laser-induced Type II scattering points in single mode fiber cores. Reflection spectra of IFPI array were demodulated in real-time using modified Bunemen frequency analysis. The demodulation algorithm, which was implemented using a photodetector-array spectrometer, achieves 64-nε dynamic strain resolution at 1-kHz spectral acquisition rate and 2-kHz maximum vibration bandwidth. Performance and stabilities of IFPI sensor array were characterized from room temperatures to 800 °C. The static strain resolution was 0.6 με and the minimum detectable dynamic strain amplitude was 23 nε/√Hz at 800 °C. The multiplex performance was also tested by measuring dynamic strain in time domain through six IFPI sensor array fabricated in one fiber. This paper presents an integrated and low-cost sensing solution to perform distributed vibration measurements in harsh environments.