Wavelength and sensitivity tunable long period gratings fabricated in fluid-cladding microfibers

We report the fabrication of long period gratings in fluid-cladding microfibers by directly focusing a femtosecond laser beam on the microfibers surface to induce periodical modification a long one side of the microfibers.A long period grating is fabricated in a water-cladding microfiber with a diameter of~5μm,which demonstrates a resonant attenuation of 28.53 dB at wavelength of 1588.1 nm with 10 pitches.When water cladding is changed to be refractive index oil of n=1.33 and alcohol solution with concentration of 5%,the resonance wavelength shifts to 1575.1 nm with resonant attenuation of 24.91 dB and 1594.1 nm with resonant attenuation of 35.9 dB,respectively.The long period grating demonstrates different temperature sensitivities of-0.524 nm/℃,-0.767 nm/℃and-1.316 nm/℃for water,alcohol solution and refractive index oil cladding microfibers,respectively,which means the alterable liquid cladding allows the availability of tunable wavelength and sensitivity.The fluid-cladding protects the microfibers from external disturbance and contamination and allows more flexibility in controlling the transmission property and sensing characteristics of long period gratings,which can be used as fiber devices and sensors for chemical,biological,and environmental applications.

Numerical and experimental analysis of long period gratings in wavelength scale elliptical microfibers

We report the fabrication of long-period gratings （LPGs） in elliptical microfibers with femtosecond laser. Based on the numerical analysis of the modes and the mode coupling condition of elliptical microfibers, an LPG is fabricated with a very short pitch of 10 μm by periodically modifying the fiber surface, which demonstrates very strong polarization-dependent resonances, a very low temperature sensitivity of a few picometers in air, and high temperature sensitivity of -1.62 nm/℃ in refractive index oil.

Broadband tunable Raman soliton self-frequency shift to mid-infrared band in a highly birefringent microstructure fiber

Raman soliton self-frequency shifted to mid-infrared band（λ 〉 2 μm） has been achieved in an air-silica microstructure fiber（MF）. The MF used in our experiment has an elliptical core with diameters of 1.08 and 2.48 μm for fast and slow axis. Numerical simulation shows that each fundamental orthogonal polarization mode has two wide-spaced λZDW and theλZDW pairs located at 701/2110 nm and 755/2498 nm along the fast and slow axis, respectively. Using 810-nm Ti：sapphire femtosecond laser as pump, when the output power varies from 0.3 to 0.5 W, the furthest red-shift Raman solitons in both fast and slow axis shift from near-infrared band to mid-infrared band, reaching as far as 2030 and 2261 nm. Also, midinfrared Raman solitons can always be generated for pump wavelength longer than 790 nm if output pump power reaches0.5 W. Specifically, with pump power at 0.5 W, the mid-infrared soliton in slow axis shifts from 2001 to 2261 nm when the pump changes from 790 nm to 810 nm. This means only a 20 nm change of pump results in 260 nm tunability of a mid-infrared soliton.

Mid-infrared supercontinuum and optical frequency comb generations in a multimode tellurite photonic crystal fiber

We numerically investigate the mid-infrared(MIR)supercontinuum(SC)and SC-based optical frequency comb(OFC)generations when the three optical modes(LP_(01),LP_(02),and LP_(12))are considered in a multimode tellurite photonic crystal fiber(MM-TPCF).The geometrical parameters of the MM-TPCF are optimized to support the multimode propagation and obtain the desired dispersion characteristics of the considered three optical modes.When the pump pulse with center wavelengthλ=2.5μm,width T=80 fs,and peak power P=18 kW is coupled into the anomalous dispersion region of the LP_(01),LP_(02),and LP_(12)modes of the MM-TPCF,the-40-dB bandwidth of the generated MIR SCs can be up to 2.56,1.39,and 1.12 octaves,respectively,along with good coherence.Moreover,the nonlinear dynamics of the generated SCs are analyzed.Finally,the MIR SCs-based OFCs are demonstrated when a train of 50 pulses at 1-GHz repetition rate is used as the pump source and launched into the MM-TPCF.

Study on polarization properties of graphene coated D-shaped fiber

The optical properties of graphene coated D-shaped single mode fiber and photonic crystal fiber are numerically analyzed. Enhancement of the graphene-light interaction is found in graphene coated D-shaped photonic crystal fiber,which introduces a tunable polarization of the D-shaped fiber by changing the chemical potential of the coated graphene.An optimal polarizer model is demonstrated with the extinction ratio of 66.26 dB/mm and the insertion loss of 9.4 dB/mm.The modulator extinction ratios of the TE mode and TM mode are 11.5 dB and 5 dB, respectively, with a device length of100 μm. This paper provides a theoretical reference for the optical property research of the graphene fiber.

A grapefruit microstructure fiber temperature sensor coated with liquid crystal based on waist-enlarged taper

In this paper, a grapefruit microstructure fiber(GMF) temperature sensor coated with liquid crystal(LC) based on waist-enlarged taper is proposed and fabricated, and its temperature sensing characteristics are analyzed. The waist-enlarged taper is formed at the fusion point between single mode fiber(SMF) and GMF. The capillary glass tube is sleeved outside GMF, LC is filled into the capillary glass tube, and its two ends are finally sealed to form a sensor. The experimental results show that when the length of GMF is 2.5 cm, the temperature sensitivity of the sensor can reach up to 195.3 pm/℃ in the range of 30-90℃, and it has a good stability for reuse. Thereby, it can be used in biochemical, industrial production and other temperature detection areas.

A compact electro-absorption modulator based on graphene photonic crystal fiber

A compact electro-absorption modulator based on graphene photonic crystal fiber is proposed. To enhance the graphene–light interaction efficiency, the innermost six air-holes of photonic crystal fiber are replaced by two large semicircular holes, and monolayer graphene is deposited on the two large semicircular holes. By optimizing the structure parameters, a strong graphene–light interaction is obtained. Moreover, the switch on–off point of the modulator is unchangeable,which is only related to the frequency of the incident light. The influence factors of this composite structure have been analyzed. The proposed modulator is compared with other graphene-based modulators, and the results show that it is filled without dielectric spacer. There are some excellent performances, such as an extinction ratio 7 dB of y-polarization mode,3-dB modulation bandwidth of 70 GHz with small footprint of 205 μm, and a consumption of energy per bit 59 pJ/bit.

Tunable Supercontinuum Generated in a Yb^3＋-Doped Microstructure Fiber Pumped by Ti：Sapphire Femtosecond Laser

We experimentally demonstrate that a tunable supercontinuum（SC） can be generated in a Yb3＋-doped microstructure fiber by the concept of wavelength conversion with a Ti：sapphire femtosecond（fs） laser as the pump.Experimental results show that an emission light around 1040 nm in an anomalous dispersion region is first generated and amplified by fs pulses in the normal dispersion region. Then, SC spectra from 1100 to 1380 nm and 630 to 840 nm can be achieved by combined effects of higher-order soliton fission and Raman soliton self-frequency shift in the anomalous dispersion region and self-phase modulation, dispersive wave, and four-wave mixing in the normal dispersion region. It is also demonstrated that the 20 nm change of pump results in a 280 nm broadband shift of soliton and the further red-shift of soliton is limited by OH-absorption at 1380 nm.

Design and research of magnetic field sensor based on grapefruit optical fiber filled with magnetic fluid

A novel magnetic field sensor was proposed based on the grapefruit optical fiber with the magnetic fluid injected into the six air holes.The sensor utilizes the Mach-Zehnder interference(MZI).The light is transmitted through the single-mode fiber to the fusion splice point and is divided into two parts.When the light passes through the cladding air hole with magnetic fluid,it interferes with the core light,and the change of the interference light is related with the change of the magnetic field.The sensor cavity length is 6 cm.It can be obtained from the experimental results that the magnetic field sensitivity reaches up to 2.243 nm/Oe with the range from 0 to 2.28 Oe.The sensor has the advantages of easy fabrication,easy installation and low cost.The findings provide new ideas for the study of ocean wind electromagnetic fields.

Miniature Fabry-Perot interferometric sensor for temperature measurement based on photonic crystal fiber

A novel miniature Fabry-Perot interferometric(FPI) temperature sensor is proposed and demonstrated experimentally. The modal interferometer is fabricated by just splicing a section of photonic crystal fiber(PCF) with a single-mode fiber(SMF). The air holes of the PCF are fully collapsed by the discharge arc during the splicing procedure to enhance the reflection coefficient of the splicing point. The transmission spectra with different temperatures are measured, and the experimental results show that the linear response of 11.12 pm/°C in the range of 30–80 °C is obtained. This sensor has potential applications in temperature measurement field.

A robust salinity sensor based on encapsulated long-period grating in microfiber

A robust fiber sensor for salinity measurement based on encapsulated long-period grating in microfiber is proposed. The long-period grating is fabricated in microfiber by inducing periodical deformation with CO2 laser, which is then encapsulated in a holey capillary tube. The encapsulation tube is designed to effectively protect the microfiber from external interference, but does not change the optical properties of the fiber and the response speed of the sensor, which makes the sensor more robust for real applications. Experimental results show that the sensor can achieve a sensitivity of 2.16 nm/% with a good linearity for concentration from 0% to 20%. It is theoretically proved that the sensitivity can be further improved by optimizing the diameter parameters. Such structure may be used as low loss evanescent-wave-coupled optical absorption, fluorescent and gain cells, photoacoustic cells, and etc.

Micro-Scanning Error Correction Technique for an Optical Micro-Scanning Thermal Microscope Imaging System

An error correction technique for the micro-scanning instrument of the optical micro-scanning thermal microscope imaging system is proposed. The technique is based on micro-scanning technology combined with the proposed second-order oversampling reconstruction algorithm and local gradient image reconstruction algorithm. In this paper, we describe the local gradient image reconstruction model, the error correction technique, down-sampling model and the error correction principle. In this paper, we use a Lena original image and four low-resolution images obtained from the standard half-pixel displacement to simulate and verify the effectiveness of the proposed technique. In order to verify the effectiveness of the proposed technique, two groups of low-resolution thermal microscope images are collected by the actual thermal microscope imaging system for experimental study. Simulations and experiments show that the proposed technique can reduce the optical micro-scanning errors, improve the imaging effect of the system and improve the system's spatial resolution. It can be applied to other electro-optical imaging systems to improve their resolution.

A temperature sensor based on the splicing of a core offset multi-mode fiber with two single mode fibers

In this paper,a temperature sensor based on the splicing of a core offset multi-mode fiber with two single mode fibers is proposed and demonstrated experimentally.The temperature sensing principle is analyzed and related experiment is performed.By controlling the core offset and splicing length of the specialty multi-mode fiber(SMMF),two sensors with different temperature sensing properties are obtained,and experimental results show that the temperature sensitivity can be up to 48.76 pm/℃ in the range of 25-95 ℃.Moreover,it has many advantages,including small size,high sensitivity,and simple structure.So it can be used in potential temperature sensing applications,such as industrial production,biomedical science,power electronics,and so on.

Brillouin scattering spectrum character extraction based on genetic algorithm and seeker optimization algorithm

A new hybrid optimization method based on genetic algorithm(GA)and seeker optimization algorithm(SOA)is presented in this paper.The hybrid algorithm optimizes SOA by using crossover and mutation operations in GA in order to improve the global search ability of SOA.Four algorithms,i.e.particle swarm optimization(PSO),SOA,GA and quantum-behaved particle swarm optimization(GA-QPSO)and GA-SOA are used to process the simulation and experimental data of Brillouin scattering spectrum(BSS)at different temperatures.The results show that GA-SOA improves the accuracy of extracting the center frequency shift and the minimum center frequency of Brillouin scattering spectrum compared with other three algorithms.The shift error is 0.203 MHz.Therefore,GA-SOA can be applied to the accurate extraction of BSS characteristics.

Oversample Reconstruction Based on a Strong Inter-Diagonal Matrix for an Optical Microscanning Thermal Microscope Imaging System

Based on a strong inter-diagonal matrix and Taylor series expansions,an oversample reconstruction method was proposed to calibrate the optical micro-scanning error. The technique can obtain regular 2 ×2 microscanning undersampling images from the real irregular undersampling images,and can then obtain a high spatial oversample resolution image. Simulations and experiments show that the proposed technique can reduce optical micro-scanning error and improve the system＇s spatial resolution. The algorithm is simple,fast and has low computational complexity. It can also be applied to other electro-optical imaging systems to improve their spatial resolution and has a widespread application prospect.

Additive Manufacturing Fiber Preforms for Structured Silica Fibers with Bismuth and Erbium Dopants

Dear Editor Silica optical fibers have attracted a lot of attention because they are widely used in communications and sensing,and forming today’s internet backbone.Driving much of the Internet-of-Things(IoT)evolution,optical fibers are expanding from a single function transmission technology to perform multiple functions and a growing need for various custom-design application-specific optical fibers.Current optical fiber manufacturing based on chemical vapor deposition(CVD)technologies together with stack-and-draw approaches used for structured optical fibers faces numerous challenges in enabling more complex geometries multimaterial composite fibers and multicore fibers.The additive manufacturing,or 3D printing,offers a solution to address all those challenges,and may potentially disrupt the optical fibers fabrication and bring in an evolution to IoT.

High-sensitivity refractive index sensors based on fused tapered photonic crystal fiber

In this paper, a novel liquid refractive index(RI) sensor based on fused tapered photonic crystal fiber(PCF) is proposed. It is fabricated by fusing and tapering a section of PCF which is spliced with two single-mode fibers(SMFs). Due to the fused biconical taper method, the sensor becomes longer and thinner, to make the change of the outside RI has more direct effects on the internal optical field of the PCF, which finally enhances the sensitivity of this sensor. Experimental results show that the transmission spectra of the sensor are red-shifted obviously with the increase of RI. The longer the tapered region of the sensor, the higher the sensitivity is. This sensor has the advantages of simple structure, easy fabrication, high performance and so on, so it has potential applications in RI measurement.

Refractive index insensitive temperature sensor based on waist-enlarged few mode fiber bitapers

A refractive index insensitive temperature sensor based on waist-enlarged few mode fiber(FMF) bitapers is presented. The first section of FMF is spliced between two single-mode fibers. In fusion process,the waist-enlarged FMF bitapers can be obtained by large current discharging repeatedly. The refractive index and temperature sensing mechanisms are analyzed. For the sensors with different sizes,the refractive index and temperature experiments have been performed. The results show that in the refractive index ranges of 1.335 0—1.346 6 and 1.348 2—1.419 3,the refractive index insensitivity is verified. In a temperature range of 31.9—90 °C,the sensor sensitivity can be up to 85.57 pm/°C. In addition,it has a compact structure. Therefore,the sensor can avoid the cross sensitivity for measuring the refractive index and temperature simultaneously.

Novel design of highly nonlinear photonic crystal fibers with flattened dispersion

Novel highly nonlinear photonic crystal fibers(HN-PCFs) with flattened dispersion are proposed by omitting 19 air holes as the fiber core.The simulation results show that the high nonlinearity and the flattened dispersion can be achieved simultaneously by employing only two types of air holes in the cladding.To reduce the confinement loss,the modified designs are presented.The confinement loss is below 0.1 dB/km at 1.55 μm,when seven layers of air-hole rings are introduced to the cladding.After modifying,the dispersion can change from-0.5 ps/(nm.km) to+0.5 ps/(nm.km) in the range from 1.35 μm to 2.06 μm,and the effective mode area is as low as 2.27 μm 2 at 1.55 μm.

A tension insensitive PbS fiber temperature sensor based on Sagnac interferometer

In this paper, a tension insensitive Pb S fiber temperature sensor based on Sagnac interferometer is proposed and demonstrated. The sensing mechanism of tension and temperature is analyzed. The relationships between the interference spectrum, temperature and tension are analyzed, respectively. The experimental temperature range is 36—70 °C. The experimental results show that the interference spectrum is red shifted, and its sensitivity is 53.89 pm/°C. In tension experiment, the tension range is 0—1 400 με. The experimental results show that there is no wavelength shift in the interference spectrum. The sensor is immune to tension cross-sensitivity compared with other sensors. It can be used for temperature testing in aerospace, chemistry and pharmacy.