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Significance: Label-free nonlinear optical microscopy has become a powerful tool for biomedical research. However, the possible photodamage risk hinders further clinical applications. Aim: To reduce these adverse effects, we constructed a new platform of simultaneous label-free autofluorescence multi-harmonic (SLAM) microscopy, featuring four-channel multimodal imaging, inline photodamage monitoring, and pulse repetition-rate tuning. Approach: Using a large-core birefringent photonic crystal fiber for spectral broadening and a prism compressor for pulse pre-chirping, this system allows users to independently adjust pulse width, repetition rate, and energy, which is useful for optimizing imaging conditions towards no/minimal photodamage. Results: It demonstrates label-free multichannel imaging at one excitation pulse per image pixel and thus paves the way for improving the imaging speed by a faster optical scanner with a low risk of nonlinear photodamage. Moreover, the system grants users the flexibility to autonomously fine-tune repetition rate, pulse width, and average power, free from interference, ensuring the discovery of optimal imaging conditions with high SNR and minimal phototoxicity across various applications. Conclusions: The combination of a stable laser source, independently tunable ultrashort pulse, photodamage monitoring features, and a compact design makes this new system a robust, powerful, and user-friendly imaging platform. [ABSTRACT FROM AUTHOR]

We present a numerical investigation of the generation of an ultra-broadband optical similariton spectrum in a photonic crystal fiber (PCF) with a carbon disulfide core through Raman amplification. The proposed PCF has an extremely high nonlinear coefficient of 4519 W−1km−1 at a telecommunication wavelength of 1550 nm and exhibits near-zero ultraflattened normal dispersion of −1.5(ps/nm/km) as well as low confinement loss of about 10−9 dB/m at 1550 nm. The numerical study was conducted using full vectorial finite-element method with a circular perfectly matched layer as a boundary condition. The similariton generated in the proposed PCF has an ultra-broadband and flat spectrum centered at 1550 nm, which spans from 1383 to 1719 nm via 14 mW input peak power and over a fiber length of only 83 cm. This continuum spectrum covers both C and L bands and can be divided into several narrow bands centered at the wavelengths of the wavelength division multiplexing channels. [ABSTRACT FROM AUTHOR]

A photonic crystal fiber (PCF) polarization rotator, featuring a large bandwidth and low insertion loss (IL) and based on the mode hybridization principle, is proposed. By reducing the size of two air holes in the 45 deg inclined direction of the fiber core, a tilted lemon-shaped fiber core is constructed. This design breaks the vertical and horizontal symmetry in the cross section of the PCF, achieving efficient mode conversion between transverse magnetic and transverse electric modes. The characteristics are numerically analyzed using a full-vectorial finite-difference method with a perfectly matched layer boundary condition. The results show that, at a working wavelength of 1.55 μm , the device achieves nearly 100% polarization conversion efficiency, an extinction ratio of 43.3 dB, and an IL of 0.025 dB, with a mere device length of 291 μm. Additionally, the operational bandwidth exceeds 825 nm (1.096 to 1.921 μm), covering the entire range of current optical fiber communications, including the O, E, S, C, and L bands. It demonstrates a good tolerance and significant potential for engineering applications in informational photonics. [ABSTRACT FROM AUTHOR]

Magnetic fluid is a good magneto-optical material. In this paper, a kind of side-etch photonic crystal long-period grating based on magnetic fluid filling is proposed. Simultaneous detection of magnetic field strength and temperature through integrated surface plasma resonance (SPR) and long period grating (LPG) sensing. The refractive index of magnetic fluid changes with the influence of magnetic field intensity and temperature. Based on the above sensing mechanism, the test results show that the magnetic field intensity sensitivity of the LPG sensor is −35.91 pm/GS , and the temperature sensitivity is 51.4 pm/°C. The temperature sensitivity of the SPR sensor is −568 pm/°C , which accurately compensates for temperature crosstalk. The calculated error rate is E(ΔB)=0.124% ; E(ΔT)=0.308%. The sensor structure provides a good application prospect for precise magnetic field intensity measurements. [ABSTRACT FROM AUTHOR]

A temperature-insensitive fiber Bragg grating (FBG) inscribed in ethanol-filled photonic crystal fibers (PCF) is designed and optimized. A structural PCF is proposed in which the diameters of the air holes of the first ring are larger than the other air holes. The structural PCF is designed and optimized via full-vector finite element method and genetic algorithms. The calculated Bragg wavelength shift (BWS) of FBG inscribed in ethanol-filled PCF is 31 pm while the temperature changes from -15°C to 55°C. When the temperature changes from 0°C to 40°C, the BWS is as small as 10 pm, and the thermal sensitivity of the Bragg wavelength is 0.25 pm/°C. [ABSTRACT FROM AUTHOR]

We propose a photonic approach to generate triangular waveforms based on nonlinear polarization rotation (NPR) effect in a highly nonlinear fiber. A continuous wave probe light is polarization rotated by an intensitymodulated control light through the NPR effect. By tuning the cascaded polarization controller, the undesired second- order harmonic is well suppressed after photodetection. A 90-deg hybrid coupler is used to compensate the phase mismatch between the fundamental and third-order harmonic components, so that triangular waveforms are generated successfully. The proposed approach is theoretically analyzed and experimentally verified. Triangular waveforms at repetition rates of 3, 4, and 5 GHz with full-duty cycle are experimentally generated. [ABSTRACT FROM AUTHOR]

A magnetic field sensor based on the magnetic fluid injected into all the cladding air holes of the solid core photonic crystal fiber (PCF) is proposed. The 8.5 cm length of the PCF filled with magnetic fluid relates to single mode fiber to compose a Mach–Zehnder interferometer. The sensor relies on the tunable refractive index of the magnetic fluid and the characteristics of the PCF. A series of magnetic field strength is applied to this sensing structure. From the experimental results, the sensitivity of the magnetic field sensor is obtained as 7.055 × 10−4 dB∕nT while a small change in wavelength shift is found. Due to a simple and easy fabrication technique, high sensitivity, and compactness, we are confident that the proposed sensor will find potential applications in measuring the magnetic field. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) is generated using circular-photonic crystal fiber (C-PCF), which is used as a key solution for higher data communication. This C-PCF steadily supports 80 OAM modes and 2 linear polarization modes from 1- to 2.5-µm wavelengths with low confinement loss (CL), very high nonlinearity, and flat dispersion. The average CL is <1 × 10-6 dB/m and the dispersion of each mode is controlled in the range of 30 to 500 ps · nm-1 · km-1. The average nonlinearity value is >1.26 × 105 W-1 km-1 at 1-µm wavelength that is essential for the generation of the supercontinuum, biomedical imaging, and several nonlinear optical applications. With all these good features, this proposed optical fiber is promising for application in fiber-basedOAM highcapacity communication systems. [ABSTRACT FROM AUTHOR]

The data rate in a single-mode fiber is approaching the capacity limit given by the Shannon theory. Mode division multiplexing, such as few modes, orbital angular momentum, and cylindrical vector beam (CVB) multiplexing, has shown great potential to further increase data capacity in both free-space and fiber communication. We propose and demonstrate high-order CVB multiplexing communication in an air-core photonics crystal fiber (PCF). The simulation results show that the 19-cell air-core PCF supports transmission of CVB modes from ±1 to ±4 orders. In the experiment, ±1- to ±4-orders CVBs are transmitted in 8.25-m-long air-core PCF with the mode purities higher than 76.5%. We demonstrate four coaxial CVB channel communication by multiplexing the ±2- and ±3-orders CVB modes. Each CVB channel carries 10-Gbit/s on-off keying signals and the measured bit error rates satisfy the forward error correction threshold. CVB communications based on air-core PCF can be used in short-distance optical communication with high capacity and low optical latency. [ABSTRACT FROM AUTHOR]

Sensitivity is of particular importance in developing liquid-core Bragg fiber sensors for refractometry instrumentations. We study the factors that influence the sensitivity of such sensors and propose to enhance it by optimizing the Bragg fiber geometry. Both theoretical analysis and experimental demonstrations have been performed to verify the feasibility. By designing and fabricating Bragg fibers with optimized bilayer thicknesses, we have enhanced the sensitivity. Furthermore, we studied the influence of fiber length on the sensitivity. Finally, we propose one possible portable sensing configuration on smartphones. [ABSTRACT FROM AUTHOR]

We present a numerical investigation of the study of the Michelson interferometer by exploring the potential of such a device to act as an all-optical logic gate when excited with ultrashort pulses of 100 fs. The input pulse is controlled by a pulse-position modulation (PPM) modulator, which has the role of controlling the value and direction of the temporal displacement from the variation of the PPM modulating factor. We present the study on the signal-to-noise ratio (SNR), which served as the basis for constructing a merit figure of the studied logic gates in order to verify their performance. We considered pulse propagation under the influence of dispersive effects, as well as the nonlinear effects in the propagation regime with no loss for the input pulses. It was possible to implement the OR logic operation in at least one interval of the modulation adjustment parameter in each of the distinct pumping power regimes, thus obtaining a total of nine intervals of OR logic gate. The situation with the best SNR = 29.36 dB, for the suggested logic gate operation, was found at the power of 150 kW with the modulation adjustment parameter ε = 43 fs. Numerical studies were done from the coupled- coupled equations solved using the fourth-order Runge-Kutta method. The implementation of fully optical logic gates tends to revolutionize digital systems in the field of data storage, such as optical memory, "replacement" of electronic circuits among other applications. [ABSTRACT FROM AUTHOR]

CO2 sensing using self-fabricated all-fiber gas cells based on a hollow-core photonic crystal fiber and chirped laser dispersion spectroscopy (CLaDS) is presented. I show that CLaDS enables significant reduction of optical fringes arising from the mutlimode nature of the air-core photonic bandgap (PBG) fiber, hence improving the gas sensing system's stability and performance. By carefully selecting modulation frequency, it is possible to match the period of interference signal (resulting from coupling between fiber's guided modes) and, therefore, suppress its contribution to the retrieved signal. CLaDS enables successful use of hollow-core PBG fibers in gas sensing applications. [ABSTRACT FROM AUTHOR]

The traditional numerical analysis in waveguide design can be time-consuming and inefficient. This is even more prominent in the THz region and with complex shapes and materials. As an alternative to overcome these drawbacks, we propose a machine learning (ML) approach to design porous-core photonic crystal fibers (PCFs) for the THz band. This method is based on an artificial neural network (ANN) model trained to predict key parameters such as the effective refractive index, effective area, dispersion, and loss values with accuracy and speed. In that sense, the network was trained to perform multiple-output regression of the above parameters. The training data for this model comes from numerical calculations that use the finite element method (FEM) to simulate and evaluate analytical expressions. Our results demonstrate the ML model's ability to capture the complex and nonlinear relationships between the input and output parameters and accurately predict the behavior of the THz PCF. Moreover, the proposed model has an inference time of ∼0.03584 s for a batch of 32 data sets, which substantially outperforms typical calculation times needed in FEM simulations for THz waveguide design. These results show that this approach is efficient and effective and has the potential to significantly accelerate the design process of PCFs for THz applications. [ABSTRACT FROM AUTHOR]

We propose a gold-coated photonic crystal fiber (PCF) polarization filter and study the influencing parameters from a structural viewpoint. In this structure, small air holes and elliptical air holes near the core are used to increase the birefringence, and we have chosen two air holes with large diameter to coat with the metal Au. The designed filter showed a loss of 403.76 dB∕cm in the y-polarization direction at the wavelength of 1.38 μm. However, the loss in the x-polarization direction was negligible, and can be selectively filtered out. Hence, light in a certain polarization direction achieves the effect of communication band filtering. We have used the finite-element method to simulate and analyze the influence of the structural parameter on fiber filter performance. The results reveal that the thickness of the metal layer, the diameter of the coated metal hole, and the asymmetry around the gold-coated hole significantly influence the position and coupling strength of the resonant response point between the surface plasmon and the core mode. In summary, the design parameters need to be optimized for maximum filtering performance of PCF. The present study opens up avenues for further research in communication band fiber filters. [ABSTRACT FROM AUTHOR]

We report a type of single-hole core photonic crystal fiber for low-loss polarization-maintaining terahertz (THz) wave guidance. Simulation results show that high birefringence at a level of 10-2 can be obtained by a design of minor position adjustment of the central air hole. Low effective material loss can be achieved because of the introduced central air hole. The strategy of the central air hole movements is also applicable for the threehole core THz photonic crystal fibers. Other transmission characteristics including single-mode condition, power fraction, confinement loss, and dispersion were discussed in detail. It is quite clear that the proposal facilitates the fabrication process due to the simple structure. [ABSTRACT FROM AUTHOR]

An ultrashort polarization splitter based on dual-core photonic crystal fibers with gold wire has been proposed. Based on the beam propagation method with anisotropic perfectly matched layers, its polarization splitter coupling length, coupling length ratio, extinction ratio (ER), and bandwidth are numerically investigated. When the gold thread is filled in the fiber, the surface of the gold wire will produce the surface plasmon polaritons, which has certain influence on the beam propagation. A polarization splitter with shorter length and greater working bandwidth can be obtained by filling the gold wire. Numerical results demonstrate that the polarization splitter possesses extremely the length of 290 µmand high ER of -56.5 dB at the wavelength of 1.55 µm. Moreover, the polarization splitter is proposed to achieve ER better than -10 dB and a bandwidth of 19.2 nm. [ABSTRACT FROM AUTHOR]

Numerical analysis and comparison of surface plasmon resonance (SPR)-based photonic crystal fiber (PCF) structures are presented. Full vectorial finite-element method (FEM) is employed to model and compare circular, elliptical, and square air hole-based PCF structures. The key propagation features, such as phase matching, sensitivity, resolution, and confinement loss behaviors of proposed structures, are reported. The effects of geometrical and design parameters are discussed and compared thoroughly. It is found that the sensitivity of circular air hole-based structure can reach up to 4200 nm/RIU, and the sensor resolution is 2.4 × 10-5 RIU. [ABSTRACT FROM AUTHOR]

We report the experimental results of ultraviolet-extended broadband supercontinuum (SC) generation in a carefully designed uniform seven-core photonic crystal fiber (PCF) pumped by Ti:sapphire femtosecond laser at 800 nm. Three different PCFs of various core diameters are fabricated to achieve group-velocity matching for ultraviolet components. A wide optical spectrum spanning down to 350 nm is obtained, which is the shortest wavelength SC generation in multicore PCF to date. High spectral flatness (10 dB) has been achieved in the entire visible window. [ABSTRACT FROM AUTHOR]

The ultraviolet to visible continuum generations are experimentally demonstrated by coupling femtosecond pump pulses around 800 nm into the normal dispersion region close to the zero-dispersion wavelength in the fundamental mode of a silica photonic crystal fiber (PCF). The ultraviolet to visible continuum is generated through a combination of nonlinear effects including degenerate four-wave mixing, cross-phase modulation, and stimulated Raman scattering. When pump pulses with the wavelength of 800 nm and the input average power of 500 mW are used, more than 40% of the incident pump power is converted to the ultraviolet to visible spectral region. The evolution of the output spectra for different PCF lengths is studied to understand the nonlinear propagation dynamics during the process of the continuum generation. The ultraviolet to visible continuum generation will find important applications in the ultraviolet and visible-based biophotonics and spectroscopy. [ABSTRACT FROM AUTHOR]

Photonic crystal fibers (PCFs) have been extensively studied to enhance communication capacity and spectral efficiency by supporting orbital angular momentum (OAM) modes. However, the structural fragility resulting from the central air hole in PCFs undermines the fiber's roundness. We introduce a hybrid PCF that replaces the central air hole with a material with a high refractive index. This innovation not only enhances the fiber's structural strength but also introduces an additional communication channel. Furthermore, this method increases the amount of OAM modes supported for transmission in the ring. Numerical results demonstrate that significant differences in refractive indices exist between adjacent vector modes, with high mode quality (ranging from 74.6% to 97.6%) and a small effective mode area (ranging from 23.5 to 59.5 μm2). The proposed hybrid PCF can transmit 20 OAM modes in the inner core and 54 OAM modes in the outer ring. The spacing between the two layers of air holes has a minimal impact on the effective mode area and mode quality. However, the effective mode area decreases and the mode quality increases with the enlargement of the diameter of air holes. In addition, a decrease in doping concentration results in lower mode quality and a larger effective mode area. Moreover, the proposed hybrid PCF can be employed in OAM-based mode division multiplexing for high-capacity short-range fiber optic communication. [ABSTRACT FROM AUTHOR]

A high-birefringence photonic crystal fiber polarization filter is proposed. The coupling theory is used to explain full and incomplete couplings. The resonance point can be adjusted to the communication band by optimizing the fiber structure parameters. Numerical simulation results indicate that the resonance strength can reach 924.96 and 710.28 dB:cm-1 at the communication wavelength of 1.31 and 1.55 µm in x- and y-polarized directions, respectively. By filling liquid analyte, the confinement loss can reach 804.52 dB:cm-1 at the wavelength of 1.55 µm. Furthermore, when the fiber length of L equals 500 µm, the peak value of the cross talk (CT) can reach 389.15 and -280.52 dB, respectively. When the length of the fiber L equals 200 μm, the bandwidth of the CT better than 20 dB is up to 120 nm at the wavelength of 1.31 µm, and the bandwidth of the CT < -20 dB is up to 140 nm at the wavelength of 1.55 µm. These properties make it a good candidate for designing types of polarization filter devices. [ABSTRACT FROM AUTHOR]

We present results of experimental and numerical investigation of supercontinuum (SC) generation in polarization-maintaining photonic crystal fiber (PCF) using chirped femtosecond pulses. The initial unchirped pump pulse source was a mode-locked Yb:KGW laser generating 52-nJ energy, 110-fs duration pulses at 1030 nm with a 76-MHz repetition rate. The nonlinear medium was a 32-cm-long polarization-maintaining PCF manufactured by NKT Photonics A/S. We demonstrated the influence of pump pulse chirp on spectral characteristics of a SC. We showed that by chirping pump pulses positively or negatively one can obtain a broader SC spectrum than in the case of unchirped pump pulses at the same peak power. Moreover, the extension can be controlled by changing the amount of pump pulse chirp. Numerical simulation results also indicated that pump pulse chirp yields an extension of SC spectrum. [ABSTRACT FROM AUTHOR]

A superfluorescent fiber source (SFS) with the low Ge-doped and Er/Ce codoped photonic crystal fiber (ECPCF) is proposed to improve the radiation resistance of SFS. The radiation effects of SFSs for an Er/Ce codoped conventional fiber (ECF) and a low Ge-doped and ECPCF are investigated in a 60Co gamma-ray environment. Results show that the low Ge-doped photonic crystal fiber exhibits better radiation tolerance than its counterpart in the Er/Ce codoped fibers, and the attenuation of the power of ECPCF-SFS is significantly smaller than that of ECF-SFS. In addition, the radiation-induced spectral variation of ECPCF-SFS with increased radiation dose is less than that of ECF-SFS. ECPCF-SFS simultaneously exhibits higher recovery performance than ECF-SFS. [ABSTRACT FROM AUTHOR]

A fiber chirped-pulse amplification system with pulse energy as high as 105 μJ is achieved at 200-kHz repetition rate using the rod-type photonic crystal fiber. The whole system’s nonlinearity accumulated in the fiber amplification is effectively suppressed, and the compressed pulse duration of 808 fs is obtained. A 500-kHz high-repetition rate KTiOPO4 Pockels cell is also applied to make the ultrafast laser pulse selection for generating pulse trains with controllable pulse number and pulse splitting without changing the pulse energy. The demonstrated pulse selection and splitting method are useful for processing of different materials and parallel processing. The pulse selection efficiency of the Pockels cell is as high as 96%. [ABSTRACT FROM AUTHOR]

This paper presents a highly sensitive photonic crystal fiber (PCF) refractive index sensor based on the surface plasmon resonance (SPR) effect operating in the telecommunication wavelengths. Gold is used as the plasmonic material due to its chemical stability and titanium dioxide (TiO2) is used to shift the resonance wavelength in the telecommunication bands. Both materials are deposited sequentially on the PCF surface, which is comparatively easy to fabricate. Numerical investigations show that the proposed sensor exhibits very high wavelength sensitivity of 10;800 nm∕RIU and amplitude sensitivity of 514 RIU−1 in the sensing range between 1.46 and 1.48. Moreover, it exhibits maximum sensor resolution of 9.25 × 10−6 RIU and high linearity over a wide sensing range. The proposed sensor can be practically realized due to its simple and straightforward structure. [ABSTRACT FROM AUTHOR]

To restrain the infrared radiation from high temperature objects to decrease the probability of being discovered by infrared detectors operating in the mid- and far-infrared atmospheric windows, we design a one-dimensional heterostructure photonic crystal (PC) using low-cost coating materials Te and ZnSe, and test its reflection spectra and radiant temperature. The tested results show that this PC has high average reflectance in 3- to 5-μm and 8- to 14-μm wavebands, which is 86.72% and 72.91%, respectively, and the corresponding emissivity is 0.072 and 0.194, respectively. The radiant temperatures of the PC are always lower than those of the background, with the maximal difference of the radiant temperature being 31.97°C corresponding to a background radiant temperature of 75.64°C. The study confirms that the deposited PC can effectively decrease the infrared radiation in mid- and far-infrared bands. [ABSTRACT FROM AUTHOR]

A tunable single polarization filter based on high-birefringence photonic crystal fiber with silver wires symmetrically filled into cladding air holes is designed. The confinement loss of the unwanted polarized mode (x-polarized mode) at 1310- and 1550-nm bands are 371 and 252 dB/cm, whereas another mode confinement loss (y-polarized mode) at the corresponding wavelength as low as 14 and 10 dB/cm, respectively. Moreover, the 20-dB bandwidth can reach 179 (at the 1310-nm band) and 71 nm (at the 1550-nm band) for a propagation distance of 1 mm. The dispersion relations and polarization characteristics are analyzed in detail utilizing the finite element method. Numerical results show that by adjusting the pitch between two adjacent air holes, the diameters of cladding air holes or silver wires near the fiber core, the resonance wavelength and resonance strength can be tuned effectively, which is beneficial for tunable polarization filter devices in the communication wave bands. [ABSTRACT FROM AUTHOR]

We demonstrate a photonic crystal fiber with near-zero flattened dispersion, ultralower effective material loss (EML), and negligible confinement loss for a broad spectrum range. The use of cyclic olefin copolymer Topas with improved core confinement significantly reduces the loss characteristics and the use of higher air filling fraction results in flat dispersion characteristics. The properties such as dispersion, EML, confinement loss, modal effective area, and single-mode operation of the fiber have been investigated using the full-vector finite element method with the perfectly matched layer absorbing boundary conditions. The practical implementation of the proposed fiber is achievable with existing fabrication techniques as only circular-shaped air holes have been used to design the waveguide. Thus, it is expected that the proposed terahertz waveguide can potentially be used for flexible and efficient transmission of terahertz waves. [ABSTRACT FROM AUTHOR]

A transverse-stress sensor with polarization filtering function based on a specially designed photonic crystal fiber (PCF) is proposed. Four ultralarge side-holes are introduced into the cladding layer, and two of them are gold-coated to enhance the stress sensitivity. The finite element method is applied to study the polarization-dependent wavelength-selective sensing characteristics at the optical communication wavelength. Results reveal that the sensor can achieve a high sensitivity in either direction that can be divided into an x-direction component and a y-direction component. Combining the advantages of side-hole structure and surface plasmon resonance technology, the proposed sensor is believed to be an excellent candidate for the transverse-stress measurement. [ABSTRACT FROM AUTHOR]

An ultrashort and high-sensitivity refractive index sensor based on dual-core photonic crystal fiber (PCF) is proposed, and the characteristics are evaluated by finite-element method. The coupling characteristics can be changed by filling different index liquids in the holes of the PCF. Also, the transmission spectrum also changes as the length of the optical fiber is set to a specific value. The average sensitivities of −8916 nm/RIU (refractive index unit) and −8588 nm/RIU are realized in the index range from 1.41 to 1.438. The corresponding linearities are 0.9936 and 0.9916, respectively. The fiber length is just 3 mm, which could be widely used in the integrated sensing system. The sensor with high sensitivity and nice linearity is significative. [ABSTRACT FROM AUTHOR]

One of the modern ways to diagnose diseases is through exhale gas of patients, which is non-invasive and therefore desirable to doctors. The composition of exhaled breath contains valuable information for identifying hyperbilirubinemia disease. The accumulation of bilirubin and its lack of excretion causes jaundice, and the production of bilirubin has a direct relationship with exhaled carbon monoxide (CO). A CO diagnostic sensor based on a photonic crystal fiber (PCF) that can detect hyperbilirubinemia has been designed. This method can be used repeatedly to detect and treated jaundice in time. The PCF is designed as a hexagon. The holes in the cladding are circular and the holes in the core are circular and oval. Finite element method is used to do all the numerical simulations. COMSOL Multiphysics software was used to obtain the results. In this setup, CO absorption wavelength is 1.567 μm, and the relative sensitivity of 64.28% is reported with confinement loss of 3.81 × 10 − 3 dB / m. [ABSTRACT FROM AUTHOR]

A v-shaped microstructured optical fiber (MOF)-based surface plasmon resonance (SPR) sensor is proposed for simultaneous measurement of temperature and refractive index (RI). Such a v-shaped structure could be coated with the gold layer conveniently, simplifying the sensor fabrication. We theoretically investigate the coupling property between the core mode and the plasmon mode, the polarized characteristics of the core modes, the behavior features of the resonance peaks and the sensing performance of the sensor at different temperatures and RIs. The simulation results demonstrate that the v-shaped structure can support two polarized resonance peaks (x- and y-polarized peaks) that have different response characteristics for the changes of temperature and RI, the temperature coefficients of x- and y-polarized peaks are 11.8 and 6.2 pm / ° C, the RI coefficients of x- and y-polarized peaks are 1408 and 1505 nm / RIU, respectively. [ABSTRACT FROM AUTHOR]

We designed an all-normal flat near-zero dispersion As2S3 photonic crystal fiber (PCF) with orderly arranged air hole rings for generating an orbital angular momentum (OAM) supercontinuum (SC). We optimized the characters of chromatic dispersion and confinement loss compared with previous studies by carefully engineering the PCF. Simulation results show that, by launching a 100 fs 250 kW chirp-free hyperbolic secant pulse at 7 μm into a 5-mm designed PCF, a 9471-nm SC of OAM1,1 forms from 757 to 10228 nm at a −40 dB level, which covers a 3.7-octave bandwidth. As the peak power increases, the higher-order OAM with a flatter chromatic dispersion profile in the normal dispersion region has a wider bandwidth. They are all highly coherent in the whole spectral range. [ABSTRACT FROM AUTHOR]

We propose and study an innovative magnetic sensor of incomplete surface defect one-dimensional (1D) photonic crystal composed of magnetic fluid, SiO2, and TiO2 as defect layers. The magnetic fluid film covers the surface of the 1D photonic crystal, whereas the magnetic fluid penetrates into the array of circular holes with period ∧ inscribed on the dielectric layers of SiO2 and TiO2 to form the incomplete surface defect layer. The sensing characteristics of a 1D photonic crystal magnetic sensor (1D-PCMS) were analyzed using the theory of rigorous coupled wave analysis. The innovative use of the angular interrogation method in magnetic field detection has investigated the effect of defect layer structure parameters, optical dielectric loss on magnetic sensor sensitivity and quality factor at a fixed wavelength. This incomplete surface defect mode 1D-PCMS structure demonstrates the feasibility of scanning angular measurements of magnetic fields and provides a reference for future research on 1D-PCMS based on angular interrogation. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid porous-core flexible photonic crystal fiber (PCF), which has the potential for low-loss guidance of broadband terahertz (THz) waves at low THz bands. Here, the cyclic olefin polymer TOPAS is used for the main material to increase the flexibility of PCF. The central hybrid porous-core area contains a circular and hexagonal design, which reduces the loss of PCF. Simulation results show that the proposed fiber has a very low confinement loss of 3.47 × 10 − 6 dB / cm at 500 GHz operating frequency. The bending loss at a bending radius of 1 cm is as low as 3.2 × 10 − 4 dB / cm. The negligible modal loss facilitates the flexible application to THz systems. Moreover, the fabrication of the proposed PCF design is compatible with widely used methods and technologies, including stacking and drawing, extrusion and drilling. This newly proposed hybrid design of the porous-core region can be considered as an improved version in the research of THz porous-core waveguides. [ABSTRACT FROM AUTHOR]

We analyzed the resonant coupling in the low-refractive-index sensor based on a directional coupler implemented in a microstructured optical fiber with a composite core and the parallel analyte channel in the form of a hollow-core waveguide. We showed the possibility of an 8-fold increase in the analyte channel radius that is equivalent to a 64-fold increase in its cross section, in comparison to the existing design. With an increase in the analyte channel radius, the resonance frequencies of the composite core mode and the satellite waveguide modes shift to longer wavelengths, while the dispersion curves of the high-order modes of the satellite waveguide tend to merge and their resonances become less pronounced than the resonances of the low-order modes. With an increase in the analyte channel radius from 2 to 16 µm, the sensor sensitivity increases by 40% and the detection limit becomes lower by a factor of 2. Such an increase in the analyte channel radius also eliminates the need in a high-pressure pump for filling the channel with analyte and thus makes this sensor much more practical than was previously thought. [ABSTRACT FROM AUTHOR]

A birefringent structured side-holes photonic crystal fiber (PCF) with high sensitivity is designed for pressure sensing. Simulation results show that the birefringence and relevant sensitivity are strongly influenced by the air-holes' sizes and the distance between the fiber core and side-hole. The modal birefringence and the polarimetric pressure sensitivity can be up to 3.943 × 10-3 and -3.67 × 10-5 MPa-1 at 1.55 μm, respectively. The proposed side-holes PCF possesses promising applications for pressure sensing. [ABSTRACT FROM AUTHOR]

A polarization splitter based on dual-core photonic crystal fiber with a gold layer is proposed. The splitter has a short length of 0.435 mm, and is capable of splitting an incident light into two orthogonal states between 1.48 and 1.72 μm. At the same time, a helpful conclusion is obtained: the variation of the thickness of the gold layer has a weak impact on splitting length and significant influence on the extinction ratio spectra. The polarization splitter can be modulated by changing the thickness of the gold layer. [ABSTRACT FROM AUTHOR]

The generation of high-quality self-similar ultrashort pulses at 1550 nm by tapering the photonic crystal fibers (PCFs) is numerically demonstrated. We taper the PCF to achieve the exponentially decreasing dispersion and exponentially increasing nonlinearity profiles, which turn out to be the fundamental requirements for generating the chirped self-similar pulses. Further, we find that the chirped solitons could also be generated with the other three possible exponential variations. Thus, for the first time, we attempt tapering the PCFs for bringing in these exponentially varying dispersion and nonlinear profiles. We carry out the detailed pulse compression studies for various decay rates of the dispersion profiles as the decay rates of dispersion depend on the initial chirp and hence on compression factor, too. The unique feature of this pulse compressor lies in the fact that the required length of the tapered PCF is about 20 times less than that of the previously reported pulse compressor operating at 850 nm. [ABSTRACT FROM AUTHOR]

Use of long period gratings (LPGs) formed in grapefruit photonic crystal fiber (PCF) with thin-film overlay coated on the inner surface of air holes for gas sensing is demonstrated. The finite-element method was used to numerically simulate the grapefruit PCF-LPG modal coupling characteristics and resonance spectral response with respect to the refractive index of thin-film inside the holey region. A gas analyte-induced index variation of the thin-film immobilized on the inner surface of the holey region of the fiber can be observed by a shift of the resonance wavelength. As an example, we demonstrate a 2,4-dinitrotoluene (DNT) sensor using grapefruit PCF-LPGs. The sensor exhibits a wavelength blue-shift of ~820 pm as a result of exposure to DNT vapor with a vapor pressure of 411 ppbv at 25°C, and a sensitivity of 2 pmppb-1v can be achieved. [ABSTRACT FROM AUTHOR]

A simple hexagonal photonic crystal fiber is proposed to simultaneously achieve ultrahigh birefringence, large nonlinear coefficient, and two zero dispersion wavelengths (ZDWs). The finite element method with circular perfectly matched layer boundary condition is used to simulate the designed structure. Simulation results show that it is possible to achieve two closely lying ZDWs of 1.08 and 1.29 µm for x-polarization with 0.88 and 1.20 µm for y-polarization modes, respectively. In addition, an ultrahigh birefringence of 3.15 × 10-2 and a high nonlinear coefficient of 58 W-1 km-1 are also obtained at the excitation wavelength of 1.55 µm. The proposed fiber can have important applications in supercontinuum generation, parametric amplification, four-wave mixing, and optical sensors design. [ABSTRACT FROM AUTHOR]

An octagonal photonic crystal fiber (O-PCF) for numerical structure design and analysis of some particular properties are presented in this paper. The proposed design is suitable for residual dispersion compensation (RDC) with polarization maintaining (PM) applications as it offers extremely high-negative flattened average chromatic dispersion (DT) and absolute dispersion variation (ΔD) of around -ð708 ± 10Þ psnm-1 km-1 and average high birefringence (B) of the order 10-2 for the wavelength limits of 1.46 to 1.67 μm (bandwidth of 210 nm that covers S + C + L+ U bands in the infrared region of the optical third window). In addition, it exhibits very low confinement loss of 10-3.5 to 10-2.5 dB/m for that bandwidth. Moreover, to evaluate the sensitivity of the fiber properties (DT and B) during fabrication, ±0.02 μm variation in the optimum parameters is also studied. [ABSTRACT FROM AUTHOR]

We experimentally demonstrated the generation of dispersive waves (DWs) at the visible wavelength by coupling femtosecond pulses into the anomalous dispersion region of the second-order mode of a homemade photonic crystal fiber. When center wavelengths of the pump pulses are located at 800 and 850 nm and input average powers Pav are increased from 300, to 400, and to 500 mW, the blue-shifted DWs can be generated during the soliton dynamics and are tunable within the wavelength range of 614 to 561 nm. Moreover, the conversion efficiency ηDW of DWs is enhanced from 5% to 21%, and the corresponding bandwidth BDW is broadened from 17 to 30 nm. It is believed that the DWs can be used as the ultrashort pulse source for visible photonics and spectroscopy. [ABSTRACT FROM AUTHOR]

A high-birefringence photonic quasi-crystal fiber (HB-PQF) based on SiO2 is proposed. The relationships between birefringence and structure parameters and between beat length and structure parameters are researched by finite difference beam propagation method. With the optimization of fiber structure parameters, the birefringence is 1.4207 x 10-2, which is two orders ofmagnitude higher than the normally used fiber when the wavelength is 1.55 µm. The radius of the fiber is 6.5 µm. The HB-PQF in a communication sensor will have important application prospects. [ABSTRACT FROM AUTHOR]

We reported the modeling result of selectively magnetic fluid infiltrated dual-core photonic crystal fiber based magnetic field sensor. Inside the cross-section of the designed photonic crystal fiber, the two fiber cores filled with magnetic fluid (Fe3O4) form two independent waveguides with mode coupling. The mode coupling under different magnetic field strengths is investigated theoretically. The sensitivity of the sensor as a function of the structural parameters of the photonic crystal fiber is calculated. The result shows that the proposed sensing device with 1 cm photonic crystal fiber length has a large sensitivity of 305.8 pm/Oe. [ABSTRACT FROM AUTHOR]

One of the ways to diagnose cancer in its early stages, which can lead to timely treatment, is to use Raman spectroscopy. The Raman spectrum of a material is created by the interaction of light with the material; it determines the exact characteristics and properties of the material, which can be used to distinguish cancerous cells from healthy ones. However, the intensity of this spectrum is weak, and the optical fibers used in these devices must have very little loss and thus have low sensitivity to bending. This paper proposes a circular photonic crystal fiber with a structure in which the losses in different bending radii are significantly reduced. The modified state of the fiber at the wavelength of 1.55 μm supports the transmission of 10 modes propagated in the core. The loss for the fundamental mode at this wavelength and bending radius of 2 mm equals 3.82 × 10 − 16 dB / m, which is considered a low value. As the bending radius decreases further, low-order modes diminish, which are more sensitive to bending than high-order modes. However, their losses are lower than in high-order modes. The critical bending radius for this fiber, with the loss reaching 0.1 dB / m, is equal to 0.096 mm. This low critical bending radius shows the low sensitivity of the proposed fiber to bending. The fiber was examined in different bending radii and wavelengths, and the parameters of this structure were presented for the lowest bending losses, which can be used in Raman spectrometers as spectrum collecting fibers and improve cancer diagnosis devices with Raman spectrum. [ABSTRACT FROM AUTHOR]

Characteristics of cladding modes in a photonic crystal fiber (PCF) with triangular air-hole lattice in the cladding are numerically analyzed using a finite element method. The transition for LP11 cladding mode to core mode with variation of the normalized wavelength has been shown. The transition of the LP01 cladding mode to the outer silica mode and reorganization of the LP0m cladding modes caused by varying the fiber radius has been investigated. By choosing the optimized fiber radius, which is located in the cladding modes' reorganization region, the sensitivity of the coupled wavelength between the core mode LP01 and cladding mode LP03 to surrounding refractive index is increased by a factor of five and reaches to 2660 nm/refractive index unit over the range of 1.40 to 1.42. The sensitivity is competitive with that of long-period grating in PCF in response to changes in refractive indices of the medium contained in the cladding air channels. [ABSTRACT FROM AUTHOR]

In order to solve for the mode intensity distributions in a photonic crystal fiber (PCF) cross section and the propagation constant for the design of fiber bandpass filters, we numerically analyze the modal distributions of the fundamental core mode and different cladding modes. Based on the simulation results, we also experimentally demonstrate a simple fabrication of bandpass filters inscribed on the PCF by inserting a π-phase shift in a 12-period long-period grating (LPG). Two rejection bands with greater than 18 dB isolation and an ultra-wide band of 85.3 nm are achieved. The phase-shifted PCF-LPGs are fabricated using a CO2 laser with point-by-point focused pulses. The proposed fiber bandpass filter is compact and is not influenced by temperature effects. [ABSTRACT FROM AUTHOR]

A simple and accurate theoretical investigation is presented, based on a novel, effective, and straight-forward ABCD matrix technique, in relation to estimation of optimum coupling efficiency between the laser diode and the photonic crystal fiber (PCF) with a hyperbolic microlens (HML) on its tip. By tailoring different parameters, such as the air-filling factor and lattice constant of the PCF, the criteria for achieving maximum coupling are predicted and reported for two different light wavelengths of practical interest. It is observed that such fiber parameters play a crucial role in predicting the optimum focal length of HML for a particular wavelength. The formulation should find application in the design of HML, for achieving a long working distance between the laser source facet and the fiber tip. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering (SBS) performances of small core tellurite photonic crystal fibers (PCF) are rigorously studied. We propose a design of tellurite PCF that is used for slow-light-based applications. We developed a two-dimensional finite element mode solver to numerically study the acoustic and optical properties of complex refractive index profiles including tellurite PCF. Our results include the calculation of Brillouin gain spectrum, Brillouin gain coefficient (gB) and Brillouin frequency shift by taking into account the contribution of the higher-order acoustic modes. Several simulations were run by varying the air-filling ratio of various PCF structures to enhance the SBS. The real scanning electron microscope image of a small core of highly nonlinear tellurite fiber is considered. Optimized results show a frequency shift of 8.43 GHz and a Brillouin gain of 9.48 x 10-11 m/W with a time delay between 21 and 140 ns. Such fibers have drawn much interest because of their capacity for increasing and tailoring the SBS gain. [ABSTRACT FROM AUTHOR]