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Whispering gallery mode (WGM) crystalline resonator is an excellent platform for acoustic sensing because the quality (Q) factor of 1011 can theoretically increase sensitivity by more than three orders of magnitude. However, ultrahigh sensitivity and wide dynamic response range cannot be achieved simultaneously, due to the extremely narrow linewidth and small linear range of ultrahigh-Q mode. Based on the multi-mode calcium fluoride (CaF2) resonator supports different Q (106∼109), fast switching with ultrahigh sensitivity and wide dynamic response range can be achieved by locking modes with different Q. The mode with Q of 5.13 × 108 shows a high sensitivity of 280 mV/Pa at 1 kHz frequency, which is better than other WGM acoustic sensors currently reported. When switching to the lower-Q of 6.24 × 106, the dynamic response range is improved by 38.3 dB. [ABSTRACT FROM AUTHOR]

Whispering gallery mode (WGM) microcavities can efficiently store and manipulate light with strong light confinement and long photon lifetime, while coupling light into and from WGMs is intrinsically hindered by their unique feature of rotational symmetry. Here, a scattering-assisted liquid crystal (LC) micropillar WGM laser is proposed. WGM lasing at the surface of the micropillar is obviously enhanced by fluorescence scattering in the core of the micropillar. Besides, weak scattering of LC molecules also builds efficient coupling channels between the laser modes and the axial transmission modes of the micropillar-based waveguide, providing an all-in-one liquid WGM laser with functions of self-seeding and self-guiding. Furthermore, based on the hysteresis characteristics of the electrically anchored LC molecules under the interaction of thermal force, an erasable read-write liquid memory device is proposed, paving the way for the application of logic-controllable WGM lasers in optical storage and optical control. [ABSTRACT FROM AUTHOR]

We experimentally proposed a helical long-period fiber grating (HLPG), which was produced by twisting a tri-hole fiber (THF) with arc discharge technology. The optical field distribution in THF-HLPG was analyzed and its transmission spectrum was studied. The simulation results prove that the light wave propagating along the fiber core can be strongly coupled into cladding modes within a short transmission distance, thus making the THF-HLPG more compact. In the experiment, a sensor with a length of 4.2 mm was manufactured by using an arc heating system. With this processing device, the air holes of the THF-HLPG can effectively avoid shrinkage during heating. Temperature, strain, torsion and curvature properties were investigated experimentally. Owing to the peculiarity of THF-HLPG, this sensor shows a high mechanical response and is suitable for identifying curvature changes with a large bending radius. The demonstrated THF-HLPG offers a promising application for precisely detecting mechanical deformation of micro-structure. [ABSTRACT FROM AUTHOR]

Optic fiber sensors for refractive index (RI) measurement have brought about widespread attention in an extensive range owing to their unique properties. In this paper, we theoretically analyzed and experimentally verified a brand-new type of long period fiber grating with two resonant peaks. This structure is obtained by side-polishing method and melting tapered method. Firstly, a single-mode fiber (SMF) is produced into staggered V-shaped grooves on symmetrical sides by high-frequency CO2 laser. It is then heated and stretched by optic fiber melting tapered system for bending the core towards two-sides grooves. The sinusoidal core is closer to external environment and exhibits a stronger refractive index modulation, which prompts this structure more sensitive to RI. The structure possesses two resonant peaks of 1382.6 nm (dip-1) and 1473.4 nm (dip-2), which can be used to measure temperature and RI simultaneously. The experimental results present that the maximum RI sensitivity of the proposed sensor exhibits 620 nm/RIU (RI unit) in dip-2. Furthermore, compact sensing area, low cost and strong mechanical property of the proposed sensor make itself more suitable for high-accuracy RI measurement. [ABSTRACT FROM AUTHOR]

We report the numerical and experimental study of a liquid crystal (LC) core periodically segmented waveguide (PSW) based mode size converter, fabricated on an ITO coated glass substrate. The effective index and mode size of the proposed device can be dynamically tuned by changing the applied voltage. The 5CB (4-cyano-4′-pentylbiphenyl) nematic LC has been used as the guiding layer, surrounded by the cladding of negative photoresist AZ15nXT. An equivalent waveguide model has been used to determine the waveguide characteristics, and we have shown that the effective index and mode size of the guided modes change with the duty cycle and the applied voltage. For a fixed duty cycle, we have experimentally demonstrated that the size of the fundamental mode can be tuned by the applied voltage. The advantage of using LC-PSW for mode size transformation is that the effective index and mode size in the fabricated waveguide can be varied by an externally applied electric field. It has also been experimentally demonstrated that the coupling efficiency between the LC-PSW and a single mode fiber can be maximized by appropriate choice of the applied voltage. The proposed mode size converter can be used in LC photonic lightwave circuits to couple the light between optical devices with maximum efficiency. [ABSTRACT FROM AUTHOR]

There are great challenges in the generation of higher-order modes for long period fiber grating (LPFG). The cross-coupling coefficient between the fundamental mode and the higher-order azimuthal modes is extremely small due to the difficulty of angular modulation with traditional methods. Therefore, how to generate higher-order orbital angular momentum (OAM) modes with a high efficiency and low insertion loss is an urgent problem to be solved. In this paper, an efficient all-fiber approach to generate fourth-order OAM mode by employing a preset twisted LPFG (PT-LPFG) in a few-mode fiber (FMF) is presented. The difficulty of generating fourth-order mode utilizing traditional single-side exposure carbon dioxide laser manufacturing method and the advantages of the PT-LPFG in enhancing the coupling coefficient are theoretically analyzed in detail. In experiment, the conversion from the fundamental mode to the fourth-order mode is achieved, and the fourth-order interference patterns demonstrated the successful generation of the OAM±4 modes. The conversion efficiency of the OAM modes is over 99% and the purity is measured to be more than 90%. To the best of our knowledge, this is the first time to generate the fourth-order OAM mode in an all-fiber system using single long-period fiber grating. [ABSTRACT FROM AUTHOR]

This work reportsa thorough analysis of intensity fluctuations of Rayleigh backscattered coherent light in single mode optical fibers. Based on the intermediate range order of silica glasses and on residual stress of optical fibers, a theoretical model was built to calculate intensity fluctuations of Rayleigh backscattered coherent light in the optical frequency domain. Numerical simulation results were compared to experimental results in time domain, strongly supporting the conclusion that Rayleigh backscattering in single mode fibers is an ergodic process exhibiting ergodicity in the time-frequency sense so that the model can be used to determine the statistical behavior of backscattered intensity fading. [ABSTRACT FROM AUTHOR]

Whispering gallery mode (WGM) resonances in radially, thermally poled glass hetero-fibers, are investigated for the first time. Upon radially oriented thermal poling, both TE and TM polarized responses undergoing a spectral “cleaning” process, namely higher-order radial modes are being suppressed, or even fully annihilated, compared to the spectral behavior of the pristine fibers. Second-harmonic generation (SHG) microscopy, Energy Dispersive X-Ray Spectroscopy (EDX) and micro-Raman (μ-Raman) measurements were conducted in order to reveal the spatial profile of the structural and optical changes introduced to the thermally poled hetero-fibers, and correlate them with the WGM spectral measurements. The specific selective, mode cleaning behavior is attributed to refractive index changes generated in proximity to the fiber's circumference, which are associated with radially-symmetric ionic re-distribution. The thermally poled hetero-fiber WGM cavities have been rigorously simulated using the finite element method, the calculated modal eigenstates and transmission spectra, further confirm the specific mode selection mechanism, introduced by the azimuthally symmetric thermal, poling process. [ABSTRACT FROM AUTHOR]

A Sagnac interferometer based optical fiber ultraviolet (UV) sensor is theoretically proposed and experimentally demonstrated in this work. A layer of ZnO film was deposited upon the surface of a silica no core fiber (NCF) using the magnetron sputtering method. Since the ZnO film was sensitive to UV, the ZnO-coated NCF was introduced into the Sagnac interferometer to help convert changes of the UV irradiance to the wavelength shifts of the interference spectrum. In comparison with the preparation of ZnO nanostructures, the ZnO film had the advantage of simple operation, high deposition rate, good adhesion and stable structure. The influence of the ZnO film thickness on the UV response was investigated, which proved that with the increase of the film thickness, the UV sensitivity decreased gradually. When the ZnO film thickness was 30 nm and the UV irradiance varied from 0 mW/cm2 to 19.95 mW/cm2, the designed sensor had the highest sensitivity of −124.9 pm/(mW/cm2) (R2 = 0.9854) and a detection accuracy of 0.4 mW/cm2. The proposed sensor has competitive sensitivity, high linearity, satisfactory repeatability and stability, which can be expected to conduct high-precision UV sensing in industrial, biomedical and military regions. [ABSTRACT FROM AUTHOR]

This paper reports on the demonstration of a single-mode flexible polymer optical interconnect for efficiently and conveniently connecting integrated photonics chips to one another (chip-to-chip) and to optical printed circuit boards (chip-to-board). The interconnect uses a low-loss partially fluorinated refractive index contrast (RIC) polymer, referred to as poly(F-SBOC), that provides for direct patterning of the desired refractive index profiles into a slab waveguide consisting of poly(F-SBOC) and a flexible fluoropolymer film (Tefzel). Using a maskless lithography system, interconnects consisting of s-bends and tapers can be printed in situ into the poly(F-SBOC) material with no need for mechanical alignment. We demonstrate the efficacy of this approach by connecting two separate ion-exchange (IOX) glass waveguide chips, achieving fiber-to-fiber total insertion losses below 6dB in some cases, through the use of grayscale tapers that are written directly into the poly(F-SBOC) material. [ABSTRACT FROM AUTHOR]

In this study, the contributions of light scattering and extra-loss mechanisms to the total losses and to the differential mode attenuation of the guided modes are quantified for three different few-mode fibers. The tested fibers have the same modal content but different refractive index profiles, namely step, trapezoidal and parabolic. It is demonstrated that a trapezoidal index profile is a good compromise between low attenuation, low differential mode attenuation while maintaining large effective index differences between modes so as to reduce mode coupling. [ABSTRACT FROM AUTHOR]

Fiber optic sensors are applied in industry, remote sensing, environmental monitoring and healthcare. A special place is occupied by tilted fiber Bragg gratings, which can significantly expand the capabilities provided by standard Bragg sensors. But these gratings have complex spectral responses, therefore, data processing becomes a critical task for achieving maximum performance. In this paper, machine learning methods for processing spectral data of a plasmonic fiber sensor based on a tilted fiber Bragg grating were applied for the first time for the measurement of small refractive index changes. The responses of two similar but not identical sensors were measured in two independent experiments. The model trained on the data of the first sensor was used to analyze data obtained with another sensor. The best resolution achieved in our experiments was $9 \times {10^{ - 6}}$ RIU. [ABSTRACT FROM AUTHOR]

We propose a competitive approach for analysis and optimization of optical fiber performance. Taking into account the standard fiber fabrication cycle, we develop a model of highly multistepped optical fiber. We introduce a fast and accurate algorithm which allows one to obtain propagation constants and transverse fields for systems with thousands of steps within seconds. The approach relies on both the exact analytical formulation of the corresponding eigenvalue problem and an original solving algorithm of linear complexity with respect to the number of steps. The model accurately represents arbitrary smooth profiles with azimuthal symmetry, which, together with low computation time, opens up ample opportunities for various optimization schemes. Obtained results are compared with those of other numerical and analytical methods. [ABSTRACT FROM AUTHOR]

Silicon photonic sensors have formed a favorable platform for various sensing applications due to the great advantages of high-performance, compactness, and large-scale integration capability. However, interrogation systems for reading out the encoded sensing information from an integrated sensor are still bulky and inflexible for various portable applications. In this paper, we propose a monolithic silicon refractive index sensor and interrogator by utilizing the cascaded microring resonators. A novel dithering and bias scan interrogation technique is designed for highly sensitive refractive index sensing with a broadband light source. The reflective index for aqueous solutions of ethanol with different concentrations are measured in the experiment, verifying that the sensitivity of the sensor is 125.1 nm/RIU and the detection accuracy of the whole system is 7.76 $\boldsymbol{\times 10^{-6}}$ RIU. This integrated photonic sensor and interrogator will be a promising solution for large-scale applications in biological and chemical sensing scenarios. [ABSTRACT FROM AUTHOR]

The nondestructive volumetric imaging technique to characterize a three-dimensional (3D) refractive index (RI) profile quantitatively is an essential tool for designing, fabricating, and evaluating photonic structures of various devices. Conventional 3D RI imaging methods usually depend on the sample rotation scheme, which finds it difficult to measure the RI distribution of sheet-like on-chip structures. In this study, we proposed an optical diffractive tomography (ODT) system by employing a galvanometric scanner to reconstruct on-chip waveguide RI profiles; the measured volumetric RI dimension was 63 × 63 × 32 μm3 in a single measurement, and the RI sensitivity was up to 0.0015. The reliable reconstruction results proved that the proposed ODT system is a promising tool for quantitatively retrieving RI profiles of photonic devices. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate that multiple tapers in optical fibers allow for quasi-distributed refractive index sensing via a high spatial resolution Brillouin Optical Frequency-Domain Analysis (BOFDA) configuration. We first characterize, theoretically and experimentally, the variation of the Brillouin frequency shift (BFS) with the diameter of the tapered fiber. Then, we characterize the dependence of the BFS from the outer refractive index in an optical fiber taper with a waist diameter of 10 µm. Finally, we show that more tapers can be realized along the same fiber, in order to provide multi-point refractive index sensing. [ABSTRACT FROM AUTHOR]

We develop an average bit error probability (BEP) expression for binary differential phase shift keying (BDPSK) signal in log-normal single-input-single-output laser free-space optical (FSO) communication system by treating the BEP under particular atmospheric turbulence as random variable and changing the integral form. By using this average BEP expression and successive polynomial approximations, we obtain an explicit three-term approximate expression of the average BEP, while the existed approximate BEP expressions are implicit. The three-term approximate BEP expression can be used to intuitively analyze the relationship between BEP and system parameters such as signal-to-noise ratio (SNR) and Rytov standard deviation. In addition, the three-term approximate BEP expression works well when the Rytov standard deviation is no larger than 0.15. In this range of Rytov standard deviation, this approximate BEP expression can accurately predict the performance of FSO communication system with the transmission distance of up to 3.9 km and the laser at a wavelength of 1550 nm. The three-term approximate BEP expression can be used directly to derive the closed-form SNR expression as a function of Rytov standard deviation. The maximum loss between closed-form and theoretical SNR expressions is only 0.50 dB. [ABSTRACT FROM AUTHOR]

We report the characterization of a cladding-pumped multicore fiber with annular erbium doping for low gain compression over the C-band (1528.8 nm to 1563.9 nm). The fiber aims to minimize saturation effects by placing the erbium doping in the cladding where the signal intensity is lower. The challenge of ensuring adequate erbium ion solubility in the cladding, without unduly raising its refractive index, was answered by using aluminophosphosilicate for the doped region. Before assembling the eight-core amplifier with fan-in and fan-out, we needed to determine the optimum fiber length to meet the gain compression target when the eight cores are loaded, for a given pump power injected in the cladding. We thus performed a thorough experimental characterization of a single core. This allowed the determination of all the relevant fiber parameters needed to do numerical simulations and predict the multicore fiber amplifier performance under the fully loaded scenario. For the first time, these numerical results are compared to the experimental results of a fully loaded multicore amplifier with cladding pumping and erbium doping in the cladding. We discuss the amplifier performance in terms of gain compression when the input power signal is varied, as would be the case in dynamic optical networks. We also examine the core-to-core gain variations and the sensitivity of the design to fabrication variations. Results show that significant reduction in gain compression of multicore cladding-pump amplifiers can be achieved with the proposed annular doping design in the cladding, although improvement in the fabrication uniformity of the core will be required for practical applications. Lastly, we discuss how this erbium doped fiber design can also lead to improved power conversion efficiency (PCE) in cladding-pumped amplifiers. [ABSTRACT FROM AUTHOR]

A general method is presented for exciting discrete modes in waveguides of arbitrary geometry. Guided modes supported by the waveguide are first calculated using a finite difference frequency domain model. High efficiency holograms to excite these discrete modes are then generated using the Direct Search hologram generation algorithm. The Direct Search algorithm is optimised such that the inherent properties of waveguide modes are exploited to give faster execution times. A nodeless antiresonant photonic crystal fibre is considered as a test geometry, in which high-purity modes are experimentally excited and in-coupling efficiencies of up to 32.8% are obtained. [ABSTRACT FROM AUTHOR]

We report on an extrinsic surface plasmon-enhanced refractometer based on cladding mode resonance excitation in a photonic crystal fiber (PCF) equipped with a straight fiber Bragg grating (FBG). First, we show that the lattice pitch and the air hole diameter of the PCF microstructure define the spectral location of the excited cladding mode resonances. Second, we demonstrate that if the PCF parameters are properly selected, those resonances are sensitive to increases in steps of 1 × 10−4 refractive index units (RIU) of the refractive index value close to that of water. To the best of our knowledge, this is the first time that the sensitivity of PCF cladding mode resonances to refractive index changes in water-based solutions is reported. We achieved experimental values of 40.3 nm/RIU in terms of wavelength sensitivity and −801 dB/RIU in terms of amplitude sensitivity. The performance of our sensor is therefore comparable to that of tilted FBGs in step-index fibers used for water refractometry, which indicates the potential of our PCF sensor for biosensing. In addition, the sensor fabrication does not require any post-processing such as etching or polishing, which allows preserving the integrity of the fiber probe. Finally, the narrow spectrum within which the PCF operates, allows envisaging multi-target detection with a single fiber probe by using cascaded wavelength-multiplexed gratings. [ABSTRACT FROM AUTHOR]

An easy-to-implement side-polished micro-structured fiber MSF)based on surface plasmon resonance (SPR) is proposed. The influence of fiber structure parameters on transmission performance is investigated by finite element method (FEM). The chemical stable gold is deposited on the side polished plane to provide the plasmonic material required for SPR. Ten cladding air-holes are used to confine light transmission and improve the coupling strength between the core mode and surface plasmon polaritons (SPP) mode. The optimization results show that the MSF integrates the characteristics of high refractive index (RI) sensing and broadband tunable polarization filtering. In addition, the RI detection range of the sensor can be further extended by depositing a $Si_{3}N_{4}$ dielectric layer on the gold film. The RI detection range of the sensor with gold coating is 1.20--1.325 with the maximum sensitivity of 12,000 nm/RIU. The RI measurement range of the sensor with gold- $Si_{3}N_{4}$ bilayer film can be extended to 1.20--1.37, and the maximum sensitivity is 10,000 nm/RIU. Furthermore, by changing the ambient RI of gold-coated fiber or the polishing depth of fiber, a broadband tunable filter with excellent performance can be obtained. Therefore, the proposed MSF not only has application value in the detection of biochemical liquid samples, but also has broad application potential in broadband communication networks, which is helpful to promote the development of miniaturization and integration of optical devices. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate the transmission spectral and surrounding refractive index (SRI) sensing performance of long-period fiber gratings (LPFGs) at 2 μm waveband. The cladding modes operating at 2 μm waveband show higher SRI sensitivity than 1.55 μm waveband. The influence of a little off resonance region of grating period, refractive index modulation and duty cycle of gratings are analyzed theoretically. Additionally, the LPFGs of LP08 mode near dispersion turning point operating at 2 μm waveband are investigated theoretically and experimentally. Mode splitting brings out with the increasing SRI. After the resonant dip splitting into two dips (dip A and dip A′), the SRI sensitivity of dip A are measured to be as high as −8495.2 nm/RIU in the SRI region of 1.445–1.457. The proposed 2-μm-waveband LPFGs makes it a potential in biochemical and environmental monitoring. [ABSTRACT FROM AUTHOR]

Terahertz (THz) technology offers multifaceted capabilities and disruptive applications in line-of-sight wireless communication, highly sensitive (bio)sensing, imaging and so on. However, the severe lack of polarization devices comparable to those for other electromagnetic frequencies is impeding moving towards practical applications. Here we demonstrate, for the first time, silicon-based integrated, high polarization extinction ratio, low insertion loss, and compact THz polarization beam splitters (PBSs) fabricated by CMOS-compatible technologies based on dielectric environment variation induced evanescent wave coupling effect. Through comprehensively analyzing working principles of conventional two-waveguide-based PBS, a high-performance three-waveguide PBS is successfully realized. Further optimizing the device parameters and working bandwidth, we achieve an integrated broadband PBS based on cascaded asymmetric directional couplers, whose polarization extinction ratios reach up to 20 dB and 32 dB for transverse electric and transverse magnetic modes, respectively, and their 10-dB bandwidth is 32 GHz for a device length of 15.5 mm. These numbers are the highest in silicon-based integrated THz PBSs so far. We believe our observations provide an effective way for exploring on-chip high-performance THz-PBSs for polarization-diverse systems, accelerating the development and application of on-chip THz photonics. [ABSTRACT FROM AUTHOR]

In this paper, a new architecture for developing plasmonic bio-sensors is proposed in which sensing is achieved through optical beam steering. The proposed structure consists of an array of nano-antennas that generate an outcoming optical beam whose direction varies when the material under test or its volume changes. This mechanism of sensing eliminates the requirement for complex instruments such as optical spectrum analyzers. For realization of the proposed bio-sensor, both 1-D and 2-D configurations for the nano-antenna array are designed and numerically studied. The full wave numerical simulation results show that the designed bio-sensor provides a very high sensitivity of $\text{33}^\circ $ per unit refractive index, and also the output light has an enough intensity to be observed by a naked eye. The final results show that although both versions have the same sensitivity, the 2-D structure can project the results with much higher intensity. It is also theoretically shown that the performance of the biosensor will be subject to the size of the array, and therefore, a practical large-scale version of the numerically studied structure would significantly outperform the simulated structure. [ABSTRACT FROM AUTHOR]

A converter that manipulates the energy exchange between two arbitrary guided modes having different mode orders and polarization states is proposed. This objective is achieved through engraving a periodic grating on a strip waveguide. Theoretical analysis, based on the coupled-mode theory, is developed to match the proposed structure. In addition, coupling analysis is addressed to provide a comprehensive view regarding the various couplings that can be achieved, including acceptable margins for perturbation designing parameters and their optimum values for each coupling. Moreover, our converter is implemented via both 3D-FDTD simulation and computational solution to examine the validity of the proposed approach and verify the coupling analysis through performing two conversions, namely TM $_{\boldsymbol{1}}$ -to-TE $_{\boldsymbol{0}}$ and TM $_{\boldsymbol{1}}$ -to-TE $_{\boldsymbol{3}}$. The first conversion is obtained with an insertion loss of $\boldsymbol{-1}\,$ dB and a crosstalk lower than $\boldsymbol{-17.5}\,$ dB at a conversion length of $\boldsymbol{9.15\,\mu }$ m. The second is executed with an insertion loss of $\boldsymbol{-1.5}\,$ dB and a crosstalk lower than $\boldsymbol{-15}\,$ dB at a conversion length of $\boldsymbol{16.937\,\mu }$ m. Furthermore, tolerance fabrication analysis is implemented to confirm the degree of stability that the proposed structure can achieve. Finally, the findings reveal that the proposed design achieved its function at a compact length and without being restricted by the hybridization approach. [ABSTRACT FROM AUTHOR]

In general, a sensor is used to monitor a single parameter only, and in many cases, a reference sensor is necessary to compensate the effect of temperature. Here, we demonstrate that a single supermode interferometer is capable of monitoring two parameters simultaneously. Said interferometer was fabricated with a segment of strongly coupled multicore fiber fusion spliced at the end of a standard single mode fiber. The free end of the multicore fiber was flat, thus, it behaved as a low reflectivity mirror whose reflection depended on the external refractive index. The reflection spectrum of our supermode interferometer consisted of well-defined periodic maxima and minima whose values and position varied when the interferometer was exposed to refractive index and temperature changes. In the Fourier domain, the changes of the interference pattern can be decoded easily. We demonstrate that the supermode interferometer here proposed can be useful to measure the thermo-optic coefficient of a sample. An important advantage of the device reported here is that the length of the multicore fiber is not determinant on the performance of the sensor. In addition, the device can be reused multiple times. [ABSTRACT FROM AUTHOR]

This study describes the design and characterization of a highly sensitive THz refractive index-based metamaterial sensor for the detection of biological samples. The proposed ultra-miniaturized THz sensor is constructed using a fan-shaped resonator enclosed within a square loop. The proposed sensor has a footprint of 0.353 $\lambda$ eff × 0.353 $\lambda$ eff where $\lambda$ eff is the wavelength calculated at the operating frequency of 4.87 THz. The sensitivity of the THz sensor is estimated using the absorption characteristics of the metamaterial. The estimated free space absorptivity is 99.9%. The proposed sensor is polarization insensitive due to the rotational symmetry and it is angularly stable up to 60°. The performance of the sensor is evaluated for different materials and biomedical samples. The results indicate that the proposed sensor has an average sensitivity of 1936 GHz/ Refractive Index Unit (RIU). Furthermore, the influence of the sample thickness is analysed, and the results are presented. From the results, it is inferred that the proposed THz sensor has a good Figure of Merit (FoM) and is suitable for chemical sensing, including the nucleic acids and other components present in the biological samples. [ABSTRACT FROM AUTHOR]

We numerically reveal the feasibility of designing multi-core fibers (MCFs) with the simple step-index (SI) profile within the standard 125-μm cladding diameter. We show that the standard cladding diameter enables us to incorporate five identical cores for C-band operation, and the number of cores can be further increased with the heterogeneous core arrangement. We then propose a novel method of allocating the non-identical cores in heterogeneous MCFs (Hetero-MCFs), where the crosstalk (XT) and critical bending radius (Rpk) are improved comparing to that of Hetero-MCFs designed by the conventional method. These new types of step-index MCFs (SI-MCFs) with standard cladding diameter have potential application in the space-division multiplexing (SDM) systems. [ABSTRACT FROM AUTHOR]

A new design of segmented cladding fiber (SCF) was proposed and fabricated in silica glass. The silica glass SCF had a central core and hexagonal wings with higher refractive indices than silica cladding. The proposed SCF was analyzed numerically in terms of the confinement loss and the effective mode area using finite element method (FEM). In experiments, SCF preform was prepared by a stack-and-draw method using two types of pre-drawn rods: silica rods for the cladding and Al-doped silica rods for the higher refractive index segments and the central core. The latter was fabricated in the powder in tube (PIT) process by optimizing the heating cycles and the bubble removal during the sintering process. Light guidance along the SCF was experimentally confirmed and its intensity patterns in the near-field at the wavelengths 0.980 μm and 1.550 μm were characterized. With the proposed SCF, all silica SCF fabrication feasibility was confirmed, which can open a new alternative for large mode area fibers. [ABSTRACT FROM AUTHOR]

In this paper, we derive the closed-form analytical expressions of the power coupling coefficients for a Laguerre-Gaussian beam coupled into a graded-index fiber with either central, offset, or tilted launch. We further generalize to an arbitrary launching condition with the radial offset and the three-dimension angular tilt simultaneously occurring. The analytical results are in good agreement with the numerical ones and can be applied to manufactured graded-index multimode fibers with an index exponent parameter from 1.8 to 2.2. Using the derived analytical expressions, the computation time shortens by at least a four-order of magnitude compared to conventional numerical methods. [ABSTRACT FROM AUTHOR]

Miniaturization of attenuated total reflectance (ATR) spectrometers has been an emerging field applying such powerful surface sensing method for in-situ spectroscopic analysis. In this work, we present a model for field propagation through ATR element under the illumination of multimode fiber (MMF) in a micro-electro-mechanical-systems (MEMS) based ATR Fourier transform infrared (FTIR) spectroscopic system. The core spectrometer is based on micro-fabricated Michelson interferometer using deep etching technology, in which the light propagates in-plane with respect to the silicon substrate. An ATR multiple reflection crystal is illuminated with infrared (IR) thermal blackbody radiation source through an MMF. The output light of the crystal is fiber-coupled to the MEMS interferometer then to an IR broadband photodetector. The optical system is modelled using scaler Fourier optics, where the fiber output field is represented by a group of spatially shifted elementary sources, to predict the ATR absorbance response, taking into account the partial spatial coherence nature of the MMF output. The model output leads to the requirements on the ATR measurement conditions and numerical aperture (NA) of the system. The model is compared to practical results of MEMS spectrometer which is experimentally characterized over the mid-infrared (MIR) wavenumber range from 5000 cm−1 to 2100 cm−1, lower limited by the used fiber and photodetector cut-off. Spectra of liquid samples are obtained using two different crystals and total internal reflection (TIR) angles showing good agreement with theoretical prediction. [ABSTRACT FROM AUTHOR]

A type of flexible hydrogel optical fiber, which consisted of a permeable core/clad structure with a step-index profile for highly efficient excitation light guiding and fluorescence emission collecting, has been judiciously designed and fabricated. The effects of refractive index and core/clad diameter on propagation loss of hydrogel optical fiber have been examined and verified theoretically and experimentally. To demonstrate the feasibility of the designed optical fiber for fluorescence sensing, in a representative experiment two types of fluorescent materials serving as a pH probe and internal reference were integrated into the core of the designed hydrogel optical fiber to obtain a ratiometric pH sensor, which eliminated the disturbance of the external environment. The prepared fluorescent sensor displayed linear responses to pH values ranging from 3.79 to 9.55 with a high sensitivity of 0.68 per pH unit, indicating the great potential of the hydrogel fiber-based sensor for on-site, real-time, and highly sensitive detection of pH in aqueous environments. [ABSTRACT FROM AUTHOR]

The apparent increase in magnetic field measurement has led to a growing demand for new sensing technologies. However, a long-lasting challenge unaddressed is how to achieve ultrahigh sensitive magnetic field measurement in both amplitude and orientation. Here, we demonstrate a compact-in-size and simple-to-implement fiber-optic magnetometer for highly sensitive vector magnetic field measurement. The sensor is based on magnetic nanoparticles coated tilted fiber Bragg grating inscribed in a commercial single mode fiber. The sensing mechanism stems from the backward coupled cladding mode resonances with narrow bandwidth (Q factor > 104), which is highly sensitive to slight surface refractive index perturbation. The switching of the magnetic field direction on the surface of fiber induces redistribution of coated magnetic nanoparticles with spatial anisotropy density and scattering by formation of magnetic chains hereafter the effective index is changed, which in turn modulates the cladding mode resonance with wavelength shift and amplitude attenuation. Through calculating the peak-to-peak intensity of target cladding modes of TFBG, the external magnetic field was precisely measured with an intensity sensitivity of 0.39 dB/mT and a direction sensitivity of 0.038 dB/rad. The inherent core mode can be used as a reference to get rid of temperature crosstalk and light source disturbance. The proposed sensor is compact in size and easy to be used in hard-to-reach place, showing good potentials for industrial applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We demonstrated a novel long-period fiber grating (LPFG) written in a tri-hole fiber (THF). The LPFG was fabricated by periodically collapsed micro holes utilizing CO2 laser irradiation integrated with pressure-assisted technology. The mode coupling between the fundamental mode and higher mode of the proposed grating is mainly caused by geometric deformation along the fiber axis. Compared with conventional LPFGs, such a grating has strong refractive index modulation due to periodic collapsed structure. We have established an approximate hypothesis to simulate the transmission spectrum of the grating. The measured spectrum is basically consistent with the simulated transmission spectrum at the resonant peak position. We investigated the sensing characteristics of the proposed LPFG, including axial strain, bending, torsion, and temperature. The sensitivity of bending, temperature and torsion are −9.56 nm/m−1, 162.8 pm/(rad/m) in clockwise and −346.9 pm/(rad/m) in counterclockwise and 86.8 pm/°C, respectively. The experimental data shows that the fabricated LPFG has potential applications in the field of external high-sensitivity strain sensing with a sensitivity of −26 pm/μϵ. [ABSTRACT FROM AUTHOR]

In the coherent Φ-OTDR system with small laser-frequency-drift less than 50 kHz, the phase change in the quiet region without external perturbation is theoretically deduced to be linear with the fiber length while its waveform in the direction of pulse sequence is highly similar with the external perturbation acting on the fiber before the quiet region. Moreover, the phase information of one position contains the perturbation information of all foregoing positions. Consequently, a simple and accurate solution of differential operation for acquiring the phase information of subsequent perturbation which is behind the initial perturbation occurring on the fiber is proposed. The fitted phase change trace between the foregoing and the subsequent perturbation is extended to the region behind the subsequent perturbation. Then, behind the subsequent perturbation, the extended phase change trace is subtracted by the fitted phase change trace containing two perturbations. Moreover, in order to afford the fitting data with less noise as much as possible, the phase changes at positions where the value of modulus of coherent Φ-OTDR is lower than the set threshold value is firstly eliminated. To verify these methods, an experiment with two PZT vibrations in this paper is described. For the subsequent PZT vibration, the sinusoidal phase signal with a frequency of 50 Hz is perfectly retrieved when the threshold is set to be 10 mV. And the discussion about threshold value shows that it is satisfactory for the accuracy of phase extraction of the subsequent perturbation if the threshold is set as or above the noise level in the form of modulus in the experiment. [ABSTRACT FROM AUTHOR]

Fiber Bragg gratings (FBGs) inscribed by using femtosecond laser point-by-point (PbP) technology typically have high birefringence due to the elliptical cross-sectional pattern of refractive index modulations (RIMs) created in the fiber core. Additionally, a highly reflective type II PbP FBG, which has a large coupling coefficient, also exhibits large insertion loss due to the limited RIM area induced by a single femtosecond laser pulse. Here we demonstrate a slit beam shaping method for femtosecond laser PbP inscription of high-quality FBGs, featuring by high reflectivity, low insertion loss, and low birefringence. The slit beam shaping method could reduce the ellipticity in the cross-sectional pattern of RIMs without reducing the cross-sectional area, leading to low birefringence and low insertion loss. The experimental results agree well with numerical calculations. Hence, a high-quality type II PbP FBG, which has high reflectivity of 99.12% (i.e., Bragg resonance attenuation of 20.52 dB), low insertion loss of 0.30 dB, and low birefringence of 1.86 × 10−6, was successfully created by use of a slit width of 0.8 mm. Moreover, an enlarged cross-sectional area was created by use of a slit width of 0.2 mm, resulting in a high ratio of 172.46 of the coupling strength coefficient to the scattering loss coefficient in the fabricated PbP FBG, which exhibits very high reflectivity of 99.99% (i.e., a strong Bragg resonance attenuation of 46.65 dB) and low insertion loss of 0.27 dB. Such high-quality FBGs will be promising in many applications, such as optical fiber communications, sensors, and lasers. [ABSTRACT FROM AUTHOR]

A geometrical optimization study of Ge-Sb-Se chalcogenide glass tapered fiber sensors for concentration sensing of methanol is presented. The effects of the geometrical parameters such as taper waist diameter and transition region lengths on the evanescent wave penetration depth are discussed based on the ray-tracing theory. Numerical analysis results demonstrate that the performance of the tapered fiber sensor can be enhanced by decreasing the transition length of the down-taper combined with increasing the transition length of the up-taper. A homemade tapering platform enables controlling the taper waist diameter and transition region length of the tapered fiber accurately. An experimental study using tapered fiber sensors with different transition region lengths is carried out to detect the concentration of methanol aqueous solution. The highest sensitivity of 0.0120 a.u./% is obtained from the tapered fiber with a down-taper transition length of 5.4 mm and an up-taper transition length of 7.9 mm, which agrees with the trend presented by the simulation results. The effects of symmetric and asymmetric tapered fiber structure on the sensing characteristics are also investigated. [ABSTRACT FROM AUTHOR]

In integrated photonics, the precise knowledge of the waveguides refractive index profile is mandatory for the modeling of photonic chips and therefore implementing innovative circuits. Usual index profile determination relies on effective index measurement of propagating modes in planar waveguides coupled with numerical fitting tools. In this paper we propose an alternative technique based on the characterization of the second harmonic generation signature of a nonlinear waveguide. We include the characterization of high-order spatial modes showing their relevance to probe both vertical and lateral distributions. We finally provide an explicit profile ready-to-use for modeling soft-proton exchanged waveguides in lithium niobate and we test its prediction capability. [ABSTRACT FROM AUTHOR]

The multi-mode vertical-cavity surface-emitting lasers (MM-VCSELs) with different photonic crystal (PhC) designs in core and cladding regions are compared for transmitting the complex formatted QAM-OFDM data over 100-m OM5 multi-mode fiber (MMF). Among the designs with using flat core/cladding (denoting as Coreflat/Cladflat) and/or PhC core/cladding (denoting as Corephc/Cladphc) for the MM-VCSEL, the Coreflat+Cladflat VCSEL with the highest threshold current and large emission aperture provides the highest differential quantum efficiency of 0.11 and output power of 2 mW, whereas the Coreflat+Cladphc2 VCSEL with two periods of cladding PhC reveals the lowest threshold current of 2 mA, the highest modulation bandwidth of 15 GHz, the lowest relative intensity noise peak of -126 dBc/Hz and background level of -143 dBc/Hz. After pre-compensation, the Coreflat+Cladphc2 VCSEL chip can achieve the highest OFDM bandwidth of 18 GHz for 72-Gbit/s transmission in the BtB case. With additionally adding 1 period of core PhC, the Corephc1+Cladphc2 VCSEL possesses a single-mode lasing spectrum to facilitate the suppression of modal dispersion after MMF propagation. Hence, the Corephc1+Cladphc2 VCSEL exhibits the same bit rate of 60 Gbit/s as compared to the Coreflat+Cladphc2 VCSEL after 100-m OM5-MMF. After improving the analog bandwidth for waveform synthesis, the data rate of Coreflat+Cladphc2 VCSEL can successfully deliver 76-Gbit/s and 72-Gbit/s single-band 16-QAM OFDM data streams for BtB and 100-m OM5 MMF conditions via the pre-leveling technology. To date, the ultimate raw data rate achieves 92-Gbit/s with employing the multi-band QAM-OFDM data format. [ABSTRACT FROM AUTHOR]

We develop an intuitive understanding for the various properties and guiding mechanism of M-type fibers. We also report the approximate expression for the V-parameter (normalized frequency) to predict the modal-cutoffs for the M-type fibers, for the first time. This work demonstrates the potential of the M-type fibers for novel applications such as tailoring the transmission spectrum of photonic bandgap fibers, bend-induced effective area reduction for non-linear applications, and all-fiberized filters. [ABSTRACT FROM AUTHOR]

Recently, few-mode Erbium-doped fiber amplifiers (FM-EDFA) for mode-division multiplexing (MDM) transmission have attracted much interest. In this paper, we propose a novel few-mode gain-flattening filter (FM-GFF) based on long-period fiber gratings (LPFG) in double-cladding few-mode fiber (DC-FMF). Due to weak coupling among all the core modes and limited number of inner-cladding modes, the phase matching condition for a specific FM-LPFG can be satisfied only between one core mode and one inner-cladding mode each time and independent gain flattening can be realized for each core mode without influencing the other ones. Moreover, both DC-FMF and transmission FMF have the same core refractive index and radius, while the inner-cladding of DC-FMF and the cladding of transmission FMF have the same refractive index, which can effectively maintain the weakly-coupled condition for their connections. We show by simulation that the output gain spectra for all the 4 or 6 linearly-polarized (LP) core modes in two kinds of DC-FMFs can be independently flattened to less than 0.6-dB in the whole C-band by the cascading LPFGs in the DC-FMF. The proposed FM-GFF scheme is beneficial for the practical design of FM-EDFAs. [ABSTRACT FROM AUTHOR]

We present novel waveguide writing concepts in bulk PMMA. The writing relies on laser induced modification tracks that are completely surrounding a waveguide core. We found the optimal parameters to construct highly reproducible, single-mode waveguides with minimal propagation losses down to 0.6 dB  cm−1. Employing the best geometry, we demonstrate 2D and 3D Y-splitters that are the building blocks for creating complex optical networks such as sensors or lab-on-chip devices in polymer materials. [ABSTRACT FROM AUTHOR]

We report on the thermal regeneration of tilted Bragg gratings (TFBGs) produced in hydrogen loaded SMF-28 optical fiber using two types of UV lasers: 266 nm femtosecond pulses and 193 nm nanosecond pulses. Different isochronal and isothermal annealing profiles are used to improve the regeneration of the cladding modes. Experimental results are in good agreement with simulation results. We show that TFBGs regenerate efficiently in the temperature range 850 °C – 900 °C when isothermal stepwise annealing profile is used. The regeneration efficiency of the cladding modes does not exceed 4.35%. The sensitivity to the surrounding refractive index is comparable to that of non-regenerated tilted Bragg gratings. These regenerated tilted Bragg gratings can be applied in several applications, like for example the follow up of high-temperature fabrication processes. [ABSTRACT FROM AUTHOR]

By filling the liquid crystal (LC) in the air holes of a photonic crystal fiber (PCF) with 8 mm length, a compact multi-band thermal optical switch is proposed here including two important telecommunication band, i.e., 1265 to 1317 nm and 1508 to 1568 nm. Within the bandwidth, the extinction ratio can be as high as 30 dB. Under the combined action of the bandgap-guiding mechanism and the interference effect, a temperature change of only 1 °C after clearing temperature (Tc) of the LC induces multi-band thermal optical switch of the LC-filled PCF. It is also noted that the transmission spectrum of the LC-filled PCF is quite sensitive to the ambient temperature. The consequent thermal sensitive is 3.9 nm/°C, making it a very good candidate for monitoring the temperature herein. [ABSTRACT FROM AUTHOR]

A compact and robust mid-infrared refractive index-based microstructured fiber sensor with high sensitivity is developed for monitoring analytes in liquids and gases. Resonant core-shell elements facilitate its exceptional performance. The theory underpinning them is explained, and the evolution of the fiber structure is presented. The material choices and details of the microstructure leading to its optimization are described. Simulation results demonstrate that the optimized fiber sensor can readily detect refractive index variations in the range from 1.0 to 1.41 and that its wavelength sensitivities are 2510 and 4303 nm/RIU for analyte detection, respectively, in gases and liquids. It is an excellent candidate for environmental monitoring applications. [ABSTRACT FROM AUTHOR]

We propose a photonic crystal fiber with a high refractive index ring, which supports up to 180 orbital angular momentum (OAM) modes without any high order radial modes from 1.5 um to 1.7 um. The finite element method is employed to numerically study the properties of the designed fiber. Further research shows that the refractive index differences of all the eigenmodes are above 2 × 10−3, which indicates that the corresponding eigenmodes can be well-separated and ensures the stable transmission of OAM modes. In addition, it is quite interesting that the high order radial modes are significantly suppressed via increasing the radius of the cladding in the design, while most of the OAM modes are without phase distortion, which is beneficial for the (de)multiplexing of the OAM modes. Particularly, the mode quality of all the eigenmodes is higher than 94.9% and the chromatic dispersion is flat with minimum value of the dispersion variation is 0.36 ps/(km·nm). Moreover, the designed fiber possesses various advantages such as low nonlinear coefficient (< 0.7 W/km) and confinement loss (on the order between 10−12 and 10−7 dB/m). The proposed fiber has shown great potential for high capacity OAM mode division multiplexing communications. [ABSTRACT FROM AUTHOR]

A fiber attached coupler for transmission bandpass whispering gallery mode (WGM) resonator is proposed and experimentally demonstrated. The coupler fabricated by femtosecond laser ablation consists of a pair of curved waveguides with a curvature radius of 50 μm, by which the WGM field is excited and collected through evanescent field coupling, respectively. As a proof of concept, a polymer microsphere functioning as the WGM cavity is embedded into a femtosecond laser ablated hole between the two curved waveguides. A passband with a full width of 0.15 nm at half maximum is observed when the microsphere is surrounded by air, verifying the feasibility of the proposed coupler for constructing a WGM cavity-based transmission narrow bandpass optical fiber filter. Benefitting from the relatively high thermo-optical coefficient, the passband of a packaged transmission bandpass WGM resonator can be thermo-optically tuned with a sensitivity of 446.6 pm/°C. [ABSTRACT FROM AUTHOR]

A compact mode converter (MC) has been demonstrated based on tapered hole-assisted dual-core fiber (HADCF). The MC is fabricated by splicing single mode fiber (SMF) and a piece of HADCF that is composed of a central single mode core (SMC), an eccentric few mode core (FMC), and a large eccentric air hole. The HADCF is tapered in a controllable manner and the fundamental mode LP01 of the central core is coupled to the high-order LP11 mode of the eccentric core as two modes satisfy the phase matching condition. The high-order LP11 mode can be converted effectively across the range from O-band to C-band by only changing the tapered length. With the decreasing taper ratio, the mode conversion wavelength has a blue shift. The measured purity of the LP11 modes in the range from 1310 to 1550 nm are always higher than 95%. In addition, the tapered HADCF-MC is insensitive to the change of the temperature and external refractive index. [ABSTRACT FROM AUTHOR]

High-sensitivity fiber-optic interferometric refractive index sensors (FIRSs) have been of interest to researchers due to their potential to fabricate specific physical, chemical, and biological sensors. Fabricating interferometers working near the dispersion turning point (DTP) is an effective approach to improve the sensitivity of FIRSs. However, the group effective refractive index (RI) difference approaching 0 and the ratio of the variation of the effective RI difference to the external RI change being −1 cannot be simultaneously realized in low RI sensing, which restricts the further improvement of sensing sensitivity. Here, dual-path Mach-Zehnder interferometers (MZIs) with unequal geometrical path length are proposed for ultrasensitive RI sensing. The dual-path MZIs contain fiber path and sample path of different geometrical lengths to form the optical path difference. The dual-path MZIs can not only show a turning point where the sensing sensitivity tends to be infinite, which is similar to the previously reported DTP, but also get the constant value −1 for the RI response factor, leading to the result that the dips within an over-500-nm band width around the turning point can achieve high sensitivity reaching 105 nm/RIU level or higher. Ultrahigh sensitivity of −1.26 × 106 nm/RIU has been experimentally demonstrated at the RI around 1.35022. The dual-path MZIs proposed here may enlighten new ideas for developing high-sensitivity FIRSs. [ABSTRACT FROM AUTHOR]