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Accurate image reconstruction in volumetric optoacoustic tomography implies the efficient generation and collection of ultrasound signals around the imaged object. Non-uniform delivery of the excitation light is a common problem in optoacoustic imaging often leading to a diminished field of view, limited dynamic range and penetration, as well as impaired quantification abilities. Presented here is an optimized illumination concept for volumetric tomography that utilizes additive manufacturing via 3D printing in combination with custom-made optical fiber illumination. The custom-designed sample chamber ensures convenient access to the imaged object along with accurate positioning of the sample and a matrix array ultrasound transducer used for collection of the volumetric image data. Ray tracing is employed to optimize the positioning of the individual fibers in the chamber. Homogeneity of the generated light excitation field was confirmed in tissue-mimicking agar spheres. Applicability of the system to image entire mouse organs ex vivo has been showcased. The new approach showed a clear advantage over conventional, single-sided illumination strategies by eliminating the need to correct for illumination variances and resulting in enhancement of the effective field of view, greater penetration depth and significant improvements in the overall image quality., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In this paper, we have presented the far field pattern in presence and absence of Kerr type nonlinearity for the first higher order mode in Dispersion shifted and dispersion flattened type optical fibers. Our analytical results are based on simple power series expressions for the first higher order (LP11) mode of aforesaid fibers, which have been formulated by Chebyshev formalism. Using the analytical expressions for the linear case, method of iteration is applied in order to predict the concerned propagation parameters in presence of Kerr type nonlinearity. We have taken some typical trapezoidal index, step W and Parabolic W fibers for our study. Our results for the far field pattern have been shown to be in excellent agreement with the exact numerical results computed by rigorous finite element technique. The simplicity and accuracy of our formalism will prove helpful to the designers for setting up of efficient low dispersion optical link. [ABSTRACT FROM AUTHOR]

Heavy metal fluoride glasses are the backbone of modern mid-infrared fiber technology. In the last few years, optical fibers made of fluorozirconate, fluoroaluminate, and fluoroindate glasses have reached the high level of reliability that is required for their widespread use in industrial and medical applications, which explains the rapid growth of this technology. In this tutorial review, the author, who was part of the team that discovered heavy metal fluoride glasses exactly fifty years ago, provides a summary of the glass synthesis process and elucidates the most important physical properties of heavy metal fluoride glasses. Finally, a brief overview of their main application areas is presented. [ABSTRACT FROM AUTHOR]

As demand for customized specialty fibers grows, standardized production methods face challenges. This article reviews industry standards and discusses potentially disruptive techniques that enable rapid prototyping and fabrication of optical fiber devices. Furthermore, we showcase laser powder deposition's (LPD) potential for additive manufacturing (AM) of customized glass structures. In the case of, for example, fiber preforms, although the feasible size is smaller than the industry standard, utilizing laser‐based manufacturing techniques for a small batch production presents an attractive avenue for rapid prototyping and expedites material and design optimization. In the realm of AM of glass, LPD offers numerous benefits, including minimal shrinkage, high densification, and the ability to tailor glass composition to achieve desired optical properties. The article reviews the latest achievements and highlights future directions in this technology. [ABSTRACT FROM AUTHOR]

We report on the properties of the Power over Fiber (PoF) transmission link using a High-Power Laser Source operating at 976 nm and using three types of optical fiber with a core diameter of 50 µm. Two step-index profile multimode optical fibers and one fiber with a gradient index were used for optical power transmission. Optical light was converted to electricity using commercially available Photovoltaic Power Convertors (PPCs) with a maximal optical input power of 1.5 W and experimental PPCs with a maximal optical input power of 4.0 W. We experimentally proved optical power transmission up to a distance of 300 m. In the case of the commercially available working PPC and using the gradient index fiber we achieved a result of 0.534 W of electrical power and using the experimental PPC we achieved 0.645 W. In the case of the step-index optical fiber, the result was 1.3 W. [ABSTRACT FROM AUTHOR]

The feasibility of achieving efficient coupling between an optical fiber and a periodically poled lithium niobate (PPLN) waveguide in an ideal optical system was theoretically analyzed using a finite element simulator. Divergence of single-mode polarization-maintaining fiber and lensed fiber in space was simulated. Direct end-face coupling simulations were conducted for a PPLN ridge waveguide with a cross-sectional size of 10 μm x 10 μm and a length of 20 mm, involving two different types of fibers. It is found that despite the lensed fiber achieving a coupling efficiency of up to 95%, its fundamental mode contribution is less than 5%. Given that PPLN frequency-doubling devices primarily rely on the laser's fundamental mode for operation, this renders the coupling efficiency of the lensed fiber relatively low in practical applications. In contrast, ordinary single-mode fiber exhibits a significantly higher fundamental mode contribution of 93. 8%, demonstrating its superior performance. Therefore, this study selects ordinary single-mode fiber for encapsulation testing with PPLN ridge waveguides. The experimental results show that when the maximum pump power output from the fiber amplifier reaches 1. 6 W, the calculated input pump power, after deducting the coupling losses between the input fiber and the waveguide, stands at 1. 2 W. The fiber-to-waveguide coupling efficiency is 75%, slightly exceeding the currently known advanced value of 72%. At a temperature of 24. 8 °C, with an input power of 1. 2 W at the fundamental wavelength of 1 560 nm, the maximum output of frequency-doubled light is 653 mW. The optical-to-optical conversion efficiency reaches 54. 4%, with a normalized conversion efficiency of 20. 2% / (W·cm²). [ABSTRACT FROM AUTHOR]

The precise evaluation of micro-crack sizes and locations is crucial for the safe operation of structures. Traditional detection techniques, however, suffer from low spatial resolution, making it difficult to accurately locate micrometer-scale cracks. A method and experimental study were proposed in this paper for identifying and locating micro-cracks using optical fiber strain sensing based on OFDR to address this issue. The feasibility of this method for micro-crack detection was verified by the combination of a polyimide-coated sensing optical fiber (PISOF) and tight sheath sensing optical fiber (TSSOF). A calculation method for micro-crack widths based on distributed optical fiber strain curves was established, and the test results of different optical fibers were compared. Through multiple verification experiments, it was found that the strain peak curves of both fiber types could accurately locate micro-cracks with a precision of 1 mm. Additionally, the crack widths could be obtained by processing the distributed strain curves using a computational model, enabling the accurate capture of micro-crack characteristics at the 10 μm level. A strong linear relationship was observed between the optical fiber stretching length and the crack width. Notably, the relative error in calculating the crack width from the strain curve of PI fiber was very small, while a linear relationship existed between the maximum strain value of the TSSOF and the crack width, allowing for the calculation of the crack width based on the maximum strain value. This further validated the feasibility of the method designed in this paper for the analysis of micro-crack characteristic parameters. [ABSTRACT FROM AUTHOR]

Laser direct writing (LDW) is versatile in structuring fibers with micro‐sized functional elements such as fiber Bragg grating (FBG) and backscattering centers by finely manipulating back and side scattering from laser‐induced refractive index modified (RIM) points. However, the side‐scattering is a lesser‐explored property in laser‐structured fibers. In this work, a concise physical model is established to understand the side‐scattering as a combined effect of microstructure and geometry of RIM points. Based on a single‐pulsed LDW method, the parametric decoupling between scattering loss (α) and coupling strength (κ) coefficients of FBGs is reported, whose cross‐section is customized to have a flattened ellipse with thoroughly positive RIM, enabling controllable reflectivity from −21.33 dB to −0.0018 dB while maintaining narrow bandwidth and low loss. Exemplarily, a designed FBG realizes ultra‐low loss of 0.008 dB with a resonance attenuation of 10.81 dB, exhibiting a record‐breaking κ/α of 2083. Using this FBG as the high‐reflective mirror of a home‐made bismuth‐doped fiber laser, narrow‐band lasing with a high optical signal‐to‐noise ratio of ≈43 dB is achieved, demonstrating flexibilities of the proposed approach in customizing both back‐ and side‐scattering in fibers and opening up wide opportunities for combining multifunctional components into optical fibers and realizing all‐fiber networks. [ABSTRACT FROM AUTHOR]

A highly sensitive and simple structure side-polished micro-structured fiber (MOF) refractive index (RI) sensor based on surface plasmon resonance is proposed and studied by the finite element method. The gold nanofilm used to stimulate plasmons is directly deposited on the inner wall of the open-loop channel of a MOF consisting of an arrangement of six cladding air-holes. The sample analyte is arranged on the outer surface of the optical fiber to improve the utilization rate of the sensor and reduce the tediousness of operation. Meanwhile, in order to increase the leakage intensity of evanescent field, three large air-holes were introduced near the fiber core. Research has shown that the proposed sensor can achieve sensing detection in the range of 1.19–1.37 RI. The maximum sensitivity in the x-polarized direction can reach 12,500 nm/RIU, and its corresponding resolution is 3.64 × 10–5 RIU. Moreover, it has a high linear response of 0.9944 within the RI detection range of 1.19–1.33. Obviously, the proposed sensor has a simple structure, wide RI detection range, and high sensitivity, which can make it widely used in biomedical and analytical chemistry. [ABSTRACT FROM AUTHOR]

A new type of anti-resonant hollow-core fiber for terahertz waveguides is proposed. By introducing central support pillars and an elliptical structure, the fiber achieves high birefringence while maintaining low confinement loss and low material absorption loss. The fiber structure is optimized through simulation using the finite element method. The optimized fiber exhibits a birefringence of up to 1.22 × 10−2 at a frequency of 1 THz, with a confinement loss of 8.34 × 10−6 dB/cm and a material absorption loss of 7.17 × 10−3 dB/cm. Furthermore, when the bending radius of the fiber is greater than 12 cm, the bending loss of the anti-resonant optical fiber at 1 THz is less than 1.36 × 10−4 dB/cm, demonstrating good bending resistance and high practical value. It is expected to play a significant role in optical communication systems. [ABSTRACT FROM AUTHOR]

We propose three hollow-core anti-resonant fibers with different nested tube structures and numerically analyse their confinement loss, single-mode performance and bending loss by using the finite element method. The results show that the nested semi-elliptical structure FD#2 and the hybrid nested FD#3 structure have excellent transmission performance. The confinement loss of FD#2 and FD#3 at 1.55 µm is 0.00037 dB/km and 0.0004 dB/km respectively, and the high order mode extinction ratio is around 1700 for both FD#2 and FD#3. The bandwidths of FD#2 and FD#3 with losses less than 0.01 dB/km are within 1.36–1.73 μm and 1.42–1.8 μm, respectively. In addition, the bending resistance of FD#2 and FD#3 is excellent, with bending losses of 0.003 dB/km and 0.001 dB/km at a bending radius of 8 cm. We also evaluated the manufacturing tolerance range of these two structures in the actual manufacturing process. The structure proposed in this paper is expected to provide new ideas for the design of nested elements, and provide more possibilities for optical fiber design suitable for large-capacity data transmission and gas sensing applications. [ABSTRACT FROM AUTHOR]

Optical fibers play a crucial role in various fields, including free-space optical communication and quantum communication. The beam coming out of the fiber facet, unfortunately, has a very high divergence angle. This divergence can be problematic, especially when coupling the light into free space or to other optical components that require collimated beams. The beam divergence can be reduced and the beam can be shaped with the help of suitable collimating optics, which serves as the coupler to couple light between fiber and free-space. However, these traditional collimating optics consists of lenses that are generally spherical in nature and are limited in their ability to shape the phase of the beam as they can provide only a spherical phase to the incoming Gaussian beam from the fiber. This limitation can be overcome by using metasurfaces, which can shape the phase of the beam in a more tailored manner. In this paper, we have designed the metalens that can be directly formed on the fiber facet and can collimate the 1550 nm beam coming out of the single mode fiber facet with the use of two different kinds of cylindrical metalens. These metalens can reduce the beam divergence and can achieve diffraction limited collimation while maintaining a transmittance of ≥ 95 %. [ABSTRACT FROM AUTHOR]

The humidity has often been measured through a single point sensor. Where, the humidity could be varied at different locations as well as depending on environmental conditions. The present paper developed the dual point humidity measuring sensor by using a polymer optical fiber (POF) based on a single illuminating fiber. The sensor's basic structure is to twist two fibers and bend them at a certain radius. However, the dual point sensor is developed through the cascading of twisted micro bend (TMB-1 and TMB-2). The twisting of fibers couples the light from one fiber to another fiber through the side coupling method. An increase in the humidity level leads to a change in the reflective index, which helps to get variation in coupled light intensity. To measure the humidity, the dual point sensors are placed into the control humidity chamber at two random positions. The power reading variation is significantly linear when the humidity level increases from 30 to 80%. The sensor has a fast response of about 1 s and a recovery time of about 4 s. Furthermore, the chemical coating is applied to improve the sensor's sensitivity. Between 30 and 80% range of humidity, the both sensors of dual point TMB-1 and TMB-2 have appropriate sensitivity and detection limits, which is about 680.8 nW/% and 763.9 nW/% and 1.37% and 1.98%, respectively. To measure the humidity at variable positions, the present dual points humidity sensor is well-stable, easy, and straightforward, which uses a less expensive method. [ABSTRACT FROM AUTHOR]

The measurement of the pole axis deviation angle of superconducting magnetic levitation rotors plays a crucial role in the field of high-precision navigation. This article conducts theoretical analysis and experimental research on the diffuse reflection intensity modulation principle of optical fiber sensors and the ideal diffuse reflection modulation model under ideal conditions. First, the structural distribution of optical fiber sensors is presented, and the principle of measuring the polar deviation angle of superconducting magnetic levitation rotors is analyzed. Then, based on the diffuse reflection intensity modulation principle, an optical fiber intensity modulation model for ideal superconducting magnetic levitation rotors is established. The correctness of this model was verified through simulation and calibration experiments, and the error between the simulation and experimental results was less than 8%. [ABSTRACT FROM AUTHOR]

Label-free and multiphoton micro-endoscopy can transform clinical histopathology by providing an in situ tool for diagnostic imaging and surgical treatment in diseases such as cancer. Key to a multiphoton imaging-based micro-endoscopic device is the optical fiber, for distortion-free and efficient delivery of ultra-short laser pulses to the sample and effective signal collection. In this work, we study a new hollow-core (air-filled) double-clad anti-resonant fiber (DC-ARF) as a high-performance candidate for multiphoton micro-endoscopy. We compare the fiber characteristics of the DC-ARF with a single-clad anti-resonant fiber (SC-ARF) and a solid core fiber (SCF). In this work, while the DC-ARF and the SC-ARF enable low-loss (<0.2 dBm−1), close to dispersion-free excitation pulse delivery (<10% pulse width increase at 900 nm per 1 m fiber) without any induced non-linearities, the SCF resulted in spectral broadening and pulse-stretching (>2000% of pulse width increase at 900 nm per 1 m fiber). An ideal optical fiber endoscope needs to be several meters long and should enable both excitation and collection through the fiber. Therefore, we performed multiphoton imaging on endoscopy-compatible 1 m and 3 m lengths of fiber in the back-scattered geometry, wherein the signals were collected either directly (non-descanned detection) or through the fiber (descanned detection). Second harmonic images were collected from barium titanate crystals as well as from biological samples (mouse tail tendon). In non-descanned detection conditions, the ARFs outperformed the SCF by up to 10 times in terms of signal-to-noise ratio of images. Significantly, only the DC-ARF, due to its high numerical aperture (NA) of 0.45 and wide-collection bandwidth (>1 µm), could provide images in the de-scanned detection configuration desirable for endoscopy. Thus, our systematic characterization and comparison of different optical fibers under different image collection configurations, confirms and establishes the utility of DC-ARFs for high-performing label-free multiphoton imaging-based micro-endoscopy. [ABSTRACT FROM AUTHOR]

A polarized light microscope (PLM) was utilized to examine the optical textures of mesophase pitch (MP) and MP-derived fibers, which aimed to reveal the arrangement and orientation characteristics of pitch molecules and to clarify the evolution and transformation mechanism of carbonaceous microcrystalline from pitch fibers to graphitized fibers. The results found that there were distinct optical textures in MP, where one side exhibited a transition from a flattening plane to a mountain-like undulating plane. This transition corresponded to the arrangement of pitch molecules, resembling stacked lamellar structures reminiscent of curved paper. Meanwhile, the optical textures of fibers revealed that the blue substance was wrapped around the red grain-like domains in the longitudinal section and confirmed that the red part belonged to the pyridine insoluble fraction of MP and the blue part belonged to its pyridine-soluble fraction. After graphitization, the red part was transformed into graphite sheets and the blue part was transformed into an amorphous carbon layer which was wrapped around the graphite sheets, forming a carbonaceous microcrystalline package-like bag. Therefore, this study provided a comprehensive interpretation of the structural evolution mechanism of MP and MP-derived fibers based on their macro-optical textures and micro-nanostructures. [ABSTRACT FROM AUTHOR]

The paper describes a new approach for determining the threshold for stimulated nonlinear scattering in active and passive optical fibers. It is proposed to fix the power of the Stokes component at a certain level, which will make it possible to determine the threshold values of laser radiation powers, which characterize the manifestation of stimulated nonlinear scattering at the expected power level, in a convenient way from the applied point of view. Verification of the obtained expressions is given. [ABSTRACT FROM AUTHOR]

Anti-resonant fiber (ARF) works well in a relatively strong magnetic field due to its weak Faraday effect, which results from the fundamental mode mainly transmitting in the air core. Accurately measuring the Faraday effect strength, i.e., the effective Verdet constant, of an ARF determines its applicable scenarios. However, the effective Verdet constant of ARF is ~3 orders of magnitude lower than that of a standard single-mode fiber, which is very difficult to measure. In this paper, we reveal that intermodal interference is the main obstacle to measuring the ultralow effective Verdet constant of ARF and propose using a narrow-band low-coherence light to suppress it. The measured effective Verdet constant of ARF is 0.423 ± 0.005 mrad/T/m at 1550 nm. [ABSTRACT FROM AUTHOR]

In this paper, a temperature sensing scheme with a miniature MZI structure based on the principle of inter-mode interference is proposed. The sensing structure mainly comprises single mode–coreless–multimode–coreless–single mode fibers (SCMCSs), which have been welded together, with different core diameters. The light beam has been expanded after passing through the coreless optical fiber and is then coupled into a multimode optical fiber. Due to the light passing through the cladding and core mode of the multimode optical fiber with different optical paths, a Mach–Zehnder interferometer is formed. Moreover, due to the thermo-optic and thermal expansion effects of optical fibers, the inter-mode interference spectrum of a multimode fiber shifts when the external temperature changes. Through theoretical analysis, it is found that the change in the length of the sensing fiber during temperature detection has less of an effect on the sensitivity of the sensing structure. During the experiment, temperature changes between 20 and 100 °C are measured at sensing fiber lengths of 1.5 cm, 2.0 cm, 2.5 cm, 3.0 cm, 3.5 cm, and 4.0 cm, respectively, and the corresponding sensitivities are 65.98 pm/°C, 72.70 pm/°C, 67.75 pm/°C, 66.63 pm/°C, 74.80 pm/°C, and 72.07 pm/°C, respectively. All the corresponding correlation coefficients are above 0.9965. The experimental results indicate that in the case of a significant change in the length of the sensing fiber, the sensitivity of the sensing structure changes slightly, which is consistent with the theory that the temperature sensitivity is minimally affected by a change in the length of the sensing fiber. Therefore, the effect of the length on sensitivity in a cascade-based fiber structure is well solved. The sensing scheme has an extensive detection range, small size, good linearity, simple structure, low cost, and high sensitivity. It has a good development prospect in some detection-related application fields. [ABSTRACT FROM AUTHOR]

Multi-band transmission is one of the key practical solutions to cope with the continuously growing demand on the capacity of optical communication networks without changing the huge existing fiber base. However, ultra-broadband communication requires the development of novel power efficient optical amplifiers operating beyond C- and L-bands, and this is a major research and technical challenge comparable to the introduction of the seminal erbium-doped fiber amplifiers that dramatically changed the optical communication sector. There are several types of optical fibers operating beyond C- and L-bands that can be used for the development of such amplifiers, specifically the fibers doped with neodymium, praseodymium, thulium, and bismuth. However, among these, Bi-doped fibers are of special interest as the most promising amplification medium because, unlike the others, different Bi-associated active centers allow amplification in an enormous band of overall width of 700 nm (1100–1800 nm). Such spectral coverage can be obtained by using different host materials, such as aluminosilicate, phosphosilicate, silica, and germanosilicate glasses. Here, we report a novel Bi-doped fiber amplifier with record characteristics for E-band amplification, including the highest power conversion efficiency among telecom-compatible E-band amplifiers reported to date. This bismuth-doped fiber amplifier (BDFA) features a maximum gain of 39.8 dB and a minimal noise figure of 4.6 dB enabled by 173 m Bi-doped fiber length. The maximum achieved power conversion efficiency of 38% is higher than that of L-band Er-doped fiber amplifiers. This performance demonstrates the high potential of BDFA for becoming the amplifier of choice in modern multi-band optical communication networks. [ABSTRACT FROM AUTHOR]

The utilization of hybrid fibers enables the amalgamation of the benefits offered by several types of fibers, while concurrently mitigating their respective limitations. The primary objective of this study is to produce a newly developed hybrid polymer composite that incorporates novel natural fibers, specifically basalt and hemp fiber mat, together with an epoxy matrix and boron carbide (B4C) filler. The objective of this study is to assess the impact of incorporating boron carbide into a polymer composite consisting of basalt and hemp fibers, specifically in relation to its mechanical and thermal shielding properties. The incorporation of boron carbide resulted in enhancements in mechanical properties, specifically an average increase of 8.7% in tensile strength (measured at 191 MPa), flexural strength (measured at 194 MPa), and impact energy (measured at 34 J). Furthermore, the thermal shield analysis demonstrated that the B4C filler possesses the capability to effectively attenuate neutrons, thereby serving as an efficient thermal insulator (with a neutron attenuation coefficient of 4.8 dB/m). The determination of the mode of failure and bonding strength of this hybrid composite can be achieved by conducting a morphological study. [ABSTRACT FROM AUTHOR]

Multiplexing techniques will be employed based on duration, polarization, and frequency to achieve the expanding demand for broadcast bandwidth. Adding time as an additional aspect to transmission networks has been put out as a flexible way to handle potential bandwidth problems. For interaction programs such as space imaging, optical fiber setup, submerged portable visual hyperlinks, onboard interconnects, information centers indoor relations, radio signals, and auditory interactions, we examine the possibilities of utilizing time as a further level of independence. Beginning in 2010, space-divisional multiplexer (SDM) studies gained popularity, partly because they had been suggested to increase the data-carrying ability of fiber optics while also boosting effectiveness through collaboration on assets. The proposed SDM transfer technologies spectrum includes identical single-mode fibers using shared amplifiers pumping laser light and the complete spatial arrangement of transceiver equipment, signal analysis, and amplifiers near a fiber with more than 100 geographical networks composed of several threads, all executing various forms. In this study, we discuss SDM study's advancements. The major categories and characteristics of innovative SDM fibers, like multiple-core fibers (MCFs), multiple-mode fibers, fewer-mode MCFs, and coupled-core MCFs, are initially described. While examining present tendencies and speculating on potential advances and uses outside optical data transfer, we provide analyses between various fiber kinds. [ABSTRACT FROM AUTHOR]

We present a technique that utilizes cascaded resonant cylindrical piezoelectric ceramics and multimode optical fibers wound around them to effectively mitigate laser speckle. By precisely driving the ceramics at their resonant frequencies and inducing comprehensive mode scrambling within the multimode fiber, we achieve a remarkable speckle suppression efficiency of up to 94%. To the best of our knowledge, this sets a new benchmark among various methods aimed at suppressing the speckle of a coherent light. Our study thoroughly explores variables influencing efficiency, including the cascading number of piezoelectric ceramics, driving voltage, fiber core diameter, and more. This method has significant promise for diverse applications that require efficient and fast control of speckle contrast. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study and the optimization of very short discrete multi-pump Raman amplifiers using Tellurite-based optical fibers. While most studies about discrete Raman amplifiers usually focus on cases where hundreds of meters of fiber are used, here we consider a multi-pumped Raman amplifier setup with extremely short dimensions, in which the length of the gain media was varied from 2.5 to 30 meters. The optimization process was developed over an broad amplification band from 1520 to 1600 nm, considering a maximum acceptable ripple of 3 dB, and using 2, 3 and 4 pumps with individual powers always below 2.5 W. The results show that it is possible to achieve net gains as high as 10 dB for gain media around 20 meters, and close to 15 dB for 30 meters. These high gains highlight the fact that Tellurite-based optical fibers can be used for the construction of very short and compact broadband amplifiers using just a few pump lasers with modest optical powers. [ABSTRACT FROM AUTHOR]

Excessively tilted fiber gratings (ExTFGs) are a type of special optical fiber grating device different from traditional fiber Bragg gratings, long period fiber gratings, and tilted fiber Bragg gratings. Due to the excessively tilted fiber fringe structure in the fiber core, ExTFGs could couple the light of the core mode into the high-order forward-propagating cladding modes, which would split into two sets of polarization dependent modes resulting in dual-peak resonances in the transmission spectrum. ExTFGs have the properties of the high refractive index sensitivity and low thermal crosstalk, which makes them very suitable for biochemical sensing applications. This paper will review the development of ExTFGs in terms of the mode coupling behavior, spectra characteristic, especially the refractive index sensitivity enhancement, biochemical modification methods of the sensor, and their applications in the bio-chemical sensing area, including pondus hydrogenii (pH) heavy metal ions, humidity, glucose, and immune sensing for various animal virus and biomarkers. Moreover, several composite sensing structures based on ExTFGs will be summarized. [ABSTRACT FROM AUTHOR]

Quantum key distribution (QKD) is a secure communication scheme for sharing symmetric cryptographic keys based on the laws of quantum physics, and is considered a key player in the realm of cyber-security. A critical challenge for QKD systems comes from the fact that the ever-increasing rates at which digital data are transmitted require more and more performing sources of quantum keys, primarily in terms of secret key generation rate. High-dimensional QKD based on path encoding has been proposed as a candidate approach to address this challenge. However, while proof-of-principle demonstrations based on lab experiments have been reported in the literature, demonstrations in realistic environments are still missing. Here we report the generation of secret keys in a 4-dimensional hybrid time-path-encoded QKD system over a 52-km deployed multicore fiber link forming by looping back two cores of a 26-km 4-core optical fiber. Our results indicate that robust high-dimensional QKD can be implemented in a realistic environment by combining standard telecom equipment with emerging multicore fiber technology. High-dimensional QKD would in principle allow for several advantages over its bidimensional counterpart, but in-the-field demonstrations are missing. Here, the authors realise 4- dimensional hybrid time-path-encoded QKD using a 52-km deployed multicore fiber link. [ABSTRACT FROM AUTHOR]

This study introduces a novel image capture and lighting techniques using a cutting-edge hybrid MEMS scanner system designed for compact microscopic imaging. The scanner comprises a tapered optical fiber waveguide and innovative aerosol-jet printed PZT (lead zirconate titanate) bimorph push-pull actuators on a stainless-steel substrate, effectively addressing issues that are commonly associated with PZT on silicon substrates such as fracture and layer separation. By leveraging nonlinear vibration, the scanner achieves a spiral scan pattern from a single signal input, in addition to the expected two-dimensional scanning and target illumination from two phase-shifted inputs. This capability is further enhanced by a novel process to taper the optical fiber, which reduces illumination scattering and tunes the fiber to the resonant frequencies of the scanner. The precisely tapered tip enables large fields of view while maintaining independent 2-axis scanning through one-degree-of-freedom actuation. Experimental validation showcases the successful generation of a spiral scan pattern with a 60 μm diameter scan area and a 10 Hz frame rate, effectively reconstructing scanned images of 5 μm lines, cross patterns (15 μm in length with a 5 μm gap), and structures of a Psychodidae wing. [ABSTRACT FROM AUTHOR]

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This article considers the deformation behavior of Panda optical fiber using different models of material behavior for the tasks of predicting residual stresses after drawing when cooling from 2000 °C to room temperature (23 °C) and indenting the fiber into an aluminum half-space at different parameters. These studies were conducted for single- and double-layer protective coatings (PCs), at different values of external load and thickness of single-layer PC. This paper determined the fields of residual stresses in the fiber formed during the drawing process. They are taken into account in modeling the fiber performance in the further process of this research. This article investigated two variants of PC behavior. The influence of behavior models and the number of covering layers on the deformation of the "fiber-half-space" system was analyzed. This paper establishes qualitative and quantitative regularities of the influence of the external load magnitude and relaxation properties of PCs on the deformation and optical characteristics of Panda optical fiber. [ABSTRACT FROM AUTHOR]

Behavior of a new step-index optical fiber having a Reuleaux-triangular-shaped core cross-section has been examined numerically. Two cases involving straight and bent optical fibers have been mainly discussed. Using James' equal-area formula, cut-off conditions of this proposed fiber have been given. Reuleaux-triangular-cored fiber can have a larger modal field area than circular-cored fiber with the same parameters when the normalized frequency is less than a certain value. As to a bent Reuleaux-triangular-cored fiber, bending orientation angle of this fiber is the most important factor determining the mode field distribution, except for the normalized frequency and bending radius. Compared with circular-cored optical fiber, the reduction of modal field area of this new fiber caused by bending can be reduced by 8.16%, and the bending loss can be reduced by 22.16%. Thus, the Reuleaux-triangular-cored fiber may be used as a bending sensor. [ABSTRACT FROM AUTHOR]

This study characterizes the transmittance behavior of structural light-transmitting concrete under natural sunlight. The experimentation involves the use of a novel test setup and a detailed analysis considering the variation and dependence on time of day, month of the year and seasonal variations. The test set consisted of 28 variations of fiber configurations, with two different diameters, spacing and bundling techniques used to increase the area of fibers while maintaining spacing to aid the placing of concrete without compromising on transmittance. The study provides a real-time observational understanding of the behavior of light-transmitting concrete, a result usually obtained by modelling and simulation. The statistical analysis helps in understanding the impact of various variables as well as their interrelationships, which can help in design optimization. Based on the behavior as well as the stipulations of standards, the applicability of the material to various structural applications has been identified. [ABSTRACT FROM AUTHOR]

Rare-earth-doped optical fibres are widely used in lasers and amplifiers. The incorporation of ytterbium and aluminium oxide in a high doping concentration has led to the fabrication of a multi-mode (MM) optical fibre. Within this research, the design, preparation and calculation for the production of a fibre with a targeted 45 μm core diameter are explored. By Energy Dispersive X-ray (EDX) analysis, the doping concentrations of the elements in the core have been measured as 60.4 at.% Al and 1 at.% Yb. Supporting micrographs are used for confirming the core/cladding ratio. Based on the atomic percentage concentration, the calculated refractive index of the multi-element core has an n = 1.61 and an NA = 0.678. Characterisation of the fibre, including absorption and emission cross-section analysis, was performed in order to prove the ability of the fibre to be used for amplification as well as lasing applications. [ABSTRACT FROM AUTHOR]

M-type optical fibers in which a core is surrounded by a thin ring layer with a higher refractive index have attracted increasing attention in recent years. One of their advantageous features is the ability to operate a non-fundamental LP02 mode possessing unusual dispersion properties, namely, a zero-dispersion wavelength (ZDW) shifted to the short wavelength region relative to the material ZDW. The LP02 mode can be selectively excited since it is predominantly localized near the core, while the fundamental LP01 and other higher modes are localized near the ring (for proper fiber parameters). In this paper, we present a comprehensive theoretical analysis of effective dispersion tailoring for the HE12 mode of highly nonlinear chalcogenide glass fibers (for which the LP mode approximation fails due to large refractive index contrasts). We demonstrate fiber designs for which ZDWs can be shifted to the spectral region < 2 μm, which is of great interest for the development of mid-IR supercontinuum sources and frequency-tunable pulse sources with standard near-IR pumping. We obtained the characteristic equation and solved it numerically to find mode fields and dispersion characteristics. We show the possibility of achieving dispersion characteristics of the HE12 mode with one, two, three, and four ZDWs in the wavelength range of 1.5–5.5 μm. We used a modified genetic algorithm (MGA) to design fibers with desired dispersion parameters. In particular, by applying an MGA, we optimized four fiber parameters and constructed a fiber for which HE12 mode dispersion is anomalous in the 1.735–5.155 μm range. [ABSTRACT FROM AUTHOR]

Research on optical amplifiers has highlighted how ionizing radiation negatively impacts the performance of erbium-doped fiber amplifiers (EDFAs), through the degradation of their gain. The amplitudes and kinetics of this degradation are mainly explained by the radiation-induced attenuation (RIA) phenomenon at the pump and signal wavelengths. In this work, the gain degradation of a radiation tolerant EDFA (exploiting a cerium-co-doped active optical fiber) induced by ionizing radiation up to 3 kGy (SiO2), at two dose rates, 0.28 Gy/s and 0.093 Gy/s, is studied through an experimental/simulation approach. Using a home-made simulation code based on the rate and power propagation equations and including the RIA effects, the radiation-dependent performance of EDFAs were estimated. The variations in the spectroscopic parameters caused by irradiation were also characterized, but our results show that they give rise to EDFA gain degradation of about 1%. To overcome the issue of overestimating the RIA during the radiation tests on the sole active rare-earth-doped fiber, a new RIA experimental setup is introduced allowing us to better consider the photobleaching mechanisms related to the pumping at 980 nm. A good agreement between experimental and simulated gain degradation dose dependences was obtained for two different irradiation conditions, thus also validating the simulation code for harsh environments applications. [ABSTRACT FROM AUTHOR]

Gold nanoparticles (AuNPs) integrated within optical fibres show great potential as sensor materials in the field of sensing, utilizing the Surface Plasmon Resonance (SPR) principle. This study presents a straightforward, cost-effective, and replicable approach for producing permanent in-situ embedding of AuNPs with nanocavities on the surface of optical fibres. The process involves chemical vapour etching (CVE), followed by gold sputtering and isothermal heat treatment. The analysis conducted using High-Resolution Transmission Electron Microscopy (HRTEM) and Energy-Dispersive X-ray Spectroscopy (EDX), confirmed the presence of nearly spherical AuNPs embedded on the surface of the optical fibre, with an average diameter of 60 ± 5 nm. Additionally, the detection of two distinct plasmon absorption peaks at 567 ± 5 nm and 620 ± 5 nm using a Fibre Optic Spectrometer further validated the presence of AuNPs. To verify the existence of nanocavities surrounding the AuNPs, Field Emission Transmission Microscopy (FESEM) was employed, along with subjecting the AuNPs embedded fibre to three different NaCl solutions of varying concentrations. The results supported the presence of a nanocavity encapsulating the AuNPs. A proposed mechanism was put forth to explain the spectral behaviour resulting from the creation of the nanocavity. This specialty optical fibre is expected to have broad applications in the development of SPR-based chemicals and biosensors, as it facilitates close interaction between the AuNPs and analytes through the nanocavity surrounding the AuNPs on the fibre surface. The fabrication process demonstrated excellent repeatability, with strong adhesion of AuNPs to the optical fibre surface, and no leaching even after repeated use. Thus, we believe that this fabrication method is an exceptional technique for permanently incorporating AuNPs into various glass-based optical waveguide materials for SPR-based sensing applications. [ABSTRACT FROM AUTHOR]

Materials synthesis in a microfluidic environment enables the flexible and controllable production of various types of nanostructures which are of great potential in the fields of chemistry, environmental science, bioengineering, and medicine. Here, we demonstrate on-chip simultaneous continuous-flow synthesis and in situ spectrum diagnosis of zinc oxide (ZnO) nanomaterials using a femtosecond-fabricated three-dimensional (3D) microchannel reactor integrated with an array of optical fiber probes. The microchannel reactor including 3D concentration gradient generators followed by 3D micromixing units provides high-efficiency manipulation of reactants with different concentrations as well as parallel reaction dynamics in an autonomous manner. The integrated optical fiber probe array allows precise and parallel spectroscopic detection in different microchannels with high spatial and temporal resolutions for screening the synthetic conditions. The synthesized ZnO nanostructures can be tailored in size, shape, and morphology by tuning the flow rates and reactant concentrations based on the spectroscopic signals detected with the fiber probe array. [ABSTRACT FROM AUTHOR]

An ultracompact optical fiber Fabry–Perot interferometric sensor is proposed and demonstrated for high‐temperature measurement. The device is simply a section of single‐mode fiber with angled end face of 45°, the reflected light from the angled fiber end face is directed sequentially to the fiber core–cladding and cladding–air interfaces, respectively, and reflected back, and finally returned to the fiber core after being reflected by the angled fiber end face again. As a result, a Fabry–Perot interferometer is constructed. Because of the thermal‐expansion and thermal‐optical effect of the fiber material, the optical path difference between the core–cladding and cladding–air surfaces is changed with temperature, hence the device is well suitable for temperature measurement. In particular, the fiber device has good high‐temperature sustainability up to 1100°C, which makes it attractive for high‐temperature measurement in a narrow or hard‐to‐reach spatial environment. [ABSTRACT FROM AUTHOR]

Optical traps, utilizing a laser to confine and manipulate microscopic particles, are widely employed in various scientific applications. We propose a miniaturized dual-beam fiber optical trap for acceleration sensing. It comprises two counter-propagating beams' output from a customized pair of single-mode fiber pigtailed focusers (SMFPF). We investigate the correlation between the misalignment and the coupling efficiency of the SMFPF pair. By maximizing the coupling efficiency, the optimal alignment is achieved. A multimode fiber (MMF) is introduced to collect and transmit side-scattered light of a trapped microsphere for motion detection. By analyzing the experimental output signal, we acquire displacement information of the trapped microspheres under both aligned and misaligned conditions. This paper provides a simple and practical solution for the alignment of dual beams and the integration of the optical traps' levitation and detection structure, which lay a solid foundation for the further miniaturization of dual-beam optical traps. [ABSTRACT FROM AUTHOR]

Here, we report on the development and application of a compact multi-core fiber optical probe for multimodal non-linear imaging, combining the label-free modalities of Coherent Anti-Stokes Raman Scattering, Second Harmonic Generation, and Two-Photon Excited Fluorescence. Probes of this multi-core fiber design avoid moving and voltage-carrying parts at the distal end, thus providing promising improved compatibility with clinical requirements over competing implementations. The performance characteristics of the probe are established using thin cryo-sections and artificial targets before the applicability to clinically relevant samples is evaluated using ex vivo bulk human and porcine intestine tissues. After image reconstruction to counteract the data's inherently pixelated nature, the recorded images show high image quality and morpho-chemical conformity on the tissue level compared to multimodal non-linear images obtained with a laser-scanning microscope using a standard microscope objective. Furthermore, a simple yet effective reconstruction procedure is presented and demonstrated to yield satisfactory results. Finally, a clear pathway for further developments to facilitate a translation of the multimodal fiber probe into real-world clinical evaluation and application is outlined. [ABSTRACT FROM AUTHOR]

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Bismuth‐doped phosphosilicate fibers have become the most promising gain medium for O‐band amplifiers. Yet scientific challenges on understanding the nature of bismuth active centers (BACs), mechanisms of bismuth cluster formation in the phosphosilicate glass network still exist. It is likely that multiple BACs with different oxidation states in different structural sites all contribute to the broad, nonsymmetric luminescence and gain spectra. Due to the progress in the fundamental understanding of bismuth‐doped phosphosilicate glass, various designs of optical amplifiers with decent performances have been demonstrated. [ABSTRACT FROM AUTHOR]

This work focuses on the methods of creating in-fiber devices, such as sensors, filters, and scatterers, using the fiber fuse effect. The effect allows for the creation of structures in a fiber core. However, it is necessary to know exactly how this process works, when the plasma spark occurs, what size it reaches, and how it depends on external parameters such as power and wavelength of radiation. Thus, this present study aims to create the possibility of predicting the consequences of optical breakdown. This paper describes a mathematical model of the optical breakdown initiation in a fiber core based on the thermal conductivity equation. The breakdown generates a plasma spark, which subsequently moves along the fiber. The problem is solved in the axisymmetric formulation. The computational domain consists of four elements with different thermophysical properties at the boundaries of which conjugation conditions are fulfilled. The term describing the heat source in the model is determined by the wavelength of radiation and the refractive indices of the core and the shell and also includes the radiation absorption on the released electrons during the thermal ionization of the quartz glass. The temperature field distributions in the optical fiber are obtained. Based on the calculations, it is possible to estimate the occurrence times of various phase states inside the fiber, in particular, the plasma spark occurrence time. [ABSTRACT FROM AUTHOR]

This paper presents results of investigation of core–clad optical fibers based on rare-earth-doped multicomponent chalcogenide glasses of the Ge–Ga(In)–As(Sb)–Se systems. Optical, emission, and lasing characteristics of such fibers in the IR spectral region demonstrate advantages of their materials over foreign analogues owing to low concentrations of regulated impurities. [ABSTRACT FROM AUTHOR]

Lasers are emerging as novel photonic tags for single‐cell labeling, anticounterfeiting, and encryption technology due to their narrow linewidth, high spectral multiplexing capacity, and superior stimuli‐responsiveness. These laser‐encoded photonic tags mostly distinguish the heterogeneity but do not yet provide both tagging and sensing of biosamples, which is highly desirable for disease screening. Here, a photonic bandgap (PBG) fiber microlaser that works as a 2D tag and an immunosensor is developed. The tubular PBG structure allows strong light–matter interaction and supports dual‐band lasing in the same optical fiber, enabling massive biosample tagging and sensitive biodetection. By encoding the random resonant peaks in the short‐wave band and multiplexing in the spatial domain, a 2D laser tag is generated with a large encoding capacity (>28500). Immunosensing of microalbumin is realized by using the periodic resonant peaks in the long‐wave band, and a limit of detection of 0.06 ng µL−1 is achieved. This work is inspiring for the development of high‐performance, multifunctional integrated devices for disease screening. [ABSTRACT FROM AUTHOR]

A novel magnetostrictive thin film fiber sensor is presented which can be utilized in an interferometric architecture. The magnetostrictive fiber sensor utilizes novel fiber and thin film technology to achieve high sensitivity in the sub-nanotesla regime. The base sensor architecture utilizes optical fiber wound in a novel low stress flat coil resembling a record, coated with a thin magnetostrictive film. Two prototype variants were fabricated, incorporating FeCo sputtered films of different thicknesses on 4 μm polyimide jacket single mode fiber. The sensors were incorporated into a fiber optic interferometric measurement apparatus to characterize time-varying magnetic field sensitivity. Device results are presented which demonstrate sensitivity is a function of film thickness. The experimental data exhibited a two-order magnitude sensitivity improvement as the thin film thickness was doubled. Sensitivity projections are made based on film thickness. [ABSTRACT FROM AUTHOR]

Scattering is one of the main challenges in scaling up photocatalytic water splitting using the most prevalent powder catalysts. This can be overcome by decoupling the reaction medium from light transmission, as in the case of optical fibers. Here we explore utilizing optical fibers coated with 5 wt% CuO supported on TiO 2 for photocatlytic H 2 production from water-methanol mixtures. CuO/TiO 2 is a well studied photo catalyst in which photoreduced Cu species are known to act as sensitizers for inducing visible light activity. Lower activity of sequentially coated systems indicates that appropriate interfaces of active Cu and TiO 2 with water are desirable. The scalability of such optical fiber-based systems along with potential in non-potable turbid water media are demonstrated. Maximum activity of 22 μmoles of H 2 in 8 h was obtained with 50 mg of catalyst coated on optical fibers, which increases linearly with increase in fiber numbers, whereas, drastic reduction in activity is observed in powder catalyst upon increasing the catalyst quantity. A one-to-one comparison of 700 mg of catalyst in powder form and coated on optical fibers indicates more than one order enhancement in activity in the optical fiber based system. In addition, ∼70% retention in activity in highly turbid non-potable water was observed as compared to powdered system which shows complete reduction in the activity by 99.99%. • 5 wt%CuO/TiO 2 coated optical fiber bundles are studied for photocatalytic hydrogen evolution via water splitting. • Change in directionality of light reduces scattering issues observed in powder catalysts on scale up. • H 2 production can be increased by merely increasing number of optical fibers. • More than one order enhancement in activity in the optical fiber when compared to powder on scale up. • Retention of ∼70% activity in turbid non-potable water. [ABSTRACT FROM AUTHOR]

We built the cross‐spectral density matrix elements analytical expression of random electromagnetic Gaussian vortex (REGV) beam propagating in gradient‐index (GRIN) fiber, which is used to study the polarization properties of the beam. The distribution of spectral degree of polarization (SDOP) and state of polarization (SOP) show the same periodicity and symmetry. The distribution law of SDOP and SOP is unchanging with the topological charge m changing and is changed by adjusting the spatial self‐correlation length σxx. The research results have guiding significance for the propagation of random electromagnetic vortex beams in optical fibers, which could be considerable value in high‐speed and large‐capacity optical fiber secure communication. [ABSTRACT FROM AUTHOR]

A highly birefringent and low-loss hollow-core anti-resonant fiber (HC-ARF) based on a hybrid guidance mechanism is proposed and investigated by using a finite element method. The hybrid guidance mechanism is caused by the anti-resonance effect and the total internal reflection effect. The proposed HC-ARF is obtained by employing twin symmetrical and mutually tangential elliptical arc anti-resonance layers (EA-ARLs) in a conventional 8-tube HC-ARF. Because of the anti-resonance mechanism and the total internal reflection mechanism in the EA-ARL, mode coupling appears between the core mode and the cladding mode. Simulation results indicate that the proposed HC-ARF can achieve birefringence as high as 10−2 in a near-infrared range of 1400 nm to 1600 nm and a low confinement loss (CL) of 7.74 × 10−4 dB/m (9.26 × 10−4 dB/m) for x- and y-polarization components of the fundamental mode (FM) at 1550 nm. In addition, the existence of the 8-tube anti-resonance structure in the cladding significantly suppresses the CL of the x-polarization component of the FM significantly, but the impact on the CL of the y-polarization FM can be ignored, which is determined mainly by the twin EA-ARLs. Furthermore, the performance of the birefringence and CL are also investigated by changing the values of other fiber structure parameters. Our proposed structure successfully shows the ability of the hybrid guidance mechanism in the application of CL manipulation of orthogonal polarization components. [ABSTRACT FROM AUTHOR]

Conductive fibers are vital for next‐generation wearable and implantable electronics. However, the mismatch of mechanical, electrical, and biological properties between existing conductive fibers and human tissues significantly retards their further development. Here, the concept of neuro‐like fibers to meet these aforementioned requirements is proposed. A new wet spinning process is established to continuously produce pure gelatin hydrogel fibers. The key is the controllable and rapid gelation of spinning solutions based on the salting‐out effect, which is inspired by the Chinese food tofu. The resultant fibers exhibit neuro‐like features of soft‐while‐strong mechanical properties, high ionic conductivity, and superior biological properties including biodegradability, biocompatibility, and edibility, which are crucial for implanted applications but seldom reported. Furthermore, all‐weather suitable neuro‐like fibers with excellent anti‐freezing and water retention properties are developed by introducing glycerol for wearable applications. The optical fiber, transient electronics, and electronic data glove made of neuro‐like fibers profoundly demonstrate their potential in biomedical applications. [ABSTRACT FROM AUTHOR]