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Free space optical technology promises to revolutionize point-to-point communications systems. By taking advantage of their vastly higher frequencies, coherent optical systems outperform their radio counterparts by orders of magnitude in achievable data throughput, while simultaneously lowering the required size, weight, and power (SWaP), making them ideal for mobile applications. However, the widespread implementation of this technology has been largely hindered by the effects of atmospheric turbulence, often necessitating complex higher-order adaptive optics systems that are largely unsuitable for deployment on mobile platforms. By employing tip/tilt beam-stabilization, we present the results of a bespoke low-SWaP optical terminal that demonstrated single-mode fiber (SMF) coupling. This was achieved by autonomously acquiring and tracking the targets using a combination of aircraft transponder and machine vision feedback to a root-mean-square (RMS) tracking error of 29.4 µrad and at angular rates of up to 0.83 deg/s. To the authors' knowledge, these works constitute the first published SMF coupled optical link to a full-sized helicopter, and we describe derived quantities relevant to the future refinement of such links. The ability to achieve SMF coupling without the constraints of complex adaptive optics systems positions this technology as a versatile quantum-capable communications solution for land-, air-, and sea-based platforms ranging across commercial, scientific, and military operators. [ABSTRACT FROM AUTHOR]

We demonstrate the use of laser diodes and multijunction photovoltaic power converters to efficiently deliver watts of electrical power for long-distance or cryogenic applications. Transmission through single-mode and multi-mode fibers at the wavelengths of 808 nm and 1470/1550 nm are studied. An electrical output power of ~0.1 W is obtained after a 5 km transmission through a standard single-mode SMF28 fiber fed with 0.25 W of optical power. An electrical output power of ~1 W is demonstrated after a 5 km transmission with a standard OM1 multi-mode fiber fed with ~2.5 W. Photovoltaic conversion efficiencies reaching Eff ~49% are obtained with an output voltage of ~5 V using commercial multijunction laser power converters. For low-temperature applications, an ultra-sensitive silicon photomultiplier (SiPM) is used to detect the residual light leaked from fibers as the temperature is decreased. Our study demonstrates that specific fiber types enable low-loss transmission compatible with cryogenic requirements and without light leakage triggering of the SiPM. A cryogenic power-over-fiber system at ~1470 nm is demonstrated with ~2 W of electrical power converted over a 10 m distance having a conversion efficiency of Eff > 65% at 77 K. [ABSTRACT FROM AUTHOR]

We propose a power-over-fiber system with a new photovoltaic converter voltage control technology consisting of intermittent operation of a light source based on capacitor voltage estimation to achieve high energy efficiency. This efficiency is made possible by ensuring the photovoltaic converter voltage stays within an appropriate range. However, detecting and controlling the voltage on the photovoltaic converter side would waste power. In our system, a controller on the light source side estimates the voltage of photovoltaic converter side from our circuit model and controls the photovoltaic converter voltage by controlling light source activation/deactivation. Therefore, it is possible to maintain high photoelectric conversion efficiency without wasting power on the photovoltaic converter side. We show that this method can significantly improve energy efficiency from 11.0% to 22.4% compared to a method that controls the voltage on the photovoltaic converter side. We also demonstrate that an optical switch with a power consumption of 130 mW can be driven with 5-mW optical input. Our power-over-fiber system is practical because its optical power is low and safe enough to be used in existing optical access networks.

In this paper, to the best of our knowledge, a new type of hollow-core anti-resonant fiber (HC-ARF) design using hybrid silica/high-index material (HIM) cladding is presented for single-polarization, high-birefringence, and endlessly single-mode operation at 2 μm wavelength. We show that the inclusion of a HIM layer in the cladding allows strong suppression of $x-$polarization, while maintaining low propagation loss and single-mode propagation for $y-$polarization. The optimized HC-ARF design includes a combination of low propagation loss, high-birefringence, and polarization-extinction ratio (PER) or loss ratio of 0.02 dB/m, 1.2 $\times \, 10^{-4}$, and >550 respectively, while the loss of the $x-$polarization is >20 dB/m. The proposed fiber may also be coiled to small bend radii while maintaining low bend-loss of $\approx$0.01–0.1 dB/m, and can potentially be used as polarization filter based on the different gap separations and bend conditions.

Introduction: Optical coherence tomography (OCT) is a pivotal imaging modality in ophthalmology for real-time, in vivo visualization of retinal structures. To enhance the capability and safety of OCT, this study focuses on the development of a micro intraocular OCT probe. The demand for minimal invasiveness and precise imaging drives the need for advanced probe designs that can access tight and sensitive areas, such as the ocular sclera.Methods: A novel OCT probe was engineered using a piezoelectric tube with quartered electrodes to drive Lissajous scanning movements at the end of a single-mode fiber. This design allows the probe to enter the eyeball through a scleral opening. Structural innovation enables the outer diameter of the endoscopic OCT probe to be adjusted from 13G (2.41 mm) to 25G (0.51 mm), accommodating various imaging field sizes and ensuring compatibility with different scleral incisions.Results: The fabricated micro intraocular OCT probe successfully performed preliminary imaging experiments on in vivo fingers. The Lissajous scanning facilitated comprehensive coverage of the target area, enhancing the imaging capabilities.Discussion: The integration of a piezoelectric tube with quartered outside electrodes into the OCT probe design proved effective for achieving precise control over scanning movements and adaptability to different surgical needs. The design characteristics and practical applications demonstrated the probe’s potential in clinical settings.

Free space optical technology promises to revolutionize point-to-point communications systems. By taking advantage of their vastly higher frequencies, coherent optical systems outperform their radio counterparts by orders of magnitude in achievable data throughput, while simultaneously lowering the required size, weight, and power (SWaP), making them ideal for mobile applications. However, the widespread implementation of this technology has been largely hindered by the effects of atmospheric turbulence, often necessitating complex higher-order adaptive optics systems that are largely unsuitable for deployment on mobile platforms. By employing tip/tilt beam-stabilization, we present the results of a bespoke low-SWaP optical terminal that demonstrated single-mode fiber (SMF) coupling. This was achieved by autonomously acquiring and tracking the targets using a combination of aircraft transponder and machine vision feedback to a root-mean-square (RMS) tracking error of 29.4 µrad and at angular rates of up to 0.83 deg/s. To the authors’ knowledge, these works constitute the first published SMF coupled optical link to a full-sized helicopter, and we describe derived quantities relevant to the future refinement of such links. The ability to achieve SMF coupling without the constraints of complex adaptive optics systems positions this technology as a versatile quantum-capable communications solution for land-, air-, and sea-based platforms ranging across commercial, scientific, and military operators.

We experimentally investigate the problem of monitoring the Kerr-nonlinearity coefficient $\gamma$ from transmitted and received data for a single-mode fiber link of 1600 km length. We compare the accuracy and speed of three different approaches. First, a standard split-step Fourier method is used to predict the output at various $\gamma$ values, which are then compared to the measured output. Second, a recently proposed nonlinear Fourier transform (NFT)-based method, which matches solitonic eigenvalues in the transmitted and received signals for various $\gamma$ values. Third, a novel fast version of the NFT-based method, which only matches the highest few eigenvalues. Although the NFT-based methods do not scale with link length, we demonstrate that the SSFM-based method is significantly faster than the basic NFT-based method for the considered link of 1600 km, and outperforms even the faster version. However, for a simulated link of 8000 km, the fast NFT-based method is shown to be faster than the SSMF-based method, although at the cost of a small loss in accuracy.

The lack of access to broadband services and speeds that allows urban and rural populations to meet the minimum requirements of today’s society creates a lack of economic opportunities and digital inequality. In contrast, regions with adequate broadband infrastructure can attract investors and become the home of tech workers while boosting the local economy. To level the field and promote the expansion and improvement of broadband infrastructure, federal and state agencies in the U.S. provide funding through programs such as the Rural Digital Opportunity Fund, and The Enabling Middle Mile Broadband Infrastructure Program among others. Here, counties assess their broadband infrastructure, identify the funding opportunity, and compete for that funding by submitting grant applications. In this research paper, we present the problems, challenges, and lessons learned during the process of assessment of the broadband infrastructure for Polk County, Florida, USA. In addition, connectivity technologies and physical layer options for broadband internet infrastructure were reviewed and we present our recommendation in terms of technology for the upgrading and expansion of the current network. In particular, we discuss the challenges in determining actual network coverage by contrasting official data of the Federal Communications Commission (FCC) with several crowd-sensing datasets such M-Lab (Measurement Lab), Ookla Open Datasets, and results from the Polk County community broadband survey. We discuss the advantages and limitations of using these publicly available datasets and open-source tools. Finally, we provide guidelines and best practices in terms of data analysis and visualization that may be used by city planners and researchers.

Coupled to endoscopic techniques, Mueller polarimetry has been proven to be promising for early detection of certain diseases which affect biological tissues of inner organs. However, the depolarization power which is one of the most informative polarimetric characteristics cannot be directly measured through a fiber endoscope. For evaluating this quantity, we propose a method based on the Lu–Chipman decomposition of the averaged sum of the Mueller matrices of neighboring pixels. The principle is well supported by numerical simulations. Depolarization powers of Spectralon and of different areas of a biological sample are also successfully evaluated. [ABSTRACT FROM AUTHOR]

Nowadays, forward Brillouin scattering in optical fibers has attracted massive research interests worldwide, owing to its great potential for applications in sensing, filtering, lasing, and all‐optical signal processing, etc. The manipulation of spectrum properties of forward stimulated Brillouin scattering (FSBS) turns out to be particularly important for various application scenarios. However, the manipulation approaches are still very limited by now. Herein, for the first time to the best of one's knowledge, a thorough investigation on the fiber geometry dependence of FSBS effect in optical fibers is presented, where the dependencies of FSBS on fiber core diameter and cladding diameter are investigated, revealing the characteristics of FSBS in terms of linewidth, frequency interval, and nonlinear coefficient, etc. It is indicated in the result that changing the size of the fiber core/cladding might give rise to considerable modification of the FSBS spectrum. The investigation paves the way to engineer FSBS in optical fibers, where acousto‐optic interaction can be manipulated and the FSBS spectrum can be tailored by adjusting the geometric size of the fiber for various applications.

Chromatic dispersion is an important fiber attribute affecting transmission performance over optical fibers. Various chromatic dispersion measurement methods have been developed primarily for single-mode fibers. In the literature, measurement techniques were also developed to characterize few-mode fibers and multi-mode fibers. These methods are often subject to some limitations. In this paper, a simple and robust measurement method for chromatic dispersion measurement of single-mode fibers, polarization--maintaining fibers, and few-mode fibers is presented using a frequency domain instrument and a vector network analyzer. The method is applied to all three types of fibers through one measurement methodology uniformly. Using a vector network analyzer, the measurement instrument obtains the complex transfer function of fiber transmission. The inverse Fourier transform of the measured complex transfer function is used to determine the group delays for each mode of the fiber. Although the sampling is highly under-sampled for the whole fiber link, through proper treatment of the data, we can de-alias the signals and obtain accurate values of the group delays of each mode. By measuring the group delays over different wavelengths, the data can yield the chromatic dispersion of each mode over the wavelength window. [ABSTRACT FROM AUTHOR]

In order to study the relationship between the macro-bending loss characteristics of the single-mode fiber and the temperature change, the thermo-optical effect and thermal expansion effect are corrected according to the theoretical formula proposed by Faustini L. Based on this formula, the macro-bending loss of SMF28 and 1060XP single-mode fiber with core-cladding-infinite coating structure under the influence of temperature is simulated and analyzed. The influence of temperature, wavelength, bending radius and other factors on the macro-bending loss of SMF28 single-mode fiber is explored, and the linear fitting results between the temperature and the two kinds of single-mode fibers are obtained. The results show that the macrobending loss increases with the decrease of bending radius, and the increase of wavelength. The loss also decreases with the increase of temperature. Under certain bending radius and wavelength, the macro-bending loss and temperature are in monotonic relationship, which means increasing the number of bending turns can effectively increase the sensitivity to temperature.

In order to study the relationship between the macro-bending loss characteristics of the single-mode fiber and the temperature change, the thermo-optical effect and thermal expansion effect are corrected according to the theoretical formula proposed by Faustini L. Based on this formula, the macro-bending loss of SMF28 and 1060XP single-mode fiber with core-cladding-infinite coating structure under the influence of temperature is simulated and analyzed. The influence of temperature, wavelength, bending radius and other factors on the macro-bending loss of SMF28 single-mode fiber is explored, and the linear fitting results between the temperature and the two kinds of single-mode fibers are obtained. The results show that the macrobending loss increases with the decrease of bending radius, and the increase of wavelength. The loss also decreases with the increase of temperature. Under certain bending radius and wavelength, the macro-bending loss and temperature are in monotonic relationship, which means increasing the number of bending turns can effectively increase the sensitivity to temperature.

When two parallel waveguides are close to each other, the power of the two adjacent waveguides will be periodically converted to form a directional coupled waveguide system (). The transverse coupling between two parallel waveguides can be described by mode coupling theory. However, in practical application, due to the limitation of equipment or manual computing capacity, it is impossible to quickly calculate the coupling efficiency of two optical fibers through the current specific formula. Through the form of MATLAB programming, it can determine whether the fiber is a single-mode fiber, solve the characteristic equation to obtain the propagation constant, integrate the coupling coefficients K12 and K21, solve the coupling efficiency in the fiber, and finally output in the form of graphics. The results show that for the directional coupler composed of two fibers, if the core radius and core refractive index change, the mismatching phase constant and coupling efficiency will change correspondingly, and the calculated results are in good agreement with the predicted results. On this background, this paper proposes the form of MATLAB programming to calculate coupling efficiency. The coupling efficiency of fiber directional couplers is analyzed. The theory gives accurate results within the actual parameters.

Coupled to endoscopic techniques, Mueller polarimetry has been proven to be promising for early detection of certain diseases which affect biological tissues of inner organs. However, the depolarization power which is one of the most informative polarimetric characteristics cannot be directly measured through a fiber endoscope. For evaluating this quantity, we propose a method based on the Lu–Chipman decomposition of the averaged sum of the Mueller matrices of neighboring pixels. The principle is well supported by numerical simulations. Depolarization powers of Spectralon and of different areas of a biological sample are also successfully evaluated.

Chromatic dispersion is an important fiber attribute affecting transmission performance over optical fibers. Various chromatic dispersion measurement methods have been developed primarily for single-mode fibers. In the literature, measurement techniques were also developed to characterize few-mode fibers and multi-mode fibers. These methods are often subject to some limitations. In this paper, a simple and robust measurement method for chromatic dispersion measurement of single-mode fibers, polarization--maintaining fibers, and few-mode fibers is presented using a frequency domain instrument and a vector network analyzer. The method is applied to all three types of fibers through one measurement methodology uniformly. Using a vector network analyzer, the measurement instrument obtains the complex transfer function of fiber transmission. The inverse Fourier transform of the measured complex transfer function is used to determine the group delays for each mode of the fiber. Although the sampling is highly under-sampled for the whole fiber link, through proper treatment of the data, we can de-alias the signals and obtain accurate values of the group delays of each mode. By measuring the group delays over different wavelengths, the data can yield the chromatic dispersion of each mode over the wavelength window.

The paper reviews the factors limiting the accuracy of locating a fiber optic cable fault when using an optical time domain reectometer (OTDR) and describes an error estimation method for typical use cases. The primary source of errors lies in the complex relationship between the length of the optical fiber (measured by OTDR), its routing, cable design depending on cable design and type of installation (i.e. duct, directly buried, aerial) as well as the spare lengths used for service purposes. The techniques which considerably improve the accuracy of the fault localization processes are presented, the importance of accurate documentation of the network and of referencing the fault location to the nearest splice instead of end of the line are discussed, as is the absence of cable helix factor in data sheets. [ABSTRACT FROM AUTHOR]

The paper reviews the factors limiting the accuracy of locating a fiber optic cable fault when using an optical time domain reflectometer (OTDR) and describes an error estimation method for typical use cases. The primary source of errors lies in the complex relationship between the length of the optical fiber (measured by OTDR), its routing, cable design depending on cable design and type of installation (i.e. duct, directly buried, aerial) as well as the spare lengths used for service purposes. The techniques which considerably improve the accuracy of the fault localization processes are presented, the importance of accurate documentation of the network and of referencing the fault location to the nearest splice instead of end of the line are discussed, as is the absence of cable helix factor in data sheets

In this scientific research article, the new Kudryashov method and the tanh-coth method, which have not been applied before, are employed to construct analytical and soliton solutions of the (2+1)-dimensional Hirota–Maccari system. The (2+1)-dimensional Hirota–Maccari system is a special kind of nonlinear Schrödinger equation (NLSEs) that models the motion of isolated waves localized in a small part of space, and is used in such various fields as fiber optics telecommunication systems, nonlinear optics, plasma physics, and hydrodynamics. In addition, the Hirota–Maccari system defines the dynamical characters of femtosecond soliton pulse propagation in single-mode fibers. Analytical solutions of the model are successfully acquired with the assistance of symbolic computation utilizing these methods. Finally, 3D, 2D, and contour graphs of solutions are depicted at specific values of parameters. It is shown that the new Kudryashov method and the tanh-coth method are uncomplicated, very effective, easily applicable, reliable, and indeed vital mathematical tools in solving nonlinear models.

Introduction: Optical coherence tomography (OCT) is a pivotal imaging modality in ophthalmology for real-time, in vivo visualization of retinal structures. To enhance the capability and safety of OCT, this study focuses on the development of a micro intraocular OCT probe. The demand for minimal invasiveness and precise imaging drives the need for advanced probe designs that can access tight and sensitive areas, such as the ocular sclera. Methods: A novel OCT probe was engineered using a piezoelectric tube with quartered electrodes to drive Lissajous scanning movements at the end of a single-mode fiber. This design allows the probe to enter the eyeball through a scleral opening. Structural innovation enables the outer diameter of the endoscopic OCT probe to be adjusted from 13G (2.41 mm) to 25G (0.51 mm), accommodating various imaging field sizes and ensuring compatibility with different scleral incisions. Results: The fabricated micro intraocular OCT probe successfully performed preliminary imaging experiments on in vivo fingers. The Lissajous scanning facilitated comprehensive coverage of the target area, enhancing the imaging capabilities. Discussion: The integration of a piezoelectric tube with quartered outside electrodes into the OCT probe design proved effective for achieving precise control over scanning movements and adaptability to different surgical needs. The design characteristics and practical applications demonstrated the probe's potential in clinical settings., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Hu, Wu, Shi, Fan, Yu and Chen.)

Fabrication and experimental validation of a newly developed copper oxide nanoparticles (CuO-NPs) and silver nanoparticles (AgNPs) immobilized SM-MM-SM-MM-SM (SMSMS) in-line Mach–Zehnder interferometer (MZI) fiber structure-based sensor using localized surface plasmon resonance (LSPR) technique are reported in this article. The AgNPs and CuO-NPs are used to improve the biocompatibility and sensitivity of the sensor. The nanoparticles (NPs) and immobilized probe were characterized by UV-Vis spectrophotometer, HR-TEM, SEM, and EDX, revealing superior performance over the existing uric acid (UA) sensors. The proposed optimized structure-based optical fiber sensors (OFSs) exhibit excellent specificity, sensitivity, and limit of detection (LoD) for the detection of UA. Moreover, the developed sensor exhibits linear profile over a very wide range: 1) $10~\mu \text{M}$ –1 mM for detection over UA presents in serum, while the normal range of UA in serum is 100– $400~\mu \text{M}$ and 2) 0.4–10 mM for detection over UA presents in urine, while the normal range of UA in urine is 1.5–4.4 mM. In addition, the developed sensor has a distinguished advantage, such as multicompatibility over the detection of UA presents in both serum and urine along with its high selectivity, reusability, reproducibility, and lowest LoD. The proposed AgNPs and CuO-NPs immobilized and the uricase functionalized SMSMS structure-based LSPR sensor has better compatibility and are potent candidates for medical applications. [ABSTRACT FROM AUTHOR]

This study presents a label-free and efficient method for the diagnosis of collagen-IV by using an optical fiber-based Mach-Zehnder interferometer (MZI) structure. The structure of MZI is fabricated by splicing a 2.3 cm long segment of single-mode fiber (SMF) between two ~ 45 cm strands of multi-mode fibers (MMF). Owing to the mismatch of core diameters, cladding of SMF guides the light. SMF’s cladding is partially etched by using 40% hydrofluoric (HF) acid so that a part of propagating energy is released that can interplay with the surroundings. The etched region is immobilized with ~ 10 nm size of gold nanoparticles (AuNPs) and ~ 50 nm size of copper oxide nanoparticles (CuO-NPs) to initiate the influence of localized surface plasmon resonance (LSPR) phenomenon. For a comparative study, two different probes are developed and analyzed. In one probe, CuO NPs are immobilized over the etched part of SMF and named as CuO-NPs probe. In other probe, a monolayer of AuNPs is sandwiched between fiber surface and CuO-NPs and termed as AuNPs/CuO-NPs probe. The characterization of NPs and developed sensor probe are performed using UV-Vis spectroscopy, high-resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), and energy distribution spectroscopy (EDS). The sensor probes are made very specific towards collagen-IV by functionalizing them with collagenase enzyme obtained from Clostridium histolyticum. A wide range of Collagen-IV solutions of concentrations from 2 $\mu \text{g}$ /ml to 40 $\mu \text{g}$ /ml are sensed through the collagenase functionalized probe. The specificity of the probes are analyzed by sensing other biomolecules present in the human body. [ABSTRACT FROM AUTHOR]

An optofluidic chip of the single-mode fiber variable optical attenuator (VOA) is proposed. This chip has a simple structure, and it utilizes microfluid and air to regulate the optical attenuation, where air acts as a shutter. Here, the single-mode characteristics of the VOA chip are discussed at 1310 nm and 1550 nm. The experiment results indicate that it is feasible to regulate optical attenuation using the proposed chip. The VOA with this chip has a large optical attenuation range (82 dB) and a very wide operation waveband from visible to near infrared wavelengths. The measured insertion loss is 0.68 dB at 1310 nm and 0.92 dB at 1550 nm, and the measured return loss is 47.8 dB at 1310 nm and is 47.67 dB at 1550 nm.

Featured Application: The research results in this paper are helpful for researchers to analyze and design the adaptive optics (AO) systems for atmospheric laser transmission and detection systems, such as free space optical communication and gas sensing lidar. The method in this paper can be used to study the residual margin allocation of AO system under different coupling efficiency thresholds. In this study, Zernike polynomials and optical fiber field theory are applied to build a mathematical model of coupling efficiency (CE) and spatial mode of aberrations. The theory built in this paper can be used to quickly calculate the CE affected by a single aberration as well as the aberrations caused by atmospheric turbulence. The aberrations are classified based on Zernike polynomials and the effects of aberrations on CE of different types and different spatial frequencies are analyzed. The influence of the effects of AO system residual errors is also analyzed. Adaptive optics (AO) equipment is applied to build a system on which the proposed theory was tested; the experimental results validate the theoretical analysis. [ABSTRACT FROM AUTHOR]

This article presents an extrinsic fiber-optic acoustic sensor applied for partial discharge (PD) detection in air. A Fabry–Perot (F-P) cavity consisting of a single-mode fiber (SMF) and a graphene oxide (GO) film, whose thickness and effective vibration diameter are approximately 500 nm and 4.377 mm, respectively, is used as this sensing core, and the manufacturing process of GO diaphragms and this sensing probe is illustrated to be simple and controllable. Performance tests indicate that this proposed sensor maintains a linear acoustic-pressure response and a flat frequency response in the range of 200 Hz to 20 kHz, while being an omnidirectional sensor and having high working stability during a ten-day test period. Additionally, PD detection results show that the minimum PD size detected by this proposed sensor in air was approximately 100 pC, which demonstrates that this proposed sensor can achieve high-sensitivity PD detection in air.

The exploration of novel polymers for temperature sensing with high sensitivity has attracted tremendous research interest. Hence, we report a polystyrene-coated optical fiber temperature sensor with high sensitivity. To enhance the temperature sensitivity, flat, thin, smooth, and air bubble-free polystyrene was coated on the edge surface of a single-mode optical fiber, where the coating thickness was varied based on the solution concentration. Three thicknesses of the polystyrene layer were obtained as 2.0, 4.1, and 8.0 μm. The temperature sensor with 2.0 μm thick polystyrene exhibited the highest temperature sensitivity of 439.89 pm °C−1 in the temperature range of 25−100 °C. This could be attributed to the very uniform and thin coating of polystyrene, along with the reasonable coefficient of thermal expansion and thermo-optic coefficient of polystyrene. Overall, the experimental results proved the effectiveness of the proposed polystyrene-coated temperature sensor for accurate temperature measurement.

The problem of small bit error rate is actual in fiber Raman amplifiers. Our experimental data on the amplified spontaneous emission (ASE) with backward pumping as a base of optical noise evaluation are presented in this paper. Raman gain spectrum for useful Stokes low-powered signal is calculated and compared with obtained spectrum of non-coherent ASE noise. It is shown that output optical signal has better gain than the amplified spontaneous emission that leads to appreciable growth of output optical signal-to-noise ratio.

We study the problem of coupling partially coherent radially polarized (PCRP) vortex beams into a single-mode optical fiber. Using the well-known concept of the cross-spectral density (CSD) matrix, we derive a general expression for the coupling efficiency of the partially coherent beam into a single-mode fiber. We adopt PCRP vortex beams for incident beams and use our general results to discuss the effects of the coherence, topological charge, and wavelength on the coupling efficiency of an optical beam focused onto a single-mode fiber with a lens. Our results should be useful for any application that requires coupling of partially coherent beams into optical fibers. [ABSTRACT FROM AUTHOR]

An all-vapor phase MCVD process has been proposed for the fabrication of fiber preforms with a Yb2O3-Al2O3-P2O5-SiO2 multicomponent glass core. We have investigated the tubular preform collapse into a rod and demonstrated approaches capable of preventing P2O5 losses in the central part of the core during the collapse process. Preforms with a flat, perfect step-index profile have been fabricated. [ABSTRACT FROM AUTHOR]

The structure, characteristics and working principle of extrinsic F-P sensor were introduced in details, and application of the extrinsic F-P sensor in gas detection system was expounded. The sensor takes hollow core photonic crystal fiber as isotopic carrier, is not affected by environmental temperature, which has characteristics such as high sensitivity, strong anti-jamming capability and low transmission loss, and can significantly improve accuracy and precision of gas concentration detection.

In this study, Zernike polynomials and optical fiber field theory are applied to build a mathematical model of coupling efficiency (CE) and spatial mode of aberrations. The theory built in this paper can be used to quickly calculate the CE affected by a single aberration as well as the aberrations caused by atmospheric turbulence. The aberrations are classified based on Zernike polynomials and the effects of aberrations on CE of different types and different spatial frequencies are analyzed. The influence of the effects of AO system residual errors is also analyzed. Adaptive optics (AO) equipment is applied to build a system on which the proposed theory was tested; the experimental results validate the theoretical analysis.

A fiber reshaping-based refractive index (RI) sensor is proposed relying on both optical intensity variation and wavelength shift. The objective of this study is to completely reshape the core and to ultimately mimic a coreless fiber, thereby creating a highly efficient multimode interference (MMI) coupler. Thus, propagation modes are permitted to leak out into the cladding and eventually escape out of the fiber, depending on the surrounding environment. Two interrogation mechanisms based on both the intensity variation and wavelength shift are employed to investigate the performance of the RI sensor, with the assistance of leaky-mode and MMI theories. By monitoring the output intensity difference and the wavelength shift, the proposed RI sensor exhibits high average sensitivities of 185 dB/RIU and 3912 nm/RIU in a broad range from 1.339 to 1.443, respectively. The operating range and sensitivity can be adjusted by controlling the interaction length, which is appealing for a wide range of applications in industry and bioscience research.

An approach to obtain single-mode operation on a semirectangular large core area Flat Fiber (FF) slab waveguide is proposed. A new design of FF with two defect eye holes is presented, and its single-mode guiding is analyzed by simulation. To validate the simulation results, a proof of concept FF with a core area > 1200\rm\ \mu \rmm^2 was fabricated, and its modal behavior was examined. It was found that higher order propagation modes could be discriminated from the large core area by introducing a high-refractive index region in the FF eye holes. Experimental results confirmed a single Gaussian mode profile (mode field diameter of \approx745\rm\ \mu m) output of this large core FF, thus validating the simulation results. [ABSTRACT FROM PUBLISHER]

An optofluidic chip of the single-mode fiber variable optical attenuator (VOA) is proposed. This chip has a simple structure, and it utilizes microfluid and air to regulate the optical attenuation, where air acts as a shutter. Here, the single-mode characteristics of the VOA chip are discussed at 1310 nm and 1550 nm. The experiment results indicate that it is feasible to regulate optical attenuation using the proposed chip. The VOA with this chip has a large optical attenuation range (82 dB) and a very wide operation waveband from visible to near infrared wavelengths. The measured insertion loss is 0.68 dB at 1310 nm and 0.92 dB at 1550 nm, and the measured return loss is 47.8 dB at 1310 nm and is 47.67 dB at 1550 nm. [ABSTRACT FROM PUBLISHER]

We study the problem of coupling partially coherent radially polarized (PCRP) vortex beams into a single-mode optical fiber. Using the well-known concept of the cross-spectral density (CSD) matrix, we derive a general expression for the coupling efficiency of the partially coherent beam into a single-mode fiber. We adopt PCRP vortex beams for incident beams and use our general results to discuss the effects of the coherence, topological charge, and wavelength on the coupling efficiency of an optical beam focused onto a single-mode fiber with a lens. Our results should be useful for any application that requires coupling of partially coherent beams into optical fibers.

Long-period fiber gratings (LPGs) are well known for their sensitivity to external influences, which make them interesting for a large number of sensing applications. For these applications, fibers with a high numerical aperture (i.e., fibers with highly germanium (Ge)-doped fused silica fiber cores) are more attractive since they are intrinsically photosensitive, as well as less sensitive to bend- and microbend-induced light attenuations. In this work, we introduce a novel method to inscribe LPGs into highly Ge-doped, single-mode fibers. By tapering the optical fiber, and thus, tailoring the effective indices of the core and cladding modes, for the first time, an LPG was inscribed into such fibers using the amplitude mask technique and a KrF excimer laser. Based on this novel method, sensitive LPG-based fiber optic sensors only a few millimeters in length can be incorporated in bend-insensitive fibers for use in various monitoring applications. Moreover, by applying the described inscription method, the LPG spectrum can be influenced and tailored according to the specific demands of a particular application.

With the rapid progress of fiber lasers, there is an increasing demand to develop optical waveguides with large mode area to reduce non-linear affects such as stimulated Raman scattering. This is most easily attained by increasing the core size. However, a larger core supports more higher-order modes which in turn worsen the beam quality. The inclusion of a ring surrounding the core (and within the cladding), combined with coiling, is one known method for removing higher-order modes. In this work, we further this approach by first coupling the core modes to the ring, and subsequently inducing losses through implementation of an absorbing material. However, such single-ring design only allows for coupling with one of the two core LP 11 modes, leaving the second unaffected. We proposed a windmill fiber design that is able to simultaneously couple to both LP 11 modes. It consists of four annular sectors that strongly couple to the targeted core modes, resulting in an effective single-mode fiber. Importantly, the windmill design is optimized to increase its tolerance to fluctuations in the refractive indices to within current fabrication limits of 1x10 -4 , and proves itself to be more effective than the single-ring design for out-coupling of higher-order modes. [ABSTRACT FROM AUTHOR]

We report on design and construction of a single-mode-fiber-based Mach-Zehnder interferometer system for measuring single photon indistinguishability from quantum dots emitting in visible spectrum (630±50 nm). The performance of the instrument was tested for its efficiency and usability in single-photon correlation measurements in a proposed experimental setup. [ABSTRACT FROM AUTHOR]

By using a Mach-Zehnder intensity modulator (MZM)-based Q-factor enhanced 40-GHz optoelectronic oscillator (OEO) loop, a self-starting pulsed encoder is demonstrated to generate 40-Gbit/s return-to-zero on-off-keying (RZ-OOK) data-stream. By nonlinearly driving the MZM with an OEO circuitry via a proper setting on the intra-loop delay, both the RF and optical output of the OEO at 40 GHz can be extracted to deliver the clock with a Q-factor of up to 2×108, and trigger the synthesizer-free 40-Gbit/s RZ-OOK data-stream. The maximal output power and the carrier-to-noise ratio of the RF output from the OEO are 20 dBm and 62 dB, respectively. Moreover, the single side-band phase noise is suppressed to -120 dBc/Hz at 100-kHz offset frequency by lengthening the single-mode fiber in the OEO loop up to 900 m. In addition, the pulsed RZ carrier with a pulsewidth of 10.5 ps provides a jitter as low as 120 fs and an on/off extinction ratio of 7.7 dB, which is particularly suitable for the RZ-OOK transmission at 40 Gbit/s. The back-to-back transmitted synthesizer-free 40-Gbit/s RZ-OOK data with a pattern length of 223-1 reveals a receiving power sensitivity of -15.2 dBm at the requested bit error rate of 10-9. [ABSTRACT FROM AUTHOR]

A tunable erbium-doped fiber (EDF) laser based on random distributed feedback through backward Rayleigh scattering in a 20-km-long single-mode fiber and a tunable fiber Fabry-Perot interferometer filter is demonstrated. It has a broad wavelength tuning range up to 40 nm (from 1525 to 1565 nm) with a narrow linewidth of ~0.04 nm. Due to the half-opened laser cavity design and the efficient gain from the pumped EDF, the measured threshold power is only 13 mW, and the pump efficiency is up to 14%. [ABSTRACT FROM PUBLISHER]

A kind of magnetic field sensor based on mode interference effect is proposed. The sensing arm consists of two special up-tapered joints formed on the traditional single-mode fiber by fusion tapering technique. Magnetic fluid is used as the cladding of the structure. The interference valley wavelength or the transmission loss of the sensing structure is sensitive to the external magnetic field, which is utilized for magnetic field sensing. The sensitivity of shift of interference valley wavelength with magnetic field can be up to 325.3 pm/mT in the range of 0-16 mT. The variation of transmission loss at valley wavelength with magnetic field has a sensitivity of -0.2121 dB/mT in the range of 2-30 mT. [ABSTRACT FROM PUBLISHER]

Although the quest for high-power, diffraction-limited, semiconductor lasers has arguably met with limited success, the drive for higher brightness diode lasers continues, driven by fiber laser systems. Using tailored refractive-index and injection-current profiles, broad-area laser beam propagation method simulations predict simultaneous suppression of free-carrier-induced self-focusing and far-field mode compression at high powers. Asymmetric gain from tailored current profiles generate unconventional, null-less modes and lead to distributed gain- and loss-filtering mechanisms for increased beam quality output. A 5.2-7.7 W of filament-free power enables high-efficiency (70%) coupling into SMF-28, resulting in 3.1-5.5 W of fiber-coupled power for erbium-doped fiber amplifiers pumping with 0.72-W/A slope efficiency. The concept is shown to be robust against the shape of the profiles, inhomogeneities in both injection current and refractive index, and thermal lensing with proper submount design. [ABSTRACT FROM PUBLISHER]

A novel localized surface plasmon resonance (LSPR) fiber sensor was proposed. This LSPR fiber sensor was primarily constructed by conducting etching of the cladding layer and core layer on a single-mode fiber, followed by plating of 1444 nanometal rings. Sensor design and relevant calculations were conducted using a semi-analytical simulation method, which integrated the exact mode solver for the cylindrical coordinate and eigenmode expansion method. It was examined that the current metallic patterns in the fiber sensor can trigger the LSPR by the electric field Er of the core mode HE11, and this is the main reason why this novel fiber sensor can obtain high performance. After performing algorithms, images showed evident excitation of the LSPR. The LSPR fiber sensor designed in this paper possesses excellent attributes of short length (185.173 μm), high resolution (approximately - 70 dB), and high sensitivity (approximately 34 257 nm/RIU). [ABSTRACT FROM PUBLISHER]