Fiber Coupling Research Articles

The use of a supercontinuum light source for the characterization of passive fiber optic components

Abstract In this article, we report the application of a commercial supercontinuum light source for testing fiber optics components in a broad optical range. We demonstrate that this kind of light can be successfully used to measure the parameters of a number of passive fiber components, such as fiber Bragg gratings, fiber couplers, wavelength division multiplexers, and fibered isolators. We also show that near the double wavelength of the pulsed laser used to pump the nonlinear fiber generating the supercontinuum, the standard optical spectrum analyzers demonstrate the false spectral peak that affects the test results and that using a simple low-cost monochromator placed at the supercontinuum source output permits the elimination of this peak. The results of experiments related to the characterization of passive fiber devices in the broad optical range, from 1 μm to more than 2 μm, are discussed in detail as possible applications of the proposed technique.

Hole-assisted three-core fiber directional coupler operated in reflection mode for vector bending measurement

We demonstrate a vector bending sensor operated in reflection mode based on a hole-assisted three-core fiber (HATCF) coupler. The sensor is built by splicing a piece of HATCF with a gold film-coated end to a single-mode fiber (SMF). The gold film on the center core end is etched with femtosecond laser and only the lights transmitted in the two suspended cores of the HATCF are reflected. The center core couples with the two suspended cores at different wavelengths, respectively. The bending direction and curvature are identified by monitoring the wavelength of the two coupled peaks. The maximum curvature sensitivity of the sensor is 15.146 nm/m−1. The temperature sensitivity is less than −58.8 pm/°C. The vector bending sensor operated in reflection mode has the advantages of a compact structure and simple transmission line, which is conducive to the miniaturization and integration of optical fiber bending sensors.

Characterization of Mode Coupling in a Few-Mode Fiber Based on Optical Frequency Domain Reflectometry

Characterization of Mode Coupling in a Few-Mode Fiber Based on Optical Frequency Domain Reflectometry

Polarization insensitive electrically reconfigurable meta-lens for the 2 µm wavelength

The conventional fiber communication band of 1.55 µm is reaching its limit attributable to the escalation in bandwidth requirements for high-speed and bulk data transmission. Researchers are exploring a 2 µm waveband for its higher capacity and low attenuation as a solution for the next generation communication technologies. Accordingly, here we report an optically engineered metasurface for this waveband for fiber coupling or lensing. The structure is polarization-insensitive and dynamically tunable between its reflective (OFF) and transmissive (ON) modes. For tunability, we incorporate a novel phase change material In3SbTe2 (IST) for its faster, non-volatile, and reversible metallic-to-insulator phase transition. The integration of indium tin oxide (ITO) as a micro-heater to electrically modulate the light by altering the phase of IST provides the device with additional functionality for point-of-care applications. Using the finite-difference-time-domain (FDTD) technique, we have achieved a modulation depth of 90%. The focusing efficiency is as high as 76% and the ON-OFF switching ratio of the optimized lens is 26 dB. The multilayer insertion of thin IST ensures uniform phase transition with switching energy as low as 232.98 nJ/µm2. Thus, with remarkable performance at 2 µm and dynamic multifunctionality, our proposed device will revolutionize the upcoming telecommunication technologies and beyond.

Anatomy and Histology of Sensorimotor Connections Between the Facial and Trigeminal Nerve in the Buccinator Muscle.

Despite indications of a close interaction between the trigeminal (CN V) and facial nerve (CN VII) within the buccinator muscle, a combination of anatomical dissection and histological analysis has not been reported. Five formalin-fixed and fresh-frozen hemifaces were dissected to reveal the buccal fat pad, the buccinator muscle, and anastomotic connections between CN V and CN VII within it. Samples were taken for histological processing and immunostaining. Branches of CN V and CN VII formed pronounced sensorimotor anastomotic connections in and surrounding the buccinator muscle. These findings were histologically evident with close intramuscular coupling of sensory and motor fibers. There was an evident but gradual shift from motor to sensory fibers in the interconnections when analyzing them from the side of CN V toward the side of CN VII and vice versa. These results further elucidate connections between CN V and CN VII and their possible role in proprioception of the facial muscles.

Overview and recent applications of the miniaturized HiPoLas ignition system

The requirements of the space industry for reusable, low cost or clustered engines have triggered high interest in a laser ignition system capable of multi-point ignition. The miniaturized SAL HiPoLas laser ignition system developed for single-point direct-plasma ignition has recently been enhanced towards a fiber coupling, multiplexing, and distribution system. A brief history of the HiPoLas development with an overview of the latest, highly miniaturized generation V ignition system is shown. Applications of the miniaturized ignition system, summarizing more than 10 years of development in the field of rocket engine laser ignition, as well as the first results of a recently developed prototype of a fiber distribution system, based on diffractive optical elements, are presented.

Optimal Design of Small-Aperture Optical Terminals for Free-Space Links

We present the generalised design of low-complexity, small-aperture optical terminals intended for kilometre-scale, terrestrial, free-space laser links between fixed and dynamic targets. The design features single-mode fibre coupling of the free-space beam, assisted by a fast-steering, tip/tilt mirror that enables first-order turbulence suppression and fine target tracking. The total power throughput over the free-space link and the scintillation index in fibre are optimised. The optimal tip/tilt correction bandwidth and range, aperture size, and focal length for a given link are derived using analytical atmospheric turbulence modelling and numerical simulations.

High-performance and fabrication-tolerant edge coupler on thin film lithium niobate based on a three-dimensional inverse taper

Thin film lithium niobate has been widely considered as a promising photonic integration platform. However, due to the non-vertical sidewall of lithium niobate etching, it is challenging to design an efficient fiber coupler for the submicron-sized lithium niobate waveguide. Here, a high-performance edge coupler to a cleaved fiber is introduced on the thin film lithium niobate platform using a sharp three-dimensional taper. The proposed taper structure, which ensures a nearly adiabatical transition of both transverse-electric (TE) and transverse-magnetic (TM) modes, is formed by two standard patterning steps for lithium niobate. A silica ridge waveguide is used as the intermediate transition structure between the taper and a high-numerical-aperture fiber. The simulated coupling losses are as low as −0.1 dB per facet, while the experimental mean values reach −0.29 and −0.24 dB for TE and TM modes, respectively. The coupling spectra exhibit a flat wavelength response, which almost coincides for both polarizations. The 0.5-dB coupling bandwidth is beyond 180 nm. Within this bandwidth, the worst coupling losses are −0.61 and −0.56 dB for TE and TM modes, respectively, and the polarization-dependent loss is below 0.17 dB. The present edge coupler also exhibits a good fabrication tolerance, and its fabrication can also facilitate wafer-scale processing without wafer dicing and end-facet polishing.

Anticrossing and Mode Coupling in Bent All-Glass Leakage Channel Microstructured Optical Fibers with Large Mode Area

This paper presents the results of a detailed theoretical study of the bending properties of original all-glass leakage channel microstructured optical fibers (LC MOFs) over a bending radius range from 3 cm to 11 cm. These LC MOFs contain two layers of fluorine-doped silica glass elements with reduced refractive index, different diameters, and different distances between them. We determined the spatial distributions of the electric field components of different modes in addition to the usual parameters such as effective refractive indices, bending losses, and spatial intensity distributions. A detailed analysis showed that three modes for each polarization have to be considered to correctly calculate the bending losses. Two pairs of these three modes couple in two distinct bending radius ranges, specifically near 3.68 cm and near 5.95 cm, and the mode coupling in these pairs is resonant. The resulting bending losses of the LC MOF for two polarizations are very close to each other and have two maxima at bending radii of 3.68 cm and 5.95 cm. However, the nature of these maxima is not resonant; they are caused by the combined influence of all three modes, each of which has specific dependencies of losses and other parameters on the bending radius that exhibit quasi-resonant behavior near the corresponding bending radii.

Resonant Cell-Based 1f Photoacoustic Gas Analyzer Immune to Light Power Fluctuation and Frequency Mismatch.

To overcome the light power fluctuation and frequency mismatch in photoacoustic spectroscopy (PAS), we proposed a self-corrected 1f-only resonant cell-based PAS (1f-RCPAS) gas analyzer. Based on the theoretical analysis of the 1f signal, a signal processing algorithm considering laser power-current nonlinearity is proposed. The 1f-only algorithm is well-tailored for the resonant systems, requiring no time-division multiplexing. The algorithm is further improved to extend the dynamic range. The T-type resonant cell incorporating a graphene sticker is utilized for effectively amplifying the acoustic signals from both the gas and solid to achieve normalization. No optical path alignment is needed. For the low resonance frequency, a digital orthogonal-vector lock-in amplifier is used, further simplifying the system setup. The gas analyzer is used to measure methane (CH4) with the near-infrared absorption peak at 1651 nm. The experiments demonstrated immunity to fiber coupling loss, laser power drift over time, and frequency mismatch caused by property differences between air and standard gases. The R2 value in the concentration calibration reaches 0.99995, and the minimum detection limit given by the Allan variance reaches 3.5 ppb at an average time of 105 s.

High-sensitivity humidity sensor based on Mach-Zehnder interferometer in seven-core fiber

A Mach-Zehnder Interferometer (MZI) based on seven-core fiber (SCF) for humidity sensing is proposed and experimentally demonstrated. The MZI is formed by sandwiched a section of SCF in between two single mode fibers (SMFs), while a fiber taper and a brief multimode fiber (MMF) act as fiber couplers to excite and couple the intermodal interference in the SCF. The optical field distribution and coupling efficiency of the MZI with different SCF lengths are analysed. The theoretical results show that the MZI is sensitive to SCF lengths, and it is expected to obtain highly sensitive humidity response. Humidity sensing experiments shows a high sensitivity of 0.6603 nm/%RH over a relative humidity (RH) range of 44–83 %RH, and the maximum measurement uncertainty introduced by the long-term stability is 0.0006 %RH. Human respiration measurement exhibits the response time and recovery time of the MZI are 0.44 s and 1.24 s respectively. The temperature effect is also experimentally investigated. Such an all-fiber MZI presents advantages of simple configuration, high sensitivity and good stability, making it has potential in humidity measurement fields.

Displaced thinned single-mode fiber Mach-Zehnder interferometer tested for temperature and curvature applications

This article explains the features and qualities of employing a thinned manufacturing fiber to assemble a Mach-Zehnder interferometer (MZI) for detecting two physical variables: temperature and curvature. The MZI comprises a filter by splicing core-offset sections of a thinned fiber (TF) cladding diameter of 80 μm in an SMF-TF-SMF configuration. The TF splices act as the arms of the MZI, while the mismatch diameter sections serve as optical fiber couplers. The optical spectrum analyzer (OSA) detects the transmitted light and analyzes its optical transmission characteristics, showing 11 peaks of modal interferences into the longitude region. As a result of the experimental arrangement, the curvature and temperature sensitivities are 0.49 nm/m−1, 0.01 nm/°C, and 0.09 nm/°C for temperatures of 0–60 °C and 70–130 °C, respectively. The proposed sensor has potential advantages for measuring refractive index, pH, torsion, curvature, and temperature.

High efficiency deep ultraviolet micro-LED and optical fiber coupling for low power charge management applications

During space gravitational wave detection missions, it is necessary to manage the charge accumulated on the surface of the test mass. A common method involves using optical fibers to transmit ultraviolet (UV) light, leveraging the photoelectric effect for charge management. An important issue in this process is achieving efficient coupling between the UV light source and the optical fiber. In this paper, for the first time we proposed to use DUV micro-LEDs as light sources for charge management systems, which will have lower power consumption. We conducted simulations to investigate the effect of the size ratio between deep ultraviolet (DUV) micro-LEDs and optical fibers on the coupling efficiency. We further employed a direct coupling method to conduct coupling experiments of DUV micro-LEDs and optical fibers with varying wavelengths and sizes. The simulation and experiment results both indicate that, under the influence of both size and divergence angle, there exists an optimal DUV micro-LED size responsible for maximum coupling efficiency between DUV micro-LEDs and optical fibers which is 80 μm when the optical fiber size is 1000 μm. Applying the 80 μm DUV micro-LED in charge management experiments demonstrated the feasibility of using DUV micro-LEDs as light sources for low power charge management systems, achieving rapid charge–discharge rates and high photoelectric current at lower driving electrical power.

All-fiber IPDA lidar for CH4 leakage monitoring using InGaAs/InP single-photon detector.

An integrated path differential absorption (IPDA) lidar for CH4 leakage monitoring is proposed and demonstrated. In the simplified all-fiber optical layout, a homemade InGaAs/InP single-photon detector (SPD) using multi-channel technique with multi-mode fiber coupling is used to increase the maximum count rate and coupling efficiency. The system is calibrated in intensity and frequency domains. Firstly, the fluctuation of the laser power is compensated. Secondly, the dead time, afterpulsing probability and dark counts of the SPD are corrected. A mean relative difference of 0.84% between SPD and PIN photodetector is achieved. Thirdly, non-linear frequency scanning of the laser is measured by homodyne detection and analyzed in joint time-frequency domain. In the symmetry-calibration process, the absorbance spectra of up and down scanning are compared. Maximum difference less than 1% with mean difference of 0.33% is achieved within a span of 4 GHz around the center of absorbance spectrum. Finally, a demonstration experiment over ten days is carried out to analyze the accuracy and stability of the system. A mean deviation of 0.03% with standard deviation of 0.46% is verified at a distance of 12 m and a time resolution of 1 s. By attenuating the laser power from 2 mW to 0.02 mW, the performance of the system is degraded to a mean deviation of 1.32% with standard deviation of 4.33%.

Fiber Bragg Grating Pulse and Systolic Blood Pressure Measurement System Based on Mach-Zehnder Interferometer.

A fiber Bragg grating (FBG) pulse and systolic blood pressure (SBP) measurement system based on the edge-filtering method is proposed. The edge filter is the Mach-Zehnder interferometer (MZI) fabricated by two fiber couplers with a linear slope of 52.45 dBm/nm. The developed system consists of a broadband light source, an edge filter, fiber Bragg gratings (FBGs), a coarse wavelength-division multiplexer (CWDM), and signal-processing circuits based on a field-programmable gate array (FPGA). It can simultaneously measure pulse pulsations of the radial artery in the wrist at three positions: Cun, Guan and Chi. The SBP can be calculated based on the pulse transit time (PTT) principle. The measurement results compared to a standard blood pressure monitor showed the mean absolute error (MAE) and standard deviation (STD) of the SBP were 0.93 ± 3.13 mmHg. The system meets the requirements of the Association for the Advancement of Medical Instrumentation (AAMI) equipment standards. The proposed system can achieve continuous real-time measurement of pulse and SBP and has the advantages of fast detection speed, stable performance, and no compression sensation for subjects. The system has important application value in the fields of human health monitoring and medical device development.

Experimental demonstration of free-space optical communication under 2 km urban atmosphere using adaptive fiber coupling

Satellite-to-ground communication, quantum communication, and intra-city communication all benefit from free-space optical communication. Coupling as much light power as possible into optical fibers is crucial for free-space optical communication, where atmospheric turbulence and platform sway will affect the coupling effect. This paper introduces a scheme for free-space optical communication utilizing a single adaptive fiber coupler, which can mitigate turbulence and other disturbances at minimal cost. Experimental results in a 2 km urban atmosphere show that our method can increase coupling power by up to 20% for the majority of the time. Moreover, the bit error rate of communication reduces from 4.53 × 10−4 to 3.80 × 10−8 after implementing the closed-loop correction and we also demonstrate video transmission. Additionally, this scheme can be combined with coherent beam combining and spatial diversity techniques, offering a comprehensive solution to both tip/tilt aberration and scintillation issues.

New method for the investigation of mode coupling in graded-index polymer photonic crystal fibers using the Langevin stochastic differential equation

The mode coupling in a graded-index polymer photonic crystal fiber (GI PPCF) with a solid core has been investigated using the Langevin equation. Based on the computer-simulated Langevin force, the Langevin equation is numerically integrated. The numerical solutions of the Langevin equation align with those of the time-independent power flow equation (TI PFE). We showed that by solving the Langevin equation, which is a stochastic differential equation, one can successfully treat a mode coupling in GI PPCFs, which is an intrinsically stochastic process. We demonstrated that, in terms of effectiveness, the Langevin equation is preferable compared to the TI PFE. The GI PPCF achieves the equilibrium mode distribution (EMD) at a coupling length that is even shorter than the conventional GI plastic optical fiber (POF). The application of multimode GI PCFs in communications and optical fiber sensor systems will benefit from these findings.

High‐resolution incoherent interference imaging without phase measurement

AbstractAiming for long‐distance, high‐resolution, passive imaging, we use fiber coupling to replace the spatial coupling of the segmented planar imaging detector for electro‐optical reconnaissance imaging system and propose a novel incoherent interference imaging method, which avoids the problem that the resolution of the imaging system is limited by the size of the silicon wafer. The method uses a phase retrieval algorithm and does not need to measure the phase in the system, effectively avoiding the intrinsic jitter problem of fiber optics. The feasibility of the method in achieving target image reconstruction was verified through simulation; furthermore, an indoor two‐dimensional discrete sampling imaging experiment for a simple four‐rod target was conducted, and an outdoor one‐dimensional feature identification experiment for a two‐rod target was also performed. The results showed that the resolution exceeds the diffraction limit and the reconstruction error of target size is less than 3.5%.

Silicon photonics efficient fiber coupler based on a metastructure with a minimum feature size of 150 nm.

We propose a silicon photonics fiber coupler consisting of a metastructure containing a through-hole array and a partially etched grating to obtain high efficiency. Using an evolution optimization algorithm, we optimized 121 structural parameters and achieved a minimum feature size of 150 nm, which greatly simplifies the fabrication using wafer-scale foundry services. We designed two structures, one achieving an 86% coupling efficiency into a single fiber, and the other exhibiting a full flat spectrum in the C band. The high-efficiency structure was fabricated using a 300-mm wafer process, and an efficiency of 78% was experimentally evaluated.

Positional Accuracy of 3D Printed Quantum Emitter Fiber Couplers

Positional Accuracy of 3D Printed Quantum Emitter Fiber Couplers