Fiber Core Size Research Articles

High-efficiency radiation-balanced Yb-doped silica fiber laser with 200-mW output.

The focus of this study was the development of a second generation of fiber lasers internally cooled by anti-Stokes fluorescence. The laser consisted of a length of a single-mode fiber spliced to fiber Bragg gratings to form the optical resonator. The fiber was single-moded at the pump (1040 nm) and signal (1064 nm) wavelengths. Its core was heavily doped with Yb, in the initial form of CaF2 nanoparticles, and co-doped with Al to reduce quenching and improve the cooling efficiency. After optimizing the fiber length (4.1 m) and output-coupler reflectivity (3.3%), the fiber laser exhibited a threshold of 160 mW, an optical efficiency of 56.8%, and a radiation-balanced output power (no net heat generation) of 192 mW. On all three metrics, this performance is significantly better than the only previously reported radiation-balanced fiber laser, which is even more meaningful given that the small size of the single-mode fiber core (7.8-µm diameter). At the maximum output power (∼2 W), the average fiber temperature was still barely above room temperature (428 mK). This work demonstrates that with anti-Stokes pumping, it is possible to induce significant gain and energy storage in a small-core Yb-doped fiber while keeping the fiber cool.

Inline Determination of the Gel Elastic Modulus During Milk Coagulation Using a Multifiber Optical Probe

AbstractFrom a state-of-the-art point of view, it is currently possible to optically monitor the enzymatic coagulation of milk for real time estimation of the elastic modulus to cut the gel at optimum gel firmness. However, European cheese industry produces a wide variety of cheeses, many of them artisanal, and has a very fragmented productive structure with many small-, medium-sized companies. Therefore, if the technology is to be successfully uptake, it must be not only accurate but very low-cost. The objective of this work was to evaluate a low-cost commercial multifiber probe, for inline optical determination of curd firmness during cheese making. Preliminary tests were carried out to select the most appropriate fiber core size and wavelength and after that coagulation trials were performed following a fully randomized factorial design with two factors, i.e., concentration of protein (3.2, 3.6 and 4.0%) and added calcium (150, 200 and 250 mg L−1), with three replicates. The observed linear increase of the least square means of the initial voltage with the protein content (V0 = 0.15[P, %] + 0.88; R2 = 0.999), will be likely synergistic with the elastic modulus prediction if the model needs to be corrected for protein. Finally, the multifiber probe allowed predicting curd firmness using the proposed model with SEP values < 7 Pa. The present work has proven that a low-cost multifiber probe is suitable for accurate, real-time prediction of curd firmness during cheese manufacture.

Design and Experimental Analysis of an Optical Fiber Coupling System for a Ground-based Telescope with Adaptive Optics

Astronomy photonics has opened up a new era for the application of astronomical optical instruments. The fiber coupling system serves as the crucial link between the telescope and photonic devices. This paper explores a beam shaping method that utilizes a coupled lens to enhance the efficiency of coupling light into an optical single mode fiber. Compared to directly coupling the telescope beam into the fiber, this approach offers improved coupling efficiency and greater adjustment tolerance. The laboratory-based optical fiber coupling system described in this study comprises an imaging component with an F/50 ratio and a fiber coupling component. Theoretical analysis indicates that optimal coupling efficiency is achieved when the diameter of the focusing spot, limited by diffraction, matches the fiber core size. Any axial error, position error, or tip-tilt error between the lens and the fiber will reduce the coupling efficiency. Experimental results confirm that the coupling system achieves an efficiency of approximately 70%, which is close to the theoretical limit of 78%. These findings underscore the effectiveness of the fiber coupling method.

A method for the controllable fabrication of optical fiber-based localized surface plasmon resonance sensors

Optical fiber-based Localized Surface Plasmon Resonance (OF-LSPR) biosensors have emerged as an ultra-sensitive miniaturized tool for a great variety of applications. Their fabrication by the chemical immobilization of gold nanoparticles (AuNPs) on the optic fiber end face is a simple and versatile method. However, it can render poor reproducibility given the number of parameters that influence the binding of the AuNPs. In order to develop a method to obtain OF-LSPR sensors with high reproducibility, we studied the effect that factors such as temperature, AuNPs concentration, fiber core size and time of immersion had on the number and aggregation of AuNPs on the surface of the fibers and their resonance signal. Our method consisted in controlling the deposition of a determined AuNPs density on the tip of the fiber by measuring its LSPR signal (or plasmonic signal, Sp) in real-time. Sensors created thus were used to measure changes in the refractive index of their surroundings and the results showed that, as the number of AuNPs on the probes increased, the changes in the Sp maximum values were ever lower but the wavelength shifts were higher. These results highlighted the relevance of controlling the relationship between the sensor composition and its performance.

Bending Induced Temperature Increases in Double-Clad Fibers for High-Power Fiber Lasers

Rare-earth-doped optical fibers are one of the most promising solid-state lasers. In these fiber lasers, a cladding-pumping scheme using large-mode-area double-clad fibers (DCFs) is utilized to increase the overall conversion efficiency of pumping light and to overcome the restriction owing to the onset of stimulated Raman scattering. On the other hand, it is extremely challenging to increase the fiber core size while retaining the excellent beam quality because fibers with large core size allow propagation of several higher-order modes (HOMs), except for the fundamental mode (FM). In order to suppress HOMs, DCFs are bent with a relatively small bend radius. Failures at bends in an optical fiber are caused by light leaking from the core when the fiber is accidentally bent tightly with a high power input. For the DCFs with core radii of 10 and 20 mm, the relationship between the bending induced temperature increases and the bend losses in the bent DCFs was investigated theoretically by the explicit finite-difference method using the thermochemical SiO_x production model. The temperature at the boundary between the inner and outer cladding layers increased with increasing optical power <I>P</I> at 1.080 mm and was a large value higher than the softening temperature <I>T</I>_s of the silica glass when <I>P</I> = 5 kW was entered to the bent DCF with a bend radius <I>R</I> of 150 mm and a core radius of 10 mm. On the other hand, the temperature at the boundary was a small value lower than the <I>T</I>_s when <I>P</I> = 10 kW was entered to the bent DCF with a large <I>R</I> of 245-275 mm. Furthermore, it was found that the initiation of the fiber fuse was fairly easy under a certain conditions where the temperature at the boundary was higher than the <I>T</I>_s.

Optical Fiber Bundle-Based High-Speed and Precise Micro-Scanning for Image High-Resolution Reconstruction.

We propose an imaging method based on optical fiber bundle combined with micro-scanning technique for improving image quality without complex image reconstruction algorithms. In the proposed method, a piezoelectric-ceramic-chip is used as the micro-displacement driver of the optical fiber bundle, which has the advantages of small volume, fast response speed and high precision. The corresponding displacement of the optical fiber bundle can be generated by precise voltage controlling. An optical fiber bundle with core/cladding diameter 4/80 μm and hexagonal arrangement is used to scan the 1951 USAF target. The scanning step is 1 μm, which is equivalent to the diffraction limit resolution of the optical system. The corresponding information is recorded at high speed through photo-detectors and a high-resolution image is obtained by image stitching processing. The minimum distinguishable stripe width of the proposed imaging technique with piezoelectric-ceramic-chip driven micro-scanning is approximately 2.1 μm, which is 1 time higher than that of direct imaging with a CCD camera whose pixel size is close to the fiber core size. The experimental results indicate that the optical fiber bundle combined with piezoelectric-ceramic-chip driven micro-scanning is a high-speed and high-precision technique for high-resolution imaging.

Construction of a dual-core hollow waveguide for visible and mid-infrared light transmission based on PTFE tubing and UV gel

This work proposes a facile approach to fabricate a dual-core hollow waveguide based on two common and low-cost raw materials, namely polytetrafluoroethylene (PTFE) tube and ultraviolet (UV) gel. A silica glass tube is nested inside a heat-shrinkable PTFE tube. After heat treatment at 350 °C, the PTFE tube shrinks and then transforms into the low-index polymer on the outside of the silica tube. The PTFE-coated silica glass tubing is filled with UV gel followed by inserting a typical Ag/AgI mid-infrared (IR) hollow optical fiber. The UV gel is cured by UV radiation, forming a solid low-index layer between the PTFE-coated silica glass tubing and the Ag/AgI hollow optical fiber. A visible laser beam can be transmitted at a loss of 0.4 dB/m through the silica annulus between the low-index PTFE and UV gel layer. A 10.6-μm-wavelength CO2 laser beam is delivered through the Ag/AgI hollow optical fiber (core size 530 μm) and the transmission loss goes up from 1.63 to 3.22 dB/m as the bending angle increases from 0° to 90°.

Design of photonic crystal fibers with flat dispersion and three zero dispersion wavelengths for coherent supercontinuum generation in both normal and anomalous regions

A photonic crystal fiber (PCF) structure with three-zero dispersion wavelengths (ZDWs) and a flat dispersion curve was designed by adjusting the structural parameters (the diameters of small air holes inserted between standard lattices in the first and third rings) and fiber core size to obtain the required shape, number of extreme points and the dynamic range of waveguide dispersion (Dw) curve. A quantitative method which was realized by fixing the positions of the ZDWs and tailoring different dispersion slopes was proposed to design another PCF with the same ZDWs and a slightly larger dispersion slope. Both of the designed PCFs were used for contrast experiments of supercontinuum (SC) generation. The simulation results show that the flat dispersion can produce the coherent SC in both normal and anomalous dispersion regions for the designed two PCFs. The degree of flatness and the wavelength range of coherent SC are determined by the great time-domain compression due to the direct soliton spectrum tunneling (DSST) resulting from the mismatch between self-phase modulation and gradually decreasing dispersion. A wide, flat, and coherent SC with a wavelength range of 1281–2200 nm and power range of −14.8 ~ −9.4 dB was obtained for the PCF with flatter dispersion by pumping in the anomalous dispersion region with 50 fs incident pulse. This study is instructive for PCF design in different application scenarios and finds a new way for the generation of wide flat coherent spectrum.

A simple approach to fiber-based tunable microcavity with high coupling efficiency

We report a simple technique for the realization of fiber-based Fabry-Pérot microcavities with large Q/V values as well as high cavity-to-fiber coupling efficiencies. The open microcavity we demonstrate consists of a flat mirror and a concave mirror on the tip of a single mode optical fiber. Combining hydrofluoric acid chemical wet etching with CO2 laser reflow, we obtain a fiber-tip atomically smooth concave surface with a diameter of 4.7 μm determined by the fiber core size. The concave surface with a spherical profile is automatically aligned with the fiber core, which enables high cavity-to-fiber power coupling efficiency (higher than 90%) with an optimal fiber-tip mirror's radius of curvature based on numerical simulation results. After distributed-Bragg-reflector coating, we have realized a wavelength-tunable open microcavity with a quality factor Q exceeding 1000 and a mode volume V of 13.6 μm3, and laser emission is demonstrated from this microcavity.

Cascaded Raman scattering based high power octave-spanning supercontinuum generation in graded-index multimode fibers

A new method to generate multi-watt-level, octave-spanning, spectrally flat supercontinua stemmed from cascaded Raman scattering in graded-index multimode fibers is reported. Formation dynamics of supercontinua are investigated by studying the effect of fiber length and core size. High power handling capacity of the graded-index multimode fibers is demonstrated by power scaling experiments. Pump pulse repetition rate is scaled from kHz to MHz while pump pulse peak power remains same and ~4 W supercontinuum is achieved with 2 MHz pump repetition rate. To the best of our knowledge, this is the highest average power and repetition supercontinuum source ever reported based on a graded-index multimode silica fiber. Spatial properties of the generated supercontinua are measured and Gaussian-like beam profiles obtained for different wavelength ranges. Numerical simulations are performed to investigate underlying nonlinear dynamics in details and well-aligned with experimental observations.

A Fast Method to Analyze the Fiber Amplifier With the GTWave Pumping Configuration

The pump propagation in the GTWave pumping structure is studied with analytical method and the beam propagation method (BPM). The power distribution function $f(\xi)=P_0(z)/[P_0(z)+P_1(z)]$ is proved to be independent of the fiber core size and the doping content. Based on this $f(\xi)$ function, the propagation of the pump power and the signal power in the fiber amplifier is quickly calculated. With our method, a BPM calculation on a long fiber can be simulated with a highly doped short fiber; thus, the total calculation time is greatly reduced. The analysis of a fiber amplifier with very complicated GTWave pumping configuration is greatly simplified and the calculation speed is accelerated without losing too much accuracy.

Anisotropic tapered polarization-maintaining large mode area optical fibers.

We demonstrate a novel type of tapered large mode area polarization-maintaining fiber. These birefringent fibers have an elliptical inner cladding and a core diameter that increases adiabatically from 8 µm to 70 µm. The polarization maintaining ability of the fiber samples was investigated by measuring the spatial distribution of polarization beat length by using optical frequency-domain reflectometry. The measurements show a clear correlation between the birefringence and the fiber core size, resulting in a modest 10-15% variation in polarization beat length along the fiber. There is no significant coupling of polarization modes or transverse modes in the tested fibers and, therefore, the linear polarization state of propagating light is preserved.

Tailoring frequency generation in uniform and concatenated multimode fibers.

We demonstrate that frequency generation in multimode parabolic-index fibers can be precisely engineered through appropriate fiber design. This is accomplished by exploiting the onset of a geometric parametric instability that arises from resonant spatiotemporal compression. By launching the output of an amplified Q-switched microchip laser delivering 400ps pulses at 1064nm, we observe a series of intense frequency sidebands that strongly depend on the fiber core size. The nonlinear frequency generation is analyzed in three fiber samples with 50μm, 60μm, and 80μm core diameters. We further demonstrate that by cascading fibers of different core sizes, a desired frequency band can be generated from the frequency lines parametrically produced in each section. The observed frequency shifts are in good agreement with analytical predictions and numerical simulations. Our results suggest that core scaling and fiber concatenation can provide a viable avenue in designing optical sources with tailored output frequencies.

Suspended Core Fibers for the Transmission of Cylindrical Vector Modes

This paper presents a study of propagation of radially and azimuthally polarized cylindrical vector modes in six-strut suspended core fibers based on finite element simulations. The study shows that large effective index differences in the order of $10^{-3}- 10^{-2}$ of these modes can be achieved in the suspended core fibers with core diameter of less than 2 μ m, material index 1.45 of silica to 2.0 of tellurite, and wavelength of 750 nm, allowing the stable propagation of the first higher order modes in doughnut shape within these fibers. The effective index difference and the field intensity of these cylindrical vector modes can be tuned by selecting appropriate fiber material and core size. The study shows that the suspended core fiber can be a competitive candidate for fiber-based high-resolution stimulated emission depletion nanoscopy application.

Optical fiber-based photocathode

We present the design of a back-illuminated photocathode for electron diffraction experiments based on an optical fiber, and experimental characterization of emitted electron bunches. Excitation light is guided through the fiber into the experimental vacuum chamber, eliminating typical alignment difficulties between the emitter metal and the optical trigger and position instabilities, as well as providing reliable control of the laser spot size and profile. The in-vacuum fiber end is polished and coated with a 30 nm gold (Au) layer on top of 3 nm of chromium (Cr), which emits electrons by means of single-photon photoemission when femtosecond pulses in the near ultraviolet (257 nm) are fed into the fiber on the air side. The emission area can be adjusted to any value between a few nanometers (using tapered fibers) and the size of a multi-mode fiber core (100 μm or larger). In this proof-of-principle experiment, two different types of fibers were tested, with emission spot diameters of 50 μm and 100 μm, respectively. The normalized thermal electron beam emittance (TE) was measured by means of the aperture scan technique, and a TE of 4.0 π nm was measured for the smaller spot diameter. Straightforward enhancements to the concept allowed to demonstrate operation in an electric field environment of up to 7 MV/m.

Supercontinuum as a light source for miniaturized endoscopes

In this work, we have successfully implemented supercontinuum based illumination through single fiber coupling. The integration of a single fiber illumination with a miniature CMOS sensor forms a very slim and powerful camera module for endoscopic imaging. A set of tests and in vivo animal experiments are conducted accordingly to characterize the corresponding illuminance, spectral profile, intensity distribution, and image quality. The key illumination parameters of the supercontinuum, including color rendering index (CRI: 72%~97%) and correlated color temperature (CCT: 3,100K~5,200K), are modified with external filters and compared with those from a LED light source (CRI~76% & CCT~6,500K). The very high spatial coherence of the supercontinuum allows high luminosity conduction through a single multimode fiber (core size~400μm), whose distal end tip is attached with a diffussion tip to broaden the solid angle of illumination (from less than 10° to more than 80°).

Studies of central wavelength of high-power all-fiber superfluorescent sources with Yb-doped double-clad fibers

The behavior patterns and dependencies of the central wavelength of high-power all-fiber superfluorescent sources (SFS) were studied based on the steady-state rate equations of Yb-doped fiber lasers. The relationships between the central wavelength and the pumping power as well as the fiber parameters including fiber length, core size, core/clad ratio, and absorption coefficient were analyzed based on the gain coefficient of the gain fiber. Experimental results from ten different fibers are presented, and the results agreed well with the simulations. The central wavelength of the ten SFSs ranged from 1034.75nm to 1072.4nm.

Enhanced supercontinuum generation in tapered tellurite suspended core fiber

We demonstrate 400-THz (0.6–3.3µm) bandwidth infrared supercontinuum generation in a 10cm-long tapered tellurite suspended core fiber pumped by nJ-level 200-fs pulses from an optical parametric oscillator. The increased nonlinearity and dispersion engineering extended by the moderate reduction of the fiber core size are exploited for supercontinuum optimization on both frequency edges (i.e., 155-THz overall gain), while keeping efficient power coupling into the untapered fiber input. The remaining limitation of supercontinuum bandwidth is related to the presence of the high absorption beyond 3µm whereas spectral broadening is expected to fully cover the glass transmission window (0.5–4.5µm).

Detailed numerical investigation of the interaction of longitudinal acoustic waves with fiber Bragg gratings in suspended-core fibers

The interaction between longitudinal acoustic waves and fiber Bragg gratings in suspended-core optical fibers is numerically investigated. The fiber core size and the air hole size are varied, and the mechanical and grating properties are simulated by means of the finite element method and the transfer matrix method, respectively. Changes of the effective index, confinement factor, silica area, strain and wavelength shift induced by the acoustic wave are evaluated, and the resultant side lobe reflectivity is estimated. A side lobe reflectivity increase of 66% compared to standard fibers is estimated, which allows reducing the modulation index or the grating length in as much as 75%. Besides, the larger reduction of the required acoustic power for achieving the acousto-optic modulation points out to more efficient modulator devices in suspended-core fibers.

Characterization on ablation performance of various surgical fibers

Laser light has been widely used as a surgical tool to treat benign prostate hyperplasia (BPH). Various surgical parameters still need to be tested and optimized to accomplish better clinical outcomes. The purpose of the current study is to investigate the effect of fiber core size on ablation performance at various laser power levels. Three optical fibers (600, 775, and 850 μm) were tested in vitro with a customized 532-nm laser system with power ranging from 120 to 180 W. Ablation performance was evaluated in light of total energy, irradiance, and volumetric energy density. Irrespective fiber size, crater dimension, and ablation volume linearly increased with the incident irradiance. However, each fiber created different geometry of ablation craters (i.e., deep columnar shape for 600 μm vs. shallow funnel shape for 850 μm). Smaller fiber size demonstrated the efficient energy usage for tissue removal. In-depth comprehension of fiber performance can help accomplish predictable and efficient surgical procedures for BPH therapy.