Double-clad Research Articles

A Coaxial Dual-Waveguide Fiber Coupler

We demonstrate a method to fabricate an in-fiber integrated multi-waveguide fiber coupler based on thermal diffusion technology to achieve stable and efficient coupling of a single-mode fiber to a multi-waveguide fiber. Simulation results show that the efficient coupling of a single-mode fiber and a multi-waveguide fiber can be realized by introducing double-clad fiber into single-mode fiber and multi-waveguide fiber. A coaxial dual-waveguide fiber coupler was fabricated based on thermal diffusion technology. The measurement results of the three-dimensional refractive index distribution of the coaxial dual-waveguide fiber coupler verified the feasibility of the thermal diffusion technology to fabricate the multi-waveguide fiber coupler. The insertion loss of the thermally diffused coaxial dual-waveguide fiber coupler was tested, and its performance was better than that of the fusion tapered coupler. The multi-waveguide fiber coupler fabricated by thermal diffusion technology allows for an all-fiber setup with coupling efficiency as high as 98%, achieves a relatively stable fundamental mode excitation in multi-waveguide fiber, requires minimal maintenance, and will accelerate the translation toward particle trapping, spectral detection, and optical communication.

FDTD modeling of excitation-balanced, mJ-level pulse amplifiers in Yb-doped double-clad optical fibers.

A finite-difference time-domain (FDTD) simulation of Yb-doped cladding-pumped, mJ-level, excitation-balanced fiber pulse amplifiers (EBFAs) is presented. In EBFAs, two pumps, one above (anti-Stokes pump, or ASP) and one below (Stokes pump, or SP) the signal wavelength, are utilized to reduce the net thermal energy generated due to the quantum defect. From the results of the FDTD simulation, detailed analyses on the fiber length optimization, excited Yb3+ population evolution, pump and signal power evolution, optical-to-optical (o-o) conversion efficiency, wall plug efficiency, as well as thermal energy generation are performed. For example, with an ASP at 990 nm and a SP at 975 nm, only 2.3 µJ of thermal energy is produced when generating a 2 mJ output pulse at 985 nm, whereas a pulse amplifier with only SP pumping rendering the same 2 mJ output gives more than 10 times the thermal energy. In the meantime, the system maintains an o-o efficiency of 8.43% and wall plug efficiency of 6.6%. The results here indicate the feasibility of the power-scaling of excitation-balanced laser systems, and the FDTD model will be beneficial for the design and optimization of such systems. The first half of this paper presents the FDTD model and provides an example calculation outlining the modeling procedure. The remaining half details the impact of varying laser parameters on system performance. These include pumping and input signal energies, repetition rates, and selection of the ASP, SP, and signal wavelengths. The results presented herein can also be extended to excitation balancing in other solid-state laser systems, such as Yb:YAG and Tm:YAG lasers.

High-power, low noise, high gain few-mode fiber amplifier

The few-mode erbium-doped fiber amplifier (FM-EDFA) is a necessary component for high-capacity long-haul mode-division multiplexing (MDM) fiber optic communication systems. In this paper, an all-fiber few-mode amplifier based on erbium-ytterbium co-doped double-clad fiber is designed and demonstrated. We have developed a fiber-optic pump beam combiner and a few-mode isolator. Combined with the two-stage cascade method, an all-fiber few-mode fiber amplifier with high-power, high-gain and low-noise is realized. Simultaneous amplification of 6 modes (LP01, LP11a, LP11b LP21a, LP21b and LP02) are achieved in the 1535–1565 nm range with average modal gain greater than 40 dB, maximum saturated output power up to 29.5 dBm, differential modal gain (DMG) of ∼3 dB and minimum noise figure (NF) of 4.69 dB.

Investigation of double-cladding heterogeneous step-index 2LP-mode multicore fiber with a two-ring layout

This paper investigates the design of C-band double-cladding 2LP mode multicore fiber (2LP-MCF) with 125-µm cladding diameter by numerical simulation, in which a two-ring layout is used to improve crosstalk and critical bend radius. The numerical results show that the crosstalk can be less than −60 and −35dB/km for 2LP mode eight-core and 10-core fibers, respectively, and it is also shown that the crosstalk can be suppressed compared with that in heterogeneous 2LP-MCFs without a double-cladding layer.

Tm3+/Ho3+ profiled co-doped core area optical fiber for emission in the range of 1.6–2.1 µm

Double-clad optical fiber with a multi-ring core profile doped with thulium and holmium fabricated by Modified Chemical Vapor Deposition Chelate Doping Technology (MCVD-CDT) is presented. The measured Tm2O3 and Ho2O3 complexes’ weight concentrations were 0.5% and 0.2% respectively. Numerical analyses show weakly guiding conditions and 42.2 µm of MFD LP01 at 2000 nm. The low NA numerical aperture (NA = 0.054) was obtained for the 20/250 µm core/cladding ratio optical fiber construction. The emission spectra in the range of 1.6–2.1 µm vs. the fiber length are presented. The full width at half maximum (FWHM) decreases from 318 to 270 nm for fiber lengths from 2 to 10 m. The presented fiber design is of interest for the development of new construction of optical fibers operating in the eye-safe spectral range.

Temperature-insensitive label-free SARS-CoV-2 spike protein detection based on complementary refractive index and temperature dependence of multi-mode interference and grating resonance

We demonstrated temperature-insensitive, label-free detection of SARS-CoV-2 spike protein (SSP) by harnessing the complementary refractive index and temperature dependence of multi-mode interference (MMI) created by a no-core fiber (NCF) and phase-matched resonance generated by a long-period fiber grating (LPFG). To combine MMI and grating resonance, primarily sensitive to the surrounding medium refractive index (SMRI) and ambient temperature, respectively, a fiber-optic transducer was fabricated by splicing an NCF segment with an LPFG inscribed on double-clad fiber. The transducer was functionalized with human ACE2 receptors to selectively capture SSP. The functionalized sensor head exhibited high SSP selectivity, with overall average wavelength displacements of ∼253.33 and ∼160.00 pm in PBS and saliva, respectively, for SSP with concentrations ranging from 1 to 104 ng/mL. These spectral shifts are associated with localized SMRI modulations on the sensor surface induced by specific binding between SSP and ACE2. We also examined the cross-reactivity of the sensor head for MERS-CoV spike protein to confirm its SSP specificity. Moreover, we proved the capability of temperature-independent SSP detection and ambient temperature measurement by scrutinizing the temperature effect on the sensor performance. Our functionalized fiber transducer showed great promise as a temperature-insensitive and portable platform for rapid SSP detection.

The Optimization of a Segmented Cladding Fiber via the Response Surface Methodology Approach for a Large Mode Area

In this work, we have proposed and optimized a segmented cladding fiber (SCF) with a large mode area (LMA) consisting of a uniform core and a double cladding. The outer cladding of the SCF consisted of a periodic alternation of high- and low-refractive-index segments, while the inner cladding consisted of a resonant layer of rods surrounded by high-refractive-index rings. The three geometrical parameters chosen as design variables were the (a) doped rod refractive index differences, (b) refractive index differences of rings, and (c) ring thickness. Using the Box–Behnken approach, we selected thirteen different design cases and modeled them numerically using the finite element method (FEM). In order to optimize the features of the proposed fiber, such as the effective mode area (EMA), we applied response surface methodology (RSM). The EMA of the optimal SCF was significantly improved and markedly enlarged to about 706 µm2 at a wavelength of 1.550 µm. Different properties of the optimized double-cladding octo-wing SCF (DC-OW-SCF), such as confinement losses in the core mode and the first higher-order mode, were studied. The DC-OW-SCF offers the advantages of feasibility in fabrication using the powder-in-tube (PIT) method and provides the possibility of utilization in compact amplifier devices and high-power fiber lasers.

High-Sensitive Double-Clad Fiber Bragg Grating Sensors for Real-Time Monitoring of Body Posture and Movement State

Musculoskeletal disorders (MSD) are prevalent in society and currently lack effective treatments. The incidence of MSD appears to be increasing as the worldwide population ages and sedentary lifestyles. Real-time monitoring of the body posture and movement state may be an effective strategy for the prevention and treatment of MSD. We proposed a real-time monitoring system based on a wearable high-sensitive double-clad fiber Bragg grating (FBG) sensors and an integrated demodulation device to recognize the body posture and movement state. The results demonstrate that the double-clad FBG sensors exhibit a high measurement sensitivity of approximately 36 pm/° in elbow joint static tests and 7.5 pm/° in knee joint static tests, which is almost 1.4 times higher than that of a single-mode FBG. Furthermore, a dataset of six gestures was created based on time and wavelength information obtained from six double-clad FBG sensors monitoring subtle deformations of muscles of the arm. The accuracy of recognizing simple and complex gestures was close to 100% and about 88%, respectively. The static and dynamic measurements demonstrate that the double-clad sensors have high accuracy, improved sensitivity, excellent flexibility and good repeatability, and this work is expected to a reliable assessment tool for clinical trials of MSD.

Mode transition phenomena into an in-fiber Mach-Zehnder interferometer

This work reports on a novel in-fiber Mach-Zehnder interferometer (MZI) device which is able to exploit the mode transition phenomena to tailor the sensitivity to external refractive index (RI). The proposed MZI structure is extremely simple: it consists of a small section of single mode double cladding fiber (DCF) in between two sections of standard single mode fiber using the core-offset splicing approach. The DCF presents a W-shaped RI profile with the outer cladding having a greater RI than the inner cladding. Therefore, the mode transition of cladding modes from outer to inner cladding of the DCF can be induced by tailoring the cladding thickness, in turn enhancing the sensitivity to surrounding refractive index. Specifically, wet chemical etching employing hydrofluoric acid is used to reduce the outer cladding diameter. Here, we report a comprehensive study of the phenomena while fiber diameter is decreased by 30 µm. Moreover, a specific study case with DCF diameter of 112 µm demonstrates a sensitivity gain of 2.5 over unetched fiber in the refractive index range of 1.33–1.39. Temperature and mechanical stress cross sensitivities are also evaluated. The suggested sensing device's advantages of considerable compactness, low manufacturing cost, simple fabrication technique, and higher sensitivity will be useful for future applications in the bio-chemical sensing and others.

Femtosecond laser-induced damage on the end surface of double-cladding fluorotellurite fiber

In this study, the damage characteristics of double-cladding fluorotellurite glass fiber endface was conducted. Double-cladding fluorotellurite fiber was prepared with a core composition of 70TeO2-20BaF2-10Y2O3 (TBY70), an inner cladding composition of 33AlF3-9BaF2-17CaF2-12YF3-8SrF2-11MgF2-10TeO2 (ABCYSMT), and an outer cladding composition of 65TeO2-25BaF2-10Y2O3 (TBY65). It was concluded that at the wavelength of 800 nm, the laser-induced damage threshold (LIDT) of inner and outer cladding were found to be 1951.8 and 1551.8 mJ/cm2, respectively, under single-pulse laser irradiation. Then, the influence of pulse number on the LIDT was studied. As the laser pulse number increased from 1 to 200, the LIDT of outer cladding decreased from 1551.8 mJ/cm2 to 695.2 mJ/cm2 due to the cumulative effect of damage crater defects. When the pulse number reached 100, the LIDT saturated, corresponding to a damage accumulation factor of 0.84. With the pulse number increasing continuously from 50 to 5000, the damage area showed no significant change, but the damage depth increased from 40.4 μm to 73.0 μm. In addition, the damage resistance of the two materials, TBY65 and ABCYSMT, was analyzed based on the ionization rate obtained from simulations.

Design mode filtering interferometer using etched double clad fiber

Abstract Mode filtering technique is one of the most desired techniques in optical fiber communication systems, especially for multiple input multiple output (MIMO) coherent optical communications that have mode-dependent losses in communication channels. In this work, a special type of optical fiber sensing head was used, where it utilizes DCF13 that is made by Thorlabs and has two numerical apertures (NA’s). One is for core and 1st cladding region, while the 2nd relates the 1st cladding to the 2nd cladding. Etching process using 40 % hydro-fluoric (HF) acid was performed on the DCF13 with variable time in minutes. Investigation of the correlation between the degree of etching and the refractive index (RI) profile of double-clad fibers for low order linearly polarized (LP) modes is the focus of this work research. The used software programs to simulate the effect of etching are Comsol Multi-Physics V6 and Optigrating V4.2.2. It is seen that the optimum case that gave highest cladding power to core power ratio is the case of etching for 10 min, which can be used for detection applications. This ratio greatly affects the sensitivity of the system.

Hollow Core Double-Clad Fiber Coupler for Nonlinear Micro-Endoscopy

We report the design, fabrication and characterization of the first hollow core double-clad fiber coupler for nonlinear micro-endoscopy. It is made of a pure silica negative curvature hollow core double-clad fiber and a highly multimode fiber fused along each other. The fabrication of such a coupler was made possible thanks to the use of a soft (borosilicate) glass multimode fiber which can be fused along the negative curvature fiber without altering its geometry, thanks to its much lower fusion temperature. A 60% power transfer from the double-clad fiber to the multimode one was measured at 500 nm. Demonstration of two-photon fluorescence and second harmonic generation imaging was performed on unlabeled biological tissues with an endoscopic head mounted on the hollow core double-clad fiber coupler to demonstrate its applicability. This device greatly simplifies the integration of nonlinear micro-endoscopes based on hollow core double clad fibers.

Robust femtosecond-written chirped and tilted fiber Bragg gratings for Raman filtering in multi-kW fiber lasers.

We present a robust chirped and tilted fiber Bragg grating (CTFBG) in a large-mode-area double-cladding fiber (LMA-DCF) written by a femtosecond (fs) laser. By implementing the fs-CTFBG into the output end of a high-power fiber laser for Raman filtering, a power handling capability of 4 kW is achieved with a Raman filtering ratio of ∼13 dB. To the best of our knowledge, this is the maximum handling power of a CTFBG for Raman filtering. The signal loss of the fs-CTFBG is 0.03 dB, which has little effect on the output laser beam quality. The air-cooled fs-CTFBG has a minimum temperature slope of 7.8°C/kW due to a self-annealing effect. This work proves the excellent performance of the fs-CTFBG, promoting the development of high-power CTFBGs.

Tunable Sensitivity in Long Period Fiber Gratings During Mode Transition With Low Refractive Index Intermediate Layer

Double-clad fibers where the second cladding has a lower refractive index than the first cladding, prove to be ideal structures for potentiating and tuning the sensitivity in long-period fiber gratings (LPFGs) operating in mode transition. When a thin film is deposited on the optical fiber, the second cladding performs acts as a barrier that initially prevents the transition to guidance in the thin film of one of the modes guided in the first cladding. Finally, the transition to guidance occurs with a sensitivity increase, in analogy to the tunnel effect observed in semiconductors. This improvement has been demonstrated both as a function of the thin film thickness and the surrounding medium refractive index, with enhancement factors of 4 and 2, respectively. This idea reinforces the performance of LPFGs, adding a new degree of freedom to the mode transition and the dispersion turning point phenomena. Moreover, the control of the variation of the effective index of cladding modes could be applied in other structures, such as tilted-fiber gratings or evanescent wave sensors.

Double-clad photonic crystal fibre for two-photon microscopy

Miniaturized and head-mountable scanning nonlinear fibre-optic microscopy offers an exciting opportunity for the visualization of brain activity on freely walking mice. One challenge in the creation of such miniaturized microscopes is the design of optical fibres capable of delivering ultrashort pulses without distortion and collecting fluorescence simultaneously and efficiently. A double-clad photonic crystal fibre (DCPCF) with 37-cell air holes arranged in hexagonal lattice in core is proposed. The full vector finite element method (FEM) is used for numerical analysis. Nearly zero dispersion of −0.34 ps/nm/km at 920 nm wavelength is finally achieved. The single-mode core with an NA of 0.53 at 920 nm wavelength is used to deliver the femtosecond excitation beam and the inner cladding with an NA of 0.54 at 532 nm is used to collect the two-photon fluorescence signal. The nearly zero dispersion, high NA and low nonlinearity will be beneficial to promote its application in two-photon microscopy.

Improving the Width of Lossy Mode Resonances (LMRs) in Double-Clad Fibers

In this work, the characteristics of lossy mode resonances (LMRs) in double-clad fibers where the refractive index (RI) of the second cladding is lower than that of the first cladding are analyzed both numerically and experimentally. In the first place, the LMRs spectra obtained with a 75 nm TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film are simulated, and it is observed that a thicker second cladding improves the width of the resonances, making them narrower. Then, two experimental cases (no second cladding, and second cladding with thickness of 1.13 µm) are assessed, showing a good agreement with the previous simulations. Finally, an experimental refractometric study is carried out in liquids (surrounding medium refractive index in the 1.34 - 1.40 range) for both fibers, calculating the full width at 1dB (FW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</sub> ), the sensitivity, and the figure of merit (FOM). The FW <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</sub> is better for the LMR obtained on the fiber with second cladding while the sensitivity is slightly greater for the fiber without second cladding. In the case of the FOM, it is higher for the double-clad fiber as the narrowing of the resonances outweighs the lower sensitivity. These results show that the performance of LMR-based optical fiber sensors can be improved by employing double-clad fibers.

Optimized segmented cladding fiber for extreme large mode area using latin hypercube sampling

In this paper a new design of octo-wing segmented cladding fiber (SCF) with a uniform core containing a ring and a non-uniform double cladding was proposed and optimized. The Latin hypercube sampling was applied to generate design cases as the input values for estimation of the objective functions as the output variables. Three physical parameters of the core assisted ring: thickness, radius, and refractive index difference were selected as the design variables. Twenty different design cases were numerically simulated by finite element method. To optimize the effective mode area (EMA) as the objective function, response surface methodology was applied. The fiber offers the single mode operation with a confinement loss ratio of over 100 between the fundamental mode (FM) and the higher order mode (HOM). Operating at the wavelength of 1.550μm, the EMA is significantly enlarged to about 884μm2. We have investigated the properties of the proposed optimized ring assisted SCF (RA-SCF) in terms of confinement losses of FM and first HOM considering a perfect matched layer. The proposed RA-SCF provides the potential applications in compact high power fiber lasers.

Influence of multi-layer laser cladding depositions and rail curvature on residual stress in light rail components

Rapid deterioration of critical rail components due to wear and fatigue is a significant challenge faced by the railway industry. Laser cladding has been applied to straight and curved light rail substrates which are particularly prone to these methods of degradation using a Stellite 21 hardfacing alloy. The influence of multi-layer cladding depositions, grinding-based finishing procedures and substrate rail grade on residual stress was analyzed using non-destructive neutron diffraction techniques. As cladding is a thermal process, microstructural changes from the heat inputs can result in a high internal stress state which reduces the loading capacity whilst a high hardness increases the likelihood of brittle failure. Stellite laser cladding depositions were found to result in low tensile residual stresses within the cladding layer, which become compressive in the heat affected zone (HAZ). Repeated thermal inputs from multi-layer cladding depositions did not negatively impact the hardness or microstructure after double layer laser cladding and increased the residual stress to 100 MPa at the cladding surface after grinding. Laser cladding on a curved rail substrate also produced increased internal stress after cladding due to higher strains resulting from the rail geometry but remained below the yield limit of the cladding and substrate material. These outcomes were compared to current literature indicating this critical combination of low internal strain and a cladding and HAZ hardness complementing the substrate material is difficult to achieve. Therefore, ensuring laser cladding is compatible with a variety of light rail components is essential in developing viable maintenance techniques to recondition critical railway infrastructure to avoid disruptive replacement procedures.

High luminescence efficiency in Er3+-doped chalcogenide fiber with square inner-cladding

High-purity chalcogenide glasses and optimal fiber structure design are essential to enhance fluorescence lifetime and pump absorption efficiency in a chalcogenide-based fiber laser. Here, Er3+-doped As2S3 glasses, with a low –OH concentration of only 0.023 cm−1 (0.01 ppm), were prepared by the novel purification method, and the 4I11/2 level lifetime of purified glasses was increased successfully from 2.3 ms in unpurified glass to 4.8 ms in the purified glass. Furthermore, a special chalcogenide preform with square inner double cladding (S-DCF) was designed and fabricated via a novel extrusion-stacking method for the first time. Compared with circle inner double cladding fiber and single cladding fiber, the pump absorption efficiency is increased by 2–5 times, which fits well with the simulation result by the three-dimensional ray tracing method. All the results mentioned above demonstrated that the elimination of –OH and the optimized structure of S-DCF can improve the luminescence efficiency apparently and promote the development of mid-infrared fiber laser based on Er3+-doped chalcogenide glass.

Friction riveting of FR4 substrates for printed circuit boards: Influence of process parameters on process temperature development and joint properties

This work investigates the influence of Friction Riveting processing conditions on FR4-PCB substrate/AA2024 rivet joints in terms of process temperature evolution, joint formation, and joint physical-chemical and mechanical properties. The joints were manufactured using 4 mm diameter AA-2024-T3 rivets and FR4 laminates of 1.5 mm thickness with single or double copper-clad layers. The evolution of process temperature evolution was recorded on the FR4 substrate surface and correlated with the resulting joint formation. Most joints obtained with double copper clad layers developed process temperatures above 300 °C, whereas joints produced with a single copper clad presented slightly lower temperatures, but still above 250 °C. Rivet anchoring was achieved for both FR4 material combinations in the configuration of a single-base laminate, as well as two and even three overlapped laminates. Thermogravimetric analyses revealed that above 300 °C intensive thermal degradation occurs on FR4 materials (with 30% mass change), followed by decomposition, resulting in non-uniform heat distribution throughout the thickness. The joint ultimate tensile force was higher for double copper-clad layers and the joints achieved within more than one laminate, showing higher anchoring efficiency.