Anti-resonant Elements Research Articles

Large-mode-area Nd-doped anti-resonant phosphate fiber for high-power single-mode 900 nm laser generation

In this work, we propose an Nd-doped double-layer anti-resonant phosphate fiber with a core diameter of 50 µm for high-power single-mode 900 nm laser generation. Double-layer interlaced anti-resonant elements were designed here to enhance the fundamental mode confinement capability of the large-mode-area Nd-doped fiber core. Moreover, a double-layer F-P etalon formed between the anti-resonant elements and the inner cladding was analyzed for the first time for fiber loss manipulation. Single-mode operation in the 890–907 nm band with confinement loss lower than 0.1 dB/m can be achieved from the designed fiber. More importantly, high confinement loss larger than 100 dB/m is achieved for all the fiber modes around 1060 nm for four-level gain competition suppression in 900 nm Nd-doped fiber laser generation. A 900 nm fiber amplifier simulation based on the designed Nd-doped phosphate fiber shows that the parasitic lasing or even amplified spontaneous emission around 1060 nm can be effectively suppressed and a high-efficiency hundred-watt laser at 900 nm can be anticipated.

Research on hollow-core terahertz fiber based on Bragg anti-resonant elements arranged in a hexagonal structure

A hollow-core terahertz fiber composed of Bragg anti-resonant elements in a hexagonal structure is proposed. The function of Bragg elements on the transmission losses and bending losses of the hollow-core terahertz fiber is investigated using finite element method. Numerical simulations demonstrate that the introduction of Bragg elements for the anti-resonant cladding can significantly reduce the confinement losses of the fiber, the appropriate choice of the outer cladding diameter can also contribute to the reduction of the total loss. The bending loss shows low dependence on the bending angle for the proposed fiber with the Bragg elements. Compared with the anti-resonant hollow-core terahertz fiber composed of only a single dielectric layer, the bending losses of the hollow-core terahertz fiber with the Bragg elements can be reduced by more than an order of magnitude.

Design of three-level Nd-doped laser fiber based on anti-resonant structure

900-nm Nd-doped fiber laser can find widespread applications including biomedical diagnosis, laser detection, and spectral analysis. The four-level gain competition of Nd<sup>3+</sup> around 1060 nm severely constrains the laser power scaling of the 900-nm three-level Nd-doped fiber laser. In this work, we propose a large-mode-area Nd-doped double-layer solid-core anti-resonant fiber with a core diameter of 27 μm for generating a high-power 900-nm laser based on the resonant and anti-resonant conditions of anti-resonant fiber. The transmission properties and mode profiles of the designed fiber are analyzed theoretically by using the full-vector finite-element method combined with an optimized mesh size. By introducing the double-layer anti-resonant elements into the active fiber and optimizing the fiber structure parameters and refractive index distribution, the high-order modes are well coupled with cladding modes. Finally, the designed fiber exhibits a confinement loss below 0.1 dB/m for fundamental mode and the confinement losses of all high-order modes are greater than 10 dB/m in 880–913 nm band. More importantly, around 1060 nm, the confinement losses of all modes can reach up to 100 dB/m, which enables the designed Nd-doped fiber to effectively suppress parasitic lasing and even amplified spontaneous emission. The Nd-doped solid-core anti-resonant fiber proposed in this study shows broad application prospects in the fields of 900-nm high-power fiber laser and amplifier. The developed chemical vapor deposition process combined with stack-and-draw technology can be adopted for the fabrication of the designed fiber. In order to ensure the optical performance of the anti-resonant fiber, it is necessary to accurately control the thickness of all anti-resonant tubes, the glass composition of the active core and background area in actual fabrication.

Polarization-maintaining large-mode-area solid-core anti-resonant fiber for high-power fiber lasers

A polarization-maintaining large-mode-area solid-core anti-resonant fiber is investigated in this work with double-layer anti-resonant elements for improved capability on fundamental mode guidance and higher order mode suppression, and structural and material asymmetry on inner-layer cladding tubes for high birefringence. Mode coupling between fundamental core mode in one polarization direction and mode on two high-index inner cladding tubes is used to enhance the fiber birefringence under large core diameter. Eventually, an optimized design of polarization-maintaining large-mode-area solid-core anti-resonant fiber with the currently largest mode field area of 3026 μm2 and birefringence of 1.20 × 10−5 can be anticipated with the confinement losses for all the higher order modes exceeding 10 dB/m. Moreover, confinement loss below 0.1 dB/m can still be obtained for the fundamental core mode in both polarization direction even under the mode-coupling effect. Active polarization-maintaining anti-resonant fiber is analyzed and the refractive index of the central rare-earth-doped region is optimized for the first time for both well guided signal and pump laser. Large pump absorption coefficient can thus be anticipated from the multimode laser core-pumped active large-mode-area fiber, which facilitates short active fiber length needed for sufficient laser gain. The fiber proposed here offers great potential for the development of high-performance fiber laser sources including high-power single-frequency fiber amplifiers and high-peak-power ultrashort pulsed fiber amplifiers.

Single-mode bend-resistant hollow-core fiber with multi-size anti-resonant elements

A novel single-ring hollow-core anti-resonant fiber (HC-ARF) with multi-size tubular anti-resonant elements (AREs) that can simultaneously provide low bending losses and robust single-mode operation is proposed. Numerical results prove that the introduction of multi-size AREs can effectively improve the suppression ability to high order modes (HOMs). The HOM extinction ratio is twice as high as that of single-ring HC-ARF with uniform AREs. Meanwhile, the bending loss is reduced by at least one order of magnitude due to the suppression of bending-induced loss peaks. A single-ring 7-ARE HC-ARF with four kinds of ARE diameters is successfully fabricated. Experimental results demonstrate that under a tight bending radius of 3.5 cm, the bending loss is as low as 0.37 dB/m @1.65 μm. The measured near-field profile through a 5 m-long fiber proves a robust single-mode performance. In order to further reduce the bending loss, we proposed the modified structure by adopting the nested multi-size AREs, achieving a lower bending loss of 0.18 dB/m @1.55 μm under an extremely tight bending radius of 0.75 cm, which is the lowest bending loss among the reported HC-ARFs under such tight bending radius as far as we know. Meanwhile, single-mode performance can be also improved due to a higher HOM extinction ratio of 938. The proposed fiber has great potential for applications in high-power laser delivery and gas-filled HC-ARF lasers.

A Negative-Curvature Hollow-Core Fiber Structure With Double Trigonal-Symmetrical Anti-Resonant Elements

We propose a novel negative-curvature hollow-core fiber structure with double trigonal-symmetrical anti-resonant elements implementing single-mode or polarization-maintaining transmission for high-power laser delivery in the 1 µm wavelength window. For the single-mode fiber, the single-mode performance with a higher-order modes extinction ratio (HOMER) as high as 4.65*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> is achieved, and it remains >1*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> with a large fault tolerance proportion of tubes size, which can increase the flexibility of fiber fabrication. For another polarization-maintaining fiber, an ultra-low confinement loss of 0.005 dB/m is obtained for polarization-maintaining transmission by optimizing the refractive index of the two transverse glass tubes, while the polarization extinction ratio and HOMER remain both >1*10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . This is the first birefringent negative-curvature hollow-core fiber with a loss below 0.01 dB/m in the 1 µm wavelength window.

Numerical modeling of a hybrid hollow-core fiber for enhanced mid-infrared guidance.

We propose a novel design of hollow-core fiber for enhanced light guidance in the mid-infrared. The structure combines an arrangement of non-touching antiresonant elements in the air core with a multilayer glass/polymer structure in the fiber's cladding. Through numerical modeling, we demonstrate that the combination of antiresonant/inhibited-coupling and photonic bandgap guidance mechanisms can decrease the optical loss of a tubular antiresonant fiber by more than one order of magnitude. More specifically, our simulations demonstrate losses of the HE11 mode in the few dB/km level, which can be tuned through mid-infrared wavelengths (5 µm-10.6 µm) by carefully optimizing the structural parameters of both structures. We also show that the hybrid hollow-core fiber design is more robust to bend-induced loss than an equivalent tubular antiresonant fiber or a Bragg/OmniGuide fiber. As a result, if successfully fabricated, the hybrid hollow-core fiber will offer low-loss, high beam-quality, effectively single-mode operation, and low bending losses, potentially solving many of the problems that affect all known mid-infrared fiber types.

Investigation of mode couplings between core and cladding of terahertz anti-resonant fibres

In this study, we numerically investigated mode couplings in terahertz (THz) anti-resonant fibres (ARFs) and found that the mode couplings were determined by the diameter ratio of the core and cladding tubes. Moreover, the optimal diameter ratio was affected by the change in the number of cladding tubes, bent radii and the refractive index of the gas filled in fibre. Mode couplings provide novel theories for fundamental mode suppression and higher-order mode suppression. In THz ARFs, taking the semielliptical tubes as anti-resonant elements can significantly improve the suppression ability of mode couplings.

Hollow core fibers for optical amplification.

Hollow core optical fibers are normally passive light transport components. In contrast, within this Letter, we numerically investigate the possibility of using them as optical amplifiers, through the adoption of a novel fiber structure. We show that optical amplification can be achieved in hollow core fibers, where the cladding region is partially doped and composed of both resonant and anti-resonant elements. A balance between loss and glass/optical mode overlap is obtained, which allows efficient amplification over a limited spectral bandwidth. We discuss the case of a thulium-doped optical amplifier based on this novel technological approach.

Approximate model for analyzing band structures of single-ring hollow-core anti-resonant fibers.

Precise knowledge of modal behavior is of essential importance for understanding light guidance, particularly in hollow-core fibers. Here we present a semi-analytical model that allows determination of bands formed in revolver-type anti-resonant hollow-core fibers. The approach is independent of the actual arrangement of the anti-resonant elements, does not enforce artificial lattice arrangements and allows determination of the effective indices of modes of preselected order. The simulations show two classes of modes: (i) low-order modes exhibiting effective indices with moderate slopes and (ii) a high number of high-order modes with very strong effective index dispersion, forming a quasi-continuum of modes. It is shown that the mode density scales with the square of the normalized frequency, being to some extent similar to the behavior of multimode fibers.

Understanding Dispersion of Revolver-Type Anti-Resonant Hollow Core Fibers

Here, we analyze the dispersion behavior of revolver-type anti-resonant hollow core fibers, revealing that the chromatic dispersion of this type of fiber geometry is dominated by the resonances of the glass annuluses, whereas the actual arrangement of the anti-resonant microstructure has a minor impact. Based on these findings, we show that the dispersion behavior of the fundamental core mode can be approximated by that of a tube-type fiber, allowing us to derive analytic expressions for phase index, group-velocity dispersion and zero-dispersion wavelength. The resulting equations and simulations reveal that the emergence of zero group velocity dispersion in anti-resonant fibers is fundamentally associated with the adjacent annulus resonance which can be adjusted mainly via the glass thickness of the anti-resonant elements. Due to their generality and the straightforward applicability, our findings will find application in all fields addressing controlling and engineering of pulse dispersion in anti-resonant hollow core fibers.

Chalcogenide-Glass Nested Anti-Resonant Nodeless Fibers in Mid-Infrared Region

We propose two types of chalcogenide-glass nested anti-resonant nodeless fibers (CNANFs) with four anti-resonant elements, in which a smaller tube is nested. One type of CNANF theoretically achieves ultra-low loss of 2.6 dB/km at the wavelength of 10.6 $\mu$ m and can be used for the delivery of a high-power CO $_2$ laser. The other CNANF is a polarization-maintaining CNANF in the broadband mid-infrared (mid-IR) region. We investigate the tradeoff between loss and birefringence in the mid-IR region and achieve a low loss of $ 1 dB/m and high birefringence of $>$ $1\times 10^{-4}$ simultaneously over a large wavelength band by adjusting the air-core size and the thickness of the vertical/horizontal outer tubes. Both CNANFs show strong capability for higher order mode suppression.

Optofluidic laser based on a hollow-core negative-curvature fiber

Abstract An optofluidic laser based on a hollow-core negative-curvature fiber (HC-NCF) is proposed and demonstrated. The submicron-thick circular capillary tubes embedded in the cladding of the HC-NCF act as antiresonant elements and are used as both a resonator and dye microfluidic channels. A stable optofluidic dye laser with a low threshold of 15.14 nJ/mm2 is achieved. The laser is compact and robust and exhibits directional output.

Low-loss single-mode hollow-core fiber with anisotropic anti-resonant elements.

A hollow-core fiber using anisotropic anti-resonant tubes in the cladding is proposed for low loss and effectively single-mode guidance. We show that the loss performance and higher-order mode suppression is significantly improved by using symmetrically distributed anisotropic anti-resonant tubes in the cladding, elongated in the radial direction, when compared to using isotropic, i.e. circular, anti-resonant tubes. The effective single-mode guidance of the proposed fiber is achieved by enhancing the coupling between the cladding modes and higher-order-core modes by suitably engineering the anisotropic anti-resonant elements. With a silica-based fiber design aimed at 1.06 µm, we show that the loss extinction ratio between the higher-order core modes and the fundamental core mode can be more than 1000 in the range 1.0-1.65 µm, while the leakage loss of the fundamental core mode is below 15 dB/km in the same range.