Effective Single-mode Operation Research Articles

Design of a large-mode-area tapered fiber amplifier for amplification of ultrashort laser pulses at 1535 nm wavelength

We design a W-type large-mode-area erbium doped tapered fiber with effective mode area varying from 1770 µm2 to 840 µm2with tolerance ± 0.0005 in refractive indices of different layers of fiber. The beam quality of the pulses is ensured by effective single-mode operation. We investigate the amplification of high power femtosecond pulses in this fiber by using nonlinear Schrodinger equation including gain term. It is solved using split-step Fourier method for both up-tapered (effective mode area increasing along the length) and down tapered (effective mode area decreasing along the length) fiber configurations at centered around 1535 nm wavelength. In both configurations of fiber, input pulse parameters like peak power, pulse width, and fiber length are calculated for maximum amplification with the least amount of pulse shape distortion. In down-tapered configuration, we numerically demonstrate the amplification of 1 kW peak power, 560 fs duration pulse to 3.6 MW peak power, 12 fs amplified pulse. In up-tapered configuration, we obtain the amplification of 2 kW, 565 fs input pulse to ∼7.9 MW peak power, ∼11 fs pulse. The fiber is useful for amplifying high-power laser pulses, and the resulting pulses can be used for laser processing, spectroscopy, and remote sensing.

Designing hybrid-guidance large mode area fiber for high-power lasers

All-solid large-mode-area antiresonant fibers possess unique characteristics of broadband, low-loss transmission of fundamental mode with effective single mode operation. Hinging on the principle, we propose a unique hybrid guidance mechanism, combining antiresonant guidance with total internal reflection-based guidance to design bend-tolerant, ultra large mode field area (>3000 µm2) fibers for high-power laser applications. Low-index rods are strategically interspersed into the antiresonant cladding to enhance the modal confinement in the fiber, resulting in lower confinement loss of the fundamental mode and its increased tolerance to bending and to the refractive index dip caused by non-uniform doping of active ions in the fiber core. We show that total internal reflection guidance greatly helps confine the fundamental mode without masking the characteristic features of antiresonant guidance, like effective single mode operation over broad spectral bandwidth. We present a detailed theoretical analysis to design a bendable, effectively single-moded passive and active hybrid-guidance antiresonant fibers, optimized to work in the 1 µm wavelength band. The design principles are universal and can be extended to shift the operating wavelength to the 2 µm range as well.

Displacement sensor based on a large-core hollow fiber and specklegram analysis

The research enterprise towards achieving high-performance hollow-core photonic crystal fibers has led to impressive advancements in the latest years. Indeed, using this family of fibers becomes nowadays an overarching strategy for building a multitude of optical systems ranging from beam delivery devices to optical sources and sensors. In most applications, an effective single-mode operation is desired and, as such, the fiber microstructure or the light launching setups are typically designed for achieving such a behavior. Alternatively, one can identify the use of large-core multimode hollow-core fibers as a promising avenue for the development of new photonic devices. Thus, in this manuscript, we propose and demonstrate the utilization of a large-core tubular-lattice fiber for accomplishing a speckle-based displacement sensor, which has been built up by inserting and suitably dislocating a single-mode fiber inside the void core of the hollow fiber. The work reported herein encompasses both simulation and experimental studies on the evolution of the multimode intensity distributions within the device as well as the demonstration of a displacement sensor with an estimated resolution of 0.7 μm. We understand that this investigation identifies a new opportunity for the employment of large-core hollow fibers within the sensing framework hence widening the gamut of applications of this family of fibers.

Small-core hollow-core nested antiresonant nodeless fiber with semi-circular tubes.

Hollow-core nested anti-resonant nodeless fibers (HC-NANFs) exhibit great performance in low loss and large bandwidth. Large core sizes are usually used to reduce confinement losses, but meanwhile, bring side effects such as high bending and coupling losses. This study proposes a small-core HC-NANF with a relatively low confinement loss. Semi-circular tubes (SCTs) are added to constitute the core boundary and reduce the fiber-core radius (R). Double NANFs tubes and single-ring tubes are added inside the SCTs to reduce loss. Simulation results show that the optimized structure with R of 5 µm has confinement loss and total loss of 0.687 dB/km and 4.27 dB/km at 1.55 µm, respectively. The bending loss is less than 10 dB/km at 1.4 ∼ 1.6 µm with a bending radius of 10 mm. The direct coupling loss with standard single mode fiber is greatly reduced to ∼ 0.125 dB compared to other HC-NANFs. The modified structure of HC-NANFs also shows a large bandwidth, effective single-mode operation, potentially high birefringence performance, and remarkable robustness of the optimized structure parameters, making it suitable for short-haul applications in laser-based gas sensing, miniaturized fiber sensing, etc.

Bend-resistant octo-wing silica segmented cladding fiber with high index rings

We have proposed a novel segmented cladding fiber (SCF) design and the bending effects were investigated in detail. The proposed SCF with large mode area properties has a uniform circular core and octo-wings of rods with higher refractive index than the silica segments in the cladding. While the high index octo-wings improved the waist of the electrical field distribution of the fundamental mode (FM), the lower index segments enabled SCF to be bend-resistant. We numerically investigated the light guiding properties through the proposed SCF by using finite element method with a perfectly matched layer. SCF was investigated in terms of effective mode area, confinement loss in bent state and it was shown that the proposed SCF is insensitive to the bending angle. By applying high index rings around the rods in octo-wings and optimizing the structure, we obtained an effective mode area as large as 715 µm2 at the wavelength of 1.55 µm with a relatively large loss ratio between FM and first higher order mode of about 137. Effective single mode operation was achieved at a tight bending radius of 10 cm over a wide wavelength range from 1 µm to 1.7 µm. The novel proposed SCF gives the advantages of fabrication feasibility by the method of powder-in-tube process. Octo-wing SCF could be applied for compact amplifiers and high power fiber lasers.

Wideband Low Loss Hollow Core Fiber With Nested Hybrid Cladding Elements

In this article, an antiresonant nodeless hollow core fiber having nested hybrid cladding elements is proposed without having large distance between core and jacket capillary. The presented anisotropic construction (elliptical nesting in circular tube) with optimized curvature factor, strut thickness, and gap between adjacent tubes lead to extremely low loss over wide band compared to existing structures (circular, circular nested, elliptical, elliptical nested). The results show the lowest confinement loss of 0.0007 dB/km at 1.06 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m with maintaining a loss of less than 0.003 dB/km over 670 nm (0.90 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m to 1.57 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m) bandwidth. In addition, the lowest surface scattering loss of 0.01 dB/km is achieved along with better bending stability and effective single mode operation. The proposed fiber may have potential opportunity in optical communication systems for its low-loss transmission over wideband region.

Amplification of femtosecond and picosecond pulses using large mode area Erbium-doped fiber at 1530 nm wavelength

We present amplification of short and ultrashort laser pulses at 1530-nm center wavelength using a large mode area Er-doped fiber (LMA-EDF). The mode area of about 1700 μm2 tremendously increases the power handling capacity of the fiber and helps in achieving up to ∼ 17 dB/m gain with 10 W power of 980-nm wavelength pump. The LMA fiber used here is a 3-layer W-type fiber designed to have effective single mode operation with higher order mode discrimination and sufficient mode spacing between the first few modes to avoid any possible mode coupling. In particular, we numerically demonstrate the amplification of 300 fs, 1 kW peak power pulses to ∼ 20 fs, ∼2.6 MW peak power pulses in 1.5-m propagation distance, and of 50 ps, 1 kW peak power pulses to ∼ 65 ps, ∼26 MW peak power pulses in 3-m propagation distance of the LMA-EDF. The generated pulses would be useful in applications such as eye-safe remote sensing and 3D laser processing of silicon.

Leakage channels enabled multi-resonant all-solid photonic bandgap fiber for effective single-mode propagation.

All-solid photonic bandgap fiber (AS-PBGF) has been fully demonstrated to be a promising candidate of large-mode-area fiber for its mode-dependent selectivity and spectral filtering mechanism. In the present work, the concepts of multiple-resonant coupling and leakage channels are taken into consideration simultaneously for mode area scaling of AS-PBGF. The single-mode performance and bending resistance of a modified structure, called leakage channels enabled multi-resonant AS-PBGF (LC-PBGF), are evaluated numerically. Robust single-mode transmission is guaranteed by a specially designed microstructure cladding with only four layers of germanium-doped rods. Multi-resonant cores in the inner layers and leakage channels in the outermost layer, resulting from missing rods in the microstructure cladding, are employed to generate modal dissipation of high-order modes under bent configuration. The missing germanium-doped rods in each layer are properly designed to eliminate the dependence on bending direction, leading to differential bending loss between fundamental mode and high-order-modes with high loss ratio. In addition, some typical derivative structures based on the LC-PBGF concept have also been proved to have great potential for effective single-mode operation.

Optimal design of large mode area all-solid-fiber using a gray relational optimization technique

The multi-objective orthogonal optimization combined with gray relational analysis method is presented for designing large mode area all-solid-fiber with effective single-mode operation in the bent state. The bending performance of fiber was carried out taking the bending loss and effective mode area as objective functions, taking the width of inner-trench, width of resonant ring, refractive index of the inner- trench, the distance from the out-trench to the resonant ring, the width of out-trench and the refractive index of the out-trench as design variables. The results show the resonance ring added in the core can enhance the coupling between the high-order mode and the cladding mode and filter out the high-order mode to ensure the single-mode operation. The trench added to the cladding can increase the refractive index difference between the core and the cladding. The bending loss of the fundamental mode is limited and the anti-bending loss performance of the fiber is enhanced. After optimization, the bending loss is 0.00953 dB/m and the effective mode area can reach 776.45 µm2. Under the same bending radius, the bending loss of the designed optical fiber was reduced by an order of magnitude, the effective mode field area increased by 7.5%, and the Loss Ratio increased double. This kind of fiber shows great potential in compact fiber communication, high power fiber lasers and amplifiers.

Segmented Cladding Fiber With a High-Index Ring in Core for Wideband Single-Mode Operation in Any Bending Orientation

A resonant-ring-assisted large mode area segmented cladding fiber (RR-SCF) with a high-index ring in the core is proposed. Numerical investigations of the effects of the high-index ring on the leakage losses of guiding modes, mode area, and electric field distribution of FM of the proposed fiber show the high-index ring not only increases the ratio of the loss between the fundamental mode (FM) and the higher-order modes (HOMs) but also improves the electric field distribution of the FM to be flat. By taking these advantages of the high-index-ring-assisted core, the effective single-mode (SM) operation is achieved in the proposed fiber bent in any orientation, which makes up the defect of the conventional RR-SCF that the SM output is affected by the bending orientation. Meanwhile, the mode area is enlarged. Furthermore, the effects of the heat load, duty cycle, bending, and operation wavelength on the fiber performance are analyzed. The results show single-mode operation in all bending orientations with a mode area larger than 824 μm2 can be achieved at a tight bending radius of 15 cm to 16 cm when the operating wavelength ranges from 1.06 μm to 1.76 μm. This fiber shows great potential in high-power fiber lasers.

Resonant-ring assisted large mode area segmented cladding fiber with high-index rings in core

This paper proposes a modified segmented cladding fiber with two-layer high-index rings in the core. The inner high-index ring helps to increase the loss ratio between the higher-order modes (HOMs) and the fundamental mode (FM) to achieve single-mode operation. The outer high-index ring is beneficial to improve the waist of the electric field distribution of the FM to enlarge the mode area. The simulation results show thanks to the joint defect of these two-layer high-index rings, the defect of the RR-SCF that cannot provide single-mode operation in all bending orientations is made up. Effective single-mode operation is achieved at a tight bending radius of 15 cm when the operating wavelength ranges from 1.06 μm to 1.58 μm. The mode area of the fiber operating at 1.06 μm is markedly enlarged to 835 μm2. This new fiber shows great potential in compact high-power fiber lasers and amplifiers.

Design and analysis for large-mode-area photonic crystal fiber with negative-curvature air ring

A large mode area photonic crystal fiber (LMA-PCF) under standard fiber size is proposed in this paper. In order to realize large mode field and low bending loss, multilayer negative curvature air ring structure is designed. The finite element method with perfectly matched layer boundaries is used to analyze the performance of PCF. The influence of various parameters of fiber structure on fiber performance is analyzed. Numerical results show that the structure can achieve large mode area with lower bending loss and smaller bending radius in the wavelength range of 1–1.8 µm. Under the condition of a five-layer six-lobed air ring with negative curvature, the mode field area reaches 1358.9 μm2. At the same time, the high-to-fundamental mode loss ratio reaches 331.5. When the bending radius is 15 cm, the bending loss of the fundamental mode is 0.21 dB/m. Within the 360° bending direction, LMA can be realized in all directions and has effective single-mode operation.

Low-loss single-mode hybrid-lattice hollow-core photonic-crystal fibre

Remarkable recent demonstrations of ultra-low-loss inhibited-coupling (IC) hollow-core photonic-crystal fibres (HCPCFs) established them as serious candidates for next-generation long-haul fibre optics systems. A hindrance to this prospect and also to short-haul applications such as micromachining, where stable and high-quality beam delivery is needed, is the difficulty in designing and fabricating an IC-guiding fibre that combines ultra-low loss, truly robust single-modeness, and polarisation-maintaining operation. The design solutions proposed to date require a trade-off between low loss and truly single-modeness. Here, we propose a novel IC-HCPCF for achieving low-loss and effective single-mode operation. The fibre is endowed with a hybrid cladding composed of a Kagome-tubular lattice (HKT). This new concept of a microstructured cladding allows us to significantly reduce the confinement loss and, at the same time, preserve truly robust single-mode operation. Experimental results show an HKT-IC-HCPCF with a minimum loss of 1.6 dB/km at 1050 nm and a higher-order mode extinction ratio as high as 47.0 dB for a 10 m long fibre. The robustness of the fibre single-modeness is tested by moving the fibre and varying the coupling conditions. The design proposed herein opens a new route for the development of HCPCFs that combine robust ultra-low-loss transmission and single-mode beam delivery and provides new insight into IC guidance.

Large mode area single mode photonic crystal fiber with ultra-low bending loss

A novel asymmetrical structure of Photonic Crystal Fibers (PCFs) has been proposed in this article. The first ring consists of three big air holes which can confine the propagating mode in the core region. The second ring consists of twelve doped rods with two different diameters which can decrease the difference of refractive index between the core and the cladding in order to provide the large mode area. The third ring consists of eleven air holes to help the higher-order-modes (HOMs) to leak out. The bending properties of proposed fiber are investigated by full vector finite element method with an anisotropic perfectly matched layer. The numerical results show that the effective mode area of the FM of the proposed fiber is as large as 595.02 μm2 with small bending radius of 10 cm at the wavelength of 1064nm. The bending loss of fundamental mode (FM) is 9.884×10−5dB/m, while large bending loss for the high-order mode (HOMs) is 14.685dB/m. Moreover, the loss ratio can reach 1.45×105which can realize the effective and stability single-mode operation. Therefore, the novel fiber has potential application in high-power fiber lasers.

Breaking the Stringent Trade-Off Between Mode Area and NA for Efficient High-Power Fiber Lasers Around 2 μm

We propose a novel approach of exploiting the LP02 mode in ultra-high NA confined-doped M-type fibers to ensure effective single-mode operation, thereby breaking the trade-off between mode-area scaling and ultra-high NA. This novel approach opens the door to highly rare-earth doped fibers suitable for power scaling applications. Therefore enabling, simultaneously scaling of power and slope efficiency of fiber lasers.

A modified leaky multi-trench fiber with managing chromatic dispersion

A new scheme of modified multi-trench fiber (MTF) with chromatic dispersion management is proposed. In this structure, the low refractive-index trenches are broken along the x and y axes by four gaps. The rod-arrays, composed of high Ge-doped rods, are arranged symmetrically in the horizontal gaps. The numerical investigations indicate that the mode area of 1051μm2 can be achieved in effective single-mode (SM) operation at 1.064μm wavelength. The high-order modes (HOMs) are well suppressed. Moreover, through optimizing the key parameters of rod-arrays, the zero chromatic dispersion is controllable and can be maintained at 1.064μm wavelength. We hope that the modified MTF can be further applied to high power fiber lasers and amplifiers.

Bend-Resistant Large Mode Area Fiber With an Azimuthally Segmented Trench in the Cladding

A bend-resistant large mode area fiber with an azimuthally segmented trench in the cladding is proposed and investigated in this paper. The loss ratio of the proposed fiber between the lowest higher order modes (HOMs) and fundamental mode can reach 1000, which is much larger than 130 of multi-trench fiber (MTF). Meanwhile, the mode area of the proposed fiber achieves 790 μm2 at core radius of 23 μm, which is the same large as MTF. Moreover, the effective mode area of our design can achieve 920 μm2 at core radius of 26 μm with effective single-mode operation at the bending radius of 20 cm. The fiber performance is stable with bending orientation at the range of [0°, 360°]. The proposed fiber ensures better HOMs suppression than the MTF. The new fiber design shows great potential in high-power fiber lasers.

Optimal Design of Large Mode Area Photonic Crystal Fibers Using a Multiobjective Gray Wolf Optimization Technique

A new multiobjective optimization framework is presented for designing large mode area photonic crystal fibers (LMA-PCFs) with effective single-mode operation in the bent state. For optimizing the structure, we utilize the multiobjective gray wolf optimizer (MOGWO) to maximize the effective mode area (EMA) and the bending loss of higher order modes (HOMs), while minimizing the fundamental mode (FM) loss. The simulation results demonstrate that this framework enables us to improve the EMA by a factor of 1.26 and increase (decrease) the bending loss of the HOMs (the FM) by a factor of 7 (17), compared to the nonoptimal design. In addition, we investigate the dependence of the optical characteristics of the optimized LMA-PCFs on the wavelength and the bending radius. We found that some optimal structures are highly wavelength dependent and are not suitable for wideband applications. Furthermore, we found that the HOM loss is very sensitive to the bending radius. The proposed framework is comprehensive and can be employed to find a broad range of optimal designs for a wide range of applications.

Bend-resistant leaky multi-trench fiber with large mode area and single-mode operation.

A novel structure of modified multi-trench fiber (MTF) with characteristics of bend-resistance and large mode-area is proposed. In this structure, each low refractive-index trench of traditional MTF is broken by two gaps up and down. Numerical investigations show that the mode field area of 840 μm2 can be achieved with effective single-mode (SM) operation when the bending radius is 15 cm. Moreover, the high order mode (HOM) suppression of the proposed design is better than that of standard MTF. The SM operation property can be enhanced with the decreases of bending radius. The proposed design shows great potential in high power fiber lasers with compact structure.

Design and analysis for a bend-resistant and large-mode-area photonic crystal fiber with hybrid cladding.

In this paper, an asymmetric large-mode-area photonic crystal fiber (LMA-PCF) with low bending loss at a smaller bending radius is designed. The finite-element method with a perfectly matched layer boundary is used to analyze the performance of the PCF. To achieve LMA-PCF with low bending loss, the air holes with double lattice constants and different sizes at the core are designed. Numerical results show that this structure can achieve low bending loss and LMA with a smaller bending radius at the wavelength of 1.55μm. The effective mode area of the fundamental mode is larger than 1000 μm2 when the bending radius is ≥10 cm. The bending loss of the fundamental mode is just 0.0113dB/m, and the difference between the fundamental and high-order modes of the bending loss is larger than 103 when the bending radius is 10cm. Simulation results show this novel PCF can achieve LMA and have effective single-mode operation when the bending orientation angle ranges in ±110°. This novel photonic crystal has potential application in high-power fiber lasers.