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Large mode area (LMA) fibers are widely used in high power fiber lasers to solve the nonlinear problems. In this work, a novel structure of LMA fiber with heterogeneous helical claddings and an inner isolation ring (HHCR) designed for 2.0 μm is proposed, and coordinate transformation simulation technique is adopted to analyze the fiber mode transmission characteristics. Fiber parameters effects on mode transmission loss and bending characteristics are both analyzed in detail. For a straight fiber, the optimized fiber parameters are Λ = 22 ~ 23 mm, θ = 13° ~ 15°, R1 = 120 μm, R2 = 150 μm, n0 (n2) = 1.439 ~ 1.4392, n1 (n3) = 1.437 ~ 1.441, d = 2 ~ 4 μm, the single mode core diameter is 60 μm and the effective mode area is 2010 μm2. For a bent fiber, the optimized fiber parameters are Λ = 24 mm, θ = 9° ~ 10°, n0 (n2) = 1.439, n1 (n3) = 1.438, R1 = 120 μm, R2 = 150 μm, d = 2 ~ 5 μm, R = 0.4 ~ 0.5 m, the single mode core diameter is 64 μm and the effective mode area is 1950 μm2. Thus, the HHCR fiber can work efficiently for LMA and single mode operation. The isolation ring can effectively reduce the fundamental mode loss and improve the output spot quality. The fiber structure is all solid and the helix pitch is long, so it is relatively easy to fabricate and use. The proposed fiber will have good applications in high-power fiber lasers. [ABSTRACT FROM AUTHOR]

A bend resistant segmented cladding fiber with high index circular multi trench in the core region is designed to attain single mode condition and large mode area (LMA). Pure silica based low refractive index trenches are introduced around the core region for reducing fabrication complexity. The high index circular multi trench is supported to avoid the mode area distortion at tight bending radius of 7.5 cm. The lasing parameters such as bending loss and mode area of the proposed fiber is investigated by varying structural parameters at the lasing wavelength of 1.06 μm. The numerical outcomes demonstrate that the proposed LMA fibre is able to operate in single mode (SM) condition with an effective mode area of 1006 µm2. Additionally, the obtained loss ratio between the basic mode (LP01) and the lowest higher order mode (LP11-odd) is 162, which confirms effective single mode operation. The proposed fibre will be crucial in the development of high power fibre laser, fibre amplifier, and high power delivery applications. [ABSTRACT FROM AUTHOR]

This work proposes a novel large mode area fiber with multi-layer holes of 2.0 μm based on electromagnetic field theory and finite element analysis. Modifying the structure parameters can reduce the fundamental mode confinement loss to less than 0.1 dB/m, and the lowest high-order mode confinement loss may be greater than 10.0 dB/m. The fiber has exceptional single-mode transmission performance due to its high loss ratio. The effective mode field area is 3849 μm2, and the single mode core diameter is 80.0 μm. To obtain a larger mode field diameter, the construction used here includes a smaller cladding diameter in the already available large mode field fiber. The cladding parameters may be changed independently, making it convenient and adaptable to obtain the best simulation results. Compared to the large-mode field fiber standard, which has excellent single mode operation performance, the loss ratio of this design is substantially larger. Additionally, it can effectively sustain single mode operation when the wavelength shifts from 1.65 to 2.10 μm, which is advantageous for developing 2.0 μm fiber lasers.

The paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with a large mode area and low bending losses. These MOFs contain two layers of fluorine-doped silica glass elements with a reduced refractive index, different diameters, and different distances between them. A numerical analysis of the properties of these MOFs was performed using the finite element method. The leakage losses for the fundamental and higher-order modes were calculated in the spectral range from 0.65 μm to 1.65 μm. Simulation results show that the proposed MOF design allows for single-mode guidance in the spectral range from 0.92 μm to 1.21 μm with a bending radius of down to 0.08 m. The measured losses of the fabricated MOF with a core diameter of 22.5 μm and a bending radius of 0.1 m were less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It is demonstrated that the segments of this MOF longer than 5 m are single-mode. [ABSTRACT FROM AUTHOR]

Evaluating the laser quality accurately is one of the most important and fundamental physical issues for laser sources, and the beam quality of lasers from the large mode area few-mode fibers have been haunted by the presence of high order mode for many years. This paper presents a modification to the M2 factor, which can be used to evaluate the mode content of fiber lasers accurately and efficiently, no matter whether the fiber modes are superposited coherently or incoherently. By mathematical derivation, the origin of the influence of relative phase on the M2 factor has been determined mathematically. A modification to the second moment of the beam intensity profile has been proposed, which eliminates the impact of uncontrollable relative phase on the second moment, and subsequently restores the one-to-one mapping between mode content and M2 factor even for coherent superposition cases. Also presented are the results of numerical simulations, which support the validity of the modified M2 factor to evaluate the mode content of the high power fiber lasers. With modified M2 factor being less than 1.1, the power fraction of LP11 mode content is unique and determined to be less than 3%.

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The paper presents the results of theoretical and experimental studies of all-glass leakage channel microstructured optical fibers (MOFs) with a large mode area and low bending losses. These MOFs contain two layers of fluorine-doped silica glass elements with a reduced refractive index, different diameters, and different distances between them. A numerical analysis of the properties of these MOFs was performed using the finite element method. The leakage losses for the fundamental and higher-order modes were calculated in the spectral range from 0.65 μm to 1.65 μm. Simulation results show that the proposed MOF design allows for single-mode guidance in the spectral range from 0.92 μm to 1.21 μm with a bending radius of down to 0.08 m. The measured losses of the fabricated MOF with a core diameter of 22.5 μm and a bending radius of 0.1 m were less than 0.1 dB/m in the spectral range from 0.9 μm to 1.5 μm. It is demonstrated that the segments of this MOF longer than 5 m are single-mode.

Large mode area (LMA) fibers are widely used in high power fiber lasers to solve the nonlinear problems. In this work, a Nd3+-doped phosphate glass LMA fiber with heterogeneous helical claddings (HHC) is fabricated successfully by rod in tube method for the first time. The performance of the preform glass was matched well and suitable for drawing the fiber, and the related fabrication method is helpful for other special structure fiber fabrication. The fiber performance such as loss and gain coefficients are both tested and compared respectively. The experiment results show that the loss coefficient α will increase with the decrease of Λ (helix pitch) or r (fiber core radius), furtherly, the net gain coefficient g will become larger with the increase of r or Λ. The experiment results are beneficial to the HHC fiber design furtherly. As the HHC fiber has an all-solid-state structure, it is easy to fabricate relatively and convenient for cutting, splicing, etc. Thus, the HHC fiber will have potential applications in the field of fiber lasers.

We report on a passively high-power Q-switched ring laser based on the active polarization-maintaining double-clad tapered Yb3+-doped fiber (APM-T-DCF) as gain media. It generates 0.24 mJ pulse trains which consist of Q-switched pulses each with the energy of about 48 µJ and up to 385 W of peak power. Q-switched regime was achieved by tuning the optical feedback polarization state and band pass filtering. For the best of our knowledge, for the first time, we demonstrate Q-switched fiber ring laser based on APM-T-DCF.

During the past few years, we have studied the granulated silica method as a versatile and cost effective way of fiber preform production and the sol-gel method. Until now, we have used the sol-gel technology together with an iterative re-melting and milling step in order to produce rare earth or transition metal doped granular material for the granulated silica method. Here, we present that the iterative re-melting (laser-assisted) and milling step is no longer needed to reach a high homogeneity. The sol-gel method also offers a high degree of compositional flexibility with respect to dopants; it further facilitates achieving high concentrations, even in cases when several dopants are used. We employed optical active doped sol-gel derived granulate for the fiber core, whereas pure or index-raised granulated silica has been employed for the cladding. Based on the powder-in-tube technique, where silica glass tubes are appropriately filled with these granular materials, fibers has been directly drawn ("fiber rapid prototyping"), or eventually after an additional optional quality enhancing vitrification step. The powder-in-tube technique is also ideally suited for the preparation of microstructured optical fibers. [ABSTRACT FROM AUTHOR]

We design an ytterbium-doped large pitch photonic quasi-crystal fiber (PQF) for amplifying high-energy ultrashort laser pulses. The proposed fiber exhibits unique properties such as very large mode area (4660 μm2), less attenuation (0.42 dB/km), high overlapping factor (0.55 for the fundamental mode), and low bending loss. These properties help amplify with high-average power ultrashort pulses with less distortion and high efficiency. To meet these requirements, we design a fiber with optimum gain filtering as well as loss filtering. From the numerical simulation of the analytical model, we confirm that the large pitch PQF plays an indispensable role in amplifying high-energy ultrashort pulses.

In this paper we present a large mode area photonic crystal fiber made of the heavy metal oxide glass CS-740, dedicated for a broadband light guidance in the visible, near- and mid-infrared regions of wavelengths from 0.4 to 4.7 μm. The fiber is effectively multi-mode in the considered wavelength range. It is composed of a ring of air-holes surrounding the core, with a high linear filling factor of 0.97. The fiber was made using a standard stack-and-draw technique. Each hole has a size of approx. 2.5 × 3.0 μm and diameter of core is 80 μm. Fiber attenuation is below 3 dB/m in the 0.9–1.7 μm wavelength range, while at 4.4 μm (mid-IR) it is approx. 5 dB/cm. Bending loss at the 1.55 μm wavelength is 0.45 dB per loop of 8 mm radius. Fiber numerical aperture is 0.53 at 1.55 μm. The effective mode area of the fundamental mode is approx. 2400 μm 2 in the wavelength range of 0.8–1.7 μm. We present a proof-of-concept demonstration that our large core photonic crystal fiber is able to efficiently collect light directly from a mid-IR quantum cascade laser without use of additional optics and can be used for pigtailing mid-IR sources and detectors. [ABSTRACT FROM AUTHOR]

During the past few years, we have studied the granulated silica method as a versatile and cost effective way of fiber preform production and the sol-gel method. Until now, we have used the sol-gel technology together with an iterative re-melting and milling step in order to produce rare earth or transition metal doped granular material for the granulated silica method. Here, we present that the iterative re-melting (laser-assisted) and milling step is no longer needed to reach a high homogeneity. The sol-gel method also offers a high degree of compositional flexibility with respect to dopants; it further facilitates achieving high concentrations, even in cases when several dopants are used. We employed optical active doped sol-gel derived granulate for the fiber core, whereas pure or index-raised granulated silica has been employed for the cladding. Based on the powder-in-tube technique, where silica glass tubes are appropriately filled with these granular materials, fibers has been directly drawn (“fiber rapid prototyping”), or eventually after an additional optional quality enhancing vitrification step. The powder-in-tube technique is also ideally suited for the preparation of microstructured optical fibers.

We report on the demonstration of a high power tunable femtosecond fiber laser based on soliton self-frequency shift in a large mode area photonic crystal fiber. The laser provides up to 13 nJ pulse energy, \sim550~mW average power, and wavelength tuning range from 1600 to 1780 nm. The pulse duration is sub-100 fs over the entire tuning range. We have also demonstrated a novel way of tuning the laser output wavelength using chirp control which has allowed us to achieve higher average output power within the tuning range. We believe the laser system will be useful for high speed deep tissue multiphoton bio-imaging and difference frequency generation to create high power tunable mid-IR radiation. [ABSTRACT FROM PUBLISHER]

This paper proposes a novel type of fiber with dual-helical-leakage-channels for large mode area and single mode operation. A coordinate transformation simulation technique is adopted to analyse the fiber performance, such as mode field area and loss coefficients, which can be modified by many fiber parameters. The simulation results show that loss coefficient L will decrease with the increase of helix pitch Λ and fiber core radius r , and increase with the increase of central angle of cladding 2 θ and wavelength λ. When θ = 12°~14°, Λ = 15 mm~16 mm, λ = 1.9 µm~2.1 µm, n 1 = n 3 = 1.438, n 0 = n 2 = n 1 + 0.001, r = 30 µm, the fiber loss coefficients are L 01 = 0.06 dB/m, L 11 = 17.9 dB/m and L 21 = 55.9 dB/m respectively, the transverse mode discrimination reaches the highest international standard of large mode area fiber, so the single mode fiber diameter is 60 µm. Especially, the helix pitch is 15 mm~16 mm, this indicates that the proposed fiber is easier to be drawn. In addition, the fiber has an all-solid-state structure, so it is convenient for cutting and splicing. The fiber may have a good application prospect in the field of high-power fiber lasers. • A large mode area fiber with dual-helical-leakage-channels is proposed. • The effects of the fiber parameters on mode transmission characteristics are analyzed. • Mode loss coefficient and mode area are both calculated. [ABSTRACT FROM AUTHOR]

• Presented a LMA-EDF to amplify the short duration high peak power pulses. • Pumped at 980 nm with 10 W pump power and achieved the gain ~17 dB/m. • LMA increases the power handling capacity and damage threshold of the fiber. • Achieved pulses of ~20 fs with ~2.6 MW peak power. • Achieved pulses of ~ 65 ps with ~26 MW peak power. 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. [ABSTRACT FROM AUTHOR]

We demonstrate an all-fiber spatiotemporal mode-locked laser based on graded-index multimode fiber. Due to the high damage threshold of graded-index multimode fiber devices, the output single-pulse energy reach up to 11.67 nJ. The beam quality at mode-locked state is significantly improved compared with continuous wave state as a result of the self-cleaning effect. By amplifying the pulse in the multimode fiber amplifier, high power pulses with high beam quality can be obtained in the all-fiber system based on graded-index MMFs. We present an approach to get high single-pulse energy in the all-fiber multimode laser with a near single-mode output beam. [ABSTRACT FROM AUTHOR]

• The double-clad polarization-maintaining tapered Yb3+ -doped fiber with core diameter is gradually increasing from 12 µm to 50 µm could be used as gain media in Q-switched ring laser. • High-power Q-switched ring laser based on active the double-clad polarization-maintaining tapered fiber generates 0.24 mJ pulse trains. • Q-switched ring laser based on active the double-clad polarization-maintaining tapered fiber allows adjusting the state of polarization and the central wavelength inside the laser cavity. We report on a passively high-power Q-switched ring laser based on the active polarization-maintaining double-clad tapered Yb3+-doped fiber (APM-T-DCF) as gain media. It generates 0.24 mJ pulse trains which consist of Q-switched pulses each with the energy of about 48 µJ and up to 385 W of peak power. Q-switched regime was achieved by tuning the optical feedback polarization state and band pass filtering. For the best of our knowledge, for the first time, we demonstrate Q-switched fiber ring laser based on APM-T-DCF. [ABSTRACT FROM AUTHOR]

We numerically demonstrate the generation of a very high peak power femtosecond pulse train by splitting a 1 ps, 100 kW pulse of central wavelength 2 μm using a few meters length of a large mode area fiber. The input 1 ps pulse during the propagation splits into multiple femtosecond pulses of very high peak powers. Concurrently, the spectral domain of the pulse also witnesses multiple peak formation. The used fiber is a W-type large mode area fiber, whose effective mode area can be varied from ~700 μm2 to ~1700 μm2 at 2 μm wavelength by varying the fiber parameters. The fiber design parameters were optimized for the efficient formation of femtosecond pulse train of peak powers ranging from ~100 kW to ~600 kW and FWHM ranging from ~100 fs to ~450 fs. [ABSTRACT FROM AUTHOR]

The heterogeneous helical cladding (HHC) fiber is a new type of extreme large mode area fiber proposed by our group firstly. In this work, Nd 3 + -doped phosphate glass HHC fibers with different core radius r (60 μ m – 175 μ m) and different helix pitch Λ (5.25 cm and 10.5 cm) were fabricated successfully. Firstly, the fiber preform glass was designed and prepared, and the performance of which was measured. Secondly, the HHC fiber preform was made by rod-in-tube method. Thirdly, the HHC fiber was obtained by the spiral drawing method, then the loss and gain performance of which were measured. Based on the experiment results, the following conclusions can be drawn: the larger the Λ (or r), the lower the loss coefficient L , and when r (or Λ) is the same, the larger the Λ (or r), the higher the net gain coefficient. This work will facilitate the further exploration and application of the HHC fiber. [ABSTRACT FROM AUTHOR]