Numerical analysis of large mode area segmented cladding fibers

This paper studies the bending effects on segmented cladding fibers (SCFs) in detail. Rod-type SCFs have offered large effective mode areas (EMAs) very successfully. The low-index segments in the design also enable the optical fibers to be bend-resistant. In this paper, the bending performance of the SCFs has been investigated by using the finite element method. The results indicate that SCFs can provide low-loss effective single-mode operation in a wide bandwidth under a bent configuration, due to the leakage losses of the higher-order modes (HOMs). A large ratio between the HOMs and the fundamental mode losses can be ensured, over a wide range of duty cycle, refractive index difference, and bending radius. Therefore, the required fabrication accuracy decreases. The mode loss ratio and EMA are independent of the bending orientation. Operating at 1550nm and 10cm bend radius, large EMA (754 μm2) is achievable with a large loss ratio (>30). The trade-offs between loss, EMA, and bending are studied. The structure has potential for compact high power fiber lasers, amplifiers, and beam delivery applications.

Segmented cladding fiber (SCF) is capable of single mode operation over an extended range of wavelengths while maintaining large mode area. In this paper we report the design of an SCF with mode area as large as 1,825 \(\upmu \hbox {m}^{2}\), suitable for delivery of high peak power femtosecond laser pulses at 1550 and 1064 nm wavelengths. An SCF with such a large-mode area is a few-moded fiber and its design requires careful choice of design parameters to have robustness against mode-coupling effects and bend loss. In this paper we address these issues and report a design of an SCF showing near distortion-free propagation of 100-fs, 53-kW peak power pulses at 1550-nm wavelength with 1,825-\(\upmu \hbox {m}^{2}\) mode area through fundamental mode. The same fiber can also deliver 250-fs, 15-kW peak power pulses at 1064-nm wavelength with 1,793-\(\upmu \hbox {m}^{2}\) mode area. The fiber has been analyzed by using the radial effective-index method in conjunction with transfer matrix method and the pulse propagation has been studied by solving the nonlinear Schroedinger equation by split-step Fourier method. Such a fiber would find applications in multiphoton microscopy and in biomedical engineering.

Multi-core fiber has aroused considerable interest as one of potential candidates for space division multiplexing that provides an additional freedom degree to increase optical fiber capacity to overcome the transmission bottleneck of current single-mode fiber optical networks. Few-mode fiber is also under intense study as a means to achieve space division multiplexing. We propose a novel dual-mode large-mode-area multi-core fiber (DMLMAMCF), which uses multi-core structure to realize few-mode condition when pursuing large mode-area. The proposed fiber consists of 5 conventional silica-based cores in the center region and 14 air hole cores surrounding the center cores. The outer circle with 12 air hole cores, which function similarly to the fluorine doping region in the bend-insensitive fiber, can mitigate the bending loss when keeping large mode area. The symmetrically distributed two cores on both sides of the center core in central region can reduce the half second-order LP11 mode consisting of two degenerate HE11 modes, TE01 mode, two degenerate HE21 modes and TM01 mode, thus leading to the remaining four vector modes, i.e. two degenerate HE11 modes and two degenerate HE21 modes. That is the reason why we call it strict dual-mode. We focus on large-mode-area properties and bending characteristics of the dual-mode. The influence of structural parameters that include corepitch Λ, refractive index difference between core and cladding Δn, and fiber core radius a, on mode characteristics and mode area of HE11 mode and HE21 mode is investigated in detail. The results reveal that it is helpful to increase the effective area of fundamental mode when we increase the value of corepitch, reduce the refractive index and fiber core radius. The effective mode area of HE11 is about 285.10 μm2 under the strict dual-mode condition. In addition, the relationship between bending loss and bending radius, and the relationship between effective mode area and bending radius of two modes are both investigated. For the HE11 mode, the least bending loss is about 5×10-5 dB/m while the least effective mode area with bending radius larger than 0.6 m is about 285.10 μm2. The HE21 mode is more sensitive to bend effect. The least bending loss is about 0.028 dB/m and the effective mode area is larger than 280.00 μm2 except for resonant coupling points. Large effective areas of both modes with low bending loss can be realized. Larger effective mode area with larger corepitch, appropriate refractive index difference and fiber core radius can be achieved. This fiber may find its usage in high power fiber lasers and amplifiers.

New progress for novel design for achieving large effective mode area fibers while maintaining single mode operation for fiber lasers and fibre amplifiers is addressed in the presentation.

Multimode fibers provide a means of scaling the peak power of ultrafast fiber lasers by orders of magnitude. While large mode area (LMA) fibers have been widely utilized in fiber amplifiers, these fibers often sacrifice practical benefits of fiber, such as flexibility, and increase system cost and complexity. In addition, the mode-field area of effectively single-mode LMA fibers is smaller than what can be achieved in multimode fibers. Recent work has shown that nonlinear interactions in multimode graded-index fiber can cause a highly multimode field to self-organize into the fundamental mode in a condensation-like process that is robust even with fiber perturbations or disorder. Building on this, we developed a series of Yb:fiber, mode-locked lasers utilizing normal-dispersion, multimode graded-index fiber. In experiments, we observe that the transition from continuous wave lasing to mode-locking is characterized by a beam cleaning process, whereby the highly multimode (speckled) beam of the continuous wave field transforms into a low-order mode beam. Remarkably, experiments and numerical simulations show that the pulses can consist not just the fundamental mode, but can even comprise multiple transverse modes. Our theoretical analysis shows this to be a consequence of a surprising kind of mode-locking – spatiotemporal mode-locking - which relies on strong intermode interactions and spatial filtering. Our initial experiments yield MW-power pulses after external compression, rivaling the best results with flexible LMA fibers. Meanwhile, simulations show that nearly GW peak powers should be possible, making spatiotemporal mode-locking extremely attractive for high-power ultrafast laser development.

Single-mode large-mode-area double-ring hollow-core anti-resonant fiber for high power delivery in mid-infrared region

This Paper proposes a novel 7-core 5-mode fiber with large mode area and low inter-core crosstalk(XT). The proposed optical fiber, by adopting a combination of air-trench structure and high-index ring, realizes large mode area and low XT. The numerical analyses reveal that the proposed fiber can stably transmit 5 LP modes in the C+L band; the effective mode area of each of the 5 LP modes is larger than 110μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ; the XT of in each of the 5 LP modes is smaller than –79 dB/km; each mode has an effective refractive index difference bigger than 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . Therefore, a low XT result can still be achieved when the mode area is large. The proposed optical fiber may be well utilized to construct a high-capacity fiber optic transmission system.

A segmented heterostructure cladding fiber designed for extreme large mode area

Regenerated fiber Bragg gratings (RFBGs) have been created in large mode area (LMA) fibers, and their key properties for their application as high-temperature sensors were experimentally characterized. UV written Type-I seed gratings in standard single-mode fibers (SMF-28) and in low numerical aperture LMA fibers with 125 μm and 250 μm cladding diameters were regenerated, and their wavelength drift characteristics at high temperatures, their temperature sensitivities, and their mechanical strengths were tested and compared with each other. We found that low dopant concentrations in the fiber cores led to lower wavelength drift rates during and after regeneration. A temperature calibration from room temperature to 800 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C showed that LMA fibers have generally similar but slightly lower temperature sensitivities than SMF-28 fibers. The tensile strength tests showed that LMA fibers with 250 μm cladding diameter have similar tensile fracture stresses as SMF-28 fibers but four times larger tensile force tolerances because of their twofold fiber diameter. Our research shows that using LMA fibers for RFBGs offers significant advantages for high-temperature sensing than their counterparts in standard fibers, which are also applicable to other silica fiber-based high-temperature sensors.

Single transverse mode fiber lasers have recently achieved output powers at the kilowatt level. These breakthroughs can be attributed to the maturation of high power diode pump technology at 980 nm and the use of large mode area (LMA) fibers. In the continous wave (cw) operation regime, LMA fibers, through their reduction of device length and increase of the effective area, have been effective in the reduction of deleterious nonlinear optical effects such as stimulated Raman scattering (SRS). The use of LMA fiber has been less effective in the suppression of stimulated Brillouin scattering (SBS), for which the threshold can be several orders of magnitude lower than for SRS. In this work we use refractive index profiles and index modifying dopant distributions for the mitigation of SBS. Our theoretical and modeling efforts led to an experimentally demonstrated increase in SBS threshold of 2.5 dB for single mode fiber and 6 dB for LMA amplifier fiber. We discuss the use of this SBS-suppressive fiber in the demonstration of a high-power, narrow linewidth fiber amplifier.

Bending characteristic of large-mode-area fibers with uniaxial crystal material cladding

Bending insensitive large mode area photonic crystal fiber

We study a new concept of polarization maintaining for large mode area silica glass fibers, where artificially anisotropic glass is used to form a core of the fiber. The fiber core is composed of interleaved subwavelength layers of high and low refractive index glasses. We obtained phase birefringence of 1.92×10−4 for the single mode fiber with the core diameter of 30 μm and the effective mode area equal to 573 μm2 and 804 μm2, for x- and y-polarization, respectively.

In this paper, we present a new and efficient technique for the exact reconstruction of large mode area (LMA) higher order mode (HOM) optical fibers by using arbitrary shape HOM electric field profiles. One known strategy to achieve LMAs for high power optical applications is by using optical fibers supporting multimode operation, and engineering these fibers such that they operate on just a single transverse mode. Although the fundamental mode is preferred in most circumstances, there is an increasing interest in operating on a single HOM instead since these can have interesting dispersion characteristics and also significantly higher ability to scale mode areas. An issue with the use of HOMs in high power applications is that often they demonstrate regions of high field intensity across the core. This leads to problems with optical damage at lower values of effective area than might otherwise be expected. In order to avoid this problem, we offer designs of optical fibers supporting mode-flattened HOM electric fields. We develop a new inverse transmission line technique for calculating directly and accurately the refractive index profiles of such fibers from knowledge of the desired HOM electric field. We demonstrate this algorithm with a number of example reconstructions of LMA HOM optical fibers supporting prescribed mode-flattened electric field profiles with ultralarge effective mode areas We expect such fiber designs to be useful in the mitigation of nonlinear effects and in thermal and damage management.

We study the application of capillary optical fibers to attain a large effective mode area for nonlinearity suppression in high-power lasers and amplifiers. The effect of the inclusion of an air capillary on the modal properties of a conventional optical fiber, such as the effective index of the guided modes, is calculated using fully vectorial analytical techniques and verified using the finite element method. It is illustrated that our design transcends the recently predicted limit on the effective mode area obtainable in conventional optical fibers at 1064 nm. The influence of bending on the mode area is investigated in detail. We further propose alternative designs, such as the depressed-clad capillary optical fiber and the trench-assisted capillary optical fiber, and we carry out simulations to show how they can be used to reduce losses in optical fibers. The variation of the fractional power contained within the capillary is calculated to determine the effect on nonlinearity suppression. We also show that by including a nanosized air capillary in the core of a photonic crystal fiber, the effective mode area can be further enhanced even in bent conditions.