Area Of Fundamental Mode Research Articles

Numerical Calculation of Effective Area of Fundamental Mode of Photonic Crystal Fibers

Numerical Calculation of Effective Area of Fundamental Mode of Photonic Crystal Fibers

Study on dual-mode large-mode-area multi-core fiber with air-hole

In this paper, a multi-core fiber with 22 air holes and 15 conventional cores is proposed. The fiber can realize dual-mode operation of fundamental mode HE11 and second-order mode HE21 under certain conditions. The relationship between performance of the fiber and the structural parameters that include core pitch, refractive index difference between core and cladding, and fiber core radius is investigated systematically. By adjusting the above parameters, the effective area of fundamental mode can be up to 1423.972 µm2 and keep a low bending loss at 1550 nm. Due to its flexibility of the structure, the proposed fiber is significant for theoretical research and practical manufacturing.

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.

Thermally Induced Mode Amplification Characteristics of Large Mode Area Segmented Cladding Fiber

A model of amplifier based on large mode area (LMA) segmented cladding fiber (SCF) is proposed. High order modes (HOMs) filtering effect and thermal-induced modal evolution characteristics are theoretically investigated by the few-mode steady state rate equations and heat conduction equation. Simulation results show that an amplifier based on ytterbium doped SCF presents much better HOMs suppression and mode beam quality than the amplifier based on conventional step-index fiber when achieving single-mode operation by coiling fiber. Furthermore, heat load generating in the core of ytterbium doped SCF can change the pre-designed refractive index profile through the thermo-optic effect, which induces evolution of mode amplification characteristics, including mode loss decreasing and mode area shrinking. And the co-pumping amplifier ytterbium doped SCF is more advantageous to get large mode area of fundamental mode at the output end, while single-mode operation is achieved much more effectively in the counter-pumping amplifier. These results can provide guidance in the design of high-power single-mode fiber amplifier based on rare-earth doped LMA-SCF.

Design and analysis of highly nonlinear, low dispersion AlGaAs-based photonic crystal fiber

Abstract An AlGaAs based standard photonic crystal fiber structure is reported for low dispersive nonlinear optics applications. By employing a finite element solver with perfectly matched layer boundary condition, the key propagation characteristics are investigated over a broad wavelength span (1–5 μm) by taking the different air filling fraction values. The proposed work provides an effective fundamental mode area of 7.30 μm2 with nonlinearity in the order of 8.325 W−1m−1 at a low absorption wavelength of 1.55 μm and zero-dispersion wavelength close to 3 μm.

Improved optical coupling based on a concave cavity lens fabricated by optical fiber facet etching

We report a low-fabrication-complexity and wideband fiber lens, which is formed by fiber facet etching and filling high refractive index UV adhesive. The optical field can be significantly shrunk by the facet lens so as to obtain improved optical coupling. Numerical simulations were carried out for different coupling conditions, on both fundamental mode and high-order mode, for a nine-mode fiber. The fundamental mode area can be reduced from 152.17 to 12.57 µm 2, and the coupling loss between the fiber lens and a photonic waveguide can be reduced to −2.9 dB with over 1000 nm 3 dB bandwidth.

Design and analysis of trench-assisted large-mode-field-area multi-core fiber with air-hole

In this paper, a trench-assisted large-mode-field-area multi-core fiber with an air-hole structure composed of 21 conventional fiber cores and 16 air holes is proposed, and it can realize dual-mode transmission. By adjusting the structural parameters, the effective mode field area of fundamental mode can be up to 1916.042 µm2 at 1550 nm. In addition, the fiber has good bending characteristics by adding a trench with a lower refractive index than the cladding. When the bending radius is 1.4 cm, the bending loss is low to 2.96 × 10–3 dB/m. This fiber is of great significance for the design of bending insensitive fiber.

All-solid mid-infrared chalcogenide photonic crystal fiber with ultralarge mode area.

In this Letter, we report, to the best of our knowledge, the largest effective single-mode mid-infrared chalcogenide (ChG) fiber. Two thermally matched ChG glasses with a large index contrast of ∼0.35 are chosen for constructing all-solid photonic crystal fiber (PCF) with two rings of "holey" microstructured cladding. Single-mode operation is confirmed in the fabricated ChG PCF with a core diameter of 91.2μm, from both near-field and far-field observation at 4μm. Ultralow numerical aperture of <0.025 has been observed at wavelengths between 2.5 and 4.0μm. The fundamental mode area is calculated to be 5200 μm2 at 4μm, where the propagation loss of the ChG PCF is measured to be 5.2dB/m. Broadband 3.5 and 7.5μm supercontinuum has been generated in a 35-cm-long ultralarge-mode-area PCF.

Dual-Mode Large-Mode-Area Multicore Fiber With Air-Hole Structure

A novel dual-mode large-mode-area multicore fiber with air-hole structure, which is a quasi-37 core structured fiber with 20 air holes and 17 cores, is proposed in this paper. This fiber can realize strict dual-mode operation, that is, the fundamental mode HE11 and the second-order mode HE21 operation. The air-hole structure makes the second-order modes TE01 and TM01 cut off. Effects of structural parameters on effective refractive index and effective area of fundamental mode are investigated systematically. By optimizing the structural parameters of the fiber, effective area of fundamental mode is up to 1557.91 μm2 and maintaining a low bending loss (less than 10−4 dB/m at R > 0.47 m) while keeping dual-mode operation. The fiber can be used for large-mode-area high-power optical fiber laser, amplifier, FTTH and so on.

Bendable large-mode-area fiber with a non-circular core.

We investigate mode-area-scaling and bending performances of a Yb-doped large-mode-area fiber with an elongated non-circular core. Such fiber can be bent in the plane of its short axis to suppress bending effects, such as mode area reduction and mode profile distortion. Meanwhile, the other orthogonal axis can be stretched for mode area scaling. Calculations show that for fibers with the same mode area, the higher the aspect ratio between the long axis and short axis, the less sensitive the fiber will be to bending effects. However, mode area scaling is limited by the increased beat length (BL) between the fundamental mode (FM) and the second-order mode, leading to mode degeneracy at higher aspect ratios. Within the 100mm BL, the FM area is scalable to 3000 μm2 in a bent fiber. To facilitate FM operation, we study mode-selective gain through confined doping. Thanks to the small bending distortions, the confined-doping approach works well in the bent large-mode-area fiber. In addition, the advantage of tandem pumping is also discussed in terms of preferential modal gain. A non-circular core fiber with a 41μm short axis and 120μm long axis was fabricated in-house. We evaluated the fiber in a linear laser cavity pumped by a 975nm laser diode. The maximum output power obtained was 191W, with slope efficiency of approximately 67% with respect to launched pump power. The output signal has good beam qualities with M2 of ∼1.5 and ∼3.1, respectively, along the short and long axis.

Dual-mode large-mode-area multi-core fiber with circularly arranged airhole cores

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.

Single-mode large-mode-area laser fiber with ultralow numerical aperture and high beam quality.

By using the chelate precursor doping technique, we report on an ytterbium-doped aluminophosphosilicate (APS) large-mode-area fiber with ultralow numerical aperture of 0.036 and effective fundamental mode area of ∼550 μm2. With a bend diameter of 600mm, the bending loss of fundamental mode LP01 was measured to be <10-3 dB/m, in agreement with the corresponding simulation results, while that of higher order mode LP11 is >100 dB/m at 1080nm. Measured in an all-fiber oscillator laser cavity, 592W single-mode laser output was obtained at 1079.64nm with high-beam quality M2 of 1.12. The results indicate that the chelate precursor doping technique is a competitive method for ultralow numerical aperture fiber fabrication, which is very suitable for developing single-mode seed lasers for high power laser systems.

Few-mode and large-mode-area fiber with circularly distributed cores

In this paper, a novel few-mode large-mode-area fiber is proposed. This type of fiber consists of 11 conventional cores and 8 air-hole cores circularly arranged around the center core. Few-mode condition equal to strict dual-mode here is available by appropriate adjusting on corepitch, relative refractive index difference and core radius. Large effective area of fundamental mode around 1403.561μm2 could be achieved by optimization of structural parameters. Bending loss less than 10−3dB/m is realized when effective area is over 1400μm2.

Rectangular-core large-mode-area photonic crystal fiber for high power applications: design and analysis

A rectangular-core large-mode-area photonic crystal fiber structure has been designed based on the principle of higher order mode filtering by introducing fluorine-doped material rods in the cladding region. The proposed structure has an effective-mode area of fundamental mode (FM) as large as 2147 μm2 at 1.064 μm with nominal loss of 1.36×10−2 dB/m and 9.34 dB/m at first higher order mode (FHOM), which confirms effective single-mode operation after ∼2.14 m propagation length. The same structure offers effective-mode area of 5688 μm2 at λ=1.55 μm with confinement loss of ∼6.3×10−2 dB/m for FM and ∼20.23 dB/m for FHOM. The loss of FHOM is above 8 dB/m from λ=1 μm to λ=1.6 μm with very small loss in FM, which offers extended single-mode operation within a broad spectral range.

Analysis of laser modes in high power unstable resonators with intra-cavity wavefront aberrations

A semi-analytical calculation framework is proposed to calculate the eigenmodes of a general laser resonator with arbitrary intra-cavity wavefront aberrations. By solving a linear eigenvalue equation in the complex space, the mode intensity distributions of the eigenmodes in high power unstable resonators are obtained, indicating that due to the aberrations the area of fundamental mode significantly shrinks to form the localized mode with extremely high power intensity. The beam quality, mode area, threshold gain, and operation reliability are compared between the traditional unstable resonator and the one with a graded reflectivity mirror. The simulation results disclose that with a graded reflectivity mirror more high-order modes participate in mode competition and the localized mode effect of fundamental mode is weakened.

Triangular-core large-mode-area photonic crystal fiber with low bending loss for high power applications.

A triangular-core large-mode-area photonic crystal fiber structure has been designed with lower bending loss. The design works on the principle of bend induced mode filtering. Effects of the design parameters have been numerically investigated using the full vectorial finite-element method. In order to improve the effective mode area of fundamental mode along with the differential bending loss, five down-doped material rods have been introduced in place of five air holes in cladding. The structure supports effective mode-area of fundamental mode as large as 794 μm2 with nominal bend loss of 0.064 dB/m at bend radius of 15cm. This structure is able to suppress all unwanted nonlinear effects and can be a potential candidate for designing compact high power delivery devices.

Leakage channel fibers with microstuctured cladding elements: A unique LMA platform

We present a novel design of leakage channel fiber (LCF) that incorporates an air-hole lattice to define the modal filtering characteristics. The approach has the potential to offer single-mode, large mode area (LMA) fibers in a single-material platform with bend loss characteristics comparable to all-solid (LCFs) whilst at the same time providing significant fabrication benefits. We compare the performance of the proposed fiber with that of rod-type photonic crystal fibers (PCFs) and all-solid LCFs offering a similar effective mode area of ~1600μm(2) at 1.05μm. Our calculations show that the proposed fiber concept succeeds in combining the advantages of the use of small air holes and the larger design space of rod-type PCFs with the improved bend tolerance and greater higher order mode discrimination of all-solid LCFs, while alleviating their respective issues of rigidity and restricted material design space. We report the fabrication and experimental characterization of a first exemplar fiber, which we demonstrate offers a single-mode output with a fundamental mode area ~1440µm(2) at 1.06µm, and that can be bent down to a radius of 20cm with a bend loss of <3dB/turn. Finally we show that the proposed design concept can be adopted to achieve larger mode areas (> 3000µm(2)), albeit at the expense of reduced bend tolerance.

Analysis of a novel dual-mode large-mode-area micro-structured fiber

A novel dual-mode large-mode-area micro-structured fiber is proposed. The characteristics of mode field distribution, effective area of fundamental mode, and bending loss based on FEM (finite element method) are analyzed. And the effects of all structured parameters on the confinement loss and effective area are discussed. Results show that this structure makes the first HOM (LP11) and the second mode(EH11) cut off, and it achieves dual-mode transmission in the fiber by adjusting the structural parameters. The effective area of a fundamental mode is approximately 700 μm2. The dual-mode large-mode-area micro-structured fiber can be used in large-capacity fiber transmission.

Design and analysis of novel multi-core dual-mode large-mode-area optical fiber

A novel double air hole multi-core dual-mode large-mode-area optical fiber is proposed. The characteristics of mode field distribution, effective area of fundamental mode, and bending loss are analyzed. And the effects of all structure parameters on the effective refractive index and effective area are discussed. This structure makes TE01 and TM01 mode cut off, and it is of dual-mode transmission in the fiber. Besides, this structure can increase the effective area. The effective area of fundamental mode is approximately 1044 μm2. It is even able to achieve single-mode operation by adjusting the structure parameters. The fabrication of the structure is simple, and design is flexible. Therefore, it is suitable for the high-speed and large-capacity passive fibers as well as for active fibers. The effective area can reach or even exceed 3512 μm2 by adjusting the structure parameters, in order to satisfy the practical demands of large-capacity high-power fiber transmission in the optical communications.

Investigation on Dispersion Property of a Novel Photonic Crystal Fiber with Square Array Air Holes

The dispersion property of a novel photonic crystal fiber with square array air holes was simulated with the vectorial beam propagation method. The impacts of the pitch and hole diameter on the effective area of fundamental mode and the dispersion parameter were evaluated. The results indicate that the effective refractive index decreases and the dispersion parameter increases with air hole diameter increasing and the pitch decreasing, the structure parameters of inner three rings air hole influences the dispersion characteristics greatly. Thus the dispersion property of the photonic crystal fiber can be tailed by the structural parameters.