Mode Field Area Research Articles

Circular-lattice microstructured optical fibers with near-zero ultra-flattened chromatic dispersion and high birefringence

In the present study, a dispersion flattened microstructured optical fiber (MOF) with extremely low dispersion value and high birefringence is proposed. The proposed MOF is composed by manipulating the number of air-holes, presents in different rings of a regular circular-lattice MOF. The dispersion and birefringence characteristics of the proposed MOF are numerically analyzed by applying finite difference field convergence approach. Moreover, the other light-guiding characteristics, e.g., fundamental mode-field pattern, effective refractive index, effective mode-field area and nonlinear characteristics of the fiber are explored as well. Numerical results show that flattened dispersion around 0 ± 1.3 ps/(nm.km) can be reached inside the spectral range 1120–2100 nm bounding a huge bandwidth of 980 nm. Numerical results also show that birefringence with the order of 10-4 can be obtained at 1550 nm.

Study on the high birefringence and low confinement loss terahertz fiber based on the combination of double negative curvature and nested claddings

A novel double negative curvature nested fiber structure is designed by adding extra circular cladding tubes to enhance the birefringence and reduce the confinement loss. The fiber structure is composed of eight circular cladding tubes and two semi-elliptical nested tubes. The transmission performances of terahertz fiber, including birefringence, confinement loss, dispersion and effective mode field area, are studied by changing the parameters of cladding tubes. In the frequency range of 1.75–2.6 THz, the broad bandwidth of 850 GHz with high birefringence (above 10−4) can be achieved. The confinement loss of y-polarization mode with the frequency of 2.575 THz can be as low as 0.00231 dB cm−1. The waveguide dispersion coefficient is between ±0.188 ps (THz cm)−1 in the frequency range of 2.0–2.475 THz. The maximum effective mode field area of x-polarization mode is 2.618 × 10−6 m2 at 2.6 THz.

Characteristics of Collimators Based on the Large-Mode-Area CMCF for Coupling Laser Beam

A new collimator based on a homemade concentric multilayer-core fiber (CMCF) is proposed and experimentally demonstrated. This collimator was fabricated using a tail fiber with large mode area and single-mode operation. By exploiting the optical transmission matrix, the propagation characteristic and coupling mechanism of this CMCF-based collimator was introduced meticulously. The coupling losses of the laser beam using this collimator in the off-axis, angular, and axial deviations were analyzed separately. In order to determine the relationship between the geometric redundancy of this collimator and the effective mode field area of the tail fiber, the corresponding mathematical model was established. Through model calculation and experiment measurement, the coupling properties of the collimator were improved effectively. Compared with the common SMF-based collimator, the declination redundancy of the CMCF-based one improved by 20%, which could make the coupling of the optical fiber collimator easier. Therefore, this collimator has potential application value in the laser diode coupling unit and high-speed optical communication system.

A large mode area fiber with dual-helical-leakage-channels

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, n1 = n3 = 1.438, n0 = n2 = n1 + 0.001, r = 30 µm, the fiber loss coefficients are L01 = 0.06 dB/m, L11 = 17.9 dB/m and L21 = 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.

Design of negative curvature fiber carrying multiorbital angular momentum modes for terahertz wave transmission

We design a novel negative curvature fiber (NCF) to support terahertz orbital angular momentum (OAM) modes. Numerical models are constructed to evaluate the transmission performances of vector modes in the frequency range of 0.40–0.80 THz. The designed fiber made from only one kind of heat-resistance resin (HTL) material can support 50–52 OAM modes with large effective index differences above 10−4 and low confinement loss (in the level of 10−15 dB/cm). Some other significant characteristics of the designed NCF including waveguide dispersion, OAM purity, and effective mode field area (Aeff), are also calculated and analyzed with their effects and applications on terahertz OAM guidance. The optimized absolute value of dispersion is between 0.33 and 0.8 ps/(THz × cm) in the range of 0.45–0.75 THz. The high mode purity (85%−95%) in the range of 0.60–0.80 THz further promotes the mode division multiplexing in OAM fiber. The proposed NCF can offer great potential in high-capacity terahertz communication based on OAM multiplexing.

Polarization-insensitive reverse-ridge AlGaAs waveguide for the mid-infrared supercontinuum generation

In this paper, a polarization-insensitive reverse-ridge AlGaAs waveguide is designed for the supercontinuum generation. In the considered wavelength range from 3.4 to 8 μm, the dispersion profiles, nonlinear coefficients, and effective mode field areas for the quasi-TE and quasi-TM modes coincide well. The simulation results show that when pump pulse with wavelength of 4.2 μm, peak power of 4.8 kW, and width of 90 fs is launched into the anomalous dispersion region of the quasi-TE and quasi-TM modes of the 3.4-mm-long waveguide, the two generated SCs overlap well. For the quasi-TE and quasi-TM modes, the SCs generated extend from 2.17 to 8.53 μm and 2.23 to 8.61 μm, respectively, spanning more than 1.95 octaves. The proposed reverse-ridge AlGaAs waveguide structure is expected to provide a possible solution for alleviating the undesired polarization effect related to the nonlinear dynamics.

Large mode area double-layer all-solid anti-resonant fiber for high-power fiber lasers

A novel double-layer all-solid anti-resonant fiber (DL-AAF) is proposed to develop large mode area (LMA) fiber for high-power fiber laser applications. Single-mode property and mode field area scaling are investigated in simulations. Numerical results show the fiber can achieve strong suppression of higher order modes (HOMs) even with a large core size. With a core diameter of 80 μm, the loss ratio defined as the ratio of the lowest loss of HOMs to that of fundamental mode can exceed 3500, and the mode field area reaches 3030 μm2 at the wavelength of 1.064 μm. By further expanding the core size, the mode filed area can be enlarged to 7481 μm2 while the fiber is ensured to be single-mode. This is a significant enhancement in mode field area scale for the current works on LMA AAF.

Design and Analysis of Trench-Assisted Low-Bending-Loss Large-Mode-Field-Area Multi-Core Fiber with an Air Hole

In this paper, a trench-assisted low-bending-loss large-mode-field-area multi-core fiber with air hole is proposed, which can achieve dual-mode transmission. The influence of structural parameters on fiber performance is analyzed systematically, and the structure of the trench, with a lower refractive index than the cladding, is also analyzed and optimized. By adjusting the structural parameters, the effective mode field area of the fundamental mode can reach 2003.24 um2 at 1550 nm, and when the bending radius is 1 cm, the bending loss is 2.57 × 10−3 dB/m. The practical implementation of the proposed fiber is feasible using existing fabrication technology and is applicable to the transmission of large-capacity optical communication systems and high-power lasers.

Numerical Simulations on Polarization Quantum Noise Squeezing for Ultrashort Solitons in Optical Fiber with Enlarged Mode Field Area

Broadband quantum noise suppression of light is required for many applications, including detection of gravitational waves, quantum sensing, and quantum communication. Here, using numerical simulations, we investigate the possibility of polarization squeezing of ultrashort soliton pulses in an optical fiber with an enlarged mode field area, such as large-mode area or multicore fibers (to scale up the pulse energy). Our model includes the second-order dispersion, Kerr and Raman effects, quantum noise, and optical losses. In simulations, we switch on and switch off Raman effects and losses to find their contribution to squeezing of optical pulses with different durations (0.1–1 ps). For longer solitons, the peak power is lower and a longer fiber is required to attain the same squeezing as for shorter solitons, when Raman effects and losses are neglected. In the full model, we demonstrate optimal pulse duration (~0.4 ps) since losses limit squeezing of longer pulses and Raman effects limit squeezing of shorter pulses.

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.

Symmetric Graphene Dielectric Nanowaveguides as Ultra-Compact Photonic Structures.

A symmetric graphene plasmon waveguide (SGPWG) is proposed here to achieve excellent subwavelength waveguiding performance of mid-infrared waves. The modal properties of the fundamental graphene plasmon mode are investigated by use of the finite element method. Due to the naturally rounded tips, the plasmon mode in SGPWG could achieve a normalized mode field area of ~10−5 (or less) and a figure of merit over 400 by tuning the key geometric structure parameters and the chemical potential of graphene. In addition, results show that the modal performance of SGPWG seems to improve over its circular counterparts. Besides the modal properties, crosstalk analysis indicates that the proposed waveguide exhibits extremely low crosstalk, even at a separation distance of 64 nm. Due to these excellent characteristics, the proposed waveguide has promising applications in ultra-compact integrated photonic components and other intriguing nanoscale devices.

Perovskite Hybrid Surface Plasmon Waveguide

A novel perovskite hybrid surface plasmon waveguide structure is designed. Its physical model is established, and its theoretical basis is introduced in detail. Based on the finite element method, the mode characteristics, quality factor, and gain threshold of the waveguide structure are analyzed with geometric parameters. The results show that the optical field constraint of the waveguide can reach a good deep sub-wavelength level under the optimal operating wavelength of 1550 nm. By adjusting the waveguide design parameters, the high-quality factor, low energy loss, low threshold limit, and ultra-small effective mode field area are obtained. Compared with the hybrid waveguide structure proposed in the current research results, this structure has stronger optical field limiting ability and microcavity binding ability. The waveguide structure can provide theoretical and technical support for the development of new efficient nanolaser devices and has a good application prospect.

Novel double negative curvature elliptical aperture core fiber for terahertz wave transmission

A novel double negative curvature terahertz fiber with elliptical tubes as cladding structure is designed and investigated. Six uniformly arranged elliptical hole tubes and six circular tubes are connected to each other as the cladding structure of the fiber. The confinement loss, bending loss, dispersion, and effective mode field area of the fiber are calculated in two different frequency regions. The simulation shows that the lowest confinement loss of 3.2 × 10−6 dB cm−1 can be achieved at 2.44 THz. The confinement loss is always in the order of 10−5–10−6 dB cm−1 in the frequency band of 2.22–2.5 THz. The waveguide dispersion of fiber remains between −0.18 and 0.05 ps/(THz × cm) in the frequency band of 2.04–2.4 THz. Moreover, the bending loss of 4.4 × 10−5 dB cm−1 can be obtained when the bending radius is 25 cm. The debut of this double negative curvature terahertz fiber, with combined advantages of ultralow confinement loss, low bending loss, low and flat dispersion and simple structure forebodes a new approach to completely release the potential of fiber in terahertz applications.

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.

Design of hollow core step-index antiresonant fiber with stepped refractive indices cladding.

With the benefits of low latency, wide transmission bandwidth, and large mode field area, hollow-core antiresonant fiber (HC-ARF) has been a research hotspot in the past decade. In this paper, a hollow core step-index antiresonant fiber (HC-SARF), with stepped refractive indices cladding, is proposed and numerically demonstrated with the benefits of loss reduction and bending improvement. Glass-based capillaries with both high (n = 1.45) and low (as low as n = 1.36) refractive indices layers are introduced and formatted in the cladding air holes. Using the finite element method to perform numerical analysis of the designed fiber, results show that at the laser wavelengths of 980 and 1064 nm, the confinement loss is favorably reduced by about 6 dB/km compared with the conventional uniform cladding HC-ARF. The bending loss, around 15 cm bending radius of this fiber, is also reduced by 2 dB/km. The cladding air hole radius in this fiber is further investigated to optimize the confinement loss and the mode field diameter with single-mode transmission behavior. This proposed HC-SARF has great potential in optical fiber transmission and high energy delivery.

Characteristic Analysis and Structural Design of Hollow-Core Photonic Crystal Fibers with Band Gap Cladding Structures.

Due to their flexible structure and excellent optical characteristics hollow-core photonic crystal fibers (HC-PCFs) are used in many fields, such as active optical devices, communications, and optical fiber sensing. In this paper, to analyze the characteristics of HC-PCFs, we carried out finite element analysis and analyzed the design for the band gap cladding structure of HC-PCFs. First, the characteristics of HC19-1550 and HC-1550-02 in the C-band were simulated. Subsequently, the structural optimization of the seven-cell HC-1550-02 and variations in characteristics of the optimized HC-1550-02 in the wavelength range 1250–1850 nm were investigated. The simulation results revealed that the optimal number of cladding layers is eight, the optimal core radius is 1.8 times the spacing of adjacent air holes, and the optimal-relative thickness of the core quartz-ring is 2.0. In addition, the low confinement loss bandwidth of the optimized structure is 225 nm. Under the transmission bandwidth of the optimized structure, the core optical power is above 98%, the confinement loss is below 9.0 × 10−3 dB/m, the variation range of the effective mode field area does not exceed 10 μm2, and the relative sensitivity is above 0.9570. The designed sensor exhibits an ultra-high relative sensitivity and almost zero confinement loss, making it highly suitable for high-sensitivity gas or liquid sensing.

Theoretical Study of Four-Core Five-Mode Microstructured Optical Fiber with Low Crosstalk and Large Mode Field Area

Theoretical Study of Four-Core Five-Mode Microstructured Optical Fiber with Low Crosstalk and Large Mode Field Area

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.

Nonlinear Impairment Scaling in Multi-Mode Fibers for Mode-Division Multiplexing

The scaling of nonlinear effects in multi-mode fibers with the mode count has been investigated by analytical studies and by numerical simulations. Increasing the core radius results in larger effective mode field areas, which reduces the impact of nonlinear effects. On the other hand, the presence of more interaction partners for intermodal nonlinear interactions due to a higher number of propagating modes strengthens the impact of nonlinear signal degradations on data transmission. Our results indicate that these two trends are mostly balanced even for large mode counts. Transmission reaches and system performance comparable to standard single-mode fibers are achievable for at least 100 modes.

Modeling and Analysis of a Pulsed Yb-Tm Fiber Laser System

Taking a comprehensive consideration of related excited state absorption, upconversion emission, and cross relaxation processes, a theoretical model concerning a pulsed Yb-Tm fiber laser is established. The criterion for stable Q-switching is derived with both excited state absorption processes and mode field area mismatch taken into account. And, detailed population transitions of major energy levels are illustrated. Impacts of pump power, mode field area ratio and saturable absorber length on the output characteristics of the pulsed laser system are investigated and analyzed. Simulations show that the excited state absorption transition 3F4→3F2,3 is the predominant factor supporting the passive Q-switching of Yb-Tm fiber lasers. Besides the pump power, the mode field area ratio and the length of the saturable absorber are also important factors affecting the output characteristics of the pulsed laser system. Proper choice of these parameters not only brings significant improvements to the pulse performance, with regards to both the pulse energy and the pulse duration, but also expands the stable operation range of the laser system. And, in practice, an effective mode field area ratio bigger than 1.65 should be exploited for the stable Q-switching operation of a pulsed Yb-Tm fiber laser.