Showing papers on "Step-index profile published in 2019"

Orbital Angular Momentum (OAM) Mode Mixing in a Bent Step Index Fiber in Perturbation Theory

Abstract: Within the framework of perturbation theory, we explore in detail the mixing of orbital angular momentum (OAM) modes due to a fiber bend in a step-index multimode fiber. Using a scalar wave equation, we develop a complete set of analytic expressions for modemixing, including those for the 2π walk-off length, which is the distance traveled within the bent fiber before an OAM mode transforms into its negative topological charge counterpart, and back into itself. The derived results provide insight into the nature of the bend effects, clearly revealing the mathematical dependence on the bend radius and the topological charge. We numerically simulate the theoretical results with applications to a few-mode fiber and a multimode fiber, and calculate bend-induced modal crosstalk with implications for mode-multiplexed systems. The presented perturbation technique is general enough to be applicable to other perturbations like ellipticity and easily extendable to other fibers with step-index-like profile as in the ring fiber.

Mid-IR supercontinuum generation in birefringent, low loss, ultra-high numerical aperture Ge-As-Se-Te chalcogenide step-index fiber

Abstract: This work reports on the fabrication and subsequent supercontinuum generation in a Ge-As-Se-Te/Ge-As-Se core/clad chalcogenide step-index fiber with an elliptical-core and an ultra-high numerical aperture of 1.88 ± 0.02 from 2.5 - 15 µm wavelength. The fiber has very low transmission loss of < 2 dB/m from 5-11 µm and a minimum loss of 0.72 ± 0.04 dB/m at 8.56 µm. Supercontinuum spanning from 2.1 µm to 11.5 µm with an average power of ∼6.5 mW was achieved by pumping a ∼16 cm fiber with a minor/major axis core diameter of 4.2/5.2 µm with 250 fs pulses at 4.65 µm wavelength and a repetition rate of 20.88 MHz. The effect of the elliptical-core was investigated by means of mechanical rotation of the fiber relative to the linear pump polarization, and it was found to cause a shift in the supercontinuum spectral edges by several hundred nanometers.

100 GHz free spectral range-tunable multi-wavelength fiber laser using single–multi–single mode fiber interferometer

Abstract: Six single–multi–single (SMS) mode fiber-based Mach–Zehnder Interferometers (MZI) with different spooling radii are demonstrated as spatial mode filtering elements for multi-wavelength laser generation. Free spectral range (FSR) tuning of the multi-wavelength output generated is realized by alternating between the six MZIs, which have different macro-bending losses. Additional FSR tuning is also realized by changing the spooling radius of the two-mode step index fiber section within the MZI setups from 80 mm to 30 mm, giving an FSR tuning range of 0.89–0.99 nm. All generated multi-wavelength outputs show high stability over a test period of 100 min. The proposed multi-wavelength lasers are highly suitable for various microwave photonics applications the field of such as microwave signal source generation and microwave photonic filtering.

High purity Ge-Sb-Se/S step index optical fibers

Abstract: Chalcogenide glasses are a promising group of materials for remote sensing applications. Two compositions from the Ge-Sb-Se/S system are investigated as core and cladding glasses in a step index fiber (SIF). Following thermomechanical and refractive index measurements, mid-infrared (MIR) light guiding is demonstrated through an 8 m length of SIF with a Ge20Sb10Se70 at. % core and Ge20Sb10Se67S3 at. % cladding. Using a single distillation procedure, Ge20Sb10Se70 at. % glass fibers are shown to have low optical loss across the 2 to 10 µm wavelength range with the lowest baseline loss shown as 0.44 dB/m at 6.4 µm.

Raman amplification of optical beam carrying orbital angular momentum in a multimode step-index fiber.

Abstract: We experimentally demonstrate 15 dB of Raman amplification of 1115 nm pulses in an orbital angular momentum mode (OAMM) with charge l=+2, S=+1 in 5 m of step-index 25 μm-diameter-core fiber. The total output reaches 4.5 kW of peak power and 68.5 μJ of energy in ∼15 ns pulses at 4 kHz repetition rate. An Yb-doped fiber source pumps the Raman amplifier at 1060 nm with 60 ns pulses. Using a spatial light modulator for modal decomposition, we measure 83% purity for the amplified target OAMM of selected polarization. To the best of our knowledge, this is the first time high energy, peak power, gain, and purity are achieved in a fiber Raman amplifier for a single OAMM.

Analysis of Dispersion Characteristics of Helical Metal Clad and Graphene Clad Circular Core Step Index Fiber

Abstract: In this paper, we have reported the study of the optical signal propagation characteristics in a nontraditional optical fiber having circular core (made of dielectric) and helical winding clad (metallic and graphene) Study has been done at different pitch angles This analysis also includes the absorption characteristic of waveguide by including imaginary part of refractive index Normal and helical boundary conditions have been applied to derive the model characteristic equation Dispersion curves and model cut-off conditions have been obtained on the basis of characteristic equation derived theoretically The dispersion curves are derived by varying the pitch angles of the winding The results obtained for different pitch angles have been analyzed; and the changes in dispersion curve have been explained The results obtained show a standard shape of dispersion curve and the numbers of guided modes are estimated on the basis of standard parameters of fiber such as core radius, reflective indices of core and cladding and operation wavelength The difference in the results obtained for metal clad and graphene clad fiber is also presented

The OAM transmission fiber based on step-index and graded-index refractive distribution

Abstract: Optical fiber is an important transmission medium in optical communication system. We have designed circular stepindex fiber and circular graded-index fiber for Orbital Angular Momentum (OAM) transmission. And we calculate the existing vector modes in fiber through the software COMSOL Multiphysics. The step-index fiber has a high refractive index ring which doped with PbS quantum dots between the core and the cladding. And the refractive index difference between the ring and the cladding reaches 0.035, which weaken the degeneracy of adjacent eigenmodes. Numerical analyses show the step-index ring core fiber can support the long-distance transmission for the OAM mode of |L|=1,5,6,7 in the wavelength range of 1530nm to 1565nm (|L| refers to the mode order of orbital angular momentum, called topological charge). In order to enhance its stability, we adjust the ring to be a graded refractive index profile, which increases the effective refractive index difference between the vector modes from the same mode order. This fiber design not only increases transmission stability, but also has a better manufacturing process than the step -index fiber. From the simulation results, we can find that the graded-index fiber with a gradation rate α =8 provides a relatively large effective refractive difference. The effective refractive difference is 1.68 × 10 −4 , 1.42 × 10 −4 and 2.44 × 10 −4 respectively corresponding to L=1, 5, and 6 at a wavelength of 1550 nm. While the effective refractive index difference is 1.65 × 10 −4 , 0.9 × 10 −4 and 1.52 × 10 −4 for the step-index optical fiber.

Mid-infrared wideband supercontinuum generation spanning up to 14 µm using dispersion-tailored all-chalcogenide step-index fiber

Abstract: We numerically study the mid-infrared supercon-tinuum generation employing a 5-cm-long dispersion-engineered all chalcogenide step-index fiber made using As2 Se3 glass as a core and Ge 11.5 As 24 Se 64.5 glass for its outer cladding. An ultrawideband supercontinuum spanning up to $14\mu m$ could be generated employing a pump source having 170-fs pulses at $5.5\mu m$ with a peak power of 10 kW.

Scaling symmetries and optimization of the refractive index profile in optical fibers

Abstract: The optical fiber refractive index profile has a significant role in the optical devices fabrication like fiber Bragg gratings (FBG) and acousto-optic devices and the way guided light interacts in the presence of external factors like strain, stress or even bends. The analysis of the fiber profile gets to be feasible throughout understanding the properties of the optical guided modes such as the effective index, group index, and the chromatic dispersion. Taking a broader look at previous publications that use standard commercial optical fibers, it becomes noticeable that the ideal core-cladding step index profile gives incompatible results when it comes to the experimental versus the simulated dispersion curves. This difference is usually caused by various factors that changes the nominal geometrical and material parameters of the fiber. The stress resulted during the fabrication process, due to thermal and drawing effects is an example of many effects that causes the alteration of these parameters. In general, any irregularity in the fiber, geometrically speaking or material-wise, will produce a coupling of the energy of one mode to the others. Hence, studying coupled modes, whether they are induced by an inscribed FBG or an acousto-optic wave, provides useful information to test the effective refractive index profile required for an accurate theoretical simulation of fiber modes. This research analyzes the characterization of the fibers refractive index profile and the effect of scaling transformation on the dispersion curves. A fiber scaling through two degrees of freedom, geometrical scaling and refractive index difference scaling, gives a significant improvement on fitting the simulated dispersion curves with the experimental ones. However, in many cases, an additional cladding alteration is also needed and shows effectiveness, especially when we analyze a wide wavelength range. According to our final results, a scaling in the geometrical properties of the core and a perturbation of linearly decreasing refractive index in the cladding both describe the fiber profile correctly. As a consequence of our modified step index model, a perfect matching between the experimental and the theoretical dispersion curves is achieved. This process of fiber profile optimization, through the fiber scaling and cladding alteration, have also proven to be efficient, comprehensive and applicable for a wide range of commercial standard fibers.

Giant Pulse at 2 μm in Polarization Maintaining Silica Step-Index Fiber

Abstract: Since the two last decades, ultrashort pulses operating around 2 μm are useful for a commercial and scientific applications, e.g. mid-infrared supercontinuum generation [1] or pumping of optical parametric oscillators. To obtain such pulses, mode-locked fiber lasers based on Thulium or Holmium doped fiber gains are usually used. An alternative consists on a nonlinear approach ‘the Raman soliton self-frequency shift’ (SSFS) [2]. It leads to a red shift of the signal allowing then to access to 2 pm. Recently, an Erbium-doped fiber laser coupled with a photonic crystal fiber (PCF) and a dispersion compensating fiber has allowed to generate sub-100 fs pulses tunable from 1700 nm to 2100 nm [3]. Here, we demonstrate experimentally the formation of a giant pulse based on the interaction between two Raman solitons in PM silica step-index fiber.

Femtosecond Optical Parametric Oscillator Based on Vector Four-Wave-Mixing in Step-Index Fiber

Abstract: We present a fiber optical parametric oscillator based on polarization-maintaining, step-index fiber. Using birefringence-induced phase-matching, we convert chirped pulses at $1\ \mu \text{m}$ to nanojoule-scale, femtosecond pulses at $0.8\ \mu \text{m}$ and $1.3\ \mu \text{m}$ .

Modal Group Velocity Mismatch Induced Intermodal Modulation Instability in Step-index Fiber

Abstract: We present detailed experimental study on noise-seeded intermodal modulation instability (IM-MI) in normal dispersion region of a conventional step-index fiber. The sharp refractive index contrast between core and cladding leads to large group velocity mismatch between the spatial modes, coaxing to efficient IM-MI and generation of multiple spectral peaks along with Raman peaks. Evolution of the spectrum with pump powers and fiber lengths are observed. Experimental findings are well supported with the theoretical framework based on bimodal-MI model considering the distinct dispersion parameters of the participating modes.

Performance of Fiber with Elevated Refractive Index at Core Axis

Abstract: This paper deals with the performance of Multimode Step Index (MSI) fiber at the bit rate of 40Gbps. A $50 \mu \mathrm{m}$ MSI fiber is designed with the profile of inner core, outer core and the cladding. Instead of analyzing the dip in refractive index at core axis, this paper proposes an idea of slightly elevating the refractive index at core axis. This is simulated by Optifiber software at the wavelength of 1550nm. The dispersion and bend forbearance of designed MSI fiber is analyzed by varying the index difference between core and cladding. Elevated Refractive Index Profile (RIP) with cladding index n clad of 1.40871 provides improved tolerance of macro bending losses at 1550nm and Elevated Refractive Index Profile (RIP) with cladding index n clad of 1.{44871 provides reduced dispersion at 1550nm.The MSI fiber with these Elevated RIPs are applied at the optical communication link at bit rate of 40 Gbps and is simulated using Optisystem simulation software.