Showing papers on "Step-index profile published in 2021"

Room temperature mid-infrared fiber lasing beyond 5 µm in chalcogenide glass small-core step index fiber

Abstract: This Letter, to the best of our knowledge, reports mid-infrared fiber lasing beyond 5 µm at room temperature for the first time, Ce3+-doped, chalcogenide glass, step index fiber employed in-band pumping with a 4.15 µm quantum cascade laser. The lasing fiber is was 64 mm long, with a calculated numerical aperture of 0.48 at the lasing wavelengths. The core glass was Ge15As21Ga1Se63 atomic % (at. %), doped with 500 parts-per-million-by-weight Ce, with a 9 µm core diameter. The cladding glass was Ge21Sb10Se69 at. % with a 190 µm outer diameter. As pump power increases continuous wave lasing corresponding to the 2F7/2→2F5/2, transition in the Ce3+ ion occurs at 5.14 µm, 5.17 µm, and 5.28 µm.

Fusion Splicing of Silica Hollow Core Anti-Resonant Fibers With Polarization Maintaining Fibers

Abstract: Fusion splicing of solid-core microstructured silica fibers has been one of the key enablers which opened practical applications of these structures in ultrafast light sources or fiber-based sensors. Anti-resonant hollow core fibers are special in this context, because their optical properties critically depend on single-micron or sub-micron thin wall membranes. This corresponds to high thermal isolation of their cladding structure and makes thermal processing of these fibers challenging. We investigate fusion splicing feasibility of a single capillary ring anti-resonant hollow core fiber made of silica glass. We begin by splicing pairs consisting of standard single mode and hollow core fibers, followed by pairs of polarization maintaining and hollow core fibers. Splice loss is determined within the 1450–2000 nm transmission window of the fiber at around 1–2 dB, depending on wavelength along the transmission window, in the transmission direction from the step index fiber into the hollow core fiber. Maintenance of polarization is verified and polarization extinction ratio of no less than 10 dB is recorded for the ARF spliced with a polarization maintaining fiber. Mechanical robustness of the fabricated splices is verified with standard pull tests returning damage thresholds of 140 g (32 kpsi) and 100 g (24 kpsi) for hollow core fibers spliced to single mode and polarization maintaining fibers, respectively.

Amplification characteristics in active tapered segmented cladding fiber with large mode area

Abstract: A kind of tapered segmented cladding fiber (T-SCF) with large mode area (LMA) is proposed, and the mode and amplification characteristics of T-SCFs with concave, linear, and convex tapered structures are investigated based on finite-element method (FEM) and few-mode steady-state rate equation. Simulation results indicate that the concave tapered structure can introduce high loss for high-order modes (HOMs) that is advantageous to achieve single-mode operation, whereas the convex tapered structure provides large effective mode area that can help to mitigate nonlinear effects. Meanwhile, the small-to-large amplification scheme shows further advantages on stripping off HOMs, and the large-to-small amplification scheme decreases the heat load density induced by the high-power pump. Moreover, single-mode propagation performance, effective mode area, and heat load density of the T-SCF are superior to those of tapered step index fiber (T-SIF). These theoretical model and numerical results can provide instructive suggestions for designing high-power fiber lasers and amplifiers.

Specialty Optical Fibers for THz Generation: A Review

Abstract: This chapter describes the state-of-the-art sources of terahertz (THz) radiation, with focus on all-optical fiber-based sources for THz generation. THz technology based on the optical fiber platform is expected to be most attractive for day-to-day applications. Though optical fibers have been considered before for low-loss guidance of THz radiation, their nonlinear effects can also be exploited for THz generation. We discuss how a glass-based legacy step index fiber can be used to make a THz source. However, high absorption losses of silica glass in the THz regime and a small overlap between the modes at the optical and THz frequencies limit the THz generation efficiency to a level below 0.01%. Next, we discuss our design of a THz source based on a plastic fiber. By exploiting the nonlinear parametric process of four-wave mixing (FWM) in an appropriately designed microstructured-core double-clad plastic fiber (MC-DCPF), both the loss and the modal overlap issues can be overcome to a great extent. The microstructure geometry of this fiber allows for fine tuning of the required phase matching condition, group-velocity dispersion, and nonlinear properties at the optical pump wavelength. By using such a MC-DCPF, we show that a THz wave at a frequency near 3 THz can be generated by using two commercially available high-power lasers. The high-power CO2 laser acts as the pump and a CO laser of much lower power acts as a seed for the FWM process. Numerical simulations reveal that more than 30 W of THz power within a bandwidth of 2.13 GHz can be generated at the end of a 65 m long fiber when 1 kW of CO2 laser power is launched together with 20 W of CO laser power. A conversion efficiency of 30% is possible for a loss-less configuration, but efficiency of > 10% is achievable even in the presence of material losses. Recent results show that further optimization of such plastic microstructured fibers can provide conversion efficiencies close to 45%. As an alternative, we have focused on the use of plastic fibers and discussed a design criterion that is promising for realizing large output powers with a relatively high efficiency.

Inverse photonic-crystal-fiber design through geometrical and material scalings

Abstract: Geometrical and material — i.e., external and internal — scaling symmetries are exploited to obtain approximated analytical expressions for the mode effective index, group index, and chromatic dispersion of a scaled fiber. Our results include material refractive index scaling that changes the numerical aperture. First, the analytical expressions are successfully tested with a conventional step index fiber in a broadband range of wavelengths, from 1 to 2 μm. Then, we establish a procedure to adapt the analytical expressions to photonic crystal fibers (PCFs) and illustrate its application in a triangular PCF with circular holes. These adapted analytical expressions show good agreement with a rigorous numerical solution of the fundamental fiber mode. Finally, we demonstrate how powerful these expressions are for the design of PCFs. In particular, we illustrate our approach designing, in four iterations or less, PCFs with flattened dispersion profile over 300 nm or high dispersion slope over 40 nm, with different chromatic dispersion values.

Experimental demonstration of the eigen modes resonant coupling in the step-index fiber with high index absorbing rods embedded into fiber cladding

Abstract: Here for the first time the technique of the resonant modes coupling is used for spectrally-selective fundamental mode suppression. In our work we considered the fiber design consisting of the low-index core surrounding by the appropriately chosen high-index absorbing rods. Mode suppression in this case happens due to the resonant core mode deformation owing to mode-anticrossing effect and its partial absorption into the rods. According to our calculations it was established that stop-band of the core fundamental mode can be easily adjusted for different practical aims by fiber bending. Furthermore, in the present work we implemented and studied passive fiber with three high-index absorbing rods incorporated into fiber cladding. The Sm was chosen as an absorbing element of the high-index rods.