Diffraction in air-clad fibres (original) (raw)
The influence of skew rays on angular losses in air-clad fibres
Optics Communications, 2006
Detailed experimental transmission properties of high numerical aperture air-clad undoped fibres are presented. Measurements of the angular transmissivity indicate a 15% lower numerical aperture (NA = 0.75) compared to theoretical predictions for the bridge thickness and wavelength ratio of d/k = 0.226. The discrepancy is attributed to skew rays that are deliberately launched and which are subject to larger loss rates through diffractive tunnelling in frustrated total internal reflection than meridional rays. Measurements of the angular propagation losses show that light launched at angles less than the critical angle for total internal reflection experiences low initial losses as well as low propagation losses. Light launched at steeper angles experiences a significant initial loss attributed to loss of skew rays within 1.5 m of propagation as well as larger general propagation losses.
Air-clad fibres for astronomical instrumentation: focal-ratio degradation
Experimental Astronomy, 2009
Focal-ratio degradation (FRD) of light launched into high-numerical aperture (NA) single-annulus all-silica undoped air-clad fibres at an NA of 0.54 is reported. The measured annular light distribution remained Gaussian after 30 m of propagation, but the angular FWHM of the output annulus doubled from 4 • after 1 m propagation to 8.5 • after 30 m, which is significantly larger than that reported of standard doped-silica fibres (NA < 0.22). No significant diffractive effects were observed. The design of air-clad fibres for broad-band, high-NA astrophotonics applications is discussed.
Simulations of Pump Absorption in Tandem-Pumped Octagon Double-Clad Fibers
IEEE Photonics Journal, 2021
The cladding pumping within a double-clad fiber structure is an effective technique to convert high-power multimode beam into high-power, almost diffraction limited beam. Since the pump efficiency is limited by the presence of higher skew-ray modes, geometrical perturbations are used to scramble the modes and to achieve a higher overlap of the electromagnetic field with the doped core. In this paper, the combined effect of fiber coiling and twisting is investigated in the double-clad fiber structure with the octagonal shape of an inner cladding. Electromagnetic field propagation through fibers with different cross-section areas, bending radii and twisting rates is numerically simulated using finite element beam propagation method. Holmium-doped double-clad fiber pumped in tandem configuration at wavelength 1950 nm is selected as an example. Nontrivial dependence of pump absorption on bending radius with the presence of local maxima is revealed. The observed impact of fiber coiling and twisting increases with the size of the cross-section area of the inner cladding.
Applied Optics, 1998
When a dielectric circular cylinder is obliquely illuminated, the scattering angle associated with the Airy caustics of the cylinder's primary rainbow depends on the tilt of the cylinder. We display records of the scattering pattern for a transparent poly͑methyl methacrylate͒ fiber ranging from small values of tilt through values of tilt that are sufficiently large for the Airy caustics from both sides of the fiber to merge in a meridional plane containing the incident wave vector and the fiber's axis. The records are compared directly with the evolution of the caustic projected onto the observation plane, and certain qualitative features of the global evolution of the caustics are confirmed. Although the observations used laser illumination, they are relevant to anticipating the scattering by sunlit transparent tilted cylinders.
Ultrahigh birefringence of elliptic core fibers with irregular air holes
Journal of Applied Physics, 2007
We have investigated experimentally and theoretically the birefringence of the elliptic core fiber with irregular air holes. The wavelength dependence of the beat length and the birefringence was measured by the wavelength scanning method. The fiber exhibits ultrahigh birefringence of 1.12 ϫ 10 −2 at 1550 nm. We also calculated the modal birefringence of the fundamental modes in the fiber by using the plane wave expansion method. The calculated birefringence is in excellent agreement with the measured one.
IEEE Transactions on Nuclear Science, 2000
Transient radiation-induced effects in air-guiding photonic crystal fibers (Air-PCF) are investigated for the first time to our knowledge. We characterize the vulnerability of this kind of waveguide by measuring the time dependent changes of their radiation-induced attenuation (RIA) at 1.55 m. We compare their radiation response to those of two silica-based single-mode fibers with germanium-doped and pure-silica cores. An X-ray pulse induces globally the same effects in all waveguides: a strong and transient increase of RIA. For higher tested doses (10 D 100 Gy), Air-PCF exhibits an interesting saturation of RIA at 0 1 dB/m. The possible mechanisms involved in this radiation response are discussed. In particular, we assume that the design of the photonic bandgap structure of the fiber, responsible for the light guidance in the air-core, could strongly affect the amplitude of RIA. The kinetics of RIA recovery depends predominantly on the point defects created at the air/silica interface and in the silica-based part of the microstructured cladding.
Theoretical study on dispersion compensation in air-core Bragg fibers
Optics express, 2002
In a previous paper we developed a matrix theory that applies to any cylindrically symmetric fiber surrounded by Bragg cladding. Using this formalism, along with Finite Difference Time Domain (FDTD) simulations, we study the waveguide dispersion for the m = 1 mode in an air-core Bragg fiber and showed it is possible to achieve very large negative dispersion values (∼-20,000 ps/(nm.km)) with significantly reduced absorption loss and non-linear effects.
Air-Guiding Photonic Bandgap Fibers: Spectral Properties, Macrobending Loss, and Practical Handling
Journal of Lightwave Technology, 2004
For development of hollow-core transmission fibers, the realizable fibers lengths, bandwidth, characterization, and compatibility with standard technology are important issues. We report record-length air-guiding fiber, spectral properties, splicing, and optical time domain reflectometer (OTDR) measurements. Furthermore, spectral macrobending loss measurements for two different designs of air-core photonic bandgap fibers are presented. While bending loss is observed, it does not limit operation for all practical bending diameters ( 5 mm).