Femtosecond laser machining of hollow-core negative curvature fibres.

Hollow core negative curvature fibres (NCFs) are a relatively new class of microstructured optical fibre with potential applications in areas such as the delivery of high power laser light and gas sensing. For sensing, it is necessary for the measurand to interact with the guided mode. To facilitate this, a novel femtosecond laser-based machining protocol has been developed that allows the precision sculpting of access slots into the NCF core along the length of the fibre. The process is a direct-write process using a digitally defined scanning strategy with no need for physical masks or additional processing such as wet etchants and/or focussed ion beam machining. Due to the inherent flexibility of the machining strategy and the high level of control over the depth of material removal, it is likely that this new technique will be transferable to a wide range of microstructured fibres.

Impact of nonlinear effects on transmission losses of hollow-core antiresonant negative curvature optical fiber.

We investigate the impact of input pulse duration and peak power of a femtosecond laser on pulse broadening and propagation losses in selected hollow-core antiresonant fiber (HC-ARF). The mixed effects of strong self-phase modulation and relatively weak Raman scattering broaden the spectral width, which in turn causes a portion of the output spectrum to exceed the transmission band of the fiber, resulting in transmission losses. By designing and setting up a gas flow control system and a vacuum system, the nonlinear behavior of the fiber filled with different pressurized gases is investigated. The experimental results show that replacing the air molecules in the fiber core with argon can weaken pulse broadening and increase the transmittable peak power by 14 MW for a given 122 MW input, while a vacuum system can reduce the nonlinearity to a larger extent, therefore enhancing the transmission of HC-ARF by at least 26 MW.

Fused silica optical fibers with graded index nanostructured core.

The ability to shape the index profile of optical fibers holds the key to fully flexible engineering of their optical properties and future applications. We present a new approach for the development of a graded index fused silica fiber based on core nanostructurization. A graded index core is obtained by means of distribution of two types of subwavelength glass rods. The proposed method allows to obtain arbitrary graded distribution not limited to the circular or any other symmetry, such as in the standard graded index fibers. We have developed a proof of concept fiber with parabolic refractive index core and showed a perfect match between its predicted, designed and measured properties. The fiber has a core composed of 2107 rods of 190 nm of diameter made of either pure fused silica or Ge-doped fused silica with 8.5% mol concentration. The proposed method breaks the limits of standard fabrication approaches used in fused silica fiber technology.

Nanostructured graded-index core chalcogenide fiber with all-normal dispersion-design and nonlinear simulations.

We propose a new approach to developing of graded-index chalcogenide fibers. Since chalcogenide glasses are incompatible with current vapor deposition techniques, the arbitrary refractive index gradient is obtained by means of core nanostructurization by the effective medium approach. We study the influence of graded-index core profile and the core diameter on the fiber dispersion characteristics. Flat, normal dispersion profiles across the mid-infrared transmission window of the assumed glasses are easily obtained for the investigated core nanostructure layouts. Nonlinear propagation simulations enable to expect 3.5-8.5 µm spectrum of coherent, pulse preserving supercontinuum. Fabrication feasibility of the proposed fiber is also discussed.

Coherent supercontinuum bandwidth limitations under femtosecond pumping at 2 µm in all-solid soft glass photonic crystal fibers.

Two all-solid glass photonic crystal fibers with all-normal dispersion profiles are evaluated for coherent supercontinuum generation under pumping in the 2.0 µm range. In-house boron-silicate and commercial lead-silicate glasses were used to fabricate fibers optimized for either flat dispersion, albeit with lower nonlinearity, or with larger dispersion profile curvature but with much higher nonlinearity. Recorded spectra at the redshifted edge reached 2500-2800 nm depending on fiber type. Possible factors behind these differences are discussed with numerical simulations. The fiber enabling the broadest spectrum is suggested as an efficient first stage of an all-normal dispersion cascade for coherent supercontinuum generation exceeding 3000 nm.

Optical fibers with gradient index nanostructured core.

We present a new approach for the development of structured optical fibers. It is shown that fibers having an effective gradient index profile with designed refractive index distribution can be developed with internal nanostructuring of the core composed of two glasses. As proof-of-concept, fibers made of two soft glasses with a parabolic gradient index profile are developed. Energy-dispersive X-ray spectroscopy reveals a possibility of selective diffusion of individual chemical ingredients among the sub-wavelength components of the nanostructure. This hints a postulate that core nanostructuring also changes material dispersion of the glasses in the core, potentially opening up unique dispersion shaping possibilities.

Coherent supercontinuum generation up to 2.3 µm in all-solid soft-glass photonic crystal fibers with flat all-normal dispersion.

Supercontinuum spanning over an octave from 900 - 2300 nm is reported in an all-normal dispersion, soft glass photonic crystal fiber. The all-solid microstructured fiber was engineered to achieve a normal dispersion profile flattened to within -50 to -30 ps/nm/km in the wavelength range of 1100 - 2700 nm. Under pumping with 75 fs pulses centered at 1550 nm, the recorded spectral flatness is 7 dB in the 930 - 2170 nm range, and significantly less if cladding modes present in the uncoated photonic crystal fiber are removed. To the best of our knowledge, this is the first report of an octave-spanning, all-normal dispersion supercontinuum generation in a non-silica microstructured fiber, where the spectrum long-wavelength edge is red-shifted to as far as 2300 nm. This is also an important step in moving the concept of ultrafast coherent supercontinuum generation in all-normal dispersion fibers further towards the mid-infrared spectral region.