Bartłomiej Siwicki

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.

Broadband supercontinuum generation in normal dispersion all-solid photonic crystal fiber pumped near 1300 nm

We report on octave-spanning supercontinuum generation under pumping with 1360 nm, 120 fs pulses, in an all-solid, all-normal dispersion photonic crystal fiber. The fiber was drawn from thermally matched oxide soft glasses with a hexagonal lattice 35 µm in diameter, 2.5 µm solid core and pitch of Λ/d = 0.9. The fiber was designed for normal dispersion broadly flattened in the 1200–2800 nm range. Experimentally recorded supercontinuum spectrum covered a 900–1900 nm bandwidth and was reconstructed with good agreement using numerical modeling. To the best of our knowledge, this is the first report of an experimentally demonstrated octave-spanning supercontinuum bandwidth, reaching as far as 1900 nm in the all-normal dispersion regime.

Extending of flat normal dispersion profile in all-solid soft glass nonlinear photonic crystal fibres

The bandwidth of coherent supercontinuum generated in optical fibres is strongly determined by the all-normal dispersion characteristic of the fibre. We investigate all-normal dispersion limitations in all-solid oxide-based soft glass photonic crystal fibres with various relative inclusion sizes and lattice constants. The influence of material dispersion on fibre dispersion characteristics for a selected pair of glasses is also examined. A relation between the material dispersion of the glasses and the fibre dispersion has been described. We determined the parameters which limit the maximum range of flattened all-normal dispersion profile achievable for the considered pair of heavy-metal-oxide soft glasses.

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.

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.

Numerical simulations of spectral broadening in all-normal dispersion photonic crystal fiber at various pump pulse conditions

Abstract. Supercontinuum (SC) generation contained in the normal dispersion range of an optical fiber has been shown to be limited primarily by the available peak power and length of the pump pulse. In this work, we numerically investigate the SC spectral width and flatness for various pump pulse conditions in a nonlinear, all-solid, soft-glass, photonic crystal fiber (PCF) with a flattened dispersion profile. We assume a range of pump pulse parameters with pulse lengths between 250 and 100 fs (60 to 150 kW of peak power), and input pulse energies between 10 and 30 nJ, numerically reaching a maximum SC width of 800 to 2600 nm. The presented theoretical study provides a guideline for the selection of a fiber laser pump source, or in other words, it enables one to expect the extent of spectral broadening in the developed, all-normal dispersion PCF, when presently available fiber laser pump pulse parameters are assumed.

Impact of steepness of pump temporal pulse profile on spectral flatness and correlation of supercontinuum in all-solid photonic crystal fibers with flattened normal dispersion

Steepness effects of pump pulse temporal profile on supercontinuum spectrum in an all-solid, normally dispersive photonic crystal fiber are investigated numerically. Utilizing a multi-shot supercontinuum spectra generated with a statistical nonlinear Schr?dinger equation model and computed spectral correlation maps, we discuss both the timescale and the spectral bandwidth of parametric interaction between self-phase modulation and optical wave-breaking components in the formation of the spectrum. Gaussian and hyperbolic secant pump pulse profiles, modelled with analytical formulas, are used to show the role of the wings of temporal pulse profile in the flatness of the developing supercontinuum from the perspective of spectral correlation. Degree of coherence of the numerical spectra is also analyzed and it is shown how steepened pump pulse slope preserves pump laser shot noise in otherwise coherent, normal dispersion supercontinuum generation. All-normal dispersion supercontinuum spectrum was also recorded experimentally in a photonic crystal fiber in the 1350?1750 nm range of wavelengths (E-U telecommunication bands), under 1550 nm pumping with 180 fs pulses and moderate in-coupled energy of 1 nJ, reproducing the bandwidth and general features of one of the numerical cases.

Supercontinuum generation in microstructured fibers made of heavy metal oxide glass

The NIR and mid IR supercontinuum generation in photonic crystal fibres are attractive light sources for sensing and spectroscopy applications, since it provides a high brightness, good beam quality source and can be offered as a compact, ready-to-use device. Most of the research work related to supercontinuum generation in photonic crystal fibres has to date been focused on silica based fibres limited to the visible and near infrared (NIR) ranges up to 2 μm. It has been shown that some groups of soft glasses can undergo multiple thermal processing steps without degradation and possess a high transmission in the mid IR range as well as a high effective nonlinearity, making them highly suited for photonic crystal fibre development in the area of NIR and mid IR supercontinuum generation. The use of the tellurite and heavy metal oxide glasses offer an interesting trade-off between high nonlinearity, transmission bandwidth, mechanical properties and fabrication requirements. Recently several successful realizations of broadband IR supercontinuum with these type of glasses have been reported in both, all-normal and anomalous, dispersion regimes.

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.