Existence of very high order guided modes of visible radiation is demonstrated for high-refractive-index-step fused silica submicron waveguide channels in microstructure fibers. Cherenkov emission of solitons produced by femtosecond laser pulses in such photonic wires is ideally suited for selective coupling into the high-order modes, which are extremely difficult to excite with the use of standard beam in-coupling optics.Output field patterns of radiation frequency-converted in smallcore waveguide channels in a microstructure fiber High-order waveguide modes [1] are interesting physical objects, which can serve as powerful tools for particle trapping [2,3], laser manipulation of biological species [4], and efficient multimode-phase-matched nonlinear-optical frequency conversion [5,6]. The number of modes in an optical fiber is controlled [1] by the waveguide parameterwhere k = 2π/λ, λ is the radiation wavelength, a is the fiber core radius,and n co and n cl are the refractive indices of the fiber core and the fiber cladding, respectively. The ratio of the fiber core size to the radiation wavelength and the index profile ∆ are thus the key factors determining the number of guided modes. In standard telecommunication fibers, the index step ∆ is most often low, providing a single-mode regime for the visible and near-infrared light with typical fiber core sizes of a few micrometers. Fiber nanotapers [7] and small-size fused silica waveguide channels in microstructure fibers [8,9] provide the limiting values of index step ∆ for fused silica waveguiding structures, allowing the existence of the maximum number of guided modes for a given a/λ ratio.
This work presents fabricated silica microstructured optical fiber with special equiangular spiral six-ray geometry, an outer diameter of 125 µm (that corresponds to conventional commercially available telecommunication optical fibers of ratified ITU-T recommendations), and induced chirality with twisting of 200 revolutions per minute (or e.g., under a drawing speed of 3 m per minute, 66 revolutions per 1 m). We discuss the fabrication of twisted microstructured optical fibers. Some results of tests, performed with pilot samples of designed and manufactured stellar chiral silica microstructured optical fiber, including basic transmission parameters, as well as measurements of near-field laser beam profile and spectral and pulse responses, are represented.
Microstructure fibers with a specially designed form birefringence are shown to be ideally suited for the creation of highly efficient frequency-tunable sources of short pulses in the visible range. We use unamplified 35-fs pulses of 820-nm Ti: sapphire laser radiation to demonstrate efficient generation of an intense anti-Stokes signal in an elliptical-core fused silica microstructure fiber. The central wavelength of this anti-Stokes signal can be switched between 490 and 510 nm by coupling the pump pulses into the fast and slow modes of the microstructure fiber.
An assortment of photonic-crystal fibers with a core area varying from 20 to 50 µm 2 is used to demonstrate efficient frequency conversion of megawatt femtosecond Cr:forsterite laser pulses with a central wavelength of 1.24 µm into the visible and infrared spectral ranges. Supercontinuum radiation with a spectrum spanning from 500 to 1800 nm and total output energy of hundreds of nanojoules is observed at the fiber output. Novel compact broadband light sources based on supercontinuum generation in photonic-crystal fibers (PCFs) [1,2] are emerging as powerful tools for optical science and ultrafast laser technologies [3]. In optical metrology, supercontinuum generation in PCFs [4] is used as a mechanism whereby frequency combs generated by modelocked femtosecond lasers are broadened to more than an octave, allowing calibration of frequency combs for high-precision measurements of frequency and time intervals, thus leading to a tremendous simplification of optical metrological measurements [5][6][7][8].In ultrafast science, supercontinua generated in PCFs give an access to the carrier-envelope phase (CEP) slip [9], thus offering an unprecedented control over few-cycle field waveform synthesis [10], allowing the generation of single attosecond pulses [11], and suggesting new approaches in time-resolved spectroscopy on the attosecond time scale [12][13][14]. Other important applications of supercontinuum generation and, more generally, nonlinearoptical transformations of laser pulses in PCFs [15] include the development of novel compact fiber-format light sources for nonlinear spectroscopy [16][17][18], microscopy [19][20][21] and optical coherence tomography [22].Small-core PCFs, typically operating in the regime of anomalous dispersion, show an excellent performance as supercontinuum sources ideally suited for nano-and subnanojoule input laser pulses. To accommodate higher input energies without the risk of laser-induced damage of the fiber material, PCFs with a larger core size are needed. Such large-mode-area (LMA) PCF components [23,24]
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