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However, further drawing speed decreases led to the intermittent interruption of the drawing process due to an unacceptably low pull force and, thereby, to the optical fiber outer diameter local deviations from the desired reference value under failures of MOF cross-section geometry parameters. Pilot samples of the above-described MOF were fabricated under a slow drawing speed of 0.9 m per minute. Here, the chirality order was improved up to 217 rpm. The first approach was implemented during the fabrication of silica few-mode MOF with typical hexagonal geometry with offset core (ratio of air hole diameter to the pitch ratio is 0.65…0.70, quality of spatial guided modes is 3…4). Therefore, the next aim was to obtain a strong improvement in the fabricating silica MOF twisting order by decreasing the drawing speed with the tower feed unit or by increasing the preform rotation speed in the tower feed unit. As a result, the performed modifications to the drawing tower provided fabrication of MOFs with weakly induced chirality from 10 rpm twisting (rotation speed is 20 revolutions per minute with MOF drawing speed of 2 m per minute) up to 66 rpm twisting (rotation speed is 200 rpm with MOF drawing speed 3 m per minute).

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After a series of experiments, the first pilot samples of twisted MOFs with a desired pitch ratio of 0.95 were successfully fabricated under excess pressure of 25 mbar and a drawing temperature of 1900 ☌. We tested various combinations of excess pressure, drawing temperature, drawing speed, and rotation speed to obtain twisted MOFs with desired non-perturbed geometry/microstructure over the drawn length. Then it is driven into the high-temperature furnace and re-drawn to optical fiber under twisting provided by the installed stepper motor. The third step contained the installation of prepared MOF cane into the feed unit of the drawing tower. This adapter provides the ability of cane rotation during the MOF drawing under the desired speed, and blocks drops in pressure between the hose and the top end of the cane due to the sealed internal space of the rotational adapter. Additionally, we developed and fabricated a special fluoroplastic rotational adapter ( Figure 1b) that directly connects an excess pressure hose to the top end of the cane and fixes it without twisting. The maximal rotation speed of the stepper motor was only 200 revolutions per minute, which provides a maximum chirality of 100 rpm under the drawing speed of 2 m per minute ( Figure 1a). Some modifications to the drawing tower were performed: we installed a stepper motor to the feed unit to add the rotation option to the drawing system. Here, the same approach proposed in was utilized: rotation of optical fiber preform during the drawing process.

We described in detail the first attempts of weakly twisted silica MOF fabrication in previously published works.

Fabrication of Twisted Silica Microstructured Optical Fibers They contain results of manufactured MOF geometry and transmission spectra measurements, while Section 3.5 is focused on measurements of beam profiles under various launching conditions and laser sources. Section 3.3 and 3.4 present successfully fabricated silica few-mode GeO2-doped six-core-MOFs with graded- and step-index refractive index profiles and twistings of 50…500 rpm. Section 3.2 is concerned with the analysis of GeO 2-doped six-core-MOF to localize the GeO2 dopant concentration, which would provide desired four-mode operation at a wavelength of λ = 1550 nm. Section 3.1 introduces a technique for the fabrication of silica GeO 2-doped MOF supporting element preforms (rod blanks). Section 2 is focused on technological issues of strongly twisted silica MOF manufacturing and describes in detail the performed required modifications of optical fiber drawing tower equipment. The rest of this paper is structured as follows.

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