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18LM-O-5
LASER-MATTER INTERACTION
Direct Laser Writing of Helical Bragg Grating in Vortex Fiber
V.V. Likhov1,2, S.L. Semjonov1, S.A. Vasiliev1, G.K. Alagashev1, A.G. Okhrimchuk1,2
1- Prokhorov General Physics Institute of the Russian Academy of Sciences, 38 Vavilova Str. Moscow, Russia 2- Mendeleev University of Chemical Technology of Russia, 9 Miusskaya Sq, Moscow, Russia
okhrim@fo.gpi.ru
Ring core fibers, also known as vortex fibers, are a perspective solution to increase capacity of optical communication systems with mode division multiplexing [1]. Modes carrying orbital angular momentum (OAM) draw attention in this concern due to low cross talk in the ring core fibers. We consider that helical structures integrated in the fiber systems are a natural solution for generating and filtering OAM modes. However, no such devices were known up to now.
Direct laser writing (DLW) by femtosecond laser pulses is widely used to pattern the refractive index of optical materials. The high spatial resolution of this 3D patterning method, comparable to the light wavelength, in combination with high-precision positioners, makes it possible to form high quality phase structures in crystals and glasses, including helical Bragg gratings (HBG) that reflect visible and near-IR light [2,3]. Here, we applied DLW with lenticular beam waist performing helical movement to inscribe third order HBG reflecting at wavelength region of 1550 nm [3]. Uniform helical pattern with refractive index modulation of about 10-3 was inscribed in the Ge-doped core of home-made ring-core fiber by femtosecond laser beam (Fig.1). Laser wavelength, pulse duration and repetition rate were 1030 nm, 180 fs and 250 Hz correspondingly. Grating length was 3 mm. Positioning of the beam waist inside the fiber core was controlled by multiphoton excitation of the blue luminescence of the germanium oxygen deficient centers (GODC) under laser pulse energy below modification threshold. HBG reflectance spectra was recorded with single frequency tunable laser light coupled into the vortex fiber core (Fig.2).
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Fig.1. Bright filed microscopic picture of HBG in ring-core fiber.
According to our calculations the investigated fiber supports twenty six cylindrical vector modes with the same radial index equaled to unity and different azimuthal indices (HEm1 and EHm1). The modes can form vortex modes with OAM as high as 7. We suppose that the written HBG provides resonance couplings of counter-propagating OAM modes (Fig.2). Different peaks within the reflection band correspond to coupling of OAM modes with different topological charges [3]. The study was supported by Russian Science Foundation, grant #22-29-01187.
[1]. H. Wang, Y. Liang, X. Zhang, S. Chen, L. Shen, L. Zhang, J. Luo, and J. Wang, "Low-Loss Orbital Angular Momentum Ring-Core Fiber: Design, Fabrication and Characterization," J. Light. Technol. 38, 6327-6333 (2020).
[2] J. He, J. He, X. Xu, B. Du, B. Xu, C. Liao, Z. Bai, and Y. Wang, "Single-mode helical Bragg grating waveguide created in a multimode coreless fiber by femtosecond laser direct writing," Photonics Res. 9, 2052 (2021).
[3] A. G. Okhrimchuk, V. V. Likhov, S. A. Vasiliev, and A. D. Pryamikov, "Helical Bragg gratings: experimental verification of light orbital angular momentum conversion," J. Light. Technol. 40, 2481-248 (2022).
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Fig.2. Reflectance spectra of HBG. Background is due to Fresnel reflection from the fiber
