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12LS-I-1
LASER SYSTEMS AND MATERIALS
Multimode and multicore fiber lasers with a cavity based on 3D fs-inscribed refractive-index structures
A.A. Wolf, A.G. Kuznetsov, A.V. Dostovalov, S.A. Babin*
Institute of Automation and Electrometry SB RAS, Novosibirsk 630090, Russia
Multimode and multicore fibers (MMF and MCFs) are treated now as an extension to conventional singlemode fibers, which involve transverse modes in optical signal generation, transmission and processing. Developments in this direction resulted in fiber-optic communication systems with increased capacity via spatial division multiplexing [1] and sensing systems with 3D sensitivity to the environmental impact [2]. MMF and MCF lasers are also explored as an alternative to large-mode area singlemode fiber lasers allowing for the power enhancement and/or nonlinear effects suppression [3], but they still have some lacks especially in terms of performance. It may be greatly improved with the use of in-fiber cavity elements analogues to fiber Bragg gratings (FBGs) in singlemode fibers, but conventional UV inscription technique has serious limitations for fabricating structures with necessary spatial resolution, especially in transverse direction. Here we review our recent results on the implementation of femtosecond IR pulses for fabricating 3D refractive-index (RI) structures in active and passive MMF and MCF of different type and development of new laser schemes with them.
First, we describe the femtosecond (fs) inscription technology with an emphasis on possibilities of point-by-point RI modification in MMFs and MCFs with high spatial resolution [4]. With this technology, fs-inscribed FBGs of special transverse structure allowing for the selection of low-order modes (LP01 or LPn) in graded-index (GI) MMF have been developed [4] and implemented in MMF Raman laser generating high-power (>50W) high-quality (M2<2) beam in 950-996 nm spectral band at pumping by highly-multimode (M2~30) 915-940 nm laser diodes (LDs). In this scheme, brightness enhancement factor ~73 at pump-to-Stokes conversion has been demonstrated [5]. It is also shown with a 2-core fiber as example, a possibility to inscribe FBGs in both cores with a shift in axial direction so that an interference between the reflected signals leads to spectral narrowing (below 0.02 nm) or multiline generation of the 2-core fiber Raman laser, see [4] and citation therein.
Further, FBGs selectively inscribed in side cores of 7-core passive MCF are used as complex reflector of a MCF Raman laser with pumping and output coupling through the central core, which is able to generate singlemode output with narrow spectrum [6]. The narrowing occurs due to the suppression of nonlinear effects in the large effective area of 7-core fiber cavity and due to the interference of signals reflected from FBGs in different cores, which have sufficient optical coupling. Another type of output power concentration in one core is demonstrated for a LD-pumped 4-core Yb-doped fiber with FBGs fs-inscribed in each core, where the core crosstalk is induced via strong bending of the fiber [7]. We also perform similar study of a LD-pumped 7-core Yb-doped fiber with highly-reflective complex mirror consisting of individual FBGs fs-inscribed in each core, where ~35 W output with narrow spectrum (<0.15 nm) is obtained. The role of core coupling and reflected signal interference is studied. We also try to form complex interferometric 3D reflector in GI MMF with random axial and transverse position of individual reflectors and study its effect on spatial and spectral features of output beam in the scheme of LD-pumped MMF Raman laser. The details of the latter studies will be presented at the conference.
This work is supported by Russian Science Foundation (№21-72-30024).
[1] D. J. Richardson, J. M. Fini, & L. Nelson, Space-division multiplexing in optical fibres, Nature Photonics, vol. 7, pp. 354-362 (2013).
[2] K. Bronnikov, A. Wolf, S. Yakushin, A. Dostovalov, O. Egorova, S. Zhuravlev, S. Semjonov, S. Wabnitz, and S. Babin, Durable shape sensor based on FBG array inscribed in polyimide-coated multicore optical fiber, Opt. Express, vol. 27, pp. 38421-38434 (2019).
[3] C. Jaregui, J. Limpert, & A. Tunnermann, High-power fibre lasers, Nature Photonics, vol. 7, pp. 861-867 (2013).
[4] A. V. Dostovalov, A. A. Wolf, M. I. Skvortsov, S. R. Abdullina, A. G. Kuznetsov, S. I. Kablukov, and S. A. Babin, Femtosecond-pulse inscribed FBGs for mode selection in multimode fiber lasers, Opt. Fib. Technol., vol. 52, 101988 (2019).
[5] A. G. Kuznetsov, S. I. Kablukov, E. V. Podivilov, S. A. Babin, Brightness enhancement and beam profiles in an LD-pumped graded-index fiber Raman laser, OSA Continuum, vol. 4, pp. 1034-1040 (2021).
[6] A. Wolf, A. Dostovalov, K. Bronnikov, M. Skvortsov, S. Wabnitz, and S. Babin, Advances in femtosecond direct writing of fiber Bragg gratings in multicore fibers: technology, sensor and laser applications, Opto-Electronic Advances, vol. 5, 210055 (2022).
[7] A. A. Wolf, M. I. Skvortsov, I. A. Lobach, A. V. Dostovalov, and S. A. Babin, Bending induced output power concentration in a core of a 4-core Yb-doped fiber laser, Opt. Exp., vol. 30, pp. 7580-7590 (2022).
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