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3The 30th International Conference on Advanced Laser Technologies ALT'23
LS-I-9
Characterisation of mid-IR light sources made of RE doped chalcogenide fibers on the base of modal approach
E. Romanova1, N. Parshina1, V. Shiryaev2
1- Saratov State University, Astrakhanskaya 83, 410012 Saratov, Russia
2- Devyatykh Institute of Chemistry of High-Purity Substances of RAS,
49 Tropinin Str., 603951 Nizhny Novgorod, Russia
Rare earth (RE) doped chalcogenide fibers are used in design of mid-IR lasers and luminescent sources [1]. Two systems of chalcogenide glasses (Ga-Ge-As-Se and Ga-Ge-Sb-Se) doped with RE elements (Pr3+, Tb3+, Dy3+) have been mostly under investigation [2-4]. These glasses have small optical losses and high refractive indices of 2.5-3.5 magnitude in the range of wavelengths 1 = 2-15 ^m [5].
In accordance with the wave theory [6], when a pump beam is launched into an active fiber coaxially with the fiber axis (Fig. 1), the guided modes, which do not have internal caustics (HEim modes) are mostly excited in the fiber. Each mode has its own wavelength-dependent intensity profile in the fiber transverse cross-section (TCS) and its own amplitude. However, in theoretical analysis of the luminescence excitation [2], pump and signal intensities are assumed constant over a TCS of a single-index fiber that is only feasible if the fiber is multimode.
In this work, the concepts of the wave theory of optical fibers that represent radiation propagating in a fiber as a set of modes, have been applied in the problem of luminescence excitation in a few-mode fiber. For a 20 ^m core made of Tb3+ doped Ga5Ge2oSbioSe65 glass with the refractive index nCo = 2.55 surrounded by a glass cladding with nc = 2.35 [4], the number of HEim modes at 1 > 3 ^m is less than 10. Propagation of the pump radiation (PR) at 1p = 2.95 ^m and luminescent radiation (LR) at 1 = 4.8 ^m in various modes has been investigated in a numerical model comprising kinetic equations for populations of three-level system of Tb3+ (Fig.2) and differential equations for PR and LR powers. Unlike the generally accepted theoretical model [2], the light intensity dependence on radial coordinate in the fiber TCS (Fig.3) has been taken into account.
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Fig. 1. Schematic of light launching into an active fiber and the fiber TCS.
2.95 nm 4.8 nm
Fig.2. System of Tb3+ energy levels with pumping at 2.95 ^m.
Fig.3. Radial profiles of HE1m modes in the fiber TCS.
Results of the computer modeling reveal that population inversion is not distributed evenly over the fiber TCS and has a radial profile, which varies in time and along the fiber. The profile shape depends on an intensity profile of a given fiber mode. Due to the levels populations depend on radial coordinate, intensity profiles of PR and LR modes propagating in the fiber experience distortion. Consequently, some part of energy leaks out the fiber as a radiation field that is an additional source of optical losses. This work was supported by the Russian Science Foundation (RSF) under Grant 21-13-00194.
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