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12LS-I-2
LASER SYSTEMS AND MATERIALS
The pulse origin in heavily erbium-doped fiber lasers: experimental evidence
and modified theory
OVButov^, A.M. Smirnovw, A.V. Dorofeenko1245
1- Kotelnikov Institute of ' Radioengineering and Electronics oof RAS, Mokhovaya 11-7, Moscow 125009, Russia 2- Moscow Institute of Physics and Technology, 19 Institutskiy pereulok, Dolgoprudny 141700, Russia 3- Faculty of Physics of Lomonosov Moscow State University, Leninskie Gory 1-2, Moscow 119991, Russia 4- Institute for Theoretical and Applied Electromagnetics RAS, 13 Izhorskaya, Moscow 125412, Russia 5- Dukhov Research Institute of Automatics, 22 Suschevskaya, Moscow 127055, Russia
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ALT'22
The comprehensive experimental and theoretical investigation of the heavily doped erbium fiber lasers operation features allowed us to determine the origin of pulse formation. The dependence of the parameters and regimes of lasing on the cavity length, erbium doping level, pump power and external temperature was investigated. Operation regimes in the case of CW pump at a 976 nm and 1490 nm wavelengths have been studied. A new approach to interpretation of self-Q-switched regime provide the explanation of the effect of switching from pulsed to CW operation regime with temperature reduce from room to liquid nitrogen (Fig.la).
A feature of Erbium-doped fiber lasers is the high probability of stable pulses operation. The parameters of the pulse mode depend both on the composition and concentration of doping in the fiber core, and on external parameters such as pump power and operating temperature.
Fig. 1 a) The diagram of laser oscillation regimes in the coordinates of pump intensity (A) and up-conversion rate (Y22). b) Energy level structure of a single Er3+ ion in silica glass. In the left part of the figure, transitions of the Er3+ ion in the lasing process are shown. In the right part of the figure, the up-conversion in ionic pair is shown. The straight and wavy solid lines show radiative and non-radiative relaxation processes, respectively. The dashed line shows a
non-radiative energy transfer process between the ions in pair.
It was shown that pulses occur due to the presence of Er3+ ion pairs in the glass structure, and a dynamic laser model describing the self-pulsed and CW regimes in the presence of ion pairs in the system was proposed [1,2]. The reason for the pulse formation is the appearance of ion pairs at high concentrations of erbium, which give an absorption effect depending on the populations of levels in the ion pair. Previously this effect was interpreted as saturating absorption. As is known, in classical laser circuits a saturating absorber (passive shutter) leads a passive Q-switched regime and pulse generation. From our point of view, passive Q-switching in heavily doped erbium lasers is realized due to this effect, which is opposite to the saturable absorption. Namely, the absorption increases, rather than saturates, as the intensity of the field increases due to the up-conversion process at the lasing wavelength [3,4] (Fig.lb). For the theoretical description were used the system of rate equations, which takes into account the up-conversion process, appended with a term responsible for the spontaneous emission process.
[1] F. Sanchez, P. Le Boudec, P.-L. François and G. Stephan, Effects of ion pairs on the dynamics of erbium-doped fiber lasers, Phys. Rev. A, vol. 48, pp. 2220-2229 (1993).
[2] F. Sanchez and G. Stephan, General analysis of instabilities in erbium-doped fiber lasers, Phys. Rev. E, vol. 53, pp. 2110-2122 (1996).
[3] A.M. Smirnov, A.P. Bazakutsa, Y.K. Chamorovskiy, I.A. Nechepurenko, A.V. Dorofeenko and O.V. Butov, Thermal Switching of Lasing Regimes in Heavily Doped Er3+ Fiber Lasers, ACS Photonics, vol. 5, № 12, pp. 5038-5046 (2018).
[4] A. Smirnov and O. Butov, Pump and thermal impact on heavily erbium-doped fiber laser generation, Optics Letters 46, pp. 8689 (2021).
