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6Er-Yb all-fiber lasers with sub-GHz pulses repetition rates based on composite active fibers
A. Zverev1*, V. Kamynin1, V. Tsvetkov1, B. Denker1, S. Sverchkov1, V. Velmiskin1, Y. Gladush2, D. Krasnikov2, A. Nasibulin2
1- Prokhorov General Physics Institute of the Russian Academy of Sciences, Russia 2- Skolkovo Institute of Science and Technology, Moscow, Russia
The intensive development in the creation of ultrashort pulse (USP) sources with high repetition rates (sub-gigahertz level) operating in the 1.5-micron spectral range is driven by their wide application in fields such as analog-to-digital converters [1], frequency comb-based spectroscopy [2], and laser material processing [3]. One method of obtaining a stable sequence of ultrashort pulses with such high repetition rates is the implementation of mode-locking in a laser with a very short cavity, based on specially heavily doped composite active fibers (which lack the drawbacks of purely phosphate fibers) and hybrid optical components.
976 nm
Freqraicy MHz . Frequency, MHz.'
Fig.1. a) Initial scheme of an erbium fiber laser with a linear cavity. b) RF spectrum of initial signal. c) A ring laser based on a hybrid element. d) The radio frequency spectrum of the ring laser signal.
Initially, to test the feasibility of using composite fiber (with an erbium ion concentration of about N ~ 1020 cm-3) as the active medium of USP sources with a high repetition rate, we tested a linear cavity (Figure 1(a)). The active fiber, only 12 cm long, was pumped by a laser diode at a wavelength of 976 nm through a multiplexer and an optical splitter. One polarization controller (PC) was used to tune the laser generation mode. Aluminum foil pressed against optical ferrules served as mirrors (SWCNT+M1 and M2) in the cavity. To achieve mode-locking, single-walled carbon nanotubes (SWCNTs) were applied to one of the ferrules. With the maximum possible reduction in the length of the resonator in this configuration, stable generation of USP with a repetition rate of 150 MHz (the radio-frequency spectrum of the output signal is shown in Figure 1(b)) at the central wavelength was obtained at a pump power of 740 mW. The pulse duration was 0.5 ps, and the average output power was 9.2 mW. Next, to increase the pulse repetition rate and reduce the required pump power, we decided to change the resonator scheme to a ring configuration (Figure 1(c)), using an 18 cm long composite fiber doped with an Er/Yb complex and a hybrid element (WIT) serving as a WDM, isolator, and optical splitter (the coupler split was 90/10 with a 10% yield). Mode-locking was also achieved with SWCNT placed between two optical connectors. In this configuration, a straightforwardly launched mode-locking regime with a fundamental pulse repetition rate of 484 MHz (the radio-frequency spectrum of the signal is shown in Figure 1(d)) was obtained at a central emission wavelength of 1542.8 nm (the spectral width at half maximum was 1.6 nm) at a pump power of 168 mW. The pulse duration and average output power were 1.6 ps and 1.5 mW, respectively.
Thus, we have carried out an optimization of the laser circuit, as a result, sources with a repetition rate of ultrashort pulses from 150 to 484 MHz have been obtained.
The work is supported by the Russian Science Foundation (#23-79-30017)
[1] G.C. Valley, Photonic analog-to-digital converters, Optics express, 15(5), 1955-1982, (2007).
[2] T. Fortier and E. Baumann, 20 years of developments in optical frequency comb technology and applications, Communications Physics, 2(1), 153, (2019).
[3] H. Kalaycioglu, P. Elahi, O. Akjaalan, F.O. Ilday, High-repetition-rate ultrafast fiber lasers for material processing, IEEE Journal of selected topics in quantum electronics, 24(3), 1-12, (2017).
