Self-Sweeping Operation of Nd-Doped Fiber Laser

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The self-induced laser frequency sweeping (self-sweeping for short) effect in fiber lasers was observed in 2011 [1]. In this case the lasers generate self-sustained pulses and its frequency drifts slowly from pulse to pulse in one direction and jumps back after reaching a border of sweeping region. The self-sweeping operation is based on formation of long-lived dynamical phase and gain gratings. The gratings are connected to modulation of population inversion inside the active medium induced by the field of a standing wave formed in a linear cavity [2]. Dynamical gratings inscribed in the cavity by previously generated longitudinal modes are kept in the active medium for a relatively long time as compared with lifetime of a single mode and affects generation of the next spectral components. Due to broad sweeping range (more than 10 nm) and simplicity, self-sweeping fiber lasers are attractive sources for applications demanding tunable radiation. As a result, the self-sweeping laser is the simplest sort of tunable laser without any tuning elements and electrical drivers for frequency tuning. Currently the self-sweeping operation was observed in almost all common fibers: ytterbium [1–2], bismuth [3], erbium [4], holmium [5] and thulium [6]. It should be noted that the self-sweeping operation has been demonstrated up to now only in quasi-three-level laser systems. In the present paper, we demonstrate for the first time self-sweeping operation in a four-level laser system based on neodymium. The Nd-doped fiber with length of 3 meter is pumped by multi-mode laser diode with wavelength of 805 nm. The cavity is formed by highly-reflective fiber loop mirror and right-angle cleaved fiber end. Two fiber Lyot filters and a 10 nm bandpass filter were used for restriction of sweeping region. The self-induced spectral dynamics is observed in the region of 1.06 μm with range of up to 1.8 nm (Fig.1a). The laser generates periodic pulses (Fig.1b) where each of them contains single longitudinal mode radiation, which is confirmed by heterodyning with single-frequency probe radiation. The laser frequency is changed from pulse to pulse by one intermode beating frequency of the laser ∼7.5 MHz. The sweeping rate is increased with pump power up to 10 nm/sec. The output power exceeds 600 mW. Self-sweeping operation for Nd-doped fiber in the new spectral range of 0.93 μm instead of standard 1.06 μm is expected.