All-Fiber Bismuth Doped Self-Sweeping Laser

Figure 1: Laser wavelength dynamics The effect of self-induced laser line sweeping (or self-sweeping) was discovered in ruby laser in 1962 [1]. It was considered as an undesirable effect due to a small sweeping range. In fiber lasers the effect with broad sweeping range (more than 10 nm) was observed almost 50 years later [2]. The self-sweeping effect is caused by the formation of dynamical phase and gain gratings induced by spatial hole burning (SHB) in population inversion of the active medium [3]. So the laser frequency is changed step-wise between the pulses by one or several intemode beating frequencies. Due to their broad tuning range and simplicity self-sweeping fiber lasers become attractive sources for applications demanding tunable radiation. In spite of the fact that the majority of publications describe the studies of the effect in an Ytterbium-doped fiber laser operating in the spectral range near 1 μm, several attempts to broaden the operating wavelength range of self-sweeping lasers by using other active fibers are known. For example, wavelength self-sweeping is successfully obtained in a Thulium-Holmium codoped fiber laser [4] demonstrating a range of 17 nm near 1.9 μm. On the other hand, in an Erbium-doped fiber laser, it appears difficult to reach a sweeping range beyond 1 nm, which prevents practical applications of the laser, albeit sweeping is principally possible [5]. Here we report on the first demonstration of single-frequency self-sweeping effect in Bismuth doped fiber laser in the range of 10 nm near 1.46 μm (figure 1). The main feature of the realized laser is quasi-continuous frequency sweeping when pulse-to-pulse frequency change (1 MHz) is almost equal to instantaneous spectral linewidth. This fact means that the realized source is similar to mode hope free tunable lasers, whereas the laser scheme consists of conventional passive elements without special tuning elements. One should note that Bi-doped active fibers may be treated as an alternative to Erbium-doped fibers in the telecom spectral range enabling operation of fiber lasers and amplifiers in the extended range of 1.15-1.78 μm [6]. We believe that self-sweeping of laser frequency is also possible in telecom window (1.5-1.6 μm) which has high potential for applications.