Objective  Tunable pulsed fiber lasers have important applications in optical fiber sensing, spectral measurement, and optical fiber communication. Currently, there are three types of pulses in fiber lasers: Q-switched pulse, mode-locked pulse, and Q-switched mode-locked (QML) pulse. QML pulse is a transition state between Q-switched pulse and mode-locked pulse. Therefore, QML pulse has the characteristics of both Q-switched pulse and mode-locked pulse. Compared with mode-locked pulse, QML pulse has higher single pulse energy and adjustable repetition frequency. Most of the fiber lasers utilize saturable absorber materials, nonlinear polarization rotation (NPR) structure, and nonlinear amplifying loop mirror (NALM) structure to achieve pulse output. Compared with saturable absorber materials and NPR structure, NALM structure has the characteristics of high environmental stability, high damage threshold, and high efficiency. Moreover, NALM structure also has filtering effect, which can realize wavelength tunability of pulsed fiber lasers. Therefore, this paper proposes a tunable multimode QML laser based on NALM structure.

Methods  A tunable multimode QML laser based on the NALM structure has been constructed (Fig.1). This laser consists of a NALM structure on the left and a linear arm on the right, connected by a 50∶50 coupler. The end of the linear arm is a fiber optic mirror. After being reflected by the mirror, the light enters the NALM structure on the left through the coupler. The incident light is split into two beams of equal intensity. The clockwise transmitted light is amplified by the gain fiber and then passes through a section of ordinary fiber, where it accumulates nonlinear phase shifts due to self-phase modulation and cross-phase modulation effects. The counterclockwise transmitted light passes through the ordinary fiber first and then is amplified. After both beams propagate one cycle within the left-side loop, due to the different nonlinear phase shifts accumulated by the two beams, interference occurs within the coupler (1:1), resulting in narrowing of the optical pulses. On the basis of achieving pulse output, by controlling the rotation angle of the polarization controller, the propagation path and optical path difference of the light waves within the NALM structure can be altered, thereby achieving tuning of the output wavelength.

Results and Discussions  The experiment successfully generated multimode QML pulses (Fig.3), and by adjusting the polarization controller, multimode QML pulses with different center wavelengths were obtained (Fig.4). The center wavelength can be tuned from 1564.1 nm to 1609.2 nm, with a tunable range of up to 45.1 nm. The optical spot patterns were all multimode distributions, and the side mode suppression ratios of the spectra were all around 40 dB, indicating good stability. To further explore the characteristics of multimode QML pulses, the pump power was increased, resulting in dual-wavelength and triple-wavelength multimode QML pulses (Fig.6). By adjusting the polarization controller, the wavelength spacing of the dual-wavelength QML pulses could be increased from 2.66 nm to 31.78 nm, with a tunable range of up to 29.1 nm.

Conclusions  The experimental results showed that stable multimode QML pulses can be obtained in the multimode cavity of the NALM structure, with a tunable range of up to 45.1 nm for the center wavelength of single-wavelength QML pulses and a tunable range of up to 29.12 nm for the wavelength spacing of dual-wavelength QML pulses. This tunable pulsed fiber laser holds great promise in fields such as optical fiber sensing, spectral measurement, and optical fiber communication.