Real-time observation of vortex mode switching in a narrow-linewidth mode-locked fiber laser

Abstract

Temporal and spatial resonant modes are always possessed in physical systems with energy oscillation. In ultrafast fiber lasers, enormous progress has been made toward controlling the interactions of many longitudinal modes, which results in temporally mode-locked pulses. Recently, optical vortex beams have been extensively investigated due to their quantized orbital angular momentum, spatially donut-like intensity, and spiral phase front. In this paper, we have demonstrated the first to our knowledge observation of optical vortex mode switching and their corresponding pulse evolution dynamics in a narrow-linewidth mode-locked fiber laser. The spatial mode switching is achieved by incorporating a dual-resonant acousto-optic mode converter in the vortex mode-locked fiber laser. The vortex mode-switching dynamics have four stages, including quiet-down, relaxation oscillation, quasi mode-locking, and energy recovery prior to the stable mode-locking of another vortex mode. The evolution dynamics of the wavelength shifting during the switching process are observed via the time-stretch dispersion Fourier transform method. The spatial mode competition through optical nonlinearity induces energy fluctuation on the time scale of ultrashort pulses, which plays an essential role in the mode-switching dynamic process. The results have great implications in the study of spatial mode-locking mechanisms and ultrashort laser applications.

Recent progress of dynamic mode manipulation via acousto-optic interactions in few-mode fiber lasers: mechanism, device and applications

“…(ω′, k′), (ω 0 , k 0 ) and (ω a , k a ) are the frequency and the wavenumber of diffracted light, incident light and acoustic wave, respectively, as shown in Figure 2B. In fiber acoustooptics, the practical applications based on acoustically induced refractive index modulation depend on different kinds of acoustic vibrations including longitudinal mode [65][66][67]84], torsion mode [113][114][115][116][117] and flexural mode [43,59,71,[107][108][109]112], as depicted in Table 1.…”

“…The lowest FAW mode F 11 mode propagating on the unjacketed silica fibers induces the periodic density change asymmetric with respect to the direction of acoustic vibration, which can be interpreted by the acoustically induced displacement vector (AIDV) ofũ. The AIDV-based microbending on the fiber medium leads to the permittivity modification δ ε composed of the geometric perturbation of δε g and the photon-elastic effect of δε p [50,112]:…”

“…The generated orthogonal HOMs are firstly rotated at 45°and then a phase difference of "π/2" ("i") is obtained by adjusting the stress of the PC appropriately. Finally, the LP 11a mode and LP 11b mode are adjusted to be the corresponding −1-order OAM mode and +1-order OAM mode [105,112]…”

“…However, pump injection switch is incapable of obtaining the intracavity perturbation and variation of the ML state. Recently the mode switching capability of the dualresonant AOMC has been proved to obtain the real-time observation of intracavity pulse evolution among different ML vortex modes [112]. Figure 14A shows the experimental setup of the narrowlinewidth vortex ML fiber laser.…”

“…[ [105][106][107][108][109][110][111] and spatial mode perturbation for spatial mode locking (ML) mechanism research [112].…”