All-fiber low-frequency shifter based on acousto-optic interaction and its heterodyne vibration response.

We demonstrate two all-fiber low-frequency shift schemes based on the acousto-optic interaction in a few-mode fiber (FMF). Two acoustically induced fiber gratings (AIFGs) are cascaded in reverse to achieve an efficient cycle conversion between LP11 and LP01 core modes in the FMF while obtaining a frequency shift of 1.8 MHz. In addition, a long-period fiber grating (LPFG) is employed to replace the AIFG, which achieves a lower frequency shift of 0.9 MHz, and its tunable wavelength range exceeds 100 nm. Both schemes show the characteristics of an upward frequency shift. Moreover, we also present a heterodyne detection system based on the above frequency shift schemes, which is verified in response to micro-vibration signals ranging from tens to hundreds of kilohertz, as well as speech signals in a lower frequency range. The experimental results show that these all-fiber frequency shift schemes have potential applications, such as in fiber optic hydrophones, laser speech detection, and fiber optic sensors.

Dynamic mode-switchable and wavelength-tunable Brillouin random fiber laser by a high-order mode pump.

We experimentally investigate two schemes of Brillouin random fiber laser (RFL) by using high-order-mode (HOM) pump in a few-mode fiber (FMF). The core-mode conversion between LP01 and LP11 modes is obtained in the FMF by cascading long period fiber gratings (LPFG) working at the same wavelength region. Different transversal modes of stimulated Brillouin scattering (SBS) can be implemented based on broadband long period fiber gratings (LPFG) and acoustically induced fiber gratings (AIFG). The RFL base on two broadband LPFGs can obtain high purity LP11 mode operating in the range of 1543 nm to 1565 nm. Moreover, the output mode can be dynamically switched between LP01 mode, LP11a mode and LP11b mode by modulating frequency shift keying (FSK) signal of the AIFG. This work has potential application prospects in the fields of mode division multiplexing systems, speckle-free imaging, free-space optical communication, laser material processing.

Vortex soliton oscillation in a mode-locked laser based on broadband long-period fiber grating.

Optical vortex beams (OVBs) have attracted much attention in diverse applications and spatiotemporal mode locking for optical soliton formation. In this Letter, a compact mode-locked (ML) vortex fiber laser is demonstrated based on a broadband long-period fiber grating near the dispersion turnaround point, where the group velocities between the core modes of ${{\rm LP}_{01}}$ and ${{\rm LP}_{11}}$ in a two-mode fiber are matched. The OVB pulses with first-order orbital angular momentum are oscillated through broadband mode conversion inside the cavity. The time-stretch dispersive Fourier transform method is also employed for the observation of vortex soliton buildup dynamics. The study of vortex soliton oscillation motivates the development towards controlling vortex modes in the ML fiber lasers.