The burgeoning advances of spatial mode conversion in few-mode fibers emerge as the investigative hotspot in novel structured light manipulation, in that, high-order modes possess a novel fundamental signature of various intensity profiles and unique polarization distributions, especially orbital angular momentum modes carrying with phase singularity and spiral wave front. Thus, control of spatial mode generation becomes a crucial technique especially in fiber optics, which has been exploited to high capacity space division multiplexing. The acousto-optic interactions in few-mode fibers provide a potential solution to tackle the bottleneck of traditional spatial mode conversion devices. Acousto-optic mode conversion controlled by microwave signals brings tremendous new opportunities in spatial mode generation with fast mode tuning and dynamic switching capabilities. Besides, dynamic mode switching induced by acousto-optic effects contributes an energy modulation inside a laser cavity through nonlinear effects of multi-mode interaction, competition, which endows the fiber laser with new functions and leads to the exploration of new physical mechanism. In this review, we present the recent advances of controlling mode switch and generation employing acousto-optic interactions in few-mode fibers, which includes acousto-optic mechanisms, optical field manipulating devices and novel applications of spatial mode control especially in high-order mode fiber lasers.
We experimentally demonstrated that the transversal vortex modes of an
all-fiber erbium-doped Brillouin laser can be dynamically switched by
using the high-order mode (HOM) of Brillouin pump (BP), which is used
to achieve the oscillation of HOM inside the ring cavity. Core-mode
conversion in a few-mode fiber (FMF) between the fundamental mode and
HOM is obtained by cascading an acoustically induced fiber grating
(AIFG) and a mode selection coupler (MSC) operating at the same
wavelength region. Through frequency shift keying (FSK) modulation of
the AIFG signal, the output transversal modes can be switched
dynamically between
L
P
01
and vortex modes, and the measured
purities of output HOM are more than 82%. Moreover, the output
Brillouin wavelength can also be tuned via altering the input
wavelength of BP and the resonant response of AIFG. We have achieved
HOM Brillouin-shifted laser output within the wavelength band from
1545–1560 nm. The output linewidth of the proposed Brillouin laser is
less than 4 kHz.
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.
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