We present a spatiotemporally mode-locked Mamyshev oscillator. A wide variety of multimode mode-locked states, with varying degrees of spatiotemporal coupling, are observed. We find that some control of the modal content of the output beam is possible through the cavity design. Comparison of simulations with experiments indicates that spatiotemporal mode locking (STML) is enabled by nonlinear intermodal interactions and spatial filtering, along with the Mamyshev mechanism. This work represents a first, to the best of our knowledge, exploration of STML in an oscillator with a Mamyshev saturable absorber.
The vapor phase chelate delivery (VPCD) technique in conjunction with the modified chemical vapor deposition (MCVD) process is adopted to fabricate fibers with customized doping profiles. The three large‐mode area (LMA) step‐index fibers with different rare‐earth doping profiles in the core region, such as uniform doping, centralized doping, and circumferential doping, are fabricated by optimizing the fabrication parameters. The fibers are tested in a cladding‐pumped amplifier configuration and their output beam qualities and signal‐to‐noise ratio (SNR) are characterized. The investigation reveals that the fiber with centralized doping in the core region exhibits lower M
2 compared with the fibers with uniform and circumferential doping, as it has a lower overlap of the higher‐order modes with the doped region. The experimental result is further affirmed through theoretically simulated results. The developed fabrication technique shows potential to fabricate specialty fibers of varied designs, where customized doping profiles are required.
Herein, the fabrication of ytterbium‐doped graded refractive‐index few‐mode fiber (GRIN‐FMF) is described for the first time following a vapor phase chelate delivery technique. Investigation is carried out to find out the best possible approach to fabricate the preform. The optimization of the process steps along with composition and associated process parameters leads to a parabolic refractive‐index profile with a profile parameter of 2 in the developed fiber. A large core of 30 μm diameter and a low numerical aperture (NA) of 0.08 with graded distribution of dopant ions is achieved with a good repeatability. Amplification gain of more than 20 dB with good beam quality for pulses with kW peak power without any distortion in temporal, spectral, and spatial profile proves the potential of the fiber for many promising applications.
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