High-power single-frequency single-mode all-solid photonic bandgap fiber laser with kHz linewidth

Abstract

There have been several demonstrations of single-frequency single-mode ytterbium-doped fiber lasers operating at a few hundred watts of power. A narrow spectral linewidth of these lasers is critical for many applications but has never been properly measured before at high powers. In this work, we report the first spectral linewidth measurement at kHz resolution of high-power single-frequency fiber lasers using a heterodyne technique and can confirm that these lasers can indeed operate at a few kHz spectral linewidth. Furthermore, we have improved the power from single-frequency single-mode all-solid photonic bandgap fiber lasers to 500 W using an improved photonic bandgap fiber.

High-power single-frequency fiber amplifiers: progress and challenge [Invited]

“…In 2015, an all-solid photonic bandgap YDF with a core diameter of ∼50 μm and with specific dopants that are favorable to decreasing the Brillouin gain was employed to amplify the single-frequency laser, while the obtained output power was just 400 W, beyond which the TMI effect would emerge despite the estimated SBS threshold being more than 1 kW [67] . Later, in 2022, this fiber was further optimized for TMI suppression, which was realized by introducing multiple cladding resonance design to the fiber for restraining the HOM content, and the output power of the corresponding SFFA was increased to 500 W [68] . It is worth noting that all the aforementioned research is based on free-space coupling the signal and pump into the main amplifier, owing to the fact that the utilized LMA-YDFs are hardly incompatible with the all-fiber architecture.…”

“…It is worth noting that all the aforementioned research is based on free-space coupling the signal and pump into the main amplifier, owing to the fact that the utilized LMA-YDFs are hardly incompatible with the all-fiber architecture. Although the free-space coupled configuration sacrifices the compact and reliable operation of the system, it allows for counter-pumping of the SFFA, enabling effective suppression of the SBS and the realization of the impressive high output power [64][65][66][67][68] .…”

Mitigating mode instabilities by controllable mode beating excitation with a photonic lantern

“…One widely accepted theoretical explanation is that there are two conditions necessary to lead to TMI: the appearance of a (thermally induced) refractive index grating (RIG) written by the mode interference pattern (MIP) and a phase shift between them [8]. Thus, TMI mitigation strategies have been proposed aiming at reducing the strength of the RIG or manipulating the phase shift, such as designing special structure fiber [9][10][11], shifting pump wavelength [12], optimizing the fiber coiling methods [13], suppressing the noise property [14], and using gain-tailoring techniques [15]. Nevertheless, the aforementioned strategies have strict requirements for the active fiber or the structure of the amplifier.…”

Large mode area saddle-shaped core Yb-doped fiber enabled monolithic high-power, high-efficiency, near-diffraction-limited MOPA laser

“…The more guided-modes for the step-index profile fiber will induce the appearance of the transverse mode instability (TMI) effect [2], accompanied by a rapid degeneration of beam quality. To deal with the contradiction between nonlinear effects and TMI at high average power level in fiber lasers, researchers have proposed and manufactured several specially-designed active optical fibers to enlarge fundamental mode field area and maintain single-mode behaviors, such as leakage-channel fiber (LCF) [3], 3C (chirally-coupled-core) fiber [4], multicore photonic crystal fiber (MCF) [5], single/multi-trench fiber (STF/MTF) [6], all-solid photonic bandgap fiber (AS-PBGF) [7] and etc. However, all the above fibers have complex transverse structure and need to more efforts to fabricate processes, which sets obstacles for their large-scale applications in multi-kW continuous-wave fiber lasers with all-fiberized format.…”