Decreasing effective reflectivity of the output coupler in the power scaling of fiber lasers

Abstract

Increasing power leads to the spectral broadening of fiber laser beams. In this effect the spectral line width of the laser beam becomes broader than that of the output coupler (OC) due to power scaling. Therefore, the effective reflectivity of the OC mirror decreases. In this paper, this phenomenon is investigated theoretically and experimentally. A Liekki Application Designer V3.3 was used for the theoretical investigation and the experiments were conducted during the fabrication of a 630W fiber laser. Given the excellent agreement between the simulation and the experimental results, a decrease in the effective reflectivity of the OC mirror was proved. The decrease in the effective reflectivity of the OC mirror changes some of the laser parameters, such as optical efficiency, longitudinal distribution of population inversion and the backward power inside the resonator. This may also lead to power instability. The effects of a decrease in the reflectivity of the OC on the mentioned parameters of the fiber laser are investigated in the present paper. Finally, recommendations for choosing appropriate OC mirror parameters in high-power (kW class) fiber lasers are suggested.

Transverse mode instability of fiber oscillators in comparison with fiber amplifiers

“…Since power scaling in a fiber laser broadens the spectral linewidth of the laser beam compared with that of the output coupler, decreased effective reflectivity of the OC FBG is applied in the simulation [33].…”

Spectral Broadening Suppressed by a Gain-Enhanced Fiber in Polarization Maintaining High-Power Systems

“…Hence, applying the appropriate strategies in suppressing the spectral broadening will make the main oscillation power amplification (MOPA) system based on the FOL seed applicable and universal. Currently, starting from optimizing the structure of the system, some experimental studies have reported on the relationship between the number of multi-longitudinal modes and spectral broadening [10], [16]- [19]. In the oscillators, for example, reducing the length of the active fiber in cavity is the most popular strategy to suppress the spectral broadening [10], [18].…”