In this paper mode instability in a 500 W ytterbium-doped fiber laser is experimentally examined by changing the pumping wavelength, spectral bandwidth of signal light, active fiber temperature and coiling radius. The magnitude of power transfer from the fundamental mode to the higher order mode due to mode instability is measured as a criterion for its incident. The experiments show that the coiling radius of the first few tens of centimeters of the active fiber plays a significant role in controlling mode instability, and shifting the pumping wavelength from 976 to 973 nm can mitigate mode instability.

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In this paper we present a novel method to reliably strip the unwanted cladding light in high-power fiber lasers. Soft metals are utilized to fabricate a high-power cladding light stripper (CLS). The capability of indium (In), aluminum (Al), tin (Sn), and gold (Au) in extracting unwanted cladding light is examined. The experiments show that these metals have the right features for stripping the unwanted light out of the cladding. We also find that the metal-cladding contact area is of great importance because it determines the attenuation and the thermal load on the CLS. These metals are examined in different forms to optimize the contact area to have the highest possible attenuation and avoid localized heating. The results show that sheets of indium are very effective in stripping unwanted cladding light.

Key words Electron, high intense, plasma accelerator, laser wake field, bubble regime, wave break. PACS 41.75.Jv, 41.75.Lx In this work we evaluate the interaction of high intense laser beam with a steepened density profile. During laser interaction with underdense plasma by freely expanding plasma regime, modification of density profile is possible. In this paper we have investigated the ultra short laser pulse interaction with nonisothermal and collisionless plasma. We consider self-focusing as an effective nonlinear phenomenon that tends to increase when the laser power is more than critical rate. By leading the expanded plasma to a preferred location near to critical density, laser reflection is obtained, so the density profile will be locally steepened. The electromagnetic fields are evaluated in this new profile. We show the amplitude and period of electrical field oscillation are increased by reducing the steepened scale length. Also our numerical results identify that by reducing the steepened scale length, the electrical field is increased to wave breaking threshold limit. This high gradient electrical field causes the effective beam loading during the wave breaking phenomenon. The wave breaking can be the initial point for other acceleration regime as cavity or channel guiding regime.

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.