A bandpass filter with adjustable bandwidth based on a press-induced long-period grating in a twisted holey fiber is presented. By twisting the holey fiber prior to the application of periodic pressure, each rejection band of the nontwisted induced long-period grating is split into two shifted rejection bands that move further apart as the twist ratio increases. This feature results in a wide bandpass filter with controllable bandwidth. A bandpass filter at 1523 nm with adjustable bandwidth from 15 to 65 nm with near-linear response and insertion loss lower than 0.7 dB is demonstrated. Additionally, the bandpass filter can be tuned over 100 nm.

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In this report, we present our analysis of the relationship between critical power and stimulated Raman scattering in Raman fiber lasers. Through our research, we have established a connection between the R.G. Smith constant at critical power and the necessary pump power required to reach the maximum power delivered by the first Stokes just prior to the generation of the second Stokes. In our experiments, two setups were successful in reaching the second Stokes generation, one utilizing a glass–air interface as the output coupler without HR mirrors and the other using HR-FBGs for both Stokes in conjunction with a glass–air interface. We found that the 1 Km 1060-XP fiber has an R.G. Smith constant of ~4.94 at critical power, which when multiplied by 2 gives ~9.88, a value close to the R.G. Smith constant (9.75) for maximum Stokes corresponding to a pump power of 5.5 W, with an approximation of ~98.6%. Our results demonstrate the importance of knowing the R.G. Smith constant at critical power in estimating the necessary pump power to achieve maximum power delivery in any Stokes component.

This study utilized solid-state lasers with a 50 ns pulse duration in a Q-switched mode of operation at wavelengths of 1.06 µm and 0.69 µm to investigate the hemispherical reflectivity of niobium. Our experimental results show that the reflectivity of niobium decreases notably as the laser fluence increases towards the plasma formation threshold for ablation at both studied wavelengths, which we attribute to changes in the absorptivity of the surface resulting from plasma formation. We also observed a significant effect of laser wavelength on the reflectivity values of the sample at low laser fluence. By determining the threshold fluence values for each wavelength, we estimated the surface temperature associated with the threshold fluence for plasma formation. Our calculations revealed discrepancies between published values for optically polished and mechanically polished niobium, which we suggest may be due to the presence of nano/micro defects, oxide films, and contaminants that amplify the wavelength-dependent effects on reflectivity. These findings have important implications for the design of optical components and laser processing techniques that use niobium, as well as for the development of accurate models of laser-material interactions. Further research is needed to fully understand the underlying mechanisms driving the observed effects and to explore potential applications of niobium in laser-based technologies.

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