Straight and linearly tapered capillaries produced by femtosecond laser micromachining

Gas-filled capillary discharge waveguides are a commonly employed medium in laser–plasma interaction applications, such as the laser wakefield accelerator, because they can simultaneously guide high-power laser pulses while acting as the medium for acceleration. In this paper, the production of both straight and linearly tapered capillaries using a femtosecond laser micromachining technique is presented. A tapered capillary is shown to possess a smooth variation in diameter (from 305 μm to 183 μm) along its entire 40 mm length, which would lead to a longitudinal plasma density gradient, thereby dramatically improving the laser–plasma interaction efficiency in applications. Efficient guiding with up to 82% energy transmission of the fundamental Gaussian mode of a low intensity, 50 fs duration laser pulse is shown for both types of capillary waveguide.