A thermal barrier apparatus includes a lens cell with a vacuum chamber, the lens cell having opposing lenses, and a one-way gas valve coupled to the lens cell to allow venting of the vacuum chamber in response to thermal expansion of gas in the vacuum chamber. Another optical fiber has a chromatic dispersion higher than a positive value x and a negative chromatic dispersion slope at a given wavelength band. All of the lobes present a common azimuth component in a common direction, and the mixer further includes a plurality of longitudinal grooves occupying at least a portion of the central body, all of the grooves presenting a common azimuth component in a common direction that is opposite to the direction of the azimuth component of the lobes.