Non-linear transmission in germanosilicate fibres at blue/green wavelengths

Abstract

Many fibre-based devices rely on the ability to transmit light at high intensities down single-mode fibres. Examples are fibre lasers, amplifiers, second-harmonic generators and simple fibre-based high power delivery systems. It is thus important to know whether any non-linear processes exist that could restrict the usefulness of the fibres used in these applications. Brown et al have reported that non-linear effects of this kind do indeed occur at Argon ion wavelengths in germanosilicate fibre, limiting the power that can be delivered to some tenths of a W over only few-metre lengths of single-mode fibre. In this paper we report the chief results of an extensive investigation into this effect in germanosilicate HiBi fibres designed to transmit single-mode blue/green light. These observed non-linearities in the transmission of CW blue/green light cannot be attributed to conventional non-linear processes (such as stimulated Raman or Brillouin scattering) because only short (few-metre) fibre lengths are needed, the thresholds are low, and the effect is insensitive to laser line-width. We have indicated that this non-linear behaviour can be explained by the creation (via two-photon absorption - TPA) and bleaching (via normal absorption) of colour-centres. TPA enables blue/green photons to reach UV energy levels that are sufficient to disrupt the glass matrix, and even though the TPA rate is very low at intensity levels where non-linear transmission is significant, the long path lengths in the fibre, added to the high likelihood of colour-centre formation by quanta at double the photon energy, mean that colour-centres created by TPA can have a dramatic effect on transmission.

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