Observation of spectral Gouy shift in femtosecond laser pulse written Volume Bragg gratings

Volume Bragg gratings (VBGs) are optical divices exhibiting a periodic refractive index structure typically inside bulk glass. Due to their characteristics they can be employed for a wide range of applications such as beam combiners for e.g. fiber or solid state lasers, frequency stabilizers of semiconductor lasers or pulse compressors in case of chirped VBGs. Here we report on the measurement of the spectral Gouy shift by the help of a femtosecond written VBG. In the theory on Gaussian beams an additional phase term occurs which pays tribute to the propagation behavior of a Gaussian beam with respect to a plane wave. Thereby the beam adapts a longitudinal phase delay within the Rayleigh length. This so called Gouy phase shift can experimentally be measured when using a passive fiber and characterizing the reflection behavior of the VBG by adopting the fiber end facet to the grating. Measuring the reflected signal by the help of a circulator while changing the distance between fiber and grating end facet one is able to observe a spectral shift of the central wavelength of the reflected signal which can be attributed to the Gouy phase. To prove this assertion we carried out numerical/theoretical calculations which fit very well with our measurements.