Photothermal Group Delay Tuning in Nonpermanently Phase-Shifted Chirped FBGs

A nonpermanent phase-shift can be locally created in a chirped fiber Bragg grating placed in contact to a thin heating wire. Changing the bias current value modifies the phase-shift (around π), which allows us to switch between a positive and negative delay variation at a specific wavelength. This mode of operation allows a wide-range tuning of the group delay in the structure. However, the tuning is rather slow and to overcome this limitation, we propose to exploit the photothermal effect. Using an optical pump emitting outside the grating spectral region, we can quickly tune the phase-shift value around π. The results demonstrate that the group delay can be all-optically tuned over a wide range and with relatively low optical powers. Experiments are confirmed by simulations. We believe that this structure may be advantageously used in microwave photonics.