Measurements of thermal effects in fibers doped with cobalt and vanadium

Thermal index changes due to nonradiative relaxation in optically pumped Co 2+-doped and Vn+-doped fibers are studied experimentally by an interferomet ric method, In both dopants these effects are shown to be very strong and t o mask any residual resonantly enhanced nonlinearity, The measured magnitud e and time constants of thermal effects are well explained by a new theoret ical model, which confirms its validity and usefulness, These measurements also demonstrate a new and simple method to differentiate between nonlinear and thermal phase shifts in doped fibers based on the dependence of the ph ase change on the pump pulsewidth. This study provides new information on t he spectroscopy of these two dopants, including the percentage of absorbed power they transform into heat (similar to 38% for Co2+ and 56% for Vn+), a nd the likely presence of clusters in Co2+-doped silica even at very low co ncentrations (8 wt ppm CoO).