Experimental evidence for strong UV transition contribution in the resonant nonlinearity of doped fibers

We report a new theoretical interpretation of the strong resonantly-enhanced third-order nonlinearity observed in fibers doped with Nd/sup 3+/, Er/sup 3+/, or Yb/sup 3+/. For each fiber, the absolute contributions of the near resonance (IR-visible) and far from resonance (UV) transitions to the nonlinearity are calculated and compared to experimental values. We show that in all cases, a large fraction of the measured nonlinearity is due to strong UV transitions. The consequences of this observation on the bandwidth, switching power and response time of switches based on this effect are discussed.