  A compact and versatile femtosecond mid-IR source is presented, based on an
optical parametric oscillator (OPO) synchronously pumped by a commercial
250-MHz Er:fiber laser. The mid-IR spectrum can be tuned in the range 2.25-2.6
\mu m (signal) and 4.1-4.9 \mu m (idler), with average power from 20 to 60 mW.
At 2.5 \mu m a minimum pulse duration of 110 fs and a power of 40 mW have been
obtained. Active stabilization of the OPO cavity length has been achieved in
the whole tuning range.
