Efficient doubling of femtosecond pulses in periodically and aperiodically poled KTP crystals is demonstrated by employing the quasiphasematched frequency conversion technique. Conversion efficiencies as high as 60% were achieved using either aperiodically or periodically poled KTP nonlinear crystals in an extracavity, single-pass configuration using a diode-pumped femtosecond Yb:KYW laser as the pump source. The temporal characteristics of the frequency-doubled pulses as a function of focusing conditions in a "thick" nonlinear crystal regime have been investigated experimentally and pulses as short as 177 fs have been generated at around 520 nm under strong focusing conditions using a KTP crystal with aperiodic poling.