We demonstrate the generation of femtosecond pulses in a synchronously pumped optical parametric oscillator (SPOPO) at the harmonics of pump repetition rate using a SPOPO cavity longer than the fundamental synchronous length. The SPOPO is based on a $1\mathrm{mm}$ crystal of periodically poled $\mathrm{Li}\mathrm{Nb}{\mathrm{O}}_3$ and pumped by a Kerr-lens mode-locked Ti:sapphire laser at $76\mathrm{MHz}$ . By increasing the SPOPO synchronous cavity length we have generated output signal pulses at successive harmonics of the pump repetition rate up to the 13th harmonic at $988\mathrm{MHz}$ , where average signal powers of $30\mathrm{mW}$ are still available for $1.45\mathrm{W}$ of pump power. The generated signal pulses at $988\mathrm{MHz}$ are near transform limited with average durations of $227\mathrm{fs}$ and a time-bandwidth product of 0.41 for $185\mathrm{fs}$ input pump pulses.