  Within a synchronously pumped optical parametric oscillator (SPOPO), the
inherent synchronism between the pump and the resonating signal is the magic to
partly transfer the coherence property of the pump to the signal. In our
demonstration, Vernier effect is observed within a femtosecond SPOPO by simply
detuning the FSR of the cavity, generating signal pulses at tunable repetition
rate from several GHz to 1 THz with a maximum 22.58 nm full width half maximum
(FWHM) bandwidth supporting 160 fs pulses covering the C- and L-bands of the
telecom wavelength region. This technique offers a simple method of active
filtering of dense frequency comb lines instead of using Fabry-P\'erot (FP)
cavities with complex locking system for astro-comb generation. Beside, as a
promising source for frequency combs with tunable and large comb-spacing, it
offers potential opportunities for applications such as high speed coherent
data transmission, line-by-line pulse shaping, optical clocks and precision
metrology.
