An optical parametric oscillator (3) is configured to provide first ultrashort output pulses comprising light of a first wavelength and second ultrashort output pulses comprising light of a second wavelength, wherein at least one of the first and second wavelengths is independently controllable. The optical parametric oscillator (3) comprises an optical resonator (10) containing first and second nonlinear optical elements (11, 12) and comprising an output coupler (13), an input system (14) configured to provide first pump pulses to the first nonlinear optical element (11) and second pump pulses to the second nonlinear optical element (12), and a control system (15).