A novel broadly tunable optical parametric oscillator is described for use in numerous applications including multi-photon microscopy. The optical parametric oscillator includes at least one sub-picosecond laser pump source configured to output a pump signal having a wavelength of about 650 nm or less and at least one type II optical parametric oscillator in optical communication with the pump source and configured to generate a single widely tunable pulsed optical signal. In one application, an optical system is in optical communication with the optical parametric oscillator and configured to direct at least a portion of the optical signal to a specimen, and at least one analyzing device is configured to receive a signal from the specimen in response to the optical signal.