Mode-locked ring laser with output pulse width of 0.4 ps

The output pulse width of a mode-locked ring laser composed of an erbium-doped fiber amplifier, Mach-Zehnder optical modulator, and optical band-pass filter depends largely on the repetition frequency and the wavelength characteristics of these optical circuit elements. In previous experiments, the output pulse width was in the order of 5 ps at a repetition frequency of 5 GHz. The principal reason was that the narrow passage band of the optical circuit elements made it extremely difficult to generate an ultra-short optical pulse. Consequently, we examined how to narrow the optical pulse width by flattening the wavelength characteristics of these optical circuit elements. Furthermore, we drove the optical modulator in the cavity using a frequency multiplier to operate at an effectively higher frequency By widening the wavelength passage band of all the devices in the optical circuit, we achieved an output pulse width of 0.4 ps at a repetition frequency of 5 GHz; the pulse peak power was more than +23 dBm, and the time-bandwidth product was 0.34. We successfully tested an ultra-short optical pulse source with an output pulse width of 0.4 ps with no external pulse compression using a mode-locked ring laser