Generation of widely tunable (65-nm) dual-wavelength picosecond pulses by harmonic and rational-harmonic mode locking of a Fabry–Perot grating-lens external-cavity laser

Abstract

A scheme to generate tunable dual-wavelength picosecond pulses by harmonic and rational-harmonic mode locking of a 1.5-µm Fabry–Perot grating-lens external-cavity laser has been proposed and experimentally demonstrated. The external cavity consists of a weak Fabry–Perot filter, a diffraction grating, a converging lens, and a totally reflecting mirror. Active mode locking has been achieved simultaneously at two different wavelengths in the semiconductor laser over a range of 65 nm. Dual-wavelength pulses with a spectral separation of 0.9 nm, a side-mode suppression ratio greater than 24 dB, and a pulse width of ∼30 ps have been generated. The relative intensity variation between the two lasing wavelengths is smaller than 1.2 dB, without any observable mode hopping. Frequency multiplication of the dual-wavelength pulse train with rational-harmonic mode locking has also been demonstrated. With a 2.5-GHz modulation frequency applied to the semiconductor laser, the pulse repetition rate was multiplied to 5.0 GHz. A total of 10.0 GHz (5.0 GHz×two wavelength channels) optical pulses was obtained. To the authors’ knowledge, this is the first report of multiple-wavelength rational-harmonic mode locking of a semiconductor laser.