Optical properties of a gain-switched laser with a sinusoidal modulation in low-coherence interferometer

The output waveform and the lasing optical spectrum of a sinusoidally modulated gain-switched Fabry-Perot diode laser (LD) is experimentally observed and investigated. Under a weak modulation, the LD maintains single-mode oscillation, with the linewidth of a single mode increasing due to carrier density fluctuation, which is independent on modulation frequency. In the vicinity of the relaxation oscillation frequency with a strong modulation, the LD outputs a regular pulse with duration of hundreds of picosecond and emits multimode spectrum with spectral half-width at half-maximum (FWHM) of approximately 2 nm, where the mode competition is suppressed and the gain is shared. As the modulation frequency further increases to twice the relaxation oscillation frequency, the laser output pulses exhibit a period doubling of the modulation frequency. The LD output pulse with spectral FWHM of 2 nm is introduced into a Michelson-type interferometer, the measured coherence length is around 184 μm with a moderate average power of 11 mW. Such a short coherence laser is suitable for application in low-coherence interferometer.