DFB laser spectra obtained by low-frequency current dither

The temporal behavior of the spectra of distributed-feedback (DFB) lasers is studied under high-bit-rate (2 Gb/s) modulation. The optical spectra are obtained by dithering the off-state laser bias current with a low-frequency (1 kHz) low-amplitude (2 mA) current. It is found that some lasers have side modes that emit only during the first ~125 ps of the pulse. During this short time interval, the intensity of the side mode emission is at least 10 dB stronger than that obtained from time averaged spectra. This technique is useful in understanding the high-speed dynamics of rapidly modulated DFB spectra