Design and Simulation of a Directly Frequency-Modulated DFB Laser

In this work we propose a simple approach to realize the direct frequency modulation of a semiconductor distributed feedback (DFB) laser. The directly frequency-modulated DFB laser (DFL) is biased by a direct current (DC) on a normal electrode along the laser cavity to define the output power. The signal current is applied on an extra short electrode placed in the center section of the DFB laser, isolated from the DC electrode. The varying of the material refractive index in the center section with the signal current introduces an effective grating phase change, thereby alters the lasing frequency. Small-signal simulation shows that a frequency modulation efficiency of 26GHz/mA from 0 to 100GHz and 17GHz/mA from 100GHZ to 200GHz can be achieved, respectively. Large-signal simulation of the waveform and eye-diagram of a frequency-shift keying (FSK) signal generated by the DFL is also performed by converting it to an intensity modulated signal through an optical slope filter.