You can find the DFB laser in the Transmitters Library\Optical Sources\ folder. It uses two models, the Transmission Line model being the most physically accurate.
The major difference between a classical Fabry Perot cavity laser and a DFB is the use of a Bragg grating instead of two reflective mirrors. The Bragg grating is a much more selective cavity design and therefor the linewidth of the will be smaller then a comparable FP cavity laser.