Spectral linewidth and FM response for a four-section DBR laser

Conclusion Theoretical expressions were presented for the spectral linewidth, effective linewidth enhancement factor and FM response of a DBR laser with two active and two passive sections. It was shown that a nearly flat red-shifted FM response over several gigahertz can be obtained by nonuniform current injection and modulation of the lower-carrier-density section in the active region. However, a high FM efficiency is also accompanied by a relatively large spectral linewidth. Wavelength tuning by using the injection currents through the passive sections enhances the red-shifted FM efficiency but also broadens the spectral linewidth. In practice, linewidth broadening could be suppressed by using a large-bandgap material as the passive waveguide in order to reduce the optical confinement factor and, consequently, the tuning efficiency.