ANALYSIS OF THE SPECTRAL LINEWIDTH OF 3-CAVITY SEMICONDUCTOR-LASER DIODES

The spectral linewidth for a semiconductor laser diode coupled to two external cavities (known as a three-cavity laser diode) is studied in the article. A closed-form expression for the linewidth of this laser is derived by analyzing the number of photons in the laser cavity. It is found that, because of the optical feedback provided by the external cavities, rite photon lifetime becomes longer than that of a solitary Fabry-Perot (FP) laser, hence reducing the value of the spectral linewidth. Our theoretical investigations reveal that the linewidth of a three-cavity laser can be reduced further by using external mirrors with high reflectivities and using anti-reflection (AR) coatings on the laser diode facets. We have also studied the effects of uncertainties in the linewidth enhancement factor alpha due to optical feedback and found that such uncertainties have negligible effects on the validity of our results.