Linewidth of monolithic semiconductor ring lasers

In this work we numerically calculate the linewidth of a single-mode monolithic Semiconductor Ring Laser (SRL) operating in the unidirectional regime. A new expression for the SRL linewidth is derived from the conventional Henry's formula, and the importance of different physical parameters is discussed. In particular, the linewidth is mainly determined by the SRL diameter, because the waveguide bending loss have a very strong dependence on the diameter. We show that circular SRLs with diameter smaller than 180 μm are unlikely to be operated CW. As a general rule, the linewidth decreases for increasing SRL diameter, and it varies from 15 MHz for 200 μm diameter down to 1 MHz for 2000 μm diameter.