Quantum-Limited Linewidth of a Bad-Cavity Laser

We show experimentally that the quantum-limited linewidth of a laser can be smaller than the conventional Schawlow-Townes limit when the gain bandwidth is smaller than the cavity loss rate. Data obtained for a HeNe 3.39 \ensuremath{\mu}m laser confirm the theoretical result derived for the linewidth of a homogeneously broadened laser in the bad-cavity limit. We show how this result can be understood in terms of the group refractive index.