Measuring the quantum-limited linewidth of a laser by using the Zeeman effect

We explore Zeeman operation of a laser as a way to measure its quantum phase noise. We measure the differential phase diffusion of a σ⁺ and a σ^- mode; the degeneracy of these modes is lifted by a longitudinal magnetic field. Experiments are performed on a high-gain HeXe gas laser, comparing quantum linewidths measured using this technique with measurements on the same laser using the well-established self-heterodyne technique. The two methods are found to be equivalent when the magnetic field used in the Zeeman technique is sufficiently large. The advantages of the Zeeman technique as compared to the self-heterodyne technique are its extreme simplicity and very modest laser-power requirement