A.M. Lindberg

Influence of transverse resonator symmetry on excess noise

Abstract We demonstrate that the quantum-limited linewidth of an unstable-cavity laser depends on the transverse symmetry of the resonator. This is due to the occurrence of excess noise associated with the nonorthogonality of the eigenmodes of an unstable-cavity laser. We show that square symmetry leads to considerably larger excess noise than circular symmetry, confirming recent predictions. Further we present measurements of the excess noise in lasers with 3, 4, 6 and 8-fold symmetry.

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 /spl sigma//sup +/ and a /spl sigma//sup -/ 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.

Higher order transverse modes of an unstable-cavity laser

We present measurements of the higher order transverse modes of an unstable-cavity HeXe gas laser. We have studied the one-dimensional strip-resonator modes and the two-dimensional square- and circular-resonator modes. The experimental intensity profiles showed generally good agreement with theoretical profiles based upon virtual-source diffraction theory. We also measured the quantum-limited linewidths and excess-noise factors of several of these modes.

Exploring the nature of excess quantum noise

We report here on a series of experiments aimed at a better understanding of excess quantum noise. Our experimental system is a miniature HeXe laser (L=5 cm, /spl lambda/=3.51 /spl mu/m). We start by comparing the quantum phase noise of a geometrically unstable and a geometrically stable HeXe cavity, with the same diffraction loss rate (realized by using a sufficiently small outcoupling mirror for the stable laser). Experimentally, we find K/sub unstable//spl sim/220 and K/sub stable//spl sim/5, showing that losses are not the key issue. In fact, these values are in good agreement with the mode-nonorthogonality theory. The data for K/sub stable/ are best fitted by the curve K/sub trans/K/sub long/, showing that the excess quantum noise can be factorized, as is expected within the paraxial approximation. We observed this factorizability also for K=K/sub trans/K/sub pol/, using a cavity with nonorthogonal transverse and polarization eigenmodes; nonorthogonality of the latter was realized by dissipative coupling of two circularly polarized modes, using a quasi-Brewster-plate in the HeXe cavity. We also report the appearance of the K factor in the intensity noise of an unstable HeXe laser (all reports so far dealt with phase noise). We measured intensity noise spectra, both below and above threshold. We extract K from these data using a phenomenological laser model in which one of the spontaneous emission decay channels, namely, the laser mode, has been given a K times larger weight than the other channels. K values deduced from intensity noise were found to be consistent with those derived from phase noise.

Excess Quantum Noise Due to Nonorthogonal Polarization Modes

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