Abstract

In the last few years the study of excess noise in non-Hermitian lasers has received a continuous and increasing interest both theoretically and experimentally [1]. In particular, enhancement of the spontaneous emission noise due to the nonorthogonality of transverse laser modes has been widely investigated in lasers with gain guiding, in unstable resonator lasers with sharp-edge or variable reflectivity mirrors, and in geometrically-stable lasers with internal apertures No attention has been payed yet, however, to the study of excess noise in misaligned optical cavities In this work it is shown in a very general framework that misalignment of optical laser cavities with transversely-varying gain or loss may lead to a substantial enhancement of the transverse excess noise We consider a general linear optical resonator with one or two tilted end mirrors containing aligned paraxial optical elements and transversely-varying gain or loss elements that can be described by a complex-valued one-way ABCD matrix. General expressions for the transverse modes, in terms of complex-valued off-axis Hermite-Gaussian modes, and corresponding eigenvalues are analytically derived by solving the Kirchhoff-Fresnel round-trip integral equation including mirror misalignments The orthogonality properties of transverse modes are lost in presence of transversely-varying gain or loss elements, and the excess noise factor K₀ for the fundamental mode is shown to be factorized, as K₀=K_alignedK_tilt, where K_aligned is the excess noise factor for the aligned cavity, and K_tilt, >1 is an enhancement factor due to mirror tilting Analytical expressions for the excess noise factors of higher-order modes will be also presented As an example, Fig.1(a) shows the behavior of the excess noise factor K₀ in case of a linear cavity with a real-valued ray matrix and a Gaussian variable reflectivity output mirror, previously studied in Ref.[2], as a function of the magnification M for an aligned cavity (dashed curve) and in presence of a small angle tilting of the other end mirror (continuous curve); Fig.1(b) shows the corresponding power losses. Notice that close to the plane-plane instability boundary M=I a large enhancement of excess noise due to mirror tilting is predicted. The physical origin of such a large increase of excess noise is traced to transverse beam walk-off induced by mirror tilting which makes it possible transient amplification of noise in the transverse plane during the convective motion This transient amplification of noise is supported by an excess gain localized close to the optical axis of the cavity.

© 2000 IEEE