Quantum noise properties of laser diode arrays

Abstract

The quantum noise properties of laser diodes has been a topic of interest essentially since the first development of such devices. A particular recent focus for such activities has been created by work, initiated by Yamamoto and co-workers, directed at generating sub-Poissonian or squeezed light using these devices. Although there was a significant early success in experimentally demonstrating appreciable noise squeezing using laser diodes it was found that not all laser diode designs are suitable for this purpose. Subsequent work in the literature has indicated that a spectrally pure laser diode should be will be of great interest in this context. It is noted , however, that great care must also be taken to control the transverse mode structure of such lasers. Recent work has highlighted the "contamination" of the lasing mode which may arise due the noise of non-lasing modes in nominally single-mode lasers. In that work it was shown that excess noise due to non-orthogonal transverse modes - so-called Petermann excess noise - appears in the laser quantum intensity noise. In our previous work we have shown that such excess noise can be eliminated by suitable choice of laser structure: namely one with high-reflectivity facet mirrors.The identified low-noise symmetric high reflectivity ( SHR) structure had a practical limitation on the available optical output power. The aim of the present paper is to examine means for removing this limitation by the use of a one-dimensional array of SHR laser diodes. An analysis is performed of quantum noise enhancement due to asymmetric coupling between elements of a laser diode array. A general formalism is presented which facilitates the estimation of the quantum noise arising due to cross contamination between lasing modes of the array. Consideration is given to limiting cases where compact expressions are obtained for the asymmetry-induced excess noise.