Optics Group -

Physics and Mathematical Physics, Univ. of Adelaide

Noise in Nonlinear Optics

People: M Hamilton, S-Y Lee

There are two areas of interest here - one is the study of the effect of laser noise on various fundamental nonlinear optical processes that occur when intense laser light interacts with atoms. There are two experiments: one looks at the formation of coherence between atomic states through optical pumping, and how that is affected by laser noise, and the other seeks to observe a new type of second order interference fringe that is predicted in two photon absorption with a noisy laser. -

The details of interactions between a non-linear system and its environment (heat bath) is a subject that can be conveniently studied with non-linear optical processes. This problem is approached here by artificially imposing noise on a laser beam to simulate the stochastic interaction of the optical system (laser plus absorber) with some environment. Two relatively simple cases in which this interaction can be nonlinear are if it is a multiphoton process or if the absorption is saturated. Interesting and often counter-intuitive phenomena can arise when the dependence of the system on the statistical properties of the noise is studied.

For example, if one were to measure the width of the two-photon absorption spectrum as a function of the width of the laser spectrum, one's initial reaction would be to say that the former depends linearly on the latter. As has now been shown, (ref) this is only true for certain sorts of noise - for others the absorption spectrum is independent of the laser spectrum, even if the spectra of the lasers with different noises have the same shape. This is because the two-photon absorption spectrum depends on a fourth order correlation function of the field, whereas the laser spectrum depends on a second order correlation.

The approach that we are currently taking is to look at the formation of atomic coherences by optical pumping and the effect that laser noise has on this. Such coherence occurs in the so-called dark-line effect (ref to be added!) where a superposition is formed between two atomic states - the amplitudes for absorption to a third state from the first two destructively interfere and so render the medium transparent.

The atomic system that we have chosen is the Samarium atom. This seems a weird and wonderful choice, but we are surprisingly restricted when we want an atom which has a resonance transition (i.e. from the ground state) that can be excited with visible light from a laser diode or dye laser. We need a transition from the ground state because we want to work with an atomic beam so that the random influences that we might get from collisions, were we to work with a discharge, are absent. We need to use a diode or dye since these can be tuned to the transitions of interest and their inherent noise can be made sufficiently low that the important laser noise is that which we add ourselves.

A further restriction is that we would like to restrict the number of states involved in the transition and this means choosing atomic levels with low angular momentum which have fewer magnetic sublevels. Rather than setting up coherences between two levels that have their own degeneracies, we would prefer to set up the coherences between magnetic sublevels of one level - a much simpler situation when it comes to describing the setup theoretically. As it happens Samarium fits the bill nicely.

One item of technology that we are using and developing is external cavity diode lasers (ECDL's). By putting a grating and a mirror so as to feed laser light back into the diode one can reduce the laser's inherent noise and greatly improve its tuning characteristics. We are working with a couple of configurations of such lasers as well as using a commercially available one.

From a more practical point of view, these rather fundamental studies are important in understanding the limits imposed by noise (and in some cases, the opportunities opened by noise) on the applications of non-linear optical techniques in frequency conversion, spectroscopy,laser cooling etc.

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queries and comments to mwh@physics.adelaide.edu.au