Spatial Correlation Effects in Multi-Transverse Mode Lasers

It has been shown by Wolf [1–3] that source correlations lead to Doppler-like frequency shifts in the spectrum of light in the far zone. Most of the theoretical and experimental activity in this regard has been confined to a restricted class of sources and mainly to the frequency domain. In contrast, changes in temporal coherence properties of the field on propagation were recently described by Chopra et al. [4, 5]. In the present paper, working in the time domain, we investigate the effects of spatial correlations in a laser oscillating in one or more transverse modes. The laser has been analyzed both theoretically and experimentally [6–9] in great detail but without paying any consideration to spatial coherence as the laser was assumed to be a perfectly spatially coherent source. Such an assumption is only valid when the laser is oscillating in one transverse mode. But spatial correlations become important in lasers oscillating in more than one transverse mode. Different transverse modes have different field distributions, they excite different spatial groups of active atoms, and therefore spatial correlations between spontaneously emitting atoms become important.