SPATIAL CORRELATION EFFECTS IN MULTI-TRANSVERSE MODE LASERS

摘要

It has been shown by Wolf that spatial coherence properties of a source affect the spectrum of emitted radiation. We have established that spatial coherence in a laser affects its output spectrum.rnWe find that the pump parameter of the laser depends on the spatial coherence which arises due to transverse mode structure of the laser field. As the spatial correlation of the laser field decreases, the pump parameter is found to increase. This results in increase of mean light intensity and decrease of mean-squared intensity fluctuations with the reduction of spatial correlation in a laser oscillating above threshold. In the other extreme, when the laser is operating near and below threshold, the output intensity is non-zero and its probability distribution is nearly exponential in intensity. In this case, we find that the probability of getting zero output increases with the decrease in spatial coherence and hence the mean intensity of the output light decreases. The intensity decreases with the increase in transverse modes as some spatial modes will not be supported by the laser cavity. The correlation function of intensity fluctuations also gets modified showing slower decay of the fluctuations present. The Fourier transform of the correlation function shows that the laser linewidth decreases with the reduction in spatial coherence. Thus the laser spectrum depends on the transverse-mode structure of the field. Similar effects are found in a two-mode laser where the pump parameter is shown to depend on the spatial correlation of the laser field.