Abstract

Modal noise^1,2,3 problems arise when lasers are used in conventional optical systems. To fully understand its nature, interaction between the various system components must be considered. Present generation optical systems convey information by intensity modulation of the source and it is generally assumed that the received signal is an attenuated replica of the transmitted signal. However, the use of a coherent source with a multipath transmission medium gives rise to multipath interference. In multimode fibre, this coherent interference between modes results in speckle patterns which can be seen in both the near and far field of the fibre. Generally the number of speckles in these patterns is proportional to the number of modes in which power is propagating. Phase changes between modes make the speckles move, i.e. change the spatial distribution of the power within the optical guide.

© 1982 Optical Society of America