Modal noise has been observed in optical fibres excited by partially coherent sources . In this paper the theory of this noise is presented . Following the basic guidelines of the speckle theory, the statistical properties of the fluctuations of the power transmitted through a circular aperture of arbitrary size are evaluated . It is found that, for fibres having an a-exponent index-profile, the maximum attainable signal-to-noise ratio, when the receiving aperture area is equal to the core area, is given by the square root of the number of the guided modes . The hypotheses on which the theory is based are discussed and an experiment, confirming the theoretical results, is presented .
Phase noise in semiconductor lasers has been investigated by many authors in the range of low frequencies (<1 MHz). In this letter we present for the first time phase noise measurements extended up to frequencies greater than 1 GHz. Experimental results showing the power spectral density Sφ̇(ω) of the instantaneous frequency φ̇(t) and the variance σ2[Δτφ(t)] of the phase shift Δτφ(t) are presented. The peculiar behavior of Sφ̇(ω), which presents a sharp peak at the same frequency of the amplitude noise peak, can account for the excessive broadening of the linewidth of single-mode injection lasers.
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