A General Relation between Frequency Noise and Lineshape of Laser Light

Lasers and especially semiconductor lasers (SCLs) are playing a major role in advanced technological and scientific tasks ranging from sensing, fundamental investigations in quantum optics and communications. The demand for ever-increasing accuracy and communication rates has driven these applications to employ phase modulation and coherent detection. The main laser attribute that comes into play is its coherence which is usually quantified by either the Schawlow-Townes (S-T) linewidth, the spectral width of the laser field, or the power spectral density (PSD) function of the laser frequency fluctuation. In this paper, we present a derivation of a general and direct relationship between these two coherence measures. We refer to the result as the Central Relation. The relation applies independently of the physical origin of the noise. Experiments are described which demonstrate the validity of the Central Relation and at the same time suggest new methods of controlling frequency noise at base band by optical filtering.