Linewidths spectral purity

Narrow-linewidth emission Laser enussion that is highly coherent or of extremely high spectral purity. Single-longitudinal-mode ertussion is inherenAy narrow-linewidth. 

The output waveform of a stable, single-frequency laser far above the threshold of oscillation may be approximated by an almost perfect sine wave with nearly constant amplitude and frequency. For a laser operating in an ideal environment, the spectral purity is measured by a linewidth which is determined by frequency fluctuations caused by random walk of the oscillation phase under the Influence of spontaneous emission (quantum) noise. In their fundamental 1958 paper, Schawlow and Townes predicted that the quantum phase noise limited line profile will be a Lorentzian with a full width between the half power points (FVIHM) that may be approximated by ... 

Most X-band EPR spectra of transition metals are recorded at low temperatures (4-lOOK) using high-purity quartz tubes (no paramagnetic impurities) with a sample volume of about 300 pL. The minimum concentration of the sample depends on the broadness of its spectrum. Because EPR spectra are recorded as a first derivative (see subsequent text), the relationship between concentration, signal amplitude and spectral linewidth can be approximated as... 

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