Role of noise in the diode-laser spectroscopy of the spectral line profile

Questions concerning precise measurements of the spectral-line-profile para meters by diode-laser spectroscopic methods were examined. The instrumental function of a distributed-feedback diode laser (lambda = 1.53 mu m), consi sting of the additive contributions of the noise due to spontaneous emissio n, frequency fluctuations, and intensity fluctuations, was investigated. An analytical formula was obtained for the spectrum of the diode-laser field formed by frequency fluctuations. The spectral density go of the frequency fluctuations, determining the width of the central part of the emission lin e profile of a diode laser, was found by two independent methods (by fittin g to a Doppler-broadened absorption line profile and by finding the intensi ty of the residual radiation and the saturated-absorption line width). The parameters Omega and Gamma of the spectral density of the frequency fluctua tions, coupled to the relaxation oscillations and determining the wing of t he diode-laser emission line profile, were determined experimentally. By ta king into account the instrumental function of the diode laser, involving s uccessive convolution with the recorded emission spectra, it was possible t o reproduce correctly the spectral line profile and to solve accurately the problem of the 'optical zero'. The role of the correlation between the int ensity noise and the diode-laser frequency was considered.