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.