A method of monitoring pressure of a gas species up to at most a
predetermined maximum pressure value is disclosed. The method includes
exposing the gas species to transmission of laser light, periodically
modulating the wavelength of the laser light over a wavelength band
including at least one absorption line of the gas species,
optoelectrically converting the transmitted laser light, thereby
generating an electric output signal, performing at least one of first
filtering the electric output signal with a filter characteristic having
a lower cut-off frequency not lower than a transition frequency and of
second filtering the electric output signal with a bandpass filter
characteristic having an upper cut-off frequency not higher than the
transition frequency and a lower cut-off frequency above the modulation
frequency of the periodic wavelength modulation. The output of at least
one of the filterings is evaluated as a pressure indicative signal.