Patent ID: 7222537

Claim:
A method of monitoring pressure of a gas species up to at most a predetermined maximum pressure value comprising exposing said gas species to transmission of laser light; periodically modulating the wavelength of said laser light over a wavelength band including at least one absorption line of said gas species; optoelectrically converting said transmitted laser light, thereby generating an electric output signal; performing at least one of first filtering said electric output signal with a filter characteristic having a lower cut-off frequency not lower than a transition frequency and of second filtering said electric output signal with a band pass filter characteristic having an upper cut-off frequency not higher than said transition frequency and a lower cut-off frequency above modulation frequency of said periodic wavelength modulation, thereby determining said transition frequency in the spectrum of said electric output signal there, where the caustic function of the pressure-dependent spectrum envelopes of said electric signal touches the envelope of said spectrum at said maximum pressure, and evaluating the output of at least one of said filterings as a pressure indicative signal.