Patent ID: 7116422

Claim:
A wavelength modulation spectroscopy method, comprising: periodically sweeping the wavelength of a light source over an interaction feature of a sample according to a sweep function; modulating the wavelength of the light source with a frequency (f0), while the wavelength is swept over the interaction feature; modulating the wavelength of the light source at a wavelength outside the interaction feature with a burst signal with a higher harmonic (Nf0) of said frequency (f0); passing the light of the light source to the sample for interacting and thereafter to a measuring detector; demodulating the measuring detector output at said higher harmonic (Nf0) of said frequency (f0); passing a portion of the light of the light source to a monitor detector; demodulating the monitor detector output at said higher harmonic (Nf0) of said frequency (f0); and calibrating the demodulated measuring detector output (S(ν)N,Meas) against the demodulated monitor detector output (S(ν)N,Mon) by the burst signal appearing in both demodulated detector outputs (S(ν)N,Meas, S(ν)N,Mon), respectively.