초록 ▼

A spectroscopic technique in which the sample under investigation is probed with a generally monochromatic beam of light which has been modulated at two distinct modulation frequencies. The double modulation produces a plurality of sidebands, and the two modulation frequencies are related to one ano

A spectroscopic technique in which the sample under investigation is probed with a generally monochromatic beam of light which has been modulated at two distinct modulation frequencies. The double modulation produces a plurality of sidebands, and the two modulation frequencies are related to one another such that a selection of sidebands falls into two groups of closely spaced component sidebands. A first group is disposed in frequency at the spectral feature of interest and serves to probe the spectral feature. The other group is disposed in frequency remote from the feature and serves as a reference group. Within each group the component sidebands are offset from one another by a characteristic offset frequency, which can be considerably less than the width of the spectral feature under investigation. After interaction with the sample the doubly modulated beam is passed on to a photodetector, which in combination with appropriate signal processing apparatus detects a signal at the characteristic offset frequency representative of the spectral feature.

대표청구항 ▼

Apparatus for detecting a spectral feature of a sample, said feature having a characteristic width, comprising: a source of a generally monochromatic beam of light having a characteristic linewidth at most comparable with the width of said spectral feature; modulation means for modulating said beam

Apparatus for detecting a spectral feature of a sample, said feature having a characteristic width, comprising: a source of a generally monochromatic beam of light having a characteristic linewidth at most comparable with the width of said spectral feature; modulation means for modulating said beam at first and second modulation frequencies related to one another so as to provide a plurality of sidebands including a first, probe group and a second, reference group of component sidebands offset from one another within each group by a characteristic offset frequency at most comparable with the width of said feature, said probe and reference groups being disposed in frequency at said spectral feature and remote from said spectral feature, respectively; a photodetector disposed to receive said beam after interaction with said sample, said photodetector providing an output signal representative of the received beam; and detection means connected to receive said output signal and detect therein a component signal at said characteristic offset frequency representative of said spectral feature.