Method and apparatus for double modulation spectroscopy
First Claim
1. 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.
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Abstract
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.
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Citations
63 Claims
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1. Apparatus for detecting a spectral feature of a sample, said feature having a characteristic width, comprising:
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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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
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13. Apparatus for detecting a spectral feature of a sample, said feature having a characteristic width, comprising:
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a source of a generally monochromatic beam of light having a characteristic linecenter and a characteristic linewidth at most comparable with the width of said spectral feature; first and second frequency modulators for frequency-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 above and below said linecenter at said spectral feature and remote from said spectral feature, respectively; means for exposing said sample to the modulated beam; a photodetector disposed so as 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 offset frequency representative of said spectral feature. - View Dependent Claims (14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)
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25. Apparatus for detecting a spectral feature of a sample, said feature having a characteristic width, comprising:
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a source of a generally monochromatic beam of light having a characteristic linecenter and a characteristic linewidth at most comparable with the width of said spectral feature; a first frequency modulator for frequency-modulating said beam at a first modulation frequency; an amplitude modulator for amplitude-modulating said beam at a second modulation frequency related to said first modulation frequency 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 above and below said linecenter at said spectral feature and remote from said spectral feature, respectively; means for exposing said sample to the modulated beam; a photodetector disposed so as 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 offset frequency representative of said spectral feature. - View Dependent Claims (26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38)
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39. Apparatus for detecting a spectral feature of sample, said feature having a characteristic width, comprising:
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a source of a generally monochromatic beam of light having a characteristic linewidth at most comparable with the width of said spectral feature and having a characteristic linecenter removed from said spectral feature by a frequency displacement greater than said characteristic spectral feature width, said beam being disposed so as to expose said sample thereto; a first oscillator for generating a first modulation frequency (ω
) approximately equal to said frequency displacement;a second oscillator for generating a second modulation frequency (2ω
±
σ
) differing from twice said first modulation frequency by an offset frequency (σ
) small in comparison with said characteristic spectral feature width;means coupled to said first and second oscillators for modulating said beam at said first and second modulation frequencies; a photodetector disposed so as to receive said beam after interaction with said sample, said photodetector providing an output signal representative of the received beam; means receiving said first and second modulation frequencies and deriving therefrom a reference signal having a frequency equal to said offset frequency; and means receiving said reference signal and said photodetector output signal and deriving therefrom the component of said photodetector output signal at said offset frequency representative of said spectral feature. - View Dependent Claims (40, 41, 42, 43, 44, 45, 46, 47)
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48. Apparatus for detecting a spectral feature of sample, said feature having a characteristic width, comprising:
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a source of a generally monochromatic beam of light having a characteristic linewidth at most comparable with the width of said spectral feature and having a characteristic linecenter removed from said spectral feature by a frequency displacement greater than said characteristic spectral feature width, said beam being disposed so as to expose said sample thereto; a first oscillator for generating a first modulation frequency (ω
) approximately equal to said frequency displacement;a second oscillator for generating an offset frequency (σ
) small in comparison with said characteristic spectral feature width;frequency synthesis means receiving said first modulation frequency (ω
) and said offset frequency (σ
) and deriving therefrom a second modulation frequency (2ω
±
σ
) differing from twice said first modulation frequency by said offset frequency (σ
);means coupled to said first oscillator and to said frequency synthesis means for modulating said beam at said first and second modulation frequencies; a photodetector disposed so as to receive said beam after interaction with said sample, said photodetector providing an output signal representative of the received beam; and means receiving said offset frequency and said photodetector output signal and deriving therefrom the component of said photodetector output signal at said offset frequency representative of said spectral feature. - View Dependent Claims (49, 50, 51)
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52. An incoherent, single-beam spectrometer for use in detecting a spectral feature of a sample, said feature having a characteristic width, comprising:
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an incoherent source of a single beam of generally monochromatic 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; means for exposing said sample to the modulated beam; a photodetector disposed so as to receive said beam after interaction with said sample, said photodetector having a predetermined bandwidth less than each of said first and second modulation frequencies and greater than said offset frequency and 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 offset frequency representative of said spectral feature. - View Dependent Claims (53, 54, 55, 56, 57, 58, 59, 60)
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61. A method of detecting a spectral feature of a sample, said spectral feature having a characteristic width, comprising the steps of:
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providing a generally monochromatic beam of light having a characteristic linewidth at most comparable with the width of said spectral feature; 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; directing said beam at said sample so as to interact therewith; photodetecting said beam after it has interacted with said sample to detect therein a signal at said characteristic offset frequency representative of said spectral feature.
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62. A method of measuring the absorption spectrum of a sample having a spectral feature of characteristic width comprising the steps of:
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providing a generally monochromatic beam of incoherent light having a characteristic linewidth at most comparable with the width of said spectral feature; 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; directing said beam at said sample so as to interact therewith; photodetecting in the beam transmitted by said sample only the spectral component frequencies thereof below a predetermined cutoff frequency less than each of said first and second modulation frequencies; and detecting within the photodetected spectral component frequencies below said cutoff a component at said offset frequency representative of the absorption by said spectral feature. - View Dependent Claims (63)
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Specification