Apparatus for fluorescence subtracted Raman spectroscopy
View Patent ↗A fluorescence subtracted Raman spectroscopy apparatus uses a wavelength modulated tunable filter as the spectrometer. The wavelength modulation results in an amplitude modulation on the detected optical signal. By using a frequency selective detection technique, the Raman signal is amplified and extracted from the fluorescence background.
1. A method for extracting a Raman signal from a fluorescence background in a Raman/fluorescence emission from a material, the method comprising:
(a) exciting the Raman/fluorescence emission from the material by a laser light source;
(b) measuring a Raman/fluorescence spectrum of the Raman/fluorescence emission using a spectrometer, wherein the spectrometer comprises a wavelength tunable optical filter for selecting a wavelength component of the Raman/fluorescence spectrum and a photo detector for measuring the Raman/fluorescence spectrum in the wavelength component selected by the wavelength tunable optical filter;
(c) modulating the wavelength component selected by the tunable filter in such a way that a modulated optical signal with a modulation amplitude representing a derivative of the Raman/fluorescence spectrum at a central wavelength of the tunable filter is detected by the photo detector;
(d) scanning the central wavelength of the tunable filter and recording the corresponding modulation amplitude for each wavelength to obtain a difference Raman spectrum; and
(e) extracting and amplifying the Raman signal in accordance with the difference Raman spectrum.
2. The method of claim 1 , wherein step (e) comprises reconstructing the original Raman spectrum from the difference Raman spectrum using curve fitting methods.
3. The method of claim 1 , wherein step (c) comprises switching the wavelength of the tunable filter between two closely spaced values at a fixed frequency.
4. The method of claim 1 , wherein the modulated optical signal is detected in step (c) using a frequency selective amplification/detection technique.
5. The method of claim 4 , wherein the frequency selective amplification/detection technique comprises a lock-in amplification technique.
6. The method of claim 1 , wherein step (d) comprises using phase information and amplitude information of the modulated optical signal for difference Raman spectrum construction.
7. The method of claim 1 , wherein the wavelength tunable optical filter is an acousto-optic tunable filter (AOTF).
8. The method of claim 1 , further comprising using the Raman signal for resonant Raman spectroscopy.
9. The method of claim 1 , further comprising using the Raman signal for Raman spectral imaging.
10. A fluorescence subtracted Raman spectroscopy apparatus for extracting a Raman signal from a fluorescence background in a Raman/fluorescence emission from a material, the spectroscopy apparatus comprising:
(a) a laser for exciting the Raman/fluorescence emission from the material;
(b) a wavelength tunable optical filter for selecting a wavelength component from the Raman/fluorescence emission;
(c) a wavelength modulation device for modulating the wavelength component of the tunable filter and producing an amplitude modulated optical signal at an output of the tunable filter; and
(d) a photo detector for detecting the amplitude modulated optical signal and for producing an output representing the fluorescence subtracted Raman signal.
11. The apparatus of claim 10 , further comprising a frequency selective amplification/detection device for difference Raman spectrum extraction.
12. The apparatus of claim 11 , wherein the frequency selective amplification/detection device comprises a lock-in amplifier.