Wavelength-modulation spectroscopy method and apparatus
In one embodiment of the spectroscopy method, the method comprises the steps of modulating the wavelength of a monochromatic radiation at a modulation amplitude and a modulation frequency; determining a first variable representative of an absorbance of an analyte in a sample; and demodulating by phase-sensitive detection the first variable at a harmonic of the modulation frequency to produce a harmonic spectrum of the analyte. In one embodiment of the spectroscopy apparatus, the apparatus comprises a laser diode integrated with a first photodetector configured to detect an intensity of a backward emission from the laser diode and act as a reference detector; a second photodetector configured to detect an intensity of laser radiation exiting a sample; and electronic circuitry coupled to the laser diode and the photodetectors, configured to acquire and process spectra of the sample. In another embodiment, the spectroscopy apparatus comprises a beam splitter configured to split the laser radiation into a first radiation portion and a second radiation portion and a first photodetector configured to detect the intensity of the first radiation portion.
1. A spectroscopy method comprising the steps of:
modulating the wavelength of a monochromatic radiation at a modulation amplitude and a modulation frequency;
determining a first variable representative of an absorbance of an analyte in a sample; and
demodulating by phase-sensitive detection said first variable at a harmonic of said modulation frequency to produce a harmonic spectrum of said analyte.
2. The method of claim 1 , wherein said monochromatic radiation is provided by a laser.
3. The method of claim 1 , wherein said first variable is provided by an absorbance of said analyte in said sample and is based on the intensity of said monochromatic radiation exiting said sample and the intensity of said monochromatic radiation entering said sample.
4. The method of claim 1 , wherein said first variable is provided by an absorbance of said analyte in said sample with a DC offset and is based on the intensity of said monochromatic radiation exiting said sample and a second variable that is proportional to the intensity of said monochromatic radiation entering said sample.
5. The method of claim 4 , wherein said second variable is determined simultaneously with said intensity of said monochromatic radiation exiting said sample.
6. The method of claim 4 , wherein said second variable is determined before said intensity of said monochromatic radiation exiting said sample is determined.
7. The method of claim 1 , wherein said harmonic is a second harmonic of said modulation frequency.
8. The method of claim 1 , wherein a magnitude of said harmonic spectrum is linearly related to a concentration of said analyte in said sample.
9. A spectroscopy apparatus comprising:
a laser diode integrated with a first photodetector, said first photodetector configured to detect an intensity of a backward emission from said laser diode and act as a reference detector;
a second photodetector configured to detect an intensity of laser radiation exiting a sample; and
electronic circuitry coupled to said laser diode and said first and second photodetectors, said electronic circuitry configured to acquire and process spectra of said sample.
10. The spectroscopy apparatus of claim 9 , wherein said laser diode is further integrated with a thermoelectric cooler and a temperature sensor.
11. The spectroscopy apparatus of claim 9 , wherein said apparatus is configured to perform at least one of:
direct absorption spectroscopy measurement and harmonic spectroscopy measurement.