Optical subtraction of molecular dispersion signals enabled by differential optical dispersion spectroscopy
An apparatus and method for differential optical dispersion using a first sample and a second sample are disclosed. The apparatus includes a single frequency chirped laser source configured to generate a single frequency chirped laser beam. A first beam splitter is configured to split the single frequency chirped laser beam into first and second optical branches, the first sample being located in the first optical branch, the second sample being located in the second optical branch. A frequency shifter is located in the second optical branch, downstream of the second sample. A second beam splitter is configured to combine the first and second optical branches and generate a chirp-modulated mixed light beam. A square law detector is configured to detect the chirp-modulated mixed light beam and generate a heterodyne beatnote signal. A demodulator is configured for detection of the heterodyne beatnote signal to generate a transmission/differential optical dispersion spectrum.
1. An apparatus for differential optical dispersion using a first sample and a second sample, the apparatus comprising:
a single frequency chirped laser source configured to generate a single frequency chirped laser beam;
a first beam splitter configured to split the single frequency chirped laser beam into first and second optical branches, the first sample being located in the first optical branch, the second sample being located in the second optical branch;
a frequency shifter located in the second optical branch, downstream of the second sample;
a second beam splitter configured to combine the first and second optical branches and generate a chirp-modulated mixed light beam;
a square law detector configured to detect the chirp-modulated mixed light beam and generate a heterodyne beatnote signal; and
a demodulator configured for demodulation of the heterodyne beatnote signal to generate a differential optical dispersion spectrum.
2. The apparatus of claim 1 further comprising a signal processer configured to perform spectral fitting.
3. The apparatus of claim 1 wherein the demodulator is configured for detection of the heterodyne beatnote signal to generate a transmission spectrum.
4. The apparatus of claim 1 wherein the first and second sample have optical transmission spectra and the demodulator is configured as an amplitude demodulator to generate an output having a magnitude that is proportional to the transmission spectrum which is affected by the sum of the optical absorptions from first and second samples.
5. The apparatus of claim 1 wherein the first and second sample have optical dispersion spectra and the demodulator is configured as a frequency demodulator and the measured differential optical dispersion spectrum have a magnitude that is proportional to the difference between the optical dispersion spectra of the first and second samples.
6. The apparatus of claim 1 wherein the first sample is a reference sample having a known concentration of a gas and the second sample has an unknown concentration of the gas.
7. The apparatus of claim 2 wherein the signal processor is further configured to analyze the measured transmission spectrum containing information about the sum of sample concentrations and differential optical dispersion spectrum containing information about the difference of sample concentrations.
8. The apparatus of claim 2 wherein the signal processor is further configured to analyze the measured transmission and dispersion spectra to retrieve sample parameters other than concentration that also affect the measured spectra.
9. The apparatus of claim 7 wherein the sample parameters include at least one of temperature, pressure and optical path length.
10. The apparatus of claim 1 wherein the frequency shifter is at least one of an acousto-optical modulator and an electro-optical modulator.
11. The apparatus of claim 1 further comprising:
a third beam splitter configured to split the single frequency chirped laser beam into a third optical branch;
a third sample located in the third optical branch; and
a second frequency shifter located in the third optical branch, downstream of the third sample.