Ring-down binning in FSR hopping mode
For cavity enhanced optical spectroscopy, the cavity modes are used as a frequency reference. Data analysis methods are employed that assume the data points are at equally spaced frequencies. Parameters of interest such as line width, integrated absorption etc. can be determined from such data without knowledge of the frequencies of any of the data points. Methods for determining the FSR index of each ring-down event are also provided.
1. A method for performing cavity enhanced optical spectroscopy, the method comprising:
providing an optical resonator that defines a first set of cavity modes having a free spectral range (FSR) and which is configured to include a sample for analysis, wherein the optical resonator is configured to passively provide relative frequency stability of the first set of cavity modes of 10% or less of the FSR in a time period of about 1 second;
providing an optical source configured to deliver optical radiation to the optical resonator;
providing a detector configured to receive an absorbance signal from the optical resonator that is responsive to optical absorption in the sample;
collecting spectrograms from the sample by sweeping a frequency of the optical source through two or more frequencies of the first set of cavity modes and recording the absorbance signal;
computing an analysis output from the spectrogram by assuming that data points in the spectrogram are evenly spaced in frequency; and
providing an FSR calibration between one or more optical source parameters and an FSR index.
2. The method of claim 1 , further comprising relating the FSR index to a frequency reference, whereby absolute frequency information for the spectrograms is provided.
3. The method of claim 1 , further comprising updating the FSR calibration over time.
4. The method of claim 3 , wherein the one or more optical source parameters include a parameter which varies with optical wavelength.
5. The method of claim 4 , wherein the optical source is a semiconductor laser and wherein the one or more optical source parameters include laser current.
6. The method of claim 5 , wherein the providing an FSR calibration comprises fitting a discrete polynomial model to measured laser current.
7. The method of claim 6 , wherein the updating the FSR calibration comprises updating coefficients of the discrete polynomial model as data is gathered during operation.
8. The method of claim 1 , wherein the optical source is configured to provide relative frequency stability of the optical radiation of 10% or less of the FSR in a time period of about 1 second.
9. The method of claim 1 , wherein the optical source is configured to provide relative frequency precision of the optical radiation of 10% or less of the FSR in a time period of about 1 second.
10. The method of claim 1 , further comprising providing frequency control of the optical source to facilitate tuning the frequency of the optical source to align with two or more of the first set of cavity modes during the collecting spectrogram.
11. The method of claim 10 , wherein the optical source is a semiconductor laser and wherein the frequency control of the optical source comprises both current control and temperature control.
12. The method of claim 1 , wherein the method for performing cavity enhanced optical spectroscopy comprises a method selected from the group consisting of: cavity ring-down spectroscopy, cavity enhanced absorption spectroscopy, and integrated cavity output spectroscopy.
13. The method of claim 1 , wherein the computing an analysis output comprises determining an integrated absorption by fitting a spectral line shape model to the spectrogram.
14. The method of claim 1 , wherein the providing the FSR calibration comprises use of a wavelength monitor to determine absolute wavelength or frequency of one or more of the cavity modes.