On-chip spectroscopic sensors with optical fringe suppression
An on-chip spectroscopic sensor includes a tunable diode laser. A laser driver for drives the tunable diode laser. An analyte test cavity receives a chemical sample and exposes the received chemical sample to light from the tunable diode laser. An optical detector detects light emerging from the analyte test cavity as a result of the laser exposure. A spectral analyzer determines a spectrum of the emerging light, matches and removes one or more known optical fringe patterns from the determined spectrum, and determines a composition or concentration of the chemical sample from the optical fringe pattern-removed spectrum.
1. A method for performing spectroscopy, comprising:
using a spectroscopic sensor to acquire spectra at a plurality of different temperatures at regular temperature increments over a temperature range;
identify temperature-dependent etalon patterns at each of the regular temperature increments within the temperature range;
store the identified temperature-dependent etalon patterns at each of the regular temperature increments within a database;
measure an ambient temperature at a time of subsequent acquisition;
use the spectroscopic sensor to acquire a subsequent spectra;
retrieve, from the database, the temperature-dependent etalon patterns for the ambient temperature at the time of subsequent acquisition;
remove the temperature-dependent etalon patterns for the ambient temperature at the time of subsequent acquisition from the acquired subsequent spectra.
2. The method of claim 1 , further comprising:
determining a composition or concentration of gas sample from the temperature-dependent etalon pattern-removed subsequent spectrum.
3. The method of claim 2 , further comprising using the determined composition or concentration of the gas sample to detect fugitive methane.
4. The method of claim 2 , wherein retrieving, from the database, the temperature-dependent etalon patterns for the ambient temperature at the time of subsequent acquisition comprises:
constructing a fitting model for the temperature-dependent etalon patterns for the ambient temperature at the time of subsequent acquisition;
applying the one constructed fitting model to the acquired subsequent spectra by performing least mean square (LMS); and
evaluating a fit residual of the applied fitting model.
5. A method for detecting fugitive methane gas, comprising:
installing a spectroscopic sensor apparatus into a calibration tool;
using the spectroscopic sensor apparatus within the calibration tool to identify a plurality of different temperature-dependent etalon patterns at regular temperature increments within a predetermined temperature range;
store the plurality of temperature-dependent etalon patterns within a database;
configure the spectroscopic sensor apparatus by constructing an etalon removal model that is configured to measure an ambient temperature at a time of subsequent acquisition, use the spectroscopic sensor apparatus to acquire a subsequent spectra, retrieve, from the database, the temperature-dependent etalon patterns for the ambient temperature at the time of subsequent acquisition, adjust the retrieved temperature-dependent etalon patterns by correcting for laser frequency ramping instabilities by allowing each of the retrieved temperature-dependent etalon patterns to be stretched/contracted;
removing the configured spectroscopic sensor apparatus from the calibration tool;
installing the configured spectroscopic sensor apparatus to a final location; and
using the spectroscopic sensor apparatus installed at the final location to detect fugitive methane.
6. The method of claim 5 , wherein using the spectroscopic sensor apparatus to detect fugitive methane includes:
acquiring the subsequent spectra causing the spectroscopic sensor apparatus;
removing background from the acquired subsequent spectrum using the constructed etalon removal model; and
identifying the presence of fugitive methane by analyzing the background-removed spectrum.
7. The method of claim 5 , wherein the predetermined temperature range is from 20° C. to 30° C. and the regular temperature intervals are 1×10 −4 ° C.
8. The method of claim 5 , wherein configure the spectroscopic sensor apparatus further includes copying the database to the spectroscopic sensor apparatus.
9. The method of claim 5 , wherein the etalon removal model allows for least-mean-squares (LMS) fitting.
10. The method of claim 6 , wherein the etalon removal model is constructed using the formula:
I t ( v ( t ))= I 0 ( v ( t ))× e −σ(v(t))NL ×fringe background
where v(t) represents laser frequency as a function of time, I t represents transmitted laser intensity or equivalently the measured spectrum, I 0 represents incident laser intensity, σ represents cross section of light absorption by a single particle, L represents optical pathlength of the sensor, and N represents analyte concentration.
11. The method of claim 5 , wherein the final location is a methane conduit or facility handling methane.
12. A method for determining ambient temperature in spectroscopy, comprising:
using a spectroscopic sensor to acquire spectra;
identifying a temperature-dependent etalon pattern from the acquired spectra;
searching a database of temperature-dependent etalon patterns indexed by temperature to find a match for the identified temperature-dependent etalon pattern; and
determining an ambient temperature based on a temperature index of the matched temperature-dependent etalon pattern.