Airborne tunable mid-IR laser gas-correlation sensor
A method and apparatus for measuring target gas concentrations in an atmosphere. The method and apparatus emit in the atmosphere a laser beam tuned to a molecular absorption line of a target gas, receive a reflected signal affected by gas absorption of the target gas in the atmosphere, divide and direct the received signal into a first optical path and a second optical path including in one of the paths a correlation gas cell filled with a predetermined concentration of the target gas, detect transmitted signals through the first optical path and the second optical path, and calculate a target gas concentration by comparing a first signal transmitted through the first optical path to a second signal transmitted through the second optical path. The apparatus includes a laser source tunable to a specific molecular absorption line of a target gas and configured to emit in the atmosphere a laser beam having a spectral bandwidth greater than a full width of the molecular absorption line of the target gas, a receiver configured to receive a reflected signal affected by gas absorption of the target gas in the atmosphere, and at least one detector configured to detect transmitted signals through the first optical path and the second optical path.
1. An apparatus for measuring target gas concentrations in an atmosphere, comprising:
a laser source tunable to an infrared molecular absorption line of a target gas, and configured to emit in the atmosphere a laser beam tuned to the infrared molecular absorption line of the target gas and having a spectral bandwidth greater than a full width of the molecular absorption line of the target gas; and
a receiver configured to receive a reflected signal affected by gas absorption of the target gas in the atmosphere, said receiver including,
a first optical path and a second optical path, the first optical path including at least one correlation gas cell filled with a predetermined concentration of the target gas, the second optical path including a reference cell filled with air or filled with a gas different than the target gas, wherein the receiver divides and directs the received reflected signal into the first optical path and the second optical path;
at least one detector configured to detect transmitted signals through the first optical path and the second optical path; and
a processor configured to calculate a target gas concentration by comparing a first signal transmitted through the first optical path to a second signal transmitted through the second optical path.
2. The apparatus of claim 1 , wherein the laser source is configured to emit a laser beam having a spectral bandwidth that exceeds a combined width of a group of molecular absorption lines.
3. The apparatus of claim 1 , wherein the laser source comprises a pulsed laser source configured to produce a laser pulse having a pulse duration, and the processor is configured to time gate the received signal to capture an entirety of the laser pulse duration at the detector.
4. The apparatus of claim 1 , wherein the processor is configured to derive a concentration of the target gas in the path length of the laser beam based on Beer's law and an absorption strength of the target gas at a wavelength of the laser beam.
5. The apparatus of claim 1 , wherein the processor is configured to accumulate a fixed number of measurements over a fixed time period.
6. The apparatus of claim 1 , further comprising:
an electronic filter disposed in an output of the at least one detector, the filter configured to reject low frequency electronic signal components from the received reflected signal.
7. The apparatus of claim 1 , further comprising:
a telescope configured to provide a common field of view (FOV) for reference and gas correlation (off/on) channels.
8. The apparatus of claim 1 , further comprising:
a plurality of correlation cells in the first optical path filled with different target gases.
9. The apparatus of claim 1 , wherein the correlation gas cell includes at least one of methane, ethane, carbon dioxide, carbon monoxide, and oxides of nitrogen.
10. The apparatus of claim 1 , wherein the correlation gas cell includes a simulant of the target gas.
11. The apparatus of claim 1 , wherein the receiver comprises:
a beam splitter configured to divide the received reflected signal into the first and second optical paths.
12. The apparatus of claim 1 , wherein the laser source comprises a tunable mid IR laser source.
13. The apparatus of claim 1 , wherein the laser source and the receiver comprises:
a light detection and ranging unit configured to determine a distance of an object from the receiver.
14. The apparatus of claim 1 , wherein the at least one detector comprises:
a thermoelectrically cooled detector.
15. The apparatus of claim 1 , wherein said laser beam is configured to have a spectral bandwidth 1-100 times the width of the molecular absorption line of the target gas.
16. The apparatus of claim 1 , wherein said laser beam is configured to have a spectral bandwidth 30-100 times the width of the molecular absorption line of the target gas.
17. The apparatus of claim 1 , wherein said laser beam is configured to have a spectral bandwidth greater than 50 times the width of the molecular absorption line of the target gas.
18. The apparatus of claim 1 , further comprising:
a mobile platform supporting the laser source and the receiver.
19. The apparatus of claim 18 , wherein the mobile platform comprises a part of an aircraft.
20. The apparatus of claim 18 ,
wherein the processor is further configured to calculate said target gas concentration for a plurality of optical paths between the aircraft and ground, and thereby produce an aerial mapping of a target gas concentration.
21. The apparatus of claim 18 , wherein the mobile platform comprises a part of a landcraft.