Patent ID: 8873061

Claim:
A system for measuring a physical parameter of a first substance having a first absorption spectrum with an absorption feature that varies based on the physical parameter, the system comprising: a second substance having a second absorption spectrum including first and second peaks respectively corresponding to first and second transitions of the second substance, the first and second peaks overlapping with one another, a point in the overlap between the first and second peaks defining an isoclinic point of the absorption spectrum of the second substance; at least one tunable-frequency laser configured to transmit a first laser beam through the first substance and to transmit a second laser beam through the second substance; a first photodetector configured to generate a first output based on an intensity of the first laser beam transmitted through the first substance; a second photodetector configured to generate a second output based on an intensity of the second laser beam transmitted through the second substance; controller circuitry configured to tune the at least one tunable-frequency laser so as to lock a first frequency of the first laser beam to the absorption feature of the first substance based on the first output, and to tune the at least one tunable-frequency laser so as to lock a second frequency of the second laser beam to the isoclinic point of the second substance based on the second output; and measurement circuitry configured to calculate the physical parameter of the first substance based on a difference between the first and second frequencies.