Pump-probe photothermal spectroscopy having passive phase detection and an optical waveguide

A method based on heterodyne interferometric photothermal spectroscopy that demodulates photothermal signals for gas measurements in optical waveguides, the method comprising:

• configuring an optical hollow-core waveguide to contain a gas sample to be measured;
• coupling a pump laser beam and a probe laser beam simultaneously into the optical waveguide that is filled with the gas sample to be measured;
• configuring a signal generator to provide wavelength modulations to a pump laser source;
• modulating phases of the probe laser beam under a photothermal effect of the gas sample in the optical waveguide;
• configuring a local oscillator to output light with a frequency shift against optical frequency of the probe laser beam that has been phase modulated;
• generating, by a photodetector, a beat note, when the probe laser beam and the light with frequency shift output from the local oscillator are simultaneously received by the photodetector, wherein the photothermal phase signal in the probe laser beam is encoded into a phase of the beat note;
• demodulating, by a digital lock-in phase detector, phases of the beat note;
• and
• demodulating, by a digital lock-in amplitude detector, harmonic signals of photothermal spectroscopy (PTS) from the demodulated phases of the beat note for gas measurements, wherein the demodulating is performed based on a signal input received from the signal generator,
• wherein the beat note from the photodetector is split into a first beat note and a second beat note for phase demodulation by digital lock-in phase detection, followed by mixing the first and second beat notes with an in-phase reference signal and a quadrature reference signal, respectively, wherein the in-phase reference signal and the quadrature reference signal have a phase difference of 90°.