US 12,422,351 B2
Pump-probe photothermal spectroscopy having passive phase detection and an optical waveguide
Wei Ren, Hong Kong (CN); Chenyu Yao, Ping Village (CN); and Mengyuan Hu, Hong Kong (CN)
Assigned to The Chinese University of Hong Kong, Hong Kong (CN)
Filed by The Chinese University of Hong Kong, Hong Kong (CN)
Filed on Jun. 30, 2022, as Appl. No. 17/810,145.
Claims priority of provisional application 63/217,096, filed on Jun. 30, 2021.
Prior Publication US 2023/0003635 A1, Jan. 5, 2023
Int. Cl. G01N 21/17 (2006.01); G01N 21/45 (2006.01)
CPC G01N 21/171 (2013.01) [G01N 21/45 (2013.01); G01N 2021/1714 (2013.01); G01N 2021/458 (2013.01)] 4 Claims
OG exemplary drawing
 
1. 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°.