Pump-probe photothermal spectroscopy having passive phase detection and an optical waveguide
A passive-phase-detection photothermal spectroscopy (PTS) system and methods are provided for gas measurements. The PTS system includes a pump laser source, a probe laser source, the pump and probe laser beams simultaneously propagating through an optical waveguide having a target gas specimen. Moreover, the PTS system can be based on a heterodyne detection scheme and includes a combiner configured to align light input from a local oscillator with the probe laser beam output from the optical waveguide to output to a photodetector that is configured to generate beat notes. A lock-in phase detector and a lock-in amplitude detector is configured to detect and measure a photothermal signal based on the beat notes received from the photodetector for gas measurements. The PTS system can also be based on a core-cladding-mode interference detection scheme and generates the core mode and cladding mode simultaneously for the probe laser in the waveguide.
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°.
2. The method of claim 1 , wherein demodulating phases of the beat note is based on reference signal inputs received from reference sources, and wherein frequency of the reference signals equals to an optical frequency difference between the probe laser beam and the light output from the local oscillator.
3. The method of claim 2 , wherein
the first and second beat notes mixed with reference signals are transmitted through a first low-pass filter and a second low-pass filter, respectively;
phases are calculated based on a digital arctangent method; and phase signals are output to the lock-in amplitude detector.
4. The method of claim 2 , further comprising configuring the lock-in amplitude detector cascaded to the lock-in phase detector to demodulate the harmonic photothermal signals from the phase of the beat note, based on a reference signal.