A Phase Noise Immune TDLAS Flow Velocimetry via Using Modulated Waveform Synchronizing

A phase noise immune tunable diode laser absorption spectroscopy (TDLAS) velocimetry method was proposed for gas flow velocity detection by using modulated waveform synchronizing. A Mach-Zehnder interferometer modulated the intensity of laser with wave number. The modulated laser was divided into a zero-velocity reference laser and a velocity measurement laser, denoted as beam 1 and beam 2. Beam 1 passed through the zero-velocity gas, while beam 2 passed through the flow. Due to the optical path differences of the two lasers and interference in the environment, phase noise exists between the two laser beams, and such phase noise distorts the readings of the flow velocities. To suppress phase noises, the peak and valley positions in these two modulated laser intensities were used to synchronize the lasers. Since the two laser beams were emitted from the same laser diode and modulated by the same interferometer, the modulated waveforms for both laser paths were the same. The synchronizing effectively suppressed the distortion from the phase noise. By extracting the absorption spectrum center positions in the two laser beams, the absorption spectrum frequency shift was obtained for a precise flow velocity. In this way, the proposed method achieved the suppression of phase noise in the flow velocimetry. Experiments verified that the flow velocimetry accuracy and the phase noise immunity of the proposed method was better than DAS.