Harmonic Double Valley Inclination: A Laser Wavelength Self-Diagnosis Identifier for a TDLAS/WMS System on a Filling Production Line

Tunable diode laser absorption spectroscopy (TDLAS) is widely used for gas concentration measurements due to its merits of rapid, noncontact, and high-precision detection. Precise control of laser emission can guarantee the accuracy of the absorption spectrum at a specific wavelength. However, in an unstable environment on a real-world pharmaceutical filling production line, it becomes difficult to ensure laser performance due to variations in temperature, pressure, and so on. In contrast to traditional methods for optimizing the internal structure of a laser emission cavity, we propose an external approach to achieve accurate wavelength control of a laser emitter. Specifically, the novel concept of the harmonic double valley inclination (HDVI) is introduced. This HDVI, mined from second harmonic signals, is an identifier that can be used to infer the working status of the laser and plan further instructions and can finally be fed back to the laser controller to achieve closed-loop control logic. Furthermore, all functions are implemented using an FPGA chip and verified on a filling production line. The results show that the stability of the output wavelength of a laser emitter can be maintained using this self-diagnosis method and that accurate oxygen concentration detection can be further ensured.