The LED-based incoherent broadband cavity-enhanced spectroscopic absorption technology features simultaneous detection of multiple species, high resolution, high sensitivity, and strong real-time performance, becoming an important means of trace gas detection technology. However, this technology also has many problems, such as the large divergence angle and poor collimation of LED, which requires efficient light source coupling systems. Additionally, the light emitted by LED is greatly affected by temperature and current, and the instability of LED temperature control will directly interfere with measurement results. We discuss the LED coupling method and the temperature control problems and propose corresponding solutions.

The high-precision miniaturized broadband cavity-enhanced absorption spectrum (HPM-BBCEAS) system is developed from a previous version in the laboratory. An LED light source with a central wavelength of 460 nm is selected as the detection light. The direct focusing of the incident end using double-bonded lenses is adopted instead of traditional fiber optic sampling coupling, which improves the coupling efficiency of the light source. Combined with a high-sensitivity resonant cavity with an optical cavity length of 322.40 mm, high-precision and miniaturized NO₂ measurement is achieved. The device adopts a self-designed LED automatic temperature control system to integrate the proportional integral derivative (PID) algorithm and the Kalman filtering algorithm and proposes an improved PID-Kalman filtering algorithm (Fig. 3). The main improvements are two-fold. First, the program can modify the PID parameters based on error changes to achieve fast and stable adjustment. Second, the Kalman filtering is added to the original PID to reduce the actual acquisition error and realize a more accurate calculated PID output value (Fig. 4). The system achieves rapid temperature control by adjusting the Peltier cooling time, reducing the light intensity fluctuations caused by temperature changes, improving the signal-to-noise ratio, and solving the problem of large system stability and detection accuracy errors caused by temperature drift.

The experimental results show that by employing the improved PID algorithm and the conventional PID algorithm for long-term temperature monitoring of the LED light source on the same system, the adjustment time of the improved PID algorithm is approximately 13 times shorter than that of the conventional PID algorithm during analyzing the adjustment time from the process of turning off the LED to turning it on and adjusting the temperature to the stable stage, with the set temperature of 28 ℃. The data is evaluated by taking the stable data of the LED for about one hour, and the results show that the temperature fluctuation range of the improved PID algorithm is 27.985-28.015 ℃ with a fluctuation range of ±0.015 ℃, while that of the conventional PID algorithm is 27.8-28.2 ℃ with a fluctuation range of ±0.2 ℃. This indicates that the control precision of the improved PID algorithm is more than ten times higher than that of the conventional PID algorithm. From the perspective of light intensity fluctuation, the light intensity fluctuation range of the improved PID algorithm is approximately (30125±25) cd, while that of the conventional PID algorithm is approximately (30125±150) cd. This reveals that the control precision and light intensity fluctuation of the improved PID algorithm are better than those of the conventional PID algorithm. When the improved PID algorithm and the conventional PID algorithm are applied to the broadband cavity-enhanced absorption spectroscopy (BBCEAS) system, the influence of temperature control precision on the system detection limit is evaluated. The evaluation results show that compared with the conventional PID algorithm, the stability and detection limit of the instrument are both improved by about ten times when the improved PID algorithm is adopted (Fig. 7).

We introduce a high-precision NO₂ analyzer based on BBCEAS technology. The analyzer adopts a cage-type coaxial integrated cavity structure consisting of a light source, a resonant cavity, and fiber optic output, greatly improving the system integration. Combined with an improved PID-Kalman filtering algorithm for the temperature control system, the stability of the entire system is greatly enhanced. The developed temperature control system can achieve precise control of the LED temperature in two minutes with a control accuracy of up to ±0.015 ℃, guaranteeing stable instrument measurement operation. Under the optical cavity length of 322.40 mm and a mirror reflectivity of 0.99985, NO₂ detection limit of 21×10^-12(60 s, 1σ) is achieved. Leveraging high-reflectivity mirrors with higher reflectivity can provide longer absorption paths and lower detection limits. The comparison of the instrument with the gas distribution system validates the accuracy of the system's NO₂ measurements. Actual atmospheric applications demonstrate that this device can capture instantaneous NO₂emissions, proving the reliability of the instrument's long-term stable operation.