A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of α_min = 7.6 × 10⁻⁹ cm⁻¹ can be obtained in a single laser scan of 10 s.

He-Ne lasers play a crucial role in ultra-precision measurement and optical sensing across various fields. For many applications based on He-Ne lasers, a higher output power is required to enhance the accuracy and signal-to-noise ratios of the associated optical measurements. However, conventional methods to increase the output power by reducing the diameter of the He-Ne laser discharge capillary inevitably result in higher diffraction losses and constrain the lasing performance. Here, we propose an approach to enhance laser pumping efficiency and output power through optimizing the ratios of He and Ne gasses. The validity of our proposal has been confirmed by both numerical simulations of He-Ne laser plasma discharge processes and experimental demonstrations, showing that the optimal gas ratio increases with the capillary diameter and total gas pressure.

In this study, frequency locking technology is investigated for high-stability microwave frequency standards based on diamond nitrogen-vacancy (NV) centers. Conventional locking methods typically utilize the side peaks induced via Zeeman splitting; however, this approach renders the frequency output highly susceptible to ambient magnetic field fluctuations. To address this limitation, a robust frequency locking method based on the central peak of the Optically Detected Magnetic Resonance (ODMR) spectrum is proposed. By systematically optimizing the bias magnetic field, the proposed method exploits the central peak’s inherent insensitivity to magnetic field variations and its narrower linewidth in environments with weak magnetic fields, thereby enhancing the quality factor of the frequency discrimination curve. The experimental results demonstrate that the proposed scheme achieves closed-loop locking of the 2.87 GHz microwave frequency, reaching short-term frequency stability (Allan deviation) of 1.73 × 10⁻⁷ at 200 s. Comparative tests under gradient magnetic fields further confirm that central-peak locking significantly suppresses frequency drift compared to side-peak methods. This study provides a vital technical pathway for the development of miniaturized, interference-resistant solid-state quantum frequency standards.

To further improve the performance of cavity-enhanced spectroscopy systems, a high-quality U-cavity system was established. In the process of the experiment, an asymmetric ripple effect, which is different from the previous etalon effect, was found, which seriously affects the performance of the spectral system. This unique phenomenon mainly manifests in the different amplitudes of the fluctuations of the spectral curves measured by the folding mirror and the end mirror in the U-cavity system. Based on multi-beam interference theory, we analyzed the characteristics of the transmission spectrum of each mirror in the presence of the etalon effect at the end mirror, and obtained the following conclusions: for the U-cavity system, the strength of the etalon effect of each mirror is inversely proportional to its transmission loss value, that is, the larger the loss, the smaller the ripple of the transmission spectrum, and vice versa. In order to eliminate this effect, the most effective way is to eliminate the etalon effect caused by the light feedback of the end mirror. After improving the system, the minimum detectable absorption coefficient of ${\alpha }_{\min }=8.33\times {10}^{-9}{\mathrm{cm}}^{-1}$ is obtained with this U-shape Optical Feedback Cavity-Enhanced Absorption Spectroscopy. These works are valuable references for the design of folded Cavity-Enhanced Absorption Spectroscopy systems and have potential for laser wavelength calibration and measurement of a mirror’s reflectance.