To improve the long-term stability of a laser gyro, a real-time loss measurement system for space triaxial laser gyro mirrors exposed to He-Ne discharge plasma is designed. The loss-change process of the mirror in plasma is experimentally studied. The influence of low- and high-temperature environments on the variation law of loss is studied. Combined with the gas discharge fluid model, the discharge characteristics of the He-Ne plasma in the cavity of the laser gyro are simulated, and the energy and distribution of electrons and ions are obtained. The loss change mechanism in the plasma environment is discussed. The research results play an important role in further improving the stability of laser gyro mirrors under the action of plasma.

Considering that the cavity ring-down and resonant measurements are both based on a passive cavity, the loss change process of a mirror in plasma cannot be measured. Therefore, a real-time loss measurement system for space triaxial laser gyro mirrors exposed to He-Ne discharge plasma was designed based on the characteristics of the orthogonal optical paths of three resonators and shared mirrors. For example, channels Ⅰ and Ⅱ shared concave mirror 3 and plane mirror 1 (Fig. 1), and the loss of channel Ⅰ was monitored using the cavity ring-down method (Fig. 2). It was found that the loss of channel Ⅰ increased when channel Ⅱ was powered on. Since concave mirror 3 is in the discharge path, the increase in loss was caused by the action of the plasma on concave mirror 3. Based on this method, the loss of concave mirror 3 before and after plasma action in the cavity was monitored. The results showed that the loss increased rapidly and tended to be stable during discharge. Once the power supply was turned off, the loss decreased dramatically, flattened out, and finally dropped to the initial value in the subsequent natural standing process. Furthermore, the variation law of the loss under low- and high-temperature conditions after power failure was studied (Table 1). The experiments showed that high temperature had a positive effect on reducing the incremental portion of loss caused by the plasma, but low temperature did not.

The loss of the mirror increases under the action of the plasma in the cavity; therefore, it is necessary to deeply analyze the parameters of the electrons and ions in the plasma, especially the energy and distribution of these particles located at the mirror. A gas-discharge fluid model is constructed in combination with the structure of the laser gyro. The simulation results show that the energies of the electrons and He⁺ are the highest at the inner surface of the cathode (Fig. 3). During the discharge process, the energy range of electron is 6.6‒10.5 eV (Fig. 4) on the mirror, and when the discharge reaches equilibrium, the energy range of electron is 2.1‒3 eV on the mirror. The electron energy is higher than the binding energies of SiO₂ and Ta₂O₅ in the discharge process, and the electron energy is equivalent to the defect absorption peak when the discharge reaches equilibrium. Therefore, the electrons will produce more defects in the mirror, leading to changes in its reflection and loss characteristics. In general, a high temperature can be applied to the mirror to eliminate defects and impurities in the mirror, such as electrons and holes, to reduce the loss. Therefore, the loss is reduced after the high-temperature experiments. However, the heat treatment commonly used for the mirror is several hundred degrees Celsius, and most defects and impurities in the mirror cannot obtain enough energy to be completely eliminated at 75 ℃, so the loss reduction is limited.

In this study, an accurate and effective real-time measurement system for space triaxial laser gyro mirrors exposed to He-Ne discharge plasma is designed. The variation law of the loss before and after plasma action is studied. The corresponding experiments are designed based on the loss-change phenomenon. It is found that high temperatures have a positive effect on the loss recovery. Finally, the energy and distribution of the electrons and ions on the surface of the mirror are simulated. The simulation results show that the energy of the electrons is high enough to cause numerous defects in the mirror. Therefore, the stability of the laser gyro mirror in the plasma can be improved by reducing the electron energy on the surface of the mirror and enhancing its anti-electron damage ability, thereby improving the long-term stability of the laser gyro.