Independent, all-weather, and high-frequency universal time measurement is crucial for establishing a comprehensive national time-monitoring system in China. This system is vital for supporting major strategic tasks such as manned spaceflight and deep-space exploration. Large-ring laser gyroscopes, which are anchored to the Earth bedrock, provide direct acquisition of signals related to the Earth rotation and are vital to universal time measurement.

To balance the angular precision and dimensions of a large-ring laser resonator, an optical-contacted laser resonator is initially designed, which comprises a central block, four extension blocks, a gain block, and four quartz tubes. Subsequently, the cavity ring-down method is utilized to measure the losses of the fundamental transverse mode (TEM₀₀) and high-order transverse modes to validate the rationality of the aperture design. The finite-element method is employed to simulate the laser plasma dynamics process, where the optimal He/Ne particle number ratio is obtained. By adjusting the gas pressure and pump current, a stable single longitudinal mode and the Earth rotation signal are achieved using the NUDT-MRLG550 laser gyroscope.

This study introduces China first large-ring laser gyroscope named NUDT-MRLG550, which is based on Zerodur glass. The gyroscope utilizes an optical-adhesive combination scheme and has a side length of 0.55 m (Fig. 1). It features a direct-current (DC) discharge gain length of 60 mm and a capillary diameter of 6 mm. The loss difference between TEM₀₀ and high-order transverse mode (TEM₀₁) is 78×10^-6, with a 3.24-mm-diameter diaphragm located at the beam waist (Fig. 2). Based on theoretical and experimental investigations on laser plasma dynamics, an optimal He/Ne particle number ratio of 25∶1 is obtained (Fig. 3), in addition to a stable output under a single longitudinal mode with a power of 4 μW and the TEM₀₀ (Fig. 4). Observations conducted at 28°N latitude successfully detect stable Earth rotation signals at a frequency of 29.6 Hz.

The design process of the first Chinese Zerodur-based large-ring laser gyroscope, NUDT-MRLG550, is presented comprehensively herein. A new design and manufacturing solution for large-ring laser gyroscopes is provided, thus addressing the technological gap in this field in China. The NUDT-MRLG550 is a two-frequency laser gyroscope with the smallest lock-in region reported thus far (the lock-in threshold is less than 30 Hz), which can compress the size of the two-frequency laser gyroscope unlocked by Earth rotation to 0.55 m×0.55 m, i.e., only one-half the size of the first-generation large laser gyroscope, C-II. Owing to its smaller size, the NUDT-MRLG550 offers the highest output power (~4 μW) in a single longitudinal mode compared with all large laser gyroscopes currently in operation. Specifically, its output power is three orders of magnitude higher than those of its counterparts, which effectively reduces the quantum noise generated. The theoretical random walk limit of the NUDT-MRLG550 is 1.6×10^-9 rad·s^-1/2.