Step 1: Analyze and Adopt Domain: Precision Optical Metrology / Laser Physics / Geodetic Engineering Persona: Senior Research Scientist in Inertial Sensing and Quantum Optics

Abstract

This research presents the design and characterization of the NUDT-MRLG550, China's first large-scale, Zerodur-based, full-inner-cavity ring laser gyroscope (RLG) with a 0.55 m side length. Developed to provide high-frequency, all-weather Universal Time (UT1) measurements—a strategic requirement for deep-space exploration and autonomous time-synchronization systems—the instrument utilizes a modular optical-contact bonding scheme to overcome the manufacturing constraints of ultra-precise angular alignment at scale.

The investigation details the optimization of He-Ne plasma dynamics through Finite Element Method (FEM) simulations, identifying a 25:1 He/Ne particle ratio as optimal for pumping efficiency. Experimental results demonstrate that operating at higher gas pressures (8 torr) effectively suppresses mode competition, ensuring stable single-longitudinal-mode and fundamental transverse-mode (TEM₀₀) operation. The NUDT-MRLG550 achieved a 4 μW output power, three orders of magnitude higher than comparable systems, and successfully detected a 29.6 Hz Earth rotation signal at 28°N latitude. With a theoretical random walk limit of $1.6 \times 10^{-9} \text{ rad} \cdot \text{s}^{-1/2}$ and a record-low lock-in threshold of $<30 \text{ Hz}$, the system represents a significant advancement in the miniaturization and sensitivity of large-scale inertial sensors.

Technical Summary: NUDT-MRLG550 Design and Performance

• [Section 1] Strategic Imperative for UT1 Monitoring: Current Universal Time (UT1) measurements rely on Very Long Baseline Interferometry (VLBI), which suffers from high costs, massive data processing requirements, and long observation cycles. Large-scale RLGs offer a complementary, high-frequency, all-weather alternative by directly measuring the Sagnac frequency shift induced by Earth's rotation.
• [Section 2] Modular Zerodur Architecture: To ensure structural stability and low thermal expansion ($<2 \times 10^{-8}/^{\circ}\text{C}$), the NUDT-MRLG550 utilizes a homogeneous microcrystalline glass (Zerodur) structure. To maintain the requisite angular precision ($<10''$ for a 0.55 m cavity), the design employs a modular "optical-contact" scheme consisting of a center block, gain blocks, extension blocks, and indium-sealed quartz tubes.
• [Section 2] Mode Suppression and Diffraction Control: The cavity utilizes a pair of 8 m radius-of-curvature spherical mirrors and high-reflectivity planar output mirrors. A 3.24 mm diameter aperture is integrated at the beam waist to increase the loss of the higher-order TEM₀₁ mode by $78 \times 10^{-6}$ relative to the TEM₀₀ mode, ensuring fundamental mode stability.
• [Section 3.1] Plasma Dynamics Optimization: FEM simulations of the He-Ne plasma discharge (1.2 mA steady-state current) determined that a 25:1 He/Ne ratio maximizes the pumping efficiency of Ne atoms from the $2^1\text{S}$ to $5\text{S}$ energy levels. This optimization reduces quantum noise and operating power consumption.
• [Section 3.2] High-Pressure Stability for Large Cavities: Unlike traditional RLGs operating at 3–5 torr, the NUDT-MRLG550 operates at 8 torr. Research indicates that higher pressure reduces atomic motion and mitigates the mode competition inherent in large cavities with narrow longitudinal mode spacing (136.5 MHz), resulting in a stable beat frequency signal.
• [Section 3.2] Sagnac Signal Observation: In a static, horizontal orientation at 28°N latitude, the system recorded a stable Earth rotation signal of 29.6 Hz. This aligns precisely with the theoretical calculation of $29.64 \text{ Hz}$ based on the $0.55 \text{ m} \times 0.55 \text{ m}$ scale.
• [Section 4] Comparative Performance Benchmarks:
  • Lock-in Threshold: The system features the world’s lowest reported lock-in threshold for a two-frequency RLG ($<30 \text{ Hz}$).
  • Output Power: Achieves $4,000 \text{ nW}$ ($4 \mu\text{W}$), significantly higher than the German G-ring ($10 \text{ nW}$) or Italian GINGERino ($1 \text{ nW}$).
  • Precision Limits: The theoretical Angle Random Walk (ARW) limit is $1.6 \times 10^{-9} \text{ rad} \cdot \text{s}^{-1/2}$, offering comparable precision to much larger instruments due to increased signal-to-noise ratios.
• [Section 4] Future Developments: Future iterations will be deployed in subterranean, constant-temperature environments using iodine-stabilized lasers or atomic clocks for active frequency stabilization to support geophysical research and seismic wave measurement.

Reviewer Recommendation

This topic is best reviewed by a panel consisting of:

1. Optical Engineers specializing in gas laser dynamics and resonator design.
2. Geodesists focused on Earth rotation parameters (ERP) and World Time monitoring.
3. Inertial Navigation Specialists familiar with Sagnac effect sensors and RLG "lock-in" physics.
4. Material Scientists expert in ultra-low expansion (ULE) materials like Zerodur and optical-bonding techniques.