1 Research Background and Problems to be Solved

All-solid-state ring-laser gyroscopes (RLGs) offer advantages such as long lifespan, strong environmental adaptability, and compact structure. However, over the past few decades, the development of all-solid-state active RLGs has faced two core challenges: intense mode competition caused by the homogeneous broadening of the gain medium, and the lock-in effect limiting sensitivity at low rotational speeds. This research aims to reduce laser mode competition in the non-planar ring oscillator (NPRO) by utilizing Nd:YAG crystal with high wavefront-distortion combined with laser feedback interferometry, thereby achieving stable bidirectional lasing operation. Additionally, by leveraging the non-degenerate characteristic of the intrinsic polarization modes in the NPRO under the influence of a magnetic field, a non-zero beat frequency is obtained, thus eliminating the lock-in effect in the RLG. Ultimately, this work presents the construction of a miniaturized all-solid-state active RLG. Relevant research results were recently published in Photonics Research, Volume 13, Issue 4, 2025.[Danqing Liu, Changlei Guo, Chunzhao Ma, Weitong Fan, Xuezhen Gong, Zhen Zhang, Wenxun Li, Jie Xu, Kui Liu, Hsien-Chi Yeh, "All-solid-state miniature laser gyroscope based on a single monolithic non-planar ring oscillator," Photonics Res. 13, 897 (2025)]

2 Theoretical and Experimental Highlights

This paper constructs a theoretical model for NPRO laser feedback interferometry, explains the operating principle of the NPRO gyroscope based on the Sagnac effect, and derives the relationship between the NPRO projected area and the angular velocity measurement.

Figure 1 Schematic diagram of the NPRO gyroscope

In the experiment, the NPRO gyroscope was subjected to rotational measurements at 10 different angular velocities using a rotating platform. The experimental result for the scale factor was 38.3 Hz/(deg s⁻¹), which is consistent with theoretical expectation.

Figure 2 Beat frequency shift versus rotation speed

The experiment also found that the wavefront distortion of the laser crystal affects the beat frequency stability: the beat frequency stability of samples with high distortion is significantly better than that of samples with low distortion.

Figure 3 Comparison of wavefront distortion in different NPRO samples. (a) High wavefront-distortion; (b) Low wavefront-distortion

In the free-running condition, the NPRO gyroscope's zero-bias-instability is 31.3 deg/h, and the angle-random-walk is 0.22 . Finally, the experiment measured the tuning coefficients of pump power, crystal temperature, and magnetic field on the beat frequency, evaluated their contributions to beat frequency noise, and identified magnetic field noise as the dominant factor.

Figure 4 Stability of the NPRO gyroscope

3 Innovations, Research Significance, and Potential Applications

This paper presents the first realization of a miniaturized all-solid-state active RLG based on a single monolithic NPRO, revealing the principle that laser crystals with higher wavefront-distortion exhibit superior beat frequency stability. A compact packaging scheme is proposed, providing design guidelines for the practical implementation of the NPRO gyroscope. Furthermore, this work establishes a new paradigm for developing all-solid-state active RLGs and offers novel solutions to address mode competition and the lock-in effect in such systems. This miniaturized all-solid-state active RLG shows promising potential for applications in aerospace, defense and industrial control, etc.

4 Peer review within the research group

Corresponding author associate professor Changlei GUO stated: "Research on monolithic NPRO lasers has spanned four decades. Previously, stable bidirectional lasing had never been observed in NPROs to our best knowledge, making them unsuitable for direct application in active RLGs. Our recent finding reveals that laser feedback interferometry can induce stable bidirectional lasing, with the beat frequency stability being correlated to the crystal's wavefront-distortion. Laser crystals with higher wavefront-distortion demonstrate superior bidirectional lasing stability, this represents the critical breakthrough for developing all-solid-state active RLGs." Associate professor Liu Kui said: "The lock-in effect has been a long-standing critical challenge in the field of laser gyroscopes, particularly prominent in all-solid-state laser gyroscopes. Despite extensive previous research on eliminating the lock-in effect in all-solid-state laser gyroscopes, the results have been limited. This experiment provides a new solution for eliminating the lock-in effect in all-solid-state laser gyroscopes through the magnetic field-induced degenerate effect of counter-propagating beams, offering strong support for the development and application of all-solid-state laser gyroscopes." Assistant professor Jie XU stated: "The miniaturized all-solid-state active laser gyroscope based on NPRO features a miniaturized, compact package and extremely low-power consumption, and is expected to provide technical support for the lightweight and intelligent upgrading of aerospace equipment in the future."

5 Future Work and Outlook

By replacing Nd:YAG crystal with neodymium-doped glass and optimizing the NPRO design, the scale factor can be increased significantly. The gyroscope's beat frequency stability can be further enhanced through active magnetic shielding and optical power feedback control. Integrated packaging will enable additional size reduction while improving environmental noise isolation, thereby facilitating its engineering applications.

Figure 5 Integrated packaging design of the NPRO gyroscope