Planetary seismology is a new discipline for imaging the internal structure of planetary objects, and it shows the law of planetary motions and determines whether they are habitable. The complete internal structure of planetary objects cannot be obtained using conventional seismometers, which can only measure three translational components in the geological movements. Thus, three rotational components in the geological movements must also be measured. Nowadays, the rotational components are frequently measured using gyroscopes. Large ring laser gyroscopes are employed to achieve ultra-high precision measurement, although they can only function at a fixed site. The fiber optic rotational seismometer (FORS), which is based on the fiber optic gyroscope (FOG), is insensitive to the translation motion and has the benefits of low noise, high sensitivity, and portability, which is ideally adequate for measuring three rotational components in planetary seismology. High precision is required for the FORS, but as a civil system, low cost is also required. The biggest challenge for a practical high-precision FORS is to satisfy the demands of high performance and low cost simultaneously. In this study, the error mechanism of depolarized FOGs is shown and a noise suppression technique is proposed. Based on the differentially depolarized FOG, a three-axis high-precision seismometer (BHFORS-3C) is developed with self-noise smaller than 4.5 nrad·s^-1·Hz－12. Long-term seismic observations demonstrate that BHFORS-3C is ready for field application and offers crucial support for the observation of 6-component planetary seismology and the accurate analysis of seismic activities.

The output spectrum of Sagnac interferometer assembly (SIA) in a depolarized FOG is the modulation of the original input spectrum. The conventional analysis approach obtains the modulated output spectrum model by calculating the transfer function of the SIA. The depolarized SIA consisted of several polarization-maintaining fibers (PMFs) and single-mode fiber (SMF) coil is a common polarization interferometer. The modulated output spectrum is actually the polarization interference spectrum, and the output spectrum model can be obtained using the Jones matrix approach. The modulated spectrum is easily affected by the environment and becomes unstable, and the fluctuation of the modulation spectrum has a considerable effect on the noise and drifts of depolarized FOGs. In this study, a spectral modulation suppression technique is proposed. A phase modulator is inserted in a PMF of the depolarizers and additional high-frequency phase modulation is applied. The spectral modulation can be efficiently suppressed, and so are the noises and drifts.

We previously demonstrated that a multifunctional integrated optical circuit can function well over a wide bandwidth (Fig. 5) and proposed a differential FOG (DFOG), which consists of two equivalent interferometric FOGs sharing a single SIA and driven by two broadband sources with various wavelengths. The DFOG has good common-mode error rejection capability and the errors because of temperature and magnetic field (Fig. 6) can be efficiently suppressed. In this study, we propose a differential depolarized FORS (DD-FORS) (Fig. 7) based on the DFOG and the depolarized FOG using additional high-frequency phase modulation. A 3-axis high-precision seismometer is developed (Fig. 8) with self-noise smaller than 4.5 nrad·s^-1·Hz－12 (Fig. 10). Long-term seismic observations have been conducted in Huainan, Lijiang, and Beijing (Fig. 9), and the result shows the reliability and portability of BHFORS-3C. The long-term formal observations in Lijiang demonstrate that BHFORS-3C has achieved reliable measurements of remote strong earthquakes and near-field weak earthquakes.

In the current study, the output spectrum model of SIA in a depolarized FOG is built based on the polarization interference principle, and the error mechanism of depolarized FOGs is shown. A noise suppression technique using additional high-frequency phase modulation in SIA is proposed and verified. A high-precision FORS based on the differential depolarized FOG is proposed and a 3-axis high-precision seismometer which has the characteristics of low cost, low noise, and adaptability to the environment is developed. The long-term observations in several places demonstrate the high reliability and portability of BHFORS-3C. Considerable observational data have been recorded and preliminary verification of the rotational seismic model has been conducted. Our study offers a practical high-precision 3-component seismic rotation observation instrument for planetary seismology.