Fiber optic gyroscope (FOG) is a novel type of gyroscope based on the Sagnac effect [1]. Its principle, process and key technology have obvious advantages compared with a traditional electromechanical gyroscope, and are widely used in aerospace, military and engineering fields [2], [3].
To enhance the integration of the gyroscope, shortening the coil has attracted a multitude of scientists’ attention. Japan Aerosp Explorat Agcy reduces the fiber length by seven times by concatenating multiple cores[4];the Beijing Institute of Automatic Control Equipment uses a small diameter fiber to implement a miniature coil the size of a coin[5];UCSB is based on Si3N4 waveguides to implement coil of length 3 m[6]; Beihang Univ is based on SiO2 waveguides to implement coil of length 2.14 m[7];Huazhong Univ Sci And Technol is based on Si waveguides to implement coil of area 38.5 cm2[8].
However, Y-waveguide phase modulator[9], [10], [11], [12] or integrated optical chip(IOC)[13], [14], [15] are commonly used in interferometric fiber optic gyroscope (I-FOG), whose modulation principle is based on temporal non-reciprocity, limited by the fiber coil transit time and eigenfrequency [16], [17], [18]. A shorter fiber coil leads to the higher frequency of modulation signal, and increases the difficulty of modulation and demodulation, especially in the research of ultra-short coil gyroscope and on-chip gyroscope, hindering the lightweight and integrated application of FOG.
In this paper, a spatial non-reciprocal phase modulator for ultra-short coil FOG is proposed by combining the electro-optical effect [19], [20] of LiNbO3 crystal with a polarizer and a Faraday rotator. It realizes the phase modulation of two polarized beams in the same time domain and different spatial domains, which fundamentally overcomes the limitations of fiber coil transit time and eigenfrequency [21], [22]. So the ultra-short coil IFOG proposed realizes the fiber coil of 5 m /10 m/20 m and the modulation frequency is still maintained at 200 Hz, which is impossible for traditional FOG. In addition, it has been demonstrated in preliminary experiments that this modulator can be mid-mounted. The modulator mid-mounted can suppress backscattering and polarization coupling noise, and the zero-bias stability is improved from 0.11878°/h to 0.09110°/h. We analyze the generation of non-reciprocal phase difference, phase modulation and phase demodulation principles based on the structural characteristics of spatial non-reciprocal phase modulator, and prove the feasibility of ultra-short coil FOG theory by experiments. This structure can minimize the fiber coil length and reduce the modulation signal frequency, and is expected to be combined with silicon-based optoelectronics integration technologies such as straight and thin-film waveguides [23], [24] to realize the integration of fiber coils and microchips, as well as phase modulators.
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The spatial non-reciprocal phase modulator is the core device of ultra-short coil FOG, and the structure is shown in Fig. 1, which consists of a polarizer, a Faraday polarizer with 45° clockwise rotation (CW 45°), a LiNbO3 crystal, a Faraday rotator with 45° counter-clockwise rotation (CCW 45°), and a polarizer.
According to the refractive index ellipsoid equation, the analysis of the primary transverse electro-optical effect [25] of LiNbO3 crystal shows that, when the applied electric field
When using the sawtooth wave as the signal modulated by the spatial non-reciprocal phase modulator, the voltage and phase are linear, hence the light intensity signal does not contain high harmonics. After amplification and filtering, it can demodulate the phase information by sampling and performing FFT [29], [30], [31], [32] to obtain the rotation angular velocity.
Assuming that the sawtooth wave signal has a frequency of 200 Hz, an amplitude of π, an initial value of -π, and a light intensity
The design based on dual polarizers requires strict alignment accuracy. Otherwise, it gives rise to additional polarization crosstalk, causing polarization error. To overcome this difficult, high precision instruments and advanced integrated assembly techniques are essential.
Limited extinction ratio of polarizers and the misalignment cause polarization crosstalk. However, applying the polarizers at both ends of the crystal, double the extinction ratio reduces the polarization error and thus
We propose a scheme to improve the proposed ultra-short coil FOG. The theory shows that the scheme can achieve all the functions of FOG, and can greatly reduce the volume and weight of FOG, predicted volume ≤ 20 mm × 20 mm × 10 mm, weight ≤ 50 g, to promote the development of integrated FOG. The comparison with the current FOG is shown in Table 3.
Since the new spatially non-reciprocal phase modulator is not limited by eigenfrequency or transit time, angular velocity measurements can be
In summary, we propose and analyze a novel ultra-short coil fiber optic gyroscope structure both theoretically and experimentally. This ultra-short coil FOG fundamentally overcomes the shortcoming that the traditional I-FOG is limited by the transit time and the eigenfrequency; it does not require additional delay coils or high-frequency modulation signals and is less affected by environmental changes, as demonstrated by the analysis and experimental results. The modulator mid-mounted can
National Natural Science Foundation of China (61735011); Science and the Key Research and Development Program of Hebei Province (19251703D).
Yuefeng Qi: Conceptualization, Methodology, Writing – original draft, Resources. WeiBin Feng: Writing – original draft, Writing – review & editing, Data curation, Investigation. FangKe Li: Conceptualization, Writing – original draft, Investigation, Supervision. Tingnan Hu: Validation. Yiqing Wang: Supervision, Validation. Nannan Wang: Visualization, Writing – review & editing.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
