**2. Fiber-optic gyroscope**

In the 1970s, FOG was first proposed and studied [6]. Then, its emergence has opened the way for the research of all solid-state sensors. It was initially considered to be devoted to medium-level applications. But over time, it has made a number of outstanding achievements in theoretical research and engineering [7]. Nowadays, FOG has reached the strategic level of performance and surpassed the ring laser gyroscope in terms of deviation noise and long-term stability. Its advantages are becoming more obvious, and its application fields are becoming more extensive. It has gradually become the key development goal for each country.

### **2.1 Development history of FOG**

In 1913, French physicist G. Sagnac presented a new theory through a considerable amount of experiments. The phase shift of two beams propagating along the closed optical path is proportional to the normal input angular rate of the closed optical path. That is the Sagnac effect [8]. Successful application of inertial navigation technology during World War II made FOG more challenging. In the early inertial navigation system, the sensor system used stable platform. With the progress of science and technology and the emergence of artificial satellite, people put forward the concept of strapdown inertial navigation, which has the characteristics of simple structure, small size, light weight, low cost, and easy maintenance. Sensitive devices are becoming more and more demanding. After World War II, gyroscopic technology has developed rapidly. In 1963, SePoy Gyroscope Company made a breakthrough in the area of optical gyroscope. The first experiment demonstrated ring laser gyroscope. Thereafter, after nearly 20 years of efforts, the inertial ring laser gyroscope has become practical. In 1983, Honeywell's ring laser gyroscope was installed in the airborne strapdown inertial navigation system of the new passenger aircraft Boeing 767 and 757. The rapid development of optical fiber communication, fiber optics, and laser technology has promoted the further development of optical rotation sensor based on Sagnac interferometer. In the mid- and late 1970s, a new type of optical gyroscope, named fiber optic gyroscope, appeared.

Scientists Macek and Davis confirmed the correctness and realizability of ring laser gyroscopes in 1963. In 1967, the French physicists G. Pincher and G. Herpner proposed the hypothesis of using optical fibers in gyroscopes [4]. In 1976, American scientists Victor Vali and Richard W. Shorthill tested the hypothesis of G. Pincher and G. Herpner, which symbolized the transition from theoretical stage to practical stage of FOG [9]. In 1978, McDonald Company developed the first practical FOG, and in 1980 Bergh et al. produced the first all-fiber optic gyroscope test prototype, making FOG a big step toward practicality [10]. In the mid-1980s, the interferometric fiber optic gyroscope was successfully developed. The development and application of optical gyroscope is an important milestone in the history

of inertial navigation technology. FOG has great value in the military field, because of its remarkable advantages, flexible structure, and broad application prospects. It has attracted the attention of universities and scientific research institutions in many countries in the world, and has invested a lot of energy in research. At the end of 1980s and the beginning of 1990s, FOG technology has been widely used. Its sensitivity has been improved by four orders of magnitude, and the angular velocity measurement accuracy has been improved from the initial 15°/h to 0.001°/h.
