Fiber Optic Gyro

Fiber optic gyroscope is the fiber angular velocity sensor, which is the most promising one among various fiber optic sensors. The fiber optic gyroscope, like the ring laser gyroscope, has the advantages of no mechanical moving parts, no warm-up time, insensitive acceleration, wide dynamic range, digital output, and small size. In addition, the fiber optic gyroscope also overcomes the fatal shortcomings of ring laser gyroscopes such as high cost and blocking phenomenon. Therefore, fiber optic gyroscopes are valued by many countries. Low-precision civilian fiber optic gyroscopes have been produced in small batches in Western Europe. It is estimated that in 1994, the sales of fiber optic gyroscopes in the American gyroscope market will reach 49%, and the cable gyroscope will take the second place (accounting for 35% of sales).

The working principle of the fiber optic gyroscope is based on the Sagnac effect. The Sagnac effect is a general related effect of light propagating in a closed-loop optical path rotating relative to inertial space, that is, two beams of light with equal characteristics emitted from the same light source in the same closed optical path propagate in opposite directions. Finally merge to the same detection point.
If there is an angular velocity of rotation relative to the inertial space around the axis perpendicular to the plane of the closed optical path, the optical path traveled by the light beams in the forward and reverse directions is different, resulting in an optical path difference, and the optical path difference is proportional to the angular velocity of rotation. . Therefore, as long as the optical path difference and the corresponding phase difference information are known, the rotational angular velocity can be obtained.

Compared with electromechanical gyroscope or laser gyroscope, fiber optic gyroscope has the following characteristics:
(1) Few parts, the instrument is firm and stable, and has strong resistance to impact and acceleration;
(2) The coiled fiber is longer, which improves the detection sensitivity and resolution by several orders of magnitude than that of the laser gyroscope;
(3) There are no mechanical transmission parts, and there is no wear problem, so it has a long service life;
(4) It is easy to adopt integrated optical circuit technology, the signal is stable, and it can be directly used for digital output and connected with the computer interface;
(5) By changing the length of the optical fiber or the number of cyclic propagation of light in the coil, different precisions can be achieved and a wide dynamic range can be achieved;
(6) The coherent beam has a short propagation time, so in principle it can be started instantly without preheating;
(7) It can be used together with the ring laser gyroscope to form sensors of various inertial navigation systems, especially the sensors of strap-down inertial navigation systems;
(8) Simple structure, low price, small size and light weight.

Classification
According to the working principle:
Interferometric fiber optic gyroscopes (I-FOG), the first generation of fiber optic gyroscopes, are currently the most widely used. It uses a multi-turn optical fiber coil to enhance the SAGNAC effect. A dual-beam toroidal interferometer composed of a multi-turn single-mode optical fiber coil can provide higher accuracy and will inevitably make the overall structure more complicated;
Resonant fiber optic gyroscope (R-FOG) is the second-generation fiber optic gyroscope. It uses a ring resonator to enhance the SAGNAC effect and cyclic propagation to improve accuracy. Therefore, it can use shorter fibers. R-FOG needs to use a strong coherent light source to enhance the resonance effect of the resonant cavity, but the strong coherent light source also brings many parasitic effects. How to eliminate these parasitic effects is currently the main technical obstacle.
Stimulated Brillouin Scattering Fiber Optic Gyroscope (B-FOG), the third-generation fiber optic gyroscope is an improvement over the previous two generations, and it is still in the theoretical research stage.
According to the composition of the optical system: integrated optical type and all-fiber type fiber optic gyroscope.
According to the structure: single-axis and multi-axis fiber optic gyroscopes.
By loop type: open loop fiber optic gyroscope and closed loop fiber optic gyroscope.

Since its introduction in 1976, the fiber optic gyroscope has been greatly developed. However, the fiber optic gyroscope still has a series of technical problems, these problems affect the accuracy and stability of the fiber optic gyroscope, and thus limit its wide range of applications. mainly includes:
(1) The effect of temperature transients. Theoretically, the two back-propagating light paths in the ring interferometer are of equal length, but this is strictly true only when the system does not change with time. Experiments show that the phase error and the drift of the rotation rate measurement value are proportional to the time derivative of temperature. This is very harmful, especially during the warm-up period.
(2) The influence of vibration. Vibration will also affect the measurement. Appropriate packaging must be used to ensure good sturdiness of the coil. The internal mechanical design must be very reasonable to prevent resonance.
(3) The influence of polarization. Nowadays, the most widely used single-mode fiber is a dual-polarization mode fiber. The birefringence of the fiber will produce a parasitic phase difference, so polarization filtering is required. Depolarization fiber can suppress polarization, but it will lead to an increase in cost.
In order to improve the performance of the top. Various solutions have been proposed. Including the improvement of the components of the fiber optic gyroscope, and the improvement of signal processing methods.