Improvement of accuracy of angular velocity detection in semiconductor fiber-optic ring laser gyroscope

Abstract

ABSTRACT A semiconductor fiber-optic ring laser gyroscope (S-FOG) consists of a semiconductor optical amplifier (SOA) and optical fiber to form a ring cavity. The fiber ring cavity enables larger sizes and smaller scattering, while the SOA gain is shared by the clockwise (CW) and counterclockwise (CCW ) propagating modes. When the S-FOG is rotated, a new beat signal called the Sagnac beat frequenc y is observed. We investigated the effect of the fiber ring cavity’s length on detection characteristics. Detection sensitivity was not depende nt on the number of laps of fiber ring cavity. However, when the cavity length became longer, the linewidth of Sagnac beat became narrower, and then accuracy of angular velocity detection improved. The relation between the linewidth ' Q of the Sagnac beat and cavity length P was proved to be ' Q | P -1.86 . Keywords : Sagnac effect, laser gyroscope, S-FOG, lin ewidth, beat frequency, scale factor, SOA 1. INTRODUCTION Since a gyroscope measures angular velocity, it is used for inertial guidance and control applications [1]. Optical gyros based on the Sagnac effect are well known for their high performance. Two types of optical gyros are commonly used. One is the fiber optic gyro (FOG), which is formed by surrounding a coil with optical fiber [2]. The other is the ring laser gyro (RLG), which is formed by a He-Ne gas lase r and a ring resonator in plural mirrors [3]. These gyros have a miniaturization limit due to the use of optical fiber for the FOG; furthermore, they have a low cost limit because a conventional RLG using a He-Ne laser requires high precision mirrors. As a new type of RLG, we proposed a semiconductor fiber-optic ring laser gyroscope (S-FOG) that consists of a semiconductor optical amplifier (SOA) and polarization-maintaining optical fiber to form a ring cavity [4]. Through experiments on S-FOG we observed the beat frequencies of counter-propagating light inside a semiconductor fiber-ring laser due to the Sagnac effect. However, we have not yet evaluated the performance of S-FOG. On performance of the RLG, it has been considered that the frequency difference between CW and CCW propagating lights do not depend on the total length of the cavity so an increased scale factor is not achieved by using long fiber optic coils in active ring laser gyros [2,3]. Even if the length of the ring cavity becomes longer, the sensitivity of the RLG does not increase. Therefore the cavity length is not related to performance of the RLG. However, we confirmed that when the cavity length becomes longer, performance of the RLG improves independent of the scale factor. In this paper, we evaluate the performances of the gyrosc ope. The linewidth of the Sagna c beat spectra is a parameter that expresses the accuracy of angular ve locity detection, and the narrow linewidth of the beat spectrum indicates high accuracy detection. Through experiments on S-FO G, we show that while detecti on sensitivity does not depend on the cavity length, the Sagnac beat spectral linewidth does. As the cavity length becomes longer, the linewidth of Sagnac beat becomes narrower, and then accuracy of angular velocity detection improves. Moreover , we show that the relation between the linewidth of the Sagnac beat and the cavity length.