Noise reduction in fiber optic gyroscopes.

摘要

Considerable research on the fiber optic gyroscope has been focused on maximizing its sensing accuracy for inertial navigation grade performance applications. Specifically, it turns out that the quality of fiber gyroscopes depends considerably on the characteristics of the light source adopted by them. The best source for fiber optic gyroscopes is known to have a large spectral bandwidth and thermal-like intensity statistics. Sources which meet these requirements are SLD (Super Luminescent Diode), ELED (Edge Light Emitting Diode), and fiber light source. Unfortunately, these all are prohibitively expensive for large volume application in the various commercial markets. Our research effort has been focused on reducing the fiber gyroscope's cost without sacrificing its performance.; In this dissertation, we implement a low cost DIFOG (Depolarized Interferometric Fiber Optic Gyroscope) using a cheap 1.3 micron wavelength multimode laser diode and a 1 km single mode fiber for the sensing loop. In normal operation, this kind of a light source exhibits very large photon fluctuations (mode partition noise) in each longitudinal mode, correlated with those of adjacent modes. These correlated photon fluctuations of the various lasing modes give rise to a large intensity noise when the light has passed through the DIFOG optical circuit. The majority of this dissertation describes how to reduce the large intensity noise converted from the mode partition noise of a multimode laser source in a DIFOG. Three noise reduction techniques have been implemented. First, coherence collapsing optical feedback is applied to the light source which results in its power spectrum being dramatically broadened, the output signal being stabilized and the aforementioned mode partition noise being greatly reduced. Secondly, a good depolarization and a noise reduction are accomplished at the same time by using a long PM fiber for the depolarizer. Lastly, we reduced the noise by adjusting the modulation depth in the PSD (Phase Sensitive Detection) signal processing scheme to an unusually high level.; In addition, we show a scheme for suppressing source intensity noise in PM fiber gyroscopes operated at the proper frequency. The intensity noise directly coming from the light source is multiplied by the raw gyro output and then added back to the raw output. This results in a noise power spectrum having a low noise at odd integer multiples of the proper frequency.