Study On The Basic Characteristics Of The Faraday Bias In Differential Laser Gyro

The ring laser gyro based on the Sagnac effect is an inertial instrument, which can measure angular rotation. It basically consists of a ring cavity around which there are two laser beams traveling in the opposite direction, one clockwise and the other counterclockwise, and the beat note of the two counter propagating beams is proportional to the rotation rate of the ring cavity relative to an inertial frame. But its development has been restricted by the "lock-in" behavior, so two approaches are adopted cooperatively to eliminate or reduce the effect of this behavior. On the one hand, the gyro is arranged in the linear area by employing a proper bias way. On the other hand, the "lock-in" region can be reduced by restraining the back scattering from the mirrors, on which thin films with ultra-high reflectivity are coated. At present differential laser gyro, in which there are an optically active crystal and a Faraday cell in the optical path of cavity, is considered to be the most promising one. But the absorption and scattering of the inserted elements will broaden the "lock-in" region, so the losses must be controlled in an acceptable range in order to ensure the sensitivity of the gyro. Therefore it's necessary to measure the weak losses of the elements with high accuracy and precision. In the thesis, the measurements of the weak losses and ultra-high reflectivity are studied utilizing cavity ring down spectroscopy (CRDS). The main works in the thesis are as follows:1. The ideal input-output properties of the gyro are obtained through analyzing its principle by use of the optical ring resonator theory. Then the "lock-in" behavior and its causes are discussed. The differential laser gyro is mainly described, and then the laser frequency splitting induced by the optically active crystal and a Faraday cell inner the cavity are discussed according to Faraday effect.2. On basis of stability conditions and influences of cavity losses, the principle of the CRDS is analyzed. The dependence of measurement error on the measurement precision of the cavity length and of the ring down time and its simulated results are given. The parameter requirements of the laser, cavity mirrors and detecting system are analyzed, which will support helps for components choice, then the experiment setup is built. In the CRDS experiment, two conditions must be satisfied: the linewidth of the laser is much smaller than the absorption linewidth of the medium and the coherent length of the laser is much smaller than the cavity length.3. By contrast with several methods of measuring reflectivity, the CRDS is proved to be the most ideal method for measuring ultrahigh reflectivity. The principle of reflectivity measurement using CRDS is analyzed in detail, and the curves between the ring down time and reflectivity are obtained by simulating the processes of cavity ring down. For the measurement objectives, three improved methods are designed: CRDS with straight cavity. CRDS with combination of straight and folded cavity, and CRDS by alternating four mirrors. The ring down curves are simulated and compared between straight cavity and folded cavity.4. The errors caused by the Faraday Cell in differential laser gyro are analyzed, and the principle of measuring weak losses of the medium in the cavity is discussed. An experiment system for measuring weak losses is designed. In order to measure its polarization absorption, a homogeneous magnetic field is added into the measurement setup along the longitudinal and transverse directions relative to the cavity axisrespectively. The ring down curves are simulated in the presence of measured samples with different lengths and different absorption coefficients in the cavity.