Poled Fiber Phase Modulator And Its Application

High speed, broad bandwidth, low loss and all-fiber phase modulators are the most important components in fiber-optic communications and sensors. It also can be used to realize high-performance all-fiber close loop gyro. But the widely used LiNbO3 crystal and PZT phase modulators can not satisfy the above requirements. So this thesis will manufacture phase modulators directly on the fiber by thermally poling. During thermally poling process, silica materials are heated to about 300℃, and applied a 2000~6000 high voltage(the electric field intensity is about 108V/m). The process will persist 10~30min. Then holding the voltage, fiber samples are cooled down to the room temperature. After above process, the electro-optic effect emerges. The electro-optic coefficient is proportional to the poling voltage and also related to poling temperature, time and components of samples. The electro-optic region lies only a 20μm thin layer beneath the anode. So the anode must be very close to the fiber core.According to the characteristics of the poling fibers, the following theoretical analysis and experiments are accomplished in this thesis:1. The propagation characteristics of the poling D-shaped fiber with complex geometrical structure are theoretically analyzed by Circular Waveguide model and scalar approximation theory for the first time. Firstly, the equivalent refractive index model of the fiber is proposed. In the fiber cladding and core, the optical fields are represented by the sum of the Bessel and modified Bessel functions, respectively. In the polymer and metal, the optical fields are written to the sum of the exponential functions. To match the boundary conditions, the modified Bessel functions are expanded to the planar wave in the interface. Then the eigenvalue equations of different polarizations are obtained by scalar approximation. At last, the relations between propagation losses and the states of polarizations, the geometric structures of the fiber, the refractive index and thickness of the polymer are calculated. The calculated results will provide theoretical instructions to design and optimize the parameters of devices in experiments.2. The propagation characteristics of the poling twin-hole fiber are theoretically analyzed for the first time. Due to the facts that the diameters of the holes and electrodes are much larger than that of the fiber core, they can be regarded as planar waveguides. Corresponding eigenvalue equations are attained by the similar method used to solve D-shaped fiber. Some important conclusions are achieved by numerically solving the eigenvalue equations: the propagation loss of x-polarization mode is much larger than that of the y-polarization mode, so the latter can be the operation mode; the air gaps (several micrometers) around the metal electrodes can decrease the propagation loss of the twin-hole fiber greatly. Different electrode materials have different propagation losses.3. The above analytical results of the propagation characteristics of the twin- hole fiber are reasonable and believable by comparing the analytical results to the calculating results through the widely used finite difference beam propagation method (FD-BPM) and experimental data.4. Based on the conclusion of the above theoretical analysis, we prepare the electrodes of poling fiber by inserting thin metal wires into the holes of the fiber. This technical project has the advantage of low propagation loss and can resist high poling voltage that can improve the electro-optic coefficient of the poled fiber devices.5. We accurately control the diameter of holes by inner erosion, use the black tungsten wires with lead coating as the electrode material and assemble a suit of microscopic and adjusting system to insert the tungsten wires into the holes. A pair of 1m long internal electrodes is obtained by careful manipulation. This length is an order higher than the present experimental result. The loss of the fiber is less than 0.5dB/m with electro-optic coefficient of 0.06pm/V. This performance can be applied to practical application.