Fundamental Research On Electro-optic Modulator Based On Poled Polymer Materials

Advances in network benefits from the development of fiber communication because 80% information is transferred by fiber. The telecommunication efficiency has increased greatly by utilizing electro-optic, photoelectric conversion and all optical networks. The frequency conversion, modulation, switch, deflection are basis devices in optical information processing, which put into effect depending on the optical signal (intensity, phase, polarization, etc.) of nonlinear optical materials along with the change of applied electric, sound, magnetic and optical field. The boosting performances of nonlinear optical materials has stimulated fast growth in optic communications era as same as the increasing of semiconductor materials have impelled the large scale development of microelectronic technique and integrated circuits The electro-optic characteristics of poled polymer and its exploratory development in electro-optic(EO) devices have been discussed. The history and the latest developments in nonlinear optical polymer materials have been introduced briefly. The necessity of the research on optics devices based on poled polymer materials is explained. The electro-optic effect of poled polymer and various measurements of electro-optic coefficient have been investigated. Many poling techniques are compared. Based on it, the theoretical and experimental research work for poled polymer electro-optic modulator has been done.1. A measurement for the electro-optic coefficient based on asymmetry Fabry-Perot cavity multi-beam interferometry is proposed.Two aluminum films were deposited on glass substrate by thermal evaporation in high vacuum to form high reflection mirror (HRM) and low reflection mirror (LRM), respectively. The reflective coefficients of the LRM and the HRM, i.e. R1 and R2, is about 0.4 and 0.9. Nonlinear polymer thin films are spin coated on LRM and poled by corona poling in order to break the internal centrosymmetry. Then the sample was turned upside down and overlaid on HRM to form the asymmetry Fabry-Perot cavity configuration. The electro-optic measurement system with obverse incidence of light was used in experiments. A 1.3μm laser beam passes through a focusing lens, polarizing beam splitter(PBS), a quarter wave plate, dichroic mirrors, objective lens(40×, 0.65NA), and focused on the sample at its normal direction. A portion of reflective beam through dichroic mirror is monitored. The other portion of reflective beam is received by a photodiode passed through polarizing beam splitter. A light chopper is added in order to change the DC to AC laser light so that the DC light intensity can be detected.When the light chopper was removed from the beam path, the modulated output signal can be detected. According to the DC light intensity and the modulated output signal, the electro-optic coefficient can be closely calculated. The previous papers do not refer to the measure technique.2. The poling technique with a shelter was proposed for nonlinear optical film for the first time. The minimum width of the shelter can be put in practice is found to be 8 micron. The results supply references to selective domain-inverted poling in order to fabricate the high efficiency electro-optic waveguide modulator. Nonlinear polymer thin films are spin coated on low reflection mirror (LRM) and corona-poled with a metal wire as shelter. The diameter of the metal wire is 120 micron and 8 micron, respectively. The asymmetry Fabry-Perot cavity configuration and high resolution electro-optic measure system are used to detect the electro-optic signals of poled polymer film. The electro-optic output signal changes periodically vertical to the metal wire. The experimental results showed that the minimum width of the shelter is 8 micron. There is a 4-micron boundary domain as a transition zone of electric field between shelter and non-shelter domain. The characteristic of poled polymer film can be investigated according to the electro-optic output signal. Also the poling uniformity of film can be investigated. It is feasible to study on the influence of domain-inverted poling on the optical performance of polymer materials.3. The hybrid DR1/SiO₂ film utilizing sol-gel technique has been prepared. Also the strip waveguide using organic/inorganic film was fabricated successfully.The reaction process of sol-gel system has been discussed theoretically. The optimal preparation for the DR1/SiO₂ film was given. The catalyzer and the ratio of water to silicon have effects on the electro-optic performance and thickness of DR1/SiO₂ film. The electro-optic coefficient of the hybrid film will be raised largely by increasing the content of organic chromophore DR1 with the given molar ratio of water to silicon. The ratio of water to silicon has also effect on the thickness of the DR1/SiO₂ film to some extent. The fabrication of electro-optic waveguide must conform to the requirements for materials. The strip waveguide using the organic/inorganic DR1/SiO₂ film was fabricated by semiconductor technology. The measurement results demonstrated that it is feasible to fabricate optic waveguide utilizing the silica film.4. The highly transparent TiO₂/SiO₂ electro-optic material with better thermal stability has been prepared for the first time.The hybrid TiO₂/SiO₂ film is spin coated on ITO glass and cured. The corona poling has been done with gradient ascend and gradient descent temperature. The electro-optic coefficient of the film r33=42pm/V was obtained at a fundamental wavelength of 1300nm and the value was unvaried for 18 months at room temperature. If the poling condition is controlled better, the electro-optic coefficient was stable at 80℃for 2500 h. The influence of mixture ratio in starting solution on the refractive index and the thickness of the poled DR1/TiO₂/SiO₂ film has been discussed. The results provide references to control the refractive index and thickness of the hybrid films and fabricate electro-optic waveguide devices.5. The phenomena of electro-optic coefficient of the hybrid film changing with the frequency of applied alternating voltage have been studied. The results showed that the electro-optic signal should be detected in high frequency.6. The matching performance between the hybrid organic/inorganic TiO₂/SiO₂ films and the cladding materials has been investigated. The exploratory work and instructive trial have been done to fabricate the poled polymer electro-optic waveguide devices utilizing the DR1/TiO₂/SiO₂ material.In experiments, the results have been obtained as follows. (1) The refractive index of the hybrid TiO₂/SiO₂ film doped with DR1 can be adjusted for the fabrication of the electro-optic waveguide device. (2)The mismatch between the hybrid TiO₂/SiO₂ film and cladding materials PMMA-GMA due to their different thermal expansion coefficients is not solved ultimately. (3) The thickness of the hybrid TiO₂/SiO₂ films is so thin that it is not suitable for being core layer by itself. The loss of the optic waveguide is too large.The improving configuration of waveguide has been designed. The buffer was added as auxiliary layer in order to improve the chemical performance matching and strain matching between the hybrid DR1/TiO₂/SiO₂ film and cladding materials PMMA-GMA. The results demonstrated that the majority of light power in waveguide can be guided to the core layer that is formed by hybrid DR1/TiO₂/SiO₂ film. As a result, the loss of optic power was decreased largely.The exploratory work established fundamental for the fabrication of high quality poled polymer electro-optic modulators and electro-optic switches.