Fabrication And Properties Of Photosensitive Azopolymer Waveguide Gratings

Azobenzene doped or covalently bonded polymers (azopolymers in short) materials have shown great potential in all aspects of photonics applications, including photon generation, photon control, photon conversion, photon storage, photonic transmission, etc. In a photonic transmission system, the tunable optical fiber devices, such as all-optical tunable lasers, all-optical tunable optical add-drop multiplexers, etc. are very important parts. In this thesis, based on reversible photoisomerization, photoinduced mass transport and photoinduced birefringence of azo moieties, the main focus is to prepare waveguide gratings, especially optical fiber gratings, which could provide the necessary theoretical and technical support for the intelligently-controlled optical fiber communications systems.The research contents of this thesis start from azopolymer material to fully research its photosensitivity. And then use point-by-point method, mask method, etc. to write azopolymer planar gratings. Furthermore, based on these techniques, using the amplitude mask method and interference method respectively, fiber gratings were inscribed on photosensitive azopolymer optical fiber, and grating characteristics were further analyzed. The main contents of this thesis are shown as follows:1. At first, the basic characteristics of azopolymer materials, including photoisomerization, photoalignment, photoinduced birefringence, photoinduced mass transport and liquid crystallinity are reviewed, and then some applications of the azopolymer materials in various areas of photonics, such as photonic production, photon regulation and control, photon conversion, photon storage, photon transmission are listed. Finally, the purpose and importance of the work is described, in order to write high birefringent and all-optical erasable fiber gratings with a non-invasive method and eventually be applied in the all-optical tunable optical devices and systems.2. The principle using the plasmon surface polaritons technology to study and characterize film refractive index and thickness was introduced, and then the photoisomerization of the azopolymer was analyzed by UV-vis spectra. Furthermore, the influence of the irradiation intensity upon the extent and rate of isomerization was studied and found that, the degree of isomerization increased with increasing of the light intensity, while the isomerization rate increases linearly with increasing of the light intensity. Then using the plasmon surface polaritons technology photoinduced refractive index change resulted from the photoisomerization was analyzed, and the relationship between the photoisomerization and photoinduced refractive index change was established from the experiments and theory. Combining with light intensity effect upon the extent and rate of the isomerization, the impact of light intensity upon the photoinduced refractive index change value and rate is discussed. At last, based on photoinduced refractive index changes of azopolymer materials, optical switching can be achieved, and then the impact of light intensity upon the optical switching was further studied.3. After the analysis of the various chemical and physical impact factors upon the photoinduced alignment and photoinduced birefringence, (E)-2-(4-((4-cyanophenyl)diazenyl)phenoxy)ethyl methacrylate(2CN) was chosen as the material to make azopolymer optical fiber. Then, in room temperature and 1atm, linearly polarized laser sources with the wavelength at the azobenzene's linear absorption band or at 532nm could be chosen to inscribe azopolymer optical fiber gratings. In future, using non-covalent polyelectrolyte, increasing the concentration of azobenzene in the azopolymer, copolymerizing with other longπ-conjugated group to get mesogenic collaborative interaction and doping noble metal nano-particles can effectively improve the photoinduced birefringence and its stability. Furthemore, the birefringence induced by linearly polarized light could be"erased"by non-polarized or circularly polarized light, realizing reversible writing and erasing of photoinduced birefringence.4. One-dimensional or two-dimensional long-period planar gratings were fabricated in azopolymer films by the point-by-point method, double-frequency grating method and amplitude mask method. Their properties including erasability, polarization, stability, surface morphology, etc. were further analyzed and studied. The results show that it is suitable to write azopolymer long-period optical fiber gratings using the polarization direct writing method and amplitude mask method.Both surface relief gratings and pure polarization gratings could be fabricated in azopolymer films by the phase mask method and interference method. The results demonstrated that it is suitable to write azopolymer short-period optical fiber gratings using these two methods.5. Azopolymer optical fiber was prepared using Teflon technique, in which it was found that the core refractive index of azo monomer-doped polymer optical fiber is smaller than the design value and the core of a fiber would be larger, due to the re-dissolution and diffusion effect of core material, while the difference between the core refractive index of azo copolymer-doped polymer optical fiber and design value was not significant, as the diffusion coefficient of azo-copolymer is far lower than that of small monomer molecules, but so far such fiber has not yet had very good optical properties.Then birefringent long period fiber gratings were fabricated in azopolymer optical fiber by the amplitude mask method. Futhermore, the reason for its formation, its difference with conventional fiber gratings and its specific application was analyzed in theory.Finally, fiber Bragg gratings were fabricated using two interference modes in azopolymer optical fiber. Thereinto, after fabrication of the fiber Bragg gratings using the polarization modulated interference light field in azopolymer optical fiber, the formed gratings can be erased by one circular polarized beam, which could be written and erased for several cycles; while using the intensity modulated interference light field, fiber gratings can also be written and erased by several cycles, but the repeatability is not good enough. This discovery laid the basis of the azopolymer optical fiber gratings used for smart adjustable optical communication device.