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Study on the Tuning Characteristics of the Long Period Fiber Gratings Coated with the Liquid Crystal

Author: WangShaoShi
Tutor: ChenJianPing
School: Shanghai Jiaotong University
Course: Electromagnetic Field and Microwave Technology
Keywords: Long period fiber grating Liquid crystal Resonant wavelength Mode Transition Mercury relay
CLC: TN253
Type: Master's thesis
Year: 2009
Downloads: 27
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Abstract


Optical switching and optical transmission are the basic technologies of the framework for the modern information network infrastructure. With the development of the wavelength division multiplexing (WDM), the number of channels in optical fiber communications is increasing. To flexibly route and exchange optical signal in WDM systems, new technologies like tunable wavelength selective optical switch should be adopted, where wavelength tuning is the key technology. As a result, it is of great importance to study further on the devices and technique of the wavelength tuning.Long period fiber grating is a new type of optical device, which can couple the light with specific wavelengths from the core mode to the cladding modes. It can play the role of band-rejection filter that can be used for wavelength selective optical switch. Since resonant wavelength is sensitive to the refractive index of environment, wavelength tuning can be realized by coating the long period fiber grating with materials, whose optical properties are also tunable. In this thesis, we use liquid crystal as the thin-film coated on the long period fiber grating, since refractive index of liquid crystal changes with the environmental temperature and electric field. Key technologies relating to the structure design and practical realization of the liquid crystal coated long period fiber grating are studied.The tuning characteristics of the long period fiber gratings depend on the characteristics of the refractive index of liquid crystal. In order to analyze the relationship between refractive index of liquid crystal and different environmental temperature and electric field, we need to accurately measure the refractive index of liquid crystal firstly. In this thesis, we use the return-reflection of the fiber end to measure the refractive index of liquid crystal, so as to collect detailed experimental data on the refractive index changes of liquid crystal under different temperature and electric field.According to the theoretical study on the tuning characteristics of the liquid crystal, we build experiment setup to study the changes on the environmental temperature and electric field, so that the spectrum and tuning characteristics of the long period fiber grating can be studied. The adopted liquid crystal is MDA-98-3699 produced by Merck Company. As the temperature is increasing from 20 C to65 C , the refractive index of liquid crystal changes from 1.477 to 1.515, which corresponds to the maximum resonant wavelength shift of the long period fiber grating about 8.53nm. As the voltage changes from 0V to 400V, the refractive index of liquid crystal increases from 1.477 to 1.48, and the corresponding maximum change of resonant wavelength is 4.79 nm. Simulation is carried out and the numerical results are in good agreement with the obtained experiment results, which prove the validity of our experiment.Finally, we do some exploratory research in two other areas. By controlling the position of the mercury ball in the mercury reed relay, we can modulate the light pass through it, which can be used for optical switch. We also study the fabrication of fiber micro ring resonator. It can couple the light with specific wavelength from the micro ring to the bare-cladding fiber, which can be used to realize frequency-selecting optical device.

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CLC: > Industrial Technology > Radio electronics, telecommunications technology > Photonics technology,laser technology > Optical waveguide and integrated optics > Fiber optic components
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