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Study On Add/Drop Multiplexing & Demultiplexing Technology In WDM Systems

Posted on:2004-01-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y L HuangFull Text:PDF
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In recent years, fiber Bragg grating (FBG) technology and optical communication has been developed quickly all over the word. Wavelength-division-multiplexing (WDM) is an attractive fiber-optic communications technique because it increases the capacity of the network. In WDM networks, each optical node needs to have various functions like dropping a wanted signal, adding a new signal or passing all of the incoming signals to the next node. OADM (optical add-drop multiplexer) and multiplexer/demultiplexer can play important roles in order to complete these functions. Beginning from summarizing different kinds of schemes of OADMs, this thesis emphasizes on the study of fixed and tunable (configurable) fiber Bragg grating OADMs based on the fiber gratings and circulators and other forms of Mach-Zehnder interferometer multiplexers/demultiplexers. The following are the main contents: 1.The spectra characteristics, dispersion and time delay of the fiber Bragg grating are analyzed and simulated with coupled-mode theory. Fiber Bragg grating tuning methods are discussed. The linearly tunable FBG and chirp tunable FBG utilizing cantilever beams are introduced in detail. The method of the use of magnetic force to strain-tune FBG is also introduced. 2. OADMs based on the FBGs and circulators are studied experimentally. They include a tunable FBG OADM with the cantilever beam tuning scheme, a reconfigurable OADM using two FBGs and a hybrid reconfigurable OADM using Mach-Zehnder interferometer (MZI) and FBG. The crosstalk of OADM is analyzed. Homodyne crosstalk performance of three kinds of OADMs is studied experimentally. For the II-type structure and III-type structure, there are significant homodyne crosstalk reduction of about 24.4dB, 39.3dB and 23.6dB, 24.6dB on the dropped and added channels, respectively, for the best proposed II-type structure and III-type structure as compared with the conventional I-type structure. 3. Temperature compensation for fiber Bragg grating is studied theoretically and experimentally by using two materials having different thermal expansion coefficient and a special material with a negative thermal expansion coefficient. The total variations in wavelength for compensating fiber Bragg gratings are 1/23 and 1/8 of the uncompensated fiber Bragg gratings, respectively. 4. Fixed wavelength OADM units and technique based on fiber Bragg grating and optical circulators are designed and made, including single-wavelength OADM units and four-wavelength OADM units. The dropping central wavelengths accord with a four-wavelength transmitter. The four wavelengths are 1550.92nm, 1551.72nm, 1552.52nm, 1553.32nm, respectively. The wavelength spacing is 0.8nm conforming to the national ITU-T Grid. The corresponding adjacent isolation is not less than 35dB. Temperature stability of the OADMs is considered. The fiber Bragg grating in the single-wavelength OADM is packaged by using special material with a negative thermal expansion coefficient. The dependence of the Bragg wavelength over the range of –20-600C is 0.0027nm/0C. In order to avoid the affection of the environmental temperature, the device for tuning fiber Bragg grating in the four-wavelength OADM is placed in a constant temperature box. The dependence of the Bragg wavelength over the range of –20-600C is 0.004nm/oC。These two kinds of packed technique have been applied Chinese invent patent. 5. Through changing the path difference of the two arms of the Mach-Zehnder interferometer by PZT, we can get the relationship between specific wavelength and the voltage applied on the PZT. The tuning range is 3.86nm while the voltage range is from –56v to 56v, and the phase change is 3.5π. 6. A novel structure of high extinction ratio multiplexer/demultiplexer based on a Mach-Zehnder interferometer and a fiber loop mirror (FLM) is carried out. The numerical simulation and experimental results show that the extinction ratio can be improved greatly in comparison with the conventional Mach-Zehnder interferometer in the same parameters. A single channel optical switch is also demonstrated by using a fiber Bragg grating (FBG) instead of the FLM. The dynamic range of greater than 37.4dB at wavelength-selective port is obtained. A novel structure of high extinction ratio multiplexer/demultiplexer based on dual-pass Mach-Zehnder interferometer is carried out. The characteristic of the transmission spectrum is improved greatly in comparison with the conventional MZI. The experimental results show good consistency with the theoretical ones.
Keywords/Search Tags:fiber Bragg grating, optical add-drop multiplexer, cantilever beam, optical circulator, multiplexer/demultiplexer, PZT, Mach-Zehnder interferometer, crosstalk, extinction ratio, temperature compensation, package, fiber loop mirror
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