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Research On Radio Over Fiber Technology Based On Novel MZ Modulators

Posted on:2010-10-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:J G ZhaoFull Text:PDF
GTID:1118360302969349Subject:Optical communication
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To meet the explosive demands of broadband wireless multimedia services (such as video, audio and Internet data), the future wireless networks have a trend of using higher Radio Frequency (RF) operating frequency. How to improve the communication performance and minimize the cost of base stations (BS) deployment are keys to success in the market. Radio over fiber (ROF) system which is a kind of analog optical link involves the use of optical components and techniques to transmit RF signals from the central stations (CS) to the distributed BS which is closer to the user terminals.To overcome electronic bottleneck, photonics approaches for generation of microwave/millimeter wave signals are preferred. These generation approaches include internal modulation, external modulation, optical heterodyning and so on. A simple and efficient technique to modulate optical fields using RF signals is employing Mach–Zehnder (MZ) modulators. The double sideband (DSB) optical RF signal is generated by conventional modulation scheme and the RF power detected at the base station suffers from a periodic degradation effect due to the fiber chromatic dispersion. An alternative modulation scheme immune to periodic degradation effect is optical single sideband with carrier (OSSB+C) modulation.In the ROF link based on OSSB+C modulation, baseband data is usually carried by two optical components. Under the influence of fiber dispersion the baseband data carried by different optical components will reach the optical detector at different time, and distortion in waveform of the base-band data carried by the RF signal recovered by the optical detector appears. The influence of the inter-band dispersion and in-band dispersion on the wave form of the baseband data is considered. According to the relationship between the waveform distortion and the bit-error-rate (BER) of the ROF link, a theoretical upper limit of the transmission capacity of the ROF link (taking BER of 10?9 as an example) is obtained.Due to some reasons, modulation of optical carrier using RF signal usually operates in the condition of small modulation depth, which corresponding to small modulation index. As a result, the carrier to sideband ratio (CSR) of the generated optical RF signals is undesirable. In this work an exact theoretical model for OSSB+C modulation scheme is developed. An analysis of the theoretical model reveals that the CSR of the optical RF signals can be adjusted by the modulation index and there is an optimum modulation index which can result in a desirable CSR. In order to deal with the problem that undesirable CSR of the optical RF signal is produced when the modulation of optical carrier using RF signal operates in the condition of small modulation depth, we improve the structure of the MZ modulator and propose a novel triple-arm MZ modulator utilizing 1×3 multimode interference (MMI) coupler. The triple-arm modulator can optimize CSR of the generated optical RF signal without changing the modulation index. The modulator contains a'direct current (DC) arm'and the CSR of the generated optical RF signal can be adjusted by controlling the DC bias applied on the'DC arm'. MMI coupler is a kind of integrated optics coupler. And incorporating the technique from integrated optics area into ROF technology is very meaningful.Optical modulators are nonlinear devices and undesired high-order harmonics (nonlinear components) are generated when the MZ modulator is driven by RF signals to generate optical RF signals. A novel four-arm MZ modulator based on 1×4 MMI coupler is proposed in this work. The proposed MZ modulator can implement OSSB+C modulation scheme with the main high-order harmonics signal suppressed, thus the linearization of the modulation scheme is enhanced. Suppression of the high-order harmonics is important for single/multi-channel ROF link. Since the modulator utilizes the optical phase relationship between the outputs of a 1x4 MMI coupler, it can operate without DC bias.The mechanism of MMI couplers is the'self-imaging'effect arising from the constructive interference between the guided modes in the multi-mode waveguide. Correspondingly, the conventional theory for design of MMI couplers is the self-imaging theory. With the development of optical communication devices made of weakly guided materials, conventional self-imaging theory has shown some shortcomings, i.e. the design error for weakly guided material MMI couplers is significant. In this work the main reasons for the design error are analyzed and an improved self-imaging theory is proposed. Two specific approaches based on the improved self-imaging theory for design of MMI device are proposed.
Keywords/Search Tags:Radio over fiber, Mach-zhender modulator, Optical single sideband and carrier modulation, Multimode interference coupler, Carrier to sideband ratio
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