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Research Of Few-mode Fiber Devices Based On Long-period Fiber Gratings

Posted on:2017-01-23Degree:MasterType:Thesis
Country:ChinaCandidate:WeiFull Text:PDF
GTID:2308330509452391Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Single-mode optical fibers(SMFs) have reached the limit of the transmission capacity due to the nonlinear effects. To reduce the nonlinear effects, the most direct solution is to increase the core diameter. When the core diameter increasing to a certain value, the fiber can transmit high-order modes, that is the few-mode optical fiber(FMF). Currently, the FMF based communication technology could be divided into two kinds. One is increasing system capacity by the mode-division multiplexing(MDM) technology. Another is single-mode operation in FMFs by selectively exciting one single mode.Firstly, we present a novel fundamental mode filter based on long-period fiber gratings(LPFGs). The MDM technology is the method that each mode in FMFs works as an individual information channel and transmits different signals. Therefore, mode converters should be used to convert between the fundamental mode in a SMF and the high-order modes in a FMF. The current mode converters are able to achieve efficient mode conversion, but few researches are about the rest of the fundamental mode being the cross-take to the high-order mode. The mode filter uses an apodized configuration of LPFG that the fundamental mode will coupled with the cladding mode in a FMF, and in virtue of the refractive index of the coating higher than the cladding index, the converted cladding mode will be dispelled. At the same time, ensure that core other high-order modes will not couple with cladding modes or the power will transfer back after coupling. Thus achieve the purpose of high fundamental mode loss and low high-order mode loss simultaneously. Furthermore, we also present an approach of cascading LPFGs with different periods to expand the operating bandwidth, furthermore analyze the operating bandwidth of the mode filter varying with the surrounding environment temperature. Numerical simulation shows that the operating bandwidth of the cascade structure can be as large as 23 nm even if the proposed filter operates in the environment temperature range of 10~40℃.We also propose a novel mode-field converter based on a two-core optical fiber with a LPFG. Single-mode operation in FMFs can be realized by selectively exciting one single mode, and need possess low cross-take. Generally, the fundamental mode in FMFs can be excited simply by the connection of a FMF with a SMF. However, such method induces large connection loss especially for large-mode area FMFs, owing to the large differences in the mode-field area and distribution between SMFs and FMFs. In addition, such method will generate unwanted modes in FMFs, which will be the cross-talk to the fundamental mode. The mode-field converter realizes low-loss coupling between two LP01 modes in single-mode core and few-mode core by applying a LPFG in few-mode core. In the few-mode core, high-order modes will also be cross-take to LP01 mode. We analyze the cross-take and compare the conversion efficiency and the cross-take when two fibers with same parameters connect directly. Numerical simulation shows that our proposed mode-field converter can realize mode-field conversion with low-loss and loss cross-take between a SMF and a FMF. When the requirement of the normalized output power of the fundamental mode in few-mode core is higher than-0.5 d B, the operating bandwidth can be as large as 36 nm, moreover the powers of the high-order modes are at least-21 d B lower than the power of the fundamental mode.
Keywords/Search Tags:few-mode optical fiber, two-core optical fiber, long-period fiber grating, mode converter, filtering mode, mode-field area, conversion efficiency, bandwidth
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