Theoretical And Technical Research About All-fiber Mode Control Device

Mode division multiplexing (MDM) is a emerging technology of optical fiber communication system. Due to the unique advantages and the potential for large communication capacity expansion, MDM has become a research hotspot in recent years. Since this technology has just appeared shortly, some key theory and technology has not been studied thoroughly.Of which the most critical part as well as a difficult part of MDM is the mode control related devices. Based on the mode theory in fiber and the coupled mode theory in fiber grating, this paper studies the principle of several all-fiber mode control devices and proposes a new all-fiber mode control device. The main contents of this paper are as follows:1?A two-mode converter based on a semicircular-distribution long period fiber grating is proposedIn this paper, we design a semicircular-distribution long period fiber grating structure. The mode converter based on this fiber grating can completely destroy the symmetry of the transverse refractive index disturbance, and realize the coupling between LP01 mode and LP11 mode.Compared to CO2-laser side-written long period fiber gratings and tilted gratings, this kind of gratings have much larger bandwidth, thus providing a better performance in mode converting.The simulation experiment is carried out to test the performance of this kind of gratings, and the simulation experiment is based on the beam propagation method. The simulation results are analyzed, and the performance of the converter based on a semicircular-distribution long period fiber grating is summarized.2?The semicircular-distribution long period fiber gratings and CO2-laser side-written long period fiber gratings are theoretically analyzed and comparedBased on the coupled mode theory, we have derived the equation of calculating the coupling bandwidth in few mode fibe gratings. The coupling bandwidth is proportional to the AC coupling coefficient.According to the distributions of the refractive indices of the semicircular-distribution long period fiber gratings and CO2-laser side-written long period fiber gratings, we have calculated the AC coupling coefficients between the LP01 mode and the LP11 mode. We find that the AC coupling coefficient between LP01 mode and LP11 mode of semicircular-distribution long period fiber gratings is more than twice that of CO2-laser side-written long period fiber gratings.Based on the equation of calculating the coupling bandwidth, we have calculated the mode conversion efficiency spectrum between the LP01 mode and the LP11 mode. With appropriate refractive index disturbance,the semicircular-distribution long period fiber grating can provide very high coupling efficiency in a very large bandwidth. This makes it possible to fabricate all fiber mode conversion devices with very large bandwidths.3?A four-mode converter based on a cascaded semicircular-distribution long period fiber grating is proposedConversion from fundmental mode to more high order modes is needed to expand the communication capacity of MDM system. By analyzing the light field distribution of the LP21 mode, we design a four-mode converter based on a cascaded semicircular-distribution long period fiber grating, which can convert the fundamental mode to the LP21 mode.This structure can provide a larger coupling coefficient between the LP01 mode and the LP21 mode than other mode converters, thus having a better performance in the conversion between the LP01 mode and the LP21 mode.