| With the popularity of broadband data services and the high-speed development of big data services such as cloud computing,the demand for the capacity of optical transmission is increasing exponentially.At present,the combination of various multiplexing technologies and high-order modulation has been widely used to increase the information capacity of communication systems,including wavelength division multiplexing(WDM),quadrature amplitude modulation(QAM),orthogonal frequency division multiplexing(OFDM),time division multiplexing and partial differential multiplexing.However,these multiplexing technologies have reached their expansion limit basically,it is still far from meeting the growing capacity demand.In order to solve this problem,space division multiplexing(SDM)technique has been presented and attracted much attention as a promising method to overcome the future capacity crunch.The orbital angular momentum(OAM)multiplexing technology is a special form of space division multiplexing,the beam has a spiral phase wavefront,and it has a circular intensity distribution due to its phase singularity at the center.The vortex beams carrying OAM are orthogonal to each other and can not interfere with each other theoretically.Therefore,OAM is expected to be applied to optical communications as a new dimension for multiplexing,which can improve the information capacity of optical communication systems greatly.In order to realize long-distance fiber transmission one of the key components is in-line fiber amplifier.Therefore,designing an optical fiber amplifier for OAM mode transmission will provide a theoretical basis for the practical application of OAM optical fiber in SDM system,and is of great significance.In this paper,we focus on the theme of OAM mode amplifier based on annular photonic crystal fiber,and designs a fiber amplifier structure for the OAM modes in the entire C-band(1530nm-1565nm).And then,the performances of fiber amplifier have been researched and analyzed in-depth.The main contents are as follows:(1)Proposed an annular photonic crystal fiber structure,it can supported 14 OAM modes in the C-band.And it could not only satisfy the condition that the effective refractive index difference of all vector modes should be larger than 10-4,but the modes is well bounded in the high refractive index region.(2)By introducing the concept of correction factor of overlap integral,the overlap factor is corrected by the normalized overlap integral between the signal and the pump optical field to make the result more accurate.(3)The characteristics of the fiber amplifier were simulated and analyzed under different conditions(the signal power,the pump power,the fiber length and the doping concentration).We analyze the curves of gain,differential model gain(DMG)and the noise figure(NF)under the different conditions,and choosing the best parameter values.Under the condition of selecting best parameters,numerical simulations suggest that the proposed OAM fiber amplifier can provide a gain larger than 20 dB for all 14 OAM modes,with the small DMG less than 0.15 dB and the noise figure lower than 3.5 dB across the C-band. |
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