Research On Asymmetric Reflective Semiconductor Optical Amplifier For WDM Access Network

Semiconductor Optical Amplifier(SOA)is one of the key solutions to realize wavelength reuse and colorlessness of optical network units and in wavelength-division multiplexing passive optical network.Restricted by the structural symmetry of the straight waveguide,the conventional SOA cannot satisfactorily erase the downstream signal while linearly amplifying the upstream signal simultaneously.Therefore,in the wavelength reuse scheme based on conventional reflective SOA(RSOA),one can often observe severe crosstalk brought by the residual downstream components in the upstream signal eyediagram,which inevitably hinders the upstream transmission performance.Focusing on solving this problem,and originating from the idea of breaking the device symmetry,this paper designs and investigates an asymmetric SOA and the integration of reflective SOA and EAM(i.e.,RSOA-EAM),in which the gain saturation characteristics are discriminated between the down-and up-stream signals.The main content of this thesis is as follows.(1)Beginning with the basic physical principles in SOAs and EAMs,the physical models are summarized,together with their numerical implementation method where the integrated RSOA-EAM is taken as an example.Taking the published works as a reference,the simulation model is verified,which provides an effective simulation tool for proposed device designs.(2)An asymmetric SOA is designed.The asymmetric SOA alters the gain saturation effect between the down-and up-stream signals by gradually narrowing the ridge width.The time-domain traveling wave model is used to analyze the gain saturation characteristics of the asymmetric SOA both in a segmented form and as a whole.The results verify that the asymmetric SOA can erase the downstream signal and linearly amplify the upstream signal as required.The number of quantum wells and the ridge width are optimized and the effects of bias current and operation wavelength are analyzed,which provide instructions for the device fabrication and operation.(3)An asymmetric RSOA-EAM is designed.The asymmetric RSOA-EAM improves the quality of the upstream signal eye diagram by ultilizing the advantages of the asymmetric SOA.Compared to the straight-waveguide symmetric RSOA-EAM,the proposed asymmetric RSOA-EAM can recover a cleaner optical carrier,and the upstream waveform after re-modulation and re-amplification has a higher integrity and extinction ratio.When restricted by the same bit error ratio,the asymmetric RSOA-EAM can relax the limit of the downstream extinction ratio by 4~7 d B.(4)The asymmetric SOA and the asymmetric RSOA-EAM are experimentally verified.A conventional wafer processing procedure has been conducted to fabricate the asymmetric SOA and the asymmetric RSOA-EAM samples.The experimental results reveal that,for an input power ranging from-16 d Bm to-3 d Bm,the downstream signal light lies in the nonlinear saturated region of the asymmetric SOA,and the upstream signal lies in the linear amplification region,while the straight waveguide SOA shows no difference between the behaviors of the down-and up-stream signals as a contrast.Benefiting from this discrimination inside the asymmetric SOA,the measured upstream eye diagram of the asymmetric RSOA-EAM has a clearer opening compared to the similar reported works,verifying the effectiveness of the device.Besides,the proposed RSOA-EAM is able to offer a small signal gain of 25 d B,a saturated output power of 10 d Bm,a 3-d B spectral width of25 nm.If forward error correction coding is applied,upstream transmission of 30 km at 5Gbit/s can be achieved.(5)A polarization-discriminated RSOA-EAM is designed.The proposed device ultilizes the close to orthogonal polarization states of the down-and up-stream signals to further reduce the crosstalk between the down-and up-stream signals.Simulation results shows that,by introducing a Faraday rotator at the rear end of the RSOA-EAM,the upstream eye diagram quality can be significately improved by tuning the Faraday rotation angle,but at a sacrifice of the upstream singal power.The optimum Faraday rotation angle is always achieved in balancing the power and integrity of the upstream signal.Compared to the RSOA-EAM without ultilizing the polarization dimension and the asymmetric RSOA-EAM,the polarization-discriminated RSOA-EAM at its optimum rotation angle can improve the quality of the upstream eye diagram by 10 d B and 6.3 d B,respectively.