光纖有線電視系統之模擬與失真評估

Abstract

光纖有線電視系統係採用直接強度調變,訊號失真主要是因雷射的共振失 真,削波失真(clipping distortion)及漏電流效應等非線性行為造成IM( intensity modulation)失真,及雷射的移頻效應和光纖的色散所造成的 FM(frequency modulation)失真。通常為了簡化分析結果 ,而將IM失真與 FM失真分開討論。在此我們將IM失真與FM失真合併分析。做IM失真分析 時,作$\epsilon S\ll 1$之近似會在低頻及高偏壓點時造成誤差,所以我 們將分析過程稍加修改,以避免誤差。在合併IM失真與FM失真時,使用半動 態(quasi-dynamic)方法分析。此種方法在系統FM失真不大時,會有較大誤 差。這是因為半動態分析法無法描述雷射的削波效應及均方根(rms)調變 深度對IM失真所造成的影響。從模擬的結果可將參數大致分為兩類,一為 與IM失真有關,如$\beta,g_0$,二為與FM失真有關,如$\beta,g_0,\alpha, \epsilon,D$。模擬的結果顯示,在設計有線電視系統時,一定要避免削波 失真。並且在短距離傳輸時, IM失真為系統非線性失真之主要來源,此時 要選擇臨界電流較小或是共振頻較高的雷射以改善訊號失真。反之,在長 距離傳輸時,FM失真則主宰系統之訊號失真,此時必須選擇阻泥因子較小或 是共振頻較高的雷射來改善移頻量。 Because CATV system uses directly intensity modulation , signal distortion is composed of IM and FM distortion . Laser IM distortion includes resonance distortion , laser clipping effect , and leakage effect etc . Laser FM distortion includes laser chirping and fiber dispersion effect . In general , we discuss IM and FM distortion individually in order to simplify analysis result . But from now on , we combine IM and FM distortion in our analysis . It will cause IM distortion analysis error in lower modulation frequency and higher bias point to approximate $\epsilon S\ll 1$ . Hence , we modify the analysis procedure to avoid error . We predict CSO by 2HD/C multiplied by IMP . We called this method ``quasi-dynamic''. It will generate larger error when FM distortion of CATV system is small . Because quasi-dynamic analysis can not describe that laser clipping effect and rms modulation index $\mu$ have an influence on IM distortion . We can divide laser parameters into two parts from our simulation results . One is related to IM distortion , as $\beta,g_0$ , and another one is related to FM distortion , as $\beta,g_0,\alpha,\epsilon,D$ . Simulation results told us , when you design a CATV system , you must avoid clipping distortion . For long distance transmission IM distortion dominates signal distortion . On the contrary , FM distortion dominates signal distortion .