雷射二極體上的啁啾效應及其在傳輸時之應用與影響

Abstract

當雷射二極體受到調變信號影響後，其雷射內部共振腔材料的等效長度會受到調變信號的影響而有些微的變化，這個變化會造成雷射輸出波長的改變，而這種動態響應的變化就叫做“啁啾效應”。 本篇論文主要在詳細探討當雷射經過調變後所產生的啁啾效應，並且就不同的調變的方法，如直接調變雷射(Directly-Modulated Laser, DML)、電吸收調變器(Electro-Absorption Modulator, EAM)、與馬赫任德調變器(Mach-Zenhder Modulator, MZM)等，進行分析與測量其啁啾效應的結果，並藉由了解其啁啾的特性對於傳輸時的影響。 直接調變雷射會產生特別嚴重的啁啾效應，當與光纖的色散互相作用下會嚴重的影響傳輸距離。因此我們所提出來的方法便是利用傳輸端所產生的頻率鍵移(Frequency Shift Keying, FSK)信號透過光濾波器轉換成開關鍵移(On-Off Keying, OOK)信號，以便在傳輸端直接抑制啁啾的效應，進而達到在傳輸更遠的距離時能有很好的傳輸品質。

Semiconductor laser change the output light intensity with an modulated voltage or current. However, the signal output phase or frequency also changes to give a dynamic chirp effect. Combined with chromatic dispersion, chirp effect typically limits the transmission distance of an optical signal. In this thesis, we focus on the chirp effect of directly-modulated semiconductor laser (DML) and external modulators such as Electro-Absorption Modulator (EAM) and Mach-Zenhder Modulator (MZM). The analyzed and measured results are used to investigate the impact of chirp to a lightwave transmission system. From experimental measurement, directly-modulated semiconductor has severe chirp effect that limits the transmission dispersion when interacting with fiber chromatic dispersion. The chirp effect is suppressed by passing a directly-modulated signal through a narrow-band optical filter to convert the frequency-modulated signal to on-off keying signal. The transmission distance is extended from 20 to over 60 km.