正交分頻多工分極化直接偵測系統應用多輸入多輸出處理程序

Abstract

面對各種寬頻服務的需求增長，新一代中短途（<1000公里）的光都會及接取網絡設計須考量頻譜效率(spectral efficiency, SE)。以數位訊號處理為基礎的光學正交頻分複用（orthogonal frequency-division multiplexing, OFDM）成為未來中短途光網路中最具吸引力的調變格式之一，因其具有高頻譜效率及對光纖色散和偏振模色散的對抗性。為了實現高接收靈敏度及高傳輸容量的系統使用了同調正交頻分複用（coherent, CO）和偏振分工複用（polarization-division-multiplexing, PDM）。但對CO-OFDM系統而言，缺點就是需要在收發兩端裝上窄線寬的雷射作為本地振盪器，和複雜的相位雜訊補償。相對而言直接接收（direct-detection, DD）則是另一種實現光學OFDM的方式(DD-OFDM)。其參考光載子與OFDM訊號一起發送，使接收器不需要額外的雷射振盪器和複雜的補償。儘管如此，當應用PDM技術來進一步提高傳輸容量和SE時多輸入多輸出（multiple-input multiple-output, MIMO）技術卻無法解調此DD-PDM-OFDM訊號。這是因為其參考光載子可能只存在於某單一的光偏極態，需要外加一個額外的偏振態動態追踪器。因此本論文中，提出一個新穎的DD-PDM-OFDM系統，並不需要在接收端加入額外的偏振態動態追踪且同時具有對抗光纖色散和偏極模色散的能力。此新穎的方式是使DD-PDM兩個偏振態的光載子能互相保持一個載波的頻率差，並使用重新設計之多輸入多輸出技術來去除極化的相互影響便可成功解調DD-PDM訊號。在模擬結果中，說明在本架構下所需的光訊噪比在達到誤碼率10-3的情況下，任何偏振態角度(state of polarization, SOP)所對應的系統代價小於3.6 dB。並且在實驗中印證與模擬的結果。最後也成功在100公里傳輸實驗下達到50Gbps傳輸量且沒有因為色散而造成任何系統代價。

As the demand of the broadband service increases, the next-generation short-haul (<~100 km) and med-haul (<~1000km) optical networks must be high spectrally efficient. It is well known that employing advanced modulation formats is one of the most promise ways to increase the capacity and spectral efficiency (SE). Due to the high SE and tolerance of the chromatic dispersion (CD), optical orthogonal frequency-division multiplexing (OFDM) technique has become an attractive approach in the near future. Moreover, coherent (CO) OFDM and polarization division multiplexing (PDM) are used to achieve high transmission capacity and receiver sensitivity. However, the drawback is that CO-OFDM receivers need a narrow-linewidth laser as a local oscillators (LO) and complex phase noise compensation algorithms. Direct-detection (DD) OFDM system is an alternative to realize optical OFDM transmission where the reference optical carrier is sent along with the OFDM band. Accordingly, DD-OFDM system do not need the optical hybrid component and LO at the receiver. Applying PDM technique to improve capacity and SE, however, the reference optical carrier may be faded at some state of polarization (SOP). Hence, a dynamic SOP tracking is required to increase cost and decrease the feasibility of DD-PDM-OFDM. This work proposes a novel DD-PDM-OFDM scheme without the need of dynamic polarization control at the polarization-diverse receiver, and the proposed scheme is robust against both CD and polarization mode dispersion (PMD). With setting the frequency difference between two polarization-orthogonal reference carriers as one OFDM subcarrier spacing, the reference optical carrier fading can be avoided, and the corresponding interference among adjacent subcarriers is eliminated by a novel MIMO processing. From the simulation results, the polarization- dependent OSNR penalty at the BER of 10-3 is <3.6 dB. Moreover, the results also show the CD and the first order PMD will not induce additional penalty. In addition, a novel DD-PDM-OFDM scheme is also experimentally demonstrated without polarization control. By inserting an empty tone every 6 subcarriers, the conditional number (CN) of the MIMO matrix is decreased to smaller than 4.8 in the experiment. Finally, 50-Gbps DD-PDM-OFDM transmission over 100-km fiber is successfully achieved without any dispersion-induced penalty. To further decrease the influence of CN, a powerful algorithm, VBLAST, is applied, and the required OSNR at the worst SOP is improved by more than 1 dB, as an empty inserted every 24 subcarriers.