未來光通訊系統所需之新穎光電元件與技術之研究---子計畫一：相位差分調制模式之全新光信號處理元件之開發

Abstract

在高速及長距離傳輸中，差分相位移鍵格式 (differential phase shift keying, DPSK) 已經逐漸 取代傳統的 on-off keying (OOK) 模式，成為下一世代光通信系統的標準傳輸模式。自2000 年以 來，在IEEE 的期刊與會議論文中就有超過400 篇文章的題目是有含有DPSK。DPSK 之所以受到 如此之重視，其中最重要的原因有二；第一：在平衡偵測之下 (balanced detection)，DPSK 模式擁 有約3 dB 優於 OOK 模式之接收靈敏度 (receiving sensitivity)，第二：因為DPSK 的信息傳載是 在相位上，因此每一位元擁有相同的功率，對於因為功率變化所引起的非線性效應，擁有較佳的 抗拒能力，特別是通道間 (inter-channel) 的交差相位調變 (cross-phase modulation, XPM)。這在遠 距離和高速傳輸系統中，更是顯得重要的一項因素。 然而，DPSK 卻同時會受到強度雜訊 (amplitude noise, AN) 及相位雜訊 (phase noise, PN) 的 干擾，這是因為在DPSK 接收端的延遲干涉計 (delay interferometer, DI) 會將相位雜訊轉成強度雜 訊，而影響到接收訊號品質。傳輸過程中的相位雜訊有兩種：線性與非線性相位雜訊 (linear and nonlinear phase noise)。線性相位雜訊跟強度雜訊一樣，主要是來自光放大器的放大自發光 (amplified spontaneous emission, ASE) 雜訊；而非線性相位雜訊則是透過光纖的克爾效應 (Kerr effect)，轉換自強度雜訊而來，這現象通常稱作Gordon- Mollenauer 效應。若要增加DPSK 的傳輸 距離，則無論相位或強度雜訊都需要加以壓制、降低。但是大多的OOK 全光再生器都不適合用 在DPSK 上，這是因為這些再生器會擾亂相位訊息。理論上，相位敏感放大器 (phase-sensitive amplifier, PSA) 可以同時減少相位、強度雜訊，但是它所需要的同調泵光 (coherent pump beam) 是 無法以符合經濟效益的前提下產生的，因為要鎖住泵光和訊號光之間的相位太過困難 (光源的頻 率約200 THz)。此外，有數種能保留相位的強度再生器被發表出來，但它們只能保留線性相位雜 訊，以及藉著壓制強度雜訊來減少非線性相位雜訊，甚至在壓抑強度雜訊的同時會產生一些不可 避免的相位擾動。這些相位雜訊始終沒有被處理，累積之後終究會破壞DPSK 訊號並限制了它的 傳輸距離。 在本計畫中，我們將設計並製造新穎的全光相位雜訊平均器 (phase noise averager, PNA)，它 的功能在平均兩個相鄰訊號的相位雜訊，很重要的一點是，這個PNA 不需要額外的同調泵光。 而且，我們將逐漸擴展PNA 的適用範圍，使其適用於其他相位調置的傳輸模式，例如 DQPSK 和 QAM 等。因此將大大提升此種元件的應用範圍。我們所展示的詳細的解析分析及數值模擬都 證實了這個PNA 具有收斂DPSK 傳輸中所產生的所有相位雜訊，且無論傳輸的距離有多長。因 此，我們將架設一個模擬長距離傳輸的re-circulating loop，將PNA 設置其中來驗證它收斂相位雜訊的效果。

Differential phase-shift keying (DPSK) format, an optical pulse appears in each bit slot with the binary data encoded as either a zero or π phase shift between adjacent bits, has emerged as an alternative to the on-off keying (OOK) format, especially for long-haul transmission. The most obvious advantage of the DPSK format with balanced detection over OOK is that its optical signal-to-noise ratio (OSNR) required to reach a given bit-error ratio (BER) is approximately 3 dB lower. DPSK signals are also less sensitive to nonlinear effects, particularly those of inter-channel cross-phase modulation (XPM), improved dispersion tolerance and high spectral efficiency. Without considering timing jitter, unlike OOK systems that are limited only by amplitude noise (AN), DPSK systems are restricted by both AN and phase noise (PN). The PN in DPSK systems will be converted to AN in the receiver using a delay interferometer (DI). In dispersion-managed systems, AN and linear PN are generated mainly from the amplified spontaneous emission (ASE) noise of optical amplifiers. The nonlinear PN is translated from AN through the fiber Kerr effect, often called the Gordon-Mollenauer effect, and dispersion-induced pattern effects through XPM in wavelength-division multiplexed (WDM) systems. Either PN or AN must be prevented from being accumulated to expand the reach of DPSK systems. Spectral inversion by phase conjugation and post nonlinear phase-shift compensators has been proposed to reduce accumulated nonlinear PN. However, these approaches are rather difficult to realize. Theoretically, a phase-sensitive amplifier (PSA) can simultaneously reduce both AN and PN, and the regeneration was experimentally demonstrated by pumping a PSA with an original undistorted DPSK signal. Even so, the coherent pump beam in a PSA is difficult to realize in the real world, owing to optical-carrier phase-locking between the pump beam and the signal beam. Several phase-preserving amplitude regeneration approaches for DPSK format have been proposed. The reduction of AN is such that the nonlinear PN caused by the Gordon-Mollenauer effect will be reduced and the transmission distance will be extended. Nevertheless, these regenerators can constrain only some of the nonlinear PN and preserve the original linear PN. They will also cause an additional PN that is transformed from AN. Therefore, the accumulated PN distorts DPSK signals and limits the transmission distance. This project proposes a novel all-optical phase noise averager (PNA) that is based on a PSA. The proposed PNA can average the PN of one bit with that of its neighboring bit coherently and does not need an extra phase-locking pump beam. These PNAs can effectively diminish differential PN and greatly extend the reach of DPSK signals. The most important and appealing feature of the proposed PNA is that, when cascaded, the chain of PNAs results in the convergence of PN. In amplitude-managed DPSK systems with repeated PNAs, the total differential PN is always less than that before the first averager and is irrelevant to the number of DPSK spans.