以主對從/自回饋/載波複用注入鎖定半同調無色二極體雷射於分波多工被動光纖網路之分析

Abstract

為了突破高密度分波多工被動光纖網路(DWDM-PON)在降低成本與提高速率所面臨之瓶頸，我們將具有高成本效益之無色費比布洛雷射(colorless FPLD)運用於DWDM-PON分別做為參考注入源與上下行注入鎖定傳輸器，藉以提供較反射式半導體光放大器(RSOA)更高三倍以上之調變頻寬與遠大於傳統FPLD之增益光譜寬度與通道數量，同時以較長的弱共振腔提供較小的縱模間距使得在增益光譜範圍內的通道間距能滿足DWDM-PON之需求。無色費比布洛雷射不僅可作為上下行傳輸器外，也能扮演一寬光頻部分同調性之注入光源，當與高同調性與非同調性光源比較，注入光源同調性對於注入鎖定無色費比布洛雷射之調變頻寬、相對強度雜訊與傳輸開關鍵控(OOK)、正交分頻多工(OFDM)訊號有相當大的影響。高同調性的注入光源不僅可降低無色費比布洛雷射之相對強度雜訊(RIN)約18 dB，同時可延展其頻率響應至8 GHz，而具部分同調特性之無色費比布洛雷射作為注入光源則因為可同時對多個傳輸器進行主對從注入鎖定而具有高成本效益，並能提供與高同調性注入光源相當之雜訊抑制與頻寬延展效果，分別可達到誤碼率為零的8-Gbit/s OOK傳輸與誤碼率僅為5.6x10-12之12.5-Gbit/s OFDM傳輸。 在用戶端若利用基於布拉格光纖光柵(fiber Bragg grating)之選模自我反射架構對無色費比布洛雷射進行自回饋注入，此架構不需要由局端提供額外注入光源即可在用戶端對傳輸器進行波長控制，並成功避免傳統的自回饋注入系統因為將反射元件設置在遠端(remote-node)所造成的模態拍頻雜訊(mode-beating noise)。如此設計可使上行2.5-Gbit/s訊號在接收功率僅為-30.5 dBm時，即可達到零誤碼率之25-km單模光纖傳輸。為了抑制串音效應而進一步提高傳輸速率，我們加入增益飽和半導體光放大器作為訊號抹除器並外加光纖迴路增強訊號抹除能力，藉由抹除反饋載波上之訊號能有效地降低上傳訊號之強度雜訊並突破目前以RSOA或FPLD進行自回饋注入所能達到的最高傳輸速率至5 Gbit/s，同時可提供23個以上的傳輸通道及展現良好的溫度穩定性。 最後，我們首度發現此一新型無色費比布洛雷射傳輸器亦能在其內部有效地對外部注入之光訊號進行格式抹除，將原先消光比高達15 dB之OOK光訊號注入此上行傳輸器後可抑制其消光比至低於2 dB，其雷射同調特性更是大大的降低一般使用SOA或RSOA作為訊號抹除器時所伴隨之強度雜訊，因此可直接再利用攜帶下行訊號的光載波進行同步訊號抹除與波長注入鎖定。基於此無色費比布洛雷射傳輸器具備低雜訊、高壓縮比之自我訊號抹除特性進行上傳時將可提供高達10 Gbit/s之OOK訊號傳輸。我們將此無色費比布洛雷射運用於全雙工載波複用之網路系統中，在局端我們以參考無色費比布洛雷射注入隨從無色費比布洛雷射之主對從注入架構傳輸10-Gbit/s下行訊號，經25公里光纖傳輸後，在接收功率為-13.5 dBm時即已達到零誤碼率傳輸。而在用戶端之隨從無色費比布洛雷射則對下行訊號進行當場格式抹除，並複用此載波達注入鎖定以上傳10-Gbit/s OOK訊號，在接收功率為-9 dBm時可達25公里零誤碼率傳輸。由於此系統僅使用具高成本效益之無色費比布洛雷射，完全不需要任何昂貴的高同調性注入光源、單模傳輸器、光或電的等化器(electrical or optical equalizer)與訊號抹除器，即可成功地突破將現有載波複用架構所能提供之傳輸速率從2.5 Gbit/s提升至10 Gbit/s，因此，無色費比布洛雷射將會是未來實現一高速且低成本之全雙工DWDM-PON所需的優選傳輸光源之一。

Recently, a promising transmitter of specially designed Febry-Perot laser diode (FPLD) with reflectance of lower than 1% and cavity length of longer than 600 um was proposed to provide a dense longitudinal-mode and wide gain spectrum with colorless feature for the dense wavelength-division-multiplex passive optical network unit (DWDM-PON). In this work, the colorless FPLD served as a universal transmitter with optical injection-locking control is respectively demonstrated by the external injection-locking, self-feedback and carrier-reusing schemes for DWDM-PON. With the external injection-locking scheme, the colorless FPLD injection-locked by three master sources with different degrees of coherence for on-off keying (OOK) and orthogonal frequency division multiplexing (OFDM) transmission are discussed first. The coherence of injection master shows impacts on noise, bandwidth and modulation performances of the colorless FPLD transmitter. By using the highly coherent master, the injection-locked colorless FPLD reduces its relative intensity noise peak at 5 GHz by 18 dB and increases its throughput frequency response by 5 dB. This enables the error-free OOK transmission at 10 Gbit/s and the 12.5-Gbit/s OFDM transmission with the lowest bit-error-rate (BER) of 1.6x10-13. In comparison, the master-to-slave injection-locked colorless FPLD pair with partial coherence can also provide an OOK transmission at 8 Gbit/s with a BER of 10-9. Such a cost-effective colorless source also supports a 16-QAM OFDM transmission at 12.5 Gbit/s with a BER of 5.6x10-12 to serve multi-channel PON in next generation. For optical network unit (ONU), a remote-control-free self-feedback colorless FPLD transmitter single-mode controlled with a fiber Bragg grating reflector based self-feedback block in ONU is successfully demonstrated and optimized for 2.5-5 Gbit/s DWDM-PON transmissions. The maximal power budget of such a single-mode lasing colorless FPLD at 2.5-Gbit/s transmission is determined as 25 dB under the feedback ratio of 90%. Between the bias of 38 and 42 mA (2-2.25 times of Ith), the trade-off between injection efficiency and the mode extinction is compromised, and the self-feedback colorless FPLD can perform a 2.5-Gbit/s error-free transmission with a receiving power sensitivity of -30.5 dBm after 25-km single-mode fiber (SMF). With the aid of a gain-saturated semiconductor optical amplifier (SOA) fiber loop inserted in the self-feedback block to enhance the data-erasing, the self-feedback single-mode colorless FPLD successfully provides an error-free transmission of up to 5 Gbit/s after 25-km SMF. The colorless FPLDs with in-situ down-stream data-erasing and carrier reusing functions are employed as down- and up-stream transmitters to demonstrate the 10-Gbit/s full-duplex on-off-keying transmission in a DWDM-PON system without using any high-coherent injection master and slave transmitters. Even with the intense injection of optical down-stream data into the up-stream slave colorless FPLD, the injection-locked up-stream slave colorless FPLD can suppress the on/off extinction ratio of the down-stream data from 15 dB to < 2 dB so as to reuse the down-stream carrier for injection-locking wavelength control without the need of data-eraser. The use of a broadband colorless FPLD master can essentially make the down-stream slave colorless FPLD approaching an error-free transmission after transmitting over 25-km fiber at a receiving power sensitivity of < -13.5 dBm. Under the down-stream data-erasing and carrier-reusing scheme, the single-mode controlled up-stream slave colorless FPLD can also provide an error-free transmission after propagating over 25 km.