高速資料中心用面射型與分佈反饋型雷射數據傳輸光源性能分析

Abstract

為了符合資料傳輸的快速發展需求，例如雲端計算、高畫質的社群影音平台和人工智慧，資料中心提供一個獲得高速且高容量資料傳輸的解決方法。光源選擇垂直共振腔面射型雷射(VCSEL)和分佈反饋式雷射(DFBLD)應用於短距離和長距離資料中心，且選擇的高速資料傳輸格式分別有開關鍵控調變(OOK)、四階脈衝強度調變(PAM-4)及正交幅度調制(QAM)正交分頻多工(OFDM)。另一方面，可見光通訊也能夠應用在資料中心內部機架間(inter-racks)的無線傳輸，可利用發光二極體建立一個新穎的小型無線傳輸資料中心。 對於可見光傳輸應用中，實現不同平台(mesa)大小的光子晶體微發光二極體(PhC-μLED)的綜合比較。在平台120 μm長的PhC-μLED擁有最大580W的光功率和外部量子效率，但也表現出最低72 MHz的3-dB調變頻寬。相反的，平台20-μm長的PhC-μLED光功率只有37 μW但擁有最高162 MHz的3-dB調變頻寬。在經過優化後，考慮光功率與類比頻寬之間的取捨將影響傳輸表現。因此PhC-μLED在60/80-μm長的平台採用OOK訊號格式且利用預補償技術皆能夠達到500 Mbit/s的傳輸容量(誤碼率<10^-12)。PhC-μLED 有著80-μm長的平台能載300-MBaud PAM-4訊號有著600 Mbit/s傳輸容量達到KP4前項錯誤更正規範。更進一步，PhC-μLED 有著60-μm長的平台利用16-QAM OFDM傳輸譨夠達到2 Gbit/s。 在短距離資料中心應用，比較質子佈植VCSEL有著不同的光子晶體設計的基本特性和經過100-m OM5多模光纖傳輸QAM-OFDM訊號。根據在纖芯(core)和纖殼(cladding)有著不同週期和孔徑大小，這些設計可以命名為Coreflat+Cladflat/Coreflat+Cladphc2/Corephc1+Cladphc2/Corephc1+Cladphc1/Corephc1-reduce+Cladphc1/Corephc1+Cladflat。Corephc1表示在core中有一週期光子晶體、Coreflat表示在core中無光子晶體、Corephc1-reduce在core中有一週期光子晶體且減少光子晶體孔徑、Cladphc2表示在cladding中有兩週期光子晶體，以此類推。首先Coreflat+Cladflat VCSEL 有著最高閥值電流和大的發光孔徑展現出最高0.11的外部量子效率和2 mW的光功率。而Coreflat+Cladphc2 VCSEL在cladding有著兩週期光子晶體顯示最低2 mA的閥值電流、最高15 GHz的3-dB類比頻寬和最低-143 dB/GHz的相對強度雜訊。而Corephc1+Cladphc2 VCSEL為core增加一週期的光子晶體和cladding增加兩週期的光子晶體在光譜上展現單模，傳輸多模光纖時能夠抑制模態色散(modal dispersion)。訊號傳輸經過優化後，Coreflat+Cladphc2 VCSEL傳輸16-QAM-OFDM訊號在背對背情況下能夠達到最高的18-GHz頻寬。在經過100-m OM5多模光纖，Coreflat+Cladphc2 和Corephc1+Cladphc2 VCSEL利用預失真(pre-leveling)技術傳輸16-QAM OFDM皆能夠達到60-Gbit/s的傳輸容量。其中Corephc1+Cladphc2能夠抑制模態色散，所以在傳輸100-m多模光纖後仍與背對背傳輸容量相同。為了更進一步提高傳輸容量，利用了位元分配(bit-loading)適當地分配QAM給OFDM子載波。因此Coreflat+Cladflat/Coreflat+Cladphc2/Corephc1+Cladphc2 PhC-VCSEL在背對背和經過100-m多模光纖情況下分別可以進一步提高傳輸容量到60.7/85/65和58.5/80.4/63.8 Gbit/s。 為了延長資料中心間的傳輸距離，選擇DFBLD搭配電致吸收調變器(EAM)利用PAM-4傳輸格式和預補償技術實現104-Gbit的傳輸容量。EAM/DFBLD發射器擁有-1.5 dBm的平均功率、出色的53 dB側模抑制比(SMSR)、25-GHz的類比頻寬和-140 dBc/Hz的相對強度雜訊。此外光接收元件也展現出30-GHz的類比頻寬。在OOK傳輸部分，電致吸收調變雷射(EML)和光接收次模組(ROSA)在背對背、2-km和10-km單模光纖傳輸情況下分別實現62、60、54 Gbit/s傳輸容量且達到無差錯標準(誤碼率<10-12)。另外，EML和ROSA在54-Gbit OOK訊號下傳輸2-/10-km單模光纖的接收功率靈敏度(sensitivity)和功率懲罰(power penalty)分別為-6.43/-6.26 dBm和0.07/0.24 dB。為了有效利用元件的頻寬限制，EML和ROSA採用PAM-4訊號格式在背對背和經過2-km單模光纖的情況下提高傳輸速率至104-Gbit/s且達到前項錯誤更正碼標準。1306-nm EML的窄線寬、高調變類比頻寬和低相對強度雜訊經過10-km公里單模光纖並傳輸預補償的48-GBaud PAM-4訊號因為低色散顯示出低的0.25 dB power penalty且符合前項錯誤更正碼標準。

To achieve the demand of rapid development in data transmission such as cloud computing, the high-quality video/audio data streaming of social networking platform and artificial intelligence, the data center provides one of solutions for obtaining high-speed and high-capacity data transmission. The high speed data format demonstrates for short- and long reach data center application with the non-return zero on-off keying (NRZ-OOK), 4-level pulse amplitude modulation (PAM-4) and 16-quadrature amplitude modulation orthogonal frequency division multiplexing (16-QAM OFDM) by vertical cavity surface emitting laser (VCSEL) and distributed feedback laser diode (DFBLD). On the other hands, the visible light communication is employed to establish an inter-rack wireless network for a novel wireless small-world data center by Light-emitting diode (LED). For high-speed visible light communication transmission application, a comprehensive comparison on the different mesa sizes of photonic crystal micro light-emitting diode (PhC-μLED) is realized. The PhC-μLED with the length of 120 μm exhibits the largest optical power of 580 μW with the highest external quantum efficiency but demonstrates the lowest 3-dB analog bandwidth of 72 MHz. By contrast, the PhC-μLED with the length of 20 μm with lowest optical power of 37 μW provides the highest 3-dB analog bandwidth of 162 MHz. After optimization, the trade-off between optical power and analog bandwidth is considered to affect performance of transmission. The PhC-μLED with mesa lengths of 60/80 μm can transmit on-off keying (OOK) data format at 500 Mbit/s with error free criterion. The device with a mesa length of 80 μm carries the 300-Mbuad PAM-4 data with the corresponding raw data rate of 600 Mbit/s for the qualified KP4-forward error correction (FEC) specification. Furthermore, the 16-QAM-OFDM transmission is employed to achieve the highest data rate of 2 Gbit/s under FEC standard for the device with mesa length of 60 μm. The proton-implant VCSEL with different photonic crystal designs are compared to transmit the 16-QAM-OFDM data over 100-m OM5 multimode fiber (MMF). The designs use flat core/cladding (indicating Coreflat/Cladflat) and/or PhC core/cladding (denoting Corephc/Cladphc). According to different periods and hole diameter in the core or cladding region, the designs of the photonic crystal are termed Coreflat+Cladflat, Coreflat+Cladphc2, Corephc1+Cladphc2, Corephc1+Cladphc1, Corephc1-reduce+Cladphc1 and Corephc1+Cladflat. The Coreflat+Cladflat VCSEL with highest threshold current and large emission aperture exhibits highest the differential quantum efficiency of 0.11 and output power of 2 mW. The Coreflat+Cladphc2 VCSEL with two periods of photonic crystal in the cladding reveals lowest threshold current of 2 mA, the highest 3-dB frequency bandwidth of 15 dB and lowest relative intensity background noise of -143 dB/GHz. The Corephc1+Cladphc2 VCSEL with additionally adding 1 period of photonic crystal in the core and 2 periods of photonic crystal in the cladding possesses optical spectra of single fundamental mode to suppress the modal dispersion after propagating MMF. After optimization, the Coreflat+Cladphc2 VCSEL chip can achieve the OFDM transmission with the highest bandwidth of 18 GHz in BtB case to meet the forward error correction (FEC) criterion. After propagating through 100-m OM5-MMF with the pre-leveled 16-QAM OFDM data, the Corephc1+Cladphc2 VCSEL with nearly modal-dispersion free still keep the same transmission bit rate of 60 Gbit/s with in the BtB condition. The bit-loading technique is used to allocate suitable QAM level for OFDM subcarriers at different frequency regions, so the Coreflat+Cladflat/Coreflat+Cladphc2/Corephc1+Cladphc2 PhC-VCSEL can improve the data rate to 60.7/85/65 and 58.5/80.4/63.8 Gbit/s under BtB and 100-m MMF condition, respectively. High-speed transmitter composed of electro-absorption modulator (EAM) and DFBLD at 1306 nm is demonstrated to carry the 104-Gbit/s PAM-4 data by using a pre-emphasized technique for intra- and inter-data center application. The EAM/DFBLD transmitter offers an average power of -1.5 dBm, an excellent side mode suppression ratio of 53 dB, an analog bandwidth of 25 GHz and a relative intensity noise of -140 dBc/Hz. In addition, the receiver module also exhibits the 30-GHz modulation bandwidth. For on-off keying data, the electro-absorption modulated laser (EML) and receiver optical sub-assembly (ROSA) support the highest transmission capacities of 62, 60 and 54 Gbit/s to meet the error-free criterion (BER<10-12) through BtB, 2- and 10-km single mode fiber (SMF), respectively. In addition, the receiving power sensitivity and power penalty are evaluated as -6.43/-6.26 dBm and 0.07/0.24 dB at 54-Gbit/s OOK data transmission after 2-/10-km SMF propagation. To efficiently employ the limited bandwidth, the data rate of PAM-4 data format is increased to 104 Gbit/s per channel under forward error correction (FEC) criterion at BtB condition. After delivering 2-km SMF, the allowable Baud rate can maintain at 52-GBaud (104 Gbit/s). The narrow-linewidth, high modulation bandwidth and low relative intensity noise of the EML at 1306 nm indicate a low power penalty of 0.25 dB at pre-emphasized 48-Gbaud PAM-4 after propagating 10-km SMF due to the extremely low chromatic dispersion.