Terabit FSO communication based on a soliton microcomb
Fig. 1. The schematic of soliton microcomb based massively parallel FSOC system. (a) The experiment scenario of the FSO communication system. (b) The schematic diagram of the transmitter terminal. (c) The schematic diagram of the receiver terminal.
Fig. 2. 1.02 Tbit/s free-space data transmission using a soliton microcomb. (a) The image of a butterfly-packaged device (left panel) and the high-index doped silica glass MRR (right panel). (b) Measured eye diagram at received power of −26 dBm for comb line (1558.093 nm) and CW laser. (c) Measured BERs for 102 optical channels. The inset shows the measured real-time BER curve (red) with 10 s counting time, as well as the accumulating BER curve (green) for the comb line of 1551.192 nm. (d) The measured optical spectra of the modulated 102 optical signals after amplified and filtered at the transmitting terminal.
Free-space optical communication (FSOC) uses laser beam as a carrier for information transmission in space. FSOC takes the merits of large communication capacity, anti-electromagnetic interference, good confidentiality, large license-free bandwidths compared with microwave communication system. The communication terminals have the advantages of small volume, easy deployment, and low power consumption, which is an ideal candidate for "last mile" information transmission. It has important application values in emergency, space-to-ground and inter-satellite communications.
It is a research hotspot to realize large capacity FSOC systems with high data rate and long transmission distance. Wavelength division multiplexing (WDM) technology is well developed to improve data transmission capacity using the large spectrum resources. Generally, many individual lasers are employed as optical carriers, whose wavelength should be carefully controlled for inter-channel guard bands. Therefore, the volume and power consumption would largely increase for a FSCO system.
To address this problem and achieve large-capacity FSO communication, the research group led by Prof. Xiao-Ping Xie and Wei Wang from the Laboratory of Photonics and Network, cooperating with Prof. Wen-Fu Zhang and associate Prof. Wei-Qiang Wang from the State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, a Tbit/s parallel FSO communication system was implemented using a soliton microcomb as a multiple wavelength laser source.
The relevant research results are published in Photonics Research, Volume. 10, Issue 12, 2022 (Wen Shao, Yang Wang, Shuaiwei Jia, Zhuang Xie, Duorui Gao, Wei Wang, Dongquan Zhang, Peixuan Liao, Brent E. Little, Sai T. Chu, Wei Zhao, Wenfu Zhang, Weiqiang Wang, and Xiaoping Xie. Terabit FSO communication based on a soliton microcomb[J]. Photonics Research, 2022, 10(12): 2802).
On-chip soliton combs (SMCs) have large repetition rate and all the comb tones are intrinsically equidistant in frequency, which are ideal integrated laser sources for massively parallel signal transmission in FSOC system. In this work, 102 comb lines of a SMC are selected as optical carriers and modulated with 10 Gbit/s differential phase-shift keying modulation signals. Figure 1 shows the setup of a FSOC system with 1.02 Tbps communication capacity and ~1 km transmission distance.
The SMC is pumped by a laser with linewidth of ~100 Hz and generated using the well-developed auxiliary laser-assisted thermal balance scheme. The SMC is long time survived by control the temperature and the frequency between pump and auxiliary lasers. The SMC has a repetition rate of ~48.97 GHz, which is multiplexed and demultiplexed using wavelength selective switches. The bit error rate (BER) performance is compared at different received power using a continuous-wave laser and a comb line (1558.093 nm) as optical carriers respectively. The measured BERs are comparable for power less than -30 dBm and Fig. 2(b) shows the received eye diagram respectively. The 1.02 Tbps parallel data transmission using 102 comb lines. Figure 2d shows the measured optical spectra of the 102 individually modulated comb lines. And the BERs results are shown in Fig. 2c, where 42 comb lines achieve of BERs are less than 10-9.
Prof. Xiaoping Xie said: "SMC has the characteristics of small volume, wide spectrum, high repetition rate and low noise. It has a prospect in the application fields of large-capacity optical communication, optical clock, precision ranging and spectroscopy. SMC is used as an on-chip multi-wavelength light source for FSOC communication in this work. It is significant to improve the communication capacity and solve the size, weight and power (SWaP) problems of an FSOC system. It provides a new scheme for the future large capacity and long distance FSOC."
微腔孤子光频梳：实现大容量自由空间光通信
图1 基于微腔孤子光频梳的大规模并行自由空间光通信系统；（a）自由空间激光通信系统场景;（b）发射端系统原理图;（c）接收端系统原理图
自由空间激光通信（FSOC）是一种利用激光束作为载波在空间进行信息传递的通信方式。相比于微波通信，它具有传输速率高、抗电磁干扰性能强、保密性好、无频谱限制等优势，且终端体积小、易于部署、功耗低，是解决信息传输“最后一英里”的理想选择，在应急通信、星地通信和星间通信等场景中具有重要的应用价值。
建立大容量、长传输距离的FSOC系统是当前的研究热点，波分复用（WDM）技术利用激光载波超宽的频谱资源，在用较低的信号调制速率保证FSOC的通信距离的情况下，同时实现了超高的通信速度。大容量WDM激光通信系统要求载波激光波长与信道相匹配，因此需要对各个载波激光器进行单独的波长控制，将极大地增加FSOC系统的功耗和体积。
为解决以上问题并实现大容量自由空间光通信，中国科学院西安光学精密机械研究所光子网络技术研究室谢小平研究员、汪伟研究员课题组与瞬态光学与光子技术国家重点实验室张文富研究员、王伟强副研究员课题组合作，利用新兴的微腔孤子光频梳代替传统的半导体可调谐激光阵列作为多载波光源，在相距1 km的自由空间光通信链路上实现了总速率为1.02 Tbps的并行数据传输。相关研究成果发表于Photonics Research 2022年第12期（Wen Shao, Yang Wang, Shuaiwei Jia, Zhuang Xie, Duorui Gao, Wei Wang, Dongquan Zhang, Peixuan Liao, Brent E. Little, Sai T. Chu, Wei Zhao, Wenfu Zhang, Weiqiang Wang, and Xiaoping Xie. Terabit FSO communication based on a soliton microcomb[J]. Photonics Research, 2022, 10(12): 2802）。
片上微腔孤子光频梳（SMC）具有超高的重复频率，并且各个梳齿具有严格相等的频率间隔，是波分复用FSOC系统的理想激光光源。该工作选择微腔孤子光频梳的102个梳齿作为载波，使用10 Gbit/s差分相移键控调制信号，在相距~1 km的两个建筑物间实现了1.02 Tbps的自由空间数据传输，实验系统如图1所示。
孤子光频梳采用一台~100 Hz线宽的激光器为泵浦，采用辅助激光热平衡的技术方案产生，并通过温度和泵浦与辅助激光频率的自动控制保证光频梳光源的稳定。系统使用波长选择开关对孤子光频梳（重复频率为49 GHz）的梳齿进行复用和解复用。
将SMC的一个梳齿（1558.093 nm）与连续波激光器分别作为载波，对比两者的信号传输性能，实验结果显示接收功率小于-30 dBm时两者的误码率在同一水平，图2（b）为两者的眼图。采用102根梳齿信道进行1.02 Tbps并行数据传输，在发射端调制后信号的光谱如图2（d）所示，相应的误码率结果如图2（c）所示，其中42根梳齿实现零误码（BERs<10-9）传输。
谢小平研究员表示：“微腔孤子光频梳具有体积小、光谱宽、重复频率高、噪声低等特性，在大容量光通信、光钟、精密测距以及光谱学等领域具有广阔的前景。这项工作将片上微腔孤子光频梳作为多波长光源应用于自由空间光通信研究中，对提升自由空间光通信的容量和解决自由空间光通信的SWaP问题具有重要意义，为未来大容量、长距离自由空间光通信发展提供了一种新途径。”