同源自零差相干光传输技术

Self-Homodyne Coherent Optical Transmission Techniques

同源自零差相干技术在信号传输时伴随传输一个同源的导频,将其在收端用作相干探测的本振光。同源自零差相干系统包含偏分复用同源自零差相干和空分复用同源自零差相干两类架构。通过同源自零差相干可以在光信号探测前实现有效的“光域载波恢复”,从而有望在系统中采用低成本非制冷的分布式反馈(DFB)激光器和波特率采样接收机。在空分架构中,信号光与导频光传输引入的相对延时会导致一个性质独特的相位噪声,其概率密度的分布特性限制了系统可容忍激光线宽与相对延时积的大小,而其微分对应频率调制噪声的有色性则提供了一种高精度、大动态范围的在线相对延时估计方法,为充分发挥同源自零差相干技术架构优势提供了可靠的保障。自动偏振控制调控技术可用于实现偏分架构中导频和信号的分离,亦可用于规避空分架构偏振分集探测时的功率衰落问题。此外,基于自动偏振控制的同源自零差相干技术为高速、对称的双向互连架构提供了一种全新的低成本解决方案,仅采用单个自动偏振控制器即可实现双向偏振锁定,可进一步降低单纤偏分双向架构的器件成本以及实现无需多入多出(MIMO-free)均衡的双纤架构,且数字信号处理(DSP)仅剩单入单出均衡和前向纠错。这使得基于自动偏振控制的同源自零差相干技术可以为高性能、低成本、DSP-free的双向高速互连提供一种极富前景的方案。本文介绍了同源自零差相干光传输系统的类型、优势、挑战及解决方案,综述了本团队在关键的自动偏振控制技术、在线相对延时估计技术,以及基于自动偏振控制的至简DSP同源自零差相干传输技术方面的系列成果。

Significance Big data services have yielded explosive growth of capacity in short-reach optical networks.The intensity modulation direct-detection (IMDD)systems with simple schemes and power-efficient digital signal processing(DSP)are typically preferred in short-reach scenes.However,they are unable to satisfy the demand for continually increasing interface speed.The Ethernet interface data rate is approaching 800Gbit/s and 1.6Tbit/s.Hence,the conventional IMDD will suffer from serious technical challenges,including dispersion-induced power fading,rapidly increasing cost,and limited sensitivity.As an alternative,coherent technology can provide high spectral efficiency,high sensitivity,and good tolerance to the chromatic and polarization-mode dispersion.However,for short-reach application,this technology is considered overly costly and power-consuming.These drawbacks originate from two main challenges.On one hand,power-consuming DSP is required for solving various impairments on the received signal in traditional coherent schemes.Moreover,with the fading of the Moore’s law,the node gain brought by new footprints of application specific integrated circuits(ASICs)tends to be marginal.Using only advanced DSP in the development of coherent technology for short-reach high-speed interconnections is quite difficult.On the other hand,traditional coherent technology is associated with complex hardware structures,especially the adoption of narrow-linewidth,frequency-stable,and tunable lasers,such as external cavity laser and integrable tunable laser assembly(ILTA).Consequently,coherent technology is still inapplicable to short-reach networks.Apart from conventional IMDD and coherent technology,many self-coherent schemes have been proposed with certain tolerance to laser linewidths and less implementation complexity(than that of the conventional systems).The Kramers-Kronig receiver (KKR)and Stokes vector receiver are two typical schemes,each receiving considerable attention.In terms of the product of analog to digital converter ADC bandwidth and number of ADCs,these schemes are more expensive while achieving the same capacity as that of the conventional coherent technology.The self-homodyne coherent (SHC)scheme has been proposed as another“coherent-lite”scheme,including polarization division multiplexing and space division multiplexing as the categories.The key feature of this scheme is that the signal lights transmit simultaneously along with their pilots in links.At receiver,coherent detection will be conduced though the remotely delivered pilot to achieve optical domain phase recovery of signal.Thus,the cost and the power consumption are reduced,and the use of low-cost and uncooled distributed feedback(DFB)laser and baud-rate-sampling receivers is realized.The advantages of the coherent technology are therefore inherited and the scheme is simplified,and hence this technology is considered one of the most promising technologies for future short-reach optical networks.Despite the excellent characteristics of SHC schemes,many key implementational issues must be solved prior to deployment.Progress In space division systems,the relative time delay will induce a unique phase noise and degrade the system performance,which may prevent use of the low-cost DFB laser in the SHC scheme.Fortunately,the derivative of such phase noise is a colored frequency modulation noise.Utilizing this characteristic,we proposed and demonstrated an in-service high-precision and large-dynamic-range estimation method of relative time delay(RTD),contributing to the realization of the SHC technique.Besides,the random birefringence in the optical fiber will lead to changes in the state of polarization (SOP)during delivery of the pilot and signal.Another implementational issue is that automatic compensations of such randomly changed SOP is required for real fiber links.By leveraging our in-house-developed adaptive polarization controller (APC),we solved both problems of polarization demultiplexing in polarization division multiplexed(PDM)SHC systems and pilot SOP locking in space division multiplexed (SDM)SHC systems.The APC technique allows further simplification of the DSP algorithms.Utilizing only one APC device and its symmetry property,we also demonstrated the first multi-input and multi-output(MIMO)-free SDM-SHC transmission and PDM-SHC transmission in bidirectional(BiDi)scenes.The APC technique paves the way for low-cost,power-efficient,high-speed BiDi optical interconnections.We present an overview of the progress that our group has realized for SHC systems,including the APC techniques,the in-service RTD estimation techniques,and the simplest BiDi SHC transmission architectures based on APC techniques.Conclusions and Prospects The SHC scheme capable of optical-domain equalization,high spectrum efficiency,and compatibility with current ASIC architecture has been demonstrated.This scheme provides a promising method for future low-cost short-to-medium-reach optical interconnections.Moreover,the SHC technology will generate new demands from other technical areas,including photonic integration,special optical fibers,and DSP.This technology will promote and accelerate innovations in multiple fields.