Applications of weakly-coupled few-mode fibers
Advantage demonstration of weakly-coupled FMF compared with SMF in different applications.
As the transmission capacity worldwide continues to grow exponentially and single-mode fiber-optic communication systems approach their capacity limit, space-division multiplexing (SDM) has attracted significant attention in recent years. By employing multiple spatial modes in few-mode fibers (FMFs) or multiple cores in multicore fibers, SDM provides a larger transmission capacity and enhanced signal processing ability compared with single mode fibers (SMFs).
However, there are unavoidable defects due to limited fabrication accuracy such as index profile fluctuation, geometry deviation, and microbending in practical fibers as well as environment changes, leading to crosstalk among fiber modes. Digital signal processing (DSP), required to recover independent information despite mode crosstalk, increases system complexity and cost.
Weakly-coupled FMFs offer a cost-effective alternative since DSP is not needed anymore at the receiver. This benefits not only mode-division multiplexing (MDM), but also many other applications, due to more degrees of freedom (DOFs).
In the article "Applications of weakly-coupled few-mode fibers" published in Chinese Optics Letters, Vol. 18, Issue 4, 2020 (Huiyuan Liu, He Wen, Guifang Li. Applications of weakly-coupled few-mode fibers [Invited][J]. Chinese Optics Letters, 2020, 18(4): 040601), the authors demonstrate several application scenarios of weakly-coupled FMFs, with only deployment of spatial DOFs or combined with other DOFs.
First, the relationship between effective area of the fundamental mode and the effective index difference between the first two modes is studied to understand the tradeoff between the reductions of nonlinear effects and linear modal crosstalk.
Second, quasi-single-mode transmission in weakly-coupled FMFs is demonstrated using only one element of spatial DOFs (one mode), with weaker nonlinear effects due to larger effective area. Larger power can be launched to increase the transmission distance.
Third, DSP-free mode-group-multiplexed transmission in weakly-coupled FMFs is demonstrated for increased fiber capacity, using multiple elements of spatial DOFs (multiple modes). Stable 3×10 Gb/s transmission over 20 km FMF is demonstrated experimentally.
To effectively deliver signal in each spatial mode channel, a photonic lantern as a low-crosstalk mode multiplexer is also needed in the transmission system. Assisted by spatial DOFs, performance improvement is demonstrated in the systems using other DOFs of light such as time and wavelength. The combining loss of upstream transmission in TDM PON system is eliminated by replacing SMFs with FMFs. Last but not least, nonlinearities in WDM microwave transmission links are alleviated assisted by using spatial modes, due to spatial orthogonality and phase walk-off between spatial modes.
"The space dimension extends both the signal space and the space in which we transmit, process, generate and detect information. The weakly-coupled few-mode fibers paves the way to the extended space." says Dr. He Wen, working in the Optical Fiber Communication group in Center for Research and Education in Optics and Lasers (CREOL), the College of Optics and Photonics at the University of Central Florida.
Currently, weakly-coupled FMFs can achieve very low crosstalk better than mode multiplexers. Future work will focus on developing mode multiplexers with better mode selectivity and lower loss to further improve system performance.
弱耦合少模光纤的应用
弱耦合少模光纤相对于单模光纤在不同应用场景下的优势表现
近些年，随着全球通信容量指数增长以及单模光纤通信系统达到容量极限，空分复用得到很大的关注。通过在少模光纤中使用多个空间模式或者在多芯光纤中使用多个纤芯，空分复用提供了相比于单模光纤更大的容量和更强的信号处理能力。
然而，在实际光纤中总会存在制造精度不足造成的缺陷，折射率曲线起伏、几何形状偏差、微弯曲、周围环境变化等都会导致光纤模式之间的串扰。为了从模式串扰中恢复各个模式的信息，需要数字信号处理技术，而这会增加系统复杂度和成本。在弱模式耦合少模光纤中，探测器端不再需要数字信号处理，因此它可以作为一个低成本的解决方案。更多的自由度不仅有益于模分复用，也有助于其他应用。
在Chinese Optics Letters 2020年第18卷第4期的封面文章中，来自美国中佛罗里达大学的研究人员演示了只利用空间自由度或者结合其他自由度下，弱模式耦合少模光纤的几种应用场景（Huiyuan Liu, He Wen, Guifang Li. Applications of weakly-coupled few-mode fibers [Invited][J]. Chinese Optics Letters, 2020, 18(4): 040601）。
首先，为了了解非线性效应减小和线性模间串扰减小的权衡，研究了基模有效面积和前两个模式有效折射率差之间的关系。
然后，演示了在弱耦合少模光纤中，只用一个空间自由度（一个模式）的准单模传输，因为更大的有效模场面积可以有效减弱非线性效应，从而获得更大的发射功率以增加传输距离。
其次，为了增加光纤容量，在弱耦合少模光纤中，用多个空间自由度（多个模式）演示了不需要数字信号处理的模组复用传输。经过实验验证，可以在20 km少模光纤中实现稳定的3×10 Gb/s的传输。为了有效传递每个空间模式信道的信号，传输系统也需要光子灯笼做为低串扰模式复用器。
在空间自由度的帮助下，利用其他自由度（如时间、波长等）的系统的表现也得到了提升。通过替换单模光纤为少模光纤，避免了在时分复用无源光网络中上行信号的合并损耗。同样重要的是，由于不同空间模式之间的空间正交性以及相位离散，波分复用微波传输链路中的非线性效应会在空间模式帮助下得以减轻。
“空间维度拓展了现有的信号表示空间，也拓展了我们传输、处理、产生/检测信息的空间，而弱模式耦合少模光纤就是一条引导走进这一未知空间的大道。”该研究组的闻和博士评论说。
现在，弱耦合少模光纤可以比模式复用器实现更低的串扰。为了进一步提高系统性能，未来工作的重点在于发展有更好模式选择性和更低损耗的模式复用器。