光纤通信新篇章：亚波长介质纤维在太赫兹通信中的潜力

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"Dispersion-limited versus power-limited terahertz communication links using solid core subwavelength dielectric fibers" 本文探讨了使用固体核心亚波长介电光纤在太赫兹（THz）通信链接中，面临散射限制与功率限制的问题。太赫兹频段（0.1-10 THz）被视为超高速通信系统的下一个关键领域。目前，大部分的研究集中在无线系统上，而基于波导或光纤的通信链路研究相对较少。尽管自由空间通信在用户移动性、多设备间简单环境的互连方面具有优势，但基于光纤的通信在某些性能方面表现更优越，如数据传输的稳定性和安全性。文章作者包括来自蒙特利尔 Polytechnique Montréal 的电气工程和工程物理系的学者，以及布朗大学工程学院的研究员。文章在2020年4月提交，7月修订，7月接受，并于8月发布，最终在10月发表。在太赫兹通信中，散射限制主要涉及信号在传播过程中的频率色散，即不同频率成分的信号速度差异导致的信号失真。这可能限制了数据传输的速度和距离。另一方面，功率限制则关注的是信号在传输过程中因损耗和非线性效应导致的功率衰减。固体核心亚波长介电光纤的设计可以减小这些负面影响，因为它们具有低色散和低损耗的特性，同时还能实现紧凑的尺寸。文章深入分析了这两种限制对太赫兹通信系统性能的影响，可能涉及的技术解决方案，以及如何通过优化光纤设计来克服这些问题。对于散射限制，可能的策略包括采用特殊的光纤结构或利用特殊的材料来控制色散。对于功率限制，可能的方法可能涉及提高光源的功率，或者利用非线性光学效应来增强信号。此外，文章还可能讨论了实验结果，展示了固体核心亚波长介电光纤在实际太赫兹通信应用中的潜力，以及与传统光纤和自由空间通信相比的优势。研究这类新型光纤技术对于推动太赫兹通信领域的进步至关重要，因为它们有望实现更高容量、更远距离的数据传输，为未来的超高速网络和物联网(IoT)应用铺平道路。该研究论文对太赫兹通信领域的散射和功率限制问题进行了详尽的探讨，同时提出了固体核心亚波长介电光纤作为潜在解决方案，对于理解并改善太赫兹通信系统的性能具有重要意义。

expanded porous polytetrafluoroethylene THz fiber link has

been established together with the resonant tunneling diode

integrated with photonic crystal waveguide for 10 Gbps and

uncompressed 4K video transmission [56]. For completeness,

we note that one promising approach for high-speed THz in-

terconnects at short distances (millimeters to few centimeters)

for on-chip and inter-chip communications is based on silicon

as the core material as such waveguides feature low transmission

loss, high RI contrast, and strong confinement, as well as

mature fabrication technology [57]. For example, in Ref.

[58], an ortho-mode sub-THz interconnect channel for planar

chip-to-chip communications using silicon dielectric wave-

guide has been investigated. By engineering the waveguide

structure, silicon-based photonic crystal waveguides were pro-

posed and demonstrated for efficient data transmission

[59–61]. Moreover, by functionalizing silicon with other ma-

terials such as graphene, active components such as THz wave-

guide-based modulators can be fabricated [62]. Additionally,

advanced shaped growth techniques [35,63] can be used to ex-

tend the use of low-loss, high RI monocrystalline materials such

as silicon or sapphire to longer THz transmission lengths up to

1 m; however, application of such waveguides in THz commu-

nications has not yet been reported. In all these works, however,

no in-depth analysis was presented as to the key reasons for the

limitations in the transmission length and maximal bit rate in

such fiber links. While, the most obvious reason for such lim-

itations is often claimed to be fiber losses, in our following

analysis, we conclude that fiber dispersion is another major fac-

tor that is often overlooked. In fact, we find that depending on

the fiber design and operation frequency, one can be in the

power-limited or dispersion-limited regime even in the case

of short several-meter-long fiber links. From the practical point

of view, in the power-limited regime, transmission is possible

up to the highest communication bit rate supported by the

available hardware even when approaching the maximal link

length when signal strength becomes comparable to the noise

level. In this regime, the eye diagram opening collapses along a

single vertical direction, while no significant degradation in its

overall form (skewedness) is observed. In the power-limited re-

gime, negative effects of the modal loss dominate over those

due to modal dispersion. In contrast, in the dispersion-

dominated regime, even for short fiber links when signal

strength is significantly larger than the noise level, one cannot

achieve maximal bit rate as allowed by the hardware. In this case,

the eye diagram shows significant asymmetry and shape contor-

tion due to modal group velocity dispersion and resultant pulse

spreading. In the dispersion-limited regime, negative effects of

the modal dispersion dominate over those due to modal loss.

In this work, we aim at deeper understanding of the limi-

tations of the fiber link quality posed by the combined effects of

the modal loss and dispersion. Without the loss of generality,

we concentrate on a pure system of rod-in-air dielectric THz

subwavelength fiber for short-range (∼10 m) communication

links with up to 6 Gbps data speeds. In fact, the rod-in-air fiber

can be used in further studies as a performance benchmark for

the more practical fibers such as rod-in-foam and suspended

core fibers. In the following, we fix the carrier frequency at

128 GHz, while using fibers of various diameters to realize

power-limited or dispersion-limited transmission regimes.

The dielectric fibers are made of low-loss PP material with three

different diameters of 1.75 mm, 0.93 mm, and 0.57 mm. Both

theoretical and experimental studies are then carried out, and a

comparative analysis of the two is presented. Experiments were

conducted using a photonics-based THz communication

system reported in Refs. [64,65]. We then demonstrate that

the limitation in the error-free link distance is mainly due to

the modal loss for the 1.75-m-diameter fiber, while for the

0.93 mm and 0.57 mm diameter fibers, the link distance is

limited due to modal dispersion. By optimizing the decision

threshold, an error-free ∼10 m-long link at 4 Gbps is achieved

with the 0.57-mm-diameter fiber, while the argument is made

for over 10 Gbps fiber links with over 10 m length when de-

signing the fiber to operate near zero dispersion frequency

(ZDF). Furthermore, study of the bending losses of the rod-

in-air fibers is presented, in which we conclude that even rel-

atively tight bends of sub-10-cm radius can be well tolerated by

such fibers. Finally, the power budget of the fiber-based link is

compared with that of the free space links, and the case is made

for the strong potential of the rod-in-air fibers in short-range

communications. To the best of our knowledge, this is the first

comprehensive study of all the major limiting factors and com-

parative advantages that relate to design and operation of short-

range fiber-assisted THz communications links.

2. THEORY OF ROD-IN-AIR DIELECTRIC TH

FIBERS

Many polymers possess almost constant RI and low absorption

losses at lower THz frequencies (<300 GHz). PP, in particular,

has one of the lowest losses over the wide THz frequency range

(<2cm

−1

below 1 THz) [66–68]. Moreover, this material is

compatible with 3D printing using the cost-effective fused dep-

osition modeling (FDM) technique that opens many exciting

opportunities in design and manufacturing of various 3D pat-

terned bulk optical components and photonic integrated cir-

cuits. Due to the importance of PP material for THz

application, in our studies we therefore used PP filaments of

three different diameters (D  1.75 mm, 0.93 mm, and

0.57 mm) as rod-in-air fibers. The filaments having smaller

diameters (0.93 mm and 0.57 mm) were extruded using an

FDM printer. Optical characterization of the fibers was then

carried out using an in-house photonics-based THz communi-

cation system detailed in Refs. [64,65] that operates at 128 GHz

carrier frequency. Complex RI of PP was measured using THz-

continuous wave spectroscopy system (se e Appendix A.1).

Mode analysis of the rod-in-air fibers was carried out using

commercial finite element software COMSOL Multiphysics.

The goal of this work is to establish limiting factors in transmis-

sion of high bit rate data streams over long distances; therefore,

modal loss, group velocity dispersion, coupling efficiency, and

bending losses are the key parameters to model.

A. Effective Index, Modal Losses, and Excitation

Efficiency

The normalized electric field distribution j Ej of the fundamen-

tal HE

mode (normalized to 1 W of carrying power) for

the PP fibers of different diameters at 128 GHz is shown in

1760 Vol. 8, No. 11 / November 2020 / Photonics Research

Research Article

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光纤通信新篇章：亚波长介质纤维在太赫兹通信中的潜力

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