「文献阅读」基于铌酸锂MZI的电光模式开关

This article was last updated on 2022-07-30 18:07:12

文献概述

AbstractContent
TitleElectro-optic mode switch based on lithium niobate Mach-Zehnder interferometer
Published2016-5-27
Platformx-cut lithium niobate
Functionswitch between the fundamental mode (TE11), and the higher-order mode (TE21) with a low driving voltage.
Footprinta total length of ~24mm
Resultmode extinction ratio of ~35dB and a 20-dB bandwidth of ~12nm @1552nm, when driven at a voltage of 1.7V at 26℃. High performance can be obtained at any wavelength in the C+L band with a driving voltage varying by no more than 3V.
PDFhere

结构图

Schematic diagrams showing (a) the perspective view and
(b) the top view of the proposed MZI mode switch.

Function of the Mode Switch.

笔记

关于模分复用系统的描述

Mode-division-multiplexing (MDM), in which each mode or mode group in a few-mode fiber is exploited as an independent channel of signals, is a promising technology for expanding the transmission capacities of optical communication systems to keep up with the rapid growth of the Internet traffic [1][2][3]. In an MDM system, mode conversion between the fundamental mode and a selected high-order mode is often needed at both the input and output ends.

模式转换器

文章的Introduction部分

以上的Introduction部分写的不错,对于模式转换器的应用平台有一个比较详细的介绍,之后在写类似论文的时候可以相互借鉴。

当然,这一部分的介绍在以下的论文中也有体现:
[1] Rulei Xiao, Yuechun Shi, Jia Li, Pan Dai, Yong Zhao, Lianyan Li, Jun Lu, and Xiangfei Chen, “On-chip mode converter based on two cascaded Bragg gratings,” Opt. Express 27, 1941 (2019). here
[2] W. K. Zhao, K. X. Chen, J. Y. Wu and K. S. Chiang, “Horizontal Directional Coupler Formed With Waveguides of Different Heights for Mode-Division Multiplexing,” IEEE Photonics Journal 9, 1 (2017). here
[3] Hanzawa N, Saitoh K, Sakamoto T, Matsui T, Tsujikawa K, Koshiba M, Yamamoto F, “Two-mode PLC-based mode multi/demultiplexer for mode and wavelength division multiplexed transmission”, Opt. Express. 21, 25752 (2013). here
[4] W. Wang, J. Y. Wu, K. X. Chen, W. Jin, and K. S. Chiang, “Ultra-broadband mode converters based on length-apodized long-period waveguide gratings”, Opt. Express 25, 14341 (2017). here
[5] W. K. Zhao, K. X. Chen and J. Y. Wu, “Ultra-short embedded long-period waveguide grating for broadband mode conversion”, Appl. Phys. B 125, 177 (2019). here

想法

这篇论文的原理其实比较简单,利用电光效应使得MZI两臂的单模相位差发生翻转,从而在combiner部分形成LP01模或LP11a模。器件的设计和制作部分就不再赘述,都是常规的制作工艺。在测试部分,作者刚开始测试了器件在不同电压下输入基模后的透射光谱,电压从0V上升到3.9V之后,相位变化实现了2π,此时两条曲线相互重合。那么在这一过程中,透射谱的阻带中心波长在漂移,实验最终的结果表明中心波长的漂移与驱动电压呈线性关系,约为67.9nm/V。

有了频谱图,自然也要有近场输出光斑图。这两个结果中间怎么连接?我觉得作者的这个表述很好,“ To confirm that the rejection band is due to the conversion from the TE11 mode to the TE21 mode, the near-field image of the output light at the wavelength 1552 nm was captured with an infrared camera (Micron Viewer 7290A).”,包括段落的最后一句话同样也是在点题,“ These results confirm that the strong rejection band obtained from the spectral measurement is indeed caused by strong mode conversion ”。

之后作者测量了该器件在不同波长下的模式开关总功率,当然,这是为了描述器件的插入损耗性能,“ No significant power change with the driving voltage was observed, which indicates little power loss in the mode conversion process”。

最后作者测量了器件的温度响应,最后说明了“As for conventional LN MZI modulators, it is necessary to apply temperature control to the mode switch to stabilize its performance against any thermal drift”,通过温度控制,避免器件由于环境的变化产生热漂移现象。

这里作者没有说明器件的模式转换效率。我觉得是基于两个原因,一个是这种典型的MZI型开关,本身的透射光谱就类似于下图,通过透射光谱不能准确得到器件的带宽(LP01-LP11a),或者说它的带宽不是很大,我们可以肯定的是该器件在透射谱中心波长位置一定实现了模式转换,但其他地方没有一个准确的位置。第二个原因是对于这样一个1×1器件,测试端如果只用一个单模光纤,是无法知道LP01-LP11a的转换效率的,因此只能输入LP11a,测量输出的LP01模。这种方法可行,但我个人认为测量结果还是不太准确。我倒是觉得可以在MZI后面接一个非对称Y分支,最后测量只看两个分支的输出谱,这样可以算出模式串扰,进而计算出模式转换效率。

MZI透射光谱

所以类似如果带宽很大的器件,我倒是觉得可以说一下MCR,如果不是,还是只说一下透射谱就可,主要是不同的器件具有不同的特性和优点。

参考文献

  1. R. J. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity limits of optical fiber networks,” J. Lightwave Technol. 28, 662 (2010). here
  2. G. F. Li, N. Bai, N. B. Zhao, and C. Xia, “Space-division multiplexing: the next frontier in optical communication,” Adv. Opt. Photon. 6, 413 (2014). here
  3. R. G. H. van Uden, R. A. Correa, E. A. Lopez, F. M. Huijskens, C. Xia, G. Li, A. Schülzgen, H. de Waardt, A. M. J. Koonen, and C. M. Okonkwo, “Ultra-high-density spatial division multiplexing with a few-mode multicore fiber,” Nat. Photonics 8, 865 (2014). here