硅纳米线光学混合器：对TE/TM偏振状态不敏感的操作

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"Silicon-nanowire-based optical hybrid with insensitive operation for TE/TM states of polarization" 本文介绍了一种基于硅纳米线的光学混合器，它能够实现对TE（横电模式）和TM（横磁模式）偏振状态的不敏感光频混频。这种光学90°混合器由一个偏振分束器-旋转器（PSR）和一个多模干涉仪（MMI）组成。PSR利用波导间的模式杂化和模式耦合，将输入的基本横电模式和横磁模式光线分别在不同的端口输出，但两个输出信号都处于基本的横电模式。然后，这两个输出场通过MMI进行进一步处理。关键词：光学混合器、横电模式、横磁模式、偏振分束器-旋转器、多模干涉仪文章的摘要指出，该光学90°混合器的设计旨在克服传统设备对偏振状态的敏感性问题，从而实现更稳定的光频率混频效果。PSR是其核心组件，它依赖于不同模式之间的相互作用，有效地将输入的TE和TM模式转换为相同模式的输出。接着，多模干涉仪MMI用于进一步处理这些输出信号，可能涉及到相位调制、功率分配或其他光信号操作。多模干涉仪是一种广泛使用的光学元件，它利用内部多个模式的干涉效应来实现光信号的复用、解复用、功率分配或相位控制。在本研究中，MMI与PSR相结合，可以处理来自不同偏振态的光信号，使得系统对输入偏振状态的变化具有抗干扰能力。此外，硅纳米线作为基础结构，因其尺寸小、集成度高、制造工艺成熟等优点，被广泛应用于光子学领域，特别是光学混合器这样的高性能光电子组件。这种基于硅纳米线的光学混合器有望在未来的光通信、光信号处理和量子信息技术等领域发挥重要作用，提供更加稳定和高效的偏振不敏感光处理解决方案。这篇论文探讨了如何通过创新设计的硅纳米线光学混合器，实现对TE和TM偏振状态的不敏感操作，这将有助于提升光子系统在各种实际应用中的性能和可靠性。

Contents lists available at ScienceDirect

Optics Communications

journal homepage: www.elsevier.com/locate/optcom

Silicon-nanowire-based optical hybrid with insensitive operation for TE/TM

states of polarization

Jin Wang

a,b,

⁎

, Yumeng Zhai

, Jinbin Mao

, Yunqing Lu

,JiXu

, Daoxin Dai

School of College of Opto-Electronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China

National Research Center for Optical Sensing/Communications Integrated Networks, Southeast University, Nanjing 210096, China

Centre for Optical and Electromagnetic Research, State Key Laboratory for Modern Optical Instrumentation, Zhejiang University, Hangzhou 310058,

China

ARTICLE INFO

Keywords:

Optical hybrid

Transverse electric mode

Transverse magnetic mode

Polarization splitter–rotator

Multimode interference coupler

ABSTRACT

An optical 90° hybrid consisting of a polarization splitter–rotator (PSR) and a multimode interferometer (MMI)

is proposed to realize insensitive optical frequency mixing for TE/TM states of polarization. The PSR is based on

the mode hybridization and mode coupling between waveguides; input light at a fundamental transverse electric

(TE) mode and a transverse magnetic (TM) mode are output at separate ports, but both output signals are in the

fundamental TE mode. These two output ﬁelds are then guided to the MMI, where the optical frequency mixing

with the light of a local oscillator takes place. This mixing is then independent of the state of polarization of the

hybrid input light. We designed such an optical hybrid based on silicon-nanowire waveguides. At a wavelength

of 1540 nm, simulation results for the hybrid show a transmission eﬃciency of 98.6% or 95.8% for TE- or TM-

polarized input light, respectively, while the transmission eﬃciencies for both polarizations are above 90% in a

30 nm wavelength range, namely [1525 nm, 1555 nm]. Further, in the wavelength range [1510 nm, 1565 nm],

the transmission imbalance between the output ﬁelds is below 0.85 dB for TE- or TM-polarized input light,

respectively, while the phase errors in both cases are less than 4°.

1. Introduction

Optical 90° hybrids are key components of coherent detection

systems [1], which play an important role in modern optical commu-

nications. The optical hybrid mixes the incoming phase-coded optical

signal with light from a local oscillator (LO) in the same state of

polarization and delivers the resulting four light signals to two pairs of

balanced detectors. By applying appropriate processing algorithms to

the converted electrical signals at a diﬀerence frequency, the amplitude

and phase of the incoming optical signal can be determined [1].

Therefore, the mixing eﬃciency and performance of the optical hybrid

are determinant factors for the performance of the coherent detection

system.

Typically, there are three methods of realizing an optical 90° hybrid

[2]. The ﬁrst is to use four 3 dB couplers, with an additional phase

delay of 90° in one branch; in turn, each 3 dB coupler can be realized

by a 2×2 multimode interference coupler (MMI). The second method is

to use a 2×4 MMI. The third method is a 90° optical hybrid composed

of two polarization-beam splitters and two 3 dB couplers. Among these

methods, the 2×4 MMI optical hybrid is the most compact, which

makes it a practical realization of densely integrated coherent receivers.

Further, it features an inherent 90° phase relation. However, the

waveguide material and structure of the MMI also exhibit birefrin-

gence. This means that optical light signals with TE/TM states of

polarization would have self-images at diﬀerent positions, leading to

polarization-dependent optical mixing [3–5]. E.g., the maximum

transmission imbalance between four hybrid outputs for TM-polarized

input light can be more than 1 dB higher than that for TE-polarized

input light [3,5].

In this work, an optical hybrid with insensitive operation for TE/

TM states of polarization was realized by cascading a polarization

splitter–rotator (PSR) and a 4×4 MMI. The PSR separates the TE- and

TM-polarized input light and also rotates the TM fundamental mode to

a TE fundamental mode. Then, the output lights of the PSR are used as

the input lights to the MMI. As the light guided to the MMI is only TE-

polarized, the performance of the frequency mixing inside the MMI

would be similar for hybrid input light at TE or TM state of

polarization. In Section 2, we introduce the structure and operation

principle of the hybrid, verifying its quadrature property by means of a

transfer matrix. In Section 3, we present the design of this optical

hybrid based on silicon-nanowire waveguides and analyze its transmis-

sion characteristics through 3D simulations.

http://dx.doi.org/10.1016/j.optcom.2016.10.047

Received 15 August 2016; Received in revised form 17 October 2016; Accepted 20 October 2016

⁎

Corresponding author at: School of College of Opto-Electronic Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210023, China.

E-mail address: jinwang@njupt.edu.cn (J. Wang).

Optics Communications 385 (2017) 124–129

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