Photonics Research Cover article for Issue 2, 2024：
Recommended reading：
As a basic property of photons, polarization state (SOP) has been widely used in communication, optical coherence tomography, medical diagnosis, remote detection, material analysis and other fields. The polarization controller is the key element in the polarization applications, which can be realized by rotating wave plate and birefringence effect, but the traditional discrete optical components have some problems such as great volume, slow speed and poor reconfigurability. This cover article presents a novel polarization controller on the silicon substrate, which is ultra-compact, large tolerance and easy to regulate. The basic principle is that the horizontal and vertical polarization components of light waves can be converted into each other by using the mode hybrid effect of ridged silicon optical waveguide. By using MZI structure and phase shifter, the energy ratio and phase difference of two polarization components can be controlled, and the conversion between arbitrary polarization states can be realized. Due to the perfect symmetry of its structure, this work has obtained the highest range of polarization extinction ratio (PER) reported so far, and has wide application prospects in related fields.
——Sun Hao, Associate Professor, Tsinghua University
Photonics ResearchYoung editorial board member
State of polarization (SOP) is the basic property of photon, including linear polarization, elliptic polarization, circular polarization, etc. Because of its unique electromagnetic wave distribution and transmission characteristics, polarized light has important applications in communication, optical coherence tomography, medical diagnosis, remote detection, material analysis and other fields. In general, polarized light can be described as:
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Where Ex and Ey are the horizontal and vertical components of the plane light wave respectively, ax and ay are their corresponding amplitude, δx and δy are the corresponding initial phase, ω is the angular frequency of the beam, and t is the time variable. The polarization state of light is usually characterized by polarization isolation PER=10log10(ay2/ax2) and polarization component phase difference δ0=δy-δx. Therefore, the polarization state can be controlled by controlling PER and δ0.
Previously, the main idea to achieve on-chip polarization control is: that the incident light passes through the polarization rotation beam splitter, and its Ex and Ey components are converted into two separate TE0 modes respectively; Then two Mach-Zehner interferometers (MZI) are used to adjust the amplitude ratio and phase difference of the two TE0 modes. Finally, a polarization rotation beam splitter is used to combine the beam output. However, due to process errors, the performance of polarization rotary beam splitters and MZI is often limited, and the PER range of on-chip polarization controllers reported so far is 20-40 dB, which is not enough to cover the surface of Poincare sphere. In order to obtain a larger polarization isolation range, it is necessary to cascade multiple MZI structures, but further increase the system complexity.
For this reason, Professor Daoxin Dai's team at Zhejiang University proposed and developed a novel on-chip polarization controller based on its solid research foundation of on-chip polarization controlling, which ingeniously utilizes the polarization mode hybrid effect of a specific size ridge silicon optical waveguide, and introduces a dual-mode MZI structure and a phase shifter to realize efficient conversion between two arbitrary polarization states. The structure has perfect symmetry, and a very large polarization isolation range of >54 dB can be obtained in the wavelength range of 90 nm (the best results reported so far). The results were published in Photonics Research, Issue 2, 2024, as a cover article.
Figure 1 (a) shows the proposed novel on-silicon polarization controller architecture, comprising a special polarizer, two thermal-optical phase shifters (PS# 1, PS# 3), and two edge couplers. Among them, the polarization rotator is composed of 1×1 MZI, polarization dependent mode converter (PDMC # 1, PDMC # 2), and its optical waveguide cross-section is shown in Figure 1(b)-(c). For incident light, its x-component and y-component are converted to the TE0 and TM0 modes of silicon optical waveguides, respectively. When light enters the polarization-dependent mode converter, its TM0 mode is efficiently converted to TE1 mode due to polarization mode hybridization, while TE0 mode is lossless. The conversion between the two polarization states can be realized by using a 1×1 MZI with phase regulation.
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Figure 1 The proposed polarization controller on silicon substrate: (a) top view; (b) phase shift waveguide cross-section; (c) MZI waveguide cross-section.
Specifically, the basic control process is as follows: for the incident beam of arbitrary polarization state, the 50%:50% splitting ratio can be obtained in both arms of MZI by adjusting the phase shifter PS #1 (φ1); Then any TE0/TM0 mode power ratio (that is, any PER) can be obtained by controlling the MZI arm phase shift difference φ2. Finally, by controlling the second phase shifter PS #1 (φ3), the phase difference δ' between the output TE0/TM0 modes can be controlled to generate arbitrary polarization states. It is worth noting that the 50%:50% splitting ratio of the two MZI arms here is the key to obtaining a high degree of polarization isolation. Fig. 2 shows the simulated transmission of different incident polarized light in the PS #1, PDMC #1 and DMPS #1 regions. Thus, for TE0 or TM0, the two arms of MZI can always obtain a splitting ratio of 50%:50%, as shown in Figure 2(a-b). For other polarized light, a splitting ratio of 50% : 50% can also be obtained by adjusting φ1, as shown in Fig. 2(c-e).
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Figure 2 Transmission of different polarized light in PS #1, PDMC1 and DMPS #1 regions: (a) TE0 mode, (b) TM0 mode, and (c) 45° line polarized light (δ=2mπ), (d) 45° line polarized light (δ=2mπ+π), (e) 45° line polarized light (δ=2mπ±0.5π) (@1550 nm)
Figure 3 shows the simulated optical field transmission results when the TE0 mode incident into the two arms of MZI with different phase difference δ. It can be found that when φ2=0, its output is pure TE0 mode, as shown in Fig. 3(a); When φ2=π, the output is pure TM0 mode, as shown in Figure 3(b); When φ2 is other values, the arbitrary ratio of TE0 and TM0 modes can be controlled, as shown in Figure 3(c). Further, by adjusting the phase shifter PS #2 (φ3), the output of an arbitrary polarization state can be obtained.
Figure 3 The simulated optical field transmission results when the TE0 modes incident into the two arms of MZI with different phase difference δ: (a) φ2=0; (b) φ2=π; (b) φ2=0.5π.
The footprint of the fabricated silicon polarization controller is only 150μm × 700μm. Fig. 4(a-c) respectively shows the polarization state information measured at the output port when the light incident with different polarizations. For horizontal/vertical linearly polarized incident light (i.e., TE0/TM0 mode), by controlling the MZI phase shifter (φ2) and PS #2 (φ3), the output light polarization state can cover the surface of the Poincare sphere, as shown in Fig. 4(a-b). For other incident light containing both x- and y- polarization components, the output light polarization state can also cover the surface of the Poincare sphere by simultaneously controlling PS #1 (φ1), MZI phase shifter (φ2) and PS #2 (φ3). Fig. 4(d) shows the measured PER results of the output light at different operating wavelengths. It can be seen that the PER of the device is larger than 54 dB in the wavelength band of 1530-1620 nm.
Figure 4 The measured device output SOP, when (a) TE0 mode, (b) TM0 mode, and (c) 45° polarized light incident into the waveguide
Professor Dai said: "The on-chip integrated polarization controller is an indispensable and critical component in an optical system, but most of the past architectures are complex and the achieved PER is small. The proposed polarization control method in this paper is very compact without the need to introduce additional polarization rotation beam splitters. Due to the perfect symmetry of its structure, the MZI arms can be guaranteed to have an ideal 3dB splitting ratio, and thus obtain a polarization extinction ratio of over 54 dB, providing a new way for on-chip light field management."
