## 4 Conclusions  
In this work, a novel MO isolator was proposed based on a high efficiency PCW and two gyromagnetic ferrites. The optimal PCW structure with ultra-wideband characteristic was realized by 3D numerical simulation and experimental verification. The MO isolator was designed by inserting two rectangular gyromagnetic ferrite sheets into the low loss PCW. By using finite element method, the isolation was optimized to be 49.49 dB for the isolator, and its relative bandwidth reaches 8.85%. Furthermore, the effects of ferrite structure parameters on the performance of MO isolator were discussed. The bandwidth of the isolator was broadened to be 24.85 to 27.15 GHz by increasing the length of the ferrite sheets. When the distance between the ferrite sheets and the boundary of PCW was optimized to be 1 mm, the designed MO isolator had an excellent transmission performance.  
It should be noted that our MO isolator is relatively simple and easily made. The excellent performance of the PCW and low material loss demonstrate that TLPCs using Al\({}_{2}\)O\({}_{3}\) posts is a promising method for generating an MO isolator or other devices, such as PCs circulators [21; 26], in the future integrated communication system. Our designed MO isolator with high isolation and wide bandwidth also provides a potential and feasible scheme for the current research hotspot of 5G communication systems. But the difficulty of processing the ferrite sheets assuredly increases due to the friability of the ferrite, especially when the thickness of the ferrite sheets is too thin, i.e., only 0.5 mm, to be processed by the existing machining partners. In our further work, we may verify the MO isolator by experiments, if we find the high precision laser processing technology similar to [27], which allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in the absence of brittle fracture.  
Figure 7: **(a)** The transmission characteristics of MO isolator change with frequency when the length L of ferrite sheets is 14, 16, 18, and 20 mm; **(b)** the transmission characteristics of MO isolator change with frequency when the distance \(d\) between ferrite sheets and the boundary of the PCW is 0.6, 0.8, 1, 1.2, and 1.4 mm; **(c)** and the isolations of MO isolator change with frequency when the distance \(d\) between ferrite sheets and the boundary of the PCW is 0.6, 0.8, 1, 1.2, and 1.4 mm.  
**Author Contributions:** Conceptualization, D.Z. and S.X.; methodology, Y.W. and B.X.; software, Y.W. and B.X.; validation, W.Y., B.X. and Z.D.; formal analysis, W.Y. and B.X.; investigation, W.Y., D.Z. and S.X.; resources, D.Z., J.P. and S.X.; data curation, Y.W. and B.X.; Writing--Original Draft preparation, Y.W.; Writing--Review and Editing, D.Z., S.X. and X.L.; visualization, Y.W. and B.X.; supervision, D.Z. and S.X.; project administration, W.Y.; funding acquisition, D.Z., S.X., X.Z., X.L. and Y.W.  
**Funding:** This work was supported partly by the National Natural Science Foundations of China (61775142, 6117006), China Postdoctoral Science Foundation (2019M653028, 2017M612726), Shenzhen basic research project on subject layout (JCYJ20170412105812811), Natural Science Foundations of Shenzhen University (2019007).  
**Acknowledgments:** The authors would like to acknowledge the Shenzhen Key Laboratory of Micro-Nano Photonic Information Technology and College of Physics and Optoelectronic Engineering of Shenzhen University.  
**Conflicts of Interest:** The authors declare no conflict of interest.