Funding for this research was provided by:
National Natural Science Foundation of China (61774021, 61376019)
National Natural Science Foundation of China (61504010)
Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), People’s Republic of China (IPOC2017ZT02)
Fundamental Research Funds for the Central Universities (2018XKJC05)
Received: 3 July 2018
Accepted: 21 September 2018
First Online: 3 October 2018
: The authors declare that they have no competing interests.
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