Intradyne and heterodyne methods are among the commonly known and widely used detection mechanisms in coherent receivers. For polarized division multiplexed quadrature amplitude modulation signals, heterodyne detection reduces half of the balanced photodetector pairs, unlike the intradyne detection. With the advancement in technology, there has been a great need to simplify the receiver structure. This have included the implementation of the single-ended instead of balanced photodetector pairs which has seriously suffered from the signal-signal beat interference induced by square-law detection.
The development of the Kramers-Kronig receiver allowing derivation of the signal phase from the amplitude waveform addressed some of the aforementioned challenges. As such, it was possible to simplify the polarization diversity heterodyne coherent receivers with only two single-ended photodetectors. Additionally, coherent receivers integrated on silicon photonic circuit have attracted significant research attention owing to their compact footprint, low power consumption, and low cost.
Recently, Professor Fan Zhang, Mr. Xiaoke Ruan, Dr. Yixiao Zhu, Mr. Zeyu Chen, Ms. Xiaoming Qiu, Ms. Fan Yang and Dr. Yanping Li from Peking University in collaboration with Dr. Ke Li from the University of Southampton fabricated a newly designed compact silicon photonic integrated circuit for polarization diversity Kramers-Kronig heterodyne coherent detection. The design comprised of an integration of two optical gratings for fiber coupling and polarization diversity, three multimode interferometers for power splitting and optical hybrid as well as two germanium single-ended photodetectors for detecting two different perpendicular polarizations. The Kramers-Kronig scheme was adopted to mitigate the signal-signal beat interference due to square-law detection. The photonic integrated circuit chip was validated with 32Gbaud polarization division multiplexed quadrature phase-shift keying and 16-ary quadrature amplitude modulation signals. Their research work is currently published in the journal, Optics Express.
The footprint of the device was only 0.68 mm x 0.9 mm, about one-sixth of that of typical silicon photonic integrated circuit intradyne receivers. With the inclusion of the Kramers-Kronig scheme, the photonic integrated circuit device was capable of detecting up to 256 Gb/s polarization division multiplexed 16-ary quadrature amplitude modulation signals with only two single-ended photodetectors. Unlike the silicon photonic integrated circuit intradyne receiver with four balanced photodetectors, the fabricated photonic integrated circuit heterodyne receiver significantly simplified the optical frontend thanks to the two single-ended photodetectors with doubled bandwidth.
It was worth noting that the heterodyne receiver could be implemented using quasi-balanced photodetectors to eliminate the signal-signal beat interference and other common mode noise through subtraction. This approach required high common-mode rejection ratio between the two respective branches. However, the Kramers-Kronig detection was used to eliminate the signal-signal beat interference in this case thus no need to consider the common-mode rejection ratio issue. This further simplified the photonic integrated circuit design.
In summary, the study is the first to successfully demonstrate polarization division multiplexed-Kramers-Kronig coherent receiver in the photonic integrated circuit for polarization diversity heterodyne coherent detection. Due to its high compactness and simplicity, Professor Fan Zhang the corresponding author in a statement to Advances in Engineering stated that the device will enhance the high-density optical interconnect.
Zhang, F., Ruan, X., Zhu, Y., Chen, Z., Qiu, X., & Yang, F. et al. (2019). Compact polarization diversity Kramers-Kronig coherent receiver on silicon chip. Optics Express, 27(17), 23654.