1. Work performed
In 2016-2018, I carried out research on multilevel amplitude regeneration based on NOLM structure: (1) develop a theory model of the NOLM interferometer and theoretical analysis on the coupling ratio, the birefringence bias and the port selection for the power-efficient multilevel amplitude regeneration; (2) set up the NOLM experiment based on the optimized parameters and obtain four plateau regions in a single NOLM unit; (3) demonstrate a PAM4 regeneration and measure the noise handling capability of each regenerative level; (4) propose the Conjugated NOLM structure to cancel out the phase distortion from the NOLM unit; (5) evaluate the transmission performance of the Conj-NOLM link and achieve 100% transmission extension; (6) use the time-interleaving technology to achieve multi-wavelength operation in the NOLM regenerator.
2. Main results achieved so far
(1) I have developed the theory model of the NOLM unit, including the key devices of this interferometer, e.g. optical coupler, HNLF, variable optical attenuator (VOA) and polarization controller. I investigated both the transmission and reflection responses by varying the coupling ratio, the extra loss and the birefringence bias. By comparing the regenerative capability I chose the optimized coupling ration of 0.9 and the initial state of the all-reflection. Then I set up the optimized NOLM unit for the power transfer function (PTF) test. To bypass the impact of the SBS effect I used the pulse signal as the pump and the strained HNLF. I obtained four power plateau regions in the NOLM unit. The results were published at ICTON 2017 and ICOCN2017, as invited talks.
(2) I demonstrated how to use the NOLM to achieve the PAM4 regeneration. According to the PTF obtained in the NOLM unit we defined the amplitude levels of PAM4 signals, enabling the effective noise suppression. Then I measured the noise handling capability of each regenerative level. By inputting into the proper PAM4 signal multiple amplitude regeneration was achieved. I obtained around 0.9dB EVM improvement. The phase distortion induced by the NOLM unit was also discussed. The result was published at Optics Express.
(3) To reduce the phase distortion from the NOLM unit we proposed a novel Conj-NOLM subsystem by cascading two NOLM units with an intermediate OPC stage. The first NOLM introduces the phase pre-distortion and less impacts on the signal’s amplitude. Then the following conjugation conversion as well as the second NOLM cancel out the phase distortion and provide the nonlinear amplitude response. I investigated the optimization condition, including the coupling ratio of the first NOLM and the gain coefficient of the optical amplifier in the OPC stage. This new structure can significantly reduce the NOLM-induced phase distortion and offer a power-efficient amplitude regeneration. The result was published at CLEO 2018 and ICTON 2018 (invited talk).
(4) I investigated the transmission performance by using the Conj-NOLM regenerator subsystem. Placing the Conj-NOLM in the middle of the link the quality of the transmitted signal can be improved by both the NOLM-based amplitude regeneration and the nonlinear compensation through the mid-span OPC. When only ASE noise is considered the reach extension can be up to 100% compared to the un-regenerative link. Moreover, when ASE noise and nonlinear phase distortion are both taken into account, the proposed regenerator can improve the Q factor at every power points. This content has been submitted to Journal of Lightwave Technology.
3. Overview of the results and their exploitation and dissemination
Through this two year researches, I have achieved a record four-regenerative-regions in a single NOLM unit and demonstrated the first experiment of the PAM4 regeneration by simultaneously suppressing amplitude noise on multiple levels. Moreover, I proposed the Conj-NOLM subsystem to combine the NOLM regenerator and the mid-span OPC, which successfully extended the transmission reach over 100%. Supported by the Marie Curie research fellowship I have published 9 high-impact papers including Optics Express, European Conference on Optical Communication and 3 invited talk. I also review the regeneration results in the book “Optical modulation and regeneration technology” (Science Press, Beijing).
