Skip to main content

Multi-wavelength regeneration technologies for advanced modulation optical signals

Periodic Reporting for period 1 - INNOVATION (Multi-wavelength regeneration technologies for advanced modulation optical signals)

Reporting period: 2016-09-15 to 2018-09-14

With the development of high-speed Internet access, mobile voice and data services, and cloud-based data centers, broadband technologies with the low-cost per transported information bit will speed for the telecommunication infrastructure market of €350 billion and 700,000 jobs in Europe by the year 2020. To meet this exponentially growing volume and bandwidth demand, the fiber communication network uses advanced modulation formats to increase the system capacity. However, when moving to high constellation orders, signals become more vulnerable to transmission impairments, e.g. amplified spontaneous noise (ASE) or Kerr nonlinearity. All-optical regeneration is a promising technology to suppress these impacts and improve the quality of the transmitted signal. Simultaneously suppressing signal distortions on multiple amplitude or phase levels is still a significant challenge in the design of the regenerator subsystem. In a coherent mixer structure phase noise suppression on 4-levels has been demonstrated. On the other hand, only 2-levels amplitude regeneration was achieved in a nonlinear-optical loop mirror (NOLM). Regenerative capability is limited by the multilevel amplitude regenerator.
INNOVATION project aims to break through the limitation of current all-optical amplitude regenerator. NOLM unit possesses the power oscillatory nature and can be used as a multilevel amplitude regenerator. However, the stimulated Brillouin scattering effects within the highly nonlinear fiber (HNLF) impede the development of the Kerr-based nonlinear response. We used the pulse signal and the strained aluminous-silicate HNLF to reduce such impact. By carefully optimizing the system parameters we have achieved four plateau regions in a single NOLM unit. Moreover, we proposed a novel subsystem-Conjugated NOLM pair-to combine the NOLM regenerator and the mid-span optical phase conjugator (OPC). Using such all-optical repeater in the long-haul transmission we have successfully extended the transmission reach up to 100%, compared to an un-regenerative link or an OPC link. All-optical regeneration schemes demonstrated in INNOVATION can deal with multi-wavelength advanced modulated signals, which supports the high-capacity long-haul backbone networks.
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
INNOVATION project has developed the effective method to optimize the NOLM-based amplitude regenerator and obtained four power plateau regions in a single NOLM. This power-efficient operation dramatically reduces the required pump level, enabling a multiple amplitude regeneration in the NOLM unit. I also proposed a novel Conj-NOLM regenerator subsystem by cascading two NOLM units with an intermediate OPC stage. This subsystem can provide both the multilevel amplitude regeneration based on NOLM and the nonlinear compensation through mid-span OPC. The transmission reach can be extended over 100% by placing this regenerator in the middle of the link. The regenerator demonstrated in INNOVATION can effectively reduce the cost and energy of signal processing per bit, and provide a regenerative system solution for high-capacity optical communication networks.
Conj-NOLM subsystem