Growing capacity demands motivate the network evolution from conventional fixed optical networks to future agile optical networks. A hitless flexible transceiver aims to adapt the transceiver configurations and utilize the instantaneous link margin can effectively better exploit the system capacity. Furthermore, as network traffic becomes more dynamic and unpredictable in future networks, hitless line rate changes implemented by switching modulation formats are expected to bring more benefits in improving capacity and saving power consumption. In this scenario, modulation format identification (MFI) is essential for reconfiguring digital signal processing (DSP) for signal recovery and de-mapping at the receiver-side (Rx). Recently, several MFI techniques based on the properties of specific standard QAM formats have been proposed. However, those techniques cannot be easily extended to more complex modulation formats, such as hybrid QAM formats or multi-dimensional format. Moreover, these MFI techniques are unable to track a fast block-by-block change of modulation format because of the high computational complexity to obtain an accurate MFI.

Prof. Songnian Fu and Ph.D student Meng Xiang, with the Wuhan National Lab for Optoelectronics, Huazhong University of Science and Technology, propose a design of a hitless coherent transceiver for agile optical networks. This transceiver uses block-wise decision-directed least-mean-square (DD-LMS) equalizer for channel tracking and a pilot symbol aided superscalar phase-locked loop (PLL) for carrier phase estimation (CPE). For the purpose of MFI, the modulation format information is encoded onto BPSK pilot symbols, which are initially used for the superscalar CPE. The proposed MFI covers a wider range of modulation formats and is more reliable especially at low optical-to-signal noise ratio (OSNR). Furthermore, the proposed hitless transceiver can support fast modulation format switching on a block by block basis.

Fig 1. Structure of the proposed hitless flexible coherent transceiver.

Fig 2. (a) Probability of correct MFI versus OSNR under B2B transmission for DP-16QAM. (b) Probability of correct MFI versus transmission distance.

Fig 3. BER and SNR versus block index for (a) interleaved DP-QPSK and DP-Hybrid QPSK/8QAM after 6400 km SSMF transmission, and (b) interleaved DP-16QAM and 128-SP-QAM after 2240 km SSMF transmission.

This paper was published at Optics Express (Vol. 24, No. 14, pp. 15642-15655, 2016) and partially supported by the 863 High Technology Plan (2015AA015502), and National Natural Science Foundation of China (61275069, 61331010).