Long Nguyen (ASTON)

Long’s research aimed to develop machine learning (ML)-based signal processing techniques for future optical transmission with the fibre optical parametric devices, such as optical phase conjugation (OPC) and fibre-optical parametric amplifiers (FOPAs). The OPC, when it is deployed in the middle of the transmission link for conjugate wave generation, can be used to simultaneously compensate for chromatic dispersion and nonlinearity. The FOPA, which has broader gain bandwidth than the current popular erbium-doped fibre amplifiers (EDFAs), can enable dense wavelength-division multiplexing (DWDM) technology that potentially expanding the fibre transmission capacity.

The efficiency of both devices, which rely on four-wave mixing (FWM) effect, however, is restricted by the nonlinear effect stimulated Brillouin scattering (SBS) which limits the maximum pump power introduced to the highly nonlinear fibre (HNLF) medium. The final FOPA power gain, for example, is dependent on the pump power, so we cannot achieve sufficient gain without increasing the SBS power threshold. The most popular solution is to modulate the pump phase by one or multiple radiofrequency (RF) sinusoidal tones (dithering) to increase the pump linewidth and avoid the overlap with the SBS bandwidth. It has been proven that the method is essential to achieve more than 20 dB required for loss compensation of 100 km fibre span.

However, the modulated pump phase also modulates the device’s complex gain and mostly distort the signal phase at a certain wavelength. It thus deteriorates the transmission quality with phase-modulated signal, e.g., quadrature amplitude modulation (QAM) signal, which has become most promising future modulation formats thanks to its bandwidth-efficient characteristic. The dithering-induced phase distortion then interacts with the optical fibre dispersion in the following spans to introduce phase-to-phase and phase-to-amplitude distortion that further distorts the signal. The effect becomes even more severe when there are multiple FOPAs cascaded along the transmission link which is the case of the future FOPA system.

Long Nguyen has been developing a DSP algorithm for detecting the dithering and compensate for the dithering-induced distortion (both in phase and amplitude) by using ML algorithms, such as least mean square (LMS) or other nonparametric methods. In the transmission with OPC, he developed a dithering-induced distortion compensation scheme with linear regression using gradient descent, assisted by blind phase search (BPS) algorithm for laser phase noise due to nonzero laser linewidths. The algorithm, which is called BPS-GD, showed significant bit-error-rate (BER) improvement over other existing compensation schemes, such as BPS and the BPS-assisted phase only compensation (BPS-GSP), as illustrated in below figure.

Figure 15. (a) BER after BPS, BPS-GSP and BPS-GD compensation versus transmission distance in the link at a pump-phase mismatch of 6 degrees. (b) BER versus pump phase mismatch after 1200-km transmission.

For FOPA transmission link, he created an online dithering-induced distortion compensation (DDC) algorithm using complex LMS for multiple cascaded FOPA stages. The below graph shows the performance improvement when using the proposed DDC algorithm across a high number of spans over the conventional phase compensation scheme. The improvement can be observed in both pump-phase modulation schemes, with 3 and 4 RF tones. He also has been developing more advanced schemes using kernel-based method, which is based on reproducing kernel Hilbert spaces theory for dithering-induced distortion prediction and compensation.

Figure 16. Performance in terms of Q²-factor between the transmission links with and without the proposed DDC algorithm

Key Publication by Long Nguyen

[1] L. H. Nguyen, S. Boscolo, A. D. Ellis, and S. Sygletos, “Machine Learning enabled compensation of Phase-to-Amplitude Distortion in OPC systems,” in CLEO 2023, Technical Digest Series (Optica Publishing Group, 2023), paper SM3I.5.

[2] L. H. Nguyen, T. T. Nguyen, A. D. Ellis, S. Sygletos and S. Boscolo, “Digital Signal Processing for Optical Phase Conjugation Assisted Coherent Systems,” 2023 23rd International Conference on Transparent Optical Networks (ICTON), Bucharest, Romania, 2023, pp. 1-4.

[3] Long H. Nguyen, Sonia Boscolo, and Stylianos Sygletos, “Online digital compensation of pump dithering induced phase and amplitude distortions in transmission links with cascaded fibre-optical parametric amplifiers,” Opt. Express 32, 13467-13477 (2024).

[4] L. H. Nguyen, S. Boscolo and S. Sygletos, “Online kernel-based phase recovery for parametrically amplified optical transmission,” 14^th International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP), Rome, Italy, 2024 (accepted).