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In this paper, we present a field-trial C-band 72Gbit/s optical on-off keying (OOK) system over 18.8km dispersion-uncompensated submarine optical cable in the South China Sea. Chromatic dispersion (CD) of 18.8km submarine optical cable causes four spectral nulls on the 36GHz bandwidth of 72Gbit/s OOK signal, which is the main obstacle for achieving an acceptable bit-error-rate (BER) performance. Decision feedback equalizer (DFE) is effective to compensate for the spectral nulls. However, DFE has a serious defect of burst-error propagation when the burst errors emerge due to the unstable submarine environment. Weighted DFE (WDFE) can be used to mitigate the burst-error propagation, but it cannot fully compensate for the spectral nulls because only a part of feedback symbols is directly decided. Fortunately, maximum likelihood sequence estimation (MLSE) can be added after the WDFE to simultaneously eliminate the resisting spectral distortions and implement optimal detection. Compared to the joint DFE and MLSE algorithm, the joint WDFE and MLSE algorithm can effectively suppress the burst-error propagation to obtain a maximum 2.9dB improvement of $\boldsymbol{Q}$ factor and eliminate the phenomenon of BER floor. In conclusion, the joint WDFE and MLSE algorithm can solve the burst-error propagation for the field-trial fiber-optic communications., Comment: Under review of Journal of Lightwave Techonlogy

An acoustooptic-mode-coupling-based scheme is proposed for excitation of whispering gallery modes (WGMs) in a silica-capillary microresonator. By exploiting acoustooptic mode coupling in a cladding-etched optical fiber, the evanescent field is strengthened by the cladding modes coupled from the core mode through axial flexural wave modulation, leading to the significant enhancement of the mode coupling between evanescent field and WGMs in a silica-capillary microresonator. WGM resonance peaks have been experimentally acquired by employing a cladding-reduced optical fiber with a diameter of 20.68 μ m, which is more robust than the coupling fibers of a few microns in diameter used in conventional WGM excitation schemes. Our proposed WGM coupling approach has desirable merits such as compactness, ease of fabrication, improved WGM coupling efficiency, and robustness, which is anticipated to find potential applications in optical filtering, microresonator laser, fiber sensor, as well as signal processing. Additionally, by controlling the electric driving signals to apply acoustic modulation, it would be convenient to achieve flexible tuning of WGMs based on acoustooptic coupling approach.