We present a novel method to estimate chromatic dispersion with ultra-low sampling rate based on training sequence. The under-sampling can be equivalent to oversampling by splicing points with periodic sequence. The functional relationship between equivalent over-sampling rate and symbol rate, actual sampling rate and sequence period is deduced. To demonstrate the feasibility of the method, the simulation of a 28 GBaud QPSK optical fiber communication system is carried, in which 76 MSa/s sampling rate is equivalent to 66.5 GSa/s sampling rate. The results show that maximum estimation error is less than 160 ps/nm after 100 km to 2300 km SSMF transmission. We also demonstrate the robustness of the proposed method to amplified spontaneous emission and nonlinear noise in a three-channel optical fiber communication system. Moreover, the proposed method has been experimentally verified with a 20.5 GBaud QPSK system, 1.25 GSa/s sampling rate is equivalent to 51.25 GSa/s sampling rate, and estimation error is less than 100 ps/nm and 150 ps/nm after 100 km and 200 km SSMF transmission, respectively. The proposed method eliminates the requirement of high speed ADC and is cost effective, which can be used for optical performance monitoring.

We design a quadrature phase-shift keying training sequence (QPSK-TS) for monitoring both optical signal-to-noise ratio (OSNR) and chromatic dispersion (CD) in a dense wavelength division multiplexing (DWDM) system. By applying fast Fourier transform on the QPSK-TS, a comb-like spectrum with discrete peaks can be obtained. OSNR can be calculated by measuring the power of the amplified spontaneous emission (ASE) noise in the gaps between these peaks. Moreover, we search the optimal order in the fractional domain to calculate the CD after applying fractional Fourier transform on the QPSK-TS signal. The proposed method shows large ASE noise and optical nonlinearity tolerance in simulation results (<1.0 dB error within 24 dB after transmission over 1000 km with 3 dBm per channel launch power, and <90 ps/nm error within 33400 ps/nm). Furthermore, the experimental results demonstrate high accuracy of the proposed method (≤1.0 dB error within 28 dB and <40 ps/nm error within 1670 ps/nm after 100 km single channel transmission).

A cost-effective chromatic dispersion (CD) estimation method is proposed, which is based on the delay-tap sampling (DTS) technique in combination with periodic training sequences (PTS). By performing equivalent sampling of PTS, DTS-based CD estimation at ultra-low sampling rate can be realized, significantly reducing the cost and high-speed requirements of analog to digital converters (ADCs). In contrast to the existing Nyquist DTS-based CD estimation method, we construct the CD lookup table in the CD over-compensated region, which can be suitable for a dynamic range of OSNR conditions. The CD lookup table built from the simulation can be used in the experimental system with similar configurations directly, indicating that the suggested method is reasonable and highly feasible. Besides, with only 1.25 GSa/s sampling rate of ADCs (about 1/30 of the Nyquist sampling rate which is 40.28 GSa/s), both 20.14 GBaud quadrature-phase-shift-keying (QPSK) simulation and experimental systems demonstrate the feasibility of the method, with a 28 ps/nm maximum CD estimation error for the simulation and a 35 ps/nm maximum CD estimation error for the experiment when OSNR is fixed to 18 dB. Furthermore, we also investigate the robustness of our method under the condition of different amplifier spontaneous emission (ASE) noise and nonlinear noise. The proposed method shows great potential for making a remarkable cost reduction in CD monitoring. INDEX TERMS Metrology, chromatic dispersion, fiber optics, fiber nonlinear optics.

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