KEYWORDS: Receivers, Signal detection, Digital signal processing, Telecommunications, Photodetectors, Transmittance, Interference (communication), Space division multiplexing
The nonlinear effects in single-mode fiber and the constraints in device technology, time division multiplexing and wavelength division multiplexing cannot increase the capacity of optical fiber communication system unlimitedly. The development of space division multiplexing technology has increased the capacity of existing optical fiber communication systems by at least an order of magnitude. With the continuous improvement of transmission rate and system bandwidth in the long-haul communication, low cost and small size are also regarded as important factors in the future. In this work, the transmission performance of the low-cost self-coherent receiver and the mature coherent receiver in the long-haul mode division multiplexing system is compared. In the 32-Gbaud 6-mode dual-polarization QPSK transmission system with a fiber length of 80 km as the single span, two receiver schemes are compared considering the same configurations of the transmitter and the optical fiber link components. Compared the use of eight photodetectors integrated in the coherent receiver, the Kramers–Kronig (KK) receiver only requires two photodetectors to demodulate the dual-polarization transmission, while the phase recovery algorithm based on Hilbert transform in the KK receiver will increase the complexity of digital signal processing. Numerical results indicate that the KK receiver scheme has the advantages of lower cost and more compact size and shows the similar performance as the coherent receiver for the transmission of less than 2500 km, despite the requirement of larger transmitted power and algorithm complexity. It can also be concluded that, self-coherent receiver based on the KK algorithm can be a complementary detection solution to the coherent receiver for next-generation long-haul transmission networks with low-cost transceivers.
We demonstrated a simultaneous residual chromatic dispersion (CD) and optical signal-to-noise ratio (OSNR) monitoring method for non-return-to-zero on-off keying signals by employing the delay-tap sampling and image processing techniques. Delay-tap sampling scatter plots reflect the signal pulse shape change and image processing can extract the contour of the scatter plots. Numerical simulation shows that the OSNR monitoring range is from 12 to 32 dB with <0.5 dB error and the CD monitoring range is from 0 to 1360 ps/nm. In the experiment, the OSNR monitoring range is from 14 to 30 dB and the CD monitoring range can go up to 1275 ps/nm.
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