We study an electronic compensator （EC） as a receiver for a 100-Gb/s polarization division multiplexing coherent optical orthogonal frequency division multiplexing （PDM-CO-OFDM） system without optical dispersion compensation.EC,including electrical dispersion compensation （EDC）,least squares channel estimation and compensation （LSCEC）,and phase compensation （PC）,is used to compensate for chromatic dispersion （CD）,phase noise,polarization mode dispersion （PMD）,and channel impairments,respectively.Simulations show that EC is highly effective in compensating for those impairments and that the performance is close to the theoretical limitation of optical signal-to-noise rate （OSNR）,CD,and PMD.Its robustness against those transmission impairments and fiber nonlinearity are also systematically studied.
Using differential detection, we perform polarization-multiplexing 160-Gb/s optical non-return-to-zero （NRZ） differential quadrature phase shift keying （DQPSK） signal transmission over 100-kin standard single mode fiber at a bit error rate （BER） of less than 10^-9. The enabling technology includes clock recovery, fine dispersion compensation, and polarization tracking for de-multiplexing. Furthermore, a hybrid clock recovery scheme is proposed. The scheme is realized with ordinary devices using an optoelectrical modulator to down-convert the clock frequency and a phase-locked loop for filtering, which can provide an indication signal that simultaneously monitors residual dispersion and tracking polarization.
A simple design procedure is used to generate photonic crystal fibers （PCFs） with ultra-flattened chromatic dispersion. Only four parameters are required, which not only considerably saves the computing time, but also distinctly reduces the air-hole quantity. The influence of the air-hole diameters of each ring of hexagonal PCFs （H-PCF, including 1-hole-missing and 7-hole-missing H-PCFs）, circular PCFs （C-PCF）, square PCFs （S-PCF）, and octagonal PCFs （O-PCF） is investigated through simulations. Results show that regardless of the cross section structures of the PCFs, the 1st ring air-hole diameter has the greatest influence on the dispersion curve followed by that of the 2nd ring. The 3rd ring diameter only affects the dispersion curve within longer wavelengths, whereas the 4th and 5th rings have almost no influence on the dispersion curve. The hole-to-hole pitch between rings changes the dispersion curve as a whole. Based on the simulation results, a procedure is proposed to design PCFs with ultra-flattened dispersion. Through the adjustment of air-hole diameters of the inner three rings and hole-to-hole pitch, a flattened dispersion of 0±0.5 ps/（nm·km） within a wavelength range of 1.239 – 2.083 μm for 5-ring 1-hole-missing H-PCF, 1.248 – 1.992 μm for 5-ring C-PCF, 1.237 – 2.21 μm for 5-ring S-PCF, 1.149 – 1.926 μm for 5-ring O-PCF, and 1.294 – 1.663 μm for 7-hole-missing H-PCF is achieved.
