A high power semiconductor laser diode with a tapered and cascaded active multimode interferometer （MMI） cavity was designed and demonstrated. An output power as high as 32 mW was obtained for the novel laser diode with a tapered and cascaded active MMI cavity, being much higher than the 9.8 mW output power of the conventional single ridge F-P laser with the same material structure and the same device length due to the larger active area; and also being higher than the 21.2 mW output power of the rectangular and cascaded active MMI laser diode with nearly the same structure, except for the shape of the MMI area. In addition, the tapered and cascaded active multimode interferometer laser showed stable single mode outputs up to the maximum output power.
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.
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.
A new blind frequency offset estimation method based on cyclic prefix and virtual subcarriers in coherent optical orthogonal frequency division multiplexing （CO-OFDM） system is presented. It is able to estimate the fractional part and integral part of frequency offset at the same time. Its estimation range is about [-3.5 GHz, 3.5 GHz]. The influence of the integral frequency offset is comprehensively analyzed in COOFDM system. Its performances in the additive white Gaussian noise （AWGN） channel and the dispersive channel are investigated, respectively. Simulation results indicate that even in the dispersive channel, when the optical signal-to-noise ratio （OSNR） is low, it can still work very well.
