Liang Dou

Peking University, Beijing, Beijing Shi, China

Are you Liang Dou?

Claim your profile

Publications (10)9.3 Total impact

  • Source
    [show abstract] [hide abstract]
    ABSTRACT: Electrical post-compensation of intrachannel nonlinearities in 10GBaud coherent QPSK transmission systems is proposed. The channel inversion method is implemented with a coarse split-step Fourier algorithm through digital signal processing at the receiver side. Simulation results show that simultaneous compensation of intrachannel nonlinearities and chromatic dispersion can be achieved with much simplified signal processing structure. The required optical signal-to-noise ratio at certain bit error ratio is remarkably reduced when the intrachannel nonlinearities induced impairments are dominant.
    Optics Communications - OPT COMMUN. 01/2009; 282(5):992-996.
  • [show abstract] [hide abstract]
    ABSTRACT: A novel nonlinear electrical equalizer (NLEE) based on Volterra theory is proposed for mitigating intra-channel nonlinearities in pseudo-linear coherent optical communication systems. According to the temporal matching condition of intra-channel nonlinear pulse interactions, we just select specific nonlinear terms and reduce the computation complexity considerably. Numerical simulations of 10GBaud coherent quadrature phase-shift keying (QPSK) transmission show that the simplified NLEE can efficiently mitigate intra-channel nonlinearities.
    Optics Communications 01/2009; 282(12):2421-2425. · 1.44 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We propose a hybrid C-band erbium-doped fiber amplifier (EDFA) and L-band Raman fiber amplifier (RFA) using a single pump laser diode. The optimum pump sharing ratio to EDFA/RFA is 1/10 with a total pump power of 660 mW. Using multiple fiber Bragg gratings (FBGs) with various reflectivities at different positions along the dispersion compensation fiber, the optimum dispersion compensation and power equalization for C + L-band channels are simultaneously realized. With an input power of −20 dBm/ch, the signal power variation among the channels is reduced from 9.8 dB to less than ±0.5 dB. Two pump reflectors are introduced to increase the pumping efficiency.
    Optics Communications 01/2009; 282(20):4087-4090. · 1.44 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: We propose a simple and stable structure to realize a tunable negative coefficients microwave photonic notch filter. A 3×3 coupler based Sagnac loop interferometer with an asymmetrically placed phase modulator is used to acquire the notches. Our system is simply implemented and with good stability. Theoretical analysis and experimental results are presented and show a good agreement. The structure is proved to be a robust filter with more than 35dB rejection deep notches.
    Optics Communications 01/2008; 281(6):1476-1479. · 1.44 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: Raman fiber amplifiers (RFAs) have become increasingly important in optical communication systems and optical networks to compensate for the fiber loss and/or splitting loss. Comparing to the conventional rare-earth doped fiber amplifiers, RFAs have flexible signal gain band and low noise figure (NF) level [1]. Recently, we reported an RFA with signal/pump double-pass the gain medium scheme by utilizing an optical circulator (OC) as a signal/pump reflector [7]. The experiments have successfully confirmed that the pumping efficiency improvement for this kind of RFA is more efficiency than other types of RFAs. What's more, the pumping efficiency improvement and the NF suppression can be realized simultaneously. Although it is crucial to numerically predict the characteristics of RFA such as signal power and noise figure (NF) versus pump wavelength, pump power, gain medium characteristic and so on, the optimum design of RFA parameters has not yet been addressed. In this paper, we preliminary describe the numerical simulation method based on a set of coupled steady-state equations to estimate the characteristics of signal/pump double-pass RFA. First we assume that the ASE level combines other noises is 30 dB lower than that of the input signal [8], so only the forward and backward signal/pump are considered. This is a typical boundary condition problem and Newton method is used. As we get the distribution of pump power and signal power along the DCF, the noise item at different frequency can be found by the relaxation method. After several iterations, all WDM channel signals and noise are convergent as predict. In this paper, in order to optimize the length of the DCF to get the best performance, we suggest a proper definition of NF called effective NF. During the simulation, the length of SMF is kept constant, so DCF length becomes the only dominant variable parameter to affect the gain and NF of the RFA. To verify the algorithm above, the parameters are set equal to those in Ref. [7], and the simulation results are in very good agreement with those of experimental data. To show the optimum DCF length under a fixed pump power and pump wavelength, we calculate results of both the signal output power and NF versus DCF length at three signal wavelengths. We find that the largest signal output power may appear at the length of DCF is 3.6 km, and the lowest NF could be found as the DCF length is 4 km. In general, DCF has a larger gain coefficient than that of the SMF, but the attenuation coefficient of the former is also higher than that of SMF. So, there is a trade-off when selecting the length of DCF. From the simulation results, we conclude that the optimum length of DCF is 3.8 +/- 0.2 km. DCF length of around 3.8 km is an ideal value to realize the best characteristics for the RFA.
    Proc SPIE 11/2007;
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: In this paper, we propose a novel signal/pump double-pass Raman fiber amplifier using fiber Brag gratings (FBGs). In order to compensate the dispersion slop mismatch among channels in lightwave system, FBGs embedded in different positions along dispersion compensated fiber are used to control the travel length of each WDM signal. Gain equalization can be achieved by optimizing the reflectivity of each FBG. Maximum output power variation among channels is less than +/-0.5 dB after appropriate optimization. Finally, a wavelength division multiplexing (WDM) system using 40-Gb/s x 8 ch non return-to-zero (NRZ) signal transmission in a 100-km transmission fiber is simulated to confirm the system performance. Using proposed dispersion compensation method, it may lead to 2 dB improvement in Q value. Such kind of RFA may find vast applications in WDM system where dispersion management is a crucial issue.
    Optics Express 10/2007; 15(19):12356-61. · 3.55 Impact Factor
  • Source
    [show abstract] [hide abstract]
    ABSTRACT: By using the coupled steady-state equations, we have numerically studied the characteristics optimization of Raman fiber amplifier (RFA) in a signal/pump double-passes-the-gain-medium scheme. The simulation results are in very good agreement with those of experimental data. Given a constant pumping power, the length of dispersion compensation fiber (DCF) in a RFA could be determined. The optimum design shows that the best length of the DCF is at around 3.8 ± 0.2 km in our study. This could provide both the highest signal output power and the lowest noise figure among all conditions we choose.
    Optics Communications 01/2007; 273(1):149-152. · 1.44 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: High energy noise-like pulses has been generated from a passively mode locking fiber laser with large normal dispersion. Super-continuum more than 600 nm can be generated directly by launching the pulses into high nonlinear fiber.
    Nano-Optoelectronics Workshop, 2007. i-NOW '07. International; 01/2007
  • [show abstract] [hide abstract]
    ABSTRACT: We propose a simple and stable structure to realize a tunable negative coefficients microwave photonic notch filter. Theoretical analysis and experimental results are presented and show a good agreement.
    Nano-Optoelectronics Workshop, 2007. i-NOW '07. International; 01/2007
  • Optical Engineering - OPT ENG. 01/2006; 45(9).