Liao Chang-Jun

Quantum Key Distribution System with Six Polarization States Encoded by Phase Modulation

An intrinsically stable quantum key distribution system (QKD) with six polarization states encoded by phase modulation is introduced. The encoder and decoder are in the same structures that consist of two polarizing Sagnac interferometers connected in tandem. The six polarization states are determined and distinguished by different sets of phase shifts induced by two respective electrically-driven integrated phase modulators. A mean visibility of interference fringes is kept stable at 97.58% for an hours performance. Theoretical and experimental analyses show that the proposed QKD system features intrinsically stability immune from environment fluctuation.

Numerical Study of the Apodization Profile Functions, Optimal Profiles and Lengths of a Linearly Chirped Fibre Bragg Grating

Starting from our definition of apodization profile functions, we discover the optimal profiles and the characteristics of defined apodization functions in sidelobe suppression. It is shown by numerical analysis that the optimal grating length is 45?mm to compensate for dispersion induced by 100?km fibres and the smoothness of the ripples in time-delay characteristics is related to the defined parameters.

MODULATION INSTABILITY OF FEMTOSECOND OPTICAL PULSES IN LONG OPTICAL FIBERS WITH MINIMUM GROUP-VELOCITY DISPERSION

A new region where modulation stability occurs in the fibers with minimum group-velocity was found due to the fourth-order dispersion effect. The spectral extent of the region is analyzed and found to be affected by initial input power and the dispersion parameter.

A novel strain method for precisely adjusting the grating chirp and center wavelength

A simple and novel strain method is used to change a uniform grating into a linearly chirped grating. Independent tuning of the chirp and the center wavelength of the fiber grating is demonstrated. A 10?Gb?s-1 transmission over 100 km of standard single-mode fiber is realized using this strain method. It is deduced that the product of the maximum dispersion compensation bandwidth and the corresponding dispersion is nearly equal to a constant by numerical simulation.

The nonlinear Schr?dinger equation and the cross-phase modulation in erbium-doped fiber amplifiers

The nonlinear Schr?dinger equation (NLSE) in erbium-doped fiber (EDF) was obtained. The cross-phase modulation (XPM) in the erbium-doped fiber amplifiers (EDFA) was studied based on this NLSE and the rate equations. A more generalized form of the propagation equation in EDFA was obtained which included the phase shifts the EDFA induced. An analytical expression was given to the XPM in the EDFA. It was found that the XPM in the EDFA dose not change very much with the wavelength except at the neighboring wavelengths (around 1531 nm) where the absorption and emission cross-sections of the erbium ions reach their maxima, and the XPMs have opposite signs on the two sides around 1531 nm. Furthermore, it was found that the XPM increases with the increase of the length of EDF.

Suppression of Raman Self-Frequency Shift of Soliton Propagation in Single Mode Optical Fiber

A novel idea is proposed to suppress the soliton self-frequency shift of femtosecond soliton pulse in dispersion-decreasing fibers. The expression of the soliton self-frequency shift as a function of the pulse width and the fiber dispersion parameter is derived. The numerical simulation is also given, and is in agreement with analytical results.

Numerical analysis of In0.53Ga0.47As/InP single photon avalanche diodes

A rigorous theoretical model for In0.53Ga0.47As/InP single photon avalanche diode is utilized to investigate the dependences of single photon quantum efficiency and dark count probability on structure and operation condition. In the model, low field impact ionizations in charge and absorption layers are allowed, while avalanche breakdown can occur only in the multiplication layer. The origin of dark counts is discussed and the results indicate that the dominant mechanism that gives rise to dark counts depends on both device structure and operating condition. When the multiplication layer is thicker than a critical thickness or the temperature is higher than a critical value, generation—recombination in the absorption layer is the dominative mechanism; otherwise band-to-band tunneling in the multiplication layer dominates the dark counts. The thicknesses of charge and multiplication layers greatly affect the dark count and the peak single photon quantum efficiency and increasing the multiplication layer width may reduce the dark count probability and increase the peak single photon quantum efficiency. However, when the multiplication layer width exceeds 1 μm, the peak single photon quantum efficiency increases slowly and it is finally saturated at the quantum efficiency of the single photon avalanche diodes.

Numerical analysis of In(0.53)Ga(0.47)As/InP single photon avalanche diodes

A rigorous theoretical model for In0.53Ga0.47As/InP single photon avalanche diode is utilized to investigate the dependences of single photon quantum efficiency and dark count probability on structure and operation condition. In the model, low field impact ionizations in charge and absorption layers are allowed, while avalanche breakdown can occur only in the multiplication layer. The origin of dark counts is discussed and the results indicate that the dominant mechanism that gives rise to dark counts depends on both device structure and operating condition. When the multiplication layer is thicker than a critical thickness or the temperature is higher than a critical value, generation—recombination in the absorption layer is the dominative mechanism; otherwise band-to-band tunneling in the multiplication layer dominates the dark counts. The thicknesses of charge and multiplication layers greatly affect the dark count and the peak single photon quantum efficiency and increasing the multiplication layer width may reduce the dark count probability and increase the peak single photon quantum efficiency. However, when the multiplication layer width exceeds 1 μm, the peak single photon quantum efficiency increases slowly and it is finally saturated at the quantum efficiency of the single photon avalanche diodes.

The analysis of the integral gated mode single photon detector

This paper critically analyses and simulates the circuit configuration of the integral gated mode single photon detector which is proposed for eliminating the transient spikes problem of conventional gated mode single photon detector. The relationship between the values of the circuit elements and the effect of transient spikes cancellation has been obtained. With particular emphasis, the bias voltage of the avalanche photodiode and the output signal voltage of the integrator have been calculated. The obtained analysis results indicate that the output signal voltage of the integrator only relates to the total quantity of electricity of the avalanche charges by choosing the correct values of the circuit elements and integral time interval. These results can be used to optimize the performance of single photon detectors and provide guides for the design of single photon detectors.

Two Methods for Extending Quantum Key Warehouse

Because the rates of quantum key distribution systems are too low, the interleaving technique and interpolation technique are used to extend the capacity of the quantum key warehouse to increase the quantum key rates of quantum secure communication systems. The simulation shows that the interleaving technique and interpolation technique can extend random sequences and that their randomness are invariable. The correlative theory and technique of digital signal processing is an effective method of extending the quantum key warehouse.