Characterization of small-signal intensity modulation of single-mode fiber grating Fabry-Perot laser source

A comprehensive study on the small-signal intensity modulation (IM) characteristics of a fiber grating Fabry-Perot (FGFP) laser is numerically investigated. The effect of external optical feedback (OFB), temperature, injection current, cavity volume, nonlinear gain compression factor, and fiber grating (FG) parameters on IM characteristics are presented. The temperature dependence (TD) of IM is calculated according to the TD of laser cavity parameters instead of using the well-known Parkove relationship. It has been shown that the optimum external fiber length (L _ext) is 3.1 cm. The optimum range of working temperature for FGFP laser is between 23 to 27 °C. We also show that by increasing the laser injection current from 10 to 60 mA, the IM peak amplitude decreased from 6.3 to 0.2 dB and the relaxation-oscillation frequency (ROF) is shifted from 1.2 GHz towards higher frequency of 5.48 GHz. In addition, the AR coating reflectivity and gain compression factor have no significant effect on the IM. The study indicates that a stable operation and excellent modulation characteristic can be obtained after optimization process.     

Improving the performance of electrical duty-cycle division multiplexing with optimum signal level spacing

Performance optimization of 3 × 10 Gbps conventional electrical-duty-cycle division multiplexing (C-E-DCDM) technique is investigated. It is shown that controlling signal level spacing can optimize its performance. Two level spacing optimization techniques, one in electrical domain and another in optical domain are examined. In general, performance of the C-E-DCDM is improved significantly using both approaches. The results show by optimization, an improvement of around 5.5 dB can be achieved for the C-E-DCDM in terms of receiver sensitivity and optical signal-to-noise ratio using both electrical and optical methods. However, chromatic dispersion tolerance in one of the optimization approaches is degraded by around 34 ps/nm for negative dispersion, while the positive dispersion tolerance improved compared to the C-E-DCDM.     

Improving the performance of three level code division multiplexing using the optimization of signal level spacing

In order to optimize the performance of three level code division multiplexing (3LCDM) at 2×20 Gb/s data rate, signal level spacing technique is investigated in this paper. The 3LCDM performance is improved considerably using both electrical and optical level spacing optimization configurations. The results demonstrate that by optimization, in conditions of the optical signal-to-noise ratio, an improvement of around 4.5 dB can be achieved in both approaches as well as 3.3 dB in the electrical configuration and 3.5 dB in the optical configuration can be accomplished for the 3LCDM in terms of the receiver sensitivity.     

Duty-cycle division multiplexing (DCDM)

A multiplexing technique, which is based on duty-cycle division, is proposed. The channel multiplexing and demultiplexing are performed electrically at the single user bit rate, which is very economic. In a three-user system (3×10 Gb/s), the simulation results show that the best receiver sensitivity value achieved is −30.1 dBm with an optical signal-to-noise ratio (OSNR) of 22.3 dB, while the chromatic dispersion tolerance ranges from 192 to 280 ps/nm. Migration from 30 to 120 Gb/s is achieved with the penalty of 6.4 and 5.2 dB in the receiver sensitivity and OSNR, respectively, for the worst user.     

The effect of nonlinear gain on the characteristics of an optically injected VCSEL and cavity solitons

Using the Lagrange method to fit the curve of maximum gain as a function of carrier density for an active region consisting of an AlGaAs/GaAs layers sandwiched between DBR layers, it is found that the curve is better approximated assuming a quadratic dependence on the carrier density. By summarizing all of the calculations into a nonlinear gain coefficient parameter, $\beta $ , in the Maxwell–Bloch equations we numerically studied the effect of nonlinear gain on the characteristics of the VCSEL and also on the cavity solitons (CSs) forming in such a device. Particularly, it is shown that with nonlinear gain a wider locked region can be achieved along with enhanced sustained relaxation oscillation amplitude. The switching on/off time of CSs is modified and there appears a considerable enhancement in their efficiency and contrast.     

Duty-cycle-division-multiplexing: Bit error rate estimation and performance evaluation

An improved estimation of bit-error-rate (BER) for electrically multiplexed duty-cycle division multiplexing (E-DCDM), which is based on the probability of error, is presented. Performance of 3 × 10 Gbit/s E-DCDM is investigated in terms of optical signal-to-noise ratio (OSNR) and dispersion tolerance. This technique requires 29.4 dB OSNR and can tolerate ±96 ps/nm chromatic dispersion for the worst user.     

Micro‐PIXE analysis of doped SiO2 fibres intended as TL dosimeters for radiation measurements

Sample elemental concentrations can be determined using the microbeam proton‐induced X‐ray emission (PIXE) technique, providing non‐destructive simultaneous low‐background multi‐element analysis. Present interest concerns analysis of Ge‐doped SiO₂ fibres intended as high spatial‐resolution thermoluminescence (TL) dosimeters for radiation measurements in place of their more typical applications in telecommunications. During fibres fabrication, defined amounts of the Ge dopant are added, the dopant more usually having a determining role in the transmission properties of the fibre. Characteristic X‐rays produced in PIXE analysis provide information on the relative distribution of elements within a sample, as in for instance Ge and Si concentrations, the Ge acting as point defect centres that promote TL. With the dopant tending to diffuse in and away from the fibre core, it is essential to define the sample matrix composition in order to accurately evaluate the X‐ray yield. This is determined in part using simultaneous Rutherford Back Scattering analysis. In present work, PIXE/Rutherford Back Scattering measurements have been employed to ascertain dopant concentrations of fibres that have been fabricated at the University of Malaya with a view to improving TL yield. Present results concern cylindrical fibres, nominally with 4%, 6% and 8% weight peak Ge concentrations and flat fibres of nominal 6% weight Ge concentration. For the cylindrical fibres, Ge dopant concentration has been found to be in the range of 2.41–4.56%, 6.44–8.29% and 10.27–12.25% weight, respectively, while for the flat fibres, the Ge concentration range is broader, at 0.07–6.55% weight. Copyright © 2014 John Wiley & Sons, Ltd.     

Absolute polar duty cycle division multiplexing over wavelength division multiplexing system

The performance of absolute polar duty cycle division multiplexing (AP-DCDM) over wavelength division multiplexing (WDM) system is presented based on the simulation results. The AP-DCDM signal has narrower bandwidth than conventional time division multiplexing (TDM) signal, which makes its implementation in WDM system advantageous. In this paper, characteristics of AP-DCDM and TDM signals in WDM system are compared at the speed of 40 Gbit/s per channel, for the minimum allowed channel spacing and the chromatic dispersion tolerance. The results clearly show that AP-DCDM performs significantly better than TDM. By using AP-DCDM, 1.28 Tbit/s (32 × 40 Gbit/s) was successfully transmitted over 320 km standard single mode fiber. Spectral efficiency of 0.64 b/s/Hz was achieved by using 10 Gbit/s transmitters and receivers without polarization multiplexing.