A large signal theory is performed to analyze linear fibre dispersion effect on the intensity modulated dual-electrode Mach-Zehnder Modulators (DE-MZMs) when used for millimetre-wave harmonic up-conversion (MWHU) through photonic mixing in Radio over Fiber (RoF) systems. Closed form expressions are derived for the directly detected photocurrent spectrum in general case of MWHU and for general operating conditions of DE-MZM such as biasing, driving level, phase shifted and unbalanced drives. The analytical expressions derived here are much more useful than just numerical simulations or measurements. They provide fast and accurate analysis and better insight into harmonic up-conversion in terms of dispersive fading, amplitude and phase ripples. Harmonic up-conversion products are given in the case of any local oscillator (LO) order. Harmonic up-conversion is analysed in terms of dispersion induced power penalty (DIPP) and intensity modulation depth (IMD) of the up-converted signal. The special cases of double sideband (DSB) and single sideband (SSB) modulation of balanced drives of DE-MZM are studied in detail. The DIPP and IMD are expressed for different bias point configurations. Optimum LO modulation index that maximizes the IMD and reduces the DIPP is also calculated. The obtained new results are compared to previously published works.
Homogeneous gain broadening suppression and super mode noise reduction in a multiwavelength active mode-locked erbium-doped fiber ring laser are investigated. By incorporating a semiconductor optical amplifier that is biased to operate just above the transparent point, the gain spectral hole burning of the erbium-doped fiber ring laser is effectively suppressed and the super mode noise is significantly reduced. Active mode locking of 8 wavelengths at room temperature with improved noise figure is demonstrated.
Erbium-doped fiber lasers can find many applications such as WDM systems, fiber-optic sensors and microwave photonics, thanks to their high power, narrow linewidth and low noise. The key factor that limits the number of stable lasing wavelengths achieved at room temperature is the strong gain homogenous broadening of erbium-doped fiber. Different approaches have been approached. These approaches include the use of a phase shifter in the fiber ring cavity to reduce the homogenous broadening, and the use of Nitrogen to cool the erbium-doped fiber with a reduced homogenous broadening. Clearly, the approach using a phase shifter increases the system cost and the approach using Nitrogen is not suitable for practical applications.
In this paper, we propose a novel approach to achieving stable multiwavelength lasing at room temperature using a semiconductor optical amplifier in the fiber ring laser cavity. In the fiber laser, the erbium-doped fiber is used as a gain medium. The semiconductor optical amplifier is used as a phase shifter. The semiconductor optical amplifier is driven by a sinusoidal wave. The refractive index change of the semiconductor optical amplifier leads to a phase shift, which effectively suppresses the homogenous broadening and cross-gain saturation. In addition, since the semiconductor optical amplifier is biased at the transparent point, no insertion loss is introduced. In the experiment, the modulation voltage to the semiconductor optical amplifier can be as low as 10 mV, which is much lower than that used in a phase modulator (10 V). Experiment is being carried out and more experimental results will be reported.