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.