Linewidth characteristics of a filterless tunable erbium doped fiber ring laser

Abstract

The linewidth characteristics of a continuous wave erbium doped fiber ring laser whose tunability is controlled by intracavity loss are studied in this work. The spectral linewidth of such a filterless laser is measured experimentally, and its characteristics are analyzed using the existing analytical model of fiber laser. The long length of the cavity results in lasing into multiple longitudinal modes, thus leading to a broad linewidth. The extent of this broadening is found to depend on the intracavity attenuation and pump powers. Under the given experimental conditions, for a fiber of length 12m, the linewidth increases up to 4.5nm corresponding to an additional cavity loss of 5dB, while it increases up to 8nm for a loss of 1.2dB for a fiber of length 4.6m. The linewidth decreases with further increase in intracavity loss for both cases. In this work, the linewidth dependence on the intracavity loss is directly linked to the spectral dependence of the threshold power for each length. The linewidth increases with pump power irrespective of the intracavity attenuation, and this is explained through the nature of the amplified spontaneous emission from the fiber at different pump powers. The experimental results and the subsequent analysis are useful for the design of a filterless tunable laser with a narrow linewidth using erbium doped fiber in the ring configuration.