Anomalous linewidth behavior in short-cavity single-frequency fiber lasers

An experimental investigation of the anomalous linewidth behavior in a distributed feedback fiber laser is presented. It is found that not only does the linewidth deviate from the Schawlow-Townes linewidth formula by increasing with pump and laser power, but it also varies significantly with the pumping configuration. These observations have potentially important implications for the design and operation of such fiber lasers.

I. Background

Short-Cavity single-frequency fiber lasers have been a topic of continued interest since the early work of Ball et al. on Er-doped distributed Bragg reflector fiber lasers [1]. Fig. 1. Threshold and laser output power characteristics. Their size, simplicity, ease of fabrication, and optical emission characteristics make them attractive for a number of applications, particularly in the sensing area. However, there have been few detailed experimental investigations on the linewidth/phase noise characteristics of these lasers, with much of the work focused on improving the efficiency and output power through fiber, cavity, or grating design [2] [3] [4]. Laser efficiencies in the tens of percent are now easily achievable [5]. An implicit assumption in many of these designs is that the attainable laser frequency noise characteristics would not be substantively compromised, clearly an important assumption as it is usually the single-frequency characteristics of these kinds of fiber lasers that make them so attractive. To our knowledge, however, there has been little evidence shown to verify the extent to which this assumption is true.