Modeling and Design of DBR Fiber Lasers for Sensor Applications

We present a comprehensive design methodology addressing DBR fiber laser sensors. Modeling such lasers requires solving several critical simulation challenges. The laser cavity provides optical feedback where light experiences hundreds of round trips. The corresponding simulation schematic represents a network of elementary building blocks where the optical feedback introduces feedback loops requiring an iterative approach. We show how the cavity characteristics can be simulated in the frequency or time domain and compare their effectiveness. The design of digital filters accurately reproducing the grating spectra is one important challenge. For DBR lasers to oscillate in single mode the grating selectivity needs to be high enough, which may require a response length exceeding reasonable simulation windows by far.Furthermore, we discuss how to simulate the entire laser including an amplifying medium. We compare approaches using a system-level amplifier model, stationary and dynamic doped fiber models. We show that for an adequate description of the laser spectrum, modal and noise characteristics the time domain approach in combination with a dynamic fiber model is desirable. The frequency domain approach is problematic due to varying noise samples in each iteration. Note, due to the slow population dynamic in doped fibers tens or hundreds of thousands of iterations may be needed to achieve stationary operation. Once this is accomplished, the laser shows a single-mode spectrum with linewidth in agreement to the quantum limit. Using our developed model we show how the laser characteristics depend on the grating frequencies and can be used for sensor applications.