Modes of Frequency-Modulated and Frequency-Shifted Feedback Lasers

The modes of a phase-modulated and frequency-shifting dispersion-free unidirectional ring cavity are shown to be, in general, linearly chirped and phase-modulated hybrid modes. Using heterodyne detection and digital signal processing techniques, incoherent ASE recirculation and laser oscillation in terms of these modes are experimentally verified in a frequency-shifted feedback fiber laser and in a frequency-modulated fiber laser, respectively. The different modal regimes appearing in both types of lasers are exemplified, and their use as tailorable waveform generators discussed.