Observation of two-wave mixing in a single-frequency fiber amplifier induced by frequency modulation

Abstract

We report on the observation of unstable two-wave mixing in a Yb-doped optical fiber amplifier induced by frequency modulation of a single-frequency laser. What is believed to be a reflection of the main signal experiences a gain much higher than that provided by the optical pumping and potentially limits power scaling under frequency modulation. We propose an explanation for the effect based on the dynamic population and refractive index gratings formed by the interference between the main signal and its slightly frequency-detuned reflection.

Theory and experiment of two-wave mixing instability in single-frequency fiber amplifiers induced by frequency modulation

“…The hypothesis underlying the present work, and qualitatively presented in [20], is that two-wave mixing can cause strong backscattering in a fiber amplifier when the signal is frequency-modulated and a weak reflection at the output facet gives rise to a counter-propagating frequency-shifted signal. Because of the delayed response of the gain material, the resulting moving IIP is spatially offset with respect to the induced RIG, which enables power transfer between the two fields.…”

“…However, the PER was typically 15 dB, and the output beam was stable with an intensity profile corresponding to the fundamental mode with no apparent distortions. Further details on the setup and its characterization are provided in [20].…”

“…In addition, there is a trend that the second maximum is lower than the first. Both of these trends are quantified in greater detail in [20].…”

“…We have recently shown that frequency modulation (FM) of a SFFL can cause an instability that manifests itself as undesired backscattering of signal light in a subsequent fiber amplifier [20].…”

“…In [20] we hypothesized that the instability induced by FM is a manifestation of two-wave mixing in the active core of the amplifier under investigation, but a detailed theory was not developed.…”