Sources of the Spontaneous Component in Probe Reflection in Brillouin Dynamic Gratings

Abstract

Spontaneous Brillouin backscattering, which accompanies the operation of Brillouin Dynamic Gratings (BDG) sensors, is investigated for both the Stokes and anti-Stokes configurations, showing levels much higher than encountered in classical Brillouin backscattering.

Brillouin Dynamic Gratings (BDGs) in Polarization Maintaining (PM) fibers have recently gained considerable interest due to their highly controlled generation mechanism and wide range of possible applications [1-2]. In BDG, two co-polarized pumps (PumpH and PumpL, Fig. 1) counter-propagate along the slow axis of a PM fiber and interfere to generate an acoustic wave in the fiber (propagating from PumpH towards PumpL), which reflects a probe (signal) wave polarized along the fast axis. For a Stokes BDG scenario, Fig. ‎1(a), the Probe is injected into the fiber from the same side as the strong writing pump PumpH and propagates in the direction of the acoustic wave. For an anti-Stokes BDG scenario, Fig. 1(b), the Probe is injected into the fiber from the opposite side and is being reflected by the counter-propagating grating into a higher frequency wave.

Figure 1: A Brillouin Dynamic Grating (the acoustic wave in the figures) formed in a PM fiber by PumpH and either a counter-propagating PumpL, or a PumpH-excited counter-propagating spontaneous Brillouin reflection (not shown). It can reflect a Stokes-wave propagating with the grating (a); or an anti-Stokes wave propagating against the grating (b).

The advantages of BDGs in fiber-optic sensing and optical signal processing are accompanied by detrimental spontaneous Brillouin reflection, which limits the performance of many Brillouin-related applications. This work presents a pioneering analytical, numerical and experimental study of this phenomenon. The main novel practical observation of this work is the substantial increase in the spontaneous backscattering of the signal of interest (i.e., the fast-polarized probe), caused by its interaction with the acoustic field generated by stimulated amplification of the spontaneous Brillouin scattering of the s-polarized pump(s). For a Stokes BDG, where the signal wave co-propagates with the high frequency pump wave [3], a symmetric situation occurs and a strong signal may also enhance the spontaneous backscattering of the pump. On the other hand, in an anti-stokes configuration, where the signal wave propagates against the high frequency pump, the anti-Stokes reflection process can only consume the spontaneously generated dynamic grating. In this latter scenario, it is shown that, eventually, as the signal reaches a critical value, the anti-Stokes reflection sharply decreases from its high BDG value to its classical one.

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