Efficient interrogation method of forward Brillouin scattering in optical fibers using a narrow bandwidth long-period grating
Luis Alberto Pomed Sánchez, A. Dı́ez, J.L. Cruz, Miguel V. Andrés
Abstract
A new technique, to the best of our knowledge, for the characterization of the effective refractive index modulation in optical fibers due to transverse acoustic mode resonances excited by electrostriction is reported. The resonances excited by an optical pulse are probed by a narrow bandwidth long-period grating (LPG) inscribed in the fiber, which is interrogated by a continuous wave (CW) beam. The LPG used in this experiment has a narrow bandwidth and high sensitivity to small mode index perturbations, allowing the measurement of index variations from below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> . Radial and torsional-radial acoustic modes were characterized up to 1.1 GHz. The linewidth of resonances was found to be much shorter than in previous reports in which long fiber lengths are typically required, obtaining Q factors as high as 5000.