28 February 2017 Observation of thermodynamic phase noise using a slow-light resonance in a fiber Bragg grating
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Abstract
Thermodynamic phase noise in passive fiber devices is generally so weak that in most devices, in particular fiber sensors, it has only been observed in fiber lengths in the range of 1 meter or much longer. Here we present a passive fiber strain sensor only 4.5 mm in length in which the noise in the frequency range of 1 kHz to ~12 kHz is limited by thermal phase noise in the fiber. The phase noise could be measured in such a short fiber by utilizing a slow-light fiber Bragg grating (FBG) resonator in which the phase noise is magnified by the resonator's slowing-down factor ng/n ≈ 370, where ng is the group index. At the same time, the usually dominant laser frequency noise was brought below the level of the phase noise by using a short fiber and a low-noise laser with a linewidth under 200 Hz. At 4 kHz, the total measured noise expressed in units of strain is 110 fε/√Hz, and the phase noise accounts for 77% of it. This sensor resolves a single-pass thermodynamic length fluctuation of only 5 x10-16 m/√Hz. These measurements provide experimental support for the dependencies of the phase noise on the fiber resonator length and group index predicted by a recent model.
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George Skolianos, Arushi Arora, Martin Bernier, Michel Digonnet, "Observation of thermodynamic phase noise using a slow-light resonance in a fiber Bragg grating", Proc. SPIE 10119, Slow Light, Fast Light, and Opto-Atomic Precision Metrology X, 1011919 (28 February 2017); doi: 10.1117/12.2261148; https://doi.org/10.1117/12.2261148
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