Abstract
Signal-dependent speckle-like noise was the dominant noise in a Brillouin grating measurement with micrometer-resolution optical low coherence reflectometry (OLCR). The noise was produced by the interaction of a Stokes signal with beat noise caused by a leaked pump light via square-law detection. The resultant signal-to-noise ratio (SNR) was calculated and found to be proportional to the square root of the dynamic range (DR) defined by the ratio of the Stokes signal magnitude to the variance of the beat noise. The calculation showed that even when we achieved a DR of 20 dB on a logarithmic scale, the SNR value was only 7 on a linear scale and the detected signal tended to fluctuate over ±14% with respect to the mean level. We achieved an SNR of 24 by attenuating the pump light power entering the balanced mixer by 55 dB, and this success enabled us to measure the Brillouin spectrum distributions of mated fiber connectors and a 3-dB fused fiber coupler with a micrometer resolution as examples of OLCR diagnosis.
Highlights
The Brillouin-enhanced four-wave mixing induced by counter-propagating pump lights and one probe light produces a backward Stokes light in a waveguide under test [1], which is assumed to be the reflection of the probe light by an acoustic wave or a dynamic Brillouin grating excited in the waveguide by the two pump lights
This paper shows theoretically and experimentally that the speckle-like noise was generated by the interaction of the Stokes signal with the beat noise via the square-law detection employed in the optical low coherence reflectometry (OLCR) system
We report that the signal-to-noise ratio (SNR) of a Brillouin grating distribution that we obtained with one measurement increased to 24 even at a micrometer spatial resolution as a result of the beat noise reduction that we realized by incorporating the polarizer
Summary
The Brillouin-enhanced four-wave mixing induced by counter-propagating pump lights and one probe light produces a backward Stokes light in a waveguide under test [1], which is assumed to be the reflection of the probe light by an acoustic wave or a dynamic Brillouin grating excited in the waveguide by the two pump lights. The phase of the acoustic wave travelling along the fiber was considered to fluctuate at frequencies of approximately several Hertz due to perturbations applied to their optical paths, and the resultant Stokes light wave, which was assumed to be the reflection of the probe light by the acoustic wave, should be coherent with the probe light wave in the same way as a Fresnel reflection This means that there were no serious optical effects inducing signal-dependent (or speckle-like) variations in the Brillouin grating measurement. In the latter half, we report that the SNR of a Brillouin grating distribution that we obtained with one measurement increased to 24 even at a micrometer spatial resolution as a result of the beat noise reduction that we realized by incorporating the polarizer. This reduction enabled us to obtain the Brillouin spectrum distributions of the mated optical connectors and a 3-dB fused fiber coupler
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