Non-reciprocity in optical fiber links: experimental evidence.

Dan Xu,Anne Amy-Klein,Olivier Lopez,Paul-Eric Pottie

doi:10.1364/oe.420661

Dan Xu, Anne Amy-Klein + Show 2 more

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https://doi.org/10.1364/oe.420661

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Abstract

Fundamental limits of fiber link are set by non-reciprocal effects that violate the hypothesis of equality between forward and backward path. Non-reciprocal noise arises technically from the set-up asymmetry, and fundamentally by the Sagnac effect when the fiber link encloses a non-zero area. As a pre-requisite for observation of Sagnac effect in fiber links, we present a study on phase noise and frequency stability contributions affecting coherent optical frequency transfer in bi-directional fiber links. Both technical and fundamental limitations of Two-Way optical frequency transfer are discussed. Our model predicts and our experiments substantially verify that the dominant noise mechanism at low Fourier frequencies is the polarization asymmetry induced by the temperature and relative humidity variations impacted on fiber links. The flicker noise floor due to the non-reciprocal noise arising from polarization mode dispersion is evidenced for the first time. We perform a post-processing approach which enables us to remove this polarization noise, improve the long-term stability and remove a frequency bias. We evaluate the uncertainty contributions of all the effects discussed for our 50 km spooled fiber link, dominated by its non-reciprocal noise induced by polarization mode dispersion with uncertainty of 1.9( ± 0.8)( ± 1.2) × 10-20. After correction, the linear drift of the residual phase is as low as 27 yoctosecond/s, leading to an uncertainty of the frequency transfer of 2.6 ( ± 39) × 10-22, confirming its potential for searching for more fundamental effects such as Sagnac effect or transient frequency variation due to dark matter.

Highlights

Optical fiber links play an important role in today global efforts towards a redefinition of the International System of Units (SI) second and coordination of international atomic time
We develop in this paper the theoretical analysis for the fundamental limits for fiber-based frequency transfer and optical clock comparison, and evaluate how much the performance of a fiber link is degraded by the path, frequency, wavelength and polarization asymmetry for the forward and backward signal transmitted on the optical fiber
The fiber spools are connected in such a way that the normal vectors to the enclosed surfaces of each spool are opposite in direction

Summary

Introduction

Optical fiber links play an important role in today global efforts towards a redefinition of the International System of Units (SI) second and coordination of international atomic time. Fiber links operated in the optical frequency domain use an ultra-stable laser as a frequency signal. This signal is degraded by the propagation delay fluctuations, induced by thermal and mechanical perturbations that act on the fiber optical length, that is both on the physical length and the fiber refraction index. Some actively or passively compensation techniques have been adopted to suppress these environmentally-induced optical phase noise [11,12,13], a fine understanding of the phase noise contributions will improve the use of fiber links and may widen its field of applications, such as study of Sagnac effect [14], search for dark matter [15,16,17] and quantum key distribution (QKD) [7]

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