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Optics Letters

Atmospheric transfer of optical and radio frequency clock signals

Benjamin Sprenger, Jie Zhang, Z. H. Lu, L. J. Wang

Published Mar 17, 2009• 62 citations
Publisher

Abstract

The phase instability induced during the transfer of radio frequency and optical clock signals through the turbulent atmosphere was measured in a rooftop experiment. Radio frequency intensity modulation of a laser to transmit signals over 100 m results in an Allan deviation of 1.31x10(-10) at 1 s. Optical transfer is more accurate at 1.68x10(-13) at 1 s. As a consequence, fiber links are more suitable for the transfer of optical frequencies over very long distances while free space transmission might find applications in short distances of less than 1 km.

DOI
10.1364/ol.34.000965

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The phase instability induced during the transfer of radio frequency and optical clock signals through the turbulent atmosphere was measured in a rooftop experiment. Radio frequency intensity modulation of a laser to transmit signals over 100 m results in an Allan deviation of 1.31x10(-10) at 1 s. Optical transfer is more accurate at 1.68x10(-13) at 1 s. As a consequence, fiber links are more suitable for the transfer of optical frequencies over very long distances while free space transmission might find applications in short distances of less than 1 km.
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The high-precision time-frequency transfer of the optical atomic clock signals over ground-to-satellite and terrestrial free-space laser paths is of great significance in the fields of fundamental and applied sciences. However, the phase noises caused by atmospheric turbulence severely degrade the measurement precision. In this paper, a new method to simulate the transmission of time-frequency signal propagating through atmospheric turbulence is investigated. An analog transmission system comparable to the practical out-field link has been demonstrated, which can provide a deep insight into the phase distortion of time-frequency signal of free-space optical communication links.
To compare the increasing number of optical frequency standards, highly stable optical signals have to be transferred over continental distances. We demonstrate optical-frequency transfer over a 1840-km underground optical fiber link using a single-span stabilization. The low inherent noise introduced by the fiber allows us to reach short term instabilities expressed as the modified Allan deviation of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ for a gate time $\ensuremath{\tau}$ of 1 s reaching $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ in just 100 s. We find no systematic offset between the sent and transferred frequencies within the statistical uncertainty of about $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$. The spectral noise distribution of our fiber link at low Fourier frequencies leads to a ${\ensuremath{\tau}}^{\ensuremath{-}2}$ slope in the modified Allan deviation, which is also derived theoretically.
We demonstrated an atmospheric radio-frequency transfer over a 50 m outdoor free-space link using a compact diode laser with an electronic phase compensation technique. With transferring a 1 GHz microwave signal in 5000 seconds, the total root-mean-square (RMS) timing fluctuation was measured to be about 560 fs, with fractional frequency instability on the order of 4 × 10-13at 1 s, and order of 4 × 10-16at 1000 s. This atmospheric frequency transfer scheme can be used to disseminate a commercial Rb or Cs clock via a free-space transmission link.
Abstract Timescale comparison between optical atomic clocks over ground-to-space and terrestrial free-space laser links will have enormous benefits for fundamental and applied sciences. However, atmospheric turbulence creates phase noise and beam wander that degrade the measurement precision. Here we report on phase-stabilized optical frequency transfer over a 265 m horizontal point-to-point free-space link between optical terminals with active tip-tilt mirrors to suppress beam wander, in a compact, human-portable set-up. A phase-stabilized 715 m underground optical fiber link between the two terminals is used to measure the performance of the free-space link. The active optical terminals enable continuous, cycle-slip free, coherent transmission over periods longer than an hour. In this work, we achieve residual instabilities of 2.7 × 10 −6 rad 2 Hz −1 at 1 Hz in phase, and 1.6 × 10 −19 at 40 s of integration in fractional frequency; this performance surpasses the best optical atomic clocks, ensuring clock-limited frequency comparison over turbulent free-space links.