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IEEE International Conference on Communication Technology (ICCT)

Portable Atmospheric Frequency Transfer over Outdoor Free-space Link with Compact Diode Laser

Jie Tian, Zhengyu Ji, Dong Hou

Published Oct 28, 2020• 0 citations
Publisher

Abstract

We demonstrated a portable atmospheric radio-frequency transfer over a 120 m outdoor free-space link using a compact diode laser as the clock carrier. Timing fluctuation and Allan Deviation are both measured to characterize the instability of transferred frequency incurred during the transfer process. In this experimental setup, all components were powered by a full-charged battery, and the experiment was conducted in an open-air environment, which is far from the laboratory-based room. With transferring a 100 MHz microwave signal in 4500 seconds, the total root-mean-square (RMS) timing fluctuation was measured to be about 22 ps, with fractional frequency instability on the order of 3 × 10-12at 1 s, and order of 4 × 10-14at 1000 s. We found that the majority of this excess timing fluctuation in our experiment is attributed to the air turbulence which could contribute a jitter at tens picosecond level in a normal windy night. This portable atmospheric frequency transfer scheme can be used to fast build an atomic clock-based frequency free-space transmission link since its stability is superior to a commercial Rb or Cs clock.

DOI
10.1109/icct50939.2020.9295691

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We demonstrated a portable atmospheric radio-frequency transfer over a 120 m outdoor free-space link using a compact diode laser as the clock carrier. Timing fluctuation and Allan Deviation are both measured to characterize the instability of transferred frequency incurred during the transfer process. In this experimental setup, all components were powered by a full-charged battery, and the experiment was conducted in an open-air environment, which is far from the laboratory-based room. With transferring a 100 MHz microwave signal in 4500 seconds, the total root-mean-square (RMS) timing fluctuation was measured to be about 22 ps, with fractional frequency instability on the order of 3 × 10-12at 1 s, and order of 4 × 10-14at 1000 s. We found that the majority of this excess timing fluctuation in our experiment is attributed to the air turbulence which could contribute a jitter at tens picosecond level in a normal windy night. This portable atmospheric frequency transfer scheme can be used to fast build an atomic clock-based frequency free-space transmission link since its stability is superior to a commercial Rb or Cs clock.
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.
We demonstrated an underwater frequency transfer technique with a green diode laser. The characteristic of the timing fluctuation and instability for the transfer technique was analyzed and simulated. With this technique, we had transferred a highly stable 100-MHz frequency signal over an underwater link with distances of 3 m, 6 m, and 9 m for 5000 s, respectively. The experimental results involving the underwater transfer of the 100-MHz radio-frequency signal shows that the rms timing fluctuations are 5.9 ps (3-m link), 6.4 ps (6-m link), and 8.4 ps (9-m link). The calculations also show that the relative Allan deviations for the 3-m, 6-m, and 9-m transmission links are 5.6 × 10−13 at 1 s and 5.3 × 10−15 at 1000 s, 5.8 × 10−13 at 1 s and 1.1 × 10−14 at 1000, and 6.8 × 10−13 at 1 s and 1.1 × 10−14 at 1000 s, respectively. The measured instabilities were lower than Rb and Cs atomic clocks, implying that the proposed frequency transfer scheme can potentially be used to transfer the signals of these atomic clocks over underwater links.
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.
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.
A global network of optical atomic clocks will enable unprecedented measurement precision in fields including tests of fundamental physics, dark matter searches, geodesy, and navigation. Free-space laser links through the turbulent atmosphere are needed to fully exploit this global network, by enabling comparisons to airborne and spaceborne clocks. We demonstrate frequency transfer over a 2.4 km atmospheric link with turbulence comparable to that of a ground-to-space link, achieving a fractional frequency stability of 6.1×10^{-21} in 300 s of integration time. We also show that clock comparison between ground and low Earth orbit will be limited by the stability of the clocks themselves after only a few seconds of integration. This significantly advances the technologies needed to realize a global timescale network of optical atomic clocks.