RF Reference Transmitted Over Fiber Optics Could Reduce Reliance on Atomic Clocks

Researchers from a consortium of Australian institutions have demonstrated that a stable radio-frequency (RF) reference can be reliably transmitted more than 300 km over a standard fiber optic telecommunications network and used to synchronize two radio telescopes. The new technology could allow scientists anywhere to access the frequency standard simply by tapping into the telecommunications network, regardless of whether they have access to an atomic clock.

The RF reference for very-long-baseline interferometry (VLBI) over a telecom optical fiber link was performed by means of an innovative phase-conjugation technique. Bidirectional optical amplification was used in parallel with live traffic, and phase perturbations in the effective optical-fiber path length were compensated.

Researchers sent a reference signal between two radio telescopes using a 155-km fiber optic telecommunications link. The new technique passively compensates for network signal fluctuations introduced by environmental factors such as temperature changes or vibrations. Courtesy of D. Smyth, CSIRO.

To keep the frequency stable during transmission, researchers sent the signal through the network to a destination and then reflected it back. The returning signal was used to determine if any changes occurred. After each round trip, any transmitted frequency shift was passively subtracted to precisely compensate for the measured changes.

For every 100 km of fiber, the round trip will take about 1 ms. Although the compensation process happens quickly, the time on the receiving end can drift during the round trips, and to solve this problem, a quartz oscillator at the remote location was used to keep the time steady between round trips.

“The frequency of the quartz oscillator will also eventually drift, so our unique process combines local stabilization with the quartz oscillator for short time lengths, with the longer — greater than round trip time — stabilization provided by the transmitted stable frequency reference technique,” said researcher Kenneth Baldwin. “This highly stable method for transmitting the frequency reference allows an atomic clock, which costs around two hundred thousand dollars, to be replaced with a system that only costs a few tens of thousand dollars.”

To demonstrate the method, researchers began with a hydrogen maser located at the CSIRO Australia Telescope Compact Array (ATCA). They imprinted the RF reference signal from the maser onto a laser beam that traveled through a 155-km AARNet fiber and several amplification stages to a second radio telescope, and back again. Once the compensation process began, the reference was picked up by the radio telescope at the other end of the connection.

The researchers used the stable frequency reference to calibrate both telescopes, which were used to examine the same object in space. Rather than the stable frequency signal limiting the performance of the telescopes, they found the limiting factor to be atmospheric differences between the two locations.

To eliminate atmospheric interference and better understand how the new method improved telescope performance, the researchers then used just one telescope antenna at the ATCA fitted with two separate receivers to take measurements. This split antenna method allowed the team to compare one receiver stabilized by the hydrogen maser with the other receiver secured using the stable frequency reference that was sent on a 310-km round trip through the fiber.

“Our experiments showed that the transmitted frequency reference was very stable, significantly more stable than the earth’s atmosphere,” said Baldwin. “Our approach of exactly replicating the stable frequency signal from one atomic clock performed at least as well as two atomic clocks, which can exhibit slight differences from each other.”

Results indicate that the technique is capable of compensating for signal fluctuations in the fiber optic network introduced by environmental factors such as temperature changes or vibrations. The demonstration was even performed over a network that was transmitting live telecommunications traffic at the same time.

“By running the experiment on optical fibers also carrying normal traffic, we showed that transmitting the stable frequency standard doesn’t affect the data or telephone calls on the other channels,” said Baldwin. “This is necessary to gain the cooperation of the telecommunications companies that own these fiber networks.”

The researchers say that their demonstration shows that the new method is ready for implementation by radio astronomers who want to avoid using multiple atomic clocks across a telescope array. The method could be used over even longer distances by using more amplifiers to boost the signal. This would also allow stable frequency references to be broadcast across a national fiber optic network, where any scientist with access to a telecommunications network could use them.

“When atomic clocks were first invented, no one thought that they would provide timing standards that would be used for GPS navigation, for example,” said Baldwin. “We hope that in the same way, easy access to frequency standards that are just as stable as those found in a national measurement laboratory will be an enabling technology for many applications that require precise timing and accurate frequency measurements.”

Institutions participating in this research include: Australia’s Academic and Research Network (AARNet), the Australian National University, the Commonwealth Scientific and Industrial Research Organization (CSIRO), the National Measurement Institute, Macquarie University and the University of Adelaide.

The research was published in Optica, a publication of OSA, The Optical Society (doi: 10.1364/OPTICA.5.000138).