For the first time, researchers have demonstrated that a stable frequency reference can be reliably transmitted more than 300 kilometers over a standard fiber optic telecommunications network and used to synchronize two radio telescopes, the company said.
Stable frequency references, which are used to calibrate clocks and instruments that make ultraprecise measurements, are usually only accessible at facilities that generate them using expensive atomic clocks. The new technology could allow scientists anywhere to access the frequency standard simply by tapping into the telecommunications network.
The ability to send stable frequency references over the telecommunications network could be particularly useful for radio telescope arrays such as the Square Kilometer Array (SKA), an international effort to build the world´s largest radio telescope using arrays in Australia and South Africa. When complete, SKA will detect faint radio waves from deep space with a sensitivity about 50 times greater than that of the Hubble telescope. Individual radio telescopes will be linked to create a total collecting area of about 1 million square meters.
Linking radio telescopes in an array requires that each telescope have access to an atomic clock to record the precise time at which a signal is detected from an object in space. Focusing all the telescopes on the same object and then calculating the slight differences in the time for the signal to reach each telescope allows researchers to combine all the observations and pinpoint the object´s location and other characteristics. Stable transmitted references could be used to calibrate the relative time at each telescope, eliminating the need for multiple atomic clocks in a radio telescope array.
For every 100 kilometers of fiber, the round trip takes about 1 millisecond. Even though the compensation process happens very quickly, the time on the receiving end can drift during the round trips. To solve this problem, a quartz oscillator at the remote location keeps the time steady between round trips.
To demonstrate their method, the researchers began with a type of atomic clock known as a hydrogen maser located at the CSIRO Australia Telescope Compact Array (ATCA). They imprinted the radio frequency reference signal from the maser onto a laser beam that then traveled through a 155-kilometer 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. They found that rather than the stable frequency signal limiting the performance of the telescopes, atmospheric differences between the two locations was the limiting factor. To eliminate atmospheric interference and better understand how the new method improved the 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 one receiver stabilized by the hydrogen maser to be compared with the other receiver stabilized using the stable frequency reference that was sent on a 310- kilometer round trip through the fiber.
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