Fiber-optic timing makes time transmission more "hardcore"! How can high-precision time be applied in the future?

Science Times reporter epic

The sun shines on the sundial, casting a clear shadow on the huge stone disk. Time flows quietly as the sundial rotates, neither fast nor slow, and never stops.

Time marks the instant and duration of an event, and is currently the most accurately measured and widely used physical quantity. Recently, the research and construction of the "high-precision ground-based timing system" led by the National Time Service Center of the Chinese Academy of Sciences has made new progress. The team has completed the equipment installation and performance testing of the 1,839-kilometer dual-fiber dual-path time transfer system from Xi'an to Taiyuan, marking that my country has built the world's first 1,000-kilometer optical fiber time transfer engineering application system with 10 picosecond stability.

From sundials and hourglasses to mechanical clocks and quartz clocks, to today's atomic clocks and the future's high-precision ground-based timing systems... the battle for time is a race against millimeters.

What does 10 picoseconds mean?

The time "tick" is what we often call 1 second. "1 picosecond is equal to 1 trillionth of a second, which is equivalent to moving the decimal point of 1 second forward by 12 places. The stability of time transmission reaches the order of 10 picoseconds, which means that the timing accuracy of 'Beijing Time' has reached 100 billionth of a second." Chen Jiang, an associate researcher at the National Time Service Center of the Chinese Academy of Sciences, told the Popular Science Times reporter.

Time is everywhere, but ordinary people often have no concept of time synchronization and find it difficult to understand the value of high-precision time. "This achievement has increased the existing time synchronization accuracy by 1,000 times," said Chen Jiang. The aircraft, warships, tanks, missiles, etc. that we are familiar with can determine their own positions and correct their directions by calculating the time it takes for the signal to reach them from the satellite.

Chen Jiang gave an example, saying that the fastest flight speed of an intercontinental ballistic missile has reached Mach 26 (8,800 meters per second). In order to accurately strike a target, the missile must also have a time accuracy of one thousandth of a second. A slight deviation of 0.3 seconds will deviate from the target by 2.6 kilometers. As a defense, the time accuracy requirement for interception missiles must reach one ten-thousandth of a second.

Make up for the shortcomings of satellite timing

Time service is to broadcast the national standard time to users through wired and wireless technical means. "Fiber-optic time service is a new high-precision time frequency signal transmission method that uses optical fiber as a transmission channel and laser as an information carrier." Chen Jiang said that the fiber-optic time transmission system is the foundation of the entire high-precision ground-based time service system and is currently the time service method with the best application prospects and the highest transmission accuracy.

At present, satellite timing systems are widely used, such as the US GPS and my country's BeiDou. "The advantages of satellite timing are wide coverage and low cost, but with the development needs, the improvement of satellite timing accuracy has also encountered bottlenecks." Chen Jiang explained that the reliability of satellite timing is relatively low. For example, satellite timing receivers need to receive navigation satellite signals from open space, which is susceptible to radio interference from the ground and low altitude, which will affect the timing accuracy and reliability.

Chen Jiang said that the accuracy of using optical fiber to transmit time and frequency signals has far exceeded that of satellite timing. "Optical fiber timing can be better compatible with optical fiber communication systems, and relying on optical fiber communication networks, a networked optical fiber timing system can be built, which greatly enhances the reliability and anti-attack capability of the timing system and meets the needs of various users."

Innovative technology to protect

In order to achieve high-precision time synchronization over a large area, the team laid a "big network" on the actual optical fiber link. This composite network connects the sites on the link in series and is based on multi-user technology, which avoids the continuous accumulation of transmission errors in time synchronization between sites.

During the transmission of optical fiber links, it is inevitable to encounter noise interference that accumulates step by step. "The team has made breakthroughs in purification technology to eliminate the introduced noise," said Chen Jiang.

To ensure the reliable operation of the entire system, a stable light source is essential. Chen Jiang revealed that the team developed a low-phase-noise ultra-stable light source as a transmission light source, and adopted the fiber-optic optical frequency cascade transmission technology to achieve high-precision optical frequency transmission between long-distance sites.

In terms of network architecture, the optical fiber timing system uses dual-line transmission, and any timing station on the link can receive time and frequency signals from both upstream and downstream directions. "Taking into account the optical fiber ring network structure in North China and South China, multiple optical fiber timing routes are set up to reduce the occurrence of timing signal interruptions caused by single-point line failures," said Chen Jiang.

Chen Jiang introduced that my country will build the world's first high-precision optical fiber timing operation system with a total line length of about 20,000 kilometers, a time synchronization accuracy of 100 picoseconds, and a frequency transmission stability of 19 orders of magnitude in the future. By then, high-precision time and frequency are expected to be carried by optical fiber on the magnificent mountains and rivers of the motherland and show their strength on more major scientific and technological platforms.