For the first time, an optical clock has traveled to space, surviving harsh rocket launch conditions and successfully operating under the microgravity that would be experienced on a satellite. This demonstration brings optical clock technology much closer to implementation in space, where it could eventually allow GPS-based navigation with centimeter-level location precision. “Our device represents a cornerstone in the development of future space-based precision clocks and metrology,” said Matthias Lezius of Menlo Systems GmbH, first author of the paper. “The optical clock performed the same in space as it had on the ground, showing that our system engineering worked very well.”
Frequency combs are an important component of optical clocks because they act like gears, dividing the faster oscillations of optical clocks into lower frequencies to be counted and linked to a microwave-based reference atomic clock. In other words, frequency combs allow the optical oscillations to be precisely measured and used to tell time. Optical clocks and frequency combs have been improving rapidly because of advances in building more portable and powerful lasers, as well as andvances in laser spectroscopy, Lezius said.
EPFL scientists have found a way to miniaturize frequency combs, realizing a new step toward miniaturization of such tools. Their device can measure light oscillations with a precision of 12 digits.
The highly accurate measurements made possible with frequency combs could be useful for many applications. Frequency combs are the “gears” necessary to run clocks ticking at optical frequencies. For example, space-based frequency combs could improve the accuracy of global remote sensing of greenhouse gases from satellites and could be used for space-based gravitational wave detectors. “Applications based on frequency combs are quite important for future space-based optical clocks, precision metrology and earth observation techniques,” said Lezius. “The space technology readiness of frequency combs is developing at a fast pace.”
For military Precision time and time interval (PTTI) technology is critical to warfighting capability, for use in both navigation and communications. There are many examples of essential military operations that depend on PTTI and could benefit from improvements in PTTI technology. These include: GPS clocks and autonomous operations, Weapon system four-dimensional coordination, GPS antijamming, Network-centric warfare, and Secure military communications. Advanced GPS satellite clocks would greatly improve ranging accuracy, reduce collateral damage, and enhance system survivability. Precise time synchronization is needed to efficiently determine the start of a code sequence in secure communications, to perform navigation, and to locate the position of signal emitters. Precise frequency control is required in communications for spectrum utilization and frequency-hopped spread-spectrum techniques.
The Global Positioning System (GPS), is a global navigation satellite system (GNSS) that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. GPS has become ubiquitous technology that provides real-time positioning, navigation and timing (PNT) data in cars, boats, planes, trains, smartphones and wristwatches, and has enabled advances as wide-ranging as driverless cars, precision munitions, and automated supply chain management.
Phones and other GPS-enabled devices pinpoint your location on Earth by contacting at least four satellites bearing atomic clocks. Each of these satellites provides a time stamp, and the system calculates your location based on the relative differences among those times. The atomic clocks used on today’s satellites are based on natural oscillation of the cesium atom — a frequency in the microwave region of the electromagnetic spectrum. Atomic clocks are extremely accurate because they are based on natural and universal atom vibrations. However, even the best atomic microwave clocks can still accumulate an error of about 1 nanosecond over a month. The accuracy of GPS Navigation is primary degraded due to Earth’s ionosphere, which interferes with the timing signals as they commute from a satellite to your GPS receiver. But the second biggest contribution of error comes from the stability of the clocks onboard the GPS satellites.
Optical clocks use atoms or ions that oscillate about 100,000 times higher than microwave frequencies, in the optical, or visible, part of the electromagnetic spectrum. The higher frequencies mean that optical clocks “tick” faster than microwave atomic clocks and could thus provide time-stamps that are 100 to 1,000 times more accurate, greatly improving the precision of GPS.
Optical Clock Technology Tested in Space for the First Time
In The Optical Society’s journal for high impact research, Optica, researchers report on a new compact, robust and automated frequency comb laser system that was key to the operation of the space-borne optical clock. Frequency combs are the “gears” necessary to run clocks ticking at optical frequencies.
Until recently, frequency combs have been very large, complex set-ups only found in laboratories. Lezius and his team at Menlo Systems, a spin-off company of Nobel Laureate T.W. Hänsch’s group at the Max Plank Institute for Quantum Optics, developed a fully automated optical frequency comb that measures only 22 by 14.2 centimeters and weighs 22 kilograms.
The new frequency comb is based on optical fibers, making it rugged enough to travel through the extreme acceleration forces and temperature changes experienced when leaving Earth. Its power consumption is below 70 watts, well within the requirements for satellite-based devices.
The researchers combined their new frequency comb with an atomic cesium clock for reference and a rubidium optical clock developed by research groups at Ferdinand Braun Institute Berlin and Humboldt University of Berlin as well as a group from the University of Hamburg that recently moved to Johannes Gutenberg University of Mainz (JGU). Airbus Defense & Space GmbH was involved in the construction, interfacing, and integration of the payload module that went into space and also provided support and equipment during the flight.
In April 2015, the entire system was flown on a research rocket for a 6-minute parabolic flight into space as part of the TEXUS program that launches from the Esrange Space Center in Sweden. Once microgravity was achieved, the system started measurements automatically and was controlled from the ground station via a low-bandwidth radio link.
“The experiment demonstrated the comb’s functionality as a comparative frequency divider between the optical rubidium transition at 384 THz and the cesium clock providing a 10 MHz reference,” said Lezius. Although the optical clock used in the demonstration had about one tenth the accuracy of atomic clocks used on GPS satellites today, the researchers are already working on a new version that will improve accuracy by several orders of magnitude.
The researchers plan to fly an improved version of the optical clock into space at the end of 2017. In that experiment, the frequency comb module will not fly under a pressurized dome in order to test how well it works in the vacuum conditions that would be experienced on a satellite. The researchers also seek to further improve the system’s resistance to harsh cosmic radiation to ensure that it can operate for several years in orbit.
Within a few years, Lezius and his team aim to have a space-qualified frequency comb module that the space community can use in future missions and applications. They are aiming for a device with a volume of about 3 liters that weighs a few kilograms and has a power consumption of approximately 10 watts.
Frequency combs—on-chip integration on track
Prof. Kippenberg’s lab showed in 2007 that optical frequency combs could be created using tiny devices called “optical microresonators”: microscopic ring-shaped structures made from very fine silicon nitride measuring a few millimeters to a few tens of microns in diameter. These structures can trap a continuous laser light and convert it into ultra-short pulses – solitons – thanks to the special nonlinear properties of the device. The solitons travel around the microresonator 200 billion times per second and the pulsed output from the microresonator creates the optical frequency comb.
Researchers at EPFL, in a project led by Victor Brasch and Erwan Lucas, created what is called a “self-referenced optical frequency comb”. This is essentially a series of densely-spaced spectral lines whose spacing is identical and known. Because they are so well defined, optical frequency combs can be used as a “ruler” for measuring the frequency – or color – of any laser beam. By comparing an unknown color to this ruler, it is possible to calculate its frequency.
However, this implies a critical step called “self-referencing”, a method that exactly determines the position of each individual tick of the frequency ruler, but demands a very long ruler – a broad spectral range, as scientists say – which is challenging to obtain.
The technology is amenable to integration with both photonic elements and silicon microchips. Establishing devices that provide a RF to optical link on a chip may catalyze a wide variety of applications such as integrated, atomic clocks and on-chip, and could contribute to making optical frequency metrology ubiquitous.
Optical strontium atomic clock sets new stability record
The accuracy and the stability of optical clocks are mainly based on the fact that the frequency of the optical radiation used is higher (by several orders of magnitude) than that of the microwave radiation which is used in cesium atomic clocks, which makes optical clocks much more precise than cesium clocks. In a strontium clock, laser cooling is used to slow an atomic gas down to temperatures near absolute zero. Then, an extremely narrow transition between long-lived eigenstates of the atoms is excited in order to stabilize the frequency of the excitation laser to that of the atoms. The simultaneous interrogation of numerous atoms leads to a particularly high signal-to-noise ratio and, thus, to high stability.
However, optical clocks do experience significant downtimes because of their higher technical complexity.
To deal with the downtimes that plague today’s optical clocks, the researchers combined a commercially available maser with a strontium optical lattice clock at PTB, Germany’s national metrology institute. The maser, which is like a laser except that it operates in the microwave spectral range, can be used as a type of reliable pendulum with limited accuracy to bridge the downtime of the optical clock. The researchers spanned the large spectral gap between the optical clock’s optical frequency and the maser’s microwave frequency with an optical frequency comb, which effectively divides the slower microwave-based “ticks” to match the faster “ticks” of the optical clock.
“We compared the continuously running maser with our optical clock and corrected the maser frequency as long as we had data available from the optical clock,” said Grebing. “During the optical clock’s downtimes, the maser runs on its own stably.”The researchers operated the maser and optical clock for 25 days, during which the optical clock ran about 50 percent of the time. Even with optical clock downtimes ranging from minutes to two days, the researchers calculated a time error of less than 0.20 nanoseconds over the 25 days.
The scientists from PTB have therefore developed a resonator whose frequency is among the most stable worldwide: with a length of 48 cm and ingenious thermal and mechanical isolation from its environment, it reaches a fractional frequency instability of 8 10-17.
DARPA’s Quantum-Assisted Sensing and Readout (QuASAR)
Quantum-Assisted Sensing and Readout (QuASAR) intends to make the world’s most accurate atomic clocks—which currently reside in laboratories—both robust and portable. QuASAR researchers have developed optical atomic clocks in laboratories with a timing error of less than 1 second in 5 billion years. Making clocks this precise portable could improve upon existing military systems such as GPS, and potentially enable entirely new radar, LIDAR and metrology applications.
Recently the program demonstrated the world’s most accurate clock with a total uncertainty of 2 parts in 10^18 , or about 10,000 times better than GPS clocks. This means that if the clock began ticking at the Big Bang nearly 14 billion years ago it would be accurate to better than one second today.
Recent advances in optical atomic systems give promise to a new generation of optical atomic clocks and quantum metrology that stands to transform numerous DoD applications. The Quantum-Assisted Sensing and Readout (QuASAR) program is building on established control and readout techniques from atomic physics to develop a suite of measurement tools that will be broadly applicable across disciplines, helping to address outstanding challenges in physics, materials, biological sciences, inertial navigation and robust global positioning systems.
Typically, the performance of measurement devices is limited by deleterious effects such as thermal noise and vibration. Notable exceptions are atomic clocks, which operate very near their fundamental limits. Driving devices to their physical limits will open new application spaces critical to future DoD systems. Indeed, many defense-critical applications already require exceptionally precise time and frequency standards enabled only by atomic clocks. The Global Positioning System (GPS) and the internet are two key examples.
Measurement systems based on atomic physics benefit from the exquisite properties of the atom. Among these are (a) precise frequency transitions, (b) the ability to initialize, control, and readout the atomic state and (c) environmental isolation. In addition, atomic properties are absolute, and do not “drift” over time. In this sense, atoms are self-calibrated, making them ideal for precision sensing.
QuASAR will push toward fundamental operating limits by developing atom and atom-like sensors that operate near the standard quantum limit (SQL), constructing hybrid quantum sensors that combine the optimal sensing and readout capabilities of disparate quantum systems and entangling multiple sensors/devices to operate below the SQL. These types of devices will find broad application across the DoD, particularly in the areas of biological imaging, inertial navigation and robust global positioning systems.
Program in Ultrafast Laser Science and Engineering (PULSE)
DARPA’s Ultrafast Laser Science and Engineering (PULSE) program is developing the technological means for engineering improved spectral sources, such as ultra-fast optical lasers—advances that in turn could facilitate more efficient and agile use of the entire electromagnetic spectrum and generate improvements in existing capabilities such as geolocation, navigation, communication, coherent imaging and radar, and perhaps give rise to entirely new spectrum-dependent capabilities.
Through precise spectral engineering in the optical domain, more efficient and agile use may be made of the entire electromagnetic spectrum. By generating and engineering waves in the optical domain, where engineers already exercise exquisite stability and control, these waveforms may be down or up-converted to the desired wavelength.
The Program in Ultrafast Laser Science and Engineering (PULSE) applies the latest in pulsed laser technology to significantly improve the precision and size of atomic clocks and microwave sources, enabling more accurate time and frequency synchronization over large distances. Recent PULSE demonstrations include synchronization of clocks with femtosecond precision across kilometers of turbulent atmosphere, corresponding to a 1,000-fold improvement over what is possible using conventional radio-frequency techniques.
These capabilities are essential to fully leverage super-accurate atomic clocks, as clocks such as those that QuASAR seeks to build are more precise than our current ability to synchronize between them. If successful, PULSE technology could enable global distribution of time precise enough to take advantage of the world’s most accurate optical atomic clocks.
Defense applications, such as geo-location, navigation, communication, coherent imaging and radar, depend on the generation and transmission of stable, agile electromagnetic radiation. Improved radiation sources—for example, lower noise microwaves or higher flux x-rays—could enhance existing capabilities and enable entirely new technologies.
PULSE will also aim to apply this technology to enable synchronization, metrology and communications applications spanning the electromagnetic spectrum, from radio frequencies to x-rays. By building on established ultrafast laser techniques, PULSE seeks to:
- Develop agile, low phase-noise, portable radio frequency oscillators;
- Demonstrate techniques compatible with worldwide distribution of the world’s most accurate optical clocks;
- Construct tabletop sources of coherent x-rays in the water window (3-5 nanometers); and
- Produce efficient, isolated attosecond pulses as a stroboscopic probe of electron dynamics in materials.
- Precision timing in distributed engagement and surveillance architectures
Recent PULSE demonstrations include synchronization of clocks with femtosecond precision across kilometers of turbulent atmosphere, corresponding to a 1,000-fold improvement over what are possible using conventional radio-frequency techniques.
The high coherence and full time synchronization demonstrated here could enable applications from time distribution to long-baseline radio astronomy. If extended to moving platforms by appropriate compensation for Doppler shifts, this technique could similarly enable applications such as precise formation flying of phased satellite arrays.
Distributed DOD Architecture requires ultra accurate timing sources
The future architecture of DOD is moving towards distributed engagement and surveillance that requires synchronized timing of distributed platforms. The ability to distribute the precise time and frequency from an optical clock to remote platforms could enable future precise navigation and sensing systems. Researchers have recently demonstrated tight, real-time synchronization of a remote microwave clock to a master optical clock over a turbulent 4 km open-air path via optical two-way time–frequency transfer.
Once synchronized, the 10 GHz frequency signals generated at each site agree to 10^−14 at 1 s and below 10^−17 at 1000 seconds. In addition, the two clock times are synchronized to 13 femtosecond over an 8-hour period. The ability to phase-synchronize 10 GHz signals across platforms supports future distributed coherent sensing, while the ability to time-synchronize multiple microwave-based clocks to a high-performance master optical clock supports future precision navigation / timing systems.
One of the application is multistatic synthetic aperture radar where an array of microwave oscillators are synchronized to a single master optical oscillator; LO, local oscillator. The master site’s clock is based on a laser stabilized to an optical cavity (optical oscillator). The remote site’s clock is based on a combined quartz oscillator and DRO. This remote microwave clock is tightly synchronized to the optical clock over a folded 4 km long air path via O-TWTFT. The time and the frequency outputs from each clock are compared in a separate measurement to verify femtosecond time offsets and high phase coherence of the synchronized signals
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