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Optical atomic clocks are at the pinnacle of time measuring devices, losing less than one second every ten billion years. Currently though, they are massive devices, weighing hundreds of kilograms. In order to have an optimal practical function that could be utilised by your average person, their size needs to be greatly reduced whilst retaining the accuracy and speed of the large-scale clocks.
Dr. Alessia Pasquazi from the EPic Lab in the School of Mathematical and Physical Sciences at the University of Sussex explains the breakthrough: “With a portable atomic clock, an ambulance, for example, will be able to still access their mapping whilst in a tunnel, and a commuter will be able to plan their route whilst on the underground or without mobile phone signal in the countryside. Portable atomic clocks would work on an extremely accurate form of geo-mapping, enabling access to your location and planned route without the need for satellite signal.
Militaries are also interested in technologies in GPS denied environments. GPS is vulnerable to jamming attacks, GNSS jammers are now being used by criminals or vehicle hijackers, as reported by the FBI: “GPS tracking devices have been jammed by criminals engaged in nefarious activity including cargo theft and illicit shipping of goods.Typical military jammers are able to affect GPS receiver for many tens of kilometers by line of sight. It’s a problem because best accuracy, availability and global coverage of PNT data is through GPS/GNSS.
One of the approach militaries are developing is integration of GPS with complementary technologies such as chip-scale atomic clocks and small inertial measurement units of the Microelectromechanical Systems (MEMS).
Scientists in the Emergent Photonics Lab (EPic Lab) at the University of Sussex have made a breakthrough to a crucial element of an atomic clock—devices which could reduce our reliance on satellite mapping in the future—using cutting-edge laser beam technology. Their development greatly improves the efficiency of the lancet (which in a traditional clock is responsible for counting), by 80% – something which scientists around the world have been racing to achieve.
Optical frequency combs serve as the clockwork of optical clocks, which are now the best time-keeping systems in existence. In optics, a frequency comb is a laser source whose spectrum consists of a series of discrete, equally spaced frequency lines. Frequency combs can be seen as many single frequency, continous-wave lasers whose emission frequencies are different but equidistant and whose emission is coherent between all the lasers.
Frequency combs can be generated by a number of mechanisms, including periodic modulation (in amplitude and/or phase) of a continuous-wave laser, four-wave mixing in nonlinear media, or stabilization of the pulse train generated by a mode-locked laser. Conventionally frequency combs generated using pulsed, mode locked lasers – the time frequency duality implies that an ultra-short pulse laser will create an optical comb in the frequency domain. In one of the schemes , a single laser is coupled into a microresonator (such as a microscopic glass disk that has whispering-gallery modes). This kind of structure naturally has a series of resonant modes with approximately equally spaced frequencies.
A major challenge has been how to make such comb sources smaller and more robust and portable. In the past 10 years, major advances have been made in the use of monolithic, chip-based microresonators to produce such combs. While the microresonators generating the frequency combs are tiny—smaller than a human hair—they have always relied on external lasers that are often much larger, expensive, and power-hungry.
Researchers at Columbia Engineering announced in Nature in Oct 2018, that they have built a Kerr frequency comb generator that, for the first time, integrates the laser together with the microresonator, significantly shrinking the system’s size and power requirements. They designed the laser so that half of the laser cavity is based on a semiconductor waveguide section with high optical gain, while the other half is based on waveguides, made of silicon nitride, a very low-loss material. Their results showed that they no longer need to connect separate devices in the lab using fiber—they can now integrate it all on photonic chips that are compact and energy efficient.
Scientists one step closer to a clock that could replace GPS and Galileo
In an optical atomic clock, the reference (the pendulum in a traditional clock) is directly derived by the quantum property of a single atom confined in a chamber: it is the electromagnetic field of a light beam oscillating hundreds of trillions of times per second. The clock counting element required to work at this speed is an optical frequency comb—a highly specialised laser emitting, simultaneously, many precise colours, evenly spaced in frequency.
Micro-combs bring down the dimension of frequency combs by exploiting tiny devices named optical microresonators. These devices have captured the imagination of the scientific community world-wide over the past ten years, with their promise of realising the full potential of frequency combs in a compact form. However, they are delicate devices, complex to operate and typically do not meet the requirement of practical atomic clocks.
“Our breakthrough improves the efficiency of the part of the clock responsible for counting by 80%. This takes us one step closer to seeing portable atomic clocks replacing satellite mapping, like GPS, which could happen within 20 years. This technology will changes people’s everyday lives as well as potentially being applicable in driverless cars, drones and the aerospace industry. It’s exciting that this development has happened here at Sussex.”
The breakthrough at the EPic Lab, detailed in a paper published in March 2019 in the journal, Nature Photonics, is the demonstration an exceptionally efficient and robust micro-comb based on a unique kind of wave called a ‘laser cavity soliton’. Dr. Pasquazi continues: “Solitons are special waves that are particularly robust to perturbation. Tsunamis, for instance, are water solitons. They can travel unperturbed for incredible distances; after the Japan earthquake in 2011 some of them even reached as far as the coast of California.
“Instead of using water, in our experiments performed by Dr. Hualong Bao, we use pulses of light, confined in a tiny cavity on a chip. Our distinctive approach is to insert the chip in a laser based on optical fibres, the same used to deliver internet in our homes.
“The soliton that travels in this combination has the benefit of fully exploiting the micro-cavities’ capabilities of generating many colours, whilst also offering the robustness and versatility of control of pulsed lasers. The next step is to transfer this chip-based technology to fibre technology—something that we’re exceptionally well-placed at the University of Sussex to achieve.”
Professor Marco Peccianti from the University of Sussex EPic Lab adds: “We are moving towards the integration of our device with that of the ultra-compact atomic reference (or pendulum) developed by Professor Matthias Keller’s research group here at the University of Sussex. Working together, we plan to develop a portable atomic clock that could revolutionise the way we count time in the future.
“Our development represents a significant step forward in the production of practical atomic clocks and we’re extremely excited by our plans, which range from partnerships with the UK aerospace industry, which could come to fruition within five years, through to portable atomic clocks that could be housed in your phone and within driverless cars and drones within 20 years.”
Columbia Engineers Build Smallest Integrated Kerr Frequency Comb Generator
A major challenge has been how to make such comb sources smallerand more robust and portable. In the past 10 years, major advances have been made in the use of monolithic, chip-based microresonators to produce such combs. While the microresonators generating the frequency combs are tiny—smaller than a human hair—they have always relied on external lasers that are often much larger, expensive, and power-hungry.
Researchers at Columbia Engineering announced in Nature in Oct 2018, that they have built a Kerr frequency comb generator that, for the first time, integrates the laser together with the microresonator, significantly shrinking the system’s size and power requirements. They designed the laser so that half of the laser cavity is based on a semiconductor waveguide section with high optical gain, while the other half is based on waveguides, made of silicon nitride, a very low-loss material. Their results showed that they no longer need to connect separate devices in the lab using fiber—they can now integrate it all on photonic chips that are compact and energy efficient.
The team knew that the lower the optical loss in the silicon nitride waveguides, the lower the laser power needed to generate a frequency comb. “Figuring out how to eliminate most of the loss in silicon nitride took years of work from many students in our group,” says Michal Lipson, Eugene Higgins Professor of Electrical Engineering, professor of applied physics, and co-leader of the team. “Last year we demonstrated that we could reproducibly achieve very transparent low-loss waveguides. This work was key to reducing the power needed to generate a frequency comb on-chip, which we show in this new paper.”
Microresonators are typically small, round disks or rings made of silicon, glass, or silicon nitride. Bending a waveguide into the shape of a ring creates an optical cavity in which light circulates many times, leading to a large buildup of power. If the ring is properly designed, a single-frequency pump laser input can generate an entire frequency comb in the ring. The Columbia Engineering team made another key innovation: in microresonators with extremely low loss like theirs, light circulates and builds up so much intensity that they could see a strong reflection coming back from the ring.
“We actually placed the microresonator directly at the edge of the laser cavity so that this reflection made the ring act just like one of the laser’s mirrors—the reflection helped to keep the laser perfectly aligned,” says Brian Stern, the study’s lead author who conducted the work as a doctoral student in Lipson’s group. “So, rather than using a standard external laser to pump the frequency comb in a separate microresonator, we now have the freedom to design the laser so that we can make the laser and resonator interact in new ways.”
All of the optics fit in a millimeter-scale area and the researchers say that their novel device is so efficient that even a common AAA battery can power it. “Its compact size and low power requirements open the door to developing portable frequency comb devices,” says Alexander Gaeta, Rickey Professor of Applied Physics and of Materials Science and team co-leader. “They could be used for ultra-precise optical clocks, for laser radar/LIDAR in autonomous cars, or for spectroscopy to sense biological or environmental markers. We are bringing frequency combs from table-top lab experiments closer to portable, or even wearable, devices.”
The researchers plan to apply such devices in various configurations for high precision measurements and sensing. In addition, they will extend these designs for operation in other wavelength ranges, such as the mid-infrared where sensing of chemical and biological agents is highly effective. In cooperation with Columbia Technology Ventures, the team has a provisional patent application and is exploring commercialization of this device.
Jean-Etienne Tremblay, and Yung-Hsiang (Jacky) Lin are investigating the use of promising materials that show even higher Kerr nonlinearity, such as Ta2O5 and chalcogenides glasses (ChG). ChG materials such as GeSbS have a Kerr nonlinearity more than 3x larger than silicon nitride and show low optical losses in the infrared region. We have demonstrated GeSbS waveguides with a low propagation loss of 0.2dB/cm. Supercontinuum were generated in 2cm long GeSbS waveguides with a pulse energy of 120pJ, showing that octave-spanning spectrum will be possible with low input power.
Alkaline-Earth vapor cell for enhanced optical atomic clocks : Enables size reduction and performance improvements to portable clocks necessary for satellite-based navigation and communication
U.S. Air Force scientists are advancing the capabilities of space-based communication and navigation by improving the size and accuracy of ultra-high precision atomic clocks using a alkaline element heated in a vapor cell in an ultrahigh vacuum chamber so that it can be used for timing with a pulsed laser.The manufacturing technology is available via patent license agreement to qualified private companies who would make, use, or sell, the device.
The precise timing that atomic clocks create enables numerous applications in communications and navigation, including the Global Positioning System. This next-gen timing source uses an optical atomic clock based on a two-photon optical transition in an alkali vapor. Such alkaline-earth atoms include calcium and strontium, for example. Alkaline-earth atoms are known to possess spectrally narrow electronic transitions that can be accessed with visible laser sources. Inside the vapor cell, the alkaline materials are heated between 400-600° Celsius.
The vapor cell assembly is made of alkaline-resistant sapphire, calcium fluoride, and europium-doped calcium fluoride. The cell is suspended in the chamber by thermal isolation springs made of alumina ceramic. The chamber is oxygen and particulate free, and evacuated to a pressure of ≤10−6 Torr. Commercial applications include finance, communications, power grid stabilization and synchronization, network synchronization, cyber security, encryption, and signal multiplexing.
References and Resources also include:
https://engineering.columbia.edu/press-releases/smallest-kerr-frequency-comb-generator
