Sending information with the help of light is the future. It requires 'light chips', made of a special glass. Scientists from the UT research institute MESA+ have now managed to equip these light chips - which were already known for their extremely low losses - with new 'active' functionalities, such as generating, strengthening, and modulating light. Their chip is capable of creating a very wide light spectrum that runs from blue to infrared, spanning wavelengths of 470 to 2130 nanometres. By doing so they have made a light chip with the largest frequency range ever. The research has been published in the scientific journal Optics Express this summer.
You can send information with the help of light. For example, think of glass fibres that you can use to send digital information from one computer to the other. The width and regularity of the light spectrum plays a central role in this. With a broader spectrum (meaning a larger variety of colours in the light) and a large number of light channels set next to each other (individual colours), you can process a larger amount of information faster.
Creating such a spectrum with many individual lasers is technically very complex, expensive and less precise. Researchers have, therefore, been looking for methods to generate the broadest possible light spectrum on a chip for a long time. University of Twente scientists have now successfully managed to create a light chip with the broadest light spectrum ever. Their chip achieves a bandwidth of 495 THz, which is over half more than the previous record. According to research leader Prof. Dr Klaus Boller this broad spectrum demonstrates the potential of the technology. "However, the most important breakthrough is that we have managed to create it with the help of materials that have already proven themselves in practice. These materials have the lowest optic losses on a chip and are, therefore, already extremely relevant. What's more, the fabrication matches the standard processes in the chip industry, making it suitable for mass production."
In order to generate the broad spectrum, the researchers shone laser light into a structure that guides light, called a waveguide, made of a glass-like material, silicon nitride, embedded in regular glass (silicon oxide). The shape and construction of the waveguide ensures that the laser light generates new colours of light as it passes through—in this case going from 4 THz to a remarkable 495 THz. One of the key challenges of the research was ensuring that thesilicon nitride did not crack during the manufacture of the waveguides. However, with a new fabrication technique, the researchers of the University of Twente managed to create a structure that is thick enough, 800 nanometres.
The spectrum created by the chip is not constant, but consists of about twelve million peaks that lie at exactly the same distance from each other. Because of this, the spectrum looks like a hair comb; which is why such spectra have been given the name frequency comb. Frequency combs, a fast-growing field of research, make it possible to not only increase the speed of optic communication techniques, but also to greatly improve the precision of atomic clocks, telescopes, and GPS equipment.
'Comb on a chip' powers new atomic clock design
This image depicts NIST physicists Scott Diddams (left) and Scott Papp with a prototype atomic clock based on a chip-scale frequency comb. Diddams is holding the silicon chip, which fits into the clock apparatus on the table. With performance improvements and further reductions in size, the technology might eventually be used to make portable tools for measuring time and frequency.
Researchers from the National Institute of Standards and Technology (NIST) and California Institute of Technology (Caltech) have demonstrated a new design for an atomic clock that is based on a chip-scale frequency comb, or a microcomb.
The microcomb clock, featured on the cover of the inaugural issue of the new journal Optica, is the first demonstration of all-optical control of the microcomb, and its accurate conversion of optical frequencies to lower microwave frequencies. (Optical frequencies are too high to count;microwave frequencies can be counted with electronics.)
The new clock architecture might eventually be used to make portable tools for calibrating frequencies of advanced telecommunications systems or providing microwave signals to boost stability and resolution in radar, navigation and scientific instruments. The technology also has potential to combine good timekeeping precision with very small size. The comb clock might be a component of future "NIST on a chip" technologies offering multiple measurement methods and standards in a portable form.
"The microcomb clock is one way we might get precision frequency metrology tools out of the lab and into real-world settings," NIST physicist Scott Diddams says.
Frequency combs produce precisely defined colors, or frequencies, of light that are evenly spaced throughout the comb's range. (The name comes from the spectrum's resemblance to the teeth of a pocket comb.) The original combs required relatively large lasers that produced rapid, extremely short pulses of light, but more recently NIST and other laboratories have developed much smaller microcombs.
A microcomb generates its set of frequencies from light that gets trapped in the periphery of a tiny silica glass disk, looping around and around the perimeter. These combs can be astonishingly stable. NIST has an ongoing collaboration in this area with Caltech researchers, who made the 2-millimeter-wide silica disk that generates the frequency comb for the new clock.
The new microcomb clock uses a laser to excite the Caltech disk to generate a frequency comb, broadens the spectrum using nonlinear fiber, and stabilizes two comb teeth (individual frequencies) to energy transitions in rubidium atoms that "tick" at optical frequencies. (Conventional rubidium atomic clocks operate at much lower microwave frequencies.) The comb converts these optical frequency ticks to the microwave domain.
Thanks to the gear-like properties of the disk and the comb, the output is also 100 times more stable than the intrinsic ticking of the rubidium atoms. According to Diddams. "A simple analogy is that of a mechanical clock: Therubidium atoms provide stable oscillations—a pendulum—and the microcomb is like a set of gears that synthesizes optical and microwave frequencies."
The center of the comb spectrum is locked to an infrared laser operating at 1560 nanometers, a wavelength used in telecommunications.
NIST researchers have not yet systematically analyzed the microcomb clock's precision. The prototype uses a tabletop-sized rubidium reference. The scientists expect to reduce the instrument size by switching to a miniature container of atoms like that used in NIST's original chip-scale atomic clock. Scientists also hope to find a more stable atomic reference.
The microcomb chip was made by use of conventional semiconductor fabrication techniques and, therefore, could be mass produced and integrated with other chip-scale components such as lasers and atomic references. NIST researchers expect that, with further research, the microcomb clock architecture can achieve substantially better performance in the future.
A quiet phase: NIST optical tools produce ultra-low-noise microwave signals
By combining advanced laser technologies in a new way, physicists at the National Institute of Standards and Technology (NIST) have generated microwave signals that are more pure and stable than those from conventional electronic sources. The apparatus could improve signal stability and resolution in radar, communications and navigation systems, and certain types of atomic clocks.
Described in Nature Photonics, NIST's low-noise apparatus is a new application of optical frequency combs, tools based on ultrafast lasers for precisely measuringoptical frequencies, or colors, of light. Frequency combs are best known as the "gears" for experimental next-generation atomic clocks, where they convert optical signals to lower microwave frequencies, which can be counted electronically.
The new low-noise system is so good that NIST scientists actually had to make two copies of the apparatus just to have a separate tool precise enough to measure the system's performance. Each system is based on a continuous-wave laser with its frequency locked to the extremely stable length of an optical cavitywith a high "quality factor," assuring a steady and persistent signal. This laser, which emitted yellow light in the demonstration but could be another color, is connected to a frequency comb that transfers the high level of stability to microwaves. The transfer process greatly reduces—to one-thousandth of the previous level—random fluctuations in the peaks and valleys, or phase, of the electromagnetic waves over time scales of a second or less. This results in a stronger, purer signal at the exact desired frequency.
The base microwave signal is 1 gigahertz (GHz, or 1 billion cycles per second), which is the repetition rate of the ultrafast laser pulses that generate the frequency comb. The signal can also be a harmonic, or multiple, of that frequency. The laser illuminates a photodiode that produces a signal at 1 GHz or any multiple up to about 15 GHz. For example, many common radar systems use signals near 10 GHz.
NIST's low-noise oscillator might be useful in radar systems for detecting faint or slow-moving objects. The system might also be used to make atomic clocks operating at microwave frequencies, such as the current international standard cesium atom clocks, , more stable. Other applications could include high-resolution analog-to-digital conversion of very fast signals, such as for communications or navigation, and radio astronomy that couples signals from space with arrival times at multiple antennas.