Researchers develop a new type of frequency comb that promises to further boost
the accuracy of timekeeping
By National Institute of Standards and Technology March 14, 2024
https://phys.org/news/2024-03-frequency-boost-accuracy-timekeeping.html
Chip-based devices known as frequency combs, which measure the frequency of
light waves with unparalleled precision, have revolutionized timekeeping, the
detection of planets outside of our solar system and high-speed optical
communication.
Now, scientists at the National Institute of Standards and Technology (NIST)
and their collaborators have developed a new way of creating the combs that
promises to boost their already exquisite accuracy and allow them to measure
light over a range of frequencies that was previously inaccessible.
The extended range will enable frequency combs to probe cells and other
biological material.
The researchers describe their work in Nature Photonics:
https://www.nature.com/articles/s41566-024-01401-6
The team includes François Leo and his colleagues from the Université Libre de
Bruxelles, Belgium, Julien Fatome of the Université de Bourgogne in Dijon,
France, and scientists from the Joint Quantum Institute, a research partnership
between NIST and the University of Maryland.
The new devices, which are fabricated on a small glass chip, operate in a
fundamentally different way from previous chip-based frequency combs, also
known as microcombs.
A frequency comb acts as a ruler for light. Just as the uniformly-spaced tick
marks on an ordinary ruler measure the length of objects, the uniformly-spaced
frequency spikes on a microcomb measure the oscillations, or frequencies, of
light waves.
Researchers typically employ three elements to build a microcomb: a single
laser, known as the pump laser; a tiny ring-shaped resonator, the most
important element; and a miniature waveguide that transports light between the
two.
Laser light that is injected into the waveguide enters the resonator and races
around the ring. By carefully adjusting the frequency of the laser, the light
within the ring can become a soliton—a solitary wave pulse that preserves its
shape as it moves.
Each time the soliton completes one round trip around the ring, a portion of
the pulse splits off and enters the waveguide.
Soon, an entire train of the narrow pulses--which resemble spikes--fills the
waveguide, with each spike separated in time by the same fixed interval, the
time it took for the soliton to complete one lap.
The spikes correspond to a single set of evenly spaced frequencies and form the
tick marks, or "teeth," of the frequency comb.
This method of generating a microcomb, though effective, can only produce combs
with a range of frequencies centered on the frequency of the pump laser.
To overcome that limitation, NIST researchers Grégory Moille and Kartik
Srinivasan, working with an international team of researchers led by Miro
Erkintalo of the University of Auckland in New Zealand and the Dodd-Walls
Centre for Photonic and Quantum Technologies, theoretically predicted and then
experimentally demonstrated a new process for producing a soliton microcomb.
Instead of employing a single laser, the new method uses two pump lasers, each
of which emits light at a different frequency. The complex interaction between
the two frequencies produces a soliton whose central frequency lies exactly in
between the two laser colors.
The method allows scientists to generate combs with novel properties in a
frequency range that is no longer limited by pump lasers. By generating combs
that span a different set of frequencies than the injected pump laser, the
devices could, for example, allow scientists to study the composition of
biological compounds.
Beyond this practical advantage, the physics that underlies this new type of
microcomb, known as a parametrically-driven microcomb, may lead to other
important advances. One example is a potential improvement in the noise
associated with the individual teeth of the microcomb.
In a comb generated by a single laser, the pump laser directly sculpts only the
central tooth. As a result, the teeth become wider the farther they lie from
the center of the comb. That's not desirable, because wider teeth can't measure
frequencies as precisely as narrower ones.
In the new comb system, the two pump lasers shape each tooth. According to
theory, that should produce a set of teeth that are all equally narrow,
improving the accuracy of measurements. The researchers are now testing whether
this theoretical prediction holds true for the microcombs they have fabricated.
The two-laser system offers another potential advantage: It produces solitons
that come in two varieties, which can be likened to having either a positive or
negative sign.
Whether a particular soliton is negative or positive is purely random because
it arises from the quantum properties of the interaction between the two lasers.
This may enable the solitons to form a perfect random number generator, which
plays a key role in creating secure cryptographic codes and in solving some
statistical and quantum problems that would otherwise be impossible to solve
with an ordinary, non-quantum computer.
More information: Grégory Moille et al, Parametrically driven pure-Kerr
temporal solitons in a chip-integrated microcavity, Nature Photonics (2024).
DOI: 10.1038/s41566-024-01401-6
Provided by National Institute of Standards and Technology
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