Terahertz radiation, which sits between microwaves and infrared light on the electromagnetic spectrum, has long been viewed as a prime candidate for the next generation of wireless communication. Its enormous bandwidth promises data rates far beyond what today’s microwave links can deliver, and its short wavelengths make it attractive for high-resolution sensing and imaging. Yet the promise has always come with a stubborn technical obstacle: producing terahertz signals that are both stable and quiet. Phase noise, the rapid random fluctuation in the timing of a signal’s oscillations, degrades the quality of any communication channel built on that signal, and at terahertz frequencies even tiny instabilities in the underlying oscillator are magnified enormously.

The conventional routes around this problem each carry significant penalties. Electronic frequency multiplication, in which a lower-frequency reference oscillator is multiplied up into the terahertz range, multiplies the phase noise along with the frequency, so the resulting signal becomes progressively noisier as it climbs the spectrum. Photonic approaches, which use two highly stable lasers beating against each other to produce a terahertz tone, can be much quieter, but they typically depend on ultrastable optical references, such as bulky and expensive cavity-stabilized lasers, together with elaborate feedback control systems that lock the lasers to one another. These requirements have made photonic terahertz synthesis difficult to package, power, and scale, keeping the technology largely confined to laboratory benches.

The new device attacks this bottleneck at its source. At its heart is a distributed-feedback laser coupled to a fiber Fabry–Pérot resonator, a compact optical cavity formed within a fiber. A small fraction of the light generated by the laser is fed back into the laser itself after circulating in the resonator, a scheme known as self-injection locking. This optical feedback pulls the laser onto a resonance of the cavity and dramatically suppresses its frequency noise, effectively borrowing the stability of the passive resonator without any active electronics. The researchers measured an instantaneous Lorentzian linewidth for the stabilized pump laser of just 0.386 hertz, an extraordinarily narrow figure that reflects how tightly the cavity disciplines the laser’s emission.

With the pump laser quieted, the platform then exploits a nonlinear optical effect to generate a frequency comb. When enough light circulates inside a suitable resonator, the Kerr nonlinearity of the material couples the pump into a set of equally spaced optical sidebands, and under the right conditions these sidebands lock into short pulses of light known as solitons. The packaged device produced stable Kerr soliton states with a fundamental repetition rate of 20.293 gigahertz, meaning the comb’s teeth are separated by exactly that frequency. Because every tooth of the comb inherits the coherence of the single stabilized pump laser, the entire comb functions as dozens of mutually coherent oscillators derived from one quiet source, all without bulky ultrastable references or complex external feedback loops.

The team put this comb to work synthesizing signals in the terahertz band. By selecting two comb lines and beating them together, they generated a carrier at 319 gigahertz and used it for free-space wireless communication. Over a five-meter link, the system transmitted data using both 16-QAM and 64-QAM modulation formats, which encode four and six bits per symbol respectively, at symbol rates of up to 15 gigabaud. The measured bit-error rates remained below the soft-decision forward-error-correction threshold, the standard benchmark that determines whether a communication link can be corrected to effectively error-free operation. Achieving 64-QAM at these rates is particularly demanding, because higher-order modulation formats squeeze more information into each symbol but leave far less margin for noise and distortion on the carrier.

Practical deployment demands more than a single good measurement, so the researchers also tested how the system behaved across repeated power cycles. They switched the device off and on again and again, and in each case the system returned to the target operating state and delivered similar communication performance. This reproducible turnkey operation addresses one of the chronic weaknesses of soliton microcomb systems, which historically have required careful, sometimes finicky tuning procedures to reach and maintain the desired soliton state. A comb that reliably restarts into its working configuration after every power cycle is far closer to something that could be installed in a base station or a sensing instrument and left to run.

Perhaps the most forward-looking result goes beyond single-carrier transmission. The same microcomb was used to generate multiple mutually coherent terahertz carriers, spaced 20.293 gigahertz apart, forming a terahertz frequency comb in its own right. Five of these multi-carrier signals were experimentally characterized, all showing narrow spectral features and preserved mutual coherence. This parallelism changes the architecture of a terahertz link in a fundamental way. Instead of forcing all the data through one extremely broadband channel, which places enormous demands on the modulator, the amplifier, and the receiver, the available information can be distributed across several narrower, coherent channels that are generated simultaneously from the same chip-scale device. Because the carriers are mutually coherent, they can in principle be combined and processed together, opening the door to coherent multi-channel schemes that multiply capacity without multiplying hardware.

The significance of the demonstration lies in the convergence of qualities that have previously been achieved only separately. Low phase noise, high-order wireless modulation, multi-carrier generation, and turnkey stability have each been shown in individual experiments, often with rack-sized setups and specialist laser systems. Here they coexist in a packaged fiber-based platform whose core components, a laser, a resonator, and a nonlinear microresonator, can in principle be miniaturized and mass-produced. The self-injection-locking architecture is the key simplification: by letting a passive cavity do the work of stabilizing the laser, the design eliminates the servo electronics, reference cavities, and optical locks that have made photonic terahertz sources cumbersome.

The researchers point to a range of applications that could follow as resonator performance and system integration improve. High-capacity multi-channel wireless communication in the terahertz band is the most immediate, with the 319-gigahertz demonstration sitting in a spectral region of growing interest for next-generation networks. Beyond communications, the same low-noise multi-carrier source could serve coherent radar systems, where narrow linewidths translate directly into finer range resolution and better target discrimination, and precision spectroscopy, where a comb of mutually coherent terahertz tones can probe molecular fingerprints across a broad bandwidth at once. Sensing platforms for security screening, industrial inspection, and environmental monitoring could all draw on the same underlying hardware. The work, published under the title describing a compact low-noise photonic-terahertz synthesizer, suggests that the terahertz frontier, long constrained by the difficulty of making clean signals in a small package, may be moving steadily toward systems compact enough to matter.

Subject of Research: Compact Kerr microcomb photonic terahertz synthesis for low-noise signal generation and high-speed wireless communication

Article Title: Compact fiber microcomb enables low-noise terahertz synthesis and high-speed wireless communication

Article References: Compact fiber microcomb enables low-noise terahertz synthesis and high-speed wireless communication. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: terahertz, Kerr microcomb, self-injection locking, fiber Fabry-Perot resonator, soliton, phase noise, wireless communication, 64-QAM, frequency comb, photonics, coherent carriers, 6G

The achievement matters because millimetre-wave and subterahertz carriers sit at the heart of technologies that define modern life and modern science alike. They carry data in next-generation wireless networks, probe the atmosphere in high-resolution radar, resolve rotational fingerprints of molecules in spectroscopy, drive the superconducting circuits of quantum computers, and synchronize the antennas of radio telescopes that image black holes. In all of these applications, the useful information capacity and measurement precision are ultimately limited by how cleanly the oscillation repeats in time. Any jitter in the zero crossings of the waveform blurs the constellation points of a communication link, softens the edges of a radar return, or washes out the fine spectral lines of a molecule under study. The spectral purity of conventional oscillators, however, is constrained by noise processes intrinsic to generating the signal directly at the carrier frequency, where the relative timing uncertainty scales unfavorably as frequency rises.

The IMRA team, led by Antoine Rolland with Scott Egbert and Brendan Heffernan contributing equally, sidestepped those limits with a strategy they call Kerr optical frequency division. The central idea is elegantly counterintuitive: instead of building a fast electronic oscillator and trying to clean it up, they start with an ultrastable reference at an enormous optical frequency — 3.3 terahertz — and divide it down to the millimetre-wave domain using the nonlinear dynamics of light itself. Division is the key operation. When a high-frequency reference with exquisitely low noise is divided by a large factor, the absolute timing noise of the resulting low-frequency signal is reduced proportionally, while the reference’s fractional stability is preserved. Optical frequencies are roughly ten thousand times higher than millimetre-wave frequencies, so dividing an optical reference down to 300 gigahertz buys an enormous noise advantage that no amount of electronic refinement at the carrier frequency could deliver.

The optical reference in this work is a dual-wavelength Brillouin laser, a device in which stimulated Brillouin scattering — the interaction of light with acoustic waves in an optical fiber — generates two closely spaced laser lines whose frequency difference is set with extraordinary precision. In earlier work, the same group showed that such lasers can be stabilized to molecular rotational transitions, giving them the kind of absolute accuracy normally associated with atomic clocks. Here, the 3.3-terahertz beat note between the two Brillouin laser lines serves as the multiterahertz reference whose noise will be divided. Brillouin lasers are prized for this role because the stimulated scattering process naturally narrows the laser linewidth, suppressing the technical noise that plagues conventional semiconductor and fiber lasers, and because the dual-wavelength geometry cancels much of the common-mode drift that would otherwise corrupt the difference frequency.

The division itself is performed by a chip-scale Kerr microcomb, a device that has become one of the most celebrated tools of modern photonics. A Kerr microcomb is a tiny optical resonator, typically a whispering-gallery ring or a waveguide racetrack etched onto a photonic chip, in which continuous-wave pump light circulates with such intensity that the Kerr nonlinearity of the material converts it into a stable train of ultrashort soliton pulses. The spectrum of these pulses is a frequency comb: a set of perfectly evenly spaced lines whose spacing equals the repetition rate of the solitons, in this case 300 gigahertz. The comb acts as a mechanical gearbox for light, linking the terahertz reference to the millimetre-wave output through an exact integer ratio. Because every tooth of the comb inherits the phase of the reference, whatever coherence the optical reference possesses is transferred, undiluted in relative terms, to the repetition rate that emerges when the comb light strikes a photodiode.

The crucial innovation is how the comb is locked to the reference. Rather than using electronic feedback loops — phase detectors, voltage-controlled oscillators, servo electronics — which would inject their own noise and limit the achievable purity, the researchers employed coherent injection locking. They injected the Brillouin laser light directly into the microresonator, and the Kerr nonlinearity of the cavity did the rest: the soliton comb spontaneously synchronized its repetition rate to the injected reference, a phenomenon related to the Kerr-induced synchronization of cavity solitons demonstrated by other groups in recent years. No electronic multiplication, no feedback control, no phase-locked loop. The division is enforced by physics rather than by circuitry, and the noise of the output reflects only the reference and the fundamental limits of the soliton dynamics, not the imperfections of a servo system chasing a fast signal.

Measuring a signal this pure posed its own formidable challenge, because no commercial instrument can resolve phase noise at the zeptosecond level at 300 gigahertz using conventional techniques. The team therefore built a cross-correlation phase-noise metrology system operating at the carrier frequency itself, using two independent photonic local oscillators as references. Cross-correlation metrology exploits a beautiful statistical principle: if two independent measurement channels observe the same device under test, the noise of the instruments themselves is uncorrelated between channels and averages away as the measurement accumulates, while the correlated noise of the device under test survives. The technique, whose foundations trace back to work at the National Institute of Standards and Technology in the 1970s, had to be extended to 300 gigahertz, a frequency at which suitable local oscillators simply did not exist until the team built them from the same dual-wavelength Brillouin laser technology that underpins the main experiment. The measured single-sideband phase noise of −152 dBc/Hz at a 1-megahertz offset from the 300-gigahertz carrier is the direct fingerprint of the 18-zeptosecond timing floor.

The numbers deserve a moment of reflection. A phase noise of −152 dBc/Hz means that, in a one-hertz bandwidth one megahertz away from the carrier, the noise power is 152 decibels below the carrier power — a ratio of roughly one part in 10^15. Expressed as timing, 18 zeptoseconds per square root hertz means that over any measurement bandwidth, the accumulated timing uncertainty of the signal’s zero crossings amounts to a few hundred attoseconds at most. For comparison, the orbital period of an electron in the ground state of a hydrogen atom is on the order of 150 attoseconds, so this oscillator keeps time well enough to resolve the fastest motions in ordinary atoms. Achieving this with a signal that emerges directly from a photodiode, ready to be amplified and radiated from an antenna, is what distinguishes the result from earlier demonstrations that required elaborate downstream processing or referenced their performance to optical rather than electrical outputs.

The architecture is also strikingly practical by the standards of precision oscillators. The microcomb is chip-scale, the Brillouin lasers are fiber-based, and the entire chain involves no cryogenic cooling, no atomic vapor cells, and no vacuum systems. The work builds on a rapid progression of results from the same laboratory, including a 3-terahertz Brillouin-driven oscillator with femtosecond-level jitter reported in 2024 and a carbonyl sulfide-stabilized terahertz source published in 2025, and it converges with parallel advances at other institutions, where microcombs referenced to optical cavities and integrated silicon photonics have pushed microwave generation toward zeptosecond timing noise over the past several years. What sets the new result apart is the combination of division factor, locking mechanism, and measured performance: a multiterahertz reference divided by Kerr soliton dynamics through pure optical injection, verified at the millimetre-wave carrier itself.

The implications ripple outward across several fields. For subterahertz wireless communication, where atmospheric absorption windows near 300 gigahertz promise enormous bandwidths but demand local oscillators of exceptional purity, the result removes a longstanding bottleneck. For ultrafast electronics, photonic millimetre-wave clocks of this quality could synchronize analog-to-digital converters and superconducting logic with timing margins previously unattainable. For radio astronomy, distributed oscillators of this class could phase-lock arrays of telescopes separated by continents, sharpening the resolution of very long baseline interferometry at 870-micron wavelengths and beyond. And for precision spectroscopy, a coherent 300-gigahertz carrier with attosecond jitter provides a synthesis engine for probing molecular and solid-state dynamics at their natural timescales. The authors, who have filed a provisional patent on the coherent injection architecture, describe Kerr optical frequency division as a general and scalable route to millimetre-wave and subterahertz carriers whose coherence is no longer constrained by the limits of direct generation. If the trajectory of microcomb photonics over the past decade is any guide, the zeptosecond regime they have opened will not remain a laboratory curiosity for long.

Subject of Research: Low-noise millimetre-wave generation via Kerr optical frequency division of a Brillouin laser reference

Article Title: Millimetre-wave generation with 18 zs Hz−1/2 timing noise floor via Kerr optical frequency division

Article References: Egbert, S. C., Heffernan, B. M., Greenberg, J., McGrew, W. F., & Rolland, A. (2026). Millimetre-wave generation with 18 zs Hz−1/2 timing noise floor via Kerr optical frequency division. Nature Photonics. https://doi.org/10.1038/s41566-026-02014-x

Image Credits: AI Generated

DOI: 10.1038/s41566-026-02014-x

Keywords: millimetre waves, Kerr microcomb, optical frequency division, Brillouin laser, timing jitter, phase noise, frequency combs, microwave photonics, injection locking, terahertz, photonic oscillators, zeptoseconds