Romain Bouchand

Photonic microwave signals with zeptosecond-level absolute timing noise

Ultralow-noise microwave signals are generated at 12 GHz by a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. The microwave signals have a fractional frequency stability below 6.5 × 10–16 at 1 s and a timing noise floor below 41 zs Hz–1/2. Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world1,2,3,4,5,6,7,8,9,10,11. In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 × 10−16 at 1 s and a timing noise floor below 41 zs Hz−1/2 (phase noise below −173 dBc Hz−1 for a 12 GHz carrier). This outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems12, telecommunications13 and time–frequency metrology2,14. The measurement methods developed here can benefit the characterization of a broad range of signals.

Spatial multiplexing of soliton microcombs

Dual-comb interferometry utilizes two optical frequency combs to map the optical field’s spectrum to a radio-frequency signal without using moving parts, allowing improved speed and accuracy. However, the method is compounded by the complexity and demanding stability associated with operating multiple laser frequency combs. To overcome these challenges, we demonstrate simultaneous generation of multiple frequency combs from a single optical microresonator and a single continuous-wave laser. Similar to space-division multiplexing, we generate several dissipative Kerr soliton states—circulating solitonic pulses driven by a continuous-wave laser—in different spatial (or polarization) modes of a MgF2 microresonator. Up to three distinct combs are produced simultaneously, featuring excellent mutual coherence and substantial repetition rate differences, useful for fast acquisition and efficient rejection of soliton intermodulation products. Dual-comb spectroscopy with amplitude and phase retrieval, as well as optical sampling of a breathing soliton, is realized with the free-running system. Compatibility with photonic-integrated resonators could enable the deployment of dual- and triple-comb-based methods to applications where they remained impractical with current technology.Up to three distinct frequency combs are simultaneously generated from an optical microresonator and a continuous-wave laser, enabling the deployment of dual- and triple-comb-based methods to applications unachievable by current technologies.

Accurate control of optoelectronic amplitude to phase noise conversion in photodetection of ultra-fast optical pulses

When illuminating a photodiode with modulated laser light, optical intensity fluctuations of the incident beam are converted into phase fluctuations of the output electrical signal. This amplitude to phase noise conversion (APC) thus imposes a stringent constraint on the relative intensity noise (RIN) of the laser carrier when dealing with ultra-low phase noise microwave generation. Although the APC vanishes under certain conditions, it exhibits random fluctuations preventing efficient long-term passive stabilization schemes. In this paper, we present a digital coherent modulation-demodulation system for automatic measurement and control of the APC of a photodetector. The system is demonstrated in the detection of ultra-short optical pulses with an InGaAs photodetector and enables stable generation of ultra-low phase noise microwave signals with RIN rejection beyond 50 dB. This simple system can be used in various optoelectronic schemes, making photodetection virtually insensitive to the RIN of the lasers. We utilize this system to investigate the impact of the radiofrequency (RF) transmission line at the output of the photodetector on the APC coefficient that can affect the accuracy of the measurement in certain cases.

Phase noise characterization of sub-hertz linewidth lasers via digital cross correlation

Phase noise or frequency noise is a key metric to evaluate the short-term stability of a laser. This property is of great interest for the applications but delicate to characterize, especially for narrow linewidth lasers. In this Letter, we demonstrate a digital cross-correlation scheme to characterize the absolute phase noise of sub-hertz linewidth lasers. Three 1542 nm ultra-stable lasers are used in this approach. For each measurement, two lasers act as references to characterize a third one. Phase noise power spectral density from 0.5 Hz to 0.8 MHz Fourier frequencies can be derived for each laser by a mere change in the configuration of the lasers. To the best of our knowledge, this is the first time showing the phase noise of sub-hertz linewidth lasers with no reference limitation. We also present an analysis of the laser phase noise performance.

Ultra-low noise microwave signal generation with an optical frequency comb

Photonic synthesis of radio frequency waveforms revived the quest for unrivalled microwave purity by its seducing ability to convey the benefits of the optics to the microwave world. In this contribution, we will present a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fiber-based frequency comb and cutting-edge photo-detection techniques. We will show the generation of the purest microwave signal with a fractional frequency stability below 6.5×10-16 at 1 s and a timing noise floor below 41 zs Hz-1/2 (phase noise below -173 dBc Hz-1 for a 12 GHz carrier). This outclasses existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems, telecommunications and time-frequency metrology. The measurements methods developed here can benefit the characterization of a broad range of signals.

Ultra-short optical pulses leading to ultra-stable photonic microwave generation

Microwave signals can be generated by photo-detecting ultra-short optical pulse trains. We demonstrate that the pulse duration, although shorter than the impulse response of photodiode, can greatly limit the phase noise of generated microwave signal.

Dispersion-induced amplitude to phase noise up-conversion

Microwave signals of extreme spectral purity are of great interest in a large variety of applications ranging from deep-space navigation systems to synchronization of large scale facilities as well as pulse-Doppler radars and precision spectroscopy. Photonic microwave generation schemes, in which a mode-locked laser is synchronized to an ultra-stable laser reference, allow unparalleled levels of phase noise through photodetection of the low-jitter optical train of pulses [1]. However, the photodetection process is known to introduce excess phase noise by transferring the baseband amplitude noise of the mode-locked laser to the phase noise of the output radio-frequency carrier [2, 3], hereby spoiling the quality of the phase transfer. Although this conversion is easily observed, complete understanding of its process and its relation with other parameters remains unattended.

Compact Low-Noise Photonic Microwave Generation From Commercial Low-Noise Lasers

We demonstrate how phase-locking a whispering gallery mode-stabilized semiconductor laser to a high-stability Erbium-based distributed feedback fiber laser makes an outstanding optical reference for photonic microwave generation using an optical frequency comb. Using only commercially available compact lasers, it allows generation of a 12-GHz microwave signal with absolute levels of phase noise below −47 dBc/Hz at 1 Hz and −170 dBc/Hz at 50 kHz from the carrier.

Ultra-low phase noise frequency-comb-based microwave generation and characterization

We report on the generation of ultra-low phase noise microwave signals at 12GHz by photodetecting the pulse train of an Erbium-doped fiber-based optical frequency comb phase locked to a ultra-narrow linewidth ultra-stable laser at With advanced photodetection techniques and homemade phase noise measurement apparatus, our experiment demonstrates generation of a microwave source with absolute phase noise below -170dBc/Hz at 10kHz and above, and below -100dBc/Hz at 1Hz from a 12GHz carrier, which pushes even further the best results reported so far.