Millimetre-Wave Comb Generated by an Optical Microcomb

TL;DR

The paper addresses the need for metrological-grade millimetre-wave baseband sources in the sub-THz window and demonstrates direct mm-wave comb generation from a photonic microcomb. It uses a laser-cavity-soliton microcomb with a $50~\mathrm{GHz}$ repetition rate and carrier-envelope-offset-free output, downconverted via photoconductive antennas to produce a coherent mm-wave baseband comb suitable for terahertz time-domain spectroscopy, even in free-running operation. Multisoliton states enable intrinsic spectral shaping of the mm-wave output, and the system maintains coherence over multi-metre optical delays, enabling robust, low-power spectroscopy with modest amplification. This metrological-compatible, multichannel approach holds promise for future integrated mm-wave sources and applications in communications, sensing, and positioning.

Abstract

Metrological-grade millimetre wave baseband comb sources covering the subterahertz window are a key building block for next-generation wireless communications, precision sensing, and positioning systems. While optical microcombs have set new benchmarks in ultra-low phase noise single-frequency microwave generation, to date, no microcomb source has directly produced a millimetre-wave baseband comb. Here, we present a 50 GHz repetition rate carrier-envelope offset estabilised millimetre-wave baseband comb source covering the sub-terahertz region, generated from an optical microcomb source. Our microresonator-filtered microcomb enables direct, coherent downconversion via photoconductive antennas, even without external amplification. The metrological-grade optical soliton source produces single-cycle, naturally zero carrier-envelope offset millimetrewave baseband combs. It supports time-domain spectroscopy without any need to temporally align the source and detection pulses, as the ultra-high phase coherence allows significant differences between the optical paths of the source and detection pulses, which we tested over 8m, finding no degradation even in freerunning operation. Finally, the multisoliton operation regime provides a simple way of spectrally tailoring the microwave output by selecting different optical soliton states.

Figures (9)

• Figure 1: Generation of a millimetre-wave comb via photoconductive conversion of a laser cavity-soliton microcomb.a Experimental setup. A microcomb laser, consisting of a nonlinear microresonator (FSR 48.9 ($/sim$ 50) GHz) filtering a fibre laser cavity loop (FSR $\sim$ 95MHz) (red region), is coupled to a time-domain spectroscopy (TDS) system (blue region) via a beam splitter feeding both transmitter and receiver photoconductive-switch antennas. A mechanical stage controls the relative pulse delay between the pulse replicas. b A typical optical spectrum of a single-soliton microcomb state. c TDS trace of a single-cycle millimetre-wave pulse obtained by feeding the TDS setup with a single-soliton microcomb (roughly mW average power), without further processing or amplification. The slight pedestal variation in the time-domain trace is attributed to system drift during mechanical scanning. d Power spectrum (PSD) of the millimetre-wave trace from c, highlighting the comb structure; the inset shows the autocorrelation. e Same as d, but for a state composed of two non-equidistant solitons, showing a distinctly different millimetre-wave trace.
• Figure 2: Long term robustness of THz-wave generation for single soliton statesa False-colour map in dB on the colour axis of the optical spectrum analyser traces during the TDS-scan, showing wavelength vs time delay step. b TDS-trace obtained converting the microcomb pulse extracted at $\sim$ time delay 290 ps. c. single cycle trace extracted from b d. Power spectral density of the waveform b.
• Figure 3: Long-term robustness and high-signal-to-noise ratio time-domain spectroscopy for a state with two non-equidistant solitons.a False-colour map in dB on the colour axis of the autocorrelation traces in time. b A typical autocorrelation trace at t $=$ 0. c Reconstructed double soliton state with periodicity 20 ps and soliton spacing 4.46 ps extracted from the autocorrelation trace. d False-colour map of the optical spectrum analyser traces in time e Experimental terahertz time-domain spectroscopy trace. f Experimental TDS trace for a single period. g Expected generated waveform converting with Eq(1) the optical pulses in c. h Reconstructed TDS trace using Eq. (2) with the generated waveform in g and the optical pulses in c. i Experimental THz spectrum.
• Figure 4: Terahertz generation from evolving soliton states.a Experimental autocorrelator evolution map showing transitions between soliton states over the acquisition time. b Experimental terahertz time-domain spectroscopy (TDS) trace, highlighting the distinct THz responses corresponding to different soliton configurations: A – double soliton with equal spacing ($\tau$ = 10 ps, green), B – double soliton with $\tau$ = 6.6 ps (grey), C – double soliton with $\tau$ = 5.6 ps (red), and D – single soliton (orange). c Insets I–IV: For each configuration, the reconstructed optical intensity profile (top), experimental THz trace (middle), and simulated THz trace (bottom) are shown.
• Figure 5: Phase noise performance of the system. a Phase noise of an unlocked (black) and repetition rate locked (red) laser cavity-soliton system. b Integrated time jitter for the locked (red) and unlocked (black) cases. c Allan deviation of a repetition rate locked LCS system. d Time‐domain millimetre-wave field comparison of the unlocked (black) and locked (vermillion) cases. e Same as d, but for a TDS measurement where the detection antenna is fed with a pulse replica delayed by 8m (1333rd replica) relative to the generating antenna.