Photonics-assisted terahertz (THz) wireless communication typically generates a low-phase-noise carrier by photomixing two phase-correlated optical tones, derived from a dissipative Kerr soliton (DKS) microcomb, on a uni-traveling-carrier photodiode (UTC-PD). Discrete fiber-bench demonstrations have already reached 64QAM and 240 Gbps, resolving the data-rate and modulation-format ceilings of the original 60/80 Gbps demonstration. What remains open is the milestone the source paper itself identified: no fully integrated hybrid photonic THz transmitter whose carrier derives from a microcomb — co-integrating the SiN comb resonator, tone generation (injection-locked or on-chip filtered comb lines), data modulator, and UTC-PD photomixer via e.g. photonic wire bonding or micro-transfer printing — has been demonstrated.

Background

Wireless communication systems are moving into the terahertz domain (roughly 100 GHz to 1 THz), where large contiguous bandwidths exist beyond the crowded microwave bands. Because electronic oscillators become noisy and inefficient at these frequencies, a leading approach is photonic carrier generation: two optical tones whose frequency difference equals the desired carrier are heterodyned ('photomixed') on a fast photodiode. The key device is the uni-traveling-carrier photodiode (UTC-PD), a photodiode in which only electrons carry the photocurrent, allowing high output power at millimeter-wave and THz frequencies. The quality of the generated carrier is set by the phase correlation of the two optical tones, so a low-noise optical source directly determines the achievable modulation order.

A particularly attractive source is the Kerr microcomb: an optical frequency comb (a set of equally spaced, mutually coherent laser lines) generated by parametric nonlinear mixing in a micron-scale ring resonator driven by a single continuous-wave pump laser. In silicon nitride (SiN) rings the stable pulsed state, a dissipative Kerr soliton (DKS), provides a comb whose spacing can lie from tens of GHz to the THz range on a chip-scale, integrable platform. Heffernan et al. (APL Photonics 8, 066106, 2023) demonstrated a 300 GHz wireless link in which two diode lasers injection-locked (i.e., their oscillation frequency and phase captured by injected light) to two lines of a DKS microcomb were photomixed on a UTC-PD, achieving 80 Gbps with a fiber-delivered local oscillator (LO) and 60 Gbps in a fully wireless dual-path configuration transmitting both data and clock, using 16/32QAM (quadrature amplitude modulation; information encoded in both amplitude and phase of the carrier) within hard-decision forward-error-correction (HD-FEC) error-vector-magnitude (EVM) limits — EVM being the rms deviation of received symbols from their ideal positions.

The paper's discussion identified four limitations: (i) throughput capped by the arbitrary-waveform-generator baud rate; (ii) receive SNR too low for modulation formats beyond 32QAM; (iii) amplitude noise from laser-diode facet reflections (mitigable by injecting the comb light through the back facet) and from LO amplification in wireless-LO operation; and (iv) the absence of any fully integrated hybrid photonic THz transmitter built around a microcomb — combining the SiN comb, diode lasers, modulator, and UTC-PD, for example by photonic wire bonding (three-dimensional on-chip waveguides connecting separate chips) or micro-transfer-printing (pick-and-place placement of micron-scale device coupons) — described as an important milestone for future progress toward the compact, low-power deployment pathway. Items (i) and (ii) have since been resolved on discrete hardware: 140 Gbit/s and then single-channel 240 Gbit/s sub-THz links by the same consortium (Maekawa et al. 2023, 2024), 64QAM via whole-comb modulation (Tetsumoto and Rolland 2023), and a compact turnkey Kerr comb enabling 64QAM at a 300 GHz-class carrier at a record 240 Gbps with near-quantum-limited carrier phase noise (Jia et al. 2025). Item (iii) is largely subsumed: photonic LO delivery eliminates the wireless-LO amplifier chain (Heffernan et al. 2023), and turnkey low-noise comb engineering reduces carrier noise. Item (iv), the integration milestone, remains open in its stated microcomb-based form: integrated THz transmitters and transceivers have been demonstrated without combs (DFB-laser-based minimalist integrated transceivers at 144 Gbps; heterogeneously bonded UTC-PD on thin-film lithium niobate at 40 Gbit/s; monolithic thin-film lithium niobate systems up to 115 GHz; hybrid InP/SiN/polymer transceiver PICs with on-chip combs at 45 GHz), but none co-integrates a DKS microcomb as the carrier source for a 300 GHz-class wireless data link.

Problem Statement

Provide and defend a concrete co-integration scheme for a fully integrated hybrid photonic THz transmitter in which the THz carrier (at a sub-THz/THz frequency, e.g. the 300 GHz band) derives from a Kerr microcomb. The scheme must co-integrate, via a hybrid or heterogeneous integration technology such as photonic wire bonding, micro-transfer printing, or wafer bonding, the core elements that in prior fiber-bench demonstrations were discrete — (1) the SiN (or equivalent) microresonator generating the dissipative Kerr soliton comb, (2) the two phase-correlated optical tones extracted from it (e.g., diode lasers injection-locked to comb lines, or on-chip filtered comb lines), (3) the data modulator, and (4) the uni-traveling-carrier photodiode photomixer with radiating element — such that no fiber-coupled comb source or bulk optical assembly remains in the carrier path. Integration of the continuous-wave pump laser is not among the required elements; the boundary of 'fully integrated' is the co-integration of the four core elements above. The scheme must be defended on the carrier path that the integration creates: it must fix the integration technology and interconnection used for each of the four elements on the common platform, and it must supply an explicit phase-noise or coherence budget, referenced to the fiber-bench demonstration of the same architecture, showing that the microcomb-derived carrier retains its low phase noise along the integrated carrier path rather than being replaced by an uncontrolled oscillator; restating the four-element list alone does not meet this bar. The application context is photonic THz wireless links whose carrier inherits the microcomb's low phase noise, and the milestone is defined by the integration boundary rather than by link performance: no data-rate, modulation-order, or SNR target is attached to it, and the rate and modulation ceilings of the 2023 discrete demonstration (beyond 80 Gbps, beyond 32QAM) and the mitigation of facet-reflection and LO-chain amplitude noise lie outside that boundary.

References

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