60 Gbps real-time wireless communications at 300 GHz carrier using a Kerr microcomb-based source Open Access

Modern society’s insistence on faster wireless data transfers necessitates increased bandwidth. However, overcrowding on currently deployed channels means that novel carrier frequencies must be developed to meet demand. The terahertz domain (which we take to primarily mean 100 GHz–1 THz in the context of wireless communications) is largely untapped. With plentiful bandwidth for high-speed communications, this spectral region could provide a solution to current congestion.¹ In fact, this regime has already been targeted for the proposed sixth generation of wireless communication standards (6G).² This target is enabled by recent advances in the creation and detection of THz radiation using electronic, photonic, and hybrid approaches.³ While all approaches have merit and are progressing quickly, hybrid (or photonics assisted) sources have already shown considerable promise for communications.^4–6 

In particular, photomixing two laser lines separated by a THz frequency on a suitably fast photodiode provides several key benefits. Generally, this method is readily integrated with existing optical fiber technology and provides the widest frequency tunability and finest frequency resolution available of any THz generation technology.⁷ Writing a data stream on an optical signal can be realized in a compact and low-cost fashion using an electro-optic modulator up to 100 GHz. For very large carrier frequencies, for instance, up to 300 GHz, plasmonic modulators may be used.⁸ Another benefit is the level of frequency stability that can be achieved. Optical frequencies can be stabilized to unmatched levels, and photomixing transfers this low noise to the THz regime. Unrivaled fractional frequency stabilities of 10⁻¹⁶ at one second have been reached in the microwave domain up to 100 GHz,^9–11 and similar techniques have recently been applied to the THz region to produce record-low phase noise.¹² Low phase noise terahertz sources generated from light with high optical signal-to-noise ratios (OSNRs) are important in communications for implementing dense modulation schemes, such as high order quadrature amplitude modulation (QAM).¹³ Here, we use the term OSNR to refer to the optical light before modulation in order to differentiate it from the signal-to-noise ratio of the THz wave at the receiver.

Generating spectrally pure THz radiation through photomixing can be accomplished in a straightforward manner using two low-noise diode lasers.^14–16 However, these lasers are quite expensive, and they are not phase and frequency stabilized with respect to one another, leading to added noise in the resulting THz waves and difficulty in implementing heterodyne detection schemes. Therefore, the benefits of using optical frequency combs (OFCs) in this application have long been recognized.¹⁵ Two lines from an OFC can be filtered and used to encode data while their shared source mitigates environmental noise at the transmitter. Previously, OFCs produced via mode-locked lasers and electro-optic modulators have been used to successfully demonstrate large wireless data transmission rates at various THz carriers.^15,17–19 Depending on the generation method, OFCs can suffer from limited power and OSNR in a given comb tooth. This, in turn, degrades the spectral purity of THz. Conventional erbium-doped fiber amplifiers (EDFAs) combined with optical filtering may alleviate this problem in some situations, but this involves multiple steps and, until very recently, was not miniaturized.²⁰ An alternative approach for amplifying comb modes is provided by optical injection locking (OIL).²¹ 

OIL is equivalent to injection locking in classical oscillators and has been investigated since the invention of the laser.²² Light from one oscillator is incident into the cavity of a second, causing stimulated emission that is locked to the phase of the first. Under the correct conditions, this enables low power single frequency sources to be amplified by up to 70 dB with little added phase noise.²³ Furthermore, with proper optimization of the injected power, the linewidth of the high-power laser will converge to the injected linewidth, making it possible to use cheap, multimode diodes for amplification.²¹ OIL has been studied previously using optical frequency combs generated from mode locked lasers,^24–26 cascaded electro-optic modulators,^27,28 and microresonators,²⁹ as well as from a theoretical perspective,^30,31 establishing the dynamics of locking, amplification, and noise transfer. The simultaneous filtering and amplification properties of OIL have made injection locking laser diodes (LDs) and OFCs increasingly popular for THz communications experiments in recent years. It was demonstrated that an OIL-based approach was more suited to communications than simple filter and amplify configurations in low optical signal-to-noise ratio situations.³² 

Many aspects of THz transmitters based on LD’s injection locked to OFCs are imminently miniaturized on photonic integrated circuits (PICs). The LDs and uni-carrier traveling photodiodes (UTC-PDs)^33,34 on which the optical tones are mixed have been integrated, along with important modulation components.²⁷ Similarly, laser integration has been demonstrated in offline communications experiments reaching 130 Gbps with a 300 GHz carrier.⁶ Creating compact and energy efficient THz transmitters will be key to their future commercialization and deployment. However, to date, these demonstrations have relied on bulky, external comb formation.

Dissipative Kerr soliton (DKS) frequency combs can be formed through non-linear optical processes in micrometer-scale ring resonators.³⁵ In a material such as silicon nitride, they can have comb mode spacing from tens of gigahertz to a terahertz, are compatible with wafer-scale fabrication, can be compact and low power,³⁶ and can be packaged with LDs, modulators, and UTCs via photonic wire bonding³⁷ or micro-transfer-printing,³⁸ making them a strong candidate for low noise THz sources. In fact, a very recent proof-of-concept experiment has shown that a Kerr comb-based source can support complex modulation formats up to 64 QAM.³⁹ However, this demonstration relied on careful dispersion compensation to coherently modulate 10 comb teeth at once in order to overcome limited OSNR on each individual tooth, and the baud rate was very limited.

Here, we introduce and characterize a source based on LD’s injection locked to a Kerr microcomb. Counter to prior work,³⁹ this source does not require dispersion compensation, simplifying the architecture for broader adoption. We then use this source to demonstrate wireless communication at 300 GHz and a configuration-dependent transfer rate of 60 or 80 gigabits per second (Gbps). We achieve 60 Gbps using real-time processing in a practical communication scheme where the transmitter and receiver are completely separated; both the data stream and local oscillator (LO) are wirelessly transferred, as pictured conceptually in Fig. 1.

The microcomb-based THz source is shown in Fig. 2(a). An external cavity diode laser (ECDL) (ECDL CTL1550, Toptica Photonics) pumped the microcomb. The pump was initially sent to a carrier-suppressed single sideband modulator (CS-SSBM) that can sweep the frequency at a rate of roughly 100 GHz/μs over a 2 GHz span to reliably initiate the microcomb in a single-soliton regime.⁴⁰ The pump was amplified and edge coupled into an on-chip bus waveguide using a lensed fiber. The DKS comb was generated in a silicon nitride ring, and a second lensed fiber acted as the output coupler. After the DKS comb was generated, the output was sent through a band stop filter (BSF) to remove the residual pump light. The DKS comb was then split into two paths. In the OIL path, only one circulator and no optical bandpass filters were used to minimize the non-common path of the split comb light. A 50:50 splitter/coupler was used on port two of the circulator. The entire comb was injected into both LDs without detriment because the spacing of the injected comb teeth was much greater than the locking bandwidth.²⁴ Approximately −30 dBm of power in the relevant comb line was injected, leading to an injection ratio of −40 dB. The side mode oscillations of the LDs were then filtered using a waveshaper. The resulting optical spectrum of the injection-locked LDs compared to the DKS is shown in Fig. 2(b). The injection-locked LDs show a 35 dB improvement in OSNR compared to the microcomb, as measured with a 0.02 nm resolution.

We tested the noise transfer of the microcomb repetition rate to the frequency difference of the LDs by sending a copy of the microcomb output directly into an electro-optic (EO) comb consisting of cascaded electro-optic modulators (EOMs). The EOMs were driven at 10.0335 GHz and created a number of sidebands that spanned the ≈301 GHz gap between laser lines. This provided a photodetected signal in the hundreds of megahertz range that was used to read out the frequency difference between adjacent comb teeth.⁴¹ We compared the linewidth of this RF beat note with and without injection locking, as shown in Fig. 2(c). In the absence of OIL, the frequency difference between the two LDs is unstable and drifts, whereas locking to a frequency comb greatly reduces the linewidth. To further reduce frequency drift and characterize the phase noise, the detected beat note from this EO comb was divided down and sent to a mixer, where it was mixed with the output of a frequency synthesizer. The synthesizer was locked to a rubidium clock standard. The output of the mixer was an error signal fed to a PID controller with a narrow bandwidth. It fed back to the DKS comb pump frequency through a voltage controlled oscillator (VCO) driving the CS-SSBM, loosely locking the repetition rate of the DKS microcomb to mitigate drift. The injection-locked laser lines could be sent to an EO comb of a similar design for readout. The readouts of the in-loop microcomb and the out-of-loop OIL LDs could be sent to a signal analyzer (PXA Signal Analyzer N9030A, Agilent Technologies) for analysis.

The resulting in-loop and out-of-loop phase noise is shown in Fig. 2(d). The out-of-loop phase noise represents the residual noise, i.e., the noise added by the injection-locking process and the associated setup. It is around −57 dBc/Hz at 100 Hz and −89 dBc/Hz at 10 kHz, while the in-loop phase noise is −87 dBc/Hz at 100 Hz and −95 dBc/Hz at 10 kHz. The error level of the in-loop phase is limited primarily because of the feedback bandwidth of the PID and the OSNR of the optical beat note used for detection. We attribute the increased noise of the out-of-loop PSD to non-common fiber and distinct EO combs used to measure the frequency difference of the laser lines. Regardless, the added phase noise is well below the phase noise of the free-running microcomb, indicating that the OIL process is not degrading the phase noise of the microcomb repetition rate.

To benchmark the performance of our source, we wirelessly transmitted data while using an LO delivered via optical fiber. Figure 3(a) shows a block diagram of the setup for the homodyne communication system used in the experiment. The output of the laser source was divided into two paths: 80% of the output is used for the transmitter and 20% for the receiver. In the transmitter, the QAM data signal was upconverted to the intermediate frequency (IF, 12.5 GHz) with an arbitrary waveform generator (AWG) and applied to an optical intensity modulator (IM). The IQ signal was filtered by a Gaussian filter with a roll-off factor of 0.35. The modulated optical signal was incident on a UTC-PD to generate and radiate the 301 GHz signal from a 25-dBi horn antenna. From the current draw on the power supply and the specifications of the UTC-PD, we estimate the radiated power to be ∼25 μW. In the receiver, a horn antenna with the same gain received the RF signal and was followed by a Schottky-barrier-diode fundamental balanced mixer (BAM). The power on the RF port of the mixer was limited to 10 μW, and the local oscillator (LO) port was limited to 0.5 mW. The mixer was pumped by the LO signal with the same frequency, which was generated with a UTC-PD and amplified with a THz amplifier. The THz amplifier was a medium power amplifier module commercially available from Fraunhofer IAF (using 35 nm metamorphic high-electron-mobility transistor (mHEMT) technology), with a typical small signal gain of 16 dB and an output power of over 8 dBm at frequencies from 275 to 325 GHz. There is no specification of the noise figure (NF). The down-converted IF signal was amplified and then analyzed with a real time oscilloscope (RTO).

We evaluated the THz source using 16 and 32 QAM modulation formats. Figure 3(b) shows the IQ constellations we attained for each. In the case of 16 QAM, the root mean square (rms) error vector magnitude (EVM) value was limited to below 12%. This is lower than the most stringent EVM limit of 15.8% outlined in the IEEE 802.15.3d standard amendment.^42,43 In the case of 32 QAM, there is currently no IEEE standard for wireless THz communications. We limited the rms EVM to 9%. The maximum symbol rate achieved was 19 Gbaud with 16 QAM modulation, while it was 16 Gbaud with 32 QAM modulation. Total net data rates were 76 and 80 Gbps with 16 QAM and 32 QAM, respectively. Since higher SNR is required for higher-order modulation, the maximum symbol rate decreases from 16 QAM to 32 QAM. The difference in the symbol rate is mainly limited by the SNR required for each modulation format.

To compare OIL with a more conventional amplification chain used in laser-based THz sources, we built the system shown in Fig. 4(a), which is based on electro-optic (EO) comb generation. A single mode external cavity diode laser (ECDL) produced the primary comb line at 1550 nm. An optical phase modulator modulated the light at 37.5-GHz to produce the EO comb. By using an optical filter (OF), two wavelengths with a frequency difference of 300 GHz were selected. To obtain a larger OSNR, a second optical filter with the same pass bands was used, and optical amplifiers were added after each optical filter to compensate for the loss. With this configuration, an optical output power of 100 mW with an OSNR ratio of 55 dB was achieved. 16 QAM data transmission was performed with the same setup shown in Fig. 3(a). Constellation diagrams for symbol rates of 5, 10, and 13 Gbaud are shown in Fig. 4. The maximum symbol rate within the HD-FEC limit was 13 Gbaud, which corresponds to the maximum data rate of 52 Gbps. The difference in the maximum symbol rates between the two systems is likely due to the amount of amplitude noise, or OSNR. This demonstrates that OIL, in general, is a convenient and cost-effective alternative to repeated filtering and amplification steps using waveshapers and EDFAs for low OSNR comb sources, as previously reported.³² 

Finally, we conducted a transmission experiment using the Kerr-comb-based source by wirelessly delivering the LO and RF signals using the configuration shown in Fig. 5. The modulation scheme was kept the same as before, with representations of the relevant spectra shown in the insets of Fig. 3(a). This configuration offers a realistic wireless link that completely separates a transmitter and a receiver. The LO signal was amplified and emitted from a horn antenna. In the receiver, the LO signal was received by a 25-dBi gain horn antenna, amplified again, and fed to the LO port of the BAM. The distance between the RF ports of the transmitter (UTC-PD) and receiver (BAM) was set to 50 cm by collimating the THz wave with PTFE plano-convex lenses, while the distance between the LO ports of the transmitter and receiver was 10 cm. In the experiment, we applied 16 QAM modulation. Figure 5(c) shows the constellation diagram with a maximum symbol rate of 15 Gbaud, or 60 Gbps. The EVM value was 11.9%, which is within the HD-FEC limit. The degraded symbol rate compared with the case of a fiber connected LO [Fig. 3(a)] is likely due to the increased amplitude noise of the LO signal after the second amplification stage.

This work represents a first proof-of-concept demonstration using a microcomb-based source to achieve large, fully-wireless, real-time data rates. As such, it opens several new paths for further research. Chief among them is the homodyne configuration used. It was chosen to avoid synchronization between the LO and transmitter and to facilitate a fundamental mixer approach. While it is out of the scope of this paper, a full link budget would be useful for future work, as would a careful study of modulation linearity, mixer saturation effects, and the effect of amplifier noise on the LO. In this study, two separate wireless paths were used for the LO and data. For practical applications, using an orthogonal polarization for the LO and a single antenna pair or double-antenna configuration could be used to realize a common path.^15,44

The data rates presented here are primarily limited by the maximum baud rate achievable with the AWG. The SNR at the receiver currently prevents us from using higher-order modulation formats. Future work will focus on increasing the data rate by using higher baud rate AWGs and exploring refinements to the THz transmitter for improved SNR. Moving forward, there are several routes available to improve the performance of the micro-comb based source itself. For instance, amplitude noise may be reduced by using a back-facet injection of the comb light. In this way, injection light is not reflected by the output facet of the LD, which can interfere with the LD light upon photodetection.²¹ While aspects of the source have demonstrable integrability,^6,27,36 a fully integrated hybrid photonic THz transmitter using a microcomb via photonic wire bonding or other methods has yet to be shown and will be an important milestone for future progress.

We have demonstrated that two laser diodes optically injection-locked to two teeth of a DKS microcomb produce a viable source for THz communications. This source leverages the inherent low-noise properties of a microcomb while improving the optical signal-to-noise ratio by greater than 35 dB. Using homodyne detection, wireless transfer rates of 80 Gbps were achieved with a wired LO, while rates of 60 Gbps were demonstrated using simultaneous transfer of both the data and clock channels. The source can plausibly be integrated, offering a path forward for compact, precision THz sources.

The authors would like to express their sincere gratitude to Keysight Technologies for providing equipment and technical support. Part of this research result was obtained from commissioned research (Grant No. 00901) by the National Institute of Information and Communications Technology (NICT), Japan.

The authors have no conflicts to disclose.

Brendan M. Heffernan: Investigation (equal); Visualization (lead); Writing – original draft (equal); Writing – review & editing (equal). Yuma Kawamoto: Investigation (equal); Writing – review & editing (equal). Keisuke Maekawa: Investigation (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). James Greenberg: Visualization (supporting); Writing – review & editing (equal). Rubab Amin: Writing – review & editing (equal). Takashi Hori: Conceptualization (equal); Project administration (equal). Tatsuya Tanigawa: Project administration (equal). Tadao Nagatsuma: Conceptualization (equal); Funding acquisition (lead); Methodology (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal). Antoine Rolland: Conceptualization (equal); Formal analysis (equal); Methodology (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.