The MRR used to generate the Kerr optical comb (Fig. 2(a)) was fabricated on a high-index doped silica glass platform using CMOS-compatible fabrication processes [31-38]. First, high-index (n = ~1.7 at 1550 nm) doped silica glass films were deposited using plasma enhanced chemical vapour deposition, then patterned by deep ultraviolet photolithography and etched via reactive ion etching to form waveguides with exceptionally low surface roughness. Finally, silica (n = ~1.44 at 1550 nm) was deposited as an upper cladding. The advantages of our platform for optical micro-comb generation include ultra-low linear loss (~0.06 dB‧cm−1), a moderate nonlinear parameter (~233 W−1‧km−1), and in particular negligible nonlinear loss up to extremely high intensities (~25 GW‧cm−2). The radius of the MRR was ~ 592 μm, corresponding to an FSR of ~0.4 nm or~48.9 GHz (Fig. 2(b)). The relatively small FSR of the MRR led to a comb spacing falling in the millimeter-wave region, bridging the gap between the frequency scales of on-chip optical micro-combs and microwave systems. The ultra-low loss of the MRR resulted in a Q factor of ~1.5 million (Fig. 2(c)). After packaging the device with fibre pigtails, the through-port insertion loss was ~1 dB, assisted by on-chip mode converters.
To generate coherent micro-combs, the CW pump power was amplified to ~30.5 dBm and the wavelength swept from blue to red. When the detuning between the pump wavelength and MRR’s cold resonance wavelength became small enough such that the intracavity power reached a threshold, modulation instability driven oscillation was initiated [25]. Primary combs were generated with a spacing of multiple FSRs, determined mainly by the intra-cavity power and dispersion. The calculated parametric gain (Fig. 3(a)) could be controlled by varying the pump wavelength detuning, which in turn resulted in different spacings of the generated primary combs (Fig. 3(b), (c)) [39, 40].
As the detuning was changed further, distinctive ‘fingerprint’ optical spectra were observed (Fig. 4) which were similar to the spectral interference between tightly packed solitons in the cavity – so called “soliton crystals” that have been reported [24]. The soliton crystal step in the measured transmission (Fig. 4(c)) and the dramatic reduction of the RF intensity noise (Fig. 4(d)) are hallmarks of coherent micro-combs. Therefore, the ~ 48.9 GHz-spaced coherent combs were able to serve as an equivalent LO source, reaching the millimetre-wave region (U band, 40 to 60 GHz) – a regime that is challenging to achieve using traditional electrical methods [15]. Moreover, the optical power of the two comb lines near ~1541 nm (selected for the photonic microwave frequency conversion in our experiment) reached over 2 dBm (Fig. 4(a), zoom-in view), and the power ratio of the pump to the comb lines was -11 dBm (Fig. 4(b)), which provided a relatively high link gain for the microcomb-based microwave signal processors [41-43].
By changing the pump power within ± 0.5 dB, a diverse range of spectra were observed, all displaying low RF noise, and all being indicative with different soliton crystal superstructures [44] (Fig. 4, Fig. 5). This not only enabled the generation of coherent micro-combs for photonic microwave frequency conversion, but also offered a range of different comb spectral shapes suited to different applications. The formation of high quality stable combs, potentially consisting of soliton crystals, was readily achievable using straightforward adiabatic pump wavelength sweeping. We found that it was not necessary to achieve a rigorous single soliton state in order to achieve high microwave frequency conversion performance — only that the chaotic regime needed to be avoided. This is important since there are a much wider range of coherent low RF noise states that are more readily accessible than just the single soliton state [31, 39]. We employed a TEC (Thermo-Electric Cooller) controller to stabilize the chip temperature and the generated soliton crystal states were stable for over 12 hours without stabilization (other than the TEC) or feedback.
To generate the equivalent photonic LO, two comb lines near ~1541 nm were filtered and modulated by RF signals (with 14 dBm power) with different frequencies ranging up to 40 GHz. The corresponding optical spectra are shown in Fig. 6 (a). Next, the optical signals were amplified to 12 dBm and converted back into electrical domain where the frequency mixing of the RF signal and the LO signal was achieved via photo-detection. Figure 6(b) shows the measured electrical spectral output of the IF signal. As the input RF frequency fRF was varied from 40 GHz to 23 GHz (from the Ka-band to the X-band), the converted IF output (fIF = fLO− fRF) varied from 8.9 GHz to 25.9 GHz (from the X-band to the Ka-band) with a power variation of < 5 dB, reflecting the broad operational bandwidth of our photonic microwave frequency converter.
Figure 7(a) shows the output electrical spectra when fRF = 26 GHz, corresponding to an IF output of 22.9 GHz. The electrical spectra in a 10-MHz span (Fig. 7(b)) exhibited a signal-to-noise ratio of > 70 dB, which further confirmed the low noise performance of the photonic microwave frequency converter. The output power ratio of the IF to RF signals reached −6.3 dB, while the conversion efficiency of the output IF relative to the input RF power was −36.4 dB, with the −30.3dB link gain included. The spurious suppression ratio (power ratio of the IF signal to the spurious signal at 2fIF − fLO) was 43.5 dB.
The phase noise performance of the input RF signal at 26 GHz and an output IF signal at 20.89 GHz was also measured using an RF spectrum analyzer (Keysight N9010A), as shown in Fig. 7(c). Here the 20.89 GHz IF tone was the converted output of a 28 GHz RF tone, instead of the measured 26 GHz RF tone, for the phase noise measurement. Yet since a 2 GHz difference in the RF tone’s frequency would not significantly vary its phase noise performance, the phase noise spectra measured here could still reflect the performance of our photonic LO. The phase noise spectrum of the IF signal shows low residue noise brought about by the photonic LO at offset frequencies from 20 kHz to 1 GHz, verifying the microcomb’s low noise performance that is promising for ultra-high frequency photonic LO generation and thus wideband microwave frequency conversion.
We note that the deterioration in the IF phase noise for offset frequencies < 20 kHz can be optimized by stabilizing the pump power [45] and setting the detuning to the quiet operation point [46]. Further optimization would need to deal with other noise sources such as the relative intensity noise of the pump and the thermal refractive noise of the resonator [47].
The frequency relationship between fRF and the measured fIF is clearly shown in Fig. 8, together with the calculated fLO = fIF + fRF = 48.9 GHz. This verifies the generation of a millimeter-wave LO using our approach and indicates the potential for ultra-wideband microwave frequency conversion from the L- to U-bands (i.e., 0 to 60 GHz). Note that our approach is capable of achieving even higher frequencies through the use of multiple-FSR-spaced comb lines. However, we were experimentally limited in our measurement capability by the bandwidth of our electrical spectrum analyzer and the RF source. As shown in Fig. 9, the IF power had a linear dependence on the input RF power (with a slope of 1.02, saturating at 10 dBm) and the optical power received at the photodetector (with a slope of 1.98), which matches closely with the theoretical predictions of 1 and 2, respectively from Eq. 3.
For applications requiring further suppression of spurious frequencies (such as 2fIF − fLO), the two comb lines can be separated into two paths, where one is modulated by a carrier-suppressed double-sideband format to generate photonic RF sidebands, and the other serves as the photonic LO sideband for frequency mixing. In addition, though the LO frequency was not tunable in this paper, recent advances in dual micro-comb generation [48-49] provide possibilities to achieve a frequency-tunable photonic LO for the microwave frequency converter. By changing the relative offsets of the two combs via thermal tuning, an almost unlimited range of photonic LO frequencies can be realized from sub-GHz to beyond even a terahertz.