Microwave oscillators with high spectral purity and low phase noise are critical in various applications including microwave spectroscopy [1], radar-based sensing [2,3], photonic microwave radar [4], coherent communication systems [5–7], and quantum computing systems [8,9]. Over the last decade, photonic microwave generation based on optical frequency division (OFD) [10] has become the preeminent approach for high performance, ultra-low phase noise microwave generation [11–13], exceeding the phase noise performance of the best-available electrical oscillators. For OFD oscillator systems, the two key ingredients are the low phase noise laser reference and the optical frequency comb, which transfers the exceptionally high fractional frequency stability of the optical reference to the microwave domain. Recently, there has been rapid progress in miniaturization and integration of photonic microwave oscillators (PMOs) using two-point optical frequency division [14–17] and electro-optical frequency division (eOFD) [14,18]. Various chip-scale or miniature optical reference cavities, including ultra-high-Q spiral resonators (Q∼107 to 108) [15,18–20], miniature Fabry–Pérot cavities (Q>109) [16,21], and discrete crystalline MgF2 resonators (Q>109) [22], have been incorporated into these OFD systems. Also chip-scale frequency combs, such as soliton microcombs [15–17,20,21] and thin-film lithium niobate (TFLN) electro-optic (EO) combs [18] have been used to perform optical frequency division and achieve low phase noise levels.

In this work, we report a significant advancement in chip-based photonic microwave oscillators. First, by locking two lasers to an ultra-high-Q silicon nitride spiral resonator with Q>200 million and cavity length of 14 m, we achieve relative laser noise suppression below the cavity absolute thermorefractive noise (TRN) limit, realizing >20dB improvement in laser phase noise (at 10 Hz offset) compared with other on-chip lasers. Second, in order to enhance the optical-to-microwave frequency division ratio (N) and reduce the oscillator phase noise by electro-optical frequency division (eOFD) [14], a broad integrated EO comb with 3 dB bandwidth up to 35 nm is generated, by driving a tandem thin-film lithium niobate (TFLN) phase modulator (PM) chip with two low Vπ PMs integrated on the same chip. The resulting PMO achieves record low phase noise performance for chip-based oscillators. Significantly, the PMO in this work features exceptional phase noise performance, planar photonic chip design, high technology readiness level, and foundry-ready processing, which are key steps toward mass-scale applications of integrated PMOs in communications, sensing, and signal processing.

Figure 1(a) shows the PMO architecture. Overall frequency stability derives from two lasers co-locked to two cavity modes (frequencies ν1, ν2) of a 14-m-long Si3N4 spiral resonator by the Pound–Drever–Hall (PDH) locking technique [23]. The relative phase noise between the co-locked lasers is divided down to the microwave domain using integrated EO combs via electro-optical frequency division. The absolute frequency noise of each locked laser contains the spiral resonator thermorefractive noise (correlated and common-mode noise between the two lasers) and the residual laser locking noise (uncorrelated noise between the two lasers). As demonstrated here, their frequency difference (ν2−ν1) exhibits frequency noise below the cavity TRN noise limit.

Figure 1(b) is a photograph of the Si3N4 spiral resonator. It has a footprint of 21mm×21mm and round trip length of 14 m. The waveguide cross-sectional dimension is 10μm×0.1μm for low optical confinement and ultra-low on-chip propagation loss (<0.2dB/m) at C band wavelengths [24,25]. The device is fabricated at a CMOS foundry. It has a loaded quality factor (Q) of 160 million and an intrinsic Q of 204 million. The combination of ultra-high-Q factor and large mode volume of the spiral resonator suppresses the laser locking noise as well as the TRN noise of the resonator. Figure 1(c) is a photograph of the TFLN chip with tandem phase modulators (PMs). To reduce the Vπ of the PMs, a dual pass (recycled) design reduces the Vπ by a factor two at a given modulation frequency [18,26]. Also, the long electrode length (26 mm) enhances the electro-optical modulation length and lowers the Vπ of the TFLN modulator [27,28].

To establish the system’s inherent frequency stability, the relative phase noise of the two lasers, when co-PDH locked to the 14-m-long spiral resonator, is first characterized. For this measurement, a RIO laser and a tunable external-cavity diode laser (both operating near 1553 nm) are co-PDH locked to the spiral resonator and then mixed on a photodetector with a 40 GHz bandwidth. The resulting photodetector beat note (around 20 GHz) is amplified and analyzed on a phase noise analyzer. The measured phase noise between the two co-PDH locked lasers is shown in Fig. 2(a) (blue curve), and reaches levels of −96dBc/Hz at 10 kHz offset and −22dBc/Hz at close-in (10 Hz) offset. For comparison, the red curve in Fig. 2(a) gives the phase noise of the two lasers when free running (no PDH lock). Locking reduces phase noise by 42 dB at 10 kHz offset, and 82 dB at 10 Hz offset. Also plotted in Fig. 2(a) is the calculated TRN noise of the 14 m spiral resonator (green curve). The relative phase noise between the co-PDH locked dual lasers is suppressed by 6–10 dB from 10 Hz to 10 kHz offset, compared with the absolute cavity TRN noise. It is also worth noting that co-PDH locking to non-silicon-chip-based resonators, such as a discrete MgF2 resonator [22] and a miniature Fabry–Pérot cavity [29], also reported relative laser phase noise below the cavity TRN noise recently. Here the major noise contribution of the co-PDH locked laser phase noise is attributed to the residual PDH locking phase noise from each laser, which has also been reported previously on a Fabry–Pérot-cavity-based co-PDH locking system [29,30]. The residual PDH locking phase noise is uncorrelated between the two lasers and therefore does not benefit from common mode noise cancellation, and is independent from the frequency separation between the two lasers. Lastly, the ultra-high-Q factor of the Si3N4 resonator suppresses the PDH locking phase noise due to the increased frequency discrimination slope for a narrow cavity resonance [23].

Dual laser references are the backbone of two-point OFD systems and eOFD systems, and their phase noise is related to the phase noise of the optical-to-microwave frequency-divided output (at fM) according to SfM=Sν2−ν1N2 within the locking bandwidth. The spiral resonator referenced lasers in this work have achieved record-low on-chip relative phase noise for offset frequencies from 10 Hz to 10 kHz, as shown in Fig. 2(b). In particular, at 10 Hz offset, the current work represents >20dB reduction of phase noise compared with prior on-chip dual laser references.

An integrated EO comb is generated using the tandem TFLN PM chip. The measured V_π for each individual TFLN PM device is 2.0 V at 37.3 GHz. Figure 3(a) shows a wide integrated EO comb spectrum with 3 dB bandwidth of 4.4 THz (35.3 nm) and 37.3 GHz line spacing generated using this chip where microwave drive power to each PM is 35 dBm. Compared with using a single PM, the tandem PM design doubles the EO comb bandwidth, resulting in a higher optical-to-microwave frequency division ratio, and 6 dB of extra OFD phase noise reduction.

Figure 4(a) gives the schematic for the chip-scale PMO. The two lasers at 1553 nm and 1580 nm are co-PDH locked to the Si3N4 spiral resonator, amplified to ∼100mW, and coupled to the TFLN PM chip using a lensed fiber. Figure 3(b) shows the optical spectrum of the co-PDH locked lasers (red) superimposed on the TFLN EO comb (blue). Two sets of EO combs are shown, each centered around their respective reference laser and having a wavelength span of 27 nm (or 3.43 THz). After the TFLN chip, the EO combs were amplified using a semiconductor optical amplifier (SOA) to 10 mW, and the central comb spectral lines are bandpass filtered and detected. The detected signal is used to generate the phase error for feedback control of the VCO via a fast servo filter. Here due to the limited tuning range of the optical bandpass filter, the integrated EO comb and the co-PDH locked laser reference were operated at a narrower span of 3.43 THz, instead of the maximum 4.4 THz span achieved in Fig. 3(a).

The optical-to-microwave frequency division ratio (N) for this PMO is 92 (division from 3.43 THz to 37.3 GHz). The insets in Fig. 4(b) show the RF spectra of the locked oscillator (resolution bandwidth 2 Hz) and the free-running VCO (resolution bandwidth 1 kHz). Clearly the spectral coherence (noise sidebands) is drastically improved (reduced) for the locked case compared with the free-running case. Figure 4(b) main panel shows the measured oscillator phase noise at 37.3 GHz (blue curve), along with the free-running VCO phase noise at 37.3 GHz (red curve) and the spiral resonator referenced dual laser phase noise (yellow). The phase noise was measured by a Rohde & Schwarz FSWP phase noise analyzer. Phase noise levels of −133dBc/Hz at a 10 kHz offset and −58dBc/Hz at a 10 Hz offset are obtained for the 37.3 GHz carrier (blue curve). The optical-to-microwave frequency division reduces the phase noise of the co-PDH locked lasers uniformly by close to 39 dB (20log10N) from 10 Hz to 10 kHz offset. The dashed green curve gives the phase noise scaled to a 10 GHz carrier [reduction by 20log10 (37.3 GHz/10 GHz) dB] for an equivalent phase noise of −144dBc/Hz at a 10 kHz offset and −69dBc/Hz at a 10 Hz offset. Also plotted in Fig. 4(b) is the dual laser phase noise scaled down by 39 dB (20log10N) (dashed black curve), assuming perfect optical-to-microwave frequency division from 3.43 THz to 37.3 GHz. The measured 37.3 GHz PMO phase noise (blue) and the projected 37.3 GHz phase noise by eOFD operation (dashed black) are in good agreement with each other, for offsets between 10 Hz and 100 kHz. This further verifies that the spiral resonator referenced dual laser phase noise by co-PDH locking is independent of the frequency separation between the two lasers. Finally, of particular note in these spectra are the close-in phase noise levels (10 Hz offset for 10 GHz carrier) which surpass other silicon-chip-based PMO results by over 10 dB [15,17,18,20]. Also, at 10 kHz offset (scaled to 10 GHz carrier), the current demonstration surpasses the other silicon-chip-based PMO results by 16 dB [17], 9 dB [15], 3 dB [18], and 2 dB [20].

In this work, the core components that provide the frequency stability of the OFD system and perform the optical frequency division are integrated photonic chips, i.e., the ultra-high-QSi3N4 spiral resonator and the TFLN modulator chip. For future work, to achieve a higher integration level and further size, weight, and power reduction, heterogeneously integrated narrow linewidth lasers [33], or hybrid integrated self-injection-locked lasers [16,24], can replace the external cavity diode lasers used in this work. This work uses an erbium doped fiber amplifier between the SiN chip and the TFLN chip, due to the high total insertion loss (21 dB, fiber to TFLN chip to fiber) that arises from non-optimized edge couplers to the lensed fiber. Further design of the TFLN edge coupler can provide a matching mode profile to the Si3N4 output waveguide, and enable direct edge coupling between the Si3N4 chip and the TFLN chip without using the EDFA. Also, resonant integrated EO combs [34,35] can be used to significantly reduce the overall power consumption to drive the EO comb generation and further reduce the phase noise levels by enhancing the EO comb bandwidth and optical-to-microwave frequency division ratio. Finally, the control and driving electronics (such as laser controllers, servo loop filters, VCO, and amplifier) can be implemented as a multi-chip module to further reduce the system size.

The TFLN EO comb in this work is generated due to straightforward phase modulation and does not require mode-locking or minimum threshold for microcomb generation, or dedicated pump laser tuning mechanisms for dissipative Kerr soliton generation used in several soliton OFD systems [15,17,20,22]. Moreover, the core integrated photonic components of this work (Si3N4 spiral resonator and low Vπ TFLN modulator) are already foundry-produced and feature a high technology readiness level, and large-volume and low cost manufacturing, which are important for the mass-scale deployment and application of integrated PMOs.

In summary, we have demonstrated a low phase noise PMO based on spiral resonator referenced lasers and integrated electro-optical frequency division. Record low on-chip laser phase noise has been achieved by co-locking two lasers to a high performance, 14-m-long ultra-high-QSi3N4 spiral resonator. A broadband integrated EO comb was generated from a tandem TFLN PM chip and was utilized to perform integrated electro-optical frequency division. The PMO in this work achieves record-low phase noise for chip-based oscillators. The exceptional phase noise performance, combined with a simple system design, high technology readiness level, and foundry-ready processing, represents a major advance for chip-based PMOs. Our demonstration is expected to open up new venues of mass-scale applications in communications, sensing, and signal processing based on chip-scale PMOs.

Acknowledgment. This work was supported by the Defense Advanced Research Projects Agency (DARPA) under contract No. HR001122C0019. The authors thank Dr. Zhiquan Yuan from NIST for helpful discussions on the TRN noise modeling.