Integrated Optical Frequency Division for Microwave and mmWave Generation

Introduction

Microwave and millimeter-wave (mmWave) signals with high spectral purity are crucial for various applications, including metrology, navigation, spectroscopy, and high-speed wireless communications. One of the most promising techniques to generate such signals is optical frequency division (OFD), which leverages optical frequency references and optical frequency combs to transfer the superior fractional frequency stability of optical signals to the microwave and mmWave domains.

In conventional OFD systems, bulk or fiber-based optical components are used, limiting their potential for miniaturization and mass-volume fabrication. However, recent advancements in integrated photonics have paved the way for chip-scale implementations of OFD, enabling the development of compact, low-cost, and high-performance microwave and mmWave sources.

In this tutorial, we will explore the concept of integrated OFD and its implementation using complementary metal-oxide-semiconductor (CMOS)-compatible silicon nitride (SiN) integrated photonics. We will discuss the key components involved, including optical reference cavities, soliton microcombs, and high-speed photodiodes, as well as the techniques employed for phase noise characterization and power measurements [1].

Optical Reference Cavities

The phase stability of an integrated OFD system is primarily derived from optical reference cavities. These cavities provide highly stable optical frequencies that serve as the foundation for the OFD process. One example of an integrated optical reference cavity is the SiN planar-waveguide-based coil cavity, as shown in Figure 1.

Figure 1: Photographs of a SiN 4-meter-long coil waveguide cavity as an optical reference, a SiN chip with waveguide-coupled ring microresonators for soliton microcomb generation, and a flip-chip bonded charge-compensated modified uni-travelling carrier photodiode (CC-MUTC PD) for mmWave generation.

This coil cavity features a 4-meter-long SiN waveguide coiled into a compact footprint. It achieves an intrinsic quality factor of 41 million and a loaded quality factor of 34 million at a wavelength of 1550 nm. The large mode volume and high quality factor of this cavity enable exceptionally low laser linewidths, reaching sub-MHz levels.

Soliton Microcombs

The heart of the integrated OFD system lies in the generation of soliton microcombs. These microcombs are produced in waveguide-coupled SiN microresonators through a process known as dissipative Kerr soliton formation. The microresonators feature a cross-section of 1.55 μm width and 0.8 μm height, with a radius of 228 μm and a free spectral range (FSR) of 100 GHz.

Figure 2: Optical spectra of soliton microcombs (blue) and reference lasers corresponding to different division ratios.

The soliton microcombs exhibit a wide optical bandwidth, typically spanning several THz, as shown in Figure 2. This broad bandwidth enables the division of optical frequencies down to the mmWave and THz domains.

Two-Point Locking for OFD

The OFD process is implemented using a technique called two-point locking. In this method, two reference lasers are stabilized to the optical reference cavity, providing a stable frequency difference. These reference lasers are then photomixed with the soliton microcomb on separate photodiodes, creating beat notes.

The beat note frequencies are subtracted on an electrical mixer, yielding the frequency and phase difference between the optical references and N times the soliton repetition rate, where N is the division ratio. This difference is then phase-locked to a low-frequency local oscillator by feedback control of the soliton pump laser frequency, effectively dividing down the frequency and phase of the optical references to the soliton repetition rate.

Figure 3: Experimental setup for the integrated OFD system, including optical reference lasers, soliton microcomb generation, two-point locking, and phase noise measurement techniques.

Figure 3 illustrates the experimental setup, which includes components such as erbium-doped fiber amplifiers (EDFAs), polarization controllers (PCs), phase modulators (PMs), band-pass filters (BPFs), and fiber-Bragg grating (FBG) filters.

Phase Noise Characterization

Measuring the phase noise of the generated microwave and mmWave signals is crucial for evaluating the performance of the integrated OFD system. For optical domain phase noise measurements, a dual-tone delayed self-heterodyne interferometry technique is employed. This method allows the extraction of instantaneous frequency and phase fluctuations of the reference lasers and the soliton repetition rate.

Figure 4: Phase noise measurement results in the optical domain, showing the superior phase noise performance of the OFD soliton repetition rate compared to the reference lasers and free-running soliton.

Figure 4 presents the phase noise measurement results in the optical domain, demonstrating the significant phase noise reduction achieved by the OFD process. The phase noise of the OFD soliton closely follows the 1/N^2 rule, validating the OFD principle.

For electrical domain phase noise characterization at mmWave frequencies, a novel mmWave to microwave frequency division (mmFD) method is developed. This technique coherently divides down the 100 GHz mmWave signal to a 20 GHz microwave signal, which can be directly measured on a phase noise analyzer.

Figure 5: (a) Simplified schematic of the mmWave to microwave frequency division (mmFD) method. (b) Typical electrical spectra of the voltage-controlled oscillator (VCO) after mmFD, phase-stabilized to the OFD soliton or free-running soliton. (c) Phase noise measurement in the electrical domain, comparing the mmWave phase noise with the reference lasers and OFD soliton repetition rate measured in the optical domain.

As shown in Figure 5, the mmFD method involves mixing the 100 GHz mmWave with a 19.7 GHz VCO signal in a harmonic RF mixer, creating higher harmonics of the VCO frequency. The frequency difference between the mmWave and the fifth harmonic of the VCO is phase-locked to a stable local oscillator, stabilizing the VCO frequency and phase to that of the mmWave. The phase noise of the VCO can then be directly measured and scaled back to represent the mmWave phase noise.

mmWave Generation and Power Measurements

To generate high-power, low-noise mmWaves, the OFD soliton microcombs are detected on a high-speed, flip-chip bonded CC-MUTC PD. This photodiode features a responsivity of 0.23 A/W at 1550 nm wavelength and a 3 dB bandwidth of 86 GHz, enabling efficient mmWave generation.

Figure 6: (a) Measured mmWave power versus photodiode photocurrent at -2 V bias, achieving a maximum output power of 9 dBm. (b) Measured mmWave phase noise at 1 and 10 kHz offset frequencies versus photodiode photocurrent, showing low phase noise maintained at high output power.

As shown in Figure 6a, the measured mmWave power increases with the photodiode photocurrent, reaching a maximum output power of 9 dBm (8 mW) at 100 GHz. Remarkably, as depicted in Figure 6b, the phase noise of the mmWave remains consistently low across different photocurrent levels, indicating that low phase noise and high power are achieved simultaneously.

Conclusion

The integrated optical frequency division technique, implemented using CMOS-compatible SiN integrated photonics, has demonstrated record-low phase noise for integrated photonic mmWave oscillators. By leveraging optical reference cavities, soliton microcombs, and high-speed photodiodes, this approach has achieved a phase noise of -114 dBc Hz^-1 at 10 kHz offset frequency (equivalent to -134 dBc Hz^-1 for a 10 GHz carrier frequency) for a 100 GHz mmWave signal, outperforming previous integrated photonic microwave and mmWave oscillators by more than two orders of magnitude.

This breakthrough paves the way for the development of compact, low-cost, and high-performance microwave and mmWave sources for various applications, including wireless communications, radar, and sensing systems. Future improvements in optical reference stability, soliton microcomb generation, and heterogeneous integration with semiconductor lasers and amplifiers hold the potential for further advancements in phase noise performance and system integration.

Reference

[2] Sun, S., Wang, B., Liu, K. et al. Integrated optical frequency division for microwave and mmWave generation. Nature (2024). https://doi.org/10.1038/s41586-024-07057-0