Lue Wu

Recent experiments have demonstrated impressive performance in AlGaAs on insulator waveguides due to their dual advantages of very large Kerr nonlinearity and low loss. Here, we study the acoustic properties of these waveguides, adding stimulated Brillouin scattering (SBS) to the already impressive library of nonlinear effects available within this platform. We observe SBS mediated by both shear and pressure acoustic waves in AlGaAs-on-insulator integrated photonic waveguides, with Brillouin gain coefficients of up to 150m ⁻¹ W ⁻¹ . We also discuss competing non-linear processes that interfere with the formation of an SBS laser.

The remarkable frequency stability of resonant systems in the optical domain (optical cavities and atomic transitions) can be harnessed at frequency scales accessible by electronics using optical frequency division. This capability is revolutionizing technologies spanning time keeping to high-performance electrical signal sources. A version of the technique called two-point optical frequency division (2P-OFD) is proving advantageous for application to high-performance signal sources. In 2P-OFD, an optical cavity anchors two spectral endpoints defined by lines of a frequency comb. The comb need not be self-referenced, which greatly simplifies the system architecture and reduces power requirements. Here, a 2P-OFD microwave signal source is demonstrated with record-low phase noise using a microcomb. Key to this advance is a spectral endpoint defined by a frequency-agile single-mode dispersive wave that is emitted by the microcomb soliton. Moreover, the system frequency reference is a compact all-solid-state optical cavity with a record Q factor. A hybridly packaged version of the system offers excellent longer term stability. The results advance integrable microcomb-based signal sources into the performance realm of much larger microwave sources.

We demonstrate that it is possible to linearly tune the repetition rate of a bright soliton comb that is generated using an Si₃N₄ microring resonator by linearly varying the frequency of an auxiliary heater laser. Hence, the auxiliary laser can be utilized as a linear active feedback element for stabilizing the repetition rate. We investigated the potential of the auxiliary laser as an actuator of the soliton repetition rate by varying the auxiliary laser frequency at different modulation rates. Within the modulation bandwidth of the laser, we find that the variation ratio, defined as the ratio of the change in the repetition rate to the change in the laser frequency, remains unchanged. This variation ratio also quantifies the correlation between the frequency drift of the auxiliary laser and the repetition rate phase noise and makes it possible to examine the impact of frequency drift on the attainable phase noise performance of the soliton microcomb. For our setup, we find that the repetition rate phase noise of the microcomb below a 1-kHz offset from the carrier is dominated by the frequency drift of the auxiliary laser, which emphasizes the importance of deploying an inherently low-phase-noise laser when auxiliary laser heating technique is utilized.

Next-generation communications, radar, and navigation systems will extend and exploit the higher bandwidth of the millimeter-wave domain for increased communication data rates as well as radar with higher sensitivity and increased spatial resolution. However, realizing these advantages will require the generation of millimeter-wave signals with low phase noise in simple and compact form-factors. Photonic integration addresses this challenge and provides a path toward simplified and portable, low-noise mm-wave generation. We leverage these advances by heterodyning two silicon photonic chip lasers, phase-locked to different axial modes of a miniature Fabry–Perot (F-P) cavity to demonstrate a simple framework for generating low-noise millimeter-waves. By reducing technical noise, we achieve common-mode rejection of the thermally driven Brownian noise such that the millimeter-wave phase noise surpasses that of the thermal limit of a single laser locked to the F-P cavity. This leads to a 118.1 GHz millimeter-wave signal with phase noise of −118dBc/Hz at 10 kHz offset, decreasing to −120dBc/Hz at 30 kHz offset. We achieve this with technologies that can be integrated into a platform less than ≈10mL. Our work overcomes fundamental thermal-mechanical noise limits intrinsic to integrated photonics, while illustrating advantages of the same for providing low-size, -weight, and -power (SWaP) mm-waves that will be enabling for multiple applications in communications and sensing.

Microcombs are advancing optical frequency comb technology towards a chip-integrable form. Here, we characterize a microwave signal source based upon the two-point optical frequency division (2P-OFD) technique. The system uses a frequency microcomb to transfer relative frequency stability of two low-noise optical oscillators to the microcomb repetition rate tone. The two optical oscillators are based on semiconductor lasers jointly stabilized to an ultra-stable Fabry-Pérot cavity. The coherence of the comb spectrum is confirmed through multiple stability comparisons between its repetition rate and comb line spectrum. The results underscore the excellent performance of microcombs as coherent links between optical and microwave frequencies, and how they enable simplified, miniaturized architectures for optical frequency division. ©2024. All rights reserved.

High- Q microresonators are indispensable components of photonic integrated circuits and offer several useful operational modes. However, these modes cannot be reconfigured after fabrication because they are fixed by the resonator’s physical geometry. In this work, we propose a Moiré speedup dispersion tuning method that enables a microresonator device to operate in any of three modes. Electrical tuning of Vernier coupled rings switches operating modality to Brillouin laser, bright microcomb, and dark microcomb operation on demand using the same hybrid-integrated device. Brillouin phase matching and microcomb operation across the telecom C-band is demonstrated. Likewise, by using a single-pump wavelength, the operating mode can be switched. As a result, one universal design can be applied across a range of applications. The device brings flexible mixed-mode operation to integrated photonic circuits.

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low-noise microwave signals are generated by the down-conversion of ultrastable optical references using a frequency comb^1–3. Such systems, however, are constructed with bulk or fibre optics and are difficult to further reduce in size and power consumption. In this work we address this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division^4,5. Narrow-linewidth self-injection-locked integrated lasers^6,7 are stabilized to a miniature Fabry–Pérot cavity⁸, and the frequency gap between the lasers is divided with an efficient dark soliton frequency comb⁹. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of −96 dBc Hz⁻¹ at 100 Hz offset frequency that decreases to −135 dBc Hz⁻¹ at 10 kHz offset—values that are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems.

Visible, high-coherence optical sources are important to a wide range of applications spanning spectroscopy to precision timing. Integration of these sources on a semiconductor chip is a necessary step if the systems that use these devices are to be made compact, portable, and low power. Here, by self-injection-locking a 1560 nm distributed feedback semiconductor laser to a high-Q silicon-nitride resonator, a high-coherence 780 nm second-harmonic signal is generated via the photogalvanic-induced second-order nonlinearity. A record-low frequency noise floor of 4Hz²/Hz is achieved for the 780 nm emission. The approach can potentially generate signals over a wide range of visible and near-visible bands, and thereby help transition many table-top systems into a fieldable form.

Photonic integrated circuits are widely used in applications such as telecommunications and data-centre interconnects^1–5. However, in optical systems such as microwave synthesizers⁶, optical gyroscopes⁷ and atomic clocks⁸, photonic integrated circuits are still considered inferior solutions despite their advantages in size, weight, power consumption and cost. Such high-precision and highly coherent applications favour ultralow-noise laser sources to be integrated with other photonic components in a compact and robustly aligned format—that is, on a single chip—for photonic integrated circuits to replace bulk optics and fibres. There are two major issues preventing the realization of such envisioned photonic integrated circuits: the high phase noise of semiconductor lasers and the difficulty of integrating optical isolators directly on-chip. Here we challenge this convention by leveraging three-dimensional integration that results in ultralow-noise lasers with isolator-free operation for silicon photonics. Through multiple monolithic and heterogeneous processing sequences, direct on-chip integration of III–V gain medium and ultralow-loss silicon nitride waveguides with optical loss around 0.5 decibels per metre are demonstrated. Consequently, the demonstrated photonic integrated circuit enters a regime that gives rise to ultralow-noise lasers and microwave synthesizers without the need for optical isolators, owing to the ultrahigh-quality-factor cavity. Such photonic integrated circuits also offer superior scalability for complex functionalities and volume production, as well as improved stability and reliability over time. The three-dimensional integration on ultralow-loss photonic integrated circuits thus marks a critical step towards complex systems and networks on silicon.

Soliton mode locking in high-Q microcavities provides a way to integrate frequency comb systems. Among material platforms, AlGaAs has one of the largest optical nonlinearity coefficients, and is advantageous for low-pump-threshold comb generation. However, AlGaAs also has a very large thermo-optic effect that destabilizes soliton formation, and femtosecond soliton pulse generation has only been possible at cryogenic temperatures. Here, soliton generation in AlGaAs microresonators at room temperature is reported for the first time, to the best of our knowledge. The destabilizing thermo-optic effect is shown to instead provide stability in the high-repetition-rate soliton regime (corresponding to a large, normalized second-order dispersion parameter D ₂/κ). Single soliton and soliton crystal generation with sub-milliwatt optical pump power are demonstrated. The generality of this approach is verified in a high-Q silica microtoroid where manual tuning into the soliton regime is demonstrated. Besides the advantages of large optical nonlinearity, these AlGaAs devices are natural candidates for integration with semiconductor pump lasers. Furthermore, the approach should generalize to any high-Q resonator material platform.