Frequency-comb-calibrated swept-wavelength interferometry

Active feedback stabilization of super-efficient microcombs in photonic molecules

Dissipative Kerr soliton (DKS) frequency combs, when generated within coupled cavities, exhibit exceptional performance concerning controlled initiation and power conversion efficiency. Nevertheless, to fully exploit these enhanced capabilities, it is necessary to maintain the frequency comb in a low-noise state over an extended duration. In this study, we demonstrate the control and stabilization of super-efficient microcombs in a photonic molecule. Our findings demonstrate that there is a direct relation between effective detuning and soliton power, allowing the latter to be used as a setpoint in a feedback control loop. Employing this method, we achieve the stabilization of a highly efficient microcomb indefinitely, paving the way for its practical deployment in optical communications and dual-comb spectroscopy applications.

A wideband, high-resolution vector spectrum analyzer for integrated photonics

The analysis of optical spectra-emission or absorption-has been arguably the most powerful approach for discovering and understanding matter. The invention and development of many kinds of spectrometers have equipped us with versatile yet ultra-sensitive diagnostic tools for trace gas detection, isotope analysis, and resolving hyperfine structures of atoms and molecules. With proliferating data and information, urgent and demanding requirements have been placed today on spectrum analysis with ever-increasing spectral bandwidth and frequency resolution. These requirements are especially stringent for broadband laser sources that carry massive information and for dispersive devices used in information processing systems. In addition, spectrum analyzers are expected to probe the device's phase response where extra information is encoded. Here we demonstrate a novel vector spectrum analyzer (VSA) that is capable of characterizing passive devices and active laser sources in one setup. Such a dual-mode VSA can measure loss, phase response, and dispersion properties of passive devices. It also can coherently map a broadband laser spectrum into the RF domain. The VSA features a bandwidth of 55.1 THz (1260-1640 nm), a frequency resolution of 471 kHz, and a dynamic range of 56 dB. Meanwhile, our fiber-based VSA is compact and robust. It requires neither high-speed modulators and photodetectors nor any active feedback control. Finally, we employ our VSA for applications including characterization of integrated dispersive waveguides, mapping frequency comb spectra, and coherent light detection and ranging (LiDAR). Our VSA presents an innovative approach for device analysis and laser spectroscopy, and can play a critical role in future photonic systems and applications for sensing, communication, imaging, and quantum information processing.

Compact lithium niobate microring resonators in the ultrahigh Q/V regime

Lithium niobate (LN) is a promising material for future complex photonic-electronic circuits, with wide applications in such fields as communications, sensing, quantum optics, and computation. LN took a great stride toward compact photonic integrated circuits (PICs) with the development of partially etched LN on insulator (LNOI) waveguides. However, integration density is still limited for future highly compact PICs, owing to the partial etching nature of their waveguides. Here, we demonstrate a fully etched LN PIC platform, which, for the first time to our knowledge, simultaneously achieves ultralow propagation loss and compact circuit size. The tightly confined fully etched LN waveguides with smooth sidewalls allow us to bring the bending radius down to 20 μm (corresponding to 1 THz free spectral range). We have achieved compact high Q microring resonators with Q/V of 8.7 × 10⁴ μm^-3, almost one order of magnitude larger than previous demonstrations. The statistical mean propagation losses of our LN waveguides is 8.5 dB/m (corresponding to a mean Q factor of 4.9 × 10⁶), even with a small bending radius of 40 μm. Our compact and ultralow-loss LN platform shows great potential in future miniaturized multifunctional integration systems. As complementary evidence to show the utility of our platform, we demonstrate soliton microcombs with an ultrahigh repetition rate of 500 GHz in LN.

Hyperparametric Oscillation via Bound States in the Continuum

Optical hyperparametric oscillation based on the third-order nonlinearity is one of the most significant mechanisms to generate coherent electromagnetic radiation and produce quantum states of light. Advances in dispersion-engineered high-Q microresonators allow for generating signal waves far from the pump and decrease the oscillation power threshold to submilliwatt levels. However, the pump-to-signal conversion efficiency and absolute signal power are low, fundamentally limited by parasitic mode competition and attainable cavity intrinsic Q to coupling Q ratio, i.e., Q_{i}/Q_{c}. Here, we use Friedrich-Wintgen bound states in the continuum (BICs) to overcome the physical challenges in an integrated microresonator-waveguide system. As a result, on-chip coherent hyperparametric oscillation is generated in BICs with unprecedented conversion efficiency and absolute signal power. This work not only opens a path to generate high-power and efficient continuous-wave electromagnetic radiation in Kerr nonlinear media but also enhances the understanding of a microresonator-waveguide system-an elementary unit of modern photonics.

Differential phase reconstruction of microcombs

Measuring microcombs in amplitude and phase provides unique insight into the nonlinear cavity dynamics, but spectral phase measurements are experimentally challenging. Here, we report a linear heterodyne technique assisted by electro-optic downconversion that enables differential phase measurement of such spectra with unprecedented sensitivity (-50 dBm) and bandwidth coverage (>110 nm in the telecommunications range). We validate the technique with a series of measurements, including single-cavity and photonic molecule microcombs.

Spectral Interferometry with Frequency Combs

In this review paper, we provide an overview of the state of the art in linear interferometric techniques using laser frequency comb sources. Diverse techniques including Fourier transform spectroscopy, linear spectral interferometry and swept-wavelength interferometry are covered in detail. The unique features brought by laser frequency comb sources are shown, and specific applications highlighted in molecular spectroscopy, optical coherence tomography and the characterization of photonic integrated devices and components. Finally, the possibilities enabled by advances in chip scale swept sources and frequency combs are discussed.

Dispersion Engineering of Silicon Nitride Microresonators via Reconstructable SU-8 Polymer Cladding

In this work, we propose a novel way to flexibly engineer the waveguide dispersion by patterning the cladding of waveguide microresonators. Experimentally, we demonstrate silicon nitride waveguides with air-, oxide-, and SU-8 polymer-cladding layers and compare the corresponding waveguide dispersion. By integrating SU-8 polymer as the outer cladding layer, the waveguide dispersion can be tuned from −143 to −257 ps/nm/km. Through the simple, conventional polymer stripping process, we reconstruct the waveguide dispersion back to that of the original air-cladded device without significantly impacting the quality factor of resonators. This work provides the potential to design the waveguide dispersion in normal and anomalous regimes within an integrated photonic circuit.

Coherent supercontinuum generation in all-normal dispersion Si3N4 waveguides

Spectral broadening of optical frequency combs with high repetition rate is of significant interest in optical communications, radio-frequency photonics and spectroscopy. Silicon nitride waveguides (Si₃N₄) in the anomalous dispersion region have shown efficient supercontinuum generation spanning an octave-bandwidth. However, the broadening mechanism in this regime is usually attained with femtosecond pulses in order to maintain the coherence. Supercontinuum generation in the normal dispersion regime is more prone to longer (ps) pulses, but the implementation in normal dispersion silicon nitride waveguides is challenging as it possesses strong requirements in propagation length and losses. Here, we experimentally demonstrate the use of a Si₃N₄ waveguide to perform coherent spectral broadening using pulses in the picosecond regime with high repetition rate. Moreover, our work explores the formation of optical wave breaking using a higher energy pulse which enables the generation of a coherent octave spanning spectrum. These results offer a new prospect for coherent broadening using long duration pulses and replacing bulky optical components.

Demonstration of in-depth analysis of silicon photonics circuits using OFDR: waveguides with grating couplers

Optical frequency domain reflectometry (OFDR) is a powerful technique to investigate backscatter in waveguides. However, its use in Si photonics circuits has so far been limited to measuring the propagation loss and group index of a waveguide. We demonstrate that the transmittance (T) and reflectance (R) of elemental devices comprising a Si photonics circuit can be determined by OFDR. An analysis of Si wire waveguides with grating couplers (GCs) is described in detail. The wavelength dependence of T and R of the GCs were determined by using a backscatter model incorporating time-equivalent multiple-reflection paths and were well reproduced by a numerical simulation.