Junqiu Liu

Coupling ideality of integrated planar high-Q microresonators

Chip-scale optical microresonators with integrated planar optical waveguides are useful building blocks for linear, nonlinear, and quantum-optical photonic devices alike. Loss reduction through improving fabrication processes results in several integrated microresonator platforms attaining quality (Q) factors of several millions. Beyond the improvement of the quality factor, the ability to operate the microresonator with high coupling ideality in the overcoupled regime is of central importance. In this regime, the dominant source of loss constitutes the coupling to a single desired output channel, which is particularly important not only for quantum-optical applications such as the generation of squeezed light and correlated photon pairs but also for linear and nonlinear photonics. However, to date, the coupling ideality in integrated photonic microresonators is not well understood, in particular, design-dependent losses and their impact on the regime of high ideality. Here we investigate design-dependent parasitic losses described by the coupling ideality of the commonly employed microresonator design consisting of a microring-resonator waveguide side coupled to a straight bus waveguide, a system which is not properly described by the conventional input-output theory of open systems due to the presence of higher-order modes. By systematic characterization of multimode high-Q silicon nitride microresonator devices, we show that this design can suffer from low coupling ideality. By performing 3D simulations, we identify the coupling to higher-order bus waveguide modes as the dominant origin of parasitic losses which lead to the low coupling ideality. Using suitably designed bus waveguides, parasitic losses are mitigated with a nearly unity ideality and strong overcoupling (i.e., a ratio of external coupling to internal resonator loss rate > 9) are demonstrated. Moreover, we find that different resonator modes can exchange power through the coupler, which, therefore, constitutes a mechanism that induces modal coupling, a phenomenon known to distort resonator dispersion properties. Our results demonstrate the potential for significant performance improvements of integrated planar microresonators for applications in quantum optics and nonlinear photonics achievable by optimized coupler designs.

Photonic chip-based soliton frequency combs covering the biological imaging window

Dissipative Kerr solitons (DKS) in optical microresonators provide a highly miniaturised, chip-integrated frequency comb source with unprecedentedly high repetition rates and spectral bandwidth. To date, such frequency comb sources have been successfully applied in the optical telecommunication band for dual-comb spectroscopy, coherent telecommunications, counting of optical frequencies and distance measurements. Yet, the range of applications could be significantly extended by operating in the near-infrared spectral domain, which is a prerequisite for biomedical and Raman imaging applications, and hosts commonly used optical atomic transitions. Here we show the operation of photonic-chip-based soliton Kerr combs driven with 1 micron laser light. By engineering the dispersion properties of a Si3N4 microring resonator, octave-spanning soliton Kerr combs extending to 776 nm are attained, thereby covering the optical biological imaging window. Moreover, we show that soliton states can be generated in normal group–velocity dispersion regions when exploiting mode hybridisation with other mode families.Dissipative Kerr solitons in optical microresonators provide excellent optical frequency comb sources for precision metrology and imaging techniques. Here, Karpov et al. demonstrate a chipscale octave-spanning soliton-based comb, operating at 1 μm wavelength that covers the biological imaging window.

Intermode Breather Solitons in Optical Microresonators

Dissipative solitons can be found in a variety of systems resulting from the double balance between dispersion and nonlinearity, as well as gain and loss. Recently, they have been observed to spontaneously form in Kerr nonlinear microresonators driven by a continuous wave laser, providing a compact source of coherent optical frequency combs. As optical microresonators are commonly multimode, intermode interactions, which give rise to avoided mode crossings, frequently occur and can alter the soliton properties. Recent works have shown that avoided mode crossings cause the soliton to acquire a single-mode dispersive wave, a recoil in the spectrum, or lead to soliton decay. Here, we show that avoided mode crossings can also trigger the formation of breather solitons, solitons that undergo a periodic evolution in their amplitude and duration. This new breather soliton, referred to as an intermode breather soliton, occurs within a laser detuning range where conventionally stationary (i.e., stable) dissipative Kerr solitons are expected. We experimentally demonstrate the phenomenon in two microresonator platforms (crystalline magnesium fluoride and photonic chip-based silicon nitride microresonators) and theoretically describe the dynamics based on a pair of coupled Lugiato-Lefever equations. We show that the breathing is associated with a periodic energy exchange between the soliton and a second optical mode family, a behavior that can be modeled by a response function acting on dissipative solitons described by the Lugiato-Lefever model. The observation of breathing dynamics in the conventionally stable soliton regime is relevant to applications in metrology such as low-noise microwave generation, frequency synthesis, or spectroscopy.

Frequency-comb-assisted broadband precision spectroscopy with cascaded diode lasers.

Frequency-comb-assisted diode laser spectroscopy, employing both the accuracy of an optical frequency comb and the broad wavelength tuning range of a tunable diode laser, has been widely used in many applications. In this Letter, we present a novel method using cascaded frequency agile diode lasers, which allows us to extend the measurement bandwidth to 37.4 THz (1355-1630 nm) at megahertz resolution with scanning speeds above 1 THz/s. It is demonstrated as a useful tool to characterize a broadband spectrum for molecular spectroscopy, and in particular it enables us to characterize the dispersion of integrated microresonators up to the 4th-order.

Ultra-smooth silicon nitride waveguides based on the Damascene reflow process: fabrication and loss origins

On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion are important to nonlinear photonic devices such as soliton microcombs, and likewise can be employed for on-chip gyroscopes, delay lines, or Brillouin lasers. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication-induced surface roughness, causing dominant scattering losses. Therefore, microresonators with ultra-high-Q-factors are, to date, attainable only in polished bulk crystalline or chemically etched silica-based devices, which pose, however, challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride (Si3N4) waveguides with unprecedentedly smooth sidewalls and tight confinement with record-low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable mean scattering Q-factors of 12×106 for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques, we differentiate the scattering and absorption loss contributions and reveal metal-impurity-related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time, to the best of our knowledge, they are identified—and play a tangible role—in absorption of integrated microresonators. Taken together, our work provides new insights into the origins of propagation losses in Si3N4 waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high-Q regime.On-chip optical waveguides with low propagation losses and precisely engineered group velocity dispersion (GVD) are important to nonlinear photonic devices such as soliton microcombs, and likewise can be employed for on chip gyroscopes, delay lines or Brillouin lasers. Yet, despite intensive research efforts, nonlinear integrated photonic platforms still feature propagation losses orders of magnitude higher than in standard optical fiber. The tight confinement and high index contrast of integrated waveguides make them highly susceptible to fabrication induced surface roughness. Therefore, microresonators with ultra-high Q factors are, to date, only attainable in polished bulk crystalline, or chemically etched silica based devices, that pose however challenges for full photonic integration. Here, we demonstrate the fabrication of silicon nitride (Si3N4) waveguides with unprecedentedly smooth sidewalls and tight confinement with record low propagation losses. This is achieved by combining the photonic Damascene process with a novel reflow process, which reduces etching roughness, while sufficiently preserving dimensional accuracy. This leads to previously unattainable mean microresonator Q factors larger than 5×10 for tightly confining waveguides with anomalous dispersion. Via systematic process step variation and two independent characterization techniques we differentiate the scattering and absorption loss contributions, and reveal metal impurity related absorption to be an important loss origin. Although such impurities are known to limit optical fibers, this is the first time they are identified, and play a tangible role, in absorption of integrated microresonators. Taken together, our work provides new insights in the origins of propagation losses in Si3N4 waveguides and provides the technological basis for integrated nonlinear photonics in the ultra-high Q regime.

Ultra-Low Power Photonic Chip-Based Soliton Frequency Combs

By improving the quality factor and the chip input optical coupling, we demonstrate single soliton formation in 1- THz-FSR and 88-GHz-FSR Si3

Soliton Kerr frequency combs with octave bandwidth in integrated Si 3 N 4 microresonators

N_{4}

Double inverse nanotapers for efficient light coupling to integrated photonic devices

microresonators with < 10 mW and < 50 m W input optical power, respectively, ideal for fully integrated chip-scale photonic devices.

Dissipative Kerr Solitons in Photonic Chip-based Microresonators

Frequency combs provide a set of equidistant laser lines and find applications in many areas [1]. Microresonator based Kerr frequency combs are a recent scheme for frequency comb generation promising chip-level integration and novel applications such as coherent terabit telecommunications [2, 3]. The underlying principle is nonlinear frequency conversion of a continuous wave pump laser in a high finesse microresonator. Recently the excitation of dissipative Kerr solitons (DKS) and dispersive wave (DW) formation within the resonator has provided a path to fully coherent Kerr frequency combs with engineered bandwidth [4, 5]. Attaining spectral coverage exceeding one octave is key for several applications like self-referencing via f-2f-interferometry. Although octave spanning Kerr combs have been reported, exciting fully coherent DKS based frequency combs with octave bandwidth has remained challenging.