Comb-based Characterization of Photonic Devices

Integrated photonics has been one of the fastest-growing fields in science. Measuring photonic devices in amplitude and phase (i.e. complex response) provides insight into their performance. Swept-wavelength interferometry is a prominent technique for the broadband characterization of the complex response. It leverages continuous advances in rapidly tunable laser sources, but is prone to systematic errors associated to the calibration of the frequency. This thesis focuses on the non-destructive characterization of ultralow-loss photonic devices using swept wavelength interferometric technique. We overcome issues associated to nonlinear tuning by calibrating the frequency of the laser with the aid of a frequency comb. We apply the concept to diverse components of relevance including microresonators and spiral waveguides. In addition, we provide an overview and comparative assessment of the state of the art in the field

High-Q Si3N4 microresonators based on a subtractive processing for Kerr nonlinear optics

Microresonator frequency combs (microcombs) are enabling new applications in frequency synthesis and metrology – from high-speed laser ranging to coherent optical communications. One critical parameter that dictates the performance of the microcomb is the optical quality factor (Q) of the microresonator. Microresonators fabricated in planar structures such as silicon nitride (Si3N4) allow for dispersion engineering and the possibility to monolithically integrate the microcomb with other photonic devices. However, the relatively large refractive index contrast and the tight optical confinement required for dispersion engineering make it challenging to attain Si3N4 microresonators with Qs > 107 using standard subtractive processing methods – i.e. photonic devices are patterned directly on the as-deposited Si3N4 film. In this work, we achieve ultra-smooth Si3N4 microresonators featuring mean intrinsic Qs around 11 million. The cross-section geometry can be precisely engineered in the telecommunications band to achieve either normal or anomalous dispersion, and we demonstrate the generation of mode-locked dark-pulse Kerr combs as well as soliton microcombs. Such high-Qs allow us to generate 100 GHz soliton microcombs, demonstrated here for the first time in Si3N4 microresonators fabricated using a subtractive processing method. These results enhance the possibilities for co-integration of microcombs with high-performance photonic devices, such as narrow-linewidth external-cavity diode lasers, ultra-narrow filters and demultiplexers

Frequency-Comb-Assisted Swept-Wavelength Interferometry

Swept-wavelength interferometry (SWI) is a highly sensitive and versatile technique implemented in a diverse array of industrial and scientific applications. SWI uses a continuously tunable laser to capture the magnitude and phase response of a device under test (DUT). The prevalent non-linear tuning of the laser calls for an auxiliary interferometer for the calibration of the laser frequency on the fly [1]. However, this approach is susceptible to environmental perturbations, and the inherent dispersion of the interferometer introduces systematic errors. Laser frequency combs can be used as optical rulers against which to calibrate tunable lasers with high- precision and, when self-referenced, with high accuracy [2]. Here, we apply this comb-based calibration approach in the context of SWI for the first time and illustrate its relevance for the characterization of high-Q microresonators

Integrated, Ultra-Compact High-Q Silicon Nitride Microresonators for Low-Repetition-Rate Soliton Microcombs

Multiple applications of relevance in photonics, such as spectrally efficient coherent communications, microwave synthesis or the calibration of astronomical spectrographs, would benefit from soliton microcombs operating at repetition rates <50GHz. However, attaining soliton microcombs with low repetition rates using photonic integration technologies represents a formidable challenge. Expanding the cavity volume results in a drop of intracavity intensity that can only be offset by an encompassing rise in quality factor. In addition, reducing the footprint of the microresonator on an integrated circuit requires race-track designs that typically result into modal coupling losses and disruptions into the dispersion, preventing the generation of the dissipative single soliton state. Here, we report the generation of sub-50GHz soliton microcombs in dispersion-engineered silicon nitride microresonators. In contrast to other approaches, the authors\u27 devices feature an optimized racetrack design that minimizes the coupling to higher-order modes and reduces the footprint size by an order of magnitude to approximate to 1mm(2). The statistical intrinsic Q reaches 19 million, and soliton microcombs at 20.5 and 14.0 GHz repetition rates are successfully generated. Importantly, the fabrication process is entirely subtractive, meaning that the devices can be directly patterned on the silicon nitride film

25 GHz soliton microcombs in high-Q Si3N4 racetrack-shaped microresonators

We demonstrate a 25 GHz single-soliton comb on a Si3N4\ua0platform fabricated using a novel subtractive method. A long cavity is carefully designed to fit within the e-beam writing field while minimizing coupling to higher-order modes

Frequency-comb-calibrated swept-wavelength interferometry

Lasers are often used to characterize samples in a non-destructive manner and retrieve sensing information transduced in changes in amplitude and phase. In swept wavelength interferometry, a wavelength-tunable laser is used to measure the complex response (i.e. in amplitude and phase) of an optical sample. This technique leverages continuous advances in rapidly tunable lasers and is widely used for sensing, bioimaging and testing of photonic integrated components. However, the tunable laser requires an additional calibration step because, in practice, it does not tune at a constant rate. In this work, we use a self-referenced frequency comb as an optical ruler to calibrate the laser used in swept-wavelength interferometry and optical frequency domain reflectometry. This allows for realizing high-resolution complex spectroscopy over a bandwidth exceeding 10 THz. We apply the technique to the characterization of low-loss integrated photonic devices and demonstrate that the phase information can disentangle intrinsic from coupling losses in the characterization of high-Q microresonators. We also demonstrate the technique in reflection mode, where it can resolve attenuation and dispersion characteristics in integrated long spiral waveguides

Overcoming the quantum limit of optical amplification in monolithic waveguides

Optical amplifiers are essential in numerous photonic applications. Parametric amplifiers, relying on a nonlinear material to create amplification, are uniquely promising as they can amplify without generating excess noise. Here, we demonstrate amplification based on the third-order nonlinearity in a single chip while, in addition, reporting a noise figure significantly below the conventional quantum limit when operated in phase-sensitive mode. Our results show the potential of nanophotonics for realizing continuous-wave parametric amplification that can enable applications in optical communications, signal processing, and quantum optics across a wide range of frequencies

Dark-pulse Kerr combs in linearly coupled microring structures

The generation of dissipative Kerr solitons in microresonators became a milestone in the quest towards a chip-scale frequency comb source. Although successful demonstrations in diverse fields have been made, one outstanding issue with dissipative Kerr solitons is their poor power conversion efficiency. This has a fundamental origin in the need to operate the soliton microcomb at a large frequency detuning. The limited conversion efficiency is a crucial aspect when considering co-integration with chip-scale laser sources or applications that demand higher power per line, such as optical communications

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