Frequency combs on chip for interferometry applications

Optical frequency combs have revolutionized the field of laser spectroscopy. A frequency comb is a type of laser that generates an array of equally spaced coherent laser lines. Indeed, the \ua0outstanding performance of frequency combs in terms of bandwidth and stability is readily attainable in bench-top systems. Integrated photonics offers a platform for the implementation of frequency combs relying on nonlinear optics processes.\ua0 This thesis explores the generation of chip-scale frequency combs based on supercontinuum and microcomb generation and its potential use for interferometry. This investigation covers the capabilities offered by supercontinuum generation in the normal dispersion regime. The spectral broadening is realized by pumping a straight waveguide with a short duration pulse meaning that the pump is a comb itself. Therefore, its performance in terms of coherence and the transferring of noise to the broadened spectra have been investigated. Microcombs can be generated\ua0 on a microresonator starting from a continuous wave laser. In this work, we study microcomb generation in the normal dispersion regime using a novel dual-cavity architecture.\ua0The appended papers describe the nonlinear processes involved in the microcomb generation. We have studied its capabilities in terms of spectral flatness and symmetry, together with the coherence attained on these combs. It is found that these capabilities make microcombs a suitable spectral sources for spectroscopy. Furthermore, the capabilities of different interferometry techniques are analyzed in terms of resolution, sensitivity and measurement time in order to perform on-chip dual-comb spectroscopy

Measurement of particle motion in optical tweezers embedded in a Sagnac interferometer

We have constructed a counterpropagating optical tweezers setup embedded in a Sagnac interferometer in order to increase the sensitivity of position tracking for particles in the geometrical optics regime. Enhanced position determination using a Sagnac interferometer has previously been described theoretically by Taylor et al. [Journal of Optics 13, 044014 (2011)] for Rayleigh-regime particles trapped in an antinode of a standing wave. We have extended their theory to a case of arbitrarily-sized particles trapped with orthogonally-polarized counterpropagating beams. The working distance of the setup was sufficiently long to optically induce particle oscillations orthogonally to the axis of the tweezers with an auxiliary laser beam. Using these oscillations as a reference, we have experimentally shown that Sagnac-enhanced back focal plane interferometry is capable of providing an improvement of more than 5 times in the signal-to-background ratio, corresponding to a more than 30-fold improvement of the signal-to-noise ratio. The experimental results obtained are consistent with our theoretical predictions. In the experimental setup, we used a method of optical levitator-assisted liquid droplet delivery in air based on commercial inkjet technology, with a novel method to precisely control the size of droplets.Comment: 14 pages, 8 figure

Superefficient microcombs at the wafer level

Photonic integrated circuits utilize planar waveguides to process light on a chip, encompassing functions like generation, routing, modulation, and detection. Similar to the advancements in the electronics industry, photonics research is steadily transferring an expanding repertoire of functionalities onto integrated platforms. The combination of best-in-class materials at the wafer-level increases versatility and performance, suitable for large-scale markets, such as datacentre interconnects, lidar for autonomous driving or consumer health. These applications require mature integration platforms to sustain the production of millions of devices per year and provide efficient solutions in terms of power consumption and wavelength multiplicity for scalability. Chip-scale frequency combs offer massive wavelength parallelization, holding a transformative potential in photonic system integration, but efficient solutions have only been reported at the die level. Here, we demonstrate a silicon nitride technology on a 100 mm wafer that aids the performance requirements of soliton microcombs in terms of yield, spectral stability, and power efficiency. Soliton microcombs are reported with an average conversion efficiency exceeding 50%, featuring 100 lines at 100 GHz repetition rate. We further illustrate the enabling possibilities of the space multiplicity, i.e., the large wafer-level redundancy, for establishing new sensing applications, and show tri-comb interferometry for broadband phase-sensitive spectroscopy. Combined with heterogeneous integration of lasers, we envision a proliferation of high-performance photonic systems for applications in future navigation systems, data centre interconnects, and ranging

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

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

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

Photonic molecule microcombs at 50 GHz repetition rate

We present a microcomb in a photonic molecule with 50 GHz repetition rate. The comb attains > 50% power conversion efficiency and displays a quiet point of operation in repetition rate with decreased phase noise

Dual-Comb Swept-Wavelength Interferometry: Theory and Experiment

Much efforts have been put to elaborate and improve different high precision measurement schemes for characterization of advanced photonic devices and optical fibers with increasing bandwidth requirements. In light of this, swept-wavelength interferometry and dual-comb spectroscopy have been extensively applied in characterization procedures. In this paper we present in detail an experimental scheme that combines these two techniques and overcomes their limitations by using a tunable laser source in order to sweep over the frequency comb spacing and capture all intermediate frequencies. We demonstrate full-field broadband measurements over 1.25 THz comb bandwidth with increased frequency resolution, which can be performed in only 5\ua0ms sweep. We also show that the nonlinearity of the laser sweep can be removed without an auxiliary interferometer in the setup

Linear Broadband Differential Phase Measurement of Soliton Microcombs

We demonstrate complex (amplitude and phase) spectral characterization of a 100 GHz repetition rate microcomb over a bandwidth exceeding C and L bands using a linear stepped heterodyne technique

Nonlinear Broadening of Electro-Optic Frequency Combs in All-Normal Dispersion Si₃N₄Waveguides

Dispersion-engineered silicon nitride waveguides are often used for broadening of frequency combs. Previous experiments have focused on anomalous dispersion waveguides pumped by femtosecond laser pulses, whereby coherent supercontinuum generation relies on soliton compression and dispersive wave emission (see e.g. [1] ). This regime of operation does not work well with high-repetition rate sources, such as microcombs or electro-optic (EO) combs, because the pulse duration is not sufficiently short to drive coherent broadening in anomalous dispersion media. Indeed, octave-spanning coherent broadening of high-repetition rate sources has been attained in anomalous dispersion silicon nitride waveguides, but this required using a pre-broadening stage in a normal-dispersion highly nonlinear fiber (HNLF) [2]. Implementing this stage in a silicon nitride waveguide is the topic we address in this contribution