Mid-infrared frequency comb via coherent dispersive wave generation in silicon nitride nanophotonic waveguides

Mid-infrared optical frequency combs are of significant interest for molecular spectroscopy due to the large absorption of molecular vibrational modes on the one hand, and the ability to implement superior comb-based spectroscopic modalities with increased speed, sensitivity and precision on the other hand. Here, we demonstrate a simple, yet effective, method for the direct generation of mid-infrared optical frequency combs in the region from 2.5 to 4.0 μm (that is, 2,500–4,000 cm−1), covering a large fraction of the functional group region, from a conventional and compact erbium-fibre-based femtosecond laser in the telecommunication band (that is, 1.55 μm). The wavelength conversion is based on dispersive wave generation within the supercontinuum process in an unprecedented large-cross-section silicon nitride (Si3N4) waveguide with the dispersion lithographically engineered. The long-wavelength dispersive wave can perform as a mid-infrared frequency comb, whose coherence is demonstrated via optical heterodyne measurements. Such an approach can be considered as an alternative option to mid-infrared frequency comb generation. Moreover, it has the potential to realize compact dual-comb spectrometers. The generated combs also have a fine teeth-spacing, making them suitable for gas-phase analysis.

Development of coherent mid-infrared source using chalcogenide photonic crystal fibers

Sida Xing

Many molecule bonds have vibration frequencies in the mid-infrared band. Thus, this band is of great interest to molecular spectroscopy, material processing and medical applications. However, many optical materials typically used for laser sources experience high losses due to the presence of such vibration bonds making the design of mid-infrared coherent sources challenging. Such task is even more difficult when considering designing all-fiber sources which offer additional compactness and robustness. The goal of this project is to address such challenge by designing all-fiber mid-infrared sources based on chalcogenide fibers and nonlinear effects. Using degenerated parametric conversion of a 2 um pump and a telecom band signal laser, one can generate an idler in the mid-infrared band. As a first step, the linear and nonlinear performances of several chalcogenide fibers were compared. The results prove the chalcogenide photonic crystal fiber design to be the ideal platform for this project as it offers high nonlinearity and low loss. Numerical simulations provide further improvement of photonic crystal geometry. Our improved designs allow for an extreme shaping of the dispersion to optimize the four-wave-mixing process within the thulium /holmium fiber laser band. In the meantime, several cavity configurations were tested to boost the thulium/holmium laser performance, that are required for pumping of our chalcogenide fibers. Polarizer-free cavity designs result more compact laser footprint, which moreover enables a higher slope efficiency. Such lasers enable precise measurements of sample fiber optical parameters. The combined efforts on fiber and laser optimization leads to efficient continuous-wave parametric conversion. Thanks to the excellent mid-infrared transmittance of chalcogenide glass, this high efficiency can be extended to longer pumps in mid-infrared. However, fiber fuse, a result of heat dissipative solitons, is the main constrain for reaching better conversion efficiency. For ultrafast applications, a sub-cm length chalcogenide photonic crystal fiber generates flat-top, linearly chirped supercontinuum in the normal dispersion region. This pulse length could be compressed to two optical cycles by linear compression. Once again, the experimental results match perfectly with the simulations. Overall, we show in this thesis that chalcogenide PCFs are promising for highly efficient and low threshold nonlinear optics in the middle infrared, offering new options for light generation in this critical wavelength band.

EPFL2019

Dispersion Properties of Photonic Crystals and Silicon Nanostructures Investigated by Fourier-Space Imaging

Jana Jágerská

State-of-the-art nanophotonic devices based on semiconductor technology use total internal reflection or the photonic bandgap effect to reduce the waveguide core dimensions down to hundreds of nanometers, ensuring strong optical confinement within the scale of the wavelength. Within the framework of this thesis, we investigate the light propagation in such devices by direct experimental reconstruction of their dispersion relation ω (k), where ω is the optical frequency and k the wave vector of the supported modes. Knowledge of the dispersion relation provides us with comprehensive information about the guided field, including the number of supported modes, their phase and group velocity as well as the higher order dispersion. As a principal characterization tool, an original experimental technique referred as Fourier-space imaging is used. It is based on far-field analysis of optical signal radiated out of the plane of the structure, which makes it possible to retrieve accurately, non-invasively and in one step the complex dispersion of both the leaky and the truly guided optical modes. The latter is feasible provided that the device is equipped with vanishingly weak grating probes that scatter a small part of the guided light into the light cone. The Fourier-space imaging technique was applied to study the optical properties of a large number of nanophotonic devices, ranging from simple nanowire waveguides to complex photonic crystal structures. In the first part of the work, silicon-on-insulator slot waveguides, coupled ridge waveguides and nanowire waveguide arrays are addressed. Besides the phase and group index dispersion, we investigate the phenomenon of mode splitting in coupled systems, being able to probe the coupling lengths with an accuracy of ±50 nm. In the case of waveguide arrays, beam steering using both thermo-optic effect and wavelength tuning was demonstrated. Concerning the photonic crystal devices, we primarily focus on the phenomenon of slow light propagation in line-defect and coupled-cavity photonic crystal waveguides. The latter represent a special type of a waveguide, which allows for substantial optical signal retardation by evanescent coupling along a chain of photonic crystal cavities. The main motivation was to accurately measure the group index of the slow light modes and recognize the main factors limiting its maximum achievable value. Among others, experimental observation of dispersion curve renormalization, enhanced out-of-plane and back-scattering as well as light localization due to residual disorder were reported. Finally, a detailed experimental study of hollow-core photonic crystal structures intended for optical sensing applications is presented.

EPFL2011

Development of coherent mid-infrared source using chalcogenide photonic crystal fibers

Sida Xing

EPFL2019

Dispersion Properties of Photonic Crystals and Silicon Nanostructures Investigated by Fourier-Space Imaging

Jana Jágerská

EPFL2011