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Register a new userRealizing normal group-velocity dispersion in free space via angular dispersion
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It has long been thought that normal group-velocity dispersion (GVD) cannot be produced in free space via angular dispersion. Indeed, conventional diffractive or dispersive components such as gratings or prisms produce only anomalous GVD. We identify the conditions that must be fulfilled by the angular dispersion introduced into a plane-wave pulse to yield normal GVD. We then utilize a pulsed-beam shaper capable of introducing arbitrary angular-dispersion profiles to symmetrically produce normal and anomalous GVD in free space, which are realized here on the same footing for the first time.
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We have observed the propagation of an approximately 35 ns long light pulse with a negative group velocity through a laser-cooled 85Rb atomic medium. The anomalous dispersion results from linear atom-light interaction, and is unrelated to long-lived ground state coherences often associated with fast light in atomic media. The observed negative group velocity (-c/360) in the Rb magneto-optical trap for a pulse attenuated by less than 50% is in good agreement with the value of dispersion measured independently by an RF heterodyne method. The spectral region of anomalous dispersion is between 15 and 40 MHz, which is an order of magnitude wider than that typically associated with ground-state coherences.
Designing integrated photonics, especially to leverage Kerr-nonlinear optics, requires accurate and precise knowledge of refractive index across the visible to infrared spectral ranges. Tantala (Ta_2O_5) is an emerging material platform for integrated photonics and nanophotonics that offers broadband ultralow loss, moderately high nonlinearity, and advantages for scalable and heterogeneous integration. We present refractive-index measurements on a thin film of tantala, and we explore the efficacy of this data for group-velocity dispersion (GVD) engineering with waveguide and ring-resonator devices. In particular, the observed spectral extent of supercontinuum generation in fabricated waveguides, and the wavelength dependence of free spectral range (FSR) in optical resonators provide a sensitive test of our integrated-photonics design process. Our work opens up new design possibilities with tantala, including with octave-spanning soliton microcombs.
We developed an original model describing the process of the frequency comb generation in the self-injection locking regime and performed numerical simulation of this process.Generation of the dissipative Kerr solitons in the self-injection locking regime at anomalous group velocity dispersion was studied numerically. Different regimes of the soliton excitation depending on the locking phase, backscattering parameter and pump power were identified. It was also proposed and confirmed numerically that self-injection locking may provide an easy way for the generation of the frequency combs at normal group velocity dispersion. Generation of platicons was demonstrated and studied in detail. The parameter range providing platicon excitation was found.
In positive phase-mismatched SHG and normal dispersion, a gaussian spatio-temporal pulse transforms spontaneously into a X-pulse, underlies spatio-temporal compression and eventually leads to stationary 3-D propagation. Experimental and numerical data are provided
Breathers are localized waves, that are periodic in time or space. The concept of breathers is useful for describing many physical systems including granular lattices, Bose-Einstein condensation, hydrodynamics, plasmas and optics. Breathers could exist in both the anomalous and the normal dispersion regime. However, the demonstration of optical breathers in the normal dispersion regime remains elusive to our knowledge. Kerr comb generation in optical microresonators provides an array of oscillators that are highly coupled via the Kerr effect, which can be exploited to explore the breather dynamics. Here, we present, experimentally and numerically, the observation of dark breathers in a normal dispersion silicon nitride microresonator. By controlling the pump wavelength and power, we can generate the dark breather, which exhibits an energy exchange between the central lines and the lines at the wing. The dark breather breathes gently and retains a dark-pulse waveform. A transition to a chaotic breather state is also observed by increasing the pump power. These dark breather dynamics are well reproduced by numerical simulations based on the Lugiato-Lefever equation. The results also reveal the importance of dissipation to dark breather dynamics and give important insights into instabilities related to high power dark pulse Kerr combs from normal dispersion microreosnators.
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