We study a semiconductor laser coupled to two mirrors at a distance. Understanding such a delayed feedback system and how it can make the laser behave chaotically could lead to better random number generators, safer communications, and more widespread use of these devices. Based on previous reports, the two delays are positioned in the long cavity regime and differ in the order of half the relaxation oscillation period (ROP) to limit the appearance of the so-called time-delay signature (TDS). We show theoretically that a change of the feedback phase has a crucial impact on the TDS and chaos bandwidth (CBW). At intermediate values, a change in the feedback phase will either suppress or enhance the TDS. For high feedback rates, where the chaotic bandwidth is much higher, the system can switch rapidly between stable and chaotic states due to small variations of the feedback phase. We show experimentally that the CBW is increased by increasing the feedback strength if the feedback phase is controlled. In summary, with two feedback loops, one can further suppress the TDS and increase the CBW, given that the feedback phases can be controlled accurately. Our results contrast with the one-delay system, for which the feedback phase has only limited impact if the feedback mirror is far from the laser.
We investigate pumping nonlinear interactions in multi-mode waveguides with focused laser pulses that contain space-time correlations. This allows for the different modes of the waveguide to be excited with different temporal envelopes. Established nonlinear processes in standard multi-mode graded-index fibers, such as beam self-cleaning or supercontinuum generation, could be significantly affected or even controlled by such novel initial conditions. We will introduce precisely how to introduce such initial conditions and show numerical results of parameter scans of different space-time couplings and relate them to more standard cases. We will also discuss implications for experiments, and analouges in other waveguide platforms, such as vortex fibers or integrated photonic waveguides.
Fiber Bragg grating (FBG) feedback has initially been investigated as a promising approach to conceal the time-delay signature in optical chaos generation. It has been shown that the laser dynamics vary greatly with respect to the FBG properties, especially to the frequency detuning between the laser emission and the Bragg wavelength. As a result, adjusting the FBG features will lead to significantly different behaviour. Here, we theoretically study the response of FBGs with different lengths but with similar reflectivity: this way, the impulse response is stretched over a longer period of time while its overall shape is maintained. This leads to a broadening of the FBG bandwidth and, thus, to a longer distribution of the feedback over time. In this work, we analyse the effects of the time-distribution variations for long gratings by simply tracking the first Hopf bifurcation and the feedback rate needed to destabilize the laser. The numerical results are generated using a modified version of the well-known Lang and Kobayashi equations. Our results show that the time-distribution of the feedback seem to have little effect in itself on the overall dynamics though it obviously affects the FBG spectra properties. We report stability oscillations of the laser behavior when long, narrow-bandwidth gratings are considered. The influence of the grating length on the specific dynamic details is investigated through the time delay signature (TDS) focusing especially on the implication of the stability oscillations on the TDS. We report that although variation of the TDS for long grating are observed the better TDS suppression is achieved with relatively short gratings.
The generation of THz-frequency radiation via nonlinear parametric frequency down-conversion has long been driven by the spectroscopy and imaging communities. As a result, little efforts have been undertaken toward the generation of high energy THz-frequency pulses. THz-frequency radiation has however recently been identified has a promising driver for strong-field physics and an emerging generation of compact particle accelerators. These accelerators require THzfrequency pulses with energies in the multi-millijoule range therefore demanding orders of magnitude improvements from the current state-of-the-art.
Much can be gained by improving the intrinsically low efficiency of the down-conversion process while still resorting to existing state-of-the-art lasers. However, the fundamental Manley-Rowe limit caps the efficiency of parametric downconversion from 1-μm wavelength lasers to sub-THz frequency to the sub-percent range.
We present methods that promise boosting the THz radiation yield obtained via parametric down-conversion beyond the Manley-Rowe limit. Our method relies on cascaded nonlinear three-wave mixing between two spectrally neighboring laser pulses in periodically poled Lithium Niobate. Owing to favorable phase-matching, the down-conversion process avalanches, resulting in spectral broadening in the optical domain. This allows in-situ coherent multiplexing of multiple parametric down-conversion stages within a single device and boosting the efficiency of the process beyond the ManleyRowe limit. We experimentally demonstrated the concept using either broadband, spectrally chirped optical pulses from a Joule-class laser or using two narrowband lasers with neighboring wavelengths. Experimental results are backed by numerical simulations that predict conversion efficiencies from 1 μm to sub-THz radiation in the multi-percent range.
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