Mode division multiplexing (MDM) is mooted as a technology to address future bandwidth issues and has been successfully demonstrated in free space using spatial modes with orbital angular momentum (OAM). To further increase the data transmission rate, more degrees of freedom are required to form a densely packed mode space. Here we move beyond OAM and demonstrate multiplexing and demultiplexing using both the radial and azimuthal degrees of freedom. We achieve this with a holographic approach that allows over 100 modes to be encoded on a single hologram, across a wide wavelength range, in a wavelength independent manner. Our results offer a new tool that will prove useful in realizing higher bit rates for next generation optical networks.

We use dispersive Fourier transformation to measure shot-to-shot spectral instabilities in femtosecond supercontinuum generation. We study both the onset phase of supercontinuum generation with distinct dispersive wave generation, as well as a highly-unstable supercontinuum regime spanning an octave in bandwidth. Wavelength correlation maps allow interactions between separated spectral components to be identified, even when such interactions are not apparent in shot-to-shot or average measurements. Experimental results are interpreted using numerical simulations. Our results show the clear advantages of dispersive Fourier transformation for studying spectral noise during supercontinuum generation.

We numerically report super-flat coherent mid-infrared supercontinuum (MIR-SC) generation in a chalcogenide ${\mathrm{As}}_{38.8}{\mathrm{Se}}_{61.2}$ photonic crystal fiber (PCF). The dispersion and nonlinear parameters of ${\mathrm{As}}_{38.8}{\mathrm{Se}}_{61.2}$ chalcogenide PCFs by varying the diameter of the air holes are engineered to obtain all-normal dispersion (ANDi) with high nonlinearities. We show that launching low-energy 50 fs optical pulses with 0.88 kW peak power (corresponding to pulse energy of 0.05 nJ) at a central wavelength of 3.7 μm into a 5 cm long ANDi-PCF generates a flat-top coherent MIR-SC spanning from 2900 to 4575 nm with a high spectral flatness of 3 dB. This ultra-wide and flattened spectrum has excellent stability and coherence properties that can be used for MIR applications such as medical diagnosis of diseases, atmospheric pollution monitoring, and drug detection.

We experimentally demonstrate an efficient information transmission technique using Laguerre Gaussian (LG) modes. This technique is based on multiplexing and demultiplexing multiple LG modes with different azimuthal and radial components. At the reception, the initially sent modes encoding the information are extracted with high fidelity using a complete decomposition allowing to identify a particular mode from a set of modes within a unique iteration. Importantly, we investigate the effects of the atmospheric turbulence on the proposed communication system. We believe that the proposed technique is promising for high-bit-rate spatial division multiplexing in optical fiber and free space communication systems.

We report more than two octave spanning mid-IR flat-top supercontinuum (SC) generation using all normal $As2S5$-borosilicate hybrid photonic crystal fiber. Our design is based on a chalcogenide $As2S5$ photonic crystal fiber (PCF), where the first ring composed of six air holes is made by borosilicate glass. By injecting 50-fs pulses with 1.6 nJ energy at $2.5μm$ in the all normal dispersion (ANDi) regime, a flat-top broadband SC extending from 1 to $5μm$ with high-spectral flatness of 8 dB is obtained in only 4-mm fiber length. To the best of our knowledge, we present the broadest flat mid-IR spectrum generated in the ANDi regime of an optical fiber. The self-phase modulation and the optical wave breaking are identified as the main broadening mechanisms. The obtained broadband light source can be potentially used in the field of spectroscopy and in high-resolution optical coherent tomography owing to the high-spectral SC flatness generated by our designed fiber.

We propose a real, highly nonlinear, $As2Se3$-based chalcogenide photonic crystal fiber in which a supercontinuum (SC) spanning more than 2 octaves is generated at $λ=2.8μm$ in the femtosecond regime. The designed PCF is characterized for ultrabroadband mid-infrared SC generation in only few millimetres of fiber length. A full modal analysis of the optical properties of the fiber is presented in terms of the effective area, the nonlinearity coefficient, and the chromatic dispersion. A second-order Sellmeier approximation is proposed to estimate the variation of the refractive index of the $As2Se3$ material as a function of wavelength. The numerical study shows that a SC spanning from $1.9to4μm$ can be generated in the chalcogenide PCF with an air-hole diameter of $1.26μm$ and a pitch of $1.77μm$. We examine the interplay of the nonlinear effects that lead to the construction of the SC as a function of the input power and the fiber length. We find that the dynamics behind the SC generation is mainly ruled by the effects of self phase modulation and stimulated Raman scattering. The intrinsic properties of the chalcogenide glasses and the microstructure provide enhanced optical properties and offer numerous applications in the infrared field.