Supermode supercontinuum generation in the cladding of a photonic crystal fiber

The optical properties of the guided modes in the core of photonic crystal fibers (PCFs) can be easily manipulated by changing the air-hole structure in the cladding. A two-dimensional photonic crystal with a silicon dioxide central core and a hexagonal arrangement of air holes is an established method for achieving endless single-mode properties. In addition to guiding the light in the core, various cladding modes exist. The structural parameters of the fiber can also be used to achieve very defined guidance properties in the cladding. We investigated the fiber parameters in the core and cladding modes through both measurements and calculations. The calculated group velocity dispersion (GVD) of different cladding modes based on the measurement of the fiber structure parameters, the hole diameter and the pitch of a hexagonal arrangement of all holes of a real PCF. Based on the scanning electron image, a calculation of the optical guiding properties of the microstructured cladding was performed. The results of the calculations were compared with a wavelength-dependent time delay measurement method using a white light interferometer. The light of a picosecond fiber laser was selectively coupled into the core and cladding of the PCF and a comparison of the conversion properties of both microstructures was made. The supercontinuum based on cladding modes enabled the power limit based on the damage threshold of the core to be exceeded. In the visible range, significantly higher spectral power densities were seen in the cladding over those of the core generated spectral distributions. Subsequently, we investigated the beam quality of the generated supercontinua by measuring the beam propagation factor with a wavefront sensor.