Lars Søgaard Rishøj

Generation of infrared supercontinuum radiation: spatial mode dispersion and higher-order mode propagation in ZBLAN step-index fibers

Using femtosecond upconversion we investigate the time and wavelength structure of infrared supercontinuum generation. It is shown that radiation is scattered into higher order spatial modes (HOMs) when generating a supercontinuum using fibers that are not single-moded, such as a step-index ZBLAN fiber. As a consequence of intermodal scattering and the difference in group velocity for the modes, the supercontinuum splits up spatially and temporally. Experimental results indicate that a significant part of the radiation propagates in HOMs. Conventional simulations of super-continuum generation do not include scattering into HOMs, and including this provides an extra degree of freedom for tailoring supercontinuum sources.

Dynamic characterization and amplification of sub-picosecond pulses in fiber optical parametric chirped pulse amplifiers

We show a first-time demonstration of amplification of 400 fs pulses in a fiber optical parametric amplifier. The 400 fs signal is stretched in time, amplified by 26 dB and compressed back to 500 fs. A significant broadening of the pulses is experimentally shown due to dispersion and limited gain bandwidth both in saturated and unsaturated gain regimes.

Break up of the azimuthal symmetry of higher order fiber modes

We investigate Bessel-like modes guided in a double cladding fiber where the outer cladding is an aircladding. For very high order LP(0X) -modes, the azimuthal symmetry is broken and the mode is no longer linearly polarized. This is observed experimentally and confirmed numerically. The effect is investigated numerically using a full vectorial modesolver and is observed to be dependent on the fiber design. The effect on the diffraction free propagation distance of the modes is investigated using a fast Fourier transform propagation routine and compared to the properties of an ideal circularly symmetric mode. The free space properties of modes suffering from break up of azimuthal symmetry are also investigated experimentally by measuring the free space propagation of a LP(016)-mode excited in the double cladding fiber.

Experimental demonstration of intermodal nonlinear effects between full vectorial modes in a few moded fiber

We experimentally investigate intermodal nonlinear interactions, such as Raman scattering and four wave mixing. The fiber used is a specially designed few moded fiber, which splits the degeneracy of the first mode group, leading to stable propagation of the two full vectorial modes, TM01 and TE01. For the Raman experiments pumping occur in either the fundamental mode or the two full vectorial modes, whereas the signal is in the fundamental mode. In all three experiments approximately 40 dB of gain is achieved using 307 W of pump peak power. When pumping in either of the full vectorial modes four wave mixing is observed.

Slow-light enhanced optical detection in liquid-infiltrated photonic crystals

Slow-light enhanced optical detection in liquid-infiltrated photonic crystals is theoretically studied. Using a scattering-matrix approach and the Wigner–Smith delay time concept, we show that optical absorbance benefits both from slow-light phenomena as well as a high filling factor of the energy residing in the liquid. Utilizing strongly dispersive photonic crystal structures, we numerically demonstrate how liquid-infiltrated photonic crystals facilitate enhanced light–matter interactions, by potentially up to an order of magnitude. The proposed concept provides strong opportunities for improving existing miniaturized absorbance cells for optical detection in lab-on-a-chip systems.

All-fiber Raman Probe using Higher Order Modes

We demonstrate the first all-fiber Raman probe utilizing higher order modes for the excitation. The spectrum of cyclohexane is measured using both the fundamental mode as well as in-fiber-generated Bessel-like modes.

Dynamic Characterization of Fiber Optical Chirped Pulse Amplification for Sub-ps Pulses

We investigate experimentally the propagation of sub-picosecond pulses in fiber optical parametric chirped pulse amplifiers, showing a significant broadening of the pulses from 450 fs up to 720 fs due to dispersion and self-phase modulation.

Intermodal Nonlinear Effects between Full Vectorial Modes in Few Moded Fiber

We experimentally investigate intermodal nonlinear mixing, such as Raman and four wave mixing. This is obtained by pumping in the fundamental mode, or either of the two full vectorial modes, TM01 and TE01 in a specialty designed few moded fiber.