Jacob Ramsay

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.

Up-conversion of a megahertz mid-IR supercontinuum

We present an cross-correlation frequency-resolved optical gating (XFROG) measurement of a megahertz IR supercontinuum generated in a step-index ZBLAN fiber. The resulting spectrogram gives the dispersion characteristics of the fiber and reveals that it has three zero-dispersion wavelengths. A comparison of the measured spectrogram with numerical simulations shows that this dispersion profile allows a notable dispersive-wave generation toward long wavelengths. Furthermore, the sum-frequency generation process in the XFROG measurement gives the possibility of measuring the IR light with fast Si-based detectors, such as CCD arrays.

Pulse-to-pulse noise reduction in infrared supercontinuum spectroscopy: polarization and amplitude fluctuations

Infrared supercontinuum sources suffer from amplitude and polarization fluctuations. These fluctuations are seeded by stochastic noise, and thus limits the applicability of such sources. Here it is shown that implementation of polarization insensitive pulse-to-pulse normalization enhances the signal-to-noise ratio up to 18 times compared to conventional 45° beam splitting. This serves as a promising approach to achieve highly sensitive supercontinuum spectroscopy.

Near-Infrared Spectroscopy Using a Supercontinuum Laser: Application to Long Wavelength Transmission Spectra of Barley Endosperm and Oil

The supercontinuum laser is a new type of light source, which combines the collimation and intensity of a laser with the broad spectral region of a lamp. Using such a source therefore makes it possible to focus the light onto small sample areas without losing intensity and thus facilitate either rapid or high-intensity measurements. Single seed transmission analysis in the long wavelength (LW) near-infrared (NIR) region is one area that might benefit from a brighter light source such as the supercontinuum laser. This study is aimed at building an experimental spectrometer consisting of a supercontinuum laser source and a dispersive monochromator in order to investigate its capability to measure the barley endosperm using transmission experiments in the LW NIR region. So far, barley and wheat seeds have only been studied using NIR transmission in the short wavelength region up to 1100 nm. However, the region in the range of 2260–2380 nm has previously shown to be particularly useful in differentiating barley phenotypes using NIR spectroscopy in reflectance mode. In the present study, 350 seeds (consisting of 70 seeds from each of five barley genotypes) in 1 mm slices were measured by NIR transmission in the range of 2235–2381 nm and oils from the same five barley genotypes were measured in a cuvette with a 1 mm path length in the range of 2003–2497 nm. The spectra of the barley seeds could be classified according to genotypes by principal component analysis; and spectral covariances with reference analysis of moisture, β-glucan, starch, protein and lipid were established. The spectral variations of the barley oils were compared to the fatty acid compositions as measured using gas chromotography–mass spectrometry (GC-MS).

Mid-infrared supercontinuum generation spanning more than 11 μm in a chalcogenide step-index fiber

Supercontinuum generation covering an ultra-broad spectrum from 1.5-11.7μm and 1.4-13.3μm is experimentally demonstrated by pumping an 85mm chalcogenide step-index fiber with 100fs pulses at a wavelength of 4.5μm and 6.3μm, respectively.