Sune Dupont

Supercontinuum generation in ZBLAN fibers—detailed comparison between measurement and simulation

We present a detailed comparison between modeling and experiments on supercontinuum (SC) generation in a commercial ZBLAN step-index fiber. Special emphasis is put on identifying accurate material parameters by incorporating measurements of the ZBLAN Raman gain, fiber dispersion, and loss. This identification of accurate parameters is of great importance to substantiate numerical simulations of SC generation in soft-glass fibers. Good agreement between measurement and simulation is obtained when pumping both in the normal and anomalous dispersion regimes.

IR microscopy utilizing intense supercontinuum light source

Combining the molecular specificity of the infrared spectral region with high resolution microscopy has been pursued by researchers for decades. Here we demonstrate infrared supercontinuum radiated from an optical fiber as a promising new light source for infrared microspectroscopy. The supercontinuum light source has a high brightness and spans the infrared region from 1400 nm to 4000 nm. This combination allows contact free high resolution hyper spectral infrared microscopy. The microscope is demonstrated by imaging an oil/water sample with 20 μm resolution.

Supercontinuum: broad as a lamp, bright as a laser, now in the mid-infrared

Based on the experience gained developing our market leading visible spectrum supercontinuum sources NKT Photonics has built the first mid-infrared supercontinuum source based on modelocked picosecond fiber lasers. The source is pumped by a ≈ 2 um laser based on a combination of erbium and thulium and use ZBLAN fibers to generate a 1.75-4.4 μm spectrum. We will present results obtained by applying the source for mid-infrared microscopy where absorption spectra can be used to identify the chemical nature of different parts of a sample. Subsequently, we discuss the possible application of a mid-IR supercontinuum source in other areas including infrared countermeasures.

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.

Stimulated Raman scattering in soft glass fluoride fibers

We have measured the absolute Raman gain spectrum in short fluoride soft glass fibers with a pump wavelength of 1650 nm. We found a peak gain of gR=4.0±2×10−14 m W−1.

Ultra-high repetition rate absorption spectroscopy with low noise supercontinuum radiation generated in an all-normal dispersion fibre

We report the use of a dispersed supercontinuum generated in an all-normal-dispersion fibre to record low-noise spectra from atmospheric molecules at least an order of magnitude faster than has been previously reported. Supercontinuum generation in standard, anomalous dispersion photonic-crystal fibres is inherently connected with large pulse-to-pulse fluctuations resulting in detrimental consequences for high resolution spectroscopy if temporal averaging is not permitted. Replacing the standard photonic-crystal fibre (PCF) with a specially designed all-normal dispersion PCF we find that a substantially superior noise performance is achieved and present its use for high repetition rate absorption spectroscopy where spectra covering hundreds of nm in spectral bandwidth can be captured of gases at hundreds of kHz repetition rates.

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.