Tunable supercontinuum source for fluorescence spectroscopy

Abstract

This thesis describes the experimental investigations concerning the integration and optimisation of a supercontinuum source into a fluorescence lifetime spectrometer. [Fluorescence lifetime spectrometers based on Time Correlated Single Photon Counting (TCSPC) have long suffered from the lack of a compact, broadband excitation source. The source should ideally emit picosecond pulses with a repetition rate adjustable up to megahertz. Supercontinuum sources are an ideal candidate.] Various commercial supercontinuum sources were evaluated. Initial work was carried out on a source with a standard endlessly single-mode photonic crystal fibre (PCF) with a short wavelength limit of 450 nm, followed by later work with a source using high- PCF (a PCF with a large air filling fraction), that had an emission down to 400 nm, and a prototype source with a tapered fibre that had emission to ~320 nm. Key parameters including pulse width, pulse position and pulse height distribution were found to be very wavelength dependent, and their behaviour is explained by theory. The measured pulse widths of the supercontinuum sources were found to be typically ~100 ps, with longer durations found at the blue extreme of the spectra. Analysis of the data showed that this was due to the broadened pulses being a superposition of two pulse sequences with different dispersion characteristics. It was shown that, by taking account of the particular optical and temporal properties of a supercontinuum source, it was possible to make high quality fluorescence lifetime measurements of standard fluorophores such as fluorescein, anthrascene and erythrosine B. A novel device was constructed and evaluated for the wavelength separation of a supercontinuum source based on wedge interference filters. Initial prototypes of the device were able to measure the fluorescence emission spectra of common fluorophores and adequately separate a supercontinuum. Further iterations of the design, employing multiple filters, allowed for the construction of a device that included bandwidth control. The device allowed the transmission bandwidth to be tuned from ~6 nm to >50 nm with a transmission of >70 % for bandwidths >8 nm. The transmission figures achieved are better than any alternative form of wavelength separation, for example devices based on acousto-optic tunable filters (AOTFs) and diffraction gratings. A novel monochromator for fluorescence studies was also constructed using a wedge interference filter simultaneously with a diffraction grating. The design had improved performance compared to a single grating based monochromator in terms of stray light, with only a small drop in throughput and no change in instrument footprint. The stray light performance was found to be comparable to that of a double monochromator over the spectral range of the filter.