QDev Seminar: Philip Dalsbecker – University of Copenhagen

QDev Seminar: Philip Dalsbecker

Lund University

Development of narrow-bandwidth filters for the suppression of scattered light for optical and ultrasound analysis of tissue

Ultrasound-modulated optical tomography (UOT) is a proposed diagnostic technique with potential as a means of discovering cancerous tumours. The technique combines ultrasound and laser radiation to probe tissue in a non-invasive, harmless way. An important aspect in UOT is the ability to distinguish the probing light from background scattered light generated by the technique, which limits its resolution and usefulness. In this thesis work, spectrally hole-burned Pr3+:Y2SiO5 crystals are used as spectral filters to block out the background light. Thus, the resolution depends on the filters, which this thesis work has aimed to improve.
As part of this work, simulations and experiments were performed to increase the ratio between transmission inside the spectral hole and attenuation outside it for such hole-burned crystals. This was done both by optimizing the pulse shape of the hole-burning pulses and by burning and probing the structure with different polarizations of light. As light polarized along the different axes of the biaxial crystal have different absorption coefficients, the response to the burning pulses varies with polarization.
The simulations showed that burning the hole with a weakly absorbed polarization and probing it with a strongly absorbed one produces sharper and more uniform spectral holes than burning and probing with the same polarization does. A set of optimal pulse parameters were also found. In the experiments, an attenuation of approximately 53 dB was found when comparing filtered light at the centre frequency of the spectral hole to filtered light with a 2 MHz higher frequency. The results corresponded to an absorption coefficient outside the spectral hole of approximately 20 cm-1 for both polarizations tested, falling short of the given literature value of 47 cm-1. A number of potential reasons are given, as well as several suggestions for future improvements. Still, the attenuation achieved improved upon previous results as presented by Zhang et al.1 (2012), which was the main objective of the thesis.