Speed­of­Light Tomography with Ultrafast Lasers

Krikor B. Ozanyan1, Paul Wright1, Mark Stringer2, Bob Miles2

1School of Electrical and Electronic Engineering, The University of Manchester, Manchester M13 9PL, United Kingdom

2Institute of Microwaves and Photonics, University of Leeds Leeds LS2 9JT, United Kingdom

ABSTRACT

Hard­field Tomography offers image contrast, conditioned by the nature of the measured path integrals across the subject. In this work we demonstrate imaging of the subject’s refractive index (real part), measured by the propagation delay of a femtosecond laser pulse. Thus, speed­of­light Tomography is the most direct refractive index imaging modality.

We first address the physics of the measured data, showing that pulse delay measurements yield line integrals of the refractive index and that contrast is achieved from the variation of the speed of light in the subject. Further we report experiments with extruded polystyrene foam phantoms of characteristic dimension ~10mm, with refractive index modulation (relative to air) of only 2%. Softening of the hard­ field propagation is suppressed by beam collimation over a base of ~0.5m. Thus only photons scattered strictly in forward direction contribute to the forward Radon transform, which is acquired from 12 angular projections with 30 line integrals in each projection. The reconstruction is performed on a 40x40 grid with standard hard­field filtered back­projection, applying non­negative constraints and periphery suppression. Some of the observed reconstruction artefacts are discussed in the light of the characteristics of the phantom and the radiation wavelength.

Particular attention is given to the appropriate processing of data in the case of THz biased­gap antenna conversion from 90fs@82MHz 800nm pulses delivered by a Ti:S laser (Spectra Physics MaiTai) . We show that for the correct measurement of the pulse delay from the transmitted complex time­domain waveform, it is necessary to account for the mechanism of THz generation by acceleration of free carriers in the gap antenna. Based on this, we propose a new, more robust rule for delay calculations.

Finally, we discuss the mapping of the refractive index image into other material parameters such as concentration and temperature, due to the dependence of the refractive index on these quantities. The corresponding sensitivity limits and their implications in the general case of speed­of­light tomography in gaseous subjects is discussed.

Keywords Hard­Field Tomography, Terahertz Tomography, refractive index, ultrafast laser