Samenvatting

Single Fiber Reflectance spectroscopy (SFR) can be used to determine tissue optical properties such as scattering and absorption. The SFR technique employs a semi-empirical modal, which contains a parameter that is dependent on the fiber numerical aperture, NA . However, the NA is a wavelength dependent quantity and also changes when the fiber is bent. In order to account for the error in SFR measurements caused by these changes, the NA will have to be characterized. This study will focus on investigating the NA dependence on wavelength and bending.
Firstly, a far field profile scan method was developed in order to determine the NA dependence on wavelength and bending. Various fiber output profiles where collected and the NA was defined as the angular value in which the profile intensity has dropped to 5% of the maximum profile intensity. Results show a disturbance of the profile scans of light that exits the fiber at angles that are larger than the 5% angle, which is caused by light that is coupled into the cladding at the fiber proximal end. This effect caused an increase in NA of a factor of two. This problem was accounted for by placing a pinhole at the fiber proximal end to prevent light from being coupled into the fiber. The effect of different pinholes at the proximal end of fibers of different diameters was investigated. Another problem where the low signals acquired when measuring at the distances defined to be in the far field region. This motivated the investigation of the variation in NA when measuring at distances that are smaller than the far field distance. These measurements where compared with a measurement performed on a 200 um fiber placed in the far field region. All results show a NA varying from 0.23 to 0.22 in a spectral range of 400-900 nm with an uncertainty of 0.02. These results all correspond to the NA of 0.22 with 0.02 inaccuracy specified by the manufacturer.
Secondly, the NA was also determined by CCD camera measurements. These measurements also showed promising results. CCD images were also taken of the fiber profile without pinhole at the proximal end to illustrate the effects of cladding modes on the profile.
Lastly, bend measurements where performed on different fibers that showed a monotonically decreasing NA at large bend radii and a wavelength dependent oscillatory behavior of the NA at small bend radii. These effects where described qualitatively by means of published results.