Measurement of the axial eye length by wavelength tuning interferometry

An alternative approach to measure intraocular optical distances in vivo is wavelength tuning interferometry. The principle of this technique is that every wavelength shift of a laser diode is associated with a phase shift of the waves reflected at the cornea and the retina. This results in a change of the interference order of the fringe system that will occur when the two wavefronts interfere. By Fourier transform of the intensity data, we cannot only measure the axial eye length but also other intraocular distances. In our approach we used an external cavity, single mode laser diode at a center wavelength of 780 nm which is tunable over 15 nm. First measurements were carried out in a model eye using the mode hop free tuning range of 9 nm, at the maximum tuning rate of 0.33 nm/s. The precision we obtained for the axial eye length was 0.04 mm. The large tuning range cannot be used for in vivo measurements because of the slow scanning rate of 0.33 nm/s. With this scanning rate the signal frequency corresponding to the axial eye length is on the order of 30 Hz. This is within the frequency range of the intensity modulations caused by fundus pulsations due to the heart beat and can therefore not be separated from them. The in vivo measurements have to be performed by tuning the wavelength of the laser with a piezoelectric transducer to achieve signal frequencies in the kHz range. Since the tuning range in this case is only 0.18 nm the resolution is about 50 times worse than that achieved in the model eye.