Adolf Friedrich Fercher

Flow velocity measurements by frequency domain short coherence interferometry

A method to measure the longitudinal flow velocity component based on phase resolved frequency domain optical coherence tomography (FDOCT) is introduced. At a center wavelength of 800nm the accessible velocity components ranges from 2 micrometers /s up to 2 mm/s. The upper limit is set by half the maximum frame rate of the CCD detector array. The lower limit is determined by the minimum resolvable phase change in the system, which is set by the system phase noise of 1 deg. First tests of the method include the velocity measurement of a mirror mounted on an oscillating piezo translator, and the flow of 8 micrometers latex spheres dispersed in water through a glass capillary.

Fourier domain OCT imaging of the human eye in vivo

An improved Fourier domain Optical Coherence Tomography (FdOCT) technique is proposed as a new kind of ophthalmic OCT, which enables non-invasive imaging of the retina, the iris and the lens in vivo without an axial mechanical scan of the reference mirror. The FdOCT tomograms of various parts of human eye in vivo, to our knowledge, are the first obtained to date. The detailed images of the human eye are reconstructed from spectral data by the differential method. The tomograms are free of the parasitic autocorrelation terms.

In-vivo dual-beam optical coherence tomography

Laser interferometry can be used to synthesize in vivo optical tomograms of the ocular tissue noninvasively. As a first example an in vivo partial coherence tomogram obtained from the optical nerve head of a human being is presented. The inherent high precision and sensitivity can make this technique into a very powerful diagnostic tool in ophthalmology.

Ocular partial-coherence interferometry

In the past interferometry has been applied in three main fields. Fringe interferometry has been used for the topographic measurement of surfaces. Interferometric length measurement techniques have been used for the measurement of distances, refractive indices, and wavelengths. In addition interferometric techniques have also been used in synthetic aperture imaging. Here we discuss various interferometric techniques which can be used in ophthalmology, describe the problems involved and present the results obtained so far.

Visible light optical coherence tomography

We demonstrate for the first time optical coherence tomography (OCT) in the visible wavelength range with unprecedented sub-micrometer axial resolution, achieved by employing a photonic crystal fiber in combination with a sub-15fs Ti:sapphire laser (FEMTOLASERS). The shaped emission spectrum produced by the photonic crystal fiber ranges from 535 nm to 700 nm (centered at ~600 nm) resulting in ~0.9 micrometers axial OCT resolution in air corresponding to ~0.6 micrometers in biological tissue. Preliminary demonstration of the sub-micrometer resolution achieved with this visible light OCT setup is demonstrated on a 2.2 micrometers thick nitrocellulose membrane. The visible wavelength range not only enables extremely high axial resolution for OCT imaging, but also offers an attractive region for spectroscopic OCT.

Ocular partial-coherence tomography

Partial coherence tomography (or optical coherence tomography) uses signals obtained with partial interferometry techniques to synthesize tomographic images. Being still under evolution this technique has already been successfully applied in the clinic. We discuss the underlying principles and present some results obtained at the fundus of the eye.

Measurement of the anterior structures of the human eye by partial coherence interferometry

In the past few years a new technique for measuring intraocular distances has been developed, which is based on interferometry using partially coherent light beams and the Doppler principle. It has been shown that this technique is capable of measuring corneal thickness, axial eye length, and retinal thickness in human eyes in vivo with unprecedented precision. This technique has now been further extended to measure the anterior chamber depth and the thickness of the lens. A precision of 10 micrometers is obtained for both intraocular distances. This is more than one order of magnitude better than with conventional techniques.

Scanning laser interferometer for fundus profile measurement of the human eye

A special interferometric technique, which uses light of low coherence length and the Doppler principle, was developed to measure intraocular distances along the vision axis of the human eye in vivo. This laser Doppler interferometry (LDI) technique has been improved to measure the fundus profile and to obtain tomographic images of the human eye fundus, especially in the area of the optic nerve head. A horizontal fundus profile of a human eye between 25 degree(s) nasal and 20 degree(s) temporal consisting of 71 measurement points was recorded in vivo. Furthermore, a vertical scan across the optic disc of the same eye at 13 degree(s) nasal, and from 5 degree(s) superior to 5 degree(s) interior, was carried out in steps of 0.5 degree(s). High accuracy is achieved.

In vivo optical coherence tomography and topography of the fundus of the human eye

The dual beam scanning partial coherence interferometry technique, which was recently developed to measure the fundus profile of the human eye in vivo, is used for tomographic and topographic imaging and for the measurement of different retinal structures, e.g., the contour of the papilla and thickness profiles of retina and retinal nerve fiber layer. In case of well defined, sharp boundaries between two layers of different refractive indices, the absolute position of these retinal structures can be determined with a precision (standard deviation) of 5 micrometers . Compared to first results with this instrument, the measurement time for horizontal and vertical scans was reduced by a factor of 10. At present, the measuring time of a vertical scan over 10 degree(s) in steps of 0.5 degree(s) takes at least 20 seconds without and 80 seconds with signal averaging of 4 longitudinal scans over 1.5 mm distance. Further improvements to 5 - 20 seconds are discussed. The longitudinal resolution of this instrument is approximately 15 micrometers (full-width-half-maximum), depending on the coherence length of the light source; this is about 5 times better than previous measurements. Topographic and tomographic scans across the optic disk reveal an increase of thickness of the retinal nerve fiber layer at the inferior and superior rim of the disk, which are in good agreement with results published in the literature.

High resolution spectroscopic optical coherence tomography in the 900-1100 nm wavelength range

We demonstrate for the first time optical coherence tomography (OCT) in the 900-1100 nm wavelength range. A photonic crystal fiber (PCF) in combination with a sub-15fs Ti:sapphire laser is used to produce an emission spectrum with an optical bandwidth of 35 nm centered at ~1070 nm. Coupling the light from the PCF based source to an optimized free space OCT system results in ~15 micrometers axial resolution in air, corresponding to ~10 micrometers in biological tissue. The near infrared wavelength range around 1100 nm is very attractive for high resolution ophthalmologic OCT imaging of the anterior and posterior eye segment with enhanced penetration. The emission spectrum of the PCF based light source can also be reshaped and tuned to cover the wavelength region around 950-970 nm, where water absorption has a local peak. Therefore, the OCT system described in this paper can also be used for spatially resolved water absorption measurements in non-transparent biological tissue. A preliminary qualitative spectroscopic Oct measurement in D2O and H2 O phantoms is described in this paper.