Interest in eye growth regulation has burgeoned with the rise in myopia prevalence world-wide. Eye length and eye shape are fundamental metrics for related research, but current in vivo measurement techniques are generally limited to the optical axis of the eye. We describe a high resolution, time domain low coherence interferometry based optometer for measuring the eye length of small animals over a wide field of view. The system is based upon a Michelson interferometer using a superluminescent diode as a source, including a sample arm and a reference arm. The sample arm is split into two paths by a polarisation beam splitter; one focuses the light on the cornea and the other focuses the light on the retina. This method has a high efficiency of detection for reflections from both surfaces. The reference arm contains a custom high speed linear motor with 25 mm stroke and equipped with a precision displacement encoder. Light reflected from the cornea and the retina is combined with the reference beam to generate low coherence interferograms. Two galvo scanners are employed to steer the light to different angles so that the eye length over a field of view of 20° × 20° can be measured. The system has an axial resolution of 6.8 μm (in air) and the motor provides accurate movement, allowing for precise and repeatable measurement of coherence peak positions. Example scans from a tree shrew are presented.
The images obtained from confocal imaging systems present less resolution than the theoretical limit due to imperfection
of the optical components and their arrangement. This imperfection deteriorates the wavefront and introduces aberrations
to the optical system. Adaptive optics (AO) systems composed of a wavefront sensor (WFS) and a deformable mirror
represent the most used solution to this problem. Such adaptive optics systems are expensive. In addition, in microscopy,
WFSs cannot be used due to stray reflections in the system and high aberrations introduced by the specimen. For these
reasons, sensor-less AO systems have been developed to control the deformable mirror (DM) using an optimization
algorithm in an iterative manner. At each iteration, the algorithm produces a new set of voltage and sends it to the mirror
so as to optimize its shape, in such a way, as to maximize the strength of the photodetector current in the imaging
system. In this paper the results of the application of three optimization techniques in the sensor-less AO are compared.
The three optimization techniques are simulated annealing (SA), genetic algorithm (GA) and particle swarm
optimization (PSO). SA and GA have been previously implemented and PSO is explained in this paper.
Two factors are of importance to optical coherence tomography (OCT), resolution and sensitivity. Adaptive optics
improves the resolution of a system by correcting for aberrations causing distortions in the wave-front. Balanced
detection has been used in time domain OCT systems by removing excess photon noise, however it has not been used in
Fourier domain systems, as the cameras used in the spectrometers saturated before excess photon noise becomes a
problem. Advances in camera technology mean that this is no longer the case and balanced detection can now be used to
improve the signal to noise ratio in a Fourier domain (FD) OCT system. An FD-OCT system, enhanced with adaptive
optics, is presented and is used to show the improvement that balanced detection can provide. The signal to noise ratios
of single camera detection and balanced detection are assessed and in-vivo retinal images are acquired to demonstrate
better image quality when using balance detection.
A detection method based on 7 spectral windows at the photodetection stage in an optical coherence tomography system was tested. We investigated its utility in two directions: spectroscopic optical coherence tomography and signal to noise ratio improvement. A diffraction grating was used in the photodetection unit to diffract light over a 16 photodetector array. Currently, this array has been configured to deliver 7 channels only by binning two adjacent photodetectors. The improvement of the signal to noise ratio has been investigated. Preliminary results of spectroscopy analysis which will be shown are B-scan OCT imaging of paint layers. The ultimate aim of the project is to perform spectroscopic analysis of the retina.