The use of optical coherence tomography (OCT) [1
] has revolutionized treatment and monitoring of retinal diseases in everyday clinical settings. Its superb axial resolution, independent from lateral resolution, allows precise in vivo
visualization and characterization of all the main cellular layers in the human retina. Unfortunately, similarly to other imaging techniques, lateral resolution and, therefore quality of the OCT imaging is reduced by imperfections in the eye's optics. This effect is more evident if one increases the size of the imaging aperture to be greater than 2 mm at the eye's pupil [9
]. Improvements in lateral resolution have been demonstrated by incorporation of wavefront correctors in various retinal imaging systems starting with a flood illuminated ophthalmoscope [10
] followed by scanning laser ophthalmoscopes [11
], and recently into many variations of OCT systems [12
]. A short overview of different configurations and corrector types implemented in those AO-OCT systems has recently been published by Pircher and Zawadzki [22
]. While each of these AO-OCT cameras reduces the degrading impact of the ocular aberrations, full compensation of both low- and high-order aberrations of the eye has not been achieved. This becomes a limiting factor when AO-OCT systems are used in clinical settings, where most patients are known to have moderate amounts of refractive error. Standard methods for refractive error correction, such as placement of trial lenses or a trombone in front of the eye cannot be easily implemented without affecting OCT detection. As a possible solution to this problem, we recently reported [19
] a novel “trial lens free” AO design that cascades two DMs for the purpose of extending the correction range (amplitude) and capabilities, compared to the systems using a single wavefront corrector. The performance of our AO subsystem has been evaluated by measuring the quality of the wavefront correction as well as characterizing the corresponding OCT images. This two-DM AO system was integrated into the ultrahigh-resolution (UHR) OCT system (AO-UHR-OCT) presented in this paper.
In UHR-OCT [23
], a ~3 μm axial resolution results from the use of an ultra broadband source. The signal-to-noise penalty that occurs due to the wider optical bandwidth in time-domain OCT is waived when an ultra broadband source is used in a Fourier-domain OCT (Fd-OCT) configuration [24
]. UHR Fd-OCT images with an axial resolution of ~3 μm were first demonstrated in 2004 [26
]. First attempts to combine UHR-OCT with AO have been reported [13
], however published retinal images did not demonstrate a clear improvement compared to standard AO-OCT. In this paper we describe the next generation of our AO-OCT instrumentation where a broader spectral bandwidth light source combined with a custom designed achromatizing lens is used to increase measured axial resolution to ~3.5 μm, without reduction in the lateral resolution and image quality. Results obtained with this improved AO-UHR-OCT system are provided and some performance measures are compared to our previous AO-OCT instrument that had a measured axial resolution of ~6.5 μm. Additionally, benefits of speckle size reduction due to increased light source spectral bandwidths and a method to further decrease speckle contrast [28
] are presented and tested.