Reflective afocal broadband adaptive optics scanning ophthalmoscope

Abstract

A broadband adaptive optics scanning ophthalmoscope (BAOSO) consisting of four afocal telescopes, formed by pairs of off-axis spherical mirrors in a non-planar arrangement, is presented. The non-planar folding of the telescopes is used to simultaneously reduce pupil and image plane astigmatism. The former improves the adaptive optics performance by reducing the root-mean-square (RMS) of the wavefront and the beam wandering due to optical scanning. The latter provides diffraction limited performance over a 3 diopter (D) vergence range. This vergence range allows for the use of any broadband light source(s) in the 450-850 nm wavelength range to simultaneously image any combination of retinal layers. Imaging modalities that could benefit from such a large vergence range are optical coherence tomography (OCT), multi- and hyper-spectral imaging, single- and multi-photon fluorescence. The benefits of the non-planar telescopes in the BAOSO are illustrated by resolving the human foveal photoreceptor mosaic in reflectance using two different superluminescent diodes with 680 and 796 nm peak wavelengths, reaching the eye with a vergence of 0.76 D relative to each other.

Keywords: (080.4035) Mirror system design; (110.1080) Active or adaptive optics; (170.4460) Ophthalmic optics and devices; (170.4470) Ophthalmology.

Fig. 1

Spot diagrams corresponding to the image plane of a 1:1 off-axis reflective afocal telescope that consists of two spherical mirrors with 800 mm radii of curvature in a planar configuration (left) and orthogonal configuration (right). The black circles correspond to the first minimum of the Airy disk.

Fig. 2

Spot diagrams corresponding to the pupil plane of a 1:1 off-axis reflective telescope that consists of two spherical mirrors in a planar configuration (left) and orthogonal configuration (right). The black circles correspond to the first minimum of the Airy disk.

Fig. 3

Afocal 1:1 telescope in which f denotes the focal length of the lenses. The three beams entering the telescope with negative, null and positive vergences are indicated with dotted, solid and dashed lines respectively.

Fig. 4

Retinal depth dependence with the vergence of the beam entering the eye (left), according to two simple model eyes. The plot on the right shows two longitudinal chromatic aberration models for the human eye.

Fig. 5

Broadband adaptive optics scanning ophthalmoscope setup flattened for clarity. PMT stands for photomultiplier, TZ for transimpedance amplifier, LD for laser diode, SLD for superluminescent diode, SH-WS for Shack-Hartmann wavefront sensor, sph for spherical mirror and F for interferometric band pass filter. The letter P indicates the pupil conjugate planes, in addition to the ones corresponding to the deformable mirror, the optical scanners and the SH-WS.

Fig. 6

Spot diagram for all 27 BAOSO configurations evaluated, grouped according to the vergence. Note that all configurations are diffraction limited for 450 nm wavelength over a 1.5° FOV. The radius of the Airy disk indicated by the black circles is 1.3 μm.

Fig. 7

Spot diagrams for all 4 pupil planes of the BAOSO for 450 nm wavelength over a 1.5° FOV, assuming a point source at the pupil plane in front of the Shack-Hartman wavefront sensor telescope. The black circles represent the Airy disk.

Fig. 8

Human photoreceptor mosaic at the foveal center, recorded with the BAOSO. The top images were recorded with a 796 nm SLD and a 0.6 Airy disk confocal pinhole size, while the bottom ones were obtained using a 680 nm SLD and a 0.4 Airy disk confocal pinhole. The images on the left display the image intensity using a linear gray scale mapping, while the images on the right are displayed using a logarithmic gray scale transformation. The scale bars are 20 μm across.

Fig. 9

Enlarged version of the photoreceptor mosaic shown in Fig. 8, showing the smallest foveal cones. The scale bars are 10 μm across.

Fig. 10

Cross section of an Airy disk and its logarithm, which is 80% wider.