Sapphire ball lens-based fiber probe for common-path optical coherence tomography and its applications in corneal and retinal imaging

Abstract

We describe a common-path swept source optical coherence tomography fiber probe design using a sapphire ball lens for cross-sectional imaging and sensing for retina vitrectomy surgery. The high refractive index (n=1.75) of the sapphire ball lens improves the focusing power and enables the probe to operate in the intraocular space. The highly precise spherical shape of the sapphire lens also reduces astigmatism and coma compared to fused nonspherical ball lenses. A theoretical sensitivity model for common-path optical coherence tomography (CP-OCT) was developed to assess its optimal performance based on an unbalanced photodetector configuration. Two probe designs-with working distances 415 and 1221 μm and lateral resolution 11 and 18 μm-were implemented with sensitivity up to 88 dB, which is significantly higher than previously reported CP-OCT probes. We assessed the performances of the fiber probes by cross-sectional imaging a bovine cornea and retina in air and in vitreous gel with a 1310 nm swept source OCT system. To the best of our knowledge, this is the first demonstration of sapphire ball lens-based CP-OCT probes directly inserted into the vitreous gel of a bovine eyeball for ocular imaging with a sensitivity approaching the theoretical limitation of CP-OCT.

Fig. 1

(Color online) Schematic of CP-SSOCT compatible with a dual-arm configuration. A, CP-SSOCT with a lensed probe. The fiber tip functions as a reference plane and there is air gap or UV epoxy between fiber tip and ball lens as spacer. B, The goal of lensed probe is for forward viewing in retina vitrectomy surgery and the probe is attached to the minute surgical forceps.

Fig. 2

(Color online) Theoretical sensitivity and experimental results. A, Theoretical sensitivity of CP-SSOCT probe is in black with backward optical coupling efficiency of 40%; experimental results are in green; CP-SSOCT with backward coupling efficiency of 86.5% (1/e² width) is in cyan; traditional dual balanced dual-arm bulky imaging head SSOCT with backward coupling efficiency of 86.5% is in blue.

Fig. 3

Cow’s cornea and retina images (H 2.75 mm by V 2.1 mm) A, Cornea image. We can clearly identify the following five layers: epithelium (1), Bowman’s layer (2), stroma (3), Descemet’s membrane, (4), and endothelium (5). B, Corneal image far away from the apex. C is the OCT cross-sectional image of optical nerve head. D is away from the optical nerve head and shows clear layer structures of retina. E and F were captured when the probe was immersed in vitreous gel with a length of 32 mm. The probe can still work properly without being damaged when the ball lens contacted the right corner part of retina in Fig. 3E on purpose. All images in Fig. 3 did not have frame average.