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. 2021 Jan 5;10(1):6.
doi: 10.1038/s41377-020-00450-0.

Toward optical coherence tomography on a chip: in vivo three-dimensional human retinal imaging using photonic integrated circuit-based arrayed waveguide gratings

Elisabet A Rank  1 , Ryan Sentosa  2 , Danielle J Harper  2 , Matthias Salas  2 , Anna Gaugutz  2 , Dana Seyringer  3 , Stefan Nevlacsil  4 , Alejandro Maese-Novo  4 , Moritz Eggeling  4 , Paul Muellner  4 , Rainer Hainberger  4 , Martin Sagmeister  5 , Jochen Kraft  5 , Rainer A Leitgeb  2 , Wolfgang Drexler  2
Affiliations

Toward optical coherence tomography on a chip: in vivo three-dimensional human retinal imaging using photonic integrated circuit-based arrayed waveguide gratings

Elisabet A Rank et al. Light Sci Appl. .

Abstract

In this work, we present a significant step toward in vivo ophthalmic optical coherence tomography and angiography on a photonic integrated chip. The diffraction gratings used in spectral-domain optical coherence tomography can be replaced by photonic integrated circuits comprising an arrayed waveguide grating. Two arrayed waveguide grating designs with 256 channels were tested, which enabled the first chip-based optical coherence tomography and angiography in vivo three-dimensional human retinal measurements. Design 1 supports a bandwidth of 22 nm, with which a sensitivity of up to 91 dB (830 µW) and an axial resolution of 10.7 µm was measured. Design 2 supports a bandwidth of 48 nm, with which a sensitivity of 90 dB (480 µW) and an axial resolution of 6.5 µm was measured. The silicon nitride-based integrated optical waveguides were fabricated with a fully CMOS-compatible process, which allows their monolithic co-integration on top of an optoelectronic silicon chip. As a benchmark for chip-based optical coherence tomography, tomograms generated by a commercially available clinical spectral-domain optical coherence tomography system were compared to those acquired with on-chip gratings. The similarities in the tomograms demonstrate the significant clinical potential for further integration of optical coherence tomography on a chip system.

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Conflict of interest statement

M.S. and J.K. are employed by AMS AG but were remunerated by the framework of project COHESION, No. 848588, funded by the Austrian Research Promotion Agency (FFG). To the best of our knowledge, the named authors have no conflicts of interest, financial or otherwise.

Figures

Fig. 1
Fig. 1. Principle structure of an arrayed waveguide grating.
Broad bandwidth light diverges laterally in the input star coupler toward the array of waveguides. There, each waveguide forwards a portion of the input light toward the output star coupler, resulting in different phase delays caused by the different optical path lengths of the individual waveguides. At the focal line on the image plane of the output star coupler, only plane waves with the same phase delay constructively interfere; therefore, each output waveguide forwards individual wavelengths
Fig. 2
Fig. 2. Characterization measurements of the two 256-channel AWGs.
Measured spectral characteristics of a AWG 1 and b AWG 2 for every eighth channel: the minimum, maximum, mean, and standard deviation of the peak powers are provided in the two figures. The thin black line is a polynomial second-order fit to the peaks. This fit shows the AWGs typical spectral envelope, which is different for the two designs. The deviation of the individual peaks from this envelope fit (peak power minus power of the envelope at the peak wavelength) is shown in the two figures below (red lines with blue crosses). The deviation of ~±0.5dB can be explained by the inaccuracy of the fiber alignment with respect to the chip. For the OCT measurements, where no fiber at the output was used, these variations are not present. Sensitivity roll-off measurements of c AWG 1 and d AWG 2 with the respective axial resolution measurements as insets: 14.5µm in air and 10.7µm in soft tissue (AWG 1) and 8.8µm in air and 6.5µm in soft tissue (AWG 2). e Scheme of the SD-OCT on-chip setup: a Superlum SLD fed broadband light to a fiber coupler, and 830μW (AWG 1, a booster amplifier and a 90/10 coupler were used) and 480μW (AWG 2, no booster amplifier and a 50/50 coupler were used) light on the eye interfered with the reference light and was coupled into the on-chip AWG. Projection optics were used to project the light from the PIC end facet onto a CCD camera. FC fiber coupler, PC polarization controller, L lens, C collimator, M mirror, AWG arrayed waveguide grating, AD achromatic doublet
Fig. 3
Fig. 3. B-scans of a healthy retina in the foveal region.
a Unaveraged and b five times averaged fovea acquired with AWG 1 at 67kHz. c Unaveraged and d five times averaged fovea acquired with AWG 1 at 34kHz. e Unaveraged and f five times averaged fovea acquired with AWG 2 at 20kHz. In areas perpendicular to the scanning beam, strong reflection induces visible side lobes. g 3D representation of the retina in the foveal region acquired with AWG 1 at 67kHz, and h corresponding OCTA image calculated from the volume using five B-scan repetitions. The black area on the right side of the angiogram corresponds to missing data due to motion correction in the lateral direction
Fig. 4
Fig. 4. In vivo measurements of a healthy retina:
in the region of the optic nerve head imaged with a–c AWG 1 at 67kHz, d–f AWG 1 at 34kHz, and g–i AWG 2 at 20kHz. All data are an average of three registered B-scans
Fig. 5
Fig. 5. OCT on a PIC system in comparison to a commercial OCT device.
Direct comparison of the tomograms acquired with the SD-OCT on a PIC system with tomograms of the same eye acquired with a Zeiss Cirrus 4000. a, d Acquired with AWG 1 at 34kHz; b, e acquired with the Zeiss Cirrus 4000; c, f acquired with AWG 2 at 20kHz. The reduced imaging depth with AWG 2 can especially be observed, as the optic disc cup has poor contrast and an aliasing effect occurs (as indicated with the green arrow) in (f). The green arrows in d and e indicate the boundary of the vitreous
Fig. 6
Fig. 6. Signal roll-off with depth compensation for AWG 2.
a Average of 100 registered B-scans; b average of three registered B-scans, where the retina was aligned so that the sclera was close to the zero delays. The green arrows in the tomograms indicate the choroid/sclera junction
Fig. 7
Fig. 7. Intra-wafer variation of the AWG 1 characteristics.
a A schematic of the wafer. Five samples at five positions (highlighted in green) were measured. The numbers in the brackets are the x and y coordinates on the wafer (x,y) starting with (0,0). The number below is the difference between the central channel wavelength and the mean wavelength of the five measured AWGs across the wafer. Dark gray boxes indicate useful AWGs on the circular wafer. In b, all five center channels are plotted. c Summarizes the central wavelength for the individual AWGs as well as the deviation from the mean wavelength of the five center channels. d Summarizes the mean value and standard deviation of the center, lowest and highest channels across the five AWGs
Fig. 8
Fig. 8. Spectra of the used light sources.
a Spectra of the Superlum SLD 1 (red, ~780–~830nm), Superlum SLD 3 (red, ~850–~900nm), and booster amplifier (black). The boosted spectrum of SLD 1 is plotted in green; the boosted spectrum of SLD 3 is plotted in blue; due to the insufficient wavelength support, the booster amplifier was not used for the AWG 2 setup, and SLD 3 without the booster amplifier was used instead. b Interference pattern of the AWG 1 setup; the envelope represents a rather flat and slightly modulated envelope, as expected from the green spectral shape in (a). c Interference pattern of the AWG 2 setup: the envelope represents the spectral shape of SLD 3 in (a) (red, ~850nm–~900nm)
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