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Ophthalmic OCT on a Chip

Periodic Reporting for period 3 - OCTCHIP (Ophthalmic OCT on a Chip)

Reporting period: 2018-09-01 to 2021-03-31

Retinal diseases like age-related macular degeneration (AMD), diabetic macula edema (DME) or glaucoma are worldwide leading causes for blindness. In ophthalmic diagnostics, direct visual inspection by means of slit lamp bio-microscopy and fundus photography are still considered the gold standard for morphological investigation of the eye, especially its posterior part, the retina. In recent years, these methods have been complemented by new optical imaging modalities including scanning laser ophthalmoscopy, (auto)fluorescent fundus imaging, and fundus angiography. However, all these techniques only provide superficial 2D morphological information, i.e. no tomographic depth information. In addition, they are invasive (contrast agent) or need pupil dilatation. Hence, there is a strong demand for noninvasive, high-resolution in-vivo imaging techniques that provide tomographic depth information in order to ensure early and reliable diagnosis of retinal diseases.

Optical coherence tomography (OCT) is an emerging non-invasive interferometric technique. Thanks to the use of coherence gating for axial feature resolution, OCT does not rely on the use of tightly focused beams and is able to deliver tomograms with good lateral (10-15 µm) as well as axial (1-10 µm) resolution in a depth range of 1-2mm. This enables non-invasive optical biopsy, i.e. the visualization of all clinically significant retinal layers without excision of tissue.

Recently there is a strong need for cost-effective OCT systems enabling point-of-care screening of retinal diseases. Current technology does not allow the required advancements of OCT for their widespread use. Silicon nitride waveguide based PIC technology and suitable packaging methods will enable realization of reliable low-cost and miniaturized ophthalmic OCT systems. Progress in the field of photonic methods and techniques for health and life sciences are expected to significantly contribute to solve main socio-economic challenges of our time. The required disruptive innovations will, to a large extent, be triggered by the emergence of new fabrication technologies. Among these, photonic integrated circuit (PIC) technologies will play a major role.
One of the most significant accomplishments of OCTCHIP so far include the simulation, design, and characterization of integrated optical waveguide test structures and photonic building blocks as well as the layout of the OCT engine and swept source photonics integrated circuits. Currently, there are three layouts of the OCT engine, which are in consideration: with a single sample port, with four sample ports, and finally with eight sample ports.

Specification and design of the transimpedance amplifiers, buffer, analog-to-digital converters and data transfer circuits have been developed and are on schedule. The first generation of electronic components was specified and designed and submitted for fabrication. Integration of electronic frontend as part of OCT PICs has started with architectural considerations, in order to perform an efficient design of multiple generations of PICs.

Performance testing of optoelectronic PIC has been started with a preparation phase by the development of a test platform for the first generation of electronic components. First tests with partial samples from the aC18 process showed that the evaluation process could be continued as planned, as soon as the complete aH18 samples will arrive.

Design and development of high-power broadband 840nm gain chips as well as of lower-cost electronics have successfully been conducted. Realization of high-power 840nm fiber-coupled swept sources and of high-power 840nm integrated swept sources showed promising first results.

For the fabrication of the OCT-chip, the highly-sensitive photo diodes, which are needed to detect the light which is coming from the eye or from the reference path, are integrated in the CMOS chip. The necessary implants and annealing steps, which are needed for the generation of the different doping areas are produced in parallel to the wells of the CMOS devices and before the fabrication of the CMOS gates. The wave-guides on the other hand are produced in a post-processing fabrication after the CMOS chip is already finished. The chip is covered by a planarizing Si oxide layer, which also serves as under cladding layer. Then the Si nitride layer for the wave-guide is deposited. This layer is structured in order to generate the different wave-guide devices, covered by an overcladding layer and finally trenches are etched in order to enable an in-plane coupling with high efficiency.

A fully functional photonic package for the OCTchip PIC has been accomplished. As is the case with all photonic packaging activities, this work package has a strong relationship and interdependency with other work packages to ensure the integrated photonic chip and associated electronic and photonic components can be assembled in a compact and cost effective package.
To date, OCTCHip has focused on solving the design challenges associated with packaging the 1 and 4 Channel OCTchip PICs, which will be used in the first system level integration tests within the project. Attention has been particularly focused on the optical and electrical interconnects and how they are formed within the package.

The main focus so far was the opto-mechanical integration of the PIC based OCT engine into an ophthalmic diagnostic system. The ophthalmic diagnostic system is based on the Zeiss Primus 200, which provides the patient interface. The beam path and mechanical concept for the sample arm is described, which of the PIC based OCT engine and the MEMS scanning unit and enables a parallel beam-scanning concept on the patient’s retina for higher frame rates. Furthermore the described reference beam path is based on a free space beam path that couples to a separate in- and outputs of the photonic integrated circuits.
The developed system electronics for data acquisition and system control provides a data transfer capability in the Gbit/s range, which is required by the envisioned parallel OCT data acquisition approach.
The ophthalmic diagnostic market is very sensitive to system costs and footprint because it has a strong private segment. During recent years, the OCT specialists of the OCTCHIP consortium have studied various stakeholder perspectives in numerous discussions with ophthalmic clinical experts and healthcare specialists in order to fully understand the boundary conditions for introducing OCT to the point-of-care diagnostic market. In a direct return-on-investment analysis they concluded that for a widespread adoption in ophthalmic point-of-care screening an OCT system in developed an emerging markets should have a target price in the range of 15-25k€, > 3x cheaper than momentary OCT systems. Further prerequisites are a small footprint, ease-of-use, and high reliability making the system virtually maintenance-free - maintaining necessary diagnostic imaging performance (>4x faster).
Envisaged ophthalmic OCT on a chip (middle) proportional to 1 Euro Cent coin; commercial OCT system