Periodic Reporting for period 3 - OCTCHIP (Ophthalmic OCT on a Chip)
Reporting period: 2018-09-01 to 2021-03-31
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.
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.