## 2 Current CORNERSTONE Platforms
### CONERSTONE Partner Highlights
#### 2.4.1 CONERSTONE Partner: Tyndall National Institute, Ireland--High Efficiency Optical Packaging
4.2 CONNERSTONE Partner: Phutung Research Institute, Nepal and University of Manchester, UK--Apodized Grating Couplers for Optical Coherence Tomography  
Medical imaging technologies such as optical coherence tomography (OCT) have a potentially significant role to play in the early diagnosis and progress monitoring of lung disorders such as chronic obstructive pulmonary disease (COPD). Globally there are an estimated 235 million people suffering from COPD at any time, with over 3 million related deaths annually and more than 90% of these in low to middle income countries [75]. The incurable nature of COPD and its increasing latter stage severity means that early diagnosis is vital in reducing the impact of the disease. One way to achieve early diagnosis in suspected COPD cases is to image the wall tissues of the respiratory tract and deep bronchi. Techniques such as OCT offer a relatively inexpensive, alternative imaging modality to X-ray computed tomography (CT) or magnetic resonance imaging (MRI), where access to such facilities may  
Figure 10: Insertion loss in dB for the apodized focusing grating couplers: (**a**) 220 nm platform optimized for the H-Fb scheme, (**b**) 340 nm platform optimized for the H-Fb scheme, and (**c**) 340 nm platform optimized for the V-Fb scheme. be limited. However, although OCT can provide the high axial resolution needed for such imaging, current systems are primarily benchtop, incorporating bulky external light sources and fiberoptic based interferometry. Translation of these bulky components to PICs offers the potential to realize a miniaturized, fully integrated OCT system on-a-chip, which provides both further cost benefits and reducing the overall system size to the millimeter range. One of the key requirements of such a PIC based OCT system is the ability to provide a focused spot at the biological sample plane, preferably without adding unnecessary complexity, i.e., by integrating focusing elements entirely within the PIC itself. Recent advances in fiber-based OCT systems have shown that the inclusion of nanophotonic meta-lenses can help suppress optical aberrations, although it is difficult to seamlessly integrate these with PIC platforms [76]. We have recently designed a fully integrated PIC based focusing element [77] that is similar to the circular grating coupler presented in [78]. Our design mitigates the need for completely circular gratings, rather the circumference of the grating structures we employ is 'broken up' into a combination of small arcs, which we refer to as \"apodization\" of the gratings, azimuthally. This essentially allows the manipulation of phase profiles into the structure as required, and more importantly facilitates a design that allows light to penetrate deeper into the circular grating structure (in a regular continuous grating structure the intensity of light decays exponentially from the entry point). This is critical because, for any integrated focusing component, a means for maximizing the intensity of light reaching the center of the structure, analogous to a conventional optical lens, is preferred. To achieve this, we apodize the gratings such that the arc lengths of the 'feed waveguides' increase exponentially from the outer portion of the circular grating structure. The 'feed waveguides' and apodized circular grating structures were realized in a single CORNERSTONE fabrication run, from a standard 220 nm SOI starting wafer with a 2 um buried oxide (BOX) layer. The design incorporates rib structures for our 'feed waveguides', etched to 120 \(\pm\) 10 nm depth for efficient delivery of the light to the circular gratings, which themselves were etched 70 \(\pm\) 10 nm. The length of the arc segments was limited only by the design rules of the particular fabrication run, which dictated a 200 nm minimum in the grating layer with a 250 nm minimum gap.