## 2 Current CORNERSTONE Platforms
### CONERSTONE Partner Highlights
#### 2.4.1 CONERSTONE Partner: Tyndall National Institute, Ireland--High Efficiency Optical Packaging
The use of DUV lithography for fabrication ensures the PIC based OCT system with fully integrated focusing element can be scaled at acceptably low cost (one of the drivers for wide scale adoption and application, where access to such imaging facilities is severely restricted), due to its compatibility with the CMOS process. Access to the CORNERSTONE program further assisted with low-cost derisking of our prototype designs. In characterizing the fabricated devices, we used a swept laser source (Santec HSL-20) with center wavelength 1300 nm, bandwidth 100 nm and measured average power output of 13 dBm. Initially, we used the swept laser as a broadband source, coupling the light into the guided mode of the 'feed waveguides' with a standard grating coupler design, which was provided in the CORNERSTONE PDK. The integrated output power from our integrated, apodized focusing device was measured, via a standard graded-index (GRIN) lens (Thorlabs GRIN2913, NA 0.46) mounted on ferruled couplers using a power meter (PM100D) via InGaAs detector (Thorlabs S154C). We determined the spatially resolved emitted power from the device in 2D arrays (laterally and vertically) by moving the GRIN lens point-by-point with a motorized xyz stage. Prior to fabrication we modeled the device designs with a full 2D-FDTD simulation (using ANSYS/Lumerical FDTD Solutions). We obtained the prerequisite extinction coefficient, which drives the arc-lengths of the gratings, and combined with the fabrication parameter limits, determined the optimized design layout shown in Figure 11. The design includes arcs that extend between the fabrication process limits, i.e., extending from 200 nm, increasing with decreasing radius until the gap between two arcs reached 250 nm. Beyond this point, the gratings merged into a circular grating pattern. The diameter of the outermost circular section was 182 um, with a grating period of 636 nm. The grating period decreased linearly to 500 nm, which allowed the diffraction angle to vary with decreasing radius, thus achieving a focused spot. The duty cycle for the periods was kept constant throughout the grating structure at 50%. Figure 12 shows the measured 2D (x-z) power distributions for (a) the apodized and (b) non-apodized grating designs. The fact that the apodization allows light to penetrate deeper into the structure clearly yields a focused spot about 310 \(\upmu\)m above the surface of the chip. For the non-apodized device, we see that this resulted in almost all of the light being diffracted within the first 15 \(\upmu\)m. Figure 12c shows the measured 2D (x-y) power distribution (at z = 310 \(\upmu\)m) revealing the spatial distribution of the focused spot, which is 10 \(\upmu\)m in diameter. While these early results are limited to integrated power distributions over a broadband of wavelengths, we are currently working on a more complete characterization of the spectral dependence of the apodized focusing grating. We were able to apodize the gratings in a circular structure, which increased the penetration depth of the light from 15 to 50 \(\upmu\)m. This result is promising in developing a fully integrated PIC based focusing element, eliminating the need for external focusing devices in applications requiring off-chip delivery of light, e.g., in OCT or LIDAR. The tight focus size of 10 \(\upmu\)m is encouraging, and the seamless integration of such a device indicates significant potential for realizing low cost, PIC based medical technologies such as on-chip OCT.  
Figure 11: Schematic of the device, showing the details of apodized grating structure. Figure 12: Side views (**a,b**) and top view (**c**) of the 2D emitted power distribution from the grating structures. Apodized gratings (**a**) allow for light to travel further into the device than the non-apodized gratings (**b**). Top view is the 2D power distribution in the (x-y) plane taken at a height, z = 310 \(\upmu\)m above the chip, showing a focus point 10 \(\upmu\)m in diameter. .4.3.