# Side-Illuminating Optical Fiber for High-Power-Density-Mediated Intraluminal Photoacoustic Imaging
## 4 Discussion and Conclusions
### Observations and Insights  
In this work, single-core, multimode fused silica fibers with high damage threshold (1 GW cm\({}^{-2}\)) were utilized to develop side-fire elongated illuminator using ArF excimer laser machining. The developed device was compact with a 1.5 mm OD or a 3 mm OD with a glass tube housing, had extended side-fire illumination demonstrated up to 30 mm in length, and was capable of carrying 18 m] per-pulse energy or 100 MW cm\({}^{-2}\) power density. As shown in Figure 4, for all the prototypes, the \(-6\) dB beam size in the longitudinal direction matched the machined window length. The fibers with 10 mm long windows showed a central peak in intensity, whereas the longer windows had steep rises in intensity within 5 mm of the window ends and tilted plateaus or dips (>10% drop within 10 mm) over the middle of the window. The peaks are likely due to scattering of the higher modes on interaction with the machined material, whether at the proximal end of the window or at the distal window end after reflection of forward propagating modes at the bevel. The decreasing intensity with distance from the peaks is likely due to the reducing energy in the higher order modes after interaction with the machined surface and therefore decreasing scattering and escaping light as a function of position. As expected, the silver deposition led to negligible light intensity detected opposite the window. The lateral window dimension was the same for all prototypes, corresponding with reasonably consistent transverse beam spread (80-110\({}^{\circ}\)), though the effect of widow width was not investigated here. It should be noted that the optical coupling used in the current setup has an NA of 0.10 which is less than the NA of the fiber of 0.39, which leads to mode under-filling. While mode filling was not investigated here, if we overfill the modes, we can expect more light interacting with the machined window region. This may result in more light escaping out from the side and less light propagating unperturbed and improved side-fire efficiency. The ablation depth in the fiber core is a crucial factor in the beam profile and side-fire efficiency, with deeper ablation corresponding with higher side-fire efficiency irrespective of window length (Table 1). For example, the 30 mm window was expected to allow more light to exit the fiber but had lower efficiency than the 20 mm fibers, corresponding with shallower ablation depths. Hence, greater ablation depths and the consequent disruption closer to the center of the core is a larger determinant of side-fire efficiency than the window size. The u-shaped ablation trench, created by three successive machining layers, was selected to retain as much of the fiber material for structural integrity while ablating into the core in order to scatter the lower modes. The cross-sectional geometry, such as the opening angle of the fabricated prototypes, is expected to affect the emitted azimuthal beam profile but was not investigated in this study. All fibers were structurally intact after machining but were fragile and could easily break during testing. In some early prototypes, the trench was encapsulated in an optically transparent epoxy to strengthen the fiber, but charring was observed after coupling to the PA imaging laser, caused by air bubbles not fully removed during the epoxy casting process. However, once they are encased in a glass tube, they maintain mechanical integrity through all the characterization and imaging experiments. Compared to our earlier prototype [45], this new design includes a bevel at the distal end of the fiber. This bevel, which redirects the reflected light into the fiber, allowing for further interaction with the machined region, resulted in a 30% increase in the side-fire ratio compared to the 10 mm window prototype with a flat-cut end. When the device has both a beveled tip and a conformal silver coating to reflect light from the fiber surface, up to 40% of the light emitted from a flat ended fiber is redirected outside of the machined fiber.