# Side-Illuminating Optical Fiber for High-Power-Density-Mediated Intraluminal Photoacoustic Imaging
## 3 Results
### Side-Fire Fiber Characterization  
The beam profiles of fibers with window lengths of 10 mm, 20 mm, and 30 mm are shown in Figure 4, and their characteristics are summarized in Table 1. The distal and proximal ends of the windows, relative to the light travel direction in the core, are indicated by yellow arrows; intensities are normalized to the peak in each dataset. Figure 4b,e, and k show the plots of the normalized intensity along the longitudinal and transverse  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Prototype\#** & **Est. Ablation Depth (\(\mu\)m)** & **2D Beam Size (mm)** & **Azimuth Angle (Degrees)** & **Side-Fire Ratio (I\({}_{u}\)/I\({}_{t}\))** & **Side-Fire Efficiency (I\({}_{u}\)/I\({}_{t}\))** \\ \hline A: 10 mm window & (i) 588 & (i) 12.29 \(\times\) 6.89 & (i) 104 & (i) 0.62 & (i) 20\% \\  & (ii) 500 & (ii) 11.3 \(\times\) 7.38 & (ii) 112 & (ii) 0.59 & (ii) 18\% \\ \hline B: 20 mm window & (i) 780 & (i) 20.6 \(\times\) 7.08 & (i) 109 & (i) 0.69 & (i) 40\% \\  & (ii) 750 & (ii) 20.1 \(\times\) 7.37 & (ii) 109 & (ii) 0.66 & (ii) 37\% \\ \hline C: 30 mm window & (i) 623 & (i) 30.07 \(\times\) 6.0 & (i) 81 & (i) 0.38 & (i) 29\% \\  & (ii) 631 & (ii) 30.79 \(\times\) 5.5 & (ii) 81 & (ii) 0.44 & (ii) 31\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of beam characteristics for fabricated prototypes.  
Figure 3: Laser machined side-fire fiber: (**a**) side view showing the machined window demarcated by black dashed lines in the 1.5 mm OD fiber and the 45\({}^{\circ}\) diced end, (**b**) top view showing the width of the cut indicated by black dashed lines, and (**c**) transverse view of the window cut along the width showing a valley created by the laser machining. (**d**) Side and (**e**) top view images of the fiber with silver coating, except in the ablated window, placed in a glass tube and with a red laser pointer coupled to it to show the light emitting from the machined window.  
directions relative to the fiber axis, indicated by the black dotted lines in the 2D beam plots. The maximum intensity in the beam profile was observed near the center of the ablated aperture for the 10 mm window prototypes. The maximum intensity was about 5 mm from the proximal window end for the 20-mm window prototypes, with the signal intensity tapering slowly to about 4 mm from the distal end then dropping quickly (Figure 4e). The beam profile for the 30 mm prototype has peaks within 4 mm of each end of the window and a 30% intensity drop in the middle; the highest intensity was measured near the distal edge. The pair of 10 mm window prototypes had substantially similar beam profiles, as did the 30 mm window pair. The pair of 20 mm window prototypes had differing slopes in the plateau over the machined window, with the beam profile of the second prototype.dropping to 70% of the peak before the steep drop at the distal edge of the window.  
The azimuth beam profile at the distal, middle, and proximal window positions for the three window lengths are shown in Figure 4c,f1. The intensity values are normalized to the maxima at each longitudinal position. The average azimuth angle span was 108\({}^{\circ}\), 109\({}^{\circ}\), and 82\({}^{\circ}\) for the 10 mm, 20 mm, and 30 mm window prototypes, respectively. The prototypes achieved a side fire ratio ranging from 38% for a 30 mm window and up to 69% for a 20 mm window. The side-fire efficiency, compared to a naive fiber with emission only at the flat distal end, was a maximum of 40% for one of the 20 mm prototypes and as low as 18% for a 10 mm window.