# Side-Illuminating Optical Fiber for High-Power-Density-Mediated Intraluminal Photoacoustic Imaging
## 2 Materials and Methods
### Fiber Emission Characterization  
Fiber characterization involved measuring the beam profile in the longitudinal and azimuth directions. Additionally, the percentage of light emitted from the side of the fiber was measured relative to the total light output from the fiber to estimate the proportion diverted through the side-fire window. The percentage of side-emitted light was also compared to that of a naive optical fiber of the same model to estimate the combined light lost in back-reflections and front leakage. To measure the longitudinal profile, we scanned an isotropic detection fiber (IP85, Medlight, Ecublens, Switzerland, tip diameter 850 \(\upmu\)m) over a 30 mm \(\times\) 30 mm area in the x-longitudinal and y-transverse directions with a step size of 500 \(\upmu\)m in both directions, recording the relative irradiance emitted by the area side-fire fiber regardless of the emission angle. The detection fiber, which was positioned at a distance of 5 mm from the side-fire fiber, was connected to a photodiode (DET10, Thorlabs, Newton, NJ, USA) to record the output voltage with an oscilloscope and was positioned at a distance of 5 mm from the side-fire fiber. The side-fire fiber was connected to a 660 nm laser source (S1 FC660, Thorlabs, Newton, NJ, USA) via a coupling assembly consisting of a collimator (F810 SMA-780, Thorlabs, Newton, NJ, USA) and a focusing lens (LA1560-B, Thorlabs, Newton, NJ, USA). This laser wavelength is close to the Q-switched PA imaging laser wavelengths, and the wavelength difference does not significantly change the refractive index, hence the scattering at the window can be expected to be similar for all the PA wavelengths. Figure 1: The micromachined side-fire optical fiber. (**a**) Side-view sketch showing the flattened region and beveled tip machined with a dicing saw. (**b**) A fiber before silver coating showing the surface ridges made by the dicing blade. (**c**) The top view sketch illustrates the laser ablation pattern along the length of the fiber (dotted lines). (**d**) A side view sketch illustrates the refocusing of the micromachining laser focus on ablating successive layers deeper into the fiber. (**e**) Transverse view sketch illustrating the successive ablations. The Numerical Aperture (NA) of the coupling assembly was 0.10 and the fiber was spatially overfilled by placing it behind the lens focus. The coupling assembly was mounted on a computer-controlled 2D motor stage for automated data collection by synchronizing the oscilloscope output with a 2D motor stage. The \"front\" beam profile corresponds to measurements taken with the light-emitting window facing the isotropic detection fiber, while the \"back\" profile corresponds to measurements when the detection fiber faces the silver-coated region. Figure 2a illustrates this setup. For azimuthal profile measurements, the side-fire fiber was fixed vertically on a mount, while the detection fiber was mounted on a rotation stage. The middle of the machined window was visually aligned to face towards the detection fiber. The distance between the two fibers was set at 1 mm, and data points were acquired at 5\({}^{\circ}\) intervals while manually rotating the detection fiber around the stationary side-fire fiber. Azimuth measurements were taken at the proximal, mid, and distal regions of the side-fire window, as shown in Figure 2b. The beam spread in the azimuth direction was calculated by the difference between the maximum and minimum angle at 50% of the peak intensity. The side-fire ratio is the ratio of the light emitted from the side of the fiber, I\({}_{\text{s}}\), relative to the total light exiting fiber I\({}_{\text{t}}\), and it evaluates the proportion of the total emitted light that is coming from the machined window. This is critical to assess how much light is lost in forward propagation. The side-fire efficiency is the side fire emission I\({}_{\text{s}}\) relative to the light emitted from a multi-mode straight-cut front-firing fiber of the same model (I\({}_{\text{f}}\)).