Bp209 ir2

A 1450 nm diode laser (Capella, Lockheed Martin-Aculight, Bothell, Washington) coupled to a 400 µm core bare fiber (NA = 0.22; Ocean Optics, Dunedin, Florida) was used for all INS experiments. The optical fiber was positioned orthogonal to the nerve surface using a micromanipulator (World Precision Instruments, Sarasota, Florida). In accordance with previous optimization studies, diode current was adjusted to deliver radiant exposure between 1.4 and 1.6 J/cm² at a pulse width of 500 μs^{37 (link)}. The stimulation radiant exposure was determined by incrementally increasing the diode current until a muscle twitch was achieved for every delivered pulse. Pulse trains lasting 10 s at a repetition rate of 2 Hz were employed for every nerve monitoring trial to minimize thermal superposition^{58 (link)}. The optical fiber was positioned to not be in contact with the tissue at distance of ~ 120 μm such that the average spot size at the tissue was 503.6 ± 16 μm (1/e² diameter) as measured by an infrared beam profiler (BP209-IR2, Thorlabs, Newton, New Jersey) and validated using the knife-edge technique^{59 (link)}.

Stimulation thresholds were determined for three spot sizes: $500\mu \mathrm{m}$ ( $504.5\pm 20\mu \mathrm{m}$ ), $800\mu \mathrm{m}$ ( $805.6\pm 18.8\mu \mathrm{m}$ ), and $1000\mu \mathrm{m}$ ( $1019\pm 29.8\mu \mathrm{m}$ ) (Fig. 2). All spot sizes were measured using an infrared beam profiler (BP209-IR2, Thorlabs, Newton, New Jersey), validated using the knife-edge technique,⁶⁰ (link) and refer to the $1/e^2$ diameter of the beam profile. Spot sizes were modulated by adjusting the distance between the bare $400\mu \mathrm{m}$ diameter optical fiber ( $\mathrm{NA}=0.22$ ; Ocean Optics, Dunedin, Florida) output and the nerve surface. For the 500, 800, and $1000\mu \mathrm{m}$ spot sizes, the average distance away from the nerve surface was $862.9\pm 21.79$ , $1874\pm 156$ , and $2646\pm 240\mu \mathrm{m}$ , respectively. The fiber probes were positioned using a micromanipulator (World Precision Instruments, Sarasota, Florida) orthogonal to the main trunk of the sciatic nerve.