Optimized Laser Stimulation of Nerves

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)}.