We present an early-generation Utah Slant Optrode Array (USOA) for infrared (IR) neural stimulation. Intrafascicular IR stimulation with the early prototype in the cat sciatic nerve produced highly selective and artifact-free responses, which outperformed extraneural IR stimulation. We characterized the light delivery and loss mechanisms of the device in order to facilitate design optimization. Fabrication of the USOA takes advantage of the extensive research in the development of the Utah Slant Electrode Array (USEA). An undoped (ρ > 20 ω ρ cm) c-Si (100) substrate was used to produce a 10 x 10 array of optrodes with lengths from 0.5 mm to 1.5 mm in a 400-μm pitch. This substrate is able to transmit IR (λ > 1.1μm) with negligible absorption losses. The optrodes were coupled to the laser source via fibers of different core diameters through in-coupling interfaces of varying refractive indices. The effect of these factors on optrode transmission efficiency was investigated. At 1550nm, transmittance for a butt-coupled 50-μm multimode fiber through a medium of index n = 1.66 was measured as 34.7%, which was the maximum value obtained. When the refractive index of the intervening medium was lowered, transmission decreased according to Fresnel reflection theory. Above 100-μm core size, transmitted power decreases by 40% with each doubling of the fiber core diameter. Transmission was also found to be dependent on the optrode length, where shorter and more tapered optrodes provided less output power. The results suggest that Fresnel, coupling, and radiation losses are the primary loss mechanisms.
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