Optical fibers are a class of waveguides traditionally used for long distance and high data rate communications (their interesting features being limited power loss and insensitivity to electromagnetic interferences). In an optical fiber, light propagates inside a central tube called the "core" which is surrounded by a "cladding". The higher refractive index of the core creates conditions for total internal reflection, which prevents photons hitting the core/cladding interface with an angle of incidence smaller than the critical angle from escaping through the cladding.
Single Mode vs. Multimode
Single-mode fibers have a smaller core diameter (8-10 microns) and are typically designed to transmit wavelengths between 1310 and 1550 nm. In communications applications, they provide higher data speeds over longer distances.
Multimode fibers have a larger core diameter (up to hundreds of microns). In communications applications, they are optimized for providing higher bandwidth over a shorter range.
For delivering light to the brain, multimode fibers are typically preferred for their larger core diameter.
If N1 is the refractive index of the core and N2 the refractive index of the cladding, then the numerical aperture (NA) of the optical fiber is:
Half-angle of divergence
The half-angle of divergence θdiv for a multimode optical fiber is:
where n is the refractive index of the tissue (1.36 for gray matter).
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- Ocean Optics offers laboratory-grade and premium-grade fiber assemblies, bulk unjacketed fiber, and useful fiber-optic termination kits.
- Thomas Recording manufactures 120 micron-diamter "Thomas Optical Quartz Fibers," which can be used with their microdrive systems or coupled to their custom LED light sources. It's unclear what advantage, if any, these would offer over standard optical fibers with a similar shape.
- Prizmatix offers diverse high NA fiber options for LEDs, including dual output and Y-shaped fibers for stimulating 2 areas of the brain. DIY kits for making cannulas are also available.
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