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Is slide 6 (Fiber Works by "Total Internal Reflection") wrong? I thought that total internal reflection (TIR) happened when trying to cross from a higher refrac

Is slide 6 (Fiber Works by "Total Internal Reflection") wrong? I thought that total internal reflection (TIR) happened when trying to cross from a higher refractive index (RI) medium to a lower one - not the other way around as depicted by the core-cladding boundary. Furthermore if the cladding has a higher RI, then the rays should become steeper upon transiting, not flatter like shown in the diagram. I always thought fibers were single material (i.e. just the core) because the surrounding air (lower RI) lets TIR happen. It certainly seems to be the case in the next slide (Demonstration Using a Laser Pointer), TIR on Wikipedia [1], and pictures of fiber optic lamps [2]. [1] https://en.wikipedia.org/wiki/Total_internal_reflection https://en.wikipedia.org/wiki/Total_internal_reflection [2] https://duckduckgo.com/?q=fiber+optic+lamp&t=canonical&iax=1&ia=images https://duckduckgo.com/?q=fiber+optic+lamp&t=canonical&iax=1... Okay after reading a bit more [3] I'm pretty sure it's wrong, and the cladding RI's must be lower. So my follow up question is... what's the point of the cladding? Why not just rely on the surrounding air? [3] https://en.wikipedia.org/wiki/Cladding_(fiber_optics) https://en.wikipedia.org/wiki/Cladding_(fiber_optics)

fiberz 9y ago
The fiber optic strand isn't suspended in air, so you need the cladding for total internal reflection. Fiber optic strands are coated (protection and color indentification), bundled and installed inside cables with various fillings and strength elements.
powerchord 9y ago
You're correct about total internal reflection. The light in the core must move more slowly than in the cladding. The majority of the light remains in the core though some travels in the cladding as well. A higher index of refraction means lower speed of light so you would expect to see the core have a higher IoR than the cladding.
kurthr 9y ago
Yes, you're right the cladding is lower index, and it is a controlled index! It (along with other layers) protects the inner core. Most data transport fiber is single mode, which requires very tight control of the ~8um 'diameter' of the high index core, but allows for lower loss/km and you don't get mixing of the different modes (think of them as different reflective angles for simplicity). If air was used then any scratch or dirt contacting the core would couple light out. In fact most multi-mode is graded index and it's much easier to get light into (8x larger core) without a specialized coupler. Note that even single mode allows DWDM- many different high-bandwidth wavelengths, but only 40nm spacing on 1560nm IR. The core, cladding, and outer protective layers are extruded and pulled to extremely precise dimensions, dispersions, and densities into multi-km rolls. The games that are played to equalize loss and delay among the wavelengths over a range of power and temperature is crazy... and that's ignoring the EDFA optically pumped fiber amplifiers, which let under sea cables work. Amazing tech!
- aidenn0 9y ago
  Has graded-index multimode caught on now? I last learned about fiber in the 90's when it was still something new.
deleted 9y ago
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fiberz 9y ago

The fiber optic strand isn't suspended in air, so you need the cladding for total internal reflection. Fiber optic strands are coated (protection and color indentification), bundled and installed inside cables with various fillings and strength elements.

powerchord 9y ago

You're correct about total internal reflection. The light in the core must move more slowly than in the cladding. The majority of the light remains in the core though some travels in the cladding as well. A higher index of refraction means lower speed of light so you would expect to see the core have a higher IoR than the cladding.

kurthr 9y ago

Yes, you're right the cladding is lower index, and it is a controlled index! It (along with other layers) protects the inner core. Most data transport fiber is single mode, which requires very tight control of the ~8um 'diameter' of the high index core, but allows for lower loss/km and you don't get mixing of the different modes (think of them as different reflective angles for simplicity). If air was used then any scratch or dirt contacting the core would couple light out. In fact most multi-mode is graded index and it's much easier to get light into (8x larger core) without a specialized coupler. Note that even single mode allows DWDM- many different high-bandwidth wavelengths, but only 40nm spacing on 1560nm IR. The core, cladding, and outer protective layers are extruded and pulled to extremely precise dimensions, dispersions, and densities into multi-km rolls. The games that are played to equalize loss and delay among the wavelengths over a range of power and temperature is crazy... and that's ignoring the EDFA optically pumped fiber amplifiers, which let under sea cables work. Amazing tech!

aidenn0 9y ago
Has graded-index multimode caught on now? I last learned about fiber in the 90's when it was still something new.

deleted 9y ago

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