# Slight confusion of optical fiber refraction

Why is cladding needed for TIR? The refractice index of air is lower than cladding, so wouldn't the critical angle be less, resulting in a higher chance of a light ray exceeding the acceptance angle?

And the other, how does a narrower core cause modal dispersion to decrease
Thank you.
if there is no cladding, pulses of light will pass into any fibres adjacent in contact, within the fibre optic cable, which you do not want.
(edited 1 month ago)
Original post by heidi017426
if there is no cladding, pulses of light will pass into any fibres adjacent in contact, within the fibre optic cable, which you do not want.

Alright, I understand that. But If we imagine just one fiber, why is cladding needed? Should air (because it has a lower refractive index) allow for TIR even more than the cladding will because it has a lower index than the cladding thus producing a lower critical angle?

Also, if you are able to, please answer the second questions 🙏
Thanks
I don’t know if you have seen them before but these lamps are an example of when cladding is not needed. I also didn’t mention that the cladding stops scratches which would alter the angle of the normal and it is good for general protection aswell
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im assuming here that you know how to calculate the critical angle. Hope this makes sense as to why it’s ‘better’ with cladding. Lmk if it doesn’t

give me a second on the other question
so on the second question, I don’t see why a thinner core would help reduce pulse broadening. I’ve done some calculations here

Since pulse broadening is caused by variation in distance of paths that light takes and then this causes the signals to arrive at different times I calculated the distance that the light would travel in the core at a given angle. Nowhere in my calculations does the diameter x become significant in determining this so it seems like there would be just as large variation in path difference for a thin core as a thick core resulting in just as significant pulse broadening. I am only an AS student so I might be missing something so if anyone else knows I am invested now too because I have come across the idea before that a thinner core reduces pulse broadening.
another thought…
Original post by heidi017426
I don’t know if you have seen them before but these lamps are an example of when cladding is not needed. I also didn’t mention that the cladding stops scratches which would alter the angle of the normal and it is good for general protection aswell
.

im assuming here that you know how to calculate the critical angle. Hope this makes sense as to why it’s ‘better’ with cladding. Lmk if it doesn’t

give me a second on the other question

OHHHHHHHHH, bro thank you. I thought we wanted as many rays of light to be TIR'd so less data is lost. That makes sense thanks.
Original post by heidi017426
another thought…

Yea i think thats it, I looked online and saw that the rays of light will enter more normally due ot a thinner core
Original post by heidi017426
so on the second question, I don’t see why a thinner core would help reduce pulse broadening. I’ve done some calculations here

Since pulse broadening is caused by variation in distance of paths that light takes and then this causes the signals to arrive at different times I calculated the distance that the light would travel in the core at a given angle. Nowhere in my calculations does the diameter x become significant in determining this so it seems like there would be just as large variation in path difference for a thin core as a thick core resulting in just as significant pulse broadening. I am only an AS student so I might be missing something so if anyone else knows I am invested now too because I have come across the idea before that a thinner core reduces pulse broadening.

Also one thing, I looked at your profile, and we are doing the same A levels and are both in year 12, andd your predicted grades are amazing. Do you have any tips
Original post by Cactphys
Also one thing, I looked at your profile, and we are doing the same A levels and are both in year 12, andd your predicted grades are amazing. Do you have any tips

sorry i don’t check on here much!
Thanks 🙂 Honestly i’ve always be quite academic but generally hard work pays off! So I do about 10-15 hours of independent study a week depending on how many shifts I have at work that week. Doing things like making/ using flash cards and past paper questions and blurting
hope that helps good luck with your A-levels next year