Maximum angle of acceptance - optical fibre

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Discussion Overview

The discussion revolves around calculating the maximum acceptance angle for light entering an optical fiber, specifically in the context of a laser experiment involving water and air. Participants explore the relationship between critical angles, refractive indices, and acceptance angles, while addressing measurement challenges.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant expresses uncertainty about calculating the maximum acceptance angle, suggesting that more information is needed beyond the critical angle and angle of refraction.
  • Another participant notes that in this configuration, water acts as the core and glass as the cladding, questioning the feasibility of having a higher refractive index in the core than in the cladding.
  • Participants discuss the critical angle between air and water, with one providing a value of approximately 48.6 degrees and asking for the known and measured values used to calculate the average critical angle.
  • A participant mentions that the acceptance angle can be calculated using ray optics and emphasizes the need for specific refractive indices to express the acceptance angle in terms of the critical angle.
  • There is a request for clarification on how the critical angle is being measured and why the acceptance angle cannot be measured.
  • One participant later indicates that they were able to measure the refractive index, resolving their previous measurement difficulties.

Areas of Agreement / Disagreement

Participants exhibit a mix of agreement and disagreement, particularly regarding the definitions and measurements of critical and acceptance angles. The discussion remains unresolved on certain technical aspects, such as the feasibility of the refractive index configuration.

Contextual Notes

Limitations include the dependence on accurate refractive index values and the potential variability in measurements due to experimental conditions. The discussion does not resolve the mathematical steps required to calculate the acceptance angle.

Petra de Ruyter
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Hi all

Tearing my hair out trying to calculate the Maximum acceptance angle for a problem. Experiment involved shining a laser into a tube of water. I have calculated the angle of refraction based on the average of two critical angles (one known and one measured manually) as 52.49 degrees. Asked to calculate the maximum angle of acceptance with just the critical and the angle of refraction. I am of the opinion that more information is needed, can someone please clarify.

Ive tried several geometric ways around this and can't get it?
Cheers
 
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So, if I am not mistaken the water acts as the core and the glass as cladding? However you know that the core must have higher refractive index than cladding which is hardly achievable for such configuration (most glasses have higher refractive index than water).
Petra de Ruyter said:
52.49 degrees
Between which media did you get this value?
 
Air and water :-)
 
Petra de Ruyter said:
Air and water :-)
The critical angle between water and air is about ##48.6^o##, I wonder what are the known and measured values of critical angle with which you obtained that averaged value. Can you tell us?
 
53.00 and 51.98 The laser that was provided was pretty weak and the angles are a bit sketchy.

Cheers
 
Regardless of that peculiarity of the given value, the acceptance angle is calculated using ray optics. The ray diagram is illustrated in the picture below.
Numerical_Aperture_Angles_D1-780.jpg

You want to express ##\theta_a## in terms of ##\theta_c##. Clearly you need to know ##n_i##, ##n_f##, and ##n_c##.
 
What do you mean by critical angle?
 
Khashishi said:
What do you mean by critical angle?
Wiki will help you with this.
 
Maybe I should have said, how are you measuring the critical angle? And why can't you measure the acceptance angle?
 
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Likes   Reactions: sophiecentaur
  • #10
Hi there. I could not measure it because I did not have the refractive index, now I do and all is well. many thanks for all your assistance.
 

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