# Need help with an optics question -- Coupling a surface mount LED into a cylindrical optical guide

BenPhysicist
Homework Statement:
Most surface mount types of LED’s have a very nearly cosine distribution of light intensity. If we desire to inject light from an LED to the end of a cylindrical optical guide, what is the coupling efficiency as a function of distance (x) from the end of the guide?

Assume that the LED is a point source and that the fiber has a diameter D
Relevant Equations:
I found that the half cone angle let's call it theta=arctan(D/2x)
Now that I know theta, I'm trying to use that to calculate the coupled light.

What unit am I looking for, is it luminous intensity? I'm thinking I have to integrate from 0 to theta but I'm not sure what the integral looks like. Any ideas?
Most surface mount types of LED’s have a very nearly cosine distribution of light intensity. If we desire to inject light from an LED to the end of a cylindrical optical guide, what is the coupling efficiency as a function of distance (x) from the end of the guide?

Assume that the LED is a point source and that the fiber has a diameter D

## Answers and Replies

shjacks45
Pretty vague question, lacks purpose. SMD LEDs come in a variety, some with lenses some wirhout. LED range from infrared to ultraviolet with corresponding changes to LED and interface.

Laser purpose ranges from illumination to indicator to laser diode. Low power diode has small chip and less diff between emitter and fiber diam. High power is larger emmitting area requiring optics to make beam fit in common fiber sizes.

The fiber type (index of refraction, etc.) Is an important variable. Two examples: on circuit board indicators with plastic "rods" from front panel to top of led on circuit board; fiber-optic communication/networking with laser coupled to graduated refractive index fiber that keeps light centered.

Multimode fiber typically uses fiber's internal reflections. Both plug fiber directly into the LED part. Since you don't mention using lenses and antireflective coatings, efficency of energy transfer from LED and fiber is not a concern.

Last edited by a moderator:
berkeman
Mentor
Agreed. I asked the OP via PM to post datasheets and diagrams, but no joy so far...

BenPhysicist
Thanks for the input guys. That's all the information I have hence why I asked for help. but I figured it out after trying a few things