# Coupling Efficiency of LED to Fiber Optic Guide

• BenPhysicist
In summary, the conversation discusses the coupling efficiency of light from an LED to the end of a cylindrical optical guide. The efficiency is affected by factors such as the type of LED, the diameter of the fiber, and the type of fiber being used. The use of lenses and antireflective coatings can also impact the efficiency of the energy transfer. Further information and datasheets would be helpful in determining the specific efficiency in a given situation.
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

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
Agreed. I asked the OP via PM to post datasheets and diagrams, but no joy so far...

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

## 1. What is the definition of coupling efficiency?

The coupling efficiency of an LED to a fiber optic guide refers to the percentage of light energy that is successfully transmitted from the LED into the fiber optic guide. It is a measure of how well the LED and fiber optic guide are able to transfer light from one to the other.

## 2. How is coupling efficiency calculated?

Coupling efficiency is typically calculated by comparing the output power of the LED to the input power of the fiber optic guide. The ratio of these two values is the coupling efficiency, expressed as a percentage.

## 3. What factors affect the coupling efficiency of an LED to a fiber optic guide?

The coupling efficiency can be affected by several factors, including the alignment of the LED and fiber optic guide, the numerical aperture of the fiber optic guide, the size and shape of the LED, and the quality of the optical interface between the two components.

## 4. How can the coupling efficiency be improved?

To improve the coupling efficiency, it is important to optimize the alignment of the LED and fiber optic guide, as well as the quality of the optical interface. Using a lens or collimator to focus the light from the LED into the fiber optic guide can also improve the coupling efficiency.

## 5. Why is coupling efficiency important in LED-fiber optic systems?

The coupling efficiency is important because it directly affects the amount of light that is transmitted through the system. A higher coupling efficiency means more light is transmitted, resulting in better performance and potentially longer transmission distances. It also impacts the overall efficiency and reliability of the system.

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