# Fibre Misalignment and Light Coupling

• irfanskp
In summary, the conversation is about estimating the loss of light when a reflective surface is tilted by a certain degree in front of a single mode fibre. The estimated loss is calculated using geometry and the NA of the light, and is found to be only a few percent. However, when using a fibre efficiency formula, the loss is surprisingly higher. The attached diagram provides more numbers for further discussion on the problem.
irfanskp
Hi,

I have got a single mode fibre (SMF) with core diameter 8.4 microns. Fibre is aligned in front of horizental reflective surface at a distance of 500 microns. Light from SMF make a spot size of 24 microns and is reflected and coupled back into SMF. However, if I tilt the reflective surface (mirror) by 1 degree how much off-set or light is lost that is coupled back into SMF. Please find attached the diagram.

Answer to the question would be highly appreciated.

Thanks

Irfan

#### Attachments

• tilt.JPG
2.1 KB · Views: 373
It should be simple to estimate this using geometry- given the NA of light exiting the fiber face, the distance to and and the tilt angle of the reflective surface, you can calculate what fraction of light is incident on the fiber face. I'd be surprised if it was more than a few percent.

Thanks Andy.
When I do simple geometery calculations I get about 18% of loss with 2 degree. However, When I use fibre efficiency formula for tilt misalignment its surprisingly huge. So, I ve got confused which one is correct. Please find attached diagram that gives few more numbers. If you have a look on that then we can discuss in further to solve the problem.

Irfan

#### Attachments

• scan.pdf
91.2 KB · Views: 198
Going by your diagram, I calculate only 0.8% of the light will re-enter the fiber (about 21 dB loss). And that doesn't take into account any reflection off the air-glass interface.

Hello Irfan,

Thank you for your question. Based on the information provided, it is difficult to give an exact answer without knowing the specific properties of your setup. However, in general, any misalignment between the fibre and the reflective surface can result in a loss of light coupling efficiency.

The amount of light lost due to a 1 degree tilt of the reflective surface can vary depending on the specific setup, but it is safe to assume that there will be a decrease in coupling efficiency. This is because the reflected light will no longer be perfectly aligned with the fibre core, resulting in some of the light missing the core and being lost.

To accurately determine the amount of light lost, you may need to perform some calculations or simulations using the specific properties of your setup. Factors such as the numerical aperture of the fibre, the angle of incidence of the light, and the reflectance of the surface can all affect the coupling efficiency.

Best,

## What is fibre misalignment?

Fibre misalignment refers to the deviation of the fiber end faces from their intended position or orientation. This can occur during the manufacturing process or during installation and can result in reduced light transmission and poor coupling efficiency.

## What are the causes of fibre misalignment?

There are several factors that can lead to fibre misalignment, including improper handling, poor connectorization, and mechanical stress. Environmental factors like changes in temperature and humidity can also cause fibers to shift out of alignment.

## How does fibre misalignment affect light coupling?

Fibre misalignment can significantly impact light coupling by reducing the amount of light that is transmitted through the fibers. This can result in decreased signal strength and data loss, ultimately affecting the performance of the optical system.

## What methods are used to measure fibre misalignment?

Fibre misalignment can be measured using techniques such as near-field scanning, interferometry, and back reflection measurements. These methods allow for precise quantification of the misalignment and can aid in identifying the cause of the misalignment.

## How can fibre misalignment be corrected?

Fibre misalignment can be corrected by using precision alignment tools and techniques, such as fusion splicing, to realign the fibers. Proper handling and installation practices can also help prevent misalignment from occurring in the first place.

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