Increase the coupling efficiency of fiber

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Increasing the beam size of a laser before coupling it to a fiber can enhance coupling efficiency because a larger beam allows for a smaller focus when passing through a lens. This smaller focus can increase the amount of light entering the fiber, especially if the fiber has a small diameter. Perfect coupling occurs when the laser output diameter matches the fiber diameter. Additionally, using an optically-clear adhesive is crucial for optimal light transmission. Understanding diffraction effects is essential, as a beam cannot be focused to a single point but rather to a small circle at the focus.
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I learn that increase of the beam size of the laser before coupling it to the fiber can increase the coupling efficiency of it.

Can I know why is this so?

Maybe the fiber is of very small diameter, and the focal lens in front of the fiber can't focus all the laser to this small diameter. But I don't understand how increase the diameter of the laser can make the focal lens to focus to a smaller spot??
 
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I'm not sure exactly what you're asking. Perfect coupling would occur if the laser output was of the same diameter as the fibres. You should also make sure that you have an optically-clear adhesive to join them.
 
Because of diffraction effects, a beam cannot be focused down to a single point. Instead, think of the beam's cross-section as a small circle at the focus.

The larger the beam diameter is before going through the lens, the smaller the beam will be at the focus. A smaller focus means (probably) more of the beam can enter the fiber.
 

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