Lens can focus the light to a smaller spot by ?

In summary, increasing the beam size of a laser before coupling it to a fiber can improve the coupling efficiency due to the numerical aperture (NA) of the lens. By filling the aperture of the lens, the full optical power is available, resulting in a smaller beam waist radius and better coupling. This relationship is due to the Fourier transform between the front focal plane and back pupil plane.
  • #1
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Lens can focus the light to a smaller spot by ...??

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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  • #2


It has to do with the "numerical aperture (NA)" of a lens. The larger you fill the aperture of a lens the smaller the minimum beam waist radius gets.
Here is a somewhat helpful site on numerical aperture's:
http://microscopy.fsu.edu/primer/anatomy/numaperture.html

Fiber's also have a NA, and even if the focussing lens had a higher NA than the fiber's you would still never achieve perfect coupling due to the diffracted nature of the intensity profile (sorry diverting away from your question).
 
  • #3


That's correct; by filling the aperture of the lens, the full optical power of the lens is available (the full NA is available). This may seem odd, but recall there is a relationship between the field at the front focal plane and the back pupil plane (they are Fourier transforms), so the larger the diameter (and the smaller the divergence) of the beam is in one space, the smaller the diameter and the larger divergence it has in the other.
 
  • #4


HIhi, thank you very much.
 

1. How does a lens focus light?

A lens focuses light by refracting, or bending, the light rays that pass through it. The shape of the lens and the material it is made of determine the degree of refraction.

2. What is the purpose of focusing light to a smaller spot?

Focusing light to a smaller spot can increase the intensity or brightness of the light in that area. This can be useful for tasks such as magnifying small objects or concentrating light for specific purposes, like in a laser.

3. Can any type of lens focus light to a smaller spot?

No, not all lenses are capable of focusing light to a smaller spot. Only convex lenses, which are thicker in the middle than at the edges, have the ability to do so.

4. How does the distance between the lens and the object affect the focus of light?

The closer the lens is to the object, the shorter the focal length and the smaller the focused spot of light will be. Conversely, the farther away the lens is from the object, the longer the focal length and the larger the focused spot of light will be.

5. Can a lens focus light to an infinitely small spot?

No, there is a limit to how small a spot of light can be focused due to the properties of light and the materials used to make lenses. This limit is known as the diffraction limit, and it is determined by the wavelength of light and the size of the lens aperture.

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