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??
Re: Lens can focus the light to a smaller spot by ...?? 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).
Re: Lens can focus the light to a smaller spot by ...?? 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.