Abu bakar, low-pass filter is a category of filters. there are in general three categories: low-pass filter, high-pass filter, and band-pass filters.
Also, their reciprocal or inverted. Such as inverted band-pass filter will be band-stop filter and so on.
Now each category could be practically designed according to certain equation. So for example low pass-filter; it might be Rectangle optical filter (ideal!), Gaussian optical fiber, Bessel optical filter….etc. you need to have a sharp slope to prevent interference and you need to have a linear phase response to preserve the received signal shape. Bessel filter has linear phase response that is why it is commonly used. Note that Bessel filter becomes Gaussian filter at high order.
Hope that helps to understand.
thanks
Mohamed Abouseif