How to fade out the ends of a saber?

I want to fade out the two ends of a saber line. How can I do that?

I already tried with playing the start_size and end_size, but if I'm set them to an equal value (e.g. 0), the whole saber line will be equal to the value I set.

So I would like to achive some kind of fade from the middle to both ends of the saber line. How can I do that?

I attach an image where I found an example of it. Thanks in advance!