You can use fresh lychee, but since fresh lychee tends to be more expensive than the canned version, I prefer to use canned lychee for this lychee syrup. However, if you do prefer to use fresh lychee, the following is the ratio I use to make the syrup. For every 200 g of fresh lychee (or fresh lychee puree), I use 100 g of sugar and 100 g water. This is a 2 : 1 : 1 ratio. The cook times will be different though, so keep an eye on your syrup for the correct consistency.