I admit I have never used poplar in a bow limb. However, knowing what I do about poplar and how poor its bow characteristics are I don't recommend you use it in the limbs. Justin
There is a species of Poplar (American Yellow Poplar) which is called Tulipwood. It's quite a good bow wood, I use it quite often for backings, but it's a really soft wood so I'd be careful about where you put it. As a backing it should be fine but don't put it on the belly or it'll just compress too much.
For forms I usually just go by eye depending on what kind of limb deign I was aiming for. If I was making a reflexed limb I'd just put two 1" blocks on either end of a plank of 2x4 then clamp the handle down till I got the desired amount of reflex, I'd do this after I'd floor tillered the belly wood so I got a nice even curve.
For a R/D form just increase the size of the blocks (the amount depends on how much reflex you want) and add a 1" block underneath the handle, then clamp down mid-limb instead of the handle.