I hate to be the bearer of bad news, but you can't "unstain" poplar. Poplar is one of the softests woods that are used in building a house. Any bleaching process that would be able to remove the dark staining of the wood, would also destroy the wood itself. Other than painting it, your only other option is to try to sand it down. I'm not certain how deep stain will penetrate poplar, but if it were me, I would replace it before I'd attempt to sand down to bare wood.
Sorry I don't have a more positive answer for you.