We often earn money from youtube channel by showing ads from adsense and suppose if someone want to earn bitcoin by showing ads on their youtube videos then is there any alternative ad network which provide ads for youtube channel?
Yes, You can.
You just have to cash out all of your ad sense earning directly to an exchange so, you can convert in Bitcoins, And I don't think that YouTube or google will add Bitcoins as payment option if it happens, Bitcoin will rock the world.