Basically using code behind there is no direct way to open link in new window, but using
target attribute of link you can achieve it, set
target to '
_blank', it will help you to open page in new window
OR
you can use JavaScript for it, window.open method will help you to do it
see below snippet
function openMe()
{
window.open("1.aspx",'_blank');
return false;
}