you can send through querystring variable
<script language="javascript" type="text/javascript">
function openpopup(url)
{
window.open(url, 'pop_window','toolbar=no,menubar=no,location=no,scrollbars=yes,height=300,width=450,left=200,top=200');
}
//call from code behind
Page.ClientScript.RegisterStartupScript(this.GetType(), "aa", "<script language='javascript'>openpopup('page1.aspx?variable="+variabledata+"');</script>");