PHPExcel using Ajax to download file on Wordpress - php

Question

I am making a report on Wordpress where i am using PhpExcel to make an excel file and download it. I am doing this by using Ajax. The problem with this is that when i get the response, the popup window doesn't display and hence can't download the file.
$.ajax({
url: ajaxurl,
dataType: 'json',
data: {poll_id: _id, action: 'admin_poll_by_id'},
method: 'POST',
success: function(rs) {
alert('Report generated');
}
});
I can guarantee that the excel file is being generated because it returns a 200 response, and displays some weird characters on chrome networks response. This is my php code before the exit.
header('Content-Type: application/vnd.ms-excel');
header('Content-Disposition: attachment;filename="' . $fileName . '"');
header('Cache-Control: max-age=0');
// If you're serving to IE 9, then the following may be needed
header('Cache-Control: max-age=1');
// If you're serving to IE over SSL, then the following may be needed
header('Content-Type: text/html; charset=UTF-8');
header('Expires: Mon, 26 Jul 1997 05:00:00 GMT'); // Date in the past
header('Last-Modified: ' . gmdate('D, d M Y H:i:s') . ' GMT'); // always modified
header('Cache-Control: cache, must-revalidate'); // HTTP/1.1
header('Pragma: public'); // HTTP/1.0
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel5');
$objWriter->save('php://output');
Any help would be appreciated