[anandat]

Thanks Steve for good tip.
Now decided to go with normal link instead of popup link.
So I changed the code to following

<div class="ask-question">
      {include file="customer/buttons/button.tpl" button_title=$lng.lbl_ask_question_about_product style="link" href="xcart_web_dir + /popup_ask.php?productid=`$product.productid`"}
    </div>

But now problem is with captcha.
Even I enter correct captcha numbers but form is still ask to enter correct captcha details again & again.

Is there any way to remove captcha requirement code from pop_ask.php ?

Or other way is to display content of popup_ask.php?productid=`$product.productid` directly into product detail page..but how to achieve that ?