Is it ok for a button to open in a new tab?

That been said, it is completely ok for a button to open in a new tab. An icon denoting so is not necessary (unless you are actually opening an external link).

1. Should links open in a new tab?
2. How do I make a button open a new tab?
3. Is it better to open a new tab or window?

Should links open in a new tab?

The only time it is recommended that you open a link in a new tab is when opening in the same screen would interrupt a process (e.g. when a user is filling out a form or viewing a video). Linking in the same tab or screen in these situations could cause the user to lose the work they've done or have to start over.

How do I make a button open a new tab?

You can also use the Ctrl + T keyboard shortcut to open a new tab or the new tab button in the top-left of the window. To open a link in a new tab, right click on the link, then select Open Link in New Tab or use the middle mouse button to click on the link.

Is it better to open a new tab or window?

A tab is more or less same as a window. A window can contain several tabs and all session data and cookies are shared across all tabs and open window. It's better to open a lot of tabs than opening several windows because too many window becomes too cluttered to handle.