How to get Laravel to return correct HTTP response to trigger JQuery $.ajax() success?

I'm doing an AJAX POST of a simple HTML form using JQuery on my front end, to my Laravel back end, and upon success want to return the correct response to trigger JQuery's $.ajax() 'success' block rather than it's 'error' block. Unfortunately whatever I try seems to trigger the 'error' block.

My route is:

Route::post('/signup', 'AccessController@signup');

Back end:

public function signup()
    {
    [main code]
    return response('OK', 200); // I also tried return response('OK', 200)->header('Content-Type', 'text/plain'); and just: return "OK";
    }

Front end:

$('#signupbtn').click(function(e)
{
$.ajax({
    headers: { 'X-CSRF-TOKEN': $('#signup-form > input[name="_token"]').val() },
    type: 'POST',
    url: 'signup',
    data: { email: $('#email').val(), name: $('#name').val() },
    dataType: 'json',
    success: function (data)
        {
        console.log(data);
        $('#signupmodal').modal('hide'); // Close sign up modal
        $('#thanksmodal').modal(); // Open thanks modal
        },
    error: function (data)
        {
        console.log('Error:', data.responseText);
        }
    });
});

Also, if I DO want to trigger the $.ajax() 'error' block, what's the correct Laravel code for that?

Thanks.