text: Approach him like you would any other man.  First, do it AWAY from his job, if possible.  Nothing is more annoying than having someone come up to you while you are trying to resolve a situation.  Second, make sure you are not asking him out BECAUSE he's a cop.  "Blue Flames" are a dime a dozen and never last long.  I've been a cop for almost ten years, dated a number of women in that time, and married the one that wasn't at all impressed I'm a cop.  Overall, though, treat him like any other man you would ask out because that what he is.