This is an excellent question. It sounds like you are looking for a primarily epidermal response, since post-inflammatory hyperpigmentation is due to over-active epidermal melanocytes (pigment producing cells). Having said that, some epidermal pigment can end up in the superficial dermis, a process called pigment incontinence. That is why pigment lightening procedures that target just the epidermis, like hydroquinone or chemical peels, sometimes fail. The Fraxel laser will treat both the epidermis and dermis, so that increases your chance of a successful outcome. On the other hand, if you treat too frequently you can stimulate those pesky melanocytes to produce more pigment again, so the pigment recurs. A good rule of thumb is to wait until any redness and swelling from the previous procedure is completely gone before retreating; in general we recommend 4 weeks between treatments. Also, if there are also any textural changes from your acne, you want to keep producing new dermal collagen for as long as possible to achieve the best improvement. That is another reason to wait 4 weeks or longer to allow the benefits to accumulate before retreating. That will save you money. Also, it is critical that you stay out of the sun during your treatments to avoid complications. In terms of laser setup, for epidermal improvement a high density of microthermal zones works best, with a correspondingly low treatment fluence to avoid complications. Good luck!