I am going to jump in here and post a different opinion on this very subject.
There is no question that fine hair is more challenging to treat than coarse hair, but the point that fine hair can not be treated with laser is not correct. The reason that this belief exists is multifactorial.
Fine hair is hard to treat and a number of lasers are incapable of treating very fine hair due to intrinsic limitations in the technology. The diode is the best example of that. But other lasers like most YAGs also fall into that trap.
Many people who do laser hair removal don’t understand the fundamental physics of what they are doing and do not understand the impact of their settings. Some of this is due to errors made by the people who were doing much of the early research. Many of the conceptual errors (such as needing long pulsewidths) have gone away, but many people who were taught those errors haven’t kept up with the research. The net result is their success on fine hair is poor.
In some people, the treatment of fine hairs results in growing of more hair and in some cases it appears to be coarser. This is really a function of two events and appears to be somewhat genetically predetermined.
a. The laser synchronizes dormant hairs so that more hairs are in the growing phase than when one begins, giving the appearance of more hair. This process can be exacerbated by the problems in 1 and 2 above.
b. The laser causes hair, that was otherwise not growing to begin to grow. This is due to the stimulative effect of heat in the skin. This is potentially where the “coarser” hair seems to come from. It is not uncommon for women who have fine hair all their lives, to suddenly begin to grow coarse hair on their chins and face as they approach menopause and beyond. Those hairs never grew before but were always there. It appears that the laser can potentially stimulate those hairs to start growing long before their time.
Those are the processes that appear to be in play with this issue. I don’t know of any mechanism by which laser energy can increase the size of a hair follicle thereby making the fine hair thicker. I am not even sure that hormonal issues can do that, though I would consider that a possibility.
What I do know is that with the correct equipment, approach, and understanding that it may take time and effort, laser hair removal can get rid of fine hair. Even on the face, which is the most difficult place to treat a woman. Though upper lips do well.