Someone asked me about cutting a 1/4" deep circular groove in a wood plaque that would fit an acrylic tube, for some sort of water/light project. I explained that my laser will not engrave that deeply, but that I could cut through one piece and laminate onto another for the same result. Then I got to thinking and tried an experiment on a scrap of 3/4" oak.
First I tried cutting circles .025 apart, with my settings used to cut through 1/8" oak. The result was amazingly precise cuts and an interesting effect but they were too far apart.
Then I ran it again at .016 apart. This time a quick scrape with a chisel removed the material nicely. The bottom was not perfectly smooth but I since engraved it after filling the innermost and outermost circles with black and that cleaned it up pretty well. The depth of the groove is just a hair over 1/8".