A holographic lab was a facility were holograms would go for maintenance.
In 2378, a repurposed EMH-Mark I informed another to report to the hololab for maintenance. While there, he suggested viewing file 47-Beta, which contained a copy of the holonovel Photons Be Free.
Chapter 6 - "Duel in the Ready Room" of Photons Be Free has Captain Jenkins order Tulak and Kymble to escort the protagonist to the hololab aboard the USS Vortex for reprogramming. (VOY: "Author, Author")