Adaptive Optics Capability in the Visible Range Telescopes with adaptive optics (AO) have been in use for several decades. To my knowledge, the lowest wavelengths utilized are in the near-infrared. It's known that Fried parameters are smaller for visible versus infrared ranges.

Are there any earth-based observatories utlizing AO in the visible range? What are the technical issues to achieve acceptable AO Strehl ratios in the visible range?

 A: As per my knowledge in visible range making workable AO compensation system is very difficult and no earth based observation telescope exits. If we assume that acceptable S=0.8, at the near-IR wavelength range (say, 1.5 um), then the strehl ratio at visible (say, 0.5 um) is 1/3*0.8. Because the S is proportional to (wavelength)^6/5.
A: They are rare because the size of the patch of atmosphere that can be corrected by a given guide star is so much smaller at shorter wavelengths and changes faster - so you are either limited to where you can observe (near a bright guide star), or you need complex and expensive laser guide star systems.
If you are purely imaging (rather than trying to use AO to feed a spectrograph) there is a common alternative. You image the object continually but only use those images where the atmosphere happened to be still enough to briefly give a good image. You can do this either by reading out the CCD in short exposures or using a separate shutter. 
You can combine this with a simpler AO system to make a "good" atmosphere into an "very good" one - eg the Magellan telescope has a visible AO system
The other big driver against a visible AO system purely for imaging is that you can always use the Space telescope instead.
