Context :

Aims : Spectroscopic , polarimetric , and high spectral resolution interferometric data covering the period 1995–2011 are analyzed to document the transition into a new phase of circumstellar disk activity in the classical Be-shell star 48 Lib .
The objective is to use this broad data set to additionally test disk oscillations as the basic underlying dynamical process .

Methods : The long-term disk evolution is described using the V / R ratio of the violet and red emission components of H \alpha and Br \gamma , radial velocities and profiles of He i and optical metal shell lines , as well as multi-band BVRI polarimetry .
Single-epoch broad-band and high-resolution interferometric visibilities and phases are discussed with respect to a classical disk model and the given baseline orientations .

Results : Spectroscopic signatures of disk asymmetries in 48 Lib vanished in the late nineties but recovered some time between 2004 and 2007 , as shown by a new large-amplitude and long-duration V / R cycle .
Variations in the radial velocity and line profile of conventional shell lines correlate with the V / R behavior .
They are shared by narrow absorption cores superimposed on otherwise seemingly photospheric He i lines , which may form in high-density gas at the inner disk close to the photosphere .
Large radial velocity variations continued also during the V / R -quiet years , suggesting that V / R may not always be a good indicator of global density waves in the disk .
The comparison of the polarization after the recovery of the V / R activity shows a slight increase , while the polarization angle has been constant for more than 20 years , placing tight limits on any 3-D precession or warping of the disk .
The broad H-band interferometry gives a disk diameter of ( 1.72 \pm 0.2 ) mas ( equivalent to 15 stellar radii ) , position angle of the disk ( 50 \pm 9 ) °and a relatively low disk flattening of 1.66 \pm 0.3 .
Within the errors the same disk position angle is derived from polarimetric observations and from photocenter shifts across Br \gamma .
The high-resolution interferometric visibility and phase profiles show a double or even multiple-component structure .
A preliminary estimate based on the size of the Br \gamma emitting region indicates a large diameter for the disk ( tens of stellar radii ) .
Overall , no serious contradiction between the observations and the disk-oscillation model could be construed .

Conclusions :