Penrose limit and string quantization in AdS_3 \times S^3
  We consider corrections to the Penrose limit of AdS_3 \times S^3 with NS-NS flux which are due to the terms next to leading order in inverse radius expansion. The worldsheet theory of a lightcone string is interacting due to the presence of quartic terms in the action. Perturbative corrections to the spectrum are shown to agree with the results from the exact quantization in AdS_3 \times S^3. 
Andrei Parnachev, David A. Sahakyan