A planar optical waveguide in erbium-doped NaY(WO 4) 2 crystal was formed by 2.6 MeV He + ions with dose of 1.5 �� 10 16 ions/cm 2 at room temperature. The dark modes were measured using the prism coupling technique. Four TE modes and five TM modes were observed in the waveguide. The refractive index profiles were analyzed using the reflectivity calculation method (RCM) before and after annealing at 300��C for 30 min. In addition, we have used the TRIM (Transport of Ions in Matter) code to simulate the damage profile in Er:NaY(WO 4) 2 by 2.6 MeV He ions which is helpful for choosing the analytic function in RCM calculations. �� 2001 Elsevier Science B.V. All rights reserved.