Simulations have been performed to evaluate the effect of a thermal-induced mask on the performance of an optical disk. For simplicity and also so as not to lose generality, we first assume that the thermal-induced mask can be represented by an ellipse with variable shape and a relative position with, respect to the center of the readout laser spot. Simulation results reveal that the optical disk exhibits" a maximum response when almost half the laser spot is covered by the mask. With a conventional single-beam readout technique, however, this can hardly be achieved. The possibility of achieving this by use of an assistant beam with a modified beam profile is discussed. We show that this method allows us to obtain a maximum response in the track direction without any degradation in the radial direction. # 1997 Optical Society of America
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