The optimum position of the optical window (OW) of illuminated lateral double-drift region (DDR) impact avalanche transit time (IMPATT) device has been determined subject to achieve the finest optical control of both DC and RF properties of the device. The OW is a tiny hole that has to be created on the oxide layer through which the light energy of appropriate wavelength can be coupled to the space charge region of the device. A nonsinusoidal voltage is assumed to be applied across the diode and the corresponding terminal current response is obtained from a two-dimensional (2-D) large-signal (L-S) simulation technique developed by the authors for illuminated lateral DDR IMPATT diode. Both the DC and L-S properties of the illuminated device based on Si, designed to operate at W-band frequencies (75-110 GHz) are obtained from the said L-S simulation. Simulation is carried out for different incident optical power levels of different wavelengths (600-1000 nm) by varying the position of the fixed sized OW on the oxide layer along the direction of electrical conduction of the device. Results show that, the most favorable optical tuning can be achieved when the OW is entirely created over the p-type depletion layer, i.e. when the photocurrent is purely electron dominated. Also the 700 nm wavelength is found to be most suitable wavelength for obtaining the maximum optical modulation of both DC and RF properties of the device.