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Optical Two-Guide Coupler Switches In GaAs: Ideal and Nonideal Operation*

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The two-waveguide coupler switch has recently been studied theoretically[1-3] and ex perimentally[4] to determine the configurations in which complete extinction in the “off” state can be achieved. Different switching characteristics are predicted for the two configurations of the switch shown in Fig. 1, which have been analyzed using an exact solution to the slab-guide model of the structure[5] and various versions of improved coupled-mode theory[5]. Each coupler has a single input guide. In case 1 [Fig. 1(a)], it is the initially excited guide 1 which forms the coupler output[1-3,5]; in case 2 [Fig. 1(b)], the coupled guide 2 is used as the output guide. Ideal operation with complete extinction in the "off state is predicted in case 1, and is shown to be impossible in case 2. The reason for this is the shape of the modes of the input and output guides, and the overlaps of these with the modes of the coupled-guide region. To obtain zero crosstalk in case 1, the waveguides must be appropriately detuned from synchronism, and the device length must result in a π phase shift between the coupler modes for this value of detuning (i.e., a detuned coupling length which is shorter than the synchronous coupling length Lc). In case 2, the coupler must be Lc long to achieve good transmission in the "on" state, and no improvement in switching characteristics can be achieved by varying the length[5]. LiNbO3 two-guide switches with both output configurations of Fig. 1 have previously been characterized experimentally[5]. In that work, however, because all the devices were fabricated to have an interaction length of Lc, imperfect nulls were observed in both configurations (as predicted theoretically). The results presented here demonstrate a two-waveguide switch in which the signal can be completely extinguished in the "off state (within measurement capabilities).

© 1988 Optical Society of America

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