Optical computers require microscopic optical switches to turn signals on and off. One device for doing so is the MachZender interferometer?

Optical computers require microscopic optical switches to turn signals on and off. One device for doing so, which can be implemented in an integrated circuit, is the Mach-Zender interferometer seen in the figure(Figure 1) on the next page. Light from an on-chip infrared laser ( λ = 1.020 μm ) is split into two... show more Optical computers require microscopic optical switches to turn signals on and off. One device for doing so, which can be implemented in an integrated circuit, is the Mach-Zender interferometer seen in the figure(Figure 1) on the next page. Light from an on-chip infrared laser ( λ = 1.020 μm ) is split into two waves that travel equal distances around the arms of the interferometer. One arm passes through an electro-optic crystal, a transparent material that can change its index of refraction in response to an applied voltage. Suppose both arms are exactly the same length and the crystal's index of refraction with no applied voltage is also 1.533
In the figure the length of the electro optic crystal is given to be 6.7 micro m
Update: What is the first index of refraction of the electro-optic crystal larger than 1.533 that changes the optical switch to be closed with the optical signal passing through
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