Optical computers require microscopic optical switches to turn signals on and of

Question

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 below. Light from an on-chip infrared laser ( = 1.000 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 1.522.

a.) With no voltage applied, is the output bright (switch closed, optical signal passing through) or dark (switch open, no signal)? Explain.

Explanation / Answer

delta L = optical path length - geo length

= (1.522 * 6.7 * 10-6) - (6.7 * 10-6)

= 3.50 mu.m

Phase difference = delta L * 2 pi / lambda

= (3.50 * 10-6 * 2 pi) / (1.00 * 10-6)

= 7 pi rad = 22 rad [output is dark as no voltage applied]

b) delta L = 6.70 * 10-6 (n - 1)

phi = delta L * 2 pi / lambda

= (n - 1) * 6.70 * 10-6 * 2 pi / 1.00 * 10-6

For bright output phi = 8 pi

(n - 1) * 6.70 * 10-6 * 2 pi / 1.00 * 10-6 = 8 pi

n - 1 = 0.597

n = 1.60

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