Page 268 - Introduction to Information Optics

Exercises 253

+V

Lightly doped GaAs

Heavily doped GaAs substrate

i

Fig. 4.P3.

the field in the thin layer, as in the case of reverse-biased transverse MQW
modulators.) Consider a wavelength of 1.15/mi.
(a) What orientation should the crystal be if a positive voltage is to
increase the index of the top layer?
(b) How much voltage is required to increase the index sufficiently to
cause the top layer to become a waveguide for the lowest-order mode
of an asymmetric waveguide? (Hint: Find the condition for a single-
mode waveguide first.)
4.16 Design a LiNb0 3 integrated optic intensity modulator using the Mach-
Zehnder interferometer shown in Fig. 4.22. Select the orientation of the
crystal and the polarization of the guided wave for the smallest half-wave
voltage V n. Assume that the active region has length L = 1 mm and
width d = 5 /mi, the wavelength is / = 633 nm, the refractive indices are
n 0 = 2.29, n e = 2.17, and the electro-optic coefficients are as given in
Problem 1.
4.17 A Franz-Keldysh modulator is built using GaAs. If the contrast ratio
between "on" and "off" is to be 10 dB and the operating wavelength is
900 nm, how thick should the GaAs device be made? (Use the data in Fig.
4.23.)
4.18 Use the data in Fig. 4.23, and ignore the loss of surface reflection. What
is the minimum insertion loss for the device in Problem 4.17?
4.19 Design a N x N (N > 4) switching array by using micromirrors. Specify
the configurations of the micromirrors.