Page 263 - Introduction to Information Optics

248 4. Switching with Optics
simultaneous amplitude modulation and frequency modulation of the laser
emission. As a result, spectral broadening and chirping exist, which usually
affect the spectral stability of the output optical signal. Electro-optic modula-
tors utilize the linear electro-optic effect, where a change in the refractive index
is induced by an externally applied electrical field. Modulation bandwidths up
to 100 GHz are potentially possible with a drive voltage of several volts. The
main disadvantages include the strong polarization dependence of the devices,
limited optical bandwidth, and difficulty in integration with semiconductor
lasers and amplifiers. Electroabsorptive modulators using MQWs can be easily
integrated with semiconductor lasers and amplifiers. Bandwidths up to 50 GHz
have been achieved and the drive voltage is less than 2 V. Main concerns
include strong wavelength dependence of the devices and insertion loss.
MEMS optical switches are optomechanical switching devices using mi-
cromechanical and optical elements. These elements are fabricated using
micromachining techniques, and the switches are usually actuated electrostati-
cally. Two schemes of MEMS switches were discussed in this chapter. Deform-
able diffraction gratings are arrays of microgratings with two states, diffraction
and nondiffraction, controlled by an external voltage. The switching device has
a response time of 20 ns, and can form large arrays for massive parallel signal
processing and optical displays. Micromirror switching arrays consist of
two-dimensional arrays of torsion mirrors. Each mirror has a size of about
16//m. The mirrors can rotate and redirect incoming optical beams to the
desired direction. Currently, to rotate a mirror by 45°, requires a driving
voltage of 80 V, and a switching time of 80 /im.

REFERENCES

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