Electro-Optics - Applied Optics Fundamentals and Device Applications - page 33

refractive index), then the light will be entirely coupled into the second guide
after a characteristic length referred to as the coupling length. This coupling
length value is definable in terms of wavelength, mode confinement, and
interwaveguide separation [42]. If the indices of refraction of the two guides
are slightly different, the net coupling may be quite small, so that application
of a voltage across the electrodes shown in Figure 2.18 will change the indi-
ces of refraction and, therefore, the propagation velocities, thus destroying
the phase matching. This determines which port (3 or 4 in Figure 2.18) the
light will be coupled into after a length L . For directional couplers, switch-
ing speeds as fast as 1 ps are possible; however, to accomplish this device,
lengths must be on the order of 1 mm. Although not a fabrication problem
it is known that the shorter the device length, the greater the applied volt-
age must be in order to obtain optical isolation [43]. For device lengths of
10 mm, control voltages of 2-8 V are required dependent on the wavelength.
For 1 mm devices, voltages of 10-30 V are required. Two other examples of
directional couplers are the X switch and the merged directional coupler
(Figures 2.19 and 2.20) [44]. Again a applied voltage will cause a change in
refractive index to occur causing changes in the coupling ratio. Switching
characteristics are direct functions of device length and switching voltage as
with previous devices.

v

L

1 3

α

2 4

∆ n 2∆ n

FIGURE 2.19
X-switch.

2

v 3

1

Θ

v 2

3

FIGURE 2.20
Y junction switch.