handbook of modern sensors

148 4 Optical Components of Sensors
Fig. 4.22. Acousto-optic modulator produces multiple diffracted beams.
4.11 Interferometric Fiber-optic Modulation
Light intensity in an optical fiber can be modulated to produce a useful output signal.
Figure 4.23 illustrates an optical waveguide which is split into two channels [13]. The
waveguides are formed in a LiNbO 3 substrate doped with Ti to increase its refractive
index. They are fabricated by a standard photolithographic lift-off technique. The
substrate is patterned using a photomask. The Ti layer is electron-beam evaporated
over the material; then, the photoresist is removed by a solvent, leaving the Ti in the
waveguide pattern. The Ti atoms later diffuse into the substrate by baking [11]. This
process results in a graded refractive index profile with a maximum difference at the
surface of about 0.1% higher than the bulk value. Light is coupled to the waveguides
through polished end faces. Electrodes are positioned in parallel to the waveguides,
which recombine at the output. Voltage across the electrodes produces a significant
phase shift in the light waves.
The optical transmission of the assembly varies sinusoidally with the phase shift
φ between two recombined signals, which is controlled by voltage V(t):
P out
P in
= 1 2
{
1 + cos
[ πV (t)
Vπ
+ B
]}
, (4.38)
where Vπ is the voltage change required for the full on–off modulation and B is
the constant which can be adjusted for the optimum operating point. When the phase
difference between the light in two waveguides before the recombination φ = 0,
the output couples to the output waveguide. When the shift φ = π, light propagates
into the substrate. The well-designed modulators can achieve a high extinction ratio
on the order of 30 dB.