Integrated Optics - Applied Optics Fundamentals and Device Applications - page 246

5.9 LithiumNiobateTechnology
Titanium diffusion into lithium niobate (LiNbO 3 ) is a process used for fabri-
cating waveguides and other devices of an optical circuit. Although LiNbO 3
is not an active semiconductor material such as GaAs and hence does not
afford the monolithic integration of sources and detectors, much of the work
in optical circuit engineering to date has been performed in this material
due to the high electro-optic and acousto-optic figures of merit, ease of pro-
cessing, high optical transparency (<0.1 dB cm −1 total loss from 0.4 to >2 μm),
chemical stability, and lack of the requirements for high-level integration
and monolithic compatibility. As the ancillary technologies continue to
mature, GaAs and semiconductor based optical circuits will continue to
supplant the LiNbO 3 technology; nevertheless, many of the emerging com-
mercial communications, to be discussed in later sections, are based in
LiNbO 3 .
Selection of the operating wavelength for a LiNbO 3 system depends upon
many of the same criteria as other material systems. 1.3 and 1.5 μm corre-
spond to the lowest absorption loss wavelengths for silica-based fibers (0.5
and ∼0.2 dB km −1 ) [39]. The variation of refractive index dispersion which
limits the potential modulation rate is also minimized at these wavelengths,
so that many of the devices currently available are designed for operation in
these regions, along with the AlGaAs source wavelength. At this time the
cost and performance of emitters shifts the balance in favor of 1.3 μm except
in cases of very long fiber lengths where the lower attenuation at 1.55 μm out-
weighs these factors. Fluoride fibers currently exist which are capable of less
than 1 dB km −1 and theoretically as low as 10 −2 dB km −1 at a wavelength of
2.55 μm. Research on InGaAs detectors indicates they make good detectors
at that wavelength [40]. Fabrication of these devices is relatively difficult and
the cost of fluoride based fiber is higher than silica fibers so the implementa-
tion of such a system appears to be downstream.

5.9.1 Electro-Optic and Photorefractive Effects

The electro-optic coefficient translates into the control voltage required for
an optical component to function. Typically LiNbO 3 devices require very
low control voltages with key relations being the following:
Change in refractive index [41]

⎛
⎜

1
n

⎞
⎟ =

Δ rE RE +

2

(5.21)

2

where
E is the applied electric field (voltage/electrode spacing)
r is the Pockels effect coefficient
R is the Kerr effect coefficient