FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

4.3 INTEGRATED OPTICAL CIRCUITS (IOCS)
The optical counterpart of the field of integrated
electronics is the field of integrated optics.
However, though integrated optical circuita have been
made, they are for the moat part not commercially
available. They are the subject of extensive research
and development efforta at a large number of laboratories.
The goal of these efforts is to commercially fabricate
in large quantities these miniaturized devices
with interconnected waveguides all on a single substrate.
The generation, detection, propagation, modulation,
switching, and coupling of light on such substrates
have been accomplished. Techniques for the
fabrication of substrates and the production of the
high resolution material distribution patterns required
for these integrated optical circuits exist. Most
integrated optical circuits and associated devicea
operate in a singlemode, therefore they are compatible
with singlemode optical fibers. In essence most LEDs,
diode lasers, and photodiodea are integrated optic
devices. Integrated optical circuits are considered
here because they are a part of second generation fiberoptic
sensor technology.
The waveguides used in integrated optical
circuits are usually of two types namely planar films,
that confine the light wavea in the vertical direction,
and planar strips, (channels) that confine lightwaves
in two dimensions. In both caaea, the waveguides are
made of a higher refractive index than the surrounding
material. Light is trapped in the layer or channel in
much the same way that it is trapped in the core of an
optical fiber. These planar waveguides can be formed
by sputtering glass on a substrate of lower refractive
index. A very uaeful type of channel waveguide can be
formed by evaporating titanium (Ti) onto lithium niobate
(LiNb03) or lithium tantalate in the desired
waveguide patterns, and diffusing the Ti into the aubstrate
thus forming a channel of higher refractive index.
This is shown in Fig. 4.23. The attenuation in
“’’”’””-r
LiNb03
SUBSTRATE
“1
OPTICAL
n2
CHANNEL&
nl>nz
Si02_
ELECTRODES—
Fig. 4.23
n1>n2>n3
(A) TITANIUM STRIP
ON LiNb03
SUBSTRATE
(B) TITANIUM DIFFUSEDAT
HIGH T INTO SUBSTRATE
FORMING CHANNEL
WAVEGUIDE
(C) PROTECTIVE LAYEROF
Si02 SPUTTERED
ON SURFACE
(D) ELECTRODES APPLIED
Steps in the fabrication of a channel waveguide.
auch channels is typically 1 dB/cm. A protective layer
of silicon dioxide (Si02) ia often sputtered on top in
order to protect the surface. Electrodes may be deposited
on the Si02.
Coupling a fiber to an integrated optic channel
has been accomplished as shown in Fig. 4.24. The
1 v “’SILICON
SINGLE-MODE FIBEti
IN ALIGNMENT
V-GROOVES IN SILICON
Fig. 4.24
An optical fiber pigtailed to a lithium
niobate (LiNb03) fiberoptic chip.
fiber is aligned and epoxied in an etched silicon (Si)
V-groove. The fiber end and the Si edge are polished
and butted against the polished edge of LiNb03. Indexmatching
liquid is used between the fiber and substrate
ends. Similar techniques are uaed to couple diode
lasers and photodetectors to the substrate. The dimensions
of channel waveguides are close to those of both
ainglemode fiber corea and the radiating areas of stripe
geometry diode lasers. For stripe geometry, the long
dimension of the channel should be oriented perpendicular
to the long dimension of the radiating area of the
diode. Insertion leases as low as a few dB can be
achieved.
Channel-to-channel couplers can be formed on
the integrated optical circuit aubstrate by paralleling
two channels in cloae proximity to each other for a
sufficient distance to achieve the desired coupling
ratio. The coupling length is defined as the distance
required for complete power transfer to occur. The
length required to achieve a apecified coupling ratio
depends on the separation between channels and the refractive
indices of the channels and subatrate. The
refractive index of LiNb03 ia a linear function of the
applied electric field. This is known aa the Pockels
effect. Thus, if electrodes are applied as shown in
Fig. 4.25, the refractive index can be varied and the
light can be awitched to the other of the two output
ports of the coupler. The direction of the electric
field is oriented oppositely in the two channels resulting
in equal and opposite refractive index changes.
When the proper value of voltage is applied, the coupling
length is made ahorter and the light will exit from
the same fiber it entered. When the voltage is not applied,
the coupling length is longer and the light will
emerge from both ports. The exit channels from this
switch may be combined to form the Mach-Zehnder interferometer
shown in Fig. 4.26. The result is an electrooptic
intensity modulator. When the light wavea recombine
they excite the fundamental mode of the exit channel.
When they arrive out of phase they tend to excite
the aecond mode but the second mode can’t propagate in
the singlemode channel, therefore it radiatea.
An acoustooptic modulator (Bragg Cell) is
ahown in Fig. 4.27. The interdigited ultrasonic transducer
is shown at the bottom. The surface acoustic wave
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