Basic Integrated Photonic Components

Basic Integrated Photonic Components

As in electronics, in integrated photonics there are some basic components common
to most of the integrated optical devices. Although in essence all these components
basically perform the same functions as their corresponding devices in conventional
optics, the operating principles are usually quite different, and thus their design has
very little to do with traditional optical components.
Although nowadays a long list of integrated photonic devices has been proposed,
modelled and fabricated, and their number is quickly increasing, the basic components
remain almost unchanged. Therefore it is possible to describe a short list of such
components, basic blocks from which much more complex integrated optical devices
can be built. We will now briefly outline some of the most common components, and
we will show the dramatic change in design concept of integrated photonic devices
compared to conventional optical components performing the same function. The main
difference in design comes from the fact that while in conventional optics the operation
principle is based on the behaviour of the light considered as plane waves or rays, in
integrated optics the modelling and performance of the devices should be treated using
the formalism of electromagnetic waves; this is because the size of the beams is of
the order of the light’s wavelength, typically in the range of microns. In fact, optical
propagation in integrated photonic devices is conveyed through optical channels with
dimensions of a few micrometres, both in depth and in width. Channel waveguides are
defined in a single plane substrate, and other related elements (electrodes, piezoelements,
heaters, etc.) are mounted on the same substrate, giving rise to a robust and
compact photonic device. Unless otherwise stated, all the basic components that we
will now describe will be based on monomode channel waveguides.
All the optical components in integrated photonics are constructed with three building
blocks. They are the straight waveguide, the bend waveguide and the power splitter.
Using these building blocks, several basic components have been developed to perform
basic optical functions. In addition, a particular function can be executed using different
elements, whose design may differ substantially. This versatility in optical element
conception is one of the special features of integrated photonic technology. Now, we
shall discuss several of these basic blocks and optical elements that perform some basic
functions common in many integrated optical devices.
• Interconnect. This basic element serves to connect optically two points of a photonic
chip (Figure 1.6a). The straight channel waveguide (Figure 1.6b), being the simplest
structure for guiding light, interconnects different elements which are aligned on the
optical chip. It can also act as a spatial filter, maintaining a Gaussian-like mode
throughout the chip architecture. In order to interconnect different elements which
are not aligned with the optical axis of the chip, a bend waveguide is needed, and
therefore a bend waveguide is often called an offset waveguide (Figure 1.6c). These
are also used to space channel waveguides at the chip endfaces, so that multiple
fibres may be attached to it.
• Power splitter 1 × 2. A power splitter 1 × 2 is usually a symmetric element which
equally divides power from a straight waveguide between two output waveguides
(Figure 1.6d). The simplest version of a power splitter is the Y-branch (Figure 1.6e),
which is easy to design and relatively insensitive to fabrication tolerances. Nevertheless,
the curvature radii of the two branches, as well as the junction, must be carefully
designed in order to avoid power losses. Also, if the two branches are separated by
tilted straight waveguides, the tilt angle must be small, typically a few degrees. A
different version of a power splitter is the multi-mode interference element (MMI,
Figure 1.6f). This name comes from the multi-modal character of the wide waveguide
region where the power split takes place. The advantage of this design is the
short length of the MMI compared to that of the Y-branch. Although the dimensions
of the MMI are not critical, allowing wide tolerances, this element must be designed
for a particular wavelength. The two power splitters which have been described are
symmetric, and thus 50% of the input power was carried by each output waveguide.
Nevertheless, asymmetric splitters can also be designed for specific purposes. In
addition, it is possible to fabricate splitters with N output waveguides, and in that
case the element is called a 1 × N splitter.
• Waveguide reflector. The waveguide reflector performs the task of reflecting back the
light in a straight waveguide (Figure 1.6g). The simplest method of performing this
task is to put a metallic mirror at the end of the channel waveguide (Figure 1.6h).
If one needs the reflection to occurs only for a particular wavelength, a multistack
dielectric mirror is used. Another way of building a waveguide reflector is

to implement a grating in a region of the straight waveguide (Figure 1.6i). The
grating is inherently a wavelength selective element, and thus the grating period
must be calculated for the specific working wavelength. The reflection coefficient
of the grating depends on the length of the grating region and on the modulation
refractive index depth. The wavelength selectivity of the grating is also used for
designing waveguide filters working under Bragg condition. Besides this, the grating
in integrated photonics can be used as an optical element for performing a wide
range of functions such as focusing, deflection, coupling and decoupling light in the
waveguide, feedback in an integrated laser, sensors, etc.
• Directional coupler. This element has two input ports and two output ports
(Figure 1.6j), and is composed of two closely spaced waveguides (Figure 1.6k).
The working principle of the coupler is based on the periodical optical power
exchange that occurs between two adjacent waveguides through the overlapping
of the evanescent waves of the propagating modes. This effect is described by

the coupled mode formalism described in Chapter 4. By setting design parameters,
including waveguide spacing and coupler length, the ratio of powers between the
two output ports may be set during the fabrication process to be between zero and 1.
• Polariser. A waveguide polariser allows to pass light having a well defined polarisation
character, either TE or TM light, by filtering one of them (Figure 1.6l).
The fabrication of a waveguide polariser is as simple as depositing a metallic film
onto a waveguide (Figure 1.6m): the light propagating along the waveguide with
its electric field perpendicular to the substrate plane (TM mode) is strongly attenuated
because of the resonant coupling with the superficial plasmon modes. In this
way, at the waveguide output, only light with TE polarisation is present. As the TE
mode also suffers some attenuation, the nature of the metal as well as the metallic
film length must be carefully chosen in order to obtain a high polarisation ratio,
while maintaining a high enough TE light power. An alternative way of obtaining a
waveguide polariser is to design a waveguide that supports only TE polarised modes.
These are obtained, for example, in lithium niobate waveguides fabricated by the
protonic exchange method. In this fabrication process, while the extraordinary index
increases, the ordinary index decreases, thus forming a waveguide that supports only
extraordinary polarised modes.
• Polarisation beam splitter. In some integrated optical devices, it is necessary to
divide the input light into its two orthogonal polarisation, TE and TM, in two separate
waveguide output ports (Figure 1.6n). Figure 1.6o shows an integrated optical
element based on a lithium niobate substrate, which performs this function: the
intersecting waveguide operates as a directional coupler whose behaviour depends
on the beat between odd-mode light and even-mode light for TE-mode and TM-mode
light, respectively [13]. The TE-mode light propagates to the cross-output port and
the TM-mode light to the parallel output port. This polarisation selectivity is based
on the birefringence of LiNbO3. The length and the width of the intersecting region
must be carefully controlled to obtain high extinction ratios of both polarisations,
for a chosen wavelength.
• Phase modulator. An integrated optical phase modulator performs a controlled shift
on the phase of a light beam (Figure 1.6p), and consists of a channel waveguide
fabricated on a substrate with the possibility of changing its refractive index by means
of an externally applied field (thermal, acoustic, electric, etc.). The most common
phase modulator is based on the electro-optic effect: an electric field applied to an
electro-optic material, such as LiNbO3, induces a change in its refractive index. If
the electric field is applied through a channel waveguide, the change in the refractive
index induces a change in the propagation constant of the propagating mode, and
therefore the light travelling through that region undergoes a certain phase shift
(Figure 1.6q). The geometry of the electrodes and the voltage control depend on
the crystal orientation and on the device structure. For high modulation frequency a
special electrode configuration is necessary, such as the travelling wave configuration
or phase reversal electrodes configuration.
• Intensity modulator. One of the most important functions of an optical chip is the
intensity modulation of light at very high frequencies (Figure 1.6r). One of the most
simple ways to perform this task is to build an integrated Mach-Zehnder interferometer
(MZI) on an electro-optic substrate (Figure 1.6s). The MZI starts with a channel
monomode waveguide, and then splits it in two symmetric branches by means of a

Y-branch. After some distance, the two branches becomes parallel. The MZI continues
with a symmetric reverse Y-branch, and ends in a straight waveguide. If the MZI
is exactly symmetric, the input light splits at the first Y-junction into the two parallel
branches, and then recombines constructively into the final straight waveguide. On
the contrary, if in one of the interferometer’s arms the light suffers a phase shift of
180◦, at the end of the second Y-branch the light coming from the two branches will
recombine out of phase, and will give rise to destructive interference, with no light
at the output. In practice, the phase shift in one arm is carried out via the electrooptic
effect, by applying a voltage across the waveguide. By adequately choosing
the crystal orientation, polarisation, electrode geometry and applied voltage, a total
phase shift of 180◦ can be obtained for a specific wavelength.
• TE/TM mode converter. In a normal situation, TE and TM modes are orthogonal, and
then the power transfer between them cannot occur. Nevertheless, TE to TM conversion
(Figure 1.6t) can be achieved by using electro-optic substrates, which must
have non-zero off-diagonal elements in the electro-optic coefficient matrix. If lithium
niobate is used as a substrate, a periodic electrode is required because this crystals is
birefringent, and therefore the TE and TM modes have different effective refractive
indices (propagation speeds) (Figure 1.6u). By combining phase modulators and a
TE/TM converter, a fully integrated polarisation controller can be built.
• Frequency shifter. Frequency shifting in integrated optics (Figure 1.6v) can be
performed by means of the acousto-optic effect. An acoustic surface wave (SAW)
generated by a piezo-electric transducer, creates a Bragg grating in the acoustooptic
substrate that interacts with the propagating light in a specially designed
region, giving rise to diffracted light that is frequency-shifted by the Doppler
effect (Figure 1.6w). This frequency shift corresponds to the frequency of the
acoustic wave.
1.6 Some