Characteristics of the Integrated Photonic Components

Characteristics of the Integrated Photonic
Components

The basic idea behind the use of photons rather than electrons to create integrated
photonic circuits is the high frequency of light (200 THz), which allows a very large
bandwidth for transporting and managing a huge amount of information. The replacement
of electronic by photonic means is forced by fundamental physical reasons that
limit the information transmission rate using purely electronic means: as the frequency
of an electrical signal propagating through a conductor increases, the impedance of
the conductor also increases, thus the propagation characteristics of the electrical cable
become less favourable. That is the reason why electrical signals with frequencies above
10 MHz must be carried by specially designed conductors, called coaxial cables, in
order to minimise the effect of a high attenuation. Figure 1.4 shows the attenuation
in a typical coaxial cable as a function of the frequency. It can be seen that for high
transmission rates (∼100 MHz), the attenuation is so high (∼5 dB/Km) that communications
based on electrical signals propagating on coaxial cables can be used in
applications where the typical distances are tens of metres (buildings), but they are

useless for distances greater than several kilometres (links between cities). In contrast,
optical signals propagate through non-conducting dielectric media, operating in the
wavelength range where the materials are highly transparent. For most optical materials
used in optical communications and photonic devices, this transparent window falls
in the visible and near-infrared range of the electromagnetic spectrum, which corresponds
to light frequency in the range 150–800 THz, 106 times the frequency used in
electrical transmission!
Integrated photonic devices based on integrated optical circuits take advantage of
the relatively short wavelength of the light in this range (0.5–2 μm), which allows the
fabrication of miniature components using channel waveguides the size of microns. The
technology required to fabricate planar lightwave circuit components of such dimensions
is therefore common in the well-established Micro-electronic technology, using
the tools and techniques of the semiconductor industry.
The basic concept in optical integrated circuits is the same as that which operates in
optical fibres: the confinement of light. A medium that possesses a certain refractive
index, surrounded by media with lower refractive indices, can act as a light trap,
where the rays cannot escape from the structure due to the phenomena of total internal
reflection at the interfaces. This effect confines light within high refractive index media,
and can be used to fabricate optical waveguides that transport light from point to point,
whether long distances (optical fibres) or in optical circuits (integrated photonic chips).
Figure 1.5 shows the basic structures for the most common waveguide geometries.
In a planar waveguide (Figure 1.5a) light is trapped by total internal reflection in a
film (dashed region), and therefore the film must have a refractive index greater than
the refractive indices corresponding to the upper and lower media. These are usually
referred to as the cover and the substrate, respectively, and the film is called the core,
because that is where most of the optical energy is concentrated.
In a channel waveguide the light propagates within a rectangular channel (the dashed
region in Figure 1.5b) which is embedded in a planar substrate. To confine light within
the channel it is necessary for the channel to have a refractive index greater than
that of the substrate, and of course, greater than the refractive index of the upper
medium, which is usually air. This type of waveguide is the best choice for fabricating
integrated photonic devices. Because the substrate is planar, the technology associated
with integrated optical circuits is also called planar lightwave circuits (PLC).
Finally, Figure 1.5c shows the geometry of an optical fibre, which can be considered
as a cylindrical channel waveguide. The central region of the optical fibre or core
is surrounded by a material called cladding. Of course, the core must have a higher

refractive index than the cladding in order to trap light within the structure after total
internal reflection.
In both channel waveguides and optical fibres the confinement of optical radiation
takes place in two dimensions, in contrast to planar waveguides where there is only light
confinement in a single direction. This fact allows light in planar waveguides to diffract
in the plane of the film, whereas in the case of channel waveguides and fibres diffraction
is avoided, forcing the light propagation to occur only along the structure’s main axis.
Three generations can be distinguished in the evolution of optical systems, from conventional
optical systems to integrated optical circuits (Table 1.2). The first generation
concerns conventional optical systems, where the optical components with sizes of the
order of centimetres were set on optical benches typically with dimensions of metres,
while the optical beams had diameters of the order of several millimetres. A second
generation in the evolution of optical systems can be called micro-optics. Its main
characteristic is the use of miniature optical components such as light emitting diodes,
diode lasers, multi-mode fibres, etc. These components are clearly a transition towards
the devices used nowadays in modern communication systems based on optical fibres.
Nevertheless, although the characteristics of micro-optic systems are satisfactory, there
are problems with the alignment and coupling between the components because of their
small size (of the order of millimetres). Furthermore, because of the critical alignment,
the various optical components are not packed together, making the optical system
unstable. The last generation in optical systems concerns integrated photonics, and
is based on optical circuits and components integrated in a single substrate. This, as
well as the small size of the optical components, is the key factor for the success
of integrated photonic systems. This technology, with unique features with respect to
previous generations, possesses important advantages in terms of choice of materials,
design, fabrication and performance characteristics. Some of the special features of
systems based on integrated photonic technology are the following:
1. Functionality based on electromagnetic optics. The key elements in an integrated
optical device are monomode channel waveguides with width and depth typically of
the order of microns, where the optical radiation propagates in a single mode. In this
way, while the optical systems of the first and second generation can be adequately

treated by ray optics because of the wide diameters of the optical beams (compared
to the wavelength of the light), integrated optical devices must be analysed
considering the propagating light as electromagnetic waves.
2. Stable alignment. A key factor in the good performance of an optical system is the
adjustment and alignment of the various optical components, which is critical and
difficult to achieve for conventional optical systems. In contrast, in integrated photonic
devices, once the optical chip has been fabricated, the alignment problem is
avoided and stability is assured. Furthermore, the device is stable against vibrations
or thermal changes. This characteristic, which is the most relevant feature in integrated
photonic devices, is assured because all the optical elements are integrated
in a single substrate.
3. Easy control of the guided modes. Because the waveguides are monomode, it is
easier to control the optical radiation flux through the electro-optic, acousto-optic,
thermo-optic or magneto-optic effects, or even by the light itself via non-linear interactions.
If the waveguides were multi-mode, this control by external fields would
be much more complicated, because of the different propagation characteristics of
each modal field.
4. Low voltage control. For devices based on light control via the electro-optic effect,
the short width of the channel waveguides allows one to drastically reduce the
distance between the control electrodes. This implies that the voltage required to
obtain a certain electric field amplitude can be considerably reduced. For example,
while the typical voltage for electro-optic control in conventional optical systems is
of the order of several KV, in integrated optic devices the voltage required is only
a few volts.
5. Faster operation. The small size of the control electrodes in an electro-optic integrated
photonic device implies low capacitance, and this allows for a faster switching
speed and higher modulation bandwidth. Typical modulations of 40 Gbit/s are easily
achieved using lithium niobate, polymers or InP-based devices.
6. Effective acousto-optic interactions. Since the field distributions of surface acoustic
waves (SAW) are located within a distance of a few wavelengths beneath the substrate
surface (tens of microns), the SAW and the optical waveguide modes overlap
strongly, giving rise to efficient acoustooptic interactions. Thus, using SAW generated
by piezotransducers, high performance integrated optical devices based on
acoustooptic effect can be developed.
7. High optical power density. Compared with conventional optical beams, the optical
power density in a monomode channel waveguide is very high, due to the
small cross-sectional area of the guide. This is of special relevance in the performance
of devices requiring high radiation intensity, such as frequency converters
(via non-linear effects) or even optical amplifiers and lasers. These devices
are therefore very efficient when designed and fabricated with integrated photonic
technology.
8. Compact and low weight. The use of a single substrate with an area of several
millimetres squared for integrating different photonic components makes the optical
chip very compact and very light weight.
9. Low cost. The development of integration techniques makes mass production possible
via lithographic techniques and mask replication; also, the planar technology
reduces the quantity of material necessary to fabricate the photonic devices. These

aspects are the basis of a low cost device and thus an easy introduction into
the market.