Integrated Photonics

Integrated Photonics
Optics can be defined as the branch of physical science which deals with the generation
and propagation of light and its interaction with matter. Light, the main subject of
optics, is electromagnetic (EM) radiation in the wavelength range extending from the
vacuum ultraviolet (UV) at about 50 nanometers to the far infrared (IR) at 1 mm. In
spite of being a very ancient science, already studied by the founder of the School of
Alexandria, Euclid, in his Optics (280 BC), during the last quarter of the past century,
the science of optics has suffered a spectacular renaissance, due to various key developments.
The first revolutionary event in modern optics was, no doubt, the invention
of the laser by T.H. Maiman in 1960 at Hughes Research Laboratories in Malibu [1],
which allowed the availability of coherent light sources with exceptional properties,

such as high spatial and temporal coherence and very high brightness. A second major
step forward came with the development of semiconductor optical devices for the
generation and detection of light, which permitted very efficient and compact optoelectronic
devices. The last push was given by the introduction of new fabrication
techniques for obtaining very cheap optical fibres, with very low propagation losses,
close to the theoretical limits (Figure 1.1).
As a result of these new developments and associated with other technologies, such as
electronics, new disciplines have appeared connected with optics: electro-optics, optoelectronics,
quantum electronics, waveguide technology, etc. Thus, classical optics,
initially dealing with lenses, mirrors, filters, etc., has been forced to describe a new
family of much more complex devices such as lasers, semiconductor detectors, light
modulators, etc. The operation of these devices must be described in terms of optics
as well as of electronics, giving birth to a mixed discipline called photonics. This new
discipline emphasises the increasing role that electronics play in optical devices, and
also the necessity of treating light in terms of photons rather than waves, in particular
in terms of matter–light interactions (optical amplifiers, lasers, semiconductor devices,
etc.). If electronics can be considered as the discipline that describes the flow of
electrons, the term “photonics” deals with the control of photons. Nevertheless, these
two disciplines clearly overlap in many cases, because photons can control the flux of
electrons, in the case of detectors, for example, and electrons themselves can determine
the properties of light propagation, as in the case of semiconductor lasers or electrooptic
modulators.
The emergence of novel photonic devices, as well as resulting in the important
connection between optics and electronics, has given rise to other sub-disciplines within
photonics. These new areas include electro-optics, opto-electronics, quantum optics,
quantum electronics and non-linear optics, among others. Electro-optics deals with
the study of optical devices in which the electrical interaction plays a relevant role in
controlling the flow of light, such as electro-optic modulators, or certain types of lasers.
Acousto-optics is the science and technology concerned with optical devices controlled
by acoustic waves, driven by piezo-electric transducers. Systems which involve light

but are mainly electronic fall under opto-electronics; these systems are in most cases
semiconductor devices, such as light-emitting diodes (LEDs), semiconductor lasers
and semiconductor-based detectors (photodiodes). The term quantum electronics is
used in connection with devices and systems that are based on the interaction of
light and matter, such as optical amplifiers and wave-mixing. The quantum nature of
light and its coherence properties are studied in quantum optics, and the processes
that involve non-linear responses of the optical media are covered by the discipline
called non-linear optics. Finally, some applied disciplines emerging from these areas
include optical communications, image and display systems, optical computing, optical
sensing, etc. In particular, the term waveguide technology is used to describe devices
and systems widely used in optical communications as well as in optical computing,
optical processing and optical sensors.
A clear example of an emergent branch of optics that combines some of the above
disciplines is the field of integrated optics, or more precisely, integrated photonics.
We consider integrated photonics to be constituted by the combining of waveguide
technology (guided optics) with other disciplines, such as electro-optics, acousto-optics,
non-linear optics and opto-electronics (Figure 1.2). The basic idea behind integrated
photonics is the use of photons instead of electrons, creating integrated optical circuits
similar to those in conventional electronics. The term “integrated optics”, first proposed
in 1960 by S.E. Miller [2], was introduced to emphasise the similarity between planar
optical circuits technology and the well-established integrated micro-electronic circuits.
The solution proposed by Miller was to fabricate integrated optical circuits through
a process in which various elements, passive as well as active, were integrated in a
single substrate, combining and interconnecting them via small optical transmission
lines called waveguides. Clearly, integrating multiple optical functions in a single
photonic device is a key step towards lowering the costs of advanced optical systems,
including optical communication networks.
The optical elements present in integrated photonic devices should include basic
components for the generation, focusing, splitting, junction, coupling, isolation, polarisation
control, switching, modulation, filtering and light detection, ideally all of them

being integrated in a single optical chip. Channel waveguides are used for the interconnection
of the various optical elements. The main goal pursued by integrated photonics
is therefore the miniaturisation of optical systems, similar to the way in which integrated
electronic circuits have miniaturised electronic devices, and this is possible
thanks to the small wavelength of the light, which permits the fabrication of circuits
and compact photonic devices with sizes of the order of microns. The integration of
multiple functions within a planar optical structure can be achieved by means of planar
lithographic production [3]. Although lithographic fabrication of photonic devices
requires materials different from those used in microelectronics, the processes are basically
the same, and the techniques well established from 40 years of semiconductor
production are fully applicable. Indeed, a lithographic system for fabricating photonic
components uses virtually the same set of tools as in electronics: exposure tools, masks,
photoresists, and all the pattern transfer process from mask to resist and then to device.