Integrated Photonics Technology

 Integrated Photonics Technology

The technology and fabrication methods associated with integrated optical circuits and
components are very varied, in addition, they depend on the substrate material with
which the optical device is fabricated. The methods most widely used in the definition
of optical circuits over a substrate are diffusion techniques (such as titanium diffusion
in lithium niobate) and deposition techniques (such as chemical vapour deposition used
for silica). Since the lateral dimensions of the optical circuits are only a few microns,
the fabrication technology needs photolithographic processes. In the case of diffusion
techniques it is possible to use photolithographic masks to define open channels through
which the diffused material enters the substrate, or, alternatively, one can deposit the
previously patterned material to be diffused directly onto the substrate. For waveguides
fabricated by deposition techniques the lateral definition of the optical circuits is usually
carried out by means of etching after the deposition of the material onto the whole
substrate surface.
Optical integration can expand in two directions: serial integration and parallel integration.
In serial integration for optical communication devices the different elements
of the optical chip are consecutively interconnected: laser and driver, modulator and
driver electronics, and detector and receiver electronics. In parallel integration, the chip
is built by bars of amplifiers, bars of detectors and wavelength (de)multiplexors. Also,
a combination of these two architectures should incorporate optical cross-connects
and add-drop modules. The highest level of integration (whether serial or parallel)
is achieved in monolithic integration, where all the optical elements including light
sources, light control, electronics and detectors are incorporated in a single substrate.
The most promising materials to achieve full monolithic integration are semiconductor
materials, in particular GaAs and InP. In hybrid integration technology, the optical chip
fabricated on a single substrate controls the optical signals, while additional elements
such as lasers or detectors are built on different substrates and are directly attached to
the integrated photonic device or interconnected by optical fibres. Examples of hybrid
technologies include dielectric substrates, such as glasses, silica or ferro-electric crystals.
The case of silica on silicon can be considered as quasi-hybrid integration, in the
sense that optical components, electronics and detector can be implemented in a single
substrate, but not the light source.
All integrated photonic devices require input/output optical signals carried by optical
fibres. Indeed, one of the most difficult tasks in packaging an integrated optical device is
attaching the fibres to the chip waveguides, known as fibre pigtails. The fibre alignment
is typically 0.1 micron or less for low power loss, where the optical chip surfaces should
be carefully polished at odd angles to eliminate back reflection from the interface.
This alignment must be maintained during the attachment and also through subsequent
thermal transitions as well as in shock and vibration-prone environments while the
device is operating.
Lithography replicates a prototype from chip to chip or from substrate to substrate.
Although a lithographic system for fabricating photonic devices uses the same tools
as in semiconductor electronics, there are some important differences. First, while in

electronics, bends and interconnections affect the maximum data rates, in photonic
circuits the major impact is on optical power throughput. Second, while electrons
strongly interact with each other, photons can exist even in the same circuit without
interacting. As a consequence, integrated circuits in electronics usually have an overall
square geometry, with multiple layers to enable the cross-over of electrical signals,
while integrated optical chips tend to have a single layer and an elongated geometry
with unidirectional flow to minimise bending of the optical path.
Although there is a great number of lithographically processable materials that can
be used to fabricate optical waveguides, only a few of them have shown the required
characteristics to develop integrated optical devices. These include a wide range of
glasses, crystals and semiconductors (Table 1.3). In particular, the substrates most
commonly used are glasses, lithium niobate, silica on silicon, III-V semiconductor
compounds and polymers. Each type of material has its own advantages and disadvantages,
and the choice of a specific substrate depends on the particular application of
the photonic device. Nowadays there exists a great variety of devices based on each
of these materials.
The glass-based integrated optical devices have the great advantage of the low cost
of the starting material and the fabrication technique, mainly performed by an ionic
exchange process [7]. The method used for producing waveguides in glass substrates

is the exchange of alkali ions from the glass matrix (usually Na+ ions) for monovalent
cations such as K+, Ag+, Cs+ or Tl+, immersing the glass substrate in a molten salt
that contains some of these ions at temperatures in the range 200–500◦C, depending
on the type of glass and the particular salt. For defining the optical circuits, a stopping
mask is deposited onto the substrate, in such a way that the ionic exchange takes place
only in the channels opened in the mask. This mask is removed after the exchange
process. The refractive index increase due to the ionic exchange depends both on
the glass composition and on the exchanged ions, and typically varies in the range
0.01 to 0.1. Since the glasses are amorphous materials, they do not present physical
properties useful for the direct control of light, and therefore they are used mainly for
the fabrication of passive devices.
One of the materials most widely used in the fabrication of integrated optical
devices is lithium niobate (LiNbO3) [8]. This is due to several characteristics of this
crystalline material. In the first place, LiNbO3 presents very interesting physical properties:
in particular, it has valuable acousto-optic, electro-optic and piezo-electric effects.
These properties allow the fabrication of functional devices such as phase modulators,
switches, directional couplers, multiplexors, etc. Besides being a birefringent material,
LiNbO3 shows high non-linear optical coefficients, and these two properties permit
very efficient frequency conversion, such as second harmonic generation and optical
parametric oscillation. Furthermore, several techniques for waveguide fabrication
in LiNbO3 are now well established, including Ti or Zn metallic diffusion, protonic
exchange, or even ion implantation. The resulting waveguides have very low losses,
typically in the range of 0.01–0.2 dB/cm. Integrated optical circuits technology based
on LiNbO3 substrates is now very well established, and a great variety of devices
based on this technology, mainly in the field of optical communications, are now
commercially available.
The main advantage of silica over silicon-based photonic waveguides is the low
price and the good optical quality of the silicon substrates, besides being a well-known
material with a long tradition, and the experience developed from micro-electronic technology.
The first step in waveguide fabrication using silicon substrates is the deposition
of a silicon dioxide layer a few microns thick, which can also be obtained by direct
oxidation of the silicon at high temperature. This layer has a double purpose: to provide
a low index region for allowing light confinement, and also to move away the
highly absorbing silicon substrate. For this reason this layer is called a buffer layer.
The waveguide core is formed by further deposition of a high index oxynitride layer,
usually via the chemical vapour deposition method (CVD) or the flame hydrolysis
deposition (FHD) method [9]. The refractive index of the oxynitride core, SiOxNy, can
be continuously varied in the range 1.45–2.1 by controlling the relative concentration
of SiO2 and Si3N4 compounds during the deposition. As the SiO2 buffer layer has a
refractive index of 1.45, a very high index contrast between the waveguide core and
the surrounding media can be obtained. The most appealing feature of silicon as a
substrate in integrated photonics is the possibility of integrating the detector and the
associated electronic in a single platform substrate.
Perhaps, second to LiNbO3 the III–V semiconductor compounds (mainly GaAs
and InP) are the substrates with greatest impact on integrated optics technology, and
are probably the materials with the most promising future in this field [10, 11]. The
importance of the III–V compounds in integrated photonics derives from the fact that
they offer the possibility of a high level of monolithic integration. Indeed, InP is a very
versatile platform that promises large-scale integration of active components (lasers and
detectors), passive components, and also electronics. The electronic technology of these
semiconductor materials is now well established, and optical waveguide fabrication is
quite straightforward by modifying the dopant concentration during the deposition
process, Al in the case of GaAs, and Ga or As in the case of InP. The main problem
concerning this technology has its roots in the relatively high losses of waveguides
made of these materials (>1 dB/cm). Nevertheless, the fabrication technology in InP
is rapidly improving, and several integrated photonic devices that show very high
performance are now available in the market, such as semiconductor optical amplifiers,
arrayed waveguide gratings or high speed modulators.
Among the materials suitable for integrated photonic technology, polymers occupy a
special position, due to the fact that they exhibit some very useful physical properties,
such as electro-optic, piezo-electric and non-linear effects, with values even higher
that those of lithium niobate crystals [12]. Also, the thermo-optic coefficient for polymers
is more than ten times higher than the corresponding coefficients for silica. The
waveguide fabrication method for polymers starts from a solution of the polymeric
material, followed by a deposition by spin coating or dip coating on a substrate. Due
to their easy processing, the polymer layers allow for great flexibility when choosing
a substrate; they are compatible with very different substrates such as glasses,
silicon dioxide, or even silicon and indium phosphide. The choice of a particular polymeric
material should take into consideration some important properties such as high
transparency, easy processing, and high physical, chemical, mechanical and thermal
stability. The main advantage of polymer-based integrated optical devices is their high
potential for use in the field of chemical and biological sensors, because the organic
groups in the polymeric compound can be designed and tailored to react against a
specific medium. Also, due to the large electro-optic coefficient showed by some polymers,
high speed and low voltage switches and modulators have been developed for
the telecommunication market, offering high performance at low cost.