Some Examples of Integrated Photonics Devices

Some Examples of Integrated Photonics Devices

The optical elements that can be found in an optical chip can be classified according
to their function as passive, functional, active and non-linear. A passive optical element
has fixed input/output characteristics, which are determined when the photonic
component is fabricated. Examples of these are the power splitter, waveguide reflector,
directional coupler, polariser, and polarisation beam splitter. Functional optical
elements are photonic components which are driven by externally applied fields (for
example, electric, acoustic or thermal). The above described phase modulator, intensity
modulator, frequency converter and electro-optic TE/TM converter fall into this
category. Although some authors call these devices active devices, we will keep the
name “active devices” for photonic components that perform functions such as optical
amplification and laser oscillation. This choice of nomenclature is due to the fact that
they use active impurities such as rare earths embedded in the waveguide structure, to
obtain light amplification (or oscillation) via a luminescence process after optical (or
electrical) pumping. The integrated optical amplifier and the integrated laser are two
examples of active devices. Finally, some integrated optical devices make use of the
non-linearity of certain materials to perform frequency doubling or optical parametric
oscillation, where the optical chip’s function is to generate new frequencies via a nonlinear
optical process. Since the efficiency of non-linear processes is proportional to the

light intensity, these devices yield a very good performance in the integrated photonic
version, because of the small transverse area of the waveguide propagating beams.
Figure 1.7 shows an example of a passive integrated photonic device, in which no
external signal is needed for its operation. This device is called an arrayed waveguide
grating (AWG, PHASAR or waveguide grating router, WGR); its function is to passively
multiplex or demultiplex signals of closely spaced wavelengths, and it is used in
fibre optical communication systems [14]. Several wavelengths coming through a single
fibre enter the AWG via any of its input waveguides. A coupler splits light between
many of the curved waveguides which define the AWG. The arrayed waveguides are
formed by waveguides having different lengths, and therefore light suffers different
phase shift for each curved waveguide. By precisely adjusting the phase shift from
each curved waveguide with respect to all the others, an interferometric pattern is set
up that results in light of different wavelengths being focused at different spatial location
on an output arc. Since the AWG distribute signals according to their wavelength,
each individual waveguide output corresponds to a specific wavelength, thereby acting
as a demultiplexor.
An example of a functional device, which also combines some passive elements
is the acousto-optic tuneable filter (AOTF) (Figure 1.8) [15]. This integrated optical
device requires an external radio-frequency (RF) control signal to selectively separate
one or more wavelength signals (drop signals). This device is fabricated with LiNbO3
and is composed of a piezo-transducer, a thin film acoustic waveguide and two polarisation
beam splitters. The multi-wavelength input signals propagate over the optical
waveguide and are divided into their perpendicular components (TE/TM) by the first
polarisation beam splitter (PBS). Surface acoustic waves (SAW), generated by applying
an RF signal to the transducer, travel through the SAW guide and cause a periodic
modulation of the optical waveguide’s refractive index. The periodic refractive index
change induces TE–TM or TM–TE conversion for the drop wavelength only. The
drop wavelength corresponds to the applied RF frequency and becomes perpendicular
to the incident light. The second PBS is then used to separate the drop wavelength

from the incident light. By using several RF signals simultaneously, it is even possible
to drop several wavelengths.
Several substrate materials compatible with integrated photonic technology are also
suitable to incorporate optically active rare earth ions, which makes it possible to
fabricate active integrated optical devices [16]. Figure 1.9 shows the arrangement of an
integrated optical amplifier based on Erbium and Ytterbium ions. It basically consists
of a straight waveguide, which has rare earth ions incorporated to it, an undoped
waveguide and a directional coupler. The input pumping at 980 nm is injected into the
undoped waveguide, and the coupler transfers the pump energy to the doped straight
waveguide. Via several radiative, non-radiative and energy transfer mechanisms which
takes place on the Erbium and Ytterbium ions, the feeble input signal at 1533 nm

The high optical non-linear coefficients of LiNbO3 crystals make this substrate
suitable for developing non-linear integrated photonic devices, such as the optical parametric
oscillator presented here. Also, for high efficiency conversion, it is necessary to fabricate a
periodically poled region along the waveguide structure
is amplified as it propagates along the straight waveguide. If a couple of dielectric
mirrors are attached at the two waveguide ends, the amplified signal can oscillate, and
therefore an integrated laser can be obtained. The end mirrors can also be replaced by
integrated gratings, acting as a true wavelength-selective reflector.
Integrated optical parametric oscillators (OPOs) in ferro-electric crystals have been
identified as the most useful tuneable non-linear frequency converters with many
applications, mainly in environmental sensing and process monitoring. These nonlinear
integrated photonic devices are based on ferro-electric materials showing high
values of second order nonlinearities, and are capable of obtaining a periodic inversion
of the ferroelectric domains. Figure 1.10 presents the design of an optical parametric
oscillator in its integrated optical version: a straight channel waveguide is fabricated
on a z-cut LiNbO3 substrate, where a periodically poled region has been patterned
perpendicular to the waveguide [17]. The two dielectric mirrors, directly attached to
the waveguide ends, allow parametric oscillation at the signal and idler frequencies,
which are created from the input pump via non-linear optical interactions. For efficient
optical parametric oscillation the crystal orientation must be adequately chosen, as well
as the periodicity of the ferro-electric domain structure.