opinion photonics
rhombus4
manufacturing uniformity, and commercial
challenges. While optical components can
be built using many materials, including
indium phosphide (InP), gallium arsenide
(GaAs), lithium niobate (LiNbO3), silicon
(Si), and silica-on-silicon, widespread use of
large-scale PICs has to-date been limited to
those built in either silica-on-silicon or InP.
4.0 Functional attributes of
photonic integration
A monolithically integrated photonic
integrated circuit consolidates many devices
and/or functions into a single photonic
material. As in electronic semiconductor
ICs, the fabrication of monolithic PICs
involves building devices into a common
substrate so that all photonic couplings
occur within the substrate and all functions
are consolidated into a single, physically
unique device.
Historically, many technologies have
experienced, through their lifecycle, initial
size reductions for a similar function. These Table 1: Different levels of photonic integration offer differing level of benefits
size reductions have generally involved depending on the degree of integration achieved (Infinera)
hybrid assembly. In a hybrid assembled PIC,
multiple single or multi-function optical
devices are assembled into a single
package, sometimes with associated packaging complexity and cost. In practice, categorizations below provide a reference
electronic ICs, and are interconnected to this has limited hybrid PICs integration on the scale of photonic integration
each other by electronic and/or optical levels in the 10s of devices. Many integrated achieved based on how many optical
couplings internal to the package. Generally, photonic devices available today, however, devices/functions are integrated:
the assembly of hybrid integrated utilize hybrid integration to consolidate
components is more complex than for packaging of both photonic and electronic Small-scale PICs (SS-PICs): These circuits
monolithically integrated PICs due to the ICs, and a number have increased range from 2 to 10 functions or components
need to interconnect multiple discrete component counts to the low 100s. integrated into a single monolithic substrate.
devices with sub-micron tolerances required Examples include lasers with integrated
for aligning optical components. Adding to To better understand the types of photonic modulator and maybe some ancillary
the packaging challenge is the fact that integration technologies, Table 1 compares components like rear-facet PIN monitor,
different materials may require different key metrics relevant to photonic integrated variable optical attenuators (VOA), or tuning
packaging designs due to differences in circuits. element. Small-scale PICs typically only
optical, mechanical, and thermal integrate a few functions for a single
characteristics. 5.0 Scale of photonic integration wavelength, or several channels of the same
In a manner similar to that developed to device (i.e., laser diode or PIN arrays).
For example, if two materials have different categorize silicon ICs, the degree of optical
coefficients of expansion, they can become integration achieved in a photonic integrated Medium-scale PICs: These circuits range
misaligned at different operating circuit can be categorized based on how from about 10 to 100 functions or
temperatures and require different thermo- many distinct devices and/or functions are components integrated into a single
electric coolers, thus compounding integrated into a single device. The monolithic substrate. This can include the
if two materials have different coefficients of expansion, they can become misaligned
at different operating temperatures and require different thermo-electric coolers, thus
compounding packaging complexity and cost
October 2009 www.compoundsemiconductor.net 27
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