Brief History of Integrated Photonics

Brief History of Integrated Photonics

For 30 years after the invention of the transistor, the processing and transmission
of information were based on electronics that used semiconductor devices for controlling
the electron flux. But at the beginning of the 1980s, electronics was slowly
supplemented by and even replaced by optics, and photons substituted for electrons as
information carriers. Nowadays, photonic and opto-electronic devices based on integrated
photonic circuits have grown in such a way that they not only clearly dominate
long-distance communications through optical fibres, but have also opened up new
fields of application, such as sensor devices, and are also beginning to penetrate in the
own field of the information processing technology. In fact, the actual opto-electronic
devices may be merely a transition to a future of all-optical computation and communication
systems.
The history of integrated photonics is analogous to that of other related technologies:
discovery, fast evolution of the devices, and a long waiting time for applications [4].
The first optical waveguides, fabricated at the end of the 1960s, were bidimensional
devices on planar substrates. In the mid-1970s the successful operation of tridimensional
waveguides was demonstrated in a wide variety of materials, from glasses to
crystals and semiconductors. For the fabrication of functional devices in waveguide
geometries, lithium niobate (LiNbO3) was rapidly recognised as one of the most
promising alternatives. The waveguide fabrication in LiNbO3 via titanium in-diffusion
was demonstrated at the AT&T Bell Laboratory, and gave rise to the development
of channel waveguides with very low losses in a material that possesses valuable
electro-optic and acousto-optic effects. In the mid-1980s the viability of waveguide
devices based on LiNbO3, such as integrated intensity modulators of up to 40 GHz,
and with integration levels of up to 50 switches in a single photonic chip had already
been demonstrated in laboratory experiments. A few years later, the standard packaging
required in telecommunication systems was obtained, and so the devices were
ready to enter the market. The rapid boom of monomode optical fibre systems which
started in the 1980s was the perfect niche market for these advanced integrated photonic
devices that were waiting in the research laboratories. Indeed, the demand for
increased transmission capacity (bandwidth) calls urgently for new integrated photonic
chips that permit the control and processing of such huge data transfer, in particular

with the introduction of technology to transmit light in multiple wavelengths (WDM,
wavelength division multiplexing).
Because of the parallel development of other materials, both dielectrics such as
polymers, glasses or silica on silicon (SiO2/Si), and semiconductors such as indium
phosphide (InP), gallium arsenide (GaAs) or even silicon (Si), a wide variety of novel
and advanced integrated photonic devices was ready to emerge on the market. During
the last two decades of the twentieth century we have moved from the development
of the new concept of integrated optical devices to a huge demand for such novel
devices to implement sophisticated functions, mainly in the optical communication
technology market. In fact, at the beginning of the twenty-first century the data transfer
created by computer-based business processes and by Internet applications is growing
exponentially, which translates into a demand for increasing transmission capacity
at lower cost, which can only be met by increased use of optical fibre and associated
advanced photonic technologies (Figure 1.3). Today fibres are typically used to transmit
bit-rates up to 10 Gbit/s, which is, however, far below the intrinsic bandwidth of an
optical fibre. Wavelength Division Multiplexing (WDM) (the transmission of several
signals through a single fibre using several wavelengths) paves the way to transmit
information over an optical fibre in a much more efficient way, by combining several
10 Gbit/s signals on a single fibre. Today there are commercial WDM systems available
with bit-rates in the range of 40 to 400 Gbit/s, obtained by combining a large number of
2.5 and 10 Gbit/s signal, and using up to 32 different wavelengths. The next frontier
in data transfer capacity points to the Terabit transmission, which can be achieved
by using Time Domain Multiplexing (TDM), an obvious multiplexing technique for
digital signals. An equivalent of TDM in the optical domain (OTDM) is also being
developed with the purpose of reaching much higher bit-rates which will require the
generation and transmission of very short pulses, in the order of picoseconds, and
digital processing in the optical domain. Clearly, all these technologies will require
highly advanced optical components, and integrated photonic devices based on planar
lightwave circuits are the right choice to meet the high performance levels required,
which allow the integration of multiple functions in a single substrate