The integrated optics ( IO ) is the branch of optical engineering that deals with the development of integrated optical systems employed. These systems are housed on a substrate and are characterized by a high level of functionality ( light sources , waveguides , beam splitters , intensity or phase modulators , filters , switches, etc.). The integrated optics are comparable to integrated circuits (ICs), but the integration density is not as high as with ICs.
The first integrated optical components on the order of a few square centimeters were developed as early as the 1970s. In the 1990s, integrated optical elements were used in data networks. The development benefits from GaAlAs and InGaAsP laser diodes, low-loss glass fibers and lithography. Integrated optical components have been used in the consumer sector since the 1990s, e.g. B. for CD players, CD-ROMs, and in optical communications.
The aim of integrated optics is to accommodate all of the functionalities required to set up an optical communication network on an integrated optical circuit and to avoid the detour via electrical signals.
Typical materials used in integrated optics are glass, silicon , polymers (especially photopolymers ) and dielectric crystals, for example lithium niobate . The latter has interesting electro-optical , acousto-optical and non-linear optical properties. In order to produce optical circuits with certain functions from this material, the crystal is doped with titanium , processed with proton exchange processes or doped with elements from the group of rare earths .
The main area of application for these components that work with ultrasonic waves is in communication systems. Wavelength filters, switches and multiplexers are produced.
Micro-optic lasers, amplifiers and doping elements
In order to manufacture laser-active components or optical amplifiers, glasses or crystals are doped with elements from the rare earth group ( praseodymium , neodymium , erbium , thulium , ytterbium ). The most interesting is erbium, since the crystals, glasses and optical waveguides doped with erbium can generate or amplify infrared radiation in the range around 1550 nm. At this wavelength, optical waveguides made of quartz glass have a minimum attenuation, which is why this wavelength range is primarily used in fiber optic networks in telecommunications .
Semiconductor lasers are often used directly as a radiation source . You can also work at 1550 nm.
Mixers and optical parametric oscillators
Mixers, frequency multipliers and optical parametric oscillators ( OPO ) are used for frequency conversion in order to generate coherent light in other frequency ranges from coherent light of one frequency. There are frequency ranges that cannot be covered with current laser sources. With a non-linear element, laser light can be converted into a different frequency range or a tunable laser can be created.
Passive integrated optical components
Passive integrated optical components are planar optical waveguide structures (PLWL, PLC) in which several passive waveguide functions are monolithically integrated on a chip. Such components are now used in large numbers in fiber optic transmission systems (FTTH). The function of these components is mainly to distribute the light signals from one transmission fiber to many fibers or to reverse them. Such branches, also called splitters, enable a tree structure as required in the PON systems (e.g. G-PON). Today, single-mode splitters 1 × N and 2 × N with up to 64 channels are commercially available and can be used in the entire transmission range of standard telecommunications fibers from 1260 to 1650 nm.
The longest known and tried and tested technology for the production of such components is based on the ion exchange process in glass ( Ken Koizumi, 1971 ). In this case, sodium ions of the glass are replaced by silver ions in a locally limited manner by means of a corresponding photolithographically produced metal mask. The silver ions cause an increase in the refractive index in the paths given by the mask and thus form the waveguide structure.
This initially purely thermal ion exchange results in surface waveguides whose geometry and transmission properties do not yet meet the technological requirements in terms of damping and environmental stability. This is achieved through a second diffusion carried out in an electric field, in which the near-surface silver ions are buried with sodium ions from a molten salt into the interior of the glass. The waveguides obtained in this way lie approx. 15 micrometers below the glass surface and show excellent transmission properties and long-term stability. Waveguides manufactured in this way have been in use in the OPAL networks of Deutsche Telekom since 1993 and show no signs of degeneration. In Germany, such waveguide components were developed by the company IOT (former subsidiary of Schott Glas and Carl Zeiss) as part of a national research and development project funded by the German government in the 1980s and are now manufactured by the company LEONI Fiber Optics GmbH.
An alternative chip technology is based on the deposition process of quartz glass or doped quartz glass layers on a substrate made of silicon or quartz glass. The waveguide structures are created by etching out of a higher refractive index layer (e.g. a germanium-doped quartz glass layer). The resulting structures are then covered by a further layer of quartz glass. Passive structures produced in this way are referred to as “Silica on Silicon” or “Silica on Silica” waveguides (SiOS). Like the ion-exchanged waveguides, the SiOS waveguides are also low-attenuation and broadband. However, because of the layer structure made of materials of different thermal expansion, they have a significantly higher polarization sensitivity, in particular at fluctuating temperatures.
The following properties are important for the splitter components for telecommunication applications (typical values for the example 1 × 8 splitter are given in brackets):
- Wavelength range: 1260 to 1650 nm
- Insertion loss: <10.8 dB †
- Uniformity: <1 dB
- Return loss: <55 dB
- polarization-dependent losses: <0.15 dB
- temperature-dependent losses: <± 0.1 dB
- Working temperature: −40 to 85 ° C
By modifying the manufacturing parameters, other waveguide properties can also be developed. Passive integrated optical waveguide chips are now also possible for wavelength ranges down to 600 nm. More complex structures such as interferometers or wavelength-dependent functions can also be implemented. Such more complex optical chips are of interest for various applications such as sensor technology, measurement technology, diagnostics, etc., because they offer the possibility of great miniaturization and considerable cost savings through integration.
- Robert G. Hunsperger: Integrated Optics: Theory and Technology . 6th edition. Springer, New York 2009, ISBN 978-0-387-89774-5 .
- D.Kip: Introduction to Integrated Optics. (PDF; 11 kB) Institute for Physics and Physical Technologies at Clausthal University of Technology, September 2002, accessed on November 10, 2012 .
- Ludwig Roß: Integrated optical components in substrate glasses . In: Glass technical reports . tape 62 , no. 8 , 1989, pp. 285 ff .
- Extra wide . Fast internet via fiber optics. In: c´t . No. 3 , 2009, p. 80 ff .