Photonic integrated circuit

The integrated optics ( IO ) is the branch of optical engineering that deals with the development of integrated optical systems. These systems are housed on a substrate and are characterized by a high degree of functionality ( light sources, waveguides, beam splitters, intensity or phase modulators, filters, switches etc). The integrated optics is similar to integrated circuit (IC), however, the integration density is not as high as in the ICs.

First integrated optical components in the order of a few square centimeters have been developed in 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. Since the 1990s occurred integrated optical component in the consumer sector, for eg CD player, CD -ROM, as well as in optical communications technology on.

The aim of integrated optics is all necessary for construction of an optical communication network functionalities to accommodate an integrated optical circuit and avoid the detour via electrical signals.

Materials

Typical materials for integrated optics include glass, silicon, polymers (especially photopolymers ) and dielectric crystals, for example Lithium Niobate. The latter has interesting electrooptic, acoustooptic, and nonlinear optical properties. To prepare optical circuits with specific features of this material, the crystal is doped with titanium, edited by proton exchange method or doped with elements from the group of rare earths.

Components

An acousto-optical circuits

The main application area of ​​this working with ultrasonic waves components are communication systems. Are prepared wavelength filters, switches and multiplexers.

Micro-optical lasers, amplifiers and doping elements

To produce laser-active components or optical amplifier, glasses or crystals are doped with the elements from the group of rare earths ( praseodymium, neodymium, erbium, thulium, ytterbium ). Most interesting is erbium, as the erbium-doped crystals, glasses and optical fibers can amplify or generate infrared radiation in the region of 1550 nm. At this wavelength optical waveguides made ​​of quartz glass having a minimum attenuation, so this wavelength range is primarily used in fiber-optic telecommunications.

Be pumped erbium -doped Lithiumniobatlaser and erbium - doped fiber amplifier with diode lasers with a wavelength of 980 nm or 1480 nm The chart on the right shows the energy levels.

The radiation source in many cases are also semiconductor laser for direct use. You can also operate at 1550 nm.

Mixer and optical parametric oscillators

Mixers, frequency multipliers and optical parametric oscillators ( OPO ) are used for frequency conversion of coherent light to generate a frequency coherent light in other frequency ranges. There are frequencies that can not be covered with the current laser sources. By a non-linear element can be laser light converted into a different frequency range or a tunable laser may be created.

Passive integrated optical components

Passive integrated optical components are planar optical waveguide structures ( PLWL, Eng. PLC ) in which a plurality of passive waveguide functions are monolithically integrated on a chip. Such devices are now used in large quantities in optical fiber transmission systems ( FTTH). The function of these components is essentially the distribution of the light signals of a transmission fiber to many fibers or their reversal. Such branching, also called splitter, allowing a tree structure, as they are in the PON systems required (eg G- PON). Today, single-mode splitter 1 × N and 2 × N with up to 64 channels are commercially available and are the entire frequency range of a standard telecom fiber from 1260 to 1650 nm can be used.

The longest known and tested technology in use for the production of such components based on the ion exchange process in glass ( Ken Koizumi, 1971). Here, sodium ions in the glass are limited locally replaced by silver ions by an appropriate photolithographically produced metal mask. Silver ions cause a refractive index increase in the paths defined by the mask, thus forming the waveguide structure.

In this first purely thermal ion exchange caused surface waveguide, not yet satisfy their geometry and transmission characteristics of the technological requirements for attenuation and environmental stability. This is achieved by a second carried out in the electric field diffusion, in burying the near-surface silver ions with sodium ions from the salt melt into the glass interior. The waveguides thus obtained are about 15 microns below the glass surface and show excellent transmission characteristics and long-term stability. In the OPAL networks of Deutsche Telekom waveguides are produced in this way since 1993 in use and show no signs of degeneration on. In Germany such waveguide components by the company IOT ( former subsidiary of Schott glass and Carl Zeiss ) were developed as part of a project funded by the German Federal Government's national research and development project in the 1980s and are now manufactured by the company LEONI Fiber Optics GmbH.

An alternative technology is based on the chip deposition of doped fused silica or fused silica layer on a substrate of silicon or quartz. Here, the waveguide structures created by etching out of a high-index layer (eg, a germanium- doped silica glass layer). The resulting structures are then covered by a further layer of quartz glass. This is referred to such kind of passive structures as "Silica on Silicon " or " silica on silica " waveguide ( SiOS ) produced. As the ion-exchanged waveguides are the SiOS waveguide also low loss and broadband. However, they have because of the layer structure made ​​of materials of different thermal expansion significantly higher polarization sensitivity, especially under fluctuating temperatures, to.

In Verzweigerkomponeten for telecommunications applications are the following properties of importance (in brackets the typical values ​​for example, the 1 x 8 splitter respectively indicated ):

• Wavelength Range: 1260-1650 nm
• Insertion loss: < 10.8 dB †
• Uniformity: <1 dB
• Return Loss: <55 dB
• Polarization dependent loss: < 0.15 dB
• Temperature-dependent losses: <± 0.1 dB
• Working temperature: -40 to 85 ° C

By modifying the processing parameters, other waveguide properties can be developed. Passive integrated optical waveguide chips are now also available for wavelength ranges down to 600 nm. More complex structures, such as interferometers or wavelength-dependent functions can be realized. Such complex optical chips are used for various applications such as sensors, measurement technology, diagnostics, etc. of interest because they offer the possibility of a strong miniaturization and significant cost savings through integration.

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