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PRESS RELEASE

05 March 2014

Light path controller in fiber optic networks

Fraunhofer Institute for Photonic Microsystems IPMS located in Dresden has developed an optical switch / variable optical power splitter device (OS/VOPS), which can enable fast and almost wear-free distribution of light signals in fiber optic networks. The technology is based on waveguides with liquid crystals integrated on a silicon backplane. This OS/VOPS device could have a wide range of applications.

Both the optical switching function and the power split ratio are controlled by actively adjusting the transmitted optical power on output channels. In this way optical resources in fiber optic networks can be used efficiently. Further measurement techniques based on multiplexed fiber optic sensors can benefit directly from the IPMS integrated optical switching solution. Light signals from sensor networks can now be monitored faster (MHz) and more economically.

Fraunhofer IPMS will present the OS/VOPS technology at the OFC Conference and Exposition to be held March 11-13, 2014 in San Francisco.

Telecommunications, environmental monitoring and industrial process control are high-demand applications, which currently benefit from the fast transfer and distribution of data over fiber optic networks. In particular, communications systems increasingly rely on optical fibers to carry information between locations encoded in a beam of light. Routers and switches are used to channel this information on a best path over intricate networks of optical fibers so that the information reaches the user quickly. Optical power splitters (OPS) are used for optical power management in telecommunications networks. OPSs ensure the important functions of splitting and combining optical signals in optical communications networks. By employing OPSs with variable power split ratio, the optical power can be redistributed in the network dynamically and in real-time, providing advantages such as improvement in network flexibility while maintaining the quality of transmission.

On the other hand, optical fibers can have sensors placed on them, the fiber serving in this case as transport medium for the sensor signal to the signal processing (interrogation) instrument. Fiber optic sensors are increasingly used for structural health monitoring, i.e. to measure changes in strain, temperature, pressure or displacement, as well as for controlling and monitoring changes in the environment. In comparison with conventional electrical sensors, they provide increased sensitivity and capability to process data harvested from sensors placed far away and in dangerous or inaccessible areas. Signals collected from sensors are processed by means of spectrometric methods in interrogation instruments.

The challenge is to provide portable, rugged, cost-effective instruments, able to monitor data acquired from large networks of multiplexed fiber optic sensors over prolonged periods of time. Making use of reliable switches able to facilitate fast transmission of sensor signals to the processing unit is essential here. To date, signals detected by the optical sensors placed on the fibers can be directed into the processing unit at a maximum rate of a few kHz, which means that about 1000 measurements in a second and about 30 billion measurements in a year can be processed in theory.

However, existing technologies such as opto-mechanical switches suffer from limited performance in terms of switching speed, restricted reliability and lifetime due to their mechanical parts as well as high costs. As an alternative, IPMS offers an integrated optical switching / variable power splitting (OS/VOPS) solution, with no mechanical parts, which is promising for both optical telecommunications and remote fiber optic sensing applications. The key features provided by this technology are: fast switching speeds, good reliability and stability of the switching process, scalability towards high channel count and integrability with other devices, large number of switching cycles as well as low insertion loss and low crosstalk.

Referring to the range of applications, technology manager Dr. Florenta Costache states: "For fiber optic sensor network monitoring, our device can ensure switching between channels at frequencies in the MHz range. This means that up to one million measurements can be processed per second. Moreover due to the absence of mechanical parts, the device operates nearly wear-free and can therefore be the technology of choice for long-term monitoring applications. In optical telecommunications networks, this device can provide dynamic switching between different channels, control of power allocated to network nodes and increased efficiency by utilizing optical resources effectively."

The OS/VOPS principle developed by IPMS makes use of an actively controllable guiding effect in electro-optical waveguides.

Dr. Costache describes the effect: "the OS/VOPS device is based on a specially developed electro-optical waveguide structure. Light is guided inside an electro-optical layer along pathways defined by structured electrodes placed on both sides of this layer. Unique features stem from employing highly transparent isotropic liquid crystal blends as waveguide core layer element. Specifically, the optical power transmitted along the waveguide structure is controlled by adjusting the electro-optical Kerr response strength in the liquid crystal layer by an applied electrical field. With this device we could demonstrate sub-microsecond switching, and continuously voltage adjustable, full variable range power splitting at low optical loss. The device has been developed for the telecommunication C-band centered around 1550 nm. Yet, if preferred, the device can be optimized to operate at any other wavelength between 400 nm and 1600 nm. We fabricate this device at the IPMS using precision silicon wafer level technology. This fulfills the prerequisites for high quality, high volume and cost efficient manufacturing."

Nowadays, many applications are sensitive to light polarization and require polarization maintaining components. Equally there are applications which demand polarization independent components, for instance when reducing fiber optic network complexity and costs is an issue. Due to the underlying electro-optical effect the propagation behavior of a light wave along the IPMS liquid crystal waveguide is dependent on its polarization state. This renders firstly the OS/VOPS polarization sensitive.

Dr. Costache's team has recently solved this limitation in their technology as well: the team currently finalizes a solution for a polarization independent OS/VOPS. The development has been possible within the framework of the project Electro-Optical Waveguides based on Liquid Crystals for Integrated Optical Switching (EOF-IOS), contract No. 13N12442, funded by the German Federal Ministry of Education and Research. The project, which will end in Sept 2014, is part of the research initiative Wissenschaftliche Vorprojekte (WiVorPro) within the program Photonic Research in Germany. Future developments in the project are focused on the usage of the polarization insensitive optical switch to directing signals for their cycling analysis in interrogation instruments. These signals will be acquired from multiplexed fiber optic sensors and transmitted along fibers to the optical switch device.

Demonstrations of the described OS/VOPS technology will be presented at the Optical Fiber Communication Exposition OFC to be held March 11-13, 2014 in San Francisco.