Multichannel Switch Improves Fiber Optic Networks

Dr. Barbara Stumpp, Freelance Writer

Fiber optic networks must be reliable, and fast fiber optic switches help make that happen.

As data transmission continues to grow, operators are being forced to trim their fiber optic networks for maximum performance and reliability, which means that data networks must be flexible and require an increasingly higher standard of safety. Continuous monitoring leads to early detection of network errors and, therefore, to higher-quality service as well as shorter network down times. To guarantee functions such as stable wavelength multiplexing and efficient power monitoring, operators are increasingly using test equipment to “light inside” the networks. Suitable test equipment and especially powerful fiber optic switches are required to realize this with high accuracy and at as many points as possible.

But optical fibers don’t only transmit data. They also are well suited as sensors because their optical properties, such as absorption, polarization or fluorescence, change depending on external influences. This process moves the light’s intensity, phase or polarization inside the fiber, which allows distinct conclusions about the influence as well as the location of this influence. With Bragg gratings written inside the fiber, temperature, strain and stress can be measured more accurately than with conventional methods. Further, several hundreds of such measuring points can be arranged in one fiber in line, one behind the other, and – as a result of wavelength multiplexing – be read out separately.

Thin, lightweight, flexible and wear-resistant optical fibers can also be integrated directly into structures and materials. Fiber optic sensors are ideally suited for use in complex machines with limited room for sensors, in oily environments and/or with strong electromagnetic fields of machine drives. Integrated into bridges, buildings or even in the blades of wind turbines, they work with unrivaled reliability, allowing engineers to monitor buildings, offshore wind farms, slip-prone slopes, embankments, rails and many other objects in the long term and in real time. Multiplexing within the fiber is no longer sufficient, especially at extremely extended sensor networks, as found in the monitoring of dams, tunnels and pipelines; in this case, the performance of several fibers must be known. Analysis of these multifiber setups is done sequentially. Because the sensing devices for these signals – known as interrogators – are quite expensive, it is an advantage to read out as many sensor fibers as possible with one interrogator using fiber optic switches.

A new switch called FiberSwitch from the Fiber Optics Business Unit of the Leoni Group meets virtually all requirements for applications in telecommunications and for sensor networks. Leoni Fiber Optics offers products at every stage of the value chain, from fused silica to preforms and drawn fibers, fiber optic cables and complete fiber optic systems with in-house-developed optical components such as optical switches, splitters and couplers.

The new FiberSwitch from the Leoni Group has potential for applications in telecommunications and for sensor networks. Here, a schematic showing its switching principle. Courtesy of Leoni.

Development of the new fiber switch was initiated by collaboration in the joint research project CONDOR (Converged Heterogeneous Metro / Access Infrastructure), supported by the German Federal Ministry of Education and Research, working with partners Alcatel-Lucent, JDSU Germany and the Karlsruhe University institutes IPQ and ITIV. For three years, Leoni researched a design for optical heterogeneous open-access networks to make them safer and more flexible.

To sequentially access high numbers of channels and to ensure fast switching, the engineers had to consider a tricky concept: “The basic principle of the new system is a piezo-driven two-dimensional movable tilting mirror,” said Dr. Peter Riedel. “Using this setup, we are able to access – very fast – a very high channel count without cascading of switches, as follows with a very low insertion loss.”

This illustration shows FiberSwitch in operation, switching one channel to another. Courtesy of Leoni.

The input and output fibers are arranged as a matrix, and the end face of each fiber is fixed to a graded index (GRIN) lens in such a way that the fiber and the lens have a common optical axis. This GRIN lens is made of a transparent material with a refractive index that decreases in the radial direction. A short rod of this material works like a normal lens, but it has exact plane surfaces on those sides at which light enters and leaves, simplifying the installation and the docking of the optical fiber. Another advantage is the smaller diameter of GRIN lenses in comparison to glass lenses. Therefore, in a given area, a much higher number of fiber and lens combinations can be placed and, as follows, the channel count per switch can be increased.

The beam emerging from the new switch’s GRIN lens reaches a piezo-controlled tilting mirror via a suitable imaging optic. The tilting mirror sends the reflected beam through the same optical system to the GRIN lens array and thus into the target fiber. The tilting mirror can be tilted up to 1.5° in two directions out of the optical axis of the imaging system. Provided a suitable optical setup, the reflected beam reaches each of the GRIN lenses of the array. The excellent angular accuracy of the feedback-loop-controlled piezo-driven mirrors ensures the highest repeatability when adjusting the tilt angle and, as follows, an efficient coupling of the light beam in the desired output channel.

“This structure allows [us] to precisely operate several hundred output channels with minimal insertion loss, and mainly independent from the channel count,” Riedel said. “Thus, a multimode switch with 100 channels (fiber type SI 100/125) has a maximum insertion loss of 1.0 dB and a switching time of less than 2 ms.”

A new fiber optic switch can serve more than 1000 channels for applications with wide-area networks and more. Courtesy of Leoni.

With these fiber switches, wide-area networks and other fiber optics applications can be operated efficiently and precisely even with channel counts of more than 1000.

“For our customers, this system offers a very affordable price ratio per channel, with minimal insertion loss,” Riedel said. “In the single-mode range, there are indeed comparable systems – but at much higher costs. In the multimode region, our system is one of the world’s most powerful.”

Meet the author

As a free technical journalist, Barbara Stumpp supports various companies in their external communication of complex products; email: bstumpp@gmx.de.