10 Mar Optimized transmission: spatial multiplexing and bandwidth reduction
Today, fiber optic networks use techniques such as wavelength division multiplexing. This involves splitting the light into different wavelengths using a spectrometric grating. The wavelenths are sent to a liquid crystal on silicon (LCoS) mirror, which forwards the signals to an optical fiber, allowing several data streams to be transported in each fiber. However, the process can only be used in a limited frequency spectrum. The Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena worked with partners on the new fiber optic technology in the ‘Wesoram’ (wavelength-selective switches for optical spatial multiplexing) and ‘Multi-Cap’ projects.
Cross-connection: one frequency on several fibers
In Wesoram, Dr. Steffen Trautmann and his team at Fraunhofer IOF have further developed the technology together with the project partners. First, the team made the switching mechanism of the LCoS switch so flexible that it enables the data stream to be forwarded to any number of fibers. After the grating has split the incoming light signal into frequencies, the liquid crystal mirror sends each frequency to a different fiber. The conventional wavelength-division multiplexing process is thus extended to a space-division multiplexing process. In addition to the ‘multiple frequencies – one fiber’ principle, the ‘one frequency – multiple fibers’ principle can also be applied.
Trautman explains: “In our project, we have succeeded in sending the signals from eight input channels to 16 output channels as required.” This cross-connection increases the capacity of the networks, as the transmission and forwarding of data streams becomes much more flexible. “This is particularly useful when data is sent over longer distances, for example between cities,” says Trautmann. According to the researchers, another advantage is that fewer optical switches are required for the fiber optic network overall. This reduces the costs of installation and ongoing operation.
Smaller data packets thanks to higher resolution
In a next step, the resolution of the optical module was increased using a newly developed grating, explain the researchers. “Currently, a spectral resolution of 100 GHz (around 0.8 nm) is the state of the art,” says Trautmann. “The mirror we developed achieves up to 25 GHz (around 0.2 nm).” Due to the higher resolution, the light frequency of the data stream is 4 times narrower and the data packets are correspondingly smaller. This means that many more data packets can be transmitted simultaneously through the optical fibers.
Expansion to multi-core fibers
Wesoram is complemented by the Multi-Cap project. According to the researchers, they are working on increasing the number of channels for parallel data transmission. Conventional optical fibers contain one data channel and one signal core, whereas multi-core fibers use several cores for data transmission. Although these cables contain many more fiber cores, they are hardly any thicker. The team at Fraunhofer IOF has developed the signal amplifiers required for multi-core fibers. These can serve up to twelve channels simultaneously and provide an amplification of more than 20 dB per channel. According to the researchers, the technology is significantly more energy-efficient, as only one amplifier module is required for twelve channels.
Multi-Cap project: The amplifier can amplify the signals in up to twelve data channels in one fiber strand. Image: Fraunhofer IOF.
Wesoram partners involved
The project partners were Adtran Networks from Meiningen and the Berlin-based company Holoeye Photonics, which specializes in optical systems and built the liquid crystal mirror. The experts at Fraunhofer IOF were responsible for the optical design, developed a beam splitter for the spectrometric grating using ultra-precision technology and integrated all components into a single device.
Both projects were funded by the Federal Ministry of Education and Research and the Association of German Engineers VDI.
Source and image: www.iof.fraunhofer.de
