FIBER FOR CONSUMER APPLICATIONS: New fiber designs extend the reach of short-range POF

OLAF ZIEMANN

Polymer optical fiber is benefiting from new modulation techniques and polymer designs that are extending data rates to 100 Gbit/s and improving POF performance for sensors, data-center interconnection, and home networks.

Polymer optical fiber (POF) is an optical waveguide with core and cladding made of polymer materials. In comparison to silica glass fiber, this technology enables much more flexible, and typically, thicker cables. In addition to its successful deployment for short-distance, medium-bit-rate communication applications, new modulation schemes and alternate polymer designs are extending its reach into longer-distance communications and interconnects for consumer networks, as well as for fiber sensor applications.

The most common type of POF is a polymethylmethacrylate (PMMA)-based fiber with a step-index profile, called step-index POF (SI-POF). This 1 mm core diameter and 0.50 numerical aperture (NA) fiber is typically called standard POF and is described by the International Electrotechnical Commission (IEC; www.iec.ch)—a not-for-profit, nongovernmental international standards organization that prepares and publishes international standards for electrical, electronic, and related technologies—in IEC document 60793 2 40, section A4a.2. The loss of SI-POF at a wavelength of 650 nm (red light) is less than 180 dB/km and the loss at 520 nm (green) is approximately 110 dB/km.

Because of its step-index profile, there are large differences between the propagation time of different modes (typically 2.5 million modes in 1-mm-diameter SI-POF), which limits the bandwidth to about 40 MHz at 100 m—suitable for short-reach applications only. The main reason for the high losses in PMMA-based POF is absorption at the carbon-hydrogen bonds. The only way for a significant reduction of the attenuation is the substitution of hydrogen by heavier atoms.¹ To do this, Asahi Glass (Tokyo, Japan) first proposed the use of a transparent fluor-polymer material for optical fibers. Its Lucina fiber has loss as low as 10 dB/km at 1300 nm. The core diameter of this class of perfluorinated graded-index plastic optical fiber (PF-GI-POF) is manufactured in the range between 50 and 200 µm, depending on the intended application. Its graded-index profile reduces the propagation delay and significantly increases the bandwidth.

Sensors

Glass-fiber sensors offer a wide range of new applications (see www.laserfocusworld.com/articles/320513). Big advantages include high sensitivity for many physical parameters (temperature, mechanical stress, and both presence and concentration of various chemicals), long reach (tens of kilometers), and immunity to electromagnetic fields and radiation. Unfortunately, the fragility and difficulty in handling the fiber along with sophisticated techniques necessary for data acquisition preclude the use of glass-fiber sensors in mass markets at this time.

Although POF-based sensors have reduced reach and sensitivity, their low cost and adequate optical transmission make them excellent candidates for many types of sensors. For example, we have developed a prototype liquid-level sensor that monitors fluid level based on a change in transmission for a bent fiber (see Fig. 1).² This sensor can be used for detecting the level of explosive or flammable liquids in a storage container. The principle behind the liquid-level sensor is the refractive-index difference between air and a liquid. Tens of fibers are arranged in a bundle, with each fiber having a tight bend at a different height. As long as this bend is in air, total internal reflection holds the light in the fiber. If the bend is surrounded by a liquid, the waveguiding mechanism is destroyed and the transmission sharply decreases.

Data communications

The most interesting consumer application for POF is its use in short-distance data transmission. About 55 different automobile models are equipped with POF-based Media Oriented System Transport (MOST) systems that use POF to connect audio, video, communication, and navigation devices within cars. This bus architecture uses red light-emitting diodes (LEDs) as sources and SI-POF as the transmission medium for the connection of electronic components such as CD players, video screens, and driver-assistance devices. The maximum bit rate over a link length of less than 10 m is 50 Mbit/s. Next-generation MOST systems will run at 150 Mbit/s using the same source and fiber components.

Bit rates up to 12 Mbit/s are used in automation field-bus systems; field busses are specially developed serial data transmission busses for automation and control applications. The use of POF in this application—with maximum link lengths of typically 70 m—has been well established over two decades. For longer distances, the use of hybrid glass/polymer fibers is an option with some new systems using industrial Ethernet technology with 125 Mbit/s physical bit rate.

The application for POF with possibly the largest potential for success is its use for in-home networks. The percentage of broadband connections is greater than 50% in many industrial countries. Currently, the typical bit rate is less than 10 Mbit/s. Powerline and WLAN, respectively, are the preferred technologies for in-home networks. Powerline Communication (PLC) is the transmission of digital data over existing power-supply cables (in the frequency range below 30 MHz with a network capacity of up to about 85 Mbit/s). Wireless local-area network (LAN), specified in the standards family IEEE 802.11, is a radio-based short-reach communication technology with up to 360 Mbit/s capacity in two license-free bands at 2.4 and 5.4 GHz.

In DMT modulation, the available frequency range is split into several carriers that are modulated with different quadrature amplitude modulation (QAM) levels, depending on the individual signal-to-noise-ratio. This enables the most efficient use of the channel capacity. Digital filters such as FF/DFE reconstruct the bit sequence by estimation of the real channel response. In case of POF, the mode dispersion due to the different transit times can be compensated for.

For an LED-based transmission using DMT, a bit rate of 1.5 Gbit/s is possible over a 50 m POF link (power limited).8 The use of LED sources enables simpler, more reliable, and burst-mode operated transceivers (in which the transmitter is switched on only for the time of real data transmission in order to save energy). The realization of more than 1 Gbit/s data transmission on 100 m SI-POF links is proof of the future usability of the standard A4a.2 SI-POF for in-home networks; in fact, some initial products operating at this higher speed are now available. Next-generation 1-mm-diameter PMMA-POF with a graded-index or a multicore structure will increase the capacity even more, without losing the advantages of simple handling and ability to be integrated with low-cost LED light sources.

POF for interconnection

Another major application for POF in the near future is in data interconnection links. The introduction of 100 Gbit/s per channel in long-distance wavelength-division-multiplexing (WDM) systems, the increasing speed of supercomputers, and the construction of new larger data centers will require new high-speed interconnection solutions.

The transmission of 40 Gbit/s over 100 m PF-GI-POF was demonstrated for the first time by Arup Polley and Stephen Ralph from the Georgia Institute of Technology (Atlanta, GA).⁸ Leveraging this work, active optical cables based on GI-POF combine the easy coupling of POF with a very robust cable and can be used for high-speed interconnects over short and medium distances (less than 100 m).⁹

Next-generation POF transceivers are anticipated to offer a data speed of at least 1 Gbit/s per link. These low-cost transceivers with LED sources pave the way for very reliable systems with low power consumption, with PF-GI-POF making possible even higher bit rates up to 100 Gbit/s.

REFERENCES

1. O. Ziemann et al., POF-Handbook — Optical Short Range Transmission Systems, Springer-Verlag Berlin and Heidelberg GmbH & Co. KG (February 2008).
2. H. Poisel et al., POF 2007 Conf., Torino, Italy, 178 (September 2007).
3. O. Ziemann et al., PRAXIS PROFILINE —Triple Play, 14 (July 2007).
4. O. Ziemann et al., POF 2008 Conf., Santa Clara, CA (August 2008).
5. O. Ziemann et al., invited paper OWB1, OFC 2008, San Diego, CA (February 2008).
6. J. Lee et al., POF 2008 Conf., Santa Clara, CA (August 2008).
7. B. Charbonnier et al., POF 2008 Conf., Santa Clara, CA (August 2008).
8. A. Polley and S. E. Ralph, IEEE Photonics Tech. Lett. 19(16) 1254 (2007).
9. D. DuToit, POF 2008 Conf., Santa Clara, CA (August 2008).

OLAF ZIEMANN is scientific director of the Polymer Optical Fiber Application Center, Wassertorstrasse 10, D-90489 Nürnberg, Germany; e-mail: [email protected]; www.pofac.de.