A challenging goal for high-speed optical communications systems is to overcome the speed limitation associated with opto-electronic conversion. Put simply, optical communications involves converting a stream of electrical data such as multiplexed telephone conversations into pulses of light that are transported along an optical fibre to a receiver that reconverts them into electrical signals. The electronics then sorts out the data and routes it to the right customer. Hence an optical communications system is dominantly composed of electronics with light used for transport.
Even in those optical transport links, electronics abound. Although the loss in a typical fiber is low, after about 50km the pulses still have decayed to a level where they must be re-amplified. In the first optical links this was done by changing the pulses back into electrical signals for boosting before reconverting them to light (OEO conversion). This approach had the advantage that the signal could also be ‘regenerated’ to restore the integrity of pulses degraded by noise, dispersion, and so on. The trouble now is that the data rate is determined solely by the speed of the electronics. The result is an electronic bottleneck in the optical communications system.
Figure 1. Bit error rate (BER) performance of a 160Gb/s to 16×10Gb/s all-optical demultiplexer (DEMUX) in a 5cm-long chalcogenide glass waveguide. B2B: Back-to-back.
The first attack on this problem came with the invention of the erbium-doped fiber amplifier (EDFA), which enabled amplification in the optical domain. Furthermore, an EDFA can simultaneously augment many parallel channels of data encoded onto light beams with slightly different frequencies in a process known as wavelength division multiplexing (WDM). This approach vastly increased the capacity of the optical link while simultaneously limiting the data rate in each channel to that of the electronics. WDM overcame the electronic bottleneck, but it did so only at high cost and complexity since each optical channel has to be regenerated by an individual OEO circuit.
The question now being asked by many researchers is, “Can some of this processing be done entirely in the optical domain?” Accomplishing such a feat would involve controlling light in a material that displays nonlinear optical properties to enable the functions—wavelength conversion, switching, and regeneration—that are vital for today's communication networks. The major difficulty is that the nonlinear optical response of most materials is weak, and it is difficult to make devices that work at sufficiently low power and high speed. A common technique is to use a semiconductor optical amplifier (SOA), which provides a large nonlinear response in a compact device. SOAs cannot really handle very high data rates, however, owing to the slow response speed of their nonlinear medium. Very recently there has been a surge of interest in silicon as a suitable material, but it, too, becomes problematic above 20Gb/s.
Research in Australia's Centre for Ultrahigh Bandwidth Devices for Optical Systems—CUDOS—focuses on a different nonlinear optical material, so-called chalcogenide glasses. These glasses contain sulfur, selenium, and tellurium bonded to network-forming elements such as germanium and arsenic. They display an instantaneous nonlinear response that allows data to be processed at almost unlimited rates. Being a glass, they can easily be processed to form low-loss (<0.1dB/cm) compact optical waveguides needed for all-optical processing. Their main disadvantage compared with semiconductors is that their nonlinearity is smaller. Nonetheless, it still appears adequate to fabricate devices that operate at powers compatible with current optical communications systems.
We and other colleagues recently demonstrated demultiplexing of a 160Gb/s data stream into 16×10Gb/s channels using a simple 5cm-long arsenic-sulfur glass waveguide. Figure 1 shows the bit error rate for all 16 demultiplexed channels at 10GB/s, showing a low 1dB power penalty.1 All-optical regeneration2 and wavelength conversion3 have also been demonstrated, completing a basic toolbox for all-light processing.
The main challenge is now to reduce the operating power of the devices while simultaneously increasing the data rates at which they operate. Fortunately, there is a clear route to lowering operating powers by a factor of ≈25–50 and a more speculative route to another factor of ≈5. In most devices a reduction in the required power immediately allows them to operate at increased data rates. The target is to demonstrate all-optical processing at rates up to 640Gb/s using pulses with peak power less than 1W—a challenging but achievable target, and one that will be difficult to realize using other materials.
The authors gratefully acknowledge funding from the Australian Research Council.
Barry Luther-Davies, Stephen Madden
Centre for Ultrahigh Bandwidth Devices for Optical Systems
Laser Physics Centre, Research School of Physical Sciences and Engineering
Australian National University
Barry Luther-Davies is a Federation Fellow and head of the Laser Physics Centre at the Australian National University. He is also a chief investigator in the Australian Research Council Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems.
Vahid Ta'eed, Mark Pelusi, Ben Eggleton
Centre for Ultrahigh Bandwidth Devices for Optical Systems
School of Physics
University of Sydney
1. M. D. Pelusi, V. G. Ta'eed, M. R. E. Lamont, S. Madden, D. Y. Choi, B. Luther-Davies, G. J. Eggleton, Ultra-high nonlinear As2, S3 planar waveguide for 160 Gb/s optical time-division demultiplexing by four-wave mixing, Photon. Technol. Lett., in press.
2. V. G. Ta'eed, M. Shokooh-Saremi, L. B. Fu, D. J. Moss, M. Rochette, I. C. M. Littler, B. J. Eggleton, Y. L. Ruan, B. Luther-Davies, Integrated all-optical pulse regenerator in chalcogenide waveguides, Opt. Lett. 30, pp. 2900-2902, 2005.
3. V. G. Ta'eed, M. R. E. Lamont, D. J. Moss, B. J. Eggleton, D.-Y. Choi, S. J. Madden, B. Luther-Davies, All-optical wavelength conversion via cross-phase modulation in chalcogenide glass rib waveguides, Opt. Express 14, pp. 11242-11247, 2006.