Heinrich Hertz Institut f�r Nachrichtentechnik Berlin GmbH [email protected]
	HHI Press Releases 1998 September 1998

Photonics Research Results:
 

640-Gbit/s WDM/TDM Experiment

A New Switch for Optical Time Division Multiplexing Systems, Demonstrated in a 640-Gbit/s WDM/TDM Experiment

The Internet has triggered an unparalleled flood of data communication in all national and international communication networks that is far too large for many transmission systems to cope with.
However, jams on data superhighways are caused not only by overburdened transmission lines, but also by the junction where switches control the flow of signals, i.e. the so-called "accesses to and exits from" the highways.

The Heinrich-Hertz-Institut (HHI) has developed an all-optical switch which was able to handle 640 Gbit/s data streams during first trials.

Project director Prof. Dr. Weber explains that this new optical switch overcomes all the major shortcomings of interferometric switches based on semiconductor laser amplifiers. This new switch developed by the HHI outperforms conventional switches in the following parameters: linearity, low noise, large optical bandwidth, high switching contrast and little crosstalk.

As early as 1996, laboratories at the HHI had successfully designed and demonstrated the functioning capability of a monolithically integrated OTDM switch which allowed targeted dropping of a 5-Gbit/s signal from a 20-Gbit/s multiplex signal, as well as the adding of a 5-Gbit/s signal to a 20-Gbit/s multiplex signal. Demultiplexing was even demonstrated at a data rate of 40 Gbit/s.

The all-optical switch which has now been developed on the principle of the so-called "cross-phase modulation" represents a major improvement in comparison to the switches which are currently used with optical time division multiplexing techniques. In these "interferometric" switches, an optical control signal induces a phase change in the semiconductor amplifier which is, however, also accompanied by a change in the amplitude of the data signals. This has a negative effect in the form of non-linear distortion of the signal and a low switching contrast.

The crucial improvement of the switch by the HHI researchers is that they substantially increased the wavelength distance between control signal and data signal and that they used a laser amplifier transparent for the wavelength of the data signal. This means that the control signal exclusively changes the phase, rather than the amplitude of the data signal. This invention (patented under DE 198 05 413.0 ) ensures excellent properties in terms of linearity, low noise, large optical bandwidth, high switching contrast and little crosstalk.

The project was sponsored by the German Federal Ministry for Education, Science, Research and Technology (Bundesministerium f�r Bildung, Wissenschaft, Forschung und Technologie (BMBF)) and by the Deutsche Forschungsgemeinschaft (DFG).
 
 
 
 

For the specialist press

Ultra-fast optical switches and sensors find the most various uses in optical signal processing, for example, as time division demultiplexers for high-rate TDM * data streams or as instruments for monitoring the quality of optical signals of a transmission system. When highest bit rates of more than 40 Gbit/s are of interest, opto-electronic approaches are no longer suitable for switching functions because of the limited speed of electronic components, hence demanding the use of all-optical switching methods. The currently most promising, all-optical switches are based on the use of non-linear effects in semiconductor optical amplifiers ( SOAs) in interferometric arrangements. For this purpose, a strong optical control pulse is launched into the SOA and the carrier density in the semiconductor material is reduced. The resultant gain reduction is accompanied by a change in the refractive index, so that the data signal experiences a change in phase. Whilst this phase change is used for interferometric switching, the change in gain is, however, undesired because it affects the contrast ratio of the switch and causes modulation of the transmitted data signals. Another disadvantage of these conventional switches is that a substantial noise component is added to the signal because the data signal is within the spectral range of the gain of the SOA. The patented, all- optical switch overcomes these drawbacks.

Fig. 1a illustrates the principle of operation of the new, gain-transparent SOA switch in its application as an add/drop multiplexer in a Sagnac interferometer arrangement. Whilst the control and data signals are in the order of 1.55 �m in conventional interferometric switches, so that an SOA for 1.55 �m is used, the key component of the new switch is an SOA which is suitable for amplifying optical signals at 1.3 �m.

The new switch is technically of great interest for demultiplexing and add/drop multiplexing applications in highest-bit-rate TDM and combined TDM/WDM* systems. Operation of the switch was already successfully demonstrated in a 640-Gbit/s demultiplexing experiment, where 8 WDM channels, each with a 10 Gbit/s data stream in the time domain, was simultaneously demultiplexed to 8 WDM channels.

The high linearity of the switch with regard to the data input power leads to a promising application potential as an optical switch and as an optical sensor (optical sampling) with a time resolution of around 1 ps. The commercial use of the invention is additionally supported by the fact that the switch, including all its components, can be monolithically integrated, so that it can be produced in large quantities at a reasonable cost in the future.
 
 

Fig. 1:

b) Schematic illustration of the spectrum for the switch

TDM: Time Division Multiplexing, WDM: Wavelength Division Multiplexing