Optical Communication Research - still full of promises!
pierre-yves.fonjallaz@acreo.se
Highlights from the European Conference on Optical Communication ECOC 2004 in Stockholm, organised by KPRC.
ECOC was a big event for Acreo and for the whole Swedish photonics community. A large attendance of high level scientists and a high quality programme were two ingredients for a very successful conference.
The European Conference on Optical Communication, ECOC, is the largest conference in this area in Europe, and second in the world after OFC (Optical Fiber Communication conference, always held in the US). This year, the 30th edition of ECOC took place in Stockholm in September, organised by the Kista Photonics Research Center (KPRC) with the great assistance of Ericsson and Telia Sonera. The KPRC is a collaboration between Acreo and KTH in the field of photonics in Kista and comprises more than 100 researchers working in the same building.
The conference attracted about 1150 scientists from all industrial countries and a number of emerging Asian countries. Following the tradition, ECOC continues to be very popular among Japanese and South-Koreans, forming together about one third of the attendance. Researchers from the US obviously seemed reluctant to travel this year. An exhibition was held in parallel with the conference, attracting about 3000 visitors. Acreo had a very popular stand with demonstration of the test bed and of components, leading to many new customer contacts.
Let us look at some particularly interesting highlights.
Fiber devices
Several presentations addressed new fiber compositions and designs. Notably, one paper from Sumitomo Electric Industries in Japan reported water-free pure silica core fiber with ultra low loss. One important characteristics of this fiber is that it is very resistant to in-diffusion of hydrogen, the usual cause for increased attenuation with ageing.
A couple of papers reported microstructured fibers with ultra low bending loss. In that case, the holes present in the cladding help in confining the light close to the core of the fiber and hence prevent leakage. Microstructured fibers are sometimes called photonic crystal fibers if the holes are periodically arranged, or just holey fibers to denote that their cladding is full of holes following the length of the fiber. About 20% of the submitted papers on fibers were related to this specific type, indicating a clear increasing importance.
One post-deadline paper presented the possibilities offered by ”crystals in crystals”, liquid crystal infiltrated in a photonic crystal fiber to actively modify, with just a voltage applied across the fiber, the spectral characteristics of the transmission (the optical bandgap). A series of papers showed how fiber gratings can be written with near infra-red light. Since the absorption occurs through the combination of several photons (basically 4 in that case), the index of refraction can be selectively and permanently modified by focusing the light at the right place in the volume of the fiber glass. Finally, Acreo presented a fiber-based Kerr switch and modulator showing that electricity can directly control the fiber properties to make very useful devices.
Integrated optics
A remarkable number of high quality contributions on integrated optics from Japan reflected their continuing large effort in that field. Certain maturity has been reached, as shown by a number of extremely well performing device structures. Silica-based waveguides have reached the fantastic low attenuation level of 0.3 dB/m allowing for ultra-long and compact photonic lightwave circuits (PLCs).
Semiconductor waveguides, in particular those based on InP, are also very much used for ultra-compact PLCs and have the advantage that they can incorporate optoelectronic active devices. The best performances are nevertheless usually obtained with hybrid integrated devices. On a silicon carrier, it is e.g. possible to combine many PLCs, integrate optoelectronic devices or wavelength convertors based on periodically-poled LiNbO3 waveguides and even electrical ICs in a very efficient manner.
As is the case in fiber optics, 2D photonic crystal made by drilling periodical holes in semiconductors is a rapidly increasing research subject. The loss in photonic crystal based waveguides (fabricated by removing hole rows) has been presently reduced to the 3 dB/cm level, which makes their practical use closer to reality. One fascinating property of such waveguides is that low-loss 90º-turns are possible to achieve. Quite complicated PLCs can therefore be designed with very small waveguide total lengths. The most interesting feature is the ability to tailor the dispersion, and the group velocity of picosecond pulses has been demonstrated to be as low as 6% of the speed of light in vacuum.
Optoelectronic semiconductor devices, transmission, networks
A really impressive optoelectronic semiconductor device demonstrated at ECOC is a laser diode using quantum dots in the active region and exhibiting performances almost independent of the temperature (20 to 70 ºC). Another one is an electro-absorption modulator (EAM) requiring only 0.79 V to switch from ON to OFF. A 100 Gbit/s error-free wavelength conversion is obtained by combining an ultra-fast photodiode (UTC-type) and a traveling-wave EAM. All these 3 examples from Japan...
In the category ”largest capacity - distance product”, one example from the postdeadline papers: Eight wavelength channels modulated at 170 Gb/s and transmitted over 430 km of standard single-mode fiber is achieved by using adaptive polarisation mode dispersion compensation. A European achievement this time. Also it should be mentioned that an 85.4 Gb/s fully electrical receiver (including electronic clock recovery) was presented by Alcatel.
The trend with higher bit-rates in the trunk network clearly continues with 40 Gb/s TDM/WDM transmission as almost fully commercial today and 160 - 640 Gb/s as the transmission research area. NTT expects to demonstrate 1000 160Gb/s WDM channels transmitted over one fiber (1000 km long) during 1 - 2 years.
On the networking side dynamic network reconfiguration (e.g. GMPLS enabled) is a very hot topic with GRID applications involving huge amounts of data transfer worldwide (with specified QoS for the data connection) as one application.