Investigation of the Absorption Lines of a Kr - Ne - Mixture as Wavelength Standards for Optical Communication Systems in the Range of 1270 nm - 1640 nm

Optical communication systems are since the last 25 years the backbone of all international data and telecom communications. Up to now, more than 10 TBit/s can be distributed via one optical fiber in long-haul systems. This was realized by the use of highly parallel multi-channel transmission via many colors, called Wavelength division multiplex (WDM). Here, the International Telecommunication Union specified the channel location into a frequency space grid with 100GHz distance in the infrared regime from 1300nm to 1640nm. To avoid interference between these densely arranged transmission channels, each transmitter output frequency has to be stabilized exactly in its center frequency to others. To supervise and stabilize the transmission comb, absolute frequency standards are very helpful to be integrated into optical cross-connect stations. Using excited state absorption lines of a Nobel gas is an easy way to generate reference frequencies, because these states are very stable and very small in linewidth. A well known method is the detection of the absorption by the opto-galvanic effect (OGE). Here, a tube is filled with an gas like Kr and the detection of the absorption will be performed by an electrical signal of the absorption cell via electrodes with high voltage supply. Many atomic transitions are near by to the ITU grid frequencies of ITU G 652 Recommendation.

In this article we show for our knowledge at the first time all absorption lines of krypton and neon mixed gas in the whole ITU transmission range between 1270nm -1640nm using optogalvanic spectroscopy. The temperature dependence of the possible wavelength drift of the detected lines was also measured using a climate chamber between 10ºC and 50ºC. The measured absorption lines are within the tolerances of the detection equipment in comparison to literature. The measured stronger lines are separated between 5 nm and 30 nm in the detected wavelength range. Hence the creation of a frequency grid using 27 pure stable absorption lines of Kr as a wavelength standard for optical communications systems using these lines would be possible. Combining the Galvatron detection of Kr lines with active feedback to a multi-channel laser source, a high stable reference source for optical WDM networks would be available in future.