An optical filter composed of a concatenation of elementary filters consisting of chirped slanted Bragg gratings written on a continuous portion of an optical waveguide, characterised in that the various gratings in the concatenation have the same angle, identical lengths and a linear chirp. The present invention also concerns a novel method for retrofitting conventional prior art single boom crane systems to convert them to dual trolley systems, while at the same time retaining the basic geometry of the prior art system so that the same number of multiple cranes may be positioned over the dock side. A frame carries a drive shaft which carries drive wheels. Upon completion of the call, the requested telephone service is deactivated. 1 to 20% by weight of a cesium compound.