only used for receiving signals but also for transmitting signals e g the transmitter wavelength signal can be used as the continuous wave signal which has a wavelength of u03bb x in that case no additional laser is needed the wavelength might even be the same as the received wavelength u03bb 0 when a counter propagating continuous wave signal with respect to signals from splitter 2 and 3 are used 0041 fig4 shows a schematic diagram of one exemplary implementation of the wavelength conversion element used in the present invention using a mach zehnder interferometer based on the use of semiconductor optical amplifiers soa based mach zehnder interferometer 16 the soa based mach zehnder interferometer 16 comprises a 2 u00d7 2 coupler 18 which is connected to output ports of two semiconductor optical amplifiers soas 17 the use of a 2 u00d7 2 coupler 18 and soas 17 is known to a person skilled in the art continuous wave signal 15 which has a wavelength of u03bb x is applied to an input port of both soas 17 splitting of signal 15 can be done using a 1 u00d7 2 coupler as is shown in fig4 or can be done using a 2 u00d7 2 coupler not shown where signal 15 is applied to one input both types.