the end face of the fiber before detector 45 the distance between coupler 45 and detector 43 has to be longer than the distance between the detector 45 and the reflectors in the wavelength selection device 46 in some cases a very high dynamic range of the device is needed this is the case for very long optical links of about 200 km in this case it is possible to increase the dynamic range by a modification of the receiver 4 u2033 as shown in fig5 this modification requires an extra optical component making the system more expensive it is therefore not necessary for most systems now referring to fig5 in which the main difference with fig4 is that the coupler 45 has been replaced by an optical circulator 55 light entering through port a goes to port b while light entering through port b goes to port c this device therefore suppresses the loss introduced by the coupler 45 of the previous embodiment the two pulses generated by the source enter port a of the circulator and go to port b the selected wavelengths of the led pulse are reflected by the wavelength selecting device 56 while the laser pulse is detected in detector 53 in this case the wavelength of the laser has to be different from all the selected wavelength of the wavelength selecting device 56 the reflected wavelengths enter port b of circulator 55.