frequency at a rate equal to that of the prescribed frequency a period associated with the receiving frequency is temporally offset from a period associated with the prescribed frequency application_number US 79417404 A description the present invention overcomes the aforementioned problems and limitations of conventional cotdr arrangements that arise from gain fluctuations associated with the long duty cycle of the cotdr probe pulses instead of a pulse generator that launches long duty cycle probe pulses the present invention employs a cw laser having an output frequency that is swept over an appropriate range as explained in more detail below the cotdr receiver has a fixed frequency local oscillator that is sensitive to a single frequency swept by the cw laser one important advantage of the present invention is that because the cotdr arrangement employs a cw laser instead of a pulsed source the optical power level u201c seen u201d by the optical amplifiers will remain constant in time when the cotdr arrangement is in use accordingly gain fluctuations and the associated signal degradations due to cross gain coupling will not arise thereby allowing the cotdr arrangement to be used when the transmission system is in service fig1 shows a simplified block diagram of an exemplary wavelength division multiplexed wdm transmission system in accordance with the present invention the.