A communications network element comprising: a network element input arranged to receive an input optical signal having a first spectral property; a network element output arranged to output an output optical signal having a second spectral property; a monitoring port arranged to output an optical monitoring signal; a first optical splitting device arranged to tap a portion of one of the input optical signal and the output optical signal to form a first signal having one of the first spectral property and the second spectral property; optical signal transforming apparatus having an input and an output, the input arranged to receive the first signal and to apply an optical transfer function to the first signal to form the optical monitoring signal, the optical transfer function being arranged to preserve the spectral property of the first signal and to apply a time-domain obfuscation to the tapped signal, the optical signal transforming apparatus being further arranged to provide the optical monitoring signal to the output and to the monitoring port, the optical signal transforming apparatus comprising: an optical through signal path between the input and the output; a second optical splitting device positioned in the optical through signal path; an optical combining device positioned in the optical through signal path; an optical feedback path connected to the second optical splitting device and the optical combining device, wherein the second optical splitting device is arranged to tap off a portion of a signal passing along the through signal path and apply it to the feedback path, and the optical combining device is arranged to recombine at least part of the tapped portion of the signal with the signal passing along the through signal path. a network element input arranged to receive an input optical signal having a first spectral property; a network element output arranged to output an output optical signal having a second spectral property; a monitoring port arranged to output an optical monitoring signal; a first optical splitting device arranged to tap a portion of one of the input optical signal and the output optical signal to form a first signal having one of the first spectral property and the second spectral property; optical signal transforming apparatus having an input and an output, the input arranged to receive the first signal and to apply an optical transfer function to the first signal to form the optical monitoring signal, the optical transfer function being arranged to preserve the spectral property of the first signal and to apply a time-domain obfuscation to the tapped signal, the optical signal transforming apparatus being further arranged to provide the optical monitoring signal to the output and to the monitoring port, the optical signal transforming apparatus comprising: an optical through signal path between the input and the output; a second optical splitting device positioned in the optical through signal path; an optical combining device positioned in the optical through signal path; an optical feedback path connected to the second optical splitting device and the optical combining device, wherein the second optical splitting device is arranged to tap off a portion of a signal passing along the through signal path and apply it to the feedback path, and the optical combining device is arranged to recombine at least part of the tapped portion of the signal with the signal passing along the through signal path. an optical through signal path between the input and the output; a second optical splitting device positioned in the optical through signal path; an optical combining device positioned in the optical through signal path; an optical feedback path connected to the second optical splitting device and the optical combining device, wherein the second optical splitting device is arranged to tap off a portion of a signal passing along the through signal path and apply it to the feedback path, and the optical combining device is arranged to recombine at least part of the tapped portion of the signal with the signal passing along the through signal path.