A portable apparatus for measuring optical powers of optical signals propagating concurrently in opposite directions in an optical transmission path between two elements, at least one of the elements being operative to transmit a first optical signal (S1) only if it continues to receive a second optical signal (S2) from the other of said elements, comprises first and second connector means for connecting the apparatus into the optical transmission path in series therewith, and propagating and measuring means connected between the first and second connector means for propagating at least the second optical signal (S2) towards the one of the elements, and measuring the optical powers of the concurrently propagating optical signals (S1, S2). The measurement results may be displayed by a suitable display unit. Where one element transmits signals at two different wavelengths, the apparatus may separate parts of the corresponding optical signal portion according to wavelength and process them separately.