Abstract: There is provided a method for optical telecommunications with which an optical signal modulated for the plane of polarization can be received for certain without lowering the threshold level and being affected by noises so that any optical telecommunications can be identified without fail. With the method, an optical signal is modulated by continuous pulses a at the transmitting side before transmission and a predetermined number (n) of pulses a are extracted within a given period of time in synchronism at the receiving side so that the presence of a modulated optical signal is acknowledged only when the number of pulses a received within the given period of time is smaller than the predetermined number (n) but greater than another predetermined number (m) (n.gtoreq.m). This method may be so modified that the absence of a modulated optical signal is acknowledged even when the above defined number of pulses are received if k or more than k (k.gtoreq.

Abstract: There is provided a method of identifying a particular optical cable out of a plurality of similar optical cables in a simple, quick and accurate manner without requiring any significant additional cost. For identifying a particular optical cable C2 out of a plurality of optical cables C1 through C3 by means of a means for transmitting optical signals 11, a signal receiving/detecting means 13 and a signal applying device 16 and by utilizing the fluctuation in the level of optical signal due to the photoelastic effect that appears when external signals are applied to a cable carrying an optical signal, external signals are applied to the outer peripheral surface of each of the optical cable at a plurality of spots to boost the level of fluctuation of optical signal so that the particular optical cable can be identified without fail.