Experimental investigation of security issues in OCDMA: a code-switching scheme

Introduction: Optical code-division multiple-access (OCDMA) is receiving increasing attention with several groups pushing to realise Gbit=s multi-user demonstrations [1–8]. Among other possible advantages provided by OCDMA, its potential for enhanced information security is frequently mentioned. This is plausible at first glance considering the OCDMA encoded signal manifests itself as noise-like waveforms that may not be accessible to an eavesdropper without knowledge of the applied code; however, this argument is worth deeper thought. Our group has previously demonstrated there is no security at all in OCDMA for a single-user system employing on-off keying (OOK) [1]. This is due to the fact that, for an OOK system, although the encoded waveform is noise-like on an ultrafast time scale, it can be detected using a simple energy detector without any knowledge of the spectral code. For example, the energy detector could be a standard receiver bandwidth limited to the bit rate which is unable to resolve the fine structure of the noise-like waveforms but integrates the energy in a bit period. Fig. 1 confirms this argument in a 10 Gbit=s spectrally phase coded system. Fig. 1a shows the properly decoded waveforms seen by an authorised user, which are recovered back to a short pulse as desired (intensity cross-correlation). When detected by a 20 GHz photodiode, a clear eye diagram is observed as expected. Fig. 1b shows the encoded noise-like waveform (intensity cross-correlation), which is distinct from a short pulse on an ultrafast time scale. However, using the 20 GHz photodiode, a similarly clear eye diagram is still observed, which could be detected by an eavesdropper with no knowledge of the code. This clearly shows that there is no security at all for such OOK single-user OCDMA systems. On the other hand, even in a multi-user network environment, there are typically still fibre links where only a single user exists and which are therefore vulnerable as pointed out by a recent systematic theoretical study on security issues in OCDMA [9, 10]. In [9, 10], a codeswitching scheme is proposed, which may provide enhanced security in OCDMA. In this Letter, we experimentally demonstrate for the first time to our knowledge this code-switching scheme for spectrally phase coded OCDMA.