Polarization-insensitive 640 Gbit/s demultiplexing for OTDM data signals is demonstrated using a 100 m polarization-maintaining highly non-linear fibre. The scheme is based on four wave mixing (FWM) in a polarization-maintaining fibre loop (PMFL) with bidirectional operation. Less than 0.2 dB polarization dependence is obtained. The FWM efficiency is about -6 dB if the passive loss of the PMFL is not included. The flatness characteristic of the FWM efficiencies allows for the OTDM demultiplexing of a high speed signal with a bandwidth of 1.2 THz. Error free performance with low penalty for the demultiplexed 10 Gbit/s signal is achieved for the polarization scrambled 640 Gbit/s data signal. BER measurements and eye-diagrams show that the demultiplexed 10 Gbit/s signals with and without polarization scrambling have almost identical performance.