Transmission of active and passive optical reference signals over a 16 channel, 534.4 km WDM optical fibre network

Active and passive optical frequency reference signals have been transmitted over a 534.4 km optical fibre network carrying live traffic and an equivalent fibre test-bed. The active references comprised an optical-transition locked fibre-laser and a fibre-grating locked distributed feedback (DFB) laser. A low-finesse etalon and molecular absorptions in an acetylene gas cell were used as passive references. No degradation of the reference wavelength was observed down to the 10^-8 level and in a self-heterodyne experiment; the upper limits for line-broadening and frequency shift of the transmitted signal are 2times10^-9, limited by the DFB characteristics. Amplified spontaneous emission (ASE) noise and interference signals were observed for active references. The passive reference signals were also degraded by the ASE noise. The technical and logistical issues associated with using a long-distance point-to-point all-optical link for frequency metrology have been discussed