Phase Transfer for Radio Astronomy Interferometers, Over Installed Fiber Networks, Using a Round-Trip Correction System

摘要

The MERLIN telescope, based at Jodrell Bank, achieves coherent operation using a frequency standard transmitted over microwave links. This system is locally known as the 'Lband Link' (LBL). The LBL uses pulses of RF carrier waves to transmit bi-directionally between two locations in a time- division-multiplexed system (TDM). Tests in the laboratory and astronomical observations have confirmed that the LBL detects changes of delay at approximately the 1-picosecond level over periods of time extending to many hours. The legacy, radio-based, LBL terminal equipment was adapted to transmit, using thermally controlled externally modulated lasers at 1550 nanometers, over an installed fiber network of up to 110 km. The TDM operation of the LBL means that the system lasers may have very close wavelengths and are transmitted over a single fiber. Phase errors due to chromatic dispersion, circulator leakage, and differences in fiber path length are, therefore, reduced or eliminated. The round-trip delay value, halved, and the one-way path delay were measured. Any differences between the two values will indicate the error in the delay measurement, or stability, of the LBL. The objective was to establish if, in an LBL over-fiber system, an rms error close to the 1-picosecond level could be achieved. The measured rms error of the LBL over-fiber system is 1 picosecond, over sampling intervals of between 1 second and 1 minute, irrespective of fiber length. This suggests that the short-term instability seen in the results is due to terminal equipment and measurement error, rather than a fiber effect. Over a 2-hour sampling interval, an LBL over 110-km fiber system has an rms error of 4 picoseconds, compared to the legacy terminal equipment that has an rms error of 3 picoseconds, over the same period.