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RF over fiber

Compensating changes of optical path length


For the distribution of an optical time signal in large-scale facilities, a stable distribution system is advantageous. However, because even fiber links which are robust against environmental variations cannot wholly prevent fluctuations, a system must compensate for such inaccuracies. This can be found in Cycle’s Wave Timing. It actively stabilizes the timing signal and compensates for changes in the optical path length caused by environmental influences. How does the system exactly work?

In contrast to Cycle’s more sophisticated PULSE Timing Distribution Systems, the Wave Timing Link does not rely on ultra-stable pulsed laser sources but works with continuous wave (CW) lasers. The Wave timing distribution system comprises a transmitter using a continuous wave (CW) laser and an electro-optic modulator (EOM). The EOM imprints the facility’s radio frequency (RF) timing signal on the CW laser signal. Then, the RF-modulated laser signal is transmitted to the facility’s remote locations via optical fiber links. At the remote locations, a receiver extracts the RF timing signal for further usage. Nevertheless, since environmental fluctuations influence the signal on its way to the receiver, the synchronization may suffer. To correct these disturbances, part of the signal is fed back to the transmitter, compared to the outgoing signal, and corrected.

For this purpose, the transmitter additionally comprises an electronic phase detector, an electronic phase shifter, and an optical delay line (ODL). The electronic phase detector measures an error signal used to correct the signal either by the ODL or by the electronic phase shifter.

Synchronization with a BOMPD

Figure 1 — Schematic of an optical F&T system using the CW laser technique. EOM: electro-optical modulator, ODL: optical delay line, ΔΦ: phase shifter.

This way, Cycle’s Wave Timing Link provides a cost-effective, robust, and highly compatible transmitter for many timing distribution tasks that do not require the cutting-edge precision of a PULSE Timing Distribution System.