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IOC Description

The IOC provides the control controls and a monitoring capability for a low drift and low phase noise continuous-wave optical link system.  The latter is used for the transmission of a 3GHz RF reference signal over long distance. The system consists of a Transmitter (Tx) and a Receiver (Rx) units that are connected with two single mode fibers (SMF) (optical links).  To achieve low phase noise performance, the RF signal is propagated through the unidirectional optical link. The low noise link is additionally stabilized against phase drifts with the bidirectional phase-compensated optical link (low drift link).  An ethernet connection between the units is also needed for data exchange and control purposes.  The IOC communicates with Tx and Rx via Telnet sessions.  

Equipment State Machine

 We will refer to this system as an RF-over-Fiber (RFoF) system.

Equipment State Machine

The control system for RFoF consists of a state machine responsible for the initialization, tuning and reliable operation of the system. The state machine consists of 6 states and its functional view is provided below.

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The state machine always starts in the Init State.  In this state, all temperature controllers are enabled, while all phase controllers are disabled.  In the Startup State, the power supply voltages, ethernet connection between Tx and Rx units and environmental conditions are checked. All units are equipped with external humidity, pressure and temperature sensors.  

After the thermal stabilization of all internal modules (Warming Up State), the Tuning State is reached. This state consists of 30 sub-states.  See attached manual for details on this.  The basic tasks that are performed in these sub-states are: starting laser, identification of MZM and VOA best working points, checking if the optical power levels are sufficient, stabilizing the optical power at different photodiodes and closing the phase and amplitude control loops for the optical and RF signals in the Tx and Rx units.

When all phase control loops are closed and the RF output power at Rx is stabilized, the state machine enters the Ready State.  The latter has two basic sub-states: Locked and Unlocked.  Sub-State Locked consists of two additional sub-states. As before, see the manual for more details on these states and sub-states.  In summary however, in Sub-State Locked, all phase loops are locked and some other actuators are in their nominal working regions (laser and fiber spool temperatures in Tx module, RF phase shifters, RF attenuators, MZM bias voltage, VOA voltage). The Locked state is indicated as an illuminated green LED diode on the front panel of Tx and Rx units and with the LOCKD caption on the LCD display.

If any of these actuators approaches its working region boundary and all the phase control loops are still closed and the output RF power at Rx unit is sufficient, then the system goes in Sub-State Semi-Locked.  This state is indicated as blinking green LED on the front panel of Tx and Rx units and as blinking LOCKD* caption on the LCD display.

In case any of the phase control loops is unlocked or the output RF power is too low, then the system goes in the Sub-State Unlocked.  If the timer exceeds the 15 minutes limit, then the state machine enters the Shutdown procedure and state.

Also, the Tx unit receives data from Rx via the ethernet connection. In case of a broken ethernet connection, the system goes from Locked to Unlocked. This may eventually cause the system to go to the Shutdown State if the connection is not reestablished within 10 minutes.Image Removed

IOC Deployment

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