The triggers for the laser are sourced from EVR:LA20:LS21. The "Event Receiver" (EVR) receives timing information from an "Event Generator" (EVG) that lives in S10. This pages describes how the laser is synchronized to the triggers that are produced by the EVR. This is necessary because the laser itself contains other timing hardware, like the SDG for the regen and the DG645s that control the SAGAs and CFR pump lasers. Because of this timing hardware downstream of the EVR, as well has all the glass of various indexes of refraction that can delay the laser, the regen laser pulses themselves must be compared to a reference trigger to determine when in time the laser leaves the regen.
Trigger System Block Diagram
The "Diode Box" is what is used to combine triggers when in goose trigger mode. It is not technically needed in regular operation, but it is easier to leave it in place. The DG545 is used as an amplifier because the diode drop across the diode (about 0.6V) from the diode box makes the signal into the pacemaker too low. The DG545 is programmed to add a small delay to the 120 Hz signal for future flexibility, this box should never be adjusted. The pacemaker is a box that measures the frequency of the incoming signals and if they differ from the expected 10 Hz of 120 Hz (channel specific) by more than 1 Hz it provides its own 10 or 120 Hz. This is to ensure the laser always gets triggers. The DG645 and Coherent SDG convert the EVR triggers into various triggers for the lasers. These boxes are very rarely adjusted.
How does the regen timing work? (Not goose triggered)
The trigger system block diagram shows how triggers are sent to the laser. The regen timing system measures the time of arrival of the regen laser pulse itself onto a diode and compares that with a trigger from the EVR to measure the real time of the laser pulse itself.
The laser timing system compares the signal from the regen diode, converted to a time by the "Laser Interface Module", to a reference trigger from the EVR that provides laser timing. The difference between "Regen Time Out" and the "Reference Trigger" (Start and Stop in the Keysight box) tells the laser timing software where the laser is in time compared to a fixed reference point in the timing system.
Laser Interface Module
The laser interface module turns the signal from the "Regen Diode" into a square wave that can be used by the Keysight 53230A. The Laser Interface Module turns the square wave on at (or near) the peak of the Regen Diode Signal. It turns the square wave off when it receives a reset signal. The reference signal is used as a convenient reset, the reference nature is not used. Here, it is simply a reset so the Laser Interface Module is ready to turn the square wave on when the next Regen Diode Signal Arrives. The length of the Regen Timing Out signal does not matter.
Keysight 53230A
The Keysight 53230A measures the time delay between the rising edge of the signals sent into the two input channels. The Keysight is capable of measuring time differences out to several minutes. Those signals are the Regen Timing Out from the Laser Interface Module and the Reference Trigger from the EVR. A gate does not need to be supplied. If a gate is supplied, the two rising edges must be within the window of the gate.
The "Raw Meas." value is stored in the SR620 "Raw Meas." box in the Vitara GUI.
"Counter Time" = "Inv & Offset" = Offset - Raw Meas. Offset seems to be totally arbitrarily set to 50 us. The Reference Trigger is set to 50 us, but the Offset PV isn't derived from the Reference channel on the EVR.
The Counter Time is the measured time between the regen (passed through the Laser Interface Module and the Keysight 53230A) and the Offset. For the above image, the measured time between Regen Timing Out and the Reference Trigger is 48755.09 ns. The Offset is set to 50000 ns. So the Counter Time is 50000 - 48744.09 = 1255.91 ns. Because the Offset is arbitrary, this value is also arbitrary. As of this writing the Regen timing TDES was -4563 ns, the Reference TDES was 50000ns, and the measured difference is 1255 ns. Evidently the laser adds about 4563 + 1255 = 5818 ns to the timing signal from the EVR.
What does the "Target Time" do?
The target time tells the timing system to make the measured counter time the target time. There are two ways the timing system can adjust the arrival time of the regen on the regen signal diode. For changes that are small compared to the Vitara frequency (68 MHz, 14 ns separation) the length of the Vitara cavity must be changed to change the arrival time of the Vitara pulse into the regen (very rough description). For changes large compared to the Vitara frequency, the triggers to the whole laser system can be adjusted and a different Vitara pulse can be used. Exactly what to change and when is calculated by the "FS Python Script" (top right of the image above). Roughly speaking, if you request a relative change in the target time of 100 ns the Vitara will change by (110 % 14) = 2 ns and the EVR triggers will be changed by 98 ns.
To be compatible across all the labs in AD, the FE Python Script is built to only handle on trigger. The regen 120 Hz trigger. That is why the FACET S20 laser uses a python GUI to control other triggers. See Adjusting Laser-Electron Beam Timing.