To prevent the seeding of narrowband/under-chirped light into the regenerative amplifier, the Legend employs a BandWidth Detector (BWD) consisting of two laterally-separated photodiodes looking parasitically at the spatially-dispersed beam in the stretcher. If the spectrum is broad enough, both diodes see a signal, and the BWD is satisfied; if the spectrum is not broad enough (e.g. the mode-lock of the oscillator is broken or someone mistakenly blocks some of the spectrum within the stretcher), one or neither of the diodes may see a signal, and the BWD trips. This removes the trigger permissives for Pockels cells which would otherwise allow the seed pulses into the regenerative amplifier, thus protecting the amplifier from the potential unsafe operating condition.

The BWD trip state latches in the fault position such that, once the narrowband condition is corrected and the BWD is satisfied again, the interlock must be reset in order to re-enable the Pockels cell permissives and return to normal operations. The most common way of doing this is by pressing the "Reset" button on the front panel of the SDG (the light will be flashing quickly when the BWD is ready for reset), as shown in the picture below. However, the need to physically press a reset button is problematic when the hutch is in NO ACCESS mode or when laser personnel are supporting an experiment remotely/off-site. As such, we have instituted a rudimentary way to reset the BWD remotely. The steps for doing so are described below. (Note that in the future this procedure may change with the introduction of an IOC for the SDG – hopefully coming in 2023 sometime.)


  1. Open the remote connection to the Vitara laptop. This is most often done opening a terminal on one of the MEC control room computers or on a remote session using FastX, NoMachine, MobaXTerm, etc. From the terminal, you can open MEC Home and the Short Pulse Laser Operator GUI, where you can select the Legend from a dropdown menu of VNC options. Alternatively, you can simply type vncvitara directly into the terminal if your .bashrc alises have been set up previously. (Use this page for help in starting a remote session and/or editing the aliases in your .bashrc file: Remote access session on NoMachine or FastX (and add aliases to your .bashrc file)). The opened screen should look like this:

    If the screen doesn't look like this because the PComm Terminal Emulator is already open, skip ahead to Step 4. If there is a problem opening the VNC connection, please consult your nearest friendly neighborhood ECS associate.
  2. Open the program PComm Terminal Emulator by clicking the icon of the two computers along the bottom toolbar (shown between the icons for Windows Media Player and the Sticky Notes applications in the picture above). Once opened, the program window should initially look like this:
  3. At the top menu, click "Port Manager" followed by "Open" (the only menu option available). In the new "Property" window, fill in the various parameters such that they match the picture below, and then press OK to open the new terminal.

    Note: most often, the only parameter that needs to be changed is the Baud rate, which needs to be set at 19200. (You'll note that the virtual Sticky Note on the desktop is a reminder of this number!)
    Note: there is a small chance that the COM port on the left might need to be changed, as occasionally the numbering will be reset (e.g. after Windows restarts or that kind of thing). If this does happen, click on the Start Menu and open the Device Manager application. Under "Ports (COM & LPT)", look for the port labeled "Prolific USB-to-Serial Comm Port" and the numbered port listed next to it (in this case COM3). This will allow you to verify which port you should select on the left. (The other port is the USB-to-Serial adapter used for communicating with the Vitara.)(Note that the name of this adapter could also change in the future if, for example, someone steals the cable and then replaces it with a different cable that has a slightly different name.)
  4. If you have a fresh terminal, it should look like this:

    (If your terminal has some stuff already in it, no big deal! It will still work. If this is concerning to you, just close this terminal by hitting the X in the top-right corner and repeat the step above for opening a fresh terminal.)
    To check if the terminal is live or not, enter a single question mark character ? and hit enter. (Note that the question mark character will not appear on your screen (depending on the settings) since the display is only for communication returned from the SDG.) If everything is working well, you will see a status report with the word Ok superimposed on top of a status report that looks something like what's shown in the picture below:

    (If nothing happens, try some basic troubleshooting – close the terminal and try step 3 again? try toggling some different settings in the PComm Terminal Emulator? make sure you've selected the correct COM port? check the connections of the USB-to-Serial cable (one of two similar devices plugged into the Vitara laptop) and of the serial cable (i.e. that connects from the USB-to-Serial adapter into the Serial port on the back of the SDG)? etc.)
    To reset the BWD, make sure the BWD is ready to be reset (as described above) and type the word reset into the terminal and hit enter. This should reset the BWD. (Note again that it can be a little difficult to tell if you're typing the letters in correctly or not, but the way you can tell you're entering in the characters appropriately is you should see the transmitted (TX) and received (RX) character counter in the bottom-right corner increase as you type.)

Hopefully this saves you a bunch of time and effort!!

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