Wavefront Measurement and Correction Procedure

Check and flatten the wavefront in laser room

This procedure assumes that the wavefront sensor (WFS) and focus imaging are already setup, and no significant modification was added to the configuration. 

Currently the WFS beampath is picked off after the first turning mirror after the DM. A f=1m lens (~f/20) combined with a f=100mm lens images the surface of the DM on the sensor.

The focus of the 1m lens is picked off by a flipping mirror and imaged by a 10X objective (currently shared with Inj Far camera) coupled to a CCD camera. 

Check WFS alignment and measure wavefront

The WFS beampath is stable if DM Near and DM Far are well aligned. The HASO #7677 is dedicated to measure the wavefront in the laser room. 

•  Check WFS and DM connection. Ensure that all cables are connected to the server and DM controller, and that the server and DM controller are communicating. The DM controller should be connected to the server by ethernet cable. If not, turn off the DM controller using the switch on the front side of the controller, then connect the cable and switch it back on. The “Lan Link” indicator LED should be on after connecting to the server. This step is not necessary unless the wavefront measurement is done in the tunnel right before.                                                                                                                                                                                                                                                                                                                                                         
• Connect to the server in the laser room:
  1. Connect to the SLAC VPN using your SLAC windows account
  2. Use a Remote desktop client to connect to the server. The computer name is PC97430. The account for logging should be the group account for the server if the purpose is to do regular operation of the HASO and DM; or an administrator account ending by “-a” if installing new software or network configuration is needed. Also add “SLAC\” in front of the user name.
• Once connected to the server, locate the WaveView and WaveTune The first one will be used to align the HASO, and the second one will be use to control the DM according to the wavefront measured by the HASO. Only use one app at a time to avoid conflict of resources.
• Check WFS alignment. This step is not necessary unless the beampath is significantly modified or optics are bumped. 
  1. For regen beam and full beam with RP attenuator, the pulse energy is about 4-5mJ, it needs to add filters of ND6 in total to avoid saturating the HASO by a single pulse. There are a total of fixed ND3 on the beam path with the ND3 flipper out, and another ND3 is attached on a camera adapter ring in front of the HASO.                                                                                                                                                                                                                           
  2. Put the alignment cross in front of the DM, and put a regular camera at the position of the HASO. Image the cross on the camera. It may need to slightly rotate the lens before the HASO to minimize imaging aberration. This will  set the imaging plane to the DM surface.                                                                                                                                                                                                                                                                                                                                                                                                                 
  3. Put the HASO back, remove the alignment cross, and open WaveView. Default settings are showed below. Click Ok to start
     •  The default timing setting is external "TTL". If internal trigger is preferred, open "Configuration", switch to internal under "Hardware" and change the exposure under "Acquisition."                                             
  4. Click the green arrow to start live imaging. Check if the beam is centered on the camera signal. Adjust the horizonal and vertical position of the HASO to center the image . Don’t use the mirrors to send the beam to the HASO, it can easily ruin the alignments.
  5. Stop running the HASO, then choose “Alignment” mode and start (A green arrow icon with red and green dot) . Meanwhile, screw in the alignment pin hole to the HASO. This requires to removal of the ND3 attached to the HASO. Remember to put equivalent filters (ND3 flipper in) on the beam path before removing this filter.
  6. Use the tip-tilt adjustment (knobs on the WFS mount) to put the dot in the blue circle. The dot will turn from red color to green color.
  7. Remove the alignment cap and put the ND3 filter back in front of the camera (ND3 filpper out). Check if the full beam image still roughly center on the CCD. If not, go back to step 2 and iterate. If signal saturates, lower the waveplate setting.

• Measure wavefront and zernike polynomials. Once aligned, stop the alignment mode and switch to live mode. Locate and open the "wavefront", "intensity" and "polynomial" windows (or any desired measurement) in the middle section of the front panel, and click start. Create a new folder with dates in C:\DM files and save each measurement as a txt file ("File - export TXT") if needed. You need to stop the acquisition while saving.
  • Check the tilts. Fine tune the tip-tilt knobs to minimize the tilts to <1um if too large. This further improves the WFS alignment.

• Close the WaveView software. All setting will be saved automatically in the session that you used.

Flatten the wavefront using closed loop.

• Open the WaveTune. The default settings are shown below. Click OK and continue. You will see the following error because the Pharao camera is not connected. It is fine to click CONTINUE. If other errors pop up, it means that the HASO or DM connection is not correct. It will most likely be the DM (wavefront corrector) as shown below. In that case, power cycle the DM controller and reconnect the ethernet cable between DM controller and the server until the error clears. This might happen when there's a power outage or after tunnel measurement.

• Set up the HASO exposure time. Use similar parameter as in the WaveView setting, but take at least 20-shot sum image to improve the SNR.

• Save the current position of the pistons as initial (in the same DM folder where you save the wavefront).

• Check if phase slope can be calculated from the HASO images. This can be done by clicking the Run arrow head in AO tab. Phase will be up dating if it works. If it doesn’t, check trigger setting and exposure time, if beam is blocked, or filters are too much (it needs at least 10% of the maximum pixel count to work). Also, if the iris in front of the DM is close, it may also cause problem to retrieve the phase. 

• Calibrate the correction function between the DM and the HASO. This step is needed if the HASO has been used in a different location, or the laser beam pointing has change significantly, i.e. if you realigned the WFS in the previous section. You can also do the calibration if you think previous calibration is not accurate enough.
  • Go to AO calib tab, and click start. You need to stop running in the AO tab to do the calibration.
  • Wait until it finishes. This may take a while depending on the laser rep-rate and number of shots summed in each data point.                                                                                                                                                                                                                           

• Flatten the wavefront using closed loop.
  • Go back to the AO tab.
  • Save the current position of the pistons if you think it may be useful. It should remain unchanged from before calibration.
  • In AO tab, make sure that correction mode is set to "Auto." Use "costum" only if you want to correct to a reference wavefront, i.e. in tunnel. The saved settings are generally good without change.
  • However, if desired, you can change the loop settings in the loop configurator. There you can set the desired maximum number of Zernike orders. For 20-shot average on the fully amplified beam, recommended setting is between 10 – 15. If you want to try higher orders, select Auto in the security check to avoid pistons are driven too far. Use finite number of closed loop correction (normally between 5 – 15). 
  • Optionally you could open the WFS window and chose "PSF" to observe the strehl ratio during the loop.                                                                                                                                                                                                           
  • Click the run arrow in the main menu, then select Close loop on. It will run for the number of loops that you have set and stop. Usually the wavefront can be flatten to 50nm RMS and difficult to go down further.
  • Check the Inj Far or other far field imaging after the DM during the loop to ensure that the loop works.
  • Save the piston positions after the loop. 
  • Depending on the results, recalibrate the pistons after the loop and repeat the loop to further optimize the wavefront if needed. Typically the strehl ration will exceed 0.9 after 1 loop run.

  •  Close the program. Wait until the following message box shows up. All settings will be save to the current session.                                                                                                                                                                                                                               

Check and save the new wavefront after loop.

• Open WaveView and save the new wavefront and Zernike polynomials. Make sure that the leading Zernike coefficients (Astigmatism, coma, trefoil, etc.) are optimized compared to before the loop. Now you have flattened the wavefront in the laser room.
• Close the program and exit remote desktop.