Jess and I have many responsibilities as the winter over lidar team at McMurdo station. These responsibilities include maintenance of the lidar systems, data analysis, outreach, and, of course, the operation of the lidar systems. Anytime the skies are clear, we’re trying to gather data. But what does lidar operation actually look like?
The first step in lidar operation is understanding the weather. We can only run the systems with clear skies, so we have to know how to use weather resources (forecasts, satellite imagery, and the McMurdo weather office) to predict if/when we might see a window of clear skies open, and for how long. The startup of the lidars takes 1-1.5 hours, which means that we need to get to the laboratory and prepare for a clear window in advance. However, if we start the systems and the skies don’t clear, then we’ve wasted some of the lifetime of the lasers.
If the window seems sufficiently long and we successfully startup the systems, then we have to watch the systems 24/7. Jess and I trade 12 hour shifts watching the lasers (lidar runs can be anywhere from a few hours to multiple days). Our longest run was a 6.5 day run at the end of February.
The data that is recorded is raw photon counts returned in each profile of the atmosphere. In the lower atmosphere, photons come back to our telescopes through Rayleigh and Mie scattering, while in the upper atmosphere they come from resonance fluorescence off metal layers. The photon counts that come back through resonance fluorescence from the metal layers are variable, however the photons that are returned through Rayleigh scattering are fairly consistent. If these counts drop, something is wrong. It could mean a loss of power in the laser, it could mean the laser beam is not aimed correctly, it could mean a problem with the receivers, or it could be as simple as a cloud or aerosol layer or other atmospheric interference.
Above: Na system data acquisition (left), showing photon counts throughout a vertical profile of the atmosphere at 3 different frequencies, total photon counts, and background. Na system frequency locking (middle), locks the laser to a specific wavelength using an Na oven. Fe system data acquisition (right), showing photon counts throughout a vertical profile of the atmosphere in one window and wavelength locking behind it.
As soon as we see a drop in photon counts we go to the laboratory. First and most important, we check for leaks in the cooling systems. If the cooling systems are leaking then the lasers could be critically damaged. If there are no leaks, then we try to figure out what’s wrong. We check the simpler and more common problems first. We try to adjust the outgoing mirror to ensure the beam is aimed directly into the center of the field of view of the telescope. If this isn’t the problem, we usually check the laser’s power. The problem could come from many different sources but usually it’s the laser. It could be one of the optics isn’t tuned quite right, or the laser cavity isn’t aligned perfectly. If adjusting the tuner and the laser cavity doesn’t help, we can increase the voltage being applied to the flashlamps (which generate the light that forms the laser). This is a reliable way to get more power out of the laser, however if we need to add a large amount of additional voltage it means there’s a deeper problem with the laser that we need to resolve. We record everything we do in run logs which are later scanned and uploaded for our own reference when troubleshooting and for when the data is used.
Above: Jess adjusts the high reflector on the 372 nm pulsed alexandrite laser (Fe system) to try to find the optimal alignment of the laser cavity. She watches the power meter and beam spot to find the optimal angle.
Those are the most common problems, however we frequently encounter other issues during operation. These lidar systems are very sophisticated, so there are many separate parts that could run into trouble. The only way we can be properly prepared for the complications that arise is by learning in depth how the lidar systems work, what each specific part does and how they connect in the intricate “lidar architecture.”
Besides troubleshooting problems, we regularly do power checks, we change the dye used in the STAR Na system, and Jess does configuration changes to switch between night mode and day mode. The laser performance depends on many different variables, so depending on the day we might have an easy day of operation or a difficult day where we’re constantly working to keep the lasers performing well.
