Research Update August 29th

This is more of a note than an update of any kind, but I haven’t been able to sit down and write about what’s been going on research wise lately. The 461 nm laser system works wonderful now as long as you know you to finesse it. We went through the whole system and after passively stabilizing the diode as well as possible and rebuilding the doubling cavity we charted out the stable cavity modes that the diode has over a series of currents and temperatures. While the diode can scan mode hop free for ~30 GHz when things are JUST right, it still doesn’t follow the doubling cavity very far when locked. However, it follows it far enough.

The problem has been that while the laser scans over the resonance in the vapor cell the doppler broadening is about 300 MHz FWHM and the lamb dip is about 190 MHz FWHM. In short I took the cell over a broad range of intensities and alignments, we looked at the linewidth of the laser on a cavity, and we actively pumped (6 L/s) on the cell while running it all with no effect on the linewidth of the lamb dip. However, I noticed today that while we were pumping out on the cell when I first turned on the Sr, as the laser scanned across the resonance, not only could I see fluorescence all the way from window to window, but the brightest part of the fluorescence was concentrated on a very small segment of atoms. However, by the time we got to looking at thing on the scope the fluorescence was limited to the spray of atoms exiting the bottom part of the cell and the brightest part of the fluorescence was very spread out. This indicates that as the cell heats a Sr vapor that is mostly only Sr flows through the cell, however, when we get to temperatures where the Sr absorption is significant something else flows with the Sr.

Basically, watching the length of the fluorescence become limited across the cell is like watching the mean free path diminish. This is not good, and it happens even while pumping out on the cell which indicates that it is something that is generated with the Sr vapor. I’m going to convert the cell into hybrid jet/vapor cell design in an effort to overcome this problem and I’m going to refurbish it in the meantime. We have some lead left over in the cell, and I’m going to cut down the Sr in the cell big time to make sure that only the cleanest Sr is present.

And…. We’re BACK!

Oh, man. What a long past couple of months it has been and for a variety of reasons. Somehow we magically had set the laser JUST right all the time we had been running it before. It is actually an extremely fickle and ornery laser system despite the diode being AR coated. I’ll just list the basic changes we made to the laser system.

1. Replaced the doubling crystal. The AR coating on one end of the original crystal had been been damaged. We don’t know why but we suspect it was related to changing the crystal’s temperature. The new crystal was cushioned in several wraps of .1 mm thick purified lead foil to act as a buffer between the crystal and the aluminum mount.

2. Rebuilt the doubling cavity. Since we were aligning the cavity to a new crystal we took a moment to improve the cavity too. We made the cavity just the right length to be at the most stable beam waist for a 125 mm focal length lens. This required the cavity to be square instead of bowtie which wasn’t an issue because the mirrors are all flat. This also made the piezo scan twice as far as the cavity length with a FSR of 1.2 GHz.

3. Modified the doubling crystal temperature controller. Since we had been worried about the crystal damage having occurred due to temperature rates we installed an adjustable voltage regulator set at ~-18 V down from -48 V to ensure that the crystal temperature would never change faster than 10 C/min.

4. Floated both the doubling cavity box and the master laser system box on Sorbothane. Although the boxes float independently the alignment to the doubling cavity is surprisingly stable. The difference in mechanical noise isolation was immediately noticeable.

5. Mounted the diffraction grating on a piece cut at the right feedback angle on a cage mounted to the laser head face. I made the cavity as short as possible.

6. Placed a wavemeter in the master beam to watch stability and wavelength along with cavity coupling and blue output power. Verified the .1 nm/1 wavenumber FWHM of the cavity temperature.

7. Mapped out wavelength resonances of the diode as a function of current and temperature.

All of the work paid off in the end. The pictures and video show fluorescence in the vapor cell today. The infrared wavelength scanned 10849.13-.18 cm^-1 to cross the atomic transition in the cell. With the cartridge heater at ~500 degrees C (cold end at ~200 C) fluorescence could be seen extending all the way across the cell, not just the central source even with chilled water running to cool the target area.

 

Here you can see fluorescence in the reflections off of both windows.

Sr fluorescence on the 461 nm resonant transition.