not a good week in research
Cycling laser suddenly died last week. Jason contacted the manufacturer to see if we can get it fixed asap. It took them a week to get back to us, and the bad news is they are very busy now and it probably gonna take them at least 2 months to just get the laser checked. Although it’s not the end of world (we can still try to trap ion using dye laser as the cycling laser and use electron gun to ionize neutral magnesium), it’s gonna limit us to do any furthur experiments. Also, the electron impact is not as clean as photoionization, and the dielectric material near the trap (top view window, trap holder, etc.) will stay charged for quite a long time which will make it very difficult to keep the ions for long peroid of time. Another news which i don’t know it’s good or bad: yesterday brian called his old superviser in NIST and found out that 2 out of the 3 fibre lasers they have in boulder also died.
Spend most of this week working on the repumper laser. Before we were using mirrors from cvi laser to build the LBO cavity, which frequency doubles 1120nm IR to 560nm green. Those mirrors are rated better than 99% reflectivity. I measured the total loss of the cavity is about 1.7%, consisted of 0.3% nonlinear loss due to the second harmonic generation and 1.4% linear loss due to mirrors. The power buildup factor is just 1/loss, which is about 60x. I was able to couple about 800mW of IR into the cavity, which gave 0.8*60=48W intra-cavity power. The single-pass efficiency is about 70e-6/W, and i got about 120mW of green.
Last month we decided to purchase a new set of high-reflective mirrors, which should be better than 99.9%. We got those mirrors at the beginning of this month and I started to change my cavity mirrors last week. They are not as good as the coating company claimed, and I measure the linear loss is 0.6% (if we assume the mirrors are the same, then the reflectivity is about 99.8%). But still, I almost doubled my intra-cavity power and got 240mW of green.
Thursday morning, my green power was lower than the usual (about 170mW) so I decided to clean the cavity mirrors. What I supposed to do is to clean one mirror and check the power, then clean another mirror. I was too lazy to do that and cleaned all four mirrors at once. After I removed the beam block, there was no any sign of green! I tried to adjust some knobs but that didn’t help. It turned out that the output coupler was loose, but it was too late; I already fucked up my whole cavity. I ended up to spend a whole day to get my cavity back, and good news is the power is up to 270mW. So sometimes making a mistake is not a bad thing.
One of the problems we have is we are not sure if the magnesium oven is working or not. Yesterday we ramped the oven current to 5.1A and hoped to see some magnesium plating. We got nothing, and the pressure went to low -4 during the process. I am not very confident that it’s gonna come back to -11.