Oh, holy cow, can we please stop being plagued by problems? Alright, I can’t remember where my last post left off and I’m too lazy to go look it up but I need to get down the basics of things that have been going on recently.
First off, lets start at the end. I found today that one of my windows on the vapor cell is coated with Strontium. I’m not sure how or why. The controllers are keeping the window flanges at around 430 C, and the windows do not have any extra insulation on them. So even though they are recessed, they are quite a bit colder. I touched a thermocouple to the metal/glass junction and the coated one read ~ 272 C while the other was harder to get a reading on but it was only slightly hotter (maybe 10-15 C hotter). Because the temperature difference is not incredibly significant I wonder if there is something else that might make it more of a preference to stick to the one viewport. I turned off the heat on the back of the cell completely and capped the coated window with some foil to boost its temp a little. We’ll see how long it takes to blow all the Sr off of it.
I had been noticing previously a decrease in the size of the error signal. I had believed this might be due to decreased amounts of Sr in the cell so I started playing with hotter temperatures on the back side. Now, it seems in retrospect that what was really happening is that temperatures in the cell were just so that the window was slowly coating. As the window coated the error signal size decreased until I boosted temperatures and finished the job. I love seeing the glow of the atoms in the cell, but maybe we’ll have to run with caps on the viewports to keep the center of the sapphire hotter and less prone to condensation. We’ll see how things look when the windows are clear and the only heat source.
There have been other window scares as well. Yesterday, the temperature controller on the MOT’s slower viewport ran away and the temperature hit 694 C – almost 250 C above the temperature rating of the flange and almost 300 C above the rating of the glass to metal seal. This window is now nick-named “lucky”, because despite this the vacuum did not break (how? I’ll never know). In order to control the cooling of the window, we cut power to the controller and quickly hooked the heater’s leads up to a variac. Despite managing to accomplish this in about 15 seconds, the thermocouple temperature dropped about 50 C. Miraculously, the vacuum was still good. Using the variac I slowly brought the temperature down to just below 400 C and left it there. I don’t want to cool it all the way down to room temperature and temp fate with opening up an overstressed glass to metal seal.
In other news, I spent all last week implementing a homodyne lock to rid the error signal of unwanted noise and junk . It appears as though the circuit I built adds a small offset to the signal but I can zero that out with alignment. The circuit is saved as something like “9khzNoiseReductionCircuit” or something to that effect. It actually runs at ~7 kHz and basically consists of an astable multivibrator which generates a square wave to control 4 digital switches. One of the switches chops the signal to the voltage controlled oscillate (VCO) which drives the acousto-optic modulator (AOM). Two other switches alternately pass the the error signal to generate one signal for when the AOM is on and a second signal for when the AOM is off. One of these signals then gets inverted and they are summed back together. A low pass filter then takes care of high frequency noise.
The circuit seemed to be doing what it was designed to do but didn’t actually work until we figured out that low frequency noise was still passing through the circuit. It turns out that the square wave was not putting out a 50/50 duty cycle and so was giving preference one of the signals generated by the chop. By adding a potentiometer to adjust the oscillator we were able to zero out all the noise. Unfortunately this achievement came at the point where other problems prevented us from moving on.
Aside from the viewport coating on the vapor cell, I noticed that the laser was only putting out about 1/2 it’s intensity in the 461 nm. The output from the tapered amplifier (TA) was still good however. I spent yesterday with the top and side off of the doubling cavity box, getting the laser power back up to snuff. Most of it turned out to be alignment both of the input beam and the cavity. As it turns out we run at about 75% of the peak 461 nm power normally. The initial hunch was that this might be due to local heating in the doubling crystal when the laser is locked, but I was able to verify that it really has to do with stability of the master laser and that with a lot of pain and patience we can regain that power for a short time until the master becomes less stable again.
