FMCG/Operating procedure

This section will cover everything up to the noise measurement, and will assume everything from the "Experimental Setup" procedure has been completed.

FPGA and Arduino "Reset"

Both the FPGA and Arduino microcontroller tend to behave somewhat strangely after a computer restart, so it is a good idea to "reset" both of them before beginning.

1. Open up the Arduino code. The file is in the upper-right corner of the desktop and is named Decoupling Code (with LCD). Click the right arrow (under the "Edit" menu to upload the code to the Arduino.
2. Once you receive an "Upload Complete" message in the program, the code is ready. The LCD screen should read all zeroes (or whatever values the user specified). However, because of a feature of the code, the outputs of the Arduino (and consequently, the current outputs of the current supplies) are all sent to the rail.
3. To truly set the currents to zero, turn any one of the Field Adjust knobs on the current supply box by one click. Then turn the knob back one click to return all supplies to zero.
4. Now open up LabVIEW 2014 using the shortcut on the bottom taskbar. Open the FPGA Magnetometer.lvproj project. LabVIEW will then load a bunch of programs which takes ~60 seconds.
5. Once the project is opened, inside the Main Programs folder, open the Set_AO_Zero_(Host).vi program. When the program is run, it will send a zero (reset) signal to all of the outputs of the FPGA. If an error is output when this code is run, something caused the computer to lose connection with the NI chassis and the computer will have to be restarted.

Absorption Scan

Before proceeding with a noise measurement, it is a good idea to measure the rubidium absorption in each of the cells.

1. On the array, switch the detectors to SUM mode. This is accomplished by moving both the 4-socket leads from the detector and the 2-socket leads that carry signals out of the room to the set of header pins denoted by a Σ or + label.
2. If not done already, turn on the TEC and LD for the probe laser diode (the right-hand SRS LDC501 controller). We have historically and recently run at 170.00 mA of laser diode drive current on the probe. With this drive current, the D2 absorption line occurs at a "temperature" of roughly 7.05 kΩ and we generally do our magnetometry at a temperature of 8.5-9.5 kΩ.
3. Now return to the rear of the rack and, after verifying the probe tapered amplifier TEC 2000 temperature controller is enabled, enable the probe tapered amplifier output by pressing the output button on the Newport Model 560B driver. The drive current on the TA sets the eventual light intensity on the detectors. Historically, we have worked with currents as high as 1200 mA, though recently we have worked at much lower values around 750 mA, which delivers ~500 μW of laser power through a hot cell to the detector.
4. Once the TA is enabled and light is being sent to the detectors, adjust the gain settings on the I-V converters appropriately. 100-200 uA/V is usually appropriate for the laser powers mentioned above.
5. At this point, the detector sum signals should be visible on the scope. The signals should be pretty constant over time. If you notice non-periodic "noise" on this signal, it's possible the beam alignment between the TA and the fiber input is poor. While monitoring the signal, go behind the rack and walk the beam using the two mirrors on the second shelf from the top.
6. Once the coupling is optimized, the laser can be scanned across the resonance to measure absorption. The detuning can be scanned by varying the temperature of the diode in one of three ways:
   1. The temperature can be set manually by pressing Set on the TEC half of the controller and entering a numerical value with the keypad followed by Enter.
   2. After Set is pressed, pressing the Live Entry button will allow the user to slowly ramp the temperature by turning the knob next to the Live Entry button.
   3. The controller can be controlled using the GPIB connector on the back. This is done using a LabVIEW program, which is what will be described below.
7. In the FPGA Magnetometer.lvproj project, open the Laser Scan v1.1 program under the Main Programs folder. By default, the program is set up to take an absorption scan with the probe beam:
   1. Differential should not be enabled.
   2. The light next to Scan Probe should be bright green.
   3. With the D2 resonance at roughtly 7 kΩ, ramping the temperature from 5 kΩ to 10 kΩ will more than cover the resonance.
   4. 200 points with a 100 ms dwell time at each point will allow the actual laser temperature to "keep up" with the set temperature as the ramp runs.
   5. A PreScan Dwell of 10 s will allow the laser temperature to stabilize at the start temperature before the ramp begins.
8. Clicking the LabVIEW Run button will begin the ramp. Results for each trace will be plotted in the XY Graph window. A .dat file containing all of the data collected (Thermistor resistance and 4 channels of voltage data) is recorded on the file in Path Out.