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I. Nonlinear Spectroscopy and Magneto-Optics

II. LabView Program Descriptions

III.Photodiode Circuit

IV. Tuning the Laser

V. Optics Tutorial

"Diode Scan for 111 lab.vi" and "B-field scan for 111 lab.vi" LabView^TM Program Descriptions

These two programs are nearly identical. The first one is used to control the laser scan parameters and data acquisitions from the analog channels ACH0-2, while the second one controls magnetic field scans and data acquisition on ACH3-4. The data from the latter two channels is combined to determine the optical rotation in the balanced polarimeter (see text). The front panel of the "Diode Scan for 111 lab.vi" program is shown in Figure A-1a.

Figure A-1a: The front panel of the laser scan control program

Each scan consists of triangular waveforms displayed on the Output Waveform graph. This waveform represents voltage that will appear at the DAC0 output of the DAQ card. Parameters of this waveform are set by the Minimum V and P-P Amplitude V controls. The first of these determines the lowest value of the waveform. The second parameter determines the width of the scan; the smaller it is, the narrower the scan. The "average" voltage of the scan is a combination of these two values, i.e. V_avg=V_min+V_pp/2. This is particularly important for the "Diode Scan" program as it allows tuning the laser to various spectral peaks.

The number of points per waveform is set by Number of points per scan, but data is only taken over half of them. (This is explained later.) When the program is run, a series of data acquisitions on ACH0-3 is taken. The Acquisitions per scan point control does determine how many times data is repeatedly taken at each scan point. Because of a difference in handling single and multiple acquisitions by National Instruments hardware and software, the minimum setting for this control is 3. Setting it to a higher number provides a way of signal averaging, since the data displayed and stored is an average of these multiple acquisitions. The Acqw. Scan rate (points/sec) control determines the time interval between successive date acquisitions (at a given point of laser frequency waveform). Most likely, there will be no need to change that from the default value (500) in the course of this experiment.

The scan can be controlled started, stopped and paused/resumed using the Start Scan, Stop, and Pause/Resume controls. The latter button is useful for stopping laser frequency scan at a particular point where a magnetic field scan will be performed. The Scan Mode selector allows choosing between the Single and the Continuous mode. The latter is analogous to the way an oscilloscope operates and is useful for tuning things. The data acquired in a scan can be written to a spreadsheet file using the Write to File control. Please create a folder for your files in the C:\User directory. You should give a name to the first file you save. The program will automatically suggest the same name to subsequent files with appended numeric file extension (this is done in the "suggestive open-read file with memory" sub-vi), so normally you just have to click the "Save" button in the file dialog.

NOTE:

• After you adjust the scan parameters, you must press Start twice. The first time you press it, the program sets the parameters and prepares for the scan by outputting the average voltage. Pressing Start again will actually start the scan. If the parameters are already set, you only need to press Start once.
• Below is a diagram (Figure A-1b) of a what the output from the Lab-view program looks like in time, as opposed to the diagram in the program, which is with respect to how the program increments the voltage at each program step. Here, V_min=0V, V_pp=2V

Figure A-1b: Output from the LabView in time