Investigation of Laser Diode Properties

Data and Results

 Measurements of Wavelength vs. Temperature and Current

Viewing the data for the wavelength vs. temperature graph and the wavelength vs. current graph, we see to very important properties of the diode laser. The first is the phenomenon of mode hopping. The wavelength vs. temperature graph shows many more mode hops, and therefore is best for illustrating the phenomenon. Looking at the size of the mode hops, it is apparent that most are relatively of the same size. To analyze the mode hops, it is necessary to make measurements of

click on graphs for larger images

this size of the wavelength change for each mode hop. The value for each mode hop is given in the following table.

Diode Laser Mode Hops
Wavelength vs. Temperature
Temperature (oC)Mode Hop Size (nm)
Wavelength vs. Current
Current (mA)Mode Hop Size (nm)

In analyzing this data, we chose to ignore the mode hop occuring at 67 mA because it was inconsistent with the rest of the data. This hop is most likely due to inconsistencies in the laser. The data points at 30o and 39oC may at first glance also look like like inconsistencies, except that they are almost exactly 2 and 4 times the size of the other mode hops. What happened at these point is that the laser simply hopped multiple modes. By averaging the size of a single mode hop, we found that the jump between modes was 0.31 nm.

Knowing that mode hops originate from inserting an additional half wavelength inside the laser cavity, it is possible to compute the number of half wavelengths and consequently, the size of the laser cavity. Calculating this requires a value for the index of refraction inside the diode laser. This index varies with temperature and diode composition, but is around 3.54. The following equations show how the number of half wavelengths inside the laser cavity is calculated:

We found that there were about 2550+-100 half wavelengths in the laser cavity. Using this number for n in the following equation gives a value for the length of the laser cavity.

For our laser, we found the length of the cavity to be on the order of 0.29+-0.02 mm.

Measurements of Power vs. Current

We next took measurements of the laser's output power for various currents at constant temperatures. The data for these measurements are pretty self explanatory and shown in the following graph.

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Note how the laser produces no output power until a threshold current is reached. This threshold is lower for lasers at lower temperatures. Also, note how the laser's power increases linearly as the current increases.

Excitation of Rubidium

In the final portion of our experiment, we used the laser to produce electron transitions in rubidium atoms. To do this, we merely found a literature value for the wavelength of photons emitted in the first transition and found a temperature and current that supplied us with that wavelength. We then used a special camera that is sensitive to infrared light to observe the dim light emitted by the rubidium. Although the observation was impressive, no data was taken, and no comment on the results is necessary other than to say that the transitions were produced with a wavelength of 794.98 nm.

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