The spectrum of a laser becomes simple to measure by means of spectroscopy. Unlike my original expectations, I saw a maximum peak at a certain wavelength (near 807nm), and little peaks similar to what you would see in a single slit diffraction pattern. I had expected to see only one peak.
As the current increased, not only did the visibility of the smaller peaks decrease, but the wavelength also changed. Very odd and unexpected. So, I measured the wavelength and the relative intensities for 13 different currents, ranging from 23.3 to 32.2 milliamps.
Spectroscopy DATA - These are the graphs of the spectrum of the laser light at varying currents
Click Here for Wavelength vs. Current and Intensity vs. Current graphs
Q: Why is there a Single peak with higher currents?
- Generally, a LASER works by "stimulating emission" of a second photon, by a single photon. The stimulated photons "bounce" back and forth between two mirrors stimulating the emission of more photons. So, the number of photons double with each "reflextion" off the mirrors. As the current increases, and therefore temp changes, certain wavelengths stimulate more photons. Higher currents and more stimulated photons, means a larger difference between the different wavelengths.
Q: Why does the maximum intensity peak slightly increase before making a drastic drop?
- This is more of a human reading error than a machine or spectroscopy fluke. Ideally, when we look at the graphs, we should be able to lable the middle peak Zero, and the other smaller peaks negative or positive relative to it. If we then watch and map these peaks over the change in current, one would expect that the entire "wave" would move down, as current increased, while the intensity doubled for each individual peak. However, at around current = 28, one can see in the Wavelength vs. Current graph that the drops, while before hand the wavelength was increasing. When we go back to labelling the peaks, we noticed that there is a certain current where the "Zero" peak, the same peak at the low current, is not the highest peak in the center anymore. Now the Plus One peak is the highest shown. So we are now measuring the wavelength of a different frequency. If we had continued to follow the same peak throughout the series of graphs, aka follow the same frequency, one would notice a steady, expected, change in wavelength vs. current.
Q: Why does the wavelength change with current? I thought a laser would produce only one specific and consistant wavelength of light.
- More important than wavelength changing with current is the fact that temperature changes with current, with in turn alters the wavelength. One can compare the spectroscopy data graphs with the idea of a standing wave in a closed system. For a closed system (a box or two secured ends of a rope), only resonant frequencies will create a standing wave. The curve which we see in the data is the "Gain". Each individual peak which makes up the Gain is a resonant frequency for the laser. Below are two standing waves:
The change in frequency is the width separating two of the peaks for the laser. n(T), is a function of temperature. Therefore, as the current increases, and the temperature in turns becomes greater, the index of refraction also changes. The difference between peaks, are now closer, or father, from one another, and at the same time, the shape of the curve has been altered due to the doubling of the photons. So, the frequency is a function of temperature, which changes the wavelengths of the resonant peaks.
Acknowledgements:
Mark Peterson
Special thanks to Janice Hudgings for her help in understanding lasers and my data.