Characterizing a Violet Diode Laser
|Trial||L (cm)||x (cm)||d (µm)|
d = 1.351 µm
In order to find the temperature the HeNe laser was replaced with the violet diode laser. 'L' and 'x' were measured to find θ and plugged it into the equation. Then, instead of filling in a wavelength, d = 1.351 was filled in. This was done at seven different distances and the seven answers were averaged together.
|Trial||L (cm)||x (cm)||λ (nm)|
λ = 407.71 nm
Part of the beauty of this laser is that is comes with a built in device to change the temperature. In order to change the temperature what needs to be done is to change the voltage, which can be done by adjusting a screw in the laser. Using two wires to connect the laser to a volt meter, the change in voltage, and therefore the change in temperature, could be changed and detected. There is also a table with the laser that shows the voltage at each different degree of temperature. Using this, it could be determined what voltage to set the laser at to reach a certain temperature and the screw could be adjusted accordingly.
|Temp (°C)||Voltage (V)|
In order to be able to observe such a small movement in a spot of the diffraction pattern (which would be caused by a change in wavelength) this set-up needed to include a linear-array photo detector and an oscilloscope. The photo detector was first set up at the 1st order, but was later switched to the 2nd when it was realized that 2nd order would create a greater dispersion, therefore giving more accuracy. The photo detector was connected to the oscilloscope, which gave the readings of the individual pixels of the light and their movements. Another component of the set-up was a lens. Because the detector was so narrow, the spot on it had to be very small. The lens was able to focus the light. Although conventionally a lens should go after the diffraction grating so that as much of the diffraction grating is covered with light as possible, therefore giving more accuracy, because of limitations of the table the set-up was on, the lens was put right before the diffraction grating. However, because the focal length of the lens was relatively large, 0.5 meters, the amount of diffraction grating covered was not affected greatly. The photo detector was 0.5 meters away from the lens and was able to detect a much smaller spot with more accuracy.
These pictures were taken as the temperature was being slightly changed. They can be used to measure an overall change in the peak by measuring the centriod of the peak. This would allow you to see the shift of the peak over a large change in temperature.
In order to find the wavelength change for the change in centroid the derivative dλ / dθ = L m / dcosθ was used. It was found that dλ / dθ = 817 µm/nm. Since the centroid told how far x had moved, it could be figured out how far λ had moved. When this was done a goodness-of-fit line was put over the points and the mode hop at around 21°C became obvious.
There were certain temperatures, such as between 20°C and 22°C were the oscilloscope didn't always show a reading of a single line of pixels, but instead showed two. This happened at points when the laser was switching between modes. Because of this, although the temperature doesn't change drastically, the wavelength could. This explains data such as why the movement from 22°C to 20°C caused double the movement of the spot as the movement from 25°C to 23°C. Because of this the final graph is expected to resemble steps, instead of a straight line.
The process of measuring the output power consisted only of a power meter. When the laser and focused into the meter at different areas it read different powers, but when it was measured through the lens, which absorbed some of the power, it read at 3.25 mWatts. This reading stayed constant as the power meter was moved closer and farther form the laser.