Investigation of Laser Diode Properties
The experimental setup for the laboratory varies slightly depending on the measurements being made, but is not difficult to change when needed. Our experiment, in its basest form, required an optical mounting table, a diode laser including temperature and current controls, and optical isolator, a photodiode, a device for measuring the wavelength of light (we used the Wavemeter Jr.), and a multimeter. For observing the excitation of rubidium atoms, we also needed a cell of rubidium and a camera capable of detecting light in the range of 790 nm, however that experiment was almost external to the rest of the lab.
The original setup required mounting the laser with the current and temperature controls attached and the optical isolator such that the laser beam traveled through the optical isolator uninhibited. We then mounted the signal collector from the Wavemeter Jr. such that the laser beam was directed into the optical fiber. Care must be taken here to ensure that the signal collector is mounted so the beam travels straight into the optical fiber. For our experiment, we were able to view the portion of the beam reflected off the signal collector and adjust the collector's orientation such that the beam was reflected 180o back toward the beam source. Once the signal collector was in place, we were ready to begin taking measurements of the output wavelength vs. current and temperature.
The second portion of our experiment required the use of a photodiode to detect the power output of the laser. We replaced the signal collector from the Wavemeter Jr. with the photodiode and attached a coaxial cable. At the other end of the cable was a 50 ohm terminator and the appropriate bnc connector to attach it to the multimeter. Using the multimeter on the 300 mV scale we were then able to measure the potential drop across the terminator and calculate the ouput current from the photodiode using Ohm's law.
The final portion of our experiment required using the Wavemeter Jr. and a rubidium cell to observe the electron transitions in rubidium. We therefore replaced the photodiode with the signal collector from the Wavemeter Jr. and performed the same setup as described for part one above. We then mounted the rubidium cell in the beam of the laser. Thus, we were able to monitor the output wavelength of the laser while passing the laser through the rubidium cell. (Note: the observation of rubidium transitions may be better performed before making measurements of the output power of the laser to cut out the step of setting up the Wavemeter Jr. a second time. The Wavemeter Jr., while a powerful tool, is not easy to set up. )
Once we had performed the setup as described above, we began by taking measurements of the output wavelength for various temperatures at a constant current of 50 mA. We began by taking measurements of the wavelength at 1 degree intervals under the assumption that we would return to the temperatures where mode hopping was observed to take additional measurements to find the exact size of the mode hops. This effort did not prove successful for two reasons. First, there is a hysteresis in the laser that prevents it from always giving the same wavelength for a given temperature. We found that when making measurements by increasing the temperature and then making making measurements by decreasing the temperature, the second series of data differed by almost 0.5 nm. The second reason this was not successful is that mode hopping does not always occur at the same temperature for a given current. While the size of the mode hops may be consistent, the actual location of the hops is very inconsistent.
Having discovered this hysteresis, we set about taking a second series of data of wavelength vs. temperature for a 50 mA current. For this series, we took data at 0.5o intervals for the range of temperatures between 25.0o and 50.0o celcius. This series of data shows quite nicely the linearity of the wavelength vs. temperature graph and the mode hops that occur. We then took another series showing the change in wavelength for 1 mA changes in current from 42 mA to 100 mA. This data too shows the linear relationship between wavelength and current, but also shows many fewer mode hops.
For the second portion of our lab, we took measurements of the laser's output power for various currents at temperatures of 5o, 25o, 30o, and 45o celcius. To get a large range of data, we used 1 mA intervals of current from 15 to 99 mA. We took data by measuring the current output of the photodiode. From this current, we used a book value conversion factor of 0.55 amps per watt to derive the output power2. This data shows that there is a distinct point at which lasing occurs that depends upon the temperature of the laser diode. Once lasing does occur, the power increases linearly with current.
The final portion of our experiment involves using the rubidium cell in the laser beam to observe the light emitted in the rubidium transition from the 5p1/2 to the 5s state. We found a literature value indicating these transitions occured at 794.9782 nm3. Consequently, our objective was to match this wavelength with the laser in order to excite the electrons in order to watch them decay back to the 5s state. For our setup, we found a wavelength of 794.98 at 4.6 kohms of resistance in the temperature control and 97.0 mA of current. This transition is outside of the visible spectrum so it is necessary to use a special viewing device to observe it. We tried two seperate cameras that are capable of viewing such radiation. The first camera was insensitive to the amount of light given off by the rubidium, however the second showed the transition clearly.