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PHYSC 3622 Experiment 2.6 13 May, 2017 Dye Lasers Purpose In this experiment, you will learn the basic working principles of a dye laser. Equipment Hanger 9 electric fuel pump, 24V DC power supply, Rhodamine 610 perchlorate organic dye in methanol, laser setup including a high-voltage power supply, a lasing chamber and two optical mirrors, photodiode, KNF vacuum pump, He-Ne laser, and oscilloscope. Background Organic dye molecules are complex molecules and have energy levels just as many other liquids and solids do. When they are dissolved in liquid solvents, such as water and alcohol, in low concentration (typically one part in ten thousand), the dye molecules are isolated from each other and are surrounded only by solvent molecules. They absorb light strongly in the ultraviolet and visible spectral regions. When laser dyes are irradiated or pumped with light at these wavelengths, the electrons in the dye molecules jump from the ground state to the excited states at higher energy levels. Because the dye molecules are sufficiently shielded from the solvent medium, their decay from the excited state to the ground state occurs predominantly through radiative decay. Consequently, the dye molecules radiate very effectively at somewhat longer wavelengths than the pump wavelengths. Organic dyes are efficient radiators and thus make good lasers. The ratio of radiating dye molecules to the total number of the excited dye molecules (exhibiting both radiative and non-radiative decay processes) is known as the quantum yield. Most laser dyes have quantum yields approaching unity. Reference book: W. T. Silfvast, "Laser Fundamentals," p.141-146, Cambridge University Press, UK, 1996. Procedure The laser setup to be used in the experiment consists of three sub-systems. (1) The dye circulation system is a closed loop consisting of an electric fuel pump operated by a DC power supply, a loop of plastic tubing and a 1-liter storage bottle. This system is used to circulate the dye solution in and out of a straight glass tube inside the lasing chamber. Circulation is needed to prevent the dye solution in the glass tube from overheating under the repeated laser pulsing. The dye solution used is perchlorated Rhodamine 610 dissolved in methanol. You may start with a dye solution at the concentration of 10-4 M and then reduce the concentration to find an optimal concentration for the dye laser. (2) The laser pumping system is used to excite the dye molecules in the lasing chamber. This is accomplished by using a thin flash tube, which is placed in parallel with the glass tube containing the dye solution. There are two electrodes located at the ends of the flash tube and they are connected to a high voltage power supply. The power supply is just a simple step-up transformer connected to a RC circuit. The maximum output of the transformer is 15kV at 10 mA and the time constant of the RC circuit is of the order of a fraction of a second (flashing time). The flash tube is connected to a vacuum pump. When air is pumped out of the tube, the voltage difference between the two electrodes exceeds the dielectric strength of dilute gas and dielectric breakdown takes place. This gives a flash needed for exciting the dye molecules. 1 PHYSC 3622 Experiment 2.6 13 May, 2017 (3) The optical system of the laser contains two mirrors and the glass tube containing the dye solution in line to form a laser cavity. The mirrors at both ends of the cavity need to be aligned in such way that at least 3 recognizable co-linear reflections are achieved in the cavity. You may use a small He-Ne laser to guide your optical alignment. The mirrors are partially transmittable (40% - 60% reflection) so that the laser beam can go through it. To align the mirrors properly, you need first to adjust the laser beam so that it can go through the middle of the lasing tube. A mirror then can be placed on the opposite side of the dye-laser cover box and be adjusted until it reflects the laser beam exactly back to the laser source. A second mirror is then placed between the cover box and the He-Ne laser. The second mirror needs to be adjusted so that it also reflects the laser beam exactly back to the laser source. Some minor adjustments to the first mirror may be necessary. Once the cavity is aligned, you should be able to see a strong laser spot coming out of the cavity. Then you can install a photodiode just in front of the He-Ne laser to detect the laser pulses (make sure that the battery of the photodiode is operational). You may use a HP oscilloscope (with a 50 terminatorto view the laser pulses from the photodiode and print out the final results. The width of the laser pulses is of the order of 5 s and the pulse height is in the range between 200mV and 500 mV. Often the dye laser can lase even if the alignment is not perfect. You may tweak the orientation of the mirrors (horizontal and vertical adjustments) to see how it affects the detected laser pulses. After the alignment of the laser, you may study how the dye concentration affects the pulse height and find an optimal concentration for the dye laser. Another interesting effect you may study is to see how the cavity length affects the pulse width. Theoretically, one expects the pulse width to increase with the cavity length. Questions In your project report, you need to discuss (1) the basic working principles of a dye laser, (2) how the flash tube works in the dye laser (what is dielectric breakdown?), and (3) experimental details including the setup and procedures. 2