Download Experiment 2-6

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.
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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 terminatorto
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.
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