Creating and Characterizing a Dye Laser Pumped by a Pulsed Nitrogen Laser

Introduction

I first became interested in dye lasers during my Introduction to Quantum Mechanics lab course. We did a lot of interesting labs involving spectroscophy and polarization but the one that most interested me was a lab in which we built a dye laser. With a very simple setup, we were able to create a laser beam through the dye solution and then once we introduced a diffraction grating we were able to change the color of that laser light just by turning the grating. The setup was so simple and elegant that I knew that there had to be more complex concepts behind how this laser was working.

This project was to built and characterize a dye laser in order to understand how a laser works and the underlying concepts in optics that govern how a dye laser must be constructed. The dye laser that I have built in the LTC has a very similar design to the laser that I first built in my lab course. There are, however, some very singular difference that made the building of this laser a challenge. I started with understanding the relation between the pump and the gain medium and how lasing could be acheived and then moved on to constructing different types of resonant cavities to increase the laser power and quality.

Background

All lasers are built on the principles of population inversion and stimulation emission. A population inversion is when there are more molecules or atoms in a excited state than there are in the ground state. Most molecules and atoms rapidly decay from an excited state to a lower level so that there are never enough molecules in the excited state to create a population inversion. Only molecules that have a long lived excited state can maintain enough molecules in an excited state to create a population inversion. A population inversion in necessary for lasing because laser light is created through stimulated emission. Stimulated emission is when a passing photon stimilates the release of an identical photon going in the same direction and in phase with the first photon. Most laser designs induce stimulated emission by bouncing the monochromatic photons back and forth between two mirrors which make up the resonant cavity.

Dye lasers are historically important because of their capability to be presicly tuned over a range of wavelengths. The reason that dye lasers are able to do this is because of the organic dye molecules that are used as the gain medium in these types of lasers. Organic dyes are large, complex molecules that have many closely spaced energy levels. Because these dye molecules have so many energy levels, they have high fluorescence gains over a broad range of wavelengths.

Molecules always fluoresce at longer wavelengths than they absorb and they always have a lower output power than input power. This means that the dye solutions must be optically pumped by a high power laser with a short wavelenghth. One type of pump that can be used is a high power continuous wave laser which will also create a continuous wave output beam. However, when using a high powered continious wave pump there are negative effects on the dye solution that is being used. One of these is the thermal effects of the pump laser light on the dye. Another major problems is the photobleaching of the dye. When the organic dye is pumped with so much continious laser light, it easily becomes photobleached and so useless as a gain medium. To solve this problem, these systems must have a flowing dye solution. In these systems, the dye solution is continuously flowing through open air where is it is being pumped by the continuous wave laser. These systems can easily become very messy and inconvenient. Alternatively, a pulsed pump source can be used. Pulsed lasers discharge short, high power pulses of laser light that reach peak powers on the order of kilowatts but with low average powers on the order of miliwatts. Because the power is distrubuted in short, infrequent pulses, the thermal and photobleaching effects are avoided and the dye solutions can simply be held in a cuvette.

Setup

The pump laser light was focused into a horizontal line just inside the face of a quartz cuvette using a quarts cylinder lens to create a line of flourescence. The optical axis for the output beam is orthogonal to the pump beam and the resonant cavity is along this axis. The cuvette can be place vertically to create lasing using the walls of the cuvette or can be tilted to ensure that the mirrors are acting as the resonant cavity and not the cuvette walls.

The available dyes were coumarin 500, coumarin 480 and rhodamine 640 and the available solvent was methanol. The solution that was used most extensively throughout this project was coumarin 500 in methanol at a 10^-3 molar concentration.

Cylinder lenses as well as cuvettes had to be tested for absorption and loss. Quartz components are required to minimize loss through absorption of the UV light. Measurements were made using a Thorlabs Det210 detector connected to a 100 kohm resistor and voltage outputs were read. The resistor was necessary to integrate the pulses and prevent saturation of the photodetector.

The measurements reveal that the 150 mm and 200 mm cylinder lenses are quartz as well as the quartz cuvette while the 100 mm cylinder lens and the plastic cuvette reported higher losses indicating that some of the UV light is being absorbed.

Lasing Threshold

The conditions for reaching the lasing threshold with this setup were studied. Given the dye solution used and the cavity conditions, this equations gives you the threshold population inversion (number of excited molecules per cubic centemeter) to achieve lasing:

Of these variables, the ones that can be most easily manipulated to increase cavity gain are the mirror reflectivities. Once the threshold population inversion is known, the threshold pump power easily be calculated using this equation:

Given a Rhodamine 590 dye in an ethanol solution, L=10 cm, R1=100% and R2=95% the necessary population inversion is No=2.2 x 10^13 and the threshold pump power is 2.5 kW not including any calculations for cavity losses. This threshold pump power is an exremely optimistic value as there are significant losses through reflection and absorption through all of the optical elements in the laser system. This is also a rather poor comparison to the laser that was built in this project becaues the reflectivities of the mirrors as well as the cavity length were much lower. In fact, lasing was acheived between just the walls of the cuvette which are only about 4% reflective. This means that the necessary pump power for this laser is many times greater than 2.5 kW. [laser sam's website]

Nitrogen Laser Pump

The optical pump was an old Laser Science Inc. VSL-337LRF nitrogen laser. The specifications for a new laser were retreived from the Laser Science Inc. website:

• Wavelength: 337.1 nm
• Rep. Rate: 1-20 Hz.
• Pulse Width: 4 ns
• Peak Power: 30 kW
• Ave. Power: 2.4 mW @ 20

Because the laser is so old, it was necessary to test for the current power and pulse width of the laser as these parameters change over time. The pulse width was determined using a Thorlabs Det210 silicon photodetector which was connected to an oscilloscope. The photodetector was becoming saturated at first because of the high peak power of the pulses and so the laser beam was tilted so that only a small portion of the laser beam was hitting the photodetector. The pulse had a very sharp rise time and was measured at around the full width at half maximum. The pulse width was measured to be 5 ns which is still very close to the specifications for a new laser.

It was also important to measure the output power of the nitrogen laser to determine whether it would be close to the threshold pump power needed to use as the optical pump for the dye laser. The average power was measured using a Molectron Detector Inc. Power Max 500A energy meter and PM3 thermal detector. The average power was measured as Pave=0.1 mW at 20 Hz. Since we also know the pulse width and repetition rate, the peak power can be calcualted using:

The peak power for this laser was only 1 kW. Much less than the calcuated pump power threshold for lasing and, not surprisingly, no lasing was observed when this laser was used as the optical pump for the dye laser.

The next step was figuring out how to increase the power of the nitrogen laser so we opened up the laser to see if there was a quick fix that we could perform. It turns out that there was a quick fix to this problem. As we were taking the laser apart, we found that the front face of the laser was covered by a seperate plating that was attatched to attenuated the light. The small circular apperature was much smaller than the actual laser beam and the inside face of the plating was damaged where the UV laser light was hitting it. Another second power measurement using a Scientech 365 power and energy meter and termal detector (model number 360001) revealed that the average power increased to Pave=0.7 mW and the peak power increased to Ppeak=7 kW.

The focused horizontal pump beam streched across the entire front face of the cuvette once the aperture was removed, whereas, before, the fluorescence line was only in the central area of the cuvette. Changing nothing other than the power of the pump light, lasing was achieved using just the parallel walls of the cuvette as the resonant cavity. For this lasing to occur the cuvette has to be orthogonal to the pump light so that the reflecting walls of the cuvette direct the light straigh back through the fluorescence area as shown in the first picture. The second picture shows that when the cuvette is tilted away, lasing ceases.

Resonant Cavity

Cylinder Lens Focal Length

The cylinder lens used to focus the light has a focal length of 200 mm in white light but will have a different focal lenght for UV light. Output laser power through the cuvette walls was measured at different distances from the cylinder lens. A Thorlabs Det110 detector connected to a 100 kohm resistor was used to measure voltage.

These measurements revealed that, for 337 nm light, the focal length of the cylinder lens is 180 mm.

Cavity Stability

The resonant cavity of a laser is very important as this is what creates the monochomatic light that characterizes laser light. The a stable cavity will directe the light back and forth through the gain medium before letting it escape through the output coupler mirror. A simple calculation tells us whether a cavity will meet this condition for stability or not.

A plane mirror has a radius that goes out to infinity. Independent of the distance between the mirrors plane parallel cavity has g1=1, g2=1, and g1g2=l which just meets the conditions for stability and is considered conditionally stable. This means that when the light is slightly off the optical axis, it will escape the cavity and the mirrors must be perfectly aligned for lasing to occur. Because a plane parallel cavity is conditionally stable, not many lasers make use of this type of cavity because of its sensitiviy to disturbances in mirror alignment.

Another type of cavity is a hemisphirical cavity which uses a concave mirror as the HR an a plane mirror as the OC. R is negative for a concave mirror and so as long as d, is shorter than the radius of the concave mirror, this type of cavity will meet the condition for stability. If the distance between the mirrors is the focal length of the concave mirror, g1g2=0.5 which is in comfortably in the region of stability. This cavity is fully stable and is not so sensitive to the aligment of the mirrors.

Both a plane parallel and hemispherical cavity were used in the making of this project. When lasing occured between just the walls of the laser, this was effectivly a plane parallel cavity with very low reflectivity mirrors. A 100% reflective HR and 98% reflective OC were introduced along the optical axis and the dye cuvette was tilted so that the lasing was occuring only between these two mirrors. This cavity was hard to align and created a very low power laser beam because of the loss introduced by the tilted cuvette and the low transmittance of the OC. The HR plane mirror was replaced by a concave mirror with a FL=500 mm and the plane mirror was placed 500 mm away.

Output Coupler

No suitable partially reflective mirror was available to use as the output couple of the resonant cavity. A neutral density filter of varying optical density was tested for reflectivity, transmittance and loss. Each section of the filter was labeled from 1 through 10 according to increasing optical densities (these are not the optical densities of the filter sections but labels for identification). Measurements were made using a 30 mW HeNe laser and Thorlab Det110 silcon detector.

The efficiency of the neutral density filter as an output coupler was also tested. The concave mirror was used as the HR and the cuvette was tilted so that lasing was occuring only between the concave mirror and the neutral density filter.

Section 5 of the neutral density filter resulted in the highest output power. This section has a 26% reflectance, 41% transmittance and 33% loss. While section 5 of the filter resulted in the highest output power for this type of setup,