Creating and Characterizing a Dye Laser
Pumped by a Pulsed N2 Laser

Yoonji Choe, John Noé, Martin G. Cohen

Wellesley College and Laser Teaching Center

Dye lasers make use of organic dye molecules in solvent solutions aas a gain medium and are important because they are a source of laser light precisely tunable over a relatively broad range of wavelengths. These features have made dye lasers an important tool in spectroscopy and medicine. A dye laser consists of three essential parts: an optical pump, the gain medium and an optically resonant cavity. A high pump power is required to create a population inverstion in the dye solution. A pulsed laser will deliver short, high peak power pulses of very modest average power, which can create the necessary population inversion while preventing photobleaching and thermal effects in the dye.

The first step in this project was constructing the dye laser. The initial gain medium was Coumarin 500 dye diluted in ethanol at a 10-3 molar concentration and contained in a 5 ml quartz cuvette. The pump laser was a 337 nm nitrogen laser [Laser Science Inc. VSL 337LRF], which provides 5 ns pulses at 5 - 20 Hz and has a specified peak power of 30 kW. The pump beam was focused to a sharp horizontal line just inside the front surface of the cuvette using a quartz cylinder lens with an 18.0 cm focal length for UV light. Initially, no lasing was observed but this was later explained by the fact that the laser was only delivering 1 kW of peak power, as calculated from the measured average power and pulse width. It was subsequently discovered that an aperture attached to the front plate of the UV laser was blocking most of its output beam. Once this plate was removed, peak power increased to 7 kW and lasing was readily observed with just the parallel cuvette walls acting as the resonant cavity. Lasing stops when a glass microscope slide is inserted in the UV beam, which suggests that the threshold pump power for lasing is not much less than 7 kW.

The effect on the power and the quality of the laser beam from employing different mirror configurations has also been studied. A plane mirror and a 50 mm FL concave mirror have been used as the high reflectivity (HR) mirror. Cavity length L = 50 mm with the concave mirror was chosen to assure resonator stability. Various optics have been tested as the output coupler (OC) mirror including reflective neutral density filters, a microscope slide and a plated beam splitter. Results will be discussed.

In the coming week, we hope to continue optimizing the laser cavity as well as characterizing different dyes and dye concentrations. I also look forward to replacing the HR mirror with a diffraction grating to allow tuning the beam across the wavelength range of the dye.

Research supported by NSF Grant Phy-0851594 and the Laser Teaching Center. Thanks to Andrzej Lipski and Jonathan Sokolov for providing materials for the dye preparation and to Dr. Metcalf's lab for providing laser dyes, quartz cuvettes and a quartz cylinder lens. Last but not least, we are indebted to Prof. Metcalf for "looking under the hood" of the UV laser.