A Coherent 699-29 ring dye laser is used, offering significant advantages over the standing wave models. In a standing wave dye laser, nodes and anti-nodes are set up in the jet, causing the output of the dye laser to drop when used in single frequency mode in conjunction with high pump laser power. As the ring dye laser uses a travelling wave laser system, pump power and hence power output can be significantly increased [67]. The schematic for this laser is shown in Figure 2.5. The 514.5 nm transition of the Ar+ pump laser is focused onto a 40 PSI jet of dye solution. This causes lasing in a ring cavity, consisting of two high finesse etalons, a Bi-Refringent Filter (BRF), an optical diode and a Brewster tipping plate. The BRF reduces the laser line width to 1 cm-1, enabling coarse frequency tuning. The action of the etalons causes the linewidth of the laser to decrease to approximately 3 MHz, and the frequency of this mode can be continuously scanned with a galvo driven tipping Brewster plate. Sections of length 10 GHz are scanned continuously by tilting the Brewster plate, the laser tuning elements are then reset and the next 10 GHz is scanned. Frequency jitter is compensated for using a reference cavity and a negative feedback mechanism controlling a piezoelectrically driven mirror (`tweeter') and the tipping Brewster plate. The wavemeter is incorporated into the laser and is accurate to cm-1, eliminating the need for the continuous recording of an iodine spectrum and the use of external etalons. The laser radiation from the ring dye laser is chopped by a mechanical chopper and aligned to the ion beam apparatus using a series of high reflectivity surface-coated mirrors and a 1m focal length lens.
A personal computer running the Coherent AUTOSCAN program controls the various tuning elements via an interface and a control box. The computer scans the laser, reads the wavemeter and collects the data from the lock-in amplifier simultaneously.