APPLICATIONS  AND  OPERATION

     Ion lasers, especially the argon laser, have numerous applications where higher power than that obtainable from HeNe lasers (which are limited to about 100 mW) is required or green-blue light is required. It has become a workhorse lab laser used for all sorts of measurement and excitation purposes, including chemical fluorescence studies and pumping CW dye and solid-state lasers. In the forensics lab, fingerprints are found to fluoresce when illuminated by argon laser light. In medical applications, the wavelength of the argon laser is absorbed readily by red blood cells but passes easily through the liquid filling the eyeball, so it is ideal for certain types of opthalmological surgery, including welding detached retinas. Other medical-lab applications include flow cytometry and DNA sequencing, where the blue and green wavelengths are important to the application.

       The powerful beams are also useful for high-speed copiers and printers, where they can be used in the same manner as an IR semiconductor laser is for small laser printers. The more energetic photons and more powerful beam allow faster printing than a semiconductor laser does (many small air-cooled argon lasers are designed precisely for this purpose). In the UV region, ion lasers can be used to expose tiny features on high-resolution printing plates and films (useful since beam quality is excellent and UV wavelengths can be focused to a much smaller spot than visible wavelengths). Other industrial purposes include the mastering of CDs and DVDs.

       The krypton laser, which can be designed for white-light output, is a favorite for laser light shows. Multiple wavelengths of the laser can be split using a prism and directed independently, or a single wavelength can be selected from the output by a high-speed AO modulator called a PCAOM. Beams are then scanned using mirrors attached to precision galvanometer movements to create patterns. As in a television set, fast scanning ensures that the eye renders the image continuous.

       For years the argon laser remained the only powerful CW source of green and blue laser light available, but at present the market is turning toward simpler (and smaller) frequency-doubled YAG lasers, which feature an output at 532 nm in the green, or frequency-tripled YAGs, which have a UV output at 355 nm. For specific applications requiring the 488-nm blue wavelength, newly developed solid-state lasers with precisely this wavelength threaten the dominance of the argon laser. Regardless of the appeal of solid-state lasers, the argon ion will remain an important source of blue-green light for years to come, especially since it features a multi wavelength output which cannot be achieved (easily, at least) with a solid-state laser.

        Operation of an older ion laser has been described as more art than science. What] with alignment of optics, the necessity to monitor gas pressure manually (in many cases via a thermocouple pressure gauge attached to the tube), perform manual gas fills via “ready” and “fill” pushbuttons as required, and monitor voltage across the pass bank and tube to keep each in a safe operating region, these lasers could hardly be called “plug and play” and usually required a skilled operator. Today, electronic controls have greatly simplified the process since most users simply need to press the start button, wait 45 seconds for the laser to preheat and ignite, and then adjust the tube current or light output for the application at hand. Standard features such as interlocks ensure water flow and temperature are adequate to protect the tube from damage due to careless operation.

     Ion lasers should not be left in storage for long periods of time, since some of the gas trapped in the tube through sputtering processes (which serve to reduce gas pressure as the tube operates) can be released back into the tube as the laser sits idle, raising the tube pressure. A high-pressure tube can be difficult, if not impossible, to start. Many manufacturers recommend operating an ion laser at least once every two weeks to keep gas pressure in the tube within a nominal range.

      Owing to the high cost of replacement, dead ion tubes are often used for rebuilding, at about half the cost of a new tube. Rebuilding is, however, much more than simply opening the tube, pumping to high vacuum, and refilling with fresh gas (called a “chop and pump” in the industry). Although a simple refill will work to revive a dead tube temporarily, the lifetime of a repumped tube is quite short compared to that of a properly rebuilt tube. Proper rebuilding of an ion tube entails replacement of the entire cathode (which is usually badly eroded by the time the gas is used), and disassembly and cleaning of the entire tube, not an easy task. Laser output is also sensitive to contamination in the tube, so extreme vacuum purity is required. Tubes must be pumped down and cycled to clean them thoroughly before fresh gas is added.