The Year of the Laser

Aug 31, 2010

History credits the Hughes Research Lab's Theodore Maiman as the first to successfully operate a laser, in 1960. Since then, according to the LaserFest website, the U.S. Patent office has granted more than 55,000 patents involving the laser.

The laser has become an important asset in mass spectrometry (MS). Indeed, its usefulness nearly rivals that of electrospray ionization (ESI) in the ability to examine the molecular underpinnings of biological systems. So it is fitting that we consider the laser from a perspective of MS practice, and speculate on its future.

The Optical Society of America (OSA) and the American Physical Society (APS) are jointly promoting LaserFest ( http://www.laserfest.org/), which they describe as "a multi-year celebration designed to commemorate the 50th anniversary of the invention of the laser." Toward that end, the partnering societies plan a range of activities including public outreach events, lectures, symposia, and educational demonstrations. Many of the proceedings will recognize and honor the accomplishments of those who discovered, developed, and applied laser technology. Among those memorialized will be Charles Townes, who figures prominently among the pioneers of laser technology. Townes developed the forerunner to the laser — the "maser" — and published, with Bell Laboratories' Arthur Schawlow, a key theoretical paper in 1958 that led to the laser's commercial development. In 1960, Townes and Schawlow were jointly awarded the first laser patent.

Laser Types

The four types of lasers are gas, solid, liquid, and semiconductor.

Low-cost, helium-neon (HeNe) lasers are ubiquitous in the educational setting and emit energy at a variety of wavelengths. The typical low-cost units operate at 633 nm and are common in education. The HeNe mixture — the gain medium — is pressurized such that helium = 1 Torr and neon = 0.1 Torr. The gas mixture is contained within a glass tube of 1 cm diameter and 0.25–1 m length. Two electrodes connect to a high-voltage dc source that generates a discharge inside the tube acting like a "pump." Two parallel mirrors are placed around the gain medium, one in front of the other, so that only mirror 1 shows the complete reflection. Mirror 2 shows a partial reflection. When electric current passes through, a continuous light wave flows inside the tube with constant frequency, developing "coherent" light waves that emanate from mirror 2.

MS systems often trigger ionization by means of a nitrogen laser. Others, though, like the solid-state diode YAG lasers (neodymium-doped ytterbium aluminum garnet), are also used. A chemical matrix protects the molecule of interest from destruction by direct laser energy and enables vaporization and ionization. Essentially a chromophore, the matrix absorbs laser energy, which, liberated from the surface (where both matrix and analyte have been applied), then transfers or assists ionization of the analyte.

Other examples of gas lasers include higher efficiency, carbon dioxide units that emit high-power beams at about 10 μm. Such devices often are used in industry for cutting and welding. Argon-ion lasers emit light in the 351–528.7 nm range. Depending upon the associated optics and type of laser tube, a variable number of lines are usable. Nevertheless, the most frequently used lines are 458, 488, and 514.5 nm. Metal-ion lasers such as helium-silver (HeAg, 224 nm) and neon-copper (NeCu, 248 nm) generate deep, ultraviolet wavelengths. The very narrow oscillation line-widths of such lasers make them useful for fluorescence-suppressed Raman spectroscopy.

In solid lasers, a ruby-like cylindrical crystal serves as the gain medium, which is surrounded by a helical xenon flash lamp that acts as a "pump." Mirrors are arranged similar to those of gas lasers. A red laser beam results on the discharge of electric current.

Liquid lasers use organic dyes as their gain medium. In semiconductor lasers, known as injection laser diodes (ILD), current causes visible light modulation from the ILD. These lasers are often used in electronic equipment.