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Laser Scanning Confocal Microscopy
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Helium-Neon Lasers

Helium-neon lasers are among the most widely utilized laser systems for a broad range of biomedical and industrial applications, and display a superior Gaussian beam quality that is virtually unrivaled by any laser. These lasers are readily available at relatively low cost, have compact size dimensions, and exhibit a long operating life (often reaching 40,000 to 50,000 hours). The low power requirements, superior beam quality (virtually a pure Gaussian profile), and simple cooling requirements (convection) make helium-neon lasers the choice system for many confocal microscopes.

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The tutorial initializes with a cutaway drawing of a helium-neon gas laser illustrated in the central window, and having the operating speed set to Medium, a level that enables the visitor to observe the slow build-up of light in the laser cavity as it is reflected back and forth through the Brewster windows and mirrors. In order to operate the tutorial, translate the Laser Wavelength slider between the various available laser spectral lines (543, 594, 612, 633, and 1523 nanometers), and observe how the color of the output beam changes with wavelength. Use the Tutorial Speed slider to adjust the speed of light oscillations within the laser cavity and the level of light emitted through the output lens.

Presented in Figure 1 is a cut-away diagram of a typical helium-neon laser system, which is constructed of glass with a large oxidized-aluminum cold cathode as the electron emitter. Operating in the abnormal glow current density gas discharge region, helium-neon lasers are generally high-voltage and low current systems, with discharge currents being limited to a few milliamperes and potentials ranging from several hundred to a thousand volts. Progressive deterioration of the oxide coating on the cathode, which ultimately leads to sputtering of aluminum, is the limiting factor in helium-neon laser operating life. Large-diameter discharge tubes typically have longer life spans than smaller tubes (40,000 hours versus about 10,000 hours, respectively).

In the past, the application of helium-neon lasers in confocal microscopy was somewhat hampered by the relatively low intensity and red emission wavelength. This difficulty has been overcome in part by the development of new lasers having additional spectral lines. The emission at 633 nanometers (termed the He-Ne line) of the common helium-neon laser has been supplemented by development of variants having emissions in the green (543 nanometers), yellow (594 nanometers), orange (612 nanometers), and near infrared (1523 nanometers) spectral regions. Typical power output values for the 633 nanometer red spectral line range from 0.5 to 10 milliwatts up to a maximum of about 75 milliwatts. The introduction of semiconductor and diode lasers having spectral lines in similar wavelength regions may ultimately lead to a reduction in the use of helium-neon lasers for confocal microscopy.

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