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Laser Fundamentals

In a few decades since the 1960s, the laser has gone from being a science fiction fantasy, to a laboratory research curiosity, to an expensive but valuable tool in esoteric scientific applications, to its current role as an integral part of everyday tasks as mundane as reading grocery prices or measuring a room for wallpaper. Any substantial list of the major technological achievements of the twentieth century would include the laser near the top. The pervasiveness of the laser in all areas of current life can be best appreciated by the range of applications that utilize laser technology. At the spectacular end of this range are military applications, which include using lasers as weapons to possibly defend against missile attack, and at the other end are daily activities such as playing music on compact disks and printing or copying paper documents. Somewhere in between are numerous scientific and industrial applications, including microscopy, astronomy, spectroscopy, surgery, integrated circuit fabrication, surveying, and communications.

Introduction to Lasers - Ordinary natural and artificial light is released by energy changes on the atomic and molecular level that occur without any outside intervention. A second type of light exists, however, and occurs when an atom or molecule retains its excess energy until stimulated to emit the energy in the form of light. Lasers are designed to produce and amplify this stimulated form of light into intense and focused beams. The word laser was coined as an acronym for Light Amplification by the Stimulated Emission of Radiation. The special nature of laser light has made laser technology a vital tool in nearly every aspect of everyday life including communications, entertainment, manufacturing, and medicine.

Laser Systems for Optical Microscopy - The lasers commonly employed in optical microscopy are high-intensity monochromatic light sources, which are useful as tools for a variety of techniques including optical trapping, lifetime imaging studies, photobleaching recovery, and total internal reflection fluorescence. In addition, lasers are also the most common light source for scanning confocal fluorescence microscopy, and have been utilized, although less frequently, in conventional widefield fluorescence investigations.

Laser Safety - The two major concerns in safe laser operation are exposure to the beam and the electrical hazards associated with high voltages within the laser and its power supply. While there are no known cases of a laser beam contributing to a person's death, there have been several instances of deaths attributable to contact with high voltage laser-related components. Beams of sufficiently high power can burn the skin, or in some cases create a hazard by burning or damaging other materials, but the primary concern with regard to the laser beam is potential damage to the eyes, which are the part of the body most sensitive to light.

John Kerr (1824-1907) - John Kerr was a Scottish physicist who discovered the electro-optic effect that bears his name and invented the Kerr cell. Pulses of light can be controlled so quickly with a modern Kerr cell that the devices are often used as high-speed shutter systems for photography and are sometimes alternately known as Kerr electro-optical shutters. In addition, Kerr cells have been used to measure the speed of light, are incorporated in some lasers, and are becoming increasingly common in telecommunications devices.

Theodore Harold Maiman (1927-Present) - Theodore Maiman is best remembered for constructing the world's first laser, a device that has transcended the field of optics to find a diversity of applications in the modern world. In May of 1960, Maiman built his prototype laser using a synthetic ruby rod silvered at both ends to reflect light. Small enough to be held in the palm of the hand, when the atoms in the rod were excited by an intense beam of light from a xenon lamp, a release of energy was initiated and an internal chain reaction occurred that caused the energy to bounce back and forth within the rod. When the energy built up to a certain level, it escaped from one end of the ruby rod to form an intense beam of monochromatic light centered at 694.3 nanometers.

Interactive Tutorials

Laser Cavity Resonance Modes and Gain Bandwidth - In a typical laser, the number of cavity resonances that can fit within the gain bandwidth is often plotted as a function of laser output power versus wavelength. This interactive tutorial explores how varying the appropriate frequencies can alter curves describing the number of cavity modes and gain bandwidth of a laser.

Laser Energy Levels - A population inversion can be produced through two basic mechanisms, either by creating an excess of atoms or molecules in a higher energy state, or by reducing the population of a lower energy state. This tutorial explores metastable states for both three-level and four-level laser systems.

Spontaneous and Stimulated Processes - One of the most important concepts necessary in understanding laser operation is the fact that quantization of energy in the atom results in discrete energy levels. In addition, transitions from one energy level to another must be possible in order for light emission to occur, and these transitions include both spontaneous and stimulated emission. This tutorial explores the concepts of spontaneous absorption and emission, as well as stimulated emission.

Stimulated Emission in a Laser Cavity - The amplification of light by stimulated emission is a fundamental concept in the basic understanding of laser action. This interactive tutorial explores how laser amplification occurs starting from spontaneous emission of the first photon to saturation of the laser cavity and the establishment of a formal equilibrium state.

Argon-Ion Lasers - As a distinguished member of the common and well-explored family of ion lasers, the argon-ion laser operates in the visible and ultraviolet spectral regions by utilizing an ionized species of the noble gas argon. Argon-ion lasers function in continuous wave mode when plasma electrons within the gaseous discharge collide with the excited laser species to produce light.

Diode Lasers - Semiconductor diode lasers having sufficient power output to be useful in optical microscopy are now available from a host of manufacturers. In general these devices operate in the infrared region, but newer diode lasers operating at specific visible wavelengths are now available. Diode lasers coupled to internal optical systems that improve beam shape have sufficient power and stability to rival helium-neon lasers in many applications. This interactive tutorial explores the properties of typical diode lasers and how specialized anamorphic prisms can be utilized for beam expansion.

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 the most superior Gaussian beam quality of 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.

Helium-Cadmium Lasers - Helium-cadmium (He/Cd) lasers are finding an increasing number of important applications in confocal microscopy due to their three primary emission spectral lines (322, 354, and 442 nanometers) in the ultraviolet and blue-violet regions. The shortest wavelength (322 nanometers) requires specialized ultraviolet transparent optics and is seldom used in microscopy, but membrane probes (such as indo-1 and fura-2) can be efficiently excited with the 354-nanometer line. The blue-violet spectral line is useful for a host of common fluorophores and fluorescent proteins in single, double, or triple labeling experiments. This interactive tutorial explores a simplified model of the helium-cadmium laser cavity operation.

Krypton-Argon Lasers - Air-cooled lasers using krypton-argon mixtures have become popular in confocal microscopy when several illumination wavelengths are required for dual or multiple-fluorophore studies. Such mixed-gas lasers are only capable of producing stable output on major lines that are well separated in the wavelength spectrum. Of the three laser lines typically utilized for confocal microscopy, the 488-nanometer and 568-nanometer lines have approximately equal power (10 to 15 milliwatts), while the 647-nanometer line has about 50 percent more (15 to 25 milliwatts). This interactive tutorial simulates the three major spectral lines produced by an krypton-argon mixed-gas laser.

Ti:Sapphire Mode-Locked Lasers - The self mode-locked Ti:sapphire pulsed laser is currently one of the preferred laser excitation sources in a majority of multiphoton fluorescence microscopy investigations. This tutorial explores the operation of Ti:sapphire lasers over a broad range of near-infrared wavelengths with variable pulse widths and an adjustable applet speed control.

Nd:YLF Mode-Locked Pulsed Lasers - An increasing number of applications, including new illumination techniques in fluorescence optical microscopy, require a reliable high average-power laser source that enables efficient frequency conversion to ultra violet and visible wavelengths. Several variants of the diode-pumped solid state laser have been developed, and of these, the Nd:YLF (neodymium: yttrium lithium fluoride) laser produces the highest pulse energy and average power in the repetition rate ranging from a single pulse up to approximately 6 kHz. This tutorial explores the operation of a Nd:YLF multi-pass slab laser side-pumped by two collimated diode-laser bars.

Pockels Cell Laser Modulators - All lasers are susceptible to noise introduced by their power supplies. Switching power supplies, which have become common because of their efficiency and small size, are particularly likely to introduce laser system ripple at frequencies ranging into the tens of kilohertz. Such interference, when it affects the light beam in confocal microscopy systems, can be especially troublesome to diagnose and remove. The beam intensity of continuous wave lasers can be stabilized by either electronic control of the tube current or through utilization of external components that modulate the light intensity. This interactive tutorial examines how the Pockels cell modulator operates to stabilize laser beam intensity.

Compact Disk Lasers - A pre-recorded compact disk is read by tracking a finely focused laser across the spiral pattern of lands and pits stamped into the disk by a master diskette. This tutorial explores how the laser beam is focused onto the surface of a spinning compact disk, and how variations between the height of pits and lands determine whether the light is scattered by the disk surface or reflected back into a detector.

Acousto-Optic Tunable Filters - Wavelength selection is of fundamental importance in many arenas of the optical sciences, including fluorescence spectroscopy and confocal microscopy. Electro-optic devices, such as the acousto-optic tunable filter (AOTF), are increasingly being employed to modulate the wavelength and amplitude of illuminating laser light in the latest generation of confocal microscopes. These filters do not suffer from the mechanical constraints, speed limitations, image shift, and vibration associated with rotating filter wheels, and can easily accommodate several laser systems tuned to different output wavelengths. In addition, acousto-optic filters do not deteriorate when exposed to heat and intense light as do fluorescence interference filters.

Selected Literature References

Reference Listing - Lasers have emerged from advanced research laboratories and military arsenals into our everyday lives as the technology advances and the fabrication costs decline. Leading researchers in the field have published a number of high-quality books and review articles on laser fundamentals and applications. This section contains a listing of selected books and book chapters from this cutting-edge field of research.

Contributing Authors

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

Matthew J. Parry-Hill, Christopher A. Burdett, Robert T. Sutter, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


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