About Lasers

The invention of the laser -Stands for Light Amplification by Stimulated Emission of Radiation- can be dated to 1958 with the publication of the scientific paper, Infrared and Optical Masers, by Arthur L. Schawlow, originally based on the theories of Albert Einstein. Then a Bell Labs researcher, Schawlow and Charles H. Townes, a consultant to Bell Labs. That paper, published in Physical Review, the journal of the American Physical Society, launched a new scientific field and opened the door to a multibillion-dollar industry. The work of Schawlow and Townes, however, can be traced back to the 1940s and early 50s and their interest in the field of microwave spectroscopy, which had emerged as a powerful tool for puzzling out the characteristics of a wide variety of molecules. Neither man was planning on inventing a device that would revolutionize a number of industries, from communications to medicine. They had something more straightforward in mind, developing a device to help them study molecular structures. Lasers were first invented in 1958, and in their early development were referred to as “a solution looking for a problem”. The first laser, based on a ruby rod, was invented in 1960 by Theodore Maiman at Hughes Aircraft Company. His scientific paper describing the invention was rejected by the leading physics publication at the time (Physical Review Letters), and his subsequent press conference was treated with scepticism. The familiar helium-neon laser was the first gas laser invented, at Bell Laboratories in 1961. Semiconductor diode lasers, now by far the most common lasers, were invented as early as 1962, but they were unreliable until technical advances in the early 80s dramatically improved their lifetimes. In 1964, William Bennett invented the argon-ion laser at Yale University. In 2000 his failing eyesight was corrected with retinal surgery using an argon-ion laser. About 500 million lasers are currently sold worldwide each year, of which 400 million are for data storage (CD and DVD players). The most powerful ultraviolet laser in the world, (the 60-terawatt Omega, at the Laboratory for Laser Energetics at the University of Rochester, New York) is used to test fusion experiments (the same nuclear energy that powers the sun). In less than a billionth of a second, the laser sends the temperature in a tiny pellet from just a few degrees above absolute zero to nearly 30 million degrees Celsius — twice as hot as the core of the sun. For this brief period of time the laser power is about 100 times the peak power of the entire U.S. power grid. An even more powerful laser is under construction at the Lawrence Livermore National Laboratory in California. Laser light can cool a gas of atoms to incredibly low temperatures, only a millionth of a degree above absolute zero. The smallest laser is an indium-gallium-arsenide semiconductor disk only 2 microns across, and only about 400 atoms thick. Lasers can commonly have their frequency (ie. colour) selected with a precision of one part in a billion. A laser can be considered as a highly collimated source of extremely intense electromagnetic radiation that is defined by three characteristics: monochromatic, directional and coherent. Due to the temporal and spatial coherence of the laser beam it can be considered as a point source of phenomenal brightness than can easily exceed the brightness of the sun. Lasers are a more directional light source than any other common fixture such as stage lights or a follow-spot. The higher the optical output power of the laser, the greater the potential hazard. Laser radiation or light is coherent electromagnetic radiation characterized by one or more specific wavelength(s), the values of which are determined by the composition of the lasing medium. Laser radiation may be emitted in the visible portion of the electromagnetic spectrum, wavelengths of 0.4 µ m and 0.7 µ m, or in the invisible infrared and ultraviolet regions. Laser radiation transmits energy which, when a laser beam strike matter, can be transmitted, absorbed, or reflected. If a material transmits a laser beam it is said to be transparent. If the beam is not transmitted the material is said to be opaque and the incident radiation is absorbed or reflected. Absorbed laser energy appears in the target material as heat. (At certain, usually short, wavelengths photochemical reactions may also occur.) Absorption and transmission are functions of the chemical and physical characteristics of the target material and the wavelength of the incident radiation. At visible wavelengths laser radiation impinging on the eye is focused on the retina and , if sufficient energy is absorbed, can cause cell destruction. At longer and shorter wavelengths, such as the far infrared and the ultraviolet, radiation striking the eye is absorbed in the cornea and the lens rather than being focused on the retina. Although these structures are less easily damaged than the retina, excessive energy absorption can cause cell damage and impairment of vision. Reflection is primarily a function of the physical character of the surface of the target material. A smooth polished surface is generally a good or specular reflector; a rough uneven surface usually is a poor reflector producing a diffuse reflection. A reflector such as a flat mirror changes the direction of an incident beam with little or no absorption. A curved mirror or surface will change the divergence angle of the impinging laser beam as well as its direction. For a diffuse reflection, the reflected energy is scattered in all directions thereby reducing the energy or power density. Generally, diffusely reflecting surfaces are favored when designing a laser experiment since their use reduces the likelihood of a specular reflection and hence enhances the safety of the experiment. Many laser pointers are in the range of 1 to 5 milliwatts (mW), a subclass of 3 called 3A. A close reading of exposure limits indicate that a 5 mW laser could cause eye damage. Why even worry about 5 mW (5 thousandths of a watt), which is less than one percent of one percent of the power of a 60 Watt incandescent bulb? First, the numbers are used differently. Light bulb wattage measures the power it uses. It only converts about 10 percent of that electrical power into light. In a laser, the power is a measure of the light output. Secondly, the light bulb gives light in all directions so you only see a small part of the whole. As you move away from the bulb, you see a quarter of the light every time the distance is doubled. A laser gives light in one small beam. If it gets into the eye, you receive all the laser’s energy, not just a fraction. Thirdly, a light bulb gives off light at many different wavelengths (different photon energies). A laser is a pure tone, only one wavelength. The coherent light will be more damaging. The laser pointer is a diode laser, really just a special type of transistor, or diode. Because of the unique features of laser light, it is magnified by 100,000 times as it passes through the eye. The light passes to the back part of the eye, the retina, which is where we perceive vision. The eye actually sees a small part of the electromagnetic spectrum that runs from short cosmic ray energies to long radiowaves. We see only from violet to red. Infrared (IR) and ultraviolet (UV) are just outside our ability to see. The eye is most sensitive to yellow-green light (550 nm). At the same power, 670 nm red light is only 3 percent as bright. CLASS IIIa LASERS: Class IIIa lasers are systems with power levels of 1 to 5 mW that normally would not produce a hazard if viewed for only momentary periods with the unaided eye. They pose severe eye hazards when viewed through optical instruments (e.g., microscopes, binoculars, or other collecting optics). Class IIIa lasers must be labeled. A warning label shall be placed on or near the laser in a conspicuous location and caution users to avoid staring into the beam or directing the beam toward the eye of individuals. Equipment, such as some visible continuous wave Helium-Neon lasers and some solid state laser pointers, are examples of Class IIIa lasers. CLASS IIIb LASERS: Class IIIb lasers are systems with power levels of 5 mW to 500 mW for continuous wave lasers or less than 10 J/cm² for a 0.25 s pulsed laser. These lasers will produce an eye hazard if viewed directly. This includes intrabeam viewing or specular reflections. Higher power lasers in this class will also produce hazardous diffuse reflections. Specific control measures covered in Class IIIb lasers shall be used in areas where entry by unauthorized personnel can be controlled. Entry into the area by personnel untrained in laser safety may be permitted by the laser operator if instructed in applicable safety requirements prior to entry and provided with required protective eye wear. Beam Hazards: The nature of laser beam damage and the threshold levels at which each type of injury may occur depends on the laser beam parameters. These include wavelength of light, energy of the beam, divergence and exposure duration. For pulsed lasers, parameters also include the pulse length, pulse repetition frequency and pulse train characteristics. The ANSI Z136.1 standard establishes Maximum Permissible Exposure (MPE) limits for laser radiation. Damage can occur to the skin, retina, lens, cornea, and conjunctive tissue surrounding the eye. For lasers over 0.5 W, the beam can ignite flammable materials and initiate a fire. Thermal burn, acoustic damage, and photochemical damage to the retina may occur from laser light in the near ultraviolet (UV), visible and near infrared (IR) regions (below 400 nm - 1400 nm). Damage occurs as the laser light enters the eye and is focused on the retina (see Fig. 1). Normal focusing of the eye amplifies the irradiance by approximately 100,000; thus, a beam of 1 mW/cm² results in an exposure of 100 W/cm² to the retina. Energy from the laser beam is absorbed by tissue in the form of heat, which can cause localized intense heating of sensitive tissues. The most likely effect of excess exposure to the retina is thermal burn that destroys retinal tissue. Since retinal tissue does not regenerate, the damage is permanent, which may result in the loss of sight in the damaged area. Safe and appropriate use of lasers is essential. Lasers are used for a multitude of purposes that range from medical to military. Most people are familiar with Red lasers. Red lasers can be found in your CD players, toys, and of course in the key chains and pointers sold both online and in many stores world wide. Evil Green lasers pointers are many times brighter than Red lasers and have the additional feature of beams that are visible in low light environments. They are however, much more expensive. Green lasers range in price from $10 and can go as high as $50,000 or more for the high mw (milliwatt) lasers.