The Invention, Construction and Function of the Laser - Essay Example

and Number of the Teacher’s Lasers The Invention, Construction and Function of the Laser A historic milestone in scientific innovation was the invention of the laser in 1960. The pioneer in laser technology was Gordon Gould who developed the laser process at Columbia University in 1958. Subsequently, Theodore Maiman from Hughes Research Laboratory “constructed the first ruby laser in 1960” (Dajnowski, Jenkins, and Lins 13). The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. Albert Einstein first identified the natural process constituting the stimulated emission of radiation of laser beams (Dajnowski et al. 13). “The invention of the laser was one of the groundbreaking scientific achievements of the twentieth century” (Lang and Barbero vii). This technology resulted in the development of new systems of communication, optical devices, space exploration, digital devices, and the mastery of nuclear energy. Lasers are oscillators functioning at optical frequencies producing monochromatic, coherent, and highly collimated intense beams of light. The frequencies of operation lie within a spectrum of far infrared to the vacuum ultraviolet or soft X-ray region (Lang and Barbero xii). This process occurs “when a beam of light passes through a specially prepared medium and intiates or stimulates the atoms within that medium to emit light” (Silfvast 1). Light is emitted from a source such as a flash lamp or diode, and the light is amplified in the lasing media which may be a gas, liquid, or solid. “The light travels between two mirrors; one of the mirrors is 100% reflective, while the other is partially translucent” (Dajnowski et al. 13). Light repeatedly passes through the lasing media until it gains the required energy level, at which point it exits through the partially reflective, translucent mirror, state Dajnowski et al. (13). The light is released in exactly the same direction and same wavelength as that of the original beam. Thus, lasers are devices that “amplify or increase the intensity of light to produce a highly directional, high-intensity beam that typically has a very pure frequency or wavelength” (Silfvast 1). The components of a typical laser device include an amplifying or gain medium, a pumping source to channel energy into the device, and an optical cavity or mirror arrangement for reflecting the beam of light back and forth through the amplifying medium for further increase and intensification. “A useful laser beam is obtained by allowing a small portion of the light to escape by passing through one of the mirrors that is partially transmitting” (Silfvast 2). Lasers range in size from one-tenth the diameter of a human hair to that of a very large building. Lasers are capable of producing powers varying from nanowatts to a billion trillion watts (1021W) for very brief releases of the beams. They produce wavelengths or frequencies of a vast spectrum; from a microwave and infrared to the “visible, ultraviolet, vacuum ultraviolet, and into the soft-X-ray spectral regions” (Silfvast 1). They produce the shortest bursts of light ever created by man, approximating five million-billionths of a second (5 x 10-15 sec). Over the last fifty years “the laser has evolved from a fundamental science phenomenon to an ubiquitous, cheap, and readily available commercial commodity” (Kutz 629). The applications of different laser technologies are found in various fields including commercial, industrial, medical, and academic areas. Lasers are a key element of modern communication systems, and are the probes that discharge the audio signals from compact disc players. They are utilized for “cutting, heat treating, cleaning, and removing materials in both the industrial and medical worlds” (Silfvast 1). In laser-guided bombs, lasers work as the targeting element; while in supermarket checkout scanners and steppers that print the microchips, they form the optical source. Laser Technology Based on Quantum Theory Laser action is explained by the well-established principles of quantum theory. Einstein stated that an active atom or molecule when stimulated by an electromagnetic wave of light, would emit photons or packets of light of wavelength equal to that of the impinging electromagnetic wave. Charles Townes was the first to utilize this stimulated emission process as an amplifier in the first MASER or Microwave Amplification by Stimulated Emission of Radiation. “The first maser was produced in ammonia vapor at a wavelength of 1.25 cm.” (DRDO 5). Townes along with Arthur Schawlow applied the maser principle to optical wavelengths, and developed the approach of using “a laser amplifier and an optical mirror cavity to provide the multiple reflections necessary for rapid growth of light signal into an intense visible beam” (DRDO 5). According to the quantum theory, every atom can have energies restricted to particular discrete states or energy levels. Usually the atoms are at the lowest or ground energy state. Absorption is the process by which the atoms in the ground state can be activated to move to one of the higher levels, “when light from a powerful source like a flash lamp or a mercury arc falls on a substance” (DRDO 5). After remaining at that level for a brief duration such as 8 to 10 seconds, the atom resumes its original ground state, while at the same time emitting a photon in a spontaneous emission process. The two processes of absorption and spontaneous emission occur in a conventional light source. In case the atom returns to its excited state, it “is struck by an outside photon having precisely the energy necessary for spontaneous emission” (DRDO 5), the outside photon being augmented by the one released by the excited atom. Further, both the photons are released from the same activated state and phase. This process known as stimulated emission is the basic and key action in laser function. Thus, the atom is “stimulated or induced to give up its photon earlier than it would have done ordinarily under spontaneous emission” (DRDO 6). The laser is thus compared to a spring that is wound up and elevated. The key required to release it is the photon with the exactly same wavelength as that of the light to be emitted (DRDO 6). In contrast to the power and intensity achieved in laser beams, scientists have also found that a “weak laser beam focused by a microscope can gently move tiny particles around, including even the organelles inside living cells” (Townes 5). These ‘optical tweezers’ act as powerful tools in biological research. Lasers are also employed in reducing the high speed of atoms, and in holding them in traps. This creates pockets of gas “with temperatures of only a few billionths of a degree above absolute zero, the lowest temperatures yet achieved by researchers” (Townes 5). An Application of Lasers: Cleaning Large Architectural Structures Laser ablation is an innovative method producing optimal results in the cleaning of several architectural projects in the field of historic preservation. The process reduces or eliminates the requirement for collection and disposal of chemical waste, blasting media and debris from the cleaning, while “producing high-quality results with mechanical or chemical damage levels far lower than those found with current chemical or mechanical methods” (Dajnowski et al. 21). Lasers have been found to be feasible, practical, and cost-effective for cleaning large-scale architectural structures. With increasingly stringent environmental regulations being put into practice, it is possible that the low environmental impact of laser technology will raise its demand in conservation works. In cleaning eight monumental bronze sculptures in Philadelphia City Hall and other sprojects, Dajnowski et al. (21) found that “materials analyses can inform and monitor the different stages of a laser-cleaning project”, with the expectation of increasing the longevity of the treatment through such monitoring and quality control. Evidence from the research indicates that lasers provide great advantages in the preservation of architectural structures of historical significance, which require cleaning of high quality and control (Dajnowski et al. 21). In recent times, increasing varieties of lasing media have been developed, as sources for light with a continually expanding range of wavelengths. This increased range provides conservators and scientists with greater control over the impact of the laser beam on sensitive substrates of surfaces that are cleaned by laser action (Dajnowski et al. 13). In the last few decades, equipment design has developed immensely to permit the cleaning of larger areas per unit of time, thereby enhancing efficiency. In determining whether it is possible to clean a number of materials safely and precisely, it is important to examine the difference in energy absorption between a material to be removed: the absorbing layer of pollution, and a substrate: the reflecting layer (Dajnowski et al. 13). In selecting an appropriate laser for the cleaning of a specific large architectural structure, it is essential to understand the ablation process affecting the substrate to be cleaned. The basic question relates to “whether the laser beam can be controlled to clean effectively and efficiently while avoiding surface damage” (Dajnowski et al. 13). Control over the laser beam is achieved by selecting a suitable wavelength, duration of dwell time, frequency of beam impact, and fluence. The laser’s wavelength is dependent on the lasing medium. This cannot be changed by the operator, hence it is essential to select an appropriate unit. Most lasers permit the selection of desired frequency and fluence of energy. The duration of the pulse is controlled by a Q-switch (quality switch). “Lasers using a normal pulse produce beam impulses of much longer duration than lasers utilizing Q-switches” (Dajnowski et al. 13-14). The cleaning spot is controlled by a specific method by each laser system. The lasers employed use “oscillating mirrors to move the cleaning spot from left to right or in a circular fashion over the substrate” (Dajnowski et al. 14). The spot oscillates from 50 to 150 times per second, forming a line. The ends of the area being cleaned are exposed to more ablation impacts, because the beam remains longer at both end points of the cleaning line. This phenomenon can be utilized to improve efficiency of the cleaning process, however, it may create lines if incorrectly applied. To prevent the occurrence of this problem, the laser head is moved constantly while cleaning. “Laser systems using a nonoscillating cleaning spot creates a dotted effect” (Dajnowski et al. 14). To avoid the appearance of this undesired surface effect, the cleaned areas have to be overlapped; however the overlapping causes a reduction in the rate of cleaning. It was found that the difficulty level is higher in creating an even surface appearance using a laser with a small cleaning spot of 1mm to 7mm, as compared to a laser using a larger spot of 15mm, or a line beam of 20mm to 50mm. According to Dajnowski et al. (14), the “sensitivity and skill of the operator are critical to the success of any laser cleaning”, in the conservation of historically significant monuments and large architectural structures. ------------------------------- Works Cited Dajnowski, Andrzej, Jenkins, Adam, and Lins, Andrew. The use of lasers for cleaning large architectural structures. APT Bulletin, 40.1 (2009): 13-23. DRDO (Defense Research and Development Organisation). Laser and its applications. Popular Science and Technology Series. 2011. 5 February 2013. http://drdo.gov.in/drdo/data/Laser%20and%20its%20Applications.pdf Kutz, J.Nathan. Mode-locked solition. SIAM Review, 48.4 (2006, December): 629-667. Lang, Gyozo G. and Barbero, Cesar A. Laser techniques for the study of electrode processes. New York: Springer, 2012. Silfvast, William T. “Lasers”. Fundamentals of Photonics. Eds. Arthur H. Guenther, Leno S. Pedrotti, and Chandrasekhar Roychoudhuri. The United States of America: University of Connecticut, 2000. 1-44. Townes, Charles H. How the laser happened: Adventures of a scientist. New York: Oxford University Press, 2002. Read More