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How Lasers Work
Lasers are everywhere and used in a wide variety of applications. They are found in barcode scanners, DVD players, used in medicine, produce dazzling laser light shows and of course, instrumental in micro manufacturing. A laser is a device which generates or amplifies light. The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. The term was coined by Gordon Gould when he was a student of Dr. Charles Townes at Columbia University in 1957. Lasers exhibit some unique characteristics. They are monochromatic which is to say they output a single wavelength or pure color with an extremely narrow linewidth. Depending on the laser type, they can have wavelengths from the ultraviolet, visible or infrared portion of the electromagnetic spectrum. Wavelength selection is important depending on the material being laser processed. As an example, UV lasers are best for drilling and cutting plastics. Lasers are highly directional where the beam can be as little as one millimeter and spreads very little over distance. In fact, lasers have been bounced off of the moon to accurately measure the distance from earth. And they are coherent where all the waves are exactly in phase with one another.
The common components of all lasers consist of an Active Medium which can be gas such as carbon dioxide or krypton fluoride in an excimer laser to generate high power UV pulses. A solid-state laser has a crystal made of ruby, neodymium-doped yttrium aluminium garnet, or Neodymium-doped yttrium lithium fluoride to name a few. The gain medium can even be a liquid although dye lasers are not used in micromachining. The pumping source or energy input can be electrical such as a HV discharge in excimer lasers or Optical using laser diodes to pump Yag or fiber lasers. Lastly, all lasers need an optical feedback which consists of a mirror or high reflector and a partially reflective mirror, more about that later. A population inversion is critical to sustaining laser operation where a large number of atoms are in an excited state. Looking at the Energy Level Diagram, an electron is pumped to a highly excited state and transitions to a metastable region. The electron will seek its natural or ground state. However, it must release its energy and does so in the form of a photon.
Now we have atoms releasing photons in all directions or spontaneous emission. Similar to a blacklight which is a UV pump source and a fluorescent dye. The dye absorbs the UV wavelength and emits a visible color in all directions. In lasers, stimulated emission is achieved by the optical cavity. Photons bounce back and forth between the mirrors. As a photon passes an atom in an excited state, it too emits a photon creating a cascading or domino effect. The output coupler, being partially reflective, permits the laser beam to exit the cavity. The chart shows the laser types commonly used in micromanufacturing. Wavelengths can be anywhere from 193nm to 10.6 microns. Average power is typically in range of a few watts to a few hundred watts. Laser pulse duration can range from microseconds all the way down to femtoseconds which is a millionth of a millionth of a second.
Thank you for viewing and stay tuned for future installments on laser applications in micromanufacturing. If you have any questions or if you want to suggest a future topic on lasers in micromachining, please contact me.