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LASER Introduction The trouble with a subject like Laser light is where to start describing how it works. Perhaps a little history lesson might be the best place. The most famous of those who first postulated the idea of a laser was non-other than Albert Einstein himself back in the 1920s and 1930s. At that time light was considered to be what is called spontaneous light. This spontaneous light is emitted by hot objects or by individual atoms in an electrical discharge. Each particle of this light, referred to as a photon, is emitted in a random fashion without any commonality with any of the other particles being emitted in the same vicinity. It is given this name, Spontaneous Light, because each photon acts in an independent and unpredictable way and its creation is known as Spontaneous Emission of Radiation. Einstein and other researchers however predicated the possibility of another type of light that they referred to as stimulated light. This was based on the idea that an excited atom could Different Photons produce an exact copy of a passing photon. Unlike the spontaneous light, where nearby photons were all different, photons produced by this Stimulated Emission of Radiation was so similar that they were indistinguishable from each other. We will see that Laser light is formed from this type of stimulated emission of radiation, in fact that is part of what the acronym LASER stands for. Spontaneous or incoherent Light Stimulated or Coherent Light Light Amplification for Stimulated Emission of Radiation. So how does an atom get excited and how does an excited atom produce a photon? We can get an atom excited by giving it some energy. This energy allows the electrons (particularly the outer electrons) to move further away from the nucleus of the atom. This is not their normal state, so before long the atom will return to its normal (or ground) state, by the electrons releasing the energy they were given. This energy is released as a photon of light. This release is spontaneous (i.e. Not triggered or stimulated by something else) and occurs by one of the electrons accelerating back and fourth and then emitting the photon. The type of photon and direction is random as it happens spontaneously. However if an excited atom is getting ready to release, as another photon is passing by, then the passing photon can trigger (stimulate) the atom to release a photon that is identical to the photon that triggered it. This is caused by a sympathetic vibration being passed from the passing photon to the electron in the excited atom, that starts to accelerate back and fourth at this same vibration frequency and causes a photon with the exact same properties as the original. When this idea was first discovered it was immediately recognized that his could allow for the possibility of a process called Light Amplification. If enough excited atoms could be put together then a passing photon could encourage the production of duplicate photons, thousands even millions or billions of them. Of course this was all theoretical, it was not until the 1950s that the detail of how to go about light amplification were finally worked out. It would be the 1960s before the first laser oscillatiors were actually constructed. These devices could emit an intense beam of light made up of photons which were all exact copies of each other. This meant that together they formed a single electro-magnetic wave. This is known as coherent light. One of the special properties of this coherent light is that it is all at a single frequency. In terms of light, its frequency is its colour. So it is mono-chromatic light, i.e. light that has only one colour. This will prove very useful in certain applications. So how do we achieve light amplification? To produce coherent light you must start with one photon and duplicate it many times. So when a photon, which on its own is not very bright, enters a laser amplifier, which is an appropriate collection of excited atoms then the light is amplified and becomes brighter because it leaves as many photons of light, all identical of course. A Laser oscillator is a device that encloses the laser medium between two specially designed mirrors. Inside the laser medium many of the excited atoms will spontaneously produce photons. If one of these photons happens to meet the specific design of the Laser system it will bounce of one of the mirrors and return in the exact direction of the other mirror. When it reaches the other mirror it will reflect back in the direction of the first mirror. This will continue indefinitely. However with each pass through the laser medium it will stimulate the emission of photons from excited atoms, and these photons will be exact duplicates of itself. As they are exact duplicates they will travel in the same Laser Medium, with Mirrors direction and the same bouncing back and fourth off of each of the mirrors will happen to them also. Of course these new photons will themselves stimulate duplicates which will follow the same process. Now if both mirrors were perfectly reflective, this process would be a bit pointless as there would be lots of these photons making a coherent light, but none could escape. Instead one of the mirrors is semi-transparent. This allows some of the photons that hit this mirror to escape. It is this escaping light that creates the usable laser light. However if we let the light escape, then eventually all of the excited atoms will have given up their energy and no more photons will be produced. So it is important that the atoms are re-excited by giving them energy. This is called pumping the laser. How the pumping is achieved is dependant on the type of laser, but it is normally achieved by passing an electrical current through the system or intense light is shone on the laser medium. Of course this intense light is incoherent light caused by spontaneous emission i.e. normal light. The light that escapes through the semi-transparent mirror is called a laser beam and it consists of duplicates of the one original photon that set it all off. Since virtually all of the photons in a laser beam are identical, they can all focus together to an extremely small spot. Laser Beam Classes of Laser Class 1 CLASS 1 LASER PRODUCT A class 1 laser is safe under all conditions of normal use. This means the maximum permissible exposure (MPE) cannot be exceeded. Class 1M LASER RADIATION DO NOT VIEW DIRECTLY WITH OPTICAL INSTRUMENTS CLASS 1M LASER PRODUCT A Class 1M laser is safe for all conditions of use except when passed through magnifying optics such as microscopes and telescopes. Class 1M lasers produce large-diameter beams, or beams that are divergent. The MPE for a Class 1M laser cannot normally be exceeded unless focusing or imaging optics are used to narrow the beam. If the beam is refocused, the hazard of Class 1M lasers may be increased and the product class may be changed. A laser can be classified as Class 1M if the total output power is below class 3B but the power that can pass through the pupil of the eye is within Class 1. Class 2 LASER RADIATION DO NOT STARE INTO BEAM CLASS 2 LASER PRODUCT A Class 2 laser is safe because the blink reflex will limit the exposure to no more than 0.25 seconds. It only applies to visible-light lasers (400–700nm). Class-2 lasers are limited to 1mW continuous wave, or more if the emission time is less than 0.25 seconds or if the light is not spatially coherent. Intentional suppression of the blink reflex could lead to eye injury. Many laser pointers and measuring instruments are class 2. Class 2M LASER RADIATION DO NOT STARE INTO BEAM OR VIEW DIRECTLY WITH OPTICAL INSTRUMENTS CLASS 2M LASER PRODUCT A Class 2M laser is safe because of the blink reflex if not viewed through optical instruments. As with class 1M, this applies to laser beams with a large diameter or large divergence, for which the amount of light passing through the pupil cannot exceed the limits for class 2. Class 3R LASER RADIATION AVOID DIRECT EYE EXPOSURE CLASS 3R LASER PRODUCT A Class 3R laser is considered safe if handled carefully, with restricted beam viewing. With a class 3R laser, the MPE can be exceeded, but with a low risk of injury. Visible continuous lasers in Class 3R are limited to 5 mW. For other wavelengths and for pulsed lasers, other limits apply. Class 3B LASER RADIATION AVOID EXPOSURE TO BEAM CLASS 3B LASER PRODUCT A Class 3B laser is hazardous if the eye is exposed directly, but diffuse reflections such as from paper or other matte surfaces are not harmful. Continuous lasers in the wavelength range from 315 nm to far infrared are limited to 0.5 W. For pulsed lasers between 400 and 700 nm, the limit is 30 mW. Other limits apply to other wavelengths and to ultrashort pulsed lasers. Protective eyewear is typically required where direct viewing of a class 3B laser beam may occur. Class-3B lasers must be equipped with a key switch and a safety interlock. Class 4 LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT Class 4 lasers include all lasers with beam power greater than class 3B. By definition, a class-4 laser can burn the skin, in addition to potentially devastating and permanent eye damage as a result of direct or diffuse beam viewing. These lasers may ignite combustible materials, and thus may represent a fire risk. Class 4 lasers must be equipped with a key switch and a safety interlock. Most industrial, scientific, military, and medical lasers are in this category. Applications of LASERs Optical Disc Systems. CD, DVD and Blu-Ray. It was Philips Electronics that first set to work on designing the Compact disc player. This was meant to be a digital optical disc of a similar size to the compact cassette tape that had been so popular. What they wanted was a contactless technology. This would mean the sensor that read the music data from the disc would not actually touch it and therefore there would be no wear and tear. Wear and tear on Vinyl records and audio cassette caused an on going degradation of the sound. But when they first went to design the CD system it was a very different world to today. For starters they knew they would require a laser. A laser that could do the job would cost thousands and would be very large. It would also likely need a specialist technician to operate it. When you work for a large company like Philips though you usually know what innovations are happening elsewhere. One such innovation was the semi-conductor laser (laser diode). This could be made very small and from very inexpensive materials, but at the time had a life span of only a few seconds. Philips went ahead with the design anyway in the hope that by the time they were ready, the laser diode would be ready for production and robust enough for use. Luckily this paid off and the laser diode was ready in time for the introduction of the CD. Another reason the Laser was so useful for the CD player was that it allowed for very inexpensive optics. The laser light would need to be focused to see the data on the CD, this required optics. One of the problems with low quality optics is that when light enters it, dispersion takes place. When a light wave arrives at an interface with a denser medium, such as the interface between air and the surface of an optical disc or a glass lens, the velocity of the wave is reduced; therefore the wavelength in the medium becomes shorter, causing the wave to leave the interface at a different angle. This is known as refraction. The ratio of velocity in a vacuum to velocity in the medium is known as the refractive index of that medium; it determines the relationship between the angles of the incident and refracted waves. If the speed of light in the medium varies with wavelength, then dispersion takes place. Dispersion is where light gets separated out into its different wavelengths (frequencies, colours) e.g. You see this when sunlight reflects off water particles and is separated into the colours of the rainbow. This can cause a lack of precision when trying to read the data on a CD. Luckily though because Laser light is mono-chromatic it only has one frequency/wavelength of light and therefore dispersion cannot occur. These inexpensive optics meant that the CD lens system would not add much to the price of the machine and would thus make it available to the ordinary consumer. In fact one of the interesting things about the CD is that they actually decided to leave some of the focusing of the light to the plastic of the CD itself. This wouldn't have worked if it wasn't mono-chromatic laser light as dispersion in the plastic of the CD would have ruined the Laser Light passing through CD preciseness of the focus. Just look at a CD yourself in any normal light and you will see the different colours of the rainbow in it. So let’s take a look at how the laser focuses on the CD. In trying to make the CD a universal disc for the masses one of the goals was that no special working environment or handling skill is required. However there is still a need to record the information at a very high density to keep the size of disc small for a reasonable playing time. High-density recording means the data on the disc is very small and to be seen precisely this requires a light of a short-wavelength. The information layer of a CD is read through the thickness of the disc. The diagram above shows that this approach causes the readout beam to enter and leave the disc surface through the largest possible area. The actual dimensions involved are shown in the figure. Despite the minute spot size of about 1.2 μm diameter, light enters and leaves through a 0.7 mm-diameter circle. As a result, surface debris such as dust or scratches must be almost 600 times larger than the readout spot before the beam is completely obscured. You should note that because the data on the CD is actually much closer to the label side, then a scratch on that side can actually remove the data. Despite this, we for some reason tend to place CDs label side down on surfaces in the belief they will be less likely to get damaged this way. Try a test with a Recordable CD that you don't mind destroying: On the label side mark it with a sharp knife or write on it with a ball point pen then hold it up to the light. As likely as not you will have removed the data and you will be able to see right through the plastic of the disk. Some of the light reflected back from the disc re-enters the aperture of the objective lens. The pickup must be capable of separating the reflected light from the incident light. The diagram below shows two systems which can be used to do this. In (a) an intensity beam-splitter a semi-silvered mirror is inserted in the optical path and reflects some of the returning light into the photosensor. This is not very efficient, as half of the replay signal is lost by transmission straight on. In the example (b) separation is by use of a polarising prism. In natural light, the electric-field component will be in many planes. Light is said to be polarised when the electric field direction is constrained to one plane only. A device known as a quarter wave plate can be used to rotate the plane of polarisation. Rotation of the plane of polarisation is a useful method of Using a Semi-silvered mirror to separate the incident light and the reflected light separating incident light (coming from the Laser) Using a Polarising Prism to separate the incident light and the reflected light and reflected light in a laser pickup. Using a quarter-wave plate, the plane of polarisation of light leaving the pickup will have been turned 45o, and on return it will be rotated a further 45o, so that it is now at right angles to the plane of polarisation of light from the source. The two can easily be separated now by a polarising prism, which acts as a transparent block to light in one plane, but as a prism to light in the other plane, such that reflected light is directed towards the sensor. The frequency of laser light used for the CD is in the infra-red range. This is at a slightly longer wavelength to the red light that the Human eye can see. In order fit more information to a disk the data would have to be made smaller. To precisely see these smaller bits of data you need a shorter wavelength (i.e. higher frequency). So when DVD was being designed, a red laser was required that had a shorter wavelength, as the data was so much smaller than on the CD in order to pack in so much of it. Later came Blu-ray. The data here was even smaller and therefore required an even shorter wavelength which required a new colour. No prizes for guessing the colour, the clue is in the name, it is of course a blue laser. Each different colour requires a different compound make-up in the semi-conductor material that Data on CD as seen under the Scanning Electron makes up the laser diode. Microscope at IT Sligo 3D scanner using Laser spot triangulation Two object positions are shown In a 3D laser scanner the triangulation laser shines a laser on the subject and uses a camera to look for the location of the laser dot. Depending on how far away the laser strikes a surface, the laser dot appears at different places in the camera's field of view. This technique is called triangulation because the laser dot, the camera and the laser emitter form a triangle. If we look at the triangle made up by ABC, the details we know are the angle formed by <CAB and the distance between A and B. By taking the point on the camera sensor we can draw a line from there through the centre of the lens, this will give us the angle formed <ABC. If you know these three pieces of information about a triangle then you can calculate the rest. Note in this exampl the grey triangle appears to be a right angle triangle but this will not always be the case, in the case of the dashed line leading to the higher object this angle is not a right angle. In most cases a laser stripe, instead of a single laser dot, is swept across the object to speed up the acquisition process. 3D Scanner using Triangulation Image author: Georg Wiora Laser Printing. Laser printing is a multipart process, but only one part of this process involves a laser. First a rotating drum made from an appropriate material, usually selenium or an organic photoconductor, is given a negative charge. A negative charge is created when an atom has more electrons that protons. This is achieved by removing energy from the atom and allowing an extra electron to fall into an orbit. Now if a laser is shone very precisely at parts of the rotating drum, its photons will give energy to these extra electrons and allow them to once again escape. This will mean that the atom is once again neutral. The atoms that have been selected by the laser, together, form the image that is to be printed. Next the toner, which is made of fine particles of dry plastic powder mixed with carbon black or coloring agents, is given a negative charge. The rotating drum is then rolled in the toner. Now most of the drum is negatively charged (the parts that do not form what we want to print). These negative charges will repel the negatively charged toner, even though they are rolled across it, because like charges repel each other. The parts that were made neutral by the laser however have no such repulsion and in fact will attract the negative toner particles, although because they are neutral the attraction is not very strong. Now that the particles are on the drum, the drum is rolled across the paper where heat (up to 200º C) and pressure are used to bond the toner particles to the paper. In some high end machines a negatively charged roller is used on the back side of the paper to help pull the toner off of the drum onto the paper. Laser Cutting The parallel rays of coherent light from the laser source often fall in range between 1.5875mm to 12.7mm in diameter. This is too wide to achieve the localised energy required to cut material. Therefore this beam is normally focused and intensified by a lens mirror to a very small spot of about 0.0254mm to create a very intense laser beam. the or a There are many different methods in Laser light shining on a material to be cut. cutting using lasers, with different types used to cut different material. Some of the methods are vaporization, melt and blow (Fusion), reactive , thermal stress cracking, scribing, cold cutting and burning stabilized laser cutting. Vaporization, Melt and blow, Reactive, Thermal stress cracking are described in brief below. Vaporisation cutting This method is normally used for non-melting material such as wood, carbon and thermoset plastics. In vaporization cutting the focused beam heats the surface of the material to boiling point and generates a keyhole. Before the keyhole is formed the surface reflects a lot of the light energy. The creation of the keyhole leads to a sudden increase in absorptivity quickly deepening the hole. As the Laser Vaporisation cutting hole deepens and the material boils, the vapor generated erodes the molten walls blowing ejecta out and further enlarging the hole. Melt and blow (also known as Laser Fusion Cutting) Melt and blow or fusion cutting uses high-pressure gas to blow molten material from the cutting area, greatly decreasing the power requirement. In the case of higher alloyed steels and aluminum, an inert gas (nitrogen, argon) is typically used as a cutting gas. Laser fusion cutting affords oxygen-free cut edges (no oxidation), which is particularly important when welding is the next process step after cutting. Today, laser fusion cutting is used industrially for material thicknesses of up to 15 mm. First the material is heated to melting point then a gas jet blows the molten material out of the kerf (up to 20 bar) avoiding the need to raise the temperature of the material any further. Materials cut with this process are usually metals. Laser melt and blow, or fusion cutting. Reactive cutting (also known as flame cutting or laser oxygen cutting) Laser Reactive cutting is like oxygen torch cutting but with a laser beam as the ignition source. The exothermic reaction of oxygen with the material (mainly steel) supports the laser cutting process by providing additional heat input. It is mostly used for cutting carbon steel in thicknesses over 1mm. This process can be used to cut very thick steel plates with relatively little laser power. The result is higher cutting speed compared to laser fusion cutting with inert gases. Thermal stress cracking Brittle materials such as glass or acrylic are particularly sensitive to thermal fracture. This sensitivity is used in thermal stress cracking. A Laser beam is focused on the surface of the material causing localised heating and thermal expansion at a very precise point. This results in a crack that can be grown and guided by moving the beam in the desired direction. The crack can be progressed relatively quickly, in order of meters per second.