laser and its applications


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LASER

The acronym laser stands for "light amplification by stimulated emission of radiation." Lasers work as a result of resonant effects. The output of a laser is a coherent electromagnetic field. In a coherent beam of electromagnetic energy, all the waves  have the same frequency and phase

In a basic laser, a chamber called a cavity is designed to internally reflect infrared (IR), visible-light, or ultraviolet (UV) waves so they reinforce each other. The cavity can contain gases, liquids, or solids. The choice of cavity material determines the wavelength of the output. At each end of the cavity, there is a mirror. One mirror is totally reflective, allowing none of the energy to pass through; the other mirror is partially reflective, allowing approximately 5 percent of the energy to pass through. Energy is introduced into the cavity from an external source, this is called pumping.

As a result of pumping, an electromagnetic field appears inside the laser cavity at the natural (resonant) frequency of the atoms of the material that fills the cavity. The waves reflect back and forth between the mirrors. The length of the cavity is such that the reflected and re-reflected wavefronts reinforce each other in phase at the natural frequency of the cavity substance. Electromagnetic waves at this resonant frequency emerge from the end of the cavity having the partially-reflective mirror. The output may appear as a continuous beam, or as a series beam  of, intense pulses.

history of laser


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Lasers are one of the most significant inventions developed during the 20th century. They have found a tremendous variety of uses in electronics, computer hardware, medicine, and experimental science.  In 1917, Albert Einstein first theorized about the process which makes lasers possible called "Stimulated Emission."

Before the Laser there was the Maser

In 1954, Charles Townes and Arthur Schawlow invented the maser (microwave amplification by stimulated emission of radiation), using ammonia gas and microwave radiation - the maser was invented before the (optical) laser. The technology is very close but does not use a visible light.

On March 24, 1959, Charles Townes and Arthur Schawlow were granted a patent for the maser. The maser was used to amplify radio signals and as an ultrasensitive detector for space research.

In 1958, Charles Townes and Arthur Schawlow theorized and published papers about a visible laser, an invention that would use infrared and/or visible spectrum light, however, they did not proceed with any research at the time.

Many different materials can be used as lasers. Some, like the ruby laser, emit short pulses of laser light. Others, like helium-neon gas lasers or liquid dye lasers emit a continuous beam of light.

TYPES OF LASER

Ruby Laser

In 1960, Theodore Maiman invented the ruby laser considered to be the first successful optical or light laser.Many historians claim that Theodore Maiman invented the first optical laser, however, there is some controversy that Gordon Gould was the first.

Gordon Gould - Laser

Gordon Gould was the first person to use the word "laser". There is good reason to believe that Gordon Gould made the first light laser. Gould was a doctoral student at ColumbiaUniversity under Charles Townes, the inventor of the maser.

Gordon Gould was inspired to build his optical laser starting in 1958. He failed to file for a patent his invention until 1959. As a result, Gordon Gould's patent was refused and his technology was exploited by others. It took until 1977 for Gordon Gould to finally win his patent war and receive his first patent for the laser.

Gas Laser

The first gas laser (helium neon) was invented by Ali Javan in 1960. The gas laser was the first continuous-light laser and the first to operate "on the principle of converting electrical energy to a laser light output." It has been used in many practical applications. As its name implies, it has a cavity filled with helium and neon gases. The output of the device is bright crimson. Other gases can be used instead of helium and neon, producing beams of different wavelengths. Argon produces a laser with blue visible output. A mixture of nitrogen, carbon dioxide, and helium produces IR

Robert Hall - Semiconductor Injection Laser

In 1962, Robert Hall created a revolutionary type of laser that is still used in many of the electronic appliances and communications systems that we use every day.

properties of laser

Monochromaticity:


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This property is due to the following two factors. First, only an EM wave of frequency n= (E2-E1)/h can be amplified, n  has a certain range which is called linewidth, this linewidth is decided by homogeneous broadening factors and inhomogeneous broadening factors, the resultant  linewidth is very small compared with normal lights. Second, the laser cavity forms a resonant system, oscillation can occur only at the resonance frequencies of this cavity. This leads to the further narrowing of the laser linewidth, the narrowing can be as large as 10 orders of magnitude! So laser light is usually very pure in wavelength, we say it has the property of monochromaticity.

Coherence:


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For any EM wave, there are two kinds of coherence, namely spatial and temporal coherence.


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Let’s consider two points that, at time t=0, lie on the same wave front of some given EM wave, the phase difference of EM wave at the two points at time t=0 is k0. If for any time t>0 the phase difference of EM wave at the two points remains k0, we say the EM wave has perfect coherence between the two points. If this is true for any two points of the wave front, we say the wave has perfect spatial coherence. In practical the spatial coherence occurs only in a limited area, we say it is partial spatial coherence.


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Now consider a fixed point on the EM wave front. If at any time the phase difference between time t and time t+dt remains the same, where "dt" is the time delay period, we say that the EM wave has temporal coherence over a time dt. If dt can be any value, we say the EM wave has perfect temporal coherence. If this happens only in a range 0<dt<t0, we say it has partial temporal coherence, with a coherence time equal to t0.


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We emphasize here that spatial and temporal coherence are independent. A partial temporal coherent wave can be perfect spatial coherent. Laser light is highly coherent, and this property has been widely used in measurement, holography, etc.


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Divergence and Directionality:

Laser beam is highly directional, which implies laser light is of very small divergence. This is a direct consequence of the fact that laser beam comes from the resonant cavity, and only waves propagating along the optical axis can be sustained in the cavity. The directionality is described by the light beam divergence angle.


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For perfect spatial coherent light, a beam of aperture diameter D will have unavoidable divergence because of diffraction. From diffraction theory, the divergence angle qd is:


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qd= b l /D


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Where l and D are the wavelength and the diameter of the beam respectively, b is a coefficient whose value is around unity and depends on the type of light amplitude distribution and the definition of beam diameter. qd is called diffraction limited divergence.


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If the beam is partial spatial coherent, its divergence is bigger than the diffraction limited divergence. In this case the divergence becomes:


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q = b l /(Sc)1/2                  where Sc is the coherence area


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Brightness:

The brightness of a light source is defined as the power emitted per unit surface area per unit solid angle. A laser beam of power P, with a circular beam cross section of diameter D and a divergence angle q, then the brightness of laser beam is:


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B=4P/(p Dq )2


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The max brightness is reached when the beam is perfect spatial coherent.


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Bmax=4P/(p l b )2


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In case of limited diffraction (q d= l b /D, D=l b /q d )

special properties of laser light

 laser is a beam of light that is very different and unique from other types of light. Here’s a look at these special properties of laser light.

1) Color of the Laser Light


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All the various kinds of light that we see in our daily lives is a kind of white light. Sunlight, light from the bulb etc, are all white light, but are not just white. If you see this light through a prism or through any glassy polished surface, you will be able to see all the colors of the rainbow. But a laser light is not the same. Laser light is just one color, be it blue, red, green, or any other color that varies according to the laser used. Thus, laser light is monochromatic and not polychromatic as other lights.

2) Organizational Property of Laser Light


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The photons of light other than laser light are not very organized and move randomly in every which way. In contrast to this, the photons released and used in laser light are very consistent and organized. The light photons in lasers travel in the same speed with the other photons and also in perfect alignment with each other. It is this property of laser light that enables lasers to be so strong and intense, as all their photons are working with each other and not against each other.

3) Invisible Laser Light


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Lasers such as infrared and ultraviolet lasers have light that is invisible to the naked eye. Such lasers are very useful for alarm systems and protective security applications. As this type of laser light cannot be seen, people are unaware of its presence, and it can go about doing its job efficiently.

4) Focus of Laser Light


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Lasers work on the principle of stimulated emissions of photons. These photons are released from their environment  from just a very small opening. Because of the manner in which the laser light is achieved, it becomes highly focused and intense. As the photons can be directed very easily, the laser light can be focused and directed efficiently, thus making lasers useful for specialized applications that require tremendous accuracy.


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Applications of lasers in industry and laser

material processing


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For nuclear decontamination and decommissioning


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In the research and development of various advanced technologies needed for decontamination and decommissioning (D&D) of nuclear facilities, laser was applied to decontamination of metal and concrete surfaces and to cutting of large metal of low level radioactive waste (LLW). (a) Laser decontamination for metal waste: Metal waste was irradiated by laser in the atmosphere of chloride gas, and contaminant was changed from oxide to chloride which is sublimable or soluble in water and could be easily removed; and also metal waste coated with gel-decontamination reagent was irradiated by laser, and contaminant could be removed through the laser-induced chemical reaction. (b) Laser decontamination for concrete surface: Concrete surface was bursted or vitrified by laser irradiation and easily removed. (c) Laser cutting: Laser cutter was applied to cutting of large metal wastes such as tanks arising from dismantling of nuclear facilities.


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Technologies for decommissioning of nuclear facilities
 

In a systems engineering approach for decommissioning, a knowledge base was developed to be applied to defining work breakdown structures, work conditions, and work precedence relations in the computer-aided decision support system for planning the decommissioning of nuclear facilities. The user interface for referring to the database was also developed to assist in defining various calculation parameters in the system. The data on JPDR dismantlement were analyzed, and the results on the development of calculation models for the project management data were studied. Data on decommissioning programs in other countries were entered into the decommissioning database.
In advanced technology development, two items were studied, remote dismantling and improvements in the measurement of radioactivity. To measure low-level radioactivity on building floors and on the inner surfaces of pipes, systems were constructed by installing radiation detectors on moving devices. Consequently, two mobile radioactive contamination detection systems were completed, one to measure the distribution of contamination on floors and the other to measure contamination inside pipes. For development of remote dismantling techniques, command packages to control manipulators were developed by classifying dismantling work into primitive work tasks for dismantlement of equipment and structures. These command packages were included in subsystems that were integrated into computer simulation systems. The control mechanism for dual-arm manipulators and the related monitoring systems were fabricated to operate these devices. In addition, the behavior of aerosols in water was analyzed for the safety of workers dismantling highly radioactive components.
In decommissioning technology development for reprocessing facilities, the following activities were pursued. The methodology for making radioactivity inventory measurements was identified, mock-up tests were performed on a large-vessel using a remote-control dismantling system, and fabrication of a flammable-waste volume-reduction system was completed. In the dismantling demonstration program, a laser decontamination device was tested to remove contaminated concrete surface layers. Data regarding this capability were collected. Data were also collected for components removed from hot caves, from solvent recovery cells, and from other areas.

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 Dry excimer laser cleaning applied to nucleardecontamination


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Excimer laser ablation is a very powerful tool of dry cleaning. This technique allows the removal or oxide or painting deposited on a material without any modifications of the chemical and physical properties of its surface. This method has been effectively used in many areas. In nuclear industry, there is a great interest to develop in developing an efficient dry decontamination process. In  laser cleaning prototype based on excimer laser ablation process is described. This prototype has been tested in nuclear facilities. It is mainly composed of a XeCl laser, a bundle of fibers for beam transmission, optical systems, collection cell with filter for ablated particle recovery, computer control of cleaning efficiency and beam displacement. Different kinds of materials, which are representative of contamination usually found in nuclear field, have been irradiated. Decontamination factors (initial activity/residual activity) higher than 15 for fixed contamination and up to 100 for unfixed contamination have been obtained. These performances demonstrate that the laser-based technique is the most efficient one for dry and fast decontamination.


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in shipping and automobile industry for metal cutting and welding


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Laser cutting

It is one of the major applications of industrial manufacturing.  Thousands of laser cutting systems are in use  around the world for cutting metal sheet, plate, tubing and formed parts.  The number of installations is growing substantially each year Laser cutting is characterized by

• Flexibility in the range of materials to which it can be applied.   Laser cutting is used to cut all types of metals, including carbon, stainless and zinc coated steel, superalloys, titanium and aluminum. Lasers can cut materials that have been coated with enamel, porcelain or ceramic without damage to the outer coating.  Laser cutting is also used for plastics and ceramics
• Precision – the narrow kerf of the laser cut with precision of modern CNC machine tools gives precision measured in the thousandths of inches or tenths of millimeters.
• Edge quality – high quality edges which can be painted without additional finishing are readily produced by laser cutting.
• Distortion-free – there is no physical contact between the workpiece and a cutting tool.
• No consumable tooling - tooling and their associated costs are all but eliminated.
• High material utilization - the narrow kerf combined with low distortion allows programmers to "nest" parts, reducing waste or scrap material remaining after processing.
• Easily automated for limited operator involvement and even ‘lights out’ operation.

Laser cutting process

Laser cutting begins when a focused high power laser beam is absorbed at the surface of the material being cut and melts the material.  Melted material is removed through the backside of the material by an assist gas, typically air, oxygen, nitrogen or argon, which is directed into the cut using a nozzle.  Oxygen is used in cases where oxidation of the edge is allowable and when cutting metals for which the reaction between the oxygen and molten metal increases the cutting rate.  Inert gases, such as nitrogen or argon, are used to produce cut edges that are oxide free. Key parameters for laser sheet metal cutting are the laser type (CO2 or Nd:YAG are the most common), average power, continuous wave(CW) or pulsed, focusing lens focal length and cutting speed.

Laser sheet metal welding

Laser welding plastics is a fairly new process. It super heats the polymer without physical contact. Most applications processes are done by, directing the beam of infrared light. Directly at the weld joint. This is done by going through one of the parts. Commonly referred to as, through transmission.


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By directing infrared beam of infrared light at the weld joint via( a laser)welding. This technique, the infrared beam, usually a laser, irradiates the joint through a part and the light is absorbed at the surface of the other. While broad band infrared beams can be used, the monochromatic lasers allow very fast heating of small areas of the part that allows the parts to be welded very rapidly, but with very small changes to the geometry part.


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Laser welding is an example of electromagnetic plastic welding process. Once radiant energy it has been directed towards polymer surface, a series of three things will happen to it, most of the light transmits through, some is absorbed, and some is reflected away. The application the process involves directing the beam of infrared light towards the weld joint through one of the parts. The part (laser) that transmits most of the energy will not heat, but the absorbing part will super heat .Most virgin, organic polymers will not absorb energy. Certain dyes and fillers such as carbon black are used. To absorb the energy at the weld joint interface. This is commonly called to as through transmission infrared (or laser) welding. Welding results when materials are heated to a molten state and fused together.


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One type of material must transmit the laser light while the other absorbs it, While converting it to heat. The great news is that the materials must be transmissive. This all depends on formulation of the pigment. Joints that require optical clarity can be done by the use of special coatings types. Thermoplastics, Laser welding, resin compatibility , resin chemistry or melt temperature differences than most all other plastic welding processes these days.


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Nd:YAG laser welding is used commercially, a wide range of C-Mn steels, stainless steels, coated steels, molybdenum, titanium, and aluminum alloys. Low heat input welding. These lasers is utilized in the electronics, domestic items, automotive sectors, the most interest has been shown more recently, to particularly for the high power CW lasers in the shipping industry. Oil and gas, R&D issues involving development of highly powered lasers of better beam quality, the use of distributed energy in the beam focus, maintenance for both thick and thin sections and weld classification.


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Light energy is generated by lasers. That can be absorbed into materials and converted to heat energy. Laser emits coherent radiation. Lasers do minimal divergence that can travel over significant distances without loss of beam quality or energy.


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Relatively new techniques in Laser welding have been compared to other plastic welding processes. Dedicated laser labs at EWI's are equipped with lasers creating and analyzing plastics welding. The laser beam used to melt the base material and filler rod, this process becomes line of sight ,as well as focal point limited process. If you cannot get a straight shot, or you can't re-line the position of the weld area, it will not work efficiently or correctly. Microscopic magnification is also is used in the laser welding process.


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Laser welding offers many advantages over traditional welding techniques. These advantages are for the most part based on the ability to deliver energy sufficient techniques for melting metals in a localized area.  The result is consistent, reliable welds with minimal distortion, a small heat affected zone (HAZ) and a narrow weld profile with excellent appearance.

Laser welding process

Laser welding occurs when a focused 0.008 to 0.04 inch (0.2 - 1 mm) diameter beam from a high power laser is absorbed at the weld joint or surface (in the case of a lap weld joint) of the material being welded.  Laser melted material flows to create the weld joint.  In most cases, the molten metal is covered with a shield gas such as nitrogen, argon or helium. 

Key parameters for laser sheet metal welding are the laser type (CO2 or Nd:YAG are the most common), average power, continuous wave (CW) or pulsed, pulse rate, pulse length (duration), focused beam diameter and welding speed.

The optimum focused beam size for a given weld and joint design is most often achieved by varying the focal length of the focusing lens, which in turn varies the power density delivered to the weld joint.  Having the correct power density is key to the quality of the resulting weld and stability of the process.  If the power density is too great, material from the weld joint will be vaporized or expelled creating a very poor weld. If the power density is too low, the weld will lack penetration. Welding speed is also adjusted to effect penetration and heat input to the material and/or bead size.

Joint designs for laser welding

Laser welding is applied to joint designs similar to electron beam welding, micro-TIG welding and other high energy density welding processes.  Where possible, joints are designed to enhance the absorption of the laser beam directing the beam into the joint. 


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 in chemical industry for material purification using laser isotope production

AVLIS - Atomic Vapor Laser Isotope Separation

 It is evident that the efficiency of the modern gas centrifuge separation technique is close to its physical limit, which follows from the mechanical properties of materials. On the other hand the problem of uranium-235 extraction from wastes of uranium separation plants is under consideration now. Thus, a search for new effective separation technologies is very important for the future of the nuclear power industry. New approaches to isotope separation might also be of interest for stable isotope production, especially for the isotopes currently being produced solely by the electromagnetic technique


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AVLIS – Atomic Vapor Laser Isotope Separation – is based on isotope shifts in the atomic absorption optical spectra. It is possible to excite selectively the atoms of the chosen isotope in vaporized isotope mixture by narrow-band laser radiation, then ionize these atoms and finally to collect ionized atoms. It takes about 6 eV to ionize a uranium atom. The IMP laboratory separation facility relies on copper vapor laser(CVL)-pumped dye lasers (DL) to generate radiation with suitable wavelengths. The investigated process scheme involves two steps of successive photoexcitation and photoionization. Recent experiments have demonstrated rather promising results on laser equipment improvement, optical scheme optimization, evaporation set-up and collection method development.

 It has been shown that the production of low-enriched (3-5 %) or high-enriched (90%) uranium-235 is industrially feasible. A pilot version of an industrial AVLIS module for uranium isotope separation is now under development. The experiments on the module will give the information for evaluating the commercial potential of the industrial application of the AVLIS technology. It is generally supported that this technology is preferable in case of low-enriched starting raw materials.

It seems reasonable to use the AVLIS method for separation and commercial production of some expensive stable isotopes, which cannot be separated by the centrifuge method. The Institute has achieved considerable progress in the development of the AVLIS method for isotopes separation of Nd, Gd, Zr, Yt and some other elements. Another interesting field of application of the AVLIS method is the production of an isotope mixture depleted in a certain undesirable isotope.

MLIS - Molecular Laser Isotope Separation

Another laser method of isotope separation is based on the effect of isotopically selective multiphoton dissociation of molecules exposed to IR-radiation. This phenomenon was discovered in Russia in 1974 and developed from scientific investigations to industrial scale production of 13C isotopes .

Multiphoton dissociation takes place if a molecule absorbs radiation quanta and its vibration states are excited sequentially until the molecule breaks apart. The molecule absorbs infrared radiation due to the interaction of its vibrating electric dipole with the oscillating electric field of the radiation. While colliding a polyatomic molecule can absorb about 40-50 photons with the total energy of 3-5 eV and then it dissociates. The crucial point is that a molecule efficiently absorbs only the light with a definite (resonant) wavelength. Since the wavelengths are determined by the masses of atoms constituting the molecule, the isotope selective excitation and dissociation can be realized separately.

The advantages of the MLIS isotope selection method are as follows:

 ·high operation selectivity a-1 >> 1 (up to 104);

 ·minimal direct expenditure of energy for separation;

 ·feasibility of selecting a single target isotope;

 ·universal separation equipment for a wide spectrum of isotopes.


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laser scalpel in medicine for bloodless surgery of delicate organs


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During the last decade, laser technology has become an effective weapon in the battle against disease. The laser has been recruited for surgery to zap trouble bodywide, particularly in hard to reach or delicate areas with small operative fields, such as the eye, fallopian tube, or mouth.


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Laser surgery uses a laser light source (laser beam) to remove tissues that are diseased or to treat blood vessels that are bleeding. Laser beams are strong beams of light produced by electrically stimulating a particular material. A solid, a liquid, or a gas is used. Alternatively, the laser is used cosmetically; it can remove wrinkles, birthmarks, or tattoos.


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The special light beam is focused to treat tissues by heating the cells until they burst. There are a number of different laser types. Each has a different use and color. The color, or the light beam, relates to the type of surgery that is being performed and the color of the tissue that is being treated.


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Laser surgery is used to:


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    * cut or destroy tissue that is abnormal or diseased without harming healthy tissue

    * shrink or destroy tumors and lesions

    * close off nerve endings to reduce postoperative pain

    * cauterize (seal) blood vessels to reduce blood loss

    * seal lymph vessels to minimize swelling and decrease spread of tumor cells

    * remove moles, warts, and tattoos

    * decrease the appearance of skin wrinkles


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Description


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Lasers can be used to perform almost any surgical procedure. In fact, general surgeons employ the various laser wavelengths and laser delivery systems to cut, coagulate, vaporize, and remove tissue. In most "laser surgeries," they actually use genuine laser devices in place of conventional surgical tools—scalpels, cryosugery probes, electrosurgical units, or microwave devices—to carry out standard procedures, like mastectomy (breast surgery). With the use of lasers, the skilled and trained surgeon can accomplish tasks that are more complex, all the while reducing blood loss, decreasing postoperative patient discomfort, decreasing the chances of infection to the wound, reducing the spread of some cancers, minimizing the extent of surgery (in some cases), and achieving better outcomes in wound healing. Also, because lasers are more precise, the laser can penetrate tissue by adjusting the intensity of the light.


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Lasers are also extremely useful in both open and laparoscopic procedures. Common surgical uses include breast surgery, removal of the gallbladder, hernia repair, bowel resection , hemorrhoidectomy , solid organ surgery, and treatment of pilonidal cyst.year. Detection of the very low alpha emission even from commercially available low-alpha lead is challenging.

.Advantages of laser surgery


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Often referred to as "bloodless surgery," laser procedures usually involve less bleeding than conventional surgery. The heat generated by the laser keeps the surgical site free of germs and reduces the risk of infection. Because a smaller incision is required, laser procedures often take less time (and cost less money) than traditional surgery. Sealing off blood vessels and nerves reduces bleeding, swelling, scarring, pain, and the length of the recovery period.


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The laser has been successfully used in a number of areas:


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Ophthalmology: Introduced into medicine as a tool for repairing leaking blood vessels in the eye, the laser is widely accepted in surgery for retinal diseases (suffered by many with diabetes), glaucoma, and removal of the secondary membrane after cataract removal.


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Obstetrics-gynecology: Lasers are in employed to remove ovarian cysts, unblock fallopian tubes to reverse infertility, and treat urinary incontinence or ectopic pregnancy. They also offer an alternative to hysterectomy for uterine bleeding - one that preserves childbearing capability.


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Surgeons opt for the laser in about half of procedures for endometriosis, a common disorder resulting when renegade tissue escapes form the uterine lining and colonizes other organs. According to many obstetrician-gynecologists, operating with lasers in the uterine cavity is a big step forward in women's medicine.


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Dermatology: Lasers are proving their worth for zapping warts, moles, precancerous lip growths, and the once-permanent tattoo. No other method has such a power of removal without blemish or pain.


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Cosmetic Surgery: Whether removing a birthmark from a baby's face or wiping away a liver spot from an aging hand, lasers have opened up worlds where cosmetic surgeons previously dared not tread.


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Urology: Urologists use lasers to zap polyps that develop into colon cancer and, in a much newer application, to vaporize tissue blocking the flow of urine in a high-tech takeoff of traditional prostatectomy, the second most common operation performed on men.


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Gastroenterology: Lasers speedily fragment gallstones trapped in the common duct between the liver and intestine, allowing their retrieval and forestalling a potentially deadly condition. The gallbladder can be removed harmlessly but not so the duct, which is needed for survival.


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Dentistry: A wide variety of dental lasers are available. Some are still in research and development. Most dental lasers have infrared beams and produce biological effects such as heating, vaporizing, coagulation, and cutting.


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The advantages of laser dentistry are that it is faster and more efficient in many cases, more antiseptic, most often bloodless, less invasive, more precise and conserving of healthy tissue, reduces postoperative discomfort, decreases stress, and reduces the need for local anesthetics.


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Laser based sensors and measurement methods in environmental sciences


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Ultra Efficient laser Spectroscopic Trace-gas sensors for Sensor Networks and Portable Chemical Analysis


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This  is primarily concerned with development of advanced miniature  laser based trace-gas sensors for distributed wireless sensor networks and portable chemical sensing in scientific and commercial applications. Infrared  laser spectroscopy can provide ultra high sensitivity and selectivity measurements of trace gas concentrations down to parts-per-billion and parts- per-trillion levels.

However, currently available trace-gas sensor  instrumentation with adequate performance for scientific studies weighs tens of kilograms, has a tabletop size, requires hundreds of watts of power, and costs greater than $50,000. This invention focuses on high-performance, ultra low  power, handheld sensor systems suitable for scalable low-cost mass-production. This prototype laser spectroscopic sensor targets oxygen in the form of a battery  powered, handheld unit, and provides high specificity with sensitivity of 0.02% in 1 sec to atmospheric (21%) concentration. Background and Motivation Sensor networks allow for acquisition of spatially resolved long-term real-time

data, which is not possible with other types of sensing technologies. For example, in environmental science ,climate change and monitoring greenhouse gas fluxes have become a forefront issue, but existing sensors do not provide sufficient spatial and time resolution. Thus,environmental sensor networks are of significant interest to monitor carbon fluxes, track hazardous gas plumes, and  monitor geochemical processes. For successful implementation of global environmental  treaties (i.e. the successor to the Kyoto Protocol) there must exist a world-wide infrastructure in order to implement “carbon credit systems” to verifiably trade carbon and  continuously monitor and localize unauthorized emissions. A common method currently used to generate chemical emissions maps is based on self reporting by industry, which does not provide  information about natural sources and sinks, and is prone to human sources of error  Satellites are capable of wide area sensing, but real-time spatial resolution and sensitivity are severely limited. Furthermore,  deployment and operational costs are exceptionally high.Another commonly used method to collect emissions source information is based on ultra-high sensitivity instrumentation installed  into vehicles (e.g. airplanes, trailers), and acquiring data in cross patterns. This method requires operation and maintenance personnel,  with spatial and time resolution limited to a point measurement at the current location of the vehicle. Additionally, for airborne platforms the sensitivity must be extremely high (ppt level) to detect ground emissions at high altitudes after the gases have spread considerably from the sources.

With our technology we target deployments of large-area wireless networks of trace-gas sensors This requires 1) low power consumption to operate on solar or battery power without having to charge batteries for hundreds of nodes spread over wide areas, 2) small size to allow deployment without any infrastructure requirements, and 3) low cost to increase the density of sensors possible within a budget. A number of methods such as gas chromatography (GC), mass spectrometry (MS), and Fourier transform infrared  spectroscopy (FTIR) provide the sensitivity and specificity required for long-term, real-time, trace-gas sensing, but typically have  considerable tradeoffs between size, cost, and power consumption vs. performance, robustness, simplicity, and autonomy.

Now there is a prototype laser based sensor platform for trace-gas sensing which weighs less than 0.4 kg, costs less than $1,500  in quantity of one, and is capable of sensitivity and specificity rivaling laboratory equipment based sensors The sensor also communicates wirelessly, and dissipates less than 0.3W of battery power. With these sensor characteristics, we can finally address the  critical applications listed above in a practical manner. These characteristics can be achieved  by performing strongly interdisciplinary research to combine the latest technologies in digital signal processing and applied mathematics, novel computer and embedded system architectures, photonics, spectroscopy, and optics, control theory, computer science  and algorithms, and computer networking. Our technology provides a highly flexible, modular system, and permits the integration of various semiconductor laser sources and detection techniques (laser absorption, photo-acoustic or polarization spectroscopy). The sensor system consists of three major elements: a digital signal processing and control core, mixed-signal circuitry and network interfaces, and laser based detection  module. Specifically, our novel core technology integrates multiple data analysis and control systems into a single microprocessor  platform including 1) three low power, low noise, ultra compact, digital lock-in amplifiers 2) high efficiency laser driver 3)  precise active laser temperature stabilization, 4) power conserving software and hardware5) interface to wireless sensor networking Hardware and 6) autonomous laser spectroscopic algorithms. The  optical systems are optimized to be rugged, low cost and quickly replicable in large numbers.

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The combination of high performance, low power consumption, low cost, and small size of this technology is unique among trace-gas  sensing methods and provides unmatched capability for large area deployments, and is reconfigurable for a variety of molecular targets.

Conclusion:

This sensors are uniquely capable of providing new tools and addressing new applications in trace-gas sensing. With this  planned  developments, anyone will have the ability to deploy a chemical mapping system to provide critical environmental  monitoring, and  medical researchers will be able to provide these sensors for patients to perform self-diagnostics. We definitely believe this technology


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Cavity-enhanced quantum-cascade laser based

instrument for carbon monoxide measurements in environmental science


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An autonomous instrument based  on off-axis integrated cavity output spectroscopy has been developed and successfully deployed for measurements of carbon monoxide in the troposphere and tropopause onboard a NASA DC-8 aircraft. The instrument (Carbon Monoxide Gas Analyzer) consists of a measure- ment cell comprised of two high-reflectivity mirrors, a continuous-wave quantum-cascade laser, gas sampling system, control and data-acquisition electronics, and data-analysis software. CO measurements were determined from high-resolution CO absorption line shapes obtained by tuning the laser  wave- length over the R(7) transition of the fundamental vibration band near 2172.8 cm. The instrument reports CO mixing ratio (mole fraction) at a 1-Hz rate based  on measured absorption, gas temperature, and pressure using Beer’s Law. During several flights in May–June 2004 and January 2005 that reached altitudes of 41,000 ft 12.5 km , the instrument recorded CO values with a precision of 0.2 ppbv (1-s averaging time) and an accuracy limited by the reference CO gas cylinder (uncertainty 1.0%). Despite moderate turbulence and measurements of particulate-laden airflows, the instrument operated con- sistently and did not require any maintenance, mirror cleaning, or optical realignment during the flights.


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For almost 30 years, tunable diode laser absorption spectroscopy techniques have been developed by dozens of groups worldwide for applications that require fast, nonintrusive measurements of trace gases, including atmospheric and environmental  monitoring, combustion, propulsion, and other industrial processes. Since Beer’s Law is used to convert the measured absorption spectra to gas mixing ratio, increasing measurement  sensitivity generally involves using longer optical paths, increasing the ability to

detect small changes in transmitted laser  intensity or cavity ringdown time, and probing absorption features with greater line strengths (typically in the mid-infrared or ultraviolet spectral regions). As a result, various strategies have been developed to achieve high sensitivity, including direct absorption spectroscopy using long-path multipass cells, wave- length (and frequency) modulation spectroscopy, cavity ringdown spectroscopy, cavity-enhanced absorption spectroscopy, integrated cavity output spectroscopy (ICOS), and Off-Axis ICOS.This new system uses development and operation of an autonomous instrument (Carbon Mon- oxide Gas Analyzer) based on Off-Axis ICOS that employs a continuous-wave quantum-cascade laser for measurements of CO onboard a DC-8 aircraft with sub-ppbv sensitivity and precision. Similar to its pre- decessor, the differential absorption CO monitor (DACOM), this instrument should prove useful in quantifying and tracking variations in CO, and characterizing the distribution of combustion products in the troposphere and tropopause. In the future, the instrument may be applied for sensitive measure- ments of other gases that absorb at other wavelengths by selecting an appropriate laser  and cavity mirrors

A new laser-based method for strain rate and vorticity measurements


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 The technique is based on the collection and direct heterodyning of coherent light scattered from individual seed particles in two adjacent locations and, thereby, allowing the determination of a component of the strain rate tensor. The beat frequency of the heterodyned light, which is proportional to the velocity difference of the two points, is analysed using a conventional LDA processor. The angle between the laser beam and the direction of the scattered light determines the measured component of velocity. Therefore, a component of the vorticity vector can be measured by using two sets of transmitting and collecting optics, focused at a single location, along with two LDA processors. The basic principle of the technique is verified through measurements on a thin rotating wire and in a laminar flow between two concentric cylinders with excellent results. In the latter experiment, the measured velocity differences between two radial locations in the flow agree very well with the exact solution.


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Laser in defence as a sensor and as weapon


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LASER WEAPONS

High Energy Laser weapons have been progressively evolving since the 1960s, a path punctuated by a series of important scientific breakthroughs and engineering milestones.

The popular view of a HEL, seen as constructing a great big laser and pointing it at a target with the intention of vapourising it, bears only vague similarity to a real HEL weapon. There are genuine technological and operational challenges involved in creating truly useful and effective weapons.

Kinetic or projectile weapons such as guns, missiles and bombs destroy targets by kinetic effects, including overpressure, projectile, shrapnel and spalling damage, and incendiary effects. The result is structural damage and fire, which can and often will cause fatal damage to a target. A kinetic weapon thus uses stored chemical energy in propellants and warhead explosives, the latter where used, and delivers this energy to a target by means of a projectile of some kind. Whether the projectile weapon is a trebuchet tossing a large rock over 300 yards, or a multimode seeker equipped long range air to air missile hitting an aircraft from 200 nautical miles away, the underpinning principle is much the same, only the implementation is different.

At the most fundamental level Directed Energy Weapons share the concept of delivering a large amount of stored energy from the weapon to the target, to produce structural and incendiary damage effects. The fundamental difference is that a Directed Energy Weapon delivers its effect at the speed of light, rather than supersonic or subsonic speeds typical of projectile weapons.

Two of the most fundamental problems seen with projectile weapons, that is getting the projectile to successfully travel a useful distance and hit the target, and then produce useful damage effects, are problems shared by Directed Energy Weapons. Having a powerful laser or microwave emitter maketh not a Directed Energy Weapon system alone.

Most contemporary literature lumps together a broad mix of weapons technologies in the Directed Energy Weapon category, including High Energy Laser (HEL) weapons, High Power Microwave (HPM) weapons, particle beam weapons and Laser Induced Plasma Channel (LIPC) weapons. The first two of these four classes of weapon are genuine Directed Energy Weapons. Particle beam weapons are best described as a form of projectile weapon, using atomic or subatomic particles as projectiles, accelerated to relativistic speeds. The LIPC is a hybrid, which uses a laser to ionise a path of molecules to the target, via which an electric charge can be delivered into the target to cause damage effects.

Of these four categories, HELs have the greatest potential in the near term to produce significant effect. HPM technology has similar potential, but has not been funded as generously and thus lags well behind lasers. LIPC has significant potential especially as a nonlethal weapon. Particle beam weapons at this time are apt to remain in the science fiction domain, as the weight and cost as yet do not justify the achievable military effect.

The next ten years will see the emergence of high energy lasers as an operational capability in defence service. These weapons will have the unique capability to attack targets at the speed of light and are likely to significantly impair the effectiveness of many weapon types, especially ballistic weapons. Constrained by propagation physics, these weapons will not provide all weather capabilities, and will perform best in clear sky dry air conditions.
DISTANCE SENSORS

Laser distance sensor


The laser distance-meter LMC-J-0310 is robustly engineered especially for industrial applications. It works at distances up to 3000 m. Up to 300m and more (depending on target reflectivity) no special reflector is needed. Measuring values are provided continuously via the integrated RS422/RS232 or analogue 4..20 mA interfaces. As option, ProfiBus and SSI interface is available. In this way the device is providing position and velocity information during the production process.

The LMC-J-0310 can be used in distance and speed measurement mode. All measuring parameters are adjustable. The high measuring speed allows the distance measurement of extremely fast moving objects.

The operation principle of the LMC-J-0310 is time delay measurement, the measuring time is adjustable between 0,5 and 1000 ms (a special version with 0,1ms is also available). Within a fraction of second one distance measurement is obtained by means of several hundreds or thousands of individual measures.

 
laser in optical communication


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An open-path optical communication system has either optical or laser sources and communicates between the source and a detector. In a first embodiment, the laser source includes a gas cell in the laser cavity to regulate laser wavelengths based on the minimum absorption between spectral lines of the gas in the cell. The laser is tuned close to a minimum absorption wavelength and the minimum absorption line locks the laser wavelength to the minimum position. In a second embodiment, the strong absorption lines of a gas in a gas cell positioned at a receiver site are used to provide channel isolation of the receiver. In a third embodiment, an atmospheric gas provides the channel isolation. In the fourth embodiment, individual wavelength channels are positioned between the absorption lines of atmospheric or non-atmospheric gases to prevent cross-talk between adjacent channels.


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The long-standing but heretofore unfulfilled need for a laser that does not require fine wavelength control is now provided by a system that incorporates, in a first embodiment, a laser having a non-filter bandwidth-defining structure preferably in the form of a gas cell positioned in the laser cavity.

The gas cell positioned inside the laser cavity enables the laser to operate at wavelengths between the maximum absorption line wavelengths of the gas in the cell. This is the spectral position or wavelength that has higher gain, i.e., the lowest absorption loss. Accordingly, the only tuning required is a coarse wavelength control such as a grating prism. The need for additional equipment providing fine wavelength control is therefore obviated.

More particularly, a gas cell containing gas with individual vibrational-rotation line spectra is positioned inside a tunable laser cavity having a resonance wavelength and the cavity resonance wavelength is positioned between adjacent absorption lines of the gas. The laser therefore operates at an absorption minimum that occurs between the absorption lines and the laser wavelength is locked to an absolute wavelength defined by the gas. Advantageously, the maximum absorption bands act as filters for the laser wavelength output.

In a second embodiment, the need to control laser wavelength to that of a receiver optical bandwidth is fulfilled by harnessing strong optical absorption lines in a preselected gas in a gas cell positioned at a receiver site upstream of the receiver. This provides an absolute wavelength reference or control for laser wavelength and receiver/detector optical bandwidth in an open path laser/optical communication system.

In a third embodiment, the need to control laser wavelength to that of the receiver optical bandwidth is fulfilled by harnessing the strong optical absorption lines in the atmosphere that are due to atmospheric oxygen.

A fourth embodiment discloses a novel method for controlling a wavelength-controlled laser to the optical bandwidth of a receiver means in an open-path communication system. The laser is tuned so that it lases at minimum absorption wavelengths positioned between strong rotational-vibrational spectral absorption lines in atmospheric gases. The strong absorption lines provide optical guard channels that prevent cross-talk between adjacent wavelength channels. An absorption line minimum locks the laser to the minimum absorption position and reliance upon optical bandwidth filters in a receiver channel is reduced. An external tuning means is employed to tune the laser to within a few nanometers of the minimum absorption wavelength so that it lases at the minimum spectral absorption lines where the laser cavity has maximum gain. Positioning an absorbing gas cell in the laser cavity of the laser forces the laser output to operate at wavelengths at the minimum of the spectral absorption lines. 

A more specific object is to provide a laser communication system laser source that operates at known absolute wavelengths defined by the minimum of absorption between gaseous absorption lines.

Another object is to provide a laser system that operates with inexpensive coarse tuning equipment and does not require expensive fine tuning equipment.

Still another object is to optimize the wavelengths used in a laser communication system to select wavelengths that use the absorption characteristics of the atmosphere or external gas cell to enhance the performance of a laser communication system detection device.

Another important object is to provide a method for providing optical guard channels that prevent cross-talk between adjacent wavelength channels.

The fourth embodiment of this invention harnesses the observation that many atmospheric and non-atmospheric gases have strong, individual and distinct rotational-vibrational spectral absorption lines. These absorption lines are used advantageously within a tunable laser cavity to force or control the laser to operate at wavelengths between the absorption line centers, i.e., where the transmission is highest. For a moderately tunable diode or other type of laser, an external tuning means such as grating, diode current, or temperature may be used to tune the laser close (within a few nanometers) to minimum absorption wavelength. The absorption line minimum locks the laser wavelength to the minimum position, i.e., the laser will lase at the minimum of the spectral absorption, where the laser cavity has the highest gain.

Multiple wavelength optical/laser open path communication systems operating through the atmosphere can operate at many simultaneous wavelength channels. If the individual wavelength channels occur between the absorption lines of non-atmospheric or atmospheric gases, then each channel is blocked from drifting (in wavelength) into the adjacent channel by the adjacent strong absorption line. As such, the strong absorption lines act like adjacent WDM optical bandwidth filters or Fabry-Perot transmission modes. The resultant optical blocking filters which still need to be used are wider in wavelength bandwidths and thus less expensive. For example, the approximately thirty (30) CO 2 gas absorption lines near 1.575 μm are separated by 2 to 3 cm −1 , i.e., about 2 to 3 Å or 0.2 to 0.3 nm. The narrow optical bandwidth filters would have a passband of 2 to 3 Å instead of a narrower bandwidth.

The fourth embodiment of this invention is therefore understood to be a method for preventing cross-talk between adjacent wavelength channels. The novel method includes the step of controlling a wavelength-controlled laser to the optical bandwidth of a receiver means in an open-path communication system by tuning the laser so that it lases at minimum absorption wavelengths positioned between strong rotational-vibrational spectral absorption lines in atmospheric gases. Strong absorption lines therefore provide optical guard channels that prevent the cross-talk. An absorption line minimum locks the laser to the minimum absorption position and reliance upon optical bandwidth filters in a receiver channel is reduced. This enables the use of less expensive optical bandwidth filters in the receiver/detector channel.

An external tuning means is used to tune the laser to within a few nanometers of the minimum absorption wavelength so that it lases at the minimum spectral absorption lines where the laser cavity has maximum gain. An absorbing gas cell positioned in the laser cavity of the laser forces the laser output to operate at wavelengths at the minimum of the spectral absorption lines.

The systems that use the gas cell absorption are appropriate for the open path systems of the incorporated disclosure, but they also may be used in direct pointlaser/optical systems and fiber optic laser/optical communication systems.

It may be appreciated by one skilled in the art that additional embodiments may be contemplated, including alternate embodiments of the laser or optical sources and the detectors.

 Short-range Free-space Optical Data Links

Technologically much less challenging are data links between metropolitan buildings (LAN-to-LAN connections), where a free-space data link over distances of hundreds of meters or even over a few kilometers can be much simpler and more cost-effective to install than any kind of cable, particularly if a road or some other kind of barrier has to be crossed, or if a connection is required for only a limited time. It is then possible e.g. to obtain fast Internet access for all buildings involved, even if only one of them has direct access to a fiber network.

The laser powers required are very moderate, as a significant fraction of the sent power can hit the receiver (e.g. a photodiode). Therefore, there are usually no significant laser safety issues, particularly if eye-safe lasers emitting in the 1.5-μm spectral region are used. However, the availability of services is smaller than with a cable, because the link may be disturbed either by atmospheric influences (e.g. heavy rain, fog, snow, or strong wind) or by flying objects such as birds. In this respect, free-space transmission is less robust than other wireless technologies such as radio links, but it has a higher potential for transmission capacity, is immune against electromagnetic interference, and does not raise concerns in the context of electro-smog. Also, it does not lead to interference between different data links, so it does not need a license to be operated, and it is superior in terms of data security, since it is more difficult to intercept a tightly collimated laser beam than a radio link. Finally, the reliability can be enhanced in various ways, e.g. with multibeam architectures, larger power margins, and backup systems, and the security can be extremely high with certain schemes of quantum cryptography.

For very short distances (e.g. some tens of meters) and moderate data rates, one does not even require a laser transmitter, because light-emitting diodes (LEDs) can be used instead.

It is even possible to establish short-range optical data connections without a direct line of sight. When ultraviolet light is used, this is strongly scattered in the atmosphere, and it is possible to receive some of that light. That technology has become more interesting with the advent of light-emitting diodes (LEDs) emitting in the deep UV (UV-C), and also of suitable semiconductor photodetectors.

Space Applications

Some space applications require large amount of data to be transferred. An examples is the transmission between different Earth-orbiting satellites (inter-satellite communications), which was first demonstrated by ESA in 2001 (ESA). It is possible to transmit tens of megabits per second or more over many thousands of kilometers, using moderate laser average powers of the order of a few watts.

Data can also be exchanged between a more remote spacecraft and a station on or near Earth. For example, planetary probes can generate a lot of image data, and a major challenge is to send large amount of data back to Earth. Until recently, radio links operating e.g. in the X band or Ka band were the only available technology. Currently, optical data links are considered particularly for the downlink, where the desired data volumes are much larger than for the uplink, and optical communications could greatly expand the transmission capacity to hundreds of kbit or even several megabits per second. The spacecraft then has a pulsed laser source (employing pulse position modulation, for example) and an optical telescope of moderate size targeting the receiver. The latter can be a large ground-based telescope or a transceiver in an Earth orbit.

The basic advantage of the optical technology over radio links is that the much shorter wavelength allows for a much more directional sending and receiving of information. In technical terms, the antenna gain can be much higher. This is particularly important for bridging interplanetary distances.

Transmission Issues

Particularly for large transmission distances, it is essential to direct the energy of the sender accurately in the form of a well-collimated laser beam in order to limit the often still very large loss of power between the sender and the receiver. In order to limit the beam divergence, it is necessary to arrange for a large beam radius from an optical source with high beam quality. Ideally, one uses a diffraction-limited source and a large high-quality optical telescope for collimating a large beam. Due to the short wavelength of light, the beam divergence of an optical transmitter can be much smaller than that of a radio or microwave source of similar size. Using a frequently used term in the area of radio transmission, the antenna gain can be much higher for optical transmitters – well over 100 dB even for moderate telescope diameters of e.g. 25 cm.

Figure 1: Simple setup for free-space optical communications. Although the transmitter signal is approximately collimated, part of the transmitted power may miss the detector.

It is also advantageous to have a high directionality on the side of the receiver: it is essential not only to collect as much of the sender's power as possible, but also to minimize disturbing influences, e.g. from background light, which introduces noise and thus reduces the data transmission capacity. Both high sensitivity and high directionality can be achieved by using a large telescope at the receiver end.

Of course, high directionality also requires high precision in the alignment of the sender and receiver. It may then be necessary to stabilize the alignment with an automatic feedback system. For ground-based receivers of signals from remote satellites, it is considered to use adaptive optics to increase the directionality further by reducing the influence of atmospheric disturbances.

An important issue is the power budget of a free-space link, including the transmitter's power and all power losses. The remaining power at the receiver largely determines the possible data transmission rate, even though this is also influenced by the modulation format, the acceptable bit error rate, and various noise sources, in particular laser noise, amplifier noise, excess noise in the receiver (e.g. an avalanche photodiode), and background light. The latter can often be efficiently suppressed with additional narrow-band optical filters, since the optical bandwidth of the signal is fairly limited, whereas background light is usually very broadband.

Severe challenges can arise from the effects of atmospheric disturbances such as clouds, dust and fog, which can cause not only strong signal attenuation but also intersymbol interference. To solve this problem, sophisticated techniques of digital signal processing have been developed, which amazingly allow for reliable high-capacity optical links even through thick clouds



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plz tell me how i download this material........


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