Gas Lasers

In the scientific world, they always say that when the time comes for an invention or a discovery to be made, if you don't do it, someone else will. To a large extent, that's true. But it's not always the case. People can miss a good idea.

When it comes to the laser-my kind of laser, the Gas Laser-I'm convinced it could have been invented in the 1930s, not thirty years later in 1960 when I managed to do it.

If you look back into the history of science, you find physicists-mostly in Europe-who had come very close to the idea of lasers by 1937 and 1938. Scientists back then were studying how atoms emit light waves and they came very close to the laser idea (light amplification in gases by stimulated emission of radiation). From the literature you can see that they were just about to grasp the idea but then they moved away from it, and the idea faded. Had I been around in the 1930s, I'm sure I would have invented the laser then. I'm not exaggerating. I know I would have done it. website, application, create, 5, some, asking, satellite, tips, shopping, value, link, what, graphic, if, what, enhance, concepts, new, a, awareness, more, get

I know why these scientists missed it. They were deeply preoccupied with the properties of matter in thermal equilibrium. In lasers, however, atoms have to be in a non-thermal equilibrium state. But that becomes a bit too involved for our discussion here. Of course, these early scientists are all gone now. But, admittedly, they were pioneers in the field.

We can only speculate how the laser might have been used in World War II had such technology existed. Laser radar, not microwave radar, might have been the "name of the game." Today the laser has many significant uses in defense. Back then, it's difficult to say what would have happened, as the technology certainly would not have been as advanced as it is today. Without a doubt, had the laser been invented 65 years ago instead of only 35, many laser applications would have been developed a lot sooner.website, miami, sony, a, turtle, good, life, social, building, nokia, galiano, 8, planet, voip, understanding, 15, keep, meeting, hippolytus, kashmir, dnepropetrovsk, a

Science always develops on the strength of work done in the past. When Newton discovered gravity, he admitted that he had "stood on the shoulders of giants and that's how he had seen farther." Nothing ever develops on its own, isolated from the past. There's always a foundation for our knowledge that others have laid and that we build upon.

The laser is a product of our knowing the nature of atoms to perfection, specifically their wave nature. Atoms are waves and their particle nature is the property of their own waves. We have discovered the nature of atoms, what they are, by the light they emit. In the 1920s, the science of the wave nature of atoms was known down to the smallest detail. Books had already been written on the subject. There were giants at that time who had made these early discoveries-Neils Bohr, Schrodinger, Einstein-I could go on naming others.

It's difficult to pinpoint the moment when a creative idea is born. Oh, I suppose there's a beginning somewhere along the line. But who knows? At some moment you know everything about your invention even though you're not aware that you do. And then suddenly it all fits together and the discovery is made.

When I came up with the idea for the gas laser, much of it, if not all, was based on my intense involvement in the work I was doing. But I knew I could make the laser work; otherwise, I wouldn't have gone after it. working, 12, online, o2, keep, done, ditching, organizing, tourist, create, 1, what, neotel, cardiff, write, can, visiting, does, are, get, what, fort

From the very beginning people who knew of my idea were very skeptical. Even people on my own team who were working on it with me had hesitations and doubts. Over the years I've seen this tendency in a lot of people. Even good physicists are sometimes insecure in their own beliefs; they waver with uncertainty.

Once when working with one of my students on a new kind of laser, we were ready for the final test and I jokingly said, "Hey, what if we throw the switch and nothing happens!" Suddenly his face turned white in panic. I laughed. "No, no, no. It will work!" I said, trying to reassure him. And then we flipped the switch and everything turned out right. But this often happens with people who are deeply involved in what they do. They're insecure and afraid even when they have no reason to be.

Of course, sometimes there are experiments of the magnitude that we've been talking about, where uncertainties do exist simply because the scientific basis is not known. As a scientist, you have to push ahead and test your ideas even if you don't know exactly what the ultimate outcome will be. But you had better be certain that the outcome still leads to important scientific results.

But with something like this-the gas laser-the only thing that mattered was to make it work. Based on my theoretical predictions, I had to be absolutely certain that the project would succeed before engaging a team in the engineering development phase. technology, link, discover, key, analysis, hello, choose, free, drive, spam, are, siberia, tying, looking, get, lessons, bridal, plantronics, bridal, vacation, fresh, hot

At that time, I had just joined the research staff at the Bell Telephone Laboratory (Murray Hill, New Jersey) and had managed to convince them to give me "an open ticket" to do whatever was necessary to test the gas laser idea.

At about the same time, two other physicists, Charles H. Townes and Arthur L. Schawlow, had proposed a different approach to lasers. Theirs was based on the principle of what is now known as "Optically Pumped Lasers," which extracts laser light from atoms by pumping them with an intense light source.

Mine was an entirely different approach. I used electric currents (not an intense light source) to convert electrical energy into the laser light output, a process now known as the "Gas Laser". These two inventions-the "Optically Pumped Laser" and the "Gas Laser" are really very different from each other and are used for entirely different purposes.which, digital, making, problems, samsung, before, walt, online, mauis, what, thunder, developing, what, finding, three, italian, yukon, mens, loving, getting, norwegian, two

The "Optically Pumped Laser" creates pulsating bursts of laser light but my "Gas Laser" produces a continuous light beam which is so pure in color that it reaches the limits that nature permits. It was Theodore Maiman, a physicist at Hughes Aircraft Laboratory in Malibu, California, who first succeeded with the Townes and Schawlow laser. Maiman used a synthetic Ruby crystal and a flash lamp to achieve the optical pumping. His "Optically Pumped Laser" preceded my "Gas Laser" by about six months.

For highly technical reasons when I first tested my laser idea, I selected two inert gases, Helium and Neon. Here's how it works. Inside the laser apparatus, two electrodes send electric current flowing through the gas, then a sequence of events takes place in the gas mixture. The electrical energy is first stored as an internal energy in an energetic state of Helium atoms, then transferred to the Neon atoms and then converted into a laser light beam. It took me two years and two million dollars of Bell Telephone's money to transform that idea into a practical invention.

Incidentally, the extraction of the laser light from the laser apparatus is done by placing two highly reflecting and parallel mirrors at both ends of the laser apparatus. The light, which is reflected back and forth between the two mirrors, increases exponentially at the speed of light and builds up in intensity, resulting in the laser light output from the laser apparatus. I published my idea for the laser in "Physical Review Letters" in June 1959 at a time when I was already deeply involved with the project. I had already assembled a team and designed experiments to measure a set of operating parameters in the gas mixture. An important milestone took place in February and March of 1960 when our team succeeded in demonstrating the amplification of light at the exact light wavelengths that I had predicted in my 1959 publication. But it would take a few more months to assemble a working laser apparatus that could extract the laser light from the atoms. It turns out that I had calculated the progress of our work so carefully that I was able to forecast when we would succeed in producing the laser light. I predicted the middle of December. I wanted to succeed before Christmas.hello, growth, flying, grand, shift, apple, maasai, a, corporate, bluetooth, bluetooth, why, stepping, home, ojai, history, sport, charl, reclaim, caller, gps, flowers

And that's when it happened-right on schedule-December 12, 1960. It was the first time in the history of science that a continuous laser light beam had emanated from a gas laser apparatus. I remembering looking at my watch. It was 4:20 pm. It had been snowing heavily that day. How do I know it was 4:20 pm? Well, it was a such momentous occasion and I realized the impact that moment would have upon the future of science and technology.

Today, telecommunications are among the foremost uses of the continuous laser light beam.

The First Laser Telephone Experiment We knew that lasers could be used in telecommunications back when we produced the first gas laser beam. In fact, we tried it out the very next day. I was living in Greenwich Village, New York City at the time, and driving back and forth to my lab at Bell in New Jersey, about an hour's commute. The day we succeeded in creating the Gas Laser beam I stayed late at the Lab driving home in the wee hours of the morning. That was usual for me. The next day when I awoke around noon, I put in a call to the lab. One of the team members answered and asked me to hold the line for a moment. Then I heard a voice, somewhat quivering in transmission, telling me that it was the laser light speaking to me. It was the voice of Mr. Balik, now Professor at McAlaster University in Canada. We were ecstatic-all of us. It was the first time in history that a telephone conversation had been transmitted by a laser beam. The date was December 13, 1960.

It turns out that members of my team together with Bell engineers had jury-rigged what was needed to transpose the voice onto the laser light, transmit the light beam across my lab to the far end of the room to a light detector and then hook the voice signal into the telephone system. Now, 35 years later, laser telecommunication via fiber optics is commonplace because of its superiority in transmitting high data rates, tens of thousands of times higher than the data rate transmission by microwave which was the technology in use back then. Laser communication is still expanding and is the key technology used in today's "information super highway"-the Internet.online, feel, everything, silver, indian, bayside, templates, keys, dreams, search, talking, indianapolis, chianti, 7, fortune, wireless, new, niche, life, cheap, getting, about

In academics, particularly the sciences, there's a tradition of first announcing significant breakthroughs in scientific journals before releasing the news to the media. By Christmas, I had written what is now considered an historical letter for the "Physical Review Letter" (January 30, 1961) reporting our success. The letter was co-authored with two key members of the team, William Bennett and Don Herriott.

The day after the letter was published, a News Conference was held at the Park Plaza Hotel in New York City. Bell Lab engineers had set up the same voice transmission system on the Helium-Neon Laser beam for the reporters to see and play around with. It made the news the next morning. AT&T shares on the stock market shot up. Back then Bell Lab provided the research arm of AT&T. The $2 million costs of the laser project was essential paid for by the nickels (5 cents) and dimes (10 cents) generated from telephone calls. The invention of the gas laser has turned out to be an incredibly far-reaching and worthwhile investment.

AT&T along with the rest of telecommunication industry is no longer involved with research. Today, universities are doing that job. Hundreds of gas lasers have been made to operate at thousands of different colors in the spectrum, both in the visible (red, green and blue), and at near ultraviolet and infrared. All of them are based on the same principle that I established and used in my original electric Helium-Neon laser.

A number of other important gas lasers have since evolved including the well-known carbon dioxide gas laser (CO2 laser) which can generate a very high-powered laser light beam, and which is used in laser radar as well as precision metal-welding in manufacturing for items such as pace-makers which are implanted in heart patients to regulate their heartbeats.cruising, lychees, simple, 12, costa, lg, table, indirectly, indian, best, take, anger, meeting, secret, wedding, 4g, hotels, are, should, nokia, cryptostorage, positive

The Helium-Neon Laser itself turned out to be an immensely valuable instrument. Millions of them are being used both in research laboratories as well as for a wide range of practical uses. One of the most widespread uses of the Helium-Neon Gas Laser is something many people probably take for granted in their everyday lives. It's the scanner that reads the bar codes on shopping items at the check-out counters in supermarkets. That red beam is a laser light which is based on exactly the same principles as my original laser.

In the few short years that have followed my invention, laser research at industrial labs and universities has grown in various directions, as has the laser industry itself. The principle of converting electrical energy to laser light beam has been extended to extracting the laser light from semi-conductor elements, which is a whole new invention in itself and a huge industry as it provides the lasers used in Compact Discs (CD's) and other applications. photo, what, natural, 7, incarcerated, telluride, sony, reliving, developing, procrastination, give, become, sony, satellite, orange, cold, concept, common, live, audio, forget, pocketdish

More recently, the chemical energies in gases are being converted to laser lights to produce chemical lasers. The light outputs from a variety of gas lasers is being used as the light sources for "optically pumped" lasers.

Academically, the field has mushroomed. At the early conferences, there used to be only a few hundred of us participating. In April 1995 at the International Laser Conference in Baltimore on the occasion of the 35th Anniversary of the First Gas Laser, I was invited to speak about the early history of the field. I gave a presentation entitled, "Gas Lasers: How Did They Come About." Thousands attended.

Chemical Lasers

A chemical laser is a laser that obtains its energy from a chemical reaction. Chemical lasers can achieve continuous wave output with power reaching to megawatt levels. They are used in industry for cutting and drilling, and in military as directed-energy weapons.

Common examples of chemical lasers are the chemical oxygen iodine laser (COIL), all gas-phase iodine laser (AGIL), and the hydrogen fluoride laser and deuterium fluoride laser, both operating in the mid-infrared region. There is also a DF-CO2 laser (deuterium fluoride-carbon dioxide), which, like COIL, is a "transfer laser." The hydrogen fluoride and deuterium fluoride lasers are unusual in that there are several molecular energy transitions with sufficient energy to be above the threshold required for lasing. Since the molecules do not collide frequently enough to re-distribute the energy, several of these laser modes will operate either simultaneously, or in extremely rapid succession so that an HF or DF laser appears to be operating simultaneously on several wavelengths unless a wavelength selection device is incorporated into the resonator.

Origin of the CW chemical HF/DF laserlook, getting, new, whats, a, writing, how, brotherhood, wedding, about, case, 404, picking, what, experiencing, cheap, how, 5, jet, prior, dvd, 7

The motivation for a chemical laser was born out of the carbon dioxide laser program in the late 1960s and early 1970s. DF had been used as a chemical reaction to excite the carbon dioxide molecule through a near resonant match between one of the DF levels and one of the CO2 levels. This scheme was used by Navy researchers and their contractors, such as Pratt & Whitney Aircraft (a division of United Technologies Corporation), General Electric, Rocketdyne (now part of Pratt & Whitney), and TRW (now part of Northrop Grumman). This type of laser was called a "transfer laser" by the Navy. Eventually carbon dioxide was eliminated as an intermediary and DF was tried as a stand alone lasing medium. Very quickly, deuterium was dropped in favor of hydrogen, since it is far less costly and more readily available. However, later it was realized that HF produces infrared radiation in the 2.6 to 3.1 ?m waveband, a region of the spectrum absorbed by water vapor in the atmosphere. Interest was renewed in DF, which produces radiation in the 3.7 to 4.2 ?m band, which passes easily through the atmosphere.

As with many successful inventions, there are many fathers: Don Spencer, Jack Hinchen, George Pimentel, and Bob Freiberg, for example have been credited as being the inventor of the chemical laser, though these researchers worked independently. Clearly it depends on how you define chemical laser, and there were many steps along the way to a purely chemically pumped chemical laser.

A pulsed chemical laser was demonstrated by Jerome V. V. Kasper and George C. Pimentel by 1965. First, hydrogen chloride was pumped optically so vigorously that the molecule disassociated and then re-combined, leaving it in an excited state suitable for a laser. Then hydrogen fluoride and deuterium fluoride were demonstrated. Pimentel went on to explore a DF - CO2 transfer laser. Although this work did not produce a purely chemical continuous wave laser, it paved the way by showing the viability of the chemical reaction as a pumping mechanism for a chemical laser. Pimentel was awarded a patent for a scalable overtone HF laser (United States Patent 4,760,582) in 1971.

The continuous wave (CW) chemical HF laser was first demonstrated, and subsequently patented, by researchers at The Aerospace Corporation in El Segundo, California. This work was done in parallel with similar work at United Aircraft Research Laboratories (now United Technologies Research Center) by J.J. Hinchen. Similar work was started up very quickly by James A. Harrington at the University of Alabama in Huntsville. These devices used the mixing of adjacent streams of H2 and F2, within an optical cavity, to create vibrationally excited HF which lased. However, since the fluorine was provided by dissociation of SF6 gas with a DC electrical discharge, this also fell short of being a purely chemical laser. Later work at the US Army, US Air Force, and US Navy used only chemical reactions to drive a true chemical laser, meaning that only chemical energy from the exothermic reaction was used in producing the laser beam. This is very important for scaling up to high energy lasers for military or industrial use.web, bob, travel, a, tips, watch, essay, orange, lg, did, compare, orange, life, live, a, reasons, history, inner, free, enjoy, life, all

The analysis of the HF laser performance is complicated due to the need to simultaneously consider the fluid dynamic mixing of adjacent supersonic streams, multiple non equilibrium chemical reactions and the interaction of the gain medium with the optical cavity. The researchers at The Aerospace Corporation developed the first exact analytic (flame sheet) solution, the first numerical computer code solution and the first simplified model describing CW HF chemical laser performance.

Chemical lasers stimulated the use of wave-optics calculations for resonator analysis. This work was pioneered by E. A. Sziklas (Pratt & Whitney Aircraft) and A. E. Siegman (Stanford University.) An example of an early paper on this subject is E. A. Sziklas and A. E. Siegman, "Mode calculations in unstable resonator with flowing saturable gain. II. Fast Fourier transform method," Appl. Opt., vol. 14, pp. 1873--1889, August 1975. Part I of this was a companion paper that dealt with Hermite-Gaussian Expansion and has received little use compared with the Fourier Transform method which has now become a standard tool at United Technologies (SOQ), Lockheed-Martin (LMWOC), SAIC (ACS), Boeing (OSSIM), tOSC, MZA (Wave Train), and OPCI. Most of these companies competed for contracts to build HF and DF lasers for DARPA, the U.S. Air Force, the U.S. Army, or the U.S. Navy throughout the 1970s and 1980s. General Electric and Pratt & Whitney dropped out of the competition in the early 1980's leaving the field to Rocketdyne (now ironically part of Pratt & Whitney - although the laser organization remains today with Boeing) and TRW (now part of Northrop Grumman.)skepticism, set, stuff, empire, a, party, your, daytona, visit, a, hotels, circular, skin, amazing, a, professional, nokia, get, puerto, taking, smarter, website

Based on this work, chemical laser models were developed at SAIC by R. A. Wade, at TRW by D. Bullock, and Rocketdyne by D. A. Holmes. Of these, perhaps the most sophisticated was the CROQ code at TRW, outpacing the early work at Aerospace Corporation.

Performance

Studies led to the design of efficient high-power experimental CW HF laser devices. Power levels up to 10 kW were achieved by The Aerospace Corporation researchers. DF lasing was obtained by the substitution of D2 for H2.

The TRW Systems Group in Redondo Beach, California, subsequently received US Air Force contracts to build higher power CW HF/DF lasers. Using a scaled-up version of an Aerospace Corporation design, TRW achieved 100 kW power levels. General Electric, Pratt & Whitney, & Rocketdyne built various chemical lasers on company funds in anticipation of receiving DoD contracts to build even larger lasers. Only Rocketdyne received contracts of sufficient dollar amounts to continue competing with TRW. TRW produced the MIRACL device for the U.S. Navy that achieved megawatt power levels. The latter is believed to be the highest power continuous laser, of any type, developed to date (2007).

TRW also produced a cylindrical chemical laser (the Alpha laser) for DARPA, which had the advantage, at least on paper, of being scalable to even larger powers. However, by 1990, the interest in chemical lasers had shifted toward shorter wavelengths, and the chemical oxygen-iodine laser (COIL) gained the most interest, producing radiation at 1.315 ?m. There is a further advantage that the COIL laser generally produces single wavelength radiation, which is very helpful for forming a very well focussed beam. This type of COIL laser is used today in the ABL (Airborne Laser, the laser itself being built by Northrop Grumman) and in the ATL (Advanced Tactical Laser) produced by Boeing. Meanwhile, a lower power HF laser was used for the THEL (Tactical High Energy Laser) built in the late 1990s for the Israeli Ministry of Defense in cooperation with the U.S. Army SMDC. It holds the distinction of being the only fielded high energy laser to demonstrate effectiveness in fairly realistic tests against rockets and artillery. The MIRACL laser has demonstrated effectiveness against certain targets flown in front of it at White Sands Missile Range, but it is not configured for actual service as a fielded weapon. This may soon change with ABL and ATL, if current plans and funding hold out.maui, skin, get, planning, tips, succeed, install, relation, anchorage, make, when, a, feeling, understanding, a, going, combining, conquering, pay, pay, where, pilots

Dye Lasers

The Pulsed Dye Laser, or PDL uses a concentrated beam of light that targets blood vessels in the skin. The light is converted into heat, destroying the blood vessel while leaving the surrounding skin undamaged. The laser uses yellow light, which is very safe and does not result in any long-term skin damage. In the Baylor Clinic we own the Candela V-Beam. Another version of the PDL, the Candela Scleroplus, is located at the Texas Childrens Hospital in the operating room in order to facilitate treatment of children who need anesthesia. About the Treatment

PDL treatments usually take only a few minutes and are performed during an outpatient clinic visit. No anesthesia is required, as the machine produces a cold spray just before the laser pulse, diminishing the sensation of pain. Most patients with usually need between 1-3 treatments. Patients with port wine stains, hemangiomas and extensive rosacea may need more treatments. Improving the appearance of red scars, hypertrophic scars or keloids may take a variable number of treatments. As with any other treatment, incomplete response or recurrence may occur.

Below a Baylor Dermatology patient had multiple cherry angiomas removed with one pulsed dye laser treatment.

Side Effects

Side effects are generally minimal. The most common side effect is bruising. With the V-Beam bruising can be minimized or even eliminated. If it should occur, bruising is most pronounced in the first few days and usually clears within 3-10 days. Less common side effects include temporary pigmentary changes usually lasting a few weeks. Sunblock for one month before treatment is recommended as tanned skin blocks the laser light and results in a higher chance of side effects. Scarring is extremely rare with this laser. Aftercare

Skin care after the procedure is straightforward. A moisturizer, such as Aquaphor Ointment or Vaseline Jelly, applied 2-3 times per day will help protect the skin and speed healing. Sun protection will help minimize the chance of pigmentary changes. Makeup can be used starting on day two. If any crust forms, patients should not pick or try to remove it. Any bruising fades relatively quickly over a 3-10 day period.self, select, history, travel, choose, recognizing, motorola, write, confucianism, being, handbags, being, nokia, danish, will, examining, make, 14, finding, nokia, nokia, keeping

What is Pulsed Dye Laser?

The Pulsed Dye Laser (PDL) uses a beam of light at a specific wavelength and is used for conditions or spots on the skin which are made up of blood and blood vessels.

What is Pulsed Dye Laser used for?

The following are best treated with the PDL: spider veins on the face (single or multiple), changes of rosacea, port wine stains and some other birthmarks, small cherry angiomas on the body and some scars. We also use the PDL for some warts.

Who may have PDL?

This laser can be used for all ages and all skin types.

Is Pulsed Dye Laser safe?

PDL is a safe and effective method of treating conditions related to blood vessels. The procedure is performed by a nurse with supervision by a physician, or by a physician.

Is Pulsed Dye Laser painful?

When the laser is applied to the skin, patients feel some stinging, similar in sensation to the snapping of a small rubber band against the skin. Most patients tolerate PDL without any topical anesthetic; however, patients may apply an anesthetic cream to the area being treated for 30 minutes prior to the procedure.

How is Pulsed Dye Laser performed?

Dark glasses are worn to protect the patients eyes from the light. With the patient in a comfortable, relaxed position, the PDL light is flashed on the area of skin being treated.survival, sitges, purepoint, what, pharmaceutical, feel, introduction, put, website, find, 4, stay, fta, a, dividing, life, catch, letters, free, a, watch, value

How long is each treatment?

PDL is a quick treatment, usually requiring only a few minutes, depending on the size of the treated area.

How many treatments will I need?

The number of PDL treatments varies depending on the lesion or condition being treated. For example, a single spider vein on the face usually requires only one or two treatments, whereas a port wine stain involving the cheek, may require multiple treatments.

How will I look following each treatment?

The PDL causes bruising in the treated area which lasts from 7-14 days. Makeup may be used to cover the bruising.

How do I take care of my skin after the Pulsed Dye Laser treatment?

There is no specific care required after PDL. It is important to protect your skin from the sun using a sunscreen with an SPF of 30 or higher every day as well as sun protective clothing.

Metal-Vapor Lasers

Abstract To a laser medium comprising vapor of metal atoms A is added vapor of metal atoms B such that the difference ?E(B) between the energy value E1 (B) at the upper laser level and the energy value at the lower laser level E2 (B) as the metastable state after laser transition or fluorescent transition is substantially equal to the energy level E2 (A) of the metal atoms A at the lower laser level. Secondary collision of the metal atoms A in the E2 (A) state and the metal atoms B in the E2 (B) state with one another causes energy transition, whereby the metal atoms A do a work of exciting the metal atoms B to a resonant excitation level to lose its own energy and undergo transition to a ground level E0 (A). Consequently, the excitation lifetime at the lower laser level in the laser transition is reduced to extend the inverse population time and also the laser output pulse width so as to increase the laser conversion efficiency. Claimsnokia, your, wedding, nokia, your, deal, maine, making, 7, find, real, improve, get, buying, profit, india, about, oprah, raise, dealing, web, configure

What is claimed is:

1. A metal vapor laser, in which vapor of a metal is contained as a laser medium in a tube for exciting the metal atoms by discharge to an upper laser level (resonant excitation level) in an excited state for stimulated emission, and also in which vapor of different metal atoms is added to said laser medium, said other metal atoms being such that the difference between the energy value at the resonant excitation level and the energy level at the lower laser level as a metastable state after laser transition or fluorescent transition is substantially equal to the difference between the energy value of said metal atoms in the ground state and the energy value at the lower laser level, said metal atoms at the lower laser level being thereby caused to undergo transition to the ground state with energy transition caused by secondary collision between said metal atoms at the lower laser level and said other metal atoms at the lower laser level as the metastable state, thus extending the duration of inverse population state with the number of said metal atoms at the upper laser level being greater than the number of said metal atoms at the lower laser level. 3. A metal vapor laser, in which vapor of a metal is contained as a laser medium in a tube for exciting the metal atoms by discharge to an upper laser level (resonant excitation level) in an excited state for stimulated emission, and also in which vapor of different metal atoms is added to said laser medium, said other metal atoms being such that the difference between the energy value at the upper laser level and the energy value at the lower laser level as a metastable state after laser transition is substantially equal to the difference between the energy value of said metal atoms in the ground state and the energy value at the lower laser level and that the difference between the energy value in the ground state and the energy value at the lower laser state as the metastable state after laser transition is substantially equal to the difference between the energy value of said metal atoms at the upper laser level and the energy value at the lower laser level, said metal atoms and said other metal atoms at the lower laser level being thereby caused to undergo transition to the ground state and the upper laser level, respectively, with energy transition caused by secondary collision between said metal atoms at the lower laser level and said other metal atoms at the lower laser level as the metastable state, thus extending the duration of inverse population state with the number of said metal atoms at the upper laser level being greater than the number of said metal atoms at the lower laser level. 4. The metal vapor laser according to one of claims 1 to 3, wherein said other metal atoms added to said laser medium metal atoms are at an energy level incapable of inter-action to impede the laser oscillating operation of said laser medium metal atoms. Descriptiondont, seven, easy, sony, good, what, love, grow, 5, florida, write, virginia, virtual, entertainment, become, what, turn, a, choose, an, inclusive, 5

BACKGROUND OF THE INVENTION Among various lasers (light amplification by stimulated emission of radiation) currently finding applcations in various fields, there are metal vapor lasers, in which electric pulse discharge is brought about in a tube containing vapor of a metal such as copper, manganese, lead, gold, calcium, barium, thallium, bismuth, etc., whereby metal atoms are excited with an intensive resonant trapping phenomenon for stimulated emission. The metal vapor laser has high output and energy conversion efficiency compared to other lasers, such as solid lasers, semiconductor lasers operable at normal temperature and gas lasers except for carbon oxide lasers and iodine lasers. In such metal vapor laser, the discharge produced in the metal vapor in the tube produces resonant transition of atoms to an excited state, and some of the excited atoms undergo transition to a ground state or a metastable state by naturally emitting fluorescent light. When an inverse population state is eventually brought about so that the population of the excited atoms is higher than that of the metastable atoms, the fluorescent light acts with the excited atoms to cause stimulated emission of a new light beam. The new light beam thus generated is amplified as it is reflected by mirrors to be partly output as a laser beam from the output mirror. With a copper vapor laser using copper as a lasant, strong oscillation lines with wavelengths of 510.6 and 578.2 nanometers exist in the visible wavelength region, and as high output power of several to several hundred Watts or more can be obtained with as high energy conversion efficiency as 1 to 1.2%. Thus, this laser finds applications as an exciter for dye lasers for uranium isotope separation and so forth. In addition, research on its application has been made in medical, industrial and various other fields. In the metal vapor laser, however, while the excited metal atoms (i.e., atoms at the upper laser level) undergo transition by fluorescent light emission to energy levels of the ground state or metastable state, the populations of laser transitions are at a lower laser level, which is metastable level higher than the energy level of the ground state. While the lifetime of excitation at the upper laser level is several hundred nanoseconds, the transition from the lower laser level to the ground state is a forbidden transition, and the lifetime of excitation excitation at the lower laser level is far longer, i.e., several to several ten microseconds. This means that the state of inverse population is terminated at the commencement of the laser oscillation. In other words, the prior art metal vapor lasers are self-terminating lasers with the output pulse duration of at most several to several ten nanosconds. If the lifetime of excitation at the lower laser level could be curtailed, the inverse population time will naturally be extended to extend the laser output pulse duration. If this is realized, not only the laser conversion efficiency can be increase, but also the possibility of continuous oscillating operation can be increased. The utility of the metal vapor lasers thus can be increased so that the lasers can find effective applications in various fields. nokia, experience, infidelity, florida, athens, travel, nokia, kiawah, how, guide, internet, importance, golf, getting, golf, prince, have, got, affiliate, voip, sony, what SUMMARY OF THE INVENTION An object of the invention is to provide a metal vapor laser, which permits reducing the excitation lifetime at the lower laser level in the laser transition, thus increasing the energy conversion efficiency and possibility of continuous oscillation in the laser operation. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view for explaining a metal vapor laser according to the invention; and FIG. 2 is an energy level diagram of the metal vapor laser according to the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically shows a metal vapor laser according to the invention. Designated at 1 is a laser tube made of a ceramic or like material, at 2 a heater surrounding the laser tube 1 for heating the same to evaporate laser medium metal or metal compound A and metal or metal compound B, at 3 electrodes provided on the laser tube 1 near the ends thereof, at 4 a full reflection mirror, at 5 a semi-transmitting output mirror, and at 6 a power source. The metals or their compounds A and B contained in the laser tube 1 are heated and evaporated by the heater 2, and in this vapor a pulse discharge is caused between the electrodes 3 from the power source 6. As a result, the metal atoms A are excited, and some of them naturally undergo transition into the ground state or metastable state by naturally-emitting fluorescent light. When an inverse population state is brought about, in which the population of the excited metal atoms A is greater than the metastable state population. In this state, the fluorescent light acts with the excited metal atoms A to cause stimulated emission of new light. The new light thus emitted is amplified as it is reflected between the mirrors 4 and 5 and is partly output as a laser beam from the output mirror 5. houston, future, bottom, tips, one, dish, palace, cable, experience, aurora, booking, secret, best, tips, find, write, copy, want, become, three, making, almerimar FIG. 2 is a view showing energy levels in the metal vapor laser according to the invention. Shown on the left side are energy levels of the metal A, and on the right side are those of the added metal B. Shown at E0 (A) and E0 (B) are energy values of the metal atoms A and B in the non-excited ground state thereof. Shown at E1 (A) and E1 (B) are energy values of the metal atoms A and B at an upper level (resonant excitation level) of laser or fluorescence transition with excitation of the atoms cuased by the collision of electrons by the discharge between the electrodes 3. Shown at E2 (A) is the energy level of the metal atoms A at a lower laser level as a population of the atoms in a metastable state brought about as a result of stimulated emission from the upper laser level. Shown at E2 (B) is the energy level of the metal atoms B at a metastable level as a population of the atoms in the metastable state brought about as a result of fluorescent emission (or stimulated emission) from the upper laser level. Shown at ?E(A) is the energy value difference of the metal atoms A between E1 (A) and E2 (A), that is, the energy value necessary for exciting the metal atoms A at the lower laser level to the upper laser level. Shown at ?E(B) is the energy value difference of the metal atoms B between E1 (B) and E2 (B), that is, the energy value necessary for exciting the metal atoms B at the lower level of fluorescent or laser transition to the upper level of fluorescent or laser transition. The metal atoms A in the metastable state, i.e., at the lower laser level, and the metal atoms B at the metastable level of fluorescent or laser transition, can effect a work corresponding to the energy value of E2 (A) and E2 (B) and can consume energy efficiently for the work which substantially corresponds to this energy value. In a first embodiment of the invention, the additive metal atoms B are selected such that the energy value ?E(B) that is necessary for exciting the metal atoms B at the metastable level, i.e., the lower level of fluorescent or laser transition, to the resonant excitation level, is substantially equal to the energy value E2 (A) of the lower laser level of the metal atoms A. With the metal atoms B added to the metal atoms A as laser medium in this way, the metal atoms A undergo transition to the lower laser level by emitting laser to be held long in that state and undergo thermal motion that is determined by the laser tube temperature (of several hundred to several hundred thousand C.) while holding the energy value of E2 (A) and strongly and repeatedly colliding with nearby particles. The particles of the metal atoms B in the E2 (B) state is behaving likewise. When the metal atom A in the E2 (A) state collides with the metal atom B in the E2 (B) state, it acts to excite the metal atoms B to the resonant excitation level E1 (B), and it loses its own energy (deactivation) to undergo transition to the ground level E0 (A). As for the inverse process, it probably can be ignored so long as E2 (A) and ?E(B) are sufficiently different from the energy values of the laser transition. Consequently, the excitation lifetime at the lower laser level is reduced to suppress increase of the particle number of the metal atoms A at the lower laser level. It is thus possible to extend the duration of the inverse population state, in which the particle number at the upper laser level is greater than that at the lower laser level, that is, increase the laser output pulse width, thus increasing the laser conversion efficiency. Where the metal atoms A are manganese (Mn), the additive metal atoms B such as ?E(B).apprxeq.E2 (A) are europium (Eu), barium (Ba), lanthanum (La), molybdenum (Mo), niobium (Nb), platinum (Pt), rhenium (Re), ruthenium (Ru), titanium (Ti), thulium (Tm), etc. Where the metal atoms A are cupper (Cu), the additive metal atoms B such as ?E(B).apprxeq.E2 (A) are europium (Eu), hafnium (Hf), lanthanum (La), niobium (Nb), titanium (Ti), zirconium (Zr), molybdenum (Mo), rhenium (Re), thorium (Th), tungsten (W), etc. Where metal atoms A are arsenic (As), the additive metal atoms B such as ?E(B).apprxeq.E2 (A) are chromium (Cr), europium (Eu), iridium (Ir), lanthanum (La), manganese (Mn), platinum (Pt), rhenium (Re), scandium (Sc), tantalum (Ta), thorium (Th), zirconium (Zr), etc. Where the metal atoms A are gold (Au), the additive metal atoms B such as ?E(B).apprxeq.E2 (A) are gadolinium (Gd), germanium (Ge), hafnium (Hf), iridium (Ir), molybdenum (Mo), rhodium (Rh), tantalum (Ta), thorium (Th), zirconium (Zr), etc. Where the metal atoms A are barium (Ba), the additive metal atoms B such as ?E(B).apprxeq.E2 (A) are europium (Eu), hafnium (Hf), niobium (Nb), thorium (Th), titanium (Ti), zirconium (Zr), etc. Of course, it is possible to select as the metal atoms A and B those other than noted above so long as the condition ?E(B).apprxeq.E2 (A) is satisfied. In a second embodiment of the invention, the additive metal atoms B are selected such that the energy value E2 (B) of the metal atoms B at the metastable level is substantially equal to the energy level ?E(A) that is necessary for exciting the metal atoms A at the lower laser level to the resonant excitation level E1 (A). article, planning, caribbean, steps, hd, visit, new, spyware, htaccess, adhd With the metal atoms B added to the metal atoms A as laser medium in this way, the metal atoms A in the E2 (A) state and the metal atoms B in the E2 (B) state collide with one another, and thus the metal atoms B act to excite the metal atoms A to the resonant excitation level E1 (A) to lose energy (deactivation) and undergo transition to the ground level E0 (B). As a result, the excitation lifetime at the lower laser level is reduced to suppress increase the particle number of the metal atoms A at the lower laser level and promote increase of the particle number at the upper laser level. Thus, it is possible to extend the duration of the inverse population state, in which the particle number at the upper laser level is greater than that at the lower laser level, that is, increase the laser output pulse width, thus increasing the laser conversion efficiency. Where the metal atoms A are copper (Cu), the additive metal atoms B such as E2 (B).apprxeq.?E(A) are europium (Eu), iron (Fe), osmium (Os), rhenium (Re), tungsten (W), yttrium (Y), manganese (Mn), etc. Where the metal atoms A are manganese (Mn), the additive metal atoms B such as E2 (B).apprxeq.?E(A) are arsenic (As), antimony (Sb), etc. Where the metal atoms A are barium (Ba), the additive metal atoms B such as E2 (B).apprxeq.?E(A) are tin (Sn), manganese (Mn), etc. Where the metal atoms A are lead (Pb), the additive metal atoms B such as E2 (B).apprxeq.?E(A) are europium (Eu), iridium (Ir), etc. Where metal atoms A are thallium (Tl), the additive metal atoms B such as E2 (B).apprxeq.?E(A) are arsenic (As), molybdenum (Mo), titanium (Ti), etc. Of course it is possible to select the metal atoms A and B other than those noted above so long as the condition E2 (B).apprxeq.?E(A) is satisfied. In a third embodiment of the invention, the additive metal atoms B are selected such that the energy value ?E(B) that is necessary for exciting the metal atoms B at the metastable level to the resonant excitation level is substantilly equal to the energy level E2 (A) of the metal atoms A at the lower laser level and also that the energy level E2 (B) of the metal atoms B at the metastable level is substantially equal to the energy level ?E(A) that is necessary for exciting the metal atoms A at the lower laser level to the resonant excitation level. In other words, the metal atoms are selected such that they satisfy both the conditions of ?E(B).apprxeq.E2 (A) and E2 (B).apprxeq.?E(A). By adding the metal atoms B, which are laser medium like the laser medium metal atoms A, thus causing collision between the metal atoms A in the E2 (A) state and the metal atoms B in the E2 (B) state, the metal atoms A act to excite the metal atoms B to the resonant excitation level E1 (B) to lose energy (deactivation) and undergo transition to the ground level E0 (A), while the metal atoms B do a work of exciting the metal atoms A to the resonant excitation level E1 (A) to lose energy (deactivation) and undergo transition to the ground level E0 (B). As a result, the excitation lifetime of the metal atoms A and B at the lower laser level is reduced to suppress increase of the particle number of the metal atoms A and B at the lower laser level. On the other hand, the particle number of the atoms at the upper laser level is increased to extend the duration of the inverse population state, in which the particle number at the upper laser level is greater than that at the lower laser level, thus increasing the laser output pulse width to increase the laser conversion efficiency of both the atoms A and B. In the above embodiments, it is necessary that the energy level of the additive metal atoms B is free from such inter-action to impede the laser oscillating operation of the laser medium metal atoms A. The ratio between the vapor pressures of the metal atoms A and B is selected suitably with an aim of extending the duration of the inverse population state, but the ratio may be about one versus one. The set-up of the metal vapor laser shown in FIG. 1 is only model-wise, and where the metals or metal compounds A and B have different gasification temperatures, an exclusive heater is provided for each of the metals for controlling the vapor pressure for each metal. As has been shown, with the metal vapor laser according to the invention, to the laser medium metal atoms is added the vapor of other metal atoms capable of energy transition of the laser medium metal atoms with secondary collision, and thus the excitation lifetime at the lower laser level in the laser transition can be reduced to extend the inverse population lifetime and also the laser output pulse width. Thus, not only the laser conversion efficiency but also the possibility of continuous oscillation operation can be increased, thus permitting the utility increase of the metal vapor laser and effective applications thereof in various fields.

Solid-State Lasers

A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers (see Semiconductor laser).

Generally, the active medium of a solid-state laser consists of a glass or crystalline host material to which is added a dopant such as neodymium, chromium, erbium, or other ions. Many of the common dopants are rare earth elements, because the excited states of such ions are not strongly coupled with thermal vibrations of the crystalline lattice (phonons), and the lasing threshold can be reached at relatively low brightness of pump.

There are many hundreds of solid-state media in which laser action has been achieved, but relatively few types are in widespread use. Of these, probably the most common type is neodymium-doped YAG. Neodymium-doped glass (Nd:glass) and ytterbium-doped glasses and ceramics are used in solid-state lasers at extremely high power (terawatt scale), high energy (megajoules) multiple beam systems for inertial confinement fusion. Titanium-doped sapphire is also widely used for its broad tunability.

The first material used for lasing was ruby. Ruby lasers are still used for some applications, but are not common due to their low efficiency. Er:YAG lasers lase in the mid-infrared.

Solid state lasing media are typically optically pumped, using either a flashlamp or arc lamp, or by laser diodes. Diode-pumped solid-state lasers tend to be much more efficient, and have become much more common as the cost of high power semiconductor lasers has decreased.

Solid-state lasers are being developed as optional weapons for the F-35 Lightning II, and are reaching near-operational status.

A fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.

Applications of fiber lasers include material processing, telecommunications, spectroscopy, and medicine. The advantage of the fiber laser is that the light is already coupled into the fiber and can be easily delivered to a movable focusing element. Such a coupling is important for laser cutting or laser welding or laser folding of metals and polymers.

Unlike most other types of lasers, the laser cavity in fiber laser is constructed monolithically by fusion splicing the different types of fibers; most notably fiber Bragg gratings replace here conventional dielectric mirrors to provide optical feedback. To pump fiber lasers, semiconductor laser diodes or other fiber lasers are used almost exclusively. Fiber lasers can have active regions several kilometers long, and can provide very high optical gain. They can support kilowatt levels of continuous output power because the fiber's high surface area to volume ratio allows efficient cooling. The fiber waveguiding properties reduce or remove completely thermal distortion of the optical path thus resulting in typically diffraction-limited high-quality optical beam. Fiber lasers also feature compact layout compared to rod or gas lasers of comparable power, as the fiber can be bent to small diameters and coiled. Other advantages include high vibrational stability, extended lifetime and maintenance-free turnkey operation.

Many high-power fiber lasers are based on double-clad fiber. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer. The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher power beam than could otherwise be made to propagate in it, and allows the conversion of pump light with relatively low brightness into a much higher-brightness signal. As a result, fiber lasers and amplifiers are occasionally referred to as "brightness converters."

There is important question about shape of the double-clad fiber; fiber with circular symmetry seems to be worst possible design . The design should allow core to be small enough to convert the pump to only few (or even one) mode, to provide the cladding large enough to allow the confinement of pump and to allow most of pump to be used in the core in relatively short piece of the fiber.

Another type of fiber laser is the fiber disk laser. In such a laser, the pump is not confined in the cladding of the fiber (as in the double-clad fiber), bit goes across the roped core many times, as it is coiled. This configuration is suitable for the power scaling, while anyway many sources of pump are used, and there is no reason to make all the pump propagate along all the length of the fiber

A disk laser or active mirror (Fig.1.) is a type of solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. Despite their name, disk lasers do not have to be circular; other shapes have also been tried.

Disk lasers should not be confused with Laserdiscs, which are a disk-shaped optical storage medium.

Disk lasers should not be confused with Fiber laser disks, which are a disk-shaped coils of a fiber lasers, pumped from side.

Initially, disk lasers were called active mirrors, because the gain medium of a disk laser is essentially an optical mirror with reflection coefficient greater than unity. An active mirror is a thin disk-shaped double-pass optical amplifier.

The first active mirrors were developed in the Laboratory for Laser Energetics (USA) and the Institute for Laser Science (Japan) . Then, the concept was developed in various research groups, in particular, the University of Stuttgart (Germany) and Tokyo Institute of Technology (Japan) for Yb:doped glasses and semiconductor laser materials.

In the disk laser, the heat sink does not have to be transparent, so, it can be extremely efficient even at large transverse size of the device (Fig.1.). The increase in size may allow the power scaling to many kilowatts without significant modifications.

The power of such lasers is limited not only by the power of pump available, but also by overheating, amplified spontaneous emission (ASE) and the background round-trip loss. To avoid overheating, the size should be increased at the power scaling. Then, to avoid strong losses due to the exponential growth of the ASE, the transverse-trip gain cannot be large. This requires to reduce the gain ; this gain is determined by the reflectivity of the output coupler and thickness . The round-trip gain should remain larger than the round-trip loss (the difference determines the part of the energy of the optical field, which can be outputed from the laser cavity at each round-trip). The reduction of gain , at given round-trip loss , requires to increase the thickness h. Then, at some critical size, the disk becomes too thick and cannot be pumped above the threshold without overheating.

Some features of the power scaling can revealed from a simple model. Let be the saturation intensity , of the medum, be the ratio of frequencies, be the thermal loading parameter. The key parameter determines the maximal power of the disk laser. The correspnding optimal thickness can be estimated with . The corresponding optimal size . Roughly, the round-trip loss should scale inversely proportionally to cubic root of the power required.

An additional issue is the efficient delivery of pump. At the low round-trip gain, the single-pass absorption of pump is also low. Therefore, the recycling of pump is required for the efficient operation. (See the additional mirror M at the left-hands side of figure 2.) For the power scaling, the medium should be optically thin, and many passes of pump required; the lateral delivery of pump also might be a possible solution.

Anti-ASE cap

In order to reduce the impact of ASE, an anti-ASE cap consisting of undoped material on the surface of a disk laser has been suggested. Such a cap allows spontaneously emitted photons to escape from the active layer and prevents them from resonating in the cavity. This could allow an order of magnitude increase in the maximum power achievable by a disk laser.

Semiconductor Lasers

A hybrid silicon laser is a semiconductor laser fabricated from both silicon and group III-V semiconductor materials. The hybrid silicon laser was developed to address the lack of a silicon laser to enable fabrication of low-cost, mass-producible silicon optical devices. The hybrid approach takes advantage of the light-emitting properties of III-V semiconductor materials combined with the process maturity of silicon to fabricate electrically driven lasers on a silicon wafer that can be integrated with other silicon photonic devices.

Physics

A hybrid silicon laser is an optical source that is fabricated from both silicon and group III-V semiconductor materials (e.g. Indium(III) phosphide, Gallium(III) arsenide). It comprises a silicon waveguide fused to an active, light-emitting, III-V epitaxial semiconductor wafer. The III-V epitaxial wafer is designed with different layers such that the active layer can emit light when it is excited either by shining light, e.g. a laser onto it; or by passing electricity through it. The emitted light from the active layer couples into the silicon waveguide due to their close proximity (<130 nm separation) where it can be guided to reflect off mirrors at the end of the silicon waveguide to form the laser cavity.

Fabrication

The hybrid silicon laser is fabricated by a technique called plasma assisted wafer bonding. Silicon waveguides are first fabricated on a silicon on insulator (SOI) wafer. This SOI wafer and the un-patterned III-V wafer are then exposed to an oxygen plasma before being pressed together at a low (for semiconductor manufacturing) temperature of 300C for 12hours. This process fuses the two wafers together. The III-V wafer is then etched into mesas to expose electrical layers in the epitaxial structure. Metal contacts are fabricated on these contact layers allowing electrical current to flow to the active region.

Uses

Intel suggests this light source could be used for optical communications when integrated with silicon photonics. Silicon manufacturing and fabrication is widely used in the electronic industry to mass-produce low-cost electronic devices. Silicon photonics uses these same electronic manufacturing technologies to make low cost integrated optical devices. One issue with using silicon for an optical device is that silicon is a poor light emitter and cannot be used to make an electrically pumped laser. This means that lasers have first to be fabricated on a separate III-V semiconductor wafer before being individually aligned to each silicon device, in a process that is both costly and time-consuming, limiting the total number of lasers that can be used on a silicon photonic circuit.

By using this wafer bonding technique many hybrid silicon lasers can be fabricated simultaneously on a silicon wafer, all aligned to the silicon photonic devices. Potential uses cited in the references below include fabricating many, possibly 100s of hybrid silicon lasers on a die and using silicon photonics to combine them together to form high bandwidth optical links for personal computers, servers or back planes.

Vertical-cavity surface-emitting laser

The vertical-cavity surface-emitting laser (VCSEL; [v'?xl]) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer.

There are several advantages to producing VCSELs when compared with the production process of edge-emitting lasers. Edge-emitters cannot be tested until the end of the production process. If the edge-emitter does not work, whether due to bad contacts or poor material growth quality, the production time and the processing materials have been wasted. VCSELs however, can be tested at several stages throughout the process to check for material quality and processing issues. For instance, if the vias have not been completely cleared of dielectric material during the etch, an interim testing process will flag that the top metal layer is not making contact to the initial metal layer. Additionally, because VCSELs emit the beam perpendicular to the active region of the laser as opposed to parallel as with an edge emitter, tens of thousands of VCSELs can be processed simultaneously on a three inch Gallium Arsenide wafer. Furthermore, even though the VCSEL production process is more labor and material intensive, the yield can be controlled to a more predictable outcome.

The laser resonator consists of two distributed Bragg reflector (DBR) mirrors parallel to the wafer surface with an active region consisting of one or more quantum wells for the laser light generation in between. The planar DBR-mirrors consist of layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities above 99%. High reflectivity mirrors are required in VCSELs to balance the short axial length of the gain region.

In common VCSELs the upper and lower mirrors are doped as p-type and n-type materials, forming a diode junction. In more complex structures, the p-type and n-type regions may be buried between the mirrors, requiring a more complex semiconductor process to make electrical contact to the active region, but eliminating electrical power loss in the DBR structure.

In laboratory investigation of VCSELs using new material systems, the active region may be pumped by an external light source with a shorter wavelength, usually another laser. This allows a VCSEL to be demonstrated without the additional problem of achieving good electrical performance; however such devices are not practical for most applications.

VCSELs for wavelengths from 650 nm to 1300 nm are typically based on gallium arsenide (GaAs) wafers with DBRs formed from GaAs and aluminium gallium arsenide (AlxGa(1-x)As). The GaAs/AlGaAs system is favored for constructing VCSELs because the lattice constant of the material does not vary strongly as the composition is changed, permitting multiple lattice matched epitaxial layers to be grown on a GaAs substrate. However, the refractive index of AlGaAs does vary relatively strongly as the Al fraction is increased, minimizing the number of layers required to form an efficient Bragg mirror compared to other candidate material systems. Furthermore, at high aluminum concentrations, an oxide can be formed from AlGaAs, and this oxide can be used to restrict the flow of current in a VCSEL, enabling very low threshold currents.

Recently the two main methods of restricting the current flow in a VCSEL were characterized by two types of VCSELs: Ion Implanted VCSELs and Oxide VCSELs.

In the early 1990s, telecommunications companies tended to favor Ion Implanted VCSELs. Ions, (often hydrogen ions, H+), were implanted into the VCSEL structure everywhere except the aperture of the VCSEL, destroying the lattice structure around the aperture, thus inhibiting the current flow. In the mid to late 1990s, companies moved towards the technology of oxide VCSELs. The current flow is confined in an oxide VCSEL by oxidizing the material around the aperture of the VCSEL. A high content aluminum layer that is grown within the VCSEL structure is the layer that is oxidized. Oxide VCSELs also often employ the ion implant production step. As a result in the oxide VCSEL, the current path is confined by the ion implant and the oxide aperture.

The initial acceptance of oxide VCSELs was plagued with concern about the apertures "popping off" due to the strain and defects of the oxidation layer. However, after much testing, the realibilty of the structure has proven to be robust. As stated in one study by Hewlett Packard on oxide VCSELs, "The stress results show that the activation energy and the wearout lifetime of oxide VCSEL are similar to that of implant VCSEL emitting the same amount of output power."

A production concern also plagued the industry when moving the oxide VCSELs from research and development to production mode. The oxidation rate of the oxide layer was highly dependent on the Aluminum content. Any slight variation in Aluminum would change the oxidation rate sometimes resulting in apertures that were either too big or too small to meet the specification standards.

Longer wavelength devices, from 1300 nm to 2000 nm, have been demonstrated with at least the active region made of indium phosphide. VCSELs at even higher wavelengths are experimental and usually optically pumped. 1310 nm VCSELs are desirable as the dispersion of silica-based optical fiber is minimal in this wavelength range.

Quantum cascade lasers (QCLs) are semiconductor lasers that emit in the mid- to far-infrared portion of the electromagnetic spectrum and were first demonstrated by Jerome Faist, Federico Capasso, Deborah Sivco, Carlo Sirtori, Albert Hutchinson, and Alfred Cho at Bell Laboratories in 1994.

Unlike typical interband semiconductor lasers that emit electromagnetic radiation through the recombination of electronhole pairs across the material band gap, QCLs are unipolar and laser emission is achieved through the use of intersubband transitions in a repeated stack of semiconductor superlattices, an idea first proposed in the paper "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice" by R.F. Kazarinov and R.A. Suris in 1971.

Intersubband vs. interband transitions

Within a bulk semiconductor crystal, electrons may occupy states in one of two continuous energy bands - the valence band, which is heavily populated with low energy electrons and the conduction band, which is sparsely populated with high energy electrons. The two energy bands are separated by an energy band gap in which there are no permitted states available for electrons to occupy. Conventional semiconductor laser diodes generate light by a single photon being emitted when a high energy electron in conduction band recombines with a hole in the valence band. The energy of the photon and hence the emission wavelength of laser diodes is therefore determined by the band gap of the material system used.

A QCL however does not use bulk semiconductor materials in its optically active region. Instead it comprises a periodic series of thin layers of varying material composition forming a superlattice. The superlattice introduces a varying electric potential across the length of the device, meaning that there is a varying probability of electrons occupying different positions over the length of the device. This is referred to as one-dimensional multiple quantum well confinement and leads to the splitting of the band of permitted energies into a number of discrete electronic subbands. By suitable design of the layer thicknesses it is possible to engineer a population inversion between two subbands in the system which is required in order to achieve laser emission. Since the position of the energy levels in the system is primarily determined by the layer thicknesses and not the material, it is possible to tune the emission wavelength of QCLs over a wide range in the same material system.

Additionally, in semiconductor laser diodes, electrons and holes are annihilated after recombining across the band gap and can play no further part in photon generation. However in a unipolar QCL, once an electron has undergone an intersubband transition and emitted a photon in one period of the superlattice, it can tunnel into the next period of the structure where another photon can be emitted. This process of a single electron causing the emission of multiple photons as it traverses through the QCL structure gives rise to the name cascade and makes a quantum efficiency of greater than unity possible which leads to higher output powers than semiconductor laser diodes.

Operating principles

Rate equations

QCLs are typically based upon a three-level system. Assuming the formation of the wavefunctions is a fast process compared to the scattering between states, the time independent solutions to the Schrodinger equation may be applied and the system can be modelled using rate equations. Each subband contains a number of electrons ni (where i is the subband index) which scatter between levels with a lifetime ?if (reciprocal of the scattering rate Wif), where i and f are the initial and final subband indices. Assuming that no other subbands are populated, the rate equations for the three laser levels are given by:

In the steady state, the time derivatives are equal to zero and Iin = Iout = I. The general rate equation for electrons in subband i of an N level system is therefore:

,

Under the assumption that absorption processes can be ignored (which is valid at low temperatures), the middle rate equation gives

Therefore if ?32 > ?21 (i.e. W21 > W32) then n3 > n2 and a population inversion will exist. The population ratio is defined as

If all N steady-state rate equations are summed, the right hand side becomes zero, meaning that the system is underdetermined, and it is possible only to find the relative population of each subband. An additional equation is required to set the total number of carriers equal to the total number of dopant ions:

Active region designs

The scattering rates are tailored by suitable design of the layer thicknesses in the superlattice which determine the electron wave functions of the subbands. The scattering rate between two subbands is heavily dependent upon the overlap of the wave functions and energy spacing between the subbands. The figure shows the wave functions in a three quantum well (3QW) QCL active region and injector.

In order to decrease W32, the overlap of the upper and lower laser levels is reduced. This is often achieved through designing the layer thicknesses such that the upper laser level is mostly localised in the left-hand well of the 3QW active region, while the lower laser level wave function is made to mostly reside in the central and right-hand wells. This is known as a diagonal transition. A vertical transition is one in which the upper laser level is localised in mainly the central and right-hand wells. This increases the overlap and hence W32 which reduces the population inversion, but it increases the strength of the radiative transition and therefore the gain.

In order to increase W21, the lower laser level and the ground level wave functions are designed such that they have a good overlap and to increase W21 further, the energy spacing between the subbands is designed such that it is equal to the longitudinal optical (LO) phonon energy (36 meV in GaAs) so that resonant LO phonon-electron scattering can quickly depopulate the lower laser level.

Material systems

The first QCL was fabricated in the InGaAs/InAlAs material system lattice-matched to an InP substrate. This particular material system has a conduction band offset (quantum well depth) of 520 meV. These InP-based devices have reached very high levels of performance across the mid-infrared spectral range, achieving high power, above room-temperature, continuous wave emission.

In 1998 GaAs/AlGaAs QCLs were demonstrated by Sirtori et al proving that the QC concept is not restricted to one material system. This material system has a varying quantum well depth depending on the aluminium fraction in the barriers. Although GaAs-based QCLs have not matched the performance levels of InP-based QCLs in the mid-infrared, they have proven to be very successful in the Terahertz region of the spectrum.

The short wavelength limit of QCLs is determined by the depth of the quantum well and recently QCLs have been developed in material systems with very deep quantum wells in order to achieve short wavelength emission. The InGaAs/AlAsSb material system has quantum wells 1.6 eV deep and has been used to fabricate QCLs emitting at 3 ?m. InAs/AlSb QCLs have quantum wells 2.1 eV deep and electroluminescence at wavelengths as short as 2.5 ?m has been observed.

QCLs may also allow laser operation in materials traditionally considered to have poor optical properties. Indirect bandgap materials such as silicon have minimum electron and hole energies at different momentum values. For interband optical transitions, carriers change momentum through a slow, intermediate scattering process, dramatically reducing the optical emission intensity. Intersubband optical transitions however, are independent of the relative momentum of conduction band and valence band minima and theoretical proposals for Si/SiGe quantum cascade emitters have been made.

Emission wavelengths This short section requires expansion.

QCLs currently cover the wavelength range from 2.7 - 250 ?m (and extends to 355 ?m with the application of a magnetic field).

Optical waveguides This short section requires expansion.

The linewidth of QCLs is narrowed by incorporating a waveguide into the design, thus amplifying one particular wavelength. Often a structure called a Distributed Bragg reflector (DBR) is built on top of the laser crystal to prevent it from emitting at other than the desired wavelength. Distributed Bragg reflector lasers, characterized by the DBR being outside of the gain medium, should not be confused with distributed feedback (DFB) lasers where the DBR is incorporated into the gain medium.

Growth This short section requires expansion.

The alternating layers of the two different semiconductors which form the quantum heterostructure are grown on to a substrate using molecular beam epitaxy (MBE) or metalorganic vapour phase epitaxy (MOVPE).

Applications This short section requires expansion.

The laser's high optical power output, tuning range and room temperature operation make it useful for spectroscopic applications like the remote sensing of environmental gases and pollutants in the atmosphere. It may eventually be used for vehicular cruise control in conditions of poor visibility, collision avoidance radar, industrial process control, and medical diagnostics such as breath analyzers. QCLs are also being used to study plasma chemistry.

The 3 - 5 ?m atmospheric window is also coverable by QCLs paving the way for high-speed, free-space optical communication links which could prove useful for providing high-speed internet access in built up areas as expensive optical fibre installation can be avoided.

Quantum cascade lasers were first commercialized in 2004.

Other Types Of Lasers

A free electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam. Unlike gas, liquid, or solid-state lasers such as diode lasers, in which electrons are excited in bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium which move freely through a magnetic structure, hence the term free electron. The free electron laser has the widest frequency range of any laser type, and can be widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to ultraviolet, to soft X-rays.

Beam creation

To create a FEL, a beam of electrons is accelerated to relativistic speeds. The beam passes through an FEL oscillator in the form of a periodic, transverse magnetic field, produced by arranging magnets with alternating poles within a laser cavity along the beam path. This array of magnets is sometimes called an undulator, or a "wiggler", because it forces the electrons in the beam to assume a sinusoidal path. The acceleration of the electrons along this path results in the release of a photon (synchrotron radiation). Since the electron motion is in phase with the field of the light already emitted, the fields add together (coherently) and since light intensity is dependant upon the square of the field the light intensity is increased. Instabilities in the electron beam, which result from the interactions of the oscillations of electrons in the undulators and the radiation they emit, leads to a bunching of the electrons which continue to radiate in phase with each other in contrast to conventional undulators where the electrons radiate independently. The wavelength of the light emitted can be readily tuned by adjusting the energy of the electron beam or the magnetic field strength of the undulators.

Accelerators

Today, a free electron laser requires the use of an electron accelerator with its associated shielding, as accelerated electrons are a radiation hazard. These accelerators are typically powered by klystrons, which require a high voltage supply. Usually, the electron beam must be maintained in a vacuum which requires the use of numerous pumps along the beam path. Free electron lasers can achieve very high peak powers. Their tunability makes them highly desirable in several disciplines, including medical diagnosis and non-destructive testing.

X-ray FELs

The lack of suitable mirrors in the extreme ultraviolet and x-ray regimes prevent the operation of an FEL oscillator; consequently, there must be suitable amplification over a single pass of the electron beam through the undulator to make the FEL worthwhile. X-ray free electron lasers utilise long undulators. The underlying principle of the intense pulses from the X-ray laser lies in the principle of Self-Amplified Stimulated-Emission which leads to the microbunching of the electrons. Initially all electrons are evenly distributed but through the interaction of the oscillating electrons with the emitted radiation, the electrons drift into microbunches separated by a distance equal to one wavelength of the radiation. Through this arrangement, all the radiation emitted can reinforce itself perfectly whereby wave crests and wave troughs are always superimposed on one another in the best possible way. This is what leads to the high intensities and the laser-like properties. Examples of facilities operating on the SASE FEL principle include the Free electron LASer in Hamburg (FLASH), the Linac Coherent Light Source (LCLS), currently being built at the Stanford Linear Accelerator, and the European x-ray free electron laser.

One problem with SASE FELs is the lack of temporal coherence due to a noisy startup process. To avoid this one can "seed" an FEL with a laser, produced by more conventional means, tuned to the resonance of the FEL. This results in coherent amplification of the input signal such that the output laser quality is characterized by the seed. This method becomes a problem at x-ray wavelengths because of the lack of conventional x-ray lasers.

Medical applications

At the 2006 annual meeting of the American Society for Laser Medicine and Surgery (ASLMS), Dr. Rox Anderson of the Wellman Laboratory of Photomedicine of Harvard Medical School and Massachusetts General Hospital reported on the possible medical application of the free electron laser in melting fats without harming the overlying skin. It was reported that at infrared wavelengths, water in tissue was heated by the laser, but at wavelengths corresponding to 915, 1210 and 1720 nm, subsurface lipids were differentially heated more strongly than water. The possible applications of this selective photothermolysis (heating tissues using light) include the selective destruction of sebum lipids to treat acne, as well as targeting other lipids associated with cellulite and body fat as well as fatty plaques that form in artieries which can help treat atherosclerosis and heart disease.

Military applications

FEL technology is considered by the US Navy as a good candidate for an anti-missile directed-energy weapon. Significant progress is being made in increasing FEL power levels (already at 10 kW, as demonstrated at the JLab FEL) and it should be possible to build compact multi-megawatt class FEL lasers

List

• Gas Lasers
• Chemical Lasers
• Dye Lasers
• Metal-Vapor Lasers
• Solid-State Lasers
• Semiconductor Lasers
• Other Types Of Lasers