Aruby laser is asolid-state laser that uses asynthetic ruby crystal as itsgain medium. The first workinglaser was a ruby laser made byTheodore H. "Ted" Maiman atHughes Research Laboratories on May 16, 1960.[1][2]
Ruby lasers produce pulses of coherentvisible light at awavelength of 694.3 nm, which is a deep red color. Typical ruby laser pulse lengths are on the order of amillisecond.
A ruby laser most often consists of a ruby rod that must bepumped with very high energy, usually from aflashtube, to achieve apopulation inversion. The rod is often placed between two mirrors, forming anoptical cavity, which oscillate the light produced by the ruby'sfluorescence, causingstimulated emission. Ruby is one of the few solid state lasers that produce light in the visible range of the spectrum, lasing at 694.3 nanometers, in a deep red color, with a very narrow linewidth of 0.53 nm.[3]
The ruby laser is athree level solid state laser. Theactive laser medium (laser gain/amplification medium) is asynthetic ruby rod that is energized throughoptical pumping, typically by axenon flashtube. Ruby has very broad and powerful absorption bands in the visual spectrum, at 400 and 550 nm, and a very long fluorescence lifetime of 3 milliseconds. This allows for very high energy pumping, since the pulse duration can be much longer than with other materials. While ruby has a very wide absorption profile, its conversion efficiency is much lower than other mediums.[3]
In early examples, the rod's ends had to be polished with great precision, such that the ends of the rod were flat to within a quarter of a wavelength of the output light, and parallel to each other within a few seconds of arc. The finely polished ends of the rod weresilvered; one end completely, the other only partially. The rod, with its reflective ends, then acts as aFabry–Pérot etalon (or aGires-Tournois etalon). Modern lasers often use rods withantireflection coatings, or with the ends cut and polished atBrewster's angle instead. This eliminates the reflections from the ends of the rod. Externaldielectric mirrors then are used to form the optical cavity.Curved mirrors are typically used to relax the alignment tolerances and to form a stable resonator, often compensating forthermal lensing of the rod.[3][4]
Ruby also absorbs some of the light at its lasing wavelength. To overcome this absorption, the entire length of the rod needs to be pumped, leaving no shaded areas near the mountings. The active part of the ruby is thedopant, which consists ofchromium ions suspended in asynthetic sapphire crystal. The dopant often comprises around only 0.05% of the crystal, but is responsible for all of the absorption and emission of radiation. Depending on the concentration of the dopant, synthetic ruby usually comes in either pink or red.[3][4]
One of the first applications for the ruby laser was in rangefinding. By 1964, ruby lasers with rotating prismq-switches became the standard for militaryrangefinders, until the introduction of more efficientNd:YAG rangefinders a decade later. Ruby lasers were used mainly in research.[5] The ruby laser was the first laser used to optically pump tunabledye lasers and is particularly well suited to excite laser dyes emitting in the near infrared.[6] Ruby lasers are rarely used in industry, mainly due to low efficiency and low repetition rates. One of the main industrial uses is drilling holes throughdiamond, because ruby's high-powered beam closely matches diamond's broad absorption band (the GR1 band) in the red.[5][7]
Ruby lasers have declined in use with the discovery of better lasing media. They are still used in a number of applications where short pulses of red light are required. Holographers around the world produceholographic portraits with ruby lasers, in sizes up to a meter square. Because of its high pulsed power and good coherence length, the red 694 nm laser light is preferred to the 532 nm green light offrequency-doubledNd:YAG, which often requires multiple pulses for large holograms.[8] Manynon-destructive testing labs use ruby lasers to create holograms of large objects such as aircraft tires to look for weaknesses in the lining. Ruby lasers were used extensively intattoo andhair removal, but are being replaced byalexandrite andNd:YAG lasers in this application.
The ruby laser was the first laser to be made functional. Built by Theodore Maiman in 1960, the device was created out of the concept of an "optical maser," amaser that could operate in the visual or infrared regions of the spectrum.
In 1958, after the inventor of the maser,Charles Townes, and his colleague,Arthur Schawlow, published an article in thePhysical Review regarding the idea of optical masers, the race to build a working model began. Ruby had been used successfully in masers, so it was a first choice as a possible medium. While attending a conference in 1959, Maiman listened to a speech given by Schawlow, describing the use of ruby as a lasing medium. Schawlow stated that pink ruby, having a lowest energy-state that was too close to the ground-state, would require too muchpumping energy for laser operation, suggesting red ruby as a possible alternative. Maiman, having worked with ruby for many years, and having written a paper on ruby fluorescence, felt that Schawlow was being "too pessimistic." His measurements indicated that the lowest energy level of pink ruby could at least be partially depleted by pumping with a very intense light source, and, since ruby was readily available, he decided to try it anyway.[9][10]
Also attending the conference wasGordon Gould. Gould suggested that, by pulsing the laser, peak outputs as high as a megawatt could be produced.[11]
As time went on, many scientists began to doubt the usefulness of any color ruby as a laser medium. Maiman, too, felt his own doubts, but, being a very "single-minded person," he kept working on his project in secret. He searched to find a light source that would be intense enough to pump the rod, and an elliptical pumping cavity of high reflectivity, to direct the energy into the rod. He found his light source when a salesman from General Electric showed him a few xenonflashtubes, claiming that the largest could ignite steel wool if placed near the tube. Maiman realized that, with such intensity, he did not need such a highly reflective pumping cavity, and, with the helical lamp, would not need it to have an elliptical shape. Maiman constructed his ruby laser at Hughes Research Laboratories, in Malibu, California.[12] He used a pink ruby rod, measuring 1 cm by 1.5 cm, and, on May 16, 1960, fired the device, producing the first beam of laser light.[13]
Theodore Maiman's original ruby laser is still operational.[14] It was demonstrated on May 15, 2010, at a symposium co-hosted inVancouver, British Columbia by the Dr. Theodore Maiman Memorial Foundation andSimon Fraser University, where Dr. Maiman was adjunct professor at the School of Engineering Science. Maiman's original laser was fired at a projector screen in a darkened room. In the center of a white flash (leakage from the xenon flashtube), a red spot was briefly visible.
The ruby lasers did not deliver a single pulse, but rather delivered a series of pulses, consisting of a series of irregular spikes within the pulse duration. In 1961, R.W. Hellwarth invented a method ofq-switching, to concentrate the output into a single pulse.[15]
In 1962,Willard Boyle, working atBell Labs, produced the first continuous output from a ruby laser. Unlike the usual side-pumping method, the light from a mercury arc lamp was pumped into the end of a very small rod, to achieve the necessary population inversion. The laser did not emit acontinuous wave, but rather a continuous train of pulses, giving scientists the opportunity to study the spiked output of ruby.[16] The continuous ruby laser was the first laser to be used in medicine. It was used by Leon Goldman, a pioneer inlaser medicine, for treatments such as tattoo removal, scar treatments, and to induce healing. Due to its limits in output power, tunability, and complications in operating and cooling the units, the continuous ruby laser was quickly replaced with more versatiledye,Nd:YAG, andargon lasers.[17]
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