Bulk solid-state lasers can generate much higher powers from a single laser in CW operation and even more peak power simply because of the larger areas and volumes involved. For efficient laser operation, it is important that 1 a large fraction of incident pump light is absorbed in the material and 2 the spatial overlap is high between the large pumped volume and the laser beam either generated or. For the simple design in Figure , the latter is accomplished by making an optical cavity out of the rod itself. Generally, all of the energy difference between pump and laser photons is taken up via nonradiative processes that generate phonons, and the measurable effect is a rise in the laser material temperature.
Unless the heat is removed, laser action will eventually stop from a variety of high-temperature effects, the most extreme being melting of the material. For the laser in Figure , heat from the rod could be removed by flowing air around the rod or, more commonly, by placing the rod inside a transparent cylinder and flowing a transparent liquid, usually water, around the outer rod surface. For the case of the rod in Figure , the center of the rod will be hotter than the cooled outside surface. All materials expand or in a few cases contract with increased temperature, and the temperature difference inside the rod leads to different levels of expansion, which in turn create stress in the material.
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Ultimately, at sufficiently high levels of power the material may fracture from this stress. Below the fracture limit, the optical properties of the material are changed. The stress changes the refractive index and polarization properties of the material through the stress-optic effect. More important, since the refractive index of materials changes with temperature, the gradient in temperature leads to a nonuniform refractive index.
The net effect of the changes is to change the properties of the laser beam passing through the material. The changes are pump-power dependent. Many of the engineering challenges and performance limitations of bulk solid-state lasers center on the issues described above. The effect that the choice of host material has on these issues is considered first. The desirable host material properties are these:. Availability in a size suited for the pumping, lasing configuration; low optical loss at the pump and laser wavelengths; ability to support active-ion doping levels high enough to absorb the pump light in the desired volume.
High mechanical strength to avoid fracture at the desired power level, high thermal conductivity, and low thermal expansion coefficient. High thermal conductivity, small change in refractive index with temperature, and small change in refractive index and polarization properties with stress. Particularly for the rare earth dopants, several hundred different host crystals have demonstrated laser operation, but only a handful have found widespread use because they best satisfy all of the requirements listed above:.
A member of the very large family of garnet-structured crystals, YAG emerged from research in the s as having the best combination of properties for hosting rare earth materials, including high thermal conductivity and good mechanical strength. Reportedly, at one point the production of YAG in New Jersey represented the largest single use of electrical power in that state.
Recent advances in ceramic YAG, which is made up of randomly oriented, micrometer-sized single crystals, have allowed fabrication of much larger materials. The successful use of ceramic materials for lasers is a recent development that required significant advances in technology to essentially eliminate the scattering of light associated with the boundaries between the microcrystals making up the ceramic. When it is multiplied by the volume concentration of laser ions, the resultant value provides a measure of the pump absorption or laser gain per unit length. Once the ion is placed in a crystal, the surrounding ions create a lower symmetry than free space, mix in other electronic states to the electronic wave functions for the ion, and greatly enhance the cross section.
This process is sometimes called activation of the transitions. For most of the host crystals for rare earths, the laser cross sections fall in a fairly limited range of values.
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The crystal YVO 4 and some related compounds have unusually low-symmetry surroundings for rare earths and induce cross sections about an order of magnitude higher than other hosts. By a fortunate coincidence, the prime Nd: YVO 4 wavelength of 1, nm overlaps that of Nd:YAG, making it possible to build specialized hybrid lasers for some applications. The fluoride-based material YLF has several unique properties, including much weaker thermal distortion effects than oxide materials and natural birefringence see below , along with favorable laser properties for some rare earth dopants.
The main drawback is a reduced mechanical strength compared to most oxides. Sapphire has thermal and mechanical properties superior to those of all other host crystals, and when undoped it finds applications ranging from high-end watch crystals to tank windows. Unfortunately, sapphire does not accommodate rare earths at any reasonable doping level, and so after ruby it found little use in solid-state lasers until the demonstration of the Ti:sapphire broadly tunable laser.
The material ZnSe and similar II-VI compounds have in the past found widespread use for IR-transmitting windows, with acceptable levels of mechanical strength and hardness. Chemical vapor deposition CVD techniques have been developed to grow large slabs of polycrystalline i. The low phonon energy of the material minimizes nonradiative decay of the longer-wavelength transitions characteristic of divalent ions in the tetragonal environment.
Glass materials represent a disordered, or amorphous, arrangement of atoms. The fine-detailed structure in the manifold levels for rare earth ions in glasses is thus smeared out inhomogeneously broadened when one measures their absorption and emission properties. This allows glass-based materials to have much broader absorption bands for pump light and broader continuous tuning ranges than crystal and ceramic hosts.
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On the other hand, the effective cross sections are lowered as well, reducing the gain and length for laser action. A big advantage for glass hosts is that they can be cast from liquid form into arbitrary and very large shapes to allow generation of very high energy pulses. The significant disadvantage of glasses for bulk lasers is their poor thermal conductivity, a direct result of their amorphous nature. A 4-MJ pulse in 24 hours leads to an average power of around 50 W, less than that of a 1-cm-long diode laser.
The choice of pumping devices lamps or diode lasers plays an important role and is discussed in detail in the next section. The properties of the laser output beam are set by the resonator configuration, as modified by any thermo-optic effects in the laser material. For materials shaped like simple cylindrical rods and cooled through the cylindrical surface, the temperature distribution can be approximated by a parabolic profile transverse to the beam , peaking in the center of the rod. The corresponding optical effect is to make a rod behave, to a first level of. The lens is positive for the majority of materials, which exhibit a positive change in the refractive index with temperature.
Additional optical distortions come about from the stress-optic effect, which converts the stress in the material from the thermal gradient into a change in refractive index. In rod configurations the stress-optic effect can also be approximated as creating a simple lens. For crystals that are naturally birefringent, such as vanadate and sapphire, the natural birefringence typically dominates stress birefringence, and one can avoid any change in, say, a linearly polarized beam by aligning the polarization with one of the crystal axes. Isotropic materials such as YAG and all glasses are, however, susceptible to stress-induced birefringence, and this is an issue since linearly polarized light is required to drive many of the nonlinear processes discussed below.
In theory, it should be possible with any bulk solid-state laser to effectively generate a large fraction of power from a single, diffraction-limited, transverse mode of the resonator, since the diameter of the single mode can be made arbitrarily large by the choice of cavity design.
To get around the limits of cavities, one can design a relatively low-power laser oscillator optimized for single-transverse-mode operation, and then amplify the oscillator in one or several amplifier stages, where the laser mode is expanded by telescopes to fill the size of the pumped region in the amplifier. This is the standard approach for most high-power systems and is termed a master oscillator power amplifier MOPA system.
Ultimately, thermo-optic effects in the laser material will limit the beam quality of the system by adding phase distortions across the area of the laser beam. While simple external lenses can correct the first-order effects in rod-geometry designs, as the thermal gradients increase with pump power, the higher-order levels of distortion become significant. In recent high-power laser systems design, so-called adaptive optics, first developed to correct for the linear phase aberration effects of atmospheric turbulence on laser beams, can be applied to correcting the more complex distortions in materials running at high powers, typically through the use of feedback loops that seek to maximize beam quality.
There are several material geometries beyond simple rods that attempt to reduce thermo-optic effects. Notably, if the heat flow is along the direction of the optical beam, the thermal gradient is as well. When the material is uniformly pumped, all sections of the beam experience the same refractive-index change and thus the beam phase-front is undistorted.
Typical designs employ thin plates, often in the shape of a disk, where the large surface can be cooled by high-velocity liquids or by contact with a transparent, high-thermal conductivity crystal. Power limits are from stress, as always, and this is minimized by the use of thin crystals. However, if the crystal becomes too thin, it will not absorb, in a single pass of light through the material, a sufficient fraction of the incident pump power. A recent design, the so-called thin-disk geometry, has been made possible through the development of diode pump lasers discussed below. Here a thin disk of material on the order of 0.
The design employs optics that arrange for the pump light to make many passes back and forth through the laser material, effectively increasing the optical path for the pump light and facilitating efficient. This arrangement would not be possible with lamp pumping.
Ultimate limits to beam quality are set by the inevitable deviation of heat flow from the desired direction along the thin axis of the material and the resultant stresses. For thin-disk lasers the mechanical distortion of the material cannot be ignored as a thermo-optic effect, and there is a limit to how large an area of the crystal can be used while still maintaining a high beam quality. One disadvantage of the thin-disk design is the relatively low gain of the system because of the short length of active material, and typical designs are with oscillators only, with low-loss laser cavities.
Another geometry used for high-power, high-beam quality systems is the zig-zag slab design. In this case the material is fabricated as a thin slab, but with a rectangular geometry. The laser beam is arranged to reflect back and forth in the thin direction hence the descriptor zig-zag , off polished faces of the slab. This has the advantage that all parts of the optical beam experience the same thermo-optic distortion in the zig-zag direction. Distortion in the other direction is minimized by designing the pump beam to be as uniform as possible.
Lamp pumping is typically done through both large faces of the slab, with cooling of the same faces by a flowing liquid. Diode pumping can be done through one face, with the other face cooled by contact with a metal heat sink. Another configuration, with the diode pump light directed along the long length of the slab, is discussed in the next section. This is the configuration used in the most powerful bulk solid-state laser as of this writing.
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Many common solid-state lasers, such as those used for materials processing, laser target designation, range-finding, and imaging at relatively short distances do not require diffraction-limited output, and resonator designs for such systems emphasize some combination of efficiency and high stability against vibration or system temperature change. The actual relation between power output and beam quality is set not only by the laser material and resonator design but also by the pumping source.
Incoherent, lamp-based pump sources were the standard for bulk solid-state lasers until the s, when development of high-power diode lasers brought about a major revolution in high-performance systems.outnatali.tk
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Lamps are still widely used for systems where cost or simplicity is important. Lamp diameters are from 3 to 19 mm, and lengths can range from 2 cm to 1 m, depending on the system and energy required. Well-designed xenon lamps are percent efficient in converting electricity to light, which they produce in a spectrum that resembles a hightemperature K blackbody with discrete emission lines in the ,nm region. As the lamp discharge current increases, the line spectrum becomes a smaller fraction of the total spectrum.
For CW lasers, one employs arc lamps, which differ in some details from flashlamps in the electrode and tube designs, as well as use of a multi-atmosphere fill pressure.
For the most widely used laser application for pumping Nd:YAG lasers, arc lamps employ krypton rather than xenon gas, since the narrow-line krypton spectrum is a better match to the absorption spectrum of that material.