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  High intensity and high power solid state laser amplifying system and methodRelated Patent Categories: Coherent Light Generators, Particular Pumping MeansThe Patent Description & Claims data below is from USPTO Patent Application 20050271111. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This is a divisional patent application of co-pending U.S. patent application Ser. No. 09/907,154, filed Jul. 16, 2001, entitled "High Intensity and High Power Solid State Laser Amplifying System and Method", which is a continuation-in-part of co-pending U.S. patent application Ser. No. 09/689,539, filed Oct. 12, 2000, entitled "Beam Correcting Laser Amplifier", which itself is based on U.S. Provisional Patent Application Ser. No. 60/159,521, filed Oct. 15, 1999, entitled "Beam Correcting Laser Amplifier". Priority is claimed to the above-identified co-pending U.S. Patent Application(s) and to the above-identified U.S. Provisional Patent Application. The disclosures of all of the above-mentioned applications are incorporated herein by reference in their entirety for all purposes. FIELD OF THE INVENTION [0002] The present invention generally relates to a system and a method for amplifying coherent light and, more specifically, to a system and a method for amplifying coherent light of a laser system. BACKGROUND OF THE INVENTION [0003] High power and high intensity laser systems are very desirable. However, such high power and high intensity lasers are very hard to obtain with a high quality beam and a short pulse duration. Laser amplifiers take an input laser beam from an external laser oscillator and amplify the input laser beam. Increasingly more intense and more powerful laser beams are achieved by increasing amplification power. However, conventional laser amplifiers have design and performance aspects that limit and even reduce achievable power and intensity gains. At high power and high intensity, heat generated by the laser pump light can create thermal optical effects and thermal stresses in laser and amplifying systems which distort the light beam, making conventional laser and amplifying systems inefficient or even inoperable. Furthermore, the energy contained in high power and high intensity laser beams can permanently damage, if not instantly vaporize, components of conventional laser and amplifying systems. [0004] A limit on high power and high intensity laser amplification is the B-integral effect. The B-integral effect describes the relationship between the refractive index of a material and the intensity of illumination. Thus, a light beam with a non-uniform intensity distribution, such as a Gaussian intensity profile, has higher indices of refraction in areas of higher light intensity. Varying illumination intensities and thus varying indices of refraction also occur due to non-uniform energy densities resulting from laser pumping sources. The refractive index of the material determines the phase velocity of light through it, and thus the effective optical path length. As a result, phase delays occur in the regions of higher intensity, distort the focus of the light beam and limit the gains in intensity and power. A varying index of refraction also alters the optical path of affected portions of the beam, causing the whole beam or portions of the beam to collapse into focus points. The B-integral effect becomes more pronounced under high power and high intensity amplification because of the greater variances in illumination levels. [0005] As a result of the B-integral effects and other sources of distortion to the light beam (such as optical imperfections in the laser path), high power and high intensity amplification in conventional laser amplifiers creates regions of heat accumulation (i.e., hot spots). Hot spots occur in areas of imperfections that disrupt the laser, dissipating energy into the surrounding regions. Hot spots may also form as a result of non-uniform pumping that causes varying levels of heat (e.g., heat gradients) to develop in different regions within the amplifier. As a region heats, it further distorts the refractive index profile in the laser amplifier, leading to still greater heat accumulation. This cycle of increasing heat and distortion continues until either the laser amplifier breaks down or destructive optical interference due to, for example, phase delays, prevents further gains in intensity and power. [0006] For at least the above reasons, conventional laser amplifier designs are prone to hot spot formation and are limited in achievable gains in intensity and power. Hot spot formation also enhances inefficiencies in conventional laser amplifiers since much of the amplifying laser light energy is lost as it is converted into waste heat. Furthermore, conventional laser amplifiers are not well designed to withstand hot spots and rapidly break down under high power and high intensity amplification, requiring expensive repair and replacement of parts. [0007] To manage these high temperatures, a means of active heat removal is generally advantageous. Conventionally, the non-optical surfaces of the laser crystal rod are cooled by the forced convection of a fluid, which is usually water. Alternatively, these surfaces can be thermally connected to a heat sink of sufficient mass to absorb the waste heat. However, due to the geometry of the active laser volume and the relatively low thermal conductivity of the laser crystal rod, high temperatures and large temperature gradients may persist. [0008] Accordingly, there is a need for systems and methods for amplifying light that effectively produces high power and high intensity laser beams, but minimizes the formation of harmful hot spots and/or is robust enough to withstand the hot spots that do develop. SUMMARY OF THE INVENTION [0009] The present invention alleviates to a great extent the disadvantages of conventional systems and methods for amplifying light. In an exemplary embodiment, the present invention provides a four pass laser amplifier that receives a polarized laser input beam from an external source, directs the input beam through four amplifying passes in an amplifier, and then allows the beam that has been so amplified to exit as a desired output. The amplifier may provide for a first polarizing beam splitter (PBS), a second PBS, a directional polarization rotator (DPR), a non-directional polarization rotator (NPR), a first reflector, a second reflector and/or a pumping module. Furthermore, although the exemplary embodiment employs four passes through the amplifier, the present invention also contemplates other even or odd numbers of amplifying passes. Furthermore, passes that are made by the laser beam through the amplifier need not be collinear. [0010] Amplification of the input beam occurs in the pumping module that is disposed in the beam path, downstream from the two PBS's and the DPR. The pumping module includes one or more pump sources (e.g., light sources) that add optical energy to the input beam to increase its intensity and power. The one or more pump sources may include, for example, a flash pump, another type of lamp and/or a laser diode. Laser diodes can be adapted to emit in a relatively small frequency band centered around a desired frequency. Accordingly, efficiency is increased since, for example, light energy is not substantially wasted at other frequencies that might not provide effective pumping. [0011] The pumping module may include an optical pathway that itself includes an elongated laser crystal rod including, for example, a solid state material such as yttrium-aluminum-garnet (YAG), doped with active materials such as, for example, neodymium (Nd), ytterbium (Yb), holmium (Ho), and/or erbium (Er). The active materials can be optically excited, for example, by light impinging on them from the laser diodes, so that certain electrons within atoms of such materials are temporarily excited (i.e., temporarily raised to elevated energy states). If the input beam impinges upon such excited materials before, for example, the electrons spontaneously revert to their normal, stable energy states, then that reversion can be triggered (i.e., stimulated), thereby causing a massive release of photons as a result of the electrons returning to the lower energy state. In other words, the laser crystal rod is pumped by the laser diodes, thereby creating corresponding excited atoms that give up quanta to the radiation field via induced emission. The stimulated or induced emission provides a phase-coherent amplification mechanism for the input beam. [0012] Individual laser diodes are used in the laser amplifiers and emit, for example, a rectangular-shaped beam of light, although other shapes are also contemplated by the present invention. These diodes may be mounted in longitudinal rows comprising diode bars. A plurality of such bars are mounted adjacent to each other (with the diodes mutually oriented in the same direction in a matrix) forming a laser diode array. These arrays are then positioned in a desired configuration opposite the laser crystal rod so that the diode light can impinge upon and pump the rod. Each laser crystal rod is paired with at least one set of laser diode arrays within an amplifier stage. Multi-stage amplifiers, including, for example, a pre-amplifier stage and amplifier stages arranged in series and/or parallel, are contemplated. [0013] In an exemplary embodiment, the present invention provides that the pumping efficiency is enhanced by using an odd number of laser diode arrays disposed around the laser crystal rod. For example, in an amplifier stage having an odd number of regularly-spaced, circumferentially disposed arrays, no two arrays are disposed opposite each other (i.e., at 180.degree. to each other) around the longitudinal axis of the laser crystal rod. In addition, reflectors disposed between the laser diode arrays can reflect pumping energy that passes unabsorbed through the laser crystal rod, back into the active medium. [0014] The present invention has an advantage in supplying substantially uniform pumping energy in the laser amplifier. Pumping uniformity may be improved by increasing the number of laser diode arrays disposed around the circumference of the laser crystal rod, thereby decreasing the radial angle between adjacent laser diode arrays. Furthermore, increasing the number of laser diode arrays increases the total amount of available pumping energy (by increasing the number of input energy sources) and enhances amplification. Accordingly, in an exemplary embodiment, the present invention may provide, for example, five or more laser diode arrays. [0015] The present invention may also provide one or more cylindrical lenses and/or mirrors that direct the emissions from the laser diodes. As a result, amplification efficiency improves since more pumping energy reaches the laser crystal rod where it can amplify the input beam. Pumping uniformity is further improved by disposing the lenses and/or mirrors so that the focal points of the pumping energy are, for example, at a distance away from the surface of and outside of the laser crystal rod. In this manner, when the pumping light reaches the laser crystal rods, it is unfocused and accordingly, dispersed evenly across the diameter of the laser crystal rod. The present invention also contemplates the use of aspherical lenses to further improve amplifier performance. [0016] In an exemplary embodiment, the present invention uses a plurality of laser crystal rods aligned along their long axes. This multi-stage configuration allows for increased amplification of the input beam by increasing the number of pumping energy sources successively amplifying the same light beam. In addition, multi-stage configurations may benefit by adjusting for distortions, for example, resulting from thermal lensing effects and/or birefringence effects. [0017] In an exemplary embodiment, the present invention provides laser diode bars and/or individual laser diodes that are selected to have substantially identical characteristics (e.g., substantially similar peak output intensities and wavelengths). The laser diode bars and/or the laser diodes are then incorporated into laser diode arrays and wired so that each laser diode receives substantially identical electrical input. Such laser diode arrays do not vary significantly in output power and average peak output wavelength. [0018] The present invention also has an advantage in that the output power of the laser diode arrays may be controlled so as to insubstantially vary. In an exemplary embodiment, the present invention provides an electrical power supply for individual laser diode arrays so that output power of the laser diode arrays in each of the stages of the laser amplifier can be empirically matched. The electrical power supplies include power controlling means (e.g., a rheostat, tunable transistor, etc.) which may be monitored and controlled manually or automatically such as, for example, by a computer system employing feedback loop circuitry using, for example, sensor circuitry. In another exemplary embodiment, the present invention provides that the electrical input to each of the arrays is limited to the level used by the least powerful laser diode array. Laser diode arrays that exhibit higher power than the least powerful laser diode array may have electrical loads placed in parallel with the higher power arrays, thereby draining the proper amount of power from the higher power arrays. Thus, all of the laser diode arrays produce substantially similar output power. [0019] In an exemplary embodiment, the present invention provides that the laser diode arrays may be oriented such that the short side of the rectangular light emitting surface of each laser diodes is disposed in parallel with the longitudinal axis of the laser crystal rod. The rectangular light emitting surface of the laser diodes has two sets of sides, the short sides and the long sides. The length of a short side is smaller than the length of a long side. In viewing the intensity patterns of light emitted from the rectangular emitting surface of a particular individual laser diode, there is a large optical angular dispersion in a direction parallel to the short sides (i.e., the short axis or the fast axis) of the rectangular light emitting surface. Conversely, there is a small optical angular dispersion in a direction parallel to the long sides (i.e., the long axis or slow axis) of the rectangular light emitting surface. Such optical angular dispersion can be approximately analyzed as a two-dimensional single slit diffraction pattern in which optical angular dispersion is approximately proportional to the wavelength of the emitted light and approximately is inversely proportional to the width of the slit in a particular dimension. By perpendicularly orienting the individual laser diodes relative to the longitudinal axis of the laser crystal rod (i.e., the short axes or the fast axes of the laser diodes are parallel to the longitudinal axis of the laser crystal rod), the dispersion away from their long axes or slow axes substantially overlaps each other and effectively smoothes out the pumping intensity impinging on the laser crystal rod. The smoothing out of the pumping intensity enhances the uniformity with which the laser crystal rod is pumped. The present invention also contemplates employing other laser diode assemblies or individual laser diodes that take advantage of the above-described smoothing effects and uniformity in pumping intensity. Further pumping uniformity may be achieved by decreasing the spacing between the laser diodes in the array. Thus, for example, the exemplary embodiment naturally provides decreased spacing between laser diodes, and thus provides increased uniformity in the pumping intensities impinging upon the laser crystal rod. In addition, the exemplary embodiment also uses a larger number of laser diodes. These latter aspects improve the overall amplification and efficiency of the laser amplifier. [0020] In an exemplary embodiment, the present invention improves the durability and robustness of the laser amplifier. The pumping source, and thus the laser amplifier, continue to operate effectively even if an isolated laser diode or an isolated laser diode bar fails. This is due, in part, to the substantial overlap of pumping intensity between substantially adjacent individual laser diodes and/or substantially adjacent laser diode bars. Thus, optical angular dispersion from the substantially adjacent laser diodes and/or the substantially adjacent laser diode bars sufficiently illuminates the portion of the laser crystal opposite the dark (damaged) bar and/or diode to compensate for the lost pumping energy. Alternatively, the pumping intensity can be selectively increased via control circuitry which may be coupled to a computer that monitors and adjusts pumping intensity (e.g., adjusts power supplied to individual laser diodes, laser bars and/or diode arrays) to compensate for the lost pumping energy in the portion of the laser crystal rod opposite the dark (damaged) laser bar and/or laser diode. [0021] In an exemplary embodiment, the present invention achieves greater uniformity in pumping energy by providing a plurality of sets of laser diode arrays along the longitudinal axes of the laser crystal rods. Each successive set of arrays is rotated with respect to the orientation of an adjacent set of arrays, so that the pumping energy reaches the laser crystal rod or laser crystal rods in each amplifier stage from different radial directions. For example, with laser amplifiers using sets of five laser diode arrays for transverse pumping, each successive set of laser diode arrays may be rotated relative to adjacent set of arrays by 36.degree.. Thus, laser diode arrays in adjacent amplifier stages are disposed in a radially staggered relative orientation along the laser crystal rod longitudinal axes. Note, however, that although such staggering yields symmetrical (and therefore optimum) distribution of the five arrays around the 360.degree. circumference, other, non-symmetrical staggering arrangements can also be used. Continue reading... Full patent description for High intensity and high power solid state laser amplifying system and method Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this High intensity and high power solid state laser amplifying system and method patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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