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  Multi-stage method and system for coherent diffractive beam combiningUSPTO Application #: 20080085128Title: Multi-stage method and system for coherent diffractive beam combining Abstract: A system or method coherently combines a large number of light beams at a single wavelength in multiple stages to form a high-power diffraction limited output beam. A two-stage system, or method based thereon, includes a master oscillator transmitting a light beam to a first phase modulation stage, which splits the beam into N beams and locks beam phases using phase correction signals from a first feedback loop. A second phase modulation stage splits each N beam into M beams and locks the phases of M beams in each N group using phase correction signals from a second feedback loop. A two-dimensional fiber array directs M×N beams to a first diffractive optical element combining the beams into N coherent beams of M beams each, and phase correction signals for the second stage are derived from a sample extracted from the N coherent beams. A second diffractive optical element combines the N coherent beams into the output beam, and phase correction signals for the first stage are derived from a sample extracted from the output. The diffractive optical elements may operate as both beam combiners and beam samplers. (end of abstract) Agent: Snell & Wilmer L.l.p. (grumman) - Costa Mesa, CA, US Inventors: Joshua E. Rothenberg, Robert R. Rice, Michael G. Wickham USPTO Applicaton #: 20080085128 - Class: 398188 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20080085128. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001]This application is related to U.S. patent application Ser. No. 11/361,352 filed Feb. 24, 2006; U.S. patent application Ser. No. ______, a.k.a. Snell & Wilmer Dkt. No. 54361-2500 entitled "Method And System For Coherent Beam Combining Using An Integrated Diffractive Beam Combiner And Sampler" filed concurrently herewith; U.S. patent application Ser. No. ______, a.k.a. Snell & Wilmer Dkt. No. 54361-2600 entitled "Method and System For Diffractive Beam Combining Using DOE Combiner With Passive Phase Control" filed concurrently herewith; and U.S. patent application Ser. No. ______, a.k.a. Snell & Wilmer Dkt. No. 54361-2700 entitled "Method And System For Hybrid Coherent And Incoherent Diffractive Beam Combining" filed concurrently herewith; all of which are fully incorporated herein by reference. BACKGROUND OF THE INVENTION [0002]1. Field of the Invention [0003]The present invention relates to combining light beams using diffractive beam combining. More specifically, the invention relates to combining a plurality of laser beams in multiple stages, where in each stage, a diffractive optical element operating as a beam combiner diffracts a low power sample of combined beams for use in active phasing. [0004]2. Description of Related Art [0005]High power lasers have many possible applications. In a military application, sufficient energy focused in a laser beam can provide an effective defensive countermeasure against ballistic projectiles. In a commercial application, a high power laser can be used to weld metal components that are too thick to be welded by conventional methods. To improve the effectiveness of the laser in any of these applications, the power transmitted by the beam may be enhanced by focusing the beam to its far-field diffraction limit, i.e., into as small an area as theoretically possible. A laser beam focused to this theoretical limit is said to be diffraction limited. Generally speaking, advancement of the art of high power lasers is impeded by physical limitations encountered when attempting to achieve this limit. [0006]Lasers by their nature are ideally diffraction limited, such that the smallest possible area of focus is limited by the product of the focal distance and the diffraction limited angle, which is the wavelength of the light divided by the aperture width. Thus, the larger the aperture, the tighter the focus. However, there are practical limitations on the size of an aperture that can be designed for any optical apparatus. Imperfections in the optics may cause degradations in the laser wavefront that affect the focus, and in high power applications, thermal variations contribute to the degradation. This limits the designer's ability to focus the beam, resulting in a focal spot somewhat greater than 1.0 times the diffraction limit (1.0.times.DL). Practically speaking, the designer's goal is to achieve a near-diffraction-limited laser (i.e., one that approaches 1.0.times.DL) that operates at as high a power level as possible. [0007]At present, the most advanced near-diffraction-limited laser beams cannot deliver enough power per unit area to serve effectively in high-power applications. In one case, an optimized beam can deliver a 3 kW beam having a diffraction limit of nearly 1.0. In another case, an optimized beam can deliver a 10 to 12 kW beam that is about 1.5 times diffraction limited. An objective of ongoing research in this field is to design a laser generator that can deliver 100 kW or more in a near-diffraction-limited beam. [0008]One method for increasing the power deliverable by lasers is to combine the power of many coherently phased beams of a common wavelength by arranging a plurality of optical fiber emitters in a two-dimensional array. A beam splitter may be placed at the output of the array to sample the constituent beams. Each of the sampled beams is directed to a phase sensor, and the measured error signals are provided to phase modulators in each beam to ensure all the beams have equal phase. However, even in the most tightly packed array, the `fill factor` of the composite beam (ratio of the composite beam power to a beam that uniformly fills the entire array aperture and has equal peak intensity) is only about 70%, due to voids that occur between fibers and also to the Gaussian shape of each beam. The end result is a reduction in brightness by the fill factor ratio--the focused composite beam has a central peak intensity equal to the fill factor ratio times the maximum intensity possible with an ideal uniform beam, with the remaining power appearing in widely separated side lobes. In other words the composite beam has a shape dramatically different than that of the constituent beams, and as a result the composite does not focus as well as the constituents. [0009]Another known method for combining beams is spectral combining, in which many incoherent beams, i.e. beams of different wavelengths, are superimposed. The beams are transmitted through a prism or grating that aligns the beams along a common path, creating, in essence, a singular beam of multiple colors. Thus the composite beam has a shape that is substantially identical to that of the constituent beams. While this technique therefore eliminates the fill factor problem associated with the two-dimensional array, other problems arise from using multiple wavelengths. For one, the complexity of the system increases as each wavelength requires a different oscillator. Furthermore, the propagation angle of each wavelength must be precisely adjusted such that its incidence on the grating is exact, otherwise the beams will misalign. More importantly, each wavelength may behave differently as the beam propagates through various media. Atmospheric absorption is a function of wavelength, therefore a spectrally combined beam directed through air is more susceptible to energy loss than a single-wavelength selected for optimal transmission efficiency. Spectral combining has been proposed, for example, in U.S. Pat. No. 6,697,192, U.S. Pat. No. 6,327,292, U.S. Pat. No. 6,208,679, and U.S. Pat. No. 6,192,062. [0010]Another proposed technique for increasing the power in a laser beam is to (coherently) combine, by constructive interference, a plurality of beams into a single coherent beam. This technique, known as coherent diffractive beam combining, is the subject of co-pending U.S. patent application Ser. No. 11/361,352 filed Feb. 24, 2006, which is incorporated by reference herein as though set forth in full. In general, the co-pending application teaches generating a plurality of input beams, all having a common wavelength, using a master oscillator. Each beam is individually amplified and transmitted through a fiber emitter, and the emitter outputs are combined into a single output beam using a diffractive optical element (DOE). The technique includes a means for actively controlling the phases of the multiple beams using feedback to optimize the efficiency of the beam combination. This may be accomplished by coupling a phase modulator to each input beam, and by coupling a phase detector to a sampling of the output beam. The sampling is obtained by placing a transmissive beam splitter in the output path that reflects a low power portion of the output to the phase detector. Using electronics, correction signals based on phase deviations detected at the output are fed back to the modulators. An exemplary means for effecting active phase control in this fashion is disclosed in U.S. Pat. No. 6,708,003, which is also fully incorporated herein by reference. Another active phase detection and control method has been demonstrated by T. M. Shay et al., Proceedings of the SPIE, Vol. 5550, pp. 313-319 (2004), which is also fully incorporated herein by reference. An advantage of this approach is that, similar to SBC, the combined output beam has a shape that is substantially identical to the composite beams and therefore eliminates the fill factor reduction in the intensity of the focused coherent output beam. However, disadvantages occur when sampling the phases of the high power combined output beams. A high power beam passing through a transmissive beam splitter causes thermal distortion that affects the phase measurement accuracy and focusability of the output beam. Also in this method, a single detector is used to measure the phases of all the constituent beams. For a very large number of combined beams the accuracy of phase measurement becomes more difficult with a single detector. SUMMARY OF THE INVENTION [0011]The present invention provides a system and a related method for multi-stage diffractive beam combining for coherently combining a large number of light beams at a single wavelength into single high-power diffraction limited beam. In a preferred embodiment, a two-stage system includes a master oscillator for transmitting a light beam at a single wavelength. At a first phase modulation stage, the light beam is split into N beams (where N is any integer) and the phases of the N beams are synchronized by phase modulators using phase correction signals provided to the phase modulators through a first feedback loop. A second phase modulation stage splits each of the N beams into M beams (where M is any integer). Amplifiers and phase modulators in the second phase modulation stage amplify the beams and synchronize the phases of each N group of M beams using phase correction signals provided to the second stage phase modulators from a second feedback loop. From the second phase modulation stage, optical fiber emitters arrange the beams into a two-dimensional M.times.N array, and each beam is directed toward a first diffractive optical element (DOE) at a slightly different propagation angle. An optic (e.g. lens or curved mirror) may be used to collimate and overlap the beams for precise incidence on the first DOE. The first DOE operates as a beam combiner to combine the beams into N coherent beams each comprising M beams. In the second feedback loop, a sample beam is extracted from the N coherent beams and directed to N separate phase detection means, each of which detects phase variations in each of the M constituent beams. Phase correction signals are derived from the detected phases and fed back to the second stage phase modulators. In one embodiment, the sample beam is diffracted from a periodic sampling grating superimposed on the surface of the first DOE. [0012]The N coherent beams output from the first DOE are preferably aligned along a single dimension and directed onto a second DOE. Relay optics may be used, if necessary, to ensure precision collimation and overlap of the beams incident on the second DOE. The second DOE also operates as a beam combiner, combining the N coherent beams into a single, diffraction limited output beam. In the first feedback loop, the output beam is sampled, and through phase detection means, phase correction signals for the first stage are derived from the sample and fed back to the first stage phase modulators. In one embodiment, the output sample is extracted using a periodic sampling grating superimposed on the surface of the second DOE. [0013]One main advantage of multi-stage coherent diffractive beam combining according to the present invention is that the maximum number of beam phases being measured by any phase detector is reduced to about the square root of the total number of beams being combined. BRIEF DESCRIPTION OF THE DRAWINGS [0014]Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. The invention will be better understood upon consideration of the specification and the accompanying drawings, in which like reference numerals designate like parts throughout the figures, and wherein: [0015]FIG. 1 is a block diagram of a multi-stage system according to the invention having two stages for coherent beam combining. [0016]FIG. 2 is a plot of one example of a periodic phase pattern in a five-beam combiner DOE for splitting a single beam into five diffraction orders. [0017]FIG. 3 is a plot showing normalized intensities of the five diffraction orders produced by the DOE of FIG. 2, when used as a beam splitter. [0018]FIG. 4 is a logarithmic plot of normalized intensities vs. diffraction orders resulting when the DOE of FIG. 2 is used to combine five ideally phased beams. [0019]FIG. 5 is a plot of another example of a periodic phase pattern in a five-beam combiner DOE having an added sinusoidal sampling grating. [0020]FIG. 6 is a logarithmic plot of normalized intensities of diffraction orders resulting when the DOE of FIG. 5 is used to combine five ideally phased beams. [0021]FIG. 7 is a block diagram of another embodiment of a multi-stage system according to the invention that employs a DOE combiner and sampler at each stage. Continue reading... Full patent description for Multi-stage method and system for coherent diffractive beam combining Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Multi-stage method and system for coherent diffractive beam combining 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. 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