Abstract: A method and apparatus for suppression of four-wave mixing using polarization control with a high power polarization maintaining fiber amplifier system. The apparatus includes a master oscillator (MO) that generates a beam; a polarization controller that receives the beam from the MO and transmits the beam with a desired polarization; a pre-amplifier that receives the beam from the polarization controller, pre-amplifies the beam, and transmits the beam; a high power fiber amplifier that receives the beam from the pre-amplifier, amplifies the beam, and transmits an output beam; and a polarization detector that detects the polarization of the output beam. The polarization detector transmits feedback to the polarization controller to ensure that the output beam components aligned with the principal birefringent axes of the high power fiber amplifier have approximately equal power.

Abstract: A high power laser system is provided having a master oscillator for generating a reference laser beam of desired beam quality, means for dividing the reference beam into multiple sub-beams, a multi-slab gain module positioned to receive the multiple sub-beams as input beams, and means for adjusting the sub-beams in phase to allow the output sub-beams to be coherently combined as a single composite output beam. Optionally, additional multi-slab gain modules similar to the first multi-slab gain module may be positioned to receive amplified output sub-beams from the first multi-slab gain module. The additional multi-slab gain modules generate further amplified output sub-beams of high aggregate power.

Abstract: A technique for suppressing stimulated Brillouin scattering (SBS) in fibers intended to handle high powers. A fiber is embedded in an elongated embedding material to form an embedded fiber structure. The embedded fiber structure is formed either as a cantilevered beam or as one or more turns around a circular or elliptical path, and then the entire structure is deformed to apply a desired strain that varies along the fiber length and results in suppression of SBS. In one embodiment, the embedded fiber structure is deformed by applying lateral and generally diametric force across the turns of the structure, resulting in changes to its curvature. In another embodiment the embedded fiber structure initially has a helical shape, which is deformed by stretching or twisting to change its radius. In either embodiment, a desired strain profile is obtained by selecting the position of the fiber with respect to a neutral axis.

Abstract: A system and method for combining plural low power light beams into a coherent high power light beam by means of a diffractive optical element operating as both a beam combiner and beam sampler. An oscillation source transmits a master signal that is split into plural beams propagating at a common wavelength. Each beam is phase locked by a corresponding phase modulator according to a phase correction signal. The beams are directed through a fiber array to the diffractive optical element to allow efficient coherent combination of the beams at a desired diffraction order. The diffractive optical element includes a periodic sampling grating for diffracting a low power sample beam representative of the combined beam. A phase detection stage detects phases of constituent beams in the sample beam from which the phase correction signals are derived and fed back to the phase modulators.

Abstract: A hybrid beam combining system or method combines a plurality of coherent and incoherent light beams into a composite high power diffraction limited beam. N oscillators each transmit light at one of N different wavelengths and each wavelength is split into M constituent beams. M beams in each of N groups are phase locked by a phase modulator using phase correction signals. The phase locked beams are amplified and coupled into an M×N fiber array. Beams emerging from the array are collimated and incident on a diffractive optical element operating as a beam combiner combining the M outputs at each N wavelength into a single beam. The N single beams are incident and spectrally combined on a grating which outputs a composite beam at a nominal 100% fill factor. A low power sample beam, taken from the N beams emerging from the diffractive optical element, is measured for phase deviations from which the phase correction signals are derived and fed back to the phase modulators.

Abstract: A system and method for combining plural low power light beams into a coherent high power light beam by means of a diffractive optical element operating as both a beam combiner and beam sampler. An oscillation source transmits a master signal that is split into plural beams propagating at a common wavelength. Each beam is phase locked by a corresponding phase modulator according to a phase correction signal. The beams are directed through a fiber array to the diffractive optical element to allow efficient coherent combination of the beams at a desired diffraction order. The diffractive optical element includes a periodic sampling grating for diffracting a low power sample beam representative of the combined beam. A phase detection stage detects phases of constituent beams in the sample beam from which the phase correction signals are derived and fed back to the phase modulators.

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.

Abstract: A hybrid beam combining system or method combines a plurality of coherent and incoherent light beams into a composite high power diffraction limited beam. N oscillators each transmit light at one of N different wavelengths and each wavelength is split into M constituent beams. M beams in each of N groups are phase locked by a phase modulator using phase correction signals. The phase locked beams are amplified and coupled into an M×N fiber array. Beams emerging from the array are collimated and incident on a diffractive optical element operating as a beam combiner combining the M outputs at each N wavelength into a single beam. The N single beams are incident and spectrally combined on a grating which outputs a composite beam at a nominal 100% fill factor. A low power sample beam, taken from the N beams emerging from the diffractive optical element, is measured for phase deviations from which the phase correction signals are derived and fed back to the phase modulators.

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.

Abstract: A system and method for combining plural low power light beams into a coherent high power light beam. Optical amplifiers transmit a plurality of light beams propagating at a common wavelength through an array of optical fiber emitters. Each constituent beam is emitted from the array at a different propagation angle, collimated, and incident on a diffractive optical element operating as a beam combiner such that incident beams when properly phased and located are combined into a coherent beam at a desired diffraction order. A beam splitter or a periodic sampling grating on the diffractive optical element directs a low power sample beam to a spatial filter passing resonant mode output back to the optical amplifiers in a ring laser configuration thereby passively synchronizing phases of the constituent beams to maximize combination efficiency of the coherent beam.

Abstract: A method for combining beams from multiple laser emitters, which may be optical fibers or bulk amplifiers, to form a composite output beam with desirable beam characteristics, as measured, for example, by Strehl ratio. Beams from the multiple emitters are interferometrically combined in the near field, and the phases of the beams are controlled to provide optimal phase coherence, and thereby to minimize losses. Various techniques are disclosed for controlling the phase angles of the emitted beams, using either a separate phase detector for each emitter beam, or a single detector for the composite output beam, or nulling detectors in spurious outputs from the beam combining optics. All of these techniques achieve an improvement in Strehl, largely because the interferometric combination of beams is independent of the array fill factor.

Abstract: A solid state laser amplifier architecture in which multiple zig-zag slab laser amplifiers (50) are stacked together, side-pumped using a common pump source (52, 54), and cooled with a common cooling system. The stack of zig-zag slabs (50) produces an array of sub-beams (62) that can be combined coherently into a single composite output beam. Variations in pump power absorption through the stack are mitigated by selection of doping levels for the slabs (50). The composite output beam is sufficiently symmetrical to be directed through conventional optics of circular cross section. Multiple stacks may be arranged in a two-dimensional array to obtain even higher output powers.

Abstract: A solid state zig-zag slab laser amplifier in which depolarization occurring at total internal reflection from opposed lateral faces of the amplifier slab is controlled by selecting a complex evanescent coating that provides a selected phase retardance that results in minimization of depolarization. Without use of the complex coating, small changes in incidence angles can result in phase retardance changes large enough to increase depolarization significantly, especially when the amplifier is operated at higher powers. Appropriate selection of the complex evanescent coating allows a desired phase retardance angle to be maintained relatively constant over a small range of angles of incidence, at a given wavelength, and therefore permits minimization of depolarization and birefringence effects.

Abstract: A laser array architecture scalable to very high powers by closely stacking fiber amplifiers, but in which the output wavelength is selectable to be in the visible or ultraviolet region, without being restricted by the wavelengths usually inherent in the choice of fiber materials. A pump signal at a fundamental frequency is amplified in the fiber amplifier array and input to an array of nonlinear crystals that function as harmonic generators, producing an output array at a desired harmonic of the fundamental frequency. A phase detection and correction system maintains the array of outputs in phase coherency, resulting in a high power output with high beam quality, at the desired frequency. The array of nonlinear crystals may a single array to produce a second harmonic output frequency, or a combination of multiple cascaded arrays configured to produce a selected higher order harmonic frequency.

Abstract: An apodized fiber Bragg grating, and a phase mask, method and system for making such a grating are disclosed. The refractive index profile of the grating has a periodic apodization phase component which is designed so that the grating fringes reflecting light in a spectral region of interest are apodized, by generating spurious reflection features outside of this spectral region of interest. Apodization is therefore provided through a phase variation of the grating as opposed to an amplitude variation. The phase component is added to the profile of the phase mask grating corrugations to obtain the phase-apodized grating.

Abstract: A laser array architecture scalable to very high powers by fiber amplifiers, but in which the output wavelength is selectable, and not restricted by the wavelengths usually inherent in the choice of fiber materials. A pump beam at a first frequency is amplified in the fiber amplifier array and is mixed with a secondary beam at a second frequency to yield a frequency difference signal from each of an array of optical parametric amplifiers. A phase detection and correction system maintains the array of outputs from the amplifiers in phase coherency, resulting in a high power output at the desired wavelength. A degenerate form of the architecture is disclosed in an alternate embodiment, and a third embodiment employs dual wavelength fiber amplifiers to obtain an output at a desired difference frequency.

Abstract: An apodized fiber Bragg grating, and a phase mask, method and system for making such a grating are disclosed. The refractive index profile of the grating has a periodic apodization phase component which is designed so that the grating fringes reflecting light in a spectral region of interest are apodized, by generating spurious reflection features outside of this spectral region of interest. Apodization is therefore provided through a phase variation of the grating as opposed to an amplitude variation. The phase component is added to the profile of the phase mask grating corrugations to obtain the phase-apodized grating.

Abstract: The invention reduces the effects of stitching errors from re-scaling or re-positioning in the fabrication of fiber Bragg gratings or the mask used in such fabrication. A first embodiment of the invention preferably uses characteristics of stitching errors to compensate for the stitching errors themselves. By increasing the number of stitching errors, errors caused by the stitching errors can be reduced. A second embodiment uses continuous writing of the desired pattern, wherein the desired pattern is snapped to a grid that can be written by the fabrication equipment. Using continuous writing eliminates stitching errors in the resulting gratings.

Abstract: The invention provides masks that form fiber Bragg gratings (FBGs) in optical fibers without stitching errors from re-scaling or re-positioning. The invention feathers the pixels of the mask lines by adding, removing, and/or displacing one or more pixels from the edges of the bars and spaces of the mask. The feathering of pixels will affect the FBG being written into the fiber. The feathering operates to shift the effective edge of the bars. This allows the achievement of much finer resolution FBGs than the pixel size of the mask.

Abstract: A phase mask, for writing fiber Bragg gratings (FBG) in an optical fiber, adjusts the amplitude and the phase of the FBG, while maintaining a constant mean index of refraction of the fiber in a single pass. Specifically, the phase mask embodies the amplitude information so that the amplitude information is an integral part of the phase mask and preferably cannot be separated from the phase information. A first embodiment employs a reflective or opaque surface, defining a window, on the substrate of a phase mask controlling the amplitude of light passing through the phase mask. Another embodiment employs a polygonal shaped grating region on a clear substrate. A third embodiment interleaves regions of grating and smooth substrate surface. Preferred embodiments employ two areas of gratings with the areas disposed: perpendicularly, out of bandwidth or out of phase, relative to each other or additive.