Northrop Grumman Systems Corporation

Trw Limited

Northrop Corporation

Stephen John Brosnan

Hiroshi Komine

Michael G Wickham

Donald Glen Heflinger

Lee O Heflinger

Joshua E Rothenberg

Hagop Injeyan

James G Ho

Peter Anthony Thielen

Mark Ernest Weber

James M Zamel

Gerald W Holleman

Jesse Peter Anderegg

Paul Thomas Epp

A fiber laser system and a related method for its use, in which one or more fiber laser amplifiers are cryogenically cooled and optimized to operate at a desirably high efficiency. Versions of the laser system using either thulium or erbium doped fibers are disclosed. In a high power version of the system, the outputs of multiple fiber lasers are coherently combined. Cooling by a selected liquefied gas, such as nitrogen, is applied to the fiber laser amplifiers and, optionally, to pump diodes and to optical elements used to combine the outputs of the fiber laser amplifiers.

High efficiency, high power cryogenic laser system

A system to remove cladding light from an optical fiber that includes a core and a cladding that surrounds the core. A volume of an index-matching material contacts an exterior surface of the cladding along a contact length of the optical fiber. The index-matching material has a refractive index that substantially matches a refractive index of the cladding at a predetermined clamping temperature and has a refractive index with a negative temperature coefficient, such that the index matching material distributively removes light from the cladding along the contact length based on the temperature of the index matching material that contacts the cladding.

System and method to remove light from cladding

Encircled far field energy is substantially increased by modifying the near field energy distribution of radiation from each fiber in an emitting array. Each beamlet output from a fiber is modified to have a generally uniform cross-sectional energy distribution, using a pair of aspheric optical elements selected for that purpose. The optical elements may be refractive or reflective. The modified beamlets combine to form a composite output beam with a generally uniform energy distribution. Preferably, the composite beam is subject to an array-wide inverse transformation to a near-Gaussian distribution, further enhancing the encircled far field energy and providing a more efficient high power laser source. Further gains in efficiency are achieved by selecting a fiber bundle pattern, lens array pattern and lens shape that together result in a high fill factor.

Method and apparatus for optimizing the target intensity distribution transmitted from a fiber coupled array

A hybrid optical/electronic wavefront sensor includes an electro-acoustical device used to upshift an optical reference signal. An optical test signal and the frequency upshifted optical reference signal are optically heterodyned to create a signal having a frequency equivalent to the beat frequency of the two signals, for example, the RF driving frequency of the Bragg cell. The optically heterodyned signal is then converted by way of a detector to an electronic signal having the same phase as the optical test signal. The output of the detector is a sinusoidal signal having the same phase as the phase of the optical test signal. This signal is filtered by way of an AC filter and mixed with a second clock signal, for example, a clock signal that is offset in frequency from the electro-acoustical drive signal by a frequency, for example, between 100 kHz and 1 MHz. These two signals are mixed by way of a mixer. The low frequency product of the mixer is passed by way of a filter and converted to a square wave by way of a comparator. The output of the comparator is applied to a simple pulse counter and used to disable the pulse counter. The pulse counter counts the clock pulses while it is enabled and is linearly related to the difference in phase between the optical test signal and the frequency upshifted signal.

Wavefront sensor using hybrid optical/electronic heterodyne techniques

An array of cylindrical end-caps with separate or integral lenses is stacked with its members in close contact, forming inter-cylinder gaps between every subset of three adjacent cylindrical lenses. Conductive fibers are disposed in the inter-cylinder gaps. Heat that would otherwise accumulate in the array is removed through the conductive fibers and transmitted to an external heat sink.

Cooled high power laser lens array

A method is provided for seeding laser system () for single longitudinal mode oscillation. The method includes coupling laser system () to be seeded for single mode output to a seed laser radiation source (). Next, the frequency capture range () and spacing () of the axial modes () of the cavity () of the laser system () are determined. A seed spectrum () is then generated from the seed laser radiation source () with a bandwidth () corresponding to the axial mode spacing (). The seed spectrum () includes a comb of discrete frequency components () with one or more of the discrete frequency components () being within the frequency capture range () of at least one of the axial modes (). The seed spectrum () is then injected into the cavity () such that at least one of the axial modes () oscillates with the seed radiation.

Robust seeding technique for single mode laser oscillation

A seed laser apparatus is disclosed. It comprises a distributed feedback laser system for transmitting a dithered optical signal having a frequency versus time characteristic that is represented by a triangular waveform and an optical medium including a plurality of optical signal paths, each path including an optical fiber and a fiber amplifier. The optical medium is characterized by stimulated Brillouin scattering (SBS) having a response time, whereby the period of the triangular waveform is equal to the round-trip transit time in the fiber or shorter than the response time of the SBS.

Distributed feedback laser apparatus for avoiding stimulated brillouin scattering

A system for coupling light emitted from a plurality of laser diodes to a single optical fiber provides increased pump light brightness to the optical fiber to provide increased power fiber amplifiers. The system takes advantage of the brightness mismatch in the fast and slow planes of a laser diode to allow capture of more than one diode's power into a single multimode pump fiber. A first cylindrical lens is disposed to collimate light from the fast planes of the diodes. A second cylindrical lens is disposed orthogonal to the optical axis and perpendicular to the first cylindrical lens and images light from the diodes in the slow plane. A collection lens is provided to image the light from the slow plane as well as collect light from the fast plane and directs light from multiple diodes into a single multimode optical fiber.

High brightness laser diode coupling to multimode optical fibers

A dual-clad fiber laser employs a multi-mode pump fiber array for introducing pump light into a dual-clad fiber array along the length of the array. Each fiber in the dual-clad fiber array includes a single mode core, a multi-mode inner cladding layer and an outer cladding layer. Each fiber in the multi-mode fiber array includes a multimode core and an outer cladding layer. The inner cladding layer and the core are the same material and the outer cladding layers are the same material. The dual-clad fiber array is a fiber ribbon wound on a bobbin. The multi-mode pump fiber array is a fiber ribbon that is wrapped around the outside of the dual-clad fiber ribbon on the bobbin. A doped silica frit is placed between the fiber ribbons, where the dopant makes the frit have an index of refraction greater than the index of refraction of the outer layers.

Parallel side-pumping of dual-clad fiber array using fused, layered structure

A high average power fiber-laser system comprises a master oscillator for generating a primary laser signal; a beam splitter array for splitting the primary laser signal into N secondary laser signals; an optical frequency shifter for shifting the frequency of the primary laser signal; a phase modulator array for providing phase compensation of the N secondary laser signals; N single-mode dual clad fiber amplifiers for amplifying the N secondary laser signals and generating an output beam; a beam sampler for sampling the wavefront of the output beam, defining an array of sampled signals; means responsive to the array of sampled signals for interferometrically combining the array of said sampled signals and the shifted primary laser signal into an array of heterodyne optical signals, each having a phase that corresponds to the state of phase of the array of sampled signals; and means responsive to the array of heterodyne optical signals for providing a plurality of feedback signals to the phase modulator array that are linearly proportioned to the state of phase over at least one wave to provide phase compensation of the secondary laser signals. This includes digital dividers for dividing electrical signals by a selected integer and an exclusive OR gate for providing a pulse train having a duty cycle that reflects the phase to be compensated.

High average power fiber laser system with high-speed, parallel wavefront sensor

A photonic crystal fiber (,) is provided for a laser/amplifier system including a guiding structure comprising a geometric array of axial passages (,) formed along the length of the fiber (,). More particularly, the guiding structure includes a central silica rod (,) which is doped with a rare earth element for providing optical gain to the laser/amplifier. A plurality of second silica rods (,) are disposed circumferentially about the central rod (,). Each of the second rods (,) includes an axial passage (,) formed therethrough along the length of the fiber (,). A reflective coating (,) is deposited on an outboard surface of the array of rods (,) to confine pumped light (,) therein. The pumped light (,) may be injected into the fiber (,) from the side by focusing it through small holes (,) in the reflective coating (,) or by reflecting it off transverse Bragg gratings (,) written into a fiber pigtail (,) coupled to the fiber (,). The mode field diameter of the fiber (,) is controlled by properly selecting the diameter and spacing of the passages (,) in the second rods (,).

Photonic crystal fiber lasers and amplifiers for high power

An optical heterodyne wavefront sensor uses a radio frequency (RF) signal for measuring an optical wavefront having a state of phase that differs throughout its aperture. It comprises a lens, optical fiber and optical frequency shifter arrangement to develop a reference optical wavefront having substantially the same phase throughout and that is shifted in frequency by an amount corresponding to the RF. A beam combiner interferometrically combines the optical wavefront and the shifted reference optical wavefront into a heterodyne optical signal at the RF frequency, each subaperture of which has a phase that corresponds to the state of phase of a like subaperture of the optical wavefront. An array of lenses focuses the optical heterodyne signal into an array of photodetectors, each responsive to a subaperture and that produces an electrical signal at the optical beat frequency corresponding to the RF and having a phase corresponding to the state of phase of the corresponding subaperture of the optical wavefront. A plurality of circuit chains, each including a digital divider, responds to the RF signal and a corresponding one of the array of electrical signals and with an exclusive OR circuit and an integrating circuit develops a like plurality of output signals that comprise voltages that linearly correspond to the state of optical phase of a subaperture of the optical wavefront. The composite output signals represent the optical state of phase of the incoming optical wavefront.

Heterodyne wavefront sensor

A heterodyne velocimeter system includes an illumination laser that illuminates a moving object with a coherent, laser beam. The laser illumination is reflected from the object as separate wavefronts (referred to as the signal light) and is passed through a range-focus lens pair, a polarizer element and through a partially reflective, partially transmissive element onto a plurality of separate light sensitive elements of a sensor which are referred to as pixels. A second source of laser illumination, such as a second laser provides a reference light beam that is directed onto a local oscillator scatter mask. The light beam from the second laser is scattered into separate wavefronts by the scatter mask and they are focussed by a lens onto the partially reflective partially transmissive element which reflects them onto the separate pixels of the sensor where they are individually mixed with the separate signal light wavefronts from the object. Thus light reflected from the other points of the moving object are similarly directed as signal light onto the pixels of the sensor and mixed with corresponding reference light wavefronts from the scatter mask to provide a two-dimensional array of emission points from local oscillator scatter mask is imaged onto the sensor to serve as the local oscillator for the reflected wavefronts at each pixel.

Doppler velocimeter

A method for injection locking a xenon chloride laser at 308.4 nm. Neon (Ne) is used as a diluent in the laser to shift slightly the gain profile of the laser towards the longer wavelengths and towards 308.4 nm. The laser is operated at a lower level of gain than is used in the prior art by using a lower level of electrical discharge than was used in the prior art. The lowered gain reduces the numerical magnitude of the difference in the gain of the laser at 308.4 nm and its gain at the highest gain modes of operation. Either a low level signal at 308.4 nm is injected into the laser to mode lock the lowered gain laser at 308.4 nm. or a frequency selection device such as etalons is inserted within the laser cavity to restrict operation to 308.4 nm. The period of the electrical discharge also is lengthened from that typically used in the prior art so as to obtain saturation of the output at 308.4 nm.

Injection locking a xenon chloride laser at 308.4 nm

A laser system for producing a laser beam in an unstable resonator whose spatial and temporal characteristics conform with those of a laser beam from a stable resonator. The laser system includes three reflectors, an optical switch and a laser gain medium positioned within an optical propagation path extending between the optical switch and the third reflector. The optical switch selectably exposes either the first or the second reflector to the aforesaid propagation path, the first and second reflectors being designed to form, in combination with the third reflector, a stable and an unstable resonator, respectively. In operation, the optical switch initially is set so as to include the first reflector in the laser resonator, thereby forming a stable resonator which produces a beam of laser light. A short time thereafter, the optical switch changes states so as to substitute the second reflector for the first, thereby forming an unstable resonator. The light from the stable resonator serves as 'seed radiation' for the unstable resonator, so that the unstable resonator produces a light beam having the same spatial and temporal characteristics as the light from the stable resonator. The system permits the use of a single laser gain medium for both resonators, and it overcomes the need for holes or other surface discontinuities in the resonator reflectors which can degrade the quality of the laser's output beam.

Growing community of inventors

San Pedro, CA, United States of America

Stephen John Brosnan