Patent Description:
Directed energy (DE) systems that direct a high energy optical beam to a target are rapidly being realized in real-world operational environments. Reliable, robust and efficient beam delivery of individual multi-kW class lasers and high energy and peak power pulsed illuminators to remote beam directors and combiners are key driving elements for DE systems. Fiber laser amplifiers have proven to be desirable as energy sources for DE systems because of their high efficiency, high power scalability and excellent beam quality. Fiber laser systems employ multiple fiber laser amplifiers that combine the amplified beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fiber amplifiers in a manner so that the beams provide a single beam output having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot, where focusing the combined beam to a small spot at a long distance (far-field) defines the quality of the beam.

There are two general approaches for scaling beam combiner systems to higher powers. One approach is known as spectral beam combining (SBC), where multiple lasers of different wavelengths are combined on a diffraction grating or other dispersive optic into a single beam. The other approach is known as coherent beam combining (CBC), where multiple mutually coherent lasers are locked in phase with one another and combined into a single beam either by overlapping in the near field using a beam splitter, or by tiling side by side to form a composite beam, a configuration that is colloquially referred to as a "phased array".

Conventional single mode and large mode area (LMA) fiber technology is limited by nonlinear effects when fiber lengths are more than a few meters at multiple-kW optical power levels. This limitation places sever size and weight constrains in high-power laser system architecture, for example, DE weapons applications, where it is desirable to deliver laser beams to distances of <NUM>'s of meters without using free space optics.

Hollow core (HC) fiber technology has proven to be an ideal solution to overcome the nonlinear effects of traditional fiber technology. Hollow core fibers guide the optical mode based on photonic bandgap principles, instead of index of refraction gradient. A hollow core fiber light guiding core consists of ><NUM>% air, which reduces the nonlinear effects of fused silica proportionally. Current state of the art hollow core fibers have shown to be capable of delivering kW level average powers and <NUM>'s kW peak powers with low loss (<<NUM>. 03dB/m, with a path to even lower) and minimal nonlinearity. Development work has mainly been focused on the fiber itself rather than the termination and ability to couple into these fibers efficiently, both critical aspects of DE systems.

The unique structure of hollow core fibers requires additional considerations because the air core at the end of the fiber is open to the environment. If the end is not sealed, even a small amount of contamination can result in thermal destruction due to absorption from the high intensity beam. However, an open hollow core fiber end that is terminated with a high quality cleave would deliver a flat wave front free-space beam. This termination would have minimum reflection and distortion due to good impedance match to air (from > <NUM>% air guiding core). If the air core of the fiber is sealed with a fused silica block, such as an end cap, the environmental contamination issue can potentially be eliminated. However, the process of jointing the fiber to the end cap introduces two additional issues to this interface. Particularly, contamination can be 'locked in" during the jointing process of glass fusing with various heating techniques, some with electrodes that emit carbon or metal, and during glass fusing, melting of both the hollow core fiber and the endcap introduces length uncertainty between the multiple air cores that make up the guide region. Subsequently, the free-space propagating beam wave front cannot be consistently reproduced with high certainty. This problem impacts performance in a fiber array where the wave fronts need to be matched.

Prior art is found in <CIT> which generally relates to a method of changing the surface of a glass substrate containing silver, by using a laser beam in order to obtain a microlens array. The microlens array can be combined with a two dimension optical fiber array obtained by forming holes in a second glass substrate, inserting the optical fibers into the holes and fixing them with ultraviolet curable plastic.

The following discussion of the embodiments of the disclosure directed to a hollow core optical fiber array launcher assembly is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. For example, the hollow core optical fiber array launcher assembly has particular application as a beam emitter in a CBC or an SBC fiber laser amplifier system. However, as will be appreciated by those skilled in the art, the hollow core optical fiber array launcher assembly may have application for other optical systems. The invention is set out in the independent claim.

<FIG> is an isometric type view and <FIG> is a side cross-sectional type view of a 1D beam combiner array assembly <NUM> that has application for both spectral and coherent beam combining for high power or direct energy laser weapon systems. The assembly <NUM> includes a monolithic array block <NUM> made of, for example, glass or other suitable optical material, and including a series of parallel, sealed and optically separated cylindrical channels <NUM> extending from a back wall <NUM> of the block <NUM> having a certain thickness to a cavity <NUM> open to a front surface <NUM> of the block <NUM>. A series of spaced apart fiber flanges <NUM> are coupled by, for example, glue to a back surface of the back wall <NUM> and a monolithic beam shaper lens array <NUM> is optically coupled by, for example, glue to the front surface <NUM> and includes an array of lenses <NUM>, where a separate one of the flanges <NUM> and a separate one of the lens <NUM> are aligned with each one of the channels <NUM>. An anti-reflection (AR) coating or nano-texture AR surface <NUM> can be provided on an input surface and an output surface of the lens array <NUM> to reduce beam reflection.

A separate hollow core fiber <NUM> extends through an orifice <NUM> in each of the flanges <NUM> and an orifice <NUM> in the back wall <NUM> of the block <NUM> and is secured thereto so that an end <NUM> of the fiber <NUM> is positioned in the channel <NUM> at a focal point of the associated lens <NUM>, where the fiber <NUM> can be of any design suitable for the purposes discussed herein and where there are eight fibers <NUM> in this embodiment. <FIG> is a cross-sectional view of one of the hollow core fibers <NUM> showing one non-limiting design. The fiber <NUM> includes a central hollow core <NUM> filled with air or other gas surrounded by a cladding layer <NUM> made up of a honeycomb array of hexagonal shaped hollow tubes <NUM> filled with the same or similar gas to provide a higher index of refraction than the gas in the core <NUM> so as to confine an optical beam <NUM> mostly within the core <NUM> as it propagates down the fiber <NUM>. The cladding layer <NUM> is surrounded by a solid glass outer protective layer <NUM>. When the optical beam <NUM> propagates down the fiber <NUM> and is emitted from the end <NUM> of the fiber <NUM> into free space into the channel <NUM>, it is focused by the lens <NUM> to be collimated in combination with the other beams to generate a combined high power output beam. The lenses <NUM> are closely space together to provide a compact output beam, where the pitch between the lenses <NUM> may be <NUM>-<NUM>. The lenses <NUM> can be configured to shape the beam <NUM> in any desirable manner including changing the spatial phase distribution, or wavefront, of the beam <NUM>, for example, to a square beam to increase beam fill factor.

It is noted that in an alternate embodiment the block <NUM> can have a single open space that all of the beams <NUM> propagate through instead of the separate channels <NUM>. However, since it is desirable from a reliability stand point to reduce as much contamination of the beams <NUM> as possible, the separate channels <NUM> would be more desirable. It is desirable that the fibers <NUM>, the array block <NUM> and the lens array <NUM> be made out of the same material so that thermal effects on the assembly <NUM> are consistent and don't affect beam quality. Further, it is desirable to make the array block <NUM> out of an optically transparent material, such as glass, so that any scattered light in the channels <NUM> propagates out of the block <NUM> and does not heat the block <NUM>.

In order to obtain high beam quality and output power, it is necessary that the orientation of the fibers <NUM> in all of the x-y-z directions, where the z direction is along the propagation direction of the beam <NUM>, be very accurate and precise so that there is no tilt to the end <NUM> of the fiber <NUM> and the end <NUM> of the fiber <NUM> is at the desired focal point location in the channel <NUM> so that the beam <NUM> is emitted from the end <NUM> of the fiber <NUM> exactly parallel to all of the other beams. The following discussion referencing <FIG> showing assembly steps of the beam combiner array assembly <NUM> is one non-limiting embodiment for assembling the beam combiner array assembly <NUM> to provide the desired precision, where many of the assembly steps can be automated and performed by robotics.

The block <NUM> and the lens array <NUM> are separately fabricated as monolithic elements by machining a separate suitable block of optical material, such as glass, in any suitable manner to form the channels <NUM> and the lenses <NUM>. The block <NUM> and the lens array <NUM> are then aligned and glued together along the front surface <NUM> of the block <NUM>, where the position of the array <NUM> will determine the position of the ends <NUM> of the fibers <NUM>, see <FIG>. Each fiber <NUM> is then slid through the orifice <NUM> in its respective flange <NUM>, where the diameter of the orifice <NUM> is slightly larger than the outer diameter of the fiber <NUM>, and the end <NUM> of each fiber <NUM> is cleaved to form a clean front end surface <NUM> for the guide optical mode to propagate out of the fiber <NUM> into free space, see <FIG>. The end <NUM> of each fiber <NUM> is then slid through its respective orifice <NUM> in the back wall <NUM> of the block <NUM> so that the fiber <NUM> extends into the respective channel <NUM>, see <FIG>. The diameter of the orifice <NUM> is larger than the diameter of the fiber <NUM> by an amount so that a robot (not shown) can be used to insert the fiber <NUM> through the orifice <NUM> so that the fiber end surface <NUM> does not touch the block <NUM> to prevent contamination.

The assembly <NUM> as it stands so far is then placed in a fixture (not shown) provided relative to laser beam diagnostic equipment <NUM> that provides a global reference and the beam <NUM> is sent down the fiber <NUM> so that it impinges the equipment <NUM>, see <FIG>. A translator <NUM> movable in the x, y and z directions is coupled to the fiber <NUM> and is moved in each direction, where the intensity of the beam <NUM> is analyzed by the equipment <NUM> to precisely position the end <NUM> of the fiber <NUM> with the proper orientation, and the fiber <NUM> is held in this position. The flange <NUM> is slid along the fiber <NUM> towards the array block <NUM> until it contacts the back wall <NUM> of the block <NUM>, and then is slid back some distance to provide a glue gap <NUM> therebetween, and the fiber <NUM> is then glued to the flange <NUM> while it is in this position using glue <NUM>, see <FIG>. Glue <NUM> is applied in the glue gap <NUM> and a final fine tune alignment between the fiber <NUM> and the block <NUM> is performed using the equipment <NUM>, and then the glue <NUM> is dried or cured so that the channel <NUM> is sealed. Each fiber <NUM> is sequentially coupled to and aligned with the block <NUM> in this manner.

The beam combiner array assembly <NUM> is a 1D array. However, multiple 1D assemblies can be stacked and coupled together to make a 2D beam combiner array assembly. For example, three of the assemblies <NUM> can be coupled together to define a <NUM> X <NUM> array assembly <NUM>, as shown in <FIG>, where the blocks <NUM> in each assembly <NUM> are glued together after being aligned by the equipment <NUM>.

As mentioned above, the beam combiner array assembly <NUM> can be used in spectra and coherent beam combining fiber laser amplifier systems. The following discussion refers to various examples of the assembly <NUM> being used in various embodiments of these types of systems.

<FIG> is a schematic block diagram of a CBC fiber laser amplifier system <NUM> that includes a seed beam source <NUM> that generates a continuous wave frequency-modulated seed beam having a center wavelength λ on a fiber <NUM>. The source <NUM> may include a master oscillator (MO), such as a single-longitudinal mode distributed feedback (DFB) diode laser oscillator, and a frequency modulator, such as an electro-optical modulator (EOM). The EOM may receive an applied voltage provided by an amplified radio frequency (RF) electrical drive signal from an RF source (not shown) that provides frequency modulation broadening, such as white noise or pseudo-random bit sequence (PRBS), so that the modulated seed beam has a linewidth that is substantially broadened, which suppresses stimulated Brillouin scattering in a downstream high power fiber amplifier. The modulated seed beam on the fiber <NUM> is split by an optical splitter <NUM> to produce a plurality of split seed beams on fibers <NUM> having the same wavelength λ, where each split seed beam is sent to a separate EOM <NUM> that provides servo-phase control of the seed beams for phase-locking purposes.

Each of the modulated seed beams is provided on a fiber <NUM> and sent to a fiber amplifier <NUM>, such as a Yb-doped fiber amplifier, where the amplifier <NUM> will typically be a doped amplifying portion of the fiber <NUM> that receives an optical pump beam (not shown). All of the amplified beams are directed onto hollow core fibers <NUM> and sent to a beam combiner array assembly <NUM> of the type discussed above that operates as a beam emitter. The emitted amplified beams <NUM> from the beam combiner array assembly <NUM> are directed as a combined amplified beam through a beam splitter <NUM> in a phase sensing assembly <NUM> that samples off a sample portion of each of the separate beams in the combined beam. The optical splitter <NUM> is configured to also generate a reference beam <NUM> that is sent to a reference beam modulator <NUM> that modulates the reference beam <NUM>. The reference beam modulator <NUM> could include, for example, an acousto-optic modulator that shifts the center frequency of the reference beam <NUM>, or an EOM that imparts a digital phase shift to the reference beam <NUM>. The modulated reference beam <NUM> is expanded by a beam expander <NUM> to provide a flat wavefront and to overlap with the combined amplified beam on the beam splitter <NUM>. The intensities of the overlapped reference and sample beams are detected by an array <NUM> of photodetectors <NUM>. The main part of the combined amplified beam is sent to a beam director telescope <NUM> that directs an output beam <NUM> to a target (not shown).

The electrical signals from the photodetectors <NUM> are used by a phase locking controller <NUM> to control the EOMs <NUM> to correct the phase of the seed beams, using, for example, a phase-locking technique, such as optical heterodyne detection (OHD), well known to those skilled in the art. However, other phase-locking techniques (not shown) can be employed that may not require a frequency shifted reference beam, or instead using a far-field generating lens that focuses the entire beam array onto a single detector, where error signals for each channel are extracted electrically using a variety of multi-dither approaches, such as, for example, a stochastic parallel gradient decent (SPGD) algorithm, well known to those skilled in the art. The controller <NUM> may receive other data and information provided by box <NUM> to determine the phase set-points, such as wavefront aberration data or beam steering set-points. More particularly, the controller <NUM> provides error signals of the phase difference between the sampled beams, and provides those error signals to the EOMs <NUM> to control the phases of the individual seed beams so that all of the seed beams are locked in phase. In other words, the EOMs <NUM> provide seed beam phase control so that a "piston" phase of the combined amplified beam is spatially uniform across the beam wavefront. This also allows the phases of the seed beams to be altered relative to each other for electronic beam steering purposes. The controller <NUM> can also impart phase control of the beams to correct for measured atmospheric anomalies where the output beam <NUM> may have wavefront aberrations that are corrected as a result of propagating through the atmospheric aberrations so that the beam <NUM> is of the desired quality when it impinges the target.

<FIG> is a schematic block diagram of an SBC fiber laser amplifier system <NUM> that includes N number of wavelength channels <NUM> each having a seed beam source <NUM> that generates a continuous wave frequency-modulated seed beam having a center wavelength λ on a fiber <NUM> for the particular channel <NUM>, where each seed beam source <NUM> generates a different beam wavelength λ<NUM> - λN. Each of the seed beams on the fibers <NUM> is sent to a fiber amplifier <NUM>, such as a Yb-doped fiber amplifier, where the amplifier <NUM> will typically be a doped amplifying portion of the fiber <NUM> that receives an optical pump beam (not shown). All of the amplified beams are directed to a one-dimensional beam combiner array assembly <NUM> that is similar to the beam combiner array assembly <NUM>, but does not provide phase locking as discussed above because all of the beams have different wavelengths. The beams <NUM> from the assembly <NUM> are reflected off of a set of collimating optics <NUM> that collimates the diverse beams <NUM> and directs them onto an SBC grating <NUM> so that all of the individual beams <NUM> impact the grating <NUM> and overlap on the same footprint. The grating <NUM> spatially diffracts the individual beam wavelengths λ<NUM> - λN and directs the individual amplified beams in the same direction as a combined output beam <NUM>.

SBC beam quality is limited by angular dispersion from the diffraction grating <NUM>. Since the individual fiber amplifiers have a finite optical linewidth owing to frequency modulation, power will be spread into different directions following diffraction from the diffraction grating <NUM>. If the span of these different directions is significant compared to the diffraction limited angle, then the beam quality will degrade. The degradation in beam quality can be minimized by decreasing the size of the beam footprint on the grating <NUM> along the combining dispersive axis, which increases the diffraction limited angle. Since the grating <NUM> is at a Fourier plane of the assembly <NUM>, this corresponds to an increase in the individual beam sizes at the assembly <NUM>. Hence, a high spatial fill factor along the combining axis of the assembly <NUM> provides improved beam quality. With an array of Gaussian beams, the fill factor at the assembly <NUM> cannot be increased without clipping the wings of the Gaussian beams leading to lost power. With an array of shaped high fill factor beams, for example, an array of hyper-Gaussian shaped beams, the spatial fill factor of the beam combiner array assembly <NUM> can be increased without incurring clipping losses, thus leading to improved beam quality without loss of power.

For the SBC configuration of the laser system <NUM>, the shape of the beams output from the beam combiner array assembly <NUM> will optimally be identical for all of the beams. However, the beam shapes may be different along the combining and the non-combining axes. Along the combining axis the beams may be shaped to provide a higher fill factor to minimize beam quality loss due to angular dispersion. Along the non-combining axis, the beams may, for example, be left unshaped to generate a near-Gaussian beam profile on the grating <NUM>. This can be advantageous to maximize power on a far-field target whose size is between 1x and 2x diffraction-limited (DL), since it minimizes power diffracted into far-field sidelobes at angles larger than 2x diffraction-limited. Alternatively, the beams may be shaped along the non-combining axis to generate a high fill factor profile on the grating <NUM>, with intensity tapering to near zero at the telescope aperture. This can be advantageous to fully fill a beam director telescope to maximize far-field peak intensity on a target whose size is less than 1x diffraction-limited while minimizing clipping losses on the telescope aperture. The dispersive impact of the grating <NUM> can be minimized on output beam quality by choosing the beam combiner array assembly <NUM> and the set of collimating optics <NUM> so that the shaped beams incident on the grating <NUM> are narrow in the dispersive direction, but wider in the orthogonal non-dispersive direction. This asymmetric configuration lowers the peak irradiance on the grating <NUM> while also minimizing the degradation of the combined beam quality.

The number of beams that can be combined in both SBC and CBC architectures is limited for other reasons. In particular, the one-dimensional linear fiber array required for SBC may be impractically large. However, the limitations of providing CBC and SBC combined beams can be increased by a hybrid fiber laser amplifier architecture that combines both CBC and SBC architectures. This can be accomplished by providing CBC combined beams in one direction and SBC combined beams in an orthogonal direction. This combined CBC and SBC architecture can then be improved with the beam fill factor provided by the beam shaper array assembly <NUM> or the beam shaper array assembly <NUM> discussed above.

Several architectures can be employed to measure the beam phases in a hybrid CBC and SBC fiber laser amplifier system. <FIG> is a schematic block diagram of a hybrid CBC and SBC fiber laser amplifier system <NUM> illustrating one such architecture, where like elements to the system <NUM> are identified by the same reference number. The system <NUM> includes N number of SBC channels <NUM>, where each channel <NUM> includes a separate grouping of the seed beam source <NUM> having wavelength λi, where i is in the range <NUM> ≤ i ≥ N, the splitter <NUM>, the EOMs <NUM> and the amplifiers <NUM> as shown in the system <NUM>. As such, the ith channel <NUM> includes a single wavelength λi seed beam that is split into M multiple seed beams that are separately amplified and of the same wavelength λi, where there are N groups of M EOMs <NUM> and all of the channels <NUM> together generate MxN seed beams that are amplified at each of the different beam wavelengths λ<NUM> - λN. All of the MxN fibers <NUM> are coupled to a beam combiner array assembly <NUM> that outputs MxN output beams <NUM>.

The MxN beams <NUM> from the beam combiner array assembly <NUM> are collimated by a cylindrical optical system <NUM> and directed onto a diffraction grating <NUM> that operates in a similar manner to the diffraction grating <NUM>. The optical system <NUM> has curvature along the dispersive SBC axis <NUM> in the plane of the page. The focal length of the optical system <NUM> is selected to ensure that all of the N different wavelengths λi are incident at the correct angles to the grating <NUM> such that all of the diffracted output beams from the grating <NUM> are co-propagating in the same direction with the highest precision possible. As a result, a combined output beam <NUM> from the grating <NUM> comprises M parallel beams tiled along the CBC axis <NUM>, which is orthogonal to the page, and will have phase control of the individual beams along one axis and spatially diffracted beams along a perpendicular axis.

Also reflected from the grating <NUM> is a weak specular <NUM>th order beam <NUM>, which is focused by a lens <NUM> onto a detector array <NUM> having individual detectors <NUM>. Because the wavelength groups comprising the beam <NUM> propagate at different angles in a linear array, the focused beams from the lens <NUM> for each wavelength group are separated along a line and can be directed to a linear array of the detectors <NUM>. Each of the N detectors <NUM> receives the overlap of the M focused CBC beams comprising each wavelength group. The intensity on each detector <NUM> is maximized by a multi-dither processor <NUM> that provides dither signals superimposed with phase-locking control signals to the corresponding EOMs <NUM> for each wavelength group using, for example, an SPGD algorithm.

<FIG> is a schematic block diagram of another hybrid CBC and SBC fiber laser amplifier system <NUM> that is similar to the amplifier system <NUM>, except for phase sensing and control features, where like elements to the system <NUM> are identified by the same reference number. In this embodiment, the splitters <NUM> provide a reference beam <NUM> to be used as a reference for each wavelength group. Each reference beam <NUM> is modulated by a modulator <NUM>, and all of the modulated reference beams <NUM> are combined by a wavelength division multiplexer (WDM) <NUM> onto a single fiber <NUM>, and then collimated by a lens <NUM> to form a large multi-color planar reference beam <NUM>. The planar reference beam <NUM> is combined by sample optics <NUM> with a small sample of the MxN beams <NUM> transmitted from the beam combiner array assembly <NUM>.

The combined reference beam <NUM> and the sampled MxN beams <NUM> are received by a 2D MxN detector array <NUM> including individual detectors <NUM>, where wavelength filters (not shown) may be employed in the array <NUM> to eliminate noise from the reference beams <NUM> having wavelengths other that the correct wavelength λi intended for a given detector <NUM>. Alternately, the detector array <NUM> can be AC-coupled to reject DC photocurrent arising from the reference beams having wavelengths other than the correct wavelength intended for a given detector <NUM>. The heterodyne interference signal from each detector <NUM> in the array <NUM> is transmitted to an OHD processor <NUM> that provides phase correction signals to the EOMs <NUM> to phase lock each group of M beams at each wavelength λi.

It is noted that although the laser amplifier systems <NUM> and <NUM> provide specific phase control approaches, this is non-limiting in that other approaches may be suitable. The essential element in the hybrid fiber laser amplifier systems <NUM> and <NUM> is that the CBC beams need to be phase locked together with the desired phase profile similar to the system <NUM>. The essential difference in the hybrid systems <NUM> and <NUM> is that the CBC beam is only in one dimension, since the beams in the other dimension are spectrally combined.

Claim 1:
A beam combiner array assembly (<NUM>) comprising:
an array block (<NUM>) including a back wall (<NUM>) having a thickness and a front surface (<NUM>), said array block (<NUM>) including a plurality of aligned channels (<NUM>) extending from the back wall (<NUM>) to the front surface (<NUM>), wherein a respective bore (<NUM>) extends through the back wall (<NUM>) and into each channel (<NUM>);
a lens array (<NUM>) including a plurality of lenses (<NUM>), said lens array (<NUM>) being mounted to the front surface (<NUM>) of the block (<NUM>) so that one of the lenses (<NUM>) is aligned with each channel (<NUM>);
a plurality of fiber flanges (<NUM>) secured to the back wall (<NUM>) of the block (<NUM>) so that a separate one of the flanges (<NUM>) is aligned with each channel (<NUM>), each flange (<NUM>) including a bore (<NUM>) extending therethrough; and
a plurality of hollow core fibers (<NUM>) where a separate one of the fibers (<NUM>) extends through the bore (<NUM>) in one of the flanges (<NUM>) and one of the bores (<NUM>) in the back wall (<NUM>) so that an end of the fiber (<NUM>) is positioned within the channel (<NUM>), wherein a beam (<NUM>) that propagates down the fiber (<NUM>) is emitted into the channel (<NUM>), focused by the lens (<NUM>) and emitted from the assembly (<NUM>) as a collimated beam, wherein a diameter of the bore (<NUM>) in the flange (<NUM>) is larger than an outer diameter of the fiber (<NUM>), wherein the bore (<NUM>) in the back wall (<NUM>) is larger than a diameter of the fiber (<NUM>), wherein a diameter of the channel (<NUM>) is larger than the diameter of the bore (<NUM>) in the flange (<NUM>) and the bore (<NUM>) in the back wall (<NUM>), and wherein the beam (<NUM>) expands as it propagates down the channel (<NUM>) from the fiber (<NUM>) to the lens (<NUM>).