Compact 2D optical fiber assembly

Abstract

A method of assembling a two-dimensional fiber array launcher assembly. The method includes providing an alignment structure including a two-dimensional alignment plate having holes at one end and a two-dimensional beam shaper having microlenses at the opposite end. An end cap with a fiber attached thereto is systematically positioned within each hole and aligned with one of the microlenses to high precision tolerances. The aligned end cap is then secured within the hole with a curable adhesive. This process continues until all of the holes have aligned end caps. If one of the end caps becomes misaligned or damaged, the adhesive can be heated and the end cap realigned or replaced.

Description

[Technical field]

[0001]

[0001] The present disclosure relates generally to a method for assembling a two-dimensional fiber array launcher assembly, and more specifically to a method for assembling a two-dimensional fiber array launcher assembly, including the steps of providing an alignment structure having a two-dimensional alignment plate with a hole at one end and a two-dimensional beam shaper having a microlens at an opposite end, systematically positioning an end cap having a fiber attached thereto within each hole, aligning the fiber with one of the microlenses to high precision tolerances, and fixing the end cap within the hole with a curable adhesive. [Background technology]

[0002]

[0002] Directed energy (DE) systems, which direct high-energy optical beams to targets, are rapidly being realized in real-world operating 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 the primary driving factors for DE systems. Fiber laser amplifiers have proven desirable as energy sources for DE systems due to their high efficiency, high-power scalability, and excellent beam quality. Fiber laser systems use multiple fiber laser amplifiers that somehow combine the amplified beams to generate higher power. The design challenge for this type of fiber laser amplifier system is to combine the beams from multiple fiber amplifiers so that the beams can produce a single beam output with uniform phase across the beam diameter and 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.

[0003]

[0003] There are two general approaches to scaling beam combiner systems to higher powers. One approach is known as spectral beam combining (SBC), where multiple lasers of different wavelengths are combined into a single beam on a diffraction grating or other scattering optics. The other approach is known as coherent beam combining (CBC), where multiple mutually coherent lasers are locked in phase with each other and combined into a single beam by superposing them in the near field using a beam splitter, or by tiling them side by side to form a composite beam, a configuration colloquially called a "phased array."

[0004]

[0004] Current state of the art for CBC and SBC typically uses one-dimensional beam combining. However, two-dimensional high power coherent beam combining is a significant improvement over one-dimensional spectral beam combining because it allows for more compact, higher power beam combining systems, thus enabling scaling up to megawatt capabilities. Due to the nature of coherent beam combining, the performance of the final combined beam is highly dependent on the placement of the beams in two-dimensional space. This leads to the requirement of being able to align and fix many (tens to hundreds) of beam channels in two-dimensional space to achieve output powers within 1 kW. With such high optical powers, the fiber-to-air interface is highly likely to reach the damage power threshold of the fiber glass. The industry standard way to address this is to attach a coreless end cap to the fiber, which allows for the expansion of the optical beam, which leads to a reduction in the optical power density at the air interface.

[0005]

[0005] Furthermore, as these types of beam combining systems scale to perhaps hundreds of fibers to reach megawatt-class power, there is a need to be able to replace individual fiber channels. In particular, if a fiber channel becomes misaligned during assembly or is damaged during operation, the performance of the system is improved by the ability to replace or correct the channel. Several designs have been used in the art, including replaceable individual units, stacked arrays, and direct Co2 laser welding to glass surfaces. However, all of these designs have not demonstrated sub-micron alignment tolerances and are susceptible to stack-up errors across multiple sub-component structures, and only Robust Electrical Laser Initiative (RELI) type units have the ability for sub-unit replacement at the expense of a large pitch between components, which increases the total size of the system and results in expensive, complex assemblies. Thus, a two-dimensional beam combining architecture is needed that provides a better solution. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a side view of a two-dimensional fiber array launcher assembly. [Diagram 2]

[0007] FIG. 13 is a front view of the 2D end cap alignment plate separated from the fiber array launcher assembly. [Diagram 3]

[0008] FIG. 1 is a front view of the two-dimensional monolithic beam shaper, separated from the fiber array launcher assembly. [Figure 4]

[0009] FIG. 1 is an isometric view of an assembled fiber array launcher assembly combined with beam diagnostics. [Diagram 5]

[0010] FIG. 1 is a schematic block diagram of a CBC fiber laser amplifier system including a fiber array launcher assembly. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007]

[0011] The following discussion of embodiments of the present disclosure directed to a method of assembling a two-dimensional 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 fiber array launcher assembly has particular application as a beam emitter in a CBC fiber laser amplifier system. However, as will be appreciated by those skilled in the art, the fiber array launcher assembly may have applications for other optical systems.

[0008]

[0012] FIG. 1 is a side view of a two-dimensional fiber array launcher assembly 10 including an alignment structure 12 having a support base 14 and a hollow body 16 having a front end 18, a rear end 20, and an opening 22 therebetween. In one embodiment, the alignment structure 12 is made from invariant steel so as to have a low coefficient of thermal expansion. An alignment plate 24 is mounted within the rear end 20 of the body 16 and includes a glass substrate 26 having a two-dimensional array of holes 28 extending therethrough. FIG. 2 is a front view of the plate 24, separated from the assembly 10. The configuration of the holes 28 is shown merely as an example in that any configuration and number of holes 28 suitable for the purposes discussed herein may be used. The alignment plate 24 may be cut with a CNC machine to allow tolerances in the order of a few microns in spacing and hole dimensions. The size of the holes 28 is carefully chosen to allow room for assembly errors and alignment. A clearance diameter between the holes 28 on the order of 50-100 microns may be a good balance between these two needs.

[0009]

[0013] Several optical fibers 32 deliver the amplified light beam 30 to the launcher assembly 10. For some high energy applications, the number could be hundreds of fibers. The end of each fiber 32 is spliced, i.e., cut, and secured, for example by fusion splicing, to the end of an optical or glass cylindrical end cap 34, where an anti-reflective coating 36 is formed on the opposite end of the end cap 34 and an angled facet on the end cap 34 is provided to reduce back reflections. The glass for the end cap 34 must be of extremely high purity to reduce absorption. The diameter of the end cap 34 is larger than the diameter of the fiber 32 and slightly smaller than the diameter of the hole 28. The larger diameter end cap 34 allows the beam 30 to expand as it propagates through the end cap 34, so that the end cap 34 protects the end of the fiber 32 against the high power density of the beam 30. Each end cap 34 is extended through one of the holes 28 and secured thereto, for example by a suitable adhesive, as discussed in more detail below.

[0010]

[0014] The monolithic glass beam shaper 40 is prepared on a base plate 42 attached to the support base 14, and the beam shaper 40 is surface-bonded to the front end 18 of the body 16, for example by a suitable adhesive, and is open to the hollow interior of the body 16, so that there are no strict tolerances on the X / Y structural dimensions. The beam shaper 40 includes a two-dimensional array of microlenses 44 formed in an optical glass plate 46 and positioned so that each microlens 44 aligns with one of the end caps 34. FIG. 3 is a front view of the beam shaper 40, separated from the assembly 10. The beam shaper 40 can be constructed using laser ablation machining, which can shape the surface of the glass in any manner. This can be done with a high level of accuracy, but with errors on the order of a few microns in placement. The Z-direction alignment is determined by setting the end caps 34 at the correct depth, so only minimal tolerances are required for its dimensions. Considering these components, there may be multiple micron errors, which far exceed the necessary requirements when stacked. However, there is only one critical alignment step during the entire assembly, and that is the position of the end cap 34 within the hole 28 .

[0011]

[0015] The beam 30 emitted from each fiber 32 propagates out through the end cap 34 and expands into free space through the hollow body 16. Due to the defined length of the body 16 and the size and position of the microlenses 44, the expanding beam 30 fills the entire microlens 44 and is collimated by the microlenses 44. An optional beam shaper correction plate 48 can be provided on the base 42 at a predefined distance from the beam shaper 40 and includes a microlens aligned with each of the microlenses 44, the correction plate 48 providing beam correction for beam alignment and manufacturing errors.

[0012]

[0016] FIG. 4 is an isometric view of the fiber array launcher assembly 10 in an assembled state. When dealing with very tight tolerances, it is important to reduce stack-up errors and the need for very high precision and high cost machining. The assembly process for the fiber array launcher assembly 10 allows for low cost and low precision parts to provide high precision part alignment, for example, by individually aligning the end cap 34 to the beam shaper 40 using high precision beam diagnostics 56. The glass alignment plate 24 and the beam shaper 40 are assembled to the ends 20 and 18 of the body 16, respectively, with relatively low tolerance alignment relative to each other. This step can be done using passive mechanical alignment, or the beam shaper 40 can be actively aligned to the end cap prior to assembly. The end cap 34 is attached to the fiber 32 using standard optics. Specifically, the fiber 32 is cut and then spliced ​​to the end cap 34. Each end cap 34 is then inserted into a respective hole 28 in the plate 24 and then moved inside the hole 28 using any suitable device 50, such as a 5-axis precision stage, in coordination with precision beam diagnostics 56 until it is in high tolerance alignment with the microlenses 44 in the beam shaper 40 to obtain sub-micron tolerances. The end caps 34 are held in their respective holes 28 in high precision alignment, and a suitable heated adhesive 52, such as a UV-curable epoxy or low viscosity adhesive, is placed in the holes 28 to flow around the end caps 34. The adhesive 52 is cured and hardened so that the end caps 34 are held in the desired alignment when released by the device 50. Laser welding may also be used to secure the end caps 34 to the plate 24. Each end cap 34 is thus systematically and individually secured to the plate 24. If the assembled end cap 34 becomes misaligned or if the fiber 32 or end cap 34 is damaged, the adhesive 52 can be reheated, for example, with a soldering iron or laser source, and the end cap 34 can be realigned or replaced.

[0013]

[0017] 5 is a schematic block diagram of a CBC fiber laser amplifier system 60 using the fiber array launcher assembly 10. The system 60 includes a seed beam source 62 that generates a continuous wave frequency modulated seed beam having a center wavelength λ on a fiber 64. The source 62 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-optic 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), such as white noise or a pseudorandom bit sequence (PRBS), that results in frequency modulation broadening, so that the modulated seed beam has a substantially broadened linewidth, which suppresses stimulated Brillouin scattering in a downstream high power fiber amplifier. The modulated seed beam on fiber 64 is split by optical splitter 66 to produce multiple split seed beams on fiber 68 having the same wavelength λ, and each split seed beam is sent to a separate EOM 70 that provides phase control of the seed beam for phase locking purposes.

[0014]

[0018] Each of the modulated seed beams is fed onto a fiber 72 and sent to a fiber amplifier 74, such as a Yb-doped fiber amplifier, which is typically a doped amplifying section of the fiber 72 that receives an optical pump beam (not shown) to generate the amplified beam 30. The amplified beam 30 is directed onto the fiber 32 and sent through the fiber array launcher assembly 10, which acts as a beam emitter. The emitted amplified beam 30 from the fiber array launcher assembly 10 is directed as a combined amplified beam through a beam splitter 76 in a phase detection assembly 78, which samples a sample portion of each of the individual beams in the combined beam. The optical splitter 66 is also configured to generate a reference beam 80, which is sent to a reference beam modulator 82, which modulates the reference beam 80. The reference beam modulator 82 may include, for example, an acousto-optic modulator that shifts the center frequency of the reference beam 80, or an EOM that imparts a digital phase shift to the reference beam 80. The modulated reference beam 80 is expanded by beam expander 84 to provide a planar wavefront and to overlap with the combined amplified beam on beam splitter 76. The intensities of the combined reference and sample beams are detected by an array 94 of photodetectors 96. The main portion of the combined amplified beam is sent to a beam director telescope 90, which directs an output beam 92 to a target (not shown).

[0015]

[0019] The electrical signal from the photodetector 96 is used by a phase lock controller 98 to control the EOM 70 to correct the phase of the seed beam, for example, using a phase lock technique such as optical heterodyne detection (OHD), well known to those skilled in the art. However, other phase lock techniques (not shown) can be used, which may not require a frequency-shifted reference beam, or instead use a far-field generating lens that focuses the entire beam array onto a single detector, and the error signal for each channel is extracted electronically, for example, using a variety of multi-dithering techniques, such as the stochastic parallel gradient descent (SPGD) algorithm, well known to those skilled in the art. The controller 98 may receive other data and information, such as wavefront aberration data or beam steering set points, which are provided to box 100 to determine the phase set points. More specifically, the controller 98 provides error signals of the phase difference between the sampled beams and provides those error signals to the EOM 70 to control the phase of the individual seed beams so that all of the seed beams are locked in phase. In other words, EOM 70 provides seed beam phase control such that the "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 varied relative to one another for purposes of electronic beam steering. Controller 98 can also provide beam phase control to correct for measured atmospheric anomalies, such that output beam 92 can have wavefront aberrations modified as a result of propagation through atmospheric aberrations, so that beam 92 is of the desired quality when it impinges on the target.

[0016]

[0020] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. Those skilled in the art will readily appreciate from such discussion, and from the accompanying drawings and claims, that various changes, modifications, and variations can be made without departing from the spirit and scope of the present disclosure, as defined in the appended claims.

Claims

1. 1. A method of assembling a two-dimensional fiber array launcher assembly, comprising: providing a plurality of fiber assemblies, each including a fiber having an end face joined to an end of an end cap, the end cap having a larger diameter than the fiber; providing an alignment structure including a hollow body having a front end and a rear end; Providing an alignment plate containing a two-dimensional array of holes; providing a beam shaper comprising a two-dimensional array of lenses, a separate lens for each hole on the alignment plate; mounting the alignment plate within the trailing end of the alignment structure; mounting the beam shaper to the front end of the alignment structure such that one of the lenses is generally aligned with each hole in the alignment plate; positioning one of the end caps within one of the holes in the alignment plate; Propagating a beam downstream of the fiber, the beam expanding through the end cap and the hollow body, collimated by each of the lenses in the beam shaper, and received by a beam diagnostic device; adjusting the position of the end cap within the hole until the beam is aligned with the lens to a desired tolerance; disposing an adhesive in the hole to secure the end cap to the alignment plate in the aligned position; repeating the steps of positioning one of the end caps within one of the holes, propagating a beam downstream of the fiber, adjusting the position of the end cap within the hole, and for each hole in the alignment plate, disposing adhesive within the hole; The method includes:

2. The method of claim 1 , wherein the adhesive is a UV curable epoxy.

3. The method of claim 1 , wherein the end caps, the alignment plate, and the beam shaper are glass.

4. The method of claim 1 , wherein the alignment structures are metal.

5. The method of claim 1 , wherein the end cap is cylindrical and the hole is circular.

6. The method of claim 1 , further comprising the step of heating the adhesive in one of the holes to realign a misaligned end cap or to replace a damaged end cap.

7. 10. The method of claim 1, wherein the fiber array launcher assembly is a beam emitter in a coherent beam combining (CBC) fiber laser amplifier system.

8. 1. A method of assembling a two-dimensional fiber array launcher assembly, comprising: providing a plurality of fiber assemblies, each including a fiber having an end face joined to an end of an end cap, the end cap having a larger diameter than the fiber; providing an alignment structure including a two-dimensional glass alignment plate having a hole at one end and a two-dimensional beam shaper having a microlens at an opposite end; positioning an end cap within one of the holes and aligning the associated fiber to one of the microlenses with high precision tolerances; adhering the aligned end caps into the holes with a curable adhesive; repeating the steps of positioning end caps and adhering the aligned end caps until all of the holes have aligned end caps; The method includes:

9. The method of claim 8 , wherein the adhesive is a UV curable epoxy.

10. The method of claim 8 , wherein the end cap is cylindrical and the hole is circular.

11. 9. The method of claim 8, further comprising the step of heating the adhesive in one of the holes to realign a misaligned end cap or to replace a damaged end cap.

12. 10. The method of claim 8, wherein the fiber array launcher assembly is a beam emitter in a coherent beam combining (CBC) fiber laser amplifier system.

13. 1. A two-dimensional fiber array launcher assembly comprising: an alignment structure including a hollow body having a front end and a rear end; an alignment plate positioned within the rear end of the body and including a two-dimensional array of holes; a beam shaper positioned within the front end of the body and including a two-dimensional array of lenses, a respective lens aligned with each hole in the alignment plate; a plurality of fiber assemblies, each including a fiber having an end face joined to an end of an end cap, the end caps having a larger diameter than the fiber, a respective end cap secured within each bore such that the fiber is aligned with an associated lens; A launcher assembly comprising:

14. The launcher assembly of claim 13 , wherein each end cap is secured within the hole by an adhesive.

15. The launcher assembly of claim 14 , wherein the adhesive is a UV curable epoxy.

16. The launcher assembly of claim 13 , wherein the end caps, the alignment plate, and the beam shaper are glass.

17. The launcher assembly of claim 13 , wherein the alignment structure is metallic.

18. The launcher assembly of claim 13 , wherein said end cap is cylindrical and said hole is circular.

19. 14. The launcher assembly of claim 13, wherein the fiber array launcher assembly is a beam emitter in a coherent beam combining (CBC) fiber laser amplifier system.

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