Optical apparatus for producing a high-brightness multi-laser radiation source

[0002] This application is a continuation-in-part of commonly-assigned application Ser. No. 09/042,885, filed on Mar. 17, 1998, now U.S. Pat. No. 6,075,912.

[0003] 1. Field of the Invention

[0004] This invention in general relates to an apparatus for coupling diverse sources of radiation into a waveguide device, such as an optical fiber or fiber laser, and, in particular, to a system using a unitary deflecting component to provide such coupling.

[0005] 2. Description of the Prior Art

[0006] There continues to be a need for ever-increasing levels of radiative power in applications such as printing, fabrication, and medical treatment. A commonly-used approach is to optically couple the emission beams from multiple radiative sources into an optical fiber or a fiber laser such that output radiation having a higher power level or more than one wavelength is emitted from the waveguide. The relevant art disclosed various configurations of such optical coupling systems. U.S. Pat. No. 5,418,880 issued to Lewis et al., for example, discloses a high-power fiber device powered by a linear array of laser diodes. The optical coupling system comprises an optical beam rotator for transforming the output of the laser diode array and transmitting the transformed output to collimating optics from which the radiation is coupling into the waveguide. In another approach, U.S. Pat. No. 5,333,077 issued to Legar et al. discloses a lens system comprising a first optical element for receiving and redirecting a laser beam from a linear diode array, for example, and a second optical element for further redirecting the laser beam to a focal point.

[0007] When greater levels of emitted power are required, discrete sources of radiation, such as laser diode devices, may be used. However, because of larger physical sizes and greater cooling requirements of these discrete devices, they cannot be emplaced in close proximity to one another and, consequently, cannot be adapted to be used in conjunction with the coupling devices designed for linear diode arrays. Rather, a plurality of separate mirrors is typically used to redirect the multiple radiation beams from discrete devices into the optical waveguide. For example, U.S. Pat. No. 4,828,357 issued to Arata et al. discloses an apparatus producing a high-power laser beam comprising a plurality of directing mirrors, a plurality of reflecting mirrors, and a central focusing mirror for focusing the resultant laser beam into one focal point. Another approach is disclosed in U.S. Pat. No. 5,210,643 issued to Fujii et al. in which an improved efficiency of combining laser beams is achieved through the use of suitably positioned dichroic mirrors and a beam splitter.

[0008] Unfortunately, the use of an arrangement of separate mirrors results in a system sensitive to misalignment and inadvertent movement, which results in complexity of alignment. Moreover, the use of multiple, separate mirrors makes it difficult to maintain the relative alignment of linearly-polarized beams at the waveguide input facet without the use of additional optical elements.

[0009] While the prior art describes a variety of optical coupling devices, there does not appear to have been disclosed in the relevant art a unitary device for coupling diverse sources of radiation into a waveguide, and it is a primary object of this invention to provide such a device.

[0010] It is another object of the present invention to provide such a device which can be used with sources of laser radiation which may differ in output power, wavelength, modulation, or polarization.

[0011] It is a further object of the present invention to provide such a device with which beams can be combined while maintaining relative alignment of the beam directions of polarization.

[0012] Other objects of the invention will be obvious, in part, and, in part, will become apparent when reading the detailed description to follow.

[0013] The present invention discloses an apparatus suitable for coupling the emissions from diverse sources of radiation into a waveguide such as an optical fiber or a fiber laser. The apparatus comprises a unitary beam deflector with a deflecting facet corresponding to each radiation source, an input afocal relay system for each irradiated facet, and an output afocal relay system disposed between the irradiated facets and the fiber. The apparatus and diverse radiation sources combine to produce a desired radiation output. Other features of the invention will be readily apparent when the following detailed description is read in connection with the drawings.

[0014] The structure and operation of the invention, together with other objects and advantages thereof, may best be understood by reading the detailed description to follow in connection with the drawings in which unique reference numerals have been used throughout for each part and wherein:

[0015] FIG. 1 is a diagrammatic perspective view of an embodiment of an apparatus for coupling radiation beams, in accordance with the present invention, illustrating the coupling of the emissions from two discrete laser devices into an optical fiber;

[0016] FIG. 2 is a diagram corresponding to the input end of the optical fiber of FIG. 1 illustrating the imaging of the laser emissions into the core of the optical fiber;

[0017] FIG. 3 is a diagrammatical plan view of an alternative embodiment of a coupling apparatus comprising a unitary beam deflector comprising six facets in accordance with the present invention illustrating the optical coupling of six sources of coherent radiation into a fiber laser;

[0018] FIG. 4 is a diagram corresponding to the input end view of the fiber laser of FIG. 3 showing the emissions of the laser sources of FIG. 3 imaged into the inner cladding of the fiber laser;

[0019] FIG. 5 is a diagrammatical perspective view of an alternative embodiment, comprising eight facets, of the unitary beam reflector of FIG. 3;

[0020] FIG. 6 is a diagrammatical plan view showing the use of a beam splitter to combine differently polarized outputs from two coupling apparatuses in accordance with the present invention;

[0021] FIG. 7 is a graph showing two curves representing pulsed output emission power levels, such as may be obtained from coupling apparatuses, where the sets of pulses are temporally offset from each other;

[0022] FIG. 8 is a diagrammatical representation of the unitary beam reflector of FIG. 5 showing the use of a half-wave plate to produce a different polarization in one of the radiation sources;

[0023] FIG. 9 is a diagram representing the step-wise variable output obtained from a plurality of radiation sources, each source having a different output power level;

[0024] FIG. 10 is a diagram representing the mixing of radiation sources of different wavelengths;

[0025] FIG. 11 is a diagrammatical representation of the cross-sectional view of a double-clad optical amplifier wherein pump radiation is input into the inner cladding and a signal beam is input into the core; and,

[0026] FIG. 12 is a diagrammatical representation of the cross-sectional view of an clad optical fiber wherein radiation beams of varying widths are input into the cladding layer.

[0027] There is shown in FIG. 1 a system 100, in accordance with the present invention, which provides for the coupling of continuous-wave or pulsed optical radiation, such as laser radiation beams 13 and 17 emitted by discrete laser devices 11 and 15, into a fiber laser or, as shown here, an optical fiber 10. Laser devices 11 and 15 emit output radiation of wavelengths λ₁and λ₂, respectively, where neither the wavelengths, the polarization, the output power levels, or pulse timing need be the same. The coupling of laser radiation beams 13 and 17 into optical fiber 10 is accomplished by means of an optical apparatus 110. Only two discrete laser devices are shown in the diagram, for purpose of clarity in illustration, and it should be understood that the present invention is not so limited and can be readily practiced with additional laser devices having one or more emission wavelengths, or with non-coherent radiation sources such as light-emitting diodes (LED) or an incandescent devices such as tungsten filament or arc lamps. Optical apparatus 110 comprises a unitary beam deflector 20, a first input optical system 31, a second input optical system 41, and an output optical system 51.

[0028] Unitary beam deflector 20 comprises two deflecting facets 22 and 24, where deflecting facet 22 is positioned in an optical path 37 of radiation beam 13 and oriented so as to deflect radiation beam 13 into optical fiber 10 along an optical path 39. Similarly, deflecting facet 24 is positioned in an optical path 47 of radiation beam 17 and is oriented so as to deflect radiation beam 17 into optical fiber 10 along an optical path 49. As can be appreciated by one skilled in the relevant art, deflecting facets 22 and 24 are preferably optically-smooth and reflective surfaces, as distinguished from a non-faceted surface region 25 of unitary beam deflector 20. Alternatively, deflecting facets 22 and 24 may comprise diffraction surfaces, or unitary beam reflector 20 may comprise an optically-transparent material and produce deflection by means of total internal reflection. It should also be noted that the distance along optical path 37 between laser device 11 and deflecting facet 22 need not be the same length as the distance along optical path 47 between laser device 15 and deflecting facet 24.

[0029] Deflecting facets 22 and 24 are substantially planar and may be formed in unitary beam deflector 20 by known methods, such as by diamond machining. It can be appreciated by one skilled in the relevant art that, while a particular method of machining may produce essentially planar deflecting facets having a slight cylindrical curvature, such facets can be also be used in practicing the present invention. In an alternative method of manufacture, unitary beam deflector can be fabricated as more than one component and formed into a unitary part by appropriate means. In a preferred embodiment, deflecting facets 22 and 24 are subsequently plated or optically coated so as to increase reflectivity at the wavelengths of the emissions of lasers 11 and 15. It should also be noted that, as it is not a requirement that radiation beams 13 and 17 lie in a common plane with optical fiber 10, the axes normal to the surfaces of deflecting facets 22 and 24 need not be coplanar.

[0030] First input optical system 31 preferably comprises an afocal relay system, an optical configuration well-known in the relevant art (see, for example, “Afocal systems,” Geometrical and Instrumental Optics,vol. 25, Academic Press, Inc., 1988, pp. 132-42). In the example provided, first input optical system is shown comprising a first lens 33 and a second lens 35 placed in optical path 37 of radiation beam 13. Lenses 33 and 35 are positive power elements having principal planes spaced apart by a distance (f₃₃+f₃₅), where f₃₃is the focal length of first lens 33 and f₃₅is the focal length of second lens 35. First lens 33 and second lens 35, either or both of which may comprise an anamorphic element, form an afocal relay system and serve to image the smallest portion of radiation beam 13 onto deflecting facet 22. As is understood by one skilled in the relevant art, for a radiation source such as a laser diode, the smallest portion of the radiation beam is at the diode output facet. For a laser diode, therefore, afocal relay optical system 31 images the diode output facet onto deflecting facet 22. Alternatively, for a single-mode radiation source, the smallest portion of the radiation beam (i.e., the waist) would be imaged onto deflecting facet 22. Similarly, a first lens 43 and a second lens 45, placed in optical path 47, serve to image the smallest portion of radiation beam 17 onto deflecting facet 24.

[0031] Output optical system 51 preferably comprises an afocal relay system and, for purpose of illustration, is here shown comprising a first lens 53 and a second lens 55 placed in optical paths 39 and 49 of deflected radiation beams 13 and 17. First lens 53 and second lens 55 serve to image deflected radiation beams 13 and 17 as a focused group of beam images 59 into a core 14 at input facet 19 of optical fiber 10. It can be appreciated that, for best coupling efficiency, deflecting facets 22 and 24 are shaped and sized to intercept essentially all radiation imaged by input afocal relay optical systems 31 and 41 such that this radiation is redirected along essentially parallel optical paths 39 and 49, respectively, into optical fiber 10.

[0032] In alternative embodiments, one or more afocal relay systems may be replaced with one or more optical systems which meet the criteria of: i) grouping the incident radiation beams within a desirable target area, and ii) insuring that the maximum extreme angle of any radiation beam falls within the acceptance angle, or numerical aperture, of the waveguide. Use of the preferable afocal relay system, however, serves to insure that there is no excess angle growth in the radiation beam.

[0033] As best seen in FIG. 2, optical fiber 10 comprises a core 14 surrounded by a cladding layer 12. Laser beam 13 is imaged into optical fiber 10 as focused beam image 61 and laser beam 17 is imaged as focused beam image 67. For best coupling efficiency, deflecting facets 22 and 24 are sized, positioned, and oriented such that focused beam images 61 and 67 fall within core 14 as shown. Note that the transverse axes of the imaged beams need not be aligned with each other, and that the beam sizes need not be identical.

[0034] In an alternative embodiment, shown in FIG. 3, an optical apparatus 200 is used to couple the radiation outputs of six discrete laser devices 111athrough 111f,having output emissions 113athrough 113frespectively, into an optical fiber or, as shown here, into inner cladding 71 of a fiber laser 70. Optical apparatus 200 comprises a unitary beam deflector 120, input afocal relay systems 131athrough 131f,and output afocal relay system 151. Unitary beam deflector 120 comprises six rectangular deflecting facets 122athrough 122f,where adjoining facets form a dihedral angle (e.g., facets 122fand 122e). In the example provided, the output radiation of discrete laser devices 111athrough 111fpropagate in the plane of FIG. 3. Accordingly, each of deflecting facets 122athrough 122fis orthogonal to this plane. The dimensions of each deflecting facet 122athrough 122fare selected so as to reflect most or all of the incident radiation. In the example shown, the height of each deflecting facet is sized to intercept and redirect the projection of the lateral component of the incident laser radiation, and the width of each deflecting facet is sized to intercept and redirect the projection of the transverse component of the incident laser radiation.

[0035] Output emission 113aof laser device 111ais imaged onto deflecting facet 122aby input afocal relay system 131a.Deflecting facet 122ais oriented so as to reflect this imaged output into fiber laser 70 along optical path 139a.Output afocal relay system 151 is positioned in optical path 139 so as to image the deflected radiation onto input end 119 as a focused beam image 161a.Each of deflecting facets 122bthrough 122fis likewise oriented to reflect output emissions 113bthrough 113ffrom laser devices 111bthrough 111frespectively into fiber laser 70, as focused beam images 161bthrough 161frespectively, as shown in FIG. 4. Preferably, focused beam images 161athrough 161fare projected into inner cladding 71. Inner cladding 71 is here shown having a rectangular cross-sectional shape, but may comprise one of a number of convex polygons (e.g., square, hexagon, rhombus, triangle) such as disclosed in commonly-assigned U.S. Pat. No. 5,533,163 issued to Muendel and incorporated herein in its entirety by reference.

[0036] FIG. 5 shows an alternative unitary beam deflector 180 comprising eight deflecting facets 181athrough 181h.Unitary beam deflector 180 maintains the relative alignment of linearly-polarized beams emitted from laser devices 171athrough 171h.Beam polarization vectors components P_Athough P_H, denoted as 173athrough 173hrespectively, are parallel to one another prior to reflection from unitary beam deflector 180 and maintain this relative alignment after reflection, as denoted by deflected output emissions 175athrough 175hhaving polarization vectors 177athrough 177h,respectively.

[0037] In a preferred embodiment, laser devices 171athrough 171hcomprise laser diodes disposed approximately 80 mm from respective deflecting facets. Deflecting facets 181athrough 181hare approximately 3.0 mm by 0.3 mm wide. An afocal relay system 190 images deflected output emissions 175athrough 175honto an fiber laser facet (not shown) as focused image beams approximately 3.0 mm by 0.05 mm. The focused image beams are preferably projected into a fiber laser comprising a square inner cladding approximately 0.2 mm by 0.2 mm. In an alternative embodiment, shown in FIG. 6, a focused beam 163_Pfrom optical apparatus 200 can be combined with a focused beam 163_Sfrom an optical apparatus 200′ into a polarization output 165 by means of a polarization beam splitter 193. Optical apparatus 200′ comprises a half-wave plate 191 in the optical path of focused beam 163_S.

[0038] Pulsed or modulated radiation outputs can be offset temporally to produce interdigitated emissions. As illustrated in FIG. 7, a second radiation output, represented by a curve 169, comprises pulses which occur Δt after corresponding pulses from a first radiation output, represented by a curve 167. It should be understood that the radiation outputs represented by curves 167 and 169 can be either the emissions from individual laser devices (e.g., laser device 171ain FIG. 5) or the output from an optical apparatus (e.g., optical apparatus 200 in FIG. 3).

[0039] In another embodiment, shown in FIG. 8, one or more individual polarization devices are disposed near radiation sources. In the example provided, a half-wave plate 195fis disposed between laser device 171fand deflecting facet 181fof unitary beam deflector 180. As can be appreciated by one skilled in the relative art, half-wave plate 195fproduces p-polarization at deflecting facet 181fas denoted by polarization vector 177f.

[0040] The combined output of any of the above systems can be step-wise increased or decreased by utilizing radiation sources of different power outputs. The diagram of FIG. 9 illustrates that, by selectively operating eight radiation sources having output power levels of one milliwatt to 128 milliwatts, for example, beam deflector 180 can be used to produce an output 197 having a power level of from zero to 256 milliwatts.

[0041] In another application of the disclosed system and devices, a plurality of radiation sources, each emitting at a different wavelength, can be mixed into a single output. The diagram of FIG. 10 illustrates eight radiation sources, with different wavelengths λ_i, incident upon beam deflector 180 to produce a combined output 199. In the manner of an example, a fiber laser can be simultaneously pumped at 915 nm and 975 nm. By mixing radiation of different wavelengths in the optical fiber, a good color gamut can be realized.

[0042] A double-clad amplifier, shown diagrammatically in cross section in FIG. 11, can be operated by utilizing a beam deflector 180 (not shown) for inputting pump radiation 211 into the inner cladding 201 of an optical fiber (here shown as having a circular cross-sectional shape), and by inputting a signal beam 213 into the fiber core 203. Alternatively, a circular inner cladding 205 can be used to more efficiently input both a relatively wide radiation beam 215 and a relatively narrow radiation beam 217.

[0043] While the invention has been described with reference to particular embodiments, it will be understood that the present invention is by no means limited to the particular constructions and methods herein disclosed and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.