Patent Application
20170212355
The present invention relates in general to focusing optical output from diode-laser bars. The invention relates in particular to focusing optical output from stacked diode-laser bars using a single-element lens.
Diode-lasers are efficient devices for converting electrical power into coherent optical power. In this respect, they represent the most efficient class of laser devices. An edge-emitting diode-laser has a diode-laser emitter grown on a single-crystal substrate. The diode-laser emitter emits laser-radiation through an end-facet in an emission direction. For high-power applications, diode-laser bars having a plurality of such diode-laser emitters provide a convenient way to scale optical power. In a diode-laser bar, individual diode-laser emitters are formed in a plurality of epitaxially grown semiconductor layers on a single-crystal substrate. A typical diode-laser bar has a linear array of such diode-laser emitters that generate a corresponding plurality of individual optical beams propagating in the emission direction.
Commercial diode-laser bars are usually “packaged” into assemblies that also include electrical connectors and a base element. The base element includes features for mechanical mounting and provides cooling. In a “conductively cooled package” the base element has sufficient mass to remove waste heat from the diode-laser bar. For higher power operation, the base element is typically water-cooled, for example through a micro-channel arrangement.
The optical beam from an individual diode-laser emitter is divergent in a slow-axis direction and highly-divergent in an orthogonal fast-axis direction. Typically, the optical beams of all the emitters along a diode-laser bar are collimated in the fast-axis direction by a single high-power cylindrical lens, called a “fast-axis collimating lens”. The fast-axis collimating lens is located directly in front of the diode-laser emitters and extends along the width of the bar to capture all of the optical power. Typically, slow-axis collimation is achieved using a single-optical element shaped into a linear array of cylindrical micro-lenses. The pitch of the lens array matches the spacing between diode-laser emitters. The slow-axis collimating element is located to intercept all of the optical power transmitted by the fast-axis collimating lens.
For further power scaling, a plurality of diode-laser bar assemblies are stacked together “vertically” to make a two-dimensional array of diode-laser emitters. Such a diode-laser stack can produce kilowatts (kW) of optical power. Optical output from the entire diode-laser stack is approximately collimated in both the fast-axis and slow-axis directions by above-discussed collimating-lens elements in front of each diode-laser bar. Electrically connecting all the diode-laser bar assemblies in series enables the whole stack to be driven from one current source. Similarly, stacking the diode-laser bar assemblies enables the whole stack to be cooled in parallel by a single source of cooling water.
Diode-laser stacks are utilized in diverse applications, including laser welding, heat treatment of metals (hardening and cladding), medical therapies, and pumping of other solid-state lasers. Many of these applications benefit from tight focusing of the optical output from the stack. For example, diode-laser stacks are an optical pump source for high-power fiber resonators and amplifiers. However, pump-light from a diode-laser stack must be focused into a pump-cladding with a diameter on the order of 100 micro-meters (μm) to 1 millimeter (mm). Ideally, a single-element lens would be used to focus the optical output from a stack to a tight focus. However, single-element lenses fabricated by conventional methods have optical aberrations. Spherical aberration, in particular, degrades the focusing of lenses that have large diameter relative to focal length. Such a large diameter lens is typically required to capture all light from a diode-laser stack. Spherical aberration accordingly limits the power scaling that can be achieved by stacking additional diode-laser bar assemblies.
Corrective phase-plates are a commonly used means for overcoming spherical aberration. Corrective phase-plates are customized by an expensive process that involves precise wave-front characterization of the optical output of a diode-laser stack, then fabrication of a specific phase-plate element. Alternatively, large-aperture aspheric lenses that are comparable in size to a diode-laser stack can be fabricated free of spherical aberration. However, such large aspheric lenses are expensive, becoming a significant fraction of the cost of a complete diode-laser stack light-source. There is need for an improved diode-laser stack producing optical output that can be tightly focused by a simple and inexpensive single-element lens.
In one aspect, optical apparatus in accordance with the present invention comprises a stacked plurality of diode-laser bars. Each diode-laser bar has an optical output beam that is substantially collimated in a fast-axis direction of the diode-laser bar by a fast-axis collimating lens. A focusing lens is provided having an optical axis and spherical aberration. The focusing lens intercepts the substantially-collimated beams of all of the diode-laser bars. The stacked diode-laser bars and the focusing lens are arranged to bring all of the substantially-collimated beams to focus on the optical axis. The substantially-collimated beams of at least one of the diode-laser bars are tilted at angles with respect to the optical axis. The respective tilt angles are selected such that all of the focused beams converge to a common focus location on the optical axis.
The term “substantially collimated” as used in this description and the appended claims recognizes that exact fast-axis collimation of an optical output beam is not possible because of aberrations, particularly spherical aberration in the fast-axis collimating lens. Exact slow-axis collimation is also not possible because a diode-laser emitter is spatially extended and highly multi-mode in a slow axis direction. Rays in a substantially-collimated beam may diverge or converge up to about 1 milli-radian (mrad) half-angle in the fast-axis direction and diverge up to about 20 mrad half-angle in the slow axis direction.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.
Referring now to the drawings, wherein like components are designated by like numerals,
Each output beam 16 is substantially collimated by a fast-axis collimating lens 18 and a slow-axis collimating lens 20, which are located as depicted in output beam 16 and close to diode-laser bar 12. Fast-axis collimating lens 18 substantially collimates output beam 16 in the fast-axis direction and slow-axis collimating lens 20 substantially collimates output beam 16 in the slow-axis direction. In some applications for which output beam 16 has sufficiently-low divergence in the slow-axis direction, slow-axis collimating lens 20 may be omitted, without departing from the spirit and scope of the present invention. Diode-laser bar 12, fast-axis collimating lens 18, and slow-axis collimating lens 20 may be discrete components or incorporated into a diode-laser bar assembly.
Prior-art designs would have all of the diode-laser bars stacked in parallel and all of the substantially-collimated beams propagating parallel to the emission direction. In contrast, apparatus 10 has the substantially-collimated beam originating from each one of the outermost diode-laser bars of the vertical stack propagating at an angle θ from the emission direction. Apparatus 10 also has the substantially-collimated beam originating from each one of the next-outermost diode-laser bars of the vertical stack propagating at an angle φ from the emission direction.
A focusing lens 24 having an optical axis 26 is arranged to intercept all substantially-collimated beams 22. Focusing lens 24 transforms each substantially-collimated beam 22 into a focused beam 28. All focused beams 28 converge to a common focus location 30 on optical axis 26. Focusing lens 24 is depicted as a plano-convex lens, having one flat surface and one spherically-shaped surface. Large plano-convex lenses are easier and cheaper to fabricate than large aspheric lenses or multi-element lenses. However, large plano-convex lenses have spherical aberration, which degrades focusing.
In prior art designs, spherical aberration would displace the focus locations for focused beams originating from diode-laser bars that are remote from the optical axis. Dashed lines in the drawing depict schematically propagation and focusing of beams in prior-art designs. For example, focused beams 32 and 34 originating from respectively the outermost diode-laser bars and the next-outermost diode-laser bars would converge at locations 36 and 38. Although focused beams originating from all of the off-axis diode-laser bars would be displaced from common focus location 30, displacements would be greater for focused beams originating from diode-laser bars further from the optical axis. In contrast, apparatus 10 has ideal focusing, whereby all focused beams 28 converge to common focus location 30. Tilting the substantially-collimated beams originating from the outermost and next-outermost diode-laser bars with respect to optical axis 26 compensates for aberrations imparted by focusing lens 24.
In contrast, prior art designs have all the fast-axis lenses aligned with the corresponding diode-laser emitters. Dashed lines in the drawing schematically depict propagation of beams in such prior-art designs. Specifically, substantially-collimated beams 44 and 46 originating respectively from the outermost diode-laser bar and the next-outermost diode-laser bars would propagate in the emission direction.
By way of example, consider a vertical stack of eighteen parallel diode-laser bars generating a plurality of output beams at a wavelength of 976 nm (nanometers). Each diode-laser bar has nineteen diode-laser emitters. There is 500 μm pitch between adjacent diode-laser emitters and 3 mm pitch between adjacent diode-laser bars. The vertical stack is therefore two-dimensional array of emitters that is much taller than it is wide. The output beams are formed into a plurality of substantially-collimated beams. All of the substantially-collimated beams are focused by a commercial fused-silica plano-convex lens (Thorlabs Part Number LA4795) having a diameter of 75 mm and a focal length of 200 mm. An application requires all of the focused beams to be transmitted through a circular aperture having a diameter of 600 μm. The aperture is centered on the principal focus of the lens, which is common focus location 30 in
The substantially-collimated beams from the outermost and next-outermost diode-laser bars respectively propagate at a distance of about 25.5 mm and 22.5 mm from the optical axis of the lens. Without aberration compensation, focused beams 32 and 34 respectively converge on the optical axis at locations 36 and 38, approximately 0.50 mm and 0.23 mm from the principal focus. A significant fraction of the optical power is stopped by the aperture and the application is thereby deprived of optical power.
In contrast, displacing the corresponding fast-axis collimating lenses laterally to tilt substantially-collimated beams 22A and 22B in
In contrast, apparatus 50 achieves aberration compensation by tilting individual diode-laser bar assemblies, including the corresponding collimating lenses, thereby tilting the substantially-collimated beams with respect to the optical axis. In
Aberration compensation can be optimized empirically. For example, in apparatus 10 of
The embodiments depicted and examples presented herein have a plurality of substantially-collimated beams tilted with respect to the optical axis of the focusing lens. However, some improvement in focusing would be achieved by tilting just one of the substantially-collimated beams in a vertical stack, provided the tilting compensates for spherical aberration imparted onto that beam by the focusing lens.
The present invention is described above in terms of a preferred embodiment and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.
This application claims priority to U.S. Provisional Patent Application No. 62/287,761, filed Jan. 27, 2016, the entire disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62287761 | Jan 2016 | US |
