BEAM-STACKING ELEMENT FOR DIODE-LASER BAR STACK

Optical apparatus includes a diode-laser bar stack having N fast-axis stacked diode-laser bars cooperative with a parallel sided transparent stacking plate. The stacking plate receives N original beams from the N diode-laser bars and converts the N beams to 2N fast-axis stacked beams having one-half of a width the original beams and one-half of a fast-axis spacing between the original beams.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to two dimensional arrays of diode-lasers. The invention relates in particular to vertical stacks of one-dimension arrays of diode-lasers (diode-laser bars).

DISCUSSION OF BACKGROUND ART

Vertical diode-laser bar stacks are now used for providing optical pump radiation for high-power (1 KW) fiber lasers. One such vertical diode-laser bar stack is schematically depicted inFIG. 1AandFIG. 1B. Details of one-example of the stack construction are depicted inFIG. 1B. Here, each diode-laser bar17is mounted on the front of a corresponding heat sink member19. The heat-sink members are clamped together between clamping and mounting blocks23A and23B. Each heat-sink member has a forward-extending portion21to which a fast-axis collimating (FAC) lens or a module including a FAC lens and a slow-axis collimating (SAC) lens array can be attached.

A 26-bar stack such as stack18, with nineteen emitters per bar, can deliver radiation having a total power of about 1.4 kW. Such diode-laser bars are designated by practitioners of the art as having a slow-axis (low divergence axis) aligned with the length of the diode-laser bar; a fast-axis (high divergence axis) perpendicular to the slow-axis, and a propagation-axis perpendicular to both the fast and slow axes. The slow-axis, fast-axis, and propagation-axis are alternatively designated as the x-axis, y-axis, and z-axis by practitioners of the art.

Referring toFIG. 1A, each of the diode-laser bars has a dedicated cylindrical fast-axis collimating (FAC) lens20, which, as the name suggests, collimates light from each emitter in the bar in the highly divergent fast-axis direction. In this example, there are twenty-six lenses20. Spaced apart from each FAC lens in the z-axis direction is an array22of cylindrical slow-axis collimating (SAC) lenses24. The number of lenses24in each array22corresponds with the number of spaced-apart emitters (diode-lasers) in each of the diode-laser bars. Here, it is assumed that there are nineteen (19) emitters in each bar. Each SAC lens is aligned with a corresponding emitter. The FAC lenses and SAC lens-arrays are held in alignment with each other by brackets26(shown on only one side inFIG. 1Afor convenience of illustration). Assemblies of FAC and SAC lenses are available from several commercial suppliers.

In such a diode-laser bar stack the vertical (fast-axis separation) of beams from adjacent diode-laser bars is limited by the thickness of the diode-laser bar substrates and the thickness of water cooled sub-mounts for the diode-laser bars. The fast-axis brightness of all combined beams from the diode-laser bar stack is limited by the fast-axis separation of the beams. The amount of the combined radiation that can be focused into an optical fiber at any particular numerical aperture is directly dependent on the total power and brightness of the radiation from the bar stack and the beam parameter product (BPP) of the focused radiation.

Generally, the approach to optimizing (minimizing) the focused BPP is to limit the height of the stack, and limit the fill-factor (the ratio of total of emitter widths to the total length of the bar). The height of the stack can be limited, for any given pitch of the diode-laser bars in the stack, simply by limiting the number of diode-laser bars in the stack. A convenient compromise has been found to be a stack of 13 bars, each 10 mm long and with a fill factor of 18%, with a pitch of about 3.3 millimeters (mm). In the fast-axis, the etendue can be reduced by reducing the height of the stack, for example, by limiting the number of the vertically stacked bars.

Various approaches have been suggested for limiting the fast-axis extent of a combined radiation output from a diode-laser bar stack using optical arrangements to reduce the fast-axis spacing between beams from a fixed number of diode-laser bars. A number of such approaches is described in detail in U.S. Pat. No. 6,993,059, assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated herein by reference.

The simplest of these approaches uses a parallel-sided glass block in front of a diode-laser bar stack and titled away from the stack in the fast-axis. The lower part of the block on a first side thereof closest the diode-laser bar stack has an array of spaced-apart reflective strips aligned with the fast-axis of the diode-laser bars. The number of reflective strips is one-half the number of diode-laser bars and the pitch of the strips is equal to the pitch of the diode-laser bars. On the opposite (second) side of the block, above the strips in the fast-axis direction, is a reflectively coated area. Beams from the lower bars in the stack pass between the strips on the first side of the block and under the coating on the second of the bloc. Beams from the upper bars in the stack pass over the array of strip through the first side of the block are reflected by the coated area on the second side and onto the reflective strips; and are reflected by the reflective strips interspersed between beams transmitted through the strips. While this approach provides a simple means of increasing fast-axis brightness, the approach does not provide for increasing the BPP of the focused combined beams.

SUMMARY OF THE INVENTION

In one aspect, optical apparatus in accordance with the present invention comprises a plurality N of diode laser-bars characterized as having a slow-axis in a length direction, a fast-axis perpendicular to the slow-axis, and a propagation-axis perpendicular to the slow-axis and the fast-axis. The diode-laser bars are stacked one above another in the fast-axis direction with a predetermined pitch P therebetween. A plurality N of fast-axis collimating lenses is provided one for each of the diode-laser bars and a plurality N of slow-axis collimating lens arrays is provided, one for each of the diode-laser bars, the diode-lasers bars. The fast-axis collimating lenses, and slow-axis collimating lenses provide a plurality N of combined-radiation beams propagating one above another in the fast-axis direction parallel to the propagation-axis direction. Each beam has a width W in the slow-axis direction. A transparent plate is located in the path of the combined radiation beams. The transparent plate has a thickness and first and second opposite surfaces parallel to each other. The surface are inclined to the fast-axis direction at a first angle, and inclined to the slow-axis direction at a second angle. The first surface of the plate faces the diode-laser bar stack. First and second internally reflective coatings partly cover respectively the first and second surfaces of the plate. The first and second internally reflective coatings are configured and the thickness of the plate and the first and second angles are selected such that the plate transmits 2N combined beams propagating one above another in the fast-axis direction parallel to each other in the propagation-axis direction, with each beam having a width less than W in the slow-axis direction, and with the beams spaced apart in the fast-axis direction by a distance of about P/2.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated by like reference numerals,FIG. 2schematically illustrates one-preferred embodiment of optical apparatus40in accordance with the present invention apparatus in accordance with the present invention including a vertical stack42of diode-laser bars and an inventive beam-processing plate50for increasing the fast-axis brightness of combined beams44from the diode-laser bar stack. The diode-laser bar stack is mounted on a base41can be considered as a simple version of the above described diode-laser bar stack with only 13 diode-laser bars stacked. Only sufficient detail of the diode-laser bar-stack is shown inFIG. 2for understanding principles of the present invention.

Only two beams44from the diode-laser bar stack are shown, for simplicity of illustration. Beams from the diode-laser bars are collimated in both the fast-axis and the slow-axis as described above with reference toFIG. 1AandFIG. 1B. All fast-axis collimating lenses20are depicted inFIG. 2, but only one slow-axis collimating lens array24is shown, again for convenience of illustration. The collimating lens-array has a number of individual collimating lenses corresponding to the number of emitters (not shown) in the diode-laser bar. Beams44have a width W between bounding rays depicted by bold lines. The beams have a fast-axis spacing (pitch) P indicated inFIG. 2as the fast-axis distance between apexes of adjacent fast-axis collimating lenses.

Beam processing plate50has parallel faces50A and50B. A base50C of plate50is bonded to slightly wedge-shaped mounting block43attached to base41. On face50A of plate50is a parallel array of strips60which are highly reflective for the diode-laser radiation, at least (internally) on the side facing into the plate. The array of strips has a pitch P corresponding to the pitch of the diode-laser bars in the stack. In the illustrated embodiment, each strip is as long as the beams44are wide. The height of strips60is sufficient to completely intercept the fast-axis height of a beam44. Spaces between strips60are wide enough to allow the fast-axis height of a beam44to pass between adjacent strips. The parallel array of strips60is aligned parallel to the x-z plane of the diode-laser bars.

Plate50is tilted (tipped) toward the fast-axis of the diode-laser bars by an angle θ, and rotated away from the slow-axis of the diode-laser bars by an angle φ. On face50B of plate50is coating66, here rectangular in shape and at least internally reflective. Coating66has a straight edge68aligned parallel to the fast-axis of the diode-laser bars. Edge68is aligned about centrally in the width of beams44within the plate. Coating66in the slow-axis direction has a width greater than half of the width of beams44. Coating66has a length in the fast-axis direction of the diode laser bars at least sufficient to intercept all beams44within plate60. It is recommended that portions of faces50A and50B not having reflective coatings66or60thereon are anti-reflection coated for the wavelength of radiation from the diode-laser bars.

The function of plate50can be followed by following the progress of a beam44from the uppermost diode-laser into, through and out thereof. One half-portion of the beam-width is intercepted by reflective coating66allowing the other half portion44A to be transmitted through face50B of the plate in the propagation-axis direction.

The half-portion44B intercepted by coating66is reflected downwards and laterally onto the uppermost reflective strip under the transmitted portion44A. The strip60reflects beam-portion44B in the propagation-axis direction such that beam-portion44B leaves plate60under transmitted beam portion44A at a level below the level of beam-portion44A in the z-axis direction.

The processing of a beam44from any other than the uppermost will require that the beam pass between two adjacent strips60as illustrated, but will otherwise be the same. Beam cross-sections are indicated by elongated dashed rectangles to assist in following the beam progress described above.

In an example of stacking plate50for a diode-laser bar stack having a pitch P of about 3.3 mm, the plate is a fused silica plate having a thickness of about 12 mm. Angle θ is about 5.9 degrees and angle φ is about 17.3 degrees. The reflective coatings are preferably multilayer dielectric coatings.

The effect of processing (stacking) plate50is to take the original number of beams from the diode-laser bar stack and create therefrom twice as many beams half as wide (W/2) as the original beam, with a separation P/2 therebetween, i.e., half of the pitch (P) of bars in stack42. The slow-axis divergence of the two beams obtained from each original beam will be essentially the same as that of the original beam.

As the slow-axis etendue of the beams stacked by the plate will be essentially half of the etendue of the original beams, this can provide for a reduced BPP of the focused beams in the slow-axis direction. The BPP in the fast-axis direction will not change appreciably, since the total width of the beam in the fast-axis direction will only increase from N times the pitch to N+½ times the pitch, where n is the number of bars.

Alternatively, each of the stacked beams can be made to have the slow-axis etendue of the original beams by increasing, i.e., doubling, the fill factor of the diode-laser bars, say from the above-discussed 18% to 36%. This can about double the total power in the beams without any reduction in BPP. In other words, the13-bar diode-laser bar stack ofFIG. 2will have about the same power-output as the prior art diode-laser bar stack ofFIGS. 1A and 1B.

It should be noted here, that while it may be preferable to have all beam portions44A and44B aligned one above the other in the fast-axis direction, the44A beams and the44B beams may be slightly displaced one from another in the slow-axis direction without significantly adversely affecting any of the above discussed advantages of the arrangement of diode-laser bar stack and inventive stacking plate50. Those skilled in the art will also recognize that coating66could be an array of parallel strips similar to strips60with the array staggered such that strips of coating66intercepted the beams passing between or over strips60.

In summary, the present invention is described above with reference to preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather the invention is limited only by the claims appended hereto.