Optical: systems and elements – Collimating of light beam
Reexamination Certificate
2002-03-01
2004-03-02
Mack, Ricky (Department: 2873)
Optical: systems and elements
Collimating of light beam
Reexamination Certificate
active
06700709
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to diode lasers and more particularly, a configuration of and a method for optical beam shaping of diode laser bars to produce an optical beam of high power and brightness, allowing for efficient coupling of the optical diode laser bar output into an optical fiber.
DESCRIPTION OF THE PRIOR ART
High power solid state lasers and particularly high power fiber lasers generally depend on the availability of optical pump beams with high optical power and brightness, i.e., the required pump power needs to be made available in as small a space as possible. Semiconductor diode laser arrays are firmly established as the main source of optical beams of high power, i.e., 1 watt and above including ultra-high powers of greater than 10 watts. With current semiconductor technology, a power approaching 40 kW can be produced by a stacked diode array with dimensions of only 5×10 cm (R. J. Beach et al., Laser Focus World, December 2001). The stacked diode array typically consists of individual diode bars of around 1 cm in length, which in turn incorporate 10-20 individual emitters separated by 0.2-1.0 mm. The laser beam originating from one individual emitter typically has a divergence of 10°×50°. The small divergence beam is in the plane of the x axis and the high divergence beam is in the plane of the y axis. As shown in
FIG. 1
, a laser diode
101
emits laser beam
102
having relatively small divergence in the direction of the x axis but high divergence in the direction of the y axis. These axes are sometimes referred to as slow and fast axis respectively. The fast axis beam is generally diffraction limited and can be collimated with a cylindrical lens aligned parallel to the slow axis of the single emitter, i.e., laser diode
101
. The slow axis beam is typically far from diffraction limited.
The brightness B of the optical beam from an individual emitter can be calculated as B=power/(emitting area×angular divergence). For an individual emitter of dimensions 1×100 &mgr;m operating at a power of 2 W we obtain B=14 MW/cm
2
. In contrast, the brightness of a 20 element diode bar with dimensions 1 &mgr;m×1 cm operating at a power level of 40 W is only about 3 MW/cm
2
, whereas the brightness of a diode stack as described above operating at a power of 40 kW is only of the order of 5 kW/cm
2
. Indeed, the stringent requirements for cooling of a diode stack require that the individual diode bars and emitter areas are substantially spaced apart, greatly increasing the emitter area and limiting the brightness of such high-power laser systems.
The brightness of diode laser beams has traditionally been increased by the implementation of beam-shaping optics, i.e., by optically combining the individual emitter beams from the diode array to generate a single optical beam which can be efficiently coupled into an optical fiber (See, e.g., U.S. Pat. No. 5,168,401 of Endriz, hereinafter Endriz '401). The brightness B of the fiber-coupled optical beam can be calculated as B≈P/(A*&pgr;NA
2
), where P is the coupled power, A is the core area and NA is the numerical aperture of the fiber.
Optical beam shaping is possible by a variety of means. As shown in
FIG. 2
, most beam-shaping methods transform the optical beams such that the beams from the individual emitters
201
,
202
and
203
form a picket fence
209
,
210
,
211
with the fast axis beams aligned parallel to each other. Since the beams
204
,
205
and
206
are diffraction limited along the fast axis (ie., in the y direction as depicted), very tight packing of the emitters along the fast axis is possible. To facilitate optical beam transformation, collimation of the fast axis beam is also implemented with a cylindrical lens
207
as shown in
FIG. 2
followed by beam shaping optics
208
. The generation of a properly aligned picket fence comprising collimated beams
209
,
210
and
211
is key to most industrially relevant beam shaping methods. We can distinguish four different classes of picket fence generation methods and systems.
A first class of methods generates the picket fence by optical rotation of each emitter beam by 90°, where the direction of the emitter beam is further deflected by around 90° after reflection from at least two reflecting surfaces. The Endriz '401 patent describes such an example. Further examples of such a method are U.S. Pat. No. 5,418,880 of Lewis et al. and U.S. Pat. No. 6,044,096 of Wolak, et al.
A second class of methods generates a picket fence by optical beam rotation based on beam-rotating prisms such as the Abbe-König prism as disclosed in U.S. Pat. No. 5,243,619 of Albers, et al. The advantage of this design is that it avoids a 90° deflection of the beam direction such that only a small displacement in the propagation direction results.
A third class of methods of generating a picket fence is based on beam deflection in a set of multi-facetted mirrors or prisms. In these methods a first multi-facetted optical structure deflects the beam to obtain some beam spacing in the y direction, while a second multi-facetted optical structure deflects the beams to overlay the beams along the x direction (see, e.g., U.S. Pat. No. 5,887,096 of Du et al. and U.S. Pat. No. 6,151,168 of Goering et al.). Such systems do not require beam rotation optics, but generally comprise a beam deflection along the propagation direction, relying on the manufacturing of expensive high precision multi-facet optics. The function of beam deflection and overlay can also be accomplished in one single optical element as disclosed by U.S. Pat. No. 5,825,551 of Neilson, et al. A limitation of the approach taken by Neilson et al. is the variation between the optical path lengths of each individual emitter beam through the beam shaping optic, which in-turn limits the focussability of the resulting beam. Note that the first three classes of beam-shaping optics use non-focussing optics.
A fourth class of methods of generating a picket fence is based on beam rotation in a transmissive optical element that comprises at least one cylindrical surface. (See, e.g., Lissotschenko et al., German Patent No. DE 19920293.) A representation of this structure is shown in FIG.
3
. As shown therein, optical element
301
comprises an array of cylindrical lenses
302
,
303
,
304
and
305
. The arrangement provides for a 90° rotation of respective laser beans such that a laser beam depicted as rays
306
a,
306
b
and
306
c
undergoes to 90° rotation in a plane perpendicular to its direction of transmission along the Z axis as it transits cylindrical lens
303
. Similarly, rotated in the x,y plane are laser beans
307
a,
307
b,
307
c
and
308
a,
308
b,
308
c
by the respective cylindrical lenses
304
and
305
. Recently, the technique by Lissotschenko et al. was extended (see U.S. Patent Application Publication No. US2002/0015558) to include also beam rotation cylindrical lenses with an additional concave curvature, where the central axis of the concave surface is orthogonal to the main cylinder axis. Such a cylindrical lens was referred to as a concave toroidal surface. A limitation with these techniques is that generally cylindrical lens arrays need to be manufactured for beam shaping of a whole diode bar, which are difficult to coat with anti-reflection coating due to the presence of large curvatures and multiple crevices in the optics. Moreover, another limitation with the technique by Lissotschenko et al. is that no beam homogenization elements are used. Without beam homogenization tight packing of individual beamlets from a diode bar along the fast axis is limited and therefore a significant reduction in brightness results.
Generally all techniques described so far are not monolithic and therefore complex alignment procedures are required, leading to high manufacturing costs. Moreover, the application of optical coating is also difficult in these devices which can limit the optical throughput.
SUMMARY OF THE INVENTION
The present invention is di
Boston Laser Inc.
Fulbright & Jaworski L.L.P.
Mack Ricky
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