BRIEF DESCRIPTION OF THE DRAWINGS
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.
FIG. 1 is a front elevation view schematically illustrating a prior-art diode-laser bar with emitters thereof misaligned in the fast-axis direction of the diode-laser bar giving rise to the effect commonly referred to as “smile”.
FIG. 2 is a side-elevation view schematically illustrating the diode-laser bar of FIG. 1 cooperative with a cylindrical lens arranged to fast-axis collimate beams from the emitters of the diode laser bar, and with a plane-mirror arranged to reflect the collimated emitted beams back toward the diode-laser bar, and schematically illustrating how a beam from an emitter not precisely aligned with the optic axis of the lens can not be reflected back into the emitter by the mirror.
FIG. 3 is a side elevation view schematically illustrating an arrangement in accordance with the present invention, similar to the arrangement of FIG. 2, but wherein the mirror of FIG. 2 is replaced by a one-axis corner reflector that causes a beam from any emitter in the diode-laser bar to be reflected in the fast-axis back into that emitter essentially independent of the alignment of the emitter with the optic axis of the lens.
FIG. 4 is a fast-axis side-elevation view schematically illustrating one preferred embodiment of a wavelength-locked diode-laser bar apparatus in accordance with the present invention including a feedback resonator for emitters of the diode-laser bar having a cylindrical lens cooperative with a fast-axis corner reflector and the rear end of emitters of the diode-laser bar, with a wavelength selective element included in the resonator between the cylindrical lens and the corner reflector.
FIG. 5 is a fast-axis side-elevation view schematically illustrating another preferred embodiment of a wavelength-locked diode-laser bar apparatus in accordance with the present invention similar to the apparatus of FIG. 4 but further including an array of slow-axis collimating lenses having one lens for each emitter of the diode-laser bar and being located between the fast-axis collimating lens and the wavelength-selective element.
FIG. 6 is a three-dimensional view schematically illustrating further details of the apparatus of FIG. 5.
FIG. 7 is a fast-axis side-elevation view schematically illustrating yet another preferred embodiment of a wavelength-locked diode-laser bar apparatus in accordance with the present invention including a feedback resonator for emitters of the diode-laser bar having a cylindrical lens cooperative with a fast-axis corner reflector and the front or output end of emitters of the diode-laser bar, with a wavelength selective element included in the resonator between the cylindrical lens and the corner reflector.
DETAILED DESCRIPTION OF THE INVENTION
Referring again to the drawings, wherein like components are designated by like reference numerals, FIG. 3 schematically illustrates an important principle of an apparatus in accordance with the present invention for wavelength locking a diode-laser bar. The apparatus of FIG. 3 is similar to the apparatus of FIG. 2 with an exception that mirror 32 of FIG. 2 is replaced by a fast-axis corner reflector 50 including reflecting surfaces 52 and 54 arranged perpendicular to each other. In this example, the apex 56 of the corner reflector (the junction of the reflecting surfaces) is aligned on optic axis 28 of cylindrical lens 26, and reflecting surfaces 52 and 54 are inclined at respectively plus and minus 45° to optic axis 28. Axis 28 is also aligned with the Z-axis of emitter 22B (and 22F).
Those skilled in the art will recognize that the term “cylindrical lens” as used in this description and the appended claims means only that the lens has optical power in one transverse axis only. Usually such a lens has a plano-convex shape, however, a cylindrical lens with two curved surfaces may be used without departing from the spirit and scope of the present invention.
As far as the beam from emitter 22B (or 22F) is concerned, outbound rays will be incident at plus and minus 45° on reflecting surfaces of the corner reflector and, accordingly, will be reflected from the corner reflector such that the an incident ray at any particular distance above axis 28 will return to lens 26 along a path coinciding with the path of an incident ray is the same distance below axis 28, and vice-versa. The reflected collimated beam as a whole, now inverted, will be focused by lens 26 into emitter 22D as is the case in the prior-art plane mirror arrangement of FIG. 2. In effect there will be two beams circulating between the emitter and the corner reflector in opposite directions.
The emitter-22D beam is not depicted in FIG. 3 in order to be able to highlight what happens to beam 40 from the misaligned emitter 22B. Here, outbound beam 40 is collimated by lens 26 (rays 40OUT) but propagates at an angle to axis 28 as discussed above for the arrangement of FIG. 2. Because the collimated beam 40OUT is incident on the reflecting surfaces at some other angle than 45°, the reflected beam 40BACK is laterally displaced in the fast axis direction but propagates parallel to the incident beam. This beam strikes cylindrical lens 26 in such a way that the beam is focused by the lens back into the emitter from which it was emitted. Provided an adequate clear aperture is selected for lens 26, this will be true for any emitter at any anticipated degree of misalignment, i.e., whatever the extent of smile in the diode-laser bar. Here again, there will effectively be two-beams propagating in opposite directions.
FIG. 4 is an elevation view schematically illustrating one preferred embodiment 60 of a wavelength locked diode-laser bar in accordance with the present invention. Here diode-laser bar 20 is supported on a heat sink 62, preferably via a heat spreader 64 of a highly thermally conductive dielectric material such a diamond, sapphire (Al2O3), aluminum nitride (AlN), or beryllium oxide (BeO). Other components are supported on precision machined (or etched) grooves and surfaces, evident in the drawing, but not numerically designated. In this example, the corner-reflector principle of FIG. 3, including corner reflector 50 with reflective surfaces 52 and 54 cooperative with cylindrical lens 26, is arranged to form an external feedback-resonator 68 cooperative with the rear facets of emitters of the diode-laser bar. No attempt is made to depict beams from individual emitters of the diode-laser bar in FIG. 4. This will be evident to those skilled in the art from the description provided above with reference to FIG. 3. A transmissive optical filter 70, such as a multilayer narrow band interference filter, is located between cylindrical lens 26 and corner reflector 50 in resonator 68 with surfaces of the corner reflector non-orthogonally inclined to collimated beams in the resonator. Filter 70 selects a narrow band of wavelengths from the gain-bandwidth of the emitters and locks the output of all emitters in the diode-laser bar to that band of wavelengths, as discussed above.
One suitable interference filter for filter 70 is a MaxLine™ Laser-Line Filter, available from manufactured by Semrock, Inc. of Rochester, N.Y. This filter is designed for 808 nm laser-diodes and has an FWHM bandwidth of about 3 nm and a peak-transmission sufficient to provide efficient feedback. In general, it is important that any filter used as filter 70 be relatively insensitive to temperature, and is mounted such that it is not significantly heated by the pump-current of the diode-laser bar.
As noted above, wavelength-locking resonator 68 in the embodiment of FIG. 4 is cooperative with emitters of the diode-laser bar via rear ends (facets) thereof. The actual output of diode-laser bar is at the end of the emitters opposite those included in the wavelength-locking resonator. The output (front) faces of the emitters are preferably coated with a partially reflecting and partially transmissive (PR) coating that is optimized for the efficiency of the emitters, as is known in the art. Rear faces of the emitters of external resonator can be coated either with anti-reflective (AR) coating, or PR coating. An AR coating provides maximum possible feedback to the laser. However, it may be found that less than 100% feedback is sufficient for locking the wavelength. In that case, a PR coating is preferred because this reduces the amount of power in the external resonator, and, accordingly, reduces optical losses in the external components, particularly the interference filter. An optimum reflectivity for a rear-face PR coating can readily be determined experimentally for a particular diode-laser bar and components of the wavelength-locking resonator.
The external components can be pre-aligned by placing them into the precision-machined (or etched) grooves in common heatsink 62 of the wavelength locked diode-laser bar assembly 60. Provided that components of corner reflector 50 is assembled separately with surfaces 52 and 54 being precisely at 90° to each other, the alignment of the corner reflector with beam with respect to the beam from any emitter of the bar, or with respect to optic axis 28 of lens 26, is not critical within the normal range of precision machining tolerances. The same is true for the nominal alignment angle of filter 70.
It should be noted, here, that the wavelength-locking arrangement of FIG. 4 is not optimum if feedback is from the external resonator is to be maximized. This is because measures are not taken to deal with beam divergence of the emitters in the slow-axis (X-axis) thereof. Because of this, while the corner reflector ensures that fast-axis rays are refocused into any emitter from which those rays originate, expansion of the beam in the slow axis will provide that less than 100% of the beam width returns to the emitter, whatever the efficiency of resonator components.
FIG. 5 and FIG. 6 schematically illustrate another preferred embodiment 80 of a wavelength locked diode-laser bar in accordance with the present invention. This embodiment is similar to the embodiment of FIG. 4 but further includes an array of slow-axis collimating lenses 72, here in a monolithic array 74 thereof, with one such lens for each emitter in the diode-laser bar. In the three-dimensional view of FIG. 6, it is assumed that there 12 emitters 22, equally spaced, in the bar. This is an arbitrary number of emitters, and, as such, should not be construed as limiting the invention to any number of emitters per bar. The slow-axis collimating lenses provide that whatever the slow-axis dimension of an incident beam at the point where the beam is intercepted by corner reflector 50, the reflected beam will be refocused to about the slow axis dimension of the emitter as the beam re-enters the emitter.
One disadvantage of preferred embodiments 60 and 80 is that diode-laser bar 20 must to be precision aligned with respect to heatspreader 64. This is in order to not damage the emitting surfaces of the diode-laser bar during a soldering process by which the diode-laser bar is bonded to the heat spreader, and in order not to leave areas of the diode-laser bar that are not in thermal communication with the heat-sink. Preferably, the edge of the diode-laser bar should overhang the heat-sink by about 5 μm. This means that heat-spreader 64 must be manufactured to very tight tolerances, and perhaps even individually fitted to the bar width.
FIG. 7 schematically illustrates yet another embodiment 90 of a wavelength-locked diode-laser bar in accordance with the present invention in which this disadvantage is not present. Here, output of diode laser bar 20 is from the side of the diode-laser bar on which the wavelength-locking external resonator 68 is located, which can be accordingly be considered the front of the diode-laser bar. A highly reflective (HR) coating preferably with reflectivity maximized for the peak transmission of the optical filter, is deposited on rear faces of the emitters of the diode-laser bar and the diode-bar mounting procedure is comparable with existing manufacturing techniques. However, since effectively two waves (beams) circulate in the external resonator in opposite directions as noted above, two outputs (Beam 1 and Beam 2) are possible.
Taking this into account, corner reflector 50 of above-described embodiments is replaced in embodiment 90 by a corner reflector 51 that has a highly reflective (HR) surface 54 at 90° to a partially reflecting and partially transmissive (at the peak wavelength of the optical filter) surface 53. Surfaces 54 and 53 face into the resonator. A surface 57 opposite surface 53 is AR coated for about the locked-wavelength, and is parallel to surface 53. Another reflective surface 55 is provided at 90° to surface 57 and preferably has a reflectivity that is maximized for the peak wavelength of the optical filter. The purpose of this surface is to direct the second output (Beam 2) parallel to the first output (Beam 1). Again, this can be achieved by assembling the components providing the three reflective surfaces of the corner reflector separately, to ensure that the surfaces are precisely orthogonal to each other.
Each of the output beams is depicted as being bounded by a solid line and a dashed line. The beams propagate side-by-side, parallel to each other, and eventually, overlap in the far field. This is perfectly suitable for laser pumping and other applications wherein only the far-field intensity distribution of radiation in a beam is important, or wherein near-field intensity distribution is not important at all. An additional benefit of embodiment 90 is that the output beam is collimated by cylindrical lens 26 of wavelength-locking feedback resonator 68. In the embodiments of FIG. 4 and FIG. 5, additional fast-axis or slow-axis collimating lenses are required if the output of the diode laser bar is to be collimated in one or both transverse axes.
Regarding precision optical assembly of corner reflectors 50 or 51, one possible method of manufacturing the corner reflector is to make it from a layer or sheet of a single crystal material such as a silicon wafer. An industry standard silicon wafer having a thickness of between about 0.6 and 1.2 mm can be coated with the desired AR, PR or HR coating and then cleaved into slabs of required size for the corner reflector components. With a proper crystallographic orientation of the wafer, for example <100>, cleavage planes of the wafer are exactly perpendicular to the surface of the wafer and to each other, surfaces of the wafer are precisely parallel to each other. Because of this, two or three cleaved silicon slabs can be assembled by optical contact, or by cementing with an appropriate bonding agent such as an epoxy to form a corner reflector assembly 50 as depicted in FIG. 4. As one silicon wafer may yield thousands of cleaved slabs, such process is quite inexpensive.
In the case of a corner reflector 51 which has a transmission function in addition to a reflection function, crystal quartz, sapphire or other transparent crystal materials can be used instead of silicon. The use of optical glass or fused silica for forming components of the corner reflector is not precluded, however such materials must be cut into a slabs using a conventional diamond saw, common in the semiconductor industry.
In above described embodiments of the present invention, an interference filter is described as a preferred wavelength-selective optical element. Those skilled in the art, however, may devise embodiments of the present invention including other wavelength selective elements without departing from the spirit and scope of the present invention. Such elements include birefringent filters, etalons, and volumetric gratings. Many of these elements have been developed for telecommunications and accordingly are usually relatively inexpensive, robust, and reliable.
In summary present invention is described above in terms of a 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.
Patent Application
20080025353
