Abstract:
An optical amplifier includes a solid state gain-element. The gain-element is pumped by pump-radiation from a diode-laser bar. The diode-laser radiation is delivered from the diode-laser bar to the gain-element entirely via a tapered light-guide which guides the radiation only in a fast-axis direction of the diode laser bar. The tapering of the light-guide reduces fast-axis divergence of the pump-radiation by about a factor of ten. The pump-radiation is delivered to the gain-element as a line of radiation homogenized in the fast-axis direction.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to optical pumping of solid-state lasers and amplifiers. The invention relates in particular to pumping such lasers and amplifiers with laser-radiation from a diode-laser bar. 
     DISCUSSION OF BACKGROUND ART 
     One particularly effective optical amplifier is described in U.S. Pat. No. 7,256,931. This amplifier includes a slab-type gain-element of a highly doped gain-medium such as neodymium-doped yttrium vanadate (Nd:YVO 4 ). The gain-element is energized by laser-radiation from a linear array of diode-lasers (semiconductor-lasers) commonly referred to by practitioners of the laser art as a diode-laser bar. 
     Diode-lasers in a diode-laser bar have emitters characterized as having a fast-axis, in which radiation is emitted with a relatively high divergence (for example up to about 35° half-angle), and a slow-axis in which radiation is emitted with relatively low divergence (for example, less than about 10° half-angle). The emitters are aligned, spaced apart, in the slow-axis direction. Typically, a diode-laser bar has a length of about 10 millimeters (mm) and width between about 1 and 2 mm. Emitter apertures have a slow-axis dimension of up to about 100 micrometers (μm) and a fast-axis dimension of about 1 μm. One preferred number of emitters per bar is 19. 
     In an optical pumping arrangement for a conventional slab-amplifier, radiation from the diode-laser bar is collimated in the fast-axis by a first, elongated, cylindrical lens referred to as a fast-axis collimating (FAC) lens. The fast-axis collimated radiation is then focused by a second cylindrical lens into a lateral surface of the gain-element. The radiation is primarily absorbed close to the lateral face to form a narrow, shallow, nominally linear gain-region at that lateral face. 
     A beam of radiation to be amplified is directed into the gain-element through a first end-face thereof into the linear gain-region at a first location such that the radiation is incident at grazing incidence, for example at an incidence-angle of about 80°. The radiation is reflected by total internal reflection (TIR) out of the gain-element through an opposite second end-face thereof. That radiation is then directed by a pair of mirrors back into the gain-element through the second end-face back into the linear-gain region, again at grazing incidence, but at a second location spaced apart from the first location. TIR directs the twice-incident (amplified) radiation out of the gain-element through the first end-face thereof. 
     Emitting apertures in a diode-laser bar are rarely exactly fast-axis aligned. Misalignment in the fast axis direction can occur in particularly when the diode-laser bar is mounted on a heat sink. This fast-axis misalignment, which can be as much as 20 μm, is whimsically referred to by practitioners of the art as “smile”. This presents a problem in the above-discussed optical pumping arrangement inasmuch as any smile in a diode-laser bar will be reproduced and magnified by the collimating and focusing lenses in the nominally linear gain-region of the gain-element. This can result in inefficiency or inconsistency of the amplifier gain. 
     If such inconsistency and inefficiency cannot be tolerated, diode-laser bars having tolerably low smile can be selected from a batch of mounted diode-laser bars. This of course can result in low yield and consequently higher cost for the selected mounted bars. There is a need for an optical pumping arrangement which is insensitive to smile in a diode-laser bar. 
     SUMMARY OF THE INVENTION 
     In one aspect, optical apparatus in accordance with the present invention comprises a gain-element and a diode-laser bar for providing pump-radiation for energizing the gain-element. The pump-radiation is delivered from the diode-laser bar in a propagation-direction with a transverse slow-axis parallel to the length of the diode-laser bar and a fast-axis perpendicular to the slow-axis. The pump-radiation is delivered from the diode-laser bar to the gain-element by transporting the radiation in a tapered light-guide. The light-guide guides the pump-radiation in the fast axis direction only and is tapered in the fast-axis direction, with a fast-axis height of the light-guide increasing progressively in the propagation direction. 
    
    
     
       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 an elevation view schematically illustrating a preferred embodiment of optical amplifier apparatus in accordance with the present invention, including a diode-laser bar mounted on a copper heat-sink, a solid-state gain-element, and a tapered light-guide formed by a pair of mirror-coated plates, and arranged to guide pump-radiation from the diode-laser bar through a sapphire heat sink into the gain-element, while reducing fast-axis divergence of the radiation. 
         FIG. 1A  is a three-dimensional view schematically illustrating further detail of the diode-laser bar, light-guide, sapphire heat-sink, and gain-element in the apparatus of  FIG. 1 . 
         FIG. 1B  is a plan-view schematically illustrating a method of forming the light-guide of  FIGS. 1 and 1A . 
         FIG. 1C  is a fragmentary plan-view schematically illustrating laser-radiation to be amplified interacting with the pump-radiation in a double pass through the gain-element of  FIG. 1 . 
         FIG. 2  is a gray-scale diagram schematically illustrating calculated spatial distribution of radiation in a line of radiation delivered by the tapered light-guide of  FIG. 1 . 
         FIG. 3  is a gray-scale diagram schematically illustrating calculated radiation-flux in the gain-element in a direction perpendicular to the line of radiation of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, wherein like features are indicated by like reference numerals,  FIG. 1  and  FIG. 1A  schematically illustrate a preferred embodiment 10 of optical amplifier apparatus in accordance with the present invention. Apparatus  10  includes a gain-element  12  in a slab or parallelepiped form. One preferred material for gain-element  12  is Nd:YVO 4 , which has a peak-gain wavelength of about 1064 nanometers (nm). A lateral face  12 A of gain-element  12  is in thermal contact with a heat-sink  14  of a highly thermally conductive, optically transparent material, such as sapphire, diamond, or silicon carbide. 
     Optical pump-radiation (pump-radiation) for energizing the gain-element is provided by a diode-laser bar (linear diode-laser array)  16  in thermal communication with a heat-sink  18 . The fast-axis, slow-axis, and propagation-axis of the bar are indicated. A tapered light-guide  20  conveys pump-radiation from diode-laser bar  16  to gain-element. Light-guide  20  is formed by mirror coated surfaces  24  of parallel-sided plates  22 . 
     The light-guide is tapered only in the fast-axis direction of the diode laser-bar. The fast-axis height of the light-guide in the slow axis direction is uniform at any point along the propagation-axis. Light-guide  20  has a minimum height at proximal end  20 A thereon. The height increases linearly to a maximum height at distal end  20 B of the light-guide. This provides that the fast-axis divergence of radiation from the laser is progressively reduced with distance propagated in the light-guide, as indicated in the drawing of  FIG. 1  by multiple reflections of the fast-axis extreme rays of the beam. In  FIG. 1  the light-guide is nearly to scale and the extreme rays are reproduced from an actual ray trace. The total fast-axis divergence is reduced by the taper-angle at each of the multiple reflections. 
     The fast-axis height of light-guide  20  at proximal end  20 A thereof is selected dependent on the spacing of the light-guide from the diode-laser bar  18 , the fast-axis divergence of the beam, and the maximum anticipated smile of a diode-laser bar, such that all radiation from the diode-laser bar enters the light-guide. Light-guide  20  guides radiation only in the fast-axis direction and the slow-axis beam width increases progressively with propagation distance consistent with the slow-axis divergence of the pump-radiation. Because of this, in this parallel-plate realization of light-guide  20 , the (slow-axis) width of plates  22  is selected such that the beam is still completely fast-axis guided at distal end  20 B of light-guide  20 , and all of the radiation from the diode-laser bar, less ordinary reflection losses, reaches gain-element  12 . 
     In one example of light-guide  20  the light-guide is tapered from a fast-axis height of 38 μm at proximal end  20 A to a fast-axis height of 203 μm. The length of the light-guide was 15 mm. This reduced the fast-axis output-divergence to about one-fifth of the fast-axis input-divergence. 
       FIG. 1B  schematically illustrates one preferred method for assembling light-guide  20  from plates  22 . Surfaces  24  of the plates are polished and optically coated. The surface figure is not critical, and can be up to about 5-waves (5λ). The coating is preferably a dielectric-enhanced silver coating having a reflectivity of at least about 97% over a relatively broad bandwidth, for example greater than one-third of an octave, about the pump-radiation wavelength. 
     While the pump-radiation can be considered to be essentially monochromatic, the broad bandwidth is required to accommodate the wide range of incidence angles in the light-guide for the fast-axis guided pump-radiation. As the reflection spectrum of the coating shifts to shorter wavelengths with increasing angle of incidence, the diode laser wavelength should be at the short wavelength end of the spectrum measured at the lowest anticipated angle of incidence. 
     On one of the polished and coated plates, precision shims  32  having a thickness corresponding to a desired proximal-end height are placed at corners of the plate at proximal end  20 A, of the light-guide. Similarly, precision shims  34  having a thickness corresponding to a desired distal-end height are placed at corners of the plate at distal end  20 B, of the light-guide. The second plate  22  can then be placed over the first-plate supported by the four shims  32  and  34  and the plates clamped or bonded together to form the light-guide. Suitable shim stock is available as stainless steel shim stock from McMaster-Carr of Elmhurst, Ill. 
     The above-described light-guide assembly method is just one exemplary method by which the light-guide may be fabricated. Those skilled in the art may devise other methods of fabricating a tapered light-guide of comparable performance without departing from the spirit and scope of the present invention. 
       FIG. 1C  schematically illustrates a version  10 A of apparatus  10  configured as a double-pass, grazing-incidence optical amplifier. Gain-element  12  is heavily doped such that most of the line of pump-radiation is confined close to surface  12 A thereby providing a narrow, shallow, linear gain-region in gain element  12 . Input radiation to be amplified is directed into the gain-region through surface  12 C of the gain element; is reflected by TIR from surface  12 A; and exits the gain-element through surface  12 E thereof. The radiation is then steered back into the gain-region by mirrors  28  and  30 , and is reflected by TIR out of the gain-element as output radiation. 
     Only sufficient details of grazing-incidence amplifier  10 A are provided for understanding principles of the present invention. A detailed description of the grazing-incidence amplifier principle is provided in above-referenced U.S. Pat. No. 7,256,931, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference. 
     Those skilled in the art will recognize, without further detailed description or illustration, that the arrangement of  FIG. 1C  could be reconfigured as a laser (oscillator) by providing feedback mirrors for the input and output beams. Another well-known laser type suitable for optical pumping with a line of radiation in accordance with the present invention is a so-called INNOSLAB laser. In this type of laser, a gain-element having a narrow, shallow linear gain-region is included in a hybrid laser-resonator which is an unstable resonator in a first transverse-axis aligned with the linear gain-region. and stable in a second transverse-axis perpendicular to the first transverse-axis. Those skilled in the art may devise other amplifier and laser arrangements suitable for optical pumping with the above described apparatus without departing from the spirit and scope of the present invention. 
     The high degree of linearity and homogeneity for a line of pump-radiation delivered by the inventive apparatus can be appreciated from the gray-scale diagram of  FIG. 2 , which depicts calculated spatial distribution in the line of radiation in the fast- and slow-axes. It can be seen that distribution is very linear, and very uniform, particularly in the fast-axis. This degree of uniformity and linearity is independent of smile in a diode-laser bar. The linearity is limited only by the precision with which light-guide  20  can be assembled. 
       FIG. 3  is a gray-scale diagram schematically illustrating calculated radiation-flux in the gain-element in a propagation-direction perpendicular to the line of radiation of  FIG. 2 . The gain-element is assumed to be a Nd:YVO 4  crystal with a neodymium-doping percentage of 1 atomic percent. The fast-axis uniformity of the radiation is again evident. 
     Those skilled in the art will recognize from the description provided above that a particularly advantageous aspect of the present invention is that the high precision of line-of-radiation delivery is achieved without any refractive focusing optics, and can be achieved with slow-axis (smile) misaligned diode-laser bars that would be unusable with prior-art line projection methods for reasons discussed above. This reduces the cost of optics and diode-laser bars. The tapered light-guide is assembled from simple flat plates, which can be polished and coated in bulk, and assembled with a relatively simple mechanical process. 
     The uniformity of the pump-radiation intensity in the gain-element is critical for obtaining a high spatial quality of the amplified beam. The inventive pumping arrangement is not only simple and cost efficient, but also acts as a pump beam homogenizer that is insensitive to smile, misalignment alignment and the like. With a conventional arrangement involving a fast-axis collimator lens, it is necessary to rely on the quality of the lens, low smile, and alignment which can be inconsistent and accordingly costly to control in a product. \ 
     In summary, the 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 herein. Rather the invention is limited only to the claims appended hereto.