Abstract:
A modular solid-state laser comprises a diode-laser pump module and a laser-enclosure. The diode-laser pump module produces a collimated beam of laser-radiation for pumping a gain-element within the laser-enclosure. The beam of pump laser-radiation is focused into the gain-element by optics located within the laser-enclosure. The diode-laser pump module can be replaced or exchanged without realigning optics located within the laser-enclosure.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to diode-laser pumping of solid-state lasers. The invention relates in particular to diode-laser pump modules that are exchangeable and replaceable in a modular solid-state laser. 
       DISCUSSION OF BACKGROUND ART 
       [0002]    Diode-lasers are efficient devices for converting electrical power into coherent optical power. For high-power applications, a plurality of diode-lasers are packaged together in a diode-laser module having common electrical connections and a common cooling base. Optical power from the individual diode-lasers is combined into a single output beam of laser-radiation that propagates from the diode-laser module. The output beam is typically multi-mode and highly divergent. Although the output beam can be used directly, a convenient way to deliver the output beam to an application is through an optical fiber attached to the diode-laser module. Such an optical fiber is commonly referred by practitioners of the art as a “transport fiber.” A transport fiber may be permanently attached as an integrated component of the diode-laser module, which is referred to as a “fiber pigtail.” Alternatively, a transport fiber may be detachable by way of connectors on an output port of the diode-laser module and on an input end of the transport fiber. 
         [0003]    Diode-laser modules have become the prevailing devices for energizing or “pumping” solid-state gain-media in laser-oscillators and laser-amplifiers. Common solid-state gain-media include crystalline, glass, and semiconductor materials, which are fabricated into gain-elements in the form of rods, slabs, discs, and fibers. A beam of pump laser-radiation is focused into a gain-element. 
         [0004]    Efficient pumping requires substantial spatial overlap between the focused beam of pump laser-radiation and a beam of laser-radiation to be amplified in the gain-element. By way of example, in an “end-pumped arrangement,” the focused pump beam is approximately cylindrical near the focus and coaxially aligned with the beam to be amplified. In a “disc geometry,” the pump beam is focused to a spot on the face of the disc that overlaps with the beam to be amplified, which is reflected from the disc. Performance of a laser-oscillator or laser-amplifier is therefore sensitive to alignment of the focused pump beam in the gain-element, which is in turn sensitive to precise alignment of the pump beam where it exits the transport fiber. 
         [0005]    Diode-lasers have finite (albeit relatively long) operating lifetimes that depend on the operating environment. Diode-lasers are electrostatic discharge (ESD) sensitive devices that are susceptible to damage by unintended voltage spikes or mishandling. Water-cooled diode-laser modules are also vulnerable to failure due to corrosion and blockages. Because of this, many commercial diode-pumped solid-state laser products have replaceable diode-laser modules to provide for performance degradation or acute failure. 
         [0006]    Products having a diode-laser module with a fiber pigtail can be designed to be disconnected at an output end of the transport fiber. However, the output end of the transport fiber has alignment tolerances close to limits achievable using conventional optical fiber connectors, as discussed above. Often the transport fiber has a “facet angle,” meaning the output face is deliberately tilted from a plane perpendicular to the geometrical axis of the transport fiber, which contributes an alignment variance. The output face is also susceptible to damage by contamination or mishandling, due to the high-intensity pump beam exiting the transport fiber. 
         [0007]    Products having a transport fiber that can be disconnected from the diode-laser module can be designed to have the output end of the transport fiber permanently fixed within the solid-state laser. However, the output port of the diode-laser module and the connector end of the transport fiber are susceptible to damage by contamination or mishandling. If the transport fiber becomes damaged, the solid-state laser requires repair as the transport fiber is an integral component. Such repairs are inconvenient and expensive, especially when the solid-state laser is permanently aligned and sealed for reliability during manufacture. 
         [0008]    There is need for an improved diode-pumped solid-state laser with a replaceable diode-laser module, having a design that minimizes vulnerability to contamination and mishandling when a diode-laser module is exchanged or replaced. Preferably, the diode-laser module can be reliably replaced without reducing performance due to misalignment of the pump-beam with the gain-element. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, a laser apparatus in accordance with the present invention comprises an optical fiber having an input end and an output end. A diode-laser delivers a beam of laser-radiation into the optical fiber through the input end. A connector-assembly body and a collimating lens are provided. The output end of the optical fiber is fixedly held in a closed end of the connector-assembly body. The beam of laser-radiation propagates out of the fixed output end of the optical fiber towards an open end of the connector-assembly body. The collimating lens is fixedly held within the connector-assembly body. The fixed collimating lens is arranged to intercept and collimate the beam of laser-radiation. A laser-enclosure is provided, which includes a focusing lens and a gain element. The collimated beam of laser-radiation propagates out through the open end of the connector-assembly body and into the laser-enclosure via an entrance-aperture therein. The focusing lens is arranged to intercept the collimated beam of laser-radiation and to focus the beam of laser-radiation into the gain-element. The focused beam of laser-radiation energizes the gain-element. The connector-assembly body is attached to the laser-enclosure and is detachable from the laser-enclosure. The fixed collimating lens is arranged such that the collimated beam of laser-radiation is collinear with a preferred alignment axis defined with respect to the connector-assembly body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    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. 
           [0011]      FIG. 1  is plan-view from above, partially in cross-section, schematically illustrating one preferred embodiment of modular laser apparatus in accordance with the present invention, comprising a diode-laser pump module connected to a laser-enclosure, the diode-laser pump module delivering an aligned beam of laser-radiation for energizing a gain-element. 
           [0012]      FIG. 2  is a plan-view from above, partially in cross-section, schematically illustrating the preferred embodiment of laser apparatus in  FIG. 1 , with the diode-laser pump module not operating and disconnected from the laser-enclosure. 
           [0013]      FIG. 3  is a plan-view from above, partially in cross-section, schematically illustrating one preferred embodiment of pump module alignment apparatus in accordance with the present invention, for aligning the diode-laser pump module in the apparatus of  FIG. 1 . 
           [0014]      FIG. 4  is a graph schematically illustrating measured beam diameter as a function of displacement in the beam-propagation direction, for six different diode-laser pump modules after an alignment procedure using the pump module alignment apparatus of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Turning now to the drawings, wherein like features are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment of modular laser apparatus  10  in accordance with the present invention. Modular laser apparatus  10  comprises a diode-laser pump module  20  that includes a diode-laser  22 , an optical fiber  24 , and a connector-assembly  26 . Optical fiber  24  has an input end  24 A attached to diode laser  22  and an output end  24 B fixedly attached to connector-assembly  26 . Diode laser  22  generates a beam of laser-radiation that is delivered through optical fiber  24  to connector-assembly  26 . The beam of laser-radiation is designated generally by a principal axis  28  and by boundary rays when propagating in free space.  28 A designates a diverging portion,  28 B designates a collimated portion, and  28 C designates a focused portion of the beam of laser-radiation. 
         [0016]    Connector-assembly  26  includes a connector-assembly body  30  (hatched) and a collimating lens  32 . Output end  24 B of optical fiber  24  is secured mechanically in a closed end  30 A of connector-assembly body  30  by a fiber-connector  34 , thereby fixing permanently the alignment of beam of laser-radiation  28  with respect to connector-assembly body  30 . Fiber-connector  34  incorporates termination of output end  24 B of optical fiber  24 , which may simply be polished flat and anti-reflection coated, or may include an endcap for high-power operation. Those skilled in the art of fiber-optic design would recognize that fiber-connector  34  may be fabricated or purchased having specifications appropriate for a specific application, without departing from the spirit and scope of the present invention. Beam of laser-radiation  28  is highly diverging as it emerges from output end  24 B of optical fiber  24  and propagates towards an open end  30 B of connector-assembly body  30 . 
         [0017]    Collimating lens  32  is arranged to intercept and collimate beam of laser-radiation  28  before diverging beam  28 A emerges from open end  30 B of connector-assembly body  30 . Collimating lens  32  is secured mechanically within connector-assembly body  30 , thereby fixing alignment of collimated beam  28 B with respect to connector-assembly body  30 . Connector-assembly  26  may also include an optional window  36  at the open end  30 B of connector-assembly body  30  for protection against particle and chemical contamination. 
         [0018]    Modular laser apparatus  10  further comprises a laser-enclosure  50  that includes a laser-enclosure body  52  (hatched), a focusing lens  54 , and a gain-element  56 . Laser-enclosure  50  supports and protects a plurality of other elements (not shown) that together make a laser-oscillator or laser-amplifier. These other elements are particular to the specific laser-oscillator or laser amplifier design and a detailed description thereof is not necessary for understanding principles of the present invention. An entrance aperture  53  in laser-enclosure  50  is defined by edges of laser-enclosure body  52 . 
         [0019]    Connector-assembly body  30  is attached to laser-enclosure body  52  such that open end  30 B is adjacent to entrance aperture  53 . Connector-assembly body  30  is precisely situated by location pins  38 . Connector-assembly  26  is thereby mechanically referenced to laser-enclosure  50 . Practitioners in the art of mechanical design would appreciate that the connector-assembly could be attached and situated on the laser-enclosure body by alternative means, without departing from the spirit and scope of the present invention. 
         [0020]    Beam of laser-radiation  28  propagates from connector-assembly  26  into laser-enclosure  50 . Focusing lens  54  is arranged to intercept collimated beam  28 B and focus the beam of laser-radiation into gain-element  56 . Focused beam  28 C is substantially absorbed by gain-element  56 , thereby energizing gain-element  56 . Here “substantially absorbed” means any residual beam of laser-radiation  58  transmitted through gain-element  56  retains only a small fraction of the power in focused beam  28 C incident on the gain-element. Laser-enclosure  50  may include an optional window  60  that transmits collimated beam  28 B and protects elements inside the laser-enclosure from contamination. 
         [0021]      FIG. 2  schematically illustrates modular laser apparatus  10  with diode-laser pump module  20  not operating and disconnected from laser-enclosure  50 . Comparing  FIGS. 1 and 2 , mechanical connection and disconnection occurs where beam of laser-radiation  28  is collimated, which has two main advantages. 
         [0022]    First, collimated beam  28 B is the most forgiving of lateral and angular misalignment. Double-arrowed dashed-line  64  represents a preferred alignment axis for collimated beam  28 B emerging from connector-assembly  26 . Principal axis  28  of collimated beam  28 B may be translated and tilted with respect to preferred alignment axis  64 , with minimal impact on location and shape of focused beam  28 C in gain-element  56 . Similarly, focused beam  28 C depends weakly on waist-location and waist-size of collimated beam  28 B. 
         [0023]    Second, collimated beam  28 B is largest and therefore least damaging to optical surfaces, especially any optical surfaces having mechanical defects or contamination. Diode-laser pump module  20  and laser-enclosure  50  are thereby less vulnerable to damage by mishandling or exposure to contaminants. In designs that include optional windows  36  and  60 , optical damage can be further mitigated by making the windows from relatively hard materials and by making external surfaces of the windows accessible for cleaning. 
         [0024]    Location pins  38  in connector-assembly body  30  and complementary location holes  62  in laser-enclosure body  52  are depicted in  FIG. 2 . Location pins situate the connector-assembly body with precise lateral location and orientation. Alternative designs allowing connector-assembly  26  to rotate about preferred alignment axis  64  would work in most applications because the present invention is insensitive to lateral and angular misalignment. 
         [0025]    Gain-element  56  is depicted in  FIGS. 1 and 2  in the form of an end-pumped rod. It is noteworthy that the present invention retains advantages of alignment insensitivity and invulnerability to contamination for other forms of the gain-element and other pumping arrangements. 
         [0026]      FIG. 3  schematically illustrates one preferred embodiment of pump module alignment apparatus  70  for aligning beam of laser-radiation  28  exiting connector-assembly  26  of diode-laser pump module  20 . Pump module alignment apparatus  70  includes pump module alignment tooling  80  described in detail herein below. Connector-assembly  26  is attached to pump module alignment tooling  80  in manner similar to modular laser apparatus  10 , with connector-assembly body  30  attached to a tooling mount  82  instead of laser-enclosure body  52 . Location pins  38  fit into location holes  62  in tooling mount  82 . The tooling mount is a permanently fixed mechanical reference that locates and orients connector-assembly  26 . 
         [0027]    Pump module alignment tooling  80  further includes wedged tooling mirrors  84  for attenuating beam of laser-radiation  28 . Wedged tooling mirrors  84  direct the attenuated beam through a tooling focusing lens  86  and into beam-diagnostic tooling  88 . Tooling focusing lens  86  may be identical to focusing lens  54  (shown in  FIG. 1 ), thereby producing a focused beam identical in shape to focused beam  28 C in modular laser apparatus  10 . Alternatively, tooling focusing lens  86  may be selected for compatibility with beam-diagnostic tooling  88 . It is straightforward to calculate differences between focused beams in modular laser apparatus  10  and pump module alignment apparatus  70  due to different specifications of focusing lenses. Focused beam  28 C has a caustic and a focus location  90 . The caustic is defined by boundary rays  92 A and  92 B of the focused beam. The caustic includes the diameter of the focused beam at focus location  90  and the shape of the focused beam about focus location  90 . 
         [0028]    The objective of an alignment procedure is consistent alignment of every diode-laser pump module  20 , by aligning collimating lens  32  to create a focused beam having a target caustic in beam-diagnostic tooling  88 , corresponding to a preferred optical and mechanical alignment. Therefore tooling mount  82 , wedged tooling mirrors  84 , tooling focusing lens  86 , and beam-diagnostic tooling  88  are mechanically fixed with respect to each other. A simple way to fix these elements is to mount them all on a common tooling plate (not shown). A reference laser (not shown) optically and mechanically referenced to tooling mount  82  may be used to maintain consistent alignment of pump module alignment tooling  80  and to facilitate replacement of any elements of the pump module alignment tooling. 
         [0029]    An exemplary alignment procedure aligns collimating lens  32  by translating it in x, y, and z-directions until caustic  92 A and  92 B of focused beam  28 C matches the target caustic. The z-direction is the propagation direction of beam of laser-radiation  28 , as indicated in the drawing. The three mutually-orthogonal translations are performed iteratively. Collimating lens  32  is then fixed permanently within connector-assembly body  30 . Tooling for aligning and fixing collimating lens  32  is not depicted in  FIG. 3 . Means for aligning and fixing a lens are well known in the art. By way of example, aligning may be performed using commercial translation stages, such as those supplied by ThorLabs of Newton, N.J. Fixing may be accomplished using soldering technology, such as the methods taught in U.S. Pat. No. 5,930,600. Fixing may also be accomplished using an adhesive. 
         [0030]      FIG. 4  is a graph depicting measured beam diameter in the x-direction, near focus location  90 , as a function of displacement in the z-direction from tooling focusing lens  86 . Here, the beam diameter was measured at 5% of the fitted peak intensity at the center of the beam.  FIG. 4  includes measurements for six diode-laser pump modules  20  after each was aligned using the exemplary alignment procedure. Beams of laser-radiation produced by the six diode-laser pump modules were highly multi-mode, having a beam-quality (M 2 ) of approximately  50 . The measurements depicted in  FIG. 4  were obtained using a 100 mm (millimeter) focal length lens ( 86  in  FIG. 3 ) and a NanoModeScan laser-beam-profiler from Ophir Photonics of North Logan, Utah ( 88  in  FIG. 3 ).  FIG. 4  illustrates minimal residual variances in focus location and caustic after the alignment procedure, which are representative of variances in the focused beam when the diode-laser pump modules are installed in modular laser apparatus  10  (shown in  FIG. 1 ). 
         [0031]    Referring again to  FIG. 1 , after an alignment procedure, diode-laser pump module  20  generates collimated beam  28 B that propagates from connector-assembly  26 . Principal axis  28  of collimated beam  28 B is collinear with preferred alignment axis  64  (shown in  FIGS. 2 and 3 ), which is optically and mechanically referenced with respect to connector-assembly body  30  and laser-enclosure body  52 . Focused beam  28 C will have a beam waist at a preferred location within gain-element  56  and will have a preferred caustic in gain-element  56 . When building a solid-state laser within laser-enclosure  50 , the solid-state laser is aligned around a volume within gain-element  56  energized by focused beam  28 C. There is no further adjustment of diode-laser pump module  20 . The location of focusing lens  54  may be adjusted during the solid-state laser build, if necessary, without affecting the exchangeability of aligned diode-laser pump module  20 . 
         [0032]    For optical fibers  24  having a facet angle on output end  24 B, diverging beam  28 A is refracted from the geometrical axis of fiber  24 , with variances in refracted angle and orientation. To compensate for these variances, another exemplary alignment procedure would include aligning and fixing fiber-connector  34  within connector-assembly body  30 . Referring again to  FIG. 3 , focused beam  28 C is aligned to a target focus location  90  and a target beam diameter at focus location  90  by translating fiber-connector  34  in the x, y, and z-directions. Principal axis  28  of collimated beam  28 B is thereby made parallel to preferred optical axis  64 . Focused beam  28 C is then aligned to a target caustic  92 A and  92 B by translating fiber-connector  34  and collimating lens  32  together in the x and y-directions, thereby making principal axis  28  of collimated beam  28 B collinear with preferred alignment axis  64 . Fiber-connector  34  and collimating lens  32  are then fixed permanently within connector assembly body  30 . 
         [0033]    In some applications, it may be preferable to separate functions of connector-assembly body  30  between a plurality of elements. For example, a first element for mounting collimating lens  32 , a second element for holding fiber-connector  34 , and a third element for sealing connector-assembly  26 . The first and second elements would be references for optical and mechanical alignment. The third element may be installed after aligning and fixing collimating lens  32  and fiber-connector  34 . 
         [0034]    For reliability and convenience, all the elements of diode-laser pump module  20  may be packaged into a common enclosure (not shown). Such an enclosure would have connectors for external electrical connection and ports for external water connection. 
         [0035]    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.