Abstract:
A method, an apparatus, and a module for producing dual beam from a single laser diode provide for means of simultaneously pumping two individual gain media with orthogonal polarizations. A beam splitter splits the emissive laser beam into two portions based on the polarization. A polarization control element or mechanism adjusts the polarization and the intensity ratio of the separated beam portions. Applications to monolithic microchip lasers include generating new wavelengths based on intracavity beam combining and mixing.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates generally to diode pumped solid-state (DPSS) lasers, in particular to design of compact and efficient pump sources, and more particularly to intracavity frequency conversion using a single laser diode for simultaneously pumping two independent gain media characteristic of polarization dependent absorption.  
       BACKGROUND OF THE INVENTION  
       [0002]     In U.S. Provisional Application No. 60/663,503, titled as “Monolithic Microchip Laser With Intracavity Beam Combining And Sum Frequency Or Difference Frequency Mixing”, Luo, Zhu, and Lu have disclosed a low-noise monolithic microchip laser, wherein intracavity beam combining and sum frequency mixing (SFM) or difference frequency mixing (DFM) are used for generation of various lasing wavelengths. According to their invention, two fundamental laser beams are generated from two lasing media pumped by two laser diodes in an integrated dual laser cavity. These two fundamental laser beams are then combined in an intracavity birefringence crystal based on the walk-off effect to generate the desired wavelength by means of intracavity SFM or DFM in a nonlinear optical (NLO) crystal.  
         [0003]     One of the advantages of the monolithic microchip laser is compact size. There still remain rooms for improvement, however. Since the walk-off angle is a function of the birefringence and the cut angle, a reduction in the separation of the pump diodes will proportionally reduce the length of the birefringence crystal, and consequently, the device size. This is the issue that will be addressed in the present invention.  
       DESCRIPTION OF RELATED ART  
       [0004]     A basic requirement for the paired pump sources employed for a monolithic microchip laser based on intracavity beam combining and SFM or DFM scheme such as the one disclosed in U.S. Provisional Application No. 60/663,503 is that their polarizations must be mutually orthogonal. This can be achieved by, e.g., splitting the light from a single emitter through polarization sensitive beam splitting elements. As a matter of fact, optical polarization beam combiners or splitters are used in many applications.  
         [0005]     For example, Nikolov, et al. in U.S. Pat. No. 6,876,784 demonstrated an optical device for combining two orthogonally polarized beams or splitting a beam into two orthogonally polarized beams using a thin film wire-grid polarizer. This and other patents referenced therein are primarily for optical fiber communication applications.  
         [0006]     As another example, in U.S. Pat. No. 6,137,820, Maag, et al. claimed an optically pumped laser, wherein a light beam is split into two components, one of which then passes through a polarization-rotating element to rotate the polarization by an angle of 90°. These two beams of parallel polarizations are individually directed to a gain medium having polarization-dependent absorption from both sides or superimposed on one face of the crystal.  
         [0007]     As another example, in United States Patent Application No. 20040258117, Nebel et al. combined two pump sources emitting light at different wavelengths and of orthogonal polarizations to enhance optical pumping of materials exhibiting polarization dependent absorption.  
         [0008]     As another example, in United States Patent Application No. 20020179912, Batchko, et.al. described a monolithic polarization-insensitive wavelength converter system comprising a polarization separator, a polarization rotator and a wavelength converter.  
         [0009]     In spite of these successes, the prior art has limited applications and, in particular, is not applicable to monolithic systems requiring two pump beams with mutually orthogonal polarizations such as the one described in U.S. Provisional Application No. 60/663,503.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an object of the present invention to provide a method and associated apparatus and module for realizing simultaneous pump of two independent gain media by dual beam from a single laser diode. In particular, these gain media can be characteristic of polarization sensitive absorption and can be oriented so that their preferable absorption directions are mutually orthogonal.  
         [0011]     It is another object of the present invention to provide a method and associated apparatus and module so that the intensity ratio of the pump beams can be properly adjusted to optimize the laser performance.  
         [0012]     A further object of the present invention is that the inventive structure can be integrated into a monolithic microchip laser using intracavity beam combining and frequency mixing such as the one described in U.S. Provisional Application No. 60/663,503.  
         [0013]     These objects can be accomplished by introducing a polarization control element or mechanism for managing the polarization orientation of the diode emission and by splitting the diode emission into two portions with mutually orthogonal polarizations.  
         [0014]     According to the present invention, diode emission is split via polarized light separating dielectric coated thin films. When a light hits such films, one component with favorable polarization is transmitted, while the other component with orthogonal polarization is reflected. At least one surface coated with such films and at lease one additional surface, which is highly reflective to the incident light wavelength and polarization, are needed to get the desired dual beam. With proper selection of the orientation and separation of these surfaces, various direction and separation of the dual beam can be obtained. Advantageously, the beam split element may be a pair of parallel mirrors with polarization-sensitive coatings and oriented with 45° relative to the light propagation direction. Alternatively, such mirrors may be replaced with appropriately coated prisms or polarized beam splitters (PBS) or a combination of the above. A further alternative is the replacement of both polarization-sensitive mirrors with a double PBS prism.  
         [0015]     Also advantageously, the polarization of the diode emission is rotated to a desired orientation through a polarization control element or mechanism. In one preferred embodiment, the polarization control is a mechanism comprising physical orientation of a free-space emitter. In another preferred embodiment, the polarization control element is a half wave plate or other polarization rotator. In yet another preferred embodiment, the polarization control element is built in the light delivery system of a fiber coupled diode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     This invention may be more completely understood by reading the following detailed description of various embodiments in connection with the accompanying drawings, in which:  
         [0017]      FIG. 1A  is a schematic illustration of a first embodiment of a dual beam pump from a single laser diode according to the present invention.  
         [0018]      FIG. 1B  is a top-view of the pump laser diode and the associated polarization control mechanism according to the first embodiment of the present invention.  
         [0019]      FIG. 2  is a schematic illustration of a second embodiment of a dual beam pump from a single laser diode according to the present invention.  
         [0020]      FIG. 3  is a schematic illustration of a third embodiment of a dual beam pump from a single laser diode according to the present invention.  
         [0021]      FIG. 4  is a schematic illustration of a fourth embodiment of a dual beam pump from a single laser diode according to the present invention.  
         [0022]      FIG. 5  is a schematic illustration of a fifth embodiment of a dual beam pump from a single laser diode according to the present invention.  
         [0023]      FIG. 6  is a flowchart of a pump beam split scheme according to the present invention.  
         [0024]      FIG. 7A  is a schematic illustration of an exemplary application of the present invention to a monolithic microchip laser for generation of 491 nm laser.  
         [0025]      FIG. 7B  shows optimized polarization orientation as a function of the optical power from the laser diode and the optimized performance of the laser demonstrated in  FIG. 7A . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Referring now to the drawings and in particular to  FIG. 1A , wherein a first embodiment of dual beam pump from a single laser diode constructed according to the present invention is shown in a schematic form. The inventive dual beam pump source  100  includes a laser diode  110 , in particular, a free-space emitter for emitting light, a beam shaping element  120  for collimating the diode emission, two coated mirrors  151  and  152  for light separation, and two additional beam shaping elements  161  and  162  for focusing the output beams. The beam shaping elements  120 ,  161  and  162  may be separated lenses or integrated micro lenses or lens arrays.  
         [0027]     Preferably, the mirror  151  is coated with polarized light separating film on at least one optical surface. In accordance with our inventive teachings, the film transmits light polarized in the preferred direction, e.g., p-component, and reflects light polarized in the other direction, e.g., s-component. The mirror  152  is highly reflective to the s-component of the light  180  emitted from the laser diode  110  and remains the polarization after reflection. These two mirrors are oriented in parallel with each other and 45° relative to the diode emission propagation direction. When the light beam  180  hits the first mirror  151 , one component  181  passes and is focused at a desired location through the lens  161  to form the first pump beam  181 . The other component  182  is reflected on the surfaces of the mirrors  151  and  152  and is focused at a desired location through the lens  162  to form the second pump beam  182 . With this configuration, the pump beams  181  and  182  are parallel to each other and are separated by √2 times the distance between the two mirrors. With proper selection of the orientation and separation of the mirrors  151  and  152 , various orientation and separation of the beams  181  and  182  can be obtained.  
         [0028]     One of the advantages of the present invention is that by rotating the diode emission polarization to an appropriate direction, the intensity ratio of the component  181  and component  182  is adjustable. When applied to a frequency conversion system, this scheme may lead to optimization of the frequency mixing efficiency. According to the first embodiment of our invention, optimized polarization orientation of the diode emission is achievable via a mechanism schematically illustrated by a top-view of the laser diode  110 , as shown in  FIG. 1B .  
         [0029]     As displayed in this graph, an emitting device  112 , which provides for the emissive beam  180 , is packaged in a housing  111 . The polarization of the emissive beam is preferably adjusted by physical rotation of the emitting device before packaging, which may be chosen from a variety of options such as C-mount with an open heat sink for compactness and versatility, high-heat-load (HHL) for increased thermal management, and TO can for straightforward incorporation of diode lasers at the production level. Single free-space diodes typically emit optical powers of 2 W to 5 W, depending on wavelength. Typically, the emitter area is in the order of 1 μm×100 μm and the polarization may be transverse electric (TE) or transverse magnetic (TM).  
         [0030]     With our inventive teachings, only one laser diode is needed to simultaneously excite two independent gain media with adjustable pump intensity ratio. Preferably, the polarization of each pump light matches the absorption characteristics of the corresponding gain medium. In case the gain medium is neodymium-doped yttrium vanadate, a pump light of 808 nm with π polarization is preferred.  
         [0031]     As can be appreciated by those skilled in the art, our inventive teachings allow for a great degree of freedom in the design practice. For example, as shown in  FIG. 2 , the polarization sensitive mirror  151  in  FIG. 1A  can be replaced by a single PBS  250  with one inclined intermediate layer  251 , which is composed of at least one polarized light separating membrane(s) and at least one adhesive layer(s) that bond the membrane(s) to the optical surface(s). Preferably, the intermediate layer  251  transmits the p-component  181  of the diode emission  180  and reflects the s-component  182 . Again, the orientation and separation of the pump beams  181  and  182  are determined by the relative orientation and separation of the intermediate layer  251  and the reflective mirror  152 .  
         [0032]     Another variation of the beam split element is shown in  FIG. 3 , wherein the parallel mirrors  151  and  152  shown in  FIG. 1A  are replaced with a double PBS  350  for splitting the incident light  180 . In the polarized beam splitter  350 , there are two parallel intermediate layers  351  and  352 , each is composed of at least one polarized light separating membrane(s) and at least one adhesive layer(s) that bond the membrane(s) to the optical surface(s). At the pump wavelength, the intermediate layers  351  and  352  transmit light component  181  polarized in the preferable direction, e.g. p-component, and reflect light component  182  polarized in the orthogonal direction, e.g., s-component.  
         [0033]     Whether to use one or two mirrors, labeled as  151  and  152  in  FIG. 1A , or use one single PBS, labeled as  250  in  FIG. 2  and one highly reflective mirror  152 , or use a double PBS, which is labeled as  350  in  FIG. 3 , or a combination of the above, is a matter of design. Of course, there are many other options, comprising at least one optical surface coated with polarized light separating membrane or film.  
         [0034]      FIG. 4  shows a fourth embodiment of the present invention, in which a fiber-coupled emitter  410  is employed as the light source, together with integrated polarization control. Fiber-coupled pump laser diodes is used as one of the most popular formats for laser diodes. Typically, the fiber core diameter is in the order of 100 μm for a single emitter laser diode and the fiber length may vary. As is well known, the light polarization tends to be randomized due to propagation through a non-polarized fiber, or fiber optic devices for de-polarization purpose. For fiber length around 1 m, the randomization becomes complete and the emissive beam  180  is randomly polarized with uniform distribution along any direction. Upon interaction with the beam split element  151 , the emissive beam is split into p-component  181  and s-component  182  with identical intensities.  
         [0035]     A fifth embodiment of the present invention is shown in  FIG. 5 , in which the polarization control element is a half wave plate  530 . Inserted in the optical path, the half wave plate  530  rotates the diode emission polarization to a desired orientation. The beam split element  150 , which, in this  FIG. 5 , is a pair of parallel mirrors  151  and  152  but can also be composed of other polarization sensitive components or their combinations as described in the forgoing text, splits the emission beam  180  into two components  181  and  182 . Advantageously, the intensity ratio of the beams  181  and  182  is adjustable according to the polarization of beam  180 .  
         [0036]     With reference now to  FIG. 6 , where is shown a flowchart of a pump beam split scheme according to the present invention. In accordance with our inventive teachings, the polarization control, which can be physical orientation of a free-space emitter or optical rotation through a half wave plate or other polarization rotator, rotates the polarization direction of the pump beam  180  emitted from the pump diode to an angle θ relative to the s-direction. Advantageously, this action may be accomplished under automatic control of an integrated computer. The beam splitter transmits the light component with polarization along the p-direction to form the first output (beam  181 ) and reflects the other light component with polarization along the s-direction to form the second output (beam  182 ). The intensity ratio of the first output to the second output is proportional to tan 2 θ, which may be optimized via an appropriate algorithm and adjustment of the polarization direction θ.  
         [0037]     As can be appreciated by those skilled in the art, our inventive teachings are particularly useful for monolithic microchip lasers employing intracavity beam combining and frequency mixing for generating laser beam at a wavelength not directly available from a laser diode or a diode pumped solid-state laser such as the one disclosed in U.S. Provisional Application No. 60/663,503. For the sake of description, an exemplary application of the present invention is demonstrated in  FIG. 7A .  
         [0038]     As shown in this  FIG. 7A , a monolithic microchip laser  700  consists of a free-space pump diode  710 , three beam shaping elements  720 ,  740 , and  750 , a double PBS  730  as beam splitter, two Nd:YVO 4  laser gain media  760  and  770 , an un-doped YVO 4  crystal  780 , and a nonlinear crystal KTP  790 . These crystals are optically bound and in physical contact for elimination of the boundary optical loss.  
         [0039]     In order to produce laser output at 491 nm, the exterior surface  761  of the gain medium  760  is coated with materials highly reflective (HR) at 1064 nm while highly transmissive (HT, T&gt;95%) at 808 nm. Similarly, the exterior side  771  of the gain medium  770  is coated HR at 914 nm and HT (T&gt;95%) at 808 nm. The coating  771  should also be HT at 1064 nm and 1342 nm to prevent these high-gain transitions from lasing. The coating  791  on another side of the microchip laser  700  is HT at 491 nm (T&gt;95%), and HR at both 914 and 1064 nm. The coating  792  between the crystals  780  and  790  is highly reflective to the mixed wavelength 491 nm and antireflective to the fundamental wavelengths 914 nm and 1064 nm. The mirrors  761  and  791  form a cavity resonating at the first fundamental wavelength 1064 nm, while the mirrors  771  and  791  form a cavity resonating at the second fundamental wavelength 914 nm.  
         [0040]     In operation, the laser diode  710  emits light with wavelength of 808 nm and the desired polarization. The light is collimated through the beam-shaping element  720  and is split into two portions  711  and  712  with mutually orthogonal polarizations due to interactions with the polarized beam splitter  730 . Preferably, the beams  712  and  712  are polarized along the π directions of their corresponding gain media  760  and  770  for favorable absorption. Upon excitation from the ground state  4 I 9/2  to the metastable state  4 F 3/2  by these two pumping sources, the laser gain media  760  and  770  produce stimulated emissions respectively at 1064 nm and 914 nm wavelengths. Two fundamental laser beams are thus formed within their respective resonators.  
         [0041]     According to our inventive teachings, the gain medium  760  is so oriented that the first fundamental beam  765  with wavelength of 1064 nm is an e ray relative to the un-doped YvO 4  crystal  780 . Similarly, the orientation of the gain medium  770  makes the second fundamental beam  775  with wavelength of 914 nm an o ray within  780 . Owing to the walk-off effect, these two beams are combined at the interface  792  between the undoped YvO 4  crystal  780  and the nonlinear optical crystal  790 . With precise control of the undoped YvO 4  crystal length, the two fundamental beams collinearly enter the nonlinear optical crystal KTP  790  and frequency mixing takes place. Through the output coupler  791 , a new laser beam  795  with the reduced wavelength 491 nm outputs. This monolithic microchip laser provides for a promising replacement of argon ion lasers.  
         [0042]     One of the advantages of the present invention is capable of producing dual beam with short separation. With integration of micro-lenses or micro-lens arrays for beam collimating/focusing, it is possible to reduce the beam separation to 1 mm or shorter. This feature is beneficial to intracavity frequency conversion based on the walk-off effects because the length of the birefringence crystal thus required is proportionally reduced, resulting in more compact structure and lower cost.  
         [0043]     Another advantage of the present invention is that by adjusting the polarization of the light emitted from the pump diode, both the pump efficiency and the wavelength conversion efficiency can be optimized. For better understanding, a numerical analysis is conducted.  
         [0044]     Plotted in  FIG. 7B  are curves representing the optimized performance of the monolithic microchip laser demonstrated in  FIG. 7A . In particular, the polarization of the laser diode  710  is so chosen as to form an angle θ relative to the π direction of the gain medium  760 , wherein the first fundamental wavelength 1064 nm is generated. With this orientation, the intensity ratio of beam  775  to beam  765  is proportional to tan 2 θ. As evidenced from this  FIG. 7B , θ decreases as the pump power increases. For sufficiently high pump power, the optimized intensity ratio approaches to one. In this case, a fiber-coupled single emitter can also be employed as the pump source. Another finding in this figure is that more than 70 mW laser output at 491 nm can be achieved from a pump diode that produces optical power of 2 W.