Abstract:
An optic device is placed in close proximity to multiple gain elements so as to selectively modify the divergence of the light from said multiple elements such that when the light is subsequently collimated and diffracted from a grating and focused into an optical fiber, it will have a predefined cross-section that matches the mode of the fiber. Using this system and method, a more efficient light transfer is achieved in an intracavity fiber coupled multigain element laser.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to and claims the benefit of U.S. Provisional Patent Application Ser. No. 60/316,806, entitled “OPTICALLY CORRECTED INTRACAVITY FIBER COUPLED MULTIGAIN ELEMENT LASER,” which is incorporated herein by reference, and the present application is related to commonly assigned and co-pending U.S. patent application Ser. No. 09/945,381, entitled “SPECTRALLY TAILORED RAMAN PUMP LASER,” filed Aug. 31, 2001, the disclosure of which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to lasers and more specifically to external cavity diode-lasers having optically corrected multi-wavelength light beams, and even more particularly to selective optically coupled lasers where the shape of the light beams match a desired mode of an optical fiber. 
     BACKGROUND OF THE INVENTION 
     Incoherently beam combined (IBC) lasers combine the output from an array of gain elements or emitters (typically consisting of semiconductor material, such as GaAlAs, GaAs, InGaAs, InGaAsP, AlGaInAs, and/or the like, which is capable of lasing at particular wavelengths) into a single output beam that may be coupled into, for example, an optical fiber. The gain elements may be discrete devices or may be included on an integrated device. Due to the geometry of IBC lasers, each gain element lases a particular wavelength. Exemplary arrangements of IBC lasers are described in U.S. Pat. No. 6,052,394 and U.S. Pat. No. 6,192,062. 
       FIG. 8  depicts a prior art arrangement of components in IBC laser  80 . IBC laser includes emitters  82 - 1  through  82 -N associated with fully reflective surface  81  and front surface  84 . Emitters  82 - 1  through  82 -N are disposed in a substantially linear configuration that is roughly perpendicular to the optical axis of collimator  85  (e.g., a lens). Collimator  85  causes the plurality of beams produced by emitters  82 - 1  through  82 -N to be substantially collimated and spatially overlapped on a single spot on diffraction grating  86 . Additionally, collimator  85  directs feedback from partially reflective component  87  via diffraction grating  86  to emitters  82 - 1  through  82 -N. 
     Diffraction grating  86  is disposed from collimator  85  at a distance approximately equal to the focal length of collimator  85 . Furthermore, diffraction grating  86  is oriented to cause the output beams from emitters  82 - 1  through  82 -N to be diffracted on the first order toward partially reflective component  87 . Partially reflective component  87  causes a portion of optical energy to be reflected. The reflected optical energy is redirected by diffraction grating  86  and collimator  85  to the respective emitters  82 - 1  through  82 -N. Diffraction grating  86  angularly separates the reflected optical beams causing a particular wavelength generated by each emitter  82 - 1  through  82 -N to return to each respective emitter  82 - 1  through  82 -N. Accordingly, diffraction grating  86  is operable to demultiplex the reflected beams from reflective component  87 . 
     It shall be appreciated that the geometry of external cavity  83  of IBC laser  80  defines the resonant wavelengths of emitters  82 - 1  through  82 -N. The center wavelength (λ i ) of the wavelengths fed back to the i th  emitter  82 -i is given by the following equation: λ i =A[sin(α i )+sin(β)]. In this equation, A is the spacing between rulings on diffraction grating  86 , α i  is the angle of incidence of the light from the i th  emitter on diffraction grating  86 , and β is the output angle on diffraction grating  86  which is common to all emitters  82 - 1  through  82 -N. As examples, similar types of laser configurations are also discussed in U.S. Pat. No. 6,208,679. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method which improves the efficiency of fiber coupling IBC lasers by utilizing optics and grating induced anamorphic magnification. 
     One embodiment of the invention uses a micro optic array placed next to the emitter array for selectively changing the ellipticity of each beam from each individual emitter, such that as each beam is diffracted from the grating, the beam is circular. This circular beam then can be focused into the fiber very efficiently. The system uses an optic that collimates each beam prior to incidence on the grating and an optic that focuses the diffracted beam into the fiber opening. Thus, the intensity profile of the beam being input into the fiber is mode matched to the fiber mode, e.g., round, and the beam can be coupled without substantial losses. 
     Additionally, each lens of the array can be made different. Accordingly, such a lens array will allow for tailoring the divergence of each individual emitter beam such that after each beam diffracts off the grating, it will be circular. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIG. 1  shows exemplary structural relationships among the elements in accordance with embodiments of the invention; 
         FIGS. 2A and 2B  show the light divergence angles from one as light rays from that element; 
         FIG. 3  shows the beam sections for the light within an element micro optic of the invention; 
         FIG. 4  shows an exemplary shape for the micro optic as used in the invention; 
         FIG. 5  shows an example of a system being affected by differential magnification; 
         FIG. 6  shows one alternative micro optic design to correct for emitter spacing; 
         FIG. 7  shows the chromatic aberration correction for an array; and 
         FIG. 8  depicts a prior art arrangement of components in an IBC laser. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to  FIG. 1 , there is shown system  10  which is a particular embodiment of this invention. In this embodiment, array  11  contains N gain elements,  19 - 1  to  19 -N. For the sake of simplicity, only the beam from a single gain element is shown. Each gain element emits light beam  110 - 1  (having cross-sectional shape  20 ) that is preferably shaped by lens component  30  into a beam having elliptical shape  102 . Lens component  30  may comprise a single lens element or multiple lens elements. Lens component  30  may comprise a micro lens array, wherein each beam  110 - 1  is shaped by a respective micro optic of lens component  30  or a rod lens. Beams  102  emerging from lens component  30  are then collimated by optic  13  into cross-sectional shape  103 - 1 . The collimated beams from optic  13  are directed onto diffraction grating  14 , which preferably diffracts the beams into a first order, more preferably, the plus first order. Diffraction grating  14  also magnifies or compresses beam shapes  103 - 1  in the process of diffraction, so that diffracted beams  104 - 1  are substantially circular in cross-section. Beams  104 - 1  are typically focused into the core of fiber  16  by optic  15 , whose focal length is chosen to match the circular beam into the desired fiber mode. 
     There is a difference in the beam shape after diffraction from diffraction grating  14  from shape  103 - 1  to shape  104 - 1 . This magnification or compression occurs principally in one axis. This axis is referred to as the slowly diverging axis, from gain elements  19 . Lens component  30  is designed to pre-compensate for the magnification or compression that occurs at diffraction grating  14 , as well as for the different divergence patterns in the perpendicular and parallel axis of beam  110 - 1 . It is this pre-compensation that makes beam cross-section  104 - 1  circular. 
     Lens component  30  prevents optic  15  from creating a focused beam that has different numerical apertures in the perpendicular and parallel axis thereby preventing diminished coupling efficiency. Thus, by pre-compensating the beam via lens component  30 , beam  110 - 1 , as it emerges from lens component  30 , takes advantage of the difference in the different axes of divergence as well as the anticipated difference in the magnification or compression experienced at diffraction grating  14  to achieve desired shape  104 - 1 . Axis  120  is normal to diffraction grating  14  and is used to define an angle of incidence (θ in ) and an exit angle (θ out ). Anamorphic magnification then is 
         cos   ⁢           ⁢     θ   in         cos   ⁢           ⁢     θ   out           
 
which equals the ratio of cross-sectional width  104 - 1  to cross-sectional width  103 - 1 .
 
     Also it should be noted that diffraction grating  14  is preferably positioned where the rays from all gain elements  19 - 1  to  19 -N substantially coincide. 
       FIG. 2A  substantially represents array  11  having laser gain elements  19 - 1  to  19 -N. Each gain element emits its own cross-sectional emission pattern  20  which is similar for each element. Note that the divergence angle in the Y axis of the pattern  20  is different from the divergence angle in the X axis. The divergence axes are said to be slow and fast. In this view the X-axis referred to as the slow axis because the beam diverges more slowly in this axis. The Y axis is said to be the fast axis because the beam diverges more quickly in this axis. Note that the difference in divergence is from the rectangular shape of the emission plane of the gain element, with the more narrow dimension of the emission plane having the fast divergence angle. 
       FIG. 2B  shows laser gain element  19 - 1 , which has been rotated with respect to the view of  FIG. 2A . The fast axis now is vertical on the page, such that gain element  19 - 1  is now in the plane of the page, and the slow axis (indicated by dashed lines in perspective) is now the beam that is perpendicular on the page. The beam is diverging from point  21  in the emission plane of gain element  19 - 1 . 
       FIG. 3  is a perspective representation that begins at point  21  (of  FIG. 2B ), which is the point on the emission plane where the beam from gain element  19 - 1  appears to emerge from the element. As the beam moves to the right, it diverges at different rates in the fast axis and the slow axis. In  FIG. 3  the fast axis is again visualized in the plane of the page and the slow axis is perpendicular to the page. The ratio of these divergences is shown in beam cross-section  302 , which arrives at input face  34  of lens component  30 - 1 , which preferably has optical power in the fast axis only. Lens component  30 - 1  in some embodiments is a portion associated with gain element  19 - 1  in lens component  30  of  FIG. 1  which may comprise a lens array. Lens component  30  may also include astigmatism correction surface(s). 
     Surface  34  is convex and forces the fast axis of the beam to converge toward back surface  33 , which is concave such that the beam then starts to diverge from virtual focal point  301 . Note that lens component  30 - 1  is preferably a positive cylindrical lens (in the fast axis), as it reduces the fast axis divergence of a diverging input beam. The basic function of lens component  30 - 1  is to create beam cross-section  303  (beam cross-section  102 ,  FIG. 1 ) at its output having a ratio of beam axes  31  and  32  and divergences  34 ,  35  that (1) it pre-compensates for the magnification or compression that the beam will experience at diffraction grating  14  ( FIG. 1 ), and (2) it corrects for different divergence rates in the fast and slow axes. 
     The radii of curvature of surface  34  and surface  33  of lens component  30  are chosen to achieve two purposes. The first is to form the beam so that it appears to diverge in both axes (slow and fast) from point  301  and also remove astigmatism; and the second is to form the beam such that the ratio of the major and minor axes of elliptical beam profile  303  are equal to the ratio of the cosines of the input angle θ in  and output angle θ out  on diffraction grating  14 , and such that the beam exits diffraction grating  14  having the form desired for maximum coupling efficiency in fiber  16 . Note that the preferred form or cross-section is round, as fibers have round cross-sections; however, other shapes, e.g. elliptical, could be used, depending upon the shape of the input pupil function of the coupling device. 
     A typical thickness for lens component  30 - 1  of this sort is on the order of 300 microns between surface  34  and surface  33 . Typical focal lengths are on the order of 20 to 100 microns. 
       FIG. 4  is a three dimensional view of lens component  30 - 1 . Note that lens component  30 - 1  can include an aspheric surface (or two) to correct for aberrations, e.g., astigmatism. 
     Optical elements  30 ,  13 ,  14 , and  15  are designed such that the beams that are coupled into fiber  16  have the same numerical aperture or transfer mode as that of the fiber. The range of angles accepted by a fiber forms a cone or numerical aperture. Optical elements  30 ,  13 , and  14  are selected so that the output beam from diffraction grating  14  has the desired cross-section. Lens  15  induces a proper converging angle onto the beam. Thus, the combination of optical elements  30 ,  13 ,  14 , and  15  cause the coupling beams to match the numerical aperture of the fiber, and optionally to match the mode shape for the fiber. 
     Note that in a preferred embodiment, the arrangement of  FIG. 1  forms a laser. The laser cavity may be formed in the system of  FIG. 1  by making surface  201  of array  11  (see  FIG. 2 ) highly reflective and including partially reflective element  18  in fiber  16  as shown in  FIG. 1 . Note that surface  202  may be substantially non-reflective or partially reflective. This embodiment is described in co-pending U.S. patent application Ser. No. 09/929,837 entitled “SYSTEM AND METHOD FOR UTILIZING AN EXTERNAL CAVITY LASER UTILIZING ETALON EMITTERS,” filed Aug. 13, 2001, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     The arrangement operates as follows. Gain elements  19 - 1 , . . . ,  19 -N begin by spontaneous emitting, which is to produce multiple spectral light that is not lasing. Each beam of spontaneous emission light is pre-shaped by lens component  30 , and collimated by lens  13 . Note that while each beam is individually collimated, the plurality of beams  110 - 1 , . . . ,  110 -N is made to converge superimposed onto diffraction grating  14 , e.g. see  FIG. 5 . Each of the beams is incident on grating  14  at a slightly different angle. The grating diffracts light based upon the wavelength of light as well as the incident angle. Since each beam is a multiple-wavelength beam, diffraction grating  14  diffracts a plurality of output beams, with each different wavelength having a different exit angle. Also, since each of the input beams has a different incident angle, then no two output beams of the same wavelength would have the same exit angle. The partially reflective surface  18 , which is located in the core of fiber  16 , is arranged at a particular angle with respect to diffraction grating  14 . Moreover, the core of the fiber only permits a range of incident angles of light to couple with the fiber. Thus, only light that is within the coupling range of the fiber and that propagates in a substantially co-axial manner from the output angle of diffraction grating  14  through lens  15  will be incident onto partially reflective surface  18  and thus be reflected back to diffraction grating  14 . Consequently, the light received and reflected by reflective surface  18  comprises a plurality of different wavelengths, with each wavelength emanating from a unique gain element  19 - 1 , . . . ,  19 -N. The reflected light from reflective surface  18  is directed back through lens  15  to diffraction grating  14 , which diffracts the light into a particular output angle, which is dependent upon the wavelength of the light. Thus, the diffracted light is provided to gain elements  19 - 1 , . . . ,  19 -N, with a particular wavelength of light being incident on each particular gain element  19 - 1 , . . . ,  19 -N, via optical elements  13  and  30 . Note that the different output angles of the diffracted beam from grating  14  translate into different lateral locations of the beams onto gain elements  19 - 1 , . . . ,  19 -N. The diffracted beams, or feedback beams, cause each gain element  19 - 1 , . . . ,  19 -N to generate stimulated emission at the wavelength of the feedback beam, i.e., “lase.” Thus, each gain element  19 - 1 , . . . ,  19 -N tends to lase at a wavelength that is dependent on its respective position in array  11  (i.e., the wavelengths have a unique relationship to position). 
       FIG. 5  illustrates a problem that is encountered in the prior art with an incoherent beam combined (IBC) laser using a large number of gain elements ( 19 - 1  to  19 - 2  shown). The two laser gain elements  19 - 1  and  19 - 2  are shown separated by distance ‘D’ large enough for illustration purposes to create different beam sizes ( 104 - 1  and  104 - 2 ) on diffraction grating  14 . In this case, only one axis, the slow axis, is shown with divergence corresponding to  110 - 1  and  110 - 2 . Beams  110 - 1  and  110 - 2  are both collimated by optic  13 , resulting in beam sizes at the output of optic  13  that are the same size. However, because these beams are incident on diffraction grating  14  at different respective angles θ in   (l)  and θ in   (2) , they illuminate different surface areas of the diffraction gratings  14 , thereby, in turn, leading to different respective output beam sizes  104 - 1  and  104 - 2 . Because these beam sizes are different, they enter the fiber with different numerical apertures, which leads to a reduction in the overall efficiency of the device. 
     This problem can also be compensated for by micro optic array  60  shown in  FIG. 6 , which is preferably a graded anamorphic lens array. The slow axis divergence of each beam is processed by a respective micro optic element that is tailored for that beam. Micro optic array  60  is no longer cylindrically symmetric as is lens component  30  ( FIG. 1 ), but rather has different curvature in both the fast axis and the slow axis. The divergence angles of each of these beams are modified such that the beam sizes are pre-compensated, so that when they are incident upon diffraction grating  14 , they emerge with the same size and shape. 
     Lens component  30 , as discussed with reference to  FIG. 1 , comprises one element operating only in one axis, whereas the elements of micro optic array  60  operate in two axes. Lens component  30 , or a similar one dimensional micro optic can be used, primarily when the fast and slow axis divergences are significantly different from one another. If those two divergences are similar to one another, as, for example, happens with a buried heterostructure gain element, then a two dimensional micro optic would prove advantageous. Accordingly, pre-compensation with a one dimensional micro optic of a nearly circular beam emitted from emitter  19 - 1  is not suitable, since the high slow axis divergence typical of a buried heterostructure and the wide emitter array would require a complicated collimated optic  13 . 
       FIG. 7  depicts another embodiment of the present invention. The various optics induce aberrations into the beams. The aberrations typically appear as field curvature  71  and chromatic aberration. Thus, light from lens  13  does not focus onto the plane of gain elements in diode array  19 , but rather at field curvature line  71 . Thus, for elements that are away from central axis  73 , the aberration increases as the distance from the central axis. The aberration causes a loss in power efficiency of the laser. Another aberration is chromatic dispersion, wherein optical elements have different indices of refraction for light of different wavelengths. Thus, light of different wavelengths travels slower or faster through optical elements depending upon their wavelengths. Specifically, redder light (longer wavelengths) tends to focus farther than bluer light (shorter wavelengths). The system of  FIG. 1  can be arranged so that these two aberrations (field curvature and chromatic aberration) compensate for each other. The system can be configured such that element array  11  (in  FIG. 1 ) is placed on one side of optical axis  73 , and with bluer emitting elements  72  (shorter wavelengths) located closer to axis  73 , and redder emitting elements  74  located further from the axis. Thus the redder elements would have shorter focal distances from the field curvature, but have longer focal lengths from chromatic dispersion, than the bluer elements. Additionally, gain elements  19  can be tilted, so that the central portion of gain element array is tangential with respect to the field curvature, to further compensate these aberrations. 
     In other embodiments, IBC laser  10  can be utilized as the excitation source for another laser or laser amplifier. IBC laser  10  can provide its output beam to excite a gain medium that is doped with appropriate materials. For example, IBC laser  10  can excite an optical fiber doped with any of the following materials: Ce, Pr, Nd, Er, Tm, Ho, and Yb. In addition, multiple doping materials can be utilized. In particular, it is advantageous to dope a gain medium with both Yb and Er. It shall be appreciated that the use of IBC laser  10  as the excitation source is advantageous for these types of applications. Specifically, IBC laser  10  is capable of providing a relatively high output power to excite the particular gain medium, because IBC laser  10  is operable to combine the output beams from a plurality of gain elements. As discussed in U.S. patent application Ser. No. 09/945,381 entitled “SPECTRALLY TAILORED RAMAN PUMP LASER,” the disclosure of which has been incorporated herein by reference, this invention will also operate for Raman gain, e.g., gain in undoped fibers. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.