Abstract:
The present invention is directed to a system and method which utilize an incoherently beam combined (IBC) laser. The IBC laser includes a plurality of emitters with each of the emitters possessing a partially reflective surface on their front facet. The partially reflective surface causes resonant wavelengths to be defined. In certain embodiments, the system and method arrange the external cavity and emitter spacings of the IBC laser such that the center feedback wavelength provided to each emitter is an etalon resonant wavelength. In other embodiments, the range of feedback wavelengths is adapted so that it is greater than the free spectral range (the separation in wavelength space between adjacent etalon resonant wavelengths).

Description:
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 tends to lase at a unique wavelength. 
     FIG. 1 depicts a prior art arrangement of components in IBC laser  10 . IBC laser includes emitters  12 - 1  through  12 -N associated with fully reflective surface  11 . Emitters  12 - 1  through  12 -N are disposed in a substantially linear configuration that is perpendicular to the optical axis of collimator  15  (e.g., a lens). Collimator  15  causes the plurality of beams produced by emitters  12 - 1  through  12 -N to be substantially collimated and spatially overlapped on a single spot on diffraction grating  16 . Additionally, collimator  15  directs feedback from partially reflective  17  via diffraction grating  16  to emitters  12 - 1  through  12 -N. 
     Diffraction grating  16  is disposed from collimator  15  at a distance approximately equal to the focal length of collimator  15 . Furthermore, diffraction grating  16  is oriented to cause the output beams from emitters  12 - 1  through  12 -N to be diffracted on the first order toward partially reflective component  17 , thereby multiplexing the output beams. Partially reflective component  17  causes a portion of optical energy to be reflected. The reflected optical energy is redirected by diffraction grating  16  and collimator  15  to the respective emitters  12 - 1  through  12 -N. Diffraction grating  16  angularly separates the reflected optical beams causing the same wavelengths generated by each emitter  12 - 1  through  12 -N to return to each respective emitter  12 - 1  through  12 -N. Accordingly, diffraction grating  16  is operable to demultiplex the reflected beams from reflective component  17 . 
     It shall be appreciated that the geometry of external cavity  13  of IBC laser  10  defines the resonant wavelengths of emitters  12 - 1  through  12 -N. The center wavelength (λ i ) of the wavelengths fed back to the i th  emitter  12 -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  16 , α i  is the angle of incidence of the light from the i th  emitter on diffraction grating  16 , and β is the output angle which is common to all emitters  12 - 1  through  12 -N. As examples, similar types of laser configurations are also discussed in U.S. Pat. No. 6,208,679. 
     To allow emitters  12 - 1  through  12 -N to operate in this type of configuration, anti-reflective coating  14  is applied to the front facet of emitters  12 - 1  through  12 -N. Anti-reflective coating  14  allows substantially all incident light to be transmitted. By applying anti-reflective coating  14 , emitters  12 - 1  through  12 -N lase at the wavelength defined by the feedback wavelengths as discussed above. Specifically, it shall be appreciated that emitters  12 - 1  through  12 -N do not operate as Fabry-Perot emitters, since anti-reflective coating  14  does not provide a partially reflective surface to create internal feedback. 
     Moreover, anti-reflective coatings of appreciable quality (possessing a reflectivity on the order of 10 −4 ) are difficult to achieve on a consistent basis. This is problematic, since anti-reflective coatings of lower quality can significantly diminish performance of an IBC laser. 
     Additionally, the use of anti-reflective components increases the difficulty of verifying the performance of components in an IBC laser. Specifically, it is desirable to verify the performance of each emitter prior to assembling the entire laser. Performance verification of an emitter array is performed by applying current through the emitters of the emitter array and measuring the output optical power over a period of time. If a very low reflectivity is applied to the front facet, the emitter array will not generate a significant amount of optical power and performance verification is not possible. As a result, the entire IBC laser must be assembled before the various components can be tested. Accordingly, this greatly increases the cost of manufacturing IBC lasers. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method which utilize an incoherently beam combined (IBC) laser. The IBC laser includes a plurality of emitters with each of the emitters possessing a partially reflective surface on their front facet. The partially reflective surface causes resonant wavelengths to be defined. In certain embodiments, the system and method arrange the external cavity and emitter spacings of the IBC laser such that the center feedback wavelength provided to each emitter is an etalon resonant wavelength. In other embodiments, the range of feedback wavelengths is adapted so that it exceeds the free spectral range (the separation in wavelength space between adjacent etalon resonant wavelengths). 
    
    
     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 DRAWING 
     For a more complete understanding of the present invention, reference is now made to following descriptions taken in conjunction with the accompanying drawing, in which. 
     FIG. 1 depicts a prior art arrangement of components in an IBC laser; 
     FIG. 2 depicts an exemplary IBC laser according to embodiments of the present invention; and 
     FIG. 3 depicts another exemplary IBC laser according to embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 depicts IBC laser  20  which is adapted according to embodiments of the present invention. IBC laser  20  includes emitters  12 - 1  through  12 -N which can advantageously be implemented on a single device or array via photo-lithographic techniques. Emitters  12 - 1  through  12 -N have a front surface and a back surface that are referred to as facets. Each facet has a reflectivity which determines how much incident light is reflected. The back facet is coated with highly reflective coating  11  causing it to reflect almost all incident light. The front facet is coated with partially reflective coating  21  causing it to reflect a portion of light and to allow transmission of a portion of light. It shall be appreciated that embodiments of the present invention are not limited to any particular implementation of emitters  12 - 1  through  12 -N. Any number of designs may be utilized including, but not limited to, edge emitters, vertical cavity surface emitting lasers (VCSELs), and grating surface emitting lasers. Accordingly, the term “emitter” is intended to mean any gain material or element capable of lasing in response to feedback. 
     The two surfaces of emitters  12 - 1  through  12 -N define an etalon. Specifically, reflective coatings  11  and  21  cause multiple internal reflections of light emitted by emitters  12 - 1  through  12 -N. The etalons defined by reflective coatings  11  and  21  of emitters  12 - 1  through  12 -N cause light at specific wavelengths to build up to high energy levels between the facets of emitters  12 - 1  through  12 -N. This is referred to as a resonance condition. The wavelengths are referred to as resonant wavelengths. The j th  resonant wavelength, λ j , of an etalon is given by 
     
       
         λ j =2 nL/j   
       
     
     where n is the index of refraction and L is the distance between the two facets. These resonant wavelengths are separated in wavelength space from one another by an amount referred to as the free spectral range which is given by: 
     
       
         λ j −λ j+1 =Δλ FSR ≈λ 2 /2 nL   
       
     
     where λ j  and λ j+1  are adjacent resonant wavelengths of light. Wavelengths λ j  and xλ j+1  are typically very nearly equal, so the wavelength λ in the above expression is an average. 
     Light at non-resonant wavelengths will not build up to very high energy levels inside the etalon. The degree to which these non-resonant wavelengths are suppressed is governed by the finesse of the etalons. High finesse etalons strongly reject all non-resonant wavelengths, while low finesse etalons provide weak rejection. High finesse etalons result from higher facet reflectivities while low finesse etalons result from low reflectivities. 
     IBC laser  20  comprises external cavity  25  which provides feedback to emitters  12 - 1  through  12 -N and combines output beams from emitters  12 - 1  through  12 -N to produce output beam  26 . External cavity  25  comprises collimator  15  and diffraction grating  16  as described in connection with FIG.  1 . Collimator  15  causes the plurality of beams produced by emitters  12 - 1  through  12 -N to be spatially overlapped on a single spot on diffraction grating  16 . Additionally, collimator  15  directs feedback from diffraction grating  16  to emitters  12 - 1  through  12 -N. 
     Diffraction grating  16  is disposed from collimator  15  at a distance approximately equal to the focal length of collimator  15 . Furthermore, diffraction grating  16  is oriented to cause the output beams from emitters  12 - 1  through  12 -N to be diffracted on the first order toward partially reflective component  17 , thereby multiplexing the output beams. Partially reflective component  17  may be embedded in an optical fiber. Partially reflective component  17  causes a portion of optical energy to be reflected, while the portion that is transmitted is output beam  26 . The reflected optical energy is redirected by diffraction grating  16  and collimator  15  to the respective emitters  12 - 1  through  12 -N. Diffraction grating  16  angularly separates the reflected optical beams causing the same wavelengths generated by each emitter  12 - 1  through  12 -N to return to each respective emitter  12 - 1  through  12 -N. Accordingly, diffraction grating  16  is operable to demultiplex the reflected beams from partially reflective component  17 . 
     Although IBC laser  20  utilizes diffraction grating  16  to provide feedback to emitters  12 - 1  through  12 -N and to multiplex their output beams, other feedback and multiplexing elements may be substituted such as arrayed waveguide gratings and Mach-Zehnder interferometers. Also, transmission diffraction gratings, prisms, holograms, and other dispersive elements may be utilized in lieu of reflective diffraction grating  16 . The particular dispersive element used in a given application will influence the geometry of external cavity  25 . 
     Moreover, the geometry of external cavity  25  of IBC laser  20  defines the resonant wavelengths of emitters  12 - 1  through  12 -N. As previously noted, the center wavelength (λ i ) of the wavelengths fed back to the i th  emitter  12  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  16 , α i  is the angle of incidence of the light from the i th  emitter on diffraction grating  16 , and β is the output angle which is common to all emitters  12 . Additionally, the range of wavelengths, Δλ i , fed back to the i th  emitter  12  is given by:          Δλ   i     =         λ   i        A                   cos        (     α   i     )           2        C   0        f                   θ   i                                
     where C 0  is of order  1  which is determined by the beam shape on diffraction grating  16 , ƒ, is the focal length of collimator  15 , and θ i  is the half angle divergence of light exiting the i th  emitter  12 -i in the plane of the array. This range of wavelengths, Δλ i , is distributed around the i th  emitter&#39;s center wavelength λ i . Most of the optical energy is fed back at the center wavelength. 
     It shall be appreciated that each emitter  12 - 1  through  12 -N is subjected to two different wavelength constraints. Specifically, the feedback provided by external cavity  25  and the resonant wavelengths defined by the etalons constrain the operation of emitters  12 - 1  through  12 -N. If the center wavelength, λ i , falls between two of the etalon resonant wavelengths (λ j  and λ j+1 ) and if the range of wavelengths (Δλ i ) is less than the free spectral range (Δλ FSR ) of the etalon, then the feedback is rejected by the etalon. In this case, IBC laser  20  is not able to control the wavelength of the i th  emitter. 
     In certain embodiments of the present invention, this problem is solved by selecting each center wavelength (λ i ) fed back to each emitter to equal a resonant wavelength (λ j ). By selecting each center wavelength (λ i ) in this manner, the external cavity feedback is allowed to control the wavelengths of emitters  12 - 1  through  12 -N. This can be achieved by selectively placing emitters  12  at specific locations on the emitter array utilizing, for example, photo-lithographic techniques. 
     In other embodiments of the present invention, this problem is solved by forcing the range of feedback wavelengths, Δλ i , to exceed the free spectral range, Δλ FSR , of the etalons. In this manner, each emitter  12 - 1  through  12 -N receives feedback including at least one of the resonant wavelengths, λ j , of the etalon regardless of λ j . This condition can be satisfied by selectively tuning the grating resolution of external cavity  25  of IBC laser  20 . In an embodiment, the grating resolution of external cavity  25  is tuned by adjusting the divergence, θ i , of the output beams from emitters  12 - 1  through  12 -N. The divergence, θ i , is tuned or adjusted by placing micro-lens component  18  in the output beams of emitters  12 - 1  through  12 -N. Micro-lens component  18  includes a series of discrete lens elements each associated with an emitter  12 -i to reduce the divergence of that emitter  12 -i. Micro-lens component  18  can be implemented utilizing, for example, photo-lithographic techniques. 
     It shall also be appreciated that the range of wavelengths, Δλ i , fed back to the i th  emitter is not solely dependent on its divergence angle, θ i , after passing through micro-lens component  18 . It is also dependent on the focal length, ƒ, of collimator  15 , the ruling density, A, and the angle of incidence, α i . In other embodiments, these parameters can also be adjusted to provide the desired range of wavelengths, Δλ i . 
     Additionally, it is advantageous to spatially filter output beams that are not co-aligned with output beam  26  exiting external cavity  25  of IBC laser  20 . By performing spatial filtering, parasitic cross-talk modes between emitters  12 - 1  through  12 -N are reduced to adapt to the modified resolution of external cavity  25 . In this example, IBC laser  20  performs spatial filtering conventionally via spatial filter  19  which may comprise slit  22  disposed between lenses  23  and  24 . Other spatial filtering mechanisms can be employed such as depicted in IBC laser  30  of FIG.  3 . For example, single lens  31  can be utilized to couple output beam  26  into fiber  32 . A partial reflector (e.g., a fiber Bragg grating  33 ) can be embedded in fiber  32  to complete external cavity  25 . Beams that are not co-aligned are not coupled into fiber  32  and, hence, are not reflected. Accordingly, the aperture of fiber  32  can accomplish the desired spatial filtering for particular applications as desired. 
     Both of the techniques described above allow IBC laser  20  to simultaneously satisfy the constraints imposed by feedback from external cavity  25  and the resonant wavelength constraints imposed by the etalons. Accordingly, IBC laser  20  provides significant advantages by allowing the use of a partially reflective surface on the front facet of emitters  12 - 1  through  12 -N. First, manufacturing difficulties associated with achieving high quality anti-reflective surfaces are eliminated. Secondly, verification of discrete components (e.g., emitters  12 - 1  through  12 -N) of IBC laser  20  can occur prior to assembly of IBC laser  20 . Accordingly, these advantages provide significant cost reductions for the manufacture of IBC laser  20 . 
     In other embodiments, IBC laser  20  can be utilized as the excitation source for another laser or laser amplifier. IBC laser  20  can provide its output beam to excite a gain medium that is doped with appropriate materials. For example, IBC laser  20  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  20  as the excitation source is advantageous for these types of applications. Specifically, IBC laser  20  is capable of providing a relatively high output power to excite the particular gain medium, because IBC laser  20  is operable to combine the output beams from a plurality of emitters. 
     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.