Abstract:
The invention provides a cascaded injection resonator for coherent beam combining of laser arrays. The resonator comprises a plurality of laser emitters arranged along at least one plane and a beam sampler for reflecting at least a portion of each laser beam that impinges on the beam sampler, the portion of each laser beam from one of the laser emitters being reflected back to another one of the laser emitters to cause a beam to be generated from the other one of the laser emitters to the beam reflector. The beam sampler also transmits a portion of each laser beam to produce a laser output beam such that a plurality of laser output beams of the same frequency are produced. An injection laser beam is directed to a first laser emitter to begin a process of generating and reflecting a laser beam from one laser emitter to another laser emitter in the plurality. A method of practicing the invention is also disclosed.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   This invention was made with Government support under Contract No. DE-AC05-00OR22725 awarded to UT-Battelle, LLC, by the U.S. Department of Energy. The Government has certain rights in this invention. 

   TECHNICAL FIELD 
   This invention relates to lasers arranged in laser arrays. 
   DESCRIPTION OF THE BACKGROUND ART 
   Broad area semiconductor lasers have extremely high electrical to optical wall-plug efficiency and low cost. As a result, they are very promising, lightweight, high-power light sources for a variety of applications. However, the poor spectral and beam quality of these lasers allows them to be used only as pumping sources for solid-state or fiber lasers. To improve their spectral and beam quality, research has focused on frequency stabilization and attempting to obtain coherent beam combinations of single mode lasers within laser arrays. To date, two main approaches have been developed: (i) optical injection by an external single mode single frequency laser (a seed laser) and (ii) external cavity stabilization implementing the external grating/mirror that redirects part of the output back to the semiconductor laser “internal” cavity. In principle, both approaches have the potential to obtain a single mode output from the laser array. However, notwithstanding partial achievements, a completely satisfactory solution for the frequency stabilization and coherent combination of individual beams produced by laser arrays has not been obtained. This is due to the inherent limitation of each of the two technologies, as explained below. 
   The scalability of the external optical injection scheme to the higher power level requires approximately one seed beam of about 25-50 mW for each Watt of output power. In order to create a coherent output beams all seed beams have to be coherent. The technical problems involve splitting the seed beam onto the array and providing the necessary power in the seed beam to produce an output beam of sufficient power. 
   In order to obtain a coherent output of an entire array in the external cavity design, a sufficiently strong coupling between the lasers in array is needed. To this end, about at least 80% of the emitted radiation has to be redirected back into the laser array. This results in a very low efficiency of the single mode output in the schemes with external cavity. 
   The technical problem is how to provide a substantial increase in the beam combining efficiency of the phase locked laser array. 
   SUMMARY OF THE INVENTION 
   The invention provides a method and circuitry for production of a single mode, single frequency, coherent output beam of increased power from a laser array. 
   The invention will significantly increase the efficiency of injection laser power and will also provide efficiently coupling between the lasers in the array. 
   One proposed configuration combines the traditional idea of single mode injection locking of a plurality of semiconductor lasers with avalanche multiplication to provide the cascaded injection. 
   In a method of the invention for providing a closed loop cascaded mode of operation for a plurality of laser emitters, a plurality of lasers are arranged along at least one plane and facing in one direction in an arrangement having first and second lateral ends. One portion of a laser beam from each of the laser emitters is reflected back to another one of the laser emitters to cause a beam to be generated from the other one of the laser emitters. Another portion of the laser beam from each of the laser emitters is transmitted to produce a laser output beam. An initial laser beam is injected into one of the plurality of lasers to begin a sequence of generating and reflecting a laser beam between laser emitters, such that the laser beam travels to others in the plurality of the laser emitters and such that a plurality of laser output beams having a single laser mode are generated from the respective laser emitters. 
   In a device of the invention, a resonator is formed by a plurality of lasers arranged along at least one plane and facing in one direction for transmitting laser beams to a beam sampler, where a portion of each beam is reflected and a portion of each beam is transmitted in a single mode of laser operation. The laser output beams can then be combined into a single mode output beam for greater power. In other embodiments either one or several high reflectivity mirrors are provided to reverse a direction of the cascaded injection, in one instance to provide a closed loop system and in other embodiments providing for an open loop system. 
   In still another embodiment, two injection lasers are provided and each laser emitter can be operated in two modes of operation. 
   The invention provides a combined output laser of greater power than the individual laser emitters with the laser operating in a single mode of laser operation having a single frequency. 
   Due to its compactness, the proposed embodiments have the potential to be implemented in an integrated circuit laser array chip. 
   The invention can be employed with semiconductor lasers or with optical fiber lasers or with solid state lasers of other types. 
   Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follows. In the description, reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a cascaded injection laser array of the present invention; 
       FIG. 2  is a schematic diagram of the apparatus of  FIG. 1  with the addition of apparatus for combining output beams and with the addition of positioning control apparatus; 
       FIG. 3  is a schematic diagram of second embodiment of the invention of  FIG. 1 ; 
       FIG. 4  is a schematic diagram of a third embodiment of  FIG. 1 ; and 
       FIG. 5  is a schematic diagram of an open loop variation of the embodiment of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , a 1×N array  10  has seven (N=7) laser semiconductor emitters  11  spaced at equal intervals along at least one plane. Preferably, these elements are parts of semiconductor devices of a commercially available type. From the description that follows it will also be apparent that the principle can be applied to a M×N stacked array, which is configurable from commercially available parts. The second emitter, labeled ‘A’ in  FIG. 1 , will be injected with an external seed beam  12  indicated by a dashed line. The seed beam  12  will lock the frequency and phase as well as the spatial mode (angle) of the laser emitter A. The output of laser emitter A passes through a collimating lens  16  and then impinges on a beam sampler  13 , which can be a semi-transparent element with a mirror surface such that a major portion of the laser beam  15  is transmitted through the beam sampler  13  to become an output beam  18  and a minor portion is reflected back to a cavity of another emitter B in this case. Preliminary test results indicate that an injection power in a range of 5˜10% of the output power is sufficient to lock the resulting laser beam  15  into the single mode of laser operation in which the laser operates at a single frequency. This determines the ratio of transmission to reflectivity by the beam sampler  13 . Such injection locking occurs in a cascaded way until the next to last emitter  11  in the array  10  is reached. The light output from this emitter  11  is reflected by a high-reflectivity mirror  14  disposed on one end of the array and standing upright and perpendicular to the array  10  and facing inwardly towards the array  10 . The laser beam  15  reflected from the second last emitter is reflected back from the beam sampler to the mirror  14  and then to the last emitter  11  in the array  10 . 
   This reverses the lateral direction of travel of the reflected laser beam  12  and it begins to travel back along the array first impinging on the beam sampler  13  and then being injected to the third-from-the-right emitter. From there it is reflected to the beam sampler  13  and then to the third-from-the-left emitter  11  and finally is reflected from the beam sampler  13  into the last emitter  11 . At the left end of the array  10  is a second upright and perpendicular inwardly facing mirror  17  which reflects the beam  15  and again reverses its lateral direction of travel. 
   As a result, the cascaded injection array  20  operates as a closed loop device and as a resonator. 
   Since the coupling between the laser emitters  11  is very effective, the phase of each laser emitter  11  will be strongly influenced by all other lasers of array  10 . It is estimated that about 0.1% of optical coupling is sufficient to induce phase locking between lasers  11  and collective behavior of the array  10 . The individual modes of each laser are reorganized by the injection beams into collective modes of the cascaded laser array  20  formed by an assembly of the elements described above. This assembly  20  may be integrated into a single semiconductor device. Far field patterns are defined from the amplitudes-phase relationships of these collective laser array modes. 
   The technical difficulties related to obtaining and manipulating an array of single mode seed beams that are essential for the classical injection locking configuration are not present in the assembly  20  of  FIG. 1 . Moreover, the required injection power is reduced by a factor equal to the number of lasers  11  in the array  10 . The injection efficiency is very important for scalability to high output power lasers. 
   In the traditional implementation of the external mirror schemes, the optical coupling between the lasers in the array is very weak. As mentioned in the background, an efficient coupling between the lasers requires about 80% of the output power to be reflected back into the semiconductor laser. In the proposed scheme, a very strong (5-15%) nearest neighbor coupling is achieved automatically. As a result, the coherent output of the cascaded injection laser array  20  can reach 85-95% of the laser array free running output. 
   For a specific laser array configuration, the positions of the beam sampler  13  and two side mirrors  14 ,  17  are determined by the incident angle of the injection beam  12 . The relationship between the incident angle of the seed beam  12  and the depth of the laser cavity  11   a  and the diameter of the beam (see  FIG. 1   a ) has been established already. The optimal angle for the injection beam varies typically between six (6) and fifteen (15) degrees, depending on the laser structure. 
   Without the invention, the cascaded laser array  20  operates in a multiple mode state. With the invention, the performance is increased by parametric optimization that achieves a single mode of laser operation of the whole array. This includes the following elements: 1) an optimal spacing for the next injected laser (in  FIG. 1  this number is 2 positions); 2) reflectivity and position of the beam sampler; 3) position of the side high reflectivity mirrors; and 4) periodical patterning of the beam sampler. 
   There is a possible problem if one of the individual lasers  11  in the array  10  fails to provide the adequate output power needed to inject the next laser emitter  11 . At small scale, this problem can be overcome by optimizing the beam sampler reflectivity and/or spatially inhomogeneous reflectivity (patterning) mentioned above. At large scale, this problem can be solved by utilizing a dynamic injection beam array that is available with recently developed beam processing tools. This array is capable of generating an arbitrary array of beams. Under this dynamic control, the failure of individual laser emitters to generate an adequate output power is compensated by injection to the next nearest neighbor laser. Thus, an individual malfunction will not affect the global synchronization stability or the overall array performance. 
   The laser output beams  18  can be combined in single collimated coherent output beam  42  as shown in  FIG. 2 . The output beams  18  are directed a lens array  30 . From there they are directed to a second beam sampler  32  which reflects a minor portion of each beam to a detector  34 . A major portion of each beam is directed through a focusing lens  36  to a spatial filter  40  of a type known in the art. The lens array  30  increases the filling factor of the array of output beams  18 , while the spatial filter  40  removes the sideband maxima in the far field pattern ( FIG. 2 ). On the other side of the filter  40 , the beams are transmitted through a lens  44  to produce a collimated coherent output beam  42 . 
   A closed loop control of the position of beam sampler  13  and a control of a variable angle of incidence of the injection beam  12  is also shown in  FIG. 2 . The angle is controlled in response to the output intensity as a feedback parameter using a detector  34  shown in  FIG. 2 . The detector  34  detects the phase and amplitude of the combined beam and provides this as an input to a personal computer (PC)  50 . The PC  50  is connected to a controller  51  which controls the position of the beam sampler  13  through a piezoelectric element  52 . The controller  51  also controls the angle of incidence of a master injection laser  54  by controlling an angle of positioning of a reflection mirror  55 . 
   The embodiments of  FIGS. 1 and 2  provide a solution for the efficient coupling between the lasers  11  in the array  10 . These embodiments reduce the external injection power requirements to the level necessary for locking just one of the lasers of the array. The latter advantage makes it possible to mount the single mode injection laser on the same chip with the laser array and to lock all other broad area lasers by using this single mode seed laser  12 . 
   The cascaded injection laser array  20  provides a closed loop resonator which is needed to obtain collective modes shown in  FIG. 2 . These collective modes do not (exist?) in the arrangements using an external injection of whole array. 
   Unlike all known injection locking and external cavity arrangements, the outside seed laser  12  injects just one laser into the array  10 . The other lasers  11  in the array  10  are injected by their respective neighbors. 
   The embodiments of  FIGS. 1 and 2  provide a simultaneous frequency/spatial-mode locking and coherent combination of all the laser array beams. The embodiments of  FIGS. 1 and 2  provide a high output efficiency by using a fraction of the output power for single mode injection and phase locking of each laser. Indeed, the fraction of the output power needed for injection does not exceed 5-15% of the free running power. This yields an output efficiency of 85-95%. The total dimensions of the proposed assembly  20  is comparable with the dimensions of the laser array itself. The distance between the array and the beam sampler surfaces is within the range of 1-5 mm, which is considerably smaller than the length of all known cascaded injection and external cavity proposals. 
   Due to these unique features, the proposed construction has the potential to be easily and inexpensively implemented on an integrated single mode laser array chip. 
     FIGS. 3 and 4  show an alternative embodiment of the invention.  FIG. 3  illustrates the most compact configuration based only on coupling to each next laser emitter  11   b  in the array  10   b  by moving the beam sampler  13   a  closer to the collimating lens  16   a . In this configuration, each laser  11   b  in the array  10   b  is injected in two opposite directions and operated in two modes to generate two lasers  15   b  instead of a single laser as in the examples described above. The side high reflectivity mirrors are replaced with beam splitters  19   a ,  19   b  which have the same ratio of reflection to transmission as the beam sampler  13   b.    
     FIG. 4  shows the version where two injection beams are used for seeding the array. The parts of the embodiment in  FIG. 4  correspond to parts with the same numbers as in  FIG. 1 , except that a “c” suffix has been added to the corresponding elements in  FIG. 4 . The one additional element is the second injection laser  9  coming in from an opposite side at an angle of incidence having an opposite incidence angle of the first injection beam  12 . This arrangement provides better outside control, though it involves more sophisticated dynamics. 
     FIG. 5  illustrates an open-loop cascaded injection array  20   d . The parts of the embodiment in  FIG. 5  correspond to elements with the same numbers as in  FIG. 1 , except that a “d” suffix has been added to the corresponding elements in  FIG. 5 . This configuration can be obtained from closed-loop cascaded injection scheme ether by shifting of the beam sampler  13   d  or by removing one of the side mirrors  17   d . This approach could be advantageous for some applications or specific array configuration. 
   Although the detailed embodiments herein have been described in terms of utilizing a semiconductor laser, the invention can be employed with semiconductor lasers or with lasers of other types such as fiber lasers, solid state lasers or other laser systems where synchronization of array can improve performance, power or thermo management. 
   This has been a description of the preferred embodiments, but it will be apparent to those of ordinary skill in the art that variations may be made in the details of these specific embodiments without departing from the scope and spirit of the present invention, and that such variations are intended to be encompassed by the following claims.