Abstract:
A beam combining device comprising at least one beam splitter and phase adjustment circuitry. The at least one beam splitter comprises a semi-reflective surface, first and second inputs, and first and second outputs operatively connected to receive first and second light beams. The semi-reflective surface has first and second sides positioned such that light entering from one input is partially reflected through one output and partially transmitted through another output. The phase adjustment circuitry adjusts the relative phases of the first and second light beams so that light transmitted through the semi reflective surface from one input may be adjusted to have a phase which can cancels or constructively adds to light reflected from another input. Light having a combined power of the light beams, or a fraction thereof, may be selectively emitted through a selective output depending upon the adjustment of phases. Also, a method of operation is claimed.

Description:
STATEMENT OF GOVERNMENT INTEREST 
       [0001]    The invention described herein may be manufactured, used, and licensed by or for the United States Government without the payment of royalties. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The power output of lasers is typically limited by catastrophic damage to the components due to the high intracavity intensities. Use of a master-oscillator/power-amplifier (MOPA) configuration allows the power to be increased, but for amplifiers of finite cross sectional area, the output power can still limited by damage at the exit face. Further increases in the power of a MOPA system can be obtained by combining multiple amplifiers in parallel. For incoherent combination, there is no fixed phase relationship between the electromagnetic field emitted by the various amplifiers. For coherent combination, there is. If, in addition, the emitted wavefronts are planar and in-phase, the divergence of the output will be smaller for coherent combination than it would be for incoherent combination. This comparison assumes the same wavelength and total aperture size in both cases. A smaller divergence for coherent combination leads to a smaller spot in the far-field, or in the focal plane of a lens. The same power in a smaller spot means a higher intensity on the sample or target. 
         [0003]    For both coherent and incoherent combination, spatial variations in the intensity of the output beams will increase the divergence. Even if the intensities are spatially uniform, if there are gaps between the multiple apertures, e.g., when circular apertures are arranged in a hexagonal pattern, the divergence will increase. For this reason, it is sometimes desirable to spatially overlap the output beams, thereby increasing the fill factor and decreasing the divergence. 
         [0004]    Electronic control of the relative phases of the multiple outputs allows for a degree of beam steering without the need for a moving mirror. Such a system is called a phased array. Beam steering with a phased array can be used to engage a moving target, for example. Usually it is done with collimated beams and no additional optics. 
       SUMMARY OF THE INVENTION 
       [0005]    A preferred embodiment beam combining device comprises: 
         [0006]    at least one beam splitter comprising a semi-reflective surface; first and second inputs, and first and second outputs; the at least one beam splitter operatively connected to receive first and second light beams at the first and second inputs, respectively; the semi-reflective surface having first and second sides positioned such that light entering the first input and illuminating the first side is partially reflected through the first output and partially transmitted through the second output; and light entering the second input illuminating the second side is partially transmitted though the first output and partially reflected through the second output: 
         [0007]    phase adjustment circuitry for adjusting the relative phases of the first and second light beams; the phases being adjustable such that when light transmitted through the semi reflective surface from the first input has a phase which cancels light reflected from the semi reflective surface from the second input, and light reflected from the semi-reflective surface from first input constructively adds to the light transmitted through the semi-reflective surface from the second input, then light having the combined power of the first and second light beams is emitted through the first output; and when light transmitted through the semi-reflective surface from the first input has a phase which constructively adds to light reflected from the semi-reflective surface from the second input, and light reflected from the semi-reflective surface from first input destructively interferes with the light transmitted through the semi-reflective surface from the second input, then light having the combined power of the first and second light beams is emitted through the second output; 
         [0008]    whereby depending upon the adjustment of phases of the first and second light beams, the power of the light outputted from either the first or second outputs may be controlled. 
         [0009]    The power emitted may be the summation of the power of the first and second light beams or fractions thereof. For example, one output may be the summation of the powers of the first and second light beams, or a fraction thereof, and the power of light outputted from the other output may be substantially zero, or a fraction thereof. Emission through the first or second output can be selected by varying the phase differential between the first and second light beams (without mechanical input). Optionally, the device may comprise a plurality of light detectors operatively connected to each output of the beam splitter to measure the power of the light from each of the outputs of the beam splitter. Optionally, the phase adjustment circuitry may operate to selectively and individually adjust each output in a range from substantially no light to the summation of the powers of the inputted light beams. Optionally, in addition to a plurality of photodetectors, feedback circuitry may be operatively connected to the phase adjustment circuitry and the photodetectors, such that the measurement detected by the plurality of photodetectors is used to control the phase of the first and second laser beams. 
         [0010]    Optionally, four beam splitters may be utilized, each having first and second inputs, a semi-reflective surface and first and second outputs. Four light beams may be inputted into the first and second inputs of the first and second beam splitters. In one embodiment, the first and second outputs of the first and second beam splitters are operatively connected to the first and second inputs of third and fourth beamsplitters, respectively; and the first and second outputs of the third and fourth beamsplitters selectively output a laser beam depending upon the phase of the four inputted light beams. Optionally, a photodetector may be associated with each output of the beamsplitters to measure the power of the light emitted from the associated output. Optionally, each photodetector may be operatively associated with a beam sampler and operatively connected to the phase adjustment circuitry, such that the phase adjustment circuitry receives inputs from the plurality of photodetectors. Optionally, the phase adjustment circuitry may further comprise a computer and a plurality of phase lock loop circuits, each of the phase lock loop circuits and the computer receiving the output of one of the plurality of photodetectors and operating to adjust the phases of the first and second light beams relative to the reference light beam. 
         [0011]    A preferred method comprises: 
         [0012]    inputting at least four laser light beams into first and second beam splitters; each of the first and second beam splitters having first and second inputs and first and second outputs; 
         [0013]    operatively connecting first and second outputs of the first and second beam splitters to first and second inputs of third and fourth beamsplitters, respectively; the third and fourth beam splitters each have two outputs; 
         [0014]    providing phase adjustment circuitry for varying the relative phase of each of the four laser beams; and 
         [0015]    by varying the phase of the at least four laser light beams, selectively emitting an output beam from one of four outputs of the third and fourth beam splitters which has the combined power of the at least four inputted beams. 
         [0016]    Other characteristics and features are described in the following drawings and detailed description of the preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The foregoing and other objects, features, and advantages of the invention will be apparent from the following more detailed description of the preferred embodiments of the invention, as illustrated in the accompanying drawings, wherein: 
           [0018]      FIG. 1  is a schematic illustration showing a system comprising a preferred embodiment 2×2 optical switch. Optical beams or fibers are shown with solid lines; electrical signals are shown with dashed lines. 
           [0019]      FIG. 2  is a block diagram showing the input and output functions of a preferred embodiment 2×2 optical switch. The solid lines represent optical beams. The dashed lines represent electrical signals or wires. 
           [0020]      FIG. 3  is a schematic illustration showing the inner components of a preferred embodiment optical switch  10 . 
           [0021]      FIG. 4  is a diagrammatic illustration of a portion of  FIG. 3  showing the interaction of input  1  and input  2  at beam splitter  22 . 
           [0022]      FIG. 5  is an isometric depiction of a preferred embodiment 4×4 optical switch. 
           [0023]      FIG. 6  is a top view of the preferred embodiment shown in  FIG. 5 . 
           [0024]      FIG. 7  is a schematic view of the preferred embodiment shown in  FIG. 5 . 
           [0025]      FIG. 8  is a diagrammatic illustration of a portion of  FIG. 7  showing the interaction of the beams from inputs  3  and  4  at the beamsplitter  103 . 
           [0026]      FIG. 9  is a diagrammatic illustration of a portion of  FIG. 7  showing the interaction of the beams at beamsplitter  104 . 
           [0027]      FIG. 10  is a diagrammatic illustration of a portion of  FIG. 7  showing the interaction of the beams at beamsplitter  101 . 
           [0028]      FIG. 11  is a schematic illustration showing a preferred embodiment of a 4×4 optical switch assembly and phase control loops used to monitor and control the relative phase difference between the four input laser beams. Optical beams or fibers are shown with solid lines. Electrical signals or wires are shown with dashed lines. 
       
    
    
       [0029]    A more complete appreciation of the invention will be readily obtained by reference to the following Description of the Preferred Embodiments and the accompanying drawings in which like numerals in different figures represent the same structures or elements. The representations in each of the figures are diagrammatic and no attempt is made to indicate actual scales or precise ratios. Proportional relationships are shown as approximates. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    The embodiments of the invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments of the invention. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments of the invention may be practiced and to further enable those with skill in the art to practice the embodiments of the invention. Accordingly, the examples should not be construed as limiting the scope of the embodiments of the invention. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the dimensions of objects and regions may be exaggerated for clarity. Like numbers refer to like elements throughout. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0031]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the full scope of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0032]    It will be understood that when an element is referred to as being “connected” or “coupled” or “operatively connected” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
         [0033]    It will be understood that, although the terms first, second, third, fourth, etc. may be used herein to describe various elements, sections, sides, or components, these elements, sections, sides, or components should not be limited by these terms. For example, when referring to first and second sides, these terms are only used to distinguish one side from another side. Thus, a first element, section, side, or component discussed below could be termed a second, third or fourth element, section, side or component without departing from the teachings of the present invention. 
         [0034]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0035]    It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed “adjacent” another feature may have portions that overlap or underlie the adjacent feature. 
       2×2 Optical Switch 
       [0036]    A system designed to make use of coherent beam combining requires the individual laser beams to (a) be locked in phase and frequency, (b) have a relative phase difference that results in the power being concentrated in a single lobe in the far-field, or in the focus of a lens, (c) have planar wavefronts, and (d) have a spatially uniform intensity. 
         [0037]    Monitoring the relative phase is typically done by interfering the beams with each other or with a reference. The former has the usual disadvantages associated with a homodyne feedback loop. The latter can be done with a heterodyne feedback loop, where the reference beam has an offset frequency relative to all the signal beams. The beat signal between the reference beam and each output beam is therefore nominally at the offset frequency. By controlling the phase of each beat signal relative to a local oscillator, one can control the relative phases of the various output beams. 
         [0038]    As shown in  FIG. 1 , a preferred embodiment assembly comprises two input laser beams X, Y, two optical phase shifters  8 A,  8 B, two optical amplifiers  3 A,  3 B, a 2×2 optical switch  10 , two possible output ports ( 11  or  12 ), two phase-locked loop circuits  7 A,  7 B, and a computer  5 . The solid lines represent optical beams or fibers. The dashed lines represent electrical signals or wires. The feedback loops ( 5 L,  5 M,  7 A,  7 B,  13 , and  14 ) control the relative phase between the two input laser beams X, Y. The system including the optical phase shifters and optical amplifiers comprises two opto-electronic feedback loops that allow the output beam to be electronically switched between the two outputs without having the speed and reliability limitations associated with a moving part. The output location containing the combined laser beam is determined by user input to a program running on computer  5 , for example. 
         [0039]    Referring to  FIG. 1 , using precise control over the relative phase difference, two input beams are interferometrically combined into one output beam at either port  11  or  12 . To do this, the optical switch  10  shown in  FIG. 1  makes use of feedback loops which causes the phase-locked loop (PLL) circuits  7 A,  7 B to adjust the relative phase difference to maintain coherence between each channel (i.e., the two input laser beams  1 A,  1 B) and a reference beam. The phase-locked loop circuits  7 A,  7 B may be, for example, a phase detector or phase comparator comprising a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs. The relative phase difference between both channels is not fixed though, so another feedback loop may be used to maintain a fixed phase relationship between the two channels. Also, a feedback mechanism (shown in dotted lines as computer program  5  and dashed lines  5 L,  5 M) is utilized to prevent the relative phase between the beams from slipping out of coherence with each other. The phase control loop comprises optical phase shifters  8 A,  8 B and optical amplifiers  3 A,  3 B discussed above. The optical amplifiers may be, for example, Ytterbium fiber amplifiers. The computer feedback loop is shown in dashed lines comprising computer  5 , ports  15 ,  16 , and connecting lines  5 L,  5 M. The 2×2 Optical Switch  10  comprises input ports  4  and  5  from the respective channels for Inputs  1  and  2  and a reference beam input port  6 . Output ports  11  and  12  may be selected for outputting the combined laser beam. Feedback ports  15 ,  16  connect via lines  5 L to a processor or computer  5  which includes a computer program and is connected to the phase-locked loop circuits  7 A,  7 B via lines  5 M. The 2×2 optical switch further comprises output ports  13 ,  14 . Port  13  outputs the reference (from port  6 ) plus the input  1  from port  4 . Port  14  outputs the reference (from port  6 ) plus input  2  from port  5 . The outputs from ports  13 ,  14  are inputted into the phase-locked loop circuits  7 A,  7 B to maintain coherence between each channel (i.e., the two input laser beams A, B) and a reference beam. The phase-locked loop circuits  7 A and  7 B are connected to each of the optical frequency shifters  8 A,  8 B to shift the frequency to maintain coherence. 
         [0040]    The optical switch  10  may be made mechanically stiff and isolated from sources of vibration, in order that the components maintain as well as possible the spacing and alignment necessary to obtain complete constructive and destructive interference at the outputs. The feedback signals from ports  13  and  14  are there to compensate for residual environmental effects that would otherwise degrade the interference. 
         [0041]      FIG. 2  is a block diagram showing the inputs to switch  10  on the left and the outputs of switch  10  on the right. The switch has two laser beam input ports  4 ,  5 , a reference beam input port  6 , two laser beam output ports  11  and  12 , two electrical feedback ports  15  and  16  for the outputs to the computer  5 , and two electrical feedback ports  13  and  14  for outputting the signal due to the interference between the reference and Input  1  and the signal due to the interference between the reference and Input  2 , respectively, to the phase-locked control circuits  7 A and  7 B. The four electrical outputs are generated by four photodiodes  23 ,  24 ,  25  and  26  (shown in  FIG. 3 ) which sample the output beams as well as monitor the interference between the reference and input beams. The reference beam (inputted from input port  6 ) is used to phase lock each channel with the phase-locked loop circuits  7 A,  7 B, and is necessary to maintain a constant relative phase difference between Input  1  and Input  2 . On the switch  10 , there are two electrical outputs labeled Beam Sample A (port  15 ) and Beam Sample B (port  16 ) that indicate the output powers which depend on the phase difference between Input  1  and Input  2 . 
         [0042]      FIG. 3  is a more detailed view of a preferred embodiment 2×2 optical switch. If inputs  1  and  2  are equal in power, beamsplitter  22  should have a reflectivity of ˜50% and a transmissivity of ˜50%. If inputs  1  and  2  are unequal in power, the reflectivity of beamsplitter  22  may be adjusted in order that the reflected portion of input  1  has the same power as the transmitted portion of input  2 . The beam samplers have a reflectivity of ˜1% and a transmissivity of ˜99%, for example. In addition, four photodiodes  23 ,  24 ,  25  and  26  (shown in  FIG. 3 ) sample the output beams as well as monitor the combination of the reference and input beams. The reference beam (inputted from input port  6 ) is used to phase lock each channel with the phase-locked loop circuits  7 A and  7 B, and is necessary to maintain a constant relative phase difference between Input  1  and Input  2 . On the switch  10 , there are two beam samples labeled Beam Sample A (port  15 ) and Beam Sample B (port  16 ) that are used to measure the actual phase difference between Input  1  and Input  2 . 
         [0043]    Referring to  FIG. 3 , Input  1  is partially reflected by a optical sampler or wedge  27  into a photodetector  25 . As an example, optical wedges may reflect about 1 to 4% of the light beam. Rather than using a wedge, an optical-quality thick glass plate tilted to the beam can also be utilized. The light which passes through the sampler or wedge  27  is reflected by mirrors  28 ,  29  into beam splitter  22 , which may be a 50/50 beam splitter. As an alternative, the beam splitter may be variable with respect to the transmittance/reflectance or accommodate variances between the intensities of input  1  and input  2 . A sampler  30  reflects a portion of the beam directed to output  12  into detector  23 . A sampler directs the portion of the beam directed to output port  11  into photo detector  24 . It can be appreciated by those of ordinary skill in the art that photodetectors  24  and  23  detect the intensity of light for output A and output B, respectively, passing through the respective ports  11  and  12 . 
         [0044]    Referring now to input  2  at port  5  in  FIG. 3 , the incoming laser beam is partially reflected by a wedge  33  into a photodetector  26 . Again, the wedge may reflect only approximately one to four percent of the light. The light which passes through the wedge  33  is reflected by mirrors  34  and  35  into beam splitter  22 . The beam splitter may be a 50/50 beam splitter. A sampler (or wedge)  30  reflects a portion of the beam directed to output port  12  into detector  23 . Another sampler or wedge  31  directs the portion of the beam directed to output port  11  into photo detector  24 . Thus, the detector  23  measures the intensity of output B and detector  24  measures the intensity of output A. 
         [0045]    Referring now to the reference input at port  6 , the reference light illuminates the beam splitter  20  which transmits 50% and reflects 50% of the light. The transmitted portion from beam splitter  20  illuminates the sampler  27  such that the reflected portion of the input  1  and the transmitted portion of the reference is measured by photodetector  25  at port  13 , for further output into the phase lock circuitry  7 A, which controls the optical phase shifters SA, which regulate the phase of the inputted laser beams as shown in  FIG. 1 . The reflected portion from beam splitter  20  passes through sampler  35  and is detected by photodetector  26  at output port  14 , for further output into the phase lock circuitry  7 B. 
         [0046]    Beamsplitter  20  divides the reference beam into two approximately equal parts. Beam sampler  27  combines a small portion of input  1  with a portion of the reference for the purpose of interfering on photodiode  25 . Beam sampler  33  combines a small portion of input  2  with a portion of the reference, for the purpose of interfering on photodiode  26 . Beamsplitter  22  reflects part of input  2  and transmits part of input  1 , allowing the two to interfere to generate output B. Beamsplitter  22  also transmits part of input  2  and reflects part of input  1 , allowing the two to interfere to generate output A. Beam sampler  30  directs a small portion of output B to photodiode  23  to measure the power of output B. Beam sampler  31  directs a small portion of output A to photodiode  24  to measure the power of output A. 
         [0047]    Referring now  FIG. 4 , which is a diagrammatic explanation of the effect of phase difference/control of input  1  and input  2  at the interface of the beam splitter  22 . Input  1  and Input  2  each illuminate the beam splitter  22 . The measurement at photodetector  23  indicates the power due to the interference of the reflected part of input  2  and the transmitted part of Input  1 . The measurement at photodetector  24  indicates the power due to the interference of the transmitted part of Input  2  and the reflected part of Input  1 . If the measurement at photodetector  23  is zero and the measurement at photodetector  24  is non-zero, there is destructive interference in the direction of Output B and constructive interference in the direction of Output A. As a result, a beam having the combined power of input  1  and input  2  will form Output A. Subsequently, the operator may change the phase difference between Input  1  and Input  2  such that the measurement at photodetector  23  is non-zero and the measurement at photodetector  24  is zero, indicating destructive interference in the direction of Output A. As a result, a beam having the combined intensity of input  1  and input  2  will form Output B. 
         [0000]    The phase is controlled by the outputs from port  13 ,  14 , which are connected, as shown in  FIG. 1  to the phase-locked loop circuits  7 A,  7 B. Also inputted into phase-locked loop circuits  7 A and  7 B are the output of the processor which operates the computer program  5 . The ports  15 ,  16  on optical switch  10  are inputted into the processor or computer  5  through lines  5 L, from which data is extracted to perform an algorithm by computer  5 . 
       Computer Control 
       [0048]    The program run by computer  5  is used to control the phase (via the electronic feedback loop) and can be implemented with, for example, LabWindows™. The program takes input from the user and the switch and sends digital characters to the phase-locked loop (PLL) circuits  7 A,  7 B, which may be, for example, via a USB connection. The user selects which port ( 11  or  12 ) will contain the output beam. The program digitizes the voltage from photodiodes  23  and  24 , and sends the signals to PLL  7 A,  7 B to adjust the phases of beams X and Y. The program algorithm (operated on by computer  5 ) uses a dither in combination with the readings from the photodiodes  23 ,  24  to decide whether or not the relative phase between the two input beams Input  1  or Input  2  should be increased or decreased. After initiating the program, the phase is continually probed until a maximum is reached, at which point the dither and the input voltage will begin to oscillate. Phase-locked loop circuitry  7 A,  7 B is configured to receive signals from the connection; therefore, an increase or decrease in the phase correlates to the computer sending certain signals to phase-locked loop circuitry  7 A,  7 B. The computer feedback loop is shown in  FIG. 1 . 
       Applications 
       [0049]    High power, low-divergence lasers have military applications, e.g. counter rockets, artillery, and mortars (CRAM), and industrial applications, e.g. welding. 
         [0050]    A preferred embodiment 2×2 optical switch has distinct advantages over single output systems for directed energy applications. An opto-electronic switch allows for very fast changing of outputs without the speed limitations associated with mechanical movement. The direct application of a 2×2 optical switch coupled with HEL is for defense against large ballistic threats aimed at manned or unmanned vehicles. In one scenario, two 1-kW amplifiers are located in the interior of a helicopter, and two delivery fibers lead to conformal exit apertures on the left and right side of the fuselage. The switch would be able to direct 2 kW into the delivery fiber leading to the left side to engage a target in that direction. A short time later, the switch could direct 2 kW to engage a target on the right side of the helicopter. 
         [0051]    The optical switch  10  could be largely monolithic; i.e., made from a single block of material. The rigidity would reduce the impact of external vibrations. The switch would need to be enclosed to eliminate dust and other contaminants that would result in optical damage. 
       4×4 Optical Switch 
       [0052]    A preferred embodiment 4×4 optical switch  100  is illustrated in  FIGS. 5 through 7 . Note that this switch is somewhat monolithic, and utilizes the same concepts from the preferred embodiment 2×2 switch of  FIGS. 2-3 . The preferred embodiment  100  enables coherent combination that can yield a higher intensity (smaller spot size) in the far field, and allows for electronic beam steering. The preferred embodiment of  FIGS. 5-7  utilizes a coherent technique that allows one to combine the power of multiple amplifiers (seeded with a common source) or multiple phase-locked lasers, and direct the output beam to any one of four output ports or tour delivery fibers. No moving parts are required, but a means for adjusting the phase of each input beam is required. Typically, this is done with an electro-optic or acousto-optic phase modulator, in conjunction with photodiodes that detect the phase via interference. 
         [0053]    The preferred embodiment illustrated in  FIG. 7  comprises four 50/50 beamsplitters  101 - 104 , each of which may be followed by two beamsamplers,  111 - 118 , and eight light detectors  121 - 128  to ascertain the distribution of light inside the switch and light leaving the switch. Alternatively, a total of four beam samplers and four detectors could be utilized, but the feedback algorithm would be more complicated. 
         [0054]    Referring now to  FIG. 7 , input  1  illuminates beam splitter  101 , and the reflected portion is sampled by sampler  114  and measured by detector  124 . The transmitted portion is sampled by sampler  111  and measured by detector  121 . The transmitted portion then illuminates beam splitter  102 , which reflects a portion in the direction of output  3  (which is sampled by sampler  115  and measured by detector  125 ) and transmits a portion in the direction of output  1  (which is sampled by sampler  116  and measured by detector  126 ). Input  2  illuminates beam splitter  101 , and the reflected portion is sampled by sampler  11  and measured by detector  121 . The transmitted portion is sampled by sampler  114  and measured by detector  124 . The transmitted portion then illuminates beam splitter  104 , which reflects a portion in the direction of output  4  (which is sampled by sampler  118  and detector  128 ) and transmits a portion in the direction of output  2  (which is sampled by sampler  117  and detector  127 ). Input  3  illuminates beam splitter  103 , and the reflected portion is sampled by sampler  113  and measured by detector  123 . The transmitted portion is sampled by sampler  112  and measured by detector  122 . The transmitted portion then illuminates beam splitter  102 , which reflects a portion in the direction of output  1  (which is sampled by sampler  116  and detector  126 ) and transmits a portion in the direction of output  3  (which is sampled by sampler  115  and detector  125 ). Input  4  illuminates beam splitter  103 , and the reflected portion is sampled by sampler  112  and measured by detector  122 . The transmitted portion is sampled by sampler  113  and measured by detector  123 . The transmitted portion then illuminates beam splitter  104 , which reflects a portion in the direction of output  2  (which is sampled by sampler  117  and detector  127 ) and transmits a portion in the direction of output  4  (which is sampled by sampler  118  and detector  128 ). 
         [0055]    Referring again to  FIG. 7 , if only input  1  were present at beam splitter  101 , half the power would be reflected and half would be transmitted. The same goes if only input  2  were present. When they are both present, and coherent with respect to one another, the relative phase will determine whether all of the combined power goes in the direction of beamsampler  111 , or all goes in the direction of beamsampler  114 . All other distributions in between are also possible. To ascertain what the distribution is, a small portion of the light incident on beamsampler  114  is directed to detector  124  and a small portion of the light incident on beamsampler  111  is directed to detector  121 . In the preferred embodiment, the electrical signals from  121  and  124  are sent to a computer. If the switch is to direct the total power to output  1 , the computer algorithm will seek to maintain a null signal on detector  124  by adjusting the relative phase between inputs  1  and  2 . By the same token, the computer algorithm will seek to maintain a null signal on detector  123  by adjusting the relative phase between inputs  3  and  4 . At this point, the total power from all four inputs is propagating toward beamsplitter  102 . To ascertain the distribution of power on the output side of beamsplitter  102 , beamsamplers  115  and  116  and photodetectors  125  and  126  are used in the same fashion as above. To direct the total power to output  1 , the computer algorithm will seek to maintain a null signal on photodetector  125 , by adjusting the average phase of inputs  1  and  2  relative to the average phase of inputs  3  and  4 . It should be clear that by adjusting the phases of the input beams, the total power can be directed to any one of the output beams. Similarly, an equal amount could be directed to each output, or any combination in between, by proper adjustment of the phases of the input beams. 
         [0056]    The interaction of the input beams is explained diagrammatically in  FIGS. 8, 9, and 10 . Referring now to  FIG. 8 , photodetector  122  measures the part of input  4  that is reflected by beamsplitter  103  and the part of input  3  that is transmitted by beamsplitter  103 . Photodetector  123  measures the part of input  4  that is transmitted by beamsplitter  103  and the part of input  3  that is reflected by beamsplitter  103 . If the signal from photodetector  122  is non-zero and the signal from photodetector  123  is zero, there is constructive interference in the direction of beamsampler  112  and destructive interference in the direction of beamsampler  113  and the total power of input  3  and  4  propagates in the direction of Beamsampler  112 . Conversely, if the signal from photodetector  122  is zero and the signal from photodetector  123  is non-zero, there is constructive interference in the direction of beamsampler  113  and destructive interference in the direction of beamsampler  112  and the total power of inputs  3  and  4  propagates in the direction of beamsampler  113 . 
         [0057]    Assuming the signal from detector  123  is non-zero, the light then illuminates beamsplitter  104  ( FIG. 9 ). Photodetector  127  measures the portion of the beam coming from point Z in  FIG. 7  that is reflected by beamsplitter  104  and the transmitted portion from point ZZ ( FIG. 7 ) that is transmitted by beamsplitter  104 . Photodetector  123  measures the part of the beam from Point Z in  FIG. 8  and the reflected portion of the beam from Point ZZ that is reflected by beamsplitter  104 . If the signal from photodetector  127  is zero and the signal from photodetector  123  is non-zero, there is destructive interference in the direction of beamsampler  117  and constructive interference in the direction of beamsampler  118 . If the measurement at photodetector  123  is zero, there is destructive interference in that direction and the total power propagates in the direction of beamsampler  117 . 
         [0058]    Referring now to  FIG. 10 , which is centered on beam splitter  101 , photodetector  121  measures the part of Input  2  that is reflected by beamsplitter  101  and the part of input  1  that is transmitted by beamsplitter  101 . Photodetector  124  measures the part of Input  1  that is reflected by beamsplitter  101  and the part of input  2  that is reflected by beamsplitter  101 . If the signal from photodetector  121  is zero and the signal from photodetector  124  is non-zero, there is destructive interference in the direction of beamsampler  111  and constructive interference in the direction of beamsampler  114  and the total power from Input  1  and  2  propagates in the direction of beamsampler  114  and point ZZ. Note that the beam formed by constructive interference continues on to  FIG. 9  which can be visualized by connecting the Points ZZ. If the measurement at photodetector  124  is zero, there is destructive interference in that direction, and the total power from Input  1  and  2  propagates in the direction of beamsampler  111 . It is noted that by selecting the phases of the inputted beams such that constructive interference occurs as outlined in  FIGS. 9 and 10 , the net result is that Output  4  will be the combination of four inputs, i.e., Inputs  1 ,  2 ,  3  and  4 . 
         [0059]    From a phase perspective, if Output  4  is the desired channel for the combination of the four beams, the following should take place: (1) the relative phases of Input  1  and Input  2 , i.e., Φ 1 −Φ 2  should be adjusted to either maximize the power on  124  or minimize the power on detector  121 ; (2) Φ 3 −Φ 4  should be adjusted to maximize detector  123  or minimize detector  122 ; (3) the difference between Φ 1 +Φ 2  (average phase of inputs  1  and  2 ) and Φ 3 +Φ 4  (average phase of inputs  3  and  4 ) should be adjusted to maximize detector  128  or minimize detector  127 . 
         [0060]    As with all varieties of coherent beam combination, the efficiency of the device will depend on the extent to which the four inputs have parallel polarizations, are diffraction-limited, i.e, plane waves, and matched in power. If the input beams originate from fiber amplifiers, they need to be collimated with high quality lenses. There are also difficulties in stabilizing about a null or a maximum with a homodyne phase-locking circuit. Typically the latter are overcome using a dithering technique, or a hill-climbing algorithm, or by going to a heterodyne technique. All these techniques are possible with this apparatus. In addition, it can function with a chirped beam to suppress stimulated Brillouin scattering in high power fiber amplifiers. 
         [0061]      FIG. 11  is a schematic illustration showing a preferred embodiment 4×4 optical switch assembly and a phase control loop used to monitor and control the relative phase difference between the four input laser beams. The wires and electrical signals are shown with dashed lines. Using precise control over the relative phase difference, four input beams are interferometrically combined into one output beam at one of the output ports  1 ,  2 ,  3 , or  4 . To do this, the optical switch  100 A makes use of a feedback loop which causes the phase-locked loop (PLL) circuits  7 A- 7 D to adjust the relative phase difference to maintain coherence between each channel (i.e., the four input laser beams) and a reference beam. There are four phase-locked loop circuits  7 A- 7 D; one for each channel. The phase-locked loop circuits  7 A- 7 D may be, for example, a phase detector or phase comparator that is a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs. The relative phase difference between all channels is maintained by the feedback loops. The phase control loop comprises optical phase shifters (OPS)  8 A-D and optical amplifiers (OAs)  3 A- 3 D, which are used to shift the phase of and amplify the inputted beams W, X, Y and Z, respectively. The computer feedback loop is shown in dashed lines comprising computer  5 , and series of connecting lines  5 L,  5 M. The 4×4 Optical Switch  100 A comprises input ports from the respective channels for inputs  1 - 4  and a reference beam input port. Output ports  1  though  4  may be selected individually for outputting the combined laser beam by adjusting the phase of the inputs  1 - 4  for constructive and/or destructive interference at the various points within the optical switch  100 A as shown in  FIG. 7  above. Optical switch  100 A comprises an additional reference port depicted as port  6  in  FIG. 3  and “reference plus input” ports (feedback ports) (depicted in  FIG. 3  as ports  13  and  14 ) shown in  FIG. 11  as “Ref.+Input . . . . ” Ports. Optical switch  100 A comprises Inputs  1  through  4 , beamsplitters  101  through  104  (shown in  FIG. 7 ), beamsamplers  111 - 118  (shown in  FIG. 7 ), photodetectors  121 - 128  (shown in  FIG. 7 ) that operate in the manner described above in conjunction with the  FIGS. 7 through 10 . 
         [0062]    Beamsamplers  126  (output  1 ).  127  (output  2 ),  125  (output  3 ) and  128  (output  4 ) sample the output ports and are connected as shown in  FIG. 7 . The outputs of beamsamplers  125  through  128  are shown by dashed lines in  FIG. 11  at the feedback ports of the 4×4 Optical Switch  100 A that are connected via lines  5 L to a processor or computer  5  which includes a computer program and is connected to the phase-locked loop circuits  7 A- 7 D via lines  5 M. The 4×4 optical switch  100 A further comprises ports which output on four separate lines the reference plus the input  1 , the reference plus the input  2 , the reference plus the input  3 , and the reference plus the input  4 . These outputs are inputted into the phase-locked loop circuits (PLL)  7 A- 7 D to maintain coherence between each channel (i.e., the four input laser beams) and a reference beam. The phase-locked loop circuits  7 A- 7 D are connected to each of the optical phase shifters  8 A- 8 D to maintain coherence. 
         [0063]    As used herein, the terminology phase-locked loop or phase lock loop (PLL) refers to a control system, subsystem, or circuit that outputs a signal having a phase related to the phase of the inputted signal. The circuit may comprise a variable frequency oscillator, which generates a periodic signal, and a phase detector that compares the phases of the signal with the phase of the input periodic signal, and adjusts the oscillator to match the phases. A feedback loop may be formed by returning the output signal back to the input signal for comparison. 
         [0064]    Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention many be practiced otherwise than as specifically described.