Abstract:
A system for selectively blocking electromagnetic energy. The system includes a first mechanism for employing a perforated component to pass a beam characterized by a first property and reject a beam characterized by a second property. A second mechanism selectively alters a beam passed by the first mechanism so that upon reflection, the beam exhibits the second property. In a specific embodiment, the first property corresponds to a first polarization, and the second property corresponds to a second polarization. In the specific embodiment, the perforated component includes a beamsplitter having a first perforated metallic plate. The second mechanism includes a quarter-wave plate having a second perforated metallic plate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to systems for directing or controlling energy. Specifically, the present invention relates to isolators for selectively blocking, redirecting, or absorbing reflected energy, such as microwave energy. 
     2. Description of the Related Art 
     Isolators are employed in various demanding applications including communications, space-based remote sensing systems, and military avionics. Such applications demand efficient and cost-effective isolators that can accommodate large amounts of reflected power without damage. 
     In relatively low-frequency applications involving radio-frequency waves, wave reflections are often attenuated via ferrite-based microstrip or waveguide circulators and isolators. Microstrip implementations of these devices have relatively low power-handling capability, while waveguide-based devices are often unacceptably lossy at multi-kilowatt power levels and frequencies beyond 10 GHz. Ultimately, the power-handling capabilities of waveguide-based isolators and circulators are limited by the dielectric-breakdown limit or air-breakdown limit, which is the electric field strength at which the dielectric or air in the waveguide is ionized. For example, at 95 GHz, a WR-8 waveguide having a cross-section measuring 80 mils (1 mil=0.001 inch) in width and 40 mils in height has a theoretical maximum continuous wave power rating of less than 2.6 kW. 
     Alternatively, low-power quasioptical isolators having capacitively-loaded linear-to-circular polarization converter grids, capacitively-loaded dipole tuner grids, and resistively-loaded absorber grids are employed. Resistive and capacitively-loaded dipole tuner grids are employed to absorb reflected energy having a predetermined polarization as described by Hollung et al. in “A Quasi-Optical Isolator,” IEEE  Microwave and Guided Wave Letters,  Volume 6, pages 205–206, published in May 1996. Unfortunately, these capacitively and resistively-loaded grids have limited power-handling capabilities. 
     Alternatively, circular polarization duplexers, such as those described by Nakajima and Watanabe in “A Quasioptical Circuit Technology for Short Millimeter-Wavelength Multiplexers,” IEEE Trans.  Microwave Theory and Techniques  MTT-29, pages 897–905, published in September 1981, are employed as isolators in low-power applications. Such duplexers employ a wire grid beamsplitter followed by a quarter-wave plate constructed from a dielectric. The quarter-wave plate and beamsplitter are configured so that reflected energy passing back through the quarter-wave plate exhibits a polarization that is reflected by the beamsplitter. Such duplexers, however, are limited to relatively low-power applications, since the dielectric quarter-wave plate has insufficient heat dissipation capabilities for many high-power applications. Furthermore, a high-power incident beam whose electric field is parallel to the wires in the wire-grid beamsplitter will induce large currents in the narrow wires of the beamsplitter, which may cause the beamsplitter to overheat and fail. 
     Isolators are particularly important in high-power Continuous Wave (CW) microwave/millimeter wave applications, which currently lack mechanisms to block reflected energy, and where reflected energy can damage or destroy microwave sources. Existing systems employing high-power millimeter-wave sources cannot protect expensive vital components from high-amplitude reflections. Unprotected millimeter-wave sources, such as gyrotron oscillators, may experience output window breakage if they are not sufficiently protected from reflected energy. Unfortunately, suitable quasioptical millimeter-wave isolators capable of handling hundreds of kilowatts of average power are typically unavailable. 
     High-power millimeter-wave sources, such as gyrotron oscillators, may have continuous-wave output power exceeding 100 kW. Such systems demand robust isolation to prevent reflected energy from damaging expensive and sensitive source components. 
     Hence, a need exists in the art for an efficient system and method that can effectively protect components from high-amplitude energy reflections, such as high-power millimeter wave reflections. There exists a further need for an efficient isolator that is not limited by dielectric-breakdown limits. There exists a further need for a beam source incorporating the efficient isolator. 
     SUMMARY OF THE INVENTION 
     The need in the art is addressed by the system for selectively blocking electromagnetic energy of the present invention. In the illustrative embodiment, the system is adapted to protect high-power millimeter wave components from reflected energy. The system includes a first mechanism for employing a perforated component to pass a beam characterized by a first property and to reject a beam characterized by a second property. A second mechanism selectively alters a beam passed by the first mechanism so that upon reflection, the beam exhibits the second property. 
     In a specific embodiment, the first property corresponds to a first polarization, and the second property corresponds to a second polarization. The perforated component includes a beamsplitter having a first perforated metallic plate. The second mechanism further includes a quarter-wave plate also having a perforated metallic plate. The beamsplitter and the quarter-wave plate have rectangular, square, elliptical, or circular perforations therethrough. The beamsplitter is sufficiently angled so that energy reflecting from the beamsplitter is directed away from the source of the beam of electromagnetic energy. 
     In the specific embodiment, the beam of electromagnetic energy is a quasioptical beam of electromagnetic energy. The source of the beam of electromagnetic energy is a gyrotron that produces a high-power beam of microwave or millimeter-wave energy. 
     The novel design of the system is facilitated by the use of thick perforated metallic plates to construct the beamsplitter and the quarter-wave plate. The use of perforated metallic plates to implement the beamsplitter and quarter-wave plate obviates the use of dielectrics, which are susceptible to breakdown and/or overheating. Unlike most dielectric materials, the metal plates have low losses at millimeter-wave frequencies and have a very high thermal conductivity, enabling them to quickly dissipate any absorbed heat. Constructing the key components from metallic plates minimizes undesirable energy loss and provides a low thermal resistance path for the removal of absorbed energy. Moreover, the elimination of dielectrics and restive grids enables isolators that can handle high power levels, such as 100 kW continuous wave power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a quasioptical system having a millimeter-wave isolator with a perforated metallic beamsplitter and a perforated metallic quarter-wave plate constructed in accordance with the teachings of the present invention. 
         FIG. 2  is a more detailed diagram illustrating the perforated metallic beamsplitter and quarter-wave plate of the isolator of  FIG. 1 . 
         FIG. 2   a  is a top view of the perforated metallic beamsplitter and quarter-wave plate of the isolator shown in  FIG. 2 . 
         FIG. 3  is an illustrative diagram of the quasioptical system of  FIG. 1  employing a gyrotron. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility. 
       FIG. 1  is a diagram illustrating a quasioptical system  10  having a millimeter-wave isolator  12  with a perforated metallic beamsplitter  16  and a perforated metallic quarter-wave plate  20  constructed in accordance with the teachings of the present invention. For clarity, various features, such as microwave amplifiers, and power supplies, have been omitted from the figures. However, those skilled in the art with access to the present teachings will know which features to implement and how to implement them to meet the needs of a given application. 
     The quasioptical isolator  12  includes the perforated metallic beamsplitter  16  and the perforated metallic quarter-wave plate  20 , which are positioned in series in an optical path between a millimeter-wave source  34  and an output load  24 . A high-power dissipation load  32  is positioned relative to the beamsplitter  16  to absorb reflected energy  30  from the beamsplitter  16 . High power quasioptical loads, such as the load  32  are often designed so that the incident beam  30  undergoes multiple reflections inside the load  32  and is further attenuated with each reflection. The beamsplitter  16  is angled so that any energy reflecting from the beamsplitter  16  is directed away from the millimeter-wave source  34 . 
     In operation, the quasioptical isolator  12  isolates and protects a millimeter-wave source  34  from reflected energy. The millimeter-wave source  34  transmits an initial quasioptical beam  14 , which is characterized by a horizontal linear polarization out of the plane of the page. In the present specific embodiment, the initial quasioptical beam  14  is a high-power millimeter wave beam. For the purposes of the present discussion, a high-power beam is a beam carrying more than 5 kW of average power. A millimeter-wave beam is a beam containing frequencies between 30 and 300 GHz. A quasioptical beam is a beam characterized by a free-space Gaussian mode (TEM 00 ). Those skilled in the art will appreciate that other types of beams may be employed without departing from the scope of the present invention. For the purposes of the present discussion, a metallic material is a metal having relatively high electrical and thermal conductivities, such as copper or aluminum. 
     The initial quasioptical beam  14  impinges on the angled perforated metallic beamsplitter  16 , which is angled at approximately 45° relative to the initial quasioptical beam  14 . In the present embodiment, the beamsplitter  16  is a perforated metallic beamsplitter having strategically sized and placed rectangular perforations, as discussed more thoroughly below. The perforations are designed and oriented to pass beams having a horizontal linear polarization (into the page), such as the initial quasioptical beam  14 , while reflecting minimal horizontally-polarized energy. Any reflected energy is directed away from the millimeter-wave source  34  due to the angle of the perforated metallic beamsplitter  16 . 
     The perforated metallic beamsplitter  16  passes the initial quasioptical beam  14 , providing a passed quasioptical beam  18  in response thereto. The passed quasioptical beam  18  is also horizontally-polarized with the electric field vector oriented out of the page. Since the beamsplitter  16  has minimal insertion loss, the passed quasioptical beam  18  is a nearly unattenuated version of the initial quasioptical beam  14 . 
     The perforated metallic quarter-wave plate  20  is positioned at an output of the perforated beamsplitter  16  and receives the passed quasioptical beam  18  as input. When properly oriented with respect to the polarization of the incident wave, the perforated quarter-wave plate  20  converts the passed quasioptical beam  18  into either a Left Hand Circularly Polarized (LHCP) beam or a Right Hand Circularly Polarized beam (RHCP)  22 , as desired, which is directed toward the output load  24 . The output load  24  may be replaced with other types of components or quasioptical circuitry, such as a quasioptical amplifier or repeater, without departing from the scope of the present invention. 
     The perforated metallic quarter-wave plate  20  may be implemented as a single plate with rectangular slots or as dual eighth-wave plates perforated with circular holes, as discussed more fully below. In the present specific embodiment, the perforated quarter-wave plate  20  includes two eighth-wave plates with circular perforations therethrough, which are sized and shaped to pass nearly all of the energy in the passed quasioptical beam  18  with minimal insertion loss, while converting the initial linear (horizontal or vertical) polarization into a circular polarization. 
     A portion of the circularly polarized beam  22  may reflect from the output load  24 . As is well known in the art, reflection from a surface will convert a LHCP beam to a RHCP beam, and will also convert a RHCP beam to a LHCP beam. Consequently, reflection of a portion of the circularly polarized beam  22  from the output load  24  yields a reflected beam  26  whose sense of rotation with respect to the direction of propagation is opposite that of the incident beam. The reflected beam  26  passes back to the perforated metallic quarter-wave plate  20 . 
     When the circularly-polarized reflected beam  26  passes back through the quarter-wave plate  20 , the quarter-wave plate  20  linearly polarizes the reflected beam  26  and rotates its polarization about the axis of the reflected beam by 90 degrees with respect to the polarization of the incident linearly-polarized quasi-optical beam  18 , resulting in a vertically-polarized beam  28  having an electric field vector that is vertical in the plane of the page. The vertically-polarized beam  28  has a polarization that is orthogonal to that of the initial quasioptical beam  14 . Consequently, the vertically-polarized beam  28  is reflected by the perforated metallic beamsplitter  16 , resulting in the reflected energy  30 , which is dissipated by the high-power dissipation load  32 . 
     Hence, the resulting linear polarization of the vertically-polarized beam  28  is orthogonal to the linear polarization of the original incident beam  14 . The vertically-polarized beam  28 , having orthogonal linear polarization, will reflect from the beamsplitter  16  and will then be dissipated via the high-power load  32 . Consequently, the reflected beam  30  is prevented from reaching and damaging the millimeter-wave source  34 . 
     With access to the present teachings, those skilled in the art will know how to construct the perforated beamsplitter  16  and quarter-wave plate  20  to meet the needs of a given application without undue experimentation. 
       FIG. 2  is a more detailed diagram illustrating the perforated metallic beamsplitter  16  and quarter-wave plate  20  of the isolator  12  of  FIG. 1 . The perforated metallic beamsplitter  16  has rectangular perforations  46  that extend completely through the surface of the beamsplitter  16 . The slot dimensions and periodic spacing are optimized to pass the initial horizontally-polarized quasioptical beam of millimeter wave electromagnetic energy  14  and are optimized to reflect the vertically-polarized millimeter-wave electromagnetic energy  30 . The exact slot dimensions and spacing are application-specific and may be changed in accordance with properties of the electromagnetic energy for which the isolator  12  will be used. The rectangular perforations  46  may be replaced with other types of perforations, such as square, elliptical, or circular perforations, without departing from the scope of the present invention. A suitable quasioptical beamsplitter is also discussed in U.S. Pat. No. 6,580,561, entitled QUASIOPTICAL VARIABLE BEAMSPLITTER, assigned to the assignee of the present invention and incorporated by reference herein. 
     In the present specific embodiment, the quarter-wave plate  20  is implemented via a first eighth-wave plate  40  in line with a second eighth-wave plate  42 . The eighth-wave plates  40 ,  42  have circular perforations  44  extending completely therethrough. The dimensions and spacing of the circular perforations  44  are optimized to convert an input horizontally-polarized beam (see beam  18  of  FIG. 1 ) into the circularly-polarized output beam  22  and are optimized to convert the reflected circularly-polarized beam  26  into the vertically-polarized beam  30 , which reflects from the beamsplitter  16 . The exact dimensions and spacing of the circular perforations  44  are application-specific and may be changed in accordance with properties of the electromagnetic energy for which the isolator  12  will be used. The construction of a suitable quarter-wave plate is also discussed in U.S. patent application Ser. No. 10/231937, entitled VARIABLE QUASIOPTICAL WAVE PLATE AND METHOD OF MAKING, now U.S. Pat. No. 6,693,605, assigned to the assignee of the present invention and incorporated by reference herein. While in the present specific embodiment, the quarter-wave plate  20  is implemented via two eighth-wave plates  40 ,  42 , one skilled in the art may implement the quarter-wave plate as a single plate perforated by an array of rectangular slots. 
     Exemplary dimensions for a representative design of a single eighth-wave plate at a frequency of 95 GHz are as follows:
 
a=Hole radius=39 mils
 
d x =Hole spacing in the x direction=103.5 mils
 
d y =Hole spacing in the y direction=118.0 mils
 
d=Plate thickness=251 mils
 
     The hole size and spacing and the plate thickness are chosen to minimize the reflected electromagnetic energy from a single plate, regardless of the incident polarization. This allows use of two plates in tandem without the need to maintain a specific distance between the plates. There is a minimum distance of separation, however, determined by the need to avoid coupling of non-propagating near fields from one plate to the other; that is, if D is the distance of separation of the two eighth-wave plates comprising a quarter-wave plate, then 
                     D   ≥       d   ⁢           ⁢   ln   ⁢           ⁢   100       2   ⁢   π   ⁢       1   -       (     d   λ     )     2               ,           [   1   ]               
where λ is the wavelength of impinging electromagnetic energy, d is the larger of the hole spacings d x  and d y , and a is the hole radius as given above. For the representative design of the present example, d=d x =118 mils and D≧276 mils.
 
     The slots of the beamsplitter  16  are oriented relative to the polarization of the incident beam  14  to facilitate nearly complete transmission of the linearly-polarized incident beam  14  and to almost totally reflect the orthogonally-polarized reflected beam  30 . Any energy reflected back to the quarter-wave plate  20  will return to the surface of the first eighth-wave plate  42  with a circular polarization having a sense of rotation with respect to the direction of propagation opposite that of the transmitted beam  22 , which will then be converted to a vertically-polarized linear polarization at the output of the second eighth-wave plate  40 . This vertically-polarized beam is then reflected by the beamsplitter  16  and is directed to the high-power dissipation load  32 . 
     In the present embodiment, the beamsplitter  16  is rotated approximately 45 degrees about an axis of rotation  50  so that the return beam  30  reflects downward. To facilitate transmission of the incident beam  14  through the beamsplitter  16 , the electric field of the incident beam  14  is parallel to the surface of the beamsplitter  16 , and is shown extending to the left in  FIG. 2  for illustrative purposes. 
     In the present specific embodiment, the thick metallic plates  16 ,  40 ,  42  are circular plates made from a suitable conductor, such as copper or aluminum. The metallic material comprising the plates  16 ,  40 ,  42  is highly conductive (both electrically and thermally), non-magnetic (permeability of the metal is approximately equal to the permeability of free space (μ=μ 0 )), will not sustain a net charge distribution (ρ=0), and is incapable of exhibiting dipole moments. Use of conductive metallic plates provides significant power-handling capability. The excellent thermal properties of perforated metallic plates enable superior high-power microwave and millimeter wave components, such as windows and beamsplitters. 
     The millimeter wave quasioptical isolator  12  obviates the need for dielectric materials incorporated into the isolator components. By omitting dielectric materials, which are often susceptible to breakdown and overheating, and employing thermally conductive metallic plates  16 ,  40 ,  42  to implement the isolator  12 , the isolator  12  exhibits power-handling capabilities that are far superior to conventional isolators. 
     The metallic plates  16 ,  40 ,  42  are sufficiently thick to provide a thermally conductive and low-resistance path to quickly remove absorbed energy. Consequently, the temperature at the center of the plates  16 ,  40 ,  42  may readily be maintained at safe levels. For very high power applications in which absorbed heat cannot be removed quickly enough by conduction alone, cooling channels can be incorporated into the metallic plates  16 ,  40 ,  42  to facilitate active cooling using water or some other suitable coolant. 
     Both the beamsplitter  16  and the quarter-wave plate  20  are periodic, frequency-selective structures. The exact dimensions of the plates  16 ,  40 ,  42  and spacing of the perforations  44 ,  46  are application-specific and may be chosen by one skilled in the art to meet the needs of a given application without undue experimentation. These parameters are chosen to yield the desired performance at the desired operating frequency. 
     Those skilled in the art may employ a method-of-moments code based on a formulation such as that described by C. C. Chen in “Transmission of Microwave Through Perforated Flat Plates of Finite Thickness,” IEEE Trans. Microwave Theory Tech. MTT-21, 1–6 (1973) to facilitate choosing the appropriate dimensions. 
     In this formulation, for an incident plane wave, the fields on each side of the beamsplitter  16  and/or eighth-wave plates  40 ,  44  are expanded in terms of a finite number of Floquet modes. Floquet modes are a set of orthonormal plane waves having the same periodicity and wavelength as the incident wave  14  in planes parallel to the surface of the plate, but propagating in different directions. The fields inside the perforations  44 ,  46  are expanded in terms of waveguide modes. By imposing the boundary conditions on the tangential electric and magnetic fields at the two surfaces of the beamsplitter  16  and/or eighth-wave plates  40 ,  44 , a matrix equation for the coefficients of the waveguide modes can be derived. When the waveguide mode coefficients are known, the amplitudes of the reflected and transmitted waves can be determined. 
     The use of a periodic array of perforations  44 ,  46  imposes constraints on the design of the beamsplitter  16  and the quarter-wave plate  20 . For the desired angle of incidence, the perforations  44 ,  46  are arranged so that grating lobes are not excited. For example, if the perforations are arranged in an isosceles-triangular pattern, grating lobes are not excited if the horizontal distance (d x ) between perforation centers and the vertical distance (d y ) between perforation centers satisfy the following equations: 
                       2   ⁢     λ     d   x         ≥     1   +     sin   ⁢           ⁢   θ         ,           ⁢       λ     d   y       ≥     1   +     sin   ⁢           ⁢   θ         ,     
     ⁢           ⁢           (     λ     d   x       )     2     +       (     λ     2   ⁢     d   y         )     2       ≥       (     1   +     sin   ⁢           ⁢   θ       )     2       ,           [   2   ]               
where λ is the wavelength of incident electromagnetic energy; and θ is the approximate angle of incidence of the electromagnetic energy on the quarter-wave plate or the beamsplitter with respect to the direction normal to the surface of the beamsplitter  16  or eighth-wave plates  40 ,  42 .
 
     For example, at a frequency of 95 GHz and an incident angle of 45° (the angle at which the incident beam impinges upon the beamsplitter), d x  and d y  must satisfy the following equations: 
                         d   x     &lt;     145.6   ⁢           ⁢   mils       ,           ⁢       d   y     &lt;     72.8   ⁢           ⁢   mils         ⁢     
     ⁢           ⁢             (     1     d   x       )     2     +       (     1     2   ⁢     d   y         )     2       ≥     1.888   ×     10     -   4       ⁢           ⁢     mils     -   2           ,             [   3   ]               
where the variables are as described above.
 
     If the perforations are arranged in a rectangular pattern, d x  and d y  must satisfy the following equations: 
                       λ     d   x       &gt;     1   +     sin   ⁢           ⁢   θ         ,           ⁢       λ     d   y       &gt;     1   +     sin   ⁢           ⁢   θ         ,           [   4   ]               
where the variables are as described above. If θ=0 (the angle at which the beam passed by the beamsplitter impinges upon the first eighth-wave plate), then
 d x &lt;124.2 mils, d y ≦124.2 mils.   [5] 
Additional theoretical background pertaining to transmission of microwaves through perforated flat surfaces is provided in the above-identified paper published in IEEE Transactions on Microwave Theory and Techniques.
 
       FIG. 2   a  is a top view of the perforated metallic beamsplitter and quarter-wave plate of the isolator shown in  FIG. 2 . The vertically-polarized reflected beam  28  output from first eighth-wave plate  40  reflects from the beamsplitter  16 , resulting in the return beam  30  (See  FIG. 2 ), which passes downward, which is into the page in  FIG. 2   a.  The return beam  30  is not shown in  FIG. 2   a,  as it is obscured by the beamsplitter  16 . 
       FIG. 3  is an illustrative diagram of the quasioptical system  10  of  FIG. 1  employing a gyrotron  34 . The millimeter-wave source  34  of  FIG. 1  is implemented as a gyrotron  34  in  FIG. 3 . Only the area near the output of the gyrotron  34  is shown in  FIG. 3 . The gyrotron  34  transmits the initial high-power quasioptical millimeter-wave beam  14  through a gyrotron output window  54 . 
     In the present specific embodiment, the gyrotron  34  is a self-contained millimeter-wave source that generates a quasioptical output beam  14 . The gyrotron  34  may be one component of a quasioptical transmitter (not shown), which could include a millimeter-wave source (e.g., a gyrotron) and a high-voltage power supply, and perhaps a computer (not shown). 
     The gyrotron  34  is a microwave vacuum electron device that generates very high power levels (both peak and average) at millimeter-wave frequencies. Gyrotrons have been constructed to generate output powers over 1 MW at frequencies well over 100 GHz. Due to the high frequency and the high power levels, the gyrotron  34  provides output into a quasioptical beam waveguide  56 . 
     The high-power quasioptical isolator  12  can be incorporated into an integrated window assembly that includes the output window  54 , the metallic beamsplitter  16 , and the metallic quarter-wave plate  20 . The output window  54  is a low-loss window that allows the high-power quasioptical millimeter wave beam to exit the gyrotron. Simultaneously, the output window  54  maintains the integrity of the vacuum inside the gyrotron, which must be maintained at an extremely low pressure (˜10 −9  torr) in order for the gyrotron to operate properly. The output window  54  may be integrated with the gyrotron  34 . 
     Moreover, the output window  54  may also be constructed from a perforated metallic plate, as described in U.S. Pat. No. 6,522,226, entitled TRANSPARENT METALLIC MILLIMETER-WAVE WINDOW, which is herein incorporated by reference. The use of a perforated metallic plate for the window  54  is relatively cost-effective. 
     Like most microwave vacuum electron devices, gyrotrons are relatively delicate and cannot tolerate high-amplitude reflections, especially long-pulse and continuous wave gyrotrons. Large amplitude reflections, if not removed quickly, often lead to failure of gyrotron output windows, such as the output window  54 . Use of the quasioptical isolator  12  protects the gyrotron  34  from such reflections. 
     High-power millimeter-wave systems producing more than 100 kW at frequencies over 100 GHz are employed in various applications, including fusion research. The gyrotron oscillator  34  produces a Gaussian-beam (quasioptical beam) output that is launched into the beam waveguide  56 , wherein the isolator  12  is mounted. Applications requiring transmission of multi-kilowatt average power levels at millimeter-wave frequencies often employ quasioptical transmission. 
     Until the development of the present invention, quasioptical millimeter-wave isolators capable of handling such high-energy beams were unavailable. Consequently, conventional gyrotron sources, which often lack mechanisms to protect vital components from high-amplitude reflections, were often damaged by reflected energy, requiring costly and time-consuming repairs. 
     At sufficiently high power densities, the electric field of the quasioptical beam  14  may cause breakdown as it passes through the slots near the center of the beamsplitter  16  or quarter-wave plate  20 , if the slots are too small. One solution involves designing the perforated metallic components  16 ,  20  so that the electric field does not reach the levels at which breakdown occurs. Alternatively, the quasioptical beam  14  is spread out before it impinges on the isolator components  16 ,  20 , thus lowering the maximum power density to an acceptable level. This may be readily performed via mirrors (not shown). Mirrors may also facilitate refocusing the resulting output beam, such as the beam  22 . If required, an additional mirror (not shown) can be used to reduce the diameter of the output beam  22  to its original value after the beam  22  has exited the isolator components  16 ,  20 . Such modifications may be readily performed by those skilled in the art to meet the needs of a given application. 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications, and embodiments within the scope thereof. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.