Abstract:
A method for attenuating an unpolarized laser beam includes separating the laser beam into two plane-polarized beams. The plane-polarized beams are polarization rotated. Each of the polarization-rotated beams is separated into two plane-polarized portions. One of the portions of one polarization-rotated beam is combined with one of the portions of the other polarization-rotated beam to provide an attenuated output-beam.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to laser beam attenuators. The invention relates in particular to laser beam attenuators including polarizing and polarization rotating elements.  
       DISCUSSION OF BACKGROUND ART  
       [0002]     In many applications of high-power lasers, particularly applications of Q-switched, high-power, pulsed lasers, it is often desirable to be able to provide variable attenuation of the laser output. Commonly-used high-power Q-switched solid state lasers often provide an output beam that is unpolarized. If laser pump power delivered to the laser is changed, for example, to change output power at a selected level, this can cause a substantial change in thermal lensing of the solid-state gain-medium and a consequent change in the quality of the output beam and the beam pointing. Both of these parameters are critical in applications that are beam-position sensitive, for example, in applications where the beam must be focused into an optical fiber. For the applications that need variable pulse energy in an output beam, there is a need to have an apparatus that that is insensitive to the degree of polarization of laser output and can provide variable pulse energy without varying the pump power to the laser thereby maintaining optimum beam quality and pointing.  
       SUMMARY OF THE INVENTION  
       [0003]     The present invention is directed to a method and apparatus for providing a laser beam of variable power. The method relies on providing a variable attenuator for selectively attenuating an output beam of a laser, rather than varying operating parameters of the laser to provide selectively variable output power.  
         [0004]     In one aspect the invention comprises separating an output beam from the laser into two beams, plane-polarized in different planes. Each of the two plane-polarized beams is divided into two portions. One of the portions of one of the plane-polarized beams is combined with one of the portions of the other plane-polarized beam to provide an attenuated output beam.  
         [0005]     In another aspect of the invention, the laser output beam is separated into first and second beams plane-polarized in respectively first and second planes in respectively first and second orientations. The polarization plane of each of the first and second plane-polarized beams is rotated to respectively third and fourth orientations different from the first and second orientations. Each of the polarization-rotated first and second beams is separated into first and second portions plane-polarized in respectively the first and second orientations. An attenuated output beam is provided by either combining the first portion of the polarization-rotated first beam with the second portion of the polarization-rotated second beam, or, alternatively, combining the second portion of the polarization-rotated first beam with the first portion of the polarization-rotated second beam.  
         [0006]     Preferably the polarization planes of the first and second plane polarized beams are rotated through the same angle. This provides that the attenuation of the beam is independent of the polarization state of the laser output beam. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the present invention.  
         [0008]      FIG. 1  schematically illustrates one preferred embodiment of an attenuator in accordance with the present invention including a Brewster-angle oriented, front-surface thin-film polarizing beamsplitter, a Brewster-angle oriented, front-surface thin-film polarizing beam-combiner, and two rotatable half-wave plates.  
         [0009]      FIG. 2  schematically illustrates one example of an arrangement for rotating a half-wave plate in the attenuator of  FIG. 1 .  
         [0010]      FIG. 3  is a graph schematically illustrating measured percentage throughput as a function of the rotation angle of the half-wave plates in one example of the attenuator of  FIG. 1 .  
         [0011]      FIG. 4  schematically illustrates another preferred embodiment of attenuator in accordance with the present invention including a 45-degree biprism polarizing beamsplitter, a 45-degree biprism polarizing beam combiner, and two rotatable half-wave plates. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates a preferred embodiment  10  of a laser beam attenuator in accordance with the present invention. Attenuator  10  includes a polarizing beamsplitter  12  having a thin film polarizer coating (not shown) on a surface  14  thereof. Beamsplitter  12  is preferably inclined at the Brewster angle to a path  18  along which an input laser beam to be attenuated is directed into the attenuator. The polarizing beamsplitter coating divides the input beam into two plane-polarized components. One of the components is plane-polarized perpendicular to the plane of incidence of the beam on the beamsplitter (here perpendicular to the plane of the drawing) and is reflected from the polarizing beamsplitter coating along a path  20 . This component is usually referred to by practitioners of the art as being S-polarized and is indicated in  FIG. 1  by an end-on arrowhead S. The other component is plane-polarized parallel to the plane of incidence of the beam on the beamsplitter (here parallel to the plane of the drawing) and is transmitted through the polarizing beamsplitter coating along a path  22 . This component is usually referred to by practitioners of the art as being P-polarized and is indicated in  FIG. 1  by an arrow P. Having the polarizing beamsplitter arranged at the Brewster angle optimizes the polarization properties of the beamsplitter coating and provides near-zero transmission loss (negligible reflection loss) at rear surface  16  of the beamsplitter. If the beamsplitter is arranged at some angle other than the Brewster angle it may be found advisable to provide an antireflection coating on surface  16 .  
         [0013]     Located in each of beam paths  20  and  22  is a half-wave plate (polarization rotator)  24 . A half-wave plate rotates the plane of polarization of a beam, dependent on the angle of the input polarization plane to the optic axis (fast or slow) of the half-wave plate. When the optic axis is aligned with the polarization plane of the input beam there is no rotation of the polarization plane. When the optic axis is aligned at ±45° (±π/4 Radians) to the beam the polarization plane is rotated by ±90°. Plates  20  and  22  can be synchronously rotated about the respective beam paths as indicated by arrows A.  
         [0014]     Path  22  extends through the half-wave plate therein to another polarizing beamsplitter  28  which also serves as a beam combiner. Polarizing beamsplitter  28  is configured similarly to polarizing beamsplitter  12 , but oriented at Brewsters angle to the path in an opposite sense. Here it is assumed that the polarizing coating is on surface  30  of the beamsplitter. If the axis of the half-wave plate is aligned with the polarization plane, light exiting the half-wave plate will still be P-polarized with respect to beamsplitter  28  and will be maximally transmitted by the beamsplitter along a path  34 . If there is an alignment of optic axis of the half wave-plate with the polarization plane that is not zero or one-hundred-eighty degrees, light exiting the half-wave plate will be rotated out of the P-orientation, and will be resolved by polarizing beamsplitter  28  into a P-polarized component that is transmitted through the beamsplitter along path  34  and an S-polarized component that is reflected from the beamsplitter to a beam dump  36  as indicated in  FIG. 1  by a dashed line. It should be noted, for completeness of explanation, that a part of the S-polarized component will be reflected from surface  32  of the beamsplitter into the beam dump.  
         [0015]     Path  20  is “folded” by a mirror  26  an directed back onto surface  30  of polarizing beamsplitter  28  at a position such that any radiation reflected by the beamsplitter out of path  20  as S-polarized radiation is directed along path  34  and combines with any radiation from path  24  transmitted by the beamsplitter as P-polarized radiation. In this regard, the polarizing beamsplitter is functioning as a beam combiner. Any radiation from path  20  transmitted by beamspitter  28  is P-polarized and makes an essentially loss free pass through surface  32  to beam dump  36  as indicated by a dashed line in  FIG. 1 .  
         [0016]     Each half-wave plate is preferably initially calibrated to determine the 100% transmission orientation for the polarization state that will be incident on the half-wave-plate. When both plates are in this orientation there will be maximum throughput into the output beam of the attenuator. For radiation at most wavelengths greater than about 400 nanometers (nm), maximum throughput will usually be close to 100%, with perhaps about 3% being lost due to scatter and absorption losses or manufacturing tolerances on the reflection and transmission of the beamsplitter coatings.  
         [0017]     In order to attenuate radiation, both half-wave plates are preferably rotated synchronously, i.e., through the same angle, from the calibrated 100% throughput orientation, to a new orientation. Here it should be noted that the term “synchronously’ as used in this description and the appended claims does not mean that the plates must be rotated simultaneously (although this is an option), but merely that rotating one will require rotation of the other. In this new orientation, the polarization plane of light transmitted by the half-wave plates is rotated, unwanted light is directed out of the attenuator to the beam dump, and the remaining light is recombined as output along path  34  by the optical processes discussed above. Variable power can thus be provided by optimizing the beam quality of a laser, operating the laser stably at a constant output power, and using the attenuator to reduce that power as described above, as required. If there are any changes in the state of polarization of the input beam, the attenuation provided by the attenuator will stay the same, whatever magnitude are the P and S polarized components resolved by polarizing beamsplitter  12 , as those components are equally attenuated. By way of example, if an input beam having an initial power W is resolved into P and S-polarized components having power a*W and b*W respectively (where a+b =1), and each is attenuated by a factor x, then the P and S-polarized components in the output beam of path  34  will have power x*a*W and x*b*W, respectively, providing a total power of x*(a+b)*W, i.e., x*W, whatever the value of a and b.  
         [0018]      FIG. 2  schematically illustrates an example of one mechanism by which the polarizers can be rotated by a computer operable actuator. Here the half-wave plate  24  is a square plate and is held centrally in a circular holder  40  having a radial arm  42  extending therefrom. Holder  40  is peripherally supported on roller bearings  44 . An actuator  45  moves arm  42  linearly, as indicated by arrow X. This causes the plate to rotate around a rotation axis  46  of the circular portion of the holder. One suitable actuator is a micrometer screw driven by an encoded shaft drive or servo motor.  
         [0019]     Axis  46  is aligned with that beam path ( 20  or  22 ) in which the half-wave plate is rotated. The rotation angle as a function of translation of the actuator in the X direction will depend on the radial distance of the actuator from axis  46 . This rotation mechanism is but one computer operable mechanism for rotating the half-wave plate. Those skilled in the art may devise other computer operable rotation mechanisms without departing from the spirit and scope of the present invention. By way of example a holder for a half-wave may be provided with peripheral gear teeth, supported on a pair of mating idler gear wheels, and rotated by a worm gear meshing with the peripheral gear teeth and driven by a servo motor.  
         [0020]      FIG. 3  is a graph is a graph schematically illustrating measured percentage throughput as at a number of different rotations (orientations) a in radians of the half-wave plates in one example of the attenuator of  FIG. 1 . The actual measured maximum transmission value is about 97% and the polarization extinction ratio along either of the beam paths is greater than 100:1. This means that the attenuator is capable of providing controllable attenuation to throughputs down to about 1% or less of the input power. It can be seen from the graph that the maximum transmission value does not coincide with the nominal (α=0.0) orientation suggested by the half-wave-plate manufacturer&#39;s indication of the optic-axis orientation. Correspondingly, maximum attenuation does not occur at exactly 45° (0.785 radians). These discrepancies can be caused by one or more factors including, but not limited to, residual or mounting stress birefringence in the polarizing beamsplitter substrates, manufacturing tolerances on the waveplates, and misalignment of the polarizing beamsplitters one with the other. This highlights the importance of calibrating each half-wave plate individually to find the actual 100% throughput orientation before connecting the mechanism for synchronous rotation of the half-wave plates.  
         [0021]     Information of the type shown by the graph can be stored in computer memory as look-up table. A computer control sequence for a laser and an attenuator can provide a sensor cooperative with a controller, with the controller being responsive to user input, and having the look-up table electronically stored therein. In response to a user-input requesting a specific output beam power from the attenuator, the controller can calculate the attenuation required (or a new value of attenuation if the beam is already being attenuated), consult the look-up table to determine the value of a required (or the actuator setting which provides that value of α) and activate the actuator to synchronously rotate both half-wave plates to the required αvalue. Alternatively, output power of the attenuator can be monitored by, and a desired output power established and maintained, by the controller comparing the monitored power with the desired power and rotating the half-wave plates until the desired power is reached, and then periodically fine-adjusting the angle of the half-wave plates, if necessary, to maintain the desired power output.  
         [0022]      FIG. 4  schematically illustrates another preferred embodiment  50  of a laser beam attenuator in accordance with the present invention. Attenuator  50  operates according to the same principle as attenuator  10  of  FIG. 1  but employs 45° biprism-type polarizing beamsplitters  52  and  58  in place of front-surface Brewster-oriented polarizing beamsplitters  12  and  28 . Polarizing beamsplitters  52  and  58  have internal surfaces  54  and  60  created by optically bonding two prisms together to form the biprism. Each internal surface includes a thin film polarizing coating (not shown). The internal surfaces are oriented at 45° to entrance and exit faces of the biprism and these faces are oriented perpendicular to beam paths. Two turning mirrors  62  and  64  are required for folding path  20  back to polarizing beamsplitter  58  to be recombined with path  22  in common path  34 . Beamsplitters  52  and  58  are sometimes referred to as cube-beamsplitters. Those skilled in the art will recognize, however, that a biprism-type polarizing beamspitter can have an internal surface that is at some angle other than 45° to an entrance or exit face, and need not be cubic.  
         [0023]     One advantage of a biprism-type polarizing beamsplitter is that polarization can be provided over a broad spectral bandwidth, for example over a three to four hundred nanometers. Front-surface polarizers by comparison are effective only over a few tens of nanometers at best. Accordingly, attenuator  50  could be used with a wide range of lasers changing only the half-wave plates for a particular laser wavelength, while in attenuator  10  beamsplitters  12  and  28  in addition must usually be configured for one particular laser wavelength. Another advantage of attenuator  50  is that the “dumped” beam-portions are combined on a common path. Those skilled in the art will recognize, without further illustration or detailed description, that an alternative embodiment of the present invention is possible, similar to the embodiment of  FIG. 4 , but wherein the combined “dumped” beam-portions of  FIG. 4  become the output beam, and the output beam portions  FIG. 4  become the “dumped” beam-portions.  
         [0024]     One disadvantage of the biprism beamsplitters is that antireflection coatings would be required to eliminate Fresnel losses at entrance and exit faces. Another disadvantage is that such biprisms often exhibit a stress birefringence (residual or due to bonding the prisms) sufficient that the “crossed” orientation of the prisms may be up to 10° or greater different from a presumed (ideal) 90° orientation of one with respect to another. This can make calibration of an attenuator such as attenuator  50  particularly difficult and may lead to maximum-throughput restrictions.  
         [0025]     In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is limited only by the claims appended hereto.