Patent Publication Number: US-2006013271-A1

Title: Variable attenuator

Description:
FIELD OF THE INVENTION  
      The present invention is related to the control of laser parameters, in particular energy density or fluence.  
     BACKGROUND ART  
      Different methods have been implemented to control the fluence of a laser system, but these methods generally include altering either pulse energy or beam diameter. One known approach is by way of a variable telescope configuration that varies the distance between a pair of lenses along the optical axis of the laser to alter the beam diameter. Another common method is to place a thin, coated optic in the path of the beam. Other attenuation and/or controlled output energy methods variously include the use of beamsplitters, linear optical absorbers, photochromatic absorbers and reflectors.  
      Polarisation characteristics have been utilised to control energy output in a number of laser applications. U.S. Pat. No. 5,383,199 describes an arrangement for optically controlling the output energy of an UV excimer laser angioplasty system. This arrangement involves placing an optically contacted thin film polariser, which is antireflection coated, in the path of the beam. A sensor is provided to detect the energy in the attenuated beam, and a controller is coupled to the sensor to control the rotation of the thin flim polariser to ultimately control the fluence of the system output beam.  
      A variable attenuator for a multi-wavelength Nd:YAG laser system is described in U.S. Pat. No. 5,703,713. The attenuator is composed of a multiple wavelength waveplate and a calcite polariser, and the angular position of both is varied to control the output energy. U.S. Pat. No. 4,398,806 describes the use of two wedge-shaped plates positioned in the path of the laser beam to polarise the incoming beam as a function of the angle of incidence. The angle of incidence is varied by rotating the plates. This system utilises Fresnel reflection near the critical angle at the second interface of the first wedge-shaped plate. The wedges are aligned in parallel, and preferably a second pair of wedges is placed in the beam path to achieve co-linearity of the output beam with the input beam.  
      U.S. Pat. No. 4,664,484 describes a variable attenuator comprising two spaced optical elements each with reflective surfaces. This attenuation system is based on reflection, whereby the incident radiation is reflected from the first window to the second, which has a metallised surface. These optical elements are moved relative to each other and rotated simultaneously around the optical axis to ensure the reflected beam is incident on the second surface at the same angle of incidence. The beam incident on the first surface is unpolarised and at least one of the elements is adapted to plane polarise the reflected beam. A second pair of reflective surfaces may be added to achieve co-linearity. Variable attenuators are also commonly used in optical fibre communications systems (see for example U.S. Pat. No. 6,149,278).  
      The use of the aforementioned methods in a high energy (ie. tens of millijoules per pulse or greater) pulsed solid state laser system is impractical because of the low damage thresholds of the optical components required to implement the respective optical configurations. Variable attenuators such as those described above that utilise coatings are also not feasible in a solid state based refractive surgery laser system, such as that described in international patent publication WO99/04317, as the reflectance of the coated optics are susceptible to changes in the angle of incidence. In these arrangements, a small change in the angle of incidence can result in very high losses, a situation that would not be suitable for a medical laser system. Dielectric mirrors (for high energy lasers) can have a small acceptance angle of &lt;5° with a very sharp drop off in the reflectance outside this range, which also makes them unsuitable. The methods described above may also induce beam expansion resulting in a varying beam profile at the working plane. Varying beam sizes can result in changes to beam propagation and spatial beam profile.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the invention to provide a method and apparatus for variably attenuating a light beam in a laser system that is suitable for incorporation in pulsed solid state laser systems of the kind used in medical applications such as refractive surgery.  
      It is a further object of the invention to provide a means to control laser fluence without affecting other parameters such as beam direction or spatial distribution to an unacceptable extent.  
      In a first aspect of the invention there is provided a method for variably controlling the energy output of a laser system including: 
          positioning in the path of a linearly polarised laser pulse of said laser system at least one optical element having a surface on which said pulse is incident and across which said pulse is at least partially transmitted;     rotating said optical element about an axis substantially parallel to, and preferably aligned with, said path to alter the polarisation of the laser pulse relative to the said surface thereby varying the energy of said transmitted pulse.        

      In the first aspect of the invention, there is further provided apparatus for variably controlling the energy output of a laser system, including: 
          a first optical element having a surface; and     means supporting said optical element for positioning thereof in the path of a linearly polarized laser pulse of said system so that said pulse is incident on said surface, and is at least partially transmitted across said surface;     wherein said supporting means is such that said optical element is rotatable about an axis substantially parallel to, and preferably aligned with, said path to alter the polarisation of the laser pulse relative to said surface thereby varying the energy of said transmitted pulse.        

      In a second aspect, the invention provides a method of variably controlling the energy output of a laser system, including: 
          positioning in the path of a linearly polarized laser pulse of said laser system at least one optical window element having parallel faces, on one of which said pulse is incident and across which the pulse is at least partially transmitted; and     rotating said optical window element to alter the polarization of the laser pulse relative to said faces, thereby varying the energy of said transmitted pulse.        

      In the second aspect, the invention further provides apparatus for variably controlling the energy output of a laser system, including: 
          an optical window element having a pair of parallel faces; and     means supporting said optical element for positioning thereof in the path of a linearly polarized laser pulse of said system so that said pulse is incident on at least one of such faces and is at least partially transmitted across said faces;        

      wherein said supporting means is such that said optical element is rotatable to alter the polarization of the laser pulse relative to said faces, thereby varying the energy of said transmitted pulse.  
      Preferably, in either or both aspects of the invention, the means supporting the optical element is a tubular member closed at one end by the optical element. Advantageously, there is a second optical element similar to the first closing the other end of the tubular member, the two elements being arranged to substantially eliminate offset of the laser pulse.  
      Said optical elements are preferably uncoated.  
      Means is preferably provided for monitoring the pulse energy downstream of the apparatus and for effecting said rotation in response to the monitored energy.  
      The invention is further directed to a laser system including means to generate a beam of laser pulses and incorporating one or both of said aspects of the invention.  
      In an advantageous application, the laser beam generating means is a solid state laser and the laser system includes means for generating, in a frequency conversion or harmonic generation process, a beam of predetermined wavelength from an output beam of said solid state laser of a wavelength different from the predetermined wavelength. The apparatus of the invention is preferably disposed to attenuate the laser beam between the solid state laser and the frequency conversion or harmonic generation means. There may typically be beam cross-section control means and/or scanning means downstream of the frequency conversion means.  
      In a particularly advantageous application, the laser system including the solid state laser comprises a laser surgical system for performing ophthalmic surgery such as corneal ablation, eg. for laser refractive correction surgery. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In order that the invention may be more fully understood, a preferred embodiment will now be described by way of example with reference to the following illustrations, in which:  
       FIG. 1  is a schematic optical diagram illustrating the underlying principle used in the invention;  
       FIGS. 2 and 3  are respective isometric views of a first embodiment of a variable attenuator according to the present invention;  
       FIG. 4  is an axial cross-section of the attenuator depicted in  FIG. 3 ;  
       FIG. 5  is a schematic optical diagram of the configuration of a second embodiment of the present invention; and  
       FIG. 6  is a plot of experimental and theoretical transmittance against rotational angle for the variable attenuator of FIGS.  2  to  4 ; and  
       FIG. 7  is an optical diagram of a solid state laser ablation system incorporating a variable attenuator between the laser and the harmonic generation module of the system. 
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION  
      With reference to  FIG. 1 , the variable attenuator takes advantage of the polarisation of the incoming laser light  10 . Two optical elements, eg in the form of windows  20 ,  21 , are separated by a predetermined distance, which will depend on the amount of space in the delivery path of the laser system, but will preferably be as short as possible. Both windows  20 ,  21 , are arranged with respect to the laser beam incident propagation path or axis  10  so that the angle of incidence of the laser beam is Brewster&#39;s angle θ B  (at which angle there is a zero reflection loss of P-polarised light). However, windows  20 ,  21  are oriented in a complementary and symmetrical manner so that their normals are coplanar but intersect (at an angle 180-2θ B ): this eliminates any offset between the incoming and outgoing beams. Another way of viewing this is to consider that the windows are at the same orientation to axis  10  but relatively rotated about an axis orthogonal to axis  10  by 180°.  
      When the windows are rotated about the axis of incident laser propagation  10 , which is equivalent to a change of the relative incident polarization, the reflection loss of these windows will change from a minimum (P-polarized Brewster angle incidence) to a maximum (S-polarized Brewster angle incidence). The windows may be set with an angle of incidence outside of Brewster&#39;s angle to change the range of attenuation. The angle of incidence should be greater than approximately 15° for the reflection loss to significantly change. Reducing the range of attenuation will result in more precise variation in the output energy.  
      Referring to FIGS.  2  to  4 , a first embodiment of attenuator  50  consists of a pair of spaced windows  100 ,  110  each having parallel faces  112 ,  113 . Windows  100 ,  110  are located at and close to the respective ends of a closed tube  120 . The tube  120  is constructed from aluminium or another suitable material and functions purely as a mount for the windows  100  &amp;  110 . Tube  120  is in two interlocking parts  122 ,  124 , one including a centre region  125 , and is held in a rotator bearing  130  by a nut  132 . The assembly is held via a support block  142  engaging rotator bearing  130 , on a standard optical mount (shown at  140  in  FIG. 3  only). In this way, the device is positioned in the laser beam delivery path so that the axis  11  of the tube is co-incident with the laser beam propagation path  10 , ie. the optic axis. The assembly may have more or fewer window elements inserted depending on the amount of attenuation variability required, but will most preferably have an even number of windows.  
      Windows  100 ,  110  are retained by screw-down clamp frames  105 ,  115  and are positioned with complementary and symmetrical orientations, as previously described, to avoid beam offset, with their normals in the same plane. The incident angles of the beam onto both windows  100 ,  110  is preferably also the same (in this case 60°, a little outside Brewster&#39;s angle) but with complementary orientations as mentioned above. Respective energy sink plates  107 ,  117  are provided to absorb light reflected at windows  100 ,  110 . Plate  107  projects to the exterior at the input end, while plate  117  covers a laterally angled opening  118  at the other end.  
      Windows  100 ,  110  are preferably fashioned from uncoated glass pieces, as coated substrates suffer from low damage thresholds. Uncoated BK7 windows for 1064 nm have a good optical quality and a high damage threshold (&gt;5 J/cm 2 ), and are inexpensive and readily available. They are therefore most preferably utilised as windows  100 ,  110  in a 1064 nm Nd:YAG system.  
      The angular deviation of a beam after transmission through a window depends on its wedge angle, while the beam offset depends on the incident angle of the laser beam and the thickness of the window. Windows  100 ,  110  are chosen to have the same thickness and are placed symmetrically, so that their beam offsets will compensate for each other. Preferably, any window used in this arrangement should have as low as practical wedge angle.  
      For the reasons noted above, a change in the incidence angle of the incoming beam will result in a variation in the amount of laser energy transmitted through elements  100 ,  110 . The output fluence of the laser system is therefore varied by synchronously rotating windows  100 ,  110  about the optical axis  10  of the incident laser beam. Rotator bearing  130  is used to set the rotation angle of windows  100 ,  110 . Bearing  130  preferably has an angular tolerance of ±0.2° with a precise range of movement. The bearing may be rotated manually by hand (as for the illustrated arrangement of  FIG. 2 ), or may be moved automatically via a motorised control system (eg. stepper motor/servo motor etc) in response to electrical control signals. In either case, worm gearing may be included in the drive. The motors are under computer control and a feedback system, such as that described in co-pending application PCT/AU01/01341, is initialised to determine the rotational requirements of bearing  130 . An energy sensor (not shown) is located downstream of the attenuator assembly  50 , and detects the energy levels of the harmonic beam. The computer controller determines an optimal target energy level and directs the motors to rotate windows  100 ,  110  in a predetermined direction to ensure the target energy is maintained. In this way, the overall fluence of the laser system is set, as the beam size remains constant. Alternatively, an open loop system, which uses tables from known values, may determine the angular position of windows  100 ,  110 .  
      A second embodiment of the present invention involves adjusting the windows around the axis of beam polarisation, and is depicted purely schematically in  FIG. 5 . In this arrangement the windows  200 ,  210  are not rotated around the beam incident axis, but are instead adjusted about axes  208 ,  209  perpendicular to the beam axis, as represented by arrows  205 ,  206  in  FIG. 5 . The two elements  200 ,  210  are mechanically linked to rotate synchronously but oppositely and maintain beam offset: arrows  205  indicate rotation that increases the angle of incidence and arrows  206  indicate rotation to decrease the angle of incidence. Windows  200 ,  210  may be supported on either side by a mounting frame (not shown) with a gear system attached to the mount.  
       FIG. 6  is a plot of theoretical and experimental transmittances against rotational angle of variable attenuator  50 . This example illustrates the attenuation range in a 1064 nm wavelength Nd:YAG laser system. As the attenuator angle increases the 1064 nm transmittance, and therefore output power decrease. Theoretically there is a maximum of 48% loss for the two windows. In practice this arrangement achieves from 5 to 50% loss. If more than 48% attenuation is required, then another two windows may be implemented into the attenuator assembly, the windows may be adjusted further away from Brewster&#39;s angle, or another attenuator assembly may be used.  
      An advantage of the above described system is that it allows optimisation of the output of the fundamental laser in a solid state harmonic generation laser system.  FIG. 7  is an optical diagram of an exemplary such system  300  configured for laser ablation, eg laser refractive correction ophthalmic surgery.  
      The system  300  includes a solid state laser  312  that emits a primary laser beam  314  in the infra-red region of the electromagnetic spectrum. Primary laser beam  314  is guided by optical elements, in this case mirrors  316 ,  317 , along an optical alignment or axis  321 , through a harmonic generation module  350  comprising a series of non-linear optical (NLO) crystals  320 ,  322 ,  324  from which emerges a multi-wavelength output beam  318 . Beam  318  comprises the original beam  314  and several harmonics generated by crystals  320 ,  322 ,  324 . The desired harmonic  326  is separated out by a prism  330 . A dichroic mirror arrangement may alternatively be used for this purpose.  
      In an application for refractive eye surgery by photo-ablation, beam  326  is directed by a beam delivery system  332  onto the cornea  334  of an eye  335 .  
      A small portion of component beam  326  is diverted by a beamsplitter  336  to a photo-detector  338  such as a photodiode for measuring and monitoring the energy of beam component  326 .  
      Controller  354 , typically a computer system, controls at least the output beam parameters of laser  312 , and the elements of the beam delivery system.  
      A particularly suitable laser  312  is a Q-switched Neodymium:YAG laser producing a 2-10 mm diameter pulsed laser beam  314  of fundamental wavelength 1064 nm. The beam  314  is collimated, resulting in a collimated harmonically generated beam downstream. A variety of other laser sources are suitable but preferred sources are Nd 3+  doped laser media such as Nd:YLF, Nd:glass and Nd:YV0 4 .  
      A particularly convenient crystal set  320 ,  322 ,  324  is as disclosed in international patent publication WO 99/04317. In this configuration, crystal  320  is a BBO crystal that uses type I or type II phase matching as a frequency doubling unit to generate a frequency doubled beam  315  of second harmonic wavelength 532 nm. Instead of a BBO crystal  320  may alternatively be a KTP, LBO, KD*P or any other suitable NLO crystal. The other two crystals  322 ,  324  are preferably CLBO crystals although other suitable crystals include BBO, and KD*P and related isomorphs. Crystal  322  converts frequency doubled beam  315  at 532 nm to a beam  323  of 4th harmonic wavelength 266 nm, utilising type I phase matching. In crystal  324 , beam components  315  and  323 , of fundamental and fourth harmonic wavelengths respectively, are frequency mixed to produce a laser beam component  326  of the fifth harmonic wavelength, 213 nm. This is effected by means of sum frequency generation, a type I phase matching interaction.  
      Further details of this process and of the crystals themselves are to be found in the aforementioned international patent publication, the disclosure of which is incorporated herein by reference.  
      Advantageously, a variable attenuator  400  similar to attenuator  50  of FIGS.  2  to  4  above is disposed in the laser beam path  314  between laser  312  and harmonic generation module  350 , and the setting of attenuator  400  is determined by controller  354  in response to inputs that include the monitored energy at photodectector  338 . The result is a change in the input energy of the fundamental wavelength pulsed laser beam eg a 1064 nm beam for an Nd:YAG laser, and resultant control of the output fluence of the harmonically generated beam. This configuration is contrary to current practice in most solid state and refractive laser settings, where the fluence control optics are usually placed at the end of the delivery system.  
      Alternatively, the attenuator may be placed in the path of any of the polarised harmonic beams, as its function is dependent on the polarisation of the beam and not the wavelength. In particular, it may be advantageous to position attenuator  400  downstream of harmonic generation module  350 , preferably prior to prism  330 .  
      It will be appreciated that, in the configuration of  FIG. 7 , the laser output is optimised in a manner that is easy to implement, and inexpensive. The configuration minimises impact on beam divergence or convergence in contrast to the effect of variable telescopes. High energy is applied only at the beginning of the delivery system, with resultant reduced damage to downstream optics, particularly in the UV range.  
      In an advantageous modification of laser system  300 , suitable optics such as one or more mirrors are placed downstream of module  350  to filter a high proportion of the non-selected harmonics, eg. other than the 213 nm fifth harmonic in the example under consideration, and so reduce the thermal energy load on the prism  330  and extend its effective life. In this configuration, attenuator  400  may be disposed between the absorbing optics at the prism.  
      In a further modification, there may be two variable attenuators in series, a manually adjusted device for course setting and a motor-driven device for fine adjustment.  
      The variable attenuator could of course also be applied to any other laser system that utilises polarised light.  
      Modification within the spirit and scope of the invention may be readily effected by a person skilled in the art. Thus, it is to be understood that this invention is not limited to the particular embodiments described by way of example herein above.