Abstract:
A system consisting of a set of spatial filters that are used to optically relay a laser beam from one position to a downstream position with minimal nonlinear phase distortion and beam intensity variation. The use of the device will result in a reduction of deleterious beam self-focusing and produce a significant increase in neutron yield from the implosion of targets caused by their irradiation with multi-beam glass laser systems.

Description:
BACKGROUND OF THE INVENTION 
     The invention described herein was made at the Lawrence Livermore Laboratory in the course of, or under, contract No. W-7405-ENG-48, between the United States Energy Research and Development Administration and the University of California. 
    
    
     This invention relates to an optical system utilized in high power laser systems, and more particularly, to the use of multiple spatial filters as elements of an optical relay line in laser systems. 
     The phenomenon of self-focusing is one of the primary factors that limits the output power of high power glass lasers. The dependence of the index of refraction on the intensity in the optical components of the laser causes intensity fluctuations on an otherwise smooth beam to be amplified. If this amplification is left unchecked it will cause a loss of focusable power, and ultimately result in damage to the components of the laser system. 
     If it were possible to maintain a spatially uniform beam intensity while propagating through the consecutive nonlinear optical components that comprise the laser chain, the beam entering any glass component would resist self-focusing within said component. The effect of the intensity dependent refractive index of the glass would then do little to enhance power depleting intensity variation. 
     SUMMARY OF THE INVENTION 
     The present invention, a set of spatial filters arranged in a particular configuration, minimizes the beam nonuniformity arising from diffraction and self focusing by acting as an optical relay which translates with minimal deviation all rays in the beam at the initiating aperture to a desired position downstream from this aperture. Due to the invention, in theory and in practice, a beam arrives, with little distortion, near the output aperture of the laser. Thus the input beam can be relayed through consecutive amplification stages with minimal nonlinear aberration. 
     Therefore, it is an object of the invention to provide a spatial filter assembly as an optical relay line. 
     A further object of the invention is to provide a means for optically relaying a laser beam from an initial position in the laser system where the beam power is low to a downstream position where the beam power is high with minimal phase distortion and beam intensity variation. 
     Another object of the invention is to provide a set of spatial filters arranged in a particular configuration which minimizes beam nonuniformity arising from diffraction and self-focusing by acting as an optical relay which translates with minimal deviation all rays in the beam at the initiating aperture to a desired position downstream from this aperture. 
     Other objects of the invention will become readily apparent from the following description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 graphically illustrates intensity distributions as an input beam propagates undisturbed over a distance; 
     FIG. 2 illustrates an embodiment of a spatial filter assembly made in accordance with the invention; and 
     FIG. 3 schematically illustrates one arm of the Argus glass laser system incorporating the invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     The invention is a spatial filter arrangement or assembly for optically relaying a laser beam from an initial position in the laser system where the beam power is low to a downstream position where the beam power is high with minimal phase distortion and beam intensity variation. In a glass laser amplifier chain, such as that utilized in the implosion of targets for known applications such as plasma generation, physics studies, or sources of neutrons, x-rays, alpha particles, etc., multiple spatial filters made in accordance with specifications of the invention can be incorporated into the laser chain and substantially increase the beam power of the laser. For example, in the two arm glass laser system known as Argus assuming that both arms produce 1×10 9  neutrons/target burst the increased beam power provided by the invention should yield an excess of 5×10 9  neutrons/target burst. 
     An input laser pulse may have a spatial intensity (and also phase variation), I (r; z=O), as shown in FIG. 1, where r denotes radial distance from the beam central axis. If one allows the beam to propagate undisturbed over a distance, L, the new intensity distribution I (r; z=L) would develop variations as shown in FIG. 1. In a geometric sense, the rays comprising the input beam (at z=O) are not all parallel to the beam axis, and thus, when extended along the propagation path, they tend to cross. The beam itself, trying to follow the directions indicated by these initial rays will develop intensity ripples as shown in the figure. The present invention, acting as an optical relay line, merely translates the entire collection of beam rays at Z=O to Z=L so that, within limits, 
     
         I(Mr; Z=L)= I(r; Z= O) 
    
     and no (new) intensity variations are manifest. That is, the final ray directions are the same as the initial ray directions, so no ripples develop. A simple magnification, M, is allowed. 
     An element of the present invention, shown in FIG. 2, also includes a pinhole 11 which is located at or near the common focal plane of two thin lenses 13 and 15 which have respective focal lengths f 1  and f 2  and are separated by a distance d=f 1  +f 2 . Adopting the ray matrix approach for paraxial rays, the radial displacement r and angular orientation u=dr/d Z  ≅θ of any ray in two displaced transverse planes π 1  and π 2  (Z=-f 1  -d 1  and Z=f 2  +d 2 ) are related by the optical transfer matrix, vis. ##EQU1## 
     If one then chooses d 1  and d 2  so that ##EQU2## then plane π 2  is merely an inverted geometrical image plane of plane π 1  with magnification 
     
         M= f.sub.2 /f.sub.1.                                       (3) 
    
     The effect, apart from an adjustable magnification, is to merely translate (and invert) the entire ray distribution at plane π 1  to plane π 2 . 
     The ray matrix and the ABCD law (see A. Yarivig, Introduction to Optical Electronics [Holt, Rinehart and Winston, N.Y., 1971] Chapters 2 and 3) may also be used to determine the diffraction effects of the system of FIG. 3. The complex propagation parameter, q, for a Laguerre-Gaussian mode expansion of an electric field component is expressed as: ##EQU3## where R is the beam radius of curvature, W is the radius of the beam waist and k=2π/λ is the beam wave number. The values of q in the planes π 1  and π 2  are related by ##EQU4## Thus, the phase of q 2  is the same as the phase of q 1 , so that the phase of each Laguerre-Gaussian mode is also the same in the two planes π 1  and π 2 . An arbitrary complex wave form in π 1  is thus merely inverted and magnified (M=-f 2  /f 1  in π 2  with no other change: the system in FIG. 3 acts as an optical relay, relaying a complex wave form from π 1  to π 2  with minimal distortion. 
     The effect of the pinhole 11 has been neglected in the ray matrix. This is arguably justified if the pinhole radius r O  is sufficiently large. In fusion layers, the pinhole radius typically satisifies 
     
         r.sub.O a/λf≈7-10, 
    
     f=focal length of input lens, 
     a=beam radius at input. 
     For example, in one recent experiment on the laser fusion system known as Argus, replicas of the spatial filter assembly were placed between apodizer 17 and rod amplifier 18, as indicated at 19, and in addition were located at positions 21, 23, and 25 in FIG. 3, with corresponding pinhole radii r 21  =r 23  =r 25  =300 microns. The effect of self-focusing was reduced to the extent that peak beam power through each arm of Argus could be increased to ≈ 2 TW (from 1-1.5 TW without the present invention). A new Argus experiment using pinhole radii of r 21  =r 23  =300 μm and r 25  =400 μm allowed peak beam power to reach 2.5 TW in each arm. Previously, Argus produced 1-2×10 9  neutrons/target burst. With the increase in peak beam power resulting from the present invention, the neutron yield will reach 5×10 9  /target burst, which represents a three order of magnitude increase in neutron yield over the laser system known as Cyclops. 
     By judicious placement of the relay elements at various positions in the optical train, the input beam at each such point can be optically relayed through various amplification stages with minimal aberration and phase distortion. The effective optical path length between relay elements can be made zero, thereby minimizing diffraction effects and reducing whole beam self-focusing. The beam power, which is currently limited by the development of self-focusing, may thus be increased, with a corresponding increase in laser fusion system performance. 
     Thus, it has been shown that the present invention provides a means which allows a laser beam to be optically relayed through various amplification stages of a high power laser system, such as for laser fusion applications, with minimal nonlinear aberration, phase distortion, diffraction, and self-focusing. The present invention clearly has application as an optical relay wherever high power laser amplification is employed and will increase the focusable power of the laser. 
     While a particular embodiment of the invention has been illustrated and described, modifications will become apparent to those skilled in the art, and it is intended to cover in the appended claims, all such modifications as come within the spirit and scope of the invention.