Abstract:
A light tunnel apparatus ( 200  or  300 ) having an output end ( 56  or  98 ), for uniformizing light (L) that travels through a light tunnel ( 30  or  80 ). The apparatus comprises a light tunnel having first and second sides ( 36, 40  or  86, 90 ), and one or more AO modulators ( 210  or  310 ) respectively arranged on at least one of the first and second sides. The AO modulators are arranged such that activating the one or more of them causes at least one of the first and second sides to be displaced. This displacement changes the path of light traveling through the light tunnel by an amount sufficient to reduce illumination non-uniformities at the output end. The light tunnel may be a hollow light tunnel ( 30 ) with reflective inner surfaces, or a solid light tunnel ( 80 ) with a refractive index. A method of uniformizing illumination using a light tunnel is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of illumination, and in particular to light tunnels used in optical systems such as illuminators to achieve uniform illumination. 
     BACKGROUND OF THE INVENTION 
     Achieving uniform illumination is necessary in numerous optical applications, and is particularly important in the fields of microscopy, and the relatively new field of photolithography. Many illumination uniformization techniques have evolved over the years to meet the increasing demands on illumination uniformity. With the advent of the laser in the 1960&#39;s, new techniques have been developed to deal with illumination non-uniformities arising from interference effects due to the coherent nature of laser light. 
     In certain applications, such as photolithography, materials processing and the like, it is desirable to illuminate an object with light having an intensity distribution that is both macroscopically and microscopically uniform. Here, macroscopic means dimensions comparable to the size of the object being illuminated and microscopic means dimensions comparable to the size of the wavelength of the illumination. In many of these applications, it is further desirable to use a pulsed laser source and to have a spatially uniform intensity distribution. However, the light output of a pulsed laser source is spatially non-uniform. Macroscopically, the light beam often has a gaussian-like cross-section (“profile”). A great deal of effort has gone into fabricating lasers that emit a beam having a more uniform profile, but even these are only uniform to +/−10% over limited areas. As a result, it is often necessary to use auxiliary optics with a pulsed laser light source to make the illumination more uniform. 
     The challenge in producing a spatially uniform intensity distribution from a laser source arises from its inherent temporal and spatial coherence. When two incoherent light beams overlap, the intensities of the two beams add. However, when two coherent beams overlap, the electric fields of the two beams add, which can produce an intensity having an interference pattern comprising fringes not present in an incoherent illumination system. As a result, the traditional methods of producing uniform illumination with incoherent sources are typically unsuitable for coherent sources like lasers. 
     With reference to FIGS. 1A and 1B, there are shown schematic cross-sectional diagrams of conventional illumination uniformizer apparatus  10  and  70  for achieving uniform macroscopic illumination. The conventional uniformizer apparatus works well for incoherent (i.e., “non-laser”) sources, but is inadequate for coherent (i.e., “laser”) sources. For many applications, apparatus  10  of FIG. 1A comprises, along an optical axis A, a laser light source  16  emitting short pulses of coherent light L (e.g., 10 ns/pulse) comprising light rays R 1  and R 2 , a condenser optical system  24 , and a hollow light tunnel  30  with an interior region  32 , upper and lower walls  36  and  40 , respectively, and corresponding highly reflective inner surfaces  36   i  and  40   i  and outer surfaces  36   o  and  40   o  respectively. Light tunnel  30  further includes an input end  50  adjacent optical system  24 , and an output end  56  at the distal end of tunnel  30  from optical system  24 . A material often used for walls  36  and  40  of hollow light tunnel  30  is quartz, which is often coated with a high-reflectivity material such as a metal or a dielectric. 
     With reference to FIG. 1B, apparatus  70  includes the same elements, except that instead of hollow light tunnel  30 , apparatus  70  includes a solid light tunnel  80  having an index of refraction n 1 , upper and lower surfaces  86  and  90 , an input end  94  and an output end  98 . A material often used for solid light tunnel  70  is fused quartz, which has a refractive index of about 1.5 in the visible wavelengths. Apparatus  10  and  70  are commonly used with incoherent sources to achieve better than +/−1% uniformity at their respective output ends  56  and  98 . 
     Because of the coherent nature of light source  16 , intersecting light rays R 1  and R 2  passing through the light tunnel produce a light intensity distribution in the form of a standing sinusoidal wave pattern P s  at the output ends  56  and  98  of light tunnels  30  and  80 , respectively. Here, two rays R 1  and R 2  and a central ray RS are shown for the sake of illustration. The period of standing wave pattern P s  is determined by the wavelength of the laser light and the angle between intersecting light rays R 1  and R 2 , between rays R 1  and RS, and between rays R 2  and RS. In practice, there are many pairs of intersecting light rays (depending on the number of reflections), with each pair producing a standing wave pattern. The length and width of light tunnels  30  and  80  define the angle between intersecting rays R 1 , R 2 , and RS and the path length difference (i.e., the phase) between the intersecting rays determines the relative position of the irradiance maxima in standing wave pattern P s . 
     A prior art technique for eliminating interference effects (e.g., standing wave pattern P s ) to achieve uniform illumination using a light tunnel is the breaking of the coherent light into packets and adding the packets incoherently, or by rotating a random diffuser between the light source and the light pipe entrance. 
     There are several U.S. patents directed to such techniques for eliminating interference effects that are relevant to light tunnel illumination systems. For example, U.S. Pat. No. 4,744,615, entitled “LASER RAY HOMOGENIZER,” describes a coherent laser ray having a possibly non-uniform spatial intensity distribution that is transformed into an incoherent light ray having a substantially uniform spatial intensity distribution by homogenizing the laser ray with a light tunnel. When the cross-section of the light tunnel is a polygon (as preferred) and the sides of the tunnel are all parallel to the axis of the tunnel (as preferred), the laser light at the exit of the light tunnel (or alternatively at any image plane with respect thereto) has a substantially uniform intensity distribution and is incoherent only when the aspect ratio of the tunnel (length divided by width) equals or exceeds the co-tangent of the input ray divergence angle theta and when W min =&gt;2RL coh , where W min  is the minimum required width for the light tunnel, L coh  is the effective coherence length of the laser light being homogenized and R is the chosen aspect ratio for the light tunnel. This approach restricts the ratio of the tunnel&#39;s length to width and consequently, the number of bounces for the light rays. However, the number of bounces affects the “macro-uniformity” of the output of the tunnel. As a result, this approach can impact the macro-uniformity at the output of the homogenizer tunnel. 
     U.S. Pat. No. 5,224,200, entitled “COHERENCE DELAY AUGMENTED LASER RAY HOMOGENIZER,” describes a system in which the geometrical restrictions on a laser ray homogenizer are relaxed by using a coherence delay line to separate a coherent input ray into several components each having a path length difference equal to a multiple of the coherence length with respect to the other components. The components recombine incoherently at the output of the homogenizer, and the resultant ray has a more uniform spatial intensity suitable for microlithography and laser pantogography. 
     U.S. Pat. No. 4,511,220, entitled “LASER TARGET SPECKLE ELIMINATOR,” describes an apparatus for eliminating the phenomenon of speckle with regard to laser light reflected from a distant target whose roughness exceeds the wavelength of the laser light. The apparatus includes a half plate wave member, a first polarizing ray splitter member, a totally reflecting right angle prism, and a second polarizing ray splitter member, all of which are in serial optical alignment, that are used in combination to convert a linearly (i.e., vertically) polarized light ray, which is emitted by a laser having a known coherence length, into two coincident, orthogonally polarized, rays that are not coherent with each other, and that have an optical path difference which exceeds the known coherence length of the emitting laser, to eliminate the speckle. 
     U.S. Pat. No. 4,521,075, entitled “CONTROLLABLE SPATIAL INCOHERENCE ECHELON FOR LASER”, describes a system for achieving very uniform illumination of a target. A ray of broadband spatially-coherent light is converted to light with a controlled spatial incoherence and focused on the target. An echelon-like grating breaks the ray up into a large number of differently delayed raylets with delay increments larger than the coherence time of the ray, and a focusing lens overlaps the raylets to produce at the target a complicated interference pattern modulated by a smooth envelope that characterizes the diffraction of an individual raylet. On long time scales, compared to the coherence time, the interference pattern averages out, leaving only the smooth diffraction envelope. This approach only works for a sufficiently long time duration and therefore limits the laser pulse length. This may not be an acceptable solution for some applications. 
     In sum, the above described prior art techniques are either too complex to apply to light tunnel systems, or are unduly restrictive in their application. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the field of illumination, and in particular to light tunnels used in optical systems such as illuminators used to achieve uniform illumination. The present invention solves the above-described uniformity problems by reducing or removing the effects of standing wave patterns by laterally shifting the standing wave pattern at the output end of the light tunnel at high speed by actively shifting the boundaries of the light tunnel using an acousto-optic (AO) modulator. 
     Accordingly, a first aspect of the present invention is a light tunnel apparatus having an output end for uniformizing light traveling through the light tunnel. The apparatus comprises a light tunnel having first and second sides, and one or more AO modulators respectively arranged on at least one of the first and second sides. The AO modulators are arranged such that their activation causes at least one of the first and second sides to be displaced. This displacement changes the path of light traveling through the light tunnel by an amount sufficient to reduce illumination non-uniformities at the output end. The light tunnel may be hollow with reflective inner surfaces, or a solid light tunnel made from transparent material with a refractive index greater than 1. 
     A second aspect of the invention is an illumination uniformizer apparatus comprising, in order along an optical axis, a light source (e.g., a laser), a condenser optical system, and the light tunnel apparatus of the present invention as described above. 
     A third aspect of the present invention is a method of uniformizing light traveling through a light tunnel having first and second sides and an output end. The method comprises the steps of first, injecting light into the light tunnel. The next step is then displacing at least one of the first and second sides by injecting acoustic energy into the light tunnel through at least one of the first and second sides. This second step may involve driving an AO modulator at a frequency of 100 MHz or greater. The light traveling through the tunnel comprises light rays having a path length which, depending on the exact nature of the path, can vary by a half wavelength or more due to the modulator. Preferably, the displacement of the one or more sides is such that interfering light rays are imparted with a path length difference greater than half the wavelength of the light rays. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross-sectional schematic diagram of a prior art light tunnel uniformizer apparatus, wherein the light tunnel is hollow; 
     FIG. 1B is a cross-sectional schematic diagram of a prior art light tunnel uniformizer apparatus, wherein the light tunnel is solid; 
     FIG. 2A is a cross-sectional schematic diagram of the light tunnel uniformizer apparatus of the present invention with a hollow light tunnel, showing the AO modulators and the paths of light rays through the light tunnel when the AO modulator is not activated; 
     FIG. 2B is a cross-sectional schematic diagram of the light tunnel uniformizer apparatus of the present invention having a hollow light tunnel, showing the AO modulators and the paths of light rays through the light tunnel when the AO modulator is activated; 
     FIG. 3A is a cross-sectional schematic diagram of the light tunnel uniformizer apparatus of the present invention with a solid light tunnel, showing the AO modulators and the paths of light rays through the light tunnel when the AO modulator is not activated; and 
     FIG. 3B is a cross-sectional schematic diagram of the light tunnel uniformizer apparatus of the present invention with a solid light tunnel, showing the AO modulators and the paths of light rays through the light tunnel when the AO modulator is activated. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to the field of illumination, and in particular to light tunnels used in optical systems such as illuminators used to achieve uniform illumination. 
     Hollow Light Tunnel Embodiment 
     With reference to FIGS. 2A and 2B, there is shown light tunnel uniformizer apparatus  200  similar to prior art apparatus  10 , except that light tunnel  30  includes, on upper and lower surfaces (i.e., first and second sides)  36   o  and  40   o,  an AO modulator  210 . AO modulators are discussed in Chapter 12 of the book “Handbook of Optics,” Volume II (Devices, Measurements and Properties), Michael Bass, Editor-in-Chief, published by McGraw-Hill, Inc., said Chapter 12 being incorporated herein by reference for its basic teachings of AO devices, and AO modulators in particular. AO modulator  210  may be purchased commercially from several suppliers, such as Isomet Corp, Springfield, Va., and NEOS Technologies, Melbourne, Fla. Further included in apparatus  200  is an AO modulator control unit  220  electrically connected to AO modulator  210  which can also be commercially purchased from Isomet Corp and NEOS Technologies. 
     Also included in apparatus  200  in a preferred embodiment of the present invention is a light-sensitive detector D movably arranged near light tunnel output end  56  and electrically connected to AO modulator control unit  220 . Detector D is movable across output end  56  to measure the light energy (e.g., irradiance) distribution at output end  56 . Detector D outputs a signal corresponding the light energy incident thereon. An exemplary detector D is a CCD array camera purchased from the COHU Corporation. 
     With reference to FIG. 2A, laser light is relayed from laser light source  16  and is injected into interior region  32  of light tunnel  30  over a range of angles via condenser optical system  24 . Three light rays RS, R 1  and R 2  are shown. Light ray RS is a straight-through ray, while light rays R 1  and R 2  are incident reflective inner surfaces  36   i  and  40   i  at given angles and are reflected therefrom at points p 1  and p 2  toward output end  56 . Typically, light tunnel surfaces  36   i  and  40   i  are made of a glass, such as fused silica, or a ceramic, and the inner surfaces are coated with a metal (e.g., aluminum or chromium) and/or a dielectric layer to obtain maximum reflectivity. Standing wave pattern P s , as described above, is formed at output end  56  from the interference between straight through ray RS and rays R 1  and R 2  that undergo a single reflection. Detector D can be moved to output end  56  to measure standing wave pattern P s  and the degree of illumination non-uniformity. 
     With reference now to FIG. 2B, AO modulator  210  is activated by an electrical signal sent from AO modulator control unit  220 . The latter, for example, may drive AO modulator  210  at a frequency of about 100 MHz or so. AO modulator  210  is designed so as to set up an acoustic wave pattern on inner surfaces  36   i  and  40   i  of light tunnel  30  in response to the electrical signal from AO modulator control unit  220  such that the physical positions of the light tunnel walls move with time. Accordingly, when activated, AO modulator  210  injects acoustic energy into light tunnel  30 , which causes walls  36  and  40  to rapidly oscillate in the Y-direction. As a result, at an instant in time, light rays R 1  and R 2  reflect from inner surfaces  36   i  and  40   i  at new positions p 1 ′ and p 2 ′ that are displaced from positions p 1  and p 2 . This, in turn, cause the paths of light rays R 1  and R 2  to change, which causes standing wave pattern P s  to shift (i.e., oscillate) about its original position. In other words, the path length differences between interfering rays RS, R 1  and R 2  are modulated dynamically, causing the standing wave pattern to rapidly shift back and forth along the Y-direction, as indicated by the double arrow. Preferably, the path length difference imparted to light rays R 1  and R 2  is greater than half the wavelength of the light rays. 
     Now, the illumination at output end  56  of light tunnel  30  is the time-integrated sum of the standing waves. By displacing walls  36  and  40  sufficiently fast (i.e., in a time much less than one temporal pulse length from laser light source  16 ) and with sufficient amplitude (e.g., &gt;1 micron), it is possible to entirely wash out standing wave pattern P s . As a result, the interference fringes commonly seen with a coherent source (such as a laser) can be significantly reduced or eliminated. For example, for laser light source  16  having a temporal pulse length of about 100 ns, AO modulator control unit  220  would drive AO modulator  210  at frequencies of 100 MHZ or greater to cause a time-varying deformation in the light tunnel walls  36  and  40  of about 10 to 20 microns in amplitude. 
     In a preferred embodiment of the present invention, detector D is moved across output end  56  to measure the illumination non-uniformity. This information is sent to AO modulator control unit  220  via an electronic signal. The frequency and amplitude of the AO modulator that provides the optimal illumination uniformity can then be determined in a closed loop fashion by measuring the illumination non-uniformity (i.e., irradiance distribution) in real-time and adjusting the frequency and amplitude of the AO modulation via AO modulation control unit  220 . 
     Solid Light Tunnel Embodiment 
     With reference now to FIGS. 3A and 3B, there is shown an illumination uniformizer apparatus  300  similar to apparatus  70  of FIGS. 1A and 1B, except that light tunnel  80  of apparatus  300  is solid and has upper and lower surfaces (i.e., first and second sides)  86  and  90 . An exemplary material for light tunnel  80  is fused quartz. Light tunnel  80  further includes a layer  306  of low-index of refraction (n c ) optical material (i.e., lower than index n 1 , i.e., n c &lt;n 1 ) on at least one of upper and lower surfaces  86  and  90 . Layer  306  is designed to preserve the total internal reflection condition that allows light to travel down light tunnel  80 . An exemplary material for the low-index layer  306  is magnesium fluoride having a refractive index of about 1.38 at visible wavelengths. An AO modulator  310  similar (if not identical) to AO modulator  210  is arranged atop low-index layer(s)  306 . Apparatus  300  further includes an AO modulator control unit  320 , similar (if not identical) to AO modulator control unit  220 . AO modulator  310  is designed to transmit acoustic waves through layer  306  and into light tunnel  80  so as to set up an acoustic standing wave pattern that causes surfaces  86  and  90  to rapidly oscillate in the Y-direction. Though FIGS. 3A and 3B show a single layer  306  and AO modulator  310  on upper surface  86 , apparatus  300  could also include another AO modulator  310  and layer  306  on lower surface  90 . 
     With reference to FIG.  3 A and apparatus  300 , as in the case for apparatus  200 , laser light L from the laser light source is injected into the light tunnel over a range of angles via condenser optical system  24 . Light rays RS and R 1  and R 2  are again shown. Light rays R 1  and R 2  travel down light tunnel  80 , and reflect off surfaces  86  and  90  at positions p 3  and p 4  due to total internal reflection. When AO modulator  310  is inactive, standing wave patten P s  results at output end  98 , in the manner described above. Also as described above, the illumination non-uniformity can be measured at output end  98  by detector D in electronic communication with AO modulation control unit  320 . 
     With reference now to FIG. 3B, AO modulator  310  is activated via an electrical signal from AO modulator control unit  320 , which causes a time-varying displacement (i.e., an oscillation) of upper and lower surfaces  86  and  90  of light tunnel  80  in the Y-direction. As a result, at a given instant in time, light rays R 1  and R 2  reflect off surfaces  86  and  90  at new positions p 3 ′ and p 4 ′ displaced from positions p 3  and p 4 . This shift imparts a path length difference between interfering rays RS, R 1  and R 2 . Preferably, the path length difference imparted to light rays R 1  and R 2  is greater than half the wavelength of the light rays. 
     Accordingly, standing wave pattern P s  that results from the interference of the straight-through ray and light rays R 1  and R 2  shifts (oscillates) in the Y-direction, as indicated by the double arrow. As in the case of apparatus  200 , the illumination at output end  98  of light tunnel  80  is the time-integrated sum of all the standing waves caused by the interference of the various light rays traveling through the light tunnel. Only three light rays (RS, R 1  and R 2 ) have been used here for the sake of illustration. By moving surfaces  86  and  90  sufficiently fast (i.e., in a time much less than the temporal pulse length) and with sufficient amplitude (e.g. a few to tens of microns), it is possible to significantly reduce or entirely wash out standing wave pattern P s . As a result, the interference fringes commonly seen with a coherent source (such as a laser) can be reduced or eliminated. 
     As discussed above in connection with apparatus  200 , in a preferred embodiment of the present invention, movable detector D is moved across output end  98  to measure the light energy and thus the illumination non-uniformity. This information is sent to AO modulator control unit  320  via an electronic signal. The frequency and amplitude of the AO modulator  310  that provides the optimal improvement in illumination uniformity can then be determined in a closed loop fashion by measuring the illumination non-uniformity (i.e., light energy distribution) in real-time with detector D while adjusting the frequency and amplitude of the AO modulator  310  with AO modulation control unit  320 . 
     As with apparatus  200 , in apparatus  300 , for laser light source  16  having a temporal pulse length of about 100 ns, AO modulator control unit  320  drives AO modulator  310  at frequencies of greater that 100 MHZ to cause a time-varying deformation in the light tunnel walls  86  and  90  of about 10 to 20 microns in amplitude. 
     Either apparatus  200  or  300  above, the AO modulator can move just one of walls  36  and  40  of hollow light tunnel  30 , or just one of surfaces  86  and  90  of solid light tunnel  80 . Alternatively the walls and surface can be moved synchronously or asynchronously. In addition, the walls or surfaces can be made to change shape. This is determined by the manner in which acousto-optic modulator(s)  210  or  310  are interfaced with the walls of the hollow light tunnel or the surfaces of the solid light tunnel. 
     For optimum effect, the frequency f of the acoustic modulation is preferably greater than the inverse of the temporal pulse length T (i.e., f&gt;T −1 ). Thus, as mentioned above, for a 10-nsec pulse, an acousto-optic modulator frequency greater than about 100-Mhz is preferable. In addition, the walls of the hollow or the surfaces of the solid light tunnel need be displaced by a distance large enough to cause a path length difference between the intersecting rays of approximately (or greater than) one-half of a wavelength. As an example, for a light tunnel having an axial length of approximately 300 mm and a width of 3 mm, the walls of the hollow light tunnel or the surfaces of the solid light tunnel need to be displaced by approximately 25 microns for light having a wavelength of 500 nm. 
     The above embodiments are described in two-dimensions for ease of illustration. It will be apparent to one skilled in the art that the present invention is generally applicable to solid and hollow light tunnels having any reasonable number of sides. For example, for a light tunnel having four sides and thus a rectangular cross-section, all four walls or surfaces can be driven with separate AO modulators at separate frequencies. More generally, for a light tunnel having a polygonal cross-section, each surface of the polygon can be driven at its own frequency and amplitude via separate AO modulator control units  220  or  320 . 
     Moreover, although the present invention has been described in the context of a coherent laser light source, the present invention may also be used with an incoherent light source, to the extent that it is capable of smoothing out illumination non-uniformities arising from effects other than the coherence of the light. 
     While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.