Abstract:
A pulse-stretcher has an optical delay loop including a beamsplitter. The beamsplitter divides an input pulse into a temporal sequence of pulse replicas, a first of which is transmitted by the beamsplitter and the remainder of which are reflected by the beamsplitter along the path of the transmitted replica. The sequence of replicas form an initially stretched pulse having a longer duration and lower peak power than the input pulse. A prism cooperative with the delay loop reflects the initially stretched pulse back into the delay loop along a path laterally displaced from the replica path. The beamsplitter divides the initially stretched pulse into a temporal sequence of pulse replicas propagating along a common path to form a finally stretched pulse, having a longer duration and a lower peak power than the initially stretched pulse. The finally stretched pulse has a sequence of power peaks. Peak power in the pulse is minimized when the beamsplitter reflectivity is selected such that the power of the first two of these peaks is equal.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     The present invention relates to optical pulse-stretchers for reducing the peak power of laser pulses while conserving pulse energy. The application relates to such a pulse-stretcher in the form of an imaging delay line.  
       DISCUSSION OF BACKGROUND ART  
       [0002]     In many applications where pulse laser radiation is used, for example, in laser material processing, laser printing, microlithography, and medical and surgical treatment, it is the energy of a pulse that is of interest rather than the peak power within the pulse. Indeed, in several such applications, too high a peak power can cause damage to whatever is being exposed to the radiation pulses, or to optical devices used to deliver the pulses. This can become particularly problematical if a higher pulse-energy would be useful, while a maximum peak power must not be exceeded. Since the duration of the laser pulse in most lasers is fixed, a pulsed laser providing higher energy would automatically increase the pulse peak power.  
         [0003]     By way of example, in UV (ultraviolet) microlithography for semiconductor device manufacture, photo-masks are formed by exposing photoresist to pulsed UV radiation from excimer lasers. The more energy per pulse that an excimer laser can deliver at any given pulse rate, the higher the manufacturing throughput will be. A high, peak pulse-intensity however can rapidly degrade optical elements of a projection system used to expose the photoresist for writing the photomask pattern therein.  
         [0004]     In order to avoid such degradation, it has become common practice in the semiconductor industry to temporally stretch a pulse delivered by an excimer laser before it is delivered to the projection system. The temporal stretching is accomplished in a way that decreases the peak pulse-intensity while conserving, to the maximum extent possible, the energy of the original (un-stretched) pulse.  
         [0005]     A commonly used pulse-stretching arrangement is an optical delay line in the form of a relay-imaging system. Such a system is illustrated schematically in  FIG. 1 . Here, stretcher  10  comprises a beamsplitter  12 , and a delay loop  14 , including a plane mirror  16  and concave mirrors  18 ,  20 , and  22 . In this drawing, the path of light through the delay loop is indicated by a single line.  
         [0006]     Mirrors  22  and  18  preferably each have a focal length f 1  and mirror  20  has a focal length f 2 , where f 1  is about equal to 2 times f 2 . The concave mirrors relay an image of an incoming pulse P 0  at the beamsplitter back onto the beamsplitter replicating the image size, position, pointing and divergence of the original pulse. Those skilled in the optical art will recognize that there is an intermediate focus (not illustrated) between mirrors  18  and  20  and also between mirrors  20  and  22 .  
         [0007]     A portion of pulse P 0  incident on beamsplitter  12  is transmitted by the beamsplitter and does not enter loop  14 . This transmitted portion, designated pulse P 1  in  FIG. 1 , is often referred to as the “prompt” pulse, as this pulse is transmitted without significant delay. Another portion of pulse is reflected by the beamsplitter and follows a path through delay loop  14  being sequentially reflected by mirrors  16 ,  18 ,  20 , and  22  before returning to the beamsplitter.  
         [0008]     Beamsplitter  12  reflects a portion (first replica) of this delayed portion of the pulse along the same path as the first-transmitted portion but delayed by a time τ, which is the round trip time in delay loop  14 . This first replica pulse is designated P 2  in  FIG. 1 . Beamsplitter  12  transmits a portion of this replica pulse which undergoes further delays and division into reflected and transmitted portions. A subsequently-delayed portion has lesser energy than a previously-delayed portion. In theory at least, the number of round trips and replica pulses is infinite. In practice, however, and in particular in a lossy system, the energy of replica pulses becomes vanishingly small after as few as three replica pulses have been produced. This energy diminution depends on the reflectivity of the beamsplitter and loss per round trip in the delay loop. In  FIG. 1 a  total of four pulse portions are shown, being prompt pulse P 1 , and increasingly delayed replicas P 2 , P 3 , and P 4 . These pulse portions or replicas, collectively, form a stretched pulse.  
         [0009]     The optimum reflectivity R of the beam splitter depends on the transmission T of the delay line and can be calculated, approximately, from a formula:  
             S   =         (     1   -   R   +         R   2     ⁢   T       1   -       (     1   -   R     )     ⁢   T           )     2           (     1   -   R     )     2     +         R   4     ⁢     T   2         1   -         (     1   -   R     )     2     ⁢     T   2                       (   1   )             
 
 where S is the stretch factor. However, the formula is only reliable if the prompt pulse P 1  and replicas P 2  through P N  are fully separated in time. For values of T between 90% and 80%, R falls in the range between about 60% and 67%. With this reflectivity, the prompt beam and the first replica have about the same intensity. All following replicas have decreasing intensity. In theory, the optimum reflectivity can be approximated by calculating the derivative (∂S/∂R), equating the derivate to equal zero, separating R and solving the resulting equation. This, however is an extremely daunting task, to say the least. Simpler is to plot equation (1) for any given value of T, and determine the value of R when S reaches a peak, i.e., when ∂S/∂R is zero. 
 
         [0010]      FIG. 2  is a graph schematically depicting computed stretching factor (S) as a function of reflectivity (R) for a range of transmission values from 80% to 100% (with values indicated on the curves). Here, it should be noted that the transmission of the delay loop itself is weakly dependent on the reflectivity of the beamsplitter.  FIG. 3  schematically illustrates computed power as a function of time for a hypothetical “sech-squared” input pulse P 0  and a stretched output-pulse in an example of the prior-art pulse-stretcher of  FIG. 1  having 100% transmission and a beamsplitter reflectivity of about 61.7% The maximum possible stretching factor S is about 3 if transmission is 100%, i.e., if the delay loop is lossless. At excimer laser UV wavelengths, the delay loop, in practice, will almost always have less than 100% transmission. Optimum performance is only obtained if the pulse duration of pulse P 1  is about equal to the delay τ of delay loop  14 . Any further delay does not further significantly reduce the peak intensity of the highest-intensity replica pulse, but merely further separates the replica pulses and creates a discontinuity in the flow of energy to whatever is being irradiated by the pulse replicas. It would be usefully to be able to further reduce peak intensity of a delivered pulse without a significant increase in the number of optical elements beyond that of the above-described pulse-stretcher.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is directed to apparatus for extending the duration and reducing peak power in an optical pulse. In one aspect apparatus in accordance with the present invention comprises an optical delay loop including a beamsplitter. The optical delay loop is in the form of an imaging optical system. The beamsplitter is arranged cooperative with the delay loop to divide the optical pulse into a first temporal sequence of pulse replicas each thereof having about the same beam dimensions, a first of which is transmitted by the beamsplitter and the remainder of which are reflected by the beamsplitter along the path of the transmitted replica. The sequence of pulse replicas propagates along a first common path. An optical arrangement is provided for directing the first sequence of pulse replicas back into the delay loop along a path laterally displaced from the first common path. The beamsplitter divides the firs sequence of pulse replicas into a second temporal sequence of pulse replicas each thereof having about the same beam dimensions and each thereof propagating along a second common path laterally displaced from the first common path to form a finally-stretched pulse. The finally-stretched pulse has a longer duration and a lower peak power than the optical pulse.  
         [0012]     The finally stretched pulse is characterized by a sequence of power peaks spaced apart in time by about the round trip period of the delay loop. Maximum peak power of the finally stretched pulse is minimized when the beamsplitter reflectivity is selected such that the power in the first and second peaks is equalized. In this condition, the power in the first and second peaks is the maximum power in the finally stretched pulse with the third and subsequent peaks having progressively less power. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     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.  
         [0014]      FIG. 1  schematically illustrates a prior art relay-imaging pulse-stretcher having a beamsplitter and a delay loop including one plane and three concave mirrors.  
         [0015]      FIG. 2  is a graph schematically illustrating computed stretching factor as a function of beamsplitter reflectivity for various values of transmission in the pulse-stretcher of  FIG. 1 .  
         [0016]      FIG. 3  is a graph schematically illustrating computed power as a function of time for an input pulse and a stretched pulse in one example of the prior-art pulse-stretcher of  FIG. 1 .  
         [0017]      FIG. 4  schematically illustrates a preferred embodiment of a relay-imaging double-pass pulse-stretcher in accordance with the present invention, including a beamsplitter cooperative with a relay-imaging delay loop and a prism arranged such that a stretched pulse exiting the delay loop after a first pass therethrough is axially displaced and redirected back into the delay loop to be further stretched by a second pass therethrough.  
         [0018]      FIG. 5  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse P 0 , and once-stretched and twice-stretched pulses from respectively first and second passes through one example of the pulse-stretcher of  FIG. 4 .  
         [0019]      FIG. 6  is a graph schematically illustrating computed optimum reflectivity as a function of delay loop transmission for the beamsplitter in the double-pass pulse-stretcher of  FIG. 4 .  
         [0020]      FIG. 7  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse twice-stretched in an example of the inventive pulse-stretcher wherein the beamsplitter has a reflectivity of 61.7%, and for the same pulse twice-stretched in an example of the inventive pulse-stretcher wherein beamsplitter  12  has a reflectivity of about 49.8%.  
         [0021]      FIG. 8  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse once stretched in a prior-art pulse stretcher, and twice-stretched in an example of the inventive pulse-stretcher wherein the beamsplitter reflectivity is specifically optimized to minimize peak power, independent of stretching.  
         [0022]      FIG. 9  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse for the input pulse after being twice-stretched in an example of the inventive pulse-stretcher wherein the beamsplitter reflectivity is specifically optimized to minimize peak power, independent of stretching.  
         [0023]      FIG. 10  schematically illustrates another preferred embodiment of a relay imaging double-pass pulse-stretcher in accordance with the present invention similar to the pulse-stretcher of  FIG. 4  but including two beamsplitters, one of which may have a different reflectivity from that of the other 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 4  schematically illustrates one preferred embodiment  30  of a double pass relay-imaging pulse-stretcher in accordance with the present invention. Pulse-stretcher  30 , in this embodiment, includes a delay loop  14  with mirrors  16 ,  18 ,  20 , and  22  arranged as described above in the pulse-stretcher of  FIG. 1  to form a relay imaging optical system of about unit (about 1:1) magnification. The optical axis of the delay loop is depicted in  FIG. 4  by a long dashed line  32 .  
         [0025]     Pulse-stretcher  30  makes use of the fact that the relay-imaging delay loop is insensitive to the position and pointing of an incident beam. After one round trip in the delay loop, the size, position, pointing, and divergence of the input beam are replicated, provided that the delay loop itself remains properly aligned. Tilting the delay loop or moving the delay loop (pulse-stretcher assembly), within certain limits, will not affect the performance of the delay loop. The limits depend, inter alia, on the optical aperture of the optical elements of the delay loop. As noted above, mirrors  22  and  18  preferably each have a focal length f 1 , and mirror  20  preferably has a focal length f 2 , where f 1  is about equal to 2 times f 2 . This being the case, mirrors  20  and  22  and mirrors  18  and  20  are preferably separated by a distance f 1 +2f 2 , and mirrors  18  and  22  are axially separated by a distance 2f 1 +f 1   2 /f 2 .  
         [0026]     In the inventive pulse-stretcher, an input beam (pulse), depicted in  FIG. 1  by a solid line, with the propagation direction of the beam indicated by single arrowheads, is directed into the delay loop, through beamsplitter  12 , displaced by a distance D from optical axis  32 . After a first pass through the beamsplitter, the input pulse replicas (not shown) exit the delay loop in the same direction, with the same displacement from the axis, on the same side of the axis, and with the same dimensions (beam cross-section).  
         [0027]     The replica pulses enter a prism  34  and emerge from the prism, after successive reflections from faces  36  and  38  thereof, displaced on the opposite side of the axis from the input put beam and propagating in the opposite direction to the original input beam. Prism  34  of course may be replaced with separate mirrors fulfilling the function of internal reflective surfaces  36  and  38  of the prism. The displaced and direction-reversed beam is depicted as a short-dashed line to assist in tracing the path of the beam through the delay loop. The propagation direction is indicated by double arrowheads.  
         [0028]     It can be considered (initially at least) that a stretched pulse of a temporal form similar to that depicted in  FIG. 2  for a prior-art pulse provides a new input pulse for a second pass through delay loop  14 . In that second pass, the once-stretched pulse is divided into a temporal sequence of replicas by the beamsplitter and the delay loop to provide a twice-stretched pulse (Output) having a longer duration and lower peak intensity than the once-stretched pulse. The twice-stretched pulse is intercepted by a mirror  39  and directed away from the optical axis.  
         [0029]     If pulse P 0  has a duration of about 24 nanoseconds (ns), which is a typical duration for excimer laser pulses, delay loop  14  will preferably have a round trip optical path of about 7.2 meters. Focal lengths f 1  and will be about 900 millimeters (mm) and 450 mm respectively. The apertures of the mirrors depend on the size of the beam and the allowed movement and tilt range of the assembly. Preferably, the apertures should be between about 3 and 4 times the beam size. By way of example for an input beam size of 3 mm×12 mm, 50 mm optics are preferred. In such an arrangement, a displacement D from the optical axis of up to about 5 mm to 10 mm is possible, while still having the delay loop function, optically, as desired. Entrance and exit beams need neither be symmetrically disposed about axis  32 , nor parallel to the axis as depicted in  FIG. 4 . The depicted arrangement, however, will provide the maximum possible lateral separation of the input and output beams consistent with proper operation of the delay loop.  
         [0030]      FIG. 5  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse P 0 , and the once-stretched and twice-stretched pulses from respectively first and second passes through pulse-stretcher  30  of  FIG. 4 . Here again, it is assumed that the stretcher has 100% transmission. Beamsplitter  12  is assumed to have a reflectivity of about 61.7%, which is about the optimum reflectivity according to prior-art assumptions. It is also assumed that the round trip delay in the delay loop is about equal to the pulse duration of the input pulse P 0 . Time is shown in arbitrary units to facilitate comparisons. The present invention is not limited to stretching pulses of any particular duration. One skilled in the art will be able to factor the graphs to determining the duration of stretched pulses generated from any assumed input-pulse duration.  
         [0031]     It can be seen that the twice-stretched pulse is stretched by comparison with the once-stretched pulse, but by a lesser factor than the once-stretched pulse is stretched by comparison with the input pulse P 0 . In the example of  FIG. 5 , the stretching factor on the first pass is almost 3.0 while the stretching factor on the second pass is between about 1.4 and 1.5. It can also be seen, however, that the peak power in the twice-stretched pulse is only reduced by about 20% compared with that of the once-stretched pulse. The reason for this is that, as the power-peaks in the once-stretched pulse are spaced apart in time essentially by the round trip delay time of delay loop  14 , the first peak of the first replica of the once-stretched pulse reinforces the second peak of the prompt portion of that pulse that is transmitted by beamsplitter  12 . This will occur whatever the delay time of delay loop  14 .  
         [0032]     In an attempt to make the inventive double-pass pulse-stretcher more effective in reducing peak power in a twice-stretched pulse, an investigation was carried out to determine if this could be accomplished by finding a reflectivity for beamsplitter  12  that is different from that indicated by prior-art teachings for a prior-art, single-pass pulse-stretcher. It was determined that if a reflectivity was selected that would make the first and second power peaks of a twice-stretched pulse about equal, that condition would provide the lowest peak power achievable in the twice-stretched pulse for any selected delay-time of the delay loop.  
         [0033]      FIG. 6  is a graph schematically illustrating computed optimum reflectivity as a function of transmission (solid curve) for beamsplitter  12  in the inventive double-pass pulse-stretcher of  FIG. 4  for a practical range of transmission values from about 75% to 100%. It can be seen that the (double-pass) optimum reflectivity for the inventive double-pass stretcher lies between about 55% and 48%. For a prior-art single-pass stretcher, the optimum reflectivity lies between about 67% and 60% for the same transmission range (see  FIG. 2 ).  
         [0034]      FIG. 7  is a graph schematically illustrating computed power as a function of time for a hypothetical, sech-squared input-pulse, twice stretched in an example of the inventive pulse-stretcher wherein beamsplitter  12  is assumed to have a reflectivity of about 61.7% (single weight curve), and twice stretched in an example of the inventive pulse-stretcher wherein beamsplitter  12  is assumed to have a reflectivity of about 49.8% (double weight curve). It is assumed, in each example, that the stretcher has 100% transmission, and that the round trip delay in delay loop  14  is about equal to the pulse duration of the input-pulse P 0 . The twice-stretched pulse is characterized by a sequence of power peaks spaced apart in time by about the round trip period of the delay loop.  
         [0035]     It can be seen from  FIG. 7  that the lower reflectivity of beamsplitter  12  provides a reduction of about 15% in the peak intensity of the twice-stretched pulse compared with the example wherein the beamsplitter has the prior-art-indicated reflectivity of 61.7 percent, and that, in general, there is a better distribution of energy between the first three peaks of the twice stretched pulse. In this “optimized” condition, the power in the first and second peaks is the maximum power in the twice-stretched pulse with the third and subsequent peaks having progressively less power. The actual temporal stretching is about the same in each example. It should be noted that as the transmission of the delay loop decreases, the reduction in power due to the new optimum reflectivity will be less, but still significant, for example, between about 10% and 12% for a transmission of about 80.  
         [0036]      FIG. 8  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse once stretched in an example of the prior-art pulse stretcher of  FIG. 1  (wherein the beamsplitter reflectivity is about 61.7%), and twice-stretched in an example of the inventive pulse-stretcher wherein the beamsplitter reflectivity is specifically optimized (here, at 49.8%) to minimize peak power after a double pass, regardless of stretching factor. Transmission is assumed to be 100%. The peak power after twice stretching a pulse in the inventive double-pass stretcher is reduced to about 64% of what the power would be after passing through a prior-art pulse-stretcher. A reduction to only about 77% of the single-pass value is obtained if prior-art guidelines are followed for determining beamsplitter reflectivity.  
         [0037]      FIG. 9  is a graph schematically illustrating computed power as a function of time for a hypothetical sech-squared input pulse P 0  (fine curve) for the input pulse after being twice-stretched in an example of the inventive pulse-stretcher (bold curve) wherein the beamsplitter reflectivity is specifically optimized to minimize peak power, independent of stretching. Transmission of the delay loop, here, is assumed to be 100%. It can be seen that the peak power in the twice-stretched (output) pulse is about 26% of the peak power of input pulse P 0 . For a loop having less than 100% transmission the peak power in the output pulse would be less than 26% of the input pulse by an amount dependent on the transmission of the loop.  
         [0038]      FIG. 10  schematically illustrates another preferred embodiment  40  of a double-pass, relay-imaging pulse-stretcher in accordance with the present invention. Pulse-stretcher  40  is similar to pulse-stretcher  30  of  FIG. 4 , with an exception that beamsplitter  12  of pulse-stretcher  30  is replaced in pulse-stretcher  40  by two different beamsplitters  12 A and  12 B on opposite sides of the optical axis of delay loop  14 . These two beamsplitters, while depicted as being on separate substrates in  FIG. 8 , may simply be zones of different reflectivity on a common substrate. Beamsplitter  12 A controls energy distribution in the once-stretched pulse, and beamsplitter  12 B controls energy distribution in the twice stretched pulse.  
         [0039]     Providing two separate beamsplitters means that one of the beamsplitters can have a different reflectivity from the other. For any given round trip loss value of the delay loop it is possible to find two different reflectivity values that will provide equal power in the first two peaks of a twice-stretched pulse. However, the power in the first two peaks is only minimized when the reflectivity of each beamsplitter is equal to the optimum value for a single beamsplitter.  
         [0040]     Those skilled in the optical art will recognize that the relay-imaging arrangement of delay loop  14  in above described embodiments of the present invention is not the only optical arrangement that will provide 1:1 imaging with preservation of beam pointing and position, and that any other such arrangement may be used without departing from the spirit and scope of the present invention. Those skilled in the art will also recognize, however, that such an arrangement will include at least two optical elements having positive optical power, an optical element, here, referring to a mirror or a lens. By way of example, a delay line may be used that replicates the beam with an inverted image. An inverted image does not present a significant problem for a beam that is symmetrical in vertical and horizontal axes. Such a delay line would require only two curved mirrors (or two lenses) with the focal lengths of the mirrors being equal. It is difficult, however, to use such an imaging arrangement in a four mirror loop, since there will be a flat mirror between the two curved mirrors, on which there is a focus. This can be avoided by using a loop with more than 4 mirrors. This becomes of interest if the delay line loop has to be several meters long or folded more than four times to reduce the physical space of the delay loop. At ultraviolet wavelengths, where some optical loss in components of the delay loop is essentially inevitable, more than four mirrors in a delay loop could significantly reduce transmission of the delay loop.  
         [0041]     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.