LASER BEAM PULSE FORMATTING METHOD
BACKGROUND OF THE INVENTION
The invention described herein arose in the course of, 5 or under, contract No. W-7405-ENG-48 and contract No. DE-AC05-840R21400 awarded by the U.S. Department of Energy.
The present invention relates generally to lasers, and more particularly to an improved method for formatting the output of pulse lasers.
New applications for laser technology are being developed at a prodigious rate. Some of the applications require the formatting of the laser output to meet specific wave shape and/or pulse duration requirements. There are many applications in which it is useful to stretch and temporarily format the pulse shape of a pulsed laser beam. One such application is in the field of X-Ray lithography. Pulse formatting techniques that 2Q can endure the thermal loading of a high average power laser beam are especially useful.
It is well known that the pulse duration of pulse laser devices may be modulated by a number of well know internal means. However, the characteristics of laser 25 pulses often are controlled externally, rather than by modulating laser excitation, or the like. Conceptually, the simplest modulation is by blocking the beam at some times, which can be done in various ways. Of course, mechanical block means is limited to relatively low 30 frequency modulation of the pulse. Further, it is frequently desirable to extend the duration of a laser pulse beyond the practical limits which may be accomplished by means of manipulation of the parameters of laser modulation. It may also be desirable to modulate a laser 35 pulse into divisional units other than simple pulses, such as a complex wave shape.
An additional problem is posed when the laser pulse to be modulated is of a high average power level. In such instances, considerable heat may be created, and 40 many, otherwise viable, means for modulating pulse beams are unable to withstand the thermal loading of such an application.
No prior art laser pulse modulation device, to the inventors' knowledge, can successfully extend the over- 45 all duration of a succession of high power laser pulses while modulating them into divisional pulses of a controlled duration. All previous means for modulating laser pulses have either, been incapable of very high frequency modulation, or else have been unable to with- 50 stand the abuses of high power laser pulse modulation, or else have not extended the overall duration of a pulse significantly, or else have not modulated a pulse according to a specified time constant into a controlled waveshape. 55
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and means for extending the overall duration of a laser pulse. 60
It is another object of the present invention to provide a method and means for modulating a laser pulse according to specific time parameters.
It is still another object of the present invention to provide a method and means for modulating a laser 65 pulse into a series of divisional complex waves.
It is yet another object of the present invention to provide a method and means for modulating and ex
tending a laser beam pulse which can withstand usage with high power lasers.
It is still another object of the present invention to provide a method and means for modulating a laser pulse which is reliable in operation.
It is yet another object of the present invention to provide a method and means for controlling the waveshape and duration of a laser pulse.
Briefly, the preferred embodiment of the present invention is an array of time delay loops, with each loop in the array consisting of a partially reflecting beam splitter and a plurality of highly reflective mirrors. The mirrors of each loop are arranged so as to accept a pulse introduced through the beam splitter and to cycle the pulse within the loop with each cycle being of a duration determined by the total length of the loop. At the end of each cycle, a portion of the pulse is emitted from the loop through the partially reflecting beam splitter, forming a divisional wave portion within the pulse. A plurality of loops, each having either similar or different time constants, can be arranged so as to modulate the laser pulse into complex wave shape pulse portions using additive waveform synthesis.
An advantage of the present invention is that a laser pulse may be extended in overall duration.
A further advantage of the present invention is that a laser pulse may be modulated at high frequency.
Yet another advantage of the present invention is that a laser pulse may be modulated according to set time parameters.
Still another advantage of the present invention is that complex wave shapes can be produced.
Yet another advantage of the present invention is that high power laser pulses can be modulated.
These and other objects and advantages of the present invention will become clear to those skilled in the art in view of the description of the best presently known mode of carrying out the invention and the industrial applicability of the preferred embodiment, as described herein and as illustrated in the several figures of the drawing.
DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic representation of a delay loop, according to the present invention;
FIG. 2 is a trace diagram representing a typical laser beam pulse waveform;
FIG. 3 is a trace diagram representing the laser pulse waveform of FIG. 2, as modified according to the present inventive method;
FIG. 4 is an example of a complex array of the inventive delay loops;
FIG. 5 is a trace diagram illustrating a simple additive waveform synthesis , according to the present method;
FIG. 6 is a complex array of the inventive display loops for producing the complex waveform of FIG. 5;
FIG. 7 is an alternative complex array of the inventive display loops; and
FIG. 8 is yet another alternative complex array of the inventive display loops.
DETAILED DESCRIPTION OF THE
INVENTION
The best presently known mode for carrying out the invention is a combination of time delay loops for modifying laser pulses according to the inventive method. The predominant expected usage of the inventive laser beam pulse formatting method is in modifying high
power laser beam pulses for scientific and industrial applications. A delay loop 10, as used to accomplish the present inventive method, is illustrated in the diagram of FIG. 1, and is designated therein by the general reference character 10. The delay loop 10 has a partially reflective beam splitter 12 and a plurality (three, in the present example) of highly reflective mirrors 14. The best presently known embodiment 10 of the present invention has a first highly reflective mirror 14a, a second highly reflective mirror 146 and a third highly reflective mirror 14c arranged such that a laser beam pulse 20 enters and exits the delay loop 10 through the beam splitter 12, as is depicted in FIG. 1. It should be noted that, in the view of FIG. 1, the laser beam pulse 20 is depicted as a line which defines the entire path of the laser beam pulse 20 as the laser beam pulse 20 traverses the inventive delay loop 10. One skilled in the art will recognize that, in practice, and at any particular point in time, the laser beam pulse may, or may not, be found instantaneously along the entire length of the path shown in FIG. 1 as the laser beam pulse 20. The actual length of the laser beam pulse 20 in relation to the depiction thereof in FIG. 1 will vary according to the duration of the pulse, and the size of the delay loop 10, which will be discussed in more detail, hereinafter.
The beam splitter 12 is a known optical element which has a mirrored side 22 that is, intentionally, imperfectly reflective, such that the laser beam pulse 20, upon striking the mirrored side 22 of the beam splitter 12 will, in part, pass therethrough and, in part, be reflected therefrom. In the best presently known embodiment 10 of the present invention the mirrored side 22 of the beam splitter 12 faces generally toward the second highly reflective mirror 146, as is depicted in the view of FIG. 1, although this is not necessary to the practice of the invention.
A variable regarding the mirrored side 22 of the beam splitter 12 is that the mirrored surface 22 is treated, using known methods, such that the proportion of the laser beam pulse 20 which is passed therethrough is 40 controlled, as compared to that portion of the laser beam pulse 20 which is reflected therefrom. In the best presently known embodiment 10 of the present invention, the mirrored surface 22 passes therethrough approximately one half of the laser beam pulse 20, al- 45 though it is within the scope of the invention that this might be varied according to the needs of the particular application.
One skilled in the art will recognize that the distance between the highly reflective mirrors 14a, 146 and 14c 50 of the delay loop 10 will be the prime factor in determining an amount of time ("t") in which the laser beam pulse 20 makes an entire circuit within the delay loop 10 from the beam splitter 12, to the first highly reflective mirror, 14a, to the second highly reflective mirror 146, to the third highly reflective mirror 14c and then returning to the beam splitter 12. Thus, two primary variables which may be controlled in the manufacture of a particular iteration of the delay loop 10 are the reflectivity ("R") of the beam splitter 12 (as previously discussed, herein) and a delay loop length 24 which, in the case of the best presently known embodiment 10 of the present invention, is the sum of the lengths of a first delay loop leg 26, a second delay loop leg 28, a third delay loop leg 30 and a fourth delay loop leg 32. As can be seen in the view of FIG. 1, the first delay loop leg 26 spans the distance between the beam splitter 12 and the first highly reflective mirror 14a, the second delay loop leg
28 spans the distance between the first highly reflective mirror 14a and the second highly reflective mirror 146, the third delay loop leg 30 spans the distance between the second high reflective mirror 146 and the third highly reflective mirror 14c and the fourth delay loop leg spans the distance between the third highly reflective mirror 14c and the beam splitter 12.
In the example of FIG. 1, the single delay loop 10 is illustrated with the laser beam pulse 20 being emitted initially from a laser source 34, and further with the laser beam pulse 20 being directed into a laser beam detection device 36 as it exits the delay loop 10. In practice, the inventive delay loop is actually used in more complex configurations, as will be described in greater detail, hereinafter.
As can be seen in the view of FIG. 1, the delay loop 10 illustrated herein is generally rectangular in shape, with corners being formed by the beam splitter 12, the first highly reflective mirror 14a, the second highly reflective mirror 146 and the third highly reflective mirror 14c. It should be noted that delay loops of other configurations (not shown) could be constructed using different quantities of highly reflective mirrors 14. For example a triangular delay loop (not shown) could be constructed using three such highly reflective mirrors 14, with proper adjustment of relative angles of the various components of the delay loop 10 and proper positioning of the delay loop 10 in relation to the laser source 34 and the laser beam detection device 36. Indeed, in some applications, such alternative configurations of the inventive delay loop 10 might be preferable.
Now beginning a discussion of the unique properties derived from the configuration of the inventive delay loop 10, it is recognized that causing the laser beam pulse 20 to be delayed by causing it to travel some additional distance between the laser source 34 and the laser beam detection device 36 is not unique to the present invention. However, the inventive delay loop 10 differs in that the unique arrangement of components provides for elongation and/or wave form modification of the laser beam pulse 20, in addition to any desired delay. After the laser beam pulse 20 is admitted into the delay loop 10 through the beam splitter 12 it circulates therein and, with each complete circuit within the delay loop 10, a portion of the laser beam pulse 20 is emitted from the delay loop 10 through the partially reflective beam splitter 12 while the remainder of the laser beam pulse 20 continues to circulate within the delay loop 10. Given the specific construction of the best presently known embodiment 10 of the present invention, as previously discussed herein, approximately one half of any remaining amount of the laser beam pulse 20 that strikes the mirrored side 22 of the beam splitter 12 will pass therethrough, and one half will be reflected therefrom to recirculate within the delay loop 10.
FIG. 2 is a trace diagram representing a typical initial laser beam pulse waveform 38 with deviation along an "X" axis 40 representing time and deviation along a "Y" axis 42 representing instantaneous intensity of the laser beam pulse 20 (FIG. 1). The initial laser beam pulse waveform 38 is the intensity/time plot of an initial portion 44 (FIG. 1) of the laser beam pulse 20, as the laser beam pulse 20 exits the laser source 34 and enters the delay loop 10. Given the example of the initial laser beam pulse waveform 38 of FIG. 2 and the best presently known embodiment 10 of the present invention disclosed in relation to FIG. 1, a resultant stretched waveform 46 is shown in FIG. 3. (While none of the
figures of the drawing are drawn to scale, in order to avoid any possible confusion it should be emphasized that, in order to illustrate the necessary detail, the comparative scale of FIG. 3 is intentionally different from that of FIG. 2, the relative intensities being explained in 5 this text.) The stretched waveform 46 is the intensity/time plot of an exit portion 48 (FIG. 1) of the laser beam pulse 20, as the laser beam pulse 20 exits the delay loop 10, with time and beam intensity plotted along the X axis 40 and the Y axis 42, respectively. 10
A "now" pulse 49, a first sub-pulse 50, a second subpulse 52, a third sub-pulse 54 and a fourth sub-pulse 56 are distributed in time along the X axis 40 of the plot of FIG. 3. A first time reference mark 58 indicates the time at which the initial laser beam pulse waveform 38 (FIG. 15 2) begins, and the "now" pulse 49, which is that portion of the initial pulse 38 which is reflected from the beam splitter 12 (FIG. 1) and which, therefore, does not enter the delay loop 10 (FIG. 1), is essentially concurrent in time (for these practical purposes). A second time refer- 20 ence mark 60 represents the time at which the first sub-pulse 50 begins. The span between the first time reference mark 58 and the second time reference mark 60 represents a time factor ("r") 62 which is the time in which the laser beam pulse 20 travels the delay loop 25 length 24 (FIG. 1) within the delay loop 10. A third time reference mark 64, a fourth time reference mark 66 and a fifth time reference mark 66 represent the origins of the second sub-pulse 52, the third sub-pulse 54 and the fourth sub-pulse 56, respectively. Each of the time 30 reference marks 58, 60, 64 66 and 68 is separated from adjacent such time reference marks 58, 60 64, 66 and 68 by the time factor 62.
Referring again to FIG. 1, as can be understood in light of the previous description of the best presently 35 known embodiment 10 of the present invention, when the laser beam pulse 20 enters the delay loop 10 through the beam splitter 12 it circulates therein. Since, as previously described, the mirrored surface 22 of the best presently known embodiment 10 of the present inven- 40 tion is constructed so as to allow approximately one half of the laser light which strikes it to pass therethrough, when the laser beam pulse 20 first strikes the mirrored surface 22 of the beam splitter 12 from within the delay loop 10, the first sub-pulse pulse 50 (FIG. 3) is emitted 45 from the delay loop 10 through the beam splitter 12, with the first sub-pulse 50 being of approximately one half the intensity of the initial laser beam pulse waveform 38. Thereafter, the portion of the laser beam pulse 20 remaining within the delay loop 10 and the second 50 sub-pulse 52 is emitted when the remaining portion of the laser beam pulse 20 strikes the beam splitter 12. This cycle is repeated creating, in turn, the third sub-pulse 54 and the fourth sub-pulse 56. One skilled in the art will recognize that this cycle will continue past the creation 55 of the fourth sub-pulse 56, and in some applications additional sub-pulses (not shown) of significant intensity will be created.
Thus far, the effects of the delay loop 10, according to the present invention, have been discussed in terms of 60 the effects of the example of the best presently known embodiment 10 of the present invention, standing alone. However, the inventors have found that the greatest utility is derived from using combinations of the delay loops 10 to produce more complex wave forms. FIG. 4 65 is an example of a first complex array 70 of four delay loops 10. In the example of FIG. 4, each of the delay loops 10 is of a different size, and thus has a different
delay loop length 24 (FIG. 1). In addition, although it cannot be seen in the view of FIG. 4, the mirrored surfaces 22 (FIG. 1) of the beam splitters 12 of each of the delay loops 10 may be constructed so as to be either more or less reflective than that described in relation to the mirrored surface 22 (FIG. 1) of the beam splitter 12 of the best presently known embodiment 10 of the present invention. One skilled in the art will recognize that, by varying the delay loop length 24 and the reflectivity of the mirror surface 22 of the beam splitter 12, a great variety of sub-pulses 50, 52, 54 and 56 can be created, varying in both relative intensity and time factor 62. Furthermore, when, as in the example of FIG. 4, the various delay loops 10 are combined to produce the varied sub-pulses 50, 52, 54 and 56 (FIG. 3), the subpulses 50, 52, 54 and 56 will be combined. Such combination adheres to the known principles of additive waveform synthesis, which principles have formerly been applied to electromagnetic wave forms. One familiar with additive waveform synthesis principles will recognize that, although there is similarity between the additive waveform synthesis of electromagnetic waves and this application of the present inventive method, the waveforms created by the present inventive method will differ substantially from those common to electromagnetic wave applications. An example illustrating the creation of a complex wave form 72 is shown in FIG. 5 and FIG. 6. FIG. 6 is a second complex array 74 of the inventive delay loops 10. In the example of FIG. 6, there are two delay loops 10 with a first delay loop 10a having a first delay loop length 24a four times as long as a second delay loop length 24b of a second delay loop 10b. Referring now to FIG. 5, an intermediate wave form 75 is produced by the first delay loop 10a (FIG. 6) and emitted toward the second delay loop 10b, according to the method discussed previously, herein, in relation to FIG. 1. The intermediate waveform 75 has a first intermediate pulse 76, a second intermediate pulse 78, a third intermediate pulse 80 and a fourth intermediate pulse 82. According to the methods previously discussed, herein, a first secondary waveform 84 is produced by the second delay loop 10b (FIG. 6) given an input of the first intermediate pulse 76 Similarly, the second intermediate pulse 78 produces a second secondary waveform 86 within the second delay loop 10b, and the third intermediate pulse 80 produces a third secondary waveform 88 within the second delay loop 10b. For the purposes of the present example, no secondary waveforms are shown resulting from the fourth intermediate pulse 82, since such would be of relatively insignificant magnitude, anyway. Given the above, one familiar with the principles of additive waveform synthesis will recognize that the secondary waveforms 84, 86 and 88 are summed to form the complex wave form 72.
It should be recognized that, in order to best illustrate the principles involved, the simplified example of FIGS. 3 and 4 is provided. For instance, the example uses a first delay loop length 24a which is an exact multiple of the second delay loop length 24b, such that each of the secondary waveforms 84,86 and 88 is perfectly in phase with the others. One skilled in the art will recognize that, were this not the case, the resultant complex wave form 72 would be much more irregular in shape than is shown in the present example of FIG. 5.
As previously stated, the delay loops 10 may be configured in multiples, according to the needs of a particular application, to produce a great variety of complex
|
