System for increasing laser pulse rate with beam splitters

A system of wholly passive or static elements for increasing laser pulse rate by interleaving the pulses of a plurality of beams of pulsed laser radiation. The system of static elements includes an array of beam splitters which are operative to receive a plurality of beams of pulsed laser radiation and provide a plurality of output beams, each sharing the radiation in each of the input beams. The invention includes a methodology for expanding the array of beam splitter elements to combine any number of input beams, of whatever differing characteristics, to provide a corresponding number of output beams, each sharing a portion of the radiation in each of the input beams.

FIELD OF THE INVENTION 
The present invention relates to the combining of beams of radiation. 
BACKGROUND OF THE INVENTION 
In high powered pulsed lasers of the type useful in isotope separation as 
described particularly for uranium enrichment in U.S. Pat. No. 3,772,519, 
it is desired for utmost efficiency to have a repetition rate measured in 
the thousands of pulses per second. Such rates may be difficult to achieve 
with present day laser technology in a single, high powered, pulsed laser, 
and it may be desired to avoid the use of rotating elements for beam 
combining as shown in U.S. Pat. No. 3,924,937. 
BRIEF SUMMARY OF THE INVENTION 
In the present invention, a technique of static elements is disclosed for 
combining a plurality of laser beams having time sequenced, pulsed 
radiation to achieve an augmented pulse rate. The technique may also be 
applied in a system for combining both time sequenced pulses and frequency 
distinct pulses for use in a system for isotope enrichment. 
In the exemplary teaching of the preferred embodiment of the present 
invention, the combining system comprises an array of beam splitter 
elements. The array typically comprises a plurality of sets of beam 
splitters which include a set of input beam splitters and a set of output 
beam splitters and may include additional sets depending upon the number 
of beams to be combined. The input set of beam splitters responds on 
opposite sides of the beam splitting dielectric coating to distinct beams 
of pulsed radiation to transmit and reflect a predetermined percentage, 
typically 50 percent, onto two corresponding output paths, each having 
half of the radiation in each input beam. These output paths are applied 
to further beam splitter elements within the array. The output set also 
consists of a number of individual beam splitters which respond to 
radiation from the array to make the final combination of beams with each 
of the output beam splitter elements providing two output beams, each with 
a portion of the radiation from all of the time sequenced input pulses, 
and thus of augmented pulse rate. 
Such a system may be provided for each of several colors of laser radiation 
in an isotope enrichment plant used for isotopically selective 
photoexcitation and a further combining system is provided for 
superposition of the respective colors, according to the teaching of 
commonly assigned U.S. patent application Ser. No. 660,649, filed Feb. 23, 
1976 Robert S. Congleton, entitled SYSTEM FOR COMBINING LASER BEAMS OF 
DIVERSE FREQUENCIES, to achieve further composite beams which are not only 
augmented in pulse rate, but include various colors of laser radiation 
employed throughout the enrichment system.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention contemplates a system for pulse rate augmenting 
including an array of beam splitter elements responsive to a plurality of 
laser beams of time sequenced, pulsed radiation. The array combines the 
beams into a plurality of output beams, each containing portions of the 
radiation of all of the input beams, thereby providing in each output beam 
a pulse rate higher, by a factor corresponding to the number of input 
beams, than can be achieved with a single laser source. 
The principle of operation of the present invention is illustrated in FIG. 
1 showing a system of four laser sources 12, 14, 16 and 18, which are 
typically monochromatic and limited in frequency and bandwith to provide 
isotopically selective photoexcitation of an isotope constituent of a 
plural isotope environment such as is illustrated in the above-identified 
U.S. Pat. No. 3,772,519 incorporated herein by reference and assigned to 
the same assignee as the present invention. Each laser has a respective 
output beam 20, 22, 24 and 26 which is applied to an array 30 of beam 
splitter elements 32, 34, 36 and 38. A synchronized trigger system 40 
synchronizes each of the lasers 12, 14, 16 and 18 for sequential pulse 
activation in accordance with the timing diagram of FIG. 2, for example. 
In preferred application of the present invention to isotope separation 
the firing sequence provides an interval over which each laser is 
triggered once to provide an output pulse in the respective beams 20-26 
where the pulse is evenly distributed throughout the interval. In this 
fashion four output beams 42, 44, 46 and 48 are provided, each containing 
a portion, in this case 25 percent, of the radiation in each of the input 
beams 20-26 and thereby having a pulse rate, as illustrated in FIG. 2, 
four times that of any individual beam 20-26. 
To achieve this function, the radiation in beams 20 and 22 is applied to 
opposite faces of beam splitter 32. Beam splitter 32, as well as beam 
splitters 34, 36 and 38, is typically 50 percent reflective, 50 percent 
transmissive such that the beam splitter 32 provides two equal output 
beams 50 and 52. These are respectively applied to the beam splitter 
elements 38 and 36, along with output beams 54 and 56 from the beam 
splitter element 34 which receives the input radiation beams 24 and 26. 
The resulting composite of the transmitted and reflected rays from the 
beam splitter 38 provide the output beams 42 and 48, while the output 
beams 44 and 46 are similarly provided from the beam splitter element 36. 
The beam splitter elements 32-38 may employ dielectric coatings to achieve 
the preferred, 50 percent transmission, 50 percent reflection 
characteristic. All such dielectric coatings are well known in the art, a 
typical and exemplary coating will consist of a multitude of alternate 
high and low index of refraction quarter wavelength layers on one surface 
of a substrate and an antireflective coating on the other. 
The laser system of FIG. 1 is shown in FIG. 3 in a modified form using 
mirrors to help to direct the laser beams throughout the array. The 
modified form of FIG. 3 permits the generalization of the pulse rate 
augmenting system of the present invention to a far greater number of 
lasers in an ordered methodology. As shown in FIG. 3, the beam splitters 
32 and 34 form a first set 60 of diagonally positioned beam splitter 
elements, each identified by the letter a, while the beam splitters 36 and 
38 form a second diagonal set 62, each identified by the small letter b. 
Mirrors 64 and 66 are provided to direct radiation from the lasers 12 and 
16 respectively to the beam splitter elements 32 and 34 in the set 60. 
Additional mirrors 68 and 70 are employed to direct the respective beams 
50 and 56 from element 32 to element 38 on the one hand and from element 
34 to 36 on the other hand. Additional mirrors 72 and 74 are employed to 
receive the output beams 48 and 46, respectively, to provide parallel 
outputs. As can be seen in the FIG. 3 embodiment, at each splitting of an 
incident laser beam the power of each component of one of the original 
input beams 20-26 is halved such that the beams 50-56 each possess 
one-half of the original power, those being the beams between the first 
and second sets of beam splitter elements 60 and 62, while the outputs 
from the second set 62 possess one-quarter of the original energy. 
Illustrated in FIG. 4 is a system for augmenting the pulse rate from 16 
pulsed lasers, each providing an input beam of laser radiation labelled 
with the designation 80 or 82. Each input beam 80 or 82 is applied to one 
beam splitter element in a first set 84 of beam splitter elements each 
labelled a. Those beams labelled 80 are first reflected by one of a set of 
mirrors 86 to strike an opposite surface of the elements a. The output 
beams resulting from splitting by the elements in the set 84 are directed 
to respective different beam splitter elements of a second set 88 of beam 
splitter elements, each labelled with a designation b. Two mirrors each 
labelled 90 are employed for the radiation shown emanating outwardly from 
the array from the end elements a in the first set 84 to direct it toward 
the last b member of the set 88 shown elsewhere in the array. Similarly, 
the outputs from each element b, in the second set 88 of elements is 
directed to respective different beam splitter elements, labelled c, in a 
third set 92 of beam splitter elements. The output radiation from the two 
end elements on each side in the set 88 are reflected by mirrors 94 for 
application to three of the beam splitter elements, c, in the set 92. The 
last set 96 of beam splitter elements, d, each receives on opposite 
surfaces output radiation from a different element in the set 88 and for 
this purpose at least one of the inputs for each of the elements in the 
set 96 is reflected by one of the mirrors in a set 98 that reorients the 
radiation from one of the elements in the array of set 92. The outputs 
from the elements, d, in the set 96 provide the system outputs, each 
containing, in the case of 50 percent elements, one-sixteenth of the 
radiation power in each of the inputs. Mirrors 100 may be used for one of 
the outputs of each of the elements, d, in the set 96 to direct them all 
into parallel paths for use throughout a system of isotope enrichment. 
A methadology which may be utilized for combining an indefinite number N of 
different laser inputs is illustrated with respect to FIG. 5. As shown 
there, each of the inputs are assumed to be provided in a parallel fashion 
as may be easily achieved using mirrors where desired. A first set of 
inputs 102 may be applied to a first set 104 of beam splitter elements, 
labelled a, on a first surface thereof and a second set of inputs 106 are 
reflected by a set of mirrors 108 for application to the opposite surfaces 
of the beam splitter elements, a, in the first set 104. The number N will 
be assumed to be one of the numbers defined by the expression 2.sup.n 
where n is an integer although it is apparent that some inputs may be 
lacking from an array of such a number N where desired. In this case, the 
number of mirrors in the set 108 will be one-half the number N and the 
number of beam splitter elements, a, in the set 104 as well as in all 
subsequent sets of beam splitter elements will also be equal to one-half 
N. 
A second set of beam splitter elements 110, labelled b, is placed to 
receive the radiation from each of the radiation output beams of the first 
set 104 of elements a. In addition, since the output beams 112 at each end 
of the first set of elements 104 will diverge from the array, a set of 
mirrors 114 is required to redirect the radiation into the array for the 
last element b in the set 110. The set 110 has N/2 elements in the set. 
The next set 116 of elements c will require the use of twice as many 
mirrors 118 as the previous set and so on until the final output set of 
beam splitter elements in which case it will be necessary to employ as 
many mirrors as there are elements in the final or output set of beam 
splitter elements. 
The system for pulse rate augmenting and power splitting illustrated above 
may be coupled with a system for laser color combining as illustrated in 
FIG. 6 and in United States patent application Ser. No. 660,649, filed 
Feb. 23, 1976, of Robert S. Congleton, for SYSTEM FOR COMBINING LASER 
BEAMS OF DIVERSE FREQUENCIES. As shown in FIG. 6, a system for combining 
four colors from lasers 120, 122, 124 and 126 of different frequencies 
employs a set of four beam splitter elements 128, 130, 132 and 134 in the 
configuration of FIG. 1, to provide four output beams 136, 138, 140 and 
142, all identical where each of the beam splitters 128-134 is a 50 
percent element. 
The system illustrated in FIG. 6 may be combined with the system for pulse 
rate augmentation as illustrated in FIG. 7 for a combined pulse rate 
augmentation and color superposition system. As shown in FIG. 7, a set of 
four pulse rate augmenting systems 146, 148, 150 and 152 are provided, 
each responsive to 16 inputs of laser radiation and, therefore, comprising 
a system as illustrated in FIG. 4. Each of the laser inputs for the system 
146 is provided from pulsed and sequenced lasers 154 and each have the 
same color or frequency, typically tuned for uranium isotopically 
selective photoexcitation. For the pulse rate augmenting system 148, 16 
lasers 156 are provided, each having the same color and time sequenced in 
the occurrence of the radiation pulses, but with the color of the lasers 
156 typically distinct from the color of the lasers 154 and tuned for a 
different excitation or energy step in the process of isotopically 
selective photoexcitation and ionization as generally in the patent above 
cited. Similarly, the systems 150 and 152 receive inputs from the sets of 
16 lasers 158 and 160. Typically, each of the frequencies for the lasers 
154, 156, 158 and 160 will be distinct corresponding to four different 
wavelengths as labelled in the drawing. While the use of four frequencies 
represents a modification of the system shown in the above-identified U.S. 
patents, less than four may also be used. 
Each of the systems 146-152 will provide 16 outputs labelled with the 
designations B1-B16 in the drawing of FIG. 7 and which will be of 
augmented pulse rate and equally divided power from each of the lasers 
154-160. A set of 16 wavelength combining beam splitter systems 162, 164 . 
. . 166 are provided in accordance with the teaching of FIG. 6 and each 
having four inputs as shown there. One output from each of the pulse rate 
augmenting systems 146-152 is applied to one input of each of the 16 color 
combining systems 162-166. The result will be that a total of 64 outputs 
will be provided from the combined color combining systems 162-166, each 
output having an augmented pulse rate and including components of each of 
the colors which may typically be employed for isotope separation. The 64 
outputs may then be employed for one or more isotope separation chambers 
of the type illustrated in the above-identified U.S. Pat. No. 3,772,519 or 
in modified chambers of the design illustrated in U.S. patent application 
Ser. No. 328,954, filed Feb. 2, 1973, now U.S. Pat. No. 3,939,354 
incorporated herein by reference and all assigned to the same assignee as 
the present application. Or several beams may be applied to the same 
module as desired. 
The above-described system is intended to be illustrative only, 
alternatives in the configuration of and design of components being 
intended to lie within the scope of the present invention. Accordingly, it 
is intended to limit the invention only in accordance with its definition 
in the following claims.