Abstract:
An optical amplification apparatus includes a polarizing beam splitter for reflecting a portion of an incident light and transmitting a remaining portion of the incident light, depending upon a polarized state of the incident light, at least two optical amplification means each including a first polarizing plate which makes polarized states of the light before and after the light reflected from the polarizing beam splitter reciprocatingly passes through the first polarizing plate, to be orthogonal to each other, an amplitude division plate for amplitude-dividing the light having passed through the first polarizing plate, into first and second lights, and optical amplifiers for respectively amplifying the first and second lights which are amplitude-divided by the amplitude division plate. The optical amplification means are located such that the light output from upstream optical amplification means is incident upon the polarizing beam splitter included in downstream optical amplification means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to an optical amplification apparatus and, more particularly, to an optical amplification apparatus which employs a plurality of optical amplification stages. 
     2. Description of the Prior Art 
     In general, a stimulated brillouin scattering-phase conjugate mirror (SBS-PCM) reflects a phase conjugated wave to compensate for the distortion of a laser beam which occurs in a laser amplification procedure. Further, since the SBS-PCM ensures easy arrangement of an optical system, the SBS-PCM is used for a light splitting laser having a high output. 
     An optical amplification system employing such an SBS-PCM is disclosed in Korean Patent No. 318520. 
     Hereafter, a conventional optical amplification system will be described with reference to  FIG. 1   a.    
       FIG. 1   a  illustrates a conventional optical amplification apparatus. 
     Referring to  FIG. 1   a , a light  505 , which is output from a laser oscillator  500 , is amplified into a gradually increasing number of laser lights while passing through a first optical amplification stage  510 , a second optical amplification stage  540 , and a third optical amplification stage  570 . In detail, the light  505  is reflected by a polarizing beam splitter (PBS), is incident upon a first optical isolator  520 , is reflected by an SBS-PCM, is transmitted through the PBS, and is incident upon a first optical amplifier  530 . Then, the light which is incident upon the first optical amplifier  530  is amplified while being transmitted through the amplifier, is reflected by an SBS-PCM, and is incident upon the PBS. The light which is incident upon the PBS is reflected and output toward a beam expander (BE)  535 . The beam expander  535  enlarges the size of the incident light and outputs the incident light to the second optical amplification stage  540 . The second optical amplification stage  540  comprises a second optical isolator  550  which is constructed in the same manner as the first optical isolator  520  of the first optical amplification stage  510  to perform the same function. The second optical amplification stage  540  additionally includes a wavefront division type beam splitter  562  positioned at the front end of a second optical amplifier  560  which forms 2×2 arrays. Here, the wavefront division type beam splitter  562  is used to deliver lights to the respective 2×2 arrays of the second optical amplifier  560 . That is to say, before the light is incident upon the second optical amplifier  560 , the light is wavefront-divided by the wavefront division type beam splitter  562 . These lights are amplified by respective amplifiers arranged along optical axes, are reflected by SBS-PCMs, are re-coupled with each other, and are incident upon a PBS. The PBS reflects and outputs the incident light to a beam expander  565 . The beam expander  565  enlarges the size of the incident light and outputs the enlarged incident light to the third optical amplification stage  570 . In the third optical amplification stage  570 , a third optical amplifier  590  forms 4×4 arrays, and a third optical isolator  580  forms 2×2 arrays. Here, wavefront division type beam splitters  582  and  592  are used to deliver lights to the respective arrays. 
     Due to the fact that the optical amplification system is constructed in this way, by adding one or more optical amplification stages as desired, output energy can be increased and repeatability can be maintained as it is without causing damage to an optical system. In this regard, when the energy density of a laser light increases due to continuous amplification, if the energy density is not prevented from increasing, damage to the optical system and a laser gain medium is caused. Thus, when it is necessary to enlarge the size of the laser light, the size of the laser gain medium must simultaneously be enlarged. This results in retardation of the cooling velocity of the laser gain medium, and actually makes it impossible to create a laser light having high repeatability. In this consideration, the optical amplification apparatus capable of re-coupling lights shown in  FIG. 1   a , which can increase the size of the laser light and maintain the size of the gain medium, is used. 
     The beam splitter employed in the optical amplification apparatus capable of re-coupling the lights as shown in  FIG. 1   a  comprises a wavefront division type beam splitter which is shown in  FIG. 1   b.    
     Referring to  FIG. 1   b , the wavefront division type beam splitter divides an incident light ‘a’ into two output lights ‘b’. Namely, in the wavefront division type beam splitter, a laser light is divided into two small-sized laser lights. 
     In addition to the wavefront division type beam splitter, an amplitude division type beam splitter has also been disclosed in the art, which is shown in  FIG. 1   c.    
     Referring to  FIG. 1   c , the amplitude division type beam splitter divides an incident light ‘a’ into two output lights ‘b’ in a manner such that one output light is transmitted through the beam splitter and the other output light is reflected by the beam splitter. That is to say, in the amplitude division type beam splitter, the size of a laser light is not changed and only energy is shared by the two output lights. In comparison with this amplitude division type, in the wavefront division type which is adopted by the conventional optical amplification apparatus, since the shape of a laser light cannot be the same as that of a light from a main resonator, difficulties are caused in that a cross-section of a gain medium of an amplifier must be formed in the shape of a laser light. The reason for this is that, when a cross-sectional shape of the gain medium is different from the shape of the laser light, amplification efficiency decreases. Further, since a spatial frequency is included in the spatial distribution of a laser light, when reflecting the laser light using an SBS-PCM, breakage of a phase conjugate phenomenon results in. Also, when re-coupling lights after amplification, in the case that phases of two lights at a position where one light meets the other light are different from each other, an intensity spike occurs, which deteriorates the spatial distribution of the lights. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an optical amplification apparatus which adopts an amplitude division type beam splitter to avoid restriction in a type of a gain medium, breakage of a phase conjugate phenomenon, occurrence of an intensity spike upon re-coupling of lights, and so forth, and which employs a beam expander so that, before incidence of a light upon an amplifier, the size of the light can be decreased to the size of the gain medium, and, after optical amplification of the light, the size of the light can be enlarged using the beam expander to reduce a laser energy density, thereby accomplishing high repeatability and a high output without experiencing a limit in energy amplification. 
     In order to achieve the above object, according to one aspect of the present invention, there is provided an optical amplification apparatus comprising a polarizing beam splitter for reflecting a portion of an incident light and transmitting a remaining portion of the incident light, depending upon a polarized state of the incident light; and at least two optical amplification means each including a first polarizing plate which makes polarized states of the light before and after the light reflected from the polarizing beam splitter reciprocatingly passes through the first polarizing plate, to be orthogonal to each other, an amplitude division plate for amplitude-dividing the light having passed through the first polarizing plate, into first and second lights, and optical amplifiers for respectively amplifying the first and second lights which are amplitude-divided by the amplitude division plate, wherein the at least two optical amplification means are located such that the light outputted from upstream optical amplification means is incident upon the polarizing beam splitter included in downstream optical amplification means. 
     According to another aspect of the present invention, the first polarizing plate is a quarter wavelength plate. 
     According to another aspect of the present invention, the amplitude division plate is a polarizing beam splitter. 
     According to another aspect of the present invention, the polarizing beam splitter divides the light which is incident from the quarter wavelength plate, into laser lights having P-polarized light and S-polarized light. 
     According to another aspect of the present invention, the optical amplifier includes an optical amplification plate for optical amplification, and Faraday rotators are arranged in front of and behind the optical amplification plate to offset birefringence of the optical amplification plate. 
     According to another aspect of the present invention, there is provided an optical amplification apparatus including a polarizing beam splitter for reflecting a portion of an incident light and transmitting a remaining portion of the incident light, depending upon a polarized state of the incident light, the apparatus comprising a first polarizing plate which makes polarized states of the light before and after the light reflected from the polarizing beam splitter reciprocatingly passes through the first polarizing plate, to be orthogonal to each other; an amplitude division plate for amplitude-dividing the light having passed through the first polarizing plate, into first and second lights; and optical amplifiers for respectively amplifying the first and second lights which are amplitude-divided by the amplitude division plate. 
     According to another aspect of the present invention, the first polarizing plate is a quarter wavelength plate. 
     According to another aspect of the present invention, the amplitude division plate is a polarizing beam splitter. 
     According to another aspect of the present invention, the polarizing beam splitter divides the light which is incident from the quarter wavelength plate, into laser lights having P-polarized light and S-polarized light. 
     According to another aspect of the present invention, the optical amplifier includes an optical amplification plate for optical amplification, and Faraday rotators are arranged in front of and behind the optical amplification plate to offset birefringence of the optical amplification plate. 
     According to another aspect of the present invention, there is provided an optical amplification apparatus comprising a polarizing beam splitter for reflecting a portion of an incident light and transmitting a remaining portion of the incident light, depending upon a polarized state of the incident light; an optical isolator including a first polarizing plate which makes polarized states of the light before and after the light reflected from the polarizing beam splitter reciprocatingly passes through the first polarizing plate, to be orthogonal to each other, and a stimulated brillouin scattering-phase conjugate mirror for reflecting the light having passed through the first polarizing plate; and at least two optical amplification means each including a second polarizing plate which makes polarized states of the light before and after the light transmitted through the polarizing beam splitter by the first polarizing plate reciprocatingly passes through the second polarizing plate, to be orthogonal to each other, an amplitude division plate for amplitude-dividing the light having passed through the second polarizing plate, into first and second lights, and optical amplifiers including stimulated brillouin scattering-phase conjugate mirrors for respectively amplifying and reflecting the first and second lights which are amplitude-divided by the amplitude division plate, wherein the at least two optical amplification means are arranged in the shape of a chain such that the light outputted from upstream optical amplification means is incident upon the polarizing beam splitter included in downstream optical amplification means. 
     According to another aspect of the present invention, the optical amplification means further includes a pre-pulser located in front of the stimulated brillouin scattering-phase conjugate mirror, for preventing a front end of a main pulse which is incident upon the stimulated brillouin scattering-phase conjugate mirror, from being cut. 
     According to another aspect of the present invention, the first polarizing plate is a quarter wavelength plate. 
     According to another aspect of the present invention, the amplitude division plate is a polarizing beam splitter. 
     According to another aspect of the present invention, the polarizing beam splitter divides the light which is incident from the quarter wavelength plate, into laser lights having P-polarized light and S-polarized light. 
     According to still another aspect of the present invention, the optical amplifier includes an optical amplification plate for optical amplification, and Faraday rotators are arranged in front of and behind the optical amplification plate to offset birefringence of the optical amplification plate. 
     According to yet another aspect of the present invention, the optical amplifier includes a phase locker for fixing a phase of a reflected wave to the stimulated brillouin scattering-phase conjugate mirror. 
     According to yet still another aspect of the present invention, the optical amplifier includes a sonic wave generator attached to the stimulated brillouin scattering-phase conjugate mirror and controls the phase of the reflected wave based on a generated sonic wave. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1   a  is a view illustrating a conventional wavefront division type optical amplification apparatus; 
         FIGS. 1   b  and  1   c  are views comparing a wavefront division type and an amplitude division type; 
         FIG. 2  is a view illustrating an optical amplification apparatus having a plurality of optical amplification stages in accordance with an embodiment of the present invention; 
         FIG. 3  is a view illustrating an amplitude division type optical amplification stage in accordance with another embodiment of the present invention; 
         FIG. 4  is a view illustrating an optical amplification apparatus having a plurality of optical amplification stages, one of which is shown in  FIG. 3 ; 
         FIG. 5  is a view illustrating a pre-pulser shown in  FIG. 4 ; and 
         FIG. 6  is a view illustrating another example of phase control by the SBS-PCM shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
       FIG. 2  is a view illustrating an optical amplification apparatus having a plurality of optical amplification stages in accordance with an embodiment of the present invention. 
     Referring to  FIG. 2 , the optical amplification apparatus comprises a laser oscillator  100  for generating a laser light, a beam expander (BE)  101  for enlarging a size of the laser light, a polarizing beam splitter (PBS)  102  for reflecting the enlarged light, a first optical amplification stage  200  for amplifying the reflected light, a BE  103  for enlarging a size of the light amplified by the first optical amplification stage  200 , a second optical amplification stage  300  for amplifying once more the light amplified by the first optical amplification stage  200 , and a BE  104  for enlarging a size of the light amplified by the second optical amplification stage  300 . The light amplified by the second optical amplification stage  300  is output to a third amplification stage (not shown). 
     In the first optical amplification stage  200 , devices for amplification of the light, such as a quarter wavelength plate  201 , PBSs  202 , BEs  203 , Faraday rotators (FRs)  204 , amplifiers  205 , FRs  206 , and a phase locker (PL)  207  are located along an optical path. Also, in the second optical amplification stage  300 , devices for amplification of the light, such as a PBS  301 , a quarter wavelength plate  302 , PBSs  303 , 45° rotators  304 , PBSs  305 , BEs  306 , FRs  307 , amplifiers  308 , FRs  309 , and a PL  310  are located along an optical path. 
     The laser light from the laser oscillator  100  which has S-polarization is enlarged in size by the BE  101  and is reflected by the PBS  102  to be incident upon the first optical amplification stage  200 . The light which is incident upon the first optical amplification stage  200  becomes a circularly polarized light after passing through the quarter wavelength plate  201 , and then, is amplitude-divided by the PBS  202  so that a portion (a P-polarized light) of the light is reflected by the PBS  202  and a remaining portion (an S-polarized light) of the light is transmitted through the PBS  202 . The P-polarized laser light and the S-polarized laser light are amplified while passing through optical paths including the BEs  203 , the FRs  204 , the amplifiers  205 , the FRs  206  and the PL  207 . Then, each of the laser lights is transmitted through or is reflected by the PBS  202  in the same polarized state. The laser lights are re-coupled with each other in front of the quarter wavelength plate  201  to be turned back into a circularly polarized light before division is implemented. The re-coupled light is transmitted through the quarter wavelength plate  201  to become a P-polarized laser light. The laser light amplified in this way is transmitted through the PBS  102  and is enlarged in size by the BE  103 . 
     The light enlarged by the BE  103  is incident upon the second optical amplification stage  300 . In the second optical amplification stage  300 , the light is amplitude-divided into four lights, and the four lights are re-coupled to be incident upon the third amplification stage (not shown). The PBS  301  of the second optical amplification stage  300  outputs the incident light to the quarter wavelength plate  302 . The PBS  303  amplitude-divides the light from the quarter wavelength plate  302 , so that P-polarized laser lights are transmitted through the PBS  303  and the S-polarized laser lights are reflected by the PBS  303  to be respectively outputted to the 45° rotators  304 . At this time, in order to amplitude-divide one light into two lights, a combination of one 45° rotator  304  and two PBSs  305  is used to constitute 2×2 arrays. When the P-polarized light and S-polatized light are transmitted through the 45° rotators  304 , their polarized states are rotated by 45°. As a consequence, these polarized lights are amplitude-divided into two lights by the PBSs  305  being the next optical devices. Since the subsequent procedure performs the same function for the same construction as the first optical amplification stage  200 , detailed description thereof will be omitted herein. When the polarized lights reciprocate through the 45° rotators  304 , because the polarized lights are rotated by +45° and −45°, the polarized states are not changed. Therefore, since the amplitude division using the 45° rotators and the PBSs can be infinitely implemented, by arranging one or more additional amplification stages after the second optical amplification stage  300 , unlimited amplification of energy is possible. 
       FIG. 3  is a view illustrating an amplitude division type optical amplification stage in accordance with another embodiment of the present invention. 
       FIG. 3  represents an example in which an SBS-PCM is applied to an amplitude division type optical amplification stage. 
     Referring to  FIG. 3 , there are illustrated a laser oscillator  10  for generating a laser light, a first optical amplification stage  30  for amplifying the laser light outputted from the laser oscillator  10 , and a beam expander  50  for enlarging a size of the light amplified by the first optical amplification stage  30 . The first optical amplification stage  30  defines a cross-shaped configuration which is centered on a PBS  31  upon which the laser light from the laser oscillator  10  is incident. An optical isolator  32  is positioned at one side of the PBS  31 , and, at an opposite side of the PBS  31 , devices for amplification of the light, such as a quarter wavelength plate  33 , a PBS  34 , a BE  35 , an FR  36 , an amplifier  37 , an FR  38 , a pre-pulser  39 , an SBS-PCM  40  and a PL  41  are located along an optical path. 
     An S-polarized laser light from the laser oscillator  10  is reflected by the PBS  31  to be incident upon the first optical isolator  32 . In the first optical isolator  32 , the S-polarized laser light is transmitted through a quarter wavelength plate  32   a  and a pre-pulser  32   b  and is reflected by an SBS-PCM  32   d . A lens  32   c  is used to focus light on the SBS-PCM  32   d . Hereafter, the lens  32   c  will be explained as it is integrated with the SBS-PCM. The light reflected by the SBS-PCM  32   d  is reversely transmitted through the pre-pulser  32   b  and the quarter wavelength plate  32   a  and then is transmitted through the PBS  31  as the P-polarized laser light. The P-polarized laser light becomes a circularly polarized light after being transmitted through a quarter wavelength plate  33  and is amplitude-divided by a PBS  34   a . That is to say, the PBS  34   a  transmits a P-polarized laser light which is a portion of the circularly polarized laser light and reflects an S-polarized laser light which is a remaining portion of the circularly polarized laser light. 
     Thereafter, the P-polarized laser light is transmitted through a BE  35   a , is decreased in size to correspond to a size of a gain medium, and is then incident upon an FR  36   a . The light transmitted through the FR  36   a  is incident upon an amplifier  37   a  to be amplified, and the amplified light is incident upon an FR  38   a . The light transmitted through the FR  38   a  is reflected by an SBS-PCM  40   a  after passing through a pre-pulser  39   a , an SBS-PCM  40   a  and the PL  41  and then again passes through the FR  36   a . At this time, since the FRs  36   a  and  38   a  which are located in front of and behind the amplifier  37  permit a difference of 90° between an incident polarization surface and a reflected polarization surface, the FRs  36   a  and  38   a  function to offset birefringence produced in the amplifier  37 . The pre-pulser  39   a  which is located in front of the SBS-PCM  40   a  will be described later with reference to  FIG. 5 . 
     The light reflected by the SBS-PCM  40   a  is transmitted in an order which is reverse to that of the incidence procedure, that is, through the pre-pulser (PP)  39   a , the FR  38   a , the amplifier  37   a , the FR  36   a  and the BE  35   a , and is returned to the PBS  34  as an amplified P-polarized laser light. 
     Meanwhile, the S-polarized laser light is amplified through the same optical path as is the P-polarized laser light, that is, while passing through a BE  35   b , an FR  36   b , an amplifier  37   b , an FR  38   b , a PP  39   b  and an SBS-PCM  40   b . The light reflected by the SBS-PCM  40   b  is transmitted in reverse order to that of the incidence procedure, that is, through the PP  39   b , the FR  38   b , the amplifier  37   b , the FR  36   b  and the BE  35   b , and is reflected by the PBS  34   b  to serve as an amplified S-polarized laser light. As a result, the P-polarized laser light is transmitted through the PBS  34   a  and the S-polarized laser light is reflected by the PBS  34   a  so that they are re-coupled with each other in front of the quarter wavelength plate  33 . Here, since the re-coupled laser light is a circularly polarized light which is the same as before division, the re-coupled light is transmitted through the quarter wavelength plate  33  and becomes an S-polarized laser beam. Accordingly, the re-coupled laser light is reflected by the PBS  31 , and, then, after being enlarged in size by the first beam expander  50 , is output. 
     An optical amplification apparatus having a plurality of such optical amplification means will be described with reference to  FIG. 4 . 
       FIG. 4  is a view illustrating an optical amplification apparatus having a plurality of optical amplification stages. 
     Referring to  FIG. 4 , the optical amplification apparatus comprises a laser oscillator  10  for generating a laser light and a first optical amplification stage  30  for amplifying the light from the laser oscillator  10 . The light amplified by the first optical amplification stage  30  is enlarged in size by a first beam expander  50 , and is then incident upon a second optical amplification stage  70  to be amplified. Thereupon, as the light passes through a second beam expander  90 , the light is enlarged in size. The first optical amplification stage  30  amplitude-divides the light into two lights. Then, after being amplified, the two lights are re-coupled to be incident upon the second optical amplification stage  70 . The second optical amplification stage  70  amplitude-divides the light into four lights. Then, after being amplified, the four lights are re-coupled to be incident upon the next optical amplification stage (not shown). Here, since the first optical amplification stage  30  was described in detail with reference to  FIG. 3  and the same reference numerals will be used to refer to the same parts, further detailed description thereof will be omitted herein. 
     Hereafter, the second optical amplification stage  70  to which the light amplified in the first optical amplification stage  30  is incident to be amplified will be described. In the second optical amplification stage  70 , the same parts as those of the first optical amplification stage  30  are assumed to perform the same functions. 
     The second optical amplification stage  70  shown in  FIG. 4  is centered on a PBS  71  upon which the laser light from the first beam expander  50  is incident. A second optical isolator  72  is positioned at one side of the PBS  71 , and, at the other side of the PBS  71  which is opposite the second optical isolator  72 , devices for amplification of the light, such as a quarter wavelength plate  73 , two PBSs  74 , two 45° rotators  75 , four PBSs  76 , four BEs  77 , four FRs  78 , four amplifiers  79 , four FRs  80 , four pre-pulsers  81 , four SBS-PCMs  82  and four PLs  83  are located along an optical path. In the second beam isolator  72 , the light reflected on the PBS  71  is amplitude-divided while passing through a quarter wavelength plate  72   a  and a PCM  72   b , is reflected on an SBS-PCM  72   e  after passing through an optical path, and is returned to the PBS  71 . The PBS  71  outputs the light, which is incident from the second optical isolator  72 , to the quarter wavelength plate  73 . The PBS  74  amplitude-divides the light from the quarter wavelength plate  73  so that a P-polarized light is transmitted through the PBS  74  and an S-polarized light is reflected on the PBS  74  to be outputted to the 45° rotator  75 . At this time, in order to amplitude-divide one light into two lights, a combination of one 45° rotator  75  and two PBSs  76  is used to constitute 2×2 arrays. When the P-polarized light and S-polarized light are transmitted through the 45° rotators  75 , their polarized states are rotated by 45°. As a consequence, these polarized lights are amplitude-divided into two lights by the PBSs  76  being the next optical devices. Since the subsequent procedure is constructed in the same manner as in the case of  FIG. 3 , the same functionality is accomplished. When the polarized lights reciprocate through the 45° rotators  75 , because the polarized lights are rotated by +45° and −45°, the polarized states are not changed. Therefore, since the amplitude division using the 45° rotators and the PBSs can be infinitely implemented, by arranging one or more additional amplification stages after the second optical amplification stage  70 , unlimited amplification of energy is possible. 
     Hereinafter, the pre-pulser which is employed in the optical amplification apparatus will be described in further detail. 
       FIG. 5  is a view illustrating the pre-pulsers shown in  FIG. 4 . 
     Referring to  FIG. 5 , the pre-pulser is located in front of the SBS-PCM in the above-described optical amplification apparatus to ensure that, before a main pulse ‘C’ having large energy is incident upon the SBS-PCM, a pre-pulse ‘B’ having small energy is incident to produce in advance a sonic wave and to thereby prevent the front end of the main pulse ‘C’ from being cut. In the case of the laser pulse which is incident upon the SBS-PCM, since damage to the front end of a pulse is caused for the sake of transmission or generation of a sonic wave, when the laser pulse is reflected, the front end of the laser pulse is cut to have an abruptly rising shape. When this reflected wave is incident upon the next SBS-PCM, it causes optical breakdown or deteriorates phase conjugation fidelity. Therefore, the pre-pulsers are located to prevent the front end of the reflected wave from being cut due to reflection by the continuous SBS-PCMs. In the embodiment shown in  FIG. 5 , as a pre-pulse for preventing pulse-shaped deformation of the reflected wave, an amplitude of a laser beam is divided using the PBSs  51  and  52  or beam splitters, and a time difference between the main pulse ‘C’ and the pre-pulse ‘B’ is determined by adjusting an optical path. Thus, the final pulse produced by the main pulse ‘C’ and the pre-pulse ‘B’ has the shape designated by the reference character ‘D’. 
     Further, in the SBS-PCMs employed in the above-described optical amplification apparatus, the PLs are simultaneously located to fix the phases of the reflected waves which are reflected on the respective SBS-PCMs. 
       FIG. 6  is a view illustrating another example of phase control by the SBS-PCM shown in  FIG. 4 . 
     Referring to  FIG. 6 , there is illustrated a method in which a sonic wave is directly generated externally of the SBS-PCM to enable phase control. An ultrasonic vibrator is attached to the SBS-PCM (SBS-cell) and is externally vibrated to generate a sonic wave in the SBS-PCM to thereby control the phase of a reflected wave. In other words, the stimulated brillouin scattering (SBS) is induced from sonic wave noise. When scattering is provoked, the phase of the reflected wave is random. Accordingly, the sonic wave functions to control this random state. Therefore, by causing a sonic wave having a frequency which corresponds to that of a stimulated brillouin scattering medium or to that of a harmonic wave, to be directly incident upon the stimulated brillouin scattering medium, the phase of the reflected wave is controlled. If the phase of an initially generated sonic wave is controlled, the phase of a subsequently generated sonic wave can be controlled, as a result of which the phase of the reflected wave can be controlled. As a consequence, before a laser beam is incident to control the phase of the initially generated sonic wave, a sonic wave is generated in advance in the stimulated brillouin scattering medium. Consequently, the subsequently provoked stimulated brillouin scattering starts from the already generated sonic wave, and as a result of this, the phases of the sonic wave and the reflected wave are controlled. In particular, by allowing the sonic wave to be generated at an exact time, it is possible to control the phase irrespective of incident energy. In order to generate the sonic wave, in addition to the ultrasonic vibrator, an acousto-optic modulator (AOM) can be employed. 
     As apparent from the above description, the optical amplification apparatus for amplifying a laser light according to the present invention provides advantages in that it realizes high repeatability and a high output. 
     Further, since the optical amplification apparatus according to the present invention adopts an amplitude division method, all disadvantages of a wavefront division method are overcome, and limitless energy amplification is possible. 
     Although a preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.