the present invention , in one embodiment thereof , is shown in fig1 . laser oscillator 10 comprises a highly reflective mirror 12 , which operates as one end of the laser oscillator 10 . continuing in the beam path from reflective mirror 12 in order are a q - switch 14 , a polarizer 16 , an iris 18 , a safety shutter 19 , a first pump cavity 20 , a 90 degree rotator 22 , a second pump cavity 24 , and then a partially reflecting mirror 26 , which operates as the terminator or output coupler for defining the laser oscillator 10 . in the embodiment shown in fig1 two pump cavities 20 and 24 are utilized with a 90 degree rotator 22 disposed there between . the 90 degree rotator 22 rotates the polarization direction of the oscillating laser beam by 90 degrees and therefore compensates for stress birefringence effects . in some embodiments of the invention , an internal telescope 13 ( fig7 ) may be constructed within laser oscillator 10 . as seen in fig7 telescope 13 may also be disposed within an oscillator of the single cavity type . telescope 13 focusing power is determined by the geometry of laser oscillator 10 for optimizing the diameter of the oscillator laser beam , and for controlling or reducing thermal lensing effects created by other elements of laser oscillator 10 . the present invention is formed by . selecting and forming iris 18 to select a particular mode to be created in laser oscillator 10 . of particular importance is the optical diameter of iris 18 . compared to the beam developed in one form of the invention , the iris utilizes a size of 5 mm or less , but preferably 2 mm or less , to reduce the laser oscillator output beam 28 to a single - transverse - mode . oscillator 10 is specifically configured for laser shock peening . oscillator 10 is effective for generating a series of laser pulses that can be sharpened by a pulse slicer . in accordance with the preferred embodiment , each laser pulse thereby created is substantially temporally shaped to optimally create the pressure pulse developed on the target ( not shown ) for the laser shock peening process . the present invention , utilizing the correct and critical size of iris 18 , has not been employed before in a laser oscillator for use with a laser peening system . use of such iris 18 dramatically increases the lifetime of the optical components both within the laser oscillator 10 and along with optical elements downstream of output coupler 26 . multi - mode configuration beams typically deteriorate optics and optical coatings and generate hot spots therein . these hot spots create optical damage to such components . typical data show that optical damage occurs within the first ten thousand laser pulses generated by the laser oscillator 10 when the appropriate sized iris is not used . the data indicates that through the use of the iris 18 of particularly less than or equal to 5 mm in diameter , and more preferably at particular sizes of 2 or 3 mm , that up to one hundred thousand shots may be obtained without significant optical damage to associated optical elements . such a small change surprisingly increases by at least a factor of 10 , the lifetime of the optical components . such an increase in the lifetime of optical components has a tremendous impact on the cost of laser processing . in another embodiment of the invention , the required single transverse mode is produced by replacing the output coupler 26 with a gradient reflective mirror . the reflectivity of the mirror is configured such that it is high at its center but falls off radially over a distance comparable to the diameter of the laser rod . the reflectivity profile of the gradient mirror can be of the gaussian or super - gaussian type . this configuration enables the center of the beam to be preferentially amplified relative to the outside part , producing a single transverse mode . in one embodiment of the invention the temporal pulse emitted by oscillator 10 is shaped by using a pulse - slicer positioned after the output coupler 26 of the oscillator . a pulse - slicer is an optical switch that produces a laser pulse with a sharply rising leading edge that is limited to less than 5 nanoseconds . it typically consists of an electro - optical q - switch and a polarizer . the q - switch rotates the polarization 10 direction of the laser beam and the polarizer analyzes the beam . initially the q - switch is off ( no voltage ) and the laser beam is rejected from the system by the polarizer . after the slowly rising part of the pulse is rejected the q - switch is activated ( voltage on ) and a beam with a fast rising edge is produced . the trailing edge of the pulse can be modified by turning the q - switch off again . in another embodiment the rising edge of the temporal pulse is limited to less than 5 nanoseconds by a phase conjugate cell ( fig2 ). a phase conjugation cell is a device that reverses the propagation direction of a laser beam and its spatial phase when the beam intensity is above a threshold value . below the threshold , the slowly rising part of the laser pulse is not reflected and hence is removed from the beam . in one embodiment the cell utilizes the nonlinear phenomena of stimulated brillouin scattering ( sbs ). brillouin scattering is the nonlinear optical phenomenon of the spontaneous scattering of light by it interaction with density variations within the medium through which it passes . the reflecting material within the cell typically consists of a gas or liquid . standard reflecting materials consist of one or more of the following carbon tetrachloride , sulfur hexafluoride , freon , and carbon disulfide . fig2 illustrates one embodiment of the phase conjugate cell 40 as a pulse slicer . in this case , the linearly polarized beam emitted by oscillator 10 passes through a faraday isolator 42 , a polarizer 44 , a quarter waveplate 46 and becomes circularly polarized . the beam is then reflected from the phase conjugate cell 40 , passes again through the quarter waveplate and is rejected from the system by the polarizer 44 . the faraday isolator 42 functions as an optical diode . photons that are reflected from the phase conjugate cell and that leak through the polarizer are prevented from returning to the oscillator by the isolator 42 . the isolator 42 consists of a faraday rotator that has a polarizer located at its entrance and exit ports . the faraday rotator utilizes a rod of terbium gallium garnet within a solenoid - generated axial magnetic field to rotate the polarization direction of a laser beam by 45 degrees . such polarizers are configured to pass the oscillator laser beam and prevent passage of a reflected laser beam . in another embodiment of the invention the coherent radiation emitted by the single mode oscillator 10 is amplified by multipass amplification . multipass amplification is the rocess by which a laser beam is amplified by passing it more han once through the same amplifier medium . an embodiment of his technique is shown in fig3 . a polarized laser beam emitted by oscillator 10 passes through a faraday isolator 42 and a polarizer 44 . the beam then passes through a first amplifier 50 , a 90 degree rotator 22 , a second amplifier 52 , a quarter waveplate 46 , and then is reflected from mirror 54 . mirror 54 can be a dielectric reflective mirror or a phase conjugate mirror . depolarization occurring in the first amplifier head is corrected by 90 degree rotator 22 and the second amplifier 52 . after reflection from mirror 54 , the beam passes again through the amplifiers 52 , 50 and is rejected by polarizer 44 into another amplifier chain ( not shown ). in another embodiment of the invention , the coherent radiation emitted by the single mode oscillator is amplified by one or more amplifiers . a 90 degree rotator and beam shaping optics , such as a telescope can be placed between amplifiers for birefringence correction . in one form of the single - transverse - mode oscillator created with the particular iris 18 to be discussed herein , it is not necessary to utilize two pump cavities . without utilizing two pump cavities 20 , 24 no such 90 degree rotator 22 would be used . such a single cavity device utilizing a porro prism 38 is shown in fig4 . the porro prism 38 in combination with an optical rotator compensates for stress birefringence . the rotator can be a phase plate or a 45 degree rotator . the laser beam exits oscillator 10 off of the polarizer 16 , after q - switch 14 is triggered . in one embodiment of the present invention , the oscillator is made to operate in a single - transverse - mode by utilizing an iris 18 sized to most preferably approximately 2 mm , or at least preferably equal to or less than 5 mm in diameter . a particular difference between laser oscillator 10 shown in fig1 and that shown in fig4 is that the elimination of the second pump cavity 24 and 90 degree rotator 22 . one advantage of this design is that the q - switch is exposed to a lower energy density . the invention further may include a means for generating a single - longitudinal - mode laser pulse . one such means is by utilizing a seed laser or alternatively using an etalon within the laser beam path . fig5 shows one means for generating a single - longitudinal - mode laser pulse by utilizing a seed laser 30 which creates a low energy , single frequency , laser beam 32 which first passes through a faraday isolator 34 and then is reflected into the oscillator cavity off of a polarizer 16 . seed laser 30 may be a solid state laser known in the art such as a diode laser , or may be a flashlamp pumped laser containing a ylf crystal . although fig5 shows the seed laser 30 entering through polarizer 16 , it may be possible to have the seed laser beam 32 enter through the back of reflective mirror 12 . inside the oscillator cavity , the number of photons at the seed laser wavelength is significantly greater than photons produced at other wavelengths by spontaneous emission . therefore , the seed photons buildup faster resulting in a single - longitudinal - mode beam at the wavelength of seed laser 30 . fig6 shows a laser oscillator utilizing an etalon 38 to generate a single - longitudinal - mode laser pulse . an etalon consists of two plane - parallel , highly reflective , optical surfaces that are separated by a fixed distance . as the optical beam passes through this component , it is reflected and transmitted by these surfaces . this leads to constructive and destructive interference effects that enable the component ( etalon 38 ) to act as a wavelength filter , eliminating all longitudinal modes except the desired one . the wavelength selectivity of the etalon 38 can be adjusted by : rotating the etalon about its vertical axis . while this invention has been described as having a preferred design , the present invention can be further modified within the spirit and scope of this disclosure . this application is therefore intended to cover any variations , uses , or adaptations of the invention using its general principles . further , this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims .