Abstract:
A technique for increasing the lifetime of optical components that are used in a laser for laser shock peening. The technique employs a properly sized iris or a gradient reflector within a laser oscillator to produce a single-transverse-mode laser beam. This technique eliminates hot spots that significantly reduce the lifetime of optical components. Utilization of the present invention can increase the lifetime of components by a factor of ten of their conventional lifetime.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the use of a coherent energy source for a high powered pulse laser, in the shock processing of materials, and more particularly to, a laser oscillator for such use, operating in a single-transverse-mode. 
     2. Description of the Related Art 
     Current lasers used in laser peening typically utilize an oscillator of the dual pump cavity type, in which the time integrated spatial profile is flat. The effect of stress birefringence in this oscillator is compensated for by a 90 degree rotator positioned between the dual pump cavities. This technique eliminates the maltese-cross pattern which is not considered desirable for laser peening. 
     A disadvantage of this type of dual-pump cavity oscillator, and almost any other large aperture oscillator, is that the output beam is in multi-transverse and multi-longitudinal-modes. The multi-mode nature of large aperture oscillators inherently generates hot spots in the beam because of the interaction of the different modes. These hot spots greatly decrease the lifetime of optical components through which the beam passes. Optical coatings utilized within the laser oscillator and associated laser system typically begin to show damage within ten thousand shots or bursts of the laser peening system, and typically must be replaced within one hundred thousand shots. This type of damage increases maintenance costs. 
     What is needed in the art is a laser peening laser utilizing a oscillator operating in a single mode to reduce damage to the associated optical components. 
     SUMMARY OF THE INVENTION 
     The present apparatus is that of a laser oscillator operating in a single-transverse mode. In one embodiment of the present invention, the oscillator is made to operate in a single-transverse mode by utilizing an iris sized to eliminate all modes but the lowest order TEMOO mode. In another embodiment a gradient reflector is used to produce a single-transverse mode. The oscillator may also include a means for compensating for stress birefringence. In addition, a telescope may be placed inside the oscillator for optimizing the diameter of the oscillator beam and for controlling thermal lensing. 
     In another embodiment of the invention, the oscillator utilizes a porro prism in combination with a retardation plate to reduce stress birefringence while further providing a means for generating a single-transverse-mode laser pulse. 
     Other embodiments of the present invention, include generating a single-transverse and single-longitudinal mode laser pulse via the addition of a seed laser or an etalon. 
     In one form of the invention thereof, a method for laser peening a workpiece, comprises generating a laser pulse from a substantially single-transverse mode oscillator, modifying the laser pulse with a pulse sharpening device, amplifying the laser pulse, and directing the laser pulse to the workpiece. 
     In another form of the invention thereof, an apparatus for laser peening a workpiece, comprises a substantially single-transverse mode laser oscillator, a laser pulse-sharpening device, means for amplifying a laser pulse, and a laser peening cell. 
     An advantage of the present invention is that a remarkable increase in the lifetime of components is created through the use of a single-transverse mode laser oscillator. Data confirms that such optical damage is not seen until approximately fifty thousand laser shots are utilized through the laser peening system. Without such single-transverse-mode utilization, such shots contain hot spots with energy densities greater than twice the normal level. 
     Another advantage of the present invention is that the lifetime of optical components may be extended at least up to five hundred thousand shots and possibly two million shots. Such extension in the component lifetime reduces maintenance costs and makes laser peening operations cost effective. 
     A further advantage of the present invention is that the TEMOO mode beam created has a higher density of laser energy at its center than a multimode beam and this produces a flatter beam spatial profile after amplification. An additional advantage of the present invention is that the TEMOO mode beam created has low divergence. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of the embodiments of the invention taken in conjunction with the accompanying drawings, a wherein: 
     FIG. 1 is a diagrammatic view of a dual-pump cavity design of the present invention; 
     FIG. 2 is a diagrammatic view of a laser oscillator in combination with an external phase conjugate cell; 
     FIG. 3 is a diagrammatic view of a laser oscillator in combination with a amplification cell; 
     FIG. 4 is a diagrammatic view of a laser oscillator in combination with a porro prism; 
     FIG. 5 is a diagrammatic view of a laser oscillator in combination with a seed laser; 
     FIG. 6 is a diagrammatic view of a laser oscillator in combination with an etalon; and 
     FIG. 7 is a diagrammatic view of a laser oscillator similar to that of FIG. 1 but utilizing a single pump cavity with the addition of an internal telescope. 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention, in one embodiment thereof, is shown in FIG.  1 . 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 FIG. 1, 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  (FIG. 7) may be constructed within laser oscillator  10 . As seen in FIG. 7, 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 (FIG.  2 ). 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. 
     FIG. 2 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 FIG. 3. 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 FIG.  4 . 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 FIG.  1  and that shown in FIG. 4 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. 
     FIG. 5 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 FIG. 5 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 . 
     FIG. 6 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.