Abstract:
The present invention provides an easily aligned, all-reflective, aberration-free pulse stretcher-compressor in a compact geometry. The stretcher-compressor device is a reflective multi-layer dielectric that can be utilized for high power chirped-pulse amplification material processing applications. A reflective grating element of the device is constructed: 1) to receive a beam for stretching of laser pulses in a beam stretcher beam path and 2) to also receive stretched amplified pulses to be compressed in a compressor beam path through the same (i.e., common) reflective multilayer dielectric diffraction grating. The stretched and compressed pulses are interleaved about the grating element to provide the desired number of passes in each respective beam path in order to achieve the desired results.

Description:
[0001] The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    In the design of short pulse laser systems (&lt;2 ps), amplification using the technique of chirped pulse amplification (CPA) has become commonplace. This technique allows high energy, short pulses to be made possible by first stretching the pulse in time, amplifying it, and then recompressing it temporally. In these CPA laser systems, it is required to stretch ultrashort-pulses (or pulses having sufficient bandwidth to become ultrashort) prior to amplification. An ultrashort pulse is one having a duration in the range of 5 femtoseconds to 50 picoseconds.  
           [0003]    The ultrashort pulse is stretched to a duration typically greater than 100 picoseconds prior to amplification by the system. Following the stretcher phase, the stretched, amplified, high energy pulse is recompressed to optimally its original temporal length. Such systems typically comprise a reflective system using a high precision grating.  
           [0004]    Early designs typically have included at least a pair of diffraction gratings, the first for stretching the ultrashort pulse prior to injection into the laser system, and the second for recompression of the high energy pulse. This leads to costly and complex systems using gratings of the same design because multiple grating stretcher-compressors require precise alignment when a change in laser wavelength is required. In addition, matched grating pairs are required for the stretcher and compressor in order to maintain spatial uniformity and to obtain the appropriate pulse duration.  
           [0005]    There is a continuing need for improvements in generating high-energy ultrashort pulses. There is also a need to simplify present systems by making them more compact, cost efficient and less burdensome in alignment sensitivity.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, the present invention provides a compact, inexpensive, single common all-reflective stretcher and compressor device for chirped-pulse amplification.  
           [0007]    The stretcher in the present all-reflective system comprises optical components that allow the entering beam to strike the all-reflective grating at different vertical displacements during the stretcher path. For example, the entering beam strikes the grating and is diffracted to a reflective focusing optic that reflects the beam to a flat reflective optic at the focal plane of the reflective focusing optic. The beam is then reflected back to the focusing element that will reflect the beam to a vertically displaced position on the reflective grating. The beam is then diffracted to a retro-reflective device such as a roof mirror that again returns the beam to a vertically displaced position of the grating. The beam undergoes additional passes to stretch the beam and exits the stretcher and is amplified before entering the compressor path of the present invention.  
           [0008]    The compressor in the present all-reflective system utilizes the same reflective grating element that is used for the stretcher. The amplified beam enters and strikes the grating and is diffracted to a retro-reflective set of optical components that allows translation along the beam path direction to enable compression adjustment of the amplified stretched pulse. The beam is then reflected back to a displaced vertical position on the reflective grating and is interleaved about positions that were used for initial compression of the pulse but is overlapped at those sites on the grating used for pulse stretching. This beam is diffracted to a retro-reflective device such as a roof mirror that again reflects the beam to the reflective grating at a vertically displaced position. The optical path then reverses itself and exits the system as a compressed amplified pulse.  
           [0009]    Further aspects of the invention include methods for stretching an ultrashort pulse and compressing an amplified stretched pulse to its former pulse-width by utilizing components that allow overlapping on a common area on a single common reflective diffraction grating.  
           [0010]    The present invention provides a cost effective compact system which combines the functionality of a stretcher and compressor employing a single high precision reflective optical grating. The system is an easily aligned, all-reflective, chirped pulse amplification device that can be utilized in commercial systems that need high power for material processing applications.  
           [0011]    These features, variations, and other advantages of the present invention will become apparent from the following detailed description, claims and drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is an optical schematic of the stretcher beam path of the present invention.  
         [0013]    [0013]FIG. 2 is a representation of the overlapping of the stretched and compressed pulses of the stretcher-compressor.  
         [0014]    [0014]FIG. 3 is an optical schematic of the compressor beam path of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    The present invention is an easily aligned, all-reflective, on-axis, aberration-free pulse stretcher-compressor with a minimum number of elements in a compact geometry. The present invention is typically an integral part of a laser system where desired power levels with necessary pulse-widths to perform high precision machining, (e.g., holes or cuts in metals, alloys, ceramics, and crystals with clearly differentiated sides and edges) is required.  
         [0016]    In the prior art, stretchers and compressors are typically separate optical devices located at different points in the laser system. Each optical device may use two gratings for a total of four gratings for the entire system. In addition, prior art stretcher-compressors routinely use transmissive optics in some part of the beam path, (i.e., the stretcher beam path or the compressor beam path). In stretcher-compressors systems of the prior art where the optical dispersive element (i.e., the grating), is all-reflective, the input beam to a stretcher and a compressor system enter from the same side of the normal to the grating. Thus, overlapping of the beams is not possible in the prior art all-reflective systems and vertical displacements are necessary. Accordingly, prior art systems typically use different sections of single stretcher-compressor diffraction gratings, which then requires a much larger stretcher-compressor device than the present invention.  
         [0017]    A stretcher-compressor device employed in the present invention, i.e., the common reflective diffraction grating, the structure and method of preparation thereof is disclosed in U.S. Pat. No. 5,907,436, the disclosure is herein incorporated by reference in its entirety. The common reflective diffraction grating of the present invention is a relatively large in dimension device, (between about 100 mm×200 mm and about 500 mm×1000 mm), that is more compact in design than other related stretcher-compressor devices because the stretched and compressed pulses of the system can share a common area on the device. The combination of the stretcher and compressor beam paths make two or more reflective passes from the grating (e.g., 12 reflections), and are interleaved along the aforementioned common area. The stretcher-compressor device, when acting as a stretcher, disperses the various frequency components that comprise each pulse from a laser source along widely varying paths through preferably all reflective optics although at least one transmissive optic may be used, and reassembles them into a well-collimated beam in the stretcher beam path. The stretcher-compressor device, when acting as a compressor, again disperses the various frequency components from each pulse from a laser source and then recompresses the various frequency components temporally into another well-collimated beam in the compressor beam path. This concept combines the functionality of the stretcher and compressor into a single reflective system using a single common grating. It minimizes costs and alignment problems by using a single grating.  
         [0018]    The common area on the stretcher-compressor device of the present invention is achieved because stretching of the pulses from the laser source and compressing of the laser source pulses after stretched amplification are incident on the grating from opposite but equal angles with respect to the normal to the grating. A second technique for beam overlapping which is particularly useful is to input the pulses for stretching and compressing into the stretcher-compressor device at unequal angles from the normal to the grating and then adjust the complimentary optical components in the compressor beam path (i.e., adjust the compressor beam path length), to match the stretcher beam path for recompression of the pulse to its original length. Thus, the present invention allows a common area on the grating to be shared by the stretched and compressed laser pulses. The input angle is chosen to be within plus or minus ten degrees from the Littrow angle, with a preferred range between about 66 and about 70 degrees, and a highly preferred angle of about 68 degrees from the normal to the grating for the grating selected and for the wavelength range with which the grating is used.  
         [0019]    The pulse stretcher section of the present invention is shown in FIG. 1. The components of the system, for example, are an input flat turning mirror  200  such as for example, a 1″ diameter mirror, a relatively large (e.g., from about 100 mm×200 mm to about 500 mm×1000 mm), common reflective diffraction (e.g., 1800 grooves/mm) multilayer dielectric grating  202 , a flat mirror  206  such as for example, a 4″ diameter mirror at the focal plane of a curved mirror  204 , typically 6″ in diameter, a roof mirror  208  that displaces the beam vertically for additional passes through the stretcher, and a retro-reflecting mirror  210  such as for example, a 1″ diameter mirror. The flat mirror  206  is positioned at the focal plane of the curved mirror  204 , thereby forming a unit magnification telescope upon retro-reflection.  
         [0020]    In the method of the invention, referring to FIG. 1, ultra-short pulse beam A is received by the flat mirror  200 , which directs the ultra-short beam along beam path B to the reflective dielectric grating  202   d  and is diffracted along beam path C to a curved spherical mirror  204   a.  The curved mirror  204  reflects a spatially dispersed beam along beam path D at a slight upward angle so that it strikes the flat mirror  206   a  at the focus of curved mirror  204 . The spatially dispersed beam is reflected along beam path E from the flat mirror  206 , back to the curved mirror  204   b,  which directs the beam along beam path F to an interleaved position on the reflective dielectric grating  202   b.  The reflective dielectric grating  202  diffracts and directs the beam to a gold roof mirror  208  along beam path G that displaces the beam vertically and redirects the beam along beam path H back to an interleaved position on the dielectric grating  202   c.  The reflective dielectric grating  202  diffracts the beam along beam path I to the curved spherical mirror  204   c.  The curved mirror  204  reflects the beam along beam path J at a slight downward angle so that it strikes the flat mirror  206   a  at the focus of curved mirror  204 . The beam is reflected along beam path K from the flat mirror  206  back to the curved mirror  204   d,  which directs the beam along beam path L to the reflective dielectric grating  202   d.  The reflective dielectric grating  202  then diffracts the beam along beam path M to the flat retro-reflecting mirror  210 . Reflection from the flat retro-reflecting mirror  210  repeats this sequence in reverse order, resulting in a stretched pulse to be received for amplification by a system.  
         [0021]    The stretched beam  202   c,  is shown in an exploded view in FIG. 2 as a spatially spread beam  25  that is overlapped, (i.e., the shared common area), with a compressed beam  27  from the compressor beam path on the dielectric diffraction grating  28 .  
         [0022]    The pulse compressor section of the present invention is shown in FIG. 3. The components of the system are, for example, an input flat turning mirror  12  such as for example a 2″ diameter mirror, the common reflective multilayer dielectric grating  14 , at least a pair of retro-reflecting mirrors on a horizontal translation stage  16 , at least a pair of flat mirrors  18 ,  20  typically 4″ in diameter, a roof mirror  22 , a pick-off mirror  24  such as for example a 2″ diameter mirror, and a flat output turning mirror  26  such as for example a 2″ diameter mirror.  
         [0023]    In the method of the invention, referring to FIG. 3, the stretched amplified beam, e.g., stretched beam, AA is received by the flat mirror  12 , which directs beam BB to the reflective dielectric grating  14   a  and beam CC is diffracted to the pair of retro-reflection mirrors  16  mounted on a horizontal translation stage. The stretcher and compressor must be perfectly matched to recompress the pulse to its original length. The pair of retro-reflection mirrors  16  mounted on a horizontal translation stage allows for this necessary distance adjustment to control the amount of negative group velocity dispersion (i.e., to control compression of the pulse). Thus, the adjustment of the pair of retro-reflection mirrors  16  mounted on a horizontal translation stage matches the compressor beam path length with the beam path length of the stretcher so that the minimum pulse width is generated. Beam DD is then directed from the pair of retro-reflection mirrors  16  to the first flat mirror  18   a.  Flat mirror  18  directs beam EE to a second flat mirror  20   a  which reflects beam FF back to an interleaved position on the dielectric grating  14   b  which then diffracts beam GG to the roof mirror  22 . Roof mirror  22  displaces the beam vertically and directs beam HH back to an interleaved position on the dielectric grating  14   c.  Dielectric grating  14  diffracts beam II to flat mirror  20   b  that then reflects beam JJ to flat mirror  18   b.  Flat mirror  18  reflects beam KK to pair of mirrors  16  mounted on a horizontal stage that then directs beam LL to diffraction grating  14   d.  The dielectric diffraction grating  14  diffracts beam MM to a pick-off mirror  24  that directs beam NN to an out put turning mirror  26 , resulting in a recompressed pulse that can be directed to a workpiece (not shown).  
         [0024]    The compressed beam  14   b,  is shown in the exploded view in FIG. 2 as a spatially spread beam  27  that is overlapped, (i.e., the shared common area), with a stretched beam  25  from the stretcher beam path on the dielectric diffraction grating  28 .  
         [0025]    Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited by the scope of the appended claims.