Abstract:
The present invention includes an apparatus and the method to scale the average power from high power ultra-short pulsed lasers, while at the same time addressing the issue of effective beam delivery and ablation, by use of an optical amplification system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates generally to the field of light amplification and, particularly to systems useful in athermal ablation. 
         [0003]    2. Description of Related Art 
         [0004]    An ultra-short pulse (USP) laser emits pulses with a temporal pulse length in the range of picoseconds (psec, 10 −12  seconds) to femtoseconds (fsec, 10 −15  seconds) resulting in a very high electric field for a short duration of time. Typical techniques for generating these ultra-short pulses are well known. Generally, large systems, such as Ti:Sapphire, are used for generating ultra-short pulses. 
         [0005]    USP phenomena were first observed in the 1970&#39;s. It was discovered that mode-locking a broad-spectrum laser could produce ultra-short pulses. As produced, an ultra-short pulse has significantly lower power compared to optical pulses having greater temporal lengths. When high-power, ultra-short pulses are desired, the pulses are intentionally lengthened temporally, or chirped, prior to amplification to avoid damaging system components. This process is referred to as chirped pulse amplification (CPA). Subsequent to chirping and amplification, the pulse is compressed temporally to obtain both high peak power and ultra-short pulse duration. 
         [0006]    Generally, ablation refers to removal of material, for example, by an erosive process. Lasers can be implemented to ablate material in a selective manner. Two broad classes of laser ablation are thermal and athermal. Thermal ablation is dependent of thermal effects, such as melting. Athermal ablation can occur when an ultra-short pulse is focused on a material as a result of the high electric fields associated with the ultra-short pulse. There are several advantages of athermal ablation over other means of material removal. Compared to conventional mechanical machining, athermal ablation permits more accurate removal without mechanical damage of surrounding material. Conventional laser machining (e.g., thermal ablation), which uses continuous wave (cw) or long-pulsed lasers (e.g., pulse durations greater than roughly 1 nsec, or nanoseconds, 10 −9  seconds) can be more precise and flexible as compared to mechanical machining, but can damage surrounding materials. Material removal by athermal ablation is especially useful for medical purposes, either in-vivo or on the outside surface (e.g., skin or tooth), as it is generally painless. 
         [0007]    Despite the advantages of athermal ablation, there is a trade-off between average pulse power and pulse quality. Higher pulse powers enable higher material removal rates, but are subject to pulse aberrations and distortions. Conversely, lower pulse powers result in low material removal rates that render the technique impractical for most applications. 
       SUMMARY OF THE INVENTION 
       [0008]    In one embodiment, a system may comprise an optical pulse stretcher, an optical splitter, an optical amplifier, and an optical pulse compressor. The optical pulse stretcher may be configured to chirp an optical pulse to produce a chirped optical pulse. The optical splitter may be configured to optically split the chirped optical pulse to produce a plurality of split optical pulses. The optical amplifier may be configured to optically amplify one of the plurality of split optical pulses to produce an optically amplified split optical pulse. The optical pulse compressor may be configured to compress the optically amplified split optical pulse to produce a compressed optically amplified split optical pulse. 
         [0009]    In another embodiment, a method may comprise optically splitting a chirped optical pulse to produce a plurality of split optical pulses, optically amplifying one of the plurality of split optical pulses to produce an optically amplified split optical pulse, and optically compressing the optically amplified split optical pulse to produce a compressed optically amplified split optical pulse. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram illustrating an ultra-short pulse laser system, according to the prior art. 
           [0011]      FIG. 2  is a block diagram illustrating one embodiment of an optical amplification system, according to various embodiments of the invention. 
           [0012]      FIG. 3  is a block diagram illustrating another embodiment of an optical amplification system, according to various embodiments of the invention. 
           [0013]      FIG. 4  is a block diagram illustrating yet another embodiment of an optical amplification system including polarization combination, according to various embodiments of the invention. 
           [0014]      FIG. 5  is a diagram illustrating a variety of delivery system configurations, according to various embodiments of the invention. 
           [0015]      FIG. 6  is a flowchart showing an exemplary process for providing a compressed optically amplified split optical pulse, according to various embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    An ultra-short pulse (USP) laser system emits optical pulses resulting in a very high electric field for an ultra-short short period of time. In this context, “ultra-short” refers to durations in the range of picoseconds (psec, 10 −12  seconds) to femtoseconds (fsec, 10 −15  seconds). Although the peak power of a USP may be high, the average power contained by the USP may be relatively low, as a result of the pulse duration being ultra-short.  FIG. 1  is a block diagram illustrating a typical USP laser system  100 , according to various embodiments of the prior art. A seed source  105  can be any light source capable of generating an optical pulse  110  with characteristics of an ultra-short pulse. Light sources with this capability may include, for example, fiber mode-locked lasers, gas lasers (e.g., helium-neon, argon, and krypton), chemical lasers (e.g., hydrogen fluoride and deuterium fluoride), dye lasers, metal vapor lasers (e.g., helium cadmium metal vapor), solid state lasers (e.g., titanium sapphire and neodymium yttrium aluminum garnet), and semiconductor lasers (e.g., gallium nitride and aluminum gallium arsenide). 
         [0017]    As discussed herein, the optical pulse  110  generated by the seed source  105  may have a small average power and require subsequent amplification for certain applications. Prior to amplification, the pulses may be temporally stretched, or “chirped,” by an optical pulse stretcher  115 . Chirping the pulse reduces the peak power and permits subsequent amplification without damage to the optical amplifiers and other system components. Temporal pulse stretching may be achieved with various grating and/or prism arrangements, although other methods exist and are known in the art. In one embodiment, the optical pulse  110  propagates through a thick slab of glass to be stretched temporally. In another embodiment, the optical pulse stretcher  115  may include an optical fiber. 
         [0018]    After a chirped optical pulse  120  is produced by the optical pulse stretcher  115 , the chirped optical pulse  120  may be amplified by an optical amplifier  125 . The optical amplifier  125  may be a component that amplifies the optical power of the pulse directly without converting it to an electrical signal. According to various embodiments, the optical amplifier  125  may be a single component or include a serial array of amplifiers, where the output of one amplifier is received directly by the input of another amplifier and so on. In other embodiments, the optical amplifier  125  may include any combination of laser amplifiers, optical fiber based optical amplifiers (e.g., doped fiber amplifier), semiconductor optical amplifiers, Raman amplifiers, and/or parametric optical amplifiers. 
         [0019]    After an optically amplified chirped optical pulse  130  is produced by the optical amplifier  125 , the optically amplified chirped optical pulse  130  may be compressed temporally by an optical pulse compressor  155 . Temporal compression of an optical pulse may be achieved using similar approaches as may be used with the optical pulse stretcher  115  (e.g., grating, prism, and/or fiber configuration). According to an exemplary embodiment, a compressed optically amplified optical pulse  160  produced by the optical pulse compressor  155  may have duration similar to the duration of the optical pulse  110  (i.e., ultra-short duration) and with a peak power increased by several orders of magnitude. Finally, a delivery system  185  may receive the compressed optically amplified optical pulse  160  and deliver it to a location. In some embodiments, the delivery system  185  may include, for example, optical fibers, focusing optics, beam modulators, and beam steerers. 
         [0020]      FIG. 2  is a block diagram illustrating one embodiment of an optical amplification system  200 , according to various embodiments of the invention. In the optical amplification system  200 , the optically amplified chirped optical pulse  130  is split by an optical splitter  235 . One skilled in the art will recognize that in some embodiments, the optical amplifier  125  may be omitted from the optical amplification system  200  such that the chirped optical pulse  120  is split by the optical splitter  235 , for example, when the chirped optical pulse  120  has sufficient power. According to various embodiments, the optical splitter  235  may include, for example, a fused fiber-based coupler or a beam splitter cube. In another embodiment, the optical splitter  235  may include a series of optical splitters. The optical splitter  235  may divide the optically amplified chirped optical pulse  130  to produce a plurality of split optical pulses  240 . Each of the plurality of split optical pulses  240  may have similar duration as the optically amplified chirped optical pulse  130 , but with reduced power. 
         [0021]    In one alternative embodiment, the optical splitter  235  may be a temporal splitter. The temporal splitter may direct different pulses from a high-repetition pulse train into different fibers. The temporal splitter may result in reduced loss of optical power at the optical splitter  235 . One skilled in the art will recognize that in some embodiments, the temporal splitter may comprise an acousto-optic switch or a series of binary switches. 
         [0022]    Subsequent to the optically amplified chirped optical pulse  130  being split by the optical splitter  235 , each of the plurality of split optical pulses  240  may be received by a separate optical amplifier  245 . The optical amplifiers  245  may have any number of physical configurations. The configuration illustrated in  FIG. 2  is a linear parallel array. According to some embodiments, the optical amplifiers  245  may be arranged in close proximity to each other. As one skilled in the art will recognize, the optical amplifiers  245  may be arranged in a substantially circular array. 
         [0023]    Each of the plurality of split optical pulses  240  may have a reduced peak power relative to that of the optically amplified chirped optical pulse  130 . To regain the power lost as a result of splitting, the plurality of split optical pulses  240  may be further amplified. In the embodiment illustrated in  FIG. 2 , the optical amplifiers  245  may include a plurality of individual optical amplifiers, each being similar to the optical amplifier  125 . The optical amplifiers  245  produce at least one optically amplified split optical pulse  250 . The optically amplified split optical pulse  250  may have increased peak power and similar duration relative to one of the plurality of split optical pulses  240 . 
         [0024]    Following amplification by the optical amplifiers  245 , the optically amplified split optical pulse  250  may be temporally compressed by an optical pulse compressor  255 . In an exemplary embodiment, the optical pulse compressor  255  may include a plurality of individual optical pulse compressors (e.g., similar to the optical pulse compressor  155 ), each of which may separately receive a pulse. A compressed optically amplified split optical pulse  260  may be produced by the optical pulse compressor  255 . The compressed optically amplified split optical pulse  260  may have duration similar to the optical pulse  110 , but with much higher peak power. The compressed optically amplified split optical pulse  260  may then be received by a delivery system  285 . 
         [0025]    The delivery system  285  may include a plurality of independent delivery systems which may each be similar to the delivery system  185 . The delivery system  285  may deliver one or more of the compressed optically amplified split optical pulses  260  to at least one location. The delivery system  285  is discussed further herein. 
         [0026]      FIG. 3  is a block diagram illustrating another embodiment of an optical amplification system  300 , according to various embodiments of the invention. The optical amplification system  300  may operate similarly to the optical amplification system  200 , while certain individual components have been substituted for other components (e.g., bulk components) as discussed herein. According to the embodiment shown in  FIG. 3 , a single optical amplifier  345  has replaced the plurality of optical amplifiers  245  of the optical amplification system  200 . According to various embodiments, the optical amplifier  345  may include a single double-clad fiber with multiple cores (e.g., photonic crystal fiber, micro-structured fiber, photonic band gap fiber, holey fiber, and Bragg fiber). In one embodiment, the optical amplifier  345  may include a single bulk amplifier. 
         [0027]    Further, in the embodiment illustrated in  FIG. 3 , a single optical pulse compressor  355  has replaced the plurality of optical pulse compressors  255  of the optical amplification system  200 . According to some embodiments, the optical pulse compressor  355  may include a bulk grating compressor. In other embodiments, the optical pulse compressor  355  may include a single volume Bragg grating. 
         [0028]    Additionally, in the embodiment illustrated in  FIG. 3 , a single delivery system  385  has replaced the plurality of delivery systems  285  of the optical amplification system  200 . According to various embodiments, the delivery system  385  serves to deliver a plurality of compressed optically amplified split optical pulses to at least one location. The delivery system  385  is discussed further herein. 
         [0029]    Various other embodiments at least include substituting or combining the components illustrated in  FIG. 2  (e.g., the optical amplifiers  245 , the optical pulse compressor  255 , and the delivery system  285 ) with the analogous components illustrated in  FIG. 3  (e.g., the optical amplifier  345 , optical pulse compressor  355 , and delivery system  385 ). For example, those skilled in the art would appreciate that an optical amplification system which included the optical amplifiers  245 , the optical pulse compressor  355 , and the delivery system  285  would embody the present invention. As mentioned herein, any combination described herein may also include integration into a planar waveguide system. 
         [0030]      FIG. 4  is a block diagram illustrating yet another embodiment of an optical amplification system  400  including polarization combination, according to various embodiments of the invention. In this embodiment, the optical splitter  235  may optically split the optically amplified chirped optical pulse  130  to produce at least one pair of split optical pulses  440 . The polarization of the optically amplified chirped optical pulse  130  may be preserved in the pair of split optical pulses  440 . This means that the polarization of the two pulses may be substantially parallel. In  FIG. 4 , parallel polarization is denoted by the symbol consisting of two parallel lines, “//”. 
         [0031]    Since each of the pair of split optical pulses  440  has a reduced power relative to the optically amplified chirped optical pulse  130 , the pair of split optical pulses  440  may be further amplified. Subsequent to the optically amplified chirped optical pulse  130  being optically split by the optical splitter  235 , each of the pair of split optical pulses  440  may be received by an optical amplifier  445 . According to the embodiment illustrated in  FIG. 4 , the optical amplifiers  445  may include individual optical amplifiers, each being similar to the optical amplifier  125 . In another embodiment, the optical amplifiers  445  may each correspond to one of the pulses of the pair of split optical pulses  440 . According to various other embodiments, the optical amplifiers  445  may include a single double-clad fiber with multiple cores (e.g., photonic crystal fiber, micro-structured fiber, photonic band gap fiber, holey fiber, or Bragg fiber). According to yet another embodiment, the optical amplifiers  445  may include a single bulk amplifier. The optical amplifiers  445  produce at least one pair of optically amplified split optical pulses  450 . Each of the pair of optically amplified split optical pulses  450  may have increased power and similar duration relative to each of the pair of split optical pulses  440 . 
         [0032]    Following optical amplification by the optical amplifiers  445 , each of the pair of optically amplified split optical pulses  450  may be temporally compressed by an optical pulse compressor  455 . Each of the optical pulse compressors  455  may include at least one optical pulse compressor similar to the optical pulse compressor  155 . A pair of compressed optically amplified split optical pulses  460  may be produced by the optical pulse compressors  455 . Each of the pair of compressed optically amplified split optical pulses  460  may have duration similar to the optical pulse  110 , but with much higher peak power. 
         [0033]    According to various embodiments, a pair of optical pulses may have approximately orthogonal polarization relative to one another to facilitate polarization combination. In the optical amplification system  400 , the polarization orientation of one of the pair of compressed optically amplified split optical pulses  460  may be rotated by approximately 90 degrees by a polarization rotator  465 . The polarization rotator may include any number of polarization rotating elements (e.g., a ½-wave plate). According to another embodiment, the polarization rotation of one of the pair of compressed optically amplified split optical pulses  460  may be achieved by physically rotating an optical fiber which contains the pulse. A pair of compressed optically amplified split optical pulses  470  results, having approximately orthogonal polarization relative to one another. In  FIG. 4 , approximately orthogonal polarization is illustrated by attributing the “//” symbol to one of the pulses of the pair of compressed optically amplified split optical pulses  470  and attributing the symbol resembling an inverted “T” to the other. 
         [0034]    Subsequent to polarization rotation, the pair of compressed optically amplified split optical pulses  470  may be polarization combined by, for example, a polarization combiner  475 . According to various embodiments, the polarization combiner  475  may be fiber-based or a bulk element. The polarization combined pulse  480  may be received by a delivery system  485 . 
         [0035]    According to various embodiments, a delivery system, such as the delivery system  285 , delivery system  385 , and delivery system  485 , may include any combination of optical fibers, focusing optics, beam modulators, and beam steerers.  FIG. 5  is a diagram illustrating a variety of delivery system configurations, according to various embodiments of the invention. The delivery systems  285 ,  385 , and  485  may be configured to focus the plurality of compressed optically amplified split optical pulses to a spot. As illustrated in  FIG. 5(   a ), a plurality of beams  510  may be focused by a lens  520  to a spot  530 . The plurality of beams  510  may or may not be synchronized, meaning that the pulses contained in the beams may impinge a target at the same time or at different times. 
         [0036]    According to other embodiments, a delivery system, such as the delivery systems  285 ,  385 , and  485 , may be configured to focus the plurality of compressed optically amplified split optical pulses to different areas, for example, as illustrated in  FIGS. 5(   b ) and ( c ). In  FIG. 5(   b ), this may be accomplished by passing the plurality of beams  510  through several independent media  540  which divert the propagation of a beam. According to one embodiment, the independent media  540  may include a glass prism. After being diverted, the lens  550  may focus the plurality of beams  510  to different areas  560 . 
         [0037]    In yet another embodiment, illustrated in  FIG. 5(   c ), each of the plurality of beams  510  are passed through a corresponding individual lens  570 , which may result in the beams being focused to different areas  580 . Focusing the plurality of compressed optically amplified split optical pulses to different areas may be a desirable approach, for example, in volume material removal applications. If beams are sufficiently separated, the average power thermal effects may be reduced. In another embodiment, the delivery system configuration  500  may be configured to independently modulate (i.e., turn on and off). According to one embodiment, the delivery system configuration  500  may be configured to independently scan the plurality of compressed optically amplified split optical pulses. 
         [0038]    In alternative embodiments, the delivery systems  285 ,  385 , and  485  may include a temporal splitter. The temporal splitter may combine different pulses from, for example, different fibers. As mentioned herein, one skilled in the art will recognize that in some embodiments, the temporal splitter may comprise an acousto-optic switch or a series of binary switches. Additionally, one skilled in the art will further recognize that a spatial or temporal optical splitter may be located at other positions in the optical amplification systems described herein (e.g., between the optical amplifier  345  and the optical pulse compressor  355 ), in accordance with some embodiments. 
         [0039]      FIG. 6  is a flowchart  600  showing an exemplary process for providing a compressed optically amplified split optical pulse, according to various embodiments of the invention. At step  610 , a chirped optical pulse (e.g., the chirped optical pulse  120 ) is optically amplified to produce an optically amplified chirped optical pulse (e.g., the optically amplified chirped optical pulse  130 ). As discussed in detail herein, step  610  may be performed by an optical amplifier, such as optical amplifier  125 . 
         [0040]    At step  620 , the optically amplified chirped optical pulse is optically split to produce a plurality of split optical pulses (e.g., the plurality of split optical pulses  240 ). As discussed in detail herein, step  620  may be performed by an optical splitter, such as the optical splitter  235 . 
         [0041]    At step  630 , at least one of the plurality of split optical pulses is optically amplified to produce an optically amplified split optical pulse (e.g., the optically amplified split optical pulse  250 ). As discussed in detail herein, step  630  may be performed by an optical amplifier, such as one of the optical amplifiers  245  and the optical amplifier  345 . 
         [0042]    At step  640 , the optically amplified split optical pulse is optically compressed to produce a compressed optically amplified split optical pulse (e.g., the compressed optically amplified split optical pulse  260 ). As discussed in detail herein, step  640  may be performed by an optical pulse compressor, such as the optical pulse compressor  255  and the optical pulse compressor  355 . 
         [0043]    At step  650 , the polarization of one of two compressed optically amplified split optical pulses is rotated by approximately 90 degrees to produce a pair of approximately orthogonally polarized compressed optically amplified split optical pulses (e.g., the pair of compressed optically amplified split optical pulses  470 ). As discussed in detail herein, step  650  may be performed by a polarization rotator, such as polarization rotator  465 . 
         [0044]    At step  660 , the pair of approximately orthogonally polarized compressed optically amplified split optical pulses is polarization combined. As discussed in detail herein, step  660  may be performed by a polarization combiner, such as polarization combiner  475 . 
         [0045]    As mentioned herein, the process shown in the flowchart  600  is exemplary. For example, steps  650  and  660  may be omitted according to some embodiments. In other embodiments, steps may be added which describe certain delivery techniques as may be implemented by delivery systems, such as the delivery systems  285 ,  385 , and  485 . 
         [0046]    Those skilled in the art would appreciate that waveguides other than optical fibers may be used for some or all components of the optical amplification systems discussed herein. Examples of other waveguides may include planar, or “chip-based,” waveguides. These waveguides may have a substantially rectangular cross-section and allow the same or similar guiding techniques to be utilized as with traditional optical fiber. 
         [0047]    The above description is illustrative and not restrictive. Many variations of the invention will become apparent to those of skill in the art upon review of this disclosure. The scope of the invention should, therefore, be determined not with reference to the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalents.