Patent Publication Number: US-11646541-B2

Title: Femtosecond laser device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 of Korean Patent Application Nos. 10-2019-0149769, filed on Nov. 20, 2019, and 10-2020-0140645, filed on Oct. 27, 2020, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure herein relates to a laser device, and more particularly, to a femtosecond laser device. 
     The development of industrial technology requires precision and high productivity in the fields in which laser pulses are used, and, to meet such requirements, femtosecond laser pulses are recently used in various fields. Femtosecond laser pulses exhibit characteristics different from those of typical laser pulses. For example, since femtosecond laser pulses are radiated to a medium only for a short time, a thermal influence or thermal deformation that may occur during typical laser processing may be avoided. Furthermore, femtosecond laser pulses make it possible to process an interior of a medium without damaging the surface of the medium. 
     SUMMARY 
     The present disclosure provides a femtosecond laser device capable of minimizing a nonlinear dispersion value of a laser pulse. 
     An embodiment of the inventive concept provides a femtosecond laser device including: a pulse oscillator configured to generate a laser pulse; a pulse width stretcher configured to stretch a width of the laser pulse; a pulse width compressor connected to the pulse width stretcher to compress the width of the laser pulse; a pulse amplifier disposed between the pulse width compressor and the pulse width stretcher to amplifier an intensity of the laser pulse; and a nonlinear pulse attenuator including an optical fiber connected between the pulse width amplifier and the pulse width stretcher, the optical fiber deformed to have a spiral shape, a stretched length, or a twist. 
     In an embodiment, the nonlinear pulse attenuator may further include a bobbin for winding the optical fiber. 
     In an embodiment, the bobbin may have an outer diameter of about 30 mm. 
     In an embodiment, the bobbin may have a length that is at least 15 times a thickness of the optical fiber. 
     In an embodiment, the bobbin may include: a first bobbin having a first diameter; and a second bobbin having a second diameter that is less than the first diameter. 
     In an embodiment, a difference between the first diameter and the second diameter may be about 10 mm or about 60 mm. 
     In an embodiment, the first bobbin and the second bobbin may have a crossed angle between center axes thereof, wherein the crossed angle may be ±π/4. 
     In an embodiment, the bobbin may include a circular tube or a circular cylinder. 
     In an embodiment, the nonlinear pulse attenuator may further include an optical fiber stretcher for stretching the optical fiber. 
     In an embodiment, the optical fiber stretcher may stretch the optical fiber so that the optical fiber has the stretched length that is about 5% of a unit length. 
     In an embodiment, the optical fiber stretcher may include: a first clamping portion fixing one side of the optical fiber; a second clamping portion facing the first clamping portion and fixing another side of the optical fiber; and a stretching portion connecting the first clamping portion to the second clamping portion. 
     In an embodiment, the stretching portion may includes: a first rod connected to the first clamping portion; a second rod connected to the second clamping portion; and a locker disposed in an overlap region between the first rod and the second rod and fixing the first rod and the second rod. 
     In an embodiment, the locker may include a bolt. 
     In an embodiment, the nonlinear pulse attenuator may further include an optical fiber twister for twisting the optical fiber. 
     In an embodiment, the optical fiber twister may include a coil spring. 
     In an embodiment, the optical fiber twister may twist the optical fiber at a twisting angle of ±π/4. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying drawings are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the inventive concept and, together with the description, serve to explain principles of the inventive concept. In the drawings: 
         FIG.  1    is a diagram illustrating an example of a femtosecond laser device according to the inventive concept; 
         FIG.  2    is a diagram illustrating an example of the pulse oscillator of  FIG.  1   ; 
         FIG.  3    is a cross-sectional view illustrating an example of the pulse width stretcher of  FIG.  1   ; 
         FIG.  4    is a diagram illustrating an example of the nonlinear pulse attenuator of  FIG.  1   ; 
         FIG.  5    is a graph illustrating a nonlinear dispersion value according to an outer diameter of a bobbin; 
         FIG.  6    is a graph illustrating a nonlinear dispersion value according to the number of turns of the optical fiber of  FIG.  4   ; 
         FIG.  7    is a diagram illustrating an example of the nonlinear pulse attenuator of  FIG.  1   ; 
         FIG.  8    is a graph illustrating a nonlinear dispersion value according to a difference between the first diameter and the second diameter of  FIG.  7   ; 
         FIG.  9    is a diagram illustrating an example of the first bobbin and the second bobbin of  FIG.  7   ; 
         FIG.  10    is a diagram illustrating a crossed angle between the first bobbin and the second bobbin of  FIG.  9   ; 
         FIG.  11    is a graph illustrating a nonlinear dispersion value according to the crossed angle between the first bobbin and the second bobbin of  FIG.  10   ; 
         FIG.  12    is a diagram illustrating an example of the nonlinear pulse attenuator of  FIG.  1   ; 
         FIG.  13    is a graph illustrating a nonlinear dispersion value according to a ratio of a stretched length to a unit length of the optical fiber of  FIG.  12   ; 
         FIG.  14    is a diagram illustrating an example of the nonlinear pulse attenuator of  FIG.  1   ; 
         FIG.  15    is a diagram illustrating a nonlinear dispersion value according to the twisting angle of  FIG.  14   ; and 
         FIG.  16    is a diagram illustrating an example of the pulse width compressor of  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the inventive concept will now be described in detail with reference to the accompanying drawings. The advantages and features of the inventive concept, and methods for achieving the advantages and features will be apparent from the embodiments described in detail below with reference to the accompanying drawings. Therefore, the inventive concept may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art, and the inventive concept is only defined by the scope of the claims. Like reference numerals refer to like elements throughout. 
     The terminology used herein is not for delimiting the embodiments of the inventive concept but for describing the embodiments of the inventive concept. The terms of a singular form may include plural forms unless otherwise specified. It will be further understood that the terms “includes”, “including”, “comprises”, and/or “comprising”, when used in this description, specify the presence of stated elements, steps, operations, and/or components, but do not preclude the presence or addition of one or more other elements, steps, operations, and/or components. Furthermore, the terms “optical fiber”, “laser pulse”, and “pulse width” used herein may be construed as general optics terminology. Reference numerals, which are presented in the order of description, are provided according to the embodiments and are thus not necessarily limited to the order. 
       FIG.  1    illustrates an example of a femtosecond laser device  10  according to the inventive concept. 
     Referring to  FIG.  1   , the femtosecond laser device  10  of an embodiment of the inventive concept may include a pulse oscillator  20 , a pulse width stretcher  30 , a nonlinear pulse attenuator  40 , a pulse amplifier  50 , and a pulse width compressor  60 . The pulse oscillator  20  may generate a laser pulse  102 . The laser pulse  102  may have a pulse width  104 . The pulse width  104  may be defined as a time interval at which an intensity and/or amplitude of the pulse  102  halves at a rising time and falling time of the pulse  102 . The intensity of the laser pulse  102  may be changed in the pulse width stretcher  30 , the pulse amplifier  50 , and the pulse width compressor  60 . The pulse width stretcher  30  may stretch the pulse width  104  by adjusting dispersion values of the laser pulse  102 . When the pulse width  104  is stretched, the intensity of the pulse  102  may reduce. The nonlinear pulse attenuator  40  may reduce a nonlinear dispersion value among the dispersion values of the laser pulse  102 . The pulse amplifier  50  may amplify the intensity of the laser pulse  102 . The pulse width compressor  60  may compress the pulse width  104  of the laser pulse  102 . For example, the intensity of the laser pulse  102  in the pulse width compressor  60  may increase at least about 105 to 106 times the intensity of the laser pulse  102  in the pulse oscillator  20 . 
       FIG.  2    illustrates an example of the pulse oscillator  20  of  FIG.  1   . 
     Referring to  FIG.  2   , the pulse oscillator  20  may be an optical fiber laser oscillator. For example, the pulse oscillator  20  may include a gain medium optical fiber  22 , a first pump light source  24 , a reflective mirror  26 , an isolator  28 , and a first optical grating  29 . 
     The gain medium optical fiber  22  may be connected between the reflective mirror  26  and the isolator  28 . The gain medium optical fiber  22  may have a gain medium (e.g., ytterbium (Yb)). 
     The first pump light source  24  may be connected to the gain medium optical fiber  22 . The first pump light source  24  may provide pump light  21  to the gain medium optical fiber  22 . The pump light  21  may have a wavelength of about 976 nm. The gain medium optical fiber  22  may obtain a gain of the laser pulse  102  using the pump light  21 . The laser pulse  10  may have a wavelength of about 1030 nm. The first pump light source  24  may include a laser diode. 
     The reflective mirror  26  may be connected to one side of the gain medium optical fiber  22 . The reflective mirror  26  may reflect the laser pulse  102  into the gain medium optical fiber  22 . The reflective mirror  26  may include a saturable absorber mirror. 
     The isolator  28  may be connected to another side of the gain medium optical fiber  22 . The isolator  28  may reflect a portion of the laser pulse  102  into the gain medium optical fiber  22 . Although not illustrated, the isolator  28  may be connected to the pulse width stretcher  30 . The isolator  28  may provide a portion of the laser pulse  102  to the pulse width stretcher  30 . 
     The first optical grating  29  may be connected to the gain medium optical fiber  22  between the isolator  28  and the reflective mirror  26 . The first optical grating  29  may include a chirped fiber Bragg grating. The first optical grating  29  may generate the laser pulse  102  using the pump light  21 . 
     The laser pulse  102  may have a frequency of about 1 MHz to about 100 MHz. The laser pulse  102  may have a full width at half maximum of about 0.1 nm to about 100 nm. The laser pulse  102  may have a pulse width of about 0.01 ps to about 100 ps. The laser pulse  102  may have output power of about 1 mW to about 1 W. 
       FIG.  3    illustrates an example of the pulse width stretcher  30  of  FIG.  1   . 
     Referring to  FIG.  3   , the pulse width stretcher  30  may be an optical fiber pulse width stretcher. For example, the pulse width stretcher  30  may include a cladding 32 and a core  33 . The cladding 32 may surround an outer circumferential surface of the core  33 . The core  33  may have a higher refractive index than that of the cladding 32. The core  33  may transmit the laser pulse  102 . The core  33  may have a Bragg grating structure. The core  33  may include high-refractive portions  34  and low-refractive portions  35 . The high-refractive portions  34  and the low-refractive portions  35  may have different refractive indices. The high-refractive portions  34  and the low-refractive portions  35  may be alternately disposed. The high-refractive portions  34  and the low-refractive portions  35  may increase a pulse width  104  and dispersion value of the laser pulse  102 . 
     The dispersion value of the laser pulse  102  may be described as below. The laser pulse  102  may be expressed as Equation 1 by Taylor-expanding a wavenumber function k(ω) for an angular frequency ω with respect to a center frequency ω 0 . Here, a coefficient of a quadratic term of Equation 1 represents a group velocity dispersion (GVD) value as expressed in Equation 2 below. The GVD value is equal to a group delay dispersion (GDD) value per unit length and a second-order dispersion value per unit length. “L” represents a travel distance of laser. Furthermore, a coefficient of a cubic term of Equation 1 represents a third-order dispersion value per unit length as expressed in Equation 3 below. A coefficient of a fourth-order term of Equation 1 represents a fourth-order dispersion value per unit length as expressed in Equation 4 below. 
     
       
         
           
             
               
                 
                   
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     For example, a second-order dispersion value k″ may be a linear dispersion value, and a third-order dispersion value k″ and a fourth-order dispersion value k″″ may be nonlinear dispersion values or high-order dispersion values. The linear dispersion value may be proportional to the pulse width  104  of the laser pulse  102 . The high-refractive portions  34  and the low-refractive portions  35  may increase the pulse width  104  of the laser pulse  102  by increasing the second-order dispersion value k″ of the linear dispersion value of the laser pulse  102 . On the contrary, the nonlinear dispersion value may abnormally deform the laser pulse  102 , thus reducing output efficiency of the laser pulse  102 . 
     Referring back to  FIG.  1   , the nonlinear pulse attenuator  40  may reduce the nonlinear dispersion value of the laser pulse  102 , thus increasing the output efficiency of the laser pulse  102 . 
       FIG.  4    illustrates an example of the nonlinear pulse attenuator  40  of  FIG.  1   . 
     Referring to  FIG.  4   , the nonlinear pulse attenuator  40  may include a nonlinear dispersion filter, a nonlinear dispersion adjuster, a high-order dispersion adjuster, or a high-order dispersion remover. For example, the nonlinear pulse attenuator  40  may include an optical fiber  42  and a bobbin  44 . 
     The optical fiber  42  may connect the pulse width stretcher  30  to the pulse amplifier  50 . The optical fiber  42  may transfer the laser pulse  102  between the pulse width stretcher  30  and the pulse amplifier  50 . For example, the optical fiber  42  may be deformed to have a coiling and/or spiral shape. 
     The bobbin  44  may wind the optical fiber  42  therearound so as to deform the optical fiber  42  in a coiling and/or spiral shape. For example, the bobbin  44  may include a circular tube or a circular cylinder. The optical fiber  42  may be wound multiple times around an outer circumferential surface of the bobbin  44  in a longitudinal direction thereof. Although not illustrated, an adhesive tape may be provided on the optical fiber  42 . The adhesive tape may fix the optical fiber  42  to the bobbin  44 . 
       FIG.  5    illustrates a nonlinear dispersion value according to an outer diameter D of the bobbin  44 . 
     Referring to  FIG.  5   , when the outer diameter D of the bobbin  44  is about 30 mm, the nonlinear dispersion value may reduce to a minimum value. The output efficiency of the laser pulse  102  may increase. When the outer diameter D of the bobbin  44  is about 10 mm and from about 50 mm to about 110 mm, the nonlinear dispersion value may increase. The output efficiency of the laser pulse  102  may reduce. 
       FIG.  6    illustrates a nonlinear dispersion value according to the number of turns of the optical fiber  42  of  FIG.  4   . 
     Referring to  FIG.  6   , when the optical fiber  42  is wound about at least 15 times around the outer circumferential surface of the bobbin  44 , the nonlinear dispersion value may reduce to a minimum value. When the number of turns of the optical fiber  42  increases up to about 15, the nonlinear dispersion value may gradually decrease. The output efficiency of the laser pulse  102  may increase. The bobbin  44  may have a length L 1  that is at least 15 times a thickness T and/or diameter of the optical fiber  42 . The bobbin  44  with the optical fiber  42  wound therearound about at least 15 times may reduce the nonlinear dispersion value. 
       FIG.  7    illustrates an example of the nonlinear pulse attenuator  40  of  FIG.  1   . 
     Referring to  FIG.  7   , the bobbins  44  of the nonlinear pulse attenuator  40  may individually wind the optical fiber  42  in a longitudinal direction of the bobbins  44 . For example, the bobbins  44  may include a first bobbin  46  and a second bobbin  48 . The first bobbin  46  may wind the optical fiber  42  with a first outer diameter D 1 . The second bobbin  48  may be disposed in parallel to the first bobbin  46 . The second bobbin  48  may wind the optical fiber  42  with a second outer diameter D 2  that is less than the first outer diameter D 1 . 
       FIG.  8    illustrates a nonlinear dispersion value according to an outer diameter difference between the first outer diameter D 1  and the second outer diameter D 2  of  FIG.  7   . 
     Referring to  FIG.  8   , when the outer diameter difference D 1 -D 2  is about 10 mm or about 60 mm, the nonlinear dispersion value may reduce to a minimum value. The output efficiency of the laser pulse  102  may increase. When the outer difference D 1 -D 2  is about 20 mm or about 80 mm, the nonlinear dispersion value may increase. The output efficiency of the laser pulse  102  may reduce. 
       FIG.  9    illustrates an example of the first bobbin  46  and the second bobbin  48  of  FIG.  7   .  FIG.  10    illustrates a crossed angle θ between the first bobbin  46  and the second bobbin  48  of  FIG.  9   . 
     Referring to  FIGS.  9  and  10   , the first bobbin  46  and the second bobbin  48  may be unparallel to each other. For example, the first bobbin  46  and the second bobbin  48  may have a crossed angle θ. The crossed angle θ may be defined between a center axis  43  of the first bobbin  46  and a center axis  45  of the second bobbin  48 . 
       FIG.  11    illustrates a nonlinear dispersion value according to the crossed angle θ between the first bobbin  46  and the second bobbin  48  of  FIG.  10   . 
     Referring to  FIG.  11   , when the crossed angle θ between the first bobbin  46  and the second bobbin  48  is ±π/4, the nonlinear dispersion value may be reduced to a minimum value. The output efficiency of the laser pulse  102  may increase. When the crossed angle θ is 0 or ±π/2, the nonlinear dispersion value may increase. The output efficiency of the laser pulse  102  may reduce. 
       FIG.  12    illustrates an example of the nonlinear pulse attenuator  40  of  FIG.  1   . 
     Referring to  FIG.  12   , the nonlinear pulse attenuator  40  may include an optical fiber stretcher  47  for stretching the optical fiber  42 . The optical fiber stretcher  47  may adjust the nonlinear dispersion value by stretching the optical fiber  42  in longitudinal direction thereof. For example, the optical fiber  42  having a unit length L 0  may be deformed so as to have a stretched length ΔL. For example, the optical fiber stretcher  47  may include a first clamping portion  472 , a second clamping portion  474 , and a stretching portion  476 . The first clamping portion  472  may fix one side of the optical fiber  42 . The second clamping portion  474  may fix another side of the optical fiber  42 . The stretching portion  476  may connect the first clamping portion  472  to the second clamping portion  474 . The stretching portion  476  may adjust a distance between the first clamping portion  472  and the second clamping portion  474 . The stretching portion  476  may include a first rod  482 , a second rod  484 , and a locker  486 . The first rod  482  may be connected to the first clamping portion  472 , and may be partially inserted into the second rod  484 . The second rod  484  may be connected to the second clamping portion  474 . The locker  486  may be provided on an overlap region between the first rod  482  and the second rod  484 . For example, the locker  486  may include a bolt. When the optical fiber  42  is stretched so as to have the stretched length ΔL, the locker  486  may fix the optical fiber  42  by fixing the first rod  482  and the second rod  484 . 
       FIG.  13    illustrates a nonlinear dispersion value according to a ratio of the stretched length ΔL to the unit length L 0  of the optical fiber  42  of  FIG.  12   . 
     Referring to  FIG.  13   , when the ratio of the stretched length ΔL to the unit length L 0  is about 5%, the nonlinear dispersion value may reduce to a minimum value. The output efficiency of the laser pulse  102  may increase. When the ratio of the stretched length ΔL to the unit length L 0  is less than or greater than about 5%, the nonlinear dispersion value may increase. The output efficiency of the laser pulse  102  may reduce. 
       FIG.  14    illustrates an example of the nonlinear pulse attenuator  40  of  FIG.  1   . 
     Referring to  FIG.  14   , the nonlinear pulse attenuator  40  may twist the optical fiber  42 . For example, the nonlinear pulse attenuator  40  may include the optical fiber  42  and an optical fiber twister  49 . The optical fiber  42  may be provided in the optical twister  49 . The optical fiber  42  may be deformed so as to be twisted. The optical fiber twister  49  may twist the optical fiber  42  in a direction of twisting angle ϕ. For example, the optical fiber twister  49  may include a coil spring. 
       FIG.  15    illustrates a nonlinear dispersion value according to the twisting angle ϕ of  FIG.  14   . 
     Referring to  FIG.  15   , when the twisting angle ϕ is ±π/4, the nonlinear dispersion value may reduce to a minimum value. The output efficiency of the laser pulse  102  may increase. When the twisting angle ϕ is 0 or ±7π/2, the nonlinear dispersion value may increase. The output efficiency of the laser pulse  102  may reduce. 
     Referring back to  FIG.  1   , the pulse amplifier  50  may be provided between the nonlinear pulse attenuator  40  and the pulse width compressor  60 . For example, the pulse amplifier  50  may include an optical fiber amplifier. Although not illustrated, the pulse amplifier  50  may include an optical fiber and a gain medium (e.g., ytterbium (Yb), erbium (Er), etc.) in the optical fiber. The pulse amplifier  50  may increase the intensity of the laser pulse  102  by about two to ten times. 
       FIG.  16    illustrates an example of the pulse width compressor  60  of  FIG.  1   . 
     Referring to  FIG.  16   , the pulse width compressor  60  may be similar to the pulse width stretcher  30 . For example, the pulse width compressor  60  may include second optical gratings  62 , a chirped mirror  64 , and a dispersion adjustment portion  66 . The second optical gratins  62  may diffract and/or disperse the laser pulse  102 . The chirped mirror  64  may be disposed on an outer periphery of the second optical gratings  62 . The chirped mirror  64  may reflect the laser pulse  102  towards the second optical gratings  62 . The dispersion adjustment portion  66  may be provided between the second optical gratings  62 . The dispersion adjustment portion  66  may reduce the pulse width  104  of the laser pulse  102  by adjusting an optical distance between the second optical gratings  62 . For example, the dispersion adjustment portion  66  may reduce the linear dispersion value and pulse width  104  of the laser pulse  102  by reducing the optical distance between the second optical gratings  62 . On the contrary, the nonlinear dispersion value of the laser pulse  102  may not be linearly reduced due to adjustment of the optical distance between the second optical gratins  62 . 
     Referring back to  FIGS.  4  to  15   , the nonlinear pulse attenuator  40  may reduce and/or remove the nonlinear dispersion value in advance before the pulse width compressor  60 , thereby increasing the output efficiency of the laser pulse  102 . 
     As described above, the femtosecond laser device according to an embodiment of the inventive concept may reduce a nonlinear dispersion value of a laser pulse using a nonlinear pulse attenuator including an optical fiber that is deformed to have a spiral shape, stretched length, or twist. 
     Although the exemplary embodiments of the present invention have been described, it is understood that the present invention should not be limited to these exemplary embodiments but various changes and modifications can be made by one ordinary skilled in the art within the spirit and scope of the present invention as hereinafter claimed.