Abstract:
The present invention relates to an apparatus for making a source material into a crystal fiber having different regions of polarization inversion. The apparatus of the present invention is similar to a laser-heated pedestal growth (LHPG) apparatus, characterized in that a first electric field generating device and a second electric field generating device are included. The first electric field generating device is used for providing a first external electric field which is used for poling the crystal fiber and inducing micro-swing of the crystal fiber. The second electric field generating device is disposed on a predetermined position above the first electric field generating device for providing a second external electric field to control and maintain the amplitude of the micro-swing. Whereby, the growth condition of the crystal fiber can be controlled precisely, and a uniformly and regularly periodic polarization inversion structure is fabricated.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method and an apparatus for fabricating a crystal fiber, and more particularly, to a method and an apparatus where two external electric fields are applied on the grown crystal fiber during the growth procedure of the crystal fiber so that the growth condition can be controlled precisely.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 1  shows a schematic diagram of a conventional apparatus for fabricating a crystal fiber. The conventional apparatus  10  is similar to a laser heated pedestal growth (LHPG) apparatus, which is used for making a source crystal rod  20  into a crystal fiber  21  having different regions of polarization inversion. The material of the source crystal rod  20  is lithium niobate (LiNbO 3 ). A molten zone  16  is formed between the tip of the source crystal rod  20  and the crystal fiber  21 . The conventional apparatus  10  comprises a laser beam generator (not shown), a beam splitter  12 , a bending mirror  13 , a paraboloidal mirror  14  and a pair of metal electrodes  18 , 19 .  
         [0005]     The laser beam generator is used for generating a laser beam  11 . The beam splitter  12  includes an outer cone  121  and an inner cone  122 . The outer cone  121  has a first conical surface  1211  and the inner cone  122  has a second conical surface  1221 , respectively. The beam splitter  12  is used for splitting the laser beam  11  into a generally annular beam  111 . The bending mirror  13  is used for reflecting the annular beam  111  from the beam splitter  12  and projecting it to the paraboloidal mirror  14 . The paraboloidal mirror  14  is used for reflecting the annular beam  111  from the bending mirror  13 , and focusing the annular beam  111  on the molten zone  16  at the tip of the source crystal rod  20 . The metal electrodes  18 , 19  are disposed near the crystal fiber  21  and are parallel to the growth direction of the crystal fiber  21  for providing an external electric field on the molten zone  16 . The metal electrodes  18 , 19  are connected to two high-voltage sources (not shown) respectively for providing a periodic alternating electric field so as to induce micro-swing during the growth procedure of the crystal fiber  21 .  
         [0006]      FIGS. 2   a  to  2   c  show the micro-swing occurred during the growth of the crystal fiber  21 , wherein  FIG. 2   b  shows the appearance of the crystal fiber  21  without being applied by any external electric field,  FIG. 2   a  shows that the crystal fiber  21  swings to the left when being applied by an external electric field, and  FIG. 2   c  shows that the crystal fiber  21  swings to the right when being applied by an external electric field. During the growth procedure of the crystal fiber  21 , when the lithium niobate crystal is heated to the melting state, negative charges will be induced and distributed on the circumferences of upper portion and lower portion of the molten zone  16  because of the ionization and precipitation of the lithium ions (Li + ). The negative charges are attracted by positive electric field and distracted by negative electric field, which causes the micro-swing during the growth procedure of the lithium niobate crystal fiber  21 . For one crystal, its displacement is defined as the amplitude of the micro-swing of the crystal fiber  21 .  
         [0007]      FIG. 3  shows a relationship between the intensity of the external electric field and the total length of the crystal fiber  21  in the conventional apparatus  10 .  FIG. 4  shows a relationship between the amplitude of the micro-swing and the total length of the crystal fiber  21  in the conventional apparatus  10 . The total length L 1  of the crystal fiber  21  is the length of the crystal fiber  21  from the molten zone  16 . If the intensity of the external electric field is constant, the value of the amplitude of the micro-swing is in direct proportion with the total length L 1  of the crystal fiber  21 . Accordingly, if the crystal fiber  21  grows freely without changing the intensity of the external electric field, the amplitude of the micro-swing of the crystal fiber  21  will increase continuously until the molten zone  16  breaks. Therefore, as shown in  FIG. 3 , when the total length L 1  of the crystal fiber  21  exceeds a particular value, the intensity of the external electric field must be reduced. Additionally, it is found that the value of the amplitude of the micro-swing of the crystal fiber  21  must be larger than the diameter of the crystal fiber  21  in order to fabricate a perfect periodic polarization inversion structure. Therefore, if an external electric field adjusted according to  FIG. 3  is applied to the crystal fiber  21 , the value of the amplitude of the micro-swing can be controlled efficiently to be larger than the diameter of the crystal fiber  21  and to be constant, as shown in  FIG. 4 .  
         [0008]     Although the value of the amplitude of the micro-swing can be controlled efficiently, the intensity of the external electric field cannot be constant and must be adjusted to a small value when the crystal fiber  21  elongates. Hence, if the intensity of the external electric field is smaller than that of the required electric field for poling, the crystal fiber  21  will not have a periodic polarization inversion structure. Therefore, the length of the periodic polarization inversion structure formed by the conventional apparatus  10  is limited.  
         [0009]     Consequently, there is an existing need for a novel, improved method and an apparatus for fabricating a crystal fiber to solve the above-mentioned problems.  
       SUMMARY OF THE INVENTION  
       [0010]     One objective of the present invention is to provide an apparatus and method for creating different regions of polarization inversion on the ferroelectric crystalline material. A significant advantage of the present invention over prior art is that it can control the growth condition of the crystal fiber precisely so as to fabricate a uniformly and regularly periodic polarization inversion structure. Additionally, the length of the periodic polarization inversion structure formed by the present invention is longer than that formed by the conventional art.  
         [0011]     Another objective of the present invention is to provide an apparatus and method for fabricating a crystal fiber that has different regions of polarization inversion and has the advantages of high quality and high coupling efficiency so that it is used for applications in wavelength converter and visible light generation.  
         [0012]     Yet another objective of the present invention is to provide a method for fabricating a crystal fiber having different regions of polarization inversion, comprising:  
         [0013]     (a) providing a source material;  
         [0014]     (b) putting the source material into a fabricating apparatus; and  
         [0015]     (c) forming the crystal fiber from the source material and applying a first external electric field and a second external electric field on the grown crystal fiber during the growth procedure of the crystal fiber, wherein the first external electric field is applied on a molten zone between the source material and the crystal fiber so as to induce micro-swing of the crystal fiber for polarization inversion, and the second external electric field is applied on a predetermined position above the first external electric field to control and maintain the amplitude of the micro-swing.  
         [0016]     Still another objective of the present invention is to provide an apparatus for making a source material into a crystal fiber having different regions of polarization inversion. The apparatus of the present invention comprises a laser beam generator, a beam splitter, a bending mirror, a paraboloidal mirror, a first electric field generating device and a second electric field generating device.  
         [0017]     The laser beam generator is used for generating a laser beam. The beam splitter is used for splitting the laser beam into a generally annular beam. The bending mirror is used for reflecting the annular beam from the beam splitter. The paraboloidal mirror is used for reflecting the annular beam from the bending mirror, and focusing the annular beam on a molten zone between the source material and the crystal fiber. The first electric field generating device is disposed near the molten zone for providing a first external electric field which is used for poling the crystal fiber and inducing micro-swing of the crystal fiber. The second electric field generating device is disposed on a predetermined position above the first electric field generating device for providing a second external electric field to control and maintain the amplitude of the micro-swing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  shows a schematic diagram of a conventional apparatus for fabricating a crystal fiber;  
         [0019]      FIGS. 2   a  to  2   c  show a micro-swing occurred during the growth of a crystal fiber, wherein  FIG. 2   b  shows the appearance of the crystal fiber without being applied by any external electric field,  FIG. 2   a  shows that the crystal fiber swings to the left when being applied by an external electric field, and  FIG. 2   c  shows that the crystal fiber swings to the right when being applied by an external electric field;  
         [0020]      FIG. 3  shows a relationship between the intensity of the external electric field and the total length of the crystal fiber in the conventional apparatus;  
         [0021]      FIG. 4  shows a relationship between the amplitude of the micro-swing and the total length of the crystal fiber in the conventional apparatus;  
         [0022]      FIG. 5  shows a schematic diagram of a chamber of an apparatus for fabricating a crystal fiber according to the present invention;  
         [0023]      FIG. 6  shows a relationship between the amplitude of the micro-swing and the height of the electrode from the bottom of the molten zone when the external electric field is maintained at 1 kV/mm;  
         [0024]      FIG. 7  shows a relationship between the intensity of the external electric field provided by the second electric field generating device and the total length of the crystal fiber according to the present invention; and  
         [0025]      FIG. 8  shows an external electric field provided by the second electric field generating device according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]      FIG. 5  shows a schematic diagram of a chamber of an apparatus for fabricating a crystal fiber according to the present invention. The apparatus  50  is similar to a laser-heated pedestal growth (LHPG) apparatus, which is used for making a source material into a crystal fiber  61  having different regions of polarization inversion. The material of the source material may be crystal (for example, a source crystal rod  60 ) or powder. The material of the source crystal rod  60  is ferroelectric and is selected from the group consisting of lithium niobate (LiNbO 3 ), lithium tantalate (LiTaO 3 ), patassium titanyl phosphate (KTP) and a dopant therein. The dopant is selected from the group consisting of the oxidation states of magnesium, zinc, yttrium, neodymium and erbium, and the mixture thereof. In the embodiment, the material of the source crystal rod  60  is lithium niobate doped with 6% mol of zinc oxide (ZnO). The apparatus  50  comprises a laser beam generator (not shown), a beam splitter  52 , a bending mirror  53 , a paraboloidal mirror  54 , a first electric field generating device  55  and a second electric field generating device  57 .  
         [0027]     The laser beam generator is used for generating a laser beam  51 . The beam splitter  52  includes an outer cone  521  and an inner cone  522 . The outer cone  5121  has a first conical surface  5211  and the inner cone  522  has a second conical surface  5221 . The beam splitter  52  is used for splitting the laser beam  51  into a generally annular beam  511 . The bending mirror  53  is used for reflecting the annular beam  511  from the beam splitter  52  and projecting it to the paraboloidal mirror  54 . The paraboloidal mirror  54  is used for reflecting the annular beam  511  from the bending mirror  53 , and focusing the annular beam  511  on the molten zone  56  at the tip of the source crystal rod  60 .  
         [0028]     The first electric field generating device  55  includes a first right electrode  551  and a first left electrode  552 , and is used for providing a first external electric field. The first external electric field is applied on the molten zone  56  to induce micro-swing of the crystal fiber  61 . The second electric field generating device  57  includes a second right electrode  571  and a second left electrode  572 , and is used for providing a second external electric field. The second external electric field is applied on a predetermined position above the first external electric field to control and maintain the amplitude of the micro-swing.  
         [0029]      FIG. 6  shows a relationship between the amplitude of the micro-swing and the height of the electrode from the bottom of the molten zone  56  when the external electric field is maintained at 1 kV/mm, wherein the positive amplitude means that the molten zone  56  swings to the right, and the negative amplitude means that the molten zone  56  swings to the left. In order to control the value of the amplitude of the micro-swing by utilizing the second electric field generating device  57  (the second right electrode  571  and the second left electrode  572 ), it is necessary to know the relationship between the amplitude of the micro-swing and the height of the electrode. In  FIG. 6 , curve A designates that the second right electrode  571  having a direct current of −1.5 kV and the second left electrode  572  having a direct current of 1.5 kV; curve B designates that the second right electrode  571  having a direct current of −3.0 kV and the second left electrode  572  having a direct current of 0 kV. Both curves A and B have the same electric potential difference of 3 KV.  
         [0030]     As shown in  FIG. 6 , when the electrode is disposed under the molten zone  56  (area C), the molten zone  56  is attracted by positive electric field and distracted by negative electric field, which causes the amplitude of the curves A and B to be all negative value. This is because when the lithium niobate crystal is heated to the melting state, free negative charges will be induced and distributed on the circumference of the molten zone  56 . But, such an effect will decrease gradually when the height of electrode increases gradually, and the crystal fiber  61  is attracted by both positive and negative electric fields. This is because that the charges with opposite electrical property to the external field are induced on the circumference of the crystal fiber  61 . Therefore, if the external electric field is positive, the crystal fiber  61  is attracted by that positive external electric field; if the external electric field is negative, the crystal fiber  61  is attracted by that negative external electric field. Accordingly, in the embodiment, the second electric field generating device  57  (the second right electrode  571  and the second left electrode  572 ) is disposed on 2 to 10 mm, preferably 5 mm, above the molten zone  56 , and the gap between the two electrodes  571 ,  572  is about 1 mm. A suitable external electric field can be generated by providing two electrodes  571 , 572  with adequate electric potential.  
         [0031]      FIG. 7  shows a relationship between the intensity of the external electric field provided by the second electric field generating device  57  and the total length of the crystal fiber  61  according to the present invention. The total length L 2  of the crystal fiber  61  is the length of the crystal fiber  61  from the molten zone  56 . As shown in the figure, the curve of the relationship is an exponential-like binding curve that approaches −0.8 kV/mm, which means that the amplitude of the micro-swing of the crystal fiber  61  can be maintained at a constant value which is larger than the diameter of the crystal fiber  61 . Preferably, the ratio of the amplitude of the micro-swing to the diameter of the crystal fiber  61  is 1.0 to 1.5.  
         [0032]      FIG. 8  shows an external electric field provided by the second electric field generating device  57  according to the present invention, wherein AC 1  is a first periodic voltage from zero to positive electric potential provided by the second right electrode  571 , AC 2  is a second periodic voltage whose phase is reverse to that of the first periodic voltage. Such an arrangement can avoid the situation that the crystal fiber  61  is attracted by both one positive electric field and one negative electric field provided by two electrodes on two sides thereof. Additionally, the first electric field generating device  55  (the first right electrode  551  and the first left electrode  552 ) provides the first external electric field whose intensity is a constant value of about 0.8 kV/mm, which can make the crystal fiber  61  have a polarization inversion structure by the periodic external electric field.  
         [0033]     The present invention also relates to a method for fabricating a crystal fiber having different regions of polarization inversion. The method comprises the following steps:  
         [0034]     (a) A source material is provided, wherein the material of the source material may be crystal (for example, a source crystal rod) or powder. The material of the source crystal rod is ferroelectric and is selected from the group consisting of lithium niobate (LiNbO 3 ), lithium tantalate (LiTaO 3 ), patassium titanyl phosphate (KTP) and a dopant therein. The dopant is selected from the group consisting of the oxidation states of magnesium, zinc, yttrium, neodymium and erbium, and the mixture thereof.  
         [0035]     (b) The source material is put into a fabricating apparatus, wherein the fabricating apparatus is a laser heated pedestal growth (LHPG) apparatus.  
         [0036]     (c) The crystal fiber is formed from the source material. During the growth procedure of the crystal fiber, a first external electric field and a second external electric field are applied on the grown crystal fiber, wherein the first external electric field is applied on a molten zone between the source material and the crystal fiber so as to induce micro-swing of the crystal fiber for polarization inversion, and the second external electric field is applied on a predetermined position, preferably 2 to 10 mm, above the first external electric field to control and maintain the amplitude of the micro-swing.  
         [0037]     Preferably, the first external electric field and the second external electric field are alternating electric fields, and the intensity of the first external electric field is a constant value. The period of the second external electric field is the same as that of the first external electric field, and the second external electric field is used to control the amplitude of the micro-swing to maintain a constant value that is larger than the diameter of the crystal fiber. Preferably, the ratio of the amplitude of the micro-swing to the diameter of the crystal fiber is 1.0 to 1.5.  
         [0038]     While several embodiments of the present invention have been illustrated and described, various modifications and improvements can be made by those skilled in the art. The embodiments of the present invention are therefore described in an illustrative but not restrictive sense. It is intended that the present invention may not be limited to the particular forms as illustrated, and that all modifications which maintain the spirit and scope of the present invention are within the scope as defined in the appended claims.