Abstract:
The present invention relates to a method for fabricating a crystal fiber having different regions of polarization inversion, comprising the following steps: (a) providing a source material; (b) putting the source material into a fabricating apparatus; and (c) forming the crystal fiber from the source material, and applying an external electric field on the grown crystal fiber during the growth procedure of the crystal fiber so as to induce micro-swing of the crystal fiber for polarization inversion, whereby poling at the time a ferroelectric crystalline body is being formed, whereas the conventional methods are designed for poling a ferroelectric crystalline body after it has been formed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and an apparatus for fabricating a crystal fiber, and more particularly, to a method and an apparatus that an external electric field is applied on the grown crystal fiber during the growth procedure of the crystal fiber so as to induce micro-swing of the crystal fiber and form regions of reversed ferroelectric polarities. 
   2. Description of the Related Art 
   Ferroelectric materials, for example, lithium niobate (LiNbO 3 ), lithium tantalate (LiTaO 3 ) and potassium titanyl phosphate (KTP), are widely used in the manufacture of optical elements because of their high nonlinear coefficient and other excellent properties. One known example is that the technique of achieving quasi-phase matching (QPM) by periodically poling can effectively generate light with sum frequency generation (SFG), second harmonic generation (SHG) or difference frequency generation (DFG). 
   The procedure of creating regions of different polarization vectors is referred to in the art as “poling”. In the present, the relative successful periodically poled method is to define a periodical electrode on the ferroelectric material and provide a high-voltage electric field (about 20 to 28 kV/mm). The periodical electrode can be made on the metal film directly, or by covering a periodical photoresist layer on the metal electrode or between the electrolytic liquid. However, the above-mentioned poling techniques are accomplished after the growth of the ferroelectric material. 
   U.S. Pat. Nos. 5,504,616, 5,714,198 and 6,013,221 defined the desired periodical electrode by complicated semiconductor process to accomplish periodically poled domains. In order to accomplish periodically poled domains, an additional high-voltage electric field or an additional chemical process is needed according to the above-mentioned methods, and the poling procedure is accomplished after the growth of the crystalline material. Additionally, the periodically poled crystal fabricated by the semiconductor process is mostly bulk material that has the disadvantages of being difficult to match with the conventional fiber and of poor mode matching. Further, the waveguide structure material made by the above-mentioned methods also has the disadvantages of poor coupling efficiency with fiber and complicated fabrication process. 
   U.S. Pat. No. 5,171,400 disclosed a method including laser heated pedestal growth (LHPG) and heat modulation so as to fabricate a lithium niobate crystal fiber that has a domain period of 2.7 μm and a diameter of 500 μm. However, such crystal fiber has the disadvantages of large variation of diameter of the crystal fiber, uneven domain period and difficulty in controlling the manufacture conditions. 
   Consequently, there is an existing need for a novel and improved method and an apparatus for fabricating a crystal fiber to solve the above-mentioned problems. 
   SUMMARY OF THE INVENTION 
   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 is applicable to poling at the time a ferroelectric crystalline body is being formed, whereas the conventional methods are designed for poling a ferroelectric crystalline body after it has been formed. 
   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. 
   Yet another objective of the present invention is to provide a method for fabricating a crystal fiber having different regions of polarization inversion, comprising:
         (a) providing a source material;   (b) putting the source material into a fabricating apparatus; and   (c) forming the crystal fiber from the source material and applying an external electric field on the grown crystal fiber during the growth procedure of the crystal fiber so as to induce micro-swing of the crystal of the crystal fiber for polarization inversion.       

   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 and an electric field generating device. 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 the molten zone between the source material and the crystal fiber. The electric field generating device is disposed near the molten zone for providing an external electric field which is used for poling the crystal fiber and inducing micro-swing of the crystal of the crystal fiber. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic diagram of a chamber of an apparatus for fabricating a crystal fiber according to the present invention; 
       FIG. 2  shows a local enlarged view of the molten zone of  FIG. 1 ; 
       FIG. 3  is a schematic diagram showing the formation of the region of polarization inversion according to the present invention; 
       FIG. 4   a  is a schematic diagram showing the distribution of the space charges on the circumference of the molten zone during the growth of the crystal fiber according to the present invention; 
       FIG. 4   b  shows a cross sectional view of the lower portion of the molten zone of  FIG. 4   a;    
       FIGS. 5   a  to  5   c  show the micro-swing occurred during the growth of the crystal fiber according to the present invention; wherein  FIG. 5   b  shows the appearance of the crystal fiber without being applied by any external electric field,  FIG. 5   a  shows that the crystal fiber swings to the left when being applied by an external electric field, and  FIG. 5   c  shows that the crystal fiber swings to the right when being applied by an external electric field; 
       FIG. 6  shows a voltage waveform of the first high-voltage source adapted in the first method for generating the external electric field according to the present invention; 
       FIGS. 7   a  and  7   b  respectively show voltage waveforms of the second and third high-voltage sources adapted in the second method for generating the external electric field according to the present invention; and 
       FIGS. 8   a  and  8   b  respectively show voltage waveforms of the fourth and fifth high-voltage sources adapted in the third method for generating the external electric field according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a schematic diagram of a chamber of an apparatus for fabricating a crystal fiber according to the present invention. The apparatus  10  is similar to a laser heated pedestal growth (LHPG) apparatus, which is used for making a source material into a crystal fiber  21  having different regions of polarization inversion. The material of the source material may be crystal (for example, a source crystal rod  20 ) or powder. The apparatus  10  comprises a laser beam generator (not shown), a beam splitter  12 , a bending mirror  13 , a paraboloidal mirror  14  and an electric field generating device. 
   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  at the tip of the source crystal rod  20 . The electric field generating device is used for providing an external electric field which is used for poling the crystal fiber  21  and inducing micro-swing of the crystal of the crystal fiber  21 . In the embodiment, the electric field generating device includes a first metal electrode  18  and a second metal electrode  19 . 
     FIG. 2  is a local enlarged view of  FIG. 1 , which shows a molten zone  16  at the tip of the source crystal rod  20 . The tip of the source crystal rod  20  can be melted to form the molten zone  16  by utilizing the laser beam  11  with desired output power from the laser beam generator. The material of the source crystal rod  20  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  20  is lithium niobate doped with 6% mol of zinc oxide (ZnO). 
   It is found that the diameter of the crystal fiber  21  must be less than 500 μm. If the diameter of the crystal fiber  21  is more than 500 μm, the micro-swing will not occur before electric breakdown. Therefore, the diameter of the crystal fiber  21  fabricated in the embodiment is less than 500 μm. Additionally, the laser beam  11  generated from the laser beam generator is CO 2  laser beam having a wavelength of 10.6 μm. 
   A ferroelectric crystalline seed is heated by the CO 2  annular laser beam  111  in the chamber and is dipped in the molten zone  16 . Such seed is withdrawn from the molten zone  16 , while the source crystal rod  20  is fed toward the same molten zone  16  so that as the seed is withdrawn, the ferroelectric crystal fiber  21  is formed at a freezing interface  23  ( FIG. 3 ). The diameter of the grown ferroelectric crystal fiber  21  is determined by the square root of the ratio of the feed speed of the source crystal rod  20  to the pull speed of the seed, and the size of the source crystal rod  20 . In operation, two metal electrodes  18 , 19  each having a diameter of about 580 μm are used for providing required electric field and micro-swing. In order to avoid blocking the CO 2  annular laser beam  111 , the metal electrodes  18 , 19  are bent so as to fit the path of the CO 2  annular laser beam  111 . Additionally, a stereo microscope (not shown) is used for defining the distance between the two metal electrodes  18 , 19  and monitoring the molten zone  16 . 
   During the procedure of growing the lithium niobate crystal fiber  21 , the lithium niobate crystal grows along X crystal axis (also called a crystal axis). Under normal growing condition, the grown lithium niobate crystal forms a bi-domain structure whose domain wall is at the center of the crystal fiber  21  due to the effect of temperature gradient and spontaneous polarization vector. However, in the present invention, the bi-domain structure can be broken so that the grown crystal fiber  21  can have periodically poled structure. 
     FIG. 3  is a schematic diagram showing the formation of the region of polarization inversion according to the present invention. As shown in the figure, the crystal grows along X crystal axis, and two metal electrodes  18 , 19  are disposed along Z crystal axis (also called c crystal axis) ( FIG. 2 ). Because the annular laser beam  111  is focused at the tip of the source crystal rod  20 , the distribution curve of an isotherm in the molten zone  16  is a symmetrical curve that is low at center and high at two sides. Therefore, the distribution curve of a solid-liquid interface  22  is also a symmetrical curve that is low at center and high at two sides. The farther the crystal fiber  21  leaves the molten zone  16 , the smoother the distribution curve of the isotherm will become, and a freezing interface  23  is defined as where the distribution curve of the isotherm is horizontal. Between the solid-liquid interface  22  and the freezing interface  23  is a Curie isotherm  24 , which is also a symmetrical curve. When the temperature of the ferroelectric material is higher than the Curie temperature, it will not have the property of spontaneous polarization, and is defined as paraelectric phase. The region between the Curie isotherm  24  and the solid-liquid interface  22  of the crystal fiber  21  is paraelectric phase. When the temperature of the ferroelectric material is lower than the Curie temperature, it will have the spontaneous polarization vector. Because the lithium niobate crystal grows along X crystal axis and the Curie isotherm  24  has a curved distribution, the spontaneous polarization vector of the lithium niobate crystal is toward ±Z crystal axis (or ±c crystal axis), and the bi-domain structure is formed accordingly. When an external electric field is applied, the bi-domain structure is broken and an effective poled region  25  is formed above the molten zone  16 . The effective poled region  25  is determined by the external electric field and the temperature gradient. The relationship between the domain period and the growth velocity of the crystal fiber  21  can be expressed as L c =V c ×T/2, wherein L c  is coherent length or domain period, V c  is growth velocity of the crystal fiber  21  and T is period of the external electric field. 
     FIGS. 4   a  and  4   b  show the distribution of the space charges on the circumference of the molten zone  16  during the growth of the crystal fiber  21  according to the present invention, wherein  FIG. 4   b  shows a cross sectional view of the lower portion of the molten zone  16 . When the lithium niobate crystal is heated to the melting state, negative charges  30  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  30  may block part of the external electric field and increase difficulty of poling. Therefore, in the present invention, the two electrodes  18 , 19  are connected to two high-voltage sources respectively so that the negative charges  30  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 . 
     FIGS. 5   a  to  5   c  show the micro-swing occurred during the growth of the crystal fiber  21  according to the present invention; wherein  FIG. 5   b  shows the appearance of the crystal fiber  21  without being applied by any external electric field,  FIG. 5   a  shows that the crystal fiber  21  swings to the left when being applied by an external electric field, and  FIG. 5   c  shows that the crystal fiber  21  swings to the right when being applied by an external electric field. It should be understood that because the micro-swing occurs, the solid-liquid interface  22  and the Curie isotherm  24  are no longer symmetrical curves, and are high at one side and low at the opposite side. Such new distribution of temperature gradient facilitates breaking the bi-domain structure of the lithium niobate crystal, and its induced pyroelectric field can compensate the part of external electric field blocked by the space charges so as to form the periodically poled structure. For one crystal, its most displacement of swing (displacement of the crystal when the crystal fiber  21  of  FIG. 5   a  swings to the appearance of  FIG. 5   c ) divided by the diameter of the crystal fiber  21  is defined as a swing ratio. In the embodiment, the value of the swing ratio is between 0.9 to 1.5. 
   In the embodiment, the source crystal rod  20  of ZnO-doped (6% mol) a-axis LiNbO 3  crystal has a cross section of 500×500 μm 2 . The ratio of the pull speed of the seed to the feed speed of the source crystal rod  20  is 9:1, and the external electric field is 1 kV/mm. Under such conditions, the crystal fiber  21  having a domain period of 16.3 μm and a diameter of 200 μm is fabricated, and the variation of the diameter of the crystal fiber  21  is less than 1%. 
     FIG. 6  shows a voltage waveform of a first high-voltage source adapted in a first method for generating the external electric field according to the present invention. In this first method, the first metal electrode  18  is connected to the ground, and the second metal electrode  19  is connected to the first high-voltage source that provides an alternating current whose waveform is alternating square wave as shown in  FIG. 6 . 
     FIGS. 7   a  and  7   b  respectively show voltage waveforms of a second and third high-voltage sources adapted in the second method for generating the external electric field according to the present invention. In this second method, the first metal electrode  18  is connected to the second high-voltage source, and the second metal electrode  19  is connected to the third high-voltage source, wherein the second high-voltage source provides an alternating current whose waveform is alternating square wave as shown in  FIG. 7   a , and the third high-voltage source provides an alternating current whose waveform is alternating square wave as shown in  FIG. 7   b . As shown in the figures, the phase of the waveform of the second high-voltage source is reverse to that of the third high-voltage source. 
     FIGS. 8   a  and  8   b  respectively show voltage waveforms of the fourth and fifth high-voltage sources adapted in a third method for generating the external electric field according to the present invention. In this third method, the first metal electrode  18  is connected to the fourth high-voltage source, and the second metal electrode  19  is connected to the fifth high-voltage source, wherein the fourth high-voltage source provides an impulse direct current whose waveform is direct impulse wave as shown in  FIG. 8   a , and the fifth high-voltage source provides an impulse direct current whose waveform is direct impulse wave as shown in  FIG. 8   b . As shown in the figures, the waveform of the fifth high-voltage source shifts one-half cycle to that of the fourth high-voltage source. 
   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.