Abstract:
A current is observed while applying a gradually increasing voltage between electrodes formed front and rear surfaces of a substrate, and then poled regions are formed by applying a DC voltage, which has a voltage value at that time or another voltage value obtained by adding a predetermined value to the voltage value (i.e., an inverted voltage value or a voltage value obtained by adding a predetermined voltage to the inverted voltage value), for a predetermined time when an inverted current is observed.

Description:
[0001]     The present application claims foreign priority under 35 USC 119 based on Japanese Patent Application No. 2005-184828, filed Jun. 24, 2005, the content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a method of manufacturing a substrate having periodically poled regions and, more particularly, a method of manufacturing a substrate having periodically poled regions, capable of suppressing a variation of a periodic structure caused due to the substrate and also attaining stable characteristics particularly even when a stoichiometric lithium tantalite substrate is used.  
         [0004]     2. Related Art  
         [0005]     A periodic electrode is formed on a surface of a lithium niobate substrate (CLN substrate) or a lithium tantalate substrate (CLT substrate) with a congruent (congruent melting) constitution and also a plane electrode is formed on a rear surface thereof, and then periodically poled regions are formed by applying the DC voltage in excess of the inverted voltage value (the voltage value at which the inverted current starts to flow) between these electrodes (see Japanese Patent No. 3059080 ([0040] to [0047])).  
         [0006]     As mentioned above, the periodically poled regions are formed on the substrate by applying the DC voltage in excess of the inverted voltage value between these electrodes.  
         [0007]     However, in the periodic structure of the periodically poled regions formed in this manner, there existed the problem such that a variation of the periodic structure is caused due to the substrate. In particular, in the recently developed stoichiometric lithium tantalite substrate, there existed the problem such that a variation of the periodic structure becomes large and thus stable characteristic cannot be obtained.  
       SUMMARY OF THE INVENTION  
       [0008]     The invention may provide a method of manufacturing a substrate having periodically poled regions, capable of suppressing a variation of a periodic structure caused due to the substrate and also attaining stable characteristics particularly even when a stoichiometric lithium tantalite substrate is used.  
         [0009]     One or more embodiments of the invention provide a method of manufacturing a substrate having periodically poled regions, comprising:  
         [0010]     observing a current while applying a gradually increasing voltage between electrodes of a singly polarized ferroelectric substrate, to front and rear surfaces of which the electrodes at least one of which is a periodic electrode are provided; and  
         [0011]     applying a DC voltage, which has a voltage value at that time when an inverted current is observed or another voltage value obtained by adding a predetermined value to the voltage value, continuously after observing the inverted current.  
         [0012]     As the result of the zealous study made by the inventors of the present invention, it was found that the inverted voltage value has a variation every substrate and this variation causes a variation of the periodic structure. In other words, when the inverted voltage value of nine sheets of MgO-doped stoichiometric lithium tantalite substrates, which were cut out from the same ingot to have the same shape, was measured actually, one sheet had the inverted voltage value of 300 V, five sheets had the inverted voltage value of 350 V, and three sheets had the inverted voltage value of 400 V. However, since the DC voltage is not normalized every substrate in the related art, the DC voltage is set to 400 V, for example, in nine sheets of substrates. Then, the DC voltage that is higher than the inverted voltage value by 100 V is applied to the substrate whose inverted voltage value is 300 V, the DC voltage that is higher than the inverted voltage value by 50 V is applied to the substrate whose inverted voltage value is 350 V, and the DC voltage that is equal to the inverted voltage value of the substrate is applied to the substrate whose inverted voltage value is 400 V. Since the polarization structure having the higher duty ratio can be formed when the applied DC voltage is increased higher than the inverted voltage value of the substrate, consequently the polarization structure formed on nine sheets of substrates is varied into three types respectively. In particular, in the stoichiometric lithium tantalite substrate, since an inverted electric field value (=inverted voltage value/substrate thickness) is very low such as 2 kV/mm, the formed polarization structure has a large variation even when a difference between the inverted voltage value and the DC voltage is shifted merely by several tens V.  
         [0013]     Therefore, in the method of manufacturing a substrate having periodically poled regions according to the present invention, the current is observed while applying a gradually increasing voltage between electrodes, and then the DC voltage having the voltage value at that time or another voltage value obtained by adding a predetermined value to the voltage value, i.e., the inverted voltage value or the voltage value obtained by adding a predetermined value to the inverted voltage value, is applied when the inverted current is observed. Accordingly, since the applied DC voltage can be set to the inverted voltage value of the substrate or the voltage value that is higher than the inverted voltage value by a predetermined value even when the inverted voltage value of the substrate is varied, the polarization structures having the same duty ratio can be formed and thus a variation of the periodic structure caused due to the substrate can be suppressed.  
         [0014]     One or more embodiments of the invention provide the method of manufacturing a substrate having periodically poled regions, in which the ferroelectric substrate is made of a lithium tantalite single crystal that has a stoichiometric composition (stoichiometry) of an C-plate or near to the stoichiometric composition.  
         [0015]     In the method of manufacturing the substrate having periodically poled regions according to the present invention, the applied DC voltage can be set to the inverted voltage value of the substrate or the voltage value that is higher than this inverted voltage value by a predetermined value even when the inverted voltage value of the substrate is varied. Therefore, the polarization structure can be formed without variation even on the stoichiometric lithium tantalite substrate the inverted voltage value of which is very low.  
         [0016]     Here, as the result of the earnest study made by the inventors of the present invention, it was found that the velocity at which the poled region spreads laterally is slower by far than the velocity estimated in the related art, and thus the substrate on which the periodically poled regions can be formed by applying the DC electric field only can be manufactured preferably by the method of the present invention.  
         [0017]     One or more embodiments of the invention provide the method of manufacturing a substrate having periodically poled regions, in which a mole ratio of Li 2 O/(Ta 2 O 5 +Li 2 O) of the ferroelectric substrate is equal to or larger than 0.495 but smaller than 0.505.  
         [0018]     In the method of manufacturing the substrate having periodically poled regions according to the present invention, as shown in the embodiment described later, the periodically poled regions can be formed preferably.  
         [0019]     One or more embodiments of the invention provide the method of manufacturing a substrate having periodically poled regions, in which a mole ratio of Li 2 O/(Ta 2 O 5 +Li 2 O) of the ferroelectric substrate is equal to or larger than 0.495 but smaller than 0.505 and at least one of Mg, Zn, Sc, and In is doped in the substrate.  
         [0020]     In the method of manufacturing the substrate having periodically poled regions according to the present invention, at least one of Mg and Zn, Sc, and In, each being equivalent to Mg, is doped. Therefore, the inverted voltage value can be reduced.  
         [0021]     According to a method of manufacturing a substrate having periodically poled regions of the present invention, a variation of a periodic structure caused due to the substrate can be suppressed, and particularly stable characteristics can also be attained even when a stoichiometric lithium tantalite substrate is used. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  shows a perspective view showing a substrate on both surfaces of which a periodic electrode and a solid electrode are formed respectively.  
         [0023]      FIG. 2  shows a top view showing the periodic electrode.  
         [0024]      FIG. 3  shows a bottom view showing the solid electrode.  
         [0025]      FIG. 4  shows an explanatory view showing a circuit that applies a DC voltage to the substrate.  
         [0026]      FIG. 5  shows a flowchart showing procedures of applying the DC voltage to the substrate (Embodiment 1).  
         [0027]     FIGS.  6 ( a )( b ) show views illustrating a change of the applied DC voltage and a change of a current value respectively.  
         [0028]      FIG. 7  shows a perspective view showing a substrate having a poled structure.  
         [0029]      FIG. 8  shows a flowchart showing procedures of applying the DC voltage to the substrate (Embodiment 2).  
         [0030]      FIG. 9  shows a flowchart showing procedures of applying the DC voltage to the substrate (Embodiment 3).  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     The present invention will be explained in more detail with reference to an illustrated embodiment hereinafter. In this case, the present invention is not limited to this embodiment.  
       EMBODIMENT 1  
       [0032]      FIG. 1  is a perspective view showing a situation that a periodic electrode  4  and a solid electrode  5  are formed on both surfaces of a substrate  1  respectively. The substrate  1  is made of a single crystal of the lithium tantalite having a stoichiometric composition of a singly polarized C plate or near to the stoichiometric composition which has a molar ratio of Li 2 O/(Ta 2 O 5 +Li 2 O) equal to or larger than 0.495 and smaller than 0.505 and is doped with MgO. A thickness of the substrate  1  is set to t=0.4 [mm].  
         [0033]     The periodic electrode  4  and the solid electrode  5  are formed as follows. 
    (1) An electrode metal such as Ta, or the like, for example, is formed on a +C face  2  and a −C face  3  of the substrate  1 .     (2) As shown in  FIG. 2 , the photolitho machining and the etching are applied to the electrode metal formed on the +C face. Thus, the periodic electrode  4  having a period p=5.5 [μm], for example, is formed.     (3) As shown in  FIG. 3 , the photolitho machining and the etching are applied to the electrode metal formed on the −C face. Thus, the solid electrode  5  having a necessary area is formed.    
 
         [0037]      FIG. 4  is an explanatory view showing a DC voltage application circuit  100  used to apply a DC voltage between the electrodes  4 ,  5  of the substrate  1 .  
         [0038]     This DC voltage application circuit  100  comprises a variable-voltage DC power source  6 , a switching circuit  7 , an ampere meter  8 , and a DC-voltage application controlling portion  9 .  
         [0039]     In this case, the substrate  1  is dipped into an insulating oil.  
         [0040]      FIG. 5  is a flowchart showing procedures of a DC voltage applying process executed by the DC voltage application circuit  100 .  
         [0041]     In step S 1 , a DC voltage Vc that the variable-voltage DC power source  6  outputs is set to a start voltage Vs (−200 V, for example).  
         [0042]     In step S 2 , the switching circuit  7  is turned on to apply the DC voltage Vc to the periodic electrode  4  and the solid electrode  5 . After the DC voltage is applied for a time τ (2.5 second, for example), the process goes to step S 3 .  
         [0043]     In step S 3 , a current value Is is read from the ampere meter  8 .  
         [0044]     In step S 4 , the output voltage Vc of the variable-voltage DC power source  6  is changed by ΔV (−10 V, for example). Then, this DC voltage Vc is applied to the periodic electrode  4  and the solid electrode  5 .  
         [0045]     In step S 5 , a current value Ic is read from the ampere meter  8 .  
         [0046]     In step S 6 , if |Ic-Is|&gt;Ith (4 μA, for example) is not satisfied, the process goes to step S 7 . In contrast, if |Ic-Is|&gt;Ith is satisfied, the process goes to step S 9 . In other words, the process goes to step S 7  until the current Ic in  FIG. 6 ( b ) is changed from the start current value Is by a threshold value Ith, while the process goes to step S 9  after the current Ic is changed from the start current value Is by the threshold value Ith.  
         [0047]     In step S 7 , the processes in steps S 5 , S 6  are repeated until a time τ has passed from step S 4 , while the process goes to step S 8  after a time has passed.  
         [0048]     In step S 8 , if the output voltage Vc of the variable-voltage DC power source  6  does not exceed an end voltage Ve (−800 V, for example), the process goes back to step S 4 . In contrast, if the output voltage Vc exceeds the end voltage Ve, the process goes to step S 17 .  
         [0049]     In step S 9 , the output voltage Vc of the variable-voltage DC power source  6  is changed by ΔV (−10 V, for example). Then, the DC voltage Vc is applied to the periodic electrode  4  and the solid electrode  5 .  
         [0050]     In step S 10 , the latest current value Ic is saved as a preceding current value Is.  
         [0051]     In step S 11 , the current value Ic is read from the ampere meter  8 .  
         [0052]     In step S 12 , if Ic-Is&gt;0 is not satisfied, the process goes to step S 13 . In contrast, if Ic-Is&gt;0 is satisfied, the process goes to step S 15 . In other words, the process goes to step S 13  until a negative peak of the current value Ic in  FIG. 6 ( b ) is sensed, while the process goes to step S 15  after the negative peak of the current value Ic is sensed.  
         [0053]     In step S 13 , the processes in steps S 10  to S 12  are repeated until a time τ has passed from step S 9 , while the process goes to step S 14  after a time τ has passed.  
         [0054]     In step S 14 , if the output voltage Vc of the variable-voltage DC power source  6  is not in excess of the end voltage Ve (−800 V, for example), the process goes back to step S 9 . In contrast, if the output voltage Vc is in excess of the end voltage Ve, the process goes to step S 17 .  
         [0055]     In step S 15 , the output voltage Vc of the variable-voltage DC power source  6  is changed by Vo (−50 V, for example). The DC voltage Vc is applied to the periodic electrode  4  and the solid electrode  5  only for a time T (20 second, for example). Then, the process goes to step S 16 .  
         [0056]     In step S 16 , an end of the process is informed. Then, the process is ended.  
         [0057]     In step S 17 , an error is informed. Then, the process is ended.  
         [0058]     A change of the DC voltage Vc is illustrated in  FIG. 6 ( a ), and a change of the current value Ic illustrated in  FIG. 6 ( b ).  
         [0059]     The substrate  1  on which periodically poled regions  10  are formed is shown in  FIG. 7 .  
         [0060]     In  FIG. 6 , the DC voltage Vc is changed stepwise. But the DC voltage Vc may be changed like a slope by shortening a time τ (1 second, for example) and reducing a voltage increment ΔV (−4 V, for example).  
         [0061]     Also, Vo=0 V may be set.  
         [0062]     Also, at least one of Zn, Sc, and In may be doped in the substrate instead of MgO. Alternately, an undoped substrate may be employed.  
         [0063]     According to Embodiment 1, since the DC voltage Vc is optimized every substrate  1 , the applied DC voltage Vc can be set to the inverted voltage value of the substrate  1  or the voltage value that is higher than the inverted voltage value by a predetermined value even when the inverted voltage of the substrate  1  is varied, so that the polarization structures having the same duty ratio can be formed. As a result, a variation of the periodic structure caused due to the substrate  1  can be suppressed and the stable characteristics can also be attained even when the stoichiometric lithium tantalite substrate is used.  
       EMBODIMENT 2  
       [0064]      FIG. 8  is a flowchart showing procedures of the DC voltage applying process executed by the DC voltage application circuit  100  according to Embodiment 2.  
         [0065]     These procedures correspond to simplified procedures in Embodiment 1, and the contents in respective steps are similar to the contents of steps to which the same reference numerals as those in steps in  FIG. 5  are affixed.  
         [0066]     That is, in Embodiment 2, the DC voltage Vc+Vo in the final stage is applied at a time point when the current value Ic in  FIG. 6 ( b ) is changed from the start current value Is by the threshold value Ith.  
       EMBODIMENT 3  
       [0067]      FIG. 9  is a flowchart showing procedures of the DC voltage applying process executed by the DC voltage application circuit  100  according to Embodiment 3.  
         [0068]     These procedures correspond to simplified procedures in Embodiment 1, and the contents in respective steps are similar to the contents of steps to which the same reference numerals as those in steps in  FIG. 5  are affixed.  
         [0069]     That is, in Embodiment 3, the DC voltage Vc+Vo in the final stage is applied at a time point when the negative peak of the current value Ic in  FIG. 6 ( b ) is sensed.  
         [0070]     The substrate having the periodically poled structure manufactured by the present invention can be utilized as the optical function element in the semiconductor-pumped solid state laser using the SHG (Second Harmonic Wave Generation) wavelength conversion technology, for example, or the like. Also, this substrate can be utilized in the optical communication field such as WDM (Wavelength Division Multiplexing) communication by executing the wavelength shift over a wide range as the QPM (Quasi Phase Matching) device, and the like.  
         [0071]     It will be apparent to those skilled in the art that various modifications and variations can be made to the described preferred embodiments of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover all modifications and variations of this invention consistent with the scope of the appended claims and their equivalents.