TE-TM mode converter

A TE-TM mode converter is provided which is capable of performing a TE-TM conversion in a wide bandwidth. In a TE-TM mode converter using an electrooptic effect, a waveguide is formed on a substrate using lithium tantalate having a birefringence of about 0.0005 or less. The direction of an optical axis of lithium tantalate forming the waveguide is approximately parallel to a primary surface of the substrate. In addition, a first electrode and a second electrode are provided on the primary surface of the substrate so as to face each other with the waveguide placed therebetween.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to TE-TM mode converters, and more particularly, relates to a TE-TM mode converter using an electrooptic effect.

2. Description of the Related Art

As a related TE-TM mode converter, a polarization controller100described in Japanese Unexamined Patent Application Publication No. 2003-202532 has been proposed.FIG. 12is a perspective view showing the appearance of the polarization controller100.

The polarization controller100shown inFIG. 12has a ferroelectric substrate101, a waveguide102, a mode conversion portion103, and plate electrodes104aand104b. The mode conversion portion103includes domain regions D1to D5disposed along a light-wave traveling direction in the waveguide102. In addition, the plate electrodes104aand104bare provided on the mode conversion portion103so as to sandwich the waveguide102.

In the polarization controller100, when a control voltage is applied to the plate electrodes104aand104b,an electric filed is generated parallel to a primary surface of the ferroelectric substrate101and perpendicular to a longitudinal direction of the waveguide102. Accordingly, the principal axis rotation of the index ellipsoid of the waveguide102occurs, and two types of light waves (TE mode and TM mode) having planes of polarization, which are perpendicular to each other, are coupled, so that a TE-TM mode conversion occurs. In addition, since the domain regions D1to D5are disposed so that the polarization directions are alternately opposite to each other, they function as a grating which performs phase matching between the two types of light waves. As a result, the polarization controller100can perform the TE-TM mode conversion with high conversion efficiency.

However, the polarization controller100performing the TE-TM mode conversion by the grating has a problem in that the bandwidth of light waves, in which the TE-TM mode conversion can be performed, is narrow. In particular, as a communication wavelength for optical communication, a light wave having a wavelength of about 1,550 nm, which belongs to the C-band (1,530 to 1,565 nm), is used in many cases. Hence, in order to perform the TE-TM mode conversion for the above light wave, it is necessary to drive the polarization controller100in a bandwidth of 35 nm. However, it has been difficult to drive the polarization controller100in a bandwidth of 35 nm.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a TE-TM mode converter capable of performing a TE-TM mode conversion in a wide bandwidth.

According to a preferred embodiment of the present invention, there is provided a TE-TM mode converter using an electrooptic effect, comprising: a substrate; and a waveguide which is composed of lithium tantalate having a birefringence of about 0.0005 or less and which is disposed on the substrate.

According to a preferred embodiment of the present invention, when lithium tantalate having a birefringence of about 0.0005 or less is used as a material for the waveguide, a TE-TM mode conversion rate for light waves in the C-band can be increased to 90% or more.

According to a preferred embodiment of the present invention, the birefringence is preferably about 0.0003 or less. As a result, the TE-TM mode conversion rate for light waves in the C-band can be increased to about 95% or more.

According to a preferred embodiment of the present invention, the TE-TM mode converter preferably further comprises a voltage application unit as a mode conversion portion to apply a control voltage to the waveguide in a direction perpendicular to a traveling direction of light. By applying the control voltage as described above, the principle axis of the index ellipsoid of the waveguide can be rotated. As a result, a TE-TM mode converter can be formed which performs a TE-TM mode conversion for light waves in a wide bandwidth, such as the C-band.

According to a preferred embodiment of the present invention, the above voltage application unit preferably includes a first electrode disposed between the substrate and the waveguide, and a second electrode disposed to face the first electrode with the waveguide interposed therebetween. When lithium tantalate having a birefringence of about 0.0005 or less is used as a substrate material for the waveguide, the domain regions D1to D5having polarization directions alternately opposite to each other, which are provided in the polarization controller100described in Japanese Unexamined Patent Application Publication No. 2003-202532, are not necessary to be formed. When the domain regions D1to D5are provided as described above, a substrate of lithium tantalate having a primary surface perpendicular to the Z axis must be used, and when the domain regions D1to D5are not provided, a substrate of lithium tantalate having a primary surface perpendicular to the Y axis may be used. As a result, electrodes applying a control voltage can be disposed on the top and the bottom of the waveguide. That is, the distance between the two electrodes can be decreased, and with a small voltage, the TE-TM mode converter can be driven.

According to the preferred embodiments of the present invention, since the waveguide is formed from LiTaO3(lithium tantalate) having a birefringence of about 0.0005 or less, a TE-TM mode converter used for a wide bandwidth including the C-band can be manufactured.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a TE-TM mode converter according to one embodiment of the present invention will be described with reference to the drawings.

FIG. 1is a perspective view showing the appearance of a TE-TM mode converter1.FIG. 2Ais a graph showing the relationship between the thickness of a waveguide and the equivalent refractive index of the TE mode.FIG. 2Bis a graph showing the relationship between the thickness of a waveguide and the equivalent refractive indexes of the TM mode. In the above graphs, the horizontal axis indicates the thickness of the waveguide, and the vertical axis indicates the equivalent refractive index of the TE mode and that of the TM mode.FIG. 3Ais a graph showing the relationship between the width of a waveguide and the equivalent refractive index of the TE mode.FIG. 3Bis a graph showing the relationship between the width of a waveguide and the equivalent refractive indexes of the TM mode. In the above graphs, the horizontal axis indicates the width of the waveguide, and the vertical axis indicates the equivalent refractive index of the TE mode and that of the TM mode.

The TE-TM mode converter1is a device performing a TE-TM mode conversion using an electrooptic effect and has a substrate2composed of LiTaO3(lithium tantalate), a first electrode3composed of Al, a lower clad layer4composed of a UV adhesive, a waveguide5composed of LiTaO3having a very small birefringence, an upper clad layer6composed of TiO2, and a second electrode7composed of Al.

The substrate2is a substrate composed of LiTaO3having a molar fraction [Li]/([Li]+[Ta]) of 0.492, a birefringence of about 0.0004, and a thickness of 0.5 mm. This substrate2is a Z-cut crystal obtained by cutting approximately perpendicular to the Z axis.

The lower clad layer4is formed on the substrate2. The thickness of the lower clad layer4is 0.2 μm or more. On the lower clad layer4, the waveguide5is formed, and the lower clad layer4functions to adhere between the substrate2and the waveguide5.

The waveguide5functions to guide a light wave in a longitudinal direction thereof and is formed on the substrate2with the lower clad layer4interposed therebetween so as to be approximately parallel to the first electrode3and the second electrode7. The waveguide5is composed of LiTaO3having a molar fraction [Li]/([Li]+[Ta]) of 0.492 and a birefringence of about 0.0004. The waveguide5has a thickness of 4 μm and a width of 3.5 μm. The thickness and the width of this waveguide5are designed so that a single-mode light wave having a wavelength of 1.55 μm is only guided and so that the birefringence has a minimum value. In particular, from the graphs shown inFIGS. 2A and 2B, the thickness of the waveguide5at which the equivalent refractive index of a single-mode TE light wave and that of a single-mode TM light wave approximately coincides with each other was obtained as 4 μm. In a manner similar to that described above, from the graphs shown inFIGS. 3A and 3B, the width of the waveguide5at which the equivalent refractive index of a single-mode TE light wave and that of a single-mode TM light wave approximately coincides with each other was obtained as 3.5 μm. In addition, when the graphs described above were prepared, TiO2was used as a material for the clad layer in contact with the top and side surfaces of the waveguide5, and the refractive index of TiO2(no=2.0595, and ne=2.1065) was calculated by a Sellmeier equation using a dispersion coefficient described in “Optical Multilayer Film Simulation Technique and Optimum Design by EXCEL VBA” authored by Masayuki Nakamura, and published from Technical Information Institute Co., Ltd.

The waveguide5is a Z-cut crystal obtained by cutting perpendicular to the Z axis as is the substrate2. In addition, the birefringence of the substrate2and that of the waveguide5may be about 0.0005 or less and more preferably about 0.0003 or less. The birefringence of the waveguide5will be described later.

The upper clad layer6is formed of a TiO2film and covers the top and the side surfaces of the waveguide5. Hence, the waveguide5is surrounded by materials having a refractive index lower than that the waveguide5so as to confine light therein.

The first electrode3and the second electrode7are formed from Al on the primary surface of the upper clad layer6to face each other with the waveguide5placed therebetween and to be approximately parallel to the waveguide5. The width of the first electrode3and that of the second electrode7are each 100 μm, and the distance therebetween is 20 μm.

In addition, in the TE-TM mode converter1, by applying a control voltage between the first electrode3and the second electrode7, the control voltage is applied to the waveguide5in a direction perpendicular to a traveling direction of a light wave. Hence, by the electrooptic effect, the principle axis rotation of the index ellipsoid of the waveguide5occurs, and the two types of waves (TE mode and TM mode) having planes of polarization perpendicular to each other are coupled together, so that the TE-TM mode conversion occurs.

In the TE-TM mode converter1having the structure as described above, since a material having a very small birefringence of about 0.0005 or less as compared to that of a commercially available material is used for the waveguide5, the TE-TM mode conversion can be performed for light waves having wavelengths in a wide waveband. Hereinafter, the TE-TM mode conversion will be described in detail.

In optical communication, a light wave having a wavelength of about 1,550 nm is generally used. This light wave belongs to a communication waveband of the C band. Hence, the TE-TM mode converter must perform a sufficient TE-TM mode conversion within a bandwidth of 35 nm which is the communication bandwidth of the C band. Accordingly, a TE-TM mode conversion rate with the change in birefringence is obtained by calculation.

First of all, in a lossless waveguide which is uniform in a propagation direction (x direction), various waveguide modes which satisfy the boundary conditions of the waveguide are present, and these modes are not coupled with each other and independently propagate powers. Based on the conditions described above, when perturbation at a certain level is applied to the waveguide, the modes under non-perturbation conditions are no longer independently present and are coupled to each other.

In this embodiment, the case is assumed in which two types of modes, TE and TM modes, propagate in two waveguides shown inFIG. 4in an x direction at propagation constants βTEand βTM, respectively.

In the above equations, CTE(x) and CTM(x) indicate the amplitudes of the respective modes, and fTEand fTMindicate field distribution functions each obtained from a normalized power flow in a cross-section. When the coupling occurs, CTE(x) and CTM(x) are not independent of each other. A mode coupling equation of the same direction coupling in which coupling is performed in the x axis propagation at a coupling coefficient κ (positive actual number) can be represented by the following equations (2) in which n indicates a grating cycle, r51indicates an electrooptic coefficient, and Eyindicates a voltage applied to the Y axis direction.

The general solutions of Equation (1) are represented as follows when CTE0and CTM0are regarded as constants.

In the above solutions, the following equations hold.

In addition, since β=k0N, and k0=2π/λhold, the following equation holds.

In the above equation, NTEand NTMindicate the equivalent refractive indexes of the TE and the TM modes, respectively. In addition, δ represented by the equation (4) indicates a phase mismatching amount.

Next, in a waveguide having a unit section L as shown inFIG. 5, when amplitudes at a position zero (0) are represented by CTE(0) and CTM(0), and amplitudes at a position L are represented by CTE(L) and CTM(L), Equation (1) is used as the boundary conditions, and C(L)=Ti·C(0) (in which [·] indicates multiplication) is obtained by rearrangement, so that transfer matrix Ti is obtained for each unit section at each wavelength in a predetermined range.

Transfer matrix T of the entire mode conversion portion can be obtained when the transfer matrixes of the individual unit sections are connected to each other using the following Equation (7).

When the CTEand CTMat the inlet and the outlet of the mode conversion portion are represented by CTE(in), CTM(in), CTE(out), and CTM(out), the following equation (8) holds.

The conversion rate χTMOf the entire mode conversion portion obtained when only a TE mode light wave is incident is the value of CTM(out) at which the CTE(in) is 1 and the CTM(in) is 0.

In addition, as the analysis from the beginning through the equation (8), the following are assumed.(A) Coupling coefficient χ when the TE mode is converted to the TM mode at a certain wavelength and that when the TM mode is converted to the TE mode are equal to each other.(B) |NTM−NTE| in a calculated wavelength range is assumed to be equal to Δn of the substrate material.(C) Reflection caused by refractive index distribution in the guide is ignored.(D) Impedance of an optical system is constant.

Based on the above assumptions (A) to (D), by the Equation (8), calculation was performed in a wavelength range of 1,300 to 1,800 nm using a birefringence of each material at a central wavelength of 1,550 nm, and the results are shown in Table 1.

Table 1 shows the bandwidths of a communication waveband at TE-TM mode conversion rates of 90% or more and 95% or more, which are obtained when commercially available LiNbO3, commercially available LiTaO3, and LiTaO3having a smaller birefringence than that of the commercially available LiTaO3are used as a waveguide material.

As shown in Table 1, it is understood that in order to obtain a TE-TM mode conversion of 90% or more in a bandwidth of 35 nm, the birefringence is about 0.0005 or less. In addition, as LiTaO3having a birefringence of about 0.0005 or less, for example, LiTaO3which satisfies no=2.1188 and ne=2.1189 may be mentioned. Furthermore, it is also understood that in order to obtain a TE-TM mode conversion of 95% or more in a bandwidth of 35 nm, the birefringence is about 0.0003 or less.

FIG. 6is a perspective view showing the appearance of a TE-TM mode converter51of a modified example of the TE-TM mode converter1according to the above embodiment.

The TE-TM mode converter51has a substrate52composed of LiTaO3(lithium tantalate), a first electrode53composed of Al, a lower clad layer54composed of a UV adhesive, a waveguide55having a very small birefringence, an upper clad layer56composed of TiO2, and a second electrode57composed of Al.

The substrate52is a substrate composed of LiTaO3having a molar fraction [Li]/([Li]+[Ta]) of 0.492, a birefringence of about 0.0004, and a thickness of 0.5 mm. This substrate52is a Y-cut crystal obtained by cutting approximately perpendicular to the Y axis.

The first electrode53is formed between the substrate52and the waveguide55so as to be approximately parallel thereto. The width of the first electrode53is 8 μm. The lower clad layer54is formed on the first electrode53. The thickness of the lower clad layer54is 0.2 μm or more. The waveguide55is formed on the lower clad layer54, and the lower clad layer54functions to adhere between the waveguide55and the first electrode53.

The waveguide55functions to guide a light wave in a longitudinal direction thereof and is formed from LiTaO3having a molar fraction [Li]/([Li]+[Ta]) of 0.492 and a birefringence of about 0.0004 on the substrate52with the first electrode53and the lower clad layer54interposed therebetween so as to be approximately parallel to the first electrode53. The thickness of the waveguide55is 4 μm which is equivalent to that of the waveguide5shown inFIG. 1, and the width of the waveguide55is 3.5 μm which is also equivalent to that of the waveguide5shown inFIG. 1.

The waveguide55is a Y-cut crystal obtained by cutting perpendicular to the Y axis, as the substrate52. In addition, the birefringence of the substrate52and that of the waveguide55are preferably about 0.0005 or less and more preferably about 0.0003 or less.

The upper clad layer56is formed from a TiO2film so as to cover the top and the side surfaces of the waveguide55. The second electrode57is formed from Al on the upper clad layer56so as to face the first electrode53with the waveguide55interposed therebetween and so as to be approximately parallel thereto. The width of the second electrode57is 8 μm.

In the TE-TM mode converter51shown inFIG. 6, since the distance between the first electrode53and the second electrode57can be decreased, the control voltage can be advantageously decreased as compared to that of the TE-TM mode converter1shown inFIG. 1. Hereinafter, the TE-TM mode converter51will be described in detail.

In a TE-TM mode converter using an electrooptic effect, by applying an electric field to a waveguide, a TE-mode light wave is coupled with a TM-mode light wave, and in addition, by using a grating, phase matching between the TE-mode light wave and the TE-mode light wave is performed. A cycle A of this grating is represented by the following equation (9) when the birefringence and the wavelength of a guided light wave are represented by Δn and λ, respectively.

According to the Equation (9), when the birefringence Δn is extremely decreased, the grating cycle Λ can be extremely increased. Hence, in this embodiment, the waveguide5is formed from LiTaO3having a birefringence of about 0.0005 or less, which is significantly smaller than that of LiTaO3or LiNbO3having a birefringence of about 0.0038 and 0.073, respectively, those being generally commercially available. Accordingly, in the TE-TM mode converter51of this embodiment, the grating cycle Λ can be significantly increased, and even when the grating is not formed, the phase matching between the TE mode light wave and the TM mode light wave can be performed. In a TE-TM mode converter100having a grating as shown inFIG. 12, a Z-cut crystal must be used for the waveguide102; however, in the TE-TM mode converter51shown inFIG. 6, a Y-cut crystal can be used for the waveguide55. Accordingly, the first electrode53and the second electrode57can be provided on the top and the bottom of the waveguide55. As a result, since the distance between the first electrode53and the second electrode57can be decreased, the TE-TM mode converter51can be driven even at a low voltage, and in addition, the size of the TE-TM mode converter51can be reduced.

For example, in the TE-TM mode converter1shown inFIG. 1, in order to uniformly apply an electric field to the waveguide5in the Y axis direction, the first electrode3and the second electrode7are disposed apart from each other at a distance of approximately 20 μm. Hence, in order to sufficiently drive this TE-TM mode converter1, a high voltage of 10.0 V must be applied between the first electrode3and the second electrode7, or by increasing the length of the TE-TM mode converter1, the length of the first electrode3and that of the second electrode7must be sufficiently increased.

In contrast, in the TE-TM mode converter51of this embodiment, the distance between the first electrode53and the second electrode57is smaller than that of the TE-TM mode converter1. Hence, compared to the TE-TM mode converter1, the TE-TM mode converter51can be driven at a low voltage. In addition, even when the voltage is not decreased, the TE-TM mode converter51can be driven even when it has a smaller length. For example, in the TE-TM mode converter51, when the electrodes have the same length as those of the TE-TM mode converter1, a voltage of 4.0 V, which is smaller than that to be applied to the TE-TM mode converter1, may only be applied between the first electrode53and the second electrode57.

In addition, when the refractive index of the substrate2or52is smaller than that of the waveguide5or55, respectively, the lower clad layer4or54may be omitted.

Next, a method for manufacturing the TE-TM mode converter1of this embodiment will be described with reference to the drawings.FIGS. 7 to 11are perspective views of the appearance of the TE-TM mode converter1in process.

First, manufacturing of a wafer of LiTaO3having a birefringence of 0.0004 used as a material for the substrate2and the waveguide5will be described. In order to obtain a molar ratio of Li2CO3to Ta2O5of about 0.550 to 0.450, 661.8 g of Li2CO3and 3,238.2 g of Ta2O5were weighed and were then charged in a fluorinated resin-made pot, followed by stirring and mixing for one hour while moisture absorption was being carefully prevented. Subsequently, calcination was performed at 1,300° C. for 8 hours, so that an intended raw material solution was obtained.

While moisture absorption was being carefully prevented, the raw material solution was charged in a double structure crucible made of Ir and was then heated, and after the crucible was rotated alternately in a clockwise direction and an anticlockwise direction for one hour at 5 rpm to homogenize the solution, single crystalline LiTaO3was grown. As for the growth conditions, the crystal rotation rate and the pulling rate were set constant to 6 rpm and 1.0 mm/h, respectively, and the growth atmosphere was nitrogen containing 0.05% to 1% of oxygen. Since the above composition was not a congruent melting composition, the crystal growth was performed for about 10 percent by weight of the raw material solution at which the composition deviation has no influence.

As a result, a LiTaO3crystal was obtained which had a diameter of 60 mm, a length of 20 mm, and a composition having a molar fraction [Li]/([Li]+[Ta]) of 0.492. The single crystal thus obtained was sandwiched by Pt plates perpendicularly to the Z axis direction and was then placed in a resistance heating furnace. After the temperature was increased to 750° C. and was held for a sufficient time, while a direct current at a current density of 0.02 mA/cm2was allowed to flow using the Pt plates as electrodes, the temperature was slowly decreased to room temperature at a rate of 20° C./h.

A plate sample was formed by cutting from a plane perpendicular to the Y axis direction, and two primary surface of the sample were then processed by a mirror polishing treatment, so that a final sample having a thickness of 0.5 mm was obtained. When refractive index measurement was performed for the plate sample by a prism coupling method, at a measurement wavelength of 1,550 nm, the birefringence Δn (=ne−no) was 0.0004. As LiTaO3having a birefringence of 0.0004, for example, LiTaO3which satisfies no=2.1189 and ne=2.1193 may be mentioned. As a result, a wafer of LiTaO3having a diameter of 50 mm and a birefringence of 0.0004 could be obtained.

In addition, it was verified by the inventor of the present invention that when the above method for manufacturing a wafer of LiTaO3is used, the birefringence with respect to a near-infrared ray of 1,000 nm or more, which is an optical communication band, can be controlled on the order of 0.00001.

Next, in a clean room, as shown inFIG. 7, by using the wafer described above as the substrate2, a UV adhesive was applied on the primary surface thereof by a spin coating method to form the lower clad layer4.

Subsequently, as shown inFIG. 8, a wafer of LiTaO3to be formed into the waveguide5was closely brought into contact with the top surface of the lower clad layer4and was irradiated by a xenon lamp while a pressure is being applied on the wafer. As a result, the lower clad layer4of the UV adhesive was cured, and the substrate2and the wafer of LiTa3to be formed into the waveguide5were adhered to each other.

Next, the primary surface of the wafer to be formed into the waveguide5was processed by chemical mechanical polishing using colloidal silica, so that as shown inFIG. 9, the thickness of the wafer was decreased to 4 μm by polishing.

Subsequently, using a dicing saw, the wafer to be formed into the waveguide5and the lower clad layer4were partly milled away to form two grooves as shown inFIG. 10. As a result, the waveguide5was formed having a width of 3.5 μm.

Next, as shown inFIG. 11, the upper clad layer6of TiO2having a thickness of 1 μm was formed on the waveguide5. Finally, as shown inFIG. 1, the first electrode3and the second electrode7were formed from Al on the upper clad layer6, so that the TE-TM mode converter1was completed.

In addition, hereinafter, manufacturing of a wafer of LiTaO3having a birefringence of 0.0001 used as a material for the substrate2and the waveguide5will also be described. In order to obtain a molar ratio of Li2CO3to Ta2O5of about 0.553 to 0.447, Li2CO3and Ta2O5were weighed and were then charged in a fluorinated resin-made pot, followed by stirring and mixing for one hour while moisture absorption was being carefully prevented. Subsequently, calcination was performed at 1,300° C. for 8 hours, so that an intended raw material solution was obtained.

While moisture absorption was being carefully prevented, the raw material solution was charged in a double structure crucible made of1rand was then heated, and after the crucible was rotated alternately in a clockwise direction and an anticlockwise direction for one hour at 5 rpm to homogenize the solution, single crystalline LiTaO3was grown. As for the growth conditions, the crystal rotation rate and the pulling rate were set constant to 6 rpm and 1.0 mm/h, respectively, and the growth atmosphere was nitrogen containing 0.05% to 1% of oxygen. Since the above composition was not a congruent melting composition, the crystal growth was performed for 10 percent by weight of the raw material solution at which the composition deviation has no influence.

As a result, a LiTaO3crystal was obtained which had a diameter of 60 mm, a length of 20 mm, and a composition having a molar fraction [Li]/([Li]+[Ta]) of 0.495. The single crystal thus obtained was sandwiched by Pt plates perpendicularly to the Z axis direction and was then placed in a resistance heating furnace. After the temperature was increased to a 750° C. and was held for a sufficient time, while a direct current at a current density of 0.02 mA/cm2was allowed to flow using the Pt plates as electrodes, the temperature was slowly decreased to room temperature at a rate of 20° C./h.

A plate sample was formed by cutting from a plane perpendicular to the Y axis direction, and two primary surface of the sample were then processed by a mirror polishing treatment, so that a final sample having a thickness of 0.5 mm was obtained. When refractive index measurement was performed for the plate sample by a prism coupling method, at a measurement wavelength of 1,550 nm, the birefringence Δn (=ne−no) was 0.0001. As LiTaO3having a birefringence of 0.0001, for example, LiTaO3which satisfies no=2.1188 and ne=2.1189 may be mentioned. As a result, a wafer of LaTiO3having a diameter of 50 mm and a birefringence of 0.0001 could be obtained.

In addition, the birefringence is approximately proportional to the composition ratio of a LiTaO3single crystal. Accordingly, when the molar ratio of Li2CO3to Ta2O5is controlled, the birefringence can be adjusted. In particular, when the composition ratio of Li to Ta is increased, the refractive index of a TM mode light wave can be decreased, and hence the birefringence can be decreased.