JP4094898B2 - Method for producing domain-inverted crystal - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention belongs to the technical field of optical crystals, and particularly relates to a method for producing a domain-inverted crystal.
[0002]
[Prior art]
The domain-inverted crystal is LiNbO._ThreeLiTaO_ThreeA periodic domain-inverted structure is formed on a ferroelectric crystal (nonlinear optical crystal) such as that so that quasi phase matching is possible. Hereinafter, the periodic polarization inversion structure is also referred to as “inversion structure”, and “polarization inversion” included in other words is also simply referred to as “inversion”.
[0003]
In the normal inversion structure, as schematically shown in FIG. 6A, a band-like inversion region R10 and a non-inversion region N10 are formed on a plate surface of the ferroelectric crystal substrate 100 with a predetermined inversion period (hereinafter referred to as “reversal period”). This structure is also inverted so that it appears alternately in parallel stripes at Λ10. The band width r10 of the inversion region R10 is determined in relation to the inversion ratio (described later). When the input light alternately passes through the inversion region R10 and the non-inversion region N10, output light (wavelength converted light) is generated by the nonlinear optical effect due to the crystal and the quasi phase matching due to the inversion structure.
For the inversion structure and various wavelength conversions using quasi-phase matching using it, the literature “Optical Second Harmonic Generation and Polarization Inversion” (Kurimura, Solid State Physics, 29 (1994) 75-82) and international publication This is described in detail in publications WO 97/15863 and the like.
[0004]
In some aspects of the inversion structure, the arrangement pattern of the inversion regions is not parallel stripes. As an example, there is a fan-out inverted structure as shown in FIG. This figure shows an example formed on a Z-cut ferroelectric crystal substrate (hereinafter also referred to as a Z plate). As shown schematically in the figure, the fan-out inverted structure has a band-shaped inversion region (a band-shaped portion shown in black) and a non-inversion region (a white band-shaped portion in between) radially (fan-shaped). The inversion regions are arranged so that the angle between the longitudinal direction and the Y axis changes sequentially, and the optical path is parallel from L10 to L20. It is characteristic that the period changes continuously when moved.
[0005]
Each inversion region and non-inversion region in the fan-out shape inversion structure has a constant inversion ratio even when the optical path is translated from L10 to L20 as shown in an ideal dimension in FIG. Should be formed as follows. In other words, the inversion region should have a band width that widens from L10 to L20 so that [inversion ratio in optical path L10 (r10 / Λ10)] = [inversion ratio in optical path L20 (r20 / Λ20)]. .
[0006]
For example, in the second harmonic generation (SHG), the fan-out inverted structure can be finely adjusted so that the period is adapted to the wavelength of the incident light by translating the optical path from L10 to L20. . Further, by selecting a large number of optical paths at the same time, it is possible to make various types of input light incident on one element and perform SHG respectively. In optical parametric oscillation (OPO), the oscillation wavelength can be continuously changed by translating the optical path.
[0007]
[Problems to be solved by the invention]
However, when the present inventors examined the state of each inversion region in detail regarding the actually manufactured fan-out shape inversion structure, it was found that the following problems existed.
As shown in FIG. 10A, the problem is that the reversal ratio is not uniform when following an arbitrary optical path L30 passing through the reversal structure, and the original reversal intended by the design. This is a problem that the wavelength conversion efficiency due to the structure is not obtained.
That is, as schematically shown in FIG. 10 (b), the band width r10 of the inversion region in the vicinity of the center of one optical path L30 and the band width rx of the inversion region in the vicinity of the end of the optical path are the latter. The band width is larger by 20 to 30%, and the inversion ratio (band width / period) that should be constant is (r10 / Λ10) <(rx / Λ10).
[0008]
FIG. 11 shows a case where a fan-out inverted sample is formed in accordance with the prior art, and [an angle θ formed between the longitudinal direction of the inversion region and the Y axis] and [inversion ratio] on the optical path when one optical path is selected. It is a graph which shows the result of having measured the relationship. However, in this sample, the inversion ratio at θ = 0 deviates within the allowable range to a value smaller than 50% due to a manufacturing error.
In the production of the sample, in accordance with a conventional manufacturing method, an electrode for applying an inversion voltage is arranged in accordance with each region to be inverted on the crystal substrate, and the other electrode side electrode arranged opposite to the back surface of the crystal substrate is used. A polarization reversal voltage was applied between them, thereby reversing the direction of spontaneous polarization of the crystal. At that time, with respect to the width of each electrode (= length in the optical path direction) arranged in each of the regions to be reversed on the crystal substrate, all electrodes at the same angle are the same width on the same optical path. It formed so that it might become. This is because it is recognized that the shape of the inversion region formed under the electrode has no correlation with the angle θ.
In the fan-out inverted structure actually obtained with such a uniform electrode width, the inversion width (= inversion ratio) increases as the angle θ increases, as is apparent from the graph of FIG.
[0009]
The problem that the inversion ratio increases as the angle θ increases as described above is not only the fan-out inversion structure, but also occurs in all inversion structures in which the inversion regions are not arranged in parallel stripes. .
[0010]
An object of the present invention is to solve the above problem, and in a reversal structure in which strip-shaped reversal regions are arranged radially, even if the angle between the longitudinal direction of the reversal region and the Y-axis direction changes, An object of the present invention is to provide a method for manufacturing an inversion structure that can suppress a change in band width.
[0011]
[Means for Solving the Problems]
The present inventors have investigated the problem of increase in the inversion ratio accompanying the increase in the angle. As a result, in the process of domain inversion performed by arranging the electrodes on the crystal substrate, the inversion region increases as the angle θ increases. It was found that there was a phenomenon that it was difficult to form as in the case of the electrode, and a larger amount protruded from the electrode, and this phenomenon was the cause of the increase in the inversion ratio.
Based on the above knowledge, the present inventors have conceived that the band width of the electrode is corrected in accordance with the angle θ, thereby canceling the above phenomenon and intending the band width of all inversion regions. The present invention has been completed. That is, the present invention has the following features.
[0012]
(1) A strip-shaped electrode corresponding to the domain-inverted region of the structure so that the periodic domain-inverted structure of the following (A) is formed on at least one plate surface of the Z-cut ferroelectric crystal substrate Having a step of arranging
  In order that all the domain-inverted regions formed by the domain-inverted processing have a band width designed for wavelength conversion, the band width of the electrode corresponding to the domain-inverted region is set to the longitudinal direction of the domain-inverted region and Y Correct the intended band width according to the angle with the axis.And the correction applied to the band width of the electrode is a correction that satisfies the following condition (B):A method for producing a domain-inverted crystal.
  (A) A band-shaped domain-inverted region and a non-inverted region having a longitudinal direction appear alternately on the plate surface of the Z-cut ferroelectric crystal substrate so as to enable wavelength conversion by quasi-phase matching. A periodic domain-inverted structure formed, wherein the domain-inverted regions are arranged such that the angle between the longitudinal direction and the Y-axis changes sequentially.
(B) When comparing the band widths of two adjacent electrodes on the same optical path that passes through the periodic polarization reversal structure of (A), the band width of the electrode with the larger angle is the other electrode. Narrower than the width of the band.
[0014]
  UpThe periodic polarization reversal structure of (A) is a fan-out shaped periodic polarization reversal structure, and the intended band width further satisfies the following condition (C)It is preferable.
  (C) When an arbitrary linear optical path that passes through the periodic domain-inverted structure in the X-axis direction is set, the bandwidths of the domain-inverted regions are equal to each other on the same optical path.
[0015]
  UpThe periodic domain-inverted structure of (A) is a periodic domain-inverted structure having domain-inverted regions that spread radially so as to overlap with an optical waveguide that is bent in an annular shape. )It is preferable.
  (D) When an arbitrary optical waveguide that passes through the periodic domain-inverted structure in an annular shape is set, at least on the same optical waveguide, the bandwidths of the domain-inverted regions are equal to each other.
[0016]
  UpThe circularly bent optical waveguide is an annular optical waveguide closed as a circle, and the radially inversion domain is a domain inversion region extending radially at equal intervals in all directions of 360 degrees.It is preferable.
[0017]
  UpThe optical waveguide bent in a ring shape is a bent portion of the optical waveguide bent in a U-shape.It is preferable.
[0018]
  UpThe periodic domain-inverted structure of (A) is two or more periodic domain-inverted structures formed on the plate surface of the ferroelectric crystal substrate. Each of the domain-inverted structures is a band-shaped domain-inverted structure. The regions and the non-inverted regions are formed so as to alternately appear in parallel stripes, and the angle between the longitudinal direction of the domain-inverted regions of each periodic domain-inverted structure and the Y-axis is the periodic polarization. The inverted structures are different from each other, and the intended band width further satisfies the following condition (E)It is preferable.
  (E) The bandwidths of the domain-inverted regions of the individual periodic domain-inverted structures are equal to each other between the domain-inverted structures.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Taking the fan-out inverted structure as an example of the inverted structure (A) above, the manufacturing method according to the present invention will be described, and other aspects of the inverted structure will be referred to as needed. FIG. 1 is a diagram schematically showing an arrangement pattern of electrodes formed on a crystal substrate in order to form a fan-out inverted structure in a manufacturing method according to the present invention. As shown in the figure, in the manufacturing method, the strip-shaped electrodes S1 and S2 corresponding to the inversion regions of the structure are formed so that the inversion structure of (A) is formed on one plate surface of the Z plate. , S3,. . . . , Sx is disposed.
In the drawing, only the electrode disposed on the right side of the electrode S1 near the center is illustrated, but the electrode may be disposed on the left side so as to have a fan-out shape.
[0020]
The important point of the manufacturing method is that the shape of each electrode formed in stripes for polarization inversion is changed according to the change in the angle θ between the longitudinal direction of the inversion region and the Y axis (that is, in FIG. 1). As the process proceeds from the electrode S1 to Sx), the band width of the electrode is corrected as a band width different from the band width of the inversion region. The correction is a correction in which the band width of the electrode is changed according to the angle so that all the inversion regions obtained as a result of the inversion processing have the band width intended by design regardless of θ.
Hereinafter, the term “angle” means “the angle formed by the longitudinal direction of the inversion region (or its electrode) and the Y axis”. The band width of the band-like inversion region is called “inversion region width”, and the band width of the band-like electrode for forming the inversion region is called “electrode width”.
[0021]
As described above, each inversion region obtained by applying an inversion voltage by correcting the electrode width in accordance with the change in the angle θ is originally designed so as to enable the target wavelength conversion. It is formed in a power dimension shape. As a result, the inversion ratio is uniform in any part on one optical path L, and the wavelength conversion efficiency is improved.
[0022]
Of the corrections to be performed on the electrode width in the present invention, a preferable correction is a correction that further satisfies the condition (B) when defined by comparison between adjacent electrodes.
That is, in the case of the fanout inverted structure, as shown in FIG. 1, when an arbitrary optical path L passing through the inverted structure in the X-axis direction is set, all the inverted region widths (W1) are set on the same optical path L. ,..., Wx) are corrected according to the angle so that they are equal to each other. At this time, when two adjacent electrode widths are compared on the same optical path L, the electrode width having the larger angle is corrected to be narrower than the electrode width having the smaller angle. By the correction satisfying the condition (B), the amount of protrusion of the inversion region that increases as the angle increases is canceled out.
[0023]
The inversion ratio is the ratio of the inversion region width in one cycle, and changes the wavelength conversion efficiency. FIG. 8 is a graph showing the relationship (theoretical value) between the inversion ratio D and the conversion efficiency η for m = 1, 2, and 3 among orders m that are arbitrary positive integers. The normalized conversion efficiency is the conversion efficiency when normalized with the maximum conversion efficiency theoretically determined as 1.
As shown in the figure, the curves indicating the relationship between D and η are different for each order. For example, in the case of the order m = 1, the graph exhibits a single peak, and the maximum conversion is achieved at the inversion ratio D = 50%. Efficiency η = 1 is obtained. As the order m increases, the number of peaks in the graph increases, and the maximum conversion efficiency decreases accordingly. Therefore, in a normal design, an inversion structure in which the inversion ratio 50% at the order m = 1 is selected as a preferable inversion ratio for obtaining the highest conversion efficiency, and the ratio between the inversion region width and the non-inversion region width is made equal. Is a preferred embodiment. The inversion region width selected in accordance with the order is the “inversion region width designed for wavelength conversion” in the present invention, but is limited to the order m = 1 and the inversion ratio 50%. In order to prevent unintentional high-order wavelength conversion from occurring, select an order other than 1 according to the purpose, and adopt an inversion ratio that deliberately deviates from the maximum conversion efficiency. Also good.
[0024]
In the present invention, the band width of the electrode is corrected according to the angle so as to be uniform in any part on the same optical path regardless of the values of the inversion area band width and the inversion ratio intended by the design. In particular, it is preferable that the condition (B) is satisfied.
[0025]
In the present invention, the resulting inversion region corrects the electrode bandwidth (so that the design-intended bandwidth for wavelength conversion is designed). In that case, the inversion region has the intended bandwidth. Is not only that the inversion region completely matches the intended bandwidth, but, as shown in FIG. 8, in each order m selected for determining the inversion ratio, the conversion efficiency is 90% of the maximum conversion efficiency. % Or more, and more preferably 95% or more, as long as the width of the inversion region has an inversion ratio.
For example, if the order m = 1 is selected in the graph of FIG. 8, the inversion ratio (50% ± 14.20) is such that the conversion efficiency η is at least 90% of the maximum conversion efficiency (η ≧ 0.9). 3%) is ensured, and more preferably, the inversion ratio (50 ± 10.1%) is 95% or more (η ≧ 0.95) of the maximum conversion efficiency. It is sufficient that the width is set.
In the case of the order m = 2, the maximum conversion efficiency is 0.5, but the range of the inversion ratio that is 90% or more (η ≧ 0.45) has two peaks. The range of inversion ratios such as% ± 7.1% and 75% ± 7.1% and 95% or more is 25% ± 5% and 75% ± 5%.
The case where the order m = 3 or more is selected can be set in the same manner as described above.
In the above, the inversion region width is defined with respect to the inversion ratio, but the actual value of the bandwidth varies depending on the wavelength of light to be subjected to wavelength conversion.
[0026]
By correcting the electrode width according to the angle, the inversion region widths obtained are all equal to each other on the same optical path L, but here, the inversion region widths are equal to each other. In addition, variations due to manufacturing errors may be included.
According to the study by the present inventors, in addition to the phenomenon that the amount of protrusion of the inversion region width increases as the angle increases, the amount of random variation in the inversion region width increases as the angle increases. In the present invention, it is only necessary to suppress the phenomenon that the amount of protrusion of the former increases, and it is only necessary that the inclination of the graph shown in FIG. 11 is corrected to be substantially horizontal as shown in FIG.
[0027]
In the present invention, when the electrode width is corrected so as to satisfy the condition (B), the electrode width may be corrected even when the angle is 0 degrees, depending on the target condition.
[0028]
In the fan-out inverted structure, the individual inverted regions themselves are not fan-shaped with a uniform width but are fan-shaped. Therefore, the longitudinal direction of the inversion region is the direction of the center line as in the case of the electrode shown in FIG.
[0029]
The angle formed between the longitudinal direction of the inversion region and the Y axis is always 90 ° or less of the two angles having a complementary angle. For example, as shown in FIG. 2, even if the longitudinal direction of the inversion region changes counterclockwise to P1, P2, and P3 and the angle between the longitudinal direction and the Y axis increases to θ1, θ2, and θ3, For P3 exceeding 90 degrees, the angle formed with the Y axis is θ3a, which is a complementary angle of θ3.
The angle formed between the longitudinal direction of the inversion region and the Y axis always takes a positive value regardless of whether the longitudinal direction changes clockwise or counterclockwise from the Y axis. The magnitude of each angle is compared with the substantial angle formed between the longitudinal direction and the Y axis.
[0030]
How narrow the electrode width is to be corrected according to the angle and so as to satisfy the above condition (B) depends on the ferroelectric crystal substrate (type, composition ratio, thickness, presence / absence of impurity addition and addition amount) ), Polarization inversion conditions (applied voltage, applied time, crystal temperature, electrode material) and the like.
In FIG. 1, in the period Λ determined by specifying one optical path L, the relationship between the angle θ and the electrode width W (θ) corrected according to θ on the same optical path is F ( θ) as a correction term
W (θ) = W (0) + F (θ) (Formula 1)
Can be expressed as
[0031]
The above (Equation 1) is an equation for determining W (θ) by adding W (0) to the correction term F (θ) according to the change in the angle θ, with W (0) as a reference dimension. The correction term F (θ) is a function of θ defined to change as θ increases. When correcting the electrode width to be narrower than W (0), F (θ) takes a negative value.
When the correction satisfying the condition of (B) is performed according to (Equation 1), the correction term F (θ) takes a negative value, and the absolute value of F (θ) increases as θ increases. To do.
The fan-out inverted structure is characterized in that the period Λ is changed by the parallel movement of the optical path L. Therefore, the electrode width of the entire Λ in the variable range may be corrected based on the above formula.
[0032]
When correcting the electrode width according to the angle, an inversion structure is experimentally formed in accordance with a conventional manufacturing method in advance, and the tendency of the amount of protrusion from the electrode as the angle increases is examined. You may determine the correction amount of each electrode.
[0033]
Next, application to other structures in which the inversion regions are not arranged in parallel stripes, such as a fan-out inversion structure, will be described.
The examples of FIGS. 3, 4, and 5 show other examples of the inversion structure of the above (A). In particular, the examples of FIGS. 3 and 4 are radial so as to overlap with the optical waveguide bent in an annular shape. It has an inversion region that spreads out. Moreover, all are the figures which looked at the Z surface of the crystal substrate.
[0034]
In the example of FIG. 3, the optical waveguide bent in an annular shape is a closed annular optical waveguide L1, and as shown in the figure, the inversion regions spread radially at equal angular intervals in 360 degrees. The inverted structure P1 is provided. When light circulates in the annular optical waveguide L1, the light passes through the radial inversion structure P1, and wavelength conversion is performed.
The entire wavelength conversion element has a structure in which this radial inversion structure P1 is disposed between two linear optical waveguides L2 and L3.
What is necessary is just to select the period of the inversion structure P1 so that various wavelength conversion is possible. Of the incident light input from the linear optical waveguide L2, the wavelength coupled to the annular optical waveguide L1 is controlled by the gap between the two optical waveguides L2 and L1. Similarly, of the light converted in the annular optical waveguide L2, the wavelength coupled to the output linear optical waveguide L3 is also controlled by the gap between the two.
In addition, since the fundamental wave light circulates by adopting an annular optical waveguide and a radial inversion structure, there is a feature that a resonator is not required at the time of OPO, and excitation efficiency is increased.
[0035]
When the radial inversion structure P1 is manufactured according to the manufacturing method of the present invention, the electrode width is set so that the band width of the inversion region to be formed satisfies the condition (D) as in the case of the fan-out shape. Correct. That is, even if the angle changes, on the same optical waveguide L1, the electrode width is set in accordance with the angle and so as to satisfy the condition (B) so that the band widths of the inversion regions are equal to each other. Is corrected.
[0036]
In the example of FIG. 4, the optical waveguide bent in a ring shape is a semicircular bent portion L4 of the optical waveguide bent in a U shape. As shown in the figure, the inversion structure has not only the parallel stripe-like structure P3 of the straight line portion but also the inversion structure P2 in which the inversion region spreads radially at equal angular intervals as a 180-degree fan shape in the bent portion. ing. With such a structure, it is possible to perform wavelength conversion while changing the direction of the optical waveguide from L5 to L6.
[0037]
FIG. 5 shows an example in which two or more inversion structures (three locations P4 to P6 in the figure) are separately formed on one crystal substrate. Each of the inversion structures P4 to P5 in the figure has a simple parallel stripe shape with the same specifications, but the angle formed between the longitudinal direction of the inversion region and the Y axis is different between the inversion structures. . In the example of the figure, the outer shape of the crystal substrate has a triangular shape (not limited to an equilateral triangle), each side is a reflection surface, incident light circulates inside the crystal substrate, and sequentially goes around the inverted structures P4 to P6. However, the wavelength is converted.
[0038]
When manufacturing a plurality of inversion structures as described above according to the manufacturing method of the present invention, as in the case of the fan-out shape, the width of the inversion region to be formed satisfies the condition (E). Correct the electrode width. That is, even if the inversion structures have different inversion region angles, the electrode widths are changed according to the angle and the above (B) so that the inversion regions have the same band width between the inversion structures. The electrode width is corrected so as to satisfy the condition (1).
[0039]
The material of the ferroelectric crystal substrate used in the present invention may be a known material, for example, LiNbO._ThreeLiTaO_Three, X_ATiOX_BO_Four(X_A= K, Rb, Tl, Cs, X_B= P, As) and the like, and those doped with various elements such as Mg. LiNbO_ThreeAnd LiTaO_ThreeMay be a congruent composition or a stoichiometric composition. Among these materials, LiNbO_ThreeAnd LiTaO_ThreeIs a preferred material, especially MgO-doped LiNbO_ThreeIs a material excellent in light damage resistance.
[0040]
In the present invention, a crystal substrate (Z plate) that is cut (Z cut) so that the Z-axis direction of the crystal is perpendicular to the substrate surface is a processing target. Even an off-cut substrate formed so that + Z appears on one plate surface and -Z appears on the other plate surface is included in the Z plate.
[0041]
In the manufacturing method of the present invention, the band width of the electrode is corrected as described above, and an inversion voltage is applied between the electrode and the electrode on the back surface of the crystal substrate to form an inversion region. As for the film method and patterning method) itself, the mode of the electrode on the back surface, the method of applying the reversal voltage, etc., a conventionally known polarization inversion crystal manufacturing technique may be referred to as appropriate.
[0042]
Examples of wavelength conversion include difference frequency generation (DFG), sum frequency generation (SFG), and optical parametric amplification (OPA) in addition to the above-described SHG and OPO.
[0043]
【Example】
According to the manufacturing method according to the present invention, the electrode width was corrected, and the fan-out inverted structure was actually manufactured.
MgO-doped LiNbO with a thickness of 0.5 mm, an optical path direction (X-axis direction) of 50 mm, and a Y-axis direction of 30 mm as a ferroelectric crystal substrate, with a unified polarization direction_ThreeA substrate was used.
The design specification of the fan-out shape to be finally obtained is so wide that the period is variable from 29 μm to 31 μm, and the inversion ratio is 50% in any optical path.
[0044]
[Preliminary experiment to determine (Equation 1) above]
According to the conventional method, a fan-out inverted structure having the above design specifications is manufactured without correcting the electrode width, and the correction term F (θ) in the above (Equation 1) is θ according to the following procedure 1 to procedure 3. As a function of
[0045]
(Procedure 1)
First, among the inversion structures obtained by the conventional method, an inversion period of 30 μm was selected as the optical path, and the inversion ratio at each angle θ was measured on the optical path.
FIG. 11 is a graph obtained by plotting the measurement results and obtaining a straight line showing a tendency for the inversion ratio D [%] to increase as the angle θ increases by the least square method.
From the straight line, the approximate expression when the period is 30 μm is
D [%] = 2.7053 [% / deg.] × θ [deg.] + 38.744 [%]
It became.
[0046]
(Procedure 2)
Next, the correction coefficient F (θ) for obtaining an inversion ratio of 50% was obtained based on the approximate expression. as a result,
F (θ)
= Λ [μm] × (11.256-2.7053 × θ) / 100
= 30 [μm] × (11256-2.7053 × θ) / 100
It became.
[0047]
(Procedure 3)
By performing the above steps 1 and 2 in the same way for other periods Λ on the fan-out, the correction term F (θ) in each period is examined, and all angles and all periods of the fan-out shape of the above specifications are corrected The above (Equation 1) showing values was established.
[0048]
The inversion structure manufactured in the preliminary experiment and the graph of FIG. 11 showing the increasing tendency of each inversion region width are also a comparative example showing the conventional fan-out inversion structure and its evaluation.
[0049]
Based on the correction formula determined above, only the non-inverted region is formed on the + Z plane of the crystal substrate so that a fan-out inverted structure is formed in the entire square region of 50 mm in the X-axis direction and 10 mm in the Y-axis direction. A covering resist pattern was formed.
In this case, the exposed region between the resist patterns is a region where the electrodes are arranged, and the band width (dimension in the X-axis direction) of the exposed region is a value according to the correction formula determined above.
[0050]
The resist pattern and the exposed region are entirely covered, and a Cr layer and an Al layer are sequentially formed to form a uniform metal electrode. A positive potential is applied to the Al layer via a liquid electrolyte, and the back surface (-Z plane). In this case, a polarization reversal voltage was applied so that the liquid electrolyte was directly brought into contact with the liquid electrolyte, and the region where the electrodes were in contact was subjected to polarization reversal to obtain a fan-out reversal structure.
[0051]
The surface of the obtained inversion structure was selectively etched using a hydrofluoric acid and nitric acid mixed solution and observed with a microscope, and the relationship between the angle θ on each optical path and the inversion ratio was examined.
FIG. 9 is a graph showing the relationship between the angle θ on the optical path and the inversion ratio using the same calculation method as that of the graph of FIG. 11 with the period of 30 μm as the optical path. As is apparent from the substantially horizontal straight line shown in the graph of FIG. 9, even if θ increases, the inversion ratio maintains a 50% line without showing an increasing tendency. Strictly speaking, random fluctuations are generated from the 50% line due to manufacturing errors, but it is important that they are within the allowable error range and that the inversion ratio does not show an increasing tendency.
In addition, the optical path of various periods was similarly examined over the entire effective region of the obtained inverted structure, but in any optical path, the inversion ratio was 50% on the same optical path and increased. The trend was not shown.
[0052]
【The invention's effect】
According to the manufacturing method of the present invention, by correcting the electrode width in accordance with the angle, even if the angle increases in the reversal structure in which the reversal regions are arranged radially like the fan-out reversal structure, Thus, the change in the bandwidth of the region can be suppressed, and the inversion ratio on one optical path can be made uniform.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing an arrangement pattern of electrodes formed on a crystal substrate to form a fan-out inverted structure in a manufacturing method according to the present invention.
FIG. 2 is a diagram for explaining an angle to be used for correcting an electrode width in the present invention.
FIG. 3 is a schematic view showing another embodiment of the inverted structure to be manufactured by the manufacturing method of the present invention. Each inversion region and each optical waveguide in the inversion structure are hatched for identification. The same applies to FIG.
FIG. 4 is a schematic view showing another embodiment of the inverted structure to be manufactured by the manufacturing method of the present invention.
FIG. 5 is a schematic view showing another embodiment of the inverted structure to be manufactured by the manufacturing method of the present invention. Each inversion region in the inversion structure is hatched.
FIG. 6 is a schematic view showing a conventional general parallel stripe-shaped inverted structure and a fan-out inverted structure. In FIG. 6A, each inversion area is hatched, and in FIG. 6B, each inversion area is indicated by a bold line.
FIG. 7 is a diagram illustrating the band width of an inversion region of an ideal fan-out inversion structure.
FIG. 8 is a graph showing the relationship between the inversion ratio D and the conversion efficiency η, and shows the relationship between D and η when the order m, which is a positive integer, is 1, 2, and 3; Yes.
FIG. 9 is a graph showing the relationship between the angle θ and the inversion ratio on one optical path of the fan-out inversion structure obtained by correcting the electrode width according to the present invention.
FIG. 10 is a diagram for explaining a problem existing in a fan-out inverted structure manufactured conventionally.
FIG. 11 is a graph showing a relationship between an angle θ and a reversal ratio on one optical path in a fanout reversal structure manufactured conventionally.
[Explanation of symbols]
1 Ferroelectric crystal substrate
S1, S2,. . . , Sx electrode
W1, Wx electrode band width
θ angle

Claims (1)

A band-shaped electrode corresponding to the domain-inverted region of the structure is arranged so that the periodic domain-inverted structure (A) below is formed on at least one plate surface of the Z-cut ferroelectric crystal substrate. Having a process,
In order that all the domain-inverted regions formed by the domain-inverted processing have a band width designed for wavelength conversion, the band width of the electrode corresponding to the domain-inverted region is set to the longitudinal direction of the domain-inverted region and Y According to the angle formed with the axis, correction is made for the intended band width , and the correction applied to the band width of the electrode is a correction that satisfies the following condition (B): A method for producing a domain-inverted crystal.
(A) A band-shaped domain-inverted region and a non-inverted region having a longitudinal direction appear alternately on the plate surface of the Z-cut ferroelectric crystal substrate so as to enable wavelength conversion by quasi-phase matching. A periodic domain-inverted structure formed, wherein the domain-inverted regions are arranged such that the angle between the longitudinal direction and the Y-axis changes sequentially.
(B) When comparing the band widths of two adjacent electrodes on the same optical path that passes through the periodic polarization reversal structure of (A), the band width of the electrode with the larger angle is the other electrode. Narrower than the width of the band.

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