JP2009205036A - Wavelength conversion element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion element, wherein conversion efficiency is heightened by making electric field peak positions of signal light and conversion light coincide with each other, and the tolerance of the width of an optical waveguide by which the signal light and conversion light can hold a single mode is made larger than those of conventional types. <P>SOLUTION: In the wavelength conversion element 10 provided with the optical waveguide 18, the optical waveguide is so constituted that a high-refractive index region 14 which is provided in a ferroelectric crystal 12, having a periodical polarization inversion structure 20 formed and having a first refractive index N1 and has a second refractive index N2; and a structural body 16, provided along a light propagation direction on an upper surface 14a of the high-refractive index region and having a third refractive index N3 are placed in a medium 11, having a fifth refractive index N5 and the first, the second, the third and the fifth refractive indices satisfy N2>N1>N5 and N1≥N3>N5. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a wavelength conversion element that outputs converted light obtained by converting the wavelength of input signal light.

  A wavelength conversion element in which an optical waveguide is formed on the surface of lithium niobate which is a ferroelectric crystal having a periodic polarization inversion structure (hereinafter also referred to as “QPM structure”) is widely known. In this type of wavelength conversion element, the optical waveguide is generally formed by a Ti diffusion method or a proton exchange method. In this wavelength conversion element, signal light to be wavelength-converted is input from one end of the optical waveguide, and converted light that has been wavelength-converted is output from the other end of the optical waveguide.

  However, the wavelength conversion element including the optical waveguide formed in this manner has a low wavelength conversion efficiency.

  This is because the equivalent refractive index distribution of the optical waveguide measured in the depth direction from the surface of the optical waveguide is asymmetric about the optical waveguide. As a result, the signal peak position and the converted light having different wavelengths have different electric field peak positions in the depth direction. This has been a cause that the conversion efficiency cannot be increased.

  To solve this problem, the equivalent refractive index distribution of the optical waveguide is symmetrical in the depth direction by bonding another lithium niobate crystal to the surface of the lithium niobate crystal provided with the optical waveguide. Is disclosed (for example, see Patent Document 1).

  Certainly, according to the technique disclosed in Patent Document 1, the electric field peak positions of the signal light and the converted light can be matched in the depth direction, and the conversion efficiency of the wavelength conversion can be increased.

  However, according to the technique of Patent Document 1, a proton exchange mask is used for producing an optical waveguide. That is, a proton exchange mask made of Ta (tantalum) is formed on the surface of the lithium niobate crystal. Then, a portion exposed from the proton exchange mask is exchanged, and a region having a higher refractive index than a region where proton exchange is not performed (hereinafter, also referred to as “high refractive index region”), that is, an optical waveguide. .

  In general, it has been necessary to use a photolithography technique to form a proton exchange mask. In the photolithographic technique, large-scale equipment such as a mask vapor deposition machine, an exposure machine, and a development apparatus is required, and the photomask needs to be updated every time the dimensions of the waveguide are changed. That is, according to the technique of Patent Document 1, although the wavelength conversion efficiency can be increased, there is a problem that the manufacturing cost increases.

  Therefore, a technique for forming an optical waveguide without using a photolithography technique has been proposed (see, for example, Patent Document 2).

In the technique disclosed in Patent Document 2, a ridge-shaped structure is formed by dicing and laser ablation without using a proton exchange mask, and proton exchange is performed on the ridge-shaped structure. As a result, the convex portion of the ridge structure functions as a high refractive region having a higher refractive index than the region where proton exchange has not been performed, that is, as an optical waveguide.
JP 2002-139755 A JP 2002-365680 A

  However, in the technique disclosed in Patent Document 2, the electric field peak position in the depth direction of the optical waveguide differs between the signal light and the converted light having different wavelengths, and the conversion efficiency of the wavelength conversion is reduced.

  Furthermore, in the technique of Patent Document 2, the periphery of the optical waveguide made of lithium niobate is surrounded by the atmosphere whose refractive index is much smaller than that of lithium niobate. As a result, the confinement of the converted light and the signal light becomes too strong in the optical waveguide. Therefore, there is a problem that the allowable error range of the optical waveguide width in which these lights can maintain a single mode is narrowed.

  The present invention has been made in view of these problems. Therefore, the object of the present invention is to (1) reduce the manufacturing cost by not using photolithography technology, (2) increase the conversion efficiency by matching the electric field peak positions of the signal light and the converted light, and ( 3) An object of the present invention is to provide a wavelength conversion element capable of making the allowable error range of the width of the optical waveguide capable of maintaining the single mode of the signal light and the converted light as compared with the conventional one.

  The inventors of this invention arrange a structure having a refractive index higher than that of the medium in which the wavelength conversion element is placed on the high refractive index region provided on the entire top surface of the ferroelectric crystal, It was conceived that a wavelength conversion element that achieves the above-described object can be obtained by forming an optical waveguide.

  Specifically, the wavelength conversion element of the present invention includes an optical waveguide. Furthermore, a high refractive index region having a second refractive index N2 larger than the first refractive index provided in the ferroelectric crystal having the first refractive index N1 formed with the periodically poled structure, and a high refractive index region The optical waveguide is composed of a structure having a third refractive index N3 provided along the direction of light propagation on the upper surface and a medium having a fifth refractive index N5 surrounding them, and has a first to fifth refractive index. Satisfy the following relationship.

  (Conditions): N2> N1> N5 and N1 ≧ N3> N5

  In this wavelength conversion element, the ferroelectric crystal is a lithium niobate crystal, the structure is a ridge structure formed of lithium niobate crystal, and the surface of the convex portion of the ridge structure faces the top surface. It is preferable that

  Further, in this wavelength conversion element, the high refractive index region is formed by thermal diffusion, the high refractive index region is disposed on the surface of the ferroelectric crystal, and the refractive index distribution is a rectangular function, a Gaussian function, The shape of the error function and the complementary error function is preferable.

  Since the wavelength conversion element of the present invention is configured as described above, (1) the manufacturing cost is reduced by not using photolithography technology, and (2) the refractive index distribution of the wavelength conversion element in the depth direction is symmetrical. By matching the electric field peak positions of the signal light and the converted light, thereby improving the conversion efficiency of the wavelength conversion, and (3) reducing the difference in refractive index between the optical waveguide and the surrounding area, The allowable error range of the width of the optical waveguide in which the light and the converted light can maintain the single mode can be made larger than the conventional one.

  Embodiments of the present invention will be described below with reference to the drawings. In each figure, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. Moreover, although the preferable structural example of this invention is demonstrated below, the material, numerical condition, etc. of each component are only a preferable example. Therefore, the present invention is not limited to the following embodiment.

(Construction)
With reference to FIG.1 and FIG.2, the wavelength conversion element of embodiment of this invention is demonstrated. FIG. 1 is a perspective view schematically showing the structure of a wavelength conversion element. FIG. 2A is a front view of the wavelength conversion element. FIG. 2B is a diagram showing the distribution of the equivalent refractive index in the width direction of the wavelength conversion element along the line AA ′ in FIG. FIG. 2C is a diagram showing the refractive index distribution in the depth direction of the wavelength conversion element along the line BB ′ in FIG.

  1 and 2, the wavelength conversion element 10 includes a high refractive index region 14 formed in the ferroelectric crystal 12, a structure 16, and an optical waveguide 18. The wavelength conversion element 10 is placed in the medium 11 having the fifth refractive index N5, for example, in the atmosphere.

  The ferroelectric crystal 12 has, for example, a rectangular plate shape, and is preferably formed using, for example, a lithium niobate crystal as a material. The ferroelectric crystal 12 has a so-called periodic polarization reversal structure 20 in which the direction of spontaneous polarization is periodically reversed.

  The periodic domain-inverted structure 20 is formed by a conventionally known method. Specifically, for example, a vertically striped metal film pattern is formed on the first main surface of a Z-plate lithium niobate crystal substrate (hereinafter also referred to as “LN substrate”) doped with MgO by photolithography. . Then, a voltage of, for example, 4 kV is applied between the first main surface of the LN substrate on which the metal film pattern is formed and the second main surface opposite to the first main surface via, for example, a saturated lithium chloride aqueous solution. As a result, the direction of spontaneous polarization of the LN substrate in the region where the metal film pattern is formed is reversed. Each spontaneous polarization region is referred to as a domain. As a result, a periodically poled structure 20 is formed in which the direction of spontaneous polarization is periodically reversed along the light propagation direction.

  The high refractive index region 14 is formed by a partial region of the ferroelectric crystal 12 on the first main surface 12 a side of the ferroelectric crystal 12. More specifically, the high refractive index region 14 extends from the first major surface 12a to a predetermined depth with respect to the thickness direction of the ferroelectric crystal 12 (the direction perpendicular to the first major surface 12a and the light propagation direction). Is formed.

  The high refractive index region 14 is a region having a higher refractive index than the remaining region of the ferroelectric crystal 12 (region excluding the high refractive index region). Here, the refractive index of the high refractive index region 14 is the second refractive index N2. The refractive index of the ferroelectric crystal 12 excluding the high refractive index region 14 (hereinafter simply referred to as “refractive index of the ferroelectric crystal”) is defined as the first refractive index N1. At this time, the magnitude relationship between the first and second refractive indexes is N2> N1.

  Here, the thickness and refractive index of the high refractive index region 14 and the second refractive index N2 are set such that the TM mode of the signal light to be wavelength-converted can maintain a single mode in the thickness direction. As an example, when the wavelength of the signal light is 1550 nm, for example, the thickness of the high refractive index region 14 is preferably about 6.9 μm, for example. Further, the second refractive index N2 is preferably about N1 + 0.0077, for example.

  The high refractive index region 14 can be formed by a conventionally known thermal diffusion method such as a Ti diffusion method or a proton exchange method. For example, when the Ti diffusion method is used, the Ti film thickness is 165 nm, the thermal diffusion temperature is 1050 ° C., and the high refractive index having the above-described thickness and the second refractive index N2 is 7 hours. Region 14 can be formed.

  Since the high refractive index region is formed from the entire surface of the first main surface 12a of the ferroelectric crystal 12 to a uniform depth, it is not necessary to use a photolithography technique for manufacturing.

  The structure 16 has a T-shaped cross section orthogonal to the light propagation direction, and extends linearly on the upper surface 14a of the high refractive index region 14 along the light propagation direction. A portion corresponding to the T-shaped vertical bar of the structure 16 is referred to as a convex portion 16a, and a portion corresponding to the horizontal bar is referred to as 16b. In addition, since the signal light and the converted light propagate so as to traverse each domain of the periodic polarization inversion structure 20 perpendicularly, the structure 16 also extends perpendicularly to each domain of the periodic polarization inversion structure 20. is doing. The width W of the structure 16 (the length in the direction parallel to the upper surface 14a of the high refractive index region 14 and perpendicular to the light propagation direction) is preferably about 10 μm, for example.

  The structure 16 is formed by dicing a material having a third refractive index N3. The third refractive index N3 needs to be larger than the fifth refractive index N5 of the medium 11 on which the wavelength conversion element 10 is placed, and needs to be equal to or lower than the first refractive index N1 of the ferroelectric crystal 12. (N1 ≧ N3> N5).

  The material that can be used as the structure 16 is not particularly limited as long as it satisfies the above-described relationship of N1 ≧ N3> N5. In the case of this embodiment, the structure 16 is made of lithium niobate crystals. Is forming.

  The surface 16c of the convex portion 16a of the structure 16 is disposed with a space d between the upper surface 14a of the high refractive index region 14. The interval d is inevitably formed when the structure 16 is attached to the high refractive index region 14 with an adhesive or the like, and the length of the interval d may be 0 (zero).

  The distance d is preferably set so that the evanescent light of the light (signal light and converted light) propagating through the high refractive index region 14 can overlap the structure 16.

  Since the distance d varies depending on the thickness of the high refractive index region 14, the second refractive index N2 of the high refractive index region 14, the fifth refractive index N5 of the medium 11, and the third refractive index N3 of the structure 16. It is difficult to make a general decision. However, according to the simulations of the inventors, in the case of this embodiment, the interval d is preferably set to a value in the range of 0 to 0.25 μm, for example. With such a configuration, the signal light and the evanescent light of the converted light sufficiently overlap the structure 16 practically.

  If the medium 11 is changed from air to polyimide (fifth refractive index N5 = 1.53), the distance d is preferably set to a value in the range of 0 to 0.4 μm, for example.

  The optical waveguide 18 has the equivalent refractive index N4 of the portion where the structure 16 exists and the structure 16 for the light passing through the high refractive index region 14 due to the third refractive index N3 of the structure 16. It is formed by the difference between the equivalent refractive index N6 of the non-existing portion. More specifically, the optical waveguide 18 is formed in a region where the surface 16c of the convex portion 16a of the structure 16 is opposed to the high refractive region 14.

  That is, the equivalent refractive index felt by the signal light and the converted light propagating through the high refractive index region 14 due to the presence of the structure 16 is greater than the second refractive index N2 of the high refractive index region 14 in the region of the optical waveguide 18. And the fourth refractive index N4 (N2> N4> N1) is larger than the first refractive index N1 of the ferroelectric crystal 12. As a result, the signal light and the converted light are confined in the optical waveguide 18.

  As will be described later, the optical waveguide 18 is a single mode waveguide in both the thickness direction and the width direction. In order to make the optical waveguide 18 a single mode waveguide in the width direction, under the conditions of this embodiment, the width of the optical waveguide 18 (and hence the width of the structure 16) is, for example, in the range of 3 to 20 μm. It is preferable to set the value within the range.

  Here, the magnitude relationship of the first to sixth refractive indexes N1 to N6 is summarized. A relationship of N1 <N2 exists between the first refractive index N1 of the ferroelectric crystal 12 and the second refractive index N2 of the high refractive index region 14.

  Further, a relationship of N5 <N1 exists between the fifth refractive index N5 of the medium 11 and the first refractive index N1 of the ferroelectric crystal 12.

  Further, the fourth refractive index N4 of the optical waveguide 18 is influenced by the third refractive index of the structure 16, so that the second refractive index N2 of the high refractive index region 14 and the first refractive index of the ferroelectric crystal 12 are increased. The refractive index is between the refractive index N1 (N1 <N4 <N2).

  As a result of these, the following relationship (1) is established between the first, second, fourth, and fifth refractive indexes N1, N2, N4, and N5.

      N2> N4> N1> N5 (1)

  In addition, in order to make the fourth refractive index N4 of the optical waveguide 18 larger than the sixth refractive index N6, the third refractive index N3 of the structure 16 needs to be larger than the fifth refractive index N5 of the medium 11. (N3> N5). In particular, as will be described later, in order to increase the tolerance in the width direction for making the optical waveguide 18 a single mode waveguide, the third refractive index N3 of the structure 16 is set to be the first refraction of the ferroelectric crystal 12. The value needs to be a value equal to or less than the rate N1 (N1 ≧ N3).

  As a result, the following magnitude relationship (2) is established between the first, third, and fifth refractive indexes N1, N3, and N5.

      N1 ≧ N3> N5 (2)

  That is, in order for the wavelength conversion element 10 of this embodiment to function, the relationship between the expressions (1) and (2) needs to be satisfied at the same time.

  Next, with reference to FIGS. 2B and 2C, the width direction equivalent refractive index distribution and the thickness direction refractive index distribution of the wavelength conversion element 10 will be briefly described.

  Referring to FIG. 2B, the distribution of the equivalent refractive index in the width direction of the wavelength conversion element 10 is from the fifth refractive index N5 of the medium 11 to the second refractive index region 14 from the point A to the point A ′. The refractive index increases to N6. Then, the portion of the high refractive index region 14 corresponding to the surface 16c of the structure 16, that is, the optical waveguide 18, further increases to the fourth refractive index N4. Furthermore, toward the point A ′, the distribution decreases in a symmetric manner with respect to the above-described equivalent refractive index distribution.

  Referring to FIG. 2C, the refractive index distribution in the depth direction of the wavelength conversion element 10 is from the fifth refractive index N5 of the medium 11 to the first refraction of the ferroelectric crystal 12 from the point B to the point B ′. The rate increases to N1. And in the part of the high refractive index area | region 14 corresponding to the surface 16c of the structure 16, it increases to 2nd refractive index N2. Furthermore, the structure 16 decreases to the third refractive index N3 (N3 = N1 in this embodiment), and decreases to the fifth refractive index of the medium 11 at the point B ′.

  In other words, referring to FIG. 2C, it can be said that the wavelength conversion element 10 has a refractive index distributed almost symmetrically about the high refractive index region 14 in the depth direction.

(Operation)
Next, the operation of the wavelength conversion element 10 will be outlined with appropriate mathematical expressions.

  The wavelength conversion element 10 has a function of emitting converted light C having a short wavelength from the other end of the optical waveguide 18 when the signal light S having a long wavelength is incident from one end of the optical waveguide 18. Here, since the periodic polarization inversion structure 20 is formed in the wavelength conversion element 10, the converted light C only when the signal light S having a specific wavelength corresponding to the period of the periodic polarization inversion structure 20 is incident. Is output.

  Hereinafter, it demonstrates using a numerical formula in detail. The following discussion was made by Toshiaki Sugawara, “LiNbO3 waveguide quasi-phase matched nonlinear optical device for optical communication”, IEICE Transactions (C), vol. J-84-C, no. 10, pp. 909-917, Oct. 2001, which are described in detail.

  When the power of the second harmonic as the converted light C is P2ω, the power P2ω is expressed by the following equation (3).

P2ω = Pω0 ² κSHG ² L ² {sin (ΔSHGL / (ΔSHGL)} (3)

  Here, Pω0 is the power of the signal light S. κSHG is a coupling coefficient. ΔSHG is a phase mismatch amount. L is the element length.

  Here, the phase mismatch amount ΔSHG is expressed by the following equation (4).

    ΔSHG = β2ω− (2βω + K) (4)

  Here, β2ω is the propagation constant of the second harmonic (converted light C). Is a number. βω is a propagation constant of the signal light S. K is the wave number of the periodically poled structure.

  In order to obtain the second harmonic as the converted light C with high conversion efficiency, ΔSHG = 0 that is a quasi phase matching condition needs to be satisfied in the equation (4).

  By the way, in the equation (4), β2ω, βω, and K can be expressed as the following equations (5) to (7), respectively, from the conventionally known relationship.

β2ω = k2ωN2ω (5)
βω = kωNω (6)
K = 2π / Λ (7)

  Here, k2ω is the wave number of the second harmonic (converted light C). N2ω is an equivalent refractive index felt by the second harmonic (converted light C). kω is the wave number of the signal light S. Nω is an equivalent refractive index felt by the signal light S. Λ is the period of the periodically poled structure 20.

  Further, as is conventionally known, k2ω and kω can be expressed by the following equations (8) and (9).

k2ω = 2π / λ2ω (8)
kω = 2π / λω (9)

  Here, λ2ω is the wavelength of the second harmonic (converted light C). λω is the wavelength of the signal light S.

  Therefore, when the conventionally known relationship of λ2ω = λω / 2 and the equations (5) to (9) are substituted into the equation (4), focusing on ΔSHG = 0, which is a quasi-phase matching condition, As a final result, the following equation (10) is obtained.

      Λ = λω / {2 (N2ω−Nω)} (10)

  That is, if the period Λ of the periodic polarization inversion structure 20 is determined so as to satisfy the expression (10), the converted light C having a large power can be obtained.

  Next, it will be described how the wavelength conversion element 10 of this embodiment improves the conversion efficiency of wavelength conversion.

  As shown in FIG. 2C, the distribution of the equivalent refractive index in the depth direction in the wavelength conversion element 10 is substantially symmetric about the optical waveguide 18. As a result, the equivalent refractive index distribution in the depth direction sensed by the signal light S and the equivalent refractive index distribution in the depth direction sensed by the converted light C are substantially equal. As a result, the electric field peak depth of the signal light S and the electric field peak depth of the converted light C substantially coincide. Therefore, the coupling coefficient κSHG in the above-described equation (3) is increased, and the conversion efficiency of wavelength conversion is improved.

  Hereinafter, it demonstrates in detail using numerical formula. The following discussion is discussed in Hideki Ishizuki, et. al. , "LiNbO3 waveguide quasi-phase matched sum frequency generation device for high-efficiency optical sampling", IEICE Transactions (C), vol. J83-C, no. 3, pp. 197-203, Mar. 2000, which are described in detail.

  When it is assumed that the waveguide mode distributions of the signal light S and the converted light C are approximated by a Gaussian function and the electric field peak positions coincide with each other, the coupling coefficient κSHG in the equation (3) is expressed as (11 ).

^{κSHG 2 = (8ω3) 2 /} {π 3 (μ0 / ε0) 3/2 d33 2 / (N1 2 N3)} × {WX3 / (WX1 2 + 2WX3 2)} × {WY3 / (WY1 2 + 2WY3 2)} (11)

Here, ω3 is the angular frequency of the second harmonic (converted light C). μ0 is the vacuum permeability. ε0 is the dielectric constant of vacuum. d33 is a nonlinear optical coefficient. N1 is an equivalent refractive index of the signal light S. N2 is an equivalent refractive index of the second harmonic (converted light C). WX3 represents the distance between two points where the peak intensity of the converted light decreases to 1 / e ² in the width direction of the second harmonic (converted light C). WX1 represents a distance between two points where the peak intensity of the converted light decreases to 1 / e ² in the width direction of the signal light S. WY3 represents the distance between two points where the peak intensity of the converted light decreases to 1 / e ² in the depth direction of the second harmonic (converted light C). WY1 represents the distance between two points where the peak intensity of the converted light decreases to 1 / e ² in the depth direction of the signal light S.

In the equation (11), the term {WX3 / (WX1 ² + 2WX3 ² )} × {WY3 / (WY1 ² + 2WY3 ² )} represents the degree of overlapping of the electric field peaks of the signal light S and the converted light C.

  Therefore, if the electric field peak positions of the signal light S and the converted light C do not match, the degree of overlap of the waveguide modes of the signal light S and the converted light C is very small.

That is, in equation ^{(11), {WX3 / (WX1 2 +} 2WX3 2)} × {WY3 / (WY1 2 + 2WY3 2)} section (hereinafter, also referred to as "overlapping section".) To decrease the coupling coefficient κSHG becomes small, and as a result, high conversion efficiency cannot be obtained.

  Hereinafter, this point will be described in more detail with reference to FIGS. FIG. 3A is a diagram schematically showing the electric field strengths of the signal light S and the converted light C in a conventional wavelength conversion element (for example, the wavelength conversion element of Patent Document 2). In FIG. 3A, the vertical axis represents the electric field strength (arbitrary unit), and the horizontal axis represents the distance in the depth direction (arbitrary unit). FIG. 3B is a diagram schematically showing an equivalent refractive index distribution in the depth direction of the wavelength conversion element in FIG. In FIG. 3B, the vertical axis represents the refractive index (arbitrary unit), and the horizontal axis represents the distance in the depth direction (arbitrary unit).

  FIG. 3C is a diagram schematically showing the electric field strengths of the signal light S and the converted light C in the wavelength conversion element 10 of this embodiment. In FIG. 3C, the vertical axis represents electric field strength (arbitrary unit), and the horizontal axis represents depth direction distance (arbitrary unit). FIG. 3D is a diagram schematically showing an equivalent refractive index distribution in the depth direction of the wavelength conversion element 10 of FIG. In FIG. 3D, the vertical axis represents the refractive index (arbitrary unit), and the horizontal axis represents the distance in the depth direction (arbitrary unit).

  First, the refractive index distribution of the conventional wavelength conversion element will be described with reference to FIG. In the conventional wavelength conversion element, an optical waveguide having a refractive index n2 (> n1) is formed near the surface of a ferroelectric crystal having a refractive index n1. The optical waveguide is in direct contact with a medium having a refractive index of n5 (<n1). That is, in the conventional wavelength conversion element, the refractive index distribution is asymmetric with respect to the depth direction.

  In the conventional wavelength conversion element having such a refractive index distribution, the equivalent refractive index to be felt is different between the signal light S and the converted light C having different wavelengths. As a result, the depth of the electric field peak differs between the signal light S and the converted light C as shown in FIG. As a result, the area of the overlapping of the waveguide mode of the signal light S and the converted light C (the hatched region in FIG. 3A) is reduced. That is, the overlapping term of the above-described equation (11) becomes small.

  On the other hand, in the wavelength conversion element 10 of this embodiment, referring to FIG. 3D, the optical waveguide 18 is a ferroelectric whose refractive index is n1 on both sides in the depth direction with respect to the high refractive index region 14. It is sandwiched between the body crystal 12 and the structure 16. As a result, the refractive index distribution is symmetric about the high refractive index region 14.

  Thereby, the equivalent refractive index to be felt is equal between the signal light S and the converted light C having different wavelengths. Therefore, as shown in FIG. 3C, the depths of the electric field peaks of the signal light S and the converted light C match.

  As a result, the area of the overlap of the waveguide mode of the signal light S and the converted light C (the hatched area in FIG. 3C) is larger than that of the conventional wavelength conversion element. That is, the overlapping term of the above-described equation (11) becomes large. That is, κSHG in the equation (11) is increased, and as a result, the conversion efficiency of wavelength conversion is improved.

  According to the evaluation by the inventors, the rate of increase in the conversion efficiency of wavelength conversion by matching the electric field peak depth in the depth direction between the signal light S and the converted light C is about several percent.

  Next, it will be described that in the wavelength conversion element 10 of this embodiment, the allowable error range of the width of the optical waveguide 18 in which the signal light S and the converted light C can maintain the single mode becomes larger than the conventional one.

  The following discussion is described in detail in Okamoto Katsunari, “Basics of Optical Waveguide”, Corona, 1992.

  First, referring to FIG. 4, the equivalent refractive index of the optical waveguide is obtained by using the conventional wavelength conversion element and the wavelength conversion element 10 of this embodiment.

  FIG. 4A is a cut end view obtained by cutting an optical waveguide of a conventional (ridge-shaped) wavelength conversion element along a plane perpendicular to the light propagation direction. FIG. 4B is a schematic diagram showing an equivalent refractive index distribution (arbitrary unit) in the width direction of the optical waveguide of the conventional wavelength conversion element. FIG. 4C is a cut end view obtained by cutting the optical waveguide 18 of the wavelength conversion element 10 of this embodiment along a plane perpendicular to the light propagation direction. FIG. 4D is a schematic diagram showing an equivalent refractive index distribution (arbitrary unit) in the width direction of the optical waveguide 18 of the wavelength conversion element 10 in this embodiment.

  In FIG. 4A, βI indicates a core region of a conventional optical waveguide. ΒII represents a cladding region of a conventional optical waveguide. In FIG. 4B, NβI represents the equivalent refractive index of the core region of the conventional optical waveguide. NβII represents the equivalent refractive index of the cladding region of the conventional optical waveguide.

  In FIG. 4C, αI indicates the core region of the optical waveguide 18 of the wavelength conversion element 10. ΑII denotes a cladding region of the optical waveguide 18 of the wavelength conversion element 10. In FIG. 4D, NαI represents the equivalent refractive index of the core region of the optical waveguide 18 of the wavelength conversion element 10. NαII represents the equivalent refractive index of the cladding region of the optical waveguide 18 of the wavelength conversion element 10.

  Furthermore, it is assumed that both the conventional wavelength conversion element and the wavelength conversion element 10 of this embodiment are formed on an LN substrate having a refractive index of ne. Further, the optical waveguide of the conventional wavelength conversion element and the optical waveguide 18 of the wavelength conversion element 10 of this embodiment increase the refractive index ne of the LN substrate by Δne by a conventionally known proton exchange method. It shall be formed by. The refractive index of the cladding region of the conventional optical waveguide is nc.

  Under such conditions, when NαI, NαII, NβI and NβII are obtained by a conventionally known equivalent refractive index method, the equivalent refractive index NβI of the core region βI of the conventional optical waveguide is expressed by the following equation (12). Given.

      nc <NβI <ne + Δne (12)

  Further, the equivalent refractive index NβII of the cladding region βII of the conventional optical waveguide is given by the following equation (13).

      NβII = nc (13)

    Further, the equivalent refractive index NαI of the core region αI of the optical waveguide 18 of this embodiment is given by the following equation (14).

      ne <NαI <ne + Δne (14)

  Further, the equivalent refractive index NαII of the cladding region αII of the optical waveguide 18 of this embodiment is given by the following equation (15).

      nc <NαII <ne + Δne (15)

  Here, specific values of NαI, NαII, NβI, and NβII are obtained in the case where TM mode light having a wavelength of 1550 nm is used. In this case, the refractive index ne of the LN substrate is 2.1381. Δne is Δne = 0.0077, assuming that the proton exchange condition is 200 ° C. for 1 hour, and then heat treatment is performed at 400 ° C. for 2.25 hours. Further, nc is a refractive index of polyimide of 1.53.

  Under these conditions, NαI, NαII, NβI, and NβII are values of the following formulas (16) to (19).

NαI = 2.1429 (16)
NαII = 2.419 (17)
NβI = 2.419 (18)
NβII = 1.53 (19)

  From the equations (16) to (19), the equivalent refractive index differences ΔNβ and ΔNα between the conventional optical waveguide and the optical waveguide 18 of this embodiment are obtained as the following equations (20) and (21). .

ΔNα = NαI−NαII = 9.8749 × 10 ⁻⁴ (20)
ΔNβ = NβI−NβII = 6.1188 × 10 ⁻¹ (21)

  From the equations (20) and (21), it can be seen that the optical waveguide 18 of this embodiment has a smaller equivalent refractive index difference in the width direction than the ridge-shaped conventional optical waveguide.

  Next, in the wavelength conversion element 10 of this embodiment in which the equivalent refractive index difference ΔNα is smaller than the equivalent refractive index difference ΔNβ of the conventional optical waveguide, the signal light S and the converted light C can maintain a single mode. The fact that the allowable error range of the width of the waveguide 18 becomes larger than the conventional one will be described.

  The following discussion is discussed in Hiroshi Nishihara, et. al. , "Optical Integrated Circuits (Revised Supplement)", Ohmsha, 1993.

  According to this document, it is shown that if the normalized frequency V of the optical waveguide is about 3 or less, the light propagating through the optical waveguide becomes a single mode.

  Based on this document, the normalized frequency Vβ of the conventional optical waveguide (FIGS. 4A and 4B) is assumed assuming that the length in the width direction of the waveguide is 4 μm and the wavelength of light is 1550 nm. Is the value of the following equation (22).

      Vβ = 24.3 (22)

  Further, based on this document, the standardized frequency Vβ of the conventional optical waveguide is 3 or less, that is, the range of the waveguide width in which the single mode is obtained is 0.5 μm or less.

  Similarly, the optical waveguide 18 of this embodiment (FIGS. 4C and 4D) is normalized assuming that the length in the width direction of the optical waveguide 18 is 8 μm and the wavelength of light is 1550 nm. When the frequency Vα is obtained, the value of the following equation (23) is obtained.

      Vα = 2.12 (23)

  Similarly, when the normalized frequency Vα of the optical waveguide 18 of this embodiment is 3 or less, that is, when the range of the waveguide width for single mode is obtained, it is 11.5 μm or less.

  From these, it can be seen that the wavelength conversion element of this embodiment has a wide allowable range in the width direction of the optical waveguide 18 in which the signal light S and the converted light C can maintain a single mode in the width direction.

(effect)
Hereinafter, the effect of the wavelength conversion element 10 of this embodiment will be described.

  (1) In the wavelength conversion element 10, the high refractive index region 14 is formed on the entire first main surface 12 a of the ferroelectric crystal 12, and the optical waveguide 18 is formed by the structure 16 disposed on the upper surface 14 a of the high refractive index region 14. Therefore, it is not necessary to use a photolithography technique. Therefore, the steps related to photolithography can be omitted, and the manufacturing cost can be suppressed.

  (2) Moreover, as shown in FIG. 3, in the wavelength conversion element 10 of this embodiment, the electric field peak positions in the depth direction of the signal light S and the converted light C can be matched. As a result, the overlap of the waveguide mode of the signal light S and the converted light C increases, and the coupling coefficient κSHG increases. As a result, the conversion efficiency of wavelength conversion is improved.

  (3) Further, as shown in FIG. 4, in the wavelength conversion element 10 of this embodiment, the equivalent refractive index difference ΔNα of the optical waveguide 18 in the width direction is smaller than that of the conventional optical waveguide. As a result, it is possible to increase the allowable range in the width direction of the optical waveguide in which the guided light can maintain a single mode. Therefore, an optical waveguide can be easily created.

(Design conditions, etc.)
(1) In this embodiment, the case where lithium niobate is used as the structure 16 has been described. However, the material constituting the structure 16 is not limited to lithium niobate. For the structure 16, a material having a refractive index larger than that of the medium 11 on which the wavelength conversion element 10 is placed and having a refractive index lower than that of the ferroelectric crystal 12 can be selected and used according to the design.

  (2) In this embodiment, the case where the medium 11 is air has been described. However, the medium 11 is not limited to air. As the medium 11, a material having a refractive index lower than that of the optical waveguide 18, for example, polyimide or fluororesin can be selected and used according to the design.

  (3) In this embodiment, the structure 16 has been described with respect to the case where the periodic domain-inverted structure is not formed. However, a periodic domain-inverted structure having the same period as that of the ferroelectric crystal 12 may be formed in the structure 16.

  (4) In this embodiment, the case where pump light is not required, that is, the case where the converted light C is the second harmonic wave has been described. However, the wavelength conversion element 10 can also be applied to wavelength conversion that requires pump light, for example, difference frequency generation or sum frequency generation.

  (5) In this embodiment, the case where the ferroelectric crystal 12 is lithium niobate has been described. However, the ferroelectric crystal 12 is not limited to lithium niobate. Other ferroelectric crystals that can form a periodically poled structure and whose refractive index can be controlled, such as lithium tantalate or quartz, may be used.

  (6) In this embodiment, the case where the structure 16 having a T-shaped cross section is formed by dicing has been described. However, the structure 16 may be formed by other processing methods that do not require photolithography, such as laser ablation.

  (7) In this embodiment, the case where the high refractive index region 14 is formed by the proton exchange method or the Ti diffusion method has been described. However, the method for forming the high refractive index region 14 is not limited to these methods. For example, a method of thermally diffusing additive elements such as Mg and Zn may be used.

  (8) When the high refractive index region 14 is formed using the proton exchange method, a general proton source such as benzoic acid or pyrophosphoric acid can be used.

It is a perspective view which shows the structure of a wavelength conversion element roughly. (A) is a front view of a wavelength conversion element. (B) is a figure which shows distribution of the equivalent refractive index regarding the width direction of the wavelength conversion element along the A-A 'line of (A). (C) is a figure which shows distribution of the refractive index regarding the depth direction of the wavelength conversion element along the B-B 'line of (A). (A) is the figure which showed typically the intensity | strength of the electric field of the signal light and conversion light in the conventional wavelength conversion element. (B) is the figure which showed typically the refractive index distribution in the depth direction of the wavelength conversion element of (A). (C) is the figure which showed typically the strength of the electric field of the signal light and the converted light in the wavelength conversion element of this embodiment. (D) is the figure which showed typically the refractive index distribution in the depth direction of the wavelength conversion element of (C). (A) is the cut end view which cut | disconnected the optical waveguide of the conventional type (ridge type) wavelength conversion element in the surface perpendicular | vertical to the light propagation direction. (B) is a schematic diagram showing an equivalent refractive index distribution in the width direction of the optical waveguide of the conventional wavelength conversion element. (C) is the cut end view which cut | disconnected the optical waveguide of the wavelength conversion element of this embodiment by the surface perpendicular | vertical to a light propagation direction. (D) is a schematic diagram showing an equivalent refractive index distribution in the width direction of the optical waveguide of the wavelength conversion element of this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Wavelength conversion element 11 Medium 12 Ferroelectric crystal 12a 1st main surface 14 High refractive index area | region 14a Upper surface 16 Structure 18 Optical waveguide 20 Periodic polarization inversion structure

Claims (3)

A wavelength conversion element comprising an optical waveguide,
The optical waveguide is
A high refractive index region having a second refractive index N2 larger than the first refractive index, provided in a ferroelectric crystal having a first refractive index N1 formed with a periodically poled structure;
A structure having a third refractive index N3 provided along the direction of light propagation on the upper surface of the high refractive index region;
The first refractive index N1 ferroelectric crystal, a high refractive index region having a second refractive index N2, and a medium having a fifth refractive index N5 surrounding a structure having a third refractive index N3,
The wavelength conversion element characterized in that the first, second, third, and fifth refractive indexes satisfy the following relationship.
N2>N1> N5 and N1 ≧ N3> N5 The ferroelectric crystal is a lithium niobate crystal,
The structure is a ridge structure formed of crystals of lithium niobate,
2. The wavelength conversion element according to claim 1, wherein the surface of the convex portion of the ridge structure is disposed to face the upper surface. The high refractive index region is formed by thermal diffusion, the high refractive index region is arranged on the ferroelectric crystal surface, and the refractive index distribution is a rectangular function, Gaussian function, error function, complementary error function. The wavelength conversion element according to claim 1, wherein the wavelength conversion element has a shape of

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