JP2004045536A - Member with optical waveguide and its manufacture method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a member with an optical waveguide formed thereon by using an ion exchange method, in which the refractive index distribution on a cross section perpendicular to the optical waveguide, especially, the refractive index distribution in the width direction of the optical waveguide can be controlled so as to be nearly a concentric circle, and to provide its manufacture method. <P>SOLUTION: The member with the optical waveguide is an optical member 1 having a ridge-shaped part functioning as the optical waveguide 2. The ridge-shaped part is constituted so that its refractive index may get larger consecutively or stepwise from the outer surface layer of the ridge-shaped part toward a center part. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a member with an optical waveguide and a method for manufacturing the same.
[0002]
[Prior art]
In recent years, there has been an increasing demand for a short wavelength laser light source due to an increase in the density of an optical disk and an increase in the accuracy of a laser printer. Attention has been paid to blue semiconductor lasers and second harmonic (SHG) devices to realize these. The SHG element is an element that generates high-output laser light by quasi-phase matching. It is made of a dielectric material such as KTiOPO ₄ (KTP), LiNbO ₃ (LN), or LiTaO ₃ (LT) which has an optical nonlinearity that generates a second harmonic having a half wavelength when a laser beam serving as a fundamental wave is transmitted. An SHG element is manufactured by forming a periodically poled structure and an ion-exchange three-dimensional waveguide on a crystal substrate or a substrate made of glass for an optical waveguide such as quartz as an optical member.
[0003]
The ion-exchange three-dimensional waveguide is usually formed in the shape of a strip on the surface of the above-mentioned LN crystal or LT crystal Z-cut plate or the like. In that case, in order to make the refractive index of the optical waveguide portion higher than that of the surroundings, a specific element is introduced into the portion from the outside. One of the typical methods for introducing the element is an ion exchange method. For example, when the optical member is a LiNbO ₃ crystal substrate, proton H ⁺ is introduced by an ion exchange method represented by the following reaction formula using benzoic acid or pyrophosphoric acid as a proton source.
^{_{LiNbO 3 + xH + → H x}} Li 1-x NbO 3 + xLi +
[0004]
The step of introducing protons by the ion exchange method is as follows.
First, as shown in FIG. 4 (a), only the pattern portion 11 to be an optical waveguide is exposed on the upper surface of the optical member (substrate) 1, and all other portions on the upper surface are mask layers 4 (hatched portions). Covered part).
Next, as shown in FIG. 4B, the liquid exchange source (benzoic acid, pyrophosphoric acid, etc. for LiNbO ₃ ) 50 is heated to reach the reaction temperature (160 ° C. to 260 ° C.). After that, the masked optical member 1 held at room temperature is immersed in the heated liquid exchange source 50 and held at the reaction temperature for a predetermined time. The ion exchange mainly proceeds while being immersed and held at the reaction temperature, and exchanges lithium ions with protons H ⁺ between the pattern portion 11 exposed from the mask layer 4 and the liquid exchange source 50 (proton H ^+). Exchange), and that portion becomes an optical waveguide.
[0005]
FIG. 6A is a schematic diagram of a refractive index distribution (a cross section perpendicular to the optical waveguide) of the optical waveguide formed by such a method (a diagram in which points having the same refractive index are connected by one line). As is clear from this figure, when focusing on the thickness direction, the refractive index is maximum near the surface of the optical member, and when focusing on the width direction, there is almost no difference in the refractive index. A member with an optical waveguide having an optical waveguide having such a distorted refractive index distribution has the following problems.
[0006]
(1) The reduction of the overlap between the fundamental wave and the SHG caused by the asymmetry of the refractive index distribution, and the reduction of the conversion efficiency to the SHG accompanying the reduction.
(2) A decrease in the conversion efficiency to SHG due to a decrease in the nonlinear optical effect due to a high concentration of proton exchange on the substrate surface and a lack of lithium ions.
[0007]
As a manufacturing method for reducing the adverse effects due to such problems, a manufacturing method using a so-called reverse proton exchange method has been proposed. FIG. 5 schematically shows the manufacturing process. That is, a mask layer 4 and a resist layer 3 are formed on the optical member 1 (FIGS. 3A to 3C), a predetermined pattern is formed, and proton exchange is performed by the above-described method. ((D)-(G) in the figure), and thereafter, the protons taken into the optical member 1 are further exchanged with ions such as lithium ions, which have a lower refractive index than the proton-exchanged portion 21. The so-called reverse proton exchange is performed (FIG. (H); a specific method will be described later). FIG. 6B is a schematic diagram of the refractive index distribution of the optical waveguide obtained by this method. Focusing on the thickness direction, as a result of the reverse proton exchange, the refractive index near the surface decreases, and as a result, an ideal distribution can be realized in which the refractive index increases once in the thickness direction and then decreases again. . However, in this method, since the refractive index in the width direction of the optical member cannot be controlled, the refractive index distribution is still distorted and far from an ideal distribution (concentric circle; see FIG. 7).
[0008]
[Problems to be solved by the invention]
The present invention provides a member with an optical waveguide that can reduce the adverse effects of the above (1) and (2), that is, the refractive index distribution in a cross section perpendicular to the optical waveguide is close to a concentric circle, particularly in the width direction of the optical waveguide. An object of the present invention is to provide a method for manufacturing a member with an optical waveguide that can control the refractive index distribution.
[0009]
[Means for Solving the Problems]
The present inventors also studied the physical structure of the optical waveguide and completed the present invention having the following features.
(1) An optical member having a ridge-type portion as an optical waveguide, wherein the refractive index of the ridge-type portion increases continuously or stepwise from the outer surface layer of the ridge-type portion toward the center. A member with an optical waveguide configured as described above.
(2) The member with an optical waveguide according to the above (1), wherein the optical member is a plate made of a ferroelectric crystal or a plate made of optical glass.
(3) a step [A] of forming a ridge-shaped portion on the optical member, a step [B] of performing proton exchange on the entire ridge-shaped portion, and an outer surface layer of the ridge-shaped portion after the steps [A] and [B]. For producing a member with an optical waveguide, comprising a step [C] of subjecting to a reverse proton exchange.
(4) The manufacturing method according to (3), wherein the steps [A] and [B] are steps in which a region where the ridge-type portion is to be formed is subjected to proton exchange and thereafter selective etching is performed.
(5) With an optical waveguide having a step [A] of forming a ridge-shaped portion on an optical member and a step [D] of ion-exchanging the outer surface layer of the ridge-shaped portion with ions that lower the refractive index of the optical member. A method for manufacturing a member.
(6) The method according to (5), wherein the optical member is a ferroelectric crystal made of LiNbO ₃ or LiTaO ₃ , and the ion that lowers the refractive index of the optical member is a magnesium ion or a lithium ion.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
The member with an optical waveguide of the present invention is an optical member having a ridge-shaped portion as an optical waveguide, and the refractive index increases continuously or stepwise from the outer surface layer of the ridge-shaped portion toward the center. It is characterized by comprising. FIG. 1 is a diagram illustrating an example of a member with an optical waveguide of the present invention. FIG. 1A is a perspective view, and FIG. 1B is a sectional view taken along line II of FIG. 1A.
[0011]
The “ridge-shaped portion” as the optical waveguide 2 is a portion that protrudes in a ridge line shape so as to function as an optical waveguide with respect to one surface of the optical member 1 (for example, a plate made of a dielectric crystal). Say. FIG. 2 is a schematic diagram of a refractive index distribution (a cross section perpendicular to the optical waveguide) of the optical waveguide 2 (a diagram in which points having the same refractive index are connected by one line). Making the optical waveguide 2 ridge-type and increasing the refractive index from the outer surface layer toward the center thereof results in controlling both the refractive index distribution in the height direction and the width direction of the optical waveguide 2. (Examples of the means will be described later). Therefore, the ridge-shaped portion as the optical waveguide 2 may have a rectangular parallelepiped structure as shown in FIG. 2A, but the optical waveguide 2 itself has a semicircular cross-sectional shape as shown in FIG. Preferably. It should be noted that the refractive index distribution as shown in FIGS. 2A and 2B is an example, and it is needless to say that achieving such a distribution is not essential for completing the present invention. For example, the change in the refractive index from the outer surface layer to the center of the ridge-shaped portion as the optical waveguide 2 may be continuous or stepwise, and the change rate is the same in each portion of the ridge-shaped portion. Or different.
[0012]
Hereinafter, a method for manufacturing a member with an optical waveguide according to the present invention, which has the above-described optical waveguide, will be further described.
[0013]
The material of the optical member used in the present invention may be any material capable of forming an optical waveguide by ion exchange, and examples thereof include a ferroelectric crystal and an optical glass.
The ferroelectric crystal, for _{example, LiNbO 3, LiTaO 3, X} A TiOX B O 4 (X A = K, Rb, Tl, Cs, X B = P, As) typical or the like, these Examples thereof include those doped with various elements such as Mg.
Examples of the optical glass include quartz glass, Pyrex (registered trademark) glass, and soda glass.
[0014]
The form of the optical member, even if it has the same shape as the element / device as the final product, exhibits a rod shape, a plate shape, etc. as a large-area substrate / wafer / base material before being divided into the element / device. You may. For example, when an optical waveguide is formed on a disk-shaped LiNbO ₃ crystal wafer and divided into individual elements, the size of the original wafer is about 0.1 mm to 1.0 mm in thickness, the wafer diameter is about Is about 2 inches to 5 inches.
[0015]
As a preferred embodiment of the optical member, a plate made of a ferroelectric crystal such as LiNbO ₃ crystal or the like (a domain-inverted crystal) in which a periodic domain-inverted structure is formed so that a wavelength conversion operation using a quasi-phase matching method can be performed. State. By applying the manufacturing method of the present invention to the plate-like object and forming the optical waveguide so as to cross the periodically poled structure, the member with the optical waveguide becomes an optical waveguide type wavelength conversion element.
[0016]
The member with an optical waveguide of the present invention is a member in which an optical waveguide is formed on the optical member, and may itself be a final product such as an element or a coupling part, and further processed and processed. It may be an intermediate member.
[0017]
As one embodiment of the manufacturing method of the present invention, after the step [A] of forming a ridge-shaped portion on the optical member, the step [B] of performing proton exchange on the entire ridge-shaped portion, and the steps [A] and [B], A step [C] of subjecting the outer surface layer of the ridge-type portion to reverse proton exchange. In this case, regardless of the order of the steps [A] and [B], the step [C] may be performed in a state where the entire ridge-shaped portion is proton-exchanged. As a preferred embodiment of the present invention, the steps [A] and [B] are such that after performing proton exchange on one surface of an optical member including a region where an optical waveguide is to be formed, selective etching is performed to form a ridge-type portion. There are methods to be performed. FIG. 3 is a diagram schematically showing a specific example of this manufacturing method.
[0018]
When the optical waveguide is formed by the method illustrated in FIG. 3, proton exchange is performed on one surface of the optical member 1 by a method described later (FIGS. 3A and 3B). A resist layer 3 is formed on the proton-exchanged portion 21, a portion to be an optical waveguide is patterned (FIGS. 9C to 10D), and a mask layer 4 is further formed (FIG. E)). After the resist layer 3 is removed together with the mask layer 4 thereon (FIG. 4F), proton exchange is performed again to form portions having different exchanged proton concentrations. That is, in FIG. 9G, a region indicated by reference numeral 21 is a portion subjected to proton exchange only once, and a region indicated by reference numeral 22 is a portion subjected to proton exchange twice. Next, after the mask layer 4 is removed (FIG. 2H), the portion 22 that has undergone twice proton exchange is mainly etched by, for example, performing etching using a mixed solution of hydrofluoric acid and nitric acid. Is possible. This is because the etching rate varies depending on the degree of proton exchange. As a result, a ridge-shaped portion as shown in FIG. 1I can be formed (the above corresponds to steps [A] and [B]).
[0019]
Further, in the step [C], the proton introduced into the outer surface layer of the ridge-type portion is exchanged with an ion such as lithium ion or magnesium ion, which lowers the refractive index than the proton-exchanged portion. Offer. As a result, the degree of proton exchange is larger near the center of the optical waveguide (reference numeral 21), and the degree of reverse proton exchange is larger near the outer surface layer (reference numeral 23), as shown in FIG. 2 can be obtained.
[0020]
In this way, exchanging the protons in the once proton-exchanged part with ions having a lower refractive index than the proton-exchanged part is called “reverse proton exchange”. Further, “reverse proton exchange on the outer surface layer of the ridge type portion” means that the degree of reverse proton exchange becomes larger as the optical waveguide 2 is closer to the outer surface layer of the ridge type portion. As such, the means for “reverse proton exchange on the outer surface layer” includes appropriately adjusting the time for immersing the optical member in a liquid exchange source of ions for lowering the refractive index, the time for annealing after the exchange, and the temperature. Is adjusted.
[0021]
In the above method, as a specific means for making the ridge-shaped portion as an optical waveguide close to a semicircular shape as shown in FIG. 2B, adjustment of the etching time with the above-mentioned mixed solution of hydrofluoric acid and nitric acid and subsequent dry etching ( (RIE, ion etching, sputter etching) and the like.
[0022]
As another embodiment of the manufacturing method of the present invention, a step [A] of forming a ridge-shaped portion on the optical member 1 and ion-exchange of the outer surface layer of the ridge-shaped portion with ions that reduce the refractive index of the optical member 1 are performed. And step [D]. In the step [D], the meaning of “to ion exchange the outer surface layer” means, as in the above step [C], the degree of ion exchange increases as the optical waveguide 2 is closer to the outer surface layer of the ridge-shaped portion. It means that
[0023]
In this embodiment, proton exchange is not essential, and the degree of ion exchange by ions that lowers the refractive index is increased nearer to the outer surface layer of the ridge-type portion, and as a result, the degree of ion exchange described in FIG. It is intended to approach such a refractive index distribution.
[0024]
The step [A] of forming the ridge-shaped portion on the optical member 1 may be performed by a method known per se, and the step [D] of performing ion exchange may employ, for example, a method of immersion in a liquid exchange source described below. However, the method is not particularly limited in both steps. The “ion that lowers the refractive index of the optical member 1” in the step [D] is not particularly limited as long as the ion has a lower refractive index than a portion that does not exchange by ion exchange with the ion. Such ions depend on the optical member 1 to be used. For example, when the optical member 1 is a ferroelectric crystal made of LiNbO ₃ or LiTaO ₃ , lithium ions, magnesium ions, and the like are exemplified.
[0025]
When ion exchange (including reverse proton exchange) is performed with protons or other ions for manufacturing an optical waveguide, a mask layer is placed in a place where ion exchange of an optical member is not desired in order to ion-exchange only a desired place. After providing 4, a method of immersing in the above-described heated liquid exchange source and maintaining the temperature at that temperature for a predetermined period of time may be mentioned. At this time, examples of the material of the mask layer 4 include a metal film, a dielectric film, and an organic thin film, and a method of forming a mask pattern includes a photolithography technique. Regarding the formation of the mask layer 4, a known technique may be referred to.
[0026]
The liquid exchange source may be selected according to the ion to be ion-exchanged and the material of the optical member, and conventional techniques may be referred to. For example, as described in the description of the related art, when proton exchange is performed on LiNbO ₃ or LiTaO ₃ , a solution of benzoic acid, pyrophosphoric acid, or the like is used. When proton exchange is performed on the optical glass, a molten salt such as AgNO ₃ , KNO ₃ , or TlNO ₃ may be used.
[0027]
Examples of a method of ion-exchanging LiNbO ₃ and LiTaO ₃ with lithium ions include a method of immersing the same in a molten salt such as LiNO ₃ and lithium benzoate. Examples of a method of performing ion-exchange with magnesium ions include MgNO ₃ and benzoic acid. And a method of forming a thin film of Mg or a Mg compound on the surface of LiNbO ₃ or LiTaO ₃ and doping Mg ions by diffusion.
[0028]
The ion exchange temperature varies depending on the combination of the optical member and the liquid exchange source, but the target temperature is approximately 200 ° C. to 350 ° C. This temperature range is a range that can be selected only as a target value, and if a certain target temperature is selected from this range, the unintended fluctuation amount is preferably close to zero.
[0029]
For example, when the LiNbO ₃ substrate is immersed in pyrophosphoric acid, the target temperature is 200 ° C. to 300 ° C., preferably 210 ° C. to 250 ° C., and it is preferable to control the temperature so as not to fluctuate as much as possible. . The amount of fluctuation and the steady-state deviation due to the temperature control method should be within ± 5 ° C., and these may be appropriately determined according to the temperature control method and the quality required for the product.
[0030]
The time for immersing the optical member in the liquid exchange source is not limited, but a practical example is about 10 seconds to 10 hours. As a more specific example, when a relatively shallow optical waveguide having a high ion exchange rate and a depth of about several μm is formed, as in the case of immersing LiNbO ₃ in pyrophosphoric acid, the immersion holding time Is 20 minutes or less, especially about 5 to 15 minutes. As another example, when the above-described reverse proton exchange is performed using a mixed solution of LiNO ₃ , KNO _3, and NaNO ₃ , the immersion holding time is 1 to 20 hours, particularly 5 to 15 hours.
[0031]
In order to end the ion exchange, the immersion may be released (the optical member may be pulled up from the liquid exchange source). If necessary, the optical member may be rotated to dry during cooling, or may be washed to remove the liquid exchange source. Further, processes necessary for forming an optical waveguide, such as annealing of an optical member after ion exchange and optical polishing of an end face, may be appropriately added, and conventional techniques may be referred to.
[0032]
【Example】
Hereinafter, the present invention will be described more specifically by showing examples, but the present invention is not limited to the description of the examples.
[0033]
[Example 1]
The optical member to be processed is a domain-inverted crystal substrate obtained by forming a periodic domain-inverted structure (polarization inversion period: 7 μm) on a MgO-added LiNbO ₃ crystal substrate, and has a thickness of 0.3 mm, a width of 10 mm and a length. It is a plate-shaped object of 10 mm. Hereinafter, description will be made with reference to FIG. 3 used again in the embodiment of the present invention.
[0034]
The entire surface of the optical member 1 (FIG. 3A) (surface on which the waveguide is formed) was subjected to proton exchange over a thickness of 0.5 μm (FIG. 3B). The proton exchange was performed by immersing the optical member 1 in a liquid exchange source made of pyrophosphoric acid heated to 240 ° C. for 30 minutes. Next, a Ta film having a thickness of 0.03 μm as the mask layer 4 was provided only on the portion where the optical waveguide is to be formed by the lift-off method (FIG. 3F). Thereafter, the proton exchange was performed again by immersing the optical member 1 in a liquid exchange source made of pyrophosphoric acid heated to 240 ° C. for 120 minutes. As a result, a portion 21 subjected to proton exchange only once and a portion 22 subjected to proton exchange twice were formed on the surface of the optical member 1 (FIG. 3 (G)).
[0035]
Thereafter, the mask layer 4 remaining on the optical member 1 was removed by etching with a 5% by weight aqueous HF solution (FIG. 3H). Further, as a selective etching, by immersing for 1 hour in an aqueous solution (70 ° C.) containing HF (concentration 50%) and HNO ₃ (concentration 60%) at a weight ratio of 1: 2, the proton exchange is performed twice. By removing the portion 22, the portion 21 subjected to proton exchange only once could be formed as a ridge-type portion (FIG. 3 (I)). Further, the sample was kept in a heating furnace at 350 ° C. for 4 hours to perform an annealing treatment (not shown), and thereafter, reverse proton exchange was performed. In the reverse proton exchange, the optical member 1 after the above-described annealing treatment is applied for 10 hours to a liquid exchange source heated to 328 ° C. containing LiNO ₃ , KNO _3, and NaNO ₃ at a weight ratio of 37.5: 44.5: 18. After immersion and holding, pulling up, it was allowed to cool at room temperature. As a result, a member with an optical waveguide having the optical waveguide 2 in which the degree of the reverse proton decreases from the outer surface layer toward the center was obtained (FIG. 3 (J)).
[0036]
【The invention's effect】
According to the manufacturing method of the present invention, not only the height (depth) direction of the optical waveguide but also the refractive index in the width direction can be controlled. (The refractive index distribution of a simple cross section is circular). As a result, (1) the reduction of the overlap between the fundamental wave and the SHG due to the improvement in the symmetry of the refractive index distribution, and the reduction of the conversion efficiency to the SHG due to the reduction, and (2) the proton exchange of the substrate surface at a high concentration, It is expected that problems such as a decrease in the conversion efficiency to SHG due to a decrease in the nonlinear optical effect due to the lack of lithium ions will be reduced.
[Brief description of the drawings]
FIG. 1 is a diagram showing an example of a member with an optical waveguide of the present invention. (A) is a perspective view, (B) is an II sectional view in (A).
FIG. 2 is a schematic diagram of a refractive index distribution in a cross section perpendicular to the optical waveguide (a diagram in which points having the same refractive index are connected by one line).
FIG. 3 is a view schematically showing a manufacturing process of a member with an optical waveguide.
FIG. 4 is a view schematically showing an important part of a process in a method for manufacturing a member with an optical waveguide.
FIG. 5 is a view schematically showing a manufacturing process of a member with an optical waveguide.
FIG. 6 is a schematic diagram of a refractive index distribution in a cross section perpendicular to the optical waveguide (a diagram in which points having the same refractive index are connected by one line).
FIG. 7 is a schematic diagram of a refractive index distribution of a cross section perpendicular to the optical waveguide in an ideal optical waveguide (a diagram in which points having the same refractive index are connected by one line).
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical member 11 Pattern part 2 Optical waveguide 21 Proton exchanged part 22 Twice proton exchanged part 23 Reverse proton exchanged part 3 Resist layer 4 Mask layer 50 Liquid exchange source

Claims (6)

An optical member having a ridge-type portion as an optical waveguide, wherein the ridge-type portion is configured such that its refractive index increases continuously or stepwise from the outer surface layer of the ridge-type portion toward the center. A member provided with an optical waveguide. The member with an optical waveguide according to claim 1, wherein the optical member is a plate made of a ferroelectric crystal or a plate made of optical glass. A step [A] of forming a ridge-shaped portion on the optical member, a step [B] of performing proton exchange on the entire ridge-shaped portion, and after the steps [A] and [B], an outer surface layer of the ridge-shaped portion is reversely protonated. A method for manufacturing a member with an optical waveguide, comprising a step [C] of exchanging. The method according to claim 3, wherein the steps [A] and [B] are steps in which proton exchange is performed on a region where the ridge-type portion is to be formed, and thereafter, selective etching is performed. Production of a member with an optical waveguide including a step [A] of forming a ridge-shaped portion on an optical member and a step [D] of performing ion exchange on the outer surface layer of the ridge-shaped portion with ions that reduce the refractive index of the optical member. Method. The optical member is a ferroelectric crystal consisting of LiNbO ₃ or LiTaO _3, ions to lower the refractive index of the optical member manufacturing method according to claim 5 which is a magnesium ion or lithium ion.

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