DE69227102T2 - Method of manufacturing a wavelength converter element - Google Patents

Description

Manufacturing method for a wavelength converter element Background of the invention Field of the invention

The present invention relates to a method for manufacturing a wavelength conversion element using a coherent light source for use in the industrial fields of optical data storage, optical information processing and applied optical measurement control.

Description of the state of the art

Domain inversion for forcibly inverting a domain in a ferroelectric material is used in optical frequency modulators using surface acoustic waves and wavelength conversion elements using inversion of a nonlinear domain in such a way that periodic domain-inverted structures are formed in a ferroelectric material. In particular, by periodically inverting the nonlinear domains of a nonlinear optical material, a second harmonic generating element with extremely high conversion efficiency can be manufactured. By using the second harmonic generating element thus manufactured to convert a beam such as a semiconductor laser, a small light source with a short wavelength can be realized, which can be used in the field of pattern printing, in the field of optical information processing, in applied optical measurement, in control and regulation, and the like. Therefore, many studies have been conducted on domain inversion.

Fig. 21 is a structural view of a conventional light wavelength conversion element. A detailed description will now be given of the generation of higher harmonic waves (wavelengths of 0.41 µm) with respect to fundamental wavelengths with a wavelength of 0.82 µm (see E. J. Lim, M. M. Fejer, R. L. Byer and W. J. Kozlovsky "Blue light generation by frequency doubling in periodically poled lithum niobate channel waveguide", Electron. Lett., 25, 731-732, (1989)).

As shown in Fig. 21, an optical waveguide 44 is formed in a LiNbO3 substrate 41, and a layer (a domain inverted structure) 45 is formed in the optical waveguide 44, the domain of the layer 45 being periodically inverted. By compensating for mismatches in propagation coefficients between the fundamental waves and the higher harmonic waves by the periodic structure of the domain inverted structure 45, higher harmonic waves can be efficiently generated. When fundamental waves P1 (43) are incident on the input surface of the optical waveguide 44, higher harmonic waves P2 (42) are efficiently generated by the optical waveguide 44, so that the above-described structure can function as an optical wavelength conversion element.

An optical wavelength conversion element of the conventional type described above has been realized substantially in a domain-inverted structure. A manufacturing process for an element of the type described above will now be described with reference to Figs. 22a-22c. In Fig. 22a, on a LiNbO₃ substrate 100 composed of nonlinear optical crystal, a Ti pattern 101 is formed at wide intervals of several millimeters by vapor deposition and a lift-off process. Then, a heat treatment is carried out at about 1100°C in a state shown in Fig. 22b so that a domain inverted structure 102 is formed in which the domain direction of the LiNbO₃ substrate 100 is inverted. Then, heat treatment is carried out in benzoic acid at 200°C for 20 minutes in a state shown in Fig. 22c and then annealed at 350°C so that an optical waveguide 103 is formed. As a result, the optical wavelength conversion element manufactured using the above benzoic acid treatment can generate higher harmonic waves P2 with a power of 940 nW with respect to the fundamental wave P1 with the wavelength of 0.82 mm under the conditions of an optical waveguide length of 1 mm and a power of the fundamental wave P1 of 14.7 mW. A conversion efficiency of 0.43%/W is achieved. If 1 W of fundamental waves is incident, higher harmonic waves of 370 mW can be delivered in the case that the length of the element is 10 mm. In this case, an element with a length of 10 mm thus shows a conversion efficiency of 37%/W cm² per W.

A report was published in Appl. Phys. Lett. by Kiyoshi Nakamura, 1990, volume 56, page 1535, reporting that a domain-inverted structure could be prepared in a LiTaO3 element. In Figs. 23a-23b, reference numeral 104 shows a LiTaO3 substrate, 105 a proton exchange layer, and 106 a domain-inverted structure. As shown in Fig. 23a, the domain-inverted structure is prepared by subjecting the LiTaO3 substrate 104 to heat treatment at 590°C in benzoic acid so that the proton exchange layer 105 is formed. Then, as shown in Fig. 23c, the LiTaO₃ substrate 104 is subjected to heat treatment at 570°C-590°C in the vicinity of the Curie point. As a result, the domain inverted structure 106 is formed on the -C surface of the LiTaO₃ substrate 104. However, there has been no thought of using the above-described structure for a wavelength conversion element.

The LiNbO3 crystals involve problems of optical damage and fluctuations in the refractive index of the waveguide because an increase in the power density of the light cannot be avoided, causing a variation in the phase matching conditions. Therefore, there arises a problem that wavelength conversion elements with stable operation and very high conversion efficiency can hardly be manufactured. Therefore, studies have been carried out on manufacturing a wavelength conversion element by forming a domain-inverted structure in a LiTaO3 crystal with resistance to the optical damage. Furthermore, the LiTaO3 crystal exhibits excellent optical properties, and in the manufacture of the crystal, the mixing of impurities can be well avoided. Since the crystal properties of LiTaO3 are better than those of LiNbO3, excellent optical damage resistance and DC drift resistance can be achieved, and therefore this advantageous optical material can be used in optical IC elements. However, there is a problem that a desired periodic domain inverted structure cannot be formed in the LiTaO₃ crystal, although a plate-shaped inverted structure can be formed by the method described above.

EP-A-0485159 describes another prior art method for manufacturing a wavelength converter element using a LiTaO₃ substrate.

The article "Blue-light generation by frequency doubling of a laser diode in a periodically domain-inverted LiTaO₃ waveguide" in IEEE Photonics Technology letters, Volume 4, No. 5, May 1992, New York US, pages 435-437 reports on a quasi phase-matched generation of the second harmonic.

The features of the preamble of claim 1 are known from the article "Fabrication of first-order periodically domain-inverted structure in LiTaO₃" in Applied Physics Letters, Volume 59, No. 13, 23 September 1991, New York US, pages 1538-1540.

Description of the invention

It is therefore an object of this invention to provide a method for producing a wavelength converter element which is designed such that a short-period domain-inverted structure is formed in a LiTaO₃ crystal, so that a conversion to short wavelengths and an efficient conversion of the wavelength become possible.

According to the invention, a method for producing a wavelength conversion device is provided with the steps:

forming a stripe-like mask on a surface of a LiTaO3 substrate obtained by cutting a plane perpendicular to the C axis of the LiTaO3 crystal; forming a proton exchange layer in unmasked areas by subjecting the LiTaO3 substrate to proton exchange of Li and H+ ions in the LiTaO3 substrate; and

Forming a domain inverted structure by subjecting the LiTaO3 substrate to a heat treatment to a temperature below the Curie point of the LiTaO3 substrate at which the LiTaO3 substrate is heat treated, thereby forming a first order periodic domain inverted structure, characterized by using a heat treatment device for increasing the temperature of the LiTaO3 substrate at a temperature rise rate of at least 75°C/sec.

The thickness D of the domain exchange layer may be in a range of 0.2 µm < D < (Λ-W) /2, where A is a predetermined period of stripes on the mask and W is a predetermined stripe width.

As a result of the above-described processes of the present invention, a periodic proton exchange layer is formed in the LiTaO3 substrate, and heat treatment is carried out at a high speed, so that the heat treatment can be carried out in a short time. As a result, domain-inverted structures with a short period can be produced, which could not be realized with conventional structures. Thus, a wavelength conversion element capable of converting short wavelengths and exhibiting high efficiency can be produced. The reason for this will now be described.

The domain-inverted structure is formed in the LiTaO3 crystal by preparing the proton exchange layer and by heat treatment. When the proton exchange is carried out, proton exchange LiTaO3 is formed, which causes a lowering of the Curie point of the region in which the proton exchange LiTaO3 is formed. By heat treating the proton exchange LiTaO3 at a temperature lower than the Curie point of the LiTaO3 crystal but higher than the Curie point of the proton exchange LiTaO3, only the proton exchange LiTaO3 reaches the Curie point. As a result, the domain of the proton exchange region is inverted. Then, selective proton exchange is performed by using a mask having proton exchange resistance to prepare a periodic proton exchange layer and by heat treatment so that the periodic domain-inverted structure is formed.

However, when the domain inverted structure having a short period is formed, the distance between the adjacent domain inverted structures is shortened. Since the proton exchange layer diffuses due to thermal diffusion, diffusion of the proton exchange layer is caused by the temperature rise in the heat treatment step and the heating step. Therefore, the domain inverted structure to be formed in the proton exchange layer is also increased, so that the problem occurs that the adjacent domain inverted structures undesirably communicate with each other in the lateral direction. As a result, the problem shown in Fig. 24b occurs that the required periodic structure cannot be formed. Therefore, the temperature rise rate in the heat treatment is increased in order to increase the heat transfer coefficient. treatment in a short time, thus avoiding the diffusion of the proton exchange layer that may occur during the heat treatment. This makes it possible to manufacture the domain-inverted structure with a short period. As a result, a wavelength conversion element with high efficiency and large output power that can perform short wavelength conversion can be manufactured.

With the method of the invention described above, there is a restriction of the relationship between the width W and the period Λ of the proton exchange layer. Therefore, the width of the proton exchange layer which will diffuse in the heat treatment is controlled. As a result, a domain-inverted structure with a short period, which could not be produced conventionally, can be produced. Thus, a wavelength conversion element which enables short wavelength conversion and high efficiency can be produced. The proton exchange layer for forming the domain-inverted structure is widened in both the depth and transverse directions by the diffusion in the proton exchange treatment. Therefore, the depth of the proton exchange layer is restricted with respect to the interval of the mask pattern, so that the lateral connection of the adjacent proton exchange layers can be avoided. Moreover, when the proton exchange layer is heat-treated to form the inverted layer, the proton exchange layer diffuses due to the heat treatment. However, a sufficiently high proton exchange density is required to produce the domain-inverted structure. Therefore, a certain proton exchange depth is necessary to ensure the proton exchange density during heat treatment. Therefore, the depth of the proton exchange layer is designed to be deeper than the depth required to produce the domain inversion. As a result, a periodic domain-inverted structure can be produced in LiTaO3. As a result, a conversion element with high efficiency and large output power that can convert short wavelengths can be produced.

In the method of the present invention described above, the proton exchange treatment is partially applied to the LiTaO3 crystal which exhibits excellent nonlinearity and sufficient optical damage resistance. Then, it is heated to a temperature below and in the vicinity of the Curie point so that the periodic domain-inverted structure can be formed in the LiTaO3 crystal. In addition, annealing is carried out at a temperature lower than 550°C so that the proton exchange density in the domain-inverted structure can be lowered while the shape of the domain-inverted structure is maintained. As a result, the reflection index (properly: refractive index) of the domain-inverted structure can be adjusted to a value close to that of the substrate so that a low-loss waveguide having the domain-inverted structure can be formed. As a result of the process steps described above, the periodic domain inversion can be produced and the difference between the reflection indices (correctly: refractive indices) between the domain-inverted structure and the substrate can be reduced. Consequently, a wavelength conversion element with low losses can be produced, which exhibits high conversion efficiency.

Other and further principal objects, features and advantages of the invention will become apparent from the following description.

Short description of the drawings

Fig. 1 is a view showing the structure of an example of a wavelength converting element according to the present invention;

Fig. 2 is a diagram showing output signals of second harmonic waves from an element in which the domain is not inverted, a domain-inverted element with a first-order period, and a domain-inverted element with a third-order period;

Fig. 3 is a diagram illustrating the relationship between the phase matching wavelength and the phase matching period;

Fig. 4a-4e are views of the wavelength converter element according to the invention;

Fig. 5 is a graph showing the relationship between the heat treatment temperature and the depth of the domain inverted structure;

Fig. 6a-6d are views showing a change in the heat treatment time and the cross-sectional shape of the domain-inverted structure depending on the heat treatment time;

Fig. 7 is a diagram showing the relationship between the heat treatment time and the width and depth of the domain inverted structure;

Fig. 8 is a graph showing the relationship between the temperature rise rate during heat treatment and the producible domain inversion period;

Figs. 9a and 9b are cross-sectional views of the wavelength converter element;

Figures 10a and 10b are cross-sectional views of the wavelength converter element;

Fig. 11 is a diagram showing the relationship between the depth of the domain inverted structure and an SHG output signal;

Fig. 12 is a diagram showing the relationship between the heat treatment temperature and the depth and width of the domain inverted structure;

Fig. 13a and 13b are diagrams showing the relationship between the width and the depth of the domain-inverted structure;

Figs. 14a-14c are cross-sectional views showing the wavelength conversion element in which a proton exchange layer is expanded;

Fig. 15 is a diagram showing the relationship between the interval of the proton exchange masks and the region in which the domain-inverted structure can be formed;

Fig. 16 is a diagram showing the relationship between the depth of the domain-inverted structure and the proton exchange layer depth D;

Fig. 17a-17h are perspective views of a manufacturing process of an optical waveguide according to the invention;

Fig. 18 is a diagram showing the SHG characteristics of the waveguide element according to the present invention;

Fig. 19 is a diagram showing the SHG characteristics of the waveguide element according to the present invention;

Fig. 20 is a structural cross-sectional view of a short wavelength laser beam source according to this invention;

Fig. 21 is a structural view of a conventional wavelength conversion element;

Figs. 22a-22c are cross-sectional views showing the steps of a manufacturing method for the conventional wavelength conversion element;

Figs. 23a-23c are cross-sectional views of a fabrication process for a conventional domain-inverted LiTaO3 structure;

Fig. 24a and 24b are cross-sectional views of the transverse extent of the domain-inverted structure.

Description of the preferred embodiment

Before describing an example of the invention, a wavelength converter element in which a domain-inverted structure is formed will first be described.

Fig. 1 is a perspective structural view showing the wavelength conversion element. Reference numeral 1 denotes a -C plate LiTaO3 substrate, 4 a domain inverted structure, 5 a proton exchange waveguide, 6 fundamental waves having the wavelength of 860 nm, and 7 a second higher harmonic wave (hereinafter referred to as "SHG light") having the wavelength of 430 nm. As shown in Fig. 1, the domain inverted structure 4 is formed with a period length of Λ and a width of W.

As shown in Fig. 1, the domain inverted structure 4 is a region in which the direction of the domain is inverted with respect to the domain inversion of the substrate. In the case of the LiTaO3 substrate 1, the direction of the domain is in the +C direction, while the direction of the domain in the domain inverted structure is -C. The period Λ of the domain inverted structure acts differently depending on the wavelength of the fundamental waves and the refractive index of the waveguide. In the case of a fundamental wave wavelength of 860 nm, the first order period is about 3.6 µm, while the third order period is 10.8 µm, which is three times the first order period (the period of the domain inversion is an odd multiple of the first order period). In the domain inversion, a phase matching state is realized, and therefore the wavelength conversion can only be performed when the period coincides with the odd multiple of the period. However, the conversion efficiency decreases with a factor of the square of the order size with an increase in the order size.

Fig. 2 illustrates the relationship between the length (I) of an element 31 whose domain is not inverted, that of the element 32 with domain inversion of the first order period, and that of the element 33 with domain inversion of the third order period, and the outgoing SHG light. As shown in Fig. 2, the SHG light outputs 32a and 33a from the first order and third order domain inverted elements increase in proportion to the square of the length (I) of the elements. The output is inversely proportional to the magnitude of the period, and the maximum efficiency is exhibited at the first order domain inversion period, namely the lowest order. Λ1 of the first order in this case can be expressed as follows, assuming that the effective refractive index of the fundamental wave (wavelength λ) is Nω. and the effective refractive index of the higher harmonic waves (wavelength I/2) N2ω is: Λ1 = λ / (2 (N2ω - Nω)), while the third order period Λ3 is three times the first order period Λ1, that is, Λ3 = 3 * Λ1. The effective refractive index is the one at which light can actually be detected.

On the other hand, blue light with a wavelength of 480 nm or less suitable for compact discs or optical storage devices is desired. Therefore, it is particularly important that a wavelength conversion element capable of generating blue light is manufactured. Fig. 3 illustrates the relationship between the domain inversion periods, such as the first-order period Λ1 (expressed as M = 1) and the third-order period Λ3 (expressed as M = 3), and the phase matching wavelength. Since the wavelength of the transmitted SHG light is half the phase matching wavelength, the fundamental wave must have a wavelength of 960 nm or less if a wavelength of 480 nm or less is desired.

The shorter the phase matching wavelength is, the shorter the period of the domain-inverted structure becomes, and accordingly, the domain-inverted structure must have a third-order period of 15 µm or less and a first-order period of 5 µm to achieve blue light with a wavelength of 480 nm or less.

An example of the invention will now be described. First, a manufacturing process for the wavelength converter element according to the invention will be described schematically.

The procedure includes the following two steps:

1. Proton exchange layers are periodically prepared on a LiTaO₃ substrate.

2. The proton exchange layers are transformed to adopt a domain-inverted structure by heat treatment.

Examples of the invention will now be described with reference to the drawings.

example 1

A manufacturing method for a wavelength conversion element with the domain-inverted structure is now described.

The inventors have studied the formation of periodic domain-inverted structures in LiTaO3, which is necessary for manufacturing a wavelength conversion element. It is already established that, as shown in the conventional example shown in Figs. 23a-23c, a layered (plate-like) domain-inverted structure can be manufactured by proton-exchanging LiTaO3 using benzoic acid before heat-treating it near the Curie point of LiTaO3. Therefore, the manufacture of the periodic domain-inverted structure was attempted by a method in which the surface of the LiTaO3 substrate was subjected to selective proton exchange to form a periodic proton exchange layer in the LiTaO3 substrate and then heat-treated.

Since the proton exchange density and the uniformity of the proton exchange layer affect the shape and uniformity of the domain-inverted structure to be produced, pyrophosphoric acid with a high proton exchange rate and the ability to form a uniform proton exchange layer was used for the proton exchange.

In order to carry out the proton exchange selectively, a mask must be used that can block the proton to a sufficient extent. Therefore, the mask is made of Ta, whose selectivity ratio can be made practically infinite, so that a selective proton exchange layer could be produced.

Figs. 4a-4e are cross-sectional views showing the manufacturing process of the wavelength conversion element according to Example 1. In Figs. 4a-4e, reference numeral 1 denotes a LiTaO3 substrate formed of a C plate, 2 a Ta mask, 3 a proton exchange layer, 4 a domain inverted structure, 8 the width W of the mask, and 9 the period Λ of the domain inverted structure (specifically, the period Λ is set to 1-20 µm and the width to N2). The manufacturing process will now be described with reference to Fig. 4.

As shown in Fig. 4a, the Ta mask 2 with a thickness of 300Å is formed on the C-plate LiTaO3 substrate 1 by the sputtering method. In Fig. 4b, a photoresist is applied on the Ta mask 2 before stripes with width W were formed by the usual photolithography method with a period Λ in the Y-transmission direction of the substrate (in this example, the period Λ was 1-20 µm and W was 1-20 µm and W was Λ/2).

In Fig. 4c, the resist pattern is transferred to the Ta mask 2 by dry etching in a CF4 atmosphere.

In Fig. 4d, the proton exchange layer 3 is formed in pyrophosphoric acid at a temperature of 260°C.

In Fig. 4e, the LiTaO3 substrate 1 is heat treated. By using a heating furnace designed to operate in a heating mode like a conventional heater, the temperature rise rate was approximately 1°C/second.

In the process described above, a periodic domain-inverted structure 4 with a period of 8-20 µm was formed. Since the processes shown in Figs. 4a-4c are well known, the processes shown in Figs. 4d and 4e will now be described in detail.

The process for preparing the proton exchange layer shown in Fig. 4d is now described.

Figs. 14d-14f are cross-sectional views showing the cross section of the proton exchange layer formed by a method of manufacturing the wavelength conversion element. In the drawings, reference numeral 1 denotes a LiTaO3 substrate, 2 a Ta mask, and 3 a proton exchange layer. By performing the heat treatment using pyrophosphoric acid, proton exchange layers 3 are formed in the unmasked areas.

Figures 14a-14c illustrate diffusion states of the proton exchange layer when the proton exchange time is extended to 20 minutes, 1 hour and 3 hours at a proton exchange temperature of 260 °C.

As shown in the drawings, the proton exchange layer 3 also expands in the depth direction and in the width direction, similarly to the domain inverted structure 4. The expansion of the proton exchange layer 3 proceeds at substantially the same speed in both the depth direction and the width direction. Therefore, in order to form the periodic proton exchange layer 3 for producing the periodic domain inverted structure 4, it is necessary to set the thickness D of the proton exchange layer 3 to half or less of the gap between the proton exchange layer 3 and the mask 2 for producing the proton exchange layer 3 (which corresponds to the difference (Λ - W) between the period and the width W of the domain inverted structure). If the proton exchange layer 3 is formed so that its thickness D is larger than the value described above, the adjacent proton exchange layers 3 are undesirably connected to each other. As a result, the desired periodic cal proton exchange layer 3 cannot be produced and thus also not the periodically domain-inverted structure 4.

The relationship between the distance between the stripes of the proton exchange mask 2 (which corresponds to the difference (Λ - W) between the period Λ and the width W of the domain-inverted structure 4) and the range in which the domain-inverted structure can be formed is shown in Fig. 15. As can be seen from Fig. 15, the proton exchange depth D must be (Λ - W)/2 or less.

Fig. 16 illustrates the relationship between the depth of the domain inverted structure 4 and that of the proton exchange layer 3. In Fig. 16, the abscissa represents the thickness of the proton exchange layer 3 and the ordinate represents the thickness of the domain inverted structure 4 having a width of 10 µm. As shown in Fig. 16, the domain inverted structure 4 cannot be formed if the thickness of the proton exchange layer 3 is 0.2 µm or less. In order to form the domain inverted structure 4, the Curie point of the region in question must be lowered during proton exchange, the lowering of the Curie point being in proportion to the proton exchange density. Therefore, in order to form the domain inverted structure 4, the proton exchange layer must be kept at a certain value or more. It is considered that when the thickness of the proton exchange layer 4 is 0.2 µm or less, the proton exchange density cannot be maintained in the case that the proton exchange layer 3 widens during the heat treatment. Therefore, the domain-inverted structure 4 cannot be manufactured.

As a result, it was found that the thickness D of the proton exchange layer 3 must satisfy the relationship 0.2 µm < D < (Λ - W) /2 µm in order to manufacture the domain inverted structure 4, where Λ is the period of the proton exchange mask and W is the width of the proton exchange mask.

By preparing the proton exchange layer 3 under the above-described circumstances, the domain inverted structure 4 can be formed in the LiTaO₃ crystal.

Fig. 5 illustrates the relationship between the heat treatment temperature for the proton exchange layer 3 and the depth of the domain-inverted structure 4, the period of which was 20 µm and which was realized by the manufacturing process for the domain-inverted structure 4 shown in Fig. 4e.

The depth of the domain-inverted structure 4 is proportional to the temperature rise. However, when the heat treatment temperature exceeds 600°C, the periodic domain-inverted structure 4 is destroyed. This is because the domain of the entire substrate is inverted because the heat treatment temperature exceeded the Curie point of the LiTaO3 substrate (the Curie point of the substrate used was 604°C). Although the lowest temperature at which the domain-inverted structure 4 can be formed is lowered as the proton exchange increases, it saturates at about 450°C.

The result clearly shows that the domain-inverted structure 4 can be produced by heat treatment at a temperature of 450-600ºC.

With the fabrication process thus designed, although the domain-inverted structure 4 with a period of 8-15 µm could form a third-order domain inversion, it was difficult to fabricate a domain-inverted structure 4 with a period of 5 µm or less, which is necessary for realizing the first-order domain inversion. The reason for this is that although the domain-inverted structure 4 was fabricated by Heat treatment of the proton exchange layer 3 at 450-600°C, the proton exchange layer 3 is widened due to heat diffusion before the heat treatment temperature reaches 450°C or more, and thereby the adjacent domain-inverted structures 4 were undesirably connected to each other in the case of manufacturing a short-period domain-inverted structure 4. Therefore, the short-period domain-inverted structure 4 could not be manufactured.

Example 2

Accordingly, the manufacturing method for the wavelength conversion element in this example is designed such that a very rapid temperature raising device is used for the heat treatment required to form the domain-inverted structure, in order to shorten the time required to raise the temperature to a required level for the heat treatment in which the domain is inverted and accordingly prevent the heat diffusion of the proton exchange layer 3, so that the transverse diffusion of the domain-inverted structure 4 formed in the proton exchange layer 3 during the heat treatment is avoided:

As a result, a short period of the domain inverted structure 4 can be formed in the LiTaO3 substrate, so that a wavelength conversion element capable of converting short wavelengths and exhibiting excellent efficiency is manufactured.

Figures 4a-4e are cross-sectional views illustrating the manufacturing process for the wavelength conversion element.

In Fig. 4a, the Ta mask 2 with a thickness of 300 Å is formed on the C-plate LiTaO3 substrate 1 by sputtering.

In Fig. 4b, a photoresist is applied to the surface of the Ta mask 2 before stripes of width W are formed on the substrate 1 with a period Λ to a length of 10 mm in the Y-transfer direction by a conventional photolithography method (the period Λ was 1-50 μm and W was Λ/2).

In Fig. 4c, the resist pattern was transferred to the Ta mask 2 by dry etching in a CF4 atmosphere.

In Fig. 4d, the proton exchange layer 3 is formed by heat treatment in pyrophosphoric acid at 260°C for 20 minutes.

In Fig. 4e, the LiTaO3 substrate 1 is heated in a heating furnace with a very rapid temperature rise.

The processes shown in Fig. 4a-4d for this example are the same as in Example 1, except that the heat treatment process shown in Fig. 4e is carried out with a very rapid temperature rise. This very rapid temperature rise during the heat treatment can avoid the undesirable transverse expansion of the domain-inverted structure 4.

Before describing the result of the production of the domain-inverted structure 4 by heat treatment of the proton exchange layer 3 with the very rapid temperature increase, the mechanism of the formation of the domain-inverted structure 4 is now described.

Mechanism of formation of domain-inverted structure 4

The result of an examination of the formed domain-inverted structure 4 is shown in Figs. 6a-6d. Figs. 6a-6d are cross-sectional views of the domain-inverted structure 30 subjected to the heat treatment at 520°C after the proton exchange for 30 seconds, 1 minute, 2 minutes and 8 minutes. As understood from Fig. 6a, the domain-inverted structure 4 was formed at the interface (between the proton exchange region and the substrate) of the proton exchange layer 3. When the heat treatment is continued, the domain-inverted structure 4 widens around the above-described layer as shown in Figs. 6b and 6c, so that a semicircular domain-inverted structure 4 is formed. That is, the domain-inverted structure 4 is formed between the proton exchange layer 3 and the substrate 1 at the position of the boundary between the proton exchange layer 3 and the substrate 1 where the electric field is strongest. The above-described formation initiates the expansion of the domain-inverted structure 4, which becomes larger as the heat treatment time increases. The expansion of the domain-inverted structure 4 is stopped when the heat treatment is continued. When the heat treatment is further carried out, the formation of microdomains is observed in the domain-inverted structure 4. The microdomains are needle-like reinverted structures formed by reinverting a layer of the domain that has been inverted, the microdomains having the same domain direction as that of the substrate 1. The microdomains cause a non-uniformity of the domain of the domain-inverted structure 4. The longer the annealing time in conventional annealing steps, the greater the non-uniformity of the layer. However, a uniform domain-inverted structure 4 can be formed by a short heat treatment of the domain-inverted structure 4.

Short-period domain-inverted structure

Fig. 7 shows the result of measuring the relationship between the heat treatment time and the measured width and depth of the domain inverted structure 4. The width of the domain inverted structure 4 diffused at a rate of about 1 µm/minute before saturation at about 2 minutes. Furthermore, it was found that although the temperature of formation of the domain inverted structure 4 as described with reference to Fig. 5 is about 450-600°C, which is close to the Curie point of the LiTaO3 substrate, the proton exchange layer 3 is undesirably expanded due to heat diffusion before reaching the above-described temperature if the temperature rise rate is too low, whereby the short-period domain inverted structure 4 is hardly formed.

Therefore, the inventors have created a method for producing the domain-inverted structure 4 by carrying out the heat treatment by RTA (Rapid Thermal Annealing). RTA is a method for heating a sample at a high speed by infrared rays and can heat from very low speeds to high speeds of 50°C/second or more while having excellent controllability. Therefore, an attempt was made to produce the periodic domain-inverted structure 4 by RTA when the proton exchange layer 3 is heat-treated in the method for producing the domain-inverted structure 4.

As a result of the RTA heat treatment, a very fast temperature rise and a short heat treatment time were obtained. Fig. 8 illustrates the obtained relationship between the temperature rise rate during the heat treatment and the shortest manufacturable period of the domain-inverted structure 4.

The temperature rise rate in the required heat treatment was obtained from the relationship between the temperature rise rate and the minimum domain inversion period shown in Fig. 8. As a result, it was found that a Ge rate of 0.5ºC/second or more was required for the third-order domain-inverted structure and 15ºC/second or more for the first-order domain-inverted structure.

As a result, in order to manufacture a wavelength conversion element with high output and high efficiency, heating had to be carried out at a temperature rise rate of 0.5ºC/second or more. In order to realize the first-order domain inversion, which enables even more efficient wavelength conversion, heating had to be carried out at a temperature rise rate of 15ºC/second or more. However, since the temperature rise rate that can be realized with an ordinary furnace is 1ºC/second or less, the first-order domain structure cannot be realized, but the third-order domain inverted structure with a period of about 10 µm can be realized. However, rapid heating of 50ºC/second or more can be carried out with RTA. As a result, a first-order domain inverted structure with a period of 4 µm or less could be manufactured. The reason for this is that the diffusion of the proton layer, which can be generated during the heat treatment of the proton exchange layer, can be restricted by the rapid temperature rise and the short time of the heat treatment. As a result, the transverse enlargement of the domain inverted structure can be restricted, thereby enabling the formation of the domain inverted structure with a short period. As a result, a wavelength conversion element with a first-order domain inversion structure with excellent efficiency could be manufactured.

Depth of the domain-inverted structure

In manufacturing an efficient wavelength conversion element, it is important to shorten the period of the inverted structure and to manufacture the domain inverted structure with the correct depth. The reason for this will be explained with reference to Figs. 9a, 9b, 10a and 10b. Fig. 10a shows a cross-sectional view along the center line of the waveguide of the wavelength conversion element shown in Fig. 1. Figs. 9b and 10b are each enlarged views showing an electric field distribution of the light propagating through the waveguide. Figs. 9a and 9b illustrate the case that the depth of the domain inverted structure is shallow with respect to the depth of the waveguide, while Figs. 10a and 10b illustrate the case that the depth of the domain inverted structure is deep with respect to the depth of the waveguide. In the drawings, reference numeral 4 denotes the domain inverted structure, 5 a proton exchange waveguide, 6 the fundamental wave light, 7 SHG light, 10 the electric field distribution of the fundamental wave light, 11 the electric field distribution of the SHG light, and 13 the overlap of the electric field distribution 10 of the fundamental wave light, the electric field distribution 11 of the SHG light, and the domain inverted structure 4. The SHG efficiency is related to the overlap 13 of the electric fields. As is clear from Figs. 9a, 9b, 10a, and 10b, in the case of a shallow depth of the domain inverted structure 4 relative to that of the proton exchange waveguide 5 with a larger overlap 13 of the electric fields, the depth of the domain inverted structure 4 becomes deeper. In proportion, the SHG output power increases. The result of calculations of the above-described relationship is shown in Fig. 11.

The relationship between the depth of the domain inverted structure 4 and the SHG conversion efficiency in a proton exchange waveguide 5 of 2.4 µm was determined. It is clear that the SHG conversion efficiency depends significantly on the depth of the domain inverted structure 4 until the depth of the domain inverted structure 4 reaches 2.4 µm. Therefore, an investigation was made on the depth of the domain inverted structure 4, and the relationship between the heat treatment temperature and the shape of the domain inverted structure 4 was measured. The proton exchange layer 3 with the width 10 µm and the depth 0.6 µm was heat treated for 30 seconds with a furnace, so that the relationships between the heat treatment temperature and the width and depth of the domain-inverted structure 4. The results are shown in Fig. 12.

As shown in Fig. 12, the domain inverted structure 4 enlarges according to a heat treatment temperature increase and also with a change in the ratio of the width of the domain inverted structure 4 and the depth thereof. That is, the higher the temperature, the depth of the domain inverted structure 4 formed increases when the width remains the same. As a result, it has been found that the high-temperature heat treatment allows a greater depth of the domain inverted structure 4 formed with the same width. The reason for this is probably that the enlargement rate of the domain inverted structure 4 in the depth direction and in the width direction has different temperature characteristics and thus the enlargement rates in the depth direction are related to the temperature.

Furthermore, it was found that the depth of the domain-inverted structure 4 also depends on the heat rise rate in the heat treatment. Figs. 13a and 13b show the relationship between the width and depth of the domain-inverted structure 4 formed by raising the temperature at a rate of 50°C/second by RTA and by heat treatment at 550°C for 50 seconds, and those of the domain-inverted structure 4 formed by using the conventional furnace at a temperature rise rate of 1°C/second and by heat treatment at 550°C for 30 seconds.

As a result of the high temperature rise by RTA, a domain inverted structure 4 with a width of 1-2 µm could be manufactured. Moreover, the depth of the domain inverted structure 4 could be made deeper with the width fixed in a range between 1 and 5 µm. The reason for this is probably that the electric field is increased due to the pyroelectric effect when the heat treatment is carried out in a short time, thereby generating a very large electric field when the temperature rises at a high rate and enhancing the enlargement of the domain inverted structure in the depth direction. For example, the required depth of the domain inverted structure with a width of 2 µm to realize a period of 4 µm is approximately 1 µm when the heat treatment is carried out at a temperature rise rate of 1ºC/second. However, a domain-inverted structure with a depth of 1.5 µm can be formed when the temperature is raised at a rate of 50ºC/second. Using the above value, the SHG conversion efficiency described by a method described with reference to Fig. 11 was calculated. As a result, the wavelength conversion element formed at a temperature rise rate of 50ºC/second can show a conversion efficiency 10 times higher than that of the wavelength conversion element formed at a temperature rise rate of 1ºC/second.

The pyroelectric effect is caused by a phenomenon in which the domain of the crystal changes due to a change in the temperature of the LiTaO3 crystal, and thereby a charge is generated on the surface of the substrate, causing an electric field. Since the charge due to the pyroelectric effect is proportional to the value of the temperature change, a large electric field can be generated when the temperature increases rapidly.

As a result, a short-period domain-inverted structure 4 could be formed on the LiTaO3 surface by the RTA heat treatment. Since the domain-inverted structure was deeper, its overlap with the waveguide mode could be increased, thereby fabricating a wavelength conversion element with higher conversion efficiency.

The heat treatment in which the domain was inverted was carried out with a Ta mask 2 formed on the outer surface of the substrate. Since the Ta mask 2 has a significantly higher infrared absorption coefficient compared to the LiTaO3 substrate, the temperature rise rate on the surface of the sample could be increased several times. As a result, a domain-inverted structure 4 with an even shorter period and deeper domain inversion depth could be manufactured. Therefore, an efficient wavelength conversion element capable of converting short wavelengths could be manufactured.

Although the growth direction in the domain-inverted structure 4 in this example is in the transfer direction Y, a comparable element can be manufactured if it is placed in the transfer direction X.

Although the substrate in this example was also made of LiTaO₃, another LiTaO₃ plate doped with MgO, Nb, Nd or the like can be used to produce a comparable element.

Although the infrared heater was used as the heater in this example, other heaters which exhibit a high temperature rise rate, such as flash lamp heaters, CO2 laser heaters or the like, can also be used to produce a similar element.

Although pyrophosphoric acid was used for ion exchange in this example, another acid may also be used, such as orthophosphoric acid, benzoic acid, sulfuric acid or the like.

Although the Ta film was used as the mask for preventing ionization in this example, another film may be used if it is acid-resistant, such as Ta₂O₅, Pt, Au, and the like.

Although the proton exchange waveguide was used as the waveguide in this example, other waveguides can also be applied, such as Ti diffusion waveguide, Nb diffusion waveguide, ion injection waveguide and the like.

Although Ta would be used to make the mask, other materials can also be used, such as Au, Pt, Ti and Ag.

Example 3

In this example, a manufacturing method for a low-loss waveguide is provided after forming the domain-inverted structure. In order to manufacture an efficient wavelength conversion element, it is necessary to form a low-loss waveguide on the domain-inverted structure 4. The method for manufacturing the domain-inverted structure 4 described in Example 1 is designed so that the proton exchange layer 3 is formed by the heat treatment to form the domain-inverted structure 4. However, when the periodic domain-inverted structure 4 is formed, a difference in refractive index is generated between the domain-inverted structure 4 and the non-inverted structure 4 due to a change in refractive index between the proton exchange layer 3 and the substrate. Therefore, there is loss when guiding waves in the waveguide formed in the domain-inverted structure 4. Consequently, a method for fabricating a waveguide with low losses on the domain-inverted structure 4 was investigated.

In Fig. 17a-17h, reference numeral 1 shows a LiTaO₃ substrate, 2 a Ta mask made of ionization-resistant mask material, 3 a proton exchange layer, 4 a domain-inverted structure, 5 a Ta mask, 6 an inversion optical waveguide and 13 a Ta- Mask made of ionization resistant mask material. In the process of this example, the LiTaO₃ crystal is heat treated at a temperature at which the domain is not inverted for the purpose of annealing to diffuse inversion treatment ions into the LiTaO₃ crystal.

The domain-inverted structure of Example 1 was prepared (see Fig. 17a-17d), and then the refractive index of the prepared domain-inverted structure 4 was measured by a prism coupling method, whereby the difference in refractive index from the substrate 1 was about 0.02. As shown in Fig. 17e, the thus prepared substrate was annealed at 450°C for 4 hours. As a result, the difference in refractive index between the domain-inverted structure 4 and the substrate 1 was 0.005 or less. Further, when annealed at 550°C or more, the domain was reinverted after its inversion, so that the domain-inverted structure 4 was destroyed.

In Fig. 17f, the Ta mask 2 is formed on the LiTaO3 substrate 1 by sputtering. After applying the resist to the surface of the Ta mask 2, waveguide stripes having a width of 6 µm are formed on the substrate in the transmission direction X of the substrate by a photolithographic process. Then, dry etching is carried out in a CF4 atmosphere to prepare the Ta mask 13. In Fig. 17g, the substrate is heat-treated in pyrophosphoric acid at 230°C for 20 minutes to subject the unmasked areas of the LiTaO3 substrate to the proton exchange treatment. As a result, the proton exchange waveguide 5 is formed. Then, the Ta mask 13 is removed in the state shown in Fig. 17h before the both end surfaces of the LiTaO3 substrate are optically polished so that the wavelength conversion element is manufactured.

The characteristics of the manufactured wavelength conversion elements were compared. Three types of samples were prepared, that is, a sample 1 which was heat-treated at 450°C for one hour after domain conversion, a sample 2 which was not heat-treated after domain conversion, and a sample 3 which was heat-treated at 550°C for one hour. Then, semiconductor laser beams having a wavelength of 800 µm and an output power of 40 mW were condensed by a condensing optical system. The emitted fundamental waves and SHG light were condensed by a lens to be measured by a power meter. The results are shown in the following table: Table 1

As is clear from the results of Samples 1 and 2 shown in Table 1, Sample 1 showed a reduction in the difference in the refractive index of the domain-inverted structure 4 by the annealing performed after the domain inversion. Therefore, the loss in the waveguide of the wavelength conversion element was reduced from 2 dB/cm (Sample 2) to 0.4 dB/cm. As a result, the waveguide loss of Sample 1 was reduced and thus the output power of the fundamental wave was increased.

In addition, the output power of SHG light was increased because the wavelength loss was reduced. As a result, a wavelength conversion element with very good conversion efficiency. Although the fundamental waves were guided in sample 3, no SHG light was generated. The reason for this is probably that the domain-inverted structure 4 was reinverted due to the heat treatment at 550ºC, so that it was destroyed.

As described above, in this example, proton exchange was partially carried out so that the periodic domain inverted layer 4 was formed. Then, annealing was carried out at 550°C or below. Therefore, a low-loss waveguide can be manufactured by the method of this example because the waveguide loss can be reduced. As a result, a waveguide conversion element with very good conversion efficiency can be manufactured.

Although this example uses pyrophosphoric acid to perform ion exchange, other acids can also be used, such as phosphoric acid, stearic acid, benzoic acid, nitric acid, hydrochloric acid, and sulfuric acid.

Although the Ta film was used as the ionization resistant mask in this example, other acid resistant films such as Ta₂O₅ and Pt and the like may also be used.

Although the substrate in this example is LiTaO3, another LiTaO3 plate with MgO doping may also be used.

The results of the examples described above are as follows:

According to Example 1, a periodic proton exchange layer was formed so that the domain inverted structure was formed by heat-treating the proton exchange layer. In this example, a wavelength conversion element with a third-order period was fabricated, but not one with a first-order period.

According to Example 2, the periodic proton exchange layer was formed on the LiTaO3 substrate and subjected to a heat treatment in which the temperature was increased at a high rate, so that a wavelength conversion element with a first-order domain-inverted structure could be manufactured.

According to Example 3, the domain-inverted structure was heat treated so that the difference in refractive index compared to the substrate was reduced.

Then, the characteristics of a wavelength conversion element manufactured by the method of the above example were evaluated.

Properties of the wavelength converter element

The characteristics of the wavelength conversion element are described below. On the domain inverted structure 4 of Example 1, the waveguide 5 of Example 3 was formed so that the wavelength conversion element with third order period was manufactured. The wavelength conversion element was manufactured by forming the proton exchange waveguide with a width of 5 µm, a depth of 2.4 µm and a length of 9 µm on the domain inverted layer 4 with a period of 10.5 µm, a width of 6.0 µm and a depth of 2.7 µm. Then, the SHG characteristics of the manufactured wavelength conversion element were measured. By using a Ti:Al₂O₃ sapphire variable wavelength laser to scan the wavelength of the incident light in a range between 840 and 880 nm, a phase-matched wavelength could be obtained.

Fig. 18 illustrates the SHG characteristics of a conventional wavelength conversion element made using LiNbO3 and the wavelength conversion element with third-order domain inversion period made using LiTaO3. The wavelength conversion element made using LiNbO3 had a problem of difficulty in achieving large output powers because the output power saturated at 10 µW due to optical damage. On the other hand, the element made using LiTaO3 did not experience any deterioration in output power due to optical damage. Furthermore, the wavelength conversion element made using LiTaO3 showed an excellent wavelength conversion of 27%/W compared with a conversion efficiency of 0.43%/W with the conventional wavelength conversion element made using LiNbO3.

In order to manufacture a wavelength conversion element with even better efficiency, a first-order period wavelength conversion element was manufactured by manufacturing the waveguide 5 of Example 3 on the domain inverted structure manufactured in Example 2. The manufactured first-order wavelength conversion element and the third-order wavelength conversion element were subjected to an extensive output characteristic test. Fig. 19 illustrates the relationship between the incident light intensity and the SHG output of the wavelength conversion element with the third-order domain inversion period and that with the first-order domain inversion period. Even with the element with the third-order domain inversion period, an output of 6 mW was obtained at an input power of 150 mW. Therefore, a wavelength conversion element with an excellent conversion efficiency of 27%/W and a sufficiently large output power could be obtained.

In addition, a maximum output power of 15 mW and a conversion efficiency of 71%/W could be achieved in the wavelength conversion element with the first-order inversion period fabricated by RTA at an input power of 145 mW, and the values thus obtained were an excellent efficiency of 2.5 times that of the wavelength conversion element with the third-order domain inversion period. The reason for this is that the domain inverted structure 4 with the first-order period could be formed by RTA and the depth of the domain inverted structure was increased. Therefore, a deep domain inverted layer could be formed despite the short period. In addition, high-efficiency conversion could be achieved by the waveguide 5 with low loss.

Example 4

Finally, a manufacturing method for a module using the wavelength converter element according to the invention is described.

Fig. 20 illustrates a short wavelength laser light source constructed by combining a semiconductor laser having a wavelength in the 0.8 µm band and the wavelength converting element of Example 1. Laser beams emitted from a semiconductor laser 31 are converged by converging optical systems 34 and 35 to make them incident on a waveguide end surface 5 of the wavelength converting element. By integrally forming to form a module, a very small wavelength light source can be manufactured. The light source thus manufactured exhibits a very large output of 2 mW and can be used as a light source for an optical disk or the like because of its small size. As a result of the short wavelength of the light source, the storage capacity of an optical disk can be greatly increased, so that a very small device can be manufactured.

As described above, the periodic proton exchange layer 3 is formed in the LiTaO₃ crystal and heat-treated to form the domain-inverted structure 4 in such a manner that the temperature rise rate is increased. As a result, the undesirable diffusion of the proton exchange layer 3 during reaching the domain inversion process temperature can be avoided. Therefore, the transverse expansion of the domain inversion in the proton exchange layer 3 can be avoided. As a result, since a very short periodic domain inverted structure 4 can be formed and short wavelength conversion can be carried out with high efficiency, a wavelength conversion element with large output can be manufactured using a LiTaO₃ crystal having excellent optical damage resistance. Therefore, there is excellent practical utility.

When the periodic proton exchange layer 3 is formed in the LiTaO₃ crystal, the depth of the proton exchange is limited to avoid undesirable interconnection of the adjacent proton exchange layers 3. Furthermore, the minimum thickness of the proton exchange with which the domain inverted structure 4 can be manufactured is limited. Although the thickness of the domain inverted structure 4 depends on the thickness of the proton exchange layer 3, the domain inverted structure can be formed even in the state that the proton exchange layer is 0.2 µm or more. As a result, by limiting the thickness of the proton exchange layer, using the LiTaO₃ crystal having a large nonlinear optical constant and excellent optical damage resistance, the domain inverted structure and the wavelength conversion element can be manufactured. Therefore, a great practical benefit can be achieved in that an efficient and stable wavelength conversion element with a large output power can be manufactured.

Further, the proton exchange treatment is partially applied to the LiTaO₃ crystal before it is heated to a temperature below the Curie point but close to the Curie point, so that the periodic domain inverted structure 4 can be formed. By additionally carrying out the heat treatment at a low temperature below 500°C to reduce the proton exchange density of the domain inverted structure 4 while maintaining the domain inverted structure, the refractive index of the domain inverted structure 4 can be brought to a value close to the refractive index substrate. As a result, a technology that was not possible conventionally can be realized in which the periodic domain inversion can be formed in the LiTaO₃ crystal. Furthermore, by reducing the difference in refractive index between the domain-inverted structure 4 and the substrate, a wavelength conversion element with low loss and excellent conversion efficiency can be manufactured. Consequently, significant practical benefits are possible.

Although the invention has been particularly described to some extent in its preferred embodiment, it is to be understood that the present description of the preferred embodiment may be varied in details of construction and combination and arrangement of parts without departing from the scope of the invention as hereinafter claimed.

Claims (4)

1. A method for manufacturing a wavelength conversion device comprising the steps: forming a stripe-like mask on a surface of a LiTaO3 substrate obtained by cutting a plane perpendicular to the C-axis of the LiTaO3 crystal; Forming a proton exchange layer in unmasked areas by subjecting the LiTaO₃ substrate to proton exchange of Li and H⁺ ions in the LiTaO₃ substrate; and Forming a domain-inverted structure by subjecting the LiTaO₃ substrate to a heat treatment to a temperature below the Curie point of the LiTaO₃ substrate at which the LiTaO₃ substrate is heat treated, thereby forming a first order periodic domain-inverted structure, characterized by using a heat treatment device for increasing the temperature of the LiTaO₃ substrate at a temperature rise rate of at least 15ºC/sec. 2. The method according to claim 1, wherein the domain exchange layer has a thickness (D) in a range given by the following formula: 0.2um < D < (λ - W)/2 ωo where A is a predetermined period of stripes on the mask and W is a predetermined width of the stripes. 3. The method according to claim 2, wherein a difference in refractive index between the domain-inverted structure and the LiTaO₃ substrate is reduced by heat-treating the LiTaO₃ substrate at 550°C or below. 4. A method of manufacturing a wavelength conversion device according to claim 1 or 2, wherein the step of manufacturing the domain inverted structure by using the heater includes a process of covering the surface of the LiTaO3 substrate with a main metal film and the heater is an infrared heater.