JP3050333B2 - Method for manufacturing second harmonic generation element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical disk device,
The second harmonic of a waveguide type, which converts a semiconductor laser light having a wavelength of about 800 nm into a blue light having a wavelength of about 400 nm, particularly for shortening the wavelength of a light source of a printer or other optical equipment. Generating element (SHG, Second Harmonic Gener
ator), a method of manufacturing the same, and components and devices for optical information processing equipment such as a bulk type optical head, an integrated optical head, a disk device, and a laser beam printer using the second harmonic generation element.

[0002]

2. Description of the Related Art Although it has been conventionally expected that the recording density of an optical recording / reproducing apparatus will be improved and the definition of a laser beam printer will be improved by shortening the wavelength of laser light, for example, the wavelength of a semiconductor laser will be increased. Conventional 800n
In order to reduce the length from m to 500 nm or less, it is necessary to change the group III-V semiconductor used in the conventional laser to the group II-VI semiconductor, which is not easy. Therefore, infrared light, for example, a semiconductor laser light having a wavelength of 800 nm (infrared light)
Has been attracting attention as a method of converting into a second harmonic having a wavelength of 400 nm using optical nonlinearity.

[0003] With such a second harmonic generation element, the second harmonic is generated.
In order to generate harmonics efficiently, it is necessary to satisfy the energy conservation law and the momentum conservation law between the fundamental wave and the second harmonic. However, since the refractive index of an optical material generally varies depending on the wavelength, there arises a problem that the law of conservation of momentum does not hold between wavelengths that satisfy the law of conservation of energy. Therefore, it is necessary to take phase matching between the fundamental wave and the second harmonic. was there. The phase matching means that innumerable second harmonic components generated in the second harmonic generation element are multiplexed in phase with each other in the process of propagating in the optical waveguide. Due to the above-mentioned phase matching, the generated second harmonic components are combined and output in directions that reinforce each other.

Several methods have been proposed as the phase matching method. For example, as shown in FIG. 4 of Japanese Patent Application Laid-Open No. 61-18964, a LiNbO ₃ single crystal substrate 41 is
An optical waveguide 42 is formed by a proton exchange method (a method of partially replacing protons with Li ions of LiNbO ₃ ), and a fundamental wave 33 polarized in a direction 35 perpendicular to the substrate surface is incident from one end face thereof, and is subjected to Cherenkov radiation. A method of extracting the second harmonic wave 44 polarized in a direction 35 perpendicular to the generated substrate surface has been proposed. In this method, the second harmonic is a radiation mode radiating out of the waveguide, and the phase matching is satisfied.

A method called angular phase matching has been reported in Proceedings of the Institute of Electronics, Information and Communication Engineers Autumn National Convention 1989, C-249. In the above angle phase matching method, as shown in FIG. 3, lithium tantalate (LiTaO ₃ )
An optical waveguide 32 formed by liquid-phase growth of magnesium-doped lithium niobate (MgO: LiNbO ₃ ) is provided on a substrate 31, and one end surface of the optical waveguide 32 is polarized in the z direction perpendicular to the substrate surface (TE polarized light). ) Is incident, and polarized light (T) from the other end surface in the x direction parallel to the substrate surface.
The second harmonic wave 34 (M-polarized light) is emitted. During the propagation of the fundamental wave 33 in the optical waveguide 32, the fundamental wave 33 is converted into a second harmonic component 34 due to the nonlinearity of the refractive index. At this time, the fundamental wave 33 and the second harmonic 3
If the propagation speeds of the four components are equal, the second harmonic component is always phase-matched and output, so that the maximum second harmonic output can be obtained.

[0006] However, since the refractive index generally changes in proportion to the frequency of light, the above-mentioned phase matching condition is not satisfied. For example, if the polarization directions of the fundamental wave 33 and the second harmonic wave 34 are both in the z direction, the above-mentioned phase matching condition cannot be satisfied, so that the polarization direction of the second harmonic wave 34 is set in the x direction as shown in FIG. A crystal having a refractive index satisfying the phase matching condition in the x direction is used. That is,
Phase matching is performed using the anisotropy of the crystal.

[0007] Also, Electronics, Letters (Elec)
5, pp. 731-732, as shown in FIG. 5, a ferroelectric substance having spontaneous polarization, for example, L
A polarization inversion layer 53 in which the spontaneous polarization direction is inverted at an equal pitch and an optical waveguide 52 formed by a proton exchange method are provided on an iNbO ₃ substrate 41, and a fundamental wave polarized in the z direction with the substrate surface from one end of the optical waveguide 52. A method has been proposed in which a second harmonic 56 is similarly incident in the z-direction from the other end. Here, the second harmonic component generated in the optical waveguide 52 due to the reversal of the spontaneous polarization is given strength and the length of the reversal pitch of the spontaneous polarization is adjusted to phase match the strong second harmonic components. And take it out.

[0008]

In the method using Cherenkov radiation shown in FIG. 4, the second harmonic wave 44 has a crescent shape, so that the wavefront aberration is large, and the second harmonic wave 44 is narrowed down to a minute light spot used in an optical disk device or the like. It was almost impossible. Further, the conventional method shown in FIG.
Since a ferroelectric material having a large nonlinear optical coefficient, such as iNbO ₃ , is used, when the wavelength of the second harmonic 34 is 500 nm or less, phase matching cannot be sufficiently performed due to the wavelength dispersion of the refractive index, and blue light cannot be matched. There was a problem that can not be obtained.

Further, since the polarization directions of the fundamental wave 33 and the second harmonic wave 34 are orthogonal to each other, the temperature coefficients of the refractive indices in the respective polarization directions are greatly different. The temperature width is narrowed to about 0.1 ° C., and the thickness accuracy of the optical waveguide 32 is 0.01 μm.
There was also a problem that the following unrealistic values were required. On the other hand, in the second harmonic generation device using the domain-inverted grating shown in FIG. 5, the second harmonic is confined in the optical waveguide, so that the emitted light can be narrowed down easily. Since the harmonics 56 are polarized in the same direction, the temperature coefficients of the respective refractive indices become substantially equal, so that the allowable temperature range of 0.2 ° C. in FIG. 3 can be improved to about 3 ° C., but it is not practically sufficient. However, there has been a problem that the temperature degradation of the conversion efficiency is excessive. An object of the present invention is to increase the allowable temperature width of the second harmonic generation element in which the polarization direction of the fundamental wave and the second harmonic is the same, and at the same time, increase the conversion efficiency. Another object of the present invention is to provide a method of manufacturing the second harmonic generation element, and further provide a light source for generating visible light, an optical head, an optical information recording / reproducing apparatus, and the like using the second harmonic generation element.

[0010]

In order to solve the above-mentioned problems, the ratio of the refractive index temperature coefficients of the optical substrate and the optical waveguide in the direction perpendicular to the substrate surface is set to fall within a range of 0.9 to 1.1. To Further, in the optical waveguide, the nonlinear optical coefficient is alternately inverted in the form of an equally-spaced lattice, the coupling coefficient representing the conversion performance from the fundamental wave to the second harmonic is K, and the input power of the fundamental wave is P when set to _0, the value of K√P ₀ is made to be in 10 m ~ ¹ or more. Further, the optical substrate is made of lithium niobate doped with magnesium, and the optical waveguide is made of lithium niobate or lithium niobate doped with less magnesium than the optical substrate.

A first step of inverting the spontaneous polarization direction of the surface of the optical substrate at equal intervals in the second harmonic generation element, and using a raw material powder of a ferroelectric metal oxide constituting the optical waveguide layer as a flux. A second step of mixing and immersing in a melt obtained by heating and melting under an atmosphere of oxygen and water vapor to liquid phase epitaxially grow a metal oxide film on the optical substrate, and polarization of a spontaneously polarized portion provided on the optical substrate surface It is manufactured by a third step of transferring the direction to the metal oxide film. Further, the above flux is formed by lithium borate (Li ₂ B ₂
O ₄ ), or lithium fluoride (LiF), or potassium fluoride (KF).

[0012]

According to the present invention, the temperature coefficient ratio of the refractive index of the optical substrate and the optical waveguide in the direction normal to the substrate is set to 0.9 to 1.1, a value close to 1, so that the fundamental wave can be reduced. By setting the coupling coefficient representing the conversion performance to the second harmonic as K and the input power of the fundamental wave as P ₀ and setting the value of K√P ₀ to a large value of 10 m to ¹ or more, the second harmonic generation element is allowed. The temperature range is expanded, and the conversion efficiency is improved. More specifically, the upper optical substrate is made of MgO: LiNbO ₃ , the optical waveguide is made of LiNbO _3, or an MgO: LiNbO ₃ material having a smaller doping amount of magnesium than that of the optical substrate is used. The ratio is obtained. In addition, a liquid crystal epitaxial growth of the metal oxide film of the optical waveguide produces a domain-inverted grating having a rectangular cross section in the optical waveguide with high precision, thereby increasing the value of the coupling coefficient K and simultaneously increasing the temperature tolerance. Spreads.

[0013]

【Example】

[Embodiment 1] FIG. 1 is a perspective view of an embodiment of a second harmonic generation device according to the present invention. In FIG. 1, the fundamental wave 5 incident on the ridge-type optical waveguide 4 and the second harmonic 6 emitted from the optical waveguide 4 are applied to the surface of the substrate 1 as in the case of the conventional device shown in FIG. Both are polarized in the vertical direction (C-axis direction). Therefore, the present invention can be applied to a structure having a flat surface as shown in FIG. However, in the following, the ridge type of FIG. 1 will be described for clarity. 5mo whose surface is + c plane
1% MgO-doped ZcutLiNbO ₃ single crystal substrate 1
A 1 mol% MgO-doped LiNbO ₃ single crystal thin film 2 having a spontaneous polarization upward is usually provided thereon. An optical waveguide 4 of the same material as the thin film 2 is provided on the thin film 2.
Inside, a domain-inverted grating 3 whose polarization direction is opposite (downward) to the thin film 2 is provided.

FIG. 2 is a sectional view of the optical waveguide 4 in FIG. In the conventional device shown in FIG.
Has a triangular waveform layered cross section, but in the present invention, in order to increase the conversion efficiency from the fundamental wave to the second harmonic, the domain-inverted portion has a rectangular cross section lattice as shown in FIG. ing. Therefore, 3 is named a domain-inverted lattice. Journa
l of Applied Physics, Vol. 40, No. 2, 720-
On page 734, M. Didomenico Jr. et al.
In a ferroelectric crystal belonging to the space group R3c such as O ₃ or LiTaO _3, it is reported that when the direction of spontaneous polarization is reversed, the sign of the nonlinear optical coefficient is also reversed. Therefore, in the optical waveguide 4, the nonlinear optical coefficient is periodically inverted due to the presence of the domain-inverted grating 3, so that a second harmonic is generated in the domain-inverted grating 3 and its polarization direction is the same as that of the fundamental wave. It becomes. In addition, by optimizing the pitch の of the domain-inverted grating 3, the second harmonic component generated in each domain-inverted grating portion can be multiplexed in phase.

However, since the optimum pitch 間 の between the domain-inverted gratings 3 changes due to the temperature change of the refractive index of each part in the optical waveguide 4, there arises a problem that the phase matching condition is broken and the conversion efficiency η is reduced. An object of the present invention is to reduce the aforementioned temperature deterioration. Hereinafter, the conversion efficiency η of the domain-inverted grating and the phase matching conditions, particularly the temperature dependence thereof, will be theoretically considered to clarify the conditions for improving the temperature characteristics.

Equations (1) and (2) are respectively shown in FIG.
1 is a general representation of a z-direction electric field component of a fundamental wave 5 and a second harmonic 6 that are plane waves (wavelength λ, angular frequency ω = 2πc / λ) polarized in the z direction in the optical waveguide 4 of FIG.

(Equation 2)

(Equation 3) Here, N (λ), N (λ / 2), etc. are the effective refractive indices for the fundamental wave 5 and the second harmonic 6 in the optical waveguide 4, respectively, c. c is the complex conjugate term (Comp
lex Conjugate). A (y) in equation (1) and equation (2)
Represents the change in the amplitude of the fundamental wave 5 and the second harmonic 6 in the y direction (traveling direction), and is represented by the coupling between the fundamental wave and the second harmonic electric field due to the optical nonlinearity of the optical waveguide 4. Stipulated.

Equations (3-1) and (3-2) are obtained from the above A
(Y) and the rate of change of C (y) in the y direction, derived from the Maxwell equation.

(Equation 4) Here, the content of the exponential term, 4π / λ (N (λ / 2) −N
(Λ)) represents the amount of phase mismatch Δβ. The integral term in Equation 4 is called an overlap integral, and is an important term that governs the second harmonic generation efficiency of the element.

Further, d (x, y, z) is a nonlinear optical coefficient, and as in the present invention, the sign of d (x, y, z) is inverted at a pitch に in the y direction. It can be expanded to a Fourier series as in equation (4).

(Equation 5) Further, when the Fourier order M = −1, the period Λ is calculated by the equation (5).
By selecting as follows, phase matching is realized.

(Equation 6)

Even if the period Λ is determined as described above, the matching condition of the equation (5) is broken due to a change in the refractive index N (λ / 2), N (λ) or the like, and the above Δβ is not zero. Conversion efficiency η from fundamental wave to second harmonic when Δβ is not zero
Is derived from Equation (3) using Jacobi's elliptic function as Equation (6).

(Equation 7) L is the element length, P ₀ is the power of the incident fundamental wave, and K is a coupling coefficient representing the conversion performance from the fundamental wave to the second harmonic, and is expressed as in equation (7).

(Equation 8)

The elliptic function of equation (6) can be expanded to a power series as in equations (8-1) and (8-2) when K√P ₀ Lν is small.

(Equation 9) From equation (8-2), it can be seen that, when Δβ = 0 where phase matching is achieved, the efficiency η is approximately proportional to the square of the coupling coefficient k and is proportional to the incident power P ₀ . Also, when the phase matching is incomplete, the second term and below of the equation (8-2) become large, the efficiency η decreases, and at the same time, the influence of temperature fluctuation of Δβ increases. Therefore, in order to reduce the temperature fluctuation of the conversion efficiency η, first, the large conversion efficiency η
It is important to adopt a structure that gives Therefore, in the present invention, the domain-inverted grating 3 having a rectangular cross-sectional shape shown in FIG. 1 is employed in place of the conventional domain-inverted layer 53 shown in FIG. In this manner, a double effect of increasing the conversion efficiency η and reducing the temperature fluctuation can be obtained.

The effect of the present invention is obtained by the above-mentioned formulas (1) to (1).
As shown in (8), this was found from the fact that the characteristics of the second harmonic generation element having the domain-inverted grating having the nonlinear optical coefficient were analyzed theoretically and considered in detail. Hereinafter, the temperature characteristics of the conversion efficiency η will be analyzed in more detail to clarify the conditions under which the temperature fluctuation range can be suppressed within a practical range. The following analysis is an example, and as an example, ZcutLiNb doped with 5 mol% of magnesium is used as the substrate 1.
Using O ₃ , magnesium is 1 mol% in the optical waveguide 4.
Using a doped LiNbO ₃ thin film, the thickness of which is 2 μm
m and the width were 3 μm. The wavelength λ of the fundamental wave 5 is 830n
m and thus the wavelength of the second harmonic is 415 nm.
The order M of the phase matching is 1. Input optical power P ₀
= 40 mW, K√P _{0 in} equations (6), (8), etc.
The value of the 88m ~ ^1.

FIG. 6 shows the temperature characteristics of the conversion efficiency η calculated from the equation (6). The parameter r is the ratio of the temperature coefficient of the refractive index in the direction (z direction) perpendicular to the substrate surface of the substrate to the temperature coefficient of the refractive index in the direction perpendicular to the substrate surface of the optical waveguide layer, as shown in equation (9). is there. It can be seen that as r is closer to 1, the conversion efficiency η is less affected by temperature, and that the effect of temperature increases rapidly as r decreases. FIG. 7 shows that the conversion efficiency η is 8
The relationship between the allowable temperature range ΔT that can be allowed when it is kept at 0% and the temperature coefficient ratio r of the refractive index (formula 9) is obtained.
From this, it can be seen that, for example, in order to obtain a temperature allowable range of 10 ° C. or more, it is necessary to set r within a range of approximately 0.9 to 1.1.

In the conventional anisotropic mode conversion shown in FIG. 3, the allowable temperature range is usually about ± 0.1 ° C.
On the other hand, in the present invention, as described above, r is 0.9 to 1.1.
As a result, it is easy to increase the allowable temperature range to ± 10 ° C., which is about 100 times or more. The temperature tolerance of ± 10 ° C. is a value that can be satisfied relatively easily when the second harmonic element is operated in an air-conditioned room. It is economically controllable and practically acceptable. FIG. 8 shows K for r = 1.
The relationship between √P ₀ and the allowable temperature range is shown. It can be seen from FIG. 8 that the larger the K√P ₀ , the larger the allowable temperature range.
If the value of P ₀ and 10M ¹ or more, can significantly expand the allowable temperature range.

As described above, the reason why the allowable temperature range is greatly expanded is as follows. (1) First of all, the ratio of the temperature coefficient of the refractive index is very close to 1. For example, in the conventional angle-matching element shown in FIG. 3, the temperature coefficient ratio of the refractive index between the substrate and the optical waveguide layer is 1
Thus, the allowable temperature range was extremely narrow. (2) Second, the value of K√P ₀ is extremely large. As is apparent from Equation 8, for the same Δβ,
The larger the value of K√P ₀ , the less the effect of Δβ on efficiency reduction is. In the conventional device, for example, in FIG. 5, K√P ₀
Is about 1 to 5 m to ¹ , so the allowable temperature range was not very wide. Since the cross-sectional shape using polarization inversion layer 3 rectangle in the present invention, on the other hand can increase the value of the K, since further loss is reduced thereby to improve the conversion efficiency, the value of the fundamental wave input P ₀ Can be increased.

FIG. 9A and FIG. 9B are manufacturing process diagrams of the second harmonic generation element using the liquid layer epitaxial growth method. First, a domain-inverted lattice of a substrate was manufactured. FIG.
A Ti film sputtered to a thickness of 5 nm on the substrate 1 to a thickness of -1 (a) by photolithography and etching at a pitch of 0.1 μm from 2.5 to 3.5 μm.
Eleven types of patterns differing by m were produced, and a pattern having an optimum pitch determined later was selected from among them. Next, as shown in FIG.
2 at 1040 ° C. for 30 minutes to form a domain inversion area 3 of about 1 μm. Note that oxygen gas and argon gas passed through pure water at 80 ° C. were used as the atmosphere gas in the heat treatment furnace 92 to prevent lithium oxide from externally diffusing.

Next, as shown in FIG. 9C, ZcutL doped with 5 mol% MgO and optically polished on the + c plane.
1 mol% MgO doped L on iNbO ₃ single crystal substrate 1
The iNbO ₃ thin film 2 was epitaxially grown to a thickness of 2.5 μm. Lithium carbonate Li ₂
When a mixed powder of CO ₃ , boric acid H ₃ BO ₃ , niobium pentoxide Nb ₂ O ₅ , magnesium oxide MgO, etc. was heated at about 1200 ° C. for 3 hours in an atmosphere of oxygen and water vapor to be uniformly melted, 1 mol% MgO-doped LiNbO _{3 of} thin film 2
Is 20 mol%, lithium borate Li as a flux material
Use _{those 2} B ₂ O ₄ was adjusted each component having the 80 mol%.

The thin film 2 is cooled to 800 ° C. at a cooling rate of 60 ° C./h, and then the substrate 1 whose + c surface is optically polished is immersed. It is produced by gradually cooling to room temperature at a rate of ° C./h. EP
When the content of Mg was examined by MA, almost 1 mo
1%. The amount of the flux material added was 7
A range of 0 to 90 mol% is desirable. The immersion time is 10 to 30 minutes for a film thickness of 0.5 to 3 μm. In addition to the lithium borate, lithium flux Li
F, potassium fluoride KF, vanadium pentoxide V ₂ O ₅ or the like can also be used.

Next, as shown in FIG.
Is annealed in an oxygen atmosphere containing water vapor to compensate for oxygen deficiency, and then a 3 μm-wide photoresist mask 93 covering the optical waveguide is provided on the thin film 2 as shown in FIG. (F) As shown in FIG.
The thin film is etched by 2 μm from above 3 by ion milling, and thereafter the photoresist is removed to form the optical waveguide 4. The ion milling device generates ions in a plasma chamber in which a plurality of permanent magnets are arranged on the outer periphery of a conical hollow vacuum vessel, and is extracted by an acceleration electrode, a deceleration electrode, a ground electrode, and the like. Due to the structure, ions having a uniform spatial density distribution can be taken out with high directivity, thereby improving the etching accuracy.

A 830 nm wavelength Ti-S laser beam polarized in a direction perpendicular to the substrate surface is incident on the optical waveguide 4 of the substrate 1, and the effective refractive index N (λ) of the TM mode having an electric field excited in the same direction. Was 2.1686. When the same measurement was performed by irradiating a dye laser having a wavelength of 415 nm, two modes were excited, and the effective refractive index N (λ / 2) of the low-order mode was 2.3016.
Further, the light propagation loss with respect to the light of 830 nm was measured by a cutback method and found to be 1 dB / cm. The first reason why the light propagation loss is low is that the thin film 2 is produced by the liquid phase epitaxial growth with high quality close to the stoichiometric composition, and the second reason is that the ion milling is performed. Is performed with high directivity, and as a result, the side wall of the optical waveguide 4 can be processed with extremely high precision.

Using the refractive indexes of the fundamental wave and the second harmonic,
When M = 1, the polarization pitch よ り is obtained from Equation (4), and is about 3.1 μm. Therefore, the pitch Λ is 3.1 μm
Was cut out to an optical waveguide length of 10 mm, a Ti-S laser beam (fundamental wave) was incident, and the generation efficiency of the second harmonic was measured. The sample was mounted on a copper block so that its temperature could be controlled by a Peltier device. When the temperature of the copper block is set to 25 ° C. and the fundamental wavelength is set so as to obtain the maximum second harmonic generation efficiency, the fundamental wave input is 40 m
A second harmonic output of 2 mW with respect to W was obtained, and the efficiency was 6.8% including Fresnel reflection loss. Since the efficiency calculated from the equation (6) is 40%, the above-mentioned 6.
8% is about 1/6 of the theoretical value. The cause of this discrepancy is:
It is considered that the light propagation loss is neglected in the equation (6). If the light propagation loss is 0.5 dB / cm, the efficiency becomes 15%.

Considering that the efficiency increases in proportion to the fundamental wave input based on the above numerical values, for example, an output of 200 mW
Is coupled to the optical waveguide 4 at a coupling efficiency of 50%, the conversion efficiency η is 17%, that is, 17 m.
As a result, a second-harmonic output of W can be obtained, and a short-wavelength light source large enough for writing and reproduction of a magneto-optical disk or a phase-change optical disk can be obtained. FIG.
Is a measurement result of the conversion efficiency η when the substrate temperature is changed by the Peltier element. From this, the conversion efficiency η
Is reduced to about 80% by about ± 1 around 25 ° C.
It was 0 ° C. This value is smaller than the temperature width at r = 1 in FIG. 6, but is considerably larger than the value for r = 0.9. The reason for this is that, in the present invention, both the substrate 1 and the optical waveguide 4 are made of MgO: LiNbO ₃ , and the temperature coefficients of the refractive indices perpendicular to the respective substrate surfaces are substantially equal.
This is probably because the value of √P ₀ was larger than that of the conventional device, and the temperature change of the phase matching condition shown in Expression (4) was reduced.

As a result, the permissible temperature range could be extended about 100 times as compared with the conventional second harmonic generation element. The second harmonic generation element of the present invention shown in FIG. 11 is mounted as shown in FIG. 12 and used as a small visible light source. The laser light of the high power semiconductor laser 111 having an output of about 100 mW and a wavelength of 830 nm is condensed on the end face of the optical waveguide portion of the second harmonic generation element 113 by the lens system 112, and the second harmonic having the wavelength of 415 nm is emitted from the emission surface 115. The light is collimated by the collimating lens system 116. The incident surface 114 is coated with an anti-reflection film, and the output surface 115 is coated with a coating for cutting a fundamental wave having a wavelength of 830 nm.

[Embodiment 3] FIG. 12 is a block diagram of an example of a write-once optical disc head on which the visible light source of FIG. 11 is mounted. The light emitted from the small visible light source 121 passes through the polarizing beam splitter 122 and is transmitted to the λ / 4 wavelength plate 123.
Is converted into circularly polarized light, and is converged on the optical disk 125 by the objective lens 124. The reflected light from the optical disk 125 is reflected by the polarization beam splitter 122, collected by a condenser lens 126, and split into two by a half mirror 127. One of the two divided lights is guided to a two-part photosensor 128 and converted into a tracking error signal of the optical disk. The other of the two divided lights is a four-divided photo sensor 1.
It is guided to 29 and converted into a focusing error signal and a reproduction signal.

[Embodiment 4] FIG. 13 is a structural view of an example of a magneto-optical disk head equipped with the visible light source of FIG. The light emitted from the visible light source 121 passes through the polarization beam splitter 132, is raised by the reflection prism 133, and is focused on the optical disk 135 by the objective lens 134. 136 is a magnetic coil for writing and erasing.
The reflected light from the disk surface 135 is reflected by the polarization beam splitter 132, passes through the λ / 2 wavelength plate 137, and is condensed by the condenser lens 138.
39 is divided into two. One of the two divided lights is input to the two-part photosensor 1310 and converted into a tracking error signal. The other of the two divided lights is guided onto a four-divided photosensor 1311 and converted into a focusing error signal and a magneto-optical reproduction signal. The visible light source 121 can be applied to a read-only optical disk or a phase-change optical disk by appropriately changing the head optical system.

Fifth Embodiment FIG. 14 shows an optical information recording / reproducing apparatus 14 using the optical head of the third or fourth embodiment.
4 is a schematic configuration diagram of FIG. The optical head 141 mounted on the actuator 142 converts optical information from the optical recording medium 145 into an electric signal and performs signal processing. In this optical information recording / reproducing apparatus, the blue light generated by the second harmonic conversion element of the present invention can be used, and the spot diameter on the disk can be reduced to 0.5 μm, so that the recording density is four times that of the conventional one. Can be enhanced. Further, since the temperature fluctuation of the second harmonic generation element is very small, a temperature control system which cannot be omitted in the conventional apparatus becomes unnecessary, thereby simplifying the system and making it economical. Since the head is reduced in size and weight, the access time can be reduced.

[0035]

The optical substrate is made of MgO: LiNbO ₃ , the optical waveguide is made of LiNbO _3, or the MgO: LiNbO ₃ material having a smaller doping amount of magnesium than that of the optical substrate is used, whereby the temperature coefficient ratio of the refractive index in the normal direction of the substrate is obtained. To 0.9
To 1.1 and can be set to a value close to 1, also the coupling coefficient representing the conversion performance from the fundamental wave to the second harmonic K, the value of K√P ₀ input power of the fundamental wave as P ₀ 10 m Since it can be set to a large value of ~ ¹ or more, the allowable temperature range is 100 times larger than that of the conventional mode conversion element utilizing the refractive index anisotropy of 0.1 ° C.
It is possible to provide a second harmonic generation element which has been enlarged to 10 ° C. which is twice or more. In addition, the liquid crystal epitaxial growth of the metal oxide film of the optical waveguide allows a highly accurate domain-inverted grating having a rectangular cross section to be formed in the optical waveguide, so that the conversion coefficient from the fundamental wave to the second harmonic can be increased. it can. Further, it is possible to provide a laser light source device having a wavelength of 400 nm for enabling short-wavelength recording on an optical disk by using the second harmonic generation element.

[Brief description of the drawings]

FIG. 1 is a perspective view of a second harmonic generation device according to the present invention.

FIG. 2 is a cross-sectional view of the second harmonic generation device of FIG.

FIG. 3 is a perspective view of a second harmonic generation element using a conventional mode phase matching method.

FIG. 4 is a perspective view of a second harmonic generation element using a conventional Cherenkov phase matching method.

FIG. 5 is a perspective view of a conventional second harmonic generation element using polarization inversion.

FIG. 6 is an example of a temperature characteristic of conversion efficiency of the second harmonic generation device according to the present invention.

FIG. 7 is a diagram showing the relationship between the temperature coefficient ratio of the refractive index of the second harmonic generation element and the allowable temperature range of the element according to the present invention.

FIG. 8 is a diagram showing the relationship between the coupling coefficient of the second harmonic generation element and the allowable temperature range according to the present invention.

FIG. 9A is a manufacturing process diagram of the second harmonic generation element according to the present invention.

FIG. 9-2 is a manufacturing step diagram of the second harmonic generation element according to the present invention.

FIG. 10 is a graph showing temperature characteristic measurement results of the second harmonic generation device according to the present invention.

FIG. 11 is an external view of a second harmonic generation device according to the present invention.

FIG. 12 is a configuration diagram of a visible light source using the second harmonic generation device according to the present invention.

13 is a configuration diagram of a write-once optical disk head using the visible light source of FIG.

FIG. 14 is a configuration diagram of a magneto-optical disk head on which the visible light source of FIG. 11 is mounted.

FIG. 15 is a configuration diagram of an optical information recording / reproducing apparatus.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 thin film 3 domain-inverted grating 4 optical waveguide 5 fundamental wave 6 second harmonic 111 laser diode 113 second harmonic generation element 121 visible light source 122, 132 polarization beam splitter 124, 134 objective lens 141 optical head 145 optical recording medium

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuo Hira 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Inside Hitachi, Ltd. Production Engineering Laboratory (72) Inventor Hideki Sato 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Hitachi, Ltd. Production Technology Laboratory (72) Inventor Takako Fukushima 292, Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Hitachi, Ltd. Production Technology Laboratory (56) References JP-A-4-97232 (JP, A) JP-A-4-104233 (JP, A) JP-A-3-287141 (JP, A) JP-A-2-63026 (JP, A) JP-A-2-187735 (JP, A) JP-A-3-191332 ( JP, A) International Publication 90/9094 (WO, A1) (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02F 1/35-1/39 H01S 3/108-3/109

Claims (2)

(57) [Claims] 1. A method of manufacturing a second harmonic generation element having an optical waveguide in which portions where nonlinear optical coefficients are alternately inverted on a surface of an optical substrate are provided at equal intervals adjacent to each other. A first step of inverting the polarization direction at equal intervals; and mixing the optical substrate having undergone the first step with a flux of a raw material powder of a ferroelectric metal oxide constituting an optical waveguide layer under an atmosphere of oxygen and water vapor. Dipping in a melt obtained by heating and melting in a second step of liquid phase epitaxial growth of the metal oxide film on the optical substrate, and the polarization direction of the spontaneously polarized portion provided on the surface of the optical substrate to the metal oxide film A method of manufacturing a second harmonic generation element, comprising a third step of transferring. 2. The second harmonic according to claim 1, wherein the flux is lithium borate (Li ₂ B ₂ O ₄ ), lithium fluoride (LiF), or potassium fluoride (KF). A method for manufacturing a wave generating element.