JPH0756203A - Formation of polarization inversion grating and optical waveguide - Google Patents

Abstract

PURPOSE:To generate SHG light with high efficiency by realizing the ideal polorization inversion grating of a rectangular shape. CONSTITUTION:A proton exchange region 16, the polarization inversion grating 12 extending outside this region and an optical waveguide 13 are simultaneously formed by a proton exchange treatment on an LiTaO3 substrate 11. A grating pattern mask is first formed on the front surface of this substrate and a metallic film over the entire rear surface thereof. The substrate is subjected to the proton exchange treatment at >=50 deg.C/min temp. gradient at the time of the treatment and >=50 deg.C/min temp. falling speed from the treatment temp. to form the proton exchange region 16 and the polarization inversion grating 12 extending outside this region. An aluminum nitride ceramic plate is used for a substrate holder of this time to electrically insulate the substrate 11. The refractive index of the proton exchange region 16 is thereafter increased by the heat treatment. The optical waveguide is thus easily realized by one time of photolithography and the SHG element having the high efficiency is realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a polarization inversion grating in a second harmonic generation element (hereinafter referred to as SHG element) using a LiTaO ₃ or LiNbO ₃ substrate which is a non-linear ferroelectric optical material. It is related to high efficiency of.

[0002]

2. Description of the Related Art In recent years, as a compact and lightweight blue light source, attention has been paid to converting a semiconductor laser having a wavelength of 830 nm into blue light having a wavelength of 415 nm, which is half that of a semiconductor type laser. For example, as described in JP-A-61-18934, an optical waveguide is formed on a LiNbO ₃ substrate by a proton exchange method (a method of partially replacing Li ions and protons of LiNbO ₃ to form an optical waveguide). It is proposed that a fundamental wave be incident on one end of the above-mentioned optical waveguide and SHG light be generated by Cherenkov radiation. This is shown in FIG. More recently, for example, Electronics Le
A method of performing phase matching using polarization reversal has been proposed, as discussed in pages 731-732 of tters, 25, 11 (1989). That is, as shown in FIG.
A periodic lattice was prepared by diffusion of Ti on an O ₃ substrate,
It is heated to 100 ℃ and the polarization of the periodic lattice layer is reversed.
After that, an optical waveguide is produced by the proton exchange method, and the fundamental wave is incident to take out SHG light. LiTaO
_When using _three substrates, for example Appl.Phys.Lett.58 (24)
(1991) As discussed on pages 2732-2734, a periodic lattice was prepared by the proton exchange method instead of Ti diffusion, heated to about 600 ° C. to invert the polarization only in the periodic lattice layer, and then the proton exchange method. A method of manufacturing an optical waveguide has also been attempted. This is shown in FIG.

[0003]

However, the above-mentioned prior art has the following problems. In the method using Cherenkov radiation shown in FIG. 2, the beam shape of the SHG light generated as shown by 22 is crescent-shaped, and the wavefront aberration is extremely large, and it is almost impossible to narrow it down to the diffraction limit. The newly proposed method for performing phase matching using polarization inversion shown in FIGS. 3 and 4 for the above example is SHG light that is collimated light.
Compared with Cherenkov radiation, it has the advantage that it is extremely easy to collect light. However, the cross-sectional shape of the domain-inverted lattice formed by the Ti diffusion method is triangular as shown at 31 and is semicircular as shown at 41 by the proton exchange method. SHG light cannot be generated with the original efficiency of the SHG element having the polarization inversion grating.
In addition, there are problems that the Ti diffusion region is prone to optical damage in which the refractive index changes due to strong light, and that the nonlinear optical coefficient decreases in the proton exchange region and the original SHG generation efficiency cannot be obtained. The conventionally used method of proton exchange treatment is a method using a glass container shown in FIG. In this case, the glass container 51 is attacked by the acid as the proton source and all the orientations of the substrate are proton-exchanged, so that the surface of the substrate is roughened or cracked due to the difference in the chemical orientation of the chemical damage due to the crystal orientation. There was a problem that would end up. Furthermore, the conventional method has a problem that the production of the polarization inversion grating and the production of the optical waveguide have to be performed in separate steps and photolithography must be performed twice or more. An object of the present invention is to manufacture a highly efficient SHG element in which an ideal rectangular polarization inversion grating and an optical waveguide are easily manufactured by one-time photolithography.

[0004]

The present invention is based on LiTa.
O ₃ , LiNbO ₃ or Mg-doped LiTaO ₃
And forming a periodic composition modulation region on the LiNbO ₃ substrate by a proton exchange heat treatment, and forming a periodic domain-inverted lattice almost in synchronization with the composition modulation region extending to a uniform composition region outside the composition modulation region, A method of forming a polarization inversion grating and an optical waveguide, characterized in that the periodic composition modulation region by proton exchange is used as an optical waveguide, and is a second harmonic generation element using the same. The present invention is characterized in that a periodically poled grating and an optical waveguide are formed using a patterned metal film mask for forming a periodically poled grating on the front surface of the substrate and a substrate having a metal film formed on the entire back surface. And Further, after the formation of the periodically poled lattice, heat treatment is performed to increase the refractive index of the proton exchange region on the surface of the substrate to form an optical waveguide. The present invention further provides that the heat treatment temperature is 200 ° C. or higher and the heat treatment time is 2
Characterized by being within 0 minutes. The present invention is further characterized in that in the heat treatment, a temperature change rate of 50 ° C./minute or more is included in one or both of a temperature increase to a treatment temperature and a temperature decrease from the treatment temperature. The present invention is further characterized in that the periodically poled grating has a single or a plurality of spike-like sharp tips. The present invention is further characterized in that in the proton exchange treatment, an acid is applied onto a substrate by utilizing surface tension and a heat treatment is performed to exchange protons only on one surface of the substrate. The present invention is further characterized in that the proton exchange treatment uses a plate heater. The present invention is further characterized in that, in the proton exchange treatment, a temperature change rate of 50 ° C./min or more is included in one or both of a temperature increase to a treatment temperature and a temperature decrease from the treatment temperature. The present invention is further characterized in that the acid is pyrophosphoric acid, phosphoric acid, benzoic acid or stearic acid. Further, the present invention is characterized in that an aluminum nitride ceramics (AlN) plate excellent in thermal conductivity and insulation is used as the substrate holder in the proton exchange treatment, and the substrate is electrically insulated by the holder. To do. Then, the present invention provides a second harmonic generation element manufactured by the above method for forming the polarization inversion grating and the optical waveguide. The object of the present invention is to realize an ideal rectangular polarization inversion lattice by producing a polarization inversion lattice extending to a uniform composition region outside the proton exchange region as shown in FIG. S is a highly efficient S
It is to manufacture an element for generating HG light by a simple process. The presence of spike-like domains in a LiTaO ₃ or LiNbO ₃ substrate has been reported, for example, in J. Appl. Phys. Vol. 46, No. 3
As is well known, as seen on page 1063 of (1975), there has hitherto been no means to control the spiked domains. The present invention intends to achieve a high-efficiency SHG device by artificially controlling the generation of this spike-shaped domain, and utilizing the property that the domain extends perpendicularly to the c-axis direction of the substrate. Therefore, we examined the conditions for the appearance of spike-shaped domain regions, and found that LiTaO ₃ or L
It was found that many spike-shaped domain regions appeared when the iNbO ₃ substrate was subjected to a relatively rapid thermal history. This is because the magnitude of the polarization changes rapidly in response to a rapid thermal change, which causes an effective electric field to be generated around the domain wall. Therefore, the spike domain is the direction of polarization. It is considered to grow along the c-axis. Further, it is considered that the similar phenomenon may generate spike-like domains not only due to thermal change but also due to stress, for example. Therefore, it was considered that the spike-like domains could be arranged periodically if the shoots of the domains were formed periodically.
It is considered that the buds in the domain may be the buds in the domain in which the magnitude or direction of polarization locally differs from that in the peripheral portion. Furthermore, by forming a metal film on the back surface of the substrate, an electric field is induced between the surface of the substrate and the metal mask portion for proton exchange, that is, in the thickness direction of the substrate, and growth of spike-shaped domains is promoted. Conceivable. Further, the proton exchange region, which becomes a sprout of the spike-shaped domain, has a higher refractive index by heat treatment as shown in FIG. 10, and can be configured as an optical waveguide. Based on the above idea, in order to form a periodically poled lattice within a uniform composition region of a LiTaO ₃ or LiNbO ₃ substrate,
When a periodic lattice pattern is formed by proton exchange, a thermal history of an appropriate rate of temperature increase / decrease is applied to form a periodic spike-like polarization-inverted lattice. If the waveguide is used, a substantially rectangular polarization inversion grating can be formed in the optical waveguide. This method has led to the idea that the domain-inverted grating and the optical waveguide can be produced only once by photolithography. Further, as shown in FIG. 7, as a method of proton exchange treatment, the surface tension of the acid, which is a proton exchange source, is used to hold the acid on the substrate surface, and a plate-type heater is used to provide a rapid thermal history to the surface of the crystal substrate. A periodic spike-like polarization inversion lattice can be formed by exchanging the protons only. Further, an aluminum nitride ceramics plate having good thermal conductivity and insulation is used as the substrate holder, and the substrate is electrically insulated by the substrate holder. As a result, it was thought that a highly efficient SHG element could be easily realized.

[0005]

EXAMPLES Examples of the present invention will be described in detail below. FIG. 1 is a configuration and operation explanatory view showing an embodiment of an SHG element according to the present invention, FIG. 6 is a view showing a method of proton exchange heat treatment, and FIGS. 7A to 7F are views showing a manufacturing process of the SHG element. Is. FIG. 8 is a photograph showing a polarization inversion lattice formed outside the proton exchange region, and FIG. 9 is a photograph showing a semicircular polarization inversion lattice. In FIG. 1, 11 is a LiTaO ₃ single crystal substrate whose surface is the −c plane, and the direction of spontaneous polarization is downward. Reference numeral 12 is an inverted polarization formed at the same time as the proton exchange region 16, and the polarization direction is upward in these portions. M. Didomenico Jr. et al. Journal of Applied
According to Physics Vol.40, No.2, pages 720 to 734, the sign of the nonlinear optical coefficient coincides with the direction of spontaneous polarization in the case of a ferroelectric crystal of space group R3c such as LiNbO ₃ or LiTaO ₃ . Therefore, it can be said that the nonlinear optical coefficients of the substrate and the optical waveguide layer of this example are also periodically inverted. Thirteen
Becomes a channel type optical waveguide, and the fundamental wave and SHG light are also confined and propagated in this portion. Reference numeral 14 is an incident fundamental wave, which is polarized in a direction perpendicular to the crystal surface. Reference numeral 15 is SHG light generated in the optical waveguide layer portion, which is also polarized in a direction perpendicular to the crystal surface. Next, the proton exchange process will be described. By proton exchange by immersing the substrate or the like in an acidic solution, H ⁺ ions penetrate into the substrate from the surface of the substrate and are exchanged with Li of the substrate to form a composition change layer. In particular, phosphoric acids have a dissociation constant of benzoic acid (C ₆ H ₅ COOH, melting point 121 ° C, boiling point 25
It is two to three orders of magnitude higher than that at 0 ° C., and the concentration of H is high, so the degree of composition change is large. Further, it is possible to perform high-temperature treatment with a liquid up to about 300 ° C., and the evaporation amount is extremely small, and controllability and workability are good. Further, since it is soluble in water, it is possible to wash the sample, the container and the jig. Pyrophosphoric acid (H
₄ P ₂ O ₇ , melting point 61 ° C., boiling point 300 ° C.) was used. In FIG. 6, a LiTaO ₃ single crystal substrate 11 whose surface is the −z plane is placed on a grounded platinum plate 62 which is a substrate holder, and a few drops of pyrophosphoric acid 63 are placed on the substrate 11 using surface tension. Hold. Place the 62 substrate holder on the plate-type heater 64 heated to the proton exchange temperature for several minutes.
Proton exchange is performed for several hours. After the proton exchange, the substrate is taken out and washed with water to remove the pyrophosphoric acid of 13. As a result, the proton exchange layer 65 is formed only on one surface of the substrate 11. All proton exchange was performed in the atmosphere. Also,
For selective proton exchange, T which is insoluble in pyrophosphoric acid
This can be done by attaching a film to the surface of the substrate and producing a lattice mask by photolithography. Here, the substrate holder 62 is made of an aluminum nitride ceramics plate having good thermal conductivity and insulating properties, so that the substrate is electrically insulated. It can be easily inferred that phosphoric acid, benzoic acid, or stearic acid can be used as the acid for proton exchange. Next, the method of forming the polarization inversion grating and the optical waveguide of the present invention will be described with reference to FIG. The figure is a cross-sectional view of an optical waveguide and a domain-inverted grating. Figure 7
As shown in (a), a LiTaO ₃ substrate 11 is prepared.
(B) A Ta film 71 is formed on the −Z surface of 11 by 30 nm sputtering. Au film 73 of 50n on the + Z surface on the back
m vapor deposition. (C) A photoresist 72 was spin-coated on the Ta film 71, and the photoresist 72 was patterned by a normal photolithography technique using a photomask in which a portion where the polarization inversion 12 was performed was opened by the optical waveguide width. The pattern period of the photomask is set to 1 to 10 μm in accordance with the period of SHG light generated, and the waveguide width is set to 2 to 6
The width is μm. (D) Using the patterned photoresist 72 as a mask, the Ta film 71 is patterned by dry etching by RIE using CF ₃ Cl gas. (E) The photoresist 72 is removed with acetone, and the proton exchange heat treatment shown in FIG. 6 is performed using pyrophosphoric acid at 260 ° C. for 30 to 60 minutes to obtain the proton exchange layer 16
At the same time that is formed, the spike-shaped polarization inversion layer 12 is formed. The substrate 11 undergoes a rapid thermal change when the substrate holder 62 is placed on or removed from the heater 64 shown in FIG. At this time, the temperature rising rate up to the proton exchange treatment temperature is 50 ° C /
The proton exchange region 16 and the spike-shaped polarization inversion lattice 12 can be manufactured by performing the heating for more than 5 minutes and the temperature lowering rate from the heat treatment temperature at 50 ° C./minute or more. (F) The Ta film 71 is etched with an aqueous solution of NaOH. Heat treatment at temperature 38
Hold time 5 minutes at 0 ° C, heating rate up to heat treatment temperature 50
℃ / min or more, the rate of temperature decrease from the heat treatment temperature is 50 ℃
/ Min or more increases the refractive index of the proton exchange part to form an optical waveguide. Further, it is possible to suppress the decrease of the nonlinear optical constant due to the proton. If the heat treatment time is 20 minutes or more, the diffusion of protons becomes too large and the loss of the optical waveguide becomes large. The depth of the fabricated polarization inversion lattice is larger than the depth of the optical waveguide and the proton exchange layer formed on the substrate surface and smaller than the substrate thickness, and its width is almost equal to the width of the proton exchange pattern. In the range of, a rectangular polarization-inverted grating could be realized.
Finally, the SHG element is manufactured by optically polishing the end face of the waveguide. A polarization inversion grating was produced by the above-described production method, and an SHG element having an element length of 1 cm was produced. When a titanium-sapphire laser was used as a light source of the fundamental wave and a fundamental wave having a wavelength of 830 nm was incident on the manufactured SHG element, blue SHG light of 415 nm was obtained. S at this time
The HG light output is 10 mW and the standardized SHG efficiency is 15
It was 0% / W · cm ² . The polarization inversion grating produced at this time had an ideal polarization inversion grating close to a rectangular shape as shown in FIG. 8, and the tip had a spike shape.
Next, for comparison, another SHG element in which only the proton exchange region was polarization-inverted was produced. Observation of the cross section of the domain-inverted grating produced at this time showed a semicircular shape as shown in FIG. Next, when the SHG light output was measured in the same manner as in the example, the SHG light output at this time was 100 nW, and the normalized SHG efficiency was 4% / W · cm ² . As a result, it was found that a spike-shaped polarization inversion lattice extending outside the proton exchange region during the proton exchange heat treatment can be easily produced by one photolithography step, and a highly efficient SHG element can be realized.

[0006]

As is apparent from the above description, according to the present invention, it is possible to form an ideal rectangular polarization inversion lattice by producing a spike-like polarization inversion lattice extending outside the proton exchange region during the proton exchange heat treatment. Can be realized 1
It is possible to easily realize an SHG element capable of generating highly efficient SHG light in a single photolithography process.

[Brief description of drawings]

FIG. 1 is a structural diagram for explaining an embodiment of the present invention.

FIG. 2 is a diagram showing a conventional SHG element using Cherenkov radiation.

FIG. 3 is a conventional SHG using a triangular polarization inversion grating.
It is a figure which shows an element.

FIG. 4 is a diagram showing a conventional SHG element using a semi-circular polarization inversion grating.

FIG. 5 is a diagram showing a conventional proton exchange method.

FIG. 6 is a diagram showing a proton exchange heat treatment method according to the present invention.

7 (a) to 7 (f) are diagrams showing a method of manufacturing a spike-shaped polarization inversion grating and an optical waveguide according to the present invention, respectively.

FIG. 8 is a photograph showing a fine pattern formed on a substrate showing a proton exchange region and a spike-shaped polarization inversion lattice.

FIG. 9 is a photograph showing a fine pattern formed on a substrate showing a semi-circular polarization inversion grating.

FIG. 10 is a diagram showing a refractive index of a proton exchange waveguide with respect to heat treatment time.

[Explanation of symbols]

11 substrate (LiTaO ₃ ) 12 polarization inversion region 13 channel type optical waveguide 14 fundamental wave incident light 15 SHG output light 16 proton exchange region 21 substrate (LiNbO ₃ ) 22 Cherenkov SHG light 31 triangular polarization inversion region 41 semicircular polarization inversion Area 51 Glass Container 52 Substrate 53 Acid 54 Constant Temperature Bath 62 Aluminum Nitride Ceramics Substrate Holder 63 Pyrophosphoric Acid 64 Plate Type Heater 65 Proton Exchange Layer 71 Ta Film 72 Photoresist 73 Au Film

Claims (11)

[Claims] 1. LiTaO ₃ , LiNbO ₃ , or Mg
A periodic composition modulation region is formed on a doped LiTaO ₃ or LiNbO ₃ substrate by a proton exchange heat treatment, and a periodic polarization inversion lattice extending in a uniform composition region outside the composition modulation region and substantially synchronized with the composition polarization region is formed. In the method of forming a polarization inversion grating and an optical waveguide in which the periodic composition modulation region is formed by proton exchange as an optical waveguide, and a patterned metal film mask for forming the periodic polarization inversion grating on the surface of the substrate, the back surface Is a method for forming a polarization inversion grating and an optical waveguide, which comprises forming a periodic polarization inversion grating and an optical waveguide using a substrate on which a metal film is formed. 2. The polarization inversion grating and the optical waveguide according to claim 1, wherein after the formation of the periodic polarization inversion grating, an optical waveguide is formed by increasing the refractive index of the proton exchange region on the substrate surface by heat treatment. Forming method. 3. The method for forming a polarization inversion grating and an optical waveguide according to claim 2, wherein the heat treatment temperature is 200 ° C. or higher and the heat treatment time is 20 minutes or less. 4. In the heat treatment, when the temperature is raised to a treatment temperature or when the temperature is lowered from the treatment temperature, either or both of them.
The method for forming a domain-inverted grating substrate according to claim 2 or 3, wherein the method includes a temperature change rate of 0 ° C / minute or more. 5. The polarization inversion grating and the optical waveguide according to claim 1, wherein the periodic polarization inversion grating has a tip having a single or a plurality of spike-like sharp points. Waveguide formation method. 6. The proton exchange treatment is characterized in that an acid is applied onto a substrate by utilizing surface tension and a heat treatment is performed to exchange protons only on one surface of the substrate.
6. The method for forming a polarization inversion grating and an optical waveguide according to any one of items 1 to 5. 7. The method of forming a polarization inversion grating and an optical waveguide according to claim 6, wherein a plate-type heater is used for the proton exchange treatment. 8. The temperature change rate of 50 ° C./minute or more is included in one or both of a temperature increase to a treatment temperature and a temperature decrease from the treatment temperature in the proton exchange treatment. 2. A method for forming a domain-inverted grating and an optical waveguide according to any one of 1. 9. The acid used is pyrophosphoric acid, phosphoric acid, benzoic acid, or stearic acid.
2. A method for forming a domain-inverted grating and an optical waveguide according to any one of 1. 10. The method for forming a polarization inversion grating and an optical waveguide according to claim 1, wherein the substrate is electrically insulated as a substrate holder in the proton exchange treatment. . 11. The method of forming a polarization inversion grating and an optical waveguide according to claim 1, wherein an AlN plate is used as the substrate holder.