KR20090073720A - A optical wavelength converter apparatus and a method thereof - Google Patents

Abstract

An optical wavelength conversion element and a manufacturing method thereof are provided to improve the conversion efficiency of the optical wavelength by minimizing the change of waveguide shape to the longitudinal direction of the optical waveguide. The optical wavelength conversion element comprises a plurality of optical waveguides(102) and a refractive index control thin film(103). A plurality of optical waveguides are perpendicularly formed in the slope direction of the thickness deviation of substrate. The refractive index control thin film is formed on the part or the whole surface among a plurality of optical waveguides. The refractive index control thin film is divided into the regions more than two according to the substrate thickness.

Description

Optical wavelength conversion device and a method of manufacturing the same

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical wavelength conversion element and a method of manufacturing the same, and more particularly, to an optical wavelength conversion element having an optical waveguide structure capable of increasing wavelength conversion efficiency and a method of manufacturing the same.

Laser light source technology using polarized inverted nonlinear chips has been tried in various ways. For example, Difference Frequency Mixig, Sum Frequency Mixing, or Optical Parametric Oscillator devices can be fabricated. Among them, a second harmonic wave generation technology, which is a special form of sum frequency, is applied to fabricate a wavelength light source in the visible light band. This refers to a technique in which a pumped light source having a low frequency is incident on a polarized inverted optical waveguide having a nonlinear characteristic and then converted to a light source having a frequency twice as high. Theoretically, the power of the second harmonic wave source is determined in proportion to the square of the incident pumping light source and the square of the length of the nonlinear chip, but the conversion efficiency cannot be 100% due to the waveguide loss, absorption loss, and optical connection loss. do. Second harmonic generation techniques using nonlinearity have been mentioned in various literatures.

The pseudophase matching period of the variable wavelength using nonlinearity is determined by applying the target wavelength to be generated, the temperature applied to the wavelength variable, and the wavelength of the input pumping light source. Determination of the quasi phase matching period can be determined by the following equations 1) and 2).

Where n is the refractive index, λp is the wavelength of the pumping light source, λs is the wavelength of the signal light source, λi is the wavelength of the idler light source, ∧g is the polarization reversal period, m is the pseudophase matching order, and Δk is the phase difference. Here, the generation of the second harmonic wave corresponds to a special case in which λs and λi have the same wavelength and use this wavelength as an input pumping light source to generate a new wavelength of λp. As shown in Equation 2), it can be seen that the refractive index must be adjusted to fix the wavelength, fix the polarization inversion period, and make Δk equal to zero. For controlling the refractive index, a method of controlling the temperature of the device is used.

In addition, as in the nonlinear conversion efficiency theory, the phase difference Δk changes the nonlinear conversion efficiency by the slope of Sin (Δk) ² / Δk ² . Therefore, when the device is manufactured to change the shape of the waveguide in the longitudinal direction in which light travels, the non-linear conversion efficiency according to the traveling direction decreases as Δk is increased to a non-zero section.

In fabricating the waveguide device, even if the machining process is precisely performed, it is almost impossible to reduce the thickness change according to the position of the substrate to a value that is independent of the device characteristics. The optical waveguide formed on the substrate due to the inclination of the surface of the substrate due to the change in thickness of the substrate is changed in the longitudinal direction in which light travels, thereby significantly reducing the conversion efficiency of the device.

 In addition, if the shape of the optical waveguide is changed in the longitudinal direction in which light travels, the temperature range of the optimized device for adjusting the refractive index is also widened, and thus there is a limit in adjusting the refractive index by temperature control.

An object of the present invention is to provide an optical wavelength conversion element having an optical waveguide structure capable of increasing the optical wavelength conversion efficiency by minimizing the waveguide shape change in the longitudinal direction of the optical waveguide and a method of manufacturing the same.

It is another object of the present invention to provide an optical wavelength conversion element and a method of manufacturing the same, which further include a refractive index conversion thin film layer for an optical waveguide, thereby reducing an operating temperature variation range of the final fabricated device by 50% or more. have.

The object of the present invention is achieved by an optical wavelength conversion element comprising a plurality of optical waveguides formed in a direction perpendicular to the inclination direction of the thickness deviation of the substrate and a refractive index control thin film layer formed on the surface of some or all of the plurality of optical waveguides. do. In this case, the refractive index controlling thin film layer may be divided into two or more regions according to the thickness of the substrate, and the materials forming the refractive index controlling thin film layer of the respective regions may be different from each other. The refractive index control thin film layer is preferably formed of a low refractive index material gradually in the downward direction of the slope of the thickness deviation of the substrate. That is, the refractive index control thin film layer may not be formed in the region of the thinnest portion of the substrate or may be formed of a low refractive index material, and may be formed of a high refractive index material in the region having the thickest thickness of the substrate. In the refractive index control thin film layer, an intermediate region between the region having the thinnest thickness of the substrate and the region having the thickest thickness of the substrate may be divided into at least one or more regions, and may be formed of an intermediate refractive index material. The intermediate region may be a plurality of regions, and in this case, it is preferable that the intermediate region is a region in which the thickness of the substrate becomes thicker, so that the refractive index of the intermediate refractive index material is higher. When the refractive index control thin film layer is divided into one or more regions, the number of the regions is preferably increased as the slope of the substrate thickness deviation increases.

Another object of the present invention is to prepare a substrate for forming an optical wavelength conversion element, a substrate preparation step, a thickness measurement step of determining the slope and direction of the thickness deviation by measuring the thickness of the substrate, forming a polarization inversion region on the substrate And a waveguide forming step of forming a polarization inversion region and forming at least one optical waveguide perpendicular to the inclination direction of the thickness deviation.

When there are a plurality of optical waveguides, it is preferable to further include a thin film layer forming step of forming a refractive index control thin film layer on the surface of some or all of the optical waveguides after the optical waveguide forming step. In the forming of the thin film layer, the refractive index controlling thin film layer may be divided into two or more regions according to the thickness of the substrate, and the materials constituting the refractive index controlling thin film layer of each region may be formed to be different from each other. At this time, in the thin film layer forming step, it is preferable to form the respective regions sequentially by using respective masks for each region of the refractive index control thin film layer.

The optical wavelength conversion element and the manufacturing method thereof according to the present invention have an advantage of maximizing the optical wavelength conversion efficiency by minimizing the waveguide shape change in the longitudinal direction of the optical waveguide. In this case, the power consumption of the pumping light source is reduced and the service life of the pumping light source is increased in manufacturing the same power optical output device as the light wavelength conversion efficiency is increased.

In addition, the optical wavelength conversion device and the method for manufacturing the same according to the present invention have an advantage of maximizing the optical wavelength conversion efficiency by reducing the use temperature variation range of the final manufactured device by forming the thin film layer by 50% or more.

1A is a side view of an optical wavelength conversion element according to an embodiment of the present invention. The substrate 101 has a constant surface slope in the machining process in the preparation step, and the optical waveguide 102 is formed in a direction perpendicular to the slope of the surface of the substrate 101. In this case, each of the waveguides 102 may generate a height difference from each other due to the inclination of the substrate 101, but the heights of the waveguides 102 do not change in the direction in which light travels in each of the waveguides 102. Through this, it is possible to minimize the shape change in the longitudinal direction of the optical waveguide 102, thereby maximizing the conversion efficiency of the optical wavelength conversion element.

FIG. 1B is a side view of a structure in which a refractive index control thin film layer 103 is formed on each waveguide 102 of FIG. 1A. In FIG. 1A, as the thickness control of the substrate 101 is formed in the cross-sectional direction of the waveguide, the wavelength conversion conversion efficiency is maximized, but this causes a problem in that the wavelength variable optimization phase matching operating temperature range between each waveguide 102 increases. . The phase matching temperature for the wavelength variability optimization is different depending on the shape of the waveguide 102, which causes a problem in that the use temperature of the final fabricated device should be applied differently. In order to solve this problem, as shown in FIG. 1B, the A, B, and C thin film layers 103 having random refractive indexes different from each other may be formed on the waveguide 102 to change the effective index of the light traveling in the waveguide 102. . The effect of changing the effective index of the waveguide 102 by the thin film layer 103 is 50% (for example, 30 ° C. to 50 ° C. of the use temperature range of 30 ° C. to 40 ° C.). To make it more effective). In FIG. 1B, A may be a material having a low refractive index such as air or MgF ₂ , B may be a material having a medium refractive index such as SiO ₂ , and C may be Ta ₂ O ₅ , TiO ₂ , Nb ₂ O _A material having a high refractive index such as ₅ may be applied. FIG. 1B is illustrative and the selection of the A, B, C thin film layer 103 material may be applied differently depending on the degree of height difference of the waveguide 102. In addition, when the height difference is small, only two thin film layers 103 may be formed. On the contrary, when the height difference is large, four thin film layers 103 may be formed to optimize the use temperature range of the device.

2 (a) to (m) is a conceptual diagram showing a method for manufacturing an optical waveguide according to an embodiment of the present invention.

In FIG. 2A, a bonding process between a nonlinear wavelength conversion element substrate and a dummy substrate is shown. The substrate is manufactured by bonding a dummy substrate (a substrate having the same substrate or similar mechanical properties) and a dummy substrate to fabricate the nonlinear wavelength conversion device, and the substrate bonding process is curable epoxy, wax, and diffusion that is cured by UV or heat treatment. ) And various methods such as direct bonding by surface treatment can be applied.

FIG. 2 (b) shows a preparation process for fabricating the polarization inversion region by processing the thickness of the bonded substrate. The process is carried out using substrate processing equipment and is carried out by grinding, polishing and lapping processes. The thickness after processing may be differently applied depending on the type of substrate used and the polarization period design.

(C) of FIG. 2 shows the step of measuring the thickness of the nonlinear conversion chip fabrication substrate processed in (b) of FIG. As a method of measuring the thickness deviation, a non-destructive thickness gauge may be applied, and a typical measurement example may be performed by applying an optical thickness gauge. In this case, as shown in FIG. 3, the thickness of the substrate is measured at various measuring points, and the equation of the circle and the cutting plane equation are obtained using the measured thickness data, and the thickness deviation inclination value and the direction thereof can be confirmed therefrom. Preferably, the calculated main axis may be used to calculate the principal axis at which the thickness variation of the processed substrate is maximized, and the slope thereof may be calculated. The cutting plane equation in the example illustrated in FIG. 3 represents a case in which a height of 2 has a height in the z-axis direction when y is 0, a value of 1 when y is 10, and 3 as a z-axis value when -10. In this case, it has a height inclination in the y-axis direction and its inclination is -1/10.

FIG. 2 (d) shows a preparation process in which the thickness deviation slope direction of the substrate measured by the method described above is found and marked on the substrate so as to know in which direction the slope of the thickness deviation is in the next process. This makes it possible to determine the direction of patterning in the next process.

2E is a step of forming an insulating film on the substrate for forming the polarization inversion region in consideration of the thickness deviation inclination direction of the substrate. The insulating film may be performed by various methods such as a photosensitive agent method, a sol-gel method and a thin film deposition method.

2 (f) shows a process step of generating a polarization inversion period using the upper insulating film as a mask. This step may vary depending on the choice of insulating film. It is generally possible to form a periodic polarization inversion in the ferroelectric when applying the photoresist method. First, a method of coating an insulating photoresist film on a substrate, forming an electrode pattern through a photolithography process, and then depositing a metal electrode thereon. The second method is to spin-coat a photoresist onto a substrate, form an electrode pattern through a photolithography process, and then form an electrode using an electrolyte solution in which LiCl is dissolved therein. It is important to uniformly coat the photoresist with a certain thickness on the ferroelectric substrate when forming the electrode structure. To form a periodic polarization inversion, a photolithography process should be performed using a mask having a pattern somewhat smaller than 50% of the period. do. The polarization inversion cycle is completed on the ferroelectric substrate by applying a voltage to the polarization period inversion electrode and the insulation pattern generated using the voltage source equipment from the outside. In order to switch the polarization state inside the ferroelectric crystal, an external electric field must be applied. Since the electric field value is a material constant determined according to the ferroelectric substrate used, it may be applied differently depending on the substrate. Here, the method of forming the electrode for polarization inversion may be applied differently depending on the axis of the ferroelectric substrate. For example, electrodes for Z-direction polarization inversion on a Z-cut substrate are fabricated at the top and bottom of the substrate. On the other hand, the electrode for the Z-direction polarization inversion on the X-cut substrate may be made parallel to the plane on which the waveguide of the substrate is formed.

FIG. 2 (g) illustrates a step of reworking the substrate having completed the formation of the polarization inversion region according to the waveguide design height before fabricating the waveguide. In this case, it is important to set the equipment so that the substrate is processed in the same direction as the thickness height deviation inclination direction of the processed substrate as described above. X-cut substrate polling may be applied in a way that eliminates this step. X-cut substrate has the advantage of being able to manufacture polarization inversion cycle after processing according to waveguide fabrication design height. This is because the electrode formation of the X-cut (or Y-cut) substrate has a structure in which the positive electrode and the negative electrode are simultaneously generated on the processing surface to poll and remove them.

FIG. 2H shows a step of forming an optical waveguide on a substrate. Formation of the optical waveguide is performed by the following process. A seed layer and a photoresist layer are sequentially formed on the ferroelectric substrate, and then a photolithography process is performed to form a photoresist pattern. A metal film such as Ni is formed in the space between the photoresist patterns and the photoresist pattern is removed. Using the metal film as a mask, the substrate is etched to a depth of about 1-10 um by a dry or wet etching method to complete the ridge optical waveguide fabrication process.

2 (i) to (m) show a refractive index control thin film layer forming process. (I) and (j) of FIG. 2 show a process and a result of depositing A material using an A material mask, and FIGS. 2 (k) and (m) show B masks and C materials sequentially. The process and results of deposition on top of the waveguide are shown. Through the refractive index control thin film layer formed in this step, it is possible to minimize the width of the phase matching optimization temperature range in each waveguide and maximize the wavelength variable conversion efficiency.

Experimental Example

A schematic diagram illustrating the effects of the A, B, and C thin film deposition is shown in FIG. 4A. 4A shows that the maximum difference (h1-h2) occurs in each waveguide in the cross-sectional direction of the optical waveguide and assumes that the value is approximately 1 micron. In addition, the maximum change value of the optimum polarization inversion period of the wavelength variable element due to such a maximum height difference is expressed as (Q1-Q2), wherein the value of the polarization inversion period calculation equation 1) described above occurs about 0.024 microns. And 2) were applied. Here, since the period of the mask for manufacturing the actual polarization inversion period is constant, in order to correct this to obtain the maximum conversion efficiency, the operating temperature of the device should be optimized. At this time, the use temperature change range was calculated by applying the theoretical formula 1) and 2) 15 ℃. When the A, B, and C thin films are deposited on the waveguide on the device, and the maximum polarization inversion period (Q1-Q2) is recalculated in consideration of the refractive index change generated at this time, it can be seen that about 0.015 micron is generated. Applying this again, it can be seen that the calculated temperature range of the device has a variation of approximately 5 ~ 7 ℃. The substrate used in this calculation is Z-cut MgO: LiNbO ₃ and the reference wavelengths are 1064nm and 532nm.

4B is a graph illustrating the effect of improving the wavelength conversion power by making the height difference of each waveguide in the cross-sectional direction of the optical waveguide. The power in the case where the height difference of the waveguide is generated in the cross-sectional direction of the waveguide and there is no height difference of the waveguide in the longitudinal direction is set to 1. It can be seen that the optical wavelength conversion power decreases exponentially as the height difference of the waveguide occurs in the longitudinal direction (thickness error-> large). It can be seen that the optical wavelength conversion power of 500% is obtained when comparing the structure of the present invention with the thickness variation in the 1 um longitudinal direction.

1A is a side view showing the arrangement of the optical waveguides according to the present invention;

1B is a side view illustrating an array shape and a refractive index control thin film layer of optical waveguides according to the present invention;

2 is a conceptual diagram of measuring a thickness gradient direction using an equation of a circle and a cutting plane equation;

3 is a process chart showing a manufacturing method of an optical wavelength conversion element according to the present invention;

Figure 4a is a schematic diagram illustrating the effect of the thin film layer deposition in the optical wavelength conversion device according to the present invention,

4B is a graph illustrating the effect of improving the wavelength conversion power by making the height difference of each waveguide in the cross-sectional direction of the optical waveguide.

Claims (8)

A plurality of optical waveguides formed in a direction perpendicular to the inclination direction of the thickness deviation of the substrate; And A refractive index control thin film layer formed on a surface of part or all of the plurality of optical waveguides Optical wavelength conversion element comprising a. The method of claim 1, The refractive index control thin film layer is divided into two or more regions according to the thickness of the substrate, the wavelength conversion element, characterized in that the material constituting the refractive index control thin film layer of each region is different. The method of claim 2, wherein the refractive index control thin film layer, And a low refractive index material gradually formed in a downward direction of the inclination of the thickness deviation of the substrate. The method of claim 2, And the number of the regions increases as the slope of the thickness variation of the substrate increases. A substrate preparation step of preparing a substrate for forming an optical wavelength conversion element; A thickness measurement step of determining a slope and a direction of the thickness deviation by measuring the thickness of the substrate; A polarization inversion region forming step of forming a polarization inversion region on the substrate; And An optical waveguide forming step of forming at least one optical waveguide perpendicular to the inclination direction of the thickness deviation Optical wavelength conversion device manufacturing method comprising a. The method of claim 5, wherein the optical waveguide is a plurality, after the step of forming the optical waveguide, A thin film layer forming step of forming a refractive index control thin film layer on the surface of part or all of the optical waveguide Optical wavelength conversion device manufacturing method further comprising. The method of claim 6, wherein in the forming of the thin film layer, The refractive index control thin film layer is divided into two or more regions according to the thickness of the substrate, the optical wavelength conversion element manufacturing method, characterized in that formed in each of the materials forming the refractive index control thin film layer is different. The method of claim 7, wherein the thin film layer forming step, And forming each of the regions sequentially by using a mask for each region of the refractive index controlling thin film layer.

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