JP2006284962A - Optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element which uses a thin plate where an optical waveguide is formed and includes a reinforcing plate bonded to the reverse surface of the thin plate, the optical element being characterized in that the reinforcing plate functions as an underclad layer and the optical element is inexpensive and has small variance in performance. <P>SOLUTION: The optical element including the thin plate 10 which is made of a single-crystal material having electrooptical effect and polished or processed with an ion beam to a thickness of 20 μm, ridge type optical waveguides 11 and 14 formed on the top surface of the thin plate, a modulating electrode for modulating light passing through the optical waveguide, and the reinforcing plate 12 bonded to the reverse surface of the thin plate is characterized in that the reinforcing plate is made of the same material with the thin plate and has a refractive index made less on its joined surface than the refractive index of the thin plate and a direct joining method is employed to join the thin plate and reinforcing plate together. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical element, and more particularly to an optical element including a thin plate formed of a material having an electro-optic effect and a reinforcing plate bonded to the thin plate.

Conventionally, in an optical communication field and an optical measurement field, an optical element such as a waveguide type optical modulator in which an optical waveguide or a modulation electrode is formed on a substrate having an electro-optic effect has been widely used.
In order to realize a wider optical modulation frequency, it is important to match the speed of the modulation signal microwave and the light wave, and various methods have been devised so far. Specific examples include thicker buffer layers, higher aspect ratios of electrodes, and ridge structures.

In Patent Document 1 or 2 below, an optical waveguide and a modulation electrode are incorporated in an extremely thin substrate (hereinafter referred to as “first substrate”) having a thickness of 30 μm or less, and has a lower dielectric constant than the first substrate. Other substrates are bonded to reduce the effective refractive index with respect to the microwave, thereby achieving speed matching between the microwave and the light wave.
JP-A 64-18121 JP 2003-215519 A

By using the thinned first substrate like this, the design flexibility of the modulator is dramatically increased, and for example, an optical modulator having a wide band and a low driving voltage can be manufactured without using a buffer layer. Become. Furthermore, from the viewpoint of reducing the propagation loss of microwaves, it is synonymous with the use of a material having a low dielectric constant for the substrate. Specifically, the first substrate is specifically set to 150 μm or less, particularly in the region of 26 GHz or more. Non-patent document 1 discloses that the radiation loss of the wave dielectric can be reduced, and is applied to the widening of the modulator.
Yamane et al., “LN Substrate Processing by Sandblasting”, Sumitomo Osaka Cement Technical Report 2003, pp49-54 (2003)

  Conventionally, when an optical waveguide is formed in an optical element such as a waveguide type optical modulator, a refractive index of the diffusion portion is made higher than that of other portions by thermally diffusing a metal such as Ti into the substrate at a high temperature. The light is confined. By thinning the substrate by polishing, it becomes possible to design and manufacture an optical modulator having a wide band and a low driving voltage as described above. However, since the mechanical strength of the substrate is reduced by thinning the substrate on which the optical waveguide is formed, it is necessary to join a reinforcing substrate to the thin plate.

  Further, if the reinforcing substrate has a lower refractive index than the material used as a thin plate, it functions as an underclad of the optical waveguide, and the light wave can be largely confined. Strongly confined waveguides can suppress crosstalk and sharply bend light, and when applied to a modulator, the distance between the waveguide and electrode can be reduced, improving the electric field efficiency. Can be expected. However, when a different material is used for this reinforcing substrate, stress is induced with a change in temperature due to the difference in linear expansion coefficient, and an unintended refractive index change due to the piezo effect occurs, so when used as a modulator, Temperature drift and DC drift will occur, resulting in poor long-term reliability.

Furthermore, an optical element that requires propagation in a single mode, such as a waveguide-type optical modulator, requires a design that strictly considers the refractive indexes of the thin plate and the underclad layer.
This requirement becomes more strict in the case of a ridge type waveguide formed by using dry etching or cutting, and its design tolerance is small.

  As an undercladding layer, conventionally, Mg: LN, Zn: LN, or stoichiometric lithium niobate (SLN) obtained by pulling a crystal from an LN melt doped with Mg or Zn, or lithium tantalate ( LT), a stoichiometric composition lithium tantalate (SLT), and the like are used.

  However, these materials have a problem that they are expensive and have a large variation in composition as compared with the conformal melt composition lithium niobate (CLN). In the crystal growth, the segregation coefficient of Mg or Zn is larger than 1 (it is easy to enter the LN crystal), so the concentration is different between the top and the bottom of the crystal. Since the Mg and Zn concentrations and the refractive index values have a strong correlation, when used as an underclad as described above, it is necessary to perform an optimum design depending on the portion of the crystal used (slice position).

  On the other hand, in CLN, the crystal is pulled up from the matching melt in which the melt and solid phase are in equilibrium and exist in the same composition, so there is little compositional deviation in principle, and the composition matches from the bottom to the top of the crystal and is growing. The composition does not easily deviate even with respect to the temperature change. Further, there is almost no composition deviation among crystal boules as well as between crystal boules, and it is provided at a low cost in the current market.

  The problem to be solved by the present invention is to solve the above-described problems, in an optical element using a thin plate on which an optical waveguide is formed, and including a reinforcing plate bonded to the thin plate, the reinforcing plate is an underclad layer. It is an object of the present invention to provide an optical element that has the above functions and that is inexpensive and has little variation in performance.

In order to solve the above-mentioned problem, in the invention according to claim 1, a thin plate having a thickness of 20 μm or less formed of a single crystal material having an electro-optic effect and obtained by polishing or ion beam processing is formed on the thin plate. In an optical element comprising a ridge-shaped optical waveguide, a modulation electrode for modulating light passing through the optical waveguide, and a reinforcing plate bonded to the thin plate,
The reinforcing plate is made of the same material as the thin plate, at least the refractive index of the joint surface of the reinforcing plate is lower than the refractive index of the thin plate, and the thin plate and the reinforcing plate are joined by a direct joining method. It is characterized by.

The optical element according to the present invention means not only a waveguide type optical modulator but also an optical element that forms an optical waveguide on a substrate made of a single crystal material having an electrooptic effect such as an optical switch. It also includes elements.
The ridge-type optical waveguide means not only a so-called “ridge” in which the portion of the optical waveguide is raised from the substrate surface other than the optical waveguide, but also grooves on both sides of the optical waveguide. It also includes an optical waveguide that is formed and constitutes a light confinement region.

  According to a second aspect of the present invention, in the optical element according to the first aspect, an impurity layer is formed on the reinforcing plate using a vacuum film forming apparatus, and the impurities are diffused to reduce the refractive index. Features.

  According to a third aspect of the present invention, in the optical element according to the first or second aspect, the thin plate is made of an LN crystal substrate or an LT crystal substrate having a coincident melt composition.

  According to the invention of claim 1, since the thin plate is formed of a single crystal material having an electro-optic effect and is obtained by polishing or ion beam processing, it is excellent in single orientation, crystal phase uniformity, chemical composition uniformity, and the like. The production reproducibility is also high. Further, the thickness of the thin plate is 20 μm or less, the same material as the thin plate is used for the reinforcing plate, at least the refractive index of the reinforcing plate is reduced from the refractive index of the thin plate, and the thin plate and the reinforcing plate Is bonded by the direct bonding method, so that the reinforcing plate has the function of the under cladding layer, there is no difference in the coefficient of linear expansion between the thin plate and the reinforcing plate, and the occurrence of thermal expansion stress is suppressed, The degree of freedom of the optical element manufacturing process can be improved, and a highly reliable optical element can be obtained.

  According to the invention of claim 2, since the impurity layer is formed on the reinforcing plate using a vacuum film forming apparatus, the amount of impurities can be controlled with high reproducibility and high accuracy. For this reason, it becomes possible to easily obtain a reinforcing plate having a refractive index strictly controlled while using the same substrate as the thin plate as the reinforcing plate.

  According to the invention of claim 3, since the thin plate is made of an LN crystal substrate or an LT crystal substrate having a coincidence melting composition, the crystal substrate having the same coincidence melting composition is used for the reinforcing plate according to the invention according to claim 1. Accordingly, it is possible to obtain a reinforcing plate that is inexpensive and has little variation in characteristics. Needless to say, in order to use as a reinforcing plate, it is necessary to diffuse impurities on the surface of the substrate.

The method for forming a single-crystal ferroelectric thin film and the diffusion of impurities into the substrate material have already been disclosed in Patent Documents 3 and 4 below.
JP-A-4-12095 JP-A-4-37697

Although the configurations according to Patent Documents 3 and 4 seem to be similar in structure to the present invention at first glance, the configuration of Patent Documents and the like is that an epitaxial film is arranged on a thin plate portion for producing an optical waveguide portion. On the other hand, as described above, the present invention uses a CLN single crystal substrate which has almost no compositional deviation in principle and has very good crystallinity as a starting material and is thinned by CMP polishing. Is decisively different. Application of a ferroelectric thin plate to an optical waveguide device requires not only crystallinity such as single orientation and crystal phase uniformity, but also chemical composition uniformity and low light propagation loss characteristics. Epitaxial films can be controlled in principle in a wide range by adjusting the liquid phase raw material, but the reproducibility is generally low, and the crystallinity of the thin film produced by this method uses the chocolate lasky method. In addition to the CLN crystal pulled up from the coincident melt composition, there is also a problem of inclusion of impurities, and long-term reliability cannot be ensured when a modulator is used. This can be easily estimated from the fact that a crystal substrate obtained from an epitaxial film is not currently used in a commercial modulator. Further, since the same material is not used as the substrate material, and a technique for strictly controlling the amount of impurities is not disclosed, a reinforcing plate with a strictly controlled refractive index as in the present invention can be obtained inexpensively and easily. It has not reached.
As described above, the inventions of Patent Documents 3 and 4 are greatly different from the present invention.

Hereinafter, the present invention will be described in detail using preferred examples.
The present invention relates to a thin plate having a thickness of 20 μm or less formed of a single crystal material having an electro-optic effect and obtained by polishing or ion beam processing, a ridge type optical waveguide formed on the thin plate, and the optical waveguide In an optical element including a modulation electrode for modulating light passing therethrough and a reinforcing plate bonded to the thin plate, the reinforcing plate uses the same material as the thin plate, and at least the refraction of the joint surface of the reinforcing plate The refractive index is lower than the refractive index of the thin plate, and the thin plate and the reinforcing plate are joined by a direct joining method.

FIG. 1 is a cross-sectional view of an optical element according to the present invention. As shown in FIG. 1A, a ridge 11 is formed on the surface of the thin plate 10 made of a material having an electro-optic effect, and the ridge 11 has a function of an optical waveguide. On the surface of the thin plate 10, a modulation electrode for modulating light passing through the optical waveguide and a buffer layer such as SiO ₂ are formed. In FIG. It is omitted.

  As the optical waveguide, in addition to the ridge 11 in FIG. 1A, as shown in FIG. 1B, it is also possible to form grooves 13 on both sides of the optical waveguide 14 to form a light confinement region. In the present invention, it is referred to as a “ridge type optical waveguide” including the forms of FIGS.

Reference numeral 12 in FIG. 1 denotes a reinforcing plate, which is bonded to the back surface of the thin plate 10.
In the present invention, the reinforcing plate 12 is a substrate made of the same material as that of the thin plate 10, and when used as a reinforcing plate, impurities are doped by thermal diffusion or the like, and the refractive index of the thin plate is formed on the reinforcing plate surface. A portion having a lower refractive index is formed and adjusted so as to function as an under cladding layer of the optical waveguide (11, 14) of the thin plate 10.

  As a material for the thin plate 10 and the reinforcing plate 12, an LN crystal (CLN) substrate or an LT crystal (CLT) substrate having a coincidence melting composition with low variation in characteristics is used. When used as a reinforcing plate, as described above, Mg, Zn, Li, Ta and the like are doped from the surface of the reinforcing plate by thermal diffusion or the like.

Next, a method for manufacturing an optical element according to the present invention will be described.
FIG. 3 shows an example of a method for manufacturing an optical element.
As shown in FIG. 3A, a CLN substrate 12 is prepared, and impurities such as Mg are deposited on the substrate using a vacuum deposition method or the like (see reference numeral 15 in FIG. 3B). The vacuum deposition method can be preferably used for controlling the value and region of the low refractive index because the adhesion amount of impurities can be controlled with high accuracy. As the vacuum film forming apparatus, an ion plating apparatus, a sputter film forming apparatus, a CVD apparatus, an MOCVD apparatus, and the like can be used in addition to the vacuum vapor deposition apparatus. Next, the substrate is heated at about 1000 to 1200 ° C. to diffuse impurities from the substrate surface toward the inside of the substrate, thereby forming a region having a refractive index lower than that of the CLN substrate before diffusion.

  In FIGS. 3B and 3C, impurities are evaporated and diffused over the entire surface of the substrate 12. In the region 16 having such a low refractive index, a thin optical waveguide is formed. It suffices if it is on the surface portion of the reinforcing plate corresponding to the region, and if necessary, impurities can be attached (evaporated) in a shape corresponding to the optical waveguide and thermally diffused.

  In addition, such a low refractive index region also functions to prevent light (stray light) from the bottom surface side to the surface side of the CLN substrate, which is a reinforcing plate, from entering the optical waveguide. It is possible to provide an optical element.

  Next, as shown in FIG. 3D, another CLN substrate 10 that becomes a thin plate is bonded onto the reinforcing plate 12. Adhesives such as Hitachi Chemical T-2000 and Nisshinbo Carbodilite Film can be used as the joining method, but the heat resistance temperature of the adhesive limits the thermal history experienced in the modulator fabrication process. It is particularly preferable to use the direct bonding method because there is a concern about the influence as a contamination source.

The bonded CLN substrate 10 is polished to a thickness of 20 μm or less using a lapping machine polishing machine or the like (see FIG. 3E). In this polishing, the reinforcing plate 12 also functions as a support member that supports the thin plate 10. Instead of polishing as a method of forming a thin plate, it is also possible to use ion beam processing, which is a method of slicing a substrate using an ion beam as shown in Non-Patent Document 2.
Tomoyuki IZUHARA, et al., "Ion-sliced Single-crystal LiNbO3 Thin Films and Their Applications", IEEE LEOS NEWSLETTER, pp4-6, August 2004

  Next, a ridge to be the optical waveguide 11 is formed on the thin plate 10 by mechanical cutting such as sandblasting or chemical etching. When forming a ridge or the like, the reinforcing plate 12 also has a role of preventing the thin plate from being damaged by a mechanical impact or thermal stress applied to the thin plate. For ridge formation, dry etching, chemical etching, micro sand blasting, machining with a blade using a dicer, or the like can be suitably used.

Finally, although not shown, a buffer layer and electrodes are formed on the thin plate 10 to complete the optical element.
Further, when forming the ridge, as shown in FIG. 2B, it is possible to form grooves by mechanical cutting or etching on both sides of the portion to become the optical waveguide to form the optical waveguide. It is.

FIG. 4 is a diagram showing another method for manufacturing an optical element.
FIGS. 4A to 4C are the same as FIG. Next, as shown in FIG. 4D, a CLN substrate 10 which is the same material as the reinforcing plate is used, and a ridge is formed on the surface of the substrate by the above-described mechanical cutting and etching, thereby forming the optical waveguide 11. (See FIG. 4 (e)). Furthermore, a buffer layer and a modulation electrode necessary for the optical element are also formed.

  An adhesive 17 such as a thermoplastic resin is applied to the surface of the CLN substrate 10 on which the ridge is formed, and the substrate is attached and fixed to the polishing jig 18 (see FIG. 4F). The substrate 10 is polished to a thickness of 20 μm or less using a lapping machine polishing machine or the like (see FIG. 4G). As described above, ion beam machining can be used instead of polishing as a method for forming a thin plate.

The thin plate 10 is removed from the polishing jig 18, and the reinforcing plate 12 and the thin plate 10 are joined by a direct joining method or the like as shown in FIG. 4 (i) to complete the optical element.
As described above, when forming a ridge, a buffer layer, a modulation electrode, or the like, a mechanical shock, thermal stress, or the like is applied. Therefore, it is possible to incorporate these various members before processing into a thin plate. However, these treatments can be performed after the reinforcing plate is attached to the thin plate.

Next, specific examples and tests related to the optical element of the present invention will be described.
Example 1
An optical element was created using the manufacturing method shown in FIG.
As the reinforcing plate 12, a Z-cut CLN (Optical Grade manufactured by CTI Co.) having a thickness of 500 μm is used, and a vacuum vapor deposition apparatus equipped with an electron beam source is used on the surface of the substrate. Vapor deposition (adhesion) was performed at 50, 65, and 100 nm. Next, the reinforcing plate 12 was heated in an electric furnace at 1000 ° C. for 5 hours to thermally diffuse Mg into the substrate.

The same material as the reinforcing plate was used as the substrate to be processed into a thin plate, and the thin plate substrate and the reinforcing plate were joined by a direct joining method with the crystal orientation aligned.
In direct bonding, each substrate is first cleaned using a surfactant and an organic solvent by applying ultrasonic waves, and then immersed for 5 minutes in a mixed chemical solution of ammonia water and hydrogen peroxide solution of Great for Electronic Industries. The surface was activated. Next, after rinsing with ultrapure water, the surface was blown with dry nitrogen to align each other's crystal orientation, and then the electric power was maintained at 300 ° C. for 1 hour in order to strengthen the adhesion. Heat treatment was performed in a furnace, and the thin plate and the reinforcing plate were bonded together.

Next, the substrate 10 on the thin plate side is transferred at a speed of 35 min ⁻¹ and a lap pressure of 12.75 to 9.81 kPa with a lapping machine polishing machine (carrier: epoxy resin containing glass fiber, wrapping agent: GC # 1200 20 wt% aq). Under these conditions, the substrate is roughly polished until the thickness of the substrate is about 50 μm, or about 20 μm thick from the finished thickness. Thereafter, precision mirror polishing is performed to a set thickness by mechanochemical polishing (CMP) using a nonwoven fabric as the pad material and colloidal silica as the processing liquid. The set thickness here was 6 μm.

Next, patterning is performed with a positive photoresist mainly composed of novolak resin, and at the same time, this is used as a mask material at the time of dry etching, using an ECR-RIE (ECR310E manufactured by Anelva) mainly composed of Ar plasma as a thin plate. A ridge was formed on the surface. The width of the manufactured ridge was about 10 μm, and the depth of the ridge was set to 3 μm. Thereafter, a buffer layer and a modulation electrode were formed to obtain an optical element. The buffer layer was formed by RF magnetron sputtering using a high-purity SiO ₂ target while mixing and introducing argon gas and oxygen gas, and the modulation electrode was formed by electrolytic plating.

(Comparative Example 1)
An optical element was obtained in the same manner as in the above example except that Mg was not deposited and diffused on the reinforcing plate.

(Comparative Example 2)
As a reinforcing plate, a Mg-added coincident melt composition lithium niobate (Mg: CLN, Mg addition amount 5 mol%, substrate refractive index 2.1924 to 2.1935@633 nm TM) substrate having a thickness of 500 μm was used, and the above-described Mg deposition In addition, an optical element was obtained in the same manner as in the above example except that no diffusion was performed.

(Test method and results)
(Evaluation result 1)
Table 1 shows the surface refractive index measurement results of the reinforcing substrate obtained in Example 1 and the Mg concentration measured by EPMA.
The refractive index was measured using a prism coupler (manufactured by Metricon), and the measurement wavelengths were 633 nm and 1550 nm, respectively. The ZAF method was used for semi-quantitative analysis measurement by EPMA.

The Mg deposition thickness can be controlled at a set value of about ± 2 nm by controlling with a crystal resonator monitor. Further, as can be understood from Table 1, since the Mg deposition thickness, the amount of change in the surface refractive index after thermal diffusion, and the Mg concentration have a linear relationship, the final refractive index is controlled at about ± 2 × 10 ^−4. It turns out that it is possible.

(Evaluation result 2)
In order to measure the optical characteristics of Example 1d and Comparative Examples 1 and 2, a near field image (NFP) was observed.
In Comparative Example 1, since there was no difference in refractive index between the upper thin plate and the reinforcing plate, the light wave was not confined and the mode diameter could not be measured. In Example 1d, a good light wave was obtained together with Comparative Example 2. Confirmed the confinement. The results of Example 1d and Comparative Example 2 are shown in FIGS.

Further, in the light intensity distribution obtained from the NFP image, a range in which the light intensity is 1 / e ^{2 of the} maximum value is determined as the mode diameter of the light wave, and the value is shown in Table 2. X and Y represent a horizontal component and a vertical component, respectively.

(Evaluation result 3)
When the propagation loss of the channel type optical waveguide obtained in Example 1d and Comparative Example 2 was measured by the cutback method, the propagation loss in the wavelength 1550 nm and TM mode was 0.5 ± 0.1 dB / cm, respectively. The difference is not confirmed. The propagation loss of a channel optical waveguide having substantially the same structure using a thin plate obtained by liquid phase epitaxial growth is reported as 1.2 dB / cm by the following Non-Patent Document 3, and the difference in crystallinity of the thin plate material It is presumed that it reflects.
Kazumo Ohno et al., “Liquid Phase Epitaxially Grown LiNbO3 Thin-Film Optical Waveguide and Fabrication and Evaluation of Traveling Waveform Optical Modulator”, IEICE Transactions Journal Vol.J77-C-I No.5 237 (1994)

From the above, it is shown that the example has the same or better characteristics as the underclad even when compared with the comparative example 2, and the example uses a commercially available low-cost CLN substrate. However, it is understood that the optical element has sufficient optical performance.
Further, when Zn, Li, and Ta were thermally diffused as impurity species, it was confirmed that the refractive index of the substrate surface was decreased in proportion to the deposited film thickness for Li and Ta as in the case of Mg. Ni and Zn have a negative refractive index change in a region where the concentration is dilute, but it has been confirmed that when the concentration exceeds a certain concentration, the refractive index change turns positive and functions as an underclad.

  According to the optical element according to the present invention, an optical element that uses a thin plate on which an optical waveguide is formed and includes a reinforcing plate bonded to the back surface of the thin plate, the reinforcing plate has a function of an under cladding layer, and is inexpensive. It is possible to provide an optical element with little variation in performance.

It is the schematic which shows the prior art example of the optical modulator which is an optical element. 1 shows a cross-sectional view of an optical element according to the present invention. It is a figure which shows the manufacturing method of the optical element which concerns on this invention. It is a figure which shows the other manufacturing method of the optical element which concerns on this invention. The near field image (NFP) in the optical element of Example 1d is shown. The near field image (NFP) in the optical element of the comparative example 2 is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,10 Thin plate 2,11,14 Optical waveguide 3 Adhesive layer 4,12 Reinforcement plate 13 Groove 15 Impurity 16 Impurity diffusion area 17 Adhesive layer 18 Fixing jig

Claims (3)

A thin plate having a thickness of 20 μm or less obtained by polishing or ion beam processing, a ridge-type optical waveguide formed on the thin plate, and a light source that passes through the optical waveguide. In an optical element including a modulation electrode for modulating light and a reinforcing plate bonded to the thin plate,
The reinforcing plate is made of the same material as the thin plate, at least the refractive index of the joint surface of the reinforcing plate is lower than the refractive index of the thin plate, and the thin plate and the reinforcing plate are joined by a direct joining method. An optical element characterized by the above.   2. The optical element according to claim 1, wherein an impurity layer is formed on the reinforcing plate by using a vacuum film forming apparatus, and the impurity is diffused to reduce a refractive index. 3. The optical element according to claim 1, wherein the thin plate is made of an LN crystal substrate or an LT crystal substrate having a coincident melt composition.

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