JP2002321947A - Optical device and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized optical device having a high reliability by realizing connecting optical elements with each other without using an organic adhesive. SOLUTION: A Faraday rotator 13 is provided with reflection preventing films 17 against glass, at both the sides. Reflection preventing films 16 against air are formed to one side of the two polarizing glasses 14, respectively and then the joining faces of the polarizing glasses 14 and those of the reflection preventing films 17 against glass of the Faraday rotator 13 are adjoined to form a jointed body 18 through alkali metal element-containing layers.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technology for integrating optical elements into an optical device without using an organic adhesive and reducing the size of the optical device.

[0002]

2. Description of the Related Art In recent years, WDM (wave division) has been developed.
Due to the increase in the wavelength of on-multiplex (wavelength division multiplexing), the degree of integration of optical communication systems is increasing. As a result, there is a strong demand for miniaturization of optical devices used therein. In many cases, an optical device is configured by joining an optical element to a fixing member and combining them. However, in this method, the fixing member hinders the miniaturization of the optical device. Therefore, a method of eliminating the fixing member and directly bonding the optical elements to each other has been studied.

The simplest method of joining optical elements is
The use of an organic adhesive. However, the organic adhesive has drawbacks in that the outgassing adversely affects the laser diode, and is susceptible to high-power laser irradiation and exposure in a high-temperature, high-humidity atmosphere, and lacks reliability.

Therefore, various methods for joining optical elements without using an organic adhesive have been studied. For example, as a method of using an inorganic bonding material, low-melting glass or solder may be used. The low-melting glass is a joining glass mainly composed of a low-melting material such as Bi ₂ O ₃ or PbO, but needs to be heated to a temperature higher than the softening point of the present glass at the time of joining.
For the purpose of reducing the size of an optical device, it is effective to join the light transmitting surfaces of the optical element. However, when the low-melting glass is softened by heating, there is a problem that the anti-reflection film applied to the optical element reacts with the low-melting glass to impair the anti-reflection function. For this reason, it is considered difficult to commercialize an optical device using a low-melting glass for joining the light transmitting surfaces.

On the other hand, since the solder has no light transmitting property, it cannot be directly disposed on the light transmitting surface. Accordingly, a joining method is adopted in which the outer frame of the light transmitting surface is selectively metallized and solder is interposed only in the metallized portion. Such a joining method requires a complicated metallization step, and has a problem that a reduction in yield and an increase in cost are unavoidable.

Further, a method of directly joining without using any joining material (JP-A-7-220923 and JP-A-200-92323)
No. 0-56265). In this method, after the surface of an optical element is subjected to a hydrophilic treatment, the hydrophilic surfaces are bonded to each other. For a semiconductor, SOI (Silicon On Ins) is used.
ulator) Practical in the wafer manufacturing process. However, when this method is applied to an optical device, there are the following problems, and it has not yet been put to practical use.

[0007] This direct joining method largely depends on the shape and physical properties of the article to be joined. For example, in the case of warpage, it is desirable that the curvature radius is several hundred meters or more. Further, it is said that the surface roughness of the article to be joined is desirably Ra = 0.3 nm or less.

[0008] Further, the difference in the thermal expansion coefficient between the objects to be bonded is greatly affected. However, there are few optical devices that satisfy the above restrictions. For example, an iron-based garnet or the like, which is one of the optical elements generally used in an optical device, has a stress distribution in a thickness direction, and thus often involves a large warp. Further, since the polarizing glass has a structure in which silver particles are dispersed in glass, it is difficult to control the surface roughness. Furthermore, the thermal expansion coefficients of these optical elements often differ greatly depending on the material, and the difference in the thermal expansion coefficient between the objects to be bonded tends to increase. For this reason, it is very difficult to apply the direct bonding technology to optical devices. As described above, at present, it is extremely difficult to easily and inexpensively manufacture an optical device in which optical elements are joined without using an organic adhesive.

[0009]

SUMMARY OF THE INVENTION Accordingly, the present invention has been made in view of such problems, and realizes a small-sized and highly reliable optical element by realizing joining of optical elements without using an organic adhesive. The main object of the present invention is to provide an optical device.

[0010]

In order to solve the above-mentioned problems, an optical device of the present invention comprises at least one surface of an optical element joined to another optical element, and transmits light through the joint surface. In a functional optical device, a layer containing an alkali metal element is provided on a joint surface of the optical element (claim 1).

As described above, if the layer containing an alkali metal element is provided on the bonding surface of the optical element constituting the optical device, sufficient bonding strength between the optical elements can be obtained without using an organic adhesive. A small and highly reliable optical device that can be easily realized, does not generate outgas and does not deteriorate the bonding surface, and can be provided at low cost.

In this case, the alkali metal elements are Li, N
It is preferable to use one or more selected from a, K, and Rb (claim 2). The thickness of the alkali metal element-containing layer provided on the bonding surface of the optical element is 0.1 n
m and less than 100 nm, and the content of the alkali metal element is preferably 0.5% by weight or more and less than 60% by weight in terms of oxide (claim 3).

Thus, the alkali metal elements are Li, N
If one or two or more selected from a, K, and Rb are used, strong bonding can be achieved. If the thickness of the alkali metal element-containing layer is 0.1 nm or more and less than 100 nm,
Sufficient bonding strength can be obtained. Further, the content of the alkali metal element-containing layer is adjusted to 0.5% by weight in terms of oxide.
When the content is less than 60% by weight, a sufficient bonding strength can be obtained, and the bonding surface of the optical element at the time of bonding does not deteriorate and the moisture resistance does not deteriorate.

In this case, it is preferable that the optical element is made of at least polarizing glass and / or bismuth-substituted iron garnet. Thus, for example, when joining a polarizing glass polarizer and a bismuth-substituted iron garnet Faraday rotator to form a joined body,
The alkali metal element-containing layer functions effectively as a bonding agent without affecting optical properties.

In this case, it is desirable that the difference in the coefficient of thermal expansion between the optical elements bonded to each other is not more than 2 × 10 ⁻⁶ / ° C. (Claim 5). An optical material having an intermediate value of the difference can be provided between the respective optical elements.

As described above, the extinction ratio of the optical element firmly bonded via the alkali metal element-containing layer may be reduced when a stress is applied. The extinction ratio deteriorates with a change in the temperature of the device, and the difference in the coefficient of thermal expansion between the optical elements becomes remarkable at 2 × 10 ⁻⁶ / ° C. or more. It is desirable to do. When the value is equal to or more than this value, deterioration of characteristics due to temperature change is prevented by providing an optical material having an intermediate value of the difference between the coefficients of thermal expansion of both optical elements, for example, a buffer glass between the optical elements. be able to.

Next, in the method for manufacturing an optical device according to the present invention, the optical elements whose bonding surfaces have been made hydrophilic are brought into close contact with each other via a solution in which an alkali metal element is dissolved, and the solution is dried and then heat-treated. In this case, the optical elements are joined to each other (claim 7). As described above, in the small and highly reliable optical device, the bonding between the bonding surfaces of the optical elements is brought into close contact with each other through a solution containing an alkali metal element dissolved therein without using an organic adhesive, and the bonding is performed by heat treatment. It can be manufactured by

In this case, when the alkali metal element is dissolved in the form of a hydroxide, the solution in which the alkali metal element is dissolved is preferably adjusted by adding a compound having a pH adjusting effect by coexistence with the alkali metal element ( Claim 8). As described above, by adding a compound having a pH adjusting function, strong alkalinity can be suppressed, and deterioration of the optical element surface at the time of bonding and a decrease in moisture resistance and bonding strength of the obtained bonded body can be prevented.

In this case, the heat treatment is desirably performed in a temperature range of 80 to 200 ° C. (claim 9), and the heat treatment can be performed in a reduced pressure atmosphere or an atmosphere containing hydrogen (claim 10). Thus, a sufficient bonding force can be obtained, for example, by performing heat treatment within the above temperature range for several hours. The heat treatment atmosphere does not matter even in the air, but a reduced pressure atmosphere or an atmosphere containing hydrogen is more preferable, and a strong bonding strength can be obtained.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited thereto. The present inventors, by joining optical elements without using an organic adhesive, in order to provide a small and highly reliable optical device at low cost, it is possible to join the light transmission surface, In addition, bonding that can provide sufficient bonding strength to optical elements having warpage, surface roughness, and a difference in coefficient of thermal expansion without damaging the antireflection film of various optical elements at the time of bonding. As a technique, it has been found that it is extremely effective to perform bonding via an alkali metal element-containing layer, and the present inventors have completed the present invention by carefully examining various conditions relating to bonding.

FIG. 1 shows the flow of the joining method of the present invention. First, after the bonding surface of the optical element is sufficiently washed (step), a hydrophilic treatment (step) is performed. For the cleaning, ordinary wet cleaning is effective. However, it is more effective to use a short wavelength ultraviolet treatment (UV treatment) or a plasma treatment in combination. In the hydrophilization treatment, ammonia peroxide (a mixture of aqueous ammonia, hydrogen peroxide, and pure water), nitric acid, or a diluted solution of hydrochloric acid or a diluted solution of these, which is generally used in a semiconductor SOI wafer process, is used. A solution to which a hydrogen peroxide solution is added is effective. Next, washing with pure water is performed to remove the hydrophilizing treatment liquid. After washing with pure water, IPA
It is desirable to dry by a steam drying method or a spin dryer to prevent uneven drying (step).

A solution containing an alkali metal element is applied (processed) to the bonding surface of the pre-processed optical element thus obtained and bonded (process). Alkali metal elements include most of 1A such as Li, Na, K, and Rb.
Group elements can be used, but Na and K are particularly easy to use. In addition, when the solution is prepared, it is easy to adjust it by adding it as a hydroxide. When the solution is composed only of the hydroxide of the alkali metal element, since the alkalinity is strong, the surface of the optical element may be deteriorated, and the moisture resistance and the bonding strength of the obtained bonded body may be reduced. . Therefore,
When the aqueous solution evaporates, SiO ₂ , TiO ₂ , Al ₂ O
_3, B ₂ O _3, compounds capable of forming a phosphate or the like, for example, it is desirable to add the silicon alkoxide or the like at the same time. These additives are not limited to the above compounds, and have an appropriate pH adjusting action (pH = 8 to 12) by coexistence with an alkali metal element.
Any compound may be used as long as it is homogeneously mixed or combined with the alkali metal element when the solvent in the solution evaporates. Further, a plurality of compounds may be mixed and used.

The concentration of the alkali metal element in the solution in which the alkali metal element is dissolved or the concentration of the alkali metal element with respect to the solid substance is determined by calculating the concentration in the finally obtained layer containing the alkali metal element in terms of oxide. Is preferably 0.5% by weight or more and less than 60% by weight.
More preferably, it is 5 to 40% by weight. If the concentration of the alkali metal element in terms of oxide is less than 0.5% by weight, sufficient bonding strength cannot be obtained. On the other hand, when the content is 60% by weight or more, the bonding surface of the optical element is deteriorated at the time of bonding, and the bonding strength and the moisture resistance are likely to be reduced. The solvent used for the above solution is desirably used alone or as a mixture of liquids mainly composed of polar molecules such as water and ammonia.

A sufficient bonding force can be obtained if the thickness of the finally obtained alkali metal element-containing layer is 0.1 nm or more, but the surface roughness (Ra) is as relatively small as about 1 nm like a polarizing glass. When joining a rough material, a more desirable result is obtained when the thickness is set to 1 nm or more. on the other hand,
When the film thickness is 100 nm or more, there is a tendency that the bonding strength decreases as the film thickness increases. Therefore, it is preferable that 0.
1 nm or more and less than 100 nm, more preferably 1n
m and less than 100 nm. In consideration of the above, the application amount or concentration of the solution in which the alkali metal element is dissolved is determined.

When the optical element bonded by the above procedure is naturally dried or vacuum dried, it is fixed with a weak bonding force (step). A necessary and sufficient bonding force can be obtained by heat-treating this at a temperature of about 80 to 200 ° C. for several hours (step). At this time, if the temperature raising rate in the heat treatment step is too fast, there is a possibility that peeling of the bonding surface occurs during the temperature raising. Therefore, it is desirable to set the heating rate to 20 ° C./h or less. Further, the atmosphere during the heat treatment is not problematic in the air, but is more preferably a reduced pressure atmosphere or an atmosphere containing hydrogen. Through the above steps, the joining of the optical elements is completed.

In an optical device, such as an optical isolator, which is easily affected by distortion of the optical element, the extinction ratio tends to decrease when stress is applied to the bonded optical element. Since the devices joined by the method of joining via the alkali metal element-containing layer are firmly fixed, a stress proportional to the difference in the coefficient of thermal expansion between the optical elements is applied as the temperature of the device changes.

In such a situation, the extinction ratio of the optical device deteriorates with a change in temperature. The temperature deterioration of the extinction ratio is caused by a difference in thermal expansion coefficient between the optical elements of 2 ×
It tends to be significant at 10 ^-6 / ° C or higher. Therefore, it is better to select one having a difference in thermal expansion coefficient of 2 × 10 ⁻⁶ / ° C. or less between the optical elements to be joined. Has a coefficient of thermal expansion of
It is also desirable to join and integrate via an optically homogeneous and transparent material.

For example, when a Faraday rotator having a coefficient of thermal expansion of 10 × 10 ^-6 / ° C. and a polarizing glass having a coefficient of thermal expansion of 6.5 × 10 ^-6 / ° C. are integrally joined, the question of the Faraday rotator and the polarizing glass is made. If an optical glass having a coefficient of thermal expansion of about 8 × 10 ⁻⁶ / ° C. is provided, it is possible to prevent characteristic deterioration due to a temperature change. For this purpose, the number of optical glasses provided between the optical elements does not need to be one, and it is more effective to provide a plurality of optical glasses by changing the coefficient of thermal expansion stepwise.

Before carrying out the above joining process,
It is preferable that an antireflection coat optimized for the refractive index of the opposing optical element or alkali metal element-containing layer be applied to the bonding surface of the optical element in advance.

[0030]

The present invention will now be described in more detail with reference to examples of the present invention.
However, the present invention is not limited to these.
There is no. (Example 1) An optical element used for bonding was a polarizer (a polarizing element).
Lath) and Faraday rotator (bismuth-substituted iron garnet)
G). Various physical properties are as follows.Polarized glass /Faraday rotator  Surface roughness (Ra (nm)): 1 / 0.5 Warpage (radius of curvature (m)): 60/20 Linear thermal expansion coefficient (× 10^-6/ ° C): 6.5 / 10 Shape (mm): 15 × 15 × 0.2t / 15 × 15 × 0.35t The above data was measured after applying an antireflection film.
Things.

A Faraday rotator (Bi-substituted iron garnet adjusted to θf = 45 ° at a wavelength of 1.31 μm) has a glass antireflection film (TiO ₂ / Al ₂ O ₃ ) on both surfaces.
/ SiO ₂ ). On the other hand, the polarizing glass has an antireflection film against air (Al ₂ O ₃ / SiO ₂ ) only on the non-bonding surface side.
Was given. These antireflection films have a wavelength of 1.31 μm.
Optimized with m.

FIG. 2 shows the structure of the optical element to be joined and integrated. The antireflection film 17 for glass is applied to both surfaces of the Faraday rotator 13. Separately, after applying an anti-reflection film 16 to one surface of two polarizing glasses 14, the bonding surface of the polarizing glass 14 and the bonding surface of the anti-reflection film 17 of the Faraday rotator are connected to each other via an alkali metal element-containing layer. Then, the joined body 18 is manufactured.

The joining procedure of these optical elements was performed according to the flow shown in FIG. The manufacturing conditions for each step are shown below. Washing: After UV (ultraviolet) treatment with a low-pressure mercury lamp, washing with pure water (US (UltraSonic, ultrasonic) washing). Hydrophilic treatment: ammonia water: hydrogen peroxide solution: pure water =
Immerse in 1: 1: 4 ammonia peroxide. Washing and drying: IPA steam drying is performed after pure water washing (US washing). Application of alkali metal element-containing solution: Na ₂ O: SiO ₂
= 2: 8 (weight ratio) aqueous solution is applied to the bonding surface. The compounds used in preparing the aqueous solution are NaOH and tetraethoxysilane. 2n as alkali metal element containing layer thickness
An amount of aqueous solution corresponding to m is applied. Lamination: Lamination before the coating liquid dries. The polarization directions of the two polarizing glasses are adjusted so as to be 45 deg with each other. Drying: After bonding, vacuum drying is performed for 24 hours. Heat treatment: performed at 110 ° C. for 10 hours in a hydrogen atmosphere at 0.2 atm. The heating rate was 4 ° C./h. The bonding is completed by this heat treatment.

The bonded body bonded under the above conditions was cut into a 1 × 1 mm chip shape by a dicer, and the bonded surface was observed. No defects such as peeling of the bonded interface were observed. Further, the chip was boiled in pure water for 1 hour, and the bonding interface was observed. No occurrence of defects such as peeling of the bonding interface was observed. From the above, it was confirmed that the bonded and integrated optical element of the present invention had necessary and sufficient strength.

Next, the joined body obtained under the above conditions was set in a cylindrical magnet to form a joined optical isolator. FIG. 3 shows a configuration example of the junction optical isolator 10. The Faraday rotator 13 is provided with anti-glass antireflection films 17 on both sides, and the non-joining surface is provided with a polarizing glass 14 provided with an anti-air antireflection film 16. The body 18 is set in the cylindrical magnet 15 to form the junction type optical isolator 10.

The forward insertion loss and the backward insertion loss of this optical isolator were measured with a laser beam having a wavelength of 1.31 μm. As a result, the forward insertion loss was 0.12 dB, and the reverse insertion loss was 48.6 dB. From these results, it was confirmed that the junction optical isolator obtained in the present invention had optically sufficient characteristics.

Example 2 A joined body was produced under the same conditions as in Example 1 except that only the thickness of the alkali metal element-containing layer was variously changed, and the bonding strength was evaluated. The thickness of the alkali metal element-containing layer was adjusted by the concentration and the amount of the alkali metal solution applied in the step of FIG. The composition of the aqueous solution used was the same as in Example 1.

As shown in Table 1, it has been confirmed that if the thickness of the alkali metal element-containing layer is 0.1 nm or more, sufficient bonding strength can be exhibited for ordinary applications. However, it was confirmed that when an optical device that withstands a bad environment such as a boiling test is required, it is desirable to set the film thickness to 1 nm or more and less than 100 nm.

[0039]

[Table 1]

Example 3 Example 1 was repeated except that the concentration of the alkali metal element in the alkali metal element-containing layer was variously changed.
A bonded body was produced under the same bonding conditions as in Example 1, and the bonding strength was evaluated in the same manner as in Example 1. The results are shown in Table 2. The alkali metal element concentration of the alkali metal element-containing layer was adjusted by the oxide conversion ratio (weight ratio) of the alkali metal element and other solid elements in the alkali metal element solution applied in the alkali metal element application step of FIG. The thickness of the alkali metal element-containing layer was set to 2 nm.

[0041]

[Table 2]

As shown in Table 2, if the alkali metal element concentration of the alkali metal element-containing layer is in the range of 0.5 to less than 60% by weight, sufficient bonding strength and production yield can be exhibited for ordinary applications. Was confirmed. However, if you need an optical device that can withstand adverse environments such as boiling tests,
It has been found that it is desirable to set the concentration to 5 to 40% by weight.

(Example 4) A junction type optical isolator having the following structure, in which a buffer glass was incorporated, was manufactured so that the difference in the coefficient of thermal expansion between the optical elements was reduced. That is, it has a configuration of polarizing glass / buffer glass / Faraday rotator / buffer glass / polarizing glass. As the buffer glass, one having a refractive index equivalent to that of the polarizing glass and a coefficient of thermal expansion of 8 × 10 ⁻⁶ / ° C. was selected. The optical elements other than the buffer glass used were exactly the same as those in Example 1.

With this configuration, the difference in the coefficient of thermal expansion between the optical elements is as follows, and the difference in the coefficient of thermal expansion of the optical isolator of the first embodiment is smaller than 3.5 × 10 ⁻⁶ / ° C. it can. Between polarizing glass and buffer glass: 1.5 × 10 ⁻⁶ / ° C., between buffer glass and Faraday rotator: 2 × 10 ⁻⁶ / ° C. In the above configuration, the refractive index of the buffer glass is the same as that of the polarizing glass. Therefore, the antireflection film of the buffer glass can be omitted.

FIG. 4 shows the structure of an optical element in which a buffer glass is provided and bonded and integrated. The antireflection film 17 for glass is applied to both surfaces of the Faraday rotator 13. Next, the two buffer glasses 12 are bonded to the bonding surface of the antireflection film 17 with an alkali metal element-containing layer interposed therebetween. Separately, an antireflection film 16 for air reflection is provided on one surface of two polarizing glasses 14.
Then, the bonding surface of the polarizing glass 14 and the bonding surface of the buffer glass 12 are bonded to each other to manufacture a bonded body 19.

Next, the joined body obtained under the above conditions was set in a cylindrical magnet to produce a joined optical isolator according to Example 1, and the temperature dependence of the reverse insertion loss was evaluated.
For comparison, the same evaluation was performed on the optical isolator having the standard configuration manufactured in Example 1. The difference between the coefficients of thermal expansion of the standard constituent optical isolators is 3.5 × 10 ⁻⁶ / ° C. between the polarizing glass and the Faraday rotator.

As shown in FIG. 5, the optical isolator having the standard configuration (Embodiment 1) has a temperature of about 1.1 ° C. at an environmental temperature of 110 ° C.
The degradation of the reverse insertion loss (corresponding to the extinction ratio) of dB was observed, whereas the degradation of the extinction ratio of the optical isolator using the buffer glass was only about 0.3 dB.
From this, it was confirmed that the temperature dependence of the extinction ratio can be significantly improved by introducing the buffer glass and reducing the difference in the coefficient of thermal expansion to 2 × 10 ⁻⁶ / ° C. The graph of FIG. 5 shows only the effect of the difference in the coefficient of thermal expansion by subtracting the temperature dependence of the extinction ratio of the isolator itself.

The present invention is not limited to the above embodiment. The above embodiment is an exemplification, and has substantially the same configuration as the technical idea described in the scope of the claims of the present invention. It is included in the technical scope of the invention.

[0049]

As described above in detail, according to the present invention, since no organic adhesive is used, various optical elements can be easily bonded and a strong bonding strength can be obtained. It is possible to provide a small and highly reliable optical device at low cost without causing deterioration of the bonding surface.

[Brief description of the drawings]

FIG. 1 is a flowchart showing an example of a joining method according to the present invention.

FIG. 2 is an explanatory view showing a configuration of an optical element to be joined in Example 1 and a joined body.

FIG. 3 is a cross-sectional view of the junction optical isolator formed in Example 1.

FIG. 4 is an explanatory diagram showing a configuration and a joined body of an optical element into which a buffer glass formed in Example 4 is introduced.

FIG. 5 is a diagram showing the relationship between the coefficient of thermal expansion between optical elements and the temperature dependence of optical characteristics (reverse insertion loss degradation (dB)).

[Explanation of symbols]

10: junction type optical isolator, 12: buffer glass, 13: Faraday rotator, 14: polarizing glass,
15: Magnet, 16: Anti-reflection coating against air, 17: Anti-reflection coating against glass, 18, 19 ... Joint.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H079 AA03 BA02 CA06 DA13 DA27 EA13 EA27 HA21 KA05 2H099 AA01 BA02 CA11 DA05 4G061 AA02 BA07 CA01 CB04 CB12 CD12 DA05 DA09 DA10 DA22 DA32 DA43

Claims (10)

[Claims] At least one surface of an optical element is bonded to another optical element, and the optical device functions by transmitting light through the bonded surface. The bonding surface of the optical element contains an alkali metal element. An optical device comprising a layer. 2. The method according to claim 1, wherein the alkali metal element is Li, Na,
The optical device according to claim 1, wherein the optical device is one or more selected from K and Rb. 3. The thickness of the alkali metal element-containing layer provided on the bonding surface of the optical element is 0.1 nm or more and less than 100 nm, and the content of the alkali metal element is 0.5% by weight or more and 60% or more in terms of oxide. The optical device according to claim 1 or 2, wherein the content is less than% by weight. 4. The optical device according to claim 1, wherein the optical element is made of at least polarizing glass and / or bismuth-substituted iron garnet. 5. The method according to claim 1, wherein a difference between thermal expansion coefficients of the optical elements bonded to each other is 2 × 10 ⁻⁶ / ° C. or less. Optical device. 6. The optical element according to claim 1, wherein an optical material having an intermediate value of a difference between thermal expansion coefficients of the optical elements bonded to each other is provided between the optical elements. The optical device according to claim 1. 7. An optical element whose bonding surface has been subjected to hydrophilic treatment is brought into close contact with each other via a solution in which an alkali metal element is dissolved,
A method for manufacturing an optical device, comprising drying the solution and heat-treating the solution to join the optical elements together. 8. The solution in which the alkali metal element is dissolved, wherein the alkali metal element is dissolved as a hydroxide,
8. The method according to claim 7, wherein a compound having a pH adjusting effect is added by coexisting with an alkali metal element.
3. The method for manufacturing an optical device according to item 1. 9. The method according to claim 7, wherein the heat treatment is performed in a temperature range of 80 to 200 ° C. 10. The method according to claim 7, wherein the heat treatment is performed in a reduced pressure atmosphere or an atmosphere containing hydrogen.