JP4267544B2 - Light modulator - Google Patents

Description

  The present invention belongs to the field of optical modulators that are high in speed, low in driving voltage, and good in production yield.

An optical waveguide and a traveling wave electrode are formed on a substrate having a so-called electrooptic effect (hereinafter, the lithium niobate substrate is abbreviated as an LN substrate) whose refractive index is changed by applying an electric field, such as lithium niobate (LiNbO ₃ ). The formed traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) is applied to a 2.5 Gbit / s, 10 Gbit / s large capacity optical transmission system because of its excellent chirping characteristics. Yes. Recently, application to an ultra large capacity optical transmission system of 40 Gbit / s is also being studied, and it is expected as a key device.

The LN optical modulator light equivalent refractive index n _o and propagating the equivalent refractive index n _m and the optical waveguide of the electric signal propagated through traveling wave electrode, the propagation loss of electric signal is decisive response band of the optical modulator play a role, among them, the equivalent refractive index n _m and the light of the equivalent refractive index n _o of the electric signal and its value is significantly different, it has become a decisive factor that limits the response band of the optical modulator . Therefore, research and development are underway to match these (speed matching between electrical signals and light, or simply speed matching).

  As for the z-cutLN substrate, a ridge structure has been devised and its effectiveness has been demonstrated (for example, see Patent Document 1). However, the actual situation is that the x-cutLN substrate does not yet have a decisively excellent structure.

  A technique for achieving speed matching between an electric signal and light in this x-cut LN substrate is disclosed in Patent Document 2. FIG. 5 is a perspective view of the first embodiment, and FIGS. 6 and 7 are sectional views taken along line A-A ′ in FIG. 5. 6 and 7 correspond to cross-sectional views of a so-called interaction portion in which an electrical signal and light interact. FIG. 7 is obtained by adding electric lines of force and the like to FIG.

  In the figure, 1 is an x-cut LN substrate, 2 is a low dielectric constant adhesive layer made of an adhesive, and 3 is a holding substrate. An optical waveguide 4 is formed by thermally diffusing Ti at 1050 ° C. for about 10 hours after depositing Ti on the x-cutLN substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Reference numerals 4a and 4b denote optical waveguides in a portion where an electrical signal and light interact (referred to as an interaction portion), that is, two arms of a Mach-Zehnder optical waveguide. The traveling wave electrode 5 includes a central conductor 5a and ground conductors 5b and 5c. 6a and 6b are electric lines of force of the electric signal, and the overlap integral of the power of the light propagating through these electric lines of force 6a and 6b and the optical waveguides 4a and 4b is the product Vπ of the half-wave voltage Vπ and the interaction length L.・ L is determined.

  As can be seen from FIG. 6, the x-cutLN substrate 1 has the optical waveguide 4, the central conductor 5a of the traveling wave electrode 5, and the ground conductors 5b and 5c formed on one surface 1a thereof, and functions as a main substrate. Yes. The low dielectric constant adhesive layer 2 serves to fix the one surface 3a of the holding base 3 having sufficient mechanical strength to the other surface 1b of the x-cutLN substrate 1.

Now, as shown in FIG. 7, when the direction parallel to one surface 1a of the x-cutLN substrate 1 is the x direction and the perpendicular direction is the y direction, the relative permittivity ε _{rx in the} x direction and the relative permittivity in the y direction ε _ry is very large at 28 and 43, respectively. Therefore, usually, it made equivalent refractive index n _m of the electric signal and degree from 3.0 to 4.2, a value larger than the light of the equivalent refractive index n _o (approximately 2.15).

Therefore, in this conventional embodiment, the low dielectric constant adhesive layer 2 has a low relative dielectric constant of 3 to 5, and the electric lines of force 6a and 6b are passed through the region of the low dielectric constant adhesive layer 2. lowering the effective refractive index n _m of the electric signals by, and to approach the optical equivalent refractive index n _o. In order to simplify the explanation, the lines of electric force existing in the air are not shown in FIG.

In order to pass the region of the low dielectric constant adhesive layer 2 through the electric lines of force 6a and 6b, it is necessary to reduce the thickness T ₁ of the x-cutLN substrate ₁ to about 10 μm. However, the mechanical strength becomes extremely weak only with the x-cutLN substrate 1 having such a thickness, and it is a fact that the x-cutLN substrate 1 is fixed to a package housing (not shown) and used as an optical modulator. It becomes impossible.

  To avoid this problem, it is necessary to increase the mechanical strength. Therefore, as described above, the holding substrate 3 having sufficient mechanical strength using the low dielectric constant adhesive layer 2 on the other surface 1b of the x-cutLN substrate 1 where the optical waveguide 4 and the traveling wave electrode 5 are not formed. By fixing the surface 3a, the mechanical strength of the entire LN optical modulator is increased. Further, the other surface 3b of the holding base 3 is fixed to a package housing (not shown) so that it can be used for communication. According to Patent Document 2, the x-cutLN substrate itself is used as the holding base 3.

JP-A-4-288518 JP 2003-215519 A

However, the prior art disclosed in Patent Document 2 has the following problems to be solved. As described above, the dielectric constant of the low dielectric constant adhesive layer 2 is as low as 3 to 5, and the equivalent refractive index n of the electric signal is obtained by passing the electric lines of force 6a and 6b through the region of the low dielectric constant adhesive layer 2. lowering _m, so that close to the light of the equivalent refractive index n _o. However, the electric lines of force 6 a and 6 b penetrate through the low dielectric constant adhesive layer 2 having a thickness T ₂ of usually about 30 μm and reach the holding base 3.

As described above, an x-cutLN substrate is used as the material of the holding base 3. Since that is the dielectric constant of the supporting body 3, like the x-cutLN substrate 1 whose traveling wave electrode 5 and the light guide 4 is formed larger, the equivalent refractive index n _m of the electric signal from the equivalent refractive index n _o of the light Also tends to be large. In particular, when the width S of the central conductor 5a of the traveling wave electrode 5 and the gap W between the central conductor 5a and the ground conductors 5b and 5c are as large as several tens of μm, the way in which the electric lines of force 6a and 6b spread is further increased. Thus, the low dielectric constant adhesive layer 2 is penetrated and reaches the holding base 3. In this case the size of the traveling wave electrode is large, the equivalent refractive index n _m of the electrical signal is more increased, there is a problem that deviates greatly from the light of the equivalent refractive index n _o.

Note that an alumina substrate or a quartz substrate may be used as the holding substrate 3 instead of the x-cutLN substrate having a large relative dielectric constant. However, they have a high relative dielectric constant is low dielectric constant as compared to the adhesive layer 2 relative dielectric constant is 3 to 5 (about 10 back and forth), is inevitable also tends equivalent refractive index n _m of the electrical signal increases .

  Furthermore, an alumina substrate or a quartz substrate substrate has a thermal expansion coefficient several orders of magnitude larger than that of the x-cutLN substrate 1. Therefore, when these are used as the holding substrate 3, stress is generated in the LN substrate with a change in temperature, and the bias voltage superimposed on the electric signal drifts, or in the worst case, the x-cut LN substrate 1 itself is It can be destroyed.

  Further, in the conventional embodiment shown in FIGS. 5 to 7, since the holding base 3 is separately used in addition to the x-cut LN substrate 1 on which the optical waveguide 4 and the traveling wave electrode 5 are formed, it is natural that In particular, the member cost of the holding base 3 and the processing cost for processing into the shape are required. Furthermore, a process for bonding and fixing the one surface 3a of the holding base 3 to the other surface 1b of the LN substrate 1 on which the optical waveguide and the traveling wave electrode are formed is required, resulting in an increase in cost. It was.

  Further, as an important problem, there is a problem that air easily enters between the low dielectric constant adhesive layer 2 and the one surface 3a of the holding base 3 during the adhesive fixing. When air enters between the low dielectric constant adhesive layer 2 and the one surface 3a of the holding substrate 3, the air that has entered between the one surface 3a of the holding substrate 3 during the subsequent high temperature test. Was thermally expanded, and the x-cutLN substrate 1 having a thickness as thin as about 10 μm and low mechanical strength was ruptured and destroyed, resulting in a decrease in yield.

In order to solve the above problems, in the optical modulator according to claim 1 of the present invention,
A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a central conductor formed on one surface side of the substrate for applying a voltage for modulating the light And a traveling wave electrode made of a ground conductor, and a low dielectric constant adhesive layer formed by bonding to the substrate on the other surface side of the substrate and made of a material having a dielectric constant lower than that of the substrate. A waveguide, an input optical waveguide for entering the light, an interactive optical waveguide for modulating the phase of the light by applying the voltage between the center conductor and the ground conductor, and the mutual In an optical modulator comprising an output optical waveguide that emits signal light generated as a result of modulation in the working optical waveguide,
The low dielectric constant adhesive layer is configured such that electric lines of force other than in the air pass through the substrate and the low dielectric constant adhesive layer among electric lines of electric signal propagating in the vicinity of the interaction optical waveguide. the lower the equivalent refractive index of the electrical signals, together with the thickness to approach the equivalent refractive index of the light propagating through the optical waveguide is set to a predetermined thickness, said substrate of said low-dielectric adhesive layer With mechanical strength to suppress damage to the substrate only with the low dielectric constant adhesive layer in a state where nothing is bonded to the reverse surface that is the surface opposite to the bonded surface ,
The thermal expansion coefficient of the low-dielectric adhesive layer is at ^{0.5 × 10 -5 / K~3 × 10} -5 / K,
Further, the opposite surface of the low dielectric constant adhesive layer is substantially flat and is fixed to the package housing on the opposite surface .

In the optical modulator according to claim 2 of the present invention,
A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a central conductor formed on one surface side of the substrate for applying a voltage for modulating the light And a traveling wave electrode made of a ground conductor, and a low dielectric constant adhesive layer formed by bonding to the substrate on the other surface side of the substrate and made of a material having a dielectric constant lower than that of the substrate. A waveguide, an input optical waveguide for entering the light, an interactive optical waveguide for modulating the phase of the light by applying the voltage between the center conductor and the ground conductor, and the mutual In an optical modulator comprising an output optical waveguide that emits signal light generated as a result of modulation in the working optical waveguide,
In the low dielectric constant adhesive layer, the electric lines of force other than in the air pass through only the substrate and the low dielectric constant adhesive layer among the electric force lines of the electric signal propagating in the vicinity of the interaction optical waveguide. electric equivalent refractive index of the air signal is lowered, together with the thickness to approach the equivalent refractive index of the light propagating through the optical waveguide is set to a predetermined thickness, the substrate of the low-dielectric adhesive layer by Has a mechanical strength that suppresses damage to the substrate only with the low dielectric constant adhesive layer in a state where nothing is adhered to the opposite surface that is the surface opposite to the surface to which the material is adhered ,
The thermal expansion coefficient of the low-dielectric adhesive layer is at ^{0.5 × 10 -5 / K~3 × 10} -5 / K,
Further, the opposite surface of the low dielectric constant adhesive layer is substantially flat and is fixed to the package housing on the opposite surface .

In the optical modulator according to claim 3 of the present invention,
3. The optical modulator according to claim 1, wherein the thickness of the substrate is 50 [mu] m or less.

In the optical modulator according to claim 4 of the present invention,
4. The optical modulator according to claim 1, wherein the other surface of the substrate is substantially flat.

In the optical modulator according to claim 5 of the present invention,
5. The optical modulator according to claim 1, wherein the substrate is an x-cut lithium niobate substrate.

In the optical modulator according to claim 6 of the present invention,
5. The optical modulator according to claim 1, wherein the substrate is a z-cut lithium niobate substrate.

In the optical modulator according to claim 7 of the present invention,
6. The optical modulator according to claim 1, wherein the thickness of the substrate, the relative dielectric constant of the low dielectric constant adhesive layer, the width and thickness of the center conductor, the thickness of the ground conductor, and the center conductor and the ground The gap between the conductor and the conductor is characterized in that each value is set so that the equivalent refractive index of the electric signal propagating through the traveling wave electrode is close to the equivalent refractive index of the light propagating through the optical waveguide. Yes.

In the optical modulator according to claim 8 of the present invention,
The optical modulator according to claim 6, wherein the thickness of the substrate, the relative dielectric constant of the low dielectric constant adhesive layer, the width and thickness of the center conductor, the thickness of the ground conductor, and the center conductor and the ground conductor The gap between them and the thickness of the buffer layer were set so that the equivalent refractive index of the electric signal propagating through the traveling wave electrode was close to the equivalent refractive index of the light propagating through the optical waveguide. It is characterized by that.

In the present invention, the low dielectric constant adhesive layer, which has been considered as a conventional adhesive layer, is made thick and a holding base is made of a strong material so that it has the function of a conventional holding substrate. A low dielectric constant adhesive layer having a low relative dielectric constant of about 3 to 5 serving as a holding base exists thickly under the substrate on which the optical waveguide and traveling wave electrode are formed. As a result, lowering the effective refractive index n _m of the electric signals, not only it is easy to approach the light of the equivalent refractive index n _o, there is no variation in the setting of the equivalent refractive index n _m of the electrical signal, the optical modulation band The yield about is significantly improved.
In addition, since the holding substrate which is conventionally manufactured by the x-cut LN substrate is not necessary, not only can the cost of the optical modulator be reduced by reducing the number of components, but also the other surface 1b of the LN substrate can be realized. The step of adhering and fixing the holding substrate through the low dielectric constant adhesive layer 2 becomes unnecessary, and the cost can be further reduced. Furthermore, since there is no particular bonding process, the yield is reduced by the entry of air between the low dielectric constant adhesive layer 2 and the one surface 3a of the holding base 3 which has been a problem when bonding and fixing. There is an advantage that it can be solved completely. The present invention has an advantage that it can be applied not only to an x-cut LN substrate but also to a z-cut LN substrate.

Hereinafter, embodiments of the present invention will be described. However, since the same numbers as those in the conventional embodiments shown in FIGS. 5 to 7 correspond to the same function units, the description of the function units having the same numbers is omitted here. To do.
[First Embodiment]

  FIG. 1 is a perspective view of the optical modulator according to the first embodiment of the present invention, and FIG. 2 is a sectional view taken along line B-B ′ of FIG. 1. As can be seen from FIGS. 1 and 2, in this embodiment, there is no holding substrate 3 made of an x-cut LN substrate having a high relative dielectric constant shown in FIGS. 5 to 7, which is a conventional embodiment. Instead, the holding base 7 is provided below the x-cutLN substrate 1. One surface 7 a of the holding base 7 is bonded to the other surface 1 b of the x-cutLN substrate 1. The holding base 7 is made of an adhesive having a low dielectric constant, like the conventional low dielectric constant adhesive layer 2 shown in FIGS. However, unlike the prior art, it is made of a material having high strength and is thick. Thereby, in addition to the function of adhesion, it also has a function of sufficient mechanical strength that the holding substrate 3 of the prior art has.

Here, x-cutLN each about 10μm thickness T ₇ of the holding base 7 of the thickness T ₁ of the substrate 1 made of a low dielectric constant adhesive, of about 500 [mu] m, the thickness T ₅ of the traveling-wave electrodes was 20 [mu] m. The holding base 7 was solidified by irradiating with ultraviolet rays after spin-coating an ultraviolet curable adhesive with a spinner. It was possible to perform overcoating by spin coating with a spinner, and the holding base 7 of about 300 μm could be easily manufactured.

In the present invention, an epoxy or acrylic adhesive is used as the holding base 7. As a method for solidifying the adhesive, ultraviolet curing, heating, or a combination thereof may be used. It should be noted that since the low dielectric constant adhesive layer is used as the holding base 7, the thermal expansion coefficient, particularly the mechanical strength, etc., that is, the adhesive that can satisfy the conditions required for the holding substrate 3 of the prior art. It is important to use materials. The adhesive material is desirably shrinkage during solidification and less than 2%, and a thermal expansion coefficient of the selected in the range of at ^{0.5 × 10 -5 / K~3 × 10} -5 / K good.

  The other surface 1b of the x-cutLN substrate 1 was made substantially flat by polishing. The same applies to substrates of other orientations such as z-cut or y-cut as the main substrate. Here, the term “substantially flat” means flat including unevenness and curvature within the range of processing such as polishing. Moreover, it is set as the form which can be used for communication by fixing the other surface 7b of the holding base 7 to a package housing not shown.

Like FIG. 7, FIG. 2 shows electric lines of force 6a and 6b. In order to simplify the description, the lines of electric force existing in the air are not shown in FIG.
As can be seen from FIG. 2, the electric lines of force 6a and 6b are all distributed inside the holding base 7 having a low relative dielectric constant after penetrating the x-cutLN substrate 1 having a high relative dielectric constant. Therefore, after the electric lines of force 6a and 6b penetrate through the x-cutLN substrate 1, they are incident on the holding base 3 made of the x-cutLN substrate having a large relative dielectric constant. On the other hand, not only is it easy to lower the equivalent refractive index n _m of the electric signal and bring it closer to the equivalent refractive index n _o of the light, but there is no variation in the setting of the equivalent refractive index n _m of the electric signal. Yield can be significantly improved.

Furthermore, when the width S of the central conductor 5a of the traveling wave electrode 5 and the gap W between the central conductor 5a and the ground conductors 5b and 5c are large as described above, the electric lines of force 6a and 6b are greatly spread. made, but the electric force lines 6a, 6b after penetration of x-cutLN substrate 1, since all are in the holding base 7 having a low dielectric constant, suppressing the equivalent refractive index n _m of the electric signal, It is possible to _approach the equivalent refractive index no of light. That is, when the size of the traveling wave electrode 5 is large, the conventional embodiment of an equivalent refractive index n _m of the electric signal tends to increase had problems can be significantly solved.

  As described above, in the present embodiment, since the holding base 3 made of the x-cutLN substrate having a high relative dielectric constant shown in FIGS. 5 to 7 does not exist, the member cost and the processing cost for preparing this are further increased. Can eliminate the process of adhering the x-cutLN substrate 1 and the holding base 3 made of the x-cutLN substrate, which has been conventionally required, and can reduce the cost due to the labor cost.

  In general, the thermal expansion coefficient of the dielectric substrate is several orders of magnitude larger than the value of the x-cutLN substrate 1, but since the adhesive is manufactured by mixing various materials, products similar to the x-cutLN substrate 1 are also available. is there. By using such an adhesive as the holding base 7, the difference in thermal expansion coefficient between the x-cutLN substrate 1 and the holding base 7 can be reduced. Therefore, there is no worry that a DC drift occurs or the element is destroyed.

  More importantly, one of the low dielectric constant adhesive layer 2 and the holding substrate 3 which has been a problem when the x-cutLN substrate 1 and the holding substrate 3 are bonded to each other in FIGS. The problem that air easily enters between the surface 3a and the surface 3a of the present invention can be naturally solved since the holding substrate 3 does not exist in the present invention.

Further, since the thickness of the x-cut LN substrate 1 is as thin as about 10 μm, the electric lines of force 6 a and 6 b tend to be confined to the x-cut LN substrate 1. As a result, the efficiency of interaction with the light propagating through the optical waveguides 4a and 4b increases, so that the product Vπ · L of the half-wave voltage Vπ and the interaction length L can be reduced, resulting in a drive voltage. Can be reduced.
[Second Embodiment]

FIG. 3 is a sectional view of an optical modulator according to the second embodiment of the present invention. In the figure, reference numeral 8 denotes a medium having a relative dielectric constant different from that of the holding base 7. For example, an adhesive having a lower relative dielectric constant than that of the holding base 7 may be used, or an adhesive having a higher relative dielectric constant may be used. . Furthermore, air may be used.
[Third Embodiment]

FIG. 4 is a cross-sectional view of an optical modulator according to a third embodiment in which the present invention is applied to a z-cut substrate. In this embodiment, it is made of a low dielectric constant material such as SiO ₂ having a low relative dielectric constant of 3 to 5 between one surface 10a of the z-cutLN substrate 10 on which the optical waveguides 4a and 4b are formed and the traveling wave electrode 5. A buffer layer 9 is inserted. The other surface 10b of the z-cutLN substrate 10 is bonded and fixed to the holding base 7 made of a low dielectric constant adhesive, as in the other embodiments of the present invention.

The buffer layer 9 both the effect of reducing the effect of preventing the light propagating the optical waveguide 4a, and 4b is absorbed in the traveling wave electrode 5, the equivalent refractive index n _m of the electric signal. Therefore, by optimizing the thickness of the buffer layer 9, z-cutLN substrate 10 and the thickness T ₁₀ 10 [mu] m or more, for example 30μm approximately, or it is possible to set any time about 50 [mu] m, the fabrication of the LN optical modulator Will be improved.

Furthermore, since the electric lines of force are confined in the z-cutLN substrate 10, the driving voltage can be reduced. Although V [pi · L and the buffer layer is thick is increased, since the present embodiment has an effect of reducing the equivalent refractive index n _m of the electrical signals, can reduce the thickness of the buffer layer 9, the drive from this point of view The voltage can be reduced.

The other surface 10b of the z-cutLN substrate 10 is substantially flat. The term “substantially flat” means that the surface is flat including unevenness and curvature within a processing range such as polishing.
[About each embodiment]

In the embodiment of the present invention, the preferred thickness of the main substrate such as the x-cutLN substrate 1 or the z-cutLN substrate 10 is about 10 μm. However, the width of the central conductor 5a of the traveling wave electrode 5 and the central conductor 5a Needless to say, other thicknesses may be used for the LN substrate depending on the gap between the first and second conductors 5b and 5c, the thickness thereof, and the material and thickness of the buffer layer 9. That is, the thickness may be 20 μm, 30 μm, 50 μm, or the like. Furthermore, the thickness T ₇ of the holding base 7 made of the low dielectric constant adhesive layer has been described by taking 500 μm as an example, but it goes without saying that it may be thinner or thicker depending on the dimensions of the traveling wave electrode. Yes.

The x-cutLN modulator shown in the above embodiment has been described as having no buffer layer such as SiO ₂ , but of course there may be a buffer layer. Since the buffer layer has the effect of reducing the equivalent refractive index n _m of the electric signals, the thickness of the x-cut substrate 1 as compared with the case without will be possible to realize a high-speed light modulation be thicker.

  Further, the electric lines of force of the electric signal other than the upper air portion are all contained in the x-cutLN substrate 1 and the holding base 7, or the z-cutLN substrate 10, the buffer layer 9, and the holding base 7. It is also possible to have a part that penetrates under them.

  A metal such as gold constituting the traveling wave electrode 5 absorbs light and increases the insertion loss of light. Therefore, if the buffer layer is formed only near the edge of the central conductor 5a and the edge of the central conductor 4a is formed on the buffer layer, the optical waveguides 4a and 4b can be brought close to the edge of the central conductor 5a. Further, as can be seen from FIG. 1, in the present invention, the optical waveguide 4 is disposed under the ground conductor 5b, and the central conductor 5a of the traveling wave electrode 5 is disposed near the interaction portion between the electric signal and the light. Therefore, if a buffer layer is formed in these portions, light insertion loss can be reduced.

  Furthermore, although a coplanar waveguide (CPW) type traveling wave electrode is assumed as an electrode, a traveling wave electrode of another structure such as an asymmetric coplanar strip (ACPS) may be used, and in the claims, a traveling wave electrode is described. However, the present invention can of course be applied to a lumped electrode in order to reduce the capacitance. Further, although an LN substrate is assumed as a substrate, other dielectric substrates such as lithium tantalate, and further a semiconductor substrate may be used. In the present invention, since the sufficiently thick holding base 7 made of the low dielectric adhesive layer is formed, the surface of the package for fixing the manufactured LN optical modulator chip may be flat. It is not necessary.

Usually, when light enters the LN optical modulator, glass beads with holes are used, and fixed to the end face of the optical waveguide 4 formed on the LN substrate through the single mode optical fiber in the holes. In the case where the adhesion area between the glass beads and the end face of the optical waveguide formed on the LN substrate is insufficient, in addition to the reinforcing plate often used on the top surface of the LN substrate, A reinforcing plate for increasing the bonding area with the glass beads may be installed on the end face of the waveguide 4 and the vicinity of the end face of the optical waveguide may be fixed with an ultraviolet curable adhesive or the like. In this case, the reinforcing plate and the holding base 7 do not need to be bonded and fixed to each other in the vicinity of the interaction portion where the electrical signal and light interact.

Furthermore, in this case, the surface of the reinforcing plate placed on the lower side of the LN substrate may not be substantially flat. In this case, it goes without saying that the thin LN substrate and a thick holding base 7 of the effect of a low dielectric constant adhesive in the present invention can reduce the effective refractive index n _m of the electric signal. Even if the reinforcing plate to which the vicinity of the end face of the optical waveguide 4 is fixed is below the LN substrate, the optical modulator can be fixed to the package as it is. At this time, the surface of the package for fixing the optical modulator may be substantially flat or may not be flat.

The perspective view of 1st Embodiment of the optical modulator of this invention. Sectional view taken along line B-B 'in FIG. Sectional drawing of 2nd Embodiment of this invention Sectional drawing of 3rd Embodiment of this invention A perspective view of a conventional optical modulator Sectional drawing in the A-A 'line of FIG. FIG. 5 is a cross-sectional view taken along line A-A ′ in FIG. 5 to explain the operation principle.

Explanation of symbols

1: x-cut LN substrate, 2: low dielectric constant adhesive layer, 3: holding substrate, 4: Ti thermal diffusion optical waveguide, 4a, 4b: two arms of Mach-Zehnder optical waveguide 4, 5: traveling wave electrode, 5a: Center conductor of traveling wave electrode 5, 5b, 5c: Ground conductor of traveling wave electrode 5, 6a, 6b: Electric field lines of electric signal, 7: Holding base made of low dielectric constant adhesive layer, 8: Low dielectric constant adhesion Medium having a relative dielectric constant different from the holding base 7 composed of layers, 9: buffer layer, 10: z-cutLN substrate

Claims (8)

A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a central conductor formed on one surface side of the substrate for applying a voltage for modulating the light And a traveling wave electrode made of a ground conductor, and a low dielectric constant adhesive layer formed by bonding to the substrate on the other surface side of the substrate and made of a material having a dielectric constant lower than that of the substrate. A waveguide, an input optical waveguide for entering the light, an interactive optical waveguide for modulating the phase of the light by applying the voltage between the center conductor and the ground conductor, and the mutual In an optical modulator comprising an output optical waveguide that emits signal light generated as a result of modulation in the working optical waveguide,
The low dielectric constant adhesive layer is configured such that electric lines of force other than in the air pass through the substrate and the low dielectric constant adhesive layer among electric force lines of an electric signal propagating in the vicinity of the interaction optical waveguide. the lower the equivalent refractive index of the electrical signals, together with the thickness to approach the equivalent refractive index of the light propagating through the optical waveguide is set to a predetermined thickness, said substrate of said low-dielectric adhesive layer With mechanical strength to suppress damage to the substrate only with the low dielectric constant adhesive layer in a state where nothing is bonded to the reverse surface that is the surface opposite to the bonded surface ,
The thermal expansion coefficient of the low-dielectric adhesive layer is at ^{0.5 × 10 -5 / K~3 × 10} -5 / K,
Further, the light modulator is characterized in that the opposite surface of the low dielectric constant adhesive layer is substantially flat, and is fixed to the package housing on the opposite surface .
A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a central conductor formed on one surface side of the substrate for applying a voltage for modulating the light And a traveling wave electrode made of a ground conductor, and a low dielectric constant adhesive layer formed by bonding to the substrate on the other surface side of the substrate and made of a material having a dielectric constant lower than that of the substrate. A waveguide, an input optical waveguide for entering the light, an interactive optical waveguide for modulating the phase of the light by applying the voltage between the center conductor and the ground conductor, and the mutual In an optical modulator comprising an output optical waveguide that emits signal light generated as a result of modulation in the working optical waveguide,
In the low dielectric constant adhesive layer, the electric lines of force other than in the air pass through only the substrate and the low dielectric constant adhesive layer among the electric force lines of the electric signal propagating in the vicinity of the interaction optical waveguide. electric equivalent refractive index of the air signal is lowered, together with the thickness to approach the equivalent refractive index of the light propagating through the optical waveguide is set to a predetermined thickness, the substrate of the low-dielectric adhesive layer by Has a mechanical strength that suppresses damage to the substrate only with the low dielectric constant adhesive layer in a state where nothing is adhered to the opposite surface that is the surface opposite to the surface to which the material is adhered ,
The thermal expansion coefficient of the low-dielectric adhesive layer is at ^{0.5 × 10 -5 / K~3 × 10} -5 / K,
Further, the light modulator is characterized in that the opposite surface of the low dielectric constant adhesive layer is substantially flat, and is fixed to the package housing on the opposite surface .
  3. The optical modulator according to claim 1, wherein the thickness of the substrate is 50 μm or less.   4. The optical modulator according to claim 1, wherein the other surface of the substrate is substantially flat.   5. The optical modulator according to claim 1, wherein the substrate is an x-cut lithium niobate substrate.   5. The optical modulator according to claim 1, wherein the substrate is a z-cut lithium niobate substrate.   The thickness of the substrate, the relative dielectric constant of the low dielectric constant adhesive layer, the width and thickness of the center conductor, the thickness of the ground conductor, and the gap between the center conductor and the ground conductor, 6. The optical modulator according to claim 1, wherein each value is set so that an equivalent refractive index of the propagating electric signal and an equivalent refractive index of the light propagating through the optical waveguide are close to each other. The thickness of the substrate, the relative dielectric constant of the low dielectric constant adhesive layer, the width and thickness of the center conductor, the thickness of the ground conductor, the gap between the center conductor and the ground conductor, and the thickness of the buffer layer, The respective values are set so that the equivalent refractive index of the electric signal propagating through the traveling wave electrode and the equivalent refractive index of the light propagating through the optical waveguide are close to each other. Light modulator.

Images