JP5853880B2 - Light modulator - Google Patents

Description

The present invention relates to an optical modulator used in the fields of optical communication and optical measurement.

  With the spread of the Internet, the amount of communication has increased dramatically, and the importance of optical fiber communication has greatly increased. Optical fiber communication is a technique for converting an electrical signal into an optical signal and transmitting the optical signal through an optical fiber, and is characterized by a wide band, low loss, and resistance to noise.

As a method for converting an electric signal into an optical signal, a direct modulation method using a semiconductor laser and an external modulation method using an optical modulator are known. Direct modulation does not require an optical modulator and is low-cost, but there is a limit to high-speed modulation, and external light modulation is used for high-speed and long-distance applications.

As an optical modulator, an optical modulator in which an optical waveguide is formed near the surface of a LiNbO ₃ (hereinafter abbreviated as LN) single crystal substrate by Ti diffusion has been put into practical use. Although a high-speed optical modulator of 40 Gb / s or more has been commercialized, the long total length of around 10 cm is a major drawback. The optical waveguide by Ti diffusion is disadvantageous in that confinement is weak and that the dielectric constant of LN is high, and sufficient miniaturization has not been realized.

On the other hand, in Patent Document 1, for example, a Mach-Zehnder having a configuration in which a c-axis oriented LN thin film is formed on a single crystal substrate made of sapphire and a buffer layer made of SiO ₂ is formed thereon. A method of manufacturing a type optical modulator is disclosed.

JP 2006-195383 A

However, the optical modulator using the LN thin film according to Patent Document 1 still has a problem that it is difficult to reduce the size. FIG. 6 shows a cross-sectional configuration of a conventional optical modulator 100 of Patent Document 1. An SiO ₂ buffer layer 23 is disposed above the optical waveguides 22a and 22b made of LN thin film, and a sapphire substrate 21 is disposed below. Since the SiO ₂ film is used as the buffer layer, as will be described later, it is difficult to satisfy simultaneously the small size, the low driving voltage, and the low light absorption loss.

The present invention has been made in consideration of the above points, and is intended to realize an optical modulator capable of reducing the size and driving voltage in an optical modulator having a low optical absorption loss under a wide band. Objective.

In the present invention, a single crystal substrate having a relative dielectric constant of 17 or less, an optical refractive index formed on the single crystal substrate is nc, a film thickness is 2 μm or less , and a main component composition is LiNbO ₃ . The dielectric constant is εb between the electro-optic film, a traveling wave electrode composed of a signal electrode and a ground electrode for applying a voltage to the electro-optic film, and the electro-optic film and the traveling wave electrode, An optical modulator comprising a buffer layer having an optical refractive index of nb and satisfying (nc−nb) εb> 3.5.

  By adopting such a film configuration, the speed of light propagating through the optical waveguide and the speed of the high frequency propagating through the traveling wave electrode are matched, that is, by matching the effective refractive index of both, a broadband optical modulator By providing a buffer layer having a larger film thickness and a smaller optical refractive index, suppressing light absorption loss and selecting a material that satisfies (nc−nb) εb> 3.5. Thus, a small and low driving voltage optical modulator can be realized.

According to the present invention, it is possible to realize an optical modulator capable of downsizing and driving voltage reduction in an optical modulator having a low optical absorption loss under a wide band.

2 is a plan view of the optical modulator of Embodiment 1. FIG. 2 is a cross-sectional view of the optical modulator of Embodiment 1. FIG. FIG. 5 is a diagram illustrating a relationship between a light absorption loss and a film thickness of a buffer layer in the first embodiment. FIG. 4 is a diagram illustrating a relationship between a film thickness of a buffer layer and 1 / (nc−nb) in the first embodiment. FIG. 3 is a diagram illustrating a relationship between an effective refractive index of a traveling wave electrode according to Embodiment 1 and a film thickness of an electro-optic film. It is sectional drawing of the optical modulator of a prior art.

Hereinafter, preferred embodiments of the present invention will be described. The subject of the present invention is not limited to the following embodiment. In addition, the constituent elements described below include those that can be easily assumed by those skilled in the art and substantially the same elements, and the constituent elements can be appropriately combined. Further, the explanatory diagram is schematic, and for convenience of explanation, the relationship between the thickness and the planar dimension may be different from the actual structure within a range in which the effect of the present embodiment can be obtained.

(Embodiment 1)
Hereinafter, embodiments will be described with reference to the drawings. FIG. 1 is a plan view of a Mach-Zehnder optical modulator according to the first embodiment. Hereinafter, it is called an optical modulator. The optical modulator 10 is a device that modulates light propagating in the optical waveguide 1 by applying a voltage to a Mach-Zehnder interferometer formed by the optical waveguide 1 having an electro-optic effect. The optical waveguide 1 is branched into two optical waveguides 1a and 1b, and one optical signal is provided on each of the optical waveguides 1a and 1b, that is, two signal electrodes 2a and 2b. It has become.

FIG. 2 is a cross-sectional view taken along line AA of the optical modulator 10 according to the first embodiment. An electro-optic film 5 is formed on the single crystal substrate 4. The electro-optic film 5 is formed of, for example, a LiNbO ₃ (LN) thin film. The c-axis of the electro-optic film 5 is oriented in the direction perpendicular to the upper surface of the single crystal substrate 4 and the film thickness is 2 μm or less. By processing the electro-optic film 5 into the ridge shape 7, the optical waveguides 1a and 1b are formed. On the electro-optic film 5, signal electrodes 2 a and 2 b are provided via a buffer layer 6.

The operation principle of the light modulation device 10 will be described. In FIG. 1, two signal electrodes 2a, 2b and ground electrodes 3a, 3b, 3c are connected by a terminating resistor 9 to form a traveling wave electrode 8, which has an absolute value relative to the two signal electrodes 2a, 2b. So-called complementary signals that are the same and have different positive and negative phases are input from the input sides 13 a and 13 b of the signal electrodes 2 a and 2 b of the optical modulator 10. At this time, the refractive index of the optical waveguides 1a and 1b changes as + Δn and −Δn, respectively, due to the electric field applied to the optical waveguide 1, and the phase difference between the optical waveguides 1a and 1b changes. The intensity-modulated signal light is output from the output waveguide 1 c on the output side 12. If the phase difference between the optical waveguides 1a and 1b is 0 °, the light is intensified. If the phase difference is π, the light is intensified.

In an optical modulator using a conventional LN (LiNbO ₃ ) single crystal substrate, it is considered common sense to use a low dielectric constant film typified by SiO _{2 as a} buffer layer. An example using layers is disclosed. As a result of intensive studies by the present inventor, it was found that this SiO ₂ buffer layer is not essential when the LN thin film is used, but rather is not appropriate. This point will be described in detail below.

The reason why the SiO ₂ buffer layer is essential when a conventional LN single crystal substrate is used for an optical waveguide will be described. The relative dielectric constant of LN is 28 when an electric field is applied parallel to the c-axis and 43 when it is applied perpendicular to the c-axis. In order to realize a broadband optical modulator, it is necessary to match the speed of light propagating through the optical waveguide with the speed of high frequency propagating through the traveling wave electrode. That is, it is necessary to match the effective refractive indexes of the optical waveguide and the traveling wave electrode. The traveling wave electrode here is a general term for electrodes. The effective refractive index of the optical waveguide made of the LN single crystal substrate is about 2.1. On the other hand, the effective relative permittivity of the traveling wave electrode is the average value of the relative permittivity of LN and air when the electrode thickness is thin and there is no buffer layer. It is about 18 if it calculates using the number. The effective refractive index of the traveling wave electrode is the square root, which is 4.2, which is a large difference of twice the effective refractive index 2.1 of the optical waveguide made of the LN single crystal substrate.

In the prior art, it is difficult to greatly change the effective refractive index of the optical waveguide. For speed matching, it is necessary to significantly reduce the effective refractive index of the traveling wave electrode. For this reason, it is essential to form an SiO ₂ buffer layer having a relative dielectric constant of 4 on the LN single crystal substrate, and an electrode having a thickness of 10 μm or more is also used. Note that the relative dielectric constant of the SiO ₂ buffer layer is 4, which is lower than the values of the relative dielectric constants 28 and 43 of LN described above. Some ideas have been proposed such as processing the LN single crystal substrate into a ridge shape or a thin plate shape. As described above, when an LN single crystal substrate is used, it is necessary to dispose a low dielectric constant buffer layer for speed matching, and an SiO ₂ film having a relative dielectric constant of 4 is widely used. .

Next, a case where an LN thin film is used for the electro-optical film 5 as in the first embodiment will be described with reference to FIGS. By processing the electro-optic film 5 into the ridge shape 7 on the electro-optic film 5, the optical waveguides 1, 1a, 1b are formed. In FIG. 2, the first embodiment has a structure of a single crystal substrate 4 / an electro-optic film 5 / a buffer layer 6 / a traveling wave electrode 8 from the bottom. The optical waveguide 1 is a part of the electro-optic film 5, and the effective refractive index of the optical waveguide 1 using the LN thin film is the same as the above and is about 2.1.

On the other hand, the effective relative dielectric constant of the traveling wave electrode 8 approximates to the average value of the relative dielectric constants of the single crystal substrate 4 and air when the thickness of the LN thin film 5 is sufficiently thin and the contribution of the LN thin film 5 is ignored. it can. For example, when a sapphire single crystal substrate is used as the single crystal substrate 4, the relative dielectric constant is 11.5 when an electric field is applied parallel to the c-axis, and 9.3 when applied perpendicular to the c-axis. When calculated using the numbers 11.5 and 9.3, the traveling wave electrode 8 has an effective relative dielectric constant of about 5.7 and an effective refractive index of 2.4, and an optical waveguide using an LN thin film. It is in good agreement with the effective refractive index of 2.1. Unlike the case of the LN single crystal substrate, in the case of the LN thin film 5 having a sufficiently thin film thickness, speed matching is almost realized without conventional contrivance, and a broadband optical modulator can be easily realized.

  When an LN thin film is used for the electro-optic film 5 as in the first embodiment, the buffer layer 6 is not necessary for the purpose of speed matching, but the buffer layer 6 is necessary to suppress light absorption by the traveling wave electrode 8. . FIG. 3 shows the calculation result of the light absorption loss by the traveling wave electrode 8 when the thickness of the buffer layer 6 is changed. It can be seen that the light absorption loss can be reduced exponentially by adjusting the thickness of the buffer layer 6 to be thicker. It can also be seen that the lower the optical refractive index nb of the buffer layer 6 is, the light absorption loss can be reduced with a thinner film thickness.

Incidentally, when the light absorption loss increases, the insertion loss of the optical modulator increases. The insertion loss is the ratio of the outgoing light intensity emitted from the direction 12 to the incident light intensity incident from the direction 11 in FIG. 1. The insertion loss is one of the important characteristics of the optical modulator. The lower the size, the better. If the insertion loss is increased, a problem that a communication error occurs is caused. Therefore, it is necessary to suppress the insertion loss. That is, it is necessary to suppress light absorption loss.

In FIG. 4, the horizontal axis plots 1 / (nc−nb), and the vertical axis plots the film thickness at which the light absorption loss is 1 dB / cm, indicating a substantially proportional relationship. The optical refractive index of the buffer layer 6 was nb, and the optical refractive index of the electro-optic film 5 was nc. In general, when the film thickness of the buffer layer 6 is db,
db ≧ k / (nc−nb) <Formula 1>
I found out that Here, k is calculated from the specification value of the loss, the wavelength, and the waveguide shape, and is a coefficient independent of nb and nc.

Next, as a general example, consider the voltage applied to the LN thin film in the optical waveguide portion. First, as a simple model, consider a capacitor having materials 1 and 2 having thicknesses d1 and d2 and relative dielectric constants ε1 and ε2 between parallel plate electrodes. The voltage V1 applied to the material 1 is V between the electrodes,
V1 = (d1 / ε1) V / (d1 / ε1 + d2 / ε2) <Expression 2>
It is. The smaller d2 / ε2 is, the larger V1 is.

  Similarly, in the first embodiment, in the configuration of the electro-optic film 5 / buffer layer 6, the voltage applied by the traveling wave electrode 8 to the electro-optic film 5 and the LN thin film 5 in the optical waveguide portion is the buffer. When the film thickness of the layer 6 is db and the relative dielectric constant is εb, the smaller db / εb is, the larger the voltage can be applied. The buffer layer 6 here corresponds to d2 / ε2 in <Expression 2>.

From the viewpoint of the voltage applied to the LN thin film, the smaller the film thickness db of the buffer layer 6, the more desirable. In order to satisfy the loss specification of <Formula 1>, db = k / (nc−nb). Dividing both sides by εb
db / εb = k / {εb (nc−nb)} <Expression 3>
Therefore, it can be understood that the larger εb (nc−nb), the smaller db / εb.

As described above, in the optical modulator 10 using the LN thin film for the electro-optic film 5, it is important to select a material having a large εb (nc−nb) as the buffer layer 6. Conventionally, a SiO ₂ film widely used as a buffer layer has nb = 1.45 and εb = 4. When the optical refractive index nc = 2.14 of LN, (nc−nb) εb = 2.8. It becomes. In the first embodiment, the buffer layer 6 satisfying (nc−nb) εb> 3.5, which is 20% or more larger than the value of 2.8 described above, is used, compared with the SiO ₂ film. Therefore, it is possible to provide an optical modulator that has a remarkable improvement effect, and that achieves a significant reduction in size and driving voltage.

Components of the first embodiment will be described with reference to FIGS. 1 and 2. In FIG. 2, the first embodiment has a structure of a single crystal substrate 4 / an electro-optic film 5 / a buffer layer 6 / a traveling wave electrode 8 from the bottom. As the single crystal substrate 4, a material having a relative dielectric constant of 17 or less is used. When the relative dielectric constant is 17 or less, the electro-optic film 5 is sufficiently thin and can be ignored. Therefore, the effective relative dielectric constant of the traveling wave electrode 8 is an average value of the single crystal substrate 4 and air having a relative dielectric constant of 1. 9 or less. Since the effective refractive index is the square root of the average value of the effective relative dielectric constant, the effective refractive index of the traveling wave electrode 8 is 3.0 or less.

The effective refractive index of the optical waveguide made of a ferroelectric material is usually in the range of 2.0 to 2.8, and is almost the same, so that the effective refractive index of the traveling wave electrode 8 is 3.0 or less, It is considered that speed matching can be achieved. For example, if the effective refractive index of the traveling wave electrode 8 is 3.0 or less, it is a part of the electro-optic film 5, and the effective refractive index of the optical waveguide 1 using the LN thin film is about 2.1. The electrode length L of the electrodes 2a and 2b is 10 mm, and the band can be 20 GHz or more.

In the first embodiment, a single crystal substrate 4 having a relative dielectric constant of around 10 is suitable, and specific examples include single crystal substrates 4 such as sapphire, Si, MgO, ZnO, and CaF ₂ . For example, when the relative dielectric constant is 10, the same calculation is performed, and the effective refractive index of the traveling wave electrode 8 is 2.3. Therefore, the optical waveguide is a part of the electro-optical film 5 and uses an LN thin film. It agrees well with an effective refractive index of 1 of about 2.1. In order to form the optical waveguide 1 using the electro-optic film 5 and propagate light, the optical refractive index nc of the electro-optic film 5 needs to be higher than the surrounding optical refractive index. Therefore, when a Si single crystal substrate having an optical refractive index of about 3.5 is used, an optical refractive index lower than the optical refractive index nc of the electro-optic film is not shown between the Si single crystal substrate 4 and the electro-optic film 5 (not shown). It is necessary to sandwich the cladding layer.

  The electro-optic film 5 needs to have a film thickness of 2 μm or less. If the film thickness is further increased, it becomes difficult to form a high-quality film, and the effective relative dielectric constant of the traveling wave electrode 8 is increased, which makes it difficult to satisfy speed matching. .

FIG. 5 is a graph showing a change in effective refractive index with respect to the film thickness of the electro-optic film. Single crystal sapphire 4 as sapphire single crystal, electro-optic film 5 as LN thin film, signal electrode width 5 μm, signal electrodes 2 a, 2 b and ground electrodes 3 a, 3 b, 3 c have a distance of 5 μm, and traveling wave electrode 8 has a film thickness The effective refractive index at 10 GHz is calculated as 5 μm. In order to make the electrode length L of the signal electrode 10 mm and the band 20 GHz or more, it is desirable that the effective refractive index of the traveling wave electrode 8 is 3.0 or less, calculated from the relational expression between velocity matching and band. If the film thickness of the LN thin film is 2 μm or less, this is satisfied.

When the electro-optic film 5 is too thin, light confinement in the electro-optic film 5 becomes weak, and light leaks to the single crystal substrate 4 and the buffer layer 6 and is guided. Even if an electric field is applied to the electro-optic film 5, the change in the effective refractive index of the optical waveguide may be reduced. Therefore, it is desirable that the electro-optic film 5 has a film thickness of about 1/3 or more of the wavelength.

As a method for forming the electro-optic film 5, it is very difficult to achieve a film thickness of 2 μm or less by a conventionally known method of attaching a bulk material to a single crystal substrate 4 in a thin plate. It is desirable to use a thin film forming method such as a CVD method, a CVD method, or a sol-gel method. As the film composition of the electro-optic film 5, in addition to LiNbO ₃ (LN), materials having an electro-optic effect such as LiTaO ₃ (LT), PZT, PLZT, and BaTiO ₃ can be used. The c-axis is oriented perpendicular to the surface of the single crystal substrate 4, and the optical refractive index changes in proportion to the electric field by applying an electric field parallel to the c-axis. Various known methods can be used as a method for realizing the c-axis orientation. For example, in the case of an LN thin film, a C-axis oriented LN epitaxial film can be realized by forming an LN thin film on a c-plane sapphire substrate.

Various methods can be used as a method of forming the optical waveguide 1 which is a part of the electro-optical film 5. A ridge-type optical waveguide processed into a ridge shape 7 as shown in FIG. 2 and an embedded optical waveguide obtained by processing an electro-optic film into a square cross section as shown in FIG. 6 can be used. Ridge type optical waveguides are desirable because they are easy to process and are advantageous in terms of voltage reduction. The buffer layer 6 may be any material that satisfies (nc−nb) εb> 3.5 as described above. For example, when the electro-optic film is an LN thin film, Al ₂ O ₃ , MgF ₃ , La ₂ O ₃ , Y ₂ O ₃ , HfO ₂ , CaF ₂ or the like can be used.

  The traveling wave electrode 8 may be made of a material having high electrical conductivity, but a metal material having high electrical conductivity such as Au, Cu, Ag, or Pt is used in order to reduce signal propagation loss at high frequencies. It is desirable to do. Regarding the shape of the traveling wave electrode 8, in particular, important parameters that affect the characteristics are the width of the signal electrodes 2a and 2b, the distance between the signal electrodes 2a and 2b and the ground electrodes 3a, 3b and 3c, and the film thickness of the traveling wave electrode 8. Therefore, it is necessary to appropriately set the impedance of the traveling wave electrode 8, the effective relative permittivity, and the propagation loss of the high frequency signal so as to match the target values.

Example 1
As Example 1, the optical modulator 10 according to Embodiment 1 was manufactured by the following procedure. In FIG. 2, a sapphire substrate is prepared as the single crystal substrate 4, and a c-axis oriented LN thin film is formed on the surface of the single crystal substrate 4 as the electro-optic film 5 by sputtering. The thickness of the LN thin film was 1.0 μm. Next, the optical waveguide 1 was formed by processing into a ridge shape 7 by a photo process and a milling process. The ridge width was 2.0 μm, and the ridge height was 0.2 μm.

Next, in FIG. 2, after forming an Al ₂ O ₃ film as the buffer layer 6, the signal electrodes 2a, 2b and the ground electrodes 3a, 3b, 3c were formed by a photo process and a gold plating process. The film thickness of the buffer layer 6 was set to 1.0 μm so that the optical absorption loss of the optical waveguide 1 with electrodes satisfies 1 dB / cm or less. The thickness of the traveling wave electrode 8 was 4 μm, the width of the signal electrodes 2a and 2b was 4 μm, and the distance G between the signal electrode and the ground electrode was 4 μm. The interval G was set so that the impedance of the traveling wave electrode was approximately 50Ω. Table 1 is a table showing data for the examples and comparative examples. As shown in Table 1, the optical refractive index and relative dielectric constant of the buffer layer 6 made of Al ₂ O ₃ are nb = 1. And εb = 10. (Nc−nb) εb = 4.4, and the condition of (nc−nb) εb> 3.5 is satisfied. The length of the signal electrode on the optical waveguide 1, that is, the electrode length L is 1 cm.

  In Example 1, the characteristics of the optical modulator 10 were evaluated using light having a wavelength of 1550 nm. As shown in Table 1, the band was 20 GHz or more and VπL = 3.7 Vcm. Here, Vπ is a half-wave voltage, which is defined by the difference between the voltage V1 at which the light output is maximum and the voltage V2 at which the light output is minimum, and means a driving voltage. VπL is a product of the drive voltage, which is a voltage applied to the signal electrode, and the electrode length L, and is an index representing the performance of the optical modulator. A smaller VπL indicates a smaller size and a lower driving voltage. Since VπL of an optical modulator using a conventional LN single crystal substrate is usually 7 Vcm or more, it can be seen that the optical modulator of Example 1 achieves a significant reduction in size and voltage.

(Comparative Example 1)
In Example 1, an SiO ₂ film is used as the buffer layer 6 instead of the Al ₂ O ₃ film, and the thickness of the buffer layer satisfies the optical absorption loss of the optical waveguide 1 with an electrode of 1 dB / cm or less. Thus, it was set to 0.8 μm. As shown in Table 1, the interval G is 3 μm, and the other conditions are the same. The optical refractive index and relative dielectric constant of the SiO ₂ film were nb = 1.45 and εb = 4. Further, (nc−nb) εb = 2.8, which does not satisfy the condition of 3.5 or more.

  When the characteristics of the optical modulator were evaluated in the same manner as in Example 1, as shown in Table 1, the band was 20 GHz or more and VπL = 4.6 Vcm. Compared with Example 1, VπL increased by about 24%. It became. That is, VπL = 4.6 Vcm is large, which is not preferable.

(Example 2)
As Example 2, the buffer layer 6 was changed from the Al ₂ O ₃ film of Example 1 to an MgF ₂ film. The film thickness of the buffer layer 6 was set to 0.7 μm so that the light absorption loss of the optical waveguide 1 with electrodes satisfied 1 dB / cm or less. The distance G between the signal electrode and the ground electrode was set to 3 μm so that the traveling wave electrode 8 had an impedance of approximately 50Ω. (Nc−nb) εb = 3.8, and the condition of (nc−nb) εb> 3.5 is satisfied. When the characteristics of the optical modulator were evaluated using light having a wavelength of 1550 nm, the band was 20 GHz or more and VπL = 3.9 Vcm.

(Example 3)
As Example 3, the buffer layer 6 was changed from the Al ₂ O ₃ film of Example 1 to a La ₂ O ₃ film. The film thickness of the buffer layer 6 was set to 1.0 μm so that the light absorption loss of the optical waveguide 1 with electrodes satisfied 1 dB / cm or less. The distance G between the signal electrode and the ground electrode was set to 8 μm so that the impedance of the traveling wave electrode 8 was approximately 50Ω. (Nc−nb) εb = 11.9, and the condition of (nc−nb) εb> 3.5 is satisfied. When the characteristics of the optical modulator were evaluated using light having a wavelength of 1550 nm, the band was 20 GHz or more and VπL = 3.9 Vcm.

(Comparative Example 2)
As Comparative Example 2, the buffer layer 6 was changed from the Al ₂ O ₃ film of Example 1 to a ZnO film. The film thickness of the buffer layer 6 was set to 1.6 μm so that the light absorption loss of the optical waveguide 1 with electrode satisfies 1 dB / cm or less. The distance G between the signal electrode and the ground electrode was set to 4 μm so that the impedance of the traveling wave electrode 8 was approximately 50Ω. When the characteristics of the optical modulator were evaluated using light having a wavelength of 1550 nm, the band was 20 GHz or more and VπL = 4.8 Vcm 2. Note that (nc−nb) εb = 2.2, and the condition of (nc−nb) εb> 3.5 is not satisfied.

Table 1 is a table showing each data for the examples and comparative examples. In both cases, the band satisfies 20 GHz or more, and the data is compared on the same earth in terms of wide band. Incidentally, the optical refractive index formed on the single crystal substrate is nc, and the relative dielectric constant is εb between the electro-optic film 5 having a film thickness of 2 μm or less, and between the electro-optic film 5 and the traveling wave electrode 8. Conventionally, an optical modulator using an LN thin film for the electro-optic film 5 has been known for the condition that the buffer layer 6 having an optical refractive index of nb is provided and (nc−nb) εb> 3.5 is satisfied. Compared with the value 2.8 of (nc−nb) εb for the buffer layer 6 of SiO ₂ , the value 3.5, which is 20% or more higher than the value 2.8, was set as the lower limit. In the examples of Table 1, there is a clear difference between SiO ₂ with a value of (nc−nb) εb of 2.8 and Al ₂ O ₃ , NgF ₃ , La ₂ O _{3 of} 3.8 or more, and light modulation Since a clear difference was also found in the value of VπL, which is one index representing the performance of the vessel, 3.5 was determined to be an appropriate value as a boundary.

Also, from Table 1, in Examples 1 to 3, VπL was as low as 4 Vcm or less, and VπL of the optical modulator using the conventional LN single crystal substrate was usually 7 Vcm or more, and thus good results were confirmed. . On the other hand, in Comparative Examples 1-2, VπL was higher by about 20% or more than Examples 1-3. In Examples 1 to 3, it was confirmed that an optical modulator having a low light absorption loss, a wide band, a small size, and a low driving voltage can be realized. Although the La ₂ O ₃ film of Example 3 has the highest (nc−nb) εb = 11.9, the gap G is 8 μm and wider than the other examples in order to adjust the impedance to 50Ω. As a result, VπL is not significantly different from the values in the first and second embodiments.

The optical modulator according to the present invention can be used for various applications in optical fiber communication and optical measurement.

DESCRIPTION OF SYMBOLS 10, 100 Optical modulator 1, 1a, 1b Optical waveguide 2a, 2b Signal electrode 3a, 3b, 3c Ground electrode 4 Single crystal substrate 5 Electro-optic film 6 Buffer layer 7 Ridge shape 8 Traveling wave electrode 9 Termination resistor 11 Input side 12 Output side 13a, 13b Input side 21 Sapphire substrates 22a, 22b Optical waveguide 23 SiO ₂ buffer layer 24a Signal electrode 24b Ground electrode

Claims (1)

A single crystal substrate having a relative dielectric constant of 17 or less;
An electro-optic film having an optical refractive index of nc, a film thickness of 2 μm or less , and a main component composition of LiNbO ₃ formed on the single crystal substrate;
A traveling wave electrode composed of a signal electrode and a ground electrode for applying a voltage to the electro-optic film;
A buffer layer having a relative dielectric constant of εb and an optical refractive index of nb between the electro-optic film and the traveling wave electrode;
(Nc−nb) An optical modulator satisfying εb> 3.5.

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