JPH0713711B2 - High speed optical modulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical modulator having a high operating speed and a low driving voltage.

(Prior Art) In a high-speed and large-capacity optical fiber communication system, particularly in a coherent optical fiber communication system, a high-performance external optical modulator with high speed and small driving voltage is useful. Examples of this type of external optical modulator include a light intensity modulator, an optical phase modulator, and an optical frequency modulator.

As an example of a conventional external optical modulator, the basic configuration of a directional coupler type optical intensity modulator is shown in FIG.

FIG. 14 shows modulation electrodes 1 and 2 consisting of a center conductor and a ground conductor.
Is a distributed constant circuit. in this case,
For example, optical waveguides 4 and 5 are formed as directional couplers on a substrate 3 having an electro-optical effect such as lithium niobate (hereinafter referred to as LNbO _{3 and referred to} as LN). The optical waveguides 4 and 5 are formed by, for example, a titanium (Ti) thermal diffusion method or a proton exchange method. Reference numeral 6 is a buffer layer for reducing the optical propagation loss of the optical waveguide due to the modulation electrodes 1 and 2, and is made of silicon dioxide (Si
O ₂ ), alumina (Al ₂ O 3) and the like. Then, when incident light 7 having a constant intensity is made incident on the optical waveguide 4 and a signal is input from the signal source 8 between the modulation electrodes 1 and 2, the light 9 whose intensity is modulated according to the signal is generated. Alternatively, the light is emitted from the optical waveguide 5.

In the case of this optical modulator, since the modulation electrodes 1 and 2 are configured as a distributed constant circuit, ideally there is no limitation on the bandwidth of an electric circuit. Also, as long as the signal wave propagating between the modulation electrodes 1 and 2 and the propagation speed of light match, there is no limitation on the bandwidth due to the influence of the time that the incident light 7 travels in the optical waveguides 4 and 5, so Used in high speed optical modulators.

However, in reality, there is a speed difference between the signal wave and the light, which limits the bandwidth. When the effective refractive index of the substrate 3 with respect to the signal wave is n _m , the effective refractive index of the optical waveguides 4 and 5 with respect to light is n _o , and the electrode length is l, the bandwidth BW caused by this speed difference is Is BW = 1.4c / (πl | n _m −n _o |) (1) However, c is the luminous flux [Reference: Theological theory (c), J64
-C, 4, P264-271,1981]. The refractive index n _m is relative to the effective dielectric constant ε _eff of the substrate 3, Given in.

In a substrate material having an electro-optical effect, the refractive index n _m for signal waves is usually larger than the refractive index n _o for light. The effective permittivity ε _eff of the substrate 3 is mainly determined by the permittivity ε _s of the substrate material, the electrode spacing g, the widths w ₁ and w ₂ , the substrate thickness d, and the like. The thickness d of the substrate is usually 0.5 to several mm due to the limitation of the ease of handling the substrate when manufacturing the optical modulator. The electrode gap g is selected to be several times the width of the optical waveguides 4 and 5, and is about 5 to 10 μm. Also electrodes 1, 2
The thickness t is selected to be about 2 to 3 μm in consideration of the conductivity of the electrode material and the resistance due to the microwave skin effect. Therefore, since the size of the electrode gap g is usually sufficiently smaller than the substrate thickness d, and the electrode thickness t is smaller than g, the effective permittivity is approximately ε _eff ≈ (ε _s +1) / 2. … (3). For example, when the substrate 3 is LN, the dielectric constant ε _s of the substrate material ε 35, the refractive index n _m ≈4.2, n _o ≈2.2, and the refractive index _nm
Since approximately doubles the size of the refractive index n _o, when the 5GHz operation, the length l of the electrodes, the front and rear 10mm selected [the above equation (1) refer to Fig.

(Problems to be Solved by the Invention) In order to make the optical modulator of FIG.
As can be seen from equation (1), the electrode
It is necessary to shorten the length l of 1,2. However, when the electrode length l is shortened, the driving voltage of the optical modulator increases, so that the modulation efficiency decreases.

The present invention has been made under such a background, and an object thereof is to provide optical modulation with high speed and low driving voltage.

(Means for Solving the Problems) The present invention relates to an optical modulator having an optical waveguide formed near the surface of a substrate, and a modulation electrode provided on the surface on which the optical waveguide is formed. The effective thickness of the above-configured modulation electrode is set to be equal to or larger than the distance between the modulation electrodes.

According to the present invention, since the effective thickness of the modulation electrode is increased, the effective dielectric constant ε of the modulation electrode with respect to the modulation signal wave is ε.
_eff can be reduced. As a result, the effective refractive index for the modulated signal wave can be made smaller than that of the conventional component, and the difference in the phase velocities between the modulated signal wave and the light is reduced, and an optical modulator that operates at high speed with a low driving voltage can be realized.

(Embodiment) FIGS. 1A and 1B are configuration diagrams of an embodiment of the present invention in which a symmetrical coplanar strip line is applied as a modulation electrode, wherein FIG. 1A is a sectional view of a central portion, and FIG. Is a plan view. The same components as those in the conventional example are represented by the same reference numerals.

In FIG. 1, reference numerals 11 and 12 are symmetrical coplanar strip lines having a width w, a gap g, a thickness t and a length l. According to this embodiment, as compared with the conventional optical modulator,
The bandwidth limitation effect due to the speed difference between the signal wave and the light is alleviated. The principle will be described below with reference to FIGS. 2 (a) and 2 (b).

FIG. 2 shows the lines of electric force and the capacitance distribution of the modulation electrode portion, where (a) shows the case of the conventional example and (b) shows the case of the embodiment of the present invention.

That is, in the case of the conventional example, since the thickness of the buffer layer 6 and the thickness t of the electrodes 1 and 2 shown in FIG. 14 are sufficiently smaller than the electrode gap g, the capacitance C between the electrodes 1 and 2 is Capacity of Ca
And the capacitance Cs between the electrode lower surface (substrate) side. The dielectric in the gap of the capacitance Ca consists of air, and its relative permittivity ε
_a is about 1.0. The substrate material is filled between the capacitors Cs, and the relative permittivity thereof is the relative permittivity ε _s of the substrate material 3. When a signal wave is input between the modulation electrodes 1 and 2, electric lines of force are generated between the modulation electrodes according to the signal wave. The ratio of the amount of electric force lines 13 generated inside the substrate 3 to the amount of electric force lines 14 generated outside the substrate 3 corresponds to the ratio of the magnitudes of the capacitances Cs and Ca. In this case, the effective permittivity ε _eff of the modulation electrode is a value between ε _a and ε _s depending on the amount of the lines of electric force 13 and 14, and is given by the equation (3). Therefore, when the substrate material is determined, the magnitude of ε _eff is uniquely determined.

On the other hand, in the case of the embodiment of the present invention, since the thickness t of the electrodes is set to be equal to or larger than the size of the gap g, the capacitance C between the electrodes 11 and 12 is equal to the capacitances Ca and Cs. And the capacitance Cg in the gap between the electrodes. Therefore, the amount of electric lines of force 14 outside the substrate increases relative to the amount of electric lines of force 13 inside the substrate 3, and the dielectric (air) relative permittivity ε _a outside the substrate is Since the relative permittivity ε _s is smaller than the relative permittivity ε _s , the effective permittivity ε ′ _eff of the modulation electrodes 11 and 12 is smaller than the above ε _eff . This means that ε ′ _eff decreases as the ratio of the amount of electric flux lines 14 outside the substrate increases.

FIG. 3 shows, in the embodiment of FIG. 1, when the substrate 3 is made of LN, the ratio w / g of the width w of the electrode and the distance g is on the horizontal axis and the effective refractive index of the electrode on the vertical axis is calculated by approximation. It represents _nm and the modulation bandwidth BW. Here, the ratio t / g of the thickness t of the electrode and the interval g is used as a parameter. In the calculation, ε = 35, and the buffer layer 6 is absent. The size of the capacitance Cg is calculated by ignoring the capacitance component of the LN substrate 3.
As it can be seen from Figure 3, small w / g, greater the t / g increases, the size of the effective refractive index n _m has a small tendency
It can be further completely coincide refractive index N _o (about 2.2) to light.

In this case, the modulated signal wave and the phase velocity of the light can be matched,
According to the equation (1), an infinite modulation bandwidth BW can be realized. Also, the desired modulation band BW is the structure w /
By combining g and t / g and the magnitude of the dielectric constant ε _s of the substrate material, it can be similarly realized by the relationship of the equation (1).

Note that the relationship between _nm and w / g, t / g shown in FIG. 3 is an approximate result and gives a guideline for structural design. Various structural designs are possible.

From the above, according to this embodiment, as compared with the conventional optical modulator, the bandwidth limiting effect due to the speed difference between the signal wave and the light is alleviated. Therefore, for example, if the electrode length 1 is the same as the conventional one, the bandwidth is increased by the equation (1), and the high speed operation can be realized without increasing the driving voltage.

Further, if the bandwidth is the same as the conventional one, the electrode length 1 can be increased, so that the driving voltage can be reduced and a high-performance optical modulator with improved modulation efficiency can be realized.

The optical modulator of this embodiment can be manufactured, for example, by the following method.

The optical waveguides (directional couplers) 4 and 5 and the buffer layer 6 formed near the surface of the substrate 3 can be manufactured by the same method as in the conventional example shown in FIG. On the substrate 3 on which the buffer layer 6 is formed, a metal film for buffer such as NiCr is adhered as needed by several hundred liters to several thousand liters by vapor deposition or the like,
Furthermore, a metal film with good conductivity such as Au can be used from several hundred Å to several thousand.
Å Attach it.

Next, a photoresist is applied to this metal surface by, for example, a spinner. At this time, for example, when using a spinner, by appropriately setting the rotation speed and time,
Since the thickness of the photoresist can be arbitrarily formed from several μm to several tens of μm, the photoresist is formed so as to have the same thickness as the electrode t to be formed or several μm or more and more than t. Further, a photoresist is exposed through a photomask having the same shape as the pattern of the electrodes 11 and 12, and by the phenomenon, the resist is removed only at the portions where the electrodes 11 and 12 are formed, and a resist pattern having the same shape as the electrode pattern is formed. . After that, a metal such as Au is formed to a thickness t by a method such as an electroplating method.

After removing the resist, the first NiCr, Au formed by chemical etching or ion beam etching.
By removing the metal film such as, a modulation electrode having an arbitrary thickness can be formed.

4A and 4B are a sectional view and a plan view of another embodiment of the present invention, in which Mach-Zehnder interferometers 15 and 16 are used as optical waveguides and asymmetric coplanar strip lines 17 and 18 are used as modulation electrodes. Is applied. Here, the width of the center conductor of the asymmetric coplanar strip line 17 is w, the distance between the center conductor and the ground conductor of the asymmetric coplanar strip line 18 is g, and the thickness of the line is t.

In FIG. 5, in this embodiment, similarly in the case of using the LN as the substrate, with respect to the effective refractive index n _m for the modulated signal wave, shows the result of calculation in Fig. 3 the same way. From FIG. 5, even closer to the effective refractive index n _m of the modulated signal wave in the effective refractive index n _o of the light in this embodiment, furthermore seen also possible that taking perfectly matched.

6 (a) and 6 (b) are a sectional view and a plan view of another embodiment of the present invention, in which the coplanar line 1 is used as a modulation electrode.
The case where 9,18 is applied is shown. FIG. 7 shows the calculation result of the effective refractive index n _m for the modulated signal wave when the LN substrate is used in this embodiment as well. It can be seen from FIG. 7 that the same effects as those of the above-described embodiment can be obtained in this embodiment as well.

FIG. 8 is a sectional view of another embodiment of the present invention in which an overhang portion 21 is formed near the surfaces of the modulation electrodes 11 and 12. In this case, the capacitance between the overhang portions 21 of the modulation electrode is further added, and the amount of the lines of electric force in this portion is relatively increased. Therefore, even if the thickness t of the electrode is relatively thin, it will be described in the above embodiment. It is possible to make the effective refractive index _nm smaller than that in the case.

FIG. 9 is a cross-sectional view of another embodiment of the present invention in which the end portions of the modulation electrodes 11 and 12 in contact with the optical waveguides 4 and 5 are configured to have an acute angle θ. As the angle θ of the electrode end becomes smaller, the lines of electric force tend to concentrate at this end, and the electric field strength near the end tends to become stronger. Therefore, by arranging the optical waveguides 4 and 5 near this end, there is an advantage that the driving voltage can be reduced by the effect of increasing the electric field strength. The smaller the angle θ, the more remarkable the effect.

FIG. 10 is a sectional view of another embodiment of the present invention. In this case, the modulation voltages 11 and 12 are the modulation electrode metal film 22 and the modulation electrode metals 11 ′ and 12 ′ having a thickness t ′ sufficiently smaller than the electrode gap g.
And the thickness of the modulation electrodes 11 and 12 is t. Since the thickness t'of the metal film 22 is sufficiently smaller than g, in the vicinity of this end, as in the embodiment of FIG.
There is an advantage that the effect of increasing the electric field strength can be obtained and the drive voltage can be reduced. Further, in the metals 11 'and 12', as in the embodiment of FIG. 8, since the overhang portion is formed, the effective refractive index _nm can be reduced even if the thickness t of the modulation electrode is thin. is there.

FIG. 11 is a sectional view of another embodiment of the present invention, in which a groove 23 is formed near the surface of the substrate 3 between the modulation electrodes 11 and 12. The relative permittivity of the groove 23 is the relative permittivity of air (ε _a ~
Becomes 1), is smaller than the dielectric constant of the substrate, the capacitance C _s of the substrate 3 shown in FIG. 2 is smaller than the C _s when no groove 23 in the embodiment of FIG. 1 is the effective refractive index n _m becomes smaller. As the depth dg of the groove 23 increases, _nm tends to decrease. (Reference: IEEE J.QE-22,6, P902-906,198
6, Japanese Patent Publication 60-11326). Therefore, according to this embodiment,
Even if the electrode thickness t is relatively small, there is an advantage that n _m can be made small.

FIG. 12 is a sectional view of another embodiment of the present invention, in which an insulator having a dielectric constant smaller than the dielectric constant ε _s of the substrate 3 is provided on the substrate 3 in the vicinity where the optical waveguides 4 and 5 are formed. 24, and the modulation electrodes 11 and 12 are formed so as to cover the insulator 24. In this case, since the thickness of the insulator 24 is t, even if the thickness of the electrode itself is smaller than that in the above embodiment,
The same effect can be obtained. Further, by forming the modulation electrodes 11 and 12 so as to cover the insulator as shown in FIG. 12, even if t is relatively thin, the effective refractive index n _m can be reduced as in the embodiment of FIG. It can be reduced. In this embodiment, the material of the insulator 24 is LN on the substrate 3.
When using, the effect of the present invention can be obtained by using, for example, SiO ₂ or Al ₂ O ₃ .

13 (a) and 13 (b) are sectional views of another embodiment of the present invention. Generally, in a material having an electro-optic effect such as LN, the electro-optic constant has anisotropy with respect to the crystal axis, and in the case of LN, the electro-optic constant in the Z (c) axis direction is X, Y.
Greater than axial. Therefore, as a substrate, for example, Z-cut
When the LN substrate of No. 1 is used, the electric field in the Z direction of the substrate has the maximum magnitude in the vicinity of directly below the end of the modulation electrode. Therefore, the first, fourth, sixth, eighth, ninth, tenth, eleventh of the above-described embodiment is The configuration shown in FIGS. 12 and 12 is advantageous from the viewpoint of minimizing the drive voltage. On the other hand, when the X-cut or Y-cut LN substrate 25 is used as in this embodiment, as shown in FIGS. 13 (a) and 13 (b), light is guided to a position substantially in the middle of the modulation electrodes 19 and 20. It is advantageous to arrange the waveguides 15 and 16. 13 (a) and 13 (b) are modulation electrodes 19 and 20.
As a case where a coplanar line is used as the above, and the modulation electrodes 19 and 20 and the optical waveguides 15 and 16 are not directly in contact with each other in the above-described embodiment, the problem of the propagation loss of the optical waveguide due to the modulation electrode is caused. Therefore, the buffer layer 6 in the above embodiment is unnecessary.

The effects of the present invention can be similarly obtained by applying various microwave planar circuits such as a slot line and a microstrip line as the modulation electrode, in addition to the embodiments described above. Further, not only the optical intensity modulator, but also an optical frequency modulator, an optical phase modulator, and an optical device such as an optical switch for which it is effective to match the phase velocities of the light and the operation signal using the electro-optical effect The present invention can be applied. Although LN has been described as an example of the substrate forming the optical waveguide, other dielectric substrates or semiconductor substrates having an electro-optical effect may be used. Although the case of using the Ti thermal diffusion waveguide as the optical waveguide has been described, it is obvious that the same effect can be obtained by using a proton exchange waveguide or a ridge waveguide. As for the electrode forming method, the case of using the plating method has been described above, but a normal metal electrode forming method such as a vapor deposition method or a sputtering method may be used.

(Effects of the Invention) As described above, in the high-speed optical modulator of the present invention, since the modulation electrode is formed thick and the effective refractive index of the modulated signal wave can be made close to the effective refractive index of light, the driving voltage It is possible to realize an optical modulator having a high speed and an extremely wide modulation band without increasing the value.

[Brief description of drawings]

1 (a) and 1 (b) are sectional views and plan views of an embodiment of the optical modulator of the present invention, and FIGS. 2 (a), 2 (b) and 3 are related to the embodiment of FIG. FIGS. 4 (a) and 4 (b) are sectional views and a plan view of another embodiment of the optical modulator of the present invention, and FIG. 5 is an embodiment shown in FIG. FIG. 6 (a) and FIG. 6 (b) are sectional views and plan views of another embodiment of the optical modulator of the present invention, and FIG. 7 is an embodiment shown in FIG. FIG. 8, FIG. 9, FIG. 9, FIG. 10, FIG. 11 and FIG. 12 are cross-sectional views of another embodiment of the optical modulator of the present invention, and FIG. 13 (a). , (B) are a sectional view and a plan view of another embodiment of the optical modulator of the present invention, and FIG. 14 is a perspective view showing a configuration example of a conventional optical modulator. DESCRIPTION OF SYMBOLS 1 ... Modulation electrode 2 ... Modulation electrode 3 ... Substrate having an electro-optic effect 4 ... Optical waveguide (directional coupler) 5 ... Optical waveguide (directional coupler) 6 ... Buffer layer, 7 ... Incident light 8 ... Signal source , 9 ... Emitted light 10 ... Termination resistor 11 ... Modulation electrode (symmetrical coplanar strip line) 11 '... Modulation electrode metal 12 ... Modulation electrode (symmetrical coplanar strip line) 12' ... Modulation electrode metal, 13 ... Electricity in substrate Lines of force 14 ... Electric lines of force outside the substrate 15 ... Optical waveguide (Mach-Zehnder interferometer) 16 ... Optical waveguide (Mach-Zehnder interferometer) 17 ... Modulation electrode (asymmetric coplanar strip line) 18 ... Modulation electrode (asymmetric coplanar strip) Line) 19 ... Modulation electrode (coplanar line) 20 ... Modulation electrode (coplanar line) 21 ... Modulation electrode overhang portion 22 ... Modulation electrode metal film, 23 ... Groove formed on substrate 24 ... Insulator or Substrate having a conductive layer 25 ... electro-optical effect

Front Page Continuation (72) Inventor Toshio Suzuki 1-6, Uchiyuki-cho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) Reference JP-A-63-49732 (JP, A)

Claims (4)

[Claims] 1. An optical waveguide is provided near one surface of a substrate,
Also, in an optical modulator having at least a modulation electrode composed of a central conductor and a ground conductor on the surface on which the optical waveguide is formed, the amount of electric force lines in the substrate generated by the signal wave of the modulation electrode. And the magnitude of the ratio to the amount of electric force lines outside the substrate, the magnitude of the effective refractive index of the modulation electrode for the signal wave is made close to the magnitude of the effective refractive index of the optical waveguide for the light. A high-speed optical modulator characterized in that the thickness of the modulation electrode is formed. 2. An insulating layer is formed on a portion of the substrate having an optical waveguide near one surface thereof, or on a surface in the vicinity thereof, and the insulating layer is formed so as to cover the insulating layer. In an optical modulator including at least a modulation electrode composed of a central conductor and a ground conductor, the amount of electric force lines inside the substrate and the amount of electric force lines outside the substrate generated by a signal wave of the modulation electrode. The thickness of the insulating layer is formed so that the magnitude of the effective refractive index of the modulation electrode with respect to the signal wave approaches the magnitude of the effective refractive index of the optical waveguide with respect to the light according to the magnitude of the ratio of A high-speed optical modulator characterized by the above. 3. A side surface of the center conductor and a side surface of the ground conductor on a side where the center conductor and the ground conductor of the modulation electrode face each other, and an end portion of a surface in contact with the substrate or a surface in contact with the substrate. The end portion on the opposite side to the side, or the angle of both end portions thereof is configured to be an acute angle.
A high-speed optical modulator according to item 2 or item 3. 4. A groove is formed in the vicinity of the surface of the substrate at an intermediate position where the center conductor and the ground conductor of the modulation electrode face each other. The high-speed optical modulator according to claim 1.