JPH0476456B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a high-speed, broadband optical modulator.

[Prior Art] FIGS. 5a and 5b show a plan view and a partially enlarged sectional view of the AA' section of a conventional optical phase modulator. In this figure, Z-cut LiNbO ₃ with electro-optic effect is shown.
A straight optical waveguide 2 is formed on a substrate 1 by thermal diffusion of Ti. On top of the substrate 1 is SiO ₂ of thickness D.
A buffer layer 3 (generally D is 2000 to 3000 Å) is formed, and a coplanar waveguide (CPW) consisting of a center electrode 4 and a ground electrode 5 is further formed thereon. These dimensions include the width of the center electrode 4, which is 2W.
is 25 μm, and the gap 2G between the center electrode 4 and the ground electrode 5 is 6 μm. These electrodes 4, 5
is collectively called a CPW electrode, and since the characteristic impedance of this CPW electrode is 21Ω, 42Ω is selected as the value of the terminating resistor 6. Drive power is supplied from a modulation microwave signal feed line 7.

In the case of this optical modulator, since the CPW electrodes 4 and 5 are configured as traveling wave electrodes, ideally there is no restriction on the bandwidth due to the electric circuit. In addition, as long as the propagation speed of the modulating microwave signal wave propagating between the CPW electrodes 4 and 5 and the light propagating in the optical waveguide 2 match, there is no restriction on the modulation band, so it can be used as a high-speed optical modulator. Operable.

However, in reality, there is a difference between the speed of a signal wave and the speed of light, and this limits the modulation band.
The microwave effective refractive index of the substrate for the signal wave is
Assuming that n _n is the effective refractive index of the optical waveguide for light, n _p is the length of the CPW electrodes 4 and 5 in the part that interacts with the optical waveguide, and c is the speed of light, the bandwidth BW generated by this speed difference is is BW=1.9c/(πl｜n _n −n _p ｜) (1) (References: Journal of the Institute of Electronics and Communication Engineers (C), J64
-C, 4, p.264-271, 1981). The microwave effective refractive index n _n is given by n _n =√ _eff (2) with respect to the effective permittivity ε _eff of the substrate.

In a substrate material having an electro-optic effect, the microwave effective refractive index n _n for signal waves is larger than the effective refractive index n _p for normal light. The effective dielectric constant ε _eff of the substrate 1 is mainly determined by the dielectric constant ε _r2 and thickness of the substrate material, the gap 2G of the CPW electrodes 4 and 5, the operating frequency, etc. The thickness of the substrate is usually 0.5 to several mm from the viewpoint of ease of handling the substrate when manufacturing the optical modulator. Usually, since the thickness of the substrate is sufficiently larger than the electrode spacing 2G, ε _eff ≈(ε _r2 +1)/2 (3).

The dielectric constant ε _r2 of LiNbO ₃ substrate 1 is ≒35, (2), (3)
From the formula, the microwave effective refractive index n _n ≒4.2. n _p
= 2.1, the microwave effective refractive index n _n is approximately twice as large as the effective refractive index n _p .

Therefore, during 5 GHz operation, the length l of the CPW electrodes 4 and 5 that interact with the optical waveguide is selected to be approximately 10 mm based on equation (1).

[Problem to be solved by the invention]

In order to operate the conventional optical modulator shown in FIG. 5 at high speed, it is necessary to shorten the length l of the CPW electrodes 4 and 5 according to the operating frequency, as can be seen from equation (1). However, if the electrode length l is shortened, the drive voltage of the optical modulator increases, which has the disadvantage of decreasing modulation efficiency.

In addition, in the conventional design, the influence of the SiO ₂ buffer layer 3 is ignored and only the effective dielectric constant ε _eff of the substrate 1 is used.
From equations (3) and (2), the microwave effective refractive index is
I was giving n _n . However, when the inventor analyzed it by applying the spectral domain method described below, it was found that SiO ₂
It has become clear that as the buffer layer 3 becomes thicker, the microwave effective refractive index n _n becomes lower. Those values are shown in FIG. 6 by dashed lines and dash-dotted lines. Therefore, in reality, the thickness D of the buffer layer is increased.
It can be seen that by bringing n _n closer to n _p , a wider band can be achieved as expressed by equation (1).

Figure 7 a, b, c are shown in Figure 5, 2W=
In order to verify the influence of the SiO ₂ buffer layer 3 in a phase modulator with a conventional electrode structure of 35 μm and 2G = 6 μm, the thickness D was set to 1000 Å, 5000 Å,
Electric field in the depth direction of the substrate E _y when changed to 10000Å
This is the result of a rigorous calculation of the intensity distribution. The figure shows the intensity of the electric field E _y at depths of 0, 1, 2, and 3 μm in the depth direction from the interface between the buffer layer and LiNbO ₃ . Although the depth parameter is not illustrated here, the deeper the depth, the smaller the absolute value of the electric field strength. Figure 7 a, b,
As can be seen from the comparison of c, as the thickness D of the buffer layer 3 is increased, the electric field strength near the edge of the center electrode 4 becomes significantly weaker. Therefore, in the conventional structure in which the waveguide 2 is located near the edge of the center electrode 4, if the thickness D of the buffer layer 3 is increased in order to reduce _n , the electric field strength remains the same unless the driving voltage is significantly increased. The disadvantage was that it was not possible to obtain Therefore, with the conventional structure, there was a problem in that a wide band/high speed and an improvement in modulation efficiency were not compatible with each other.

The present invention has been made against this background, and its object is to provide an optical modulator that can operate at high speed while minimizing a decrease in modulation efficiency.

[Means to solve the problem]

The present invention comprises a substrate having an electro-optic effect and having at least one optical waveguide, a buffer layer formed on the substrate, and a center electrode and a ground electrode formed on the buffer layer. In an optical modulator configured with a coplanar wave guide electrode, the effective microwave refractive index of the substrate for a modulating microwave signal applied to the coplanar wave guide electrode is equal to the effective refractive index of light propagating through the optical waveguide. The thickness of the buffer layer is set to be close to that of the optical waveguide, the width of the center electrode is approximately equal to the width of the optical waveguide, and the optical waveguide is arranged below the center electrode.

As described above, the present invention differs from the prior art in that the buffer layer is thicker and the center electrode is narrower.

[Effect]

In the present invention, by increasing the thickness D of the buffer layer, the effective microwave refractive index n _n of the substrate for the modulating microwave signal can be reduced,
Therefore, the bandwidth expressed by equation (1) can be increased. Furthermore, in the present invention, since the width 2W of the center electrode is made approximately equal to the width of the optical waveguide, a feature is that deterioration in electric field strength can be suppressed to a small level even if the thickness of the buffer layer is increased. Therefore, high-speed optical modulation is possible while suppressing an increase in the drive power of the modulation microwave signal.

〔Example〕

FIG. 1 is a diagram illustrating an optical phase modulator according to a first embodiment of the present invention, and FIGS. 1A and 1B are a plan view and a partially enlarged sectional view of the AA' section. The difference from the conventional example is that the width 2W of the center electrode 4 is narrower, and is approximately equal to the width of the optical waveguide 2. Also, the gap width 2G is a larger value than 2W. Furthermore, the thickness of the buffer layer has become extremely thick. That is, in this embodiment, the center electrode 4
The width 2W of the gap is 8 μm, the width 2G of the gap is 34 μm, the electrode length l is 10 mm, and the width of the optical waveguide 2 is 6 μm. Furthermore, the thickness of the buffer layer is set to 5000 Å.

In the embodiment shown in FIG. 1, the analysis result of the relationship between the microwave effective refractive index n _n of the substrate and the thickness D of the buffer layer 3 with respect to the modulating signal wave is shown by a solid line in FIG. This analysis uses the spectral domain method (Kawano: “Hybrid-mode analysis of a
broadside−coupled microstrip line”, IEE
Proc.Pt.H, vol.131, pp.21-24, 1984),
The influence of buffer layer 3 is calculated with high accuracy. As can be seen from this figure, as in the conventional example, as the thickness D of the buffer layer increases, n _n decreases and approaches the effective refractive index n _p of the optical waveguide. the result,
The difference in phase velocity between microwaves and light is improved, enabling high-speed, broadband operation.

On the other hand, _FIG .
Similarly to the figure, the intensity of the electric field E _y in the substrate depth direction at positions 0, 1, 2, and 3 μm in the substrate depth direction from the interface between the buffer layer 3 and the LiNbO ₃ substrate 1 is shown. Note that the scale of the figure is the same as that of FIG. 7 for both the vertical and horizontal axes. As you can see from the figure,
In the CPW electrode of this example as well, it can be seen that the intensity of the electric field E _y near the edge of the center electrode 4 becomes weaker as the thickness D of the buffer layer 3 increases, but the electric field intensity decreases as the thickness D of the buffer layer increases. Deterioration is significantly smaller than in the conventional example.

Therefore, it is preferable for the thickness D of the buffer layer to be thick as shown in FIG. 6, but on the other hand, it is not preferable to make it thick as shown in FIG. 2 because the electric field strength becomes small. Therefore, the value of D is determined from the balance between the two.

In this embodiment, since the center of the optical waveguide 2 is located directly below the center of the center electrode 4, the buffer layer 3
Even if the thickness D is increased, the increase in driving power can be kept sufficiently small. In other words, by adopting the configuration of the present invention, high-speed, wide-band operation is possible while suppressing an increase in drive power.

In addition, configurations that are seemingly similar to this configuration are Mizuochi, Izutsu,
Literature by Sueda et al. (IEICE Technical Report OQE87-163, pp.29~
36, February 1988, but in this paper, the buffer layer is simply used to suppress the loss of light due to the resonator electrode, and the improvement of the phase velocity mismatch between microwave and light is Not thought about. Therefore, the thickness of the buffer layer is made as thin as 1100 Å for the sole purpose of reducing the driving voltage. This configuration was created because the conformal mapping method, which cannot handle the influence of the buffer layer, was used as the analysis method for the electric field formed by the electrodes, and also because the thickness D of the buffer layer 3 and the electric field E _y were It is presumed that this was because there was no recognition that strength and strength were interrelated.
Furthermore, although this resonant electrode type optical modulator is a high-speed, narrow-band optical modulator that is intended to operate only at a certain resonant frequency, it is different from the baseband operation having a wide band that is the object of the present invention. Therefore, no improvement has been made to the above-mentioned phase velocity mismatch.

In the above embodiment, the case where the optical waveguide 2 is a straight optical waveguide, that is, the case of a phase modulator has been explained, but if a Matsuhatsu Enda type optical waveguide is used as the optical waveguide, a Matsuhatsu Enda intensity optical modulator can be constructed. A second embodiment thereof is shown in FIG. In the figure, a is a plan view, and b is a partially enlarged cross-sectional view of the AA' cross section of a. The difference between this configuration and FIG. 1 is that the other optical waveguide 2 is placed below the edge of the electrode 5.

FIG. 4a is a modulation characteristic showing the effect of the first embodiment of the present invention. In this example, the wavelength is 1.5μ.
These are the measurement results of the modulation characteristics of the m-band phase modulator,
The modulation band (optical 3 dB band, that is, the 3 dB degradation band of received optical power) is 12 GHz, and the half-wave voltage is
9V, driving power per unit band is 15mW/GHz
It is. Despite this high speed and wide band, the half wavelength voltage is low, and the CPW electrodes 4 and 5
Since the characteristic impedance is designed to be close to 50Ω, the drive power is also low.

FIG. 4b is a modulation characteristic showing the effect of the second embodiment of the present invention. In this example, the wavelength is 1.5μ.
These are the measurement results of the M-band Matsuha Tsuender intensity optical modulator.The modulation band is 12GHz, the half-wave voltage is 8.2V, and the driving power per unit band is 13mW/GHz, which provides excellent modulation characteristics similar to phase modulators. I was able to do that.

Next, we will develop wavelength
Another example of the Matsuhatsu Enda intensity optical modulator for the 1.5 μm band is shown. Regarding the structure of the CPW electrode, the center electrode width 2W and the gap width 2G are each 8μ.
m and 15 μm, and the length l of the CPW electrode in the portion that interacts with the light guide path was 27 mm. Further, the thickness D of the buffer layer was 12000 Å, and the width of the waveguide was 6 μm. The characteristics of the Matsuhatsu Enda intensity optical modulator with this structure are a modulation band of 9 GHz, a half-wave voltage of 5 V,
Drive power 63mW (drive power per unit band 7m
W/GHz).

In the structure of this embodiment, the thickness D of the buffer layer is further increased compared to the embodiment shown in FIG. In addition, the design is such that the increase in drive voltage caused by thickening the buffer layer is avoided by increasing the electrode length l.

In the above example, LiNbO ₃ was used as the substrate, but other substrates having electro-optic effects such as
LiTaO ₃ or the like may also be used.

Furthermore, in the above embodiment, the width of the center electrode 4 is 2W (8μ
m) is approximately equal to the width (6 μm) of the optical waveguide 2, but the range of 2W is 2σ to 4σ relative to the spot size σ of the light propagating through the optical waveguide 2.
This means that the range is suitable. In the above example (width of optical waveguide 6 μm), the spot size σ
is approximately 3 μm, and therefore the range of the width 2W of the center electrode is preferably 6 to 12 μm (2σ to 4σ). That is, as shown in FIG.
The main power component of the light propagating in the optical waveguide 2 must be contained within the electric field region formed by the electric field, and the electric field region must not be wider than the main power component of the light. be.

〔Effect of the invention〕

As explained above, in the present invention, a CPW electrode is used in which the width of the center electrode 4 is approximately equal to the width of the optical waveguide, and the thickness of the buffer layer is extremely thick compared to the conventional one, so that a significant increase in driving power is avoided. This has the advantage that high-speed, broadband optical modulation can be performed without any interference.

[Brief explanation of the drawing]

FIG. 1 shows an optical phase modulator according to a first embodiment of the present invention, in which FIG. 1a is a plan view and FIG. 1b is a partially enlarged sectional view of the AA' section. FIG. 2 shows the electric field strength distribution in the depth direction of the substrate in the first embodiment. FIG. 3 shows a Matsuhatsu Enda intensity optical modulator according to a second embodiment of the present invention. Figure 4 shows the first and second embodiments of the present invention.
2 is a modulation index characteristic showing the effect of the embodiment. Fifth
The figure shows a conventional example of an optical phase modulator. Figure 6 shows the microwave effective refractive index of the substrate depending on the thickness of the buffing layer.
n Shows the change in _n . FIG. 7 shows the electric field strength distribution in the depth direction of the substrate in the conventional example shown in FIG. 1... LiNbO ₃ substrate, 2... Optical waveguide, 3...
SiO ₂ buffer layer, 4... Center electrode, 5... Earth electrode, 6... Termination resistor, 7... Microwave signal feed line for modulation.

Claims (1)

[Claims] 1. A substrate having an electro-optic effect and having at least one optical waveguide, a buffer layer formed on the substrate, and a center electrode and a ground electrode formed on the buffer layer. and a coplanar wave guide electrode, wherein the effective microwave refractive index of the substrate with respect to the modulating microwave signal applied to the coplanar wave guide electrode is the effective refractive index of the light propagating through the optical waveguide. The thickness of the buffer layer is set to be close to the refractive index, the width of the center electrode is approximately equal to the width of the optical waveguide, and the optical waveguide is arranged below the center electrode. optical modulator.