JPH0760226B2 - Light modulator - Google Patents

Description

The present invention relates to an optical modulator, and more particularly to an optical modulator having a high operating speed.

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

As an example of a conventional external optical modulator, a semiconductor optical phase modulator (reference: Applied Physics Lette) whose perspective view is shown in FIG.
rs, vol.51, pp.83-85, 1987). Here, the modulation electrode 1 is a traveling wave electrode made of a metal layer such as Au or Cu, and a symmetric coplanar strip (symmetric CPS) is used, and an electro-optic effect is used as a light modulation principle. Incident light 5 propagates through a ridge-shaped GaAs (gallium arsenide) portion 2 which is an optical waveguide. The GaAs portion 2 is formed on an SI (semi-insulating) GaAs substrate 4 with Al _X Ga _1-X As as a buffer layer 3 interposed therebetween.

In the case of this optical modulator, since the electrode 1 is configured as a traveling wave electrode, ideally there is no limitation on the bandwidth of an electric circuit. Further, as long as the signal wave propagating through the electrode 1 and the propagation speed of light coincide with each other, there is no limitation on the bandwidth due to the influence of the time during which the incident light 5 travels in the optical waveguide 2. Therefore, in general, optical modulation for high-speed operation Used for vessels.

However, in reality, there is a speed difference between the signal wave and the light, which limits the bandwidth. Signal wave and each n _m the refractive index of GaAs portion 2 to light, n _o, and the length of the electrode indicated as l, bandwidth BW caused by the speed difference, BW = 1.4 / (πl | n m -n _o |) (1) where c is the speed of light (Reference: Theory of Communication (C), J64-C.4, P264-271,19
81). The refractive index n _m is relative to the effective dielectric constant ε _eff of the GaAs part 2. Given in.

Here n _o is about 3.4 in the 1.3μm band, ε _eff is approximately Becomes Note that ε _s is the dielectric constant of GaAs on which the traveling wave electrode 1 is formed and is about 12. Therefore, n _m becomes about 2.5, which causes a large difference from n _o, and is band-limited by the phase velocity difference.

[Problems to be Solved by the Invention] In order to speed up the operation of the optical modulator shown in FIG.
As can be seen from the equation (1), it is necessary to shorten the length 1 of the electrode 1 according to the operating frequency. However, when the electrode length 1 is shortened, the driving voltage of the optical modulator increases, so that the modulation efficiency is lowered.

The present invention has been made under such a background, and an object thereof is to provide a high speed optical modulator.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides an optical modulator including an optical waveguide made of a semiconductor formed on a substrate and a traveling wave electrode for modulation. Near the region where the waveguide and the signal wave propagating through the modulation electrode interact with each other, at least one layer having a dielectric constant higher than that of air is present at a position including the electromagnetic field of the signal wave, and It is characterized in that it is formed with a thickness such that the refractive index approaches the effective refractive index of light.

Further, the present invention is an optical modulator comprising an optical waveguide made of a semiconductor formed on a substrate and a traveling wave electrode for modulation,
At least one layer having a dielectric constant higher than that of air is present in the vicinity of a region where the optical waveguide and the signal wave propagating through the modulation electrode interact with each other, and at a position including an electromagnetic field of the signal wave. Is formed to have a thickness such that the effective refractive index thereof approaches the effective refractive index of light, and a metal conductor layer is formed on the layer having a high dielectric constant.

[Operation] According to the present invention, since the layer having a high dielectric constant is provided in the vicinity of the traveling wave electrode, the effective dielectric constant ε _eff of the modulation electrode for the modulation signal wave can be increased. As a result, the effective refractive index _nm for the modulated signal wave can be made larger than that of the conventional component, so that the difference in the phase velocities between the modulated signal wave and the light becomes small, and an optical modulator capable of high-speed operation can be realized.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a sectional view of an embodiment of the present invention. The same components as those of the conventional example shown in FIG. In FIG. 1, reference numeral 6 is an overlay composed of a semiconductor or a dielectric material. The thickness of this overlay 6 is D. Electrode 1,1
The electric field due to is applied parallel to the <011> axis of the crystal axis.

According to this embodiment, compared with the conventional semiconductor optical modulator,
The band limitation effect due to the difference in the phase velocities of the modulated signal wave and the light is alleviated. The description will be given below.

FIG. 2 shows the thickness D of the overlay 6 in the embodiment of FIG.
This is the calculation result of the refractive index _nm of the modulated signal wave when the thickness is increased. In the calculation, the electrode width is 20 μm and the gap between the electrodes is 6 μm.
m and calculated by the conformal mapping method. As can be seen, as the thicker D, n _m is asymptotic to n _o.
That is, when the thickness D of the overlay 6 is 0 (conventional example), the electromagnetic fields of the modulation signal propagating through the modulation electrodes are air and Ga.
Although dispersed in the As portion 2, the electromagnetic field existing in the air of the modulation signal decreases as the thickness of the overlay 6 increases, so that n _m increases. In this figure, the overlay 6 is assumed to be Al _X Ga _1-X As. The dielectric constant of Al _X Ga _1-X As is almost equal to that of GaAs 12 regardless of the value of x. Therefore, there is D in which n _m and n _o are almost completely matched, that is, the speed of the light and the modulated signal can be matched. Further, in the present embodiment can match the n _o and n _m be selected an optimum thickness D be varied materials and composition of the example overlay 6.

FIG. 3 shows another embodiment of the present invention. In this example, the shield conductor 7 is further formed on the overlay 6 in FIG. 1, and the same effect can be realized with an overlay having a thickness sufficiently smaller than that in the case of FIG.

FIG. 4 shows still another embodiment of the present invention. In this example, an asymmetric coplanar strip (asymmetric CP
S) is used. Reference numeral 8 denotes a buffer layer, which uses the same Al _X Ga _1-X As as the buffer layer 3. The electric field is applied parallel to the <100> axis of the crystal axis. Also in this case, if the overlay 6 is, for example, Al _X Ga _1-X As or the like,
The same effect as the embodiment shown in the figure is obtained. Further, a similar effect can be obtained by disposing a shield layer as shown in FIG.

Although Al _X Ga _1-X As is used as the material of the overlay 6 in the above example, the same effect can be expected if the optimum thickness D is selected even if the material or composition is changed.

Furthermore, if a material with a higher dielectric constant is used, the overlay 6
The effect can be obtained even if the thickness D is reduced. Also,
In the embodiment shown in FIGS. 1, 3, and 4, the overlay 6 is formed on the entire surface. For example, the symmetry shown in FIG.
Like CPS, partially, namely electrode 1 and optical waveguide 2
The overlay 6 may be formed between the electrode and a part of the electrode. This method can also be applied to the case of the asymmetric CPS shown in FIG.

FIG. 6 shows still another embodiment of the present invention. In this configuration, the gap between the electrodes is small in the upper part of the overlay, and the capacitance in this part is large. Therefore, the same effect can be obtained even if the overlay is thin.

It is obvious that the present invention can be applied to the case of using other modulation electrodes, for example, microstrip line or slot line, and also applicable to other optical control circuits such as intensity modulator and optical switch. Is.

[Effects of the Invention] As described above, in the optical modulator according to the present invention, the modulation electrode propagates in the vicinity of the region where the modulation signal propagating in the modulation electrode interacts with the light propagating in the optical waveguide. At least one layer having a high dielectric constant is formed so that the signal wave and the light propagating through the optical waveguide have a close propagation velocity.
Therefore, the effective refractive index n _m of the modulated signal wave is
Since it can approach n _o , an extremely wide modulation band can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention, FIG. 2 is a diagram showing a relationship between an overlay thickness and an effective refractive index of microwaves, and FIGS. 3 to 6 are embodiments of the present invention. FIG. 7 is a perspective view of a conventional semiconductor optical phase modulator. 1 ... Modulation electrode, 2 ... Optical waveguide, 3 ... Buffer layer, 4 ...
Substrate, 5 ... Incident light, 6 ... Overlay, 7 ... Shield layer,
8 ... Buffer layer.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiromichi Jumonji 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (56) Reference JP-A-49-113648 (JP, A) JP 61-47929 (JP, A) JP 63-49732 (JP, A) Actual development 59-122557 (JP, U) Actual 60-104821 (JP, U) Applied Physics Letters, Vol. 51, p. 83-85

Claims (2)

[Claims] 1. An optical modulator including an optical waveguide made of a semiconductor formed on a substrate and a traveling wave electrode for modulation, wherein a region where the optical waveguide and a signal wave propagating through the modulation electrode interact with each other. At least one layer having a dielectric constant higher than that of air is formed in the vicinity of the signal wave including the electromagnetic field of the signal wave, and the effective refractive index of the signal wave is formed to have a thickness close to that of light. An optical modulator characterized in that. 2. An optical modulator comprising an optical waveguide made of a semiconductor formed on a substrate and a traveling wave electrode for modulation, and a region where the optical waveguide and a signal wave propagating through the modulation electrode interact with each other. In the vicinity of the electromagnetic wave of the signal wave, at least one layer having a dielectric constant higher than that of air is formed in a thickness such that the effective refractive index of the signal wave approaches the effective refractive index of light. And an optical modulator in which a metal conductor layer is formed on the layer having a high dielectric constant.