JPH0675256A - Waveguide type optical modulator - Google Patents

Abstract

PURPOSE:To provide the optical modulator of low loss and low voltage driving by forming a single mode waveguide structure consisting of an org. high-polymer material exhibiting a high primary electrooptical effect. CONSTITUTION:This optical modulator has the structure disposed with a coplanar electrode 11 consisting of metal on the uppermost part (or oxide silicon film of the lower part) and is constituted by using the org. high-polymer material exhibiting the primary electrooptical effect as the material of core layers 8, optimizing the difference in the specific refractive index from the materials of the ambient clad layers 7, 9, 10 to constitute the single mode waveguides and providing the surface of the silicon oxide layer (or upper clad layer) in the lower part with another grounding electrode 6. The confinement of electric fields is thereby increased and the modulation operation with the low voltage is executed. Then, the org. high-polymer material having the high primary electrooptical effect is used as the material of the waveguide layer and the difference in the specific refractive index from the material of the ambient clad layers is optimized to form the single mode waveguide. In addition, the grounding electrode 6 is separately provided in order to increase the confinement of the electric fields of coplanar line type electrodes 11. Further, the spacing between the upper and lower electrodes is narrowed and, therefore, the modulation operation is executed with the extremely low driving voltage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical modulator, and more particularly to an optical modulator which applies an electro-optical effect and is driven at a low voltage.

[0002]

The electro-optic effect is a phenomenon in which the refractive index of an optical medium changes when an electric field is applied to the medium.
There are a linear electro-optic effect (Pockels effect) caused by the second-order optical nonlinearity and a second-order electro-optic effect (Kerr effect) caused by the third-order optical nonlinearity. Practically, since the second-order nonlinear constant is several orders of magnitude larger than the third-order nonlinear constant, the electro-optic effect utilizing the second-order nonlinearity is often used. A waveguide type optical modulator utilizing this effect is incorporated in an optical integrated circuit and can be applied to high-speed external modulation of a semiconductor laser or the like. Therefore, an optical modulator having a high operation speed (wide modulation band) is strongly demanded. . Examples of this type of external modulator include a light intensity modulator and an optical phase modulator.

As an example of a conventional external modulator, a light intensity modulator using an inorganic ferroelectric crystal LiNbO ₃ (LN) whose perspective view is shown in FIG. 5 (reference: Izutsu et al., IEICE Transactions, J64 -Vol. C, No. 4, 264, 1981).
Here, the modulation electrode 1 is a traveling wave electrode made of a metal layer such as Au or Cu, and a symmetrical or asymmetrical flat strip line is used. The optical waveguide portion 2 is a portion in which Ti ions and the like are internally diffused in the LN crystal 3 cut out in an appropriate orientation to increase the refractive index. The incident light is branched into two boats at the Y branch near the entrance, and the light propagating in one boat interacts with the microwave applied by the electrode 1 to cause a phase change. The light wave having the phase change merges with the light wave propagating through another boat (without the phase change) at the Y branch near the exit, and the superposed light wave causes the intensity change due to the phase change.

In the case of this optical modulator, since the electrode 1 is constructed as a traveling wave electrode, ideally, there is no limitation on the bandwidth of an electric circuit. Further, as long as the propagation speeds of the microwave propagating through the electrode 1 and the light wave are the same, there is no limitation on the bandwidth due to the influence of the time during which the incident light travels through the optical waveguide 2. To be done.

In reality, however, there is a speed difference between the microwave and light, which limits the bandwidth. The equivalent refractive index of the LN crystal for microwave and light is n _m ,
_Assuming that n _{o is} the length of the electrode on the waveguide and l is the bandwidth BW caused by this velocity difference,

[0006] BW = 1.4c / (π1 | n m -n o |) (1) where, c is the speed of light

[0007] Here, n _o is approximately 2.3 in the 1.3 μm band, and n _m can be approximately estimated by the following equation and is approximately 4.2.

N _m = [(1 + ε _g ) / 2] ^1/2 (2) where ε _g is the dielectric constant of the crystal forming the waveguide.

As described above, in the conventional optical modulator, since the dielectric constant (about 34) of the crystal itself that constitutes the waveguide is large, the equivalent refractive index of the microwave greatly exceeds the refractive index of the crystal,
Bandwidth is limited by the phase velocity difference between microwaves and light waves. Further, in order to improve this velocity mismatch, it is necessary to shorten the interaction length (electrode length) l of the microwave and the light wave, as can be seen from the equation (1). However, when the electrode length 1 is shortened, the driving voltage increases, and the modulation efficiency decreases.

On the other hand, a nonlinear optical material made of an organic polymer material has a smaller dielectric constant than an inorganic material, and it is expected that the above-mentioned problem of speed mismatch can be greatly improved, and in addition, a thin film is required. It is expected to be used as a high-performance optical modulator material because it has excellent workability and has a large second-order nonlinear optical coefficient χ (2) in a wide wavelength range.

[0011]

As described above, the organic polymer material has various advantages as a material for an optical modulator, but in reality, it has a single mode waveguide structure effective for improving the modulation efficiency. As for the optical modulator in which the electrode structure is optimized so that the velocity matching between the microwave and the light is achieved, there are few examples using an organic polymer material. Further, there has been no report of a waveguide type optical modulator using such an organic polymer material and having a low loss and driven at a low voltage of 5 V or less.

The present invention has been made under such a background, and an object thereof is to solve the above problems of the existing optical modulator and to provide an optical modulator of low loss and low voltage drive. Especially.

[0013]

According to the present invention, in an optical modulator having a structure in which a coplanar electrode made of a metal is disposed on the uppermost (or lower silicon oxide film), an organic modulator showing a primary electro-optical effect is obtained. A polymer material is used for the core layer material, the relative refractive index difference with the surrounding cladding layer material is optimized to form a single mode waveguide, and another ground electrode is placed on the lower silicon oxide film (or upper cladding layer). Is provided to strengthen the confinement of the electric field and perform the modulation operation at a low voltage.

The conventional technique has a single-mode waveguide structure made of an organic polymer material exhibiting a large first-order electro-optical effect, and a silicon oxide film (or a lower silicon oxide film) in addition to the coplanar line type electrode. The difference is that a ground electrode is arranged on the upper clad layer).

[0015]

The optical modulator having a structure in which both the coplanar line type electrode and the ground electrode of the present invention are arranged, compared with the conventional optical modulator having only symmetrical or asymmetrical coplanar electrodes as shown in FIG. , Because the electric field of the modulating electric signal is largely confined and the electric field and the guided light overlap each other,
It is easy to reduce the drive voltage.

FIG. 1 shows the result of calculation of the relationship between the overlap integral Γ of the microwave and the light wave propagating through the coplanar line type electrode and the air gap g between the upper cladding layer and the upper ground electrode. In the calculation, it is assumed that the coplanar electrode is formed on the lower silicon oxide film, the thickness of the waveguide is 2 μm, and the total thickness of the waveguide and the upper and lower cladding layers is 10 μm. As is clear from the figure, Γ increases as the gap g decreases, and Γ becomes maximum when the ground electrode is formed on the upper clad layer (g = 0 μm). Since the overlap integral Γ is inversely proportional to the drive voltage, the larger Γ, the lower the drive voltage. The same effect of reducing the driving voltage can be recognized even when the coplanar electrode is formed on the upper clad layer and the ground electrode is formed on the lower silicon oxide film.

Further, in the optical modulator of the present invention, an organic polymer material exhibiting a primary electro-optical effect is used as the core layer material, and the relative refractive index difference with the surrounding clad layer material is optimized to make the single mode waveguide. I am trying. Therefore, even if the distance between the upper and lower electrodes is narrowed, it is possible to suppress the optical loss due to the metal forming the electrodes to a range that does not pose a practical problem, so that the low voltage driving which is the object of the present invention can be performed. Become. In the optical modulator of the present invention, the thickness of the waveguide made of the organic polymer is 2 μm or less, and the thickness of the waveguide and the upper and lower clad layers is 10 μm or less. FIG. 2 shows the relationship between the dimensions w and d of the rectangular optical waveguide satisfying the single mode and the relative refractive index difference Δn ₂ between the waveguide and the upper and lower clad layers in contact with it.
It is the result calculated by the equivalent refractive index method. Here, Δn ₃ is the relative refractive index difference between the waveguide and the left and right cladding layers in contact with it. As can be seen from the figure, the film thickness of the waveguide is 2 μm or less,
When the total thickness of the waveguide and the upper and lower clad layers is 10 μm or less, the relative refractive index difference Δn ₃ between the waveguide and the left and right clad layers is 0.5% or less, and the clad between the waveguide and the upper and lower clad layers is 0.5% or less. If the relative refractive index difference Δn ₂ of the layer is 2%, the waveguide width w is 4 to 6 μm, which is the optimum mode matching with the polarization maintaining fiber having a small diameter (5 μm diameter), and the waveguide becomes a single mode waveguide. .

In the optical modulator of the present invention, since the dielectric constant (about 3.6) of the organic polymer itself which constitutes the waveguide is small, the equivalent refractive index of microwave becomes almost equal to the refractive index of the organic polymer. , The band is not limited by the phase velocity difference between the microwave and the light wave. Therefore, it is not necessary to shorten the interaction length (electrode length) l of the microwave and the light wave.

Further, in the optical modulator of the present invention, the central conductor width of the coplanar line type electrode and the gap between the ground electrode in the horizontal direction can be optimized to achieve impedance matching with the coaxial cable, and a wide frequency band. Can be used as an electric signal for modulation.

As a polymer material in which a substance having a large second-order optical non-linear susceptibility used in the present invention is dissolved or bonded, a polymer material can be easily made into a low loss and has a high degree of processing freedom. Methyl methacrylate (PMMA)
Acrylic ester resins, polystyrene resins, deuterium-substituted products of these resins, or resins containing a fluorine atom as described in Japanese Patent Application No. 3-269728, and Japanese Patent Application No. No. 184080 or Japanese Patent Application No. 4-20074, a material in which an organic dye compound composed of a stilbene compound, an azo compound or an azomethine compound is dispersed in a polymer or bonded to a side chain of the polymer. Can be used.

In the optical modulator of the present invention, the material used for the clad layers on the upper, lower, left and right sides of the waveguide is such that it has a smaller refractive index than the above organic polymer material constituting the core layer. Although not particularly limited, for example, an epoxy-based or acrylic-based ultraviolet curable resin can be used.

When the above-mentioned organic polymer used in the optical modulator of the present invention is subjected to polarization treatment, the optical modulator is sandwiched between a pair of electrodes and the glass transition temperature of the organic polymer (about 1
(100 to 150 ° C.) or higher, and a high voltage is applied between the electrodes to perform electric field orientation of the organic polymer. After that, the orientation is frozen by gradually cooling while applying the high voltage. By doing so, the primary electrochemical effect can be imparted to the core portion of the waveguide to be exhibited.

Next, the waveguide type optical modulator of the present invention will be specifically described with reference to Examples.

[0024]

Embodiment 1 FIG. 3 is a sectional view of a waveguide type optical intensity modulator of the present invention. As shown in FIG. 3, a lower ground electrode 6 made of gold is formed on the silicon oxide film 5 by a plating method on a silicon substrate 4 on which a silicon oxide film 5 is formed by a thermal oxidation method, and then epoxy-based ultraviolet curing is performed. The resin was applied to a thickness of 4 μm to form the lower clad layer 7. After coating the organic polymer of Chemical formula 1 on this with a thickness of 2 μm by spin coating,
By using a Mach-Zehnder interferometer type waveguide pattern mask, by the method described in Japanese Patent Application No. 3-254352,
A channel waveguide layer 8 having a size of 2 × 5 μm was formed. Next, an epoxy-based ultraviolet curable resin is applied thickly on this, and is etched to the same height as the waveguide by reactive ion etching to form the clad layers 9 on the left and right of the waveguide.
Was formed. At this time, the epoxy-based ultraviolet curable resin used for the left and right clad layers had a relative refractive index difference of 0.3% from that of the channel waveguide layer 8. The upper clad layer 10 is formed by applying an epixy ultraviolet curing resin to a thickness of 4 μm.
Was formed. Here, the epoxy-based UV curable resin used for the upper and lower clad layers 7 and 10 had a relative refractive index difference of 2% from that of the channel waveguide layer 8. Further, on the upper clad layer 10, a central conductor width of 20 μm, a thickness of 5 μm, and a gap 1 made of gold are formed so as to overlap with one arm of the Mach-Zehnder interferometer type channel waveguide 8.
A 0 μm coplanar line type electrode 11 was formed by a plating method. Coplanar line type electrode 1 overlapping with channel waveguide
The length of 1 was 15 mm.

This waveguide type optical modulator was subjected to polarization treatment by parallel plate electrodes, and then laser light having a wavelength of 1.55 μm was incident from the end face to measure a near-field image of the emitted light.
It was confirmed that only a single mode propagated. Furthermore, when microwaves were input via the coaxial cable, it was found that the light intensity could be modulated up to 20 GHz with a driving voltage of about 4V.

[0026]

[Chemical 1]

[0027]

[Embodiment 2] FIG. 4 is a sectional view of a waveguide type optical intensity modulator of the present invention. As shown in FIG. 4, a coplanar line type electrode 14 made of gold and having a central conductor width of 7 μm, a thickness of 5 μm and a gap of 8 μm was formed on a silicon substrate 12 on which a silicon oxide film 13 was formed by a thermal oxidation method by a plating method. . An epoxy-based ultraviolet curable resin was applied to the above to a thickness of 4 μm to form the lower clad layer 15. After the organic polymer of Chemical formula 2 was applied thereon to a thickness of 2 μm by a spin coating method, a Mach-Zehnder interferometer type waveguide pattern mask was overlaid and exposed with ultraviolet rays to expose a channel guide having dimensions of 2 × 5 μm. Waveguide layer 16 and clad layers 1 on the left and right of this waveguide layer
7 and 7 were formed. At this time, adjust the amount of UV exposure,
The relative refractive index difference between the channel waveguide layer 16 and the left and right cladding layers 17 was set to 0.2% to 0.5%. In addition, a coplanar line type electrode 1 overlapping the channel waveguide 1
The length of 4 was 15 mm. Next, an epoxy-based UV-curable resin is applied to this to a thickness of 4 μm to form an upper clad layer 1
8 was formed. Here, the upper and lower clad layers 15 and 1
The epoxy-based ultraviolet curable resin used in No. 8 had a relative refractive index difference of 2% from the channel waveguide layer 16. Further, an upper ground electrode 19 was formed on the upper clad layer 18 by a plating method.

This waveguide type optical modulator was polarized by a parallel plate electrode, and then laser light having a wavelength of 1.55 μm was made incident from the end face to measure a near-field image of the emitted light.
It was confirmed that only a single mode propagated. Furthermore, when microwaves were input through the coaxial cable, it was found that the light intensity could be modulated up to 20 GHz with a driving voltage of about 3V.

[0029]

[Chemical 2]

[0030]

As described above, in the waveguide type optical modulator according to the present invention, an organic polymer material having a large primary electro-optic effect is used as the channel waveguide layer material, and the relative refractive index with the surrounding cladding layer material is increased. A ground electrode is additionally provided in order to optimize the index difference to form a single mode waveguide and to increase the electric field confinement of the coplanar line type electrode. Further, since the space between the upper and lower electrodes is narrowed, the modulation operation can be realized with an extremely low drive voltage.

[Brief description of drawings]

FIG. 1 is an overlapping characteristic diagram of a microwave and a light wave of a waveguide type optical modulator of the present invention.

FIG. 2 is a waveguide parameter characteristic diagram which gives a single mode condition of the waveguide type optical modulator of the present invention.

FIG. 3 is a configuration of a waveguide type optical intensity modulator according to the present invention.

FIG. 4 is a configuration of a waveguide type optical intensity modulator according to the present invention.

FIG. 5 is a perspective view of a conventional waveguide type optical intensity modulator.

[Explanation of symbols]

 1 Modulation Electrode 2 Optical Waveguide Section 3 LN Crystal 4 Silicon Substrate 5 Silicon Oxide Film 6 Lower Ground Electrode 7 Lower Cladding Layer 8 Channel Waveguide Layer 9 Cladding Layer 10 Upper Cladding Layer 11 Coplanar Waveguide Type Electrode 12 Silicon Substrate 13 Silicon Oxide Film 14 Coplanar line type electrode 15 Lower clad layer 16 Channel waveguide layer 17 Clad layer 18 Upper clad layer 19 Upper ground electrode

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Haruki Ozawaguchi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. A channel waveguide made of a material obtained by polarization-processing a polymer in which a substance having a large second-order nonlinear susceptibility is dissolved or bound to a silicon substrate on which a silicon oxide film is formed. And a waveguide having a structure in which a clad layer having a refractive index lower than that of the waveguide layer is disposed on the left and right and above and below the waveguide layer, and a coplanar line type electrode made of metal is disposed on the upper clad layer. 1. A waveguide type optical modulator, wherein a lower ground electrode made of metal is disposed on a lower silicon oxide film in the type optical modulator. 2. The thickness of the waveguide is 2 μm or less, the thickness of the waveguide and the upper and lower clad layers is 10 μm or less, and the relative refractive index difference between the waveguide and the left and right clad layers is 0.2. % ~
0.5%, the relative refractive index difference between the waveguide and the upper and lower clad layers is set to 2% or more, and the width of the central body of the lower coplanar line type electrode is about 2 of the film thickness of the waveguide and the clad layer combined. The waveguide type optical modulator according to claim 1, wherein the number is doubled. 3. A metal coplanar line type electrode is provided on a silicon substrate having a silicon oxide film formed thereon,
A channel waveguide layer made of a material in which a polymer having a high second-order nonlinear susceptibility dissolved or bound thereto is polarized, and the waveguides are formed on the left, right, top, and bottom of the waveguide layer. In a waveguide type optical modulator having a structure in which a clad layer having a refractive index lower than that of the layer is arranged, an upper ground electrode made of metal is arranged on the upper clad layer. . 4. The thickness of the waveguide is 2 μm or less, the thickness of the waveguide and upper and lower cladding layers is 10 μm or less, and the relative refractive index difference between the waveguide and the left and right cladding layers is 0.2. % ~
0.5%, the relative refractive index difference between the waveguide and the upper and lower clad layers is set to 2% or more, and the width of the central body of the lower coplanar line type electrode is about 2 of the film thickness of the waveguide and the clad layer combined. 4. The waveguide type optical modulator according to claim 3, wherein the number is doubled.