JPH05119287A - Waveguige type optical device - Google Patents

Abstract

PURPOSE:To provide a high speed modulator which is produced without difficulty, produced as the extension of a general electrode producing stage and does not need the special production of a shielding surface. CONSTITUTION:As to the structure of the high speed modulator, the modulator is constituted of electrooptical crystal having at least one produced waveguide 2, 3 or 4, a buffer layer 5, at least a set of electrodes 6, 7 or 8, a dielectric medium 9 on the electrode, a metallic layer 10 on the dielectric medium, and a metallic contact 11 between the metallic layer on the medium 9.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to optical waveform modulators / switches in a variety of systems, including high speed optical communications, optical switching networks, optical information processing, and optical image processing.

[0002]

2. Description of the Related Art Optical waveform modulators / switches are the most important elements for realizing various systems including high-speed optical communication, optical switching networks, optical information processing, optical image processing and the like. Optical waveform modulators have been made by various manufacturing methods and in some substrates of interest. However, most of the research on optical waveform devices
It includes a LiNbO ₃ substrate and a GaAs substrate. L
The indiffusion of titanium into the iNbO ₃ substrate provides a convenient and relatively simple way to fabricate low loss strip waveguides in the substrate with good electro-optical properties. The important parameters of the waveguide modulator are drive power, modulation band, and insertion loss. The parameters of the modulation band and the driving power have a trade-off relationship. Research on waveguide modulators has focused on optimizing this trade-off relationship.

The bandwidth of a waveguide modulator depends primarily on the electrode type, electrode geometry, and relative permittivity of the substrate. Traveling wave electrodes are widely used for broadband applications. The idea is to make the electrodes look like an extension of the drive transmission line. In that case, the electrodes must have the same characteristic impedance as that of the source or cable. The modulation speed in this case is limited by the difference in transit time (or phase speed or effective index) for the optical microwave. In order to increase the bandwidth, it is necessary to reduce the microwave effective refractive index n _m (from the value 4.2), whereby the microwave effective refractive index n ₀ (compared to the case of the LiNbO ₃ substrate). It approaches the typical value 2.2).

One way to achieve this is to use an air layer created by using a metal screen. This is described in the next paper. "New travel
ing-wave electrode Mach-Z
ehender optical modulator
with 20 GHz bandwidth and and
4.7 V driving voltage at
1.52 μm wavelength ”, Elec
tronics Letters, Vol. 25, N
o. 20, pp 1382-1383 (1989).

The basic structure of a conventional modulator is shown in FIG. FIG. 6A is a perspective view of the modulator, and FIG. 6B is A-.
It is an A'line sectional view. This modulator operates on a z-cut, y-propagating LiNbO ₃ crystal wafer 1 as two Y-branch waveguides (on both sides of the input and the output, as a power distributor at input 2 and as a combiner at output 3). And the phase shifter portion 4 are manufactured by internally diffusing a Ti strip. The SiO ₂ buffer layer 5 is coated on the waveguide, and when the electrode is formed on the waveguide, TM
Reduce mode loss.

The incident wave separated into two equal components by the input 3 dB coupler propagates through the two arms of the phase shifter unit 4. If no phase shift is introduced between the interferometer arms, the two incident components will combine in phase,
Propagation continues without attenuation at the output 3 dB coupler.
For a phase shift of π, the two components experience destructive interference at the output coupler and the transmitted light at this output is minimal. This phase shift is achieved by applying a voltage to the coplanar waveguide (traveling waveguide) electrode structure. This coplanar waveguide electrode structure comprises a signal electrode 6 and two ground electrodes 7, 8 having a characteristic impedance of 50 ohms. A special shielding surface 12 is manufactured separately and placed on the electrodes. On the output side of the electrode structure, a 50 ohm resistor is connected.

[0007]

The bandwidth of a traveling wave modulator is limited by the phase velocity mismatch between the microwave and the light wave. Therefore, it is necessary to reduce the microwave effective index, so that the difference between the microwave effective index and the light wave effective index is reduced. In this example, the microwave effective index is reduced by passing most of the microwave through the air layer above the traveling wave electrode structure. In order to achieve this, it is necessary to fabricate a metallized special shield with precisely sized grooves, which requires special and complex techniques, increases the manufacturing process and reduces the allowable manufacturing tolerances. ..

Therefore, a simple manufacturing process is facilitated,
As a result of the extension of the general electrode manufacturing process, a new structure of the high speed modulator that does not require an extra special shield is required.

An object of the present invention is to provide a waveguide type optical device which meets such a demand.

[0010]

These problems are solved by the present invention described below. The present invention provides an optical optical crystal comprising at least one fabricated waveguide, a buffer layer, at least one set of electrodes, a dielectric on the electrodes, and a ground electrode structure on the dielectric. It is a corrugated device. The dielectric layer can be coated without much difficulty as an extension of the electrode fabrication process. By controlling the dielectric layer thickness and the dielectric material, the 600 GHz band (subject to microwave loss limitation only) can achieve a traveling wave coplanar electrode with a wavelength of 10 mm.

[0011]

In order to increase the bandwidth of the modulator, it is necessary to reduce the microwave effective index, so that the difference between the microwave effective index and the light wave effective index is reduced. In the present invention, the microwave effective index is reduced by passing a dielectric layer overlying the traveling wave electrode structure to most of the microwave. By controlling the thickness and the relative permittivity of the dielectric medium, the microwave effective index can be reduced to be approximately equal to the light wave effective index. A very fast modulator is thus realized.

Thus, the main advantage of the present invention is that a high speed modulator which can be manufactured by a simple and relatively simple manufacturing process, and as an extension of the general electrode manufacturing process, does not require extra special shielding. It is possible.

[0013]

FIG. 1 shows the structure of a waveguide type optical device of the present invention. FIG. 1A is a perspective view of a waveguide type optical device, and FIG.
(B) is a sectional view taken along the line AA '. A titanium metal film having a width of 5 to 12 μm is deposited on the LiNbO ₃ crystal wafer 1 and internally diffused in the crystal at 900 to 1100 ° C. for 5 to 12 hours to give a width of 5 to 12 μm and a depth of 3-1.
Waveguides 2, 3 and 4 of 0 μm are formed. It consists of two Y-branch waveguides (on the input and output sides, which act as a power distributor at the input end 2 and a combiner at the output end 3) and a phase shifter section 4. The SiO ₂ buffer layer 5 is coated on the waveguide so as to reduce the TM mode loss when the electrode is formed on the waveguide. The thickness of the buffer layer is 0.3 to 10 μm.

This coplanar waveguide electrode structure has a width of 8 to 30.
μm and a length of 10 to 50 mm, and two ground electrodes 7 and 8 (width 100 to 5000 μm, length 10 to 10 μm).
50 mm, characteristic impedance 50 ohms).

This coplanar electrode can be manufactured by a general electrode manufacturing process described below. First, Si
On the O ₂ buffer layer 5, a layer of Cr-Au metal (Cr is 10
0-300 Å, Au 1000-300
0 angstrom) is deposited. On top of that, 2-20
A μm photoresist layer is spin-coated to form an electrode pattern in the photoresist layer. Next, thickness 3-1
A 5 μm coplanar electrode is formed by electroplating. A dielectric coating layer 9 having a thickness of 0.5 to 10 μm is deposited on this electrode, and a metal layer 10 is deposited thereon. Thus, the deposition of the dielectric on the coplanar electrode is an extension of the general electrode manufacturing process. Removal of photoresist and unnecessary Cr-
By dry etching of Au, a coplanar electrode structure having a dielectric coating layer on the electrode is formed. A metal contact 11 is formed between the metal layer 10 on the ground electrodes 7 and 8 and the metal layer 10 on the signal electrode 6. A 50 ohm resistor is connected to the output side of the electrode structure.

In FIG. 1A, the dielectric coating layer 9 and the metal layer 10 are omitted for the sake of simplicity.

This structure is analyzed using the excess relaxation method, and the capacitance, effective microwave index, characteristic impedance, band, etc. are calculated. FIG. 2 shows the calculated microwave index n _m as a function of the dielectric coating thickness. The horizontal axis shows the dielectric coating layer thickness D in the unit of μm, and the vertical axis shows the microwave index _nm . This microwave index is determined by the dielectric coating layer (eg, air, BaF ₂ , Mg).
Light wave index n when the thickness of F ₂ , SiO ₂ ) is about 4 μm
_It is almost equal to ₀ = 2.2.

FIG. 3 shows the calculated value of the characteristic impedance Z as a function of the thickness of the dielectric coating layer. The horizontal axis represents the dielectric layer thickness in μm units, and the vertical axis represents the characteristic impedance Z in Ω units.

FIG. 4 shows the calculated bandwidth as a function of the dielectric coating thickness D. The horizontal axis shows the dielectric layer thickness D in the unit of μm, and the vertical axis shows the bandwidth when the electrode length is 1 cm.
Shown in Hz.

What can be said from these values is that according to the present invention, a very high speed modulator can be realized. A bandwidth of 600 GHz (only with microwave loss limitation) can achieve a 1 cm long traveling wave coplanar electrode when a 3-5 μm thick BaF ₂ dielectric layer is deposited.

The present invention also includes an asymmetric traveling wave electrode guided device, and FIG. 5 shows an embodiment of a modulator / switch. 5A is a perspective view and FIG. 5B is A-.
FIG. 3 is a sectional view taken along the line A ′, and the same reference numerals as those in FIG. In FIG. 5A, the dielectric coating layer 9 and the metal layer 10 are omitted to simplify the drawing.

[Brief description of drawings]

FIG. 1 shows a waveguide type optical device according to an embodiment of the present invention.

The [2] Calculated for microwave index n _m, a diagram as a function of the dielectric coating layer thickness.

FIG. 3 shows the calculated characteristic impedance as a function of dielectric coating layer thickness.

FIG. 4 shows calculated bandwidth as a function of dielectric coating thickness.

FIG. 5 is a diagram showing an embodiment of the present invention in an asymmetrical traveling structure.

FIG. 6 shows a conventional type of modulator.

[Explanation of symbols]

 1 Wafer 2, 3, 4 Waveguide 5 Buffer Layer 6 Signal Electrode 7, 8 Ground Electrode 9 Dielectric Cover Layer 10 Metal Layer 11 Metal Contact

Claims (1)

[Claims] 1. At least one optical waveguide formed on a crystal substrate having an electro-optical effect, a buffer layer formed on the optical waveguide, at least two electrodes formed on the optical waveguide, and at least two electrodes formed on the buffer layer. A waveguide type optical device comprising a dielectric layer and a metal layer formed on the dielectric layer, wherein the metal layers are electrically short-circuited to each other.