JP5099347B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device having a substrate thickness of 15 μm or less.

Conventionally, in the optical communication field and the optical measurement field, an optical waveguide element in which an optical waveguide is formed on a substrate having an electro-optic effect, such as an optical switch or an optical modulator, is often used as a means for controlling an optical wave.
In order to increase the processing speed of the optical waveguide device, the optical waveguide device is used to perform speed matching between the light wave propagating through the optical waveguide and the electric signal for controlling the light wave, and to secure a power source capable of high-speed driving. Therefore, it is required to reduce the driving voltage of the above.

In Patent Document 1, for an optical waveguide element having a cross-sectional shape as shown in FIG. 1, while achieving a speed matching condition and a low electrode propagation loss with a thin electrode film thickness, a driving voltage Vπ · L (Vπ is a half wavelength) It is disclosed that the voltage L can reduce the length of the electrode (action part) to which the drive voltage is applied.
JP 2004-219600 A

  According to Patent Document 1, when an optical waveguide 2 is formed on a substrate 1 having an electro-optic effect, and a signal electrode 3 and a ground electrode 4 for controlling a light wave propagating through the optical waveguide 2 are arranged as shown in FIG. The ratio W / G of the width W of the signal electrode 3 to the gap G between the ground electrode 4 and the signal electrode 3 is set to 0.8 or more, so that the electrode propagation loss and the speed matching with the low-thickness electrode can be realized, It has been disclosed that a drive voltage can be achieved. Preferably, the above-described effect becomes more remarkable when the thickness T of the substrate body 1 is 20 μm or less. However, when the technique disclosed in Patent Document 1 is used, when the width W of the electrode is increased, the impedance of the signal line is lowered, and impedance mismatch is generated.

  On the other hand, in order to achieve speed matching, the substrate of the optical waveguide element is thinned. Generally, when the substrate is thinned, light confinement becomes stronger. For this reason, the width of the optical waveguide required to maintain the single mode is narrower than that of the optical waveguide element having a thick substrate. That is, when the thickness of the substrate is 100 μm or more, the width of the optical waveguide that maintains the single mode is about 6 to 10 μm. However, in the case of an optical waveguide element having a substrate thickness of 15 μm or less, it is possible to guide in a single mode even if the width of the optical waveguide is 6 μm or less.

In an optical waveguide device having a substrate thickness of 15 μm or less, it is desirable to reduce the distance between the signal electrode and the ground electrode in order to reduce the drive voltage. However, when the gap G, which is the distance between the electrodes, is reduced, the optical waveguide and the electrodes are close to each other, and light waves propagating through the optical waveguide are absorbed or scattered by the electrodes, which causes an increase in light propagation loss. Moreover, when the gap G is reduced, the impedance of the signal line is further reduced.
Further, when the interval between the plurality of optical waveguides is narrowed, the light wave is transferred as in the case of the directional coupler, and so-called crosstalk occurs.

  The problem to be solved by the present invention is to solve the above-described problems, and even when the thickness of the substrate having the electro-optic effect is 15 μm or less, the drive voltage Vπ is reduced, the optical loss is suppressed, and the impedance matching is performed. An optical waveguide device capable of achieving the above is provided.

In order to solve the above-mentioned problem, the invention according to claim 1 is made of a material having an electro-optic effect, having a thickness of 15 μm or less, and a high refractive index material doped into the substrate from the surface of the substrate. In an optical waveguide device having an optical waveguide to be formed and a control electrode for modulating and controlling a light wave propagating through the optical waveguide, the control electrode includes a signal electrode and a ground electrode arranged with the optical waveguide interposed therebetween. The ratio W / G between the width W of the signal electrode and the gap G between the signal electrode and the ground electrode is 0.7 or less, and the width W of the signal electrode is 15 μm or less Ri, the gap G between the signal electrode and the ground electrode is characterized der Rukoto than 18 [mu] m.

  The invention according to claim 2 is the optical waveguide element according to claim 1, wherein the distance d between the end of the optical waveguide and the end of the control electrode located closest to the end is 2 μm or more. It is characterized by being.

  According to the invention of claim 1, a substrate made of a material having an electro-optic effect and having a thickness of 15 μm or less, and an optical waveguide formed by doping a high refractive index material into the substrate from the surface of the substrate, An optical waveguide element having a control electrode for modulating and controlling a light wave propagating through the optical waveguide, wherein the control electrode is composed of a signal electrode and a ground electrode arranged with the optical waveguide interposed therebetween, and the signal electrode Since the ratio W / G between the width W of the signal and the gap G between the signal electrode and the ground electrode is 0.7 or less, the effective refractive index of the microwave is lowered, and the speed matching between the light wave and the microwave is achieved. Therefore, impedance matching can be realized while reducing the driving voltage. Furthermore, since the width W of the signal electrode is 15 μm or less, impedance matching can be realized more easily.

Further, according to the first aspect of the present invention, since the gap G between the signal electrode and the ground electrode is 18 μm or more, a suitable impedance of 35 to 55Ω can be realized.

  According to the invention of claim 2, since the distance d between the end of the optical waveguide and the end of the control electrode located closest to the end is 2 μm or more, the absorption and scattering of light waves by the electrode are suppressed. As a result, an increase in optical loss can be suppressed.

Hereinafter, the present invention will be described in detail using preferred examples.
FIG. 2 is a diagram showing an example of the optical waveguide device of the present invention.
The optical waveguide device of the present invention is made of a material having an electro-optic effect, and has a substrate 1 having a thickness T of 15 μm or less, and an optical waveguide formed by doping a high refractive index material into the substrate from the surface of the substrate 2 and a control electrode for modulating and controlling a light wave propagating through the optical waveguide, the control electrode includes a signal electrode 3 and a ground electrode 4 arranged with the optical waveguide 2 interposed therebetween. And the ratio W / G between the width W of the signal electrode and the gap G between the signal electrode and the ground electrode is 0.7 or less.

  With this configuration, it is possible to achieve impedance matching while reducing the drive voltage. In addition, it is effective for speed matching between light waves and microwaves, reduction of light loss due to electrodes, and suppression of crosstalk.

  As the substrate 1 having an electro-optic effect, for example, lithium niobate, lithium tantalate, and combinations thereof can be used. In particular, lithium niobate (LN) or lithium tantalate (LT) crystals having a high electro-optic effect are preferably used.

The optical waveguide can be formed by doping a high refractive index material such as Ti into the substrate from the substrate surface by a thermal diffusion method or a proton exchange method.
Note that the high refractive index material in the present invention means not only a material having a higher refractive index than the material constituting the substrate, but also by doping a specific material into the substrate so that the refractive index of the doped region is other than that. In the case where the refractive index is higher than that of the region, the specific material is also included in the high refractive index material.

As shown in FIG. 2, the control electrode is disposed so as to sandwich the optical waveguide 2 in order to control the modulation of the light wave propagating through the optical waveguide 2.
The control electrode can be formed on the front surface or the back surface of the substrate 1 by forming a Ti / Au electrode pattern, a gold plating method, or the like. The control electrode includes a signal electrode that propagates a modulation signal (AC signal or DC signal) and a ground electrode that is disposed around the signal electrode.

  In the optical waveguide device of the present invention, a buffer layer such as SiO 2 is not formed between the substrate 1 and the control electrode. The presence of the buffer layer effectively prevents the light wave propagating through the optical waveguide from being absorbed or scattered by the control electrode, and also guides the microwave applied from the control electrode and the inside of the optical waveguide. It also contributes to velocity matching with light waves. However, in the present invention, there is concern about an increase in driving voltage due to the presence of the buffer layer, absorption or scattering by the electrode is avoided by the arrangement relationship between the optical waveguide and the electrode, and the substrate is thinned for speed matching. It supplements with.

  In the optical waveguide device of the present invention, the distance d between the end portion of the optical waveguide shown in FIG. 2 and the end portion of the control electrode closest to the end portion is set to 2 μm or more, so Absorption and scattering are suppressed, and an increase in light loss is suppressed. When the distance d is less than 2 μm, the influence of light loss due to the electrodes becomes significant. Also, it should be noted that the driving voltage increases as the distance d increases.

  The gap G between the signal electrode and the ground electrode of the optical waveguide element shown in FIG. 2 is preferably 18 μm or more in order to realize a suitable impedance range of 35 to 55Ω, as will be described later.

  Next, with respect to the width W of the signal electrode of the optical waveguide element, by reducing W / G to 0.7 or less as in the present invention, the impedance decrease due to the decrease in the gap G can be reduced. By compensating for the increase, impedance matching is realized. In particular, the impedance can be increased more effectively by setting the width W of the signal electrode to 15 μm or less. However, in order to prevent crosstalk between the optical waveguides, it is necessary to secure a distance of about 5 times the wavelength of the propagating light wave. For this reason, in the case of a light wave having a wavelength of 1.5 μm that is usually used for communication, a safety factor of 20% is secured and the distance between the optical waveguides is 9 μm or more. If the distance d is 2 μm and the optical waveguide width is 3 μm, the electrode width W needs to be 2 μm or more.

  By setting the height of the control electrode of the light control element to 15 μm or more, the effective refractive index of the microwave can be lowered, and speed matching between the light wave and the microwave can be realized.

Next, in order to investigate the relationship between the ratio W / G and impedance matching for the optical waveguide element of the present invention, a simulation was performed on the optical waveguide element shown in FIG. 2 under the following conditions.
・ Substrate material: Lithium niobate ・ Substrate thickness: 15 μm
・ Optical waveguide width: 3μm
Control electrode (signal electrode and ground electrode) height: 18-30 μm
-Position of optical waveguide: intermediate between signal electrode and ground electrode-Width W of signal electrode: 2-16 μm
・ Gap G between signal electrode and ground electrode: 15 to 20 μm

FIG. 3 shows impedance values when the width W of the signal electrode and the gap G between the signal electrode and the ground electrode are changed.
The unit of W and G is “μm”, and the unit of impedance value, which is a number in the frame of the table of FIG. 3, is “Ω”.
The numbers in the frame, indicated by the numerical range of “32 to 40”, are the impedance values when the lower limit value is 30 μm and the upper limit value is 20 μm when the control electrode height is 20 μm. Impedance value.
Moreover, what the number in a frame showed as a single value is an impedance value in case the height of a control electrode is 20 micrometers.

  From FIG. 3, the impedance matching range (impedance value of 35 to 55Ω) is easily changed when the control electrode height is 20 to 30 μm, and the ratio W / G is easily present at 0.7 or less. Understood. In addition, when the gap G is 18 μm or more, it is possible to always achieve a suitable impedance value (35 to 55Ω) even if the height of the control electrode changes in the range of 20 to 30 μm. Even when the height of the control electrode is 18 μm, it has been confirmed that when the ratio W / G is 0.7 or less and the gap G is 18 or more, a suitable impedance value can be realized. .

  According to the present invention, there is provided an optical waveguide device capable of reducing the driving voltage Vπ, suppressing optical loss, and achieving impedance matching even when the thickness of the substrate having the electro-optic effect is 15 μm or less. Can do.

It is explanatory drawing which shows each parameter of the optical waveguide element disclosed by patent document 1. FIG. It is explanatory drawing which shows each parameter of the optical waveguide element of this invention. It is a table | surface which shows the impedance value with respect to ratio W / G.

1 Substrate 2 Optical waveguide 3 Signal electrode 4 Ground electrode

Claims (2)

A substrate made of a material having an electro-optic effect and having a thickness of 15 μm or less;
An optical waveguide formed by doping a high refractive index material into the substrate from the surface of the substrate;
In an optical waveguide device having a control electrode for modulating and controlling a light wave propagating through the optical waveguide,
The control electrode is composed of a signal electrode and a ground electrode arranged with the optical waveguide interposed therebetween,
The ratio W / G between the width W of the signal electrode and the gap G between the signal electrode and the ground electrode is 0.7 or less;
Width W of the signal electrode state, and are less than 15μm,
Gap G, an optical waveguide device characterized der Rukoto than 18μm between the signal electrode and the ground electrode.   2. The optical waveguide device according to claim 1, wherein a distance d between an end portion of the optical waveguide and an end portion of the control electrode closest to the end portion is 2 μm or more. element.

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