JP2003295141A - Waveguide type optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To secure a high modulation efficiency with a simple circuit constitution by reducing microwave loss due to electrode resistance. <P>SOLUTION: Considering that the microwave loss can be reduced while maintaining the high modulation efficiency by applying a modulation electric field through an electrode having the reduced current resistance, an electrode 16 consisting of a wide area 16a and a narrow area 16b is arranged to be parallel with the waveguide 12 resonating light so that the wide area 16a has a period matching with a half wavelength of the microwave oscillated from an oscillation part 18. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical resonator and an optical frequency comb generator, and relates to a standard light source with high coherence at multiple wavelengths such as optical communication, optical CT, optical frequency standard, or coherence between wavelengths. It is applied to the fields that need a light source that can also be used.

[0002]

2. Description of the Related Art Optical resonators are applied to various optical application techniques such as optical frequency measurement and minute signal detection in optical communication. Especially with the recent development of optoelectronics,
In order to meet the demands for laser light control for frequency-division communication and frequency measurement of absorption lines distributed over a wide range, waveguide-type optical modulators that modulate light resonated in a waveguide are widely used. There is.

A conventional configuration example of such a waveguide type optical modulator will be described in detail. FIG. 6 shows a conventional waveguide type optical modulator 7. This waveguide type optical modulator 7 includes a substrate 71, a waveguide 72, a buffer layer 73,
Incident side reflection film 74, emission side reflection film 75, and electrode 76
And a power supply unit 77.

The substrate 71 is grown by, for example, a pulling method.
3-4 inch diameter LiNbO _ThreeAnd GaAs, etc.
The crystal is cut into a wafer. This cutout
On the formed substrate 71, due to proton exchange, titanium diffusion, etc.
In order to grow the waveguide 72, mechanical polishing, chemical polishing, etc.
There is also a case where the treatment of.

The waveguide 72 is formed of Ti on the substrate 71.
Etc. are formed by doping. This waveguide 72
Has a higher refractive index than other layers including the substrate 71 for propagating incident light. Buffer layer 73
Is made of, for example, SiO ₂ and is laminated on the waveguide 72.

Incident side reflection film 74 and emission side reflection film 75
Are so-called reflection mirrors respectively formed on the end faces of the waveguide 72, and resonate by reflecting the light propagating through the waveguide 72 back and forth. By the way, this incident side reflection film 7
4 and the emitting side reflection film 75 are, for example, a dielectric multilayer film having a reflectance close to 100%, and can be obtained by alternately laminating thin films having different refractive indexes. In addition, in order to take out the resonated light to the outside, the reflectance of the emitting side reflection film 75 may be set to be slightly lower than 100%.

The electrode 76 is arranged on the upper part of the buffer layer to apply phase modulation to the light propagating through the waveguide 72, and has a microstrip line structure such as a coplanar strip. By feeding microwaves to the electrode 76 formed of the microstrip line via the feeding portion 77 formed of, for example, a coaxial cable, an electric field corresponding to the voltage and the electrode width can be generated below the electrode 76. . Since the generated electric field changes the refractive index of the waveguide 72, it is possible to modulate the phase of the light that resonates in the waveguide 72.

Here, in order to perform modulation in a wide band, FIG.
A traveling wave type electrode 76 as shown in FIG. The electrode 76 is designed so that the velocity of light propagating in the waveguide 72 and the velocity of microwaves supplied from an oscillator (not shown) and propagating in the electrode 76 can be matched.

On the other hand, when a wide band characteristic is not particularly required in the above-mentioned modulation, the structure shown in FIG. 6 makes it possible to generate a high electric field by resonating the microwave. Since the higher modulation efficiency can be obtained by using the crystal forming the waveguide 72 over a long region, the electrode 76 is parallel to the waveguide 72 and matches the length of the waveguide 72. To be extended.

By the way, if the electrode 76 is extended too long, the resistance of the electrode 76 itself becomes excessive and the loss of the microwave supplied from the oscillator (not shown) becomes large.

FIG. 8 shows the result of calculating the loss of the microwave propagating in the electrode 76 in the structure in which the electrode 76 is extended as shown in FIG. In the calculation shown in FIG. 8, the influence of power feeding by the power feeding unit 77 is negligible for the voltage distribution, current distribution, electric field distribution, and resistance loss distribution of the electrodes. It is assumed that both ends of the electrode 76 are open, and a resonance state occurs when the wavelength λ of the microwave propagating in the electrode 76 satisfies the relationship of 2L = Nλ with respect to the electrode 76 having the length L. In order to perform the calculation assuming that the modulation efficiency is higher, the velocity of the microwave propagating in the electrode 76 and the velocity of light propagating in the waveguide 72 are equal.

Here, the distance from the light incident side reflection film is x
In order to assume that the wavelength λ of the microwave is equal to the length L of the electrode 72, N is set to 2, and the maximum voltage is V _0.
Then, the distribution of the voltage V shown in FIG. 8A is V = V ₀
It is represented by Cos (2πx / L) Sin (ωt) (By the way, the component of Sin (ωt) representing time change is 1). The electric field intensity at the electrode 76 is also uniform because the electrode width is uniform, so that it is similar to the voltage distribution as shown in FIG. The distribution of the current I at the electrode 76 is, as shown in FIG. 8 (c), I = I ₀ Cos
It is represented by (2πx / L) Sin (ωt + φ). Here, I ₀ is a factor determined by the characteristic impedance Z of the electrode 76 and is represented by I ₀ = V ₀ / Z. Also φ
Is the phase difference between current and voltage.

Further, the resistance loss Ls on the electrode 76 shown in FIG. 8 (d) is proportional to the square of the current, and Ls = Ls ₀ Cos
It is represented by (2πx / L) ² . Here, Ls ₀ = RI ²
/ 2, and R is the electric resistance per unit length of the electrode 76. The resistance loss Ls shown in FIG. 8 (d) increases in a cycle that coincides with the half wavelength λ / 2 of the microwave. Therefore, it is necessary to control the length of the electrode 76 to be equal to or less than the half wavelength λ / 2 of the microwave. By the way, in order to ensure high modulation efficiency by utilizing the crystal forming the waveguide 72 over a long region, as shown in FIG. 9, a plurality of rows of electrodes 76 shortened to a half wavelength λ / 2 of the microwave or less are used. It had to be installed.

[0014]

However, in the configuration in which the electrodes 76 are arranged in a row as shown in FIG. 9, it is necessary to provide a plurality of power feeding portions 77, so that a plurality of microwaves supplied from an oscillator (not shown) are used. Since it has to be divided and the phase between the divided microwaves needs to be controlled, there is a problem that the circuit configuration becomes complicated. Further, such a complicated circuit configuration may rather cause an increase in microwave loss, and eventually cause deterioration in modulation efficiency.

Therefore, the present invention has been devised in view of the above-mentioned problems, and it is a waveguide type optical system that can reduce microwave loss due to electrode resistance and can secure high modulation efficiency with a simple circuit configuration. It is intended to provide a modulator.

[0016]

The present inventor has noticed that by applying a modulation electric field through an electrode having a reduced current resistance, it is possible to reduce microwave loss while maintaining high modulation efficiency. Then, an electrode composed of a wide region and a narrow region is provided in parallel with the waveguide that resonates the light so that the wide region has a period corresponding to the half wavelength of the microwave oscillated from the oscillating means. Invented a waveguide type optical modulator.

That is, in order to solve the above-mentioned problems, the waveguide type optical modulator according to the present invention has an oscillating means for oscillating a microwave having a predetermined wavelength and a refractive index when an electric field is applied. , Which is composed of an electro-optic crystal that changes the phase of the incident light and modulates the phase of the incident light according to the wavelength of the microwave, and is arranged in parallel with the light modulating means, and includes a wide region and a narrow region. An electrode is provided for applying an electric field to the light modulating means based on the microwave oscillated by the oscillating means, and the electrode has the wide width in a cycle corresponding to a half wavelength of the microwave oscillated by the oscillating means It is characterized in that a region is provided.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing the configuration of a waveguide type optical modulator 1 to which the present invention is applied. This waveguide type optical modulator 1
Is a substrate 11, a waveguide 12, a buffer layer 13, an incident side reflection film 14, an emission side reflection film 15, an electrode 16,
The power feeding unit 17 and the oscillating unit 18 are provided.

The substrate 11 is grown by, for example, a pulling method.
3-4 inch diameter LiNbO _ThreeAnd GaAs, etc.
The crystal is cut into a wafer. This cutout
On the formed substrate 11, it is possible to use proton exchange, titanium diffusion, etc.
Mechanical polishing, chemical polishing, etc. to grow the waveguide 12.
There is also a case where the treatment of.

The waveguide 12 is formed of Ti on the substrate 11.
Etc. are formed by doping. This waveguide 12
Has a higher refractive index than other layers including the substrate 11 for propagating the incident light. Buffer layer 13
Is made of, for example, SiO ₂ and is laminated on the waveguide 12.

Incident side reflection film 14 and emission side reflection film 15
Are so-called reflection mirrors respectively formed on the end faces of the waveguide 12, and resonate by reflecting the light propagating through the waveguide 12 back and forth. By the way, this incident side reflection film 1
4 and the emission side reflection film 15 are, for example, a dielectric multilayer film having a reflectance close to 100%, and are obtained by alternately laminating thin films having different refractive indexes. Since the resonated light is extracted to the outside, the reflectance of the emitting side reflection film 15 is set to be slightly lower than 100%.

The waveguide type optical modulator 1 to which the present invention is applied can also be applied to a structure in which the incident side reflection film 14 and the emission side reflection film 15 are not provided. As a result, the light incident on the waveguide 12 is transmitted as it is without being reflected back and forth inside the waveguide.

The electrode 16 is arranged on the upper part of the buffer layer to apply phase modulation to the light propagating through the waveguide 12, and has a microstrip line structure such as a coplanar strip. By supplying the microwave oscillated from the oscillation unit 18 to the electrode 16 formed of the microstrip line through the power supply unit 17 formed of, for example, a coaxial cable, the microwave is oscillated according to the voltage and the electrode width below the electrode 16. Generated electric field can be generated. Since the generated electric field changes the refractive index of the waveguide 12, it is possible to modulate the phase of the light that resonates in the waveguide 12. By the way, a ground is provided around the electrode 16.

Since it is possible to obtain a higher modulation efficiency by using the crystal forming the waveguide 12 over a long region, the electrode 16 should be parallel to the waveguide 12 and the length of the waveguide 12. Can be extended to match. In addition, the electrode 16 has a wide region (hereinafter,
This region has a wide region 16a) and a narrow region (hereinafter, this region is referred to as a narrow region 16b). By the way, the wide region is set wider than the width of the waveguide in the example of the AA ′ cross-sectional view, and has a width of, for example, about 100.
It has a size of μm. The narrow region 16b is set to have a length substantially equal to the width of the waveguide and has a width of about 10 μm.

Thus, the wide area 16a and the narrow area 1
By alternately providing 6b, the electric resistance of the electrode 16 itself can be changed for each region. In particular, as in the example shown in FIG. 1, the electrode width of the wide region 16a is set to the narrow region 16a.
If the electrode width of b is 10 times, the resistance can be reduced to 1/10.

Next, the loss of the microwave propagating in the electrode 16 composed of the wide area 16a and the narrow area 16b will be described. FIG. 2 shows the loss of microwaves propagating in the electrode 16 and the like.

In the calculation shown in FIG. 2, it is assumed that the influence of the power feeding by the power feeding section 17 can be ignored for the voltage distribution, current distribution, electric field distribution, and resistance loss distribution of the electrodes. Further, it is assumed that both ends of the electrode 16 are open, and the electrode 16 having the length L is in a resonance state when the wavelength λ of the microwave propagating in the electrode 16 satisfies the relationship of 2L = Nλ. In order to perform the calculation assuming that the modulation efficiency is higher, the velocity of the microwave propagating in the electrode 16 and the velocity of the light propagating in the waveguide 12 are equal. Further, the electrode spacing is adjusted so that the characteristic impedance and the microwave velocity of the entire electrode 16 are the same as those of the electrodes of the conventional waveguide type optical modulator configured by only the narrow region. That is, it is adjusted so that the microwave is in resonance.

Here, the distance from the light incident side reflection film is x
In order to assume the case where the wavelength λ of the microwave is equal to the length L of the electrode 72, N is set to 2. Furthermore, the maximum voltage is V
_When it is set to ₀ , the distribution of the voltage V shown in FIG.
It is represented by ₀ Cos (2πx / L) Sin (ωt) (by the way, the component of Sin (ωt) representing time change is 1). That is, since it is assumed that the characteristic impedance and the microwave velocity of the electrode 16 as a whole are the same as those of the electrode composed of only the narrow region, the voltage distribution has the same tendency as that of the conventional waveguide type optical modulator. Becomes

As for the distribution of the current I in the electrode 16, as shown in FIG. 2 (c), I = I ₀ Cos (2πx
/ L) Sin (ωt + φ). Where I ₀ is
It is a factor determined by the characteristic impedance Z of the electrode 76 and is represented by I ₀ = V ₀ / Z, and φ is a phase difference between current and voltage. Since the current distribution shown in FIG. 2 (c) depends on the tendency of the voltage, it has the same tendency as that of the conventional waveguide type optical modulator. Since the voltage distribution and the current distribution shown in FIG. 2 are calculated on the premise of the resonance state, the voltage V and the current I whose polarities are inverted are actually distributed.

In addition, x = L / 4,3 at which the current flows most
The voltage is low in the region near L / 4. Therefore, even if the current loss is reduced at the expense of the modulation in the area, in other words, even if the modulation efficiency in the area is reduced, it is possible to reduce the influence on the overall modulation efficiency. Therefore, the region near x = L / 4, 3L / 4 is set as the wide region. Thereby, the current resistance can be reduced, so that the microwave loss can be reduced.

Incidentally, the resistance loss Ls on the electrode 16 is proportional to the square of the current, and Ls = Ls ₀ Cos (2πx / L)
It is represented by ² . Here is _Ls 0 = ^RI 2/2, when the electric resistance per unit length of the R electrode 76, the microwave loss in the wide region 16a composed of 10 times the electrode width of the reduced width region 16b, as shown in FIG. It is expressed as 2 (d). As a result of reducing the current resistance by providing the wide region 16a,
As shown in Fig. 2 (d), the half wavelength λ / 2 of the microwave
The microwave loss can be greatly reduced with a cycle that corresponds to.

Further, since the width of the electrode 16 is widened, the number of lines of electric force per unit area is reduced, and therefore, FIG.
As shown in (b), the electric field becomes small in the wide region 16a.

That is, while the microwave loss can be reduced, the electric field generated by the microwaves through the electrode 16 is lowered, and as a result, the modulation efficiency is lowered.

FIG. 3 shows the calculation results of the microwave loss on the electrode 16 and the change in the modulation efficiency, assuming that the voltage applied to the electrode 16 is constant. In FIG. 3, the horizontal axis B / L indicates the wide area 1
The ratio of the length B of 6a to the length L of the entire resistor 16 is shown. As the ratio of the wide region 16a is increased, the microwave loss due to the decrease in electric resistance is decreased, and the electric field strength is also locally decreased as shown in FIG. 2B, so that the modulation efficiency is also decreased. However, the degree of decrease in modulation efficiency is slower than the degree of decrease in microwave loss. That is, it is suggested that by providing the wide region 16a, it is possible to reduce the microwave loss while suppressing the reduction rate of the modulation efficiency to be low.

Further, in FIG. 4, it is assumed that the characteristic impedance of the entire electrode 16 is controlled so that the input power can be entirely consumed inside the waveguide type optical modulator 1. The result of calculation of the modulation efficiency is shown assuming that it is due to the resistance of the electrode 16. In FIG. 4 showing the modulation efficiency when the power is constant,
When the value of B / L is about 0.65, the maximum modulation efficiency is 1.8.
It turns out that This suggests that the modulation efficiency decreases when standardized by the voltage, but the modulation efficiency increases when standardized by the electric power supplied to the entire waveguide type optical modulator 1. Is. Of course, the maximum value of the modulation efficiency can be further improved by adjusting the widths of the wide region 16a and the narrow region 16b.

As described in detail above, the waveguide type optical modulator 1 according to the present invention can apply a modulation electric field through the electrode 16 having the wide region 16a with reduced current resistance. , Microwave loss can be reduced. In addition, the wide area 16a corresponding to the wavelength of the microwave
By controlling the position of the narrow region 16b and the narrow region 16b, it is possible to reduce the loss of microwaves and further suppress the deterioration of the modulation efficiency, and it is possible to improve the modulation efficiency when the power is standardized. . Also, the microwave wavelength λ
In addition to the case where is equal to the length L of the electrode, it is possible to suppress the microwave loss by providing a plurality of wide regions with a period matched with the half wavelength λ / 2 of the microwave.

That is, in the waveguide type optical modulator 1 to which the present invention is applied, even if the superconducting material is not used as the material of the electrode,
By using a normal metal electrode such as Au, the loss of microwaves can be reduced and high modulation efficiency can be secured. Further, even when high modulation efficiency is expected by utilizing the crystal forming the waveguide 72 over a long region, the microwave loss is reduced by providing the wide region 16a without providing the plurality of feeding portions 17. be able to. Therefore, the waveguide type optical modulator 1 to which the present invention is applied can solve the problem that the circuit configuration becomes complicated.

The present invention is not limited to the above embodiment. The electrode width of the wide region 16a is not limited to 10 times the electrode width of the narrow region 16b, and may be any number. Further, the shape of the wide region 16a is not limited to the embodiment, and it may be possible to deal with it by providing a taper or the like. Further, not only the case of controlling the wide region 16a, but also the configuration aiming at the same effect by controlling the shape of the narrow region 16b is included.

Further, the structure of the electrodes applied to the present invention can be applied to the optical phase modulator and also to the optical intensity modulator. Further, in the present invention, the loss of microwaves is suppressed by providing the wide region 16a and the narrow region 16b in which the width of the electrode 16 is changed due to the configuration.
The same effect can be expected by changing the thickness of the electrode 16 instead of the width.

A waveguide type optical modulator 1 to which the present invention is applied
Further, the configuration of the electrode 16 as shown in FIG. 5 may be adopted. Note that the power supply unit 17 is omitted in FIG.

In FIG. 5A, the ground provided around the electrode 16 is provided at the bottom of the substrate 11. The example shown in FIG. 5B is a case where a two-electrode coplanar line is used. Further, the example shown in FIG. 5C is a case where a coplanar strip line is used as the electrode 16. Further, the example shown in FIG. 5D is a case where a slot line with a mesh is used as the electrode 16. In the slot line with a mesh, the capacitance can be reduced while maintaining a narrow electrode interval, but the resistance of the mesh portion becomes large on the contrary. Therefore, in the high current region, the mesh portion is omitted and the electrode interval is increased to cope with the problem.

[0043]

As described above in detail, in the waveguide type optical modulator to which the present invention is applied, the electrode having the wide region and the narrow region is provided so as to be parallel to the waveguide for resonating the light. With a simple circuit configuration, it is possible to reduce microwave loss while maintaining high modulation efficiency.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a configuration example of a waveguide type optical modulator to which the present invention is applied.

FIG. 2 is a diagram showing a loss and the like of microwaves propagating in an electrode.

FIG. 3 is a diagram showing a calculation result of a microwave loss on an electrode and a change in modulation efficiency when it is assumed that a voltage applied to the electrode is constant.

FIG. 4 is a diagram showing the modulation efficiency when the power is constant.

FIG. 5 is a diagram illustrating electrodes in a waveguide type optical modulator to which the present invention is applied.

FIG. 6 is a diagram showing a conventional configuration example of a waveguide type optical modulator.

FIG. 7 is a diagram for explaining a case where a traveling wave type electrode is used.

FIG. 8 is a diagram showing the loss of microwaves propagating in the electrodes of the conventional waveguide type optical modulator.

FIG. 9 is a diagram for explaining a case where a plurality of rows of electrodes shortened to a half wavelength λ / 2 or less of microwaves are provided in a row.

[Explanation of reference numerals] 1 waveguide type optical modulator, 11 substrate, 12 waveguide, 1
3 buffer layer, 14 incident side reflection film, 15 emission side reflection film, 16 electrode, 16a wide area, 16b narrow area,
17 power feeding section, 18 oscillating section

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Osamu Nakamoto             3-3-4 Kamata, Ota-ku, Tokyo (72) Inventor Shigeyoshi Misawa             2-5-8 Denenchofu, Ota-ku, Tokyo (72) Inventor Yoshinori Nakayama             3-43-9 Okusawa, Setagaya-ku, Tokyo F-term (reference) 2H079 AA02 AA12 BA03 CA24 DA03                       EA03 EB12                 2K002 AA02 AB12 AB40 BA06 CA03                       DA06 EA03 EB05 HA03

Claims (3)

[Claims] 1. An oscillating means for oscillating a microwave having a predetermined wavelength, and an electro-optic crystal whose refractive index changes when an electric field is applied. The phase of incident light is set to the wavelength of the microwave. And a light modulating means arranged to be parallel to the light modulating means, comprising a wide area and a narrow area, and applying an electric field to the light modulating means based on the microwave oscillated from the oscillating means. The waveguide type optical modulator, wherein the wide area is provided in the electrode at a period according to a half wavelength of the microwave oscillated from the oscillating means. 2. A resonance means, which comprises an incident side reflection film and an emission side reflection film which are parallel to each other, resonates light incident through the incident side reflection film, and the light modulation means comprises the incident side reflection film. 2. A film and the emitting side reflection film are respectively formed at a light incident end and a light emitting end, and modulate the phase of the light resonated in the resonance means according to the wavelength of the microwave. Waveguide type optical modulator. 3. The waveguide type optical modulator according to claim 1, wherein the wide region is provided in a region of the electrode where a large current flows and a low voltage is applied.