JP2003005142A - Optical waveguide type optical modulator and optical waveguide type optical frequency comb generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently generate an optical frequency comb. SOLUTION: An optical resonator 10 constituted by forming an incidence-end reflecting film 14A and a projection-end reflecting film 14B on two opposite end surfaces of an electrooptic crystal substrate 12 where an optical waveguide 11 passing beam light L1 causing optical resonance is formed is built in a cavity microwave resonator 20 surrounded with metal.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Conventionally, an optical frequency comb generator (Optical Frequency Comb
Comb Generator) is used. That is, when two laser lights are heterodyne-detected and the difference frequency is measured, the band is limited by the band of the light receiving element and is about several tens of GHz, so a wideband heterodyne is used by using an optical frequency comb generator. I am trying to build a detection system. The optical frequency comb generator generates several hundred sidebands of the incident laser light at equal frequency intervals, and the frequency stability of the generated sidebands is almost the same as that of the original laser light. Therefore, by heterodyne-detecting this sideband wave and the laser light to be measured, a wide-band heterodyne detection system over several THz can be constructed.

As the optical frequency comb generator, for example, an optical waveguide type optical frequency comb generator is used.

A conventional optical waveguide type optical frequency comb generator is
An optical resonator with mirrors formed on both end faces of a broadband optical waveguide type optical phase modulator is provided, and the optical waveguide type optical phase modulator is driven by an integral multiple of the free spectral range of the optical resonator to modulate laser light. Then the optical frequency comb was generated.

[0005]

However, in the conventional optical waveguide type optical frequency comb generator having the structure as described above,
A large amount of electric power is required to drive the broadband optical waveguide type optical phase modulator.

In order to reduce the power consumption, it is considered necessary to use a narrow band optical waveguide type optical phase modulator in which the electrodes have a microwave resonator structure. However, in the case where a resonator having an electrode structure of an optical waveguide type optical phase modulator without using an external metal structure is produced, for example, as shown in FIG.
In the optical modulator 300 using the substrate 302 of Y-cut LiNbO ₃ having a two-electrode structure in which No. 01 is formed, the electrodes 303A, 3
When the end of 03B is opened and microwaves are reflected at the open end to make an optical resonator, the electrode 3
Since 03A is thin and has a large electric resistance, Joule heat is generated and the microwave loses energy, so that high-quality microwave resonance cannot be obtained. That is, a high Q value cannot be obtained. Thinning the electrode 303A is a structure for obtaining impedance matching, for matching the speed of light transmitted through the electrode of the microwave and the speed of light, and for confining the electric field in a narrow range to obtain high modulation efficiency. It was not suitable for obtaining a high Q value. Further, when the electrode is made of a superconducting material or the like, a high Q value can be obtained, but it is not practical because it needs to be cooled to the liquid nitrogen temperature.

Therefore, in view of the conventional problems as described above, an object of the present invention is to provide an optical waveguide type optical modulator and an optical waveguide type optical frequency comb generator which are driven at low power.

[0008]

By the way, in a conventional bulk type optical frequency comb generator, as shown in FIG. 13, an electro-optic crystal substrate 410 forming a bulk type phase modulator is inserted into a cavity microwave resonator 420. High efficiency has been obtained by adopting a structure that is driven by.

In order to solve the above-mentioned conventional problems, the present invention reduces the electric resistance loss of microwave propagation by coupling with a cavity microwave resonator surrounded by a metal,
Further, the electric field is confined in a narrow range, high modulation efficiency is obtained, and the speed of light transmitted through the microwave electrode is matched with the speed of light, and as a result, a practical resonator type optical modulator having a high Q value is realized.

That is, the optical waveguide type optical modulator according to the present invention is characterized in that the electro-optical crystal substrate on which the optical waveguide for passing the light beam for optical modulation is formed is built in the cavity microwave resonator. And

In this optical waveguide type optical modulator, for example, an electrode coupled to the above-mentioned cavity microwave resonator is formed on an electro-optic crystal substrate so as to sandwich the optical waveguide, and the above electrode is referred to as a cavity microwave resonator. To combine.

In the above optical waveguide type optical modulator,
The electrodes are formed in a mesh shape, for example.

Further, in the above-mentioned optical waveguide type optical modulator, the cavity microwave resonator has, for example, a structure in which the speed of light transmitted through the electrode of the microwave is matched with the speed of light.

In the optical waveguide type optical frequency comb generator according to the present invention, the electro-optic crystal substrates on which the optical waveguide for transmitting the beam light causing the optical resonance is formed are opposed to each other.
It is characterized in that an optical resonator having an incident end reflection film and an emission end reflection film formed on one end face is built in a cavity microwave resonator.

[0015]

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

In the present invention, as shown in the basic structure of FIG. 1, by applying the cavity microwave resonator structure of the bulk type optical frequency comb generator to the optical waveguide type optical comb generator, a low power optical waveguide is obtained. A waveguide type optical comb generator (two-electrode structure) is realized.

That is, in the optical waveguide type optical comb generator 30 shown in FIG. 1, the electro-optic crystal substrate 12 on which the optical waveguide 11 for passing the beam light L1 causing optical resonance is formed.
The optical resonator 10 has an incident-end reflecting film 14A and an emitting-end reflecting film 14B formed on two opposite end faces thereof, and a cavity microwave resonator 20 surrounded by a metal. In the cavity microwave resonator 20.

The cavity microwave resonator 20 propagates a light beam L1 which causes optical resonance and applies an electric field to the optical waveguide 11 corresponding to each light beam L1. By resonating with the microwave signal supplied to the antenna, an electric field corresponding to the microwave signal is applied to the optical waveguide 11.

The electro-optic crystal substrate 12 is an optical material substrate such as lithium niobate (LiNbO ₃ ) capable of phase modulating light with a voltage.

A light beam L1 as a fundamental wave is incident on the optical crystal substrate 12 via an incident end reflection film 14A, and is resonated by a Fabry-Perot etalon composed of an incident end reflection film 14A and an emission end reflection film 14B. An optical waveguide 11 is formed in which a part of the light beam L1 is emitted as an optical frequency comb LC1 via the emitting end reflection film 14B.

The incident end reflection film 14A and the emission end reflection film 1
4B is a reflecting mirror having a slight transmittance,
Chromium, gold, aluminum or a dielectric multilayer film is deposited on the two end faces of the electro-optic crystal substrate 12 facing each other.

Since the optical waveguide 11 formed on the electro-optic crystal substrate 12 is built in the cavity microwave resonator 20, the cavity microwave resonator 20 resonates with a microwave signal, so that An electric field corresponding to the microwave signal is applied, and the refractive index changes according to the microwave signal. Thereby, the optical waveguide 11 functions as an optical phase modulator that performs optical phase modulation according to the microwave signal on the light beam L1 as the fundamental wave that is incident through the incident end reflection film 14A.

In the optical waveguide type optical comb generator 30 in which the optical resonator 10 having such a structure is built in the cavity microwave resonator 20, the optical waveguide 11 is provided via the incident end reflection film 14A.
The light beam L1 as the fundamental wave incident on the optical beam can be phase-modulated in accordance with the microwave signal, the phase of the light beam L1 is modulated, and the optical frequency comb is transmitted through the emitting end reflection film 14B. LC1 can be emitted.

In the case where the microwave propagation mode in the cavity microwave resonator 20 is the TE mode,
The electric field is applied to the crystal in the vertical direction in the figure, but the electric field is applied almost uniformly to the entire crystal, and it is not the characteristic that the electric field is concentrated in the partial region where light passes, which is an advantage of the optical waveguide modulator.
Therefore, it is considered that the electric field is not concentrated on the optical waveguide 11 and the modulation does not have sufficiently low power. Further, there arises a problem of how to install the electro-optic modulator formed on the planar substrate in the cavity microwave resonator 20. Since an actual optical waveguide is formed on a thin flat substrate, it is difficult to form the structure shown in FIG.

Therefore, in a practical optical waveguide type optical comb generator 130, the optical waveguide 111 of the optical resonator 110 is formed on a thin flat electro-optic crystal substrate 112 as shown in FIG. Then, the optical resonator 110 is inserted into the cavity microwave resonator 120 suitable for the planar substrate so that the electrodes 115A and 115B are formed on the electro-optic crystal substrate 112 and coupled with the cavity microwave resonator structure.

Then, as shown in FIG.
Cavity microwave resonator 120 sandwiched between a and 120b
Is the width on the opposite side of the optical waveguide 111.
The thickness is made sufficiently larger than the thickness of 12 so that a flat substrate can be easily installed. With such a configuration, even if the electro-optic crystal substrate 112 is thin, it can be easily installed. Further, the cavity microwave resonator 120 includes an open cavity microwave resonator 120A shown in FIG. 3A and a non-open cavity microwave resonator 120B shown in FIG. 3B. The microwave resonator 120A tends to the low frequency and the non-open type microwave resonator 120B tends to the high frequency, but in principle it may be different.

Here, as the electrodes 115A and 115B, full-surface electrodes are used. FIG. 4 shows the calculation result of the electric field strength distribution at this time. It can be seen that the electric field is concentrated in the center (the part where light passes). The use of the electrode having such a structure makes it possible to reduce the current loss with respect to the current from the cavity microwave resonance part because the current passes through the electrode having a wide area.

In the above, the electro-optic crystal substrate 11 is previously prepared.
It has been explained that the formation of the auxiliary electrodes 115A and 115B on the electrode 2 helps to concentrate the electric field. However, an electric field also leaks to the opposite side of the optical waveguide 111 of the cavity microwave resonator 120 to some extent. The leakage of the electric field occurs up to the range of the thickness of the electro-optic crystal substrate 112. The leakage electric field can be reduced by reducing the thickness of the electro-optic crystal substrate 112. As a result, a large electric field concentration can be obtained. However, thinning the electro-optical crystal substrate 112 may deteriorate mechanical strength, and it is necessary to minimize the thinning of the electro-optical crystal substrate 112.

Therefore, by adopting a net-like electrode structure as shown in FIGS. 5A, 5B, and 5C as the electrodes 115A and 115B, the unnecessary lines of electric force are partially reduced. And gradually reduce the capacitance.

That is, the mesh electrodes 115A, 115
As shown in FIG. 6, B acts as a line for passing a current from the cavity microwave resonator 120 for increasing the electric field strength of a portion through which light passes, but electric lines of force are generated only under the electrodes, Electric lines of force do not occur in the space part of. Therefore, on average, the number of lines of electric force is reduced, and the electric field concentration can be increased in the portion through which light passes. The effect of the periodic structure can be eliminated by making the spacing between the mesh electrodes sufficiently smaller than the wavelength of the microwave. The range of forming the mesh-like electrode is sufficient within the range from the portion through which light passes to the thickness of the electro-optic crystal substrate 112.

Further, in this structure, the microwave is transmitted to the electrode 11
It is possible to match the speed of light transmitted through 5A and 115B with the speed of light, confine the electric field in a narrow range, and obtain high modulation efficiency. The electrostatic capacity of the electrodes 115A and 115B is 2
FIG. 7 and FIG. 8 show the phase velocity of the microwave when the distribution is about 50 pF / m as the equivalent refractive index.

In FIG. 7, H = 0.015 m, W = 0.00
The equivalent refractive index characteristics A and B in the case where the open cavity microwave resonator 120A and the non-open cavity microwave resonator 120B of 04 m are adopted are shown. The speed of light is 2.18 when expressed by the equivalent refractive index in the crystal, and is expressed by a straight line in FIG. 7. At approximately 10 GHz, the equivalent refractive index of microwaves matches the equivalent refractive index of light, and at this time, the speed of microwaves transmitted through the electrodes 115A and 115B and the speed of light match. This frequency can be changed by changing the difference between the open cavity microwave resonator 120A and the non-open cavity microwave resonator 120B and the values of H and W. For example, FIG. 8 shows a case where the open microwave cavity resonator 120B has a W
It shows the equivalent refractive index characteristics when the value of is changed.
It can be seen that the range in which the equivalent refractive index is 2.18 can be changed from 5 GHz to 20 GHz by changing W.

The optical waveguide type optical comb generator 13 described above
0, the cavity microwave resonator 120 sandwiched between the metal flat plates 120a and 120b was used. However, in the optical waveguide type optical comb generator 230 shown in FIGS. Electrode 215 in which a part of the metal flat plate constituting 220 is formed on electro-optic crystal substrate 212
The optical waveguide 21 has a structure in which A and 215B are combined.
The other side of 1 is completely free. The optical waveguide 211 side is provided with electrodes 215A, 2 on the surface of the electro-optic crystal substrate 212.
A cavity microwave resonator 220 is formed between 15B and the recessed metal block 220A provided. In this cavity microwave resonator 220, a microwave input for excitation is directly coupled to the inside of the cavity by a coaxial line,
An excitation cavity is formed.

The optical waveguide type optical comb generator 230 having such a structure is used in the cavity microwave resonator 2 sandwiched between metal flat plates.
20 is easy to create, and it is advantageous for downsizing. In this optical waveguide type optical comb generator 230, the surface of the cavity microwave resonator 220 perpendicular to the light traveling direction is an open end. This is to excite a mode that has an open end at the end face as a mode required for the optical frequency comb generator. In this case, assuming that the size of the cavity is adjusted so that the speed of light transmitted through the electrode of the microwave and the speed of light are matched, this cavity microwave resonance is an integral multiple of the free spectrum region of the optical resonator 210. This will match the resonance frequency of the device 220.

Furthermore, in the optical waveguide type optical comb generator 230 having such a structure, as shown in FIG. 11, the metal block 220 closed on one side of the cavity microwave resonator 220.
An opening 220a is provided in A and the cavity microwave resonator 220 is provided.
By adopting an open structure, it becomes possible to easily apply a DC bias.

[0036]

As described above in detail, in the waveguide type optical modulator according to the present invention, the electro-optic crystal substrate on which the optical waveguide for passing the light beam for optical modulation is formed is a cavity microwave resonator. The electric resistance loss due to the propagation of microwaves can be reduced by incorporating it in and coupling it.

Further, in the optical waveguide optical modulator according to the present invention, the electrodes formed on the electro-optic crystal substrate so as to sandwich the optical waveguide are combined with the cavity microwave resonator. A high modulation efficiency can be obtained by confining the electric field in a narrow range.

Further, in the optical waveguide type optical modulator according to the present invention, the electric field concentration on the optical waveguide portion through which light passes can be increased by forming the electrode in a mesh shape.

Further, in the optical waveguide type optical modulator according to the present invention, the cavity microwave resonator has a structure in which the speed of light transmitted through the electrode of the microwave and the speed of light are matched, and thus a high Q value is practically used. Optical modulator can efficiently perform optical modulation.

That is, according to the present invention, by coupling with a cavity microwave resonator surrounded by a metal, the electric resistance loss of microwave propagation is reduced, and the electric field is confined in a narrow range to obtain high modulation efficiency. To provide an optical waveguide type optical modulator capable of efficiently performing optical modulation by a practical resonator type optical modulator having a high Q value by matching the speed of light transmitted through a microwave electrode with the speed of light. You can

Further, in the optical waveguide type optical frequency comb generator according to the present invention, the incident end reflections are made on the two opposite end faces of the electro-optic crystal substrate on which the optical waveguide for passing the beam light causing the optical resonance is formed. An optical resonator consisting of a film and a reflection film at the output end is built in and coupled to a cavity microwave resonator to reduce the electrical resistance loss of microwave propagation and efficiently generate an optical frequency comb. be able to.

[0042]

[Brief description of drawings]

FIG. 1 is a perspective view schematically showing the basic configuration of an optical waveguide type optical frequency comb generator according to the present invention.

FIG. 2 is an enlarged perspective view showing a main part of a practical optical waveguide type optical comb generator.

FIG. 3 is an external perspective view of an open cavity microwave resonator and a open cavity microwave resonator having a practical structure.

FIG. 4 is an electric field intensity distribution in the optical waveguide type optical comb generator having the above practical structure, in which an electrode coupled to a cavity microwave resonator is formed on an electro-optic crystal substrate so as to sandwich the optical waveguide. It is a figure which shows the calculation result of.

FIG. 5 is a diagram schematically showing a mesh electrode structure adopted in the optical waveguide type optical comb generator having the above-mentioned practical structure.

FIG. 6 is a diagram schematically showing lines of electric force passing through an electro-optic crystal substrate in an optical waveguide type optical comb generator adopting the mesh electrode structure.

FIG. 7 is a characteristic diagram showing an equivalent refractive index characteristic of an electro-optic crystal substrate when an open type cavity microwave resonator and a non-open type cavity microwave resonator are adopted.

FIG. 8 is a characteristic diagram showing equivalent refractive index characteristics of an electro-optic crystal substrate when the value of W is changed in the open-type cavity microwave resonator.

FIG. 9 is a diagram schematically showing another configuration example of the optical waveguide type optical comb generator according to the present invention.

FIG. 10 is a sectional view of an essential part of the optical waveguide type optical comb generator.

FIG. 11 is a cross-sectional view of essential parts showing a modified example of the optical waveguide type optical comb generator.

FIG. 12 is a diagram schematically showing a configuration of a conventional optical waveguide type optical comb generator.

FIG. 13 is a diagram schematically showing a configuration of a conventional bulk type optical frequency comb generator.

[Explanation of symbols]

10,110,210 Optical resonator, 11,111,21
DESCRIPTION OF SYMBOLS 1 Optical waveguide, 12, 112, 212 Electro-optic crystal substrate, 14A Incident end reflection film, 14B Emission end reflection film, 2
0,220 cavity microwave resonator, 30,130,2
30 optical waveguide type optical comb generator, 115A, 115B,
215A, 215B electrode, 120a, 120b metal plate, 120 cavity microwave resonator, 120A open cavity microwave resonator, 120B non-open cavity microwave resonator, 220A metal block, 220a opening

   ─────────────────────────────────────────────────── ─── 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 Bang Bang Widy Tomoko             3034-3 Tsuda-cho, Midori-ku, Yokohama-shi, Kanagawa F-term (reference) 2H047 KA03 MA03 NA02 QA03 RA08                       TA01 TA11 TA31                 2H079 AA02 BA04 CA04 CA24 EA03                       EB01 EB04 HA11

Claims (5)

[Claims] 1. An optical waveguide type optical modulator, characterized in that an electro-optic crystal substrate having an optical waveguide through which a light beam for optical modulation is formed is built in a cavity microwave resonator. 2. An electrode coupled to a cavity microwave resonator is formed on an electro-optic crystal substrate so as to sandwich an optical waveguide,
2. The optical waveguide type optical modulator according to claim 1, wherein the electrode is coupled to a cavity microwave resonator. 3. The optical waveguide type optical modulator according to claim 2, wherein the electrodes are formed in a mesh shape. 4. The optical waveguide type optical modulator according to claim 3, wherein the cavity microwave resonator has a speed at which the microwave is transmitted through the electrode and a speed at which light is matched. 5. An optical resonator in which an incident end reflection film and an emission end reflection film are formed on two facing end faces of an electro-optic crystal substrate on which an optical waveguide for transmitting a beam of light that causes optical resonance is formed. An optical waveguide type optical frequency comb generator, characterized in that is built in a cavity microwave resonator.